UNCLASSIFIED NON-PROPRIETARY - Distribution Statement A

Final Report

Chrome Replacements

for

Internals and Small Parts

Contract #: F33615-95-C5615

Subcontract #: 4400002135

Project #: 01-0824-38-5025-000

Principal Investigator: Keith O. Legg

 :DUZLFN /DQH /LEHUW\YLOOH ,/  86$  )D[  ZZZURZDQFDWDO\VWFRP
Executive Summary

This document is a Technology Analysis assessing the current state of the art in coatings and

surface treatments that could be used to replace hard chrome plating on internal diameters (IDs)

and heat-sensitive aircraft components. Its purpose is to determine which technologies and

materials are viable options for the Joint Strike Fighter (JSF), and estimate what developments

will be needed to bring these options to production at original equipment manufacturers (OEMs)

and overhaul and repair (O&R) sites, most of which are military depots. The analysis included

assessment of written information, including scientific and technical data and trade literature,

combined with extensive discussions with manufacturing engineers, development engineers and

scientists, depot personnel, and equipment and service providers.

In the Part A we summarize how hard chrome is currently used on aircraft components,

illustrating with components that are typical of ID usage. In Part B we outline the most important

criteria for hard chrome replacement in both the OEM and O&R environments, the most critical

of which is an O&R ability to strip and rebuild worn components. We summarize the

technologies we have considered, showing which ones are most likely to fit the criteria in the

short (2 year), intermediate (3-5 year), and long term (>5 year). Part C provides details of each

of the technologies we have considered, which include

• thermal spray (including HVOF, plasma, nanoparticle)

• electroplating and electroless plating (including alloys and composites)

• chemical and physical vapor deposition (including plasma and combustion CVD, and

sputtering)

• laser processing (including laser cladding and alloying)

• weld coating (including micro-arc deposition and explosive cladding)

• heat treatment and ion implantation (including gas and plasma nitriding and carburizing,

and beam and plasma souce implantation).

We conclude that the only method that is fully developed and available immediately is

electroless nickel and its composites. However, since Ni, like Cr, is on the EPA-17 toxic

substances list, this method is not recommended, except as a stopgap measure.

For the short term, alloy plating and Co-composite plating are both possible options. These

methods are the closest drop-ins for existing hard chrome plating. However, most of the

commonly-used electroplated alloys are Ni-based, and composites pose quite severe uniformity

and handling problems, especially in the O&R environment. The better (cleaner and higher

performance) solution in the short term, we believe, is a combination of plasma spray and

electrospark deposition, both of which can be used to deposit WC-Co, the material of choice for

OD chrome replacement. This combination should solve almost all existing usage, except

landing gear main cylinder IDs.

This shortfall can be solved in the intermediate term by the use of small and nanoparticle spray

methods, which should allow plasma spray to meet all the criteria for both OEM and O&R use. A

second intermediate term solution is ID sputter coating, provided that the coating materials

useful for rebuild have adequate properties, the method can be adapted to existing IVD

equipment in use at OEMs and depots, and the deposition rates and costs are reasonable.

In the long term two CVD methods have promise - combustion CVD and plasma CVD - but it is

not clear at this point that the materials that could be deposited by these methods will meet the

requirements for component rebuilding.
Acknowledgements

We would like to thank the many people whose assistance and input have contributed to this

study, including Jean Hawkins (NAVAIR), Don Parker (NADEP-Jacksonville), Bruce Sartwell

(NRL), Roger Eybel (Messier-Dowty), Steven Gaydos (Boeing, St. Louis), Bruce Bodger (Sulzer

Metco), Jean-Gabriel Legoux (NRC), John Quets (Praxair Surface Technologies), Michael

Feldstein (Surface Technology, Inc.), John Kelley (ASAP, Inc.), and Steven Rolander (SAIC),

and all the members of the US and Canadian HCAT teams.
i

Table of Contents

PART A - HARD CHROME USAGE.................................................................................................... 1

1. Introduction............................................................................................................................. 2

2. Current usage of ID hard chrome............................................................................................ 3

2.1. Typical ID Coated Components........................................................................................ 3

2.1.1. Introduction................................................................................................................ 3

2.1.2. General items ............................................................................................................ 3

2.1.3. Specific components from Messier-Dowty and Boeing ............................................... 5

2.1.4. Summary of component coating requirements ......................................................... 10

PART B - SUMMARY OF REPLACEMENT CRITERIA AND OPTIONS ....................................................... 12

3. OEM and O&R requirements ................................................................................................ 13

4. Hard chrome replacement criteria......................................................................................... 13

5. Summary of options.............................................................................................................. 14

PART C - PROCESS SUMMARIES................................................................................................... 18

6. Process Summary - Thermal Spray ...................................................................................... 19

6.1. Introduction .................................................................................................................... 20

6.2. Process description ........................................................................................................ 20

6.2.1. High Velocity Oxy-Fuel (HVOF) ............................................................................... 21

6.2.2. Plasma spray ........................................................................................................... 22

6.3. State of development ..................................................................................................... 22

6.3.1. ID coating ................................................................................................................ 23

6.3.2. Coating materials and properties.............................................................................. 25

6.4. Capabilities and limitations............................................................................................. 2 5

6.5. Other issues ................................................................................................................... 26

6.6. Developments needed.................................................................................................... 26

6.7. Recommendations ......................................................................................................... 28

6.7.1. HVOF ...................................................................................................................... 28

6.7.2. Plasma spray ........................................................................................................... 28

7. Process Summary - Electroplating ........................................................................................ 30

7.1. Introduction .................................................................................................................... 31

7.2. Process Description........................................................................................................ 32

7.2.1. Trivalent chrome plating........................................................................................... 32

7.2.2. Composite electroplating.......................................................................................... 33

7.2.3. Alloy electroplating................................................................................................... 33
ii

7.2.4. Brush plating............................................................................................................ 34

7.2.5. State of development............................................................................................... 35

7.3. Coating materials and properties .................................................................................... 35

7.4. Capabilities .................................................................................................................... 35

7.5. Limitations...................................................................................................................... 36

7.6. Fit with OEMs and depots............................................................................................... 36

7.7. Developments needed.................................................................................................... 37

7.8. Recommendations ......................................................................................................... 37

8. Process Summary - Electroless nickel .................................................................................. 38

8.1. Introduction .................................................................................................................... 39

8.2. Process description ........................................................................................................ 40

8.2.1. State of development............................................................................................... 41

8.2.2. Coating materials and properties.............................................................................. 41

8.3. Electroless Ni composites............................................................................................... 42

8.4. Fit with OEMs and depots............................................................................................... 44

8.5. Typical costs and cost factors......................................................................................... 44

8.6. Advantages and capabilities ........................................................................................... 44

8.7. Disadvantages and limitations ........................................................................................ 44

8.8. Developments needed.................................................................................................... 45

8.9. Recommendations ......................................................................................................... 46

9. Process Summary - Chemical Vapor Deposition (CVD) ........................................................ 47

9.1. Introduction .................................................................................................................... 48

9.2. Process description ........................................................................................................ 49

9.2.1. Thermal CVD........................................................................................................... 49

9.2.2. Metal-organic CVD (MOCVD) .................................................................................. 49

9.2.3. Plasma and Laser CVD (PCVD and LACVD)............................................................ 50

9.2.4. Combustion CVD (CCVD) ........................................................................................ 51

9.3. State of development ..................................................................................................... 52

9.4. Coating materials and properties .................................................................................... 52

9.5. Capabilities .................................................................................................................... 53

9.6. Limitations...................................................................................................................... 53

9.7. Developments needed.................................................................................................... 54

9.8. Recommendations ......................................................................................................... 54

10. Process Summary - Physical Vapor Deposition (PVD) ........................................................ 55

10.1. Introduction .................................................................................................................. 56

10.2. Process description ...................................................................................................... 56

10.2.1. ID sputtering........................................................................................................... 57
iii

10.2.2. Plasma jet.............................................................................................................. 57

10.2.3. Exploding wire........................................................................................................ 58

10.2.4. High rate chrome deposition by PVD...................................................................... 58

10.2.5. Ion beam assisted deposition (IBAD, IBED)............................................................ 59

10.2.6. State of development............................................................................................. 59

10.2.7. Coating materials and properties............................................................................ 60

10.3. Capabilities................................................................................................................... 62

10.4. Limitations.................................................................................................................... 62

10.5. Fit with OEMs and depots............................................................................................. 63

10.6. Typical costs and cost factors....................................................................................... 63

10.7. Developments needed.................................................................................................. 64

10.8. Recommendations........................................................................................................ 65

11. Process Summary - Laser Processing................................................................................. 66

11.1. Introduction .................................................................................................................. 67

11.2. Process description ...................................................................................................... 68

11.2.1. Laser cladding........................................................................................................ 68

11.2.2. Laser Induced Surface Improvement (LISI) ............................................................ 68

11.2.3. QQC Diamond ....................................................................................................... 69

11.2.4. Laser glazing and peening...................................................................................... 69

11.2.5. State of development............................................................................................. 69

11.2.6. Coating materials and properties............................................................................ 69

11.3. Capabilities and advantages......................................................................................... 69

11.4. Limitations.................................................................................................................... 70

11.5. Fit with OEMs and depots............................................................................................. 70

11.6. Developments needed.................................................................................................. 71

11.7. Recommendations........................................................................................................ 71

12. Process Summary - Weld Coating ...................................................................................... 72

12.1. Introduction .................................................................................................................. 73

12.2. Process description ...................................................................................................... 74

12.2.1. Micro-arc welding (Electrospark deposition, ESD) .................................................. 74

12.2.2. Explosive bonding (cladding) ................................................................................. 75

12.2.3. State of development............................................................................................. 75

12.3. Coating materials and properties .................................................................................. 75

12.4. Capabilities................................................................................................................... 76

12.5. Limitations.................................................................................................................... 76

12.6. Fit with OEMs and depots............................................................................................. 77

12.7. Typical costs and cost factors....................................................................................... 78
iv

12.8. Developments needed.................................................................................................. 78

12.9. Recommendations........................................................................................................ 78

13. Process Summary - Heat treating and Ion Implantation....................................................... 79

13.1. Introduction .................................................................................................................. 80

13.2. Process description ...................................................................................................... 81

13.2.1. Heat treating .......................................................................................................... 81

13.2.2. Ion implantation ..................................................................................................... 81

13.2.3. State of development............................................................................................. 81

13.3. Capabilities................................................................................................................... 82

13.4. Limitations.................................................................................................................... 82

13.5. Fit with OEMs and depots............................................................................................. 82

13.6. Typical costs and cost factors....................................................................................... 83

13.7. Developments needed.................................................................................................. 83

13.8. Recommendations........................................................................................................ 83
Page 1

PART A - HARD CHROME USAGE

In this section we give an overview of current chrome usage, including examples of typical

components that currently use ID chrome plating.
Page 2

1. Introduction

Hard chrome plating is presently used throughout the aerospace industry. At the original

equipment manufacturer (OEM) level, usage is primarily for wear resistance, some corrosion

resistance, and accurate sizing of components. For IDs, thin dense chrome is frequently used

for wear and corrosion resistance, especially in landing gear. At the overhaul and repair (O&R)

level the primary use of hard chrome is to rebuild worn components, on both ODs and IDs. The

environmental problems of chrome lie primarily in hexavalent chrome air emissions from plating

tanks. Secondarily, chrome-contaminated waste must be cleaned up before discharge, making

rinse water and chrome stripping a secondary source of pollution at military depots.

High Velocity Oxy-Fuel thermal spray (HVOF) has been used in aircraft engines for some years.

The method is now being adopted widely in industry in general, and the aerospace industry in

particular, as a replacement for electrolytic hard chrome plating (EHC). For the majority of

aerospace items HVOF is expected to be cost effective and to perform well. Airlines such as

Lufthansa, United, and Delta are flight testing HVOF-coated components, and are beginning to

adopt HVOF for O&R of landing gear and other items. Manufacturers, such as Boeing

Commercial, have used various kinds of HVOF coatings for problem solving on quite a number

of components.

HVOF is ideal for line-of-sight applications, including the outer diameters (ODs) of long tubes

such as hydraulic and landing gear pistons. It can also be used for shallow internal diameters

(IDs) - of depth approximately equal to the diameter - where the flame can be angled into the ID

at 45° or more to the surface (and in some cases it is possible to coat deeper by angling as close

as 20° to the surface). This means that there are some chrome plating applications that do not

lend themselves to HVOF. These include not only IDs of long tubes, such as hydraulic outer

cylinders, but also OD coating on small or thin-walled items (especially those made of low

temperature alloys such as aluminum) that tend to overheat (above the tempering temperature).

The consequences of overheating tend to be loss of tensile or fatigue strength, which is very

serious for critical items such as landing gear.

Items not amenable to HVOF coating presently account for about 20% to 30% of DoD depot

maintenance work volume, and a replacement process must be found if the Joint Strike Fighter

(JSF) is to be chrome-free. Even ID chrome can itself be a problem - thin dense chrome

deposition is a difficult process, prone to failure, that can be done by only a few suppliers.

For the DoD depots, HVOF is being demonstrated and validated by the Hard Chrome

Alternatives Team (HCAT). The HCAT includes GE Aircraft Engines, Boeing, NAVAIR, and

O&R depots from all three services, including NADEPs-Jacksonville, Cherry Point, and North

Island, Ogden and Oklahoma City ALCs, NTS-McClellan AFB, and Corpus Christi Army Depot.

All three Canadian landing gear manufacturers - Menasco, Messier-Dowty, and Heroux - are

teaming with various laboratories and vendors in a Canadian HCAT (C-HCAT), which is being

funded by Technology Partnerships Canada (TRP) and the Canadian Department of National

Defence (DND). The Program Lead for the HCAT is Bruce Sartwell of the Naval Research Lab,

while the C-HCAT is a joint effort with leads at each of the major contractors. The HCAT

Technical Lead is Keith Legg, who is also designated to be the technical coordinator and data

custodian for the C-HCAT.

The purpose of this program has been to evaluate commercial and pre-commercial chrome

alternative technologies and identify those that show the greatest promise for those applications

where HVOF cannot be used - primarily IDs, and secondarily heat-sensitive components. Each

technology is covered in a Technology Summary in a manner designed to make it possible to

compare them directly. At the beginning of this report we identify the types of components that

are presently not amenable to HVOF and summarize the criteria we have used to assess the

alternatives.
Page 3

2. Current usage of ID hard chrome

2.1. Typical ID Coated Components

2.1.1. Introduction

Since the JSF has not yet been designed, we do not know exactly what will be the geometry and

materials in the final aircraft. However, by looking at what is currently in general use for military

aircraft, we have identified a fairly broad range of different types of items that are frequently

chrome plated, but that are not easily amenable to HVOF. In addition, engineers at Boeing and

Messier-Dowty have identified specific items on existing systems that cannot be readily HVOF

coated.

2.1.2. General items

The following geometries are generally problematic for HVOF:

• Holes (blind and through) in hydraulics, including landing gear and actuators

• Pins such as those used in landing gear and actuators

• Lugs and other items where the coating is external, but access to the area is difficult

2.1.2.1. Blind and through holes in hydraulic cylinders

Shallow holes or shallow areas within deep holes can be coated with standard HVOF methods.

Only on the largest aircraft are the landing

gear cylinders large enough in diameter to

accommodate a standard HVOF ID gun,

which can coat inside holes only above

about 11” ID. Outer cylinder IDs of both

landing gear and actuators are often

chrome plated for wear resistance (against

seals) and for corrosion resistance against

water contamination in the fluid. In many

cases the coating material used is thin

dense chrome rather than standard

chrome plate. Unlike standard chrome

plate, thin dense chrome is quite difficult

to plate and there are very few vendors capable of doing it.

Approx

1 dia

Approx

2.5 dia 20o

 45o

Figure 1. Small ID coating by HVOF.
Page 4

Figure 2 shows a cross section that is quite common in landing gear outer cylinders (although

many are simpler than this, with only a single ID). In this case the coating is used only on the

working surfaces, and is specified as uncoated on

transition areas. When chrome plating cylinders of this

type, Heroux uses conformal anodes with insulators to

shield the areas where coating is not required. This avoids

the complications and waste streams involved in masking

difficult areas within the ID. Any replacement coating must

either work without shields or it must be able to be

shielded easily. Alternatively (and with more difficulty for

existing parts) it may be possible to specify that the

coating be permitted on the non-working surfaces.

Most of these items are basically cylindrical, although

hydraulic actuators are often set into a block that contains

many different actuators and other hydraulic components

(Figure 3). Landing gear typically are several feet long

and have large IDs, whereas hydraulics are typically no more than 12 inches long with an inch or

so ID. Landing gear outer cylinders are usually made of 300M (or, for the Navy, AerMet 100)

high strength steel or 7075 aluminum alloys. Hydraulic actuator outer cylinders are typically

7075 aluminum or 4340 high strength steel.

Some landing gear pins are ID chrome

plated. This is not often done, and when it is,

it is usually to solve a problem such as

internal corrosion or fretting and wear at the

ends from retainers that fit into the ID. In

most cases, however, these items are grease

packed to prevent corrosion and end caps are

locked in place to prevent wear. In this case,

the main demand on the coating is at the

ends of the ID, where a protective coating

can be deposited with standard thermal spray guns. The ID may be able to be treated by other

methods for corrosion. These items represent the smallest IDs we have identified, and because

these types of items are seldom ID chromed, we do not expect this to be a problem for the JSF,

with proper design.

Chrome plate

No chrome plate

4 - 8” LG

1 - 2” hydr

24 - 60” LG

6 - 12” hydr

Figure 2. Complex landing gear outer cylinder cross-section schematic.

Figure 3. Hydraulic actuator

block schematic.

wear wear corrosion 1” ID

4”

Figure 4. Landing gear pin - ID chromed.
Page 5

2.1.2.2. Other difficult geometries

Not all exterior surfaces will be easy to reach with an HVOF gun. Some items have areas that

can be readily chrome plated but that will be difficult to access with typical HVOF equipment

For example, lugs on landing gear outer cylinders are frequently chrome plated on the outer flat

face to resist wear by retaining caps (Figure 5). In most cases these lugs are positioned so that

they can be reached fairly easily and coated with a robotic gun. In some cases, however, the

face to be coated is either angled toward the cylinder, making it difficult to reach, or is close to

other lugs or parts of the component. In either case, the geometry is complicated and the space

in which we can work is limited to about an inch. In most cases, dual lugs are chromed only on

the outside, but occasionally we have seen lugs chromed in more difficult areas, where the

surface to be coated is partially obscured, as it is in the right hand side of Figure 5.

2.1.3. Specific components from Messier-Dowty and Boeing

Internal chrome plating is most prevalent on landing gear components and hydraulic actuators.

A number of different types of these components have been identified with Messier-Dowty and

Boeing as posing various problems for standard HVOF coating, either because they require ID

coating or because access to the area to be coated is difficult. This is by no means an

exhaustive catalog of components, but is meant to illustrate the major categories of problems

that might be encountered.

Cylinder

Chrome

Lug Lugs

Chrome

Cylinder

Figure 5. Schematic of landing gear lugs.
Page 6

2.1.3.1. Actuators and Landing gear cylinders

A typical fighter main landing gear outer cylinder is shown in

Figure 7. This 4.9” ID item has only one open end and must be

coated over a depth of about 2 feet. Note also that the coating

is specified to break at the internal ID step, which is quite

common with this type of item and requires proper masking.

A much larger landing gear outer cylinder (for the C-17) is

shown in Figure 6. In this case, not only is the ID large, but the

chrome is required close to the open end, making it fairly easy

to HVOF spray at an angle from the open end. In this case the

spray angle is 50° to the axis, making a high quality coating

possible.

Figure 8 shows a rather typical flight surface actuator - the aileron actuator from an F-18. The

2.5” ID bore has a total depth of about 7” and is accessible only from one end. The chrome is

resin sealed for corrosion resistance.

Flight surface actuators are frequently smaller in diameter than that of Figure 8 - with some

smaller actuators having IDs of about 1”.

Landing gear and landing gear actuators built by Messier-Dowty frequently call out standard hard

chrome (0.005” thick plate) on the OD and thin dense chrome (0.0005 - 0.0008” thick on the ID).

8”

4.75”

6.3”

Cr 0.001-0.002”

Figure 6. C-17 NLG cylinder bore,

300M steel. Manufactured by B.F.

Goodrich. Courtesy Boeing.

4.9”

23.5”

Cr 0.002”

Figure 7. F-15E MLG outer cylinder bore. B.F. Goodrich,

300M steel. Courtesy Boeing.

2.5”

5.7”

Figure 8. F/A-18 E/F aileron servocylinder, manufactured by HR Textron - Courtesy

Boeing.
Page 7

The inner cylinder of a high strength steel actuator of this type is shown in Figure 9. The OD

chrome plate can be readily replaced by HVOF coatings. Its purpose is to provide a wear

resistant surface to bear against the guide bushing and to prevent wear of the outer cylinder

seals. The thin dense chrome on the inside is for a combination of corrosion resistance and

wear resistance against internal seals.

The ID chrome cannot be replaced by HVOF WC-Co using current HVOF equipment. It could

be replaced by plasma sprayed WC-Co, since the ID will accommodate a standard plasma spray

ID gun. However, in order to make the internal coating a drop-in replacement, the coating would

have to be less than 0.001” thick, with low enough porosity to prevent corrosion or leak-by, and

the gun would need to coat both the walls and the end. Since the wall thickness is only 0.12”,

heat from the gun would have to be dissipated effectively, which could presumably be done by

cooling both the inside and the outside of the cylinder.

A larger landing gear actuator inner cylinder is shown in Figure 10. It is very similar to that of

Figure 9, but is about twice the size. Any method used to coat this item internally would require

a reach of about 6 feet, and would need to coat approximately 1,100 square inches uniformly

and cost-effectively. Again, the OD chrome is readily replaced by HVOF WC-Co.

Thin dense Cr

entire inside

3.5” ID

16”

0.120 wall

OD Cr

0.004”

Figure 9. Landing gear actuator inner cylinder, (300M high strength steel). Courtesy

Messier-Dowty.
Page 8

We should also note that in both of these actuators the ID is a blind hole. Some coating

methods have more difficulty with blind holes - for example, thermal sprays must have overspray

powder (i.e. powder that is not melted and bounces back off the part) rapidly swept out of the

area to avoid their entrainment in the coating. This is fairly easy to do with open-ended tubes,

but difficult with blind holes. For PVD coatings the present design of post magnetron systems

requires access from both ends, although new designs are in development to overcome this

limitation.

2.1.3.2. Landing Gear Pins

In contrast to these large items,

landing gear pins are some of the

smallest landing gear components

that are frequently chrome plated.

In most cases pins are chromed

only on the outside to resist wear

and galling. In some cases,

however, the IDs of pins must be

coated to protect them from wear

against locking devices and end

caps, and to provide additional

corrosion protection. Figure 11

shows such an internally chromed

pin. This is the smallest diameter

that we have found for landing gear

components.

The 1.21” diameter recesses on the

ends of such a pin could be readily

coated by an HVOF gun angled into

the ID. The 1.02” ID is more difficult

to HVOF spray. However, the area

could be reached with a torch angled at 30° to the axis of the pin. While this is far from an ideal

impingement angle and does reduce the quality of the coating (increased porosity and reduced

adhesion), coating quality is frequently adequate under these conditions, since high wear is not

an issue. If needed for corrosion prevention, the deepest HVOF coating could be polymersealed, which is often done with chrome plating. Therefore we believe that this component could

be HVOF coated inside and out.

Thin dense Cr

entire inside

6.8” ID

0.236 - 350” wall

52”

OD Cr

0.004”

Figure 10. Landing gear actuator inner cylinder - courtesy Messier-Dowty (300M steel).

1.02” ID 1.21” ID

2.90”

0.0025” Cr

0.002” Cr

Figure 11. 300M pin for MLG shock strut piston -

courtesty Messier-Dowty.
Page 9

In contrast to the pin shown in Figure 11, the F/A-18

nose landing gear torque arm pin shown in Figure 12 is

the same diameter, but longer, and is unlikely to be ID

coated reliably with current HVOF equipment, since the

spray angle required to reach the center would be

about 20° to the axis. GEAE reports that in some

cases HVOF spray can be done at this angle, but the

reliability of such a coating within the narrow confines

of this component would be suspect.

2.1.3.3. Other components

Most other components do not have deep IDs such as

those found in actuators and landing gear. However,

some do have small IDs, areas that are difficult to

reach, or areas where heat dissipation could be a

problem. In most cases these components do

not pose serious problems for HVOF coating, but

in some, standard HVOF equipment is not

capable of coating them easily or reliably.

Error! Reference source not found. is an

example of an item that, while not trivial to

HVOF spray, could be coated with existing

equipment. The four lug holes, which require

0.002” of chrome plate, can be coated by rotating

the component in front of the flame, although

care must be taken to prevent overheating.

Some landing gear components of this general

shape, however, require chrome plate on the

faces of the lugs, as shown by the arrows on the

right of the figure. In these cases, access is quite

difficult, and standard HVOF equipment would

not be able to reach these areas easily.

Another lug is shown in Figure 14. In this case,

HVOF coating of the sleeve ID would be

relatively simple. However, ID coating of the lug

hole would be much more difficult because of the

depth of the hole and the fact that it is almost

closed on one end. Plasma spray coating of this

part would be possible, since the diameter

exceeds 3”.

A different type of situation is illustrated in

Figure 15. The area is readily accessible from

below provided the rest of the part does not

interfere with the gun. Access from above would

be restricted by the ID “overhang”, which shields

the top of the ID coating area. This type of

component could be satisfactorily thermal spray

coated using an ID plasma spray gun brought in

from above.

1.04” 1.4”

5”

Cr 0.002”

Figure 12. F/A-18 E/F NLG torque

arm pin, Aermet 100. Manufactured

by Messier-Dowty. Courtesy

Boeing.

Cr 0.001-0.002”

4.4”

0.8” ID

0.5” ID

Figure 13. F-15 Torque arm lugs. 300M

steel, Menasco/Goodrich. Courtesy

Boeing.

Cr 0.002-0.0035

cutouts

5.13 ID

2.5”

Figure 14. C-17 MLG swivel pin.
Page 10

This item illustrates a further

consideration in ID coating.

Note that there are cutouts

which are required to be free

of coating, and must,

therefore, be shielded to

prevent their being sprayed.

While this shielding is

somewhat easier than that

typically used for chrome

plating, its design and

fabrication (typically of thin

steel sheet) is something that

needs to be considered in

designing a part for ID spray

coating.

2.1.4. Summary of component coating requirements

The primary parameters of the different types of components are summarized in Table 1,

together with applicability of non-HVOF thermal sprays. The fact that HVOF can only be used

for components above 11” ID restricts the current HVOF method to only the largest landing gear,

or to shallow IDs whose depth is no greater than about one diameter. Plasma spray could be

used for almost all landing gear outer cylinder IDs, but is currently too porous. This problem

might well be solvable by the use of small- or nano-particle spray methods (which should also

permit coatings to be thinner).

2.438” ID

3.7”

4.68” ID

4.7”

Cr 0.0015”

LUG SLEEVE

Cr 0.0015”

Figure 15. C-17 MLG brace lugs. 300M, Goodrich .

Courtesy Boeing.

Table 1. Components and most widely applicable thermal spray coating technologies.

Component Typical Typical size HVOF shortfall Other thermal spray

methods

materials range Plasma

spray

Nano

spray

Landing gear

cylinders

300M, Aermet

100, 7075

aluminum

5”-12” ID

24” - 60” long

Diameter too

small, depth

too large

Too

porous

?

May be

too large

Actuators 4340 1”-3” ID

6”-12” long

Diameter too

small, depth

too large

 

Pins 300M, 4340 1”-2” ID

3”-6” long

Diameter too

small

Too

small



Other 300M, 4340,

7075

aluminum

Variable Partially

hidden lugs,

thin walls, etc

 
Page 11

Some other general observations on coating requirements are also of importance:

• While OEM coating thicknesses are indicated in the figures above, depot rebuild may

require thicker coatings - up to 0.010” in final thickness (0.020” pre-ground thickness).

Therefore, coating technologies must span a wider range (0.0003” - 0.020”) than is

typical with OD coatings.

• Extreme wear and corrosion resistance are usually not required for ID coatings - i.e. their

mechanical and chemical properties are not generally as stringent as for ODs.

• Very high density (low porosity) coatings are required for landing gear cylinder IDs to

prevent gas leakage at their high operating pressures. This precludes standard plasma

sprays.
Page 12

PART B - SUMMARY OF REPLACEMENT CRITERIA

AND OPTIONS

In this section we summarize the differences between OEM and O&R use, the criteria that a

viable ID chrome replacement must meet, and our findings on which options are the most viable

or the most easily brought to production for ID chrome replacement.
Page 13

3. OEM and O&R requirements

The requirements for OEM and O&R chrome replacement are somewhat different. One of the

primary differences, although difficult to quantify, is that at the OEM the processing environment

is quite well defined, whereas in O&R operations many different components must be processed,

each with its own unique problems. Some of the major differences are summarized in Table 2.

One issue to consider very carefully is whether a process for OEM can also be used for O&R.

Although in principle there is no reason that O&R processes cannot be different from OEM

processes, it makes validation and acceptance doubly expensive and time-consuming if two

different processes must be validated, rather than a single one accepted by OEMs, DoD

stakeholders, and depots alike. This is an especially important issue in view of the wide range of

coating requirements for ID chrome replacement, from 0.0003” to 0.015” thick. Some processes

are good for OEM use but cannot be used for rebuild, while others cannot reliably deposit a

coating thinner than about 0.001”.

4. Hard chrome replacement criteria

The following is drawn from the Brooks AFB Statement of Need describing the basic

requirements for a chrome replacement1

:

“A coating process (or processes) is needed that will meet the requirements of chromium without

the environmental and health hazards associated with chromic acid. Ideally, the process would

not use any EPA 17 chemicals. The process must not cause hydrogen embrittlement. Fatigue

loss should be no worse than electro-deposited chromium. The deposit should exhibit adhesion

to steel equivalent to electro-deposited chromium. The deposit must be machinable or grindable

to produce surface finishes of approximately 8 rms. The deposit must be easily strippable. Good

corrosion protection would be a plus. The finished surface must have low friction characteristics

and must not gall. The process should be relatively easy to control. It should not require large

amounts of capital to install and should fit into existing space.”

Replacing hard chrome involves a great deal more than meeting the technical requirements for

wear, corrosion, fatigue, etc. To be viable in the aerospace community the replacement must fit

into the way the industry works at both the OEM and O&R level (commercial shops and military

 

1

 “USAF FY96/97 Environment, Safety & Occupational Health Research, Development &

Acquisition Strategic Plan”, Published by the Human Systems Center, Brooks AFB (1996).

Table 2. Some differences between OEM and O&R chrome replacement.

OEM O&R

Limited numbers of different components,

processed in significant quantities on a regular

basis. Standard production lines

Many different components, sporadic work

loads

New substrate material; only new coatings

need be stripped in case of processing errors

Old, dirty substrates; old coating must be

stripped prior to recoat. Coating must

withstand component cleaning and servicing

Coating thickness typically 0.0003” (thin dense

chrome) to 0.004”

Coating thickness up to 0.020” as-coated,

0.010” finished, for rebuild

Approvals from OEM engineers Approvals from OEMs, NAVAIR, ATCOM,

Single Item Managers, Program Managers
Page 14

depots). Technical performance cannot be considered in isolation from other technical issues

such as stripping and finishing, from environmental and safety issues, from issues of complexity

and cost, or from the more “political” issues of acceptance and validation. The most important

criteria that an alternative must meet to replace chrome successfully on IDs are summarized in

Table 3, page 16. There are many more detailed issues, but these will in general be different for

each different application.

The HCAT team started out with the approach that, to be viable, a hard chrome alternative must

meet the same performance standards as hard chrome in all critical areas (as we have indicated

in Table 3). As a practical matter, however, the team has found that the alternative must exceed

the performance of hard chrome. If it does not, there is no strong driver to specify the

replacement, since all changes of this type involve both cost and risk. In general, environmental

drivers are very weak, especially in an industry as complex as aerospace, where responsibility

for change is diffused and decision makers are often not directly affected by their decisions.

However an alternative will be strongly and rapidly embraced by engineers and other

stakeholders if the replacement provides a clear technical and cost benefit, and especially if it

provides some critical capability that chrome lacks.

5. Summary of options

The options for chrome replacement are summarized in Table 4, page 17. The assessment of

each technology is based on how well it meets the criteria of Section 4 and on what it would take

to bring it to production on IDs and heat-sensitive components. Our overall assessments are as

follows:

For immediate use - The only immediately viable option is electroless nickel. Electroless nickel

composites are also good technical options but will need development and validation, and may

not be suitable for O&R. However, because Ni is on the EPA-17 list we do not recommend this

approach except if needed for the short term.

Short term development - For the short term (2 years) there are several options:

• In electroplating technology, the cobalt-based composites (of which the only one we

have found to be in production is Praxair’s Tribomet) are an option that avoids the use

of EPA-17 materials. This method is already in production for some components, but

there are unknowns for chrome replacement, such as whether it would require heat

treating. However, Boeing’s experience with composites indicates severe potential

problems when using composites in production, especially for periodic (O&R), rather

than continuous (OEM) use.

• Plasma spray can already be done in IDs > 3” with ease, and > 1.5” with more difficulty,

which would solve most of the requirements for hydraulics. However, it would not solve

the needs of landing gear main cylinders, which require less porous coatings. The

advantage of this approach is that it can be used to deposit the same materials as are

being deposited on ODs by HVOF, namely WC-Co and WC-CoCr.

• Electrospark microarc welding looks promising, with some caveats concerning fatigue

due to the heat-affected zone and cracking of coatings due to repeated thermal cycling

during deposition. The method can only be used for small IDs, and can deposit WC-Co

and Cr3C2-Co, so that it could be used in tandem with plasma spray.

Intermediate term development - For the intermediate term (OEM production in 2 years, O&R

in about 5 years) there are two main options:

• Small- or nano-particle plasma spray looks very promising to overcome the problems

inherent in standard plasma spray, making coatings denser, smoother, and suitable for

use in landing gear, and reducing component heating.
Page 15

• Post magnetron PVD of metals and alloys may be an option if the materials perform

satisfactorily and if the equipment can be properly integrated into the IVD systems

currently in use at OEMs and depots.

Long term development - Chemical vapor deposition (CVD) is one of the best means of

depositing thick coatings on even the smallest IDs, but it is generally entirely unsuitable for most

structural components because of high deposition temperature and worker safety. However,

over the long term (OEM production in 5 years, O&R in about 8 years) there are two possible

CVD methods that might become viable:

• Combustion CVD is a particularly simple approach, but is rather limited in materials that

can be deposited (primarily oxides). Not enough is known about it at present, but it

would have low capital cost and is quite flexible.

• Plasma CVD is now being done over large areas for ODs and complex shapes. It could

be adapted to IDs, where it might be possible to deposit materials that would be both

hard enough and thick enough for chrome replacement. This is likely to be a relatively

high capital cost approach.

The details of all the technologies we have evaluated are given in Part C.
Page 16

Table 3. Hard chrome replacement criteria.

Issue Criteria Notes

ID coating requirements

Hole dimensions Actuators: 1” min. dia. by up to 24” long

Landing gear, large actuators: 3-15” dia

by up to 60” long

Hole geometry Open, blind, some internal grooves

Coating thickness OEM: 0.003”

OEM thin dense Cr: 0.0003”

O&R: 0.003-0.015”

Smoothness 16μ” Ra typical, some replacements may

need to be 4μ” Ra

Highly useful to be able to be able to deposit thin

coatings to replace thin dense Cr, without need for

grinding. (As-deposited Ra<8 μ”.)

Deposition temperature High strength steels: <250C

Aluminum alloys: <150C

Critical issue is time-at-temperature. Critical issue

is fatigue reduction due to changed surface

microstructure.

Technical issues

Wear resistance and

hardness

Match performance of chrome on actual

components

Critical issue is wear life (wear rate x thickness) in

service, and avoidance of seal wear in hydraulics

Corrosion resistance Must match chrome - primarily B117 salt

fog

Microcracks make chrome a poor corrosion

inhibitor - may require sealer or Ni underlay.

Hydrogen embrittlement None This is a critical flight safety issue

Fatigue Fatigue debit must not exceed chrome Navy particularly concerned with NaCl and SO2

atmospheres - critical flight safety issue.

Producibility

Reproducibility Process must be stable Both OEM and O&R environments

Process window Within day-to-day operating parameters Simple, reasonable QC needed

Cost Total production cost comparable to 2 x

chrome.

Life-cycle cost < chrome

Reasonable capital cost

Production cost needs to include cleaning,

masking, finishing, heat treating, waste disposal,

etc.

OEM and O&R fit

Stripping Must be able to be stripped - safe

chemicals, water jet, etc

Strippability is crucial to O&R.

Field and O&R chemical

stability

Must withstand O&R cleaning,

chemicals, hydraulic fluid, etc.

Must not deteriorate when put through O&R

process

Environment/safety Must be environmentally benign and safe

for workers

Note that O&R operations are more diverse and

less easily controlled

Acceptance issues

Specifications AMS and/or aircraft company

specifications needed

Cannot be specified and put on drawings without

specs.

Proprietary technology Cannot be proprietary to one company If possible, should be able to be done at general

O&R site to avoid sending out
Table 4. Summary of ID chrome replacement options.

Technology Principle Company Capabilities/Notes StaThermal sprayHVOF Powder + high temperature flame e.g. Sulzer Metco, Praxair, TAFA 11” ID, WC-Co, alloys ProPlasma spray Powder + plasma e.g. Sulzer Metco, Praxair 1.5” ID, WC-Co, alloys, metals ProSmall/ nanoparticle thermalspray

Small particles + plasma or flame SUNYSB, ONR, U. Conn, NU. ONR (Larry

Kabacoff), DARPA funding

Dense coatings, WC-Co, oxides. May makesmaller guns possible

Res

Weld coatingElectrospark (ESD) Microarc weld Advanced Surfaces and Processes, Batelle

PNL

<0.5” ID, 120” long nanophase WC-Co,alloys. Smal diameter only

Dev

Explosive clad Explosive bonding Sigmabond Technologies Metals, WC-Co ResPVD/CVDPost-magnetron Sputtering from high current rod Surface Solutions, Praxair (ATP program),

Army Benet Labs

Metals, alloys, nitrides; Ta (Army) Dev

rese

CVD, MOCVD, plasma CVD Deposition from gas Various Very small, long holes. High

temperature/dangerous precursors

Pro

Combustion CVD Precursors combined in flame MicroCoating Corp Compounds (oxides, etc). VOC solvents

used

ResDev

Plasma CVD Precursors deposited by plasma Metroline Oxides, nitrides, metals ProHollow cathode evaporation Small internal hollow cathode U. Uppsala (Sweden) Metals, nitrides. Low build-up ResLaser deposition Laser evaporation, alloying, and

CVD

QQC Diamond Diamond. No build-up Res

Laser Induced Surf.Improvement (LISI)

Laser alloying Surface Treatment Technology, University

of Tennessee

Alloys with surface material (not coating).No build-up

Res

Plasma nitride Nitriding at about 500C e.g. Advanced Heat Treat Surface treatment (no build-up, high

temperature)

Pro

Wet platingElectroless Ni and Nicomposites

Electroless Ni-P/B (+ SiC, Teflon,diamond, etc), Amplate

Various Ni-P, Ni-B, and particle-filled alloys Pro

Brush plating Ni electroplate Various Cr, Ni, other metals ProAlloy plating Electroplating of simple alloys Various Ni-W-B, Co-W ProCo-composites, Tribomet Co alloy composite plate Praxair Co alloy, proprietary ProImmediate (/immed) Short term (/short) Intermediate term (/inter) Long term possibilities (/long)
Page 18

PART C - PROCESS SUMMARIES

In the following subsections we present detailed Process Summaries for the processes we have

considered, which include descriptions of processes and evaluation of how they fit with the

criteria for OEM and O&R chrome replacement. Different levels of detail are incorporated

depending on the viability of each method for ID chrome replacement. For the most promising

methods we have included significantly more detail on cost issues and technical performance

than for those technologies that are not viable replacements.
Page 19

6. Process Summary - Thermal Spray
Page 20

6.1. Introduction

Thermal sprays are standard hard coating and repair methods for aerospace components that

are now in widespread use at OEMs, commercial O&R shops, and DoD depots. Their primary

aerospace uses are

• turbine engines (shafts, turbine blades, thermal barrier coatings, erosion coatings,

abradable coatings, anti-fretting coatings)

• landing gear, flap and slat tracks, and for spot problem-solving in high wear areas

• chrome replacement for landing gear inner cylinders, axles and pins

6.2. Process description

The basic thermal spray process consists of firing a powder comprising particles in the typical

size range of 20-60 microns (entrained in a high-velocity carrier gas stream) through a source of

heat. The heat softens the powder particles, which hit the substrate at high speed and flatten in

pancake-shaped “splats”. As they overlay each other these splats form a continuous coating,

often merging the boundaries of the individual splats into a coherent coating material. The

method can be used for spraying alloys, ceramics, or cermets such as WC-Co. In the latter

case, only the matrix Co softens, leaving the WC particles embedded in the coating. The two

highest quality thermal spray methods, which are the only two relevant to our purposes, are

HVOF and plasma spray.

Table 5. Summary information - Thermal Spray

Process name Thermal spray (plasma spray, HVOF)

Description Powder fired through flame or plasma softens and builds up coating

Coating materials WC-Co, Cr3C2-NiCr, oxides (e.g. Al2O3, Cr2O3), alloys (e.g. Stellite,

Tribaloy)

Temperature 150 - 400C surface temperature of part during deposition

Minimum ID 1.5” (plasma spray), 11” (HVOF)

Maximum depth 26” standard. Can go to almost any depth

Stage of development Production

Primary advantages Direct fit with existing thermal spray chrome replacements (methods

and materials).

Capable of OEM and rebuild use.

Direct fit with current O&R methods.

Primary disadvantages Plasma spray not as dense as HVOF.

Difficulties with coating quality in blind holes.

Heat removal in confined spaces.
Page 21

6.2.1. High Velocity Oxy-Fuel (HVOF)

HVOF coatings are the highest quality thermal spray coatings. The principle of HVOF coating is

to use a supersonic flame, which raises the particles to high (although usually sub-sonic)

velocity. When they hit the substrate these high-velocity particles form a very coherent, low

porosity coating. There are two basic forms of HVOF equipment -

1. Continuous flame HVOF (equipment made by companies such as TAFA and SulzerMetco). Continuous flame equipment is the only commercially available HVOF

equipment. In the standard commercial HVOF gun, oxygen and fuel (usually hydrogen,

acetylene, or kerosene) are mixed and burned in a combustion chamber, forming a

stream of hot gas that comes out of the end of the gun. Powder is injected axially into

the flame, where it heats and softens on its way to the substrate surface. The stand-off

(gun to substrate distance) must be sufficient to heat the particles adequately before they

strike the substrate.

2. Detonation Gun (D-Gun) and Super D-Gun (explosive equipment and process

proprietary to Praxair). In the D-gun the fuel and oxygen are mixed and ignited by a

spark. The consequent explosion projects the powder and hot gas at high velocity onto

the substrate.

HVOF can be used to deposit

most alloys and cermets

such as WC-Co, WC-CoCr,

Cr3C2-NiCr, etc. The flame

temperature is insufficient for

most ceramics, such as

oxides.

Because the flame produces

a great deal of heat (and the

particles themselves are hot)

the substrate temperature

must be moderated by air-jet

cooling using jets set up

around the object being

coated. The gun is also

moved over the surface (or the surface under the gun) at a speed sufficient to prevent heat

build-up in any area.

The method tends to be very forgiving, with a wide process window (especially with regard to the

stand-off, which is important for coating complex parts). Coatings are usually hard, with

excellent adhesion and very low porosity (typically >98% density). For this reason they can be

machined without chipping to form a smooth, non-porous surface. HVOF coatings are the only

thermal sprays with sufficiently low porosity to prevent leak-past in high pressure landing gear

cylinders (typically 7,000 psi).

Because the flame is supersonic, the process must in general be done in a sound-proof room

typical of other thermal spray processes, but with better acoustic insulation. The coating is

normally applied using a standard industrial robot. Overspray (powder that misses or bounces

off the substrate) must be caught in a standard bag-house filter system.

The process requires the following steps

1. Gross cleaning (oil, grease, paint, rust removal). Waste stream - cleaning chemicals

typical of all parts cleaning and depainting.

2. Grit blasting to roughen the surface and give a strong mechanical lock between

substrate and coating. Waste stream - grit, which may be contaminated with metal

particles, including chrome alloys.

Figure 16. HVOF gun (Sulzer-Metco Diamond-Jet).
Page 22

3. Masking - usually mechanical (metal strips)

4. HVOF spraying. Waste stream - overspray powder, CO2 and H2O combustion products.

5. Grinding (usually diamond or borazon wheel). Waste stream - spray material

contaminated coolant.

6. Superfinishing, if needed (vibratory stone honing for a very smooth surface). Waste

stream - spray material contaminated coolant.

The HVOF process uses very high volumes of process gases - typically hydrogen and oxygen. A

full day’s spraying would typically use 16 standard tanks of hydrogen (used 8 at a time). In some

DoD locations storage of this amount of flammable gas is considered a hazard, and in others

HVOF spray rooms are required to be explosion-proofed with special vent systems.

HVOF coatings are currently being used on landing gear and other airframe components by

Boeing Commercial and on slat and flap tracks by Messier-Dowty. They are being qualified for

use on landing gear by Lufhansa, Delta, and United Airlines. The Hard Chrome Alternatives

Team (HCAT) is demonstrating and validating them for O&R chrome replacement on landing

gear, propeller hubs, hydraulics, and other aircraft parts serviced in DoD depots, while the

Canadian-HCAT is doing the same for landing gear manufactured in Canada.

6.2.2. Plasma spray

Plasma spray is more commonly used than HVOF, since it is cheaper and easier to do, and is

more versatile in the materials it can deposit. Coating quality is high, but coatings tend to be

more porous and not as strongly bonded as HVOF coatings.

In this case, the heat source is a

thermal arc struck between an

electrode and the body of the gun.

Typically this arc runs at about 60 V

and 600 A. Since the temperature in

such an arc is very high (thousands

of degrees C), it can be used to spray

almost any material, including oxide

ceramics. Gas and particle velocities

are much lower than in HVOF, and

consequently the quality of the

coating (in terms of adhesion and

porosity) is lower. Typical density is

95% of theoretical.

In most plasma spray locations the

plasma spray set-up is very similar to that used for HVOF - a spray booth with some acoustic

isolation, air ducting and a bag house to trap overspray, robotic application, and air jet cooling.

The plasma spray process and preparation is similar to that used for HVOF, and the waste

streams are consequently essentially the same, with the exception of their being no combustion

products. The powder is generally carried on a stream of dry argon or nitrogen, while the arc gas

is commonly argon.

6.3. State of development

Both HVOF and plasma spray are fully developed commercial processes in widespread industrial

and aerospace use. Equipment and powder can be obtained commercially from a number of

vendors. There are numerous commercial, military, and aerospace specifications for thermal

spray processes, the most recent being AMS 2447 for HVOF coating.

+

+

substrate

Arc gas

-

Power Supply

External Powder Feed Internal Powder Feed

Figure 17. Plasma spray schematic.
Page 23

The coating material most commonly used for chrome replacement with both thermal spray and

HVOF is WC-Co (usually WC-17Co or WC-12Co). HCAT has chosen WC-17Co because it is

easier to spray and less brittle than WC-12Co, while having similar performance. While its wear

and fatigue properties are excellent (see below), WC-10Co4Cr has superior corrosion resistance.

For this reason C-HCAT and Lufthansa have chosen to use WC-CoCr. Note, however, that the

chrome in the coating is not hexavalent, but metallic (as it is in stainless steel), and there is no

evidence that it becomes hexavalent on stripping.

At this point, all data on performance, producibility, and cost show that HVOF is at least as good

as (and usually better than) hard chrome. Validation and flight testing on both HVOF WC-Co

and WC-CoCr to replace hard chrome in OEM and O&R use is under way at Boeing, Lufthansa,

Delta, United Airlines, HCAT, and C-HCAT.

6.3.1. ID coating

There are fully qualified and commercially used thermal spray ID coating guns and processes for

both HVOF and plasma spray. Equipment for both methods is available commercially from

manufacturers such as Sulzer Metco and Praxair.

6.3.1.1. HVOF ID coating

Standard HVOF guns can be used to coat shallow IDs by angling the gun into the end of the

hole. Holes can easily be coated to a depth of about one diameter in this manner, although

success has been reported of coating to depths of more than two diameters by angling the gun to

within about 20° of the inner surface. In this way it is possible to ID coat a through hole with a

depth of 4 or more diameters.

For deeper holes, one must insert the gun into the hole. However, because of the large stand-off

and the physical size of the equipment, HVOF is limited to diameters of at least 13” and depths

of 24” with the Diamond Jet ID gun. This is too large even for landing gear main cylinders.

GM has developed an HVOF system for spraying the cylinder IDs in aluminum engine blocks

with a wear-resistant steel, but the method is not in commercial production at this point. SulzerMetco has installed a commercial engine block arc-spraying line for Volkswagen in Germany,

which seems to be superceding the GM approach.

6.3.1.2. Plasma spray ID coating

Plasma spray is currently used for coating IDs in the

aircraft industry. GE Aircraft Engines uses a Sulzer Metco

gun to coat a small (approximately 1.5” diameter) slot for

turbine blades. Praxair is currently replacing chrome plate

with plasma sprayed Tribaloy in the IDs of the blade

dampers on all the Navy’s CH-53 7-bladed heavy-lift

helicopters. These dampers are approximately 2.5” ID by

7” long and 3.25” ID by 8” long. Tribaloy is being used

because it was already tested. WC-Co has since been

optimized for this type of application and it would quite

probably have been used had it been ready in time.

Sulzer-Metco makes a number of ID guns for plasma

spray coating, as well as a rotary power and gas

manipulator called a RotoPlasma 500. The commercially available equipment is summarized

in Table 6.

Figure 18. CH-53 helicopter.
Page 24

There are a number of caveats with ID thermal spray coating.

1. Sulzer-Metco generally recommends their SM-F210 gun with a RotoPlasma unit for IDs

greater than about 3”. They generally recommend better materials rather than coatings

for smaller IDs, because smaller ID coatings are more difficult to do.

2. In general the quality of ID coating is not as good as standard OD coating, especially in

terms of the higher porosity typical of ID coatings.

3. Blind holes are more difficult to spray because the overspray (particles that are not fully

softened and bounce off the surface) creates a “dust storm” inside the hole, embedding

unmelted particles into the coating and increasing its porosity. This problem is not

serious for through holes since particles tend to be swept out in the gas stream. It is also

not a problem for HVOF since the gas flows are much higher and the holes larger in

diameter.

4. The bottoms of blind holes cannot be coated, except with the use of an angled spray gun

head. Most heads only allow coating at right angles to the axis.

5. Because the coatings are more porous, high pressure gas can leak through the coating

beneath the seal, making them unsuitable for use in landing gear main cylinders.

6. The RotoPlasma unit is very heavy (about 100kg) and requires a heavy duty robot to

manipulate it, rather than the small robot typically used for thermal spray.

6.3.1.3. Nanophase and small particle thermal spray

Thermal spray of small or nanophase particles may offer solutions to some of the shortcomings

of present-day ID plasma spray coatings, especially coating quality and equipment size.

The recent general materials interest in nanophase materials has expanded to thermal spray.

R&D of nanophase thermal spray is being funded at the University of Connecticut, Rutgers,

and the State University of New York at Stony Brook by Larry Kabacoff of ONR. Typically,

20 nm particles are agglomerated into 20 micron particles, which are large enough to be fed and

injected in standard plasma spray or HVOF guns. These agglomerates break apart in the

plasma or flame, releasing the nanometer powder particles. DARPA is funding development

work at Northwestern University on small particle plasma spray (SPPS)2

, in which the particle

size is generally in the 1- 10 micron range. These particles require special feeders and injectors

to place them into the outer regions of plasma, where they will be heated correctly. (Particles

entering the core of the plasma are overheated and vaporized.)

 

2

 Grant # MDA972-93-1-0006, “Hard chrome coatings: advanced technologies for waste

elimination”.

Table 6. ID plasma guns.

Gun type ID (in) Depth

(in)

Power

(kW)

Notes

SM-F210 2.4 26 12 Sulzer Metco. Matches to modern Rotoplas unit

11MB 1.75 24 25 Sulzer Metco

SM-F100 4 22 20 Sulzer Metco

7-MST 1.625* 13* 24 Sulzer Metco. Extension to 9MB plasma gun.

Used by GEAE for dovetail slots.

Micro-mini 1.5 Praxair. Not currently in commercial production.

* According to Sulzer-Metco engineers, it may be possible to use this gun for 1” ID by 18” deep.
Page 25

Nanophase and small particle plasma spray coatings are still in the development phase, but

results suggest that they are equal to or better than HVOF coatings:

• Small particle/nanophase coatings generally have higher hardness and fracture

toughness (higher strain to fracture) than standard coatings.

• They are less porous (similar to HVOF coatings).

• Their adhesion is comparable with that of HVOF coatings.

• Their surface is generally smooth because of the small grain size.

6.3.2. Coating materials and properties

A great many materials can be thermally sprayed. HVOF cannot spray most oxides and other

ceramics because their melting points are too high, but it can spray most metals, alloys, and

cermets such as WC-Co and WC-CoCr. Plasma spray, with its hotter plasma, can spray almost

any ceramic, metal, or cermet.

6.4. Capabilities and limitations

Thermal spray coatings are heavily used in the aircraft (especially turbine engine) industry for

both OEM and maintenance applications. The primary capabilities and limitations of the method

are summarized in Table 8.

Table 7. Typical materials properties - thermal spray WC-Co

Property Value Notes

Hardness 1100 - 1400 HV Depends on spray conditions

Maximum thickness None Usually limited by cost to 0.020”

Corrosion resistance Similar to Cr Somewhat better than hard chrome, but not as good

as hard chrome with Ni underlay. Depends on

thickness. No microcracks as-deposited

Wear resistance 2 - 3 times hard

chrome

Smooth surface finish important

Lubricity Not known

Porosity 1% - 5% HVOF Higher for plasma spray

Stress and effect on

fatigue

Compressive,

usually

Little or no fatigue debit in steels, some debit in

aluminum.

Hydrogen

embrittlement

None Even when using hydrogen as fuel
Page 26

6.5. Other issues

Beside the coating method and properties, other issues are critical to use of thermal spray

coatings in OEM and O&R. Some of the most important issues are summarized in Table 9.

6.6. Developments needed

In order to use plasma spray or HVOF to replace chrome on internals we need three primary

improvements:

1. Smaller guns that can fit better into smaller diameters and have reduced stand-offs from

the surface being coated.

Table 8. Thermal spray capabilities and limitations

Application Capabilities Limitations

Substrate materials Can be used on most steels and

aluminum alloys

Outside surfaces Rapid coating of large items and

areas. Typical coating time 15

mins per landing gear piston (> 60

sq in/min).

Line-of-sight. Adequate spray

distance (stand off).

Inside surfaces 1.5” minimum plasma spray ID, 11”

minimum HVOF ID. Smaller IDs

can be coated to a depth of 1 or 2

diameters from outside the hole.

Heat dissipation. Removal of

overspray for coating quality.

Table 9. Other issues for thermal spray coatings.

Item Comment

Stripping Chemical stripping easy for WC-Co and WC-CoCr. Can be very

difficult for some coatings, such as Tribaloy. HVOF coatings cannot

be water-jet stripped.

Finishing Plasma spray coatings can chip. Diamond wheel needed for WC-Co.

Finish may need to be <8 Ra (better than chrome). Praxair often

specifies vibratory stone superfinishing.

Process stability/control Good. HVOF has wider process window than plasma spray. Strongly

dependent on powder properties, requiring careful attention to supply

source and reliability.

Chemical stability Good for most materials. Still under evaluation.

NDI Flourescent penetrant and Barkhausen work well. Magnetic particle

does not appear to work as well as with chrome.

Installation cost $100,000 - 250,000 for equipment, $100,000 robot, $250,000 room

Processing cost Moderate - 50% - 150% cost of chrome, depending on application.

Elimination of hydrogen embrittlement baking is major saver of cost

and time-in-process.
Page 27

2. Reduction or dissipation of the heat load. On ODs heat is removed by air (or even

water) sprays, but in IDs the heat is much more difficult to remove. The best

improvement would be to reduce the heat generated in the first place.

3. Higher quality, lower porosity ID coatings suitable for high pressure landing gear main

cylinder hydraulics. HVOF coatings meet the requirements, but plasma sprays usually

do not.

Today’s standard guns and powders are inadequate on all of these counts. 1 and 2 above could

both be improved by the development of smaller guns. At this point plasma spray guns are

almost small enough for deposition on all critical IDs, while HVOF guns remain far too big for

almost any ID expected on the JSF. Both HVOF and plasma spray guns could doubtless be

reduced in size, but there is a fundamental limitation imposed by the need to have enough space

to accelerate and heat the particles adequately.

The use of smaller guns risks making 3 above more difficult to attain - again because of powder

velocity and heating rate limitations. Because small particles have lower thermal mass

(requiring less energy to heat them) and a higher surface area/volume ratio (permitting more

efficient heating in the plasma) it ought to be possible to design smaller guns for spraying small

particle powders. These guns would dissipate less energy (less heat removal needed) and

permit a smaller stand-off (smaller gun to surface distance). Such guns should fit within smaller

diameters and would probably produce higher quality coatings than those typical of today’s ID

guns and standard powders.

Simple physics allows us to determine roughly how powder temperature and velocity depend on

particle size and spray distance (see box). We find that, as we reduce the powder size, we

can reduce the spray distance while maintaining particle temperature and velocity.

Kinetic energy = Drag force X Spray distance

(1⁄2) ρpowder (4/3) π r3 v

2

powder= (1⁄2)πr

2

ρgas v

2

gas D

for vgas >> vpowder, where ρ is the density (of the gas or particle), r is the powder diameter, v is the

velocity (of the gas or particle), and D is the stand-off distance.

This leads to

vpowder = √{(3ρgasv

2

gasD)/(8ρpowderr)}

i.e. vpowder ~ √(D/r)

Thus, as the particle becomes smaller, the spray distance can be reduced while maintaining the

same velocity.

Similarly, for a particle being heated through its surface, by contact with hot gas and absorption

of radiation, we have:

Heat input = 4Jπr

2

= ρpowder (4/3)πr

3

 C ∆T/∆t

where J is the heat flux per unit area per second, C is the specific heat of the powder, and ∆T/∆t

is the rate of temperature rise of the powder particle.

This leads to

∆T = (3J)/(ρpowderrC)

i.e. ∆T/∆t ~ 1/r

Thus, as the particle becomes smaller, its temperature rises more rapidly.
Page 28

However, if we are to use small powders reliably, several developments will be needed:

1. The gun and powder must be matched to ensure that the powder is adequately heated

without being vaporized.

2. A reliable method must be found to feed the powder into the gun. Present day powder

feeders can feed powder down to 10μm particle size, and in some cases down to 1μm.

This is likely to be sufficient, provided small powders can be made with good flow

characteristics (primarily a function of particle shape). If we need smaller particles new

commercial powder feeders will be needed.

3. The alternative is to use agglomerated nanopowders, such as the nanophase WC-Co

sold commercially by Nanodyne, which can be fed with a standard commercial powder

feeder. The question with these powders is how quickly and reliably they would break

apart in the plasma, and whether their break-up would be complete, and rapid enough to

permit proper particle heating in the limited space between gun and wall. As in 1 above,

the powder and gun will need to be properly matched.

6.7. Recommendations

For the widest use at both the OEM and O&R levels we recommend the development of ID

plasma spray, which will require the development of smaller plasma guns and methods

for spraying smaller particles.

6.7.1. HVOF

In many ways an ideal solution would be to reduce the size of HVOF equipment to permit a

miniature gun to fit within a 1” ID tube. The gun need only carry gas and a coolant (air or water)

and the gas flow could be made as small as desired. The problem, however, is that a minimum

stand-off (gun-to-substrate distance) is required to soften the powder and accelerate it to a high

enough velocity. While the size of the gun could be significantly reduced, at this point the HVOF

equipment manufacturers are not in a position to be able to meet the requirements for ID HVOF

coating down to 1” ID from the current 11” ID.

Therefore, we do not recommend HVOF as a viable ID coating technology.

6.7.2. Plasma spray

The advantage of plasma spray over HVOF is the higher energy density in the plasma, which

makes it possible to heat the powder rapidly in a small distance. The disadvantages of plasma

spray are the bulk of the current equipment (necessitated by the need to carry a large arc current

sufficient to be self-sustaining), and the lower quality of plasma coatings (especially their

porosity, which prevents their use in high-pressure hydraulic landing gear). On the other hand,

existing plasma spray equipment can already be used in diameters as small as 1.5”, and new

developments in small and nanoparticle thermal spray are producing much better material that

would be easier to spray with a smaller gun (since current guns tend to vaporize the material).

Furthermore, small particles produce much smoother coatings that may eliminate the need to

grind IDs, producing a large saving in cost and manufacturing complexity.

Discussions with equipment manufacturers and thermal spray experts indicate that a

combination a of miniaturized plasma gun and small or nanosized powder could well permit the

technology to be used reliably down to diameters of 1” ID.

Plasma spray would also fit well with the depots, more of whom have plasma spray equipment

than HVOF equipment.

IDs 3” and above can already be plasma sprayed commercially, and are likely to work for many

wear applications and lower pressure hydraulics. However, for high pressure landing gear
Page 29

hydraulics, small or nanoparticle plasma spray will need to be developed to reduce porosity and

leakage. (Note that is only on gas-over-fluid landing gear hydraulics, where IDs are generally

large and gas leakage must be prevented, that porosity is such a critical issue. Actuators, which

are much smaller in diameter, operate at lower pressure and do not have such critical

requirements.) This would make the method efficient for coating even the largest ID

components, such as landing gear pistons and actuators. This would account for most of the

ID chrome usage in aircraft, especially in landing gear.

ID plasma spray is likely not to be completely universal, however. The presence of a high

temperature plasma source inside an ID will inevitably lead to significant wall heating, becoming

more serious the smaller the ID. The heat will be minimized by the lower power of the miniature

gun, and high pressure cooling gas will carry away both the excess heat and unmelted particles

(which is especially critical in blind holes). However, for smaller IDs in heat-sensitive alloys,

such as 7075 aluminum (which is commonly used for landing gear and hydraulic parts) plasma

spray may not be possible, even with a miniature gun. This may leave a small percentage of

components that cannot be processed by thermal spray.

For the JSF, however, the better solution is likely to be designing those few (if any) components

to avoid the need for ID coating, e.g. by choosing a different alloy. Today’s components are

designed to use chrome, and to take its limitations into account. There is no reason why

tomorrow’s components cannot be similarly designed to use thermal sprays, taking their

limitations similarly into account.

At this point we have no reliable estimates of the reduction in chrome plating by the use of

plasma spray technology. From the components we have evaluated and seen in O&R shops we

estimate, very roughly, that plasma spray could be used to coat more than 90% of the IDs.

Given that IDs are 20 - 30% of the total, this would mean that more than 97% of normally

chromed components could be coated by thermal spray. However, since these small IDs require

less chrome, this would eliminate more than 97% of the chrome mass deposited for IDs.

Together with better materials for those situations where thermal spray cannot be used, this

approach would eliminate chrome plating in its entirety.
Page 30

7. Process Summary - Electroplating
Page 31

7.1. Introduction

The obvious drop-in replacement for hard chrome plating is another, cleaner electroplating

technology. There are a number of alternative technologies, including:

• Trivalent chrome - deposition of hard chrome with relatively benign trivalent solutions

• Molten salt bath electroplating - non-aqueous electroplating

• Alloy and composite electroplating - incorporating alloying elements or hard particles into

a softer, non-chrome matrix

• Brush plating - local deposition with a pad rather than a large tank.

Since a parallel technology study is presently being done by CTC for Wright Labs, concentrating

on electroplating, we will not repeat the same information here. Our general concern with

electroplating is that, although it is cleaner than traditional hard chrome plating, it still involves

significant environmental, personnel safety, and waste volume issues that non-plating methods

avoid.

Most of the electroplating processes are very similar to chrome plating. However, there are a

number of composite electroplates that incorporate particles of hard materials such as carbides

or oxides to enhance the hardness of the coating. Some of the broad plating technologies are

described below.

Table 10. Summary information - Electroplating

Process name Electroplating

Description Deposition from solution - tank and pad

Coating materials Metals, alloys, composites

Temperature Less than 100C

Minimum ID < 1”

Maximum depth Unlimited (more problematic for composites)

Stage of development Production; some methods in limited or niche production

Primary advantages Direct drop-in

ID coating relatively simple

Composites offer large range of coating design, including hard and

lubricious coatings

Primary disadvantages Most hard electroplates Ni-based (EPA-17 material)

Generally high levels of contaminated waste and rinse water

Composites especially difficult to use in O&R
Page 32

7.2. Process Description

7.2.1. Trivalent chrome plating

Under the EPA’s Clean Air Act rules trivalent chrome plating is subject to significantly less

stringent regulation than the hexavalent process, since it does not produce a hexavalent mist.

The primary mist elimination method is a surfactant additive that prevents the formation of large

bubbles on the liquid surface, and the primary test method is measuring the surface tension.

Traditionally trivalent chrome has only been used for decorative coating because deposition is

slow and self-limiting to less than 0.001” in thickness. (It is interesting to note that the German

Umweltbundesamt actually prefers hexavalent chrome plating, considering it is easier to control

emissions and institute zero-discharge processes for the hexavalent method than to deal with the

wastewater issues associated with a trivalent process that uses chelating agents to hold

chemicals in solution.)

Recently there has been significant progress in devising methods to make the trivalent method

faster and trivalent chrome coatings thicker. None have yet gained market acceptance.

Both Chris Johnson of NIST3

 and M. El-Sharif of Glasgow Caledonian University4 have

developed trivalent chrome solutions that appear to overcome the thickness and deposition rate

limitations.

Faraday Technology Inc. has combined the new NIST

trivalent plating solution with a pulse plating method

(rather than the traditional DC plating method) into a

commercial trivalent chrome process5,6. The shape of the

current pulses is critical to the proper operation of the

process. Normal electroplating would use a DC cathodic

bias on the substrates. The pulse method gives a long

cathodic pulse to plate the Cr, followed by a short anodic

pulse to convert the nascent hydrogen produced during

the cathodic pulse to molecular hydrogen, which

maintains a low pH and prevents the precipitation of

chromic hydroxide, which would slow the deposition rate.

This process has a deposition rate of about 3 mils/hr

(similar to or higher than is typical of hard chrome). The

current efficiency for this process is about 35% instead of

the 15% typical of hexavalent hard chrome plating.

The chemistry of the Faraday Technology baths is given

in Table 11.

 

3

 US Patent # 5,415,763 (1995): “Methods and electrolyte compositions for electrodepositing

chromium coatings”.

4

 M. El-Sharif, 18th AESF/EPA Pollution Prevention and Control Conference Proceedings,

Orlando (1997), p 143.

5

 US Patent # 5,196,109 (1993): “Trivalent chromium electrolytes and plating processes

employing same”.

6

 Renz et al., 18th AESF/EPA Pollution Prevention and Control Conference Proceedings, Orlando

(1997), p 115.

Table 11. Faraday Technology

Cr3+ electroplating bath

chemistry.

CrCl3.6H2O 125 gm/l

KCr(SO4)2.12H2O 25 gm/l

NH4NH2SO3 178 gm/l

NH4Cl 80 gm/l

H3BO3 30 gm/l

HCOOH 30 ml/l

pH 2

Temperature 20 - 60C
Page 33

7.2.2. Composite electroplating

There are various types of composite

electroplate, which incorporate small

diameter powder into the coating.

Recently both Boeing and McClellan

AFB (Elwin Jang) tested the proprietary

Takada process for chrome replacement

of items such as hydraulic pistons. This

plating method uses SiC particles in a

Ni-W matrix. After extensive testing the

method was abandoned as too difficult

to use reliably in production.

Praxair has a proprietary composite

electroplate, with the tradename of

Tribomet, which comprises a high

percentage of 4 - 8μm Cr3C2 particles in a cobalt matrix7

. There are two formulations - T104,

with 10 - 25 wt% Cr3C2, and T104CS, with 30 - 40 wt% Cr3C2 (see Figure 19). These materials

are designed for high temperature turbine engine use, and are specified to require a 900C heat

treatment, which greatly increases the hardness and forms an oxidation-resistant cobalt oxide

glaze on the surface.

As-deposited Tribomet coatings are quite soft - 300 HV for T104C and 450 HV for T104CS,

compared with 800 - 1,000 HV for hard chrome. After high temperature annealing the T104CS

reaches 750 HV. Tribomet can be used on IDs down to 9.5 mm (0.375”) diameter by the use of

an internal anode.

7.2.3. Alloy electroplating

While most electroplates are elements (Ni, Cr, etc.) it is possible to create wear-resistant

surfaces by using alloys such as Ni-W-B, Co-W, W-Co-B, or Ni-W. The boron-containing

materials tend to be amorphous8

. Most of these alloy electroplates are available commercially,

but from a limited number of vendors. Boeing is currently investigating the use of alloy rather

than composite electroplates in order to avoid some of the problems posed by suspending

particles in the plating solution.

A recent modification of this approach is multilayer alloy plating9

, which is the electroplating

analog of nanolayer PVD coating. The idea behind this approach is that interfacial interactions

between layers of different materials can produce much harder, more wear-resistant coatings

than either material alone. The thicknesses of the individual layers must be on the order of

dislocation loop dimensions (typically 2 - 20 nm). In order to be able to plate an alloy of this type

the constituents must plate out at potentials that differ by at least 100 mV, and plating is usually

done by current or voltage pulsing. (It is possible, of course, to use two baths, but such a

method would be highly inefficient.)

 

7

 US Patent # 5,558,758 (1994): “Electrodeposited composite coatings”.

8

 US Patent # 4,529,668 (1984) “Electrodeposition of amorphous alloys and products so

produced”.

9

 See, for example, D.T. Schwartz, “Multiple-layer alloy plating”, ASM Handbook, Vol. 5, 274

(1994).

Figure 19. Tribomet T104CS coating cross

section. 200x
Page 34

7.2.4. Brush plating

Brush plating (or selective plating)

has been in use for over 30 years in

the US and is included in over 100

government, military and industrial

specifications. The principle of the

method is that the plating solution is

held in a pad rather than a tank, and

plating takes place only beneath the

pad (Figure 20). The head is usually

hand-held, although it can be set up

for more controlled automatic

plating. Brush plating is primarily

used for localized manual repair,

and can be used to deposit almost

any (non-composite) electroplate, including sulfamate nickel, which is widely used for build-up

and repair. It is used in a wide variety of repair and touch-up applications, which range from

highly visible large volume uses such as building domes and church steeples (up to 15,000 sq ft

of Cu, Ni and Au) to very small such as electronic parts. Because it uses a pad rather than a

tank of liquid, it is relatively easy to coat IDs, and masking is not usually an issue. While the

volume of brush plating solutions is very small compared with large tanks of chromic acid, the

solutions tend to be more concentrated.

Industrial applications include rotogravure rolls and other printing rolls, molds for plastics

manufacture, printed circuit boards, steel mill coating lines, and bearing and seal surfaces in

turbines. Aerospace and military applications include space shuttle manifolds, missile launch

rails, aircraft engine components, aircraft landing gears and wheels, and aircraft exterior skins.

Handle

+

Power

supply

-

Anode

Workpiece

Solution

Anode wrap (pad)

Coating

Figure 20. Brush plating schematic.

Table 12. Summary information - Brush plating

Process name Selective Plating/Brush Plating

Description Selected area electroplating in which the plating solution, or

electrolyte, is portable and brought to the area to be coated using a

handheld stylus. The workpiece is the cathode, the stylus the anode.

Coating materials Most metals but particularly Cd, Co, Cu, Cr,, Ga, Au, In, Ir, Fe, Pb,

Ni, Pd, Pt, Rd, Ag, brass, bronze, Co alloys, Tin alloys

Temperature <100C

Minimum ID At least as small as for Cd touchup in ID of oil air strut on landing

gear accommodating press fit of a bronze bearing

Maximum depth Unlimited

Stage of development Commercially available

Primary advantages Portability; higher deposition rate (than bath plating); precise

thickness control; post-machining not required on thicknesses up to

0.254 mm (on a smooth substrate); can also perform other related

functions such as electrostripping, anodizing, electromilling,

electroetching, electropolishing

Primary disadvantages Limited adhesion for titanium, tantalum and tungsten; need

conductive substrate
Page 35

7.2.5. State of development

Most of the simple bath and brush electroplating methods are in commercial use. The main

exceptions are:

• trivalent chrome, which is still in the development and validation stage, and has not yet

come into commercial use,

• layered alloy plating, which is quite new and complex to use, and

• hard alloy plating, which is available commercially, but in a limited way.

7.3. Coating materials and properties

Because of the wide variety of coating materials that can be electrodeposited, the properties are

very broad.

7.4. Capabilities

The clear advantage of electroplating methods is that they are closest to a “drop-in” replacement

for chrome plating. They can be used for IDs by the use of rod anodes placed into the ID.

Tribomet, for example, can be used on IDs down to 0.375” by using an internal anode. While

many of the electroplates are nickel-based, electroplates such as Co-W, W-Co-B, and Tribomet

avoid the use of EPA-17 materials.

Selective plating is often used for hardfacing repair, usually with Cr or an alloy such as Co-W. It

is used primarily to electroplate small areas of large parts, parts that are too large to immerse in

plating tanks, and for repair or touch up on any size components that are expensive to strip for

full tank immersion. The strongest advantage of the brush or selective plating process is the

avoidance of masking, which is a large part of the total plating cost, primarily because of the

labor time involved.

Table 13. Typical materials properties - Electroplating

Property Brush plating Tribomet Trivalent chrome

Hardness 250 to 575 HV for

sulfamate nickel

coatings

300 HV, 450 HV asdeposited; 750 HV

heat treated

700 - 850 HV

Maximum thickness up to 0.030” 0.030” 0.020”

Corrosion resistance good - similar to EHC

Wear resistance good similar to plasma

Tribaloy and EHC

similar to plasma

Tribaloy and EHC

Porosity 25% less porous than

EHC

- similar to EHC

Adhesion very good most

alloys, poor for Ti, W,

Ta

Stress and effect on

fatigue

unknown unknown unknown

Hydrogen embrittlement Yes unknown unknown
Page 36

7.5. Limitations

Rinse water and waste volumes - All aqueous plating processes involve the use of copious

amounts of rinse water to remove electroplating solution. There is always some drag-out from

plating tanks, except in the case of brush plating, where the item is not put into a tank.

Materials - All electroplates that are based on Ni have serious environmental and worker safety

concerns since Ni is an EPA-17 material.

Composite coatings - In order to match the hardness and wear resistance of chrome, most

electroplates require the addition of hard particles. All composite coatings, including Takada and

Tribomet, have the problems inherent in using particles in a plating process (also discussed

under “Electroless Nickel”). Boeing has found that it is extremely difficult to maintain the

particles in the proper concentration throughout the bath in the production environment. This

leads to variable properties, especially over large or complex objects. Furthermore, when the

tank is not in use the particles tend to settle and cannot be put back into suspension. This is an

especially serious problem for the O&R environment, where production tends to be sporadic.

We expect that this problem is likely to be more difficult with blind holes, where powders may

deposit in dead spaces or may not properly reach some internal areas. Composites generally

require heat treatment to achieve good hardness, which also limits their usefulness (see below).

Tribomet - As-deposited the coatings are quite soft (up to 450 HV). The treatment needed to

increase the hardness to that of EHC involves heat treatment at 900C, which is too high for most

structural materials. It is possible that these coatings can be used without the heat treat, but at

this time no information is available on their performance.

Brush plating - Parts with complex geometries can be plated faster and more efficiently using

tank coatings. The method does not lend itself well to the efficient plating of small components.

Large numbers of small parts are better plated using barrel plating. It is difficult to obtain

adhesion on Ti alloys, which are frequently used in newer landing gear.

Trivalent chrome - The process is as yet too new to have solid information on producibility and

performance. It is likely that trivalent plating of aluminum will pose the same difficulties that are

encountered with hexavalent chrome solutions, where adhesion is very unreliable. Trivalent

solutions avoid the serious problems of air emissions, but they do not avoid the problems of

disposal of stripping solutions or worker exposure to chrome dust during grinding.

Heat treating - Most composite coatings seem to require heat treatment to reach acceptable

hardness levels. As with electroless nickel this limits their use on heat sensitive structural alloys.

The problem is especially acute with Tribomet, which requires a 900C heat treatment, although it

is possible that performance would be adequate without the heat treatment.

Hydrogen embrittlement - Hydrogen embrittlement is a concern for all aqueous plating

processes, since they tend to release nascent hydrogen at the substrate. While the hydrogen

can be removed by heat treating, this raises the cost of the coating process, especially for large

components such as landing gear.

7.6. Fit with OEMs and depots

The basic fit of electroplating processes (especially bath plating methods) with OEMs and depots

is very good. The details of tank chemistry and operation will differ, of course, from that of hard

chrome, but most operations are familiar with a number of different plating baths.

However, the composite coatings have some significant problems that would limit their

usefulness, especially in the depots, which have been discussed in Section 7.5 above. The

primary concern over settling out of particles from the bath is especially acute in the depots,

where production is less continuous. The particles must be continuously held in solution.
Page 37

7.7. Developments needed

If composite coatings are to be brought to production, simple and reliable methods will need to

be found to maintain the particles in suspension during periods when the plating bath is not in

use. In principle this is not a serious problem since it should simply require that the bath

continue to be agitated at all times. Even with this, however, some material is likely to drop out

of suspension, especially in “dead areas” where there is little movement of the fluid, and this will

slowly deplete material from an unused tank.

Uniformity of particulates throughout the bath, and especially inside blind holes and other difficult

areas will need to be ensured by proper agitation and/or direction of fluid flow. It will be

important to ensure that particulates remain uniform within holes (especially within blind holes)

and do not drop out of suspension in dead areas of the component (including areas where plating

is not desired.

Methods will need to be developed to measure and maintain particle concentration throughout

the bath, both to ensure uniformity and to ensure that the bath is still usable after quiescent

periods.

Many composite coatings require heat treatments at temperatures that are too high for most

structural alloys. Whether or not these heat treatments are really needed, or whether they can

be done at lower temperatures needs to be determined. An alternative is to develop heat

treating methods that affect only the coating, such as laser or flash lamp methods. In order to

avoid overheating the substrate, most of these methods allow only momentary heating, however,

which is unlikely to permit the extensive diffusion and precipitation that seems to be required.

7.8. Recommendations

Of the electrodeposition methods the most viable would appear to be trivalent chrome and

composite coatings. Trivalent chrome is an obvious drop-in alternative, while the various hard

alloys and composites would appear to be the only coatings that offer equivalent or superior

performance to hard chrome.

Trivalent chrome - Of the different plating methods trivalent chrome is the obvious drop-in

replacement for hexavalent chrome. However, it is too early to know how well it will perform or

what production processing will really be like. It also has the same environmental and worker

safety issues as hard chrome in the grinding and stripping processes. Therefore it is not

recommended.

Alloy plating - Some of the alloys look quite promising. Boeing, for example, is beginning to

investigate the use of alloys such as Ni-W as ID chrome replacements. Again, Ni-based

materials are probably best avoided. However, this does leave Co-based alloys such as Co-W

and Co-W-B. These alloys avoid the problems inherent in composite electroplating. We

recommend that alloy plating be evaluated, provided that the chosen alloy solution does

not contain nickel or itself pose serious environmental or worker safety concerns.

Composite plating - Given the poor experiences with composite plating from Boeing we do not

believe that it is a viable replacement at this time, especially for depot use, where the production

problems are likely to be more serious than at the OEMs. However, since composites provide a

high level of flexibility to electroplating, allowing us to design very wear resistant or very hard

electroplates, it would be worthwhile evaluating how the problems of dealing with suspended

particles might be overcome, especially at the depot level.
Page 38

8. Process Summary - Electroless nickel
Page 39

8.1. Introduction

Electroless nickel is a well-defined commercial process available in commercial shops and DoD

depots. Because the process is very sensitive to bath chemistry, temperature, and

contamination experience with it is somewhat spotty, with some aerospace companies (such as

Boeing St Louis) using it on a regular basis and others (such as Pratt and Whitney) regarding it

as unacceptable. Because Ni is also on the EPA “Top 17” list, there is a wide divergence of

opinion for and against its use.

In its Statement of Needs10, San Antonio ALC states

“Electroless nickel deposits can achieve either excellent wear resistance or good corrosion

protection by varying the amount of phosphorus or boron in the coating. Because nickel has been

identified as an EPA 17 hazardous chemical, it would be advantageous to identify an alternative

coating.”

In discussing chrome plating alternatives, Brooks AFB11 states

“Electroless nickel is advertised as a potential replacement for chromium. We do not like

electroless nickel due to the difficulty in maintaining the bath, the slow plating rate, the frequent

bath disposals required, and the difficulty in stripping nickel when the parts return for overhaul.

Potential substitutes are nickel-tungsten-silicon carbide electroplating, high energy thermally

deposited coatings, and a proprietary process call Amplate which is a nickel-tungsten-boron

electrodeposit.”

 

10 #617 HMRPP

11 Statement of Need #251

Table 14. Summary information - electroless nickel.

Process name Electroless nickel plating

Description Autocatalytic deposition of Ni with up to 11% P or B without use of

electric current

Coating materials Ni-P, Ni-B. Sometimes SiC, diamond, PTFE, other particles added to

form composites

Temperature 10 - 90C deposition, 300 - 400C heat treatment

Minimum ID None

Maximum depth None

Stage of development Commercial. Composite coatings less fully commercialized

Primary advantages Fully commercial

Existing commercial and MIL specs for aircraft

Primary disadvantages Ni is a Toxic-17 material

Sensitivity to bath chemistry - difficult to control

Finite bath life - spent bath must be disposed as toxic waste

Hydrogen embrittlement
Page 40

Amplate is also, of course, a nickel-based alloy. An electroplate called the Takada process,

which is similar to Amplate, but with SiC additions, has been tested as a chrome replacement by

Boeing and at Sacramento ALC, but rejected due to difficulties in scale-up.

There is a strong divergence of opinion on the usefulness of electroless nickel even absent

concern over environmental issues. Most of this divergence appears to result from the different

experiences of users. For example, Boeing Commercial avoids electroless nickel, while Boeing

Defense uses it quite heavily; Messier-Dowty considers it expensive and unreliable for landing

gear, while Heroux uses it quite extensively.

8.2. Process description

Nickel is deposited by the autocatalytic chemical reduction of nickel ions onto the surface of the

part without the use of electric current. In the most common type of bath, the hypophosphite

bath (used to deposit Ni-P), the chemical reaction sequence is believed to be12

:

 catalyst

(H2PO2)

-

 + H2O H+ + (HPO3)

2- + 2Habs

 heat

Ni2+ + 2Habs → Ni + 2H+

(H2PO2)

-

 + Habs → H2O + OH- + P

(H2PO2)

-

 + H2O → H+ + (HPO3)

2- + H2

In the first reaction hydrogen is evolved at the surface of the part being plated, some of which is

absorbed at the surface. This hydrogen reduces Ni ions at the surface, depositing Ni metal, and

reduces some of the hypophosphite to deposit phosphorus. Most of the hypophosphite is

oxidized to orthophosphite, with evolution of more hydrogen.

Electroless nickel solutions typically contain the following types of materials:

1. A source of Ni (usually Ni sulfate)

2. A reducing agent

3. Complexing agents (chelators) to control the amount of free nickel

4. Buffering agents to maintain the pH

5. Accelerators to increase the reaction rate

6. Inhibitors to control reduction reactions

The most commonly used hypophosphite bath typically operates at a temperature of about 90C.

As the reaction proceeds, byproducts such as orthophosphites build up in the bath and interfere

with the deposition.

Instead of hypophosphite, the bath can be based on aminoborane or borohydride, which creates

Ni-B. These baths operate at lower temperatures and produce coatings of higher as-deposited

hardness, but typically lower corrosion resistance due to increased crystallinity.

There are numerous composite coatings based on electroless nickel containing particles such as

SiC, diamond, or PTFE to provide better hardness or lubricity.

 

12 ASM Handbook, Vol 5 (1996)
Page 41

8.2.1. State of development

Electroless nickel has been in commercial use since the 1950’s. It is widely available

commercially, most commonly as Ni-P, but also as Ni-B and as electroless nickel composites.

Many of the DoD repair depots and aerospace manufacturers have electroless nickel capabilities

and there are a number of commercial and MIL specs for these coatings, such as AMS 2404C

and MIL-C-26074E.

Many aerospace suppliers and depots are experienced with electroless nickel.

8.2.2. Coating materials and properties

Ni-P

In NiP the phosphorus can vary from less than 1% to about 11%. As-deposited, the material has

a hardness of about 500-600 HV (48-52 HRC). The material has good corrosion and wear

resistance, but both properties depend strongly on the heat treatment. Heat treating above 220C

precipitates nickel phosphide, which increases the hardness, but reduces the corrosion

resistance because of its introduction of grain boundaries and microcracking in the Ni matrix.

Heat treating at 400C for an hour raises the hardness to 1000-1100 HV, similar to that of hard

chrome, but reduces the corrosion resistance by a factor of 100.

The deposition rate is typically about 1mil/hr (comparable with chrome plate), which falls as the

bath ages.

Ni-B

Ni-B is less widely used as it appears to be more difficult to deposit. It is harder as-deposited,

and can be harder still after heat-treating, when Ni boride precipitates. However, the material is

not amorphous, and is consequently generally less corrosion-resistant as-deposited. It can

assume a nodular form which gives it a lower contact area and lower friction (Cemkote).

Table 15. Typical materials properties - electroless Ni

Property Value Notes

Hardness - Ni-P 500 - 700 HV as-deposited

900 - 1,100 HV after 400C heat treatment

Hardness - Ni-B 650 - 750 HV as-deposited

up to 1200 HV after 400C heat treatment

Maximum thickness >10 mils 1 mil typical for corrosion service

Corrosion resistance 0.6 - 50 mil/yr 10% HCl, depending on heat treat, from none to

400C

Wear resistance Good If heat treated, similar to chrome

Lubricity Good Similar to chrome

Porosity Low As-deposited is usually non-porous. Microcracks on

heat treating

Stress and effect on

fatigue

Variable to

detrimental

Usually tensile, become more tensile on heat treat.

Would tend to reduce fatigue life

Hydrogen

embrittlement

Yes Hydrogen evolved during deposition
Page 42

8.3. Electroless Ni composites

There are a great many electroless nickel composite coatings, obtained by incorporating

particles into the Ni layer. The advantage of composites is the ability to build in better lubricity or

hardness. Their disadvantage is the added complexity, especially in maintaining uniform

deposition of the particles in complex and internal geometries.

Commercially available composite coatings contain either diamond, silicon carbide, tungsten

carbide, chromium carbide, boron carbide, aluminum oxide or PTFE particulates. Maximum

particle size for most composite coatings is 10 microns. The primary goal behind the additives is

to enhance existing electroless nickel properties.

The additives are generally insoluble particulates codeposited with the electroless nickel coating.

Particulate matter stabilizers (pms) must be added, in precise amounts, to avoid incompatibility

between the plating bath, which is inherently unstable and surface-area dependent, and the high

loading of insoluble particulates. Stablilizer technology is described in US patents 4997686;

5145517 and 5300330. Particle densities of up to 40% by volume can be produced, but

densities of 15 -25% are suitable for most applications. Each type of particle requires different

bath parameters to optimize coating properties.

Because the particulates are fairly uniformly distributed throughout, composite electroless nickel

coatings are considered to be regenerative - i.e. their properties are maintained as the coating

wears down. Although most composite coatings contain micron-size particles, some

development work is also being done with nanoparticles. Particle shape is also a consideration

when designing optimum coatings and deposition conditions. Typically, coating thicknesses

range from about 7 to a maximum of 50 microns, with most coatings in the 12 to 25 microns

range. Some general properties for several types of composite EN coatings are summarized

below:

Table 16. SiC composite electroless nickel (SiC-EN)

Property Value

hardness 1200 VHN (after heat treatment)

thickness 0.001 to 0.003 inches

coefficient of friction 0.42 (EHC is 0.43; EN is 0.47)

wear resistance 3 times more wear resistant than EHC

abrasion resistance (std Taber test) minimal weight loss
Page 43

Clearly, composite electroless nickel coatings can be designed with a wide range of properties

for different applications. For example, the hard composites such as diamond- and SiC-EN can

be used for wear applications such as textile fiber guides, spinnerettes, injection mold dies,

extrusion dies, and equipment in food processing, paper processing and packaging machinery

industries. Lubricious, but softer, PTFE-EN and PTFE+SiC-EN have found applications in

hydraulic cylinders (which is of obvious interest in aircraft, since hydraulic actuators and landing

gear are the primary applications for hard chrome), bearings, industrial spray nozzles, industrial

valves, and components for fuel systems and transmissions.

It should be noted that in general, the corrosion resistance of composite electroless nickel

coatings is not as good as that of the standard electroless nickel coatings. The large amounts of

occluded materials, or particulates, in the coating reduces alloy passivity and corrosion

resistance. For all types of electroless nickel, as-deposited coatings are more corrosion resistant

than heat-treated coatings, since heat treating transforms the amorphous deposit to crystalline Ni

Table 17. PTFE composite electroless nickel (PTFE-electroless nickel)

Property Value

hardness (as-deposited) 250 HV

hardness (heat treated) 400 HV

thickness 0.2 - 1 mil (6 to 25 microns) for lubricity and good release;

0.5 - 2 mils (13 to 50 microns) for corrosion resistance

corrosion resistance non-crystalline microstructure provides corrosion

resistance. 1 mil thick coating demonstrates salt spray

resistance 500-1000 hrs per ASTM B-117 tests

wear resistance good - PTFE provides lubricity, HT hardness provides

good wear/abrasion resistance

friction coefficient (vs steel) 0.1 - 0.3

Table 18. Combined hard, lubricious electroless nickel (PTFE+SiC-EN)

Property Value

Hardness 1000 HV with heat treatment

Thickness Variable - dependent on application

Corrosion resistance Superior to hard chrome; suitable for mild corrosive

environments. 1-mil thick coating demonstrates salt spray

resistance 700 hrs per ASTM B-117.

Table 19. Diamond composite electroless nickel (Diamond-EN)

Property Value

Wear resistance (Taber wear test

for heat treated D-EN)

11.4 cubic mils/hr (compared with 16.5 as-plated and 46.2

for EHC)

Wear resistance (DuPont

accelerated yarnline wear test)

1.26 cubic mils/hr (compared with 966.60 for EHC)
Page 44

and Ni3P, and cracking of the coating is more likely. For certain types of composite coatings, the

presence of such particulates as diamond or intermetallics exacerbates this problem. The ASM

Handlbook (V. 5, p 309) points out that the presence of phosphides (from the basic Ni-P EN

chemistry), Ni and various particulates, creates a strong galvanic couple that can accelerate

corrosive attack in situations where electroless nickel coated and bare metal are in proximity.

8.4. Fit with OEMs and depots

The fit of electroless nickel and composites with both OEMs and O&R sites is quite good. One

of the advantages of the process is that it is a bath process, in common with chrome plating.

This makes it familiar to plating shops. However, the electroless nickel process is considerably

more complex than chrome plating because of the far higher level of control required by the

process. As a practical matter, this need for control often leads to process unreliability (often put

in terms of “black art”), which OEMs and depots report as a persistent problem. The proprietary

nature of electroless nickel solutions makes it difficult to troubleshoot the process, whether it is

done in-plant or with a vendor.

8.5. Typical costs and cost factors

The cost of electroless nickel on a per-part basis is generally similar to that of chrome plate.

Because of the requirement of heat treating for hardness and for embrittlement relief, costs of

post-deposition treatment are also similar. Life-cycle cost is likely to be similar - perhaps

somewhat higher or lower, depending upon the hardness of the final coating, and its wear

resistance in specific applications.

8.6. Advantages and capabilities

Uniformity - Since electroless nickel is not dependent on the use of electric fields, the method

gives good uniformity and is ideal for coating inside tubes. Consequently it is often used to plate

the insides of cylinders. Coatings can be built up to greater than 10 mils if needed for rebuild.

However, deposition rate is quite slow - usually no more than 1 mil per hour (similar to chrome).

Specifications - A very strong advantage of electroless nickel is the fact that there are existing

commercial and military specifications, making it far quicker and less expensive to adopt than

many other technologies. The primary specifications used in the aircraft industry are AMS 2404

and MIL 1501, which are broad specifications rather than detailed methodologies. The existence

of these specifications, however, makes the process of acceptance for specific components far

easier and quicker than it would be for a completely new process.

Commercial availability - Commercial bath solutions already exist from a number of suppliers.

However, most of these solutions are proprietary, using a basic chemistry, with proprietary

additions. There are a number of vendors who offer the process commercially, albeit far fewer

than the number of chrome plating shops. Ni-P is much more readily available that Ni-B.

8.7. Disadvantages and limitations

Environmental - The primary disadvantage is the fact that Ni is also on the Toxic 17 list.

Although there is presently far less stringent regulation of Ni, it is generally expected that

regulators will turn their attention to Ni once the more serious chrome plating problems are

solved.

Electroless nickel baths have a finite life, after which the bath must be dumped and disposed of

as toxic waste. The bath life is dependent on how the bath chemistry is maintained and on the

materials being processed, but life is usually limited to about 10 metal turnovers. Baths must

also be dumped if they become contaminated with certain metals. For chrome replacement the

most critical contaminant is likely to be chromium. Sirius Technology, which sells electroless
Page 45

nickel plating solutions, states that for a low-P bath, 3 ppm of Cr3+ creates unacceptable

deposits, while 0.2 ppm of Cr6+ stops deposition completely, leading to the need to dump the

bath in either case. Given that any existing component being internally plated will have chrome

plated surfaces (which might not always be completely stripped), the risk of chrome

contamination is high.

Reliability - The second major disadvantage is reliability (especially adhesion strength), which

seems primarily to be related to the difficulty of maintaining proper substrate cleaning and

deposition conditions. The method is very sensitive to bath chemistry and temperature,

including bath contaminants. We suspect that this sensitivity is the source of the variable

experience reported by different users. Long term process reliability remains an issue that must

be treated seriously, especially when the method is to be used in the depot environment where

the workload is constituted by large numbers of different components.

The precipitation of phosphides and borides introduces strains into the nickel lattice that can

result in cracking, porosity, and consequent premature corrosion. Non-uniform precipitation and

segregation has sometimes been found to produce non-uniform hardness, which can create

problems in wear applications.

Technical - Hydrogen embrittlement of high strength steels is a concern for aerospace users.

The deposition process releases hydrogen at the substrate, but unlike chrome, in which

microcracks allow the hydrogen to escape, electroless nickel forms a less porous layer, requiring

some surface area to to be left unplated to allow the escape of hydrogen during heat treating.

As-deposited, Ni-P is relatively soft (500 HV), while Ni-B is harder (750 HV), but still softer than

chrome (1,000 HV). Heat treating for hardness will be a problem for many high strength steels

and aluminum alloys, especially when treatment temperatures are in the region of 400C for up to

8 hours (Cemkote). (This heat treatment also, of course, eliminates hydrogen to prevent

hydrogen embrittlement.) This is not usually a problem for corrosion-resistant use of electroless

nickel, since the best corrosion resistance is obtained in the as-deposited form. However, to

obtain hardness and wear resistance comparable to chrome, as well as to avoid embrittlement,

heat treating will generally be required.

8.8. Developments needed

The electroless nickel process itself is fully developed, and has been in use for 30 years, but has

failed to gain wide acceptance in the aerospace industry, although it is used for niche products.

The primary reason for the failure of electroless nickel to penetrate widely into the aerospace

market appears to be the difficulty of maintaining a consistent plating (from comments made by

Boeing, Messier-Dowty, and others). Consistency is a problem both among different vendors,

and often within a single shop.

Process control - Therefore, if electroless nickel is to be used very widely, methods will need to

be developed and made commercially available to monitor and control the bath chemistry,

temperature, and performance on a continuous basis, as well as to control the heat treatment

temperature and time. These cannot be sophisticated laboratory methods, but will have to be

suitable for use in the general shop environment of repair depots and manufacturing plants.

Some companies have developed computerized methods for production use in their own plants,

but these methods are not commercially available.

The use of composites would make such control methods even more important, since one must

then control bath chemistry more closely (to prevent deposition onto bath particulates) and one

must control the particulates themselves. Methods must be found to maintain the filler powders

in a uniform concentration and to obtain proper entrainment in the coating. This is clearly a

concern for more difficult geometries such as IDs. Difficulty in maintaining SiC in suspension

was the primary reason for Boeing’s failure to adopt the Takada process (which is an electroplate

with SiC additions).
Page 46

Specifications - Although AMS and MIL specifications (as well as internal prime contractor

specifications) already exist, they need to be supplemented with much tighter specifications that

will guarantee the properties and consistency of the material. Additional specs will be needed for

the various composite electroless nickel coatings.

8.9. Recommendations

The primary advantage of electroless nickel is that it is a well-established process, already used

by some aerospace OEMs and repair depots, and already demonstrated to be capable of plating

inside IDs less than 1” in diameter. It already has aerospace specifications, standards, and

repair procedures. It would require very little development, but would require demonstration and

validation for JSF parts. Its long history of unreliability with some OEMs and depot engineers

would require that its reliability be ensured through proper specification and process control, and

clearly demonstrated, not just at a single vendor, but in typical depot overhaul situations.

However, the fact that Ni is on the EPA Toxic 17 list makes it only somewhat less of an

environmental problem than hard chrome. Indeed, it is generally expected that Ni plating

regulation will become more stringent over the next 10 years or so. This has already begun to

happen in California, where the local air quality management district for the Los Angeles area

has been deciding whether to regulate nickel plating. NAMF, the National Association of Metal

Finishers, has responded with a “Nickel Defense Fund”, just as they did earlier with the “Chrome

Defense Fund” to alleviate the severity of chrome regulation and minimize its impact on the

electroplating industry.

Because of environmental concerns, we view electroless nickel as at best a stopgap

alternative to chrome plating. If dry alternatives or clean electroplates cannot be put into

production early enough to meet JSF’s goals, then electroless nickel or electroless nickel

composites could be a viable alternative, if approved by JSF engineers. Because of the

difficulties of process control, they might be used at the OEM level, but not at the O&R level,

since we could presumably have more environmentally acceptable alternatives in place by the

time aircraft are returned in great numbers from the field for servicing.

In order for electroless nickel to garner broad acceptance as a chrome replacement it will be

essential to overcome its real and perceived unreliability. This might best be done by developing

and commercializing production methods for monitoring and controlling surface preparation and

bath chemistry. These methods must work in both OEM and O&R environments, and may need

to be put into place in conjunction with more chemically stable plating solutions. A number of

vendors claim to have more stable chemistries, but these are proprietary. For electroless nickel

to be used on the full range of JSF components not amenable to HVOF, these solutions must be

available commercially, or equivalent non-proprietary chemistries will need to be developed.

If composite electroless nickel coatings are to be used, control of the particulate distribution will

be critical, since this has already proved to be a sticking point in composite electroplating

processes.

Table 20. Summary of developments needed for electroless nickel.

Concern Development needed

Unreliable adhesion and

coating properties

Computerized monitoring and control of substrate surface

preparation and bath chemistry.

Non-proprietary solution chemistries to enhance stability.

Uniform particle distribution

in composites

Improved methods to measure and control particle distribution in

solution and on the surface.
Page 47

9. Process Summary - Chemical Vapor Deposition (CVD)
Page 48

9.1. Introduction

Chemical Vapor Deposition (CVD) has been used for many years in production for deposition of

coatings on externals and internals. It is based on the principle of bringing materials (called

precursors) into the deposition area in the gas phase, and breaking them down and reacting

them (usually with heat or a plasma assistance) to deposit the coating. Because the material is

initially in gaseous form it can penetrate very small and very long IDs. In fact one form of CVD,

called Chemical Vapor Infiltration (CVI) is used in production to fill the interstices in fiber

composites, where the holes are only microns wide and aspect ratios exceed 100:1. CVD

materials can be made extremely thick - e.g. CVD is used to grow ZnS infrared window material

several inches thick and several feet in diameter.

CVD can produce a wide variety of coatings, including metals and compounds. However, the

thermodynamics of the process generally do not permit it to be used for alloy deposition or for

the deposition of composites such as WC-Co.

Most CVD processes are done in a sealed vacuum chamber, with the exception of combustion

CVD (CCVD), which can be done in air. The reason for this is that most CVD precursors are

toxic and/or dangerous (pyrophoric, highly acidic, extremely reactive with water or oxygen, etc.).

Although there are ways of controlling the CVD process, trapping harmful byproducts, and even

of closed loop recycling of precursors, the process is not frequently used outside specialist

companies with well-trained personnel.

Table 21. Summary information - CVD

Process name CVD

Description Deposition by breaking down gas phase precursors using heat or

plasma energy

Coating materials Metals (e.g. Ni, Ta, W) and compounds (oxides, nitrides, carbides,

sulfides), including multilayers. Not alloys or composites.

Temperature Typically 1,000C with simple precursors, 100-500C with

metalorganics (MOCVD)

Minimum ID Microns

Maximum depth Very large - aspect ratios >100:1

Stage of development Production

Primary advantages Capable of high thickness, large aspect ratio holes

Many different coating materials

Primary disadvantages Thermal CVD - temperature too high for structural alloys

MOCVD - dangerous, often unstable precursors
Page 49

9.2. Process description

9.2.1. Thermal CVD

Thermal CVD is the most widely used method for

coating such items as WC-Co metal-cutting inserts,

which are capable of withstanding the typical

1,000C processing temperature. A typical

arrangement is shown in Figure 21. The substrates

in the chamber are usually heated inductively or in

a furnace. Byproducts and unused precursors are

trapped (often cryogenically) and neutralized and

precipitated for disposal. The pump output is

vented through a stack.

For metal coatings carbonyls and halides are

commonly used:

Ni(CO)4 → Ni + 4CO, or

WF6 + 3H2 → W + 6HF

Ceramics use similar types of reactions, e.g.

2TiCl4 + 2NH3 +H2 → 2TiN + 8HCl

Diamond is also deposited by CVD by reactions

such as

CH4 + H2 + O2 → C(diamond) + H2 + 2H2O,

although ratios of reactants vary and the chemistry is poorly understood.

Note that in all these cases, which are typical of the CVD process, the precursors and/or the

byproducts are corrosive, pyrophoric, flammable, or toxic (extremely so in the case of nickel

carbonyl). Careful handling of materials and proper treatment of process exhaust is essential for

worker safety and environmental protection.

The reaction temperature can be reduced by using more reactive species; e.g. the deposition

temperature can be reduced to about 600C by using TiCl + TiCl2 + TiCl3, etc. formed in-situ by

passing chlorine over hot Ti13. Because of the high processing temperatures thermal CVD

is unsuitable for structural components, although it can be (and frequently is) used for turbine

blades and other hot section engine components.

9.2.2. Metal-organic CVD (MOCVD)

Many metal-organics are less stable than the halides, permitting the reaction to be run at much

lower temperatures - typically 400 - 500C, although some reactions can take place only a little

above room temperature. Precursors are typically alkyls and hydrides, such as triethylaluminum,

arsine, and silane. Precursors must be either bought or synthesized for the correct reactions.

They are often very expensive and difficult to handle.

The method is most heavily used in semiconductor and optical applications, where the

mechanical strength and adhesion of the coating are not as critical as they are in wear. Because

of cost and concern over mechanical performance MOCVD has not been used for making

tribological coatings.

 

13 Liburdi Engineering, Toronto, Canada

Precursor 1

Precursor 2

Trap Pump Air

Neutralizer

SUBSTRATES

Figure 21. Typical thermal CVD

system.
Page 50

9.2.3. Plasma and Laser CVD (PCVD and LACVD)

Instead of heat, the reactions can be driven by other energy sources such as plasmas or lasers.

The advantage of these approaches is that it is possible to maintain the bulk of the component at

low temperature while heating only the surface region where the reaction occurs.

In LACVD the most common approach is to use the laser as a localized heat source to deposit

material beneath the beam. In photo-laser CVD optical excitation is the driving mechanism14,

which avoids substrate heating, but deposition rates are generally low. Plasma CVD is more

widely used than laser CVD, especially for large substrates.

A plasma CVD method has been patented by Energy Conversion Devices15 for use in IDs (see

Figure 22). This method uses microwave energy to excite a plasma within the tube and is

claimed to be capable of depositing high quality coatings (such as optical quality SiO2) at

temperatures of about 200C. However, since it depends on the tube being transparent to

microwaves, it cannot be used for metal components, making it useless for structural aircraft

parts. It would in principle be possible to create a microwave plasma by using the ID of the

component itself as a waveguide, provided a standing wave pattern was not set up along the ID

causing strong variations in plasma density (and hence coating thickness).

A large-area plasma CVD method is currently used commercially for the deposition of a clear,

abrasion-resistant SiO2 layer (trade name Vitrinite) on such items as brass hardware, polymer

sheet, and aluminum alloy wheels. The method is based on a technology developed by Dow

Chemical16, and is done by Metroline Surfaces, Inc. using tetramethyldisiloxane (TMDSO) liquid

precursor that is vaporized into the chamber together with oxygen. The resulting material is not

pure SiO2, but a less brittle material SiO1.8-2.4 C0.3-1.0 H0.7-4.0.

Vitrinite coatings can be made as thick as about 2 mils, but could not be used for heavy

rebuild. They offer good corrosion resistance and abrasion resistance to relatively soft materials

such as brass and polymers, but they do not have the hardness of chrome or WC-Co. Note that

most plasma CVD coatings are quite thin (one or two mils at most), making them more useful at

the OEM than at the O&R level.

In this PCVD process there are multiple plasma sources throughout the chamber, which is

typically 6’ dia by 12’ long. These sources create plasma throughout the chamber, from which

the coating deposits onto the substrates. Clearly, a similar method could be used to create

 

14 See, for example, A. Watanabe, M. Mukaida, Y. Imai, K. Osato, T. Kameyama, and K.

Fukuda, J. Materials Sci., Vol.28, pp.5363-5368, (1993).

15 US Patent #5,223,308, “Low temperature plasma enhanced CVD process within tubular

members” Energy Conversion Devices (1993).

16 Patent numbers 5,298,587 (1994); 5,320,875 (1994); 5,433,786 (1995); 5,494,712 (1996).

Figure 22. Plasma CVD inside a tube, Energy Conversion Devices

Patent # 5,223,308.
Page 51

plasma inside a tube, using plasma sources such as post magnetrons, a single rf or dc biased

electrode, or a pair of electrodes, or even a high-current electrode such as that used in the

Surface Solutions PVD ID sputtering method.

Another major use of PCVD is diamond and diamond-like carbon (DLC) coating, using various

combinations of methane, acetylene, hydrogen, and oxygen as precursors. Several companies

sell microwave PCVD systems for this type of process17.

There are many ways of creating a plasma for PCVD, but the methods for ID coating must have

plasma generators that fit inside the component. A plasma generator (such as an arc or high

frequency probe) could be inserted into the ID to create a localized plasma, or an rf or dc rod

electrode could be used to form a plasma along the entire length of the ID. Plasma can even be

generated at atmospheric pressure using a corona discharge18.

9.2.4. Combustion CVD (CCVD)

Because of its high

deposition temperature

thermal CVD is unsuitable for

most aerospace alloys.

CCVD is similar to thermal

spray, but the material being

sprayed is in gaseous rather

than powder form. For

example, tetramethylsilane

can be used for CCVD

deposition of SiO2

18.

There have been one or two

reports of doing CCVD using

a flame spray or plasma

spray gun. One variation on

the method has been recently

developed at Georgia Tech19.

The precursor material (which is the source of the metallic elements of the coating) is dissolved

in a volatile organic solvent, which is pumped into an atomizer nozzle. When the resulting fuel is

burned in air the precursor decomposes and the resulting compound (usually an oxide) deposits

on the surface. Although oxides are obviously the easiest materials to deposit, nitrides and other

compounds can be deposited under the correct reducing conditions.

In common with other CVD processes, CCVD is not line-of-sight and should therefore be capable

of coating internals. Unlike other CVD processes, substrates can be held below 100C during

deposition. The substrate generally needs to be either in the flame or not too far beyond it, but

the temperature can be held down by moving the flame across the surface (or the surface across

the flame) The method therefore avoids coefficient of thermal expansion (CTE) mismatch

stresses, which arise in thermal CVD from the difference in thermal expansion as the

temperature falls from the deposition temperature. However, internal coating stress is not known

and is likely to be a limiting factor in coating thickness, especially for ceramic coatings, as is

commonly the case with PVD ceramic coating methods.

 

17 For example, AsTex, Inc; Advanced Refractory Technologies

18 Fraunhofer Institute, Braunschweig, Germany

19 The main proponent of Combustion CVD is MicroCoating Technologies in Atlanta, who have

exclusive rights to Patent # 5,652,021, owned by Georgia Tech, issued in July 1997.

Pump

Organic solution

Atomizing gas

Pilot

Flow controller

Atomizer

CCVD flame

Substrate

Figure 23. MicroCoatings Technologies' Combustion CVD.
Page 52

9.3. State of development

Thermal CVD is a widely used commercial process, but its high process temperature makes it

unsuitable for structural components.

Metal-organic CVD is in wide commercial use, but not for mechanical coatings where adhesion

and mechanical strength are critical. Precursor cost and availability and safety concerns

preclude its use in O&R.

Plasma CVD is commercially available for limited applications and materials. If the right coating

material were used it could be a viable method, although coating thickness may not be enough

for its use in O&R.

Laser CVD is still in the development stage and used mostly for semiconductor and optical

applications. The Kansai Research Institute in Japan does offer laser CVD systems for

commercial use.

The CCVD process is particularly interesting because of its simplicity and lack of dangerous

precursors (although it does use volatile organics, which are burned in the process). It is

currently under research and commercial development, and is being tested for a very wide range

of coating materials and applications, including corrosion, wear, electrical insulation, fuel cells,

catalysis, and high temperature superconductivity. MicroCoating Technologies has not

attempted to carry out internal coating with the technique. There is therefore no data on how

well it would work for this application, although we would expect the CVD technology to be quite

effective for internals. At this point there is insufficient data to provide a clear decision on the

suitability of the method for internal coating.

Although MicroCoating Technologies advertises the coating as a chrome replacement, there is

no publicly available data that confirms its suitability for this purpose. Silica and alumina are

usually somewhat harder than hard chrome (and therefore more abrasion resistant). The

coatings appear to be considerably more corrosion resistant than chromate conversion coatings

(on aluminum) or hard chrome. However, scratch-corrosion testing does not appear to have

been done.

MicroCoating had completed a successful Phase I program and now has a Phase II SBIR

program with the Navy (contract # N00014-97-C-0265, awarded in April 1997) for development

of chromate conversion replacement and thermal oxidation resistant coatings. These coatings

are based on silica and alumina20, although the company also has looked at mullite, YSZ, and

chromia. Initial data shows that, as barrier coatings, silica and alumina have good hightemperature (800-900C) oxidation resistance and salt-fog corrosion resistance (1344 hours). No

comparative data with chrome have been given (chrome would not be used at these high

temperatures), and there is no indication of mechanical performance. The coatings are usually

in the micron thickness range (from 10 nm to 5 μm). We would not expect the method to

produce good mechanical coatings several mils in thickness because typical oxide ceramic

coatings are usually too highly stressed and too brittle, especially at high thickness. This would

limit the use of the method to OEMs.

9.4. Coating materials and properties

The CVD method is extremely versatile in terms of the number of different materials that can be

deposited. CVD works best for depositing simple compounds such as nitrides, carbides, and

oxides (including mixed oxides, which are difficult to do by thermal CVD but can be done by

CCVD) and metals such as Mo and Ni. Since it is done in air, CCVD is best at depositing oxides.

Alloys cannot be deposited by CVD, although it is possible that the CCVD process may be able

 

20 A. Hunt et. Al. “Thin film coatings for aircraft via CCVD processing”, Proceedings -

Aerospace/Airline Plating and Metal Finishing Forum, p 139-157, San Antonio (1998).
Page 53

to deposit alloy chemistries but in amorphous or nanocrystalline form, probably with quite

different properties than those of the bulk alloys.

High quality wear coatings (nitrides and carbides) are almost exclusively deposited by thermal

CVD. There is little or no data on the properties or performance of wear materials produced

other CVD methods. In general we should expect most plasma or combustion CVD coatings

deposited at low temperatures to have relatively poor adhesion, just as PVD coatings have,

unless the process involves scrupulous surface cleaning and ion bombardment. Laser CVD

coatings could in principle be fused with the underlying material to produce a very good bond,

but there is very little information available on the mechanical properties of such coating.

9.5. Capabilities

Dimensions - Thermal and MOCVD can be done in holes as small as a few microns in diameter

and aspect ratios of greater than 100:1. PCVD is more limited by the need to create a plasma in

the ID, and is likely to be limited to 0.5” or so in diameter, depending on the design of the plasma

generator. Since CCVD has not been attempted in IDs there is no data on the subject, but it

should be possible to coat IDs greater than about 0.5”, since, unlike the thermal spray methods,

the CCVD flame does not need to be aimed at the surface. (However, adhesion in these

circumstances is questionable since the method would not provide energy directly to the

surface.) LACVD could similarly coat small IDs, provided only that a mirror can be placed within

the component.

Coating materials - Almost any metal or simple compound (nitride, oxide, or carbide) can be

deposited, but the method is not suitable for alloys, composites, or complex compounds. CCVD

may, however, be capable of depositing alloy chemistries by the simple expedient of dissolving

more than one metal salt in the solvent, although this has not been demonstrated.

Substrate materials - Thermal CVD is limited to high temperature materials, such as turbine

blade alloys, which can withstand 1,000C process temperatures. PCVD and CCVD are capable

of very low process temperatures (as low as 100C), allowing them to be used on any structural

material.

9.6. Limitations

Precursors - Many CVD precursors are toxic, flammable, pyrophoric, or highly acidic, posing

serious potential hazards to personnel. While they can be handled safely in the controlled

environments of semiconductor plants and CVD coating shops, they are not likely to be easily

accepted for use in the much more diverse and open environment of DoD depots.

Materials - While CVD methods can deposit a very wide range of materials, the materials

suitable for chrome replacement are much more limited. CVD methods cannot be used for

depositing composites such as WC-Co, but they can be used for depositing hard metals or

oxides and nitrides. However, the mechanical performance of hard coatings deposited by

methods other than thermal CVD is as yet very poorly documented. The materials most easily

deposited by CCVD are oxides, which are likely to pose severe problems for stripping, since

most are impervious to all but the strongest acids.

Equipment - While there is commercial equipment available for thermal CVD, there is very little

commercially available equipment for PCVD, LACVD, or CCVD. Metroline manufactures PCVD

equipment specifically for Vitrinite deposition. However, there appears to be no commercially

available equipment for ID CVD deposition.

Thickness - Thermal CVD is used to fabricate bulk objects such as infrared windows. However,

it is far from clear how thick potential chrome replacement coatings can be made when

deposited by PCVD or CCVD. In most cases, other CVD methods are limited by internal stress

or low deposition rates. For example, while thickness and rate data are difficult to obtain,

practitioners of these processes consider 0.002” (PCVD) or even 0.0002” (CCVD) to be “thick”. It
Page 54

is likely, therefore, that these methods will be limited to OEM use, and will not be reasonable

methods for O&R rebuilding.

9.7. Developments needed

A great deal of development would be needed to bring any of the CVD processes into use for ID

coating of structural components. Developments need to focus on materials, processes, and

equipment.

Materials - An adequate chrome replacement material needs to be defined that is compatible

with safe, readily available, and inexpensive precursors as well as meeting the technical

requirements for chrome replacement.

Processes - Processes must be developed that can be done both in small and large

components, and can safely be used in both OEM and O&R environments. CCVD is probably

the most likely candidate to achieve this aim since it does not entail flooding the component with

precursor gas.

Equipment - ID coating equipment would need to be developed. Small CCVD torches do not

seem to pose any serious difficulty. PCVD using a small plasma head operating with precursors

injected through the plasma might also be a viable option. In both cases, the design would need

to limit the use of precursors and attempt to ensure that virtually all the precursor material is

consumed in the process, which will make the equipment much safer to operate.

9.8. Recommendations

Of the above methods, only a limited number are suitable for structural components, and fewer

still are suitable for use in depot maintenance. As shown in Table 22, only PCVD and CCVD

would appear to be potentially suitable for ID chrome replacement. Of these, CCVD appears to

be the simplest, but it may also be the most limited in the materials it can deposit.

Given the lack of development and lack of knowledge on these processes, we do not

recommend them for ID coating at this time. However, PCVD and CCVD might be viable

OEM processes, although it is doubtful that they can be used for O&R.

Table 22. Summary of CVD suitability.

CVD

technology

OEM

suitability

Depot

suitability

Notes

Thermal CVD No No High temperature.

Plasma CVD Yes Yes No commercial processes or equipment for IDs.

Significant development needed.

Laser CVD Perhaps Perhaps No commercial processes or equipment for IDs.

Significant development needed. Laser power and

surface heating are concerns.

CCVD Yes Yes Needs significant testing and development.

Note: All CVD methods require the right choice of coating material for performance and

thickness, and attention to availability and safety of precursors.
Page 55

10. Process Summary - Physical Vapor Deposition (PVD)
Page 56

10.1. Introduction

PVD is a vacuum process, usually, but not always, having a base pressure in the 10-6 Torr region

(high vacuum). In general the use of vacuum chambers imposes limitations on component sizes

and requires the utmost cleanliness to prevent contamination.

We consider here only those methods that are designed for ID coating.

10.2. Process description

PVD is the general name for a large number of vacuum processes that start with the coating

material in a solid (or in rare cases a liquid) form, vaporize it, and deposit it onto the surface.

The primary PVD methods are

• Evaporation - usually by electron beam or from a resistive boat (can be very fast)

• Arc - spark erosion of the deposition material (moderately fast, high ion bombardment,

generally rougher surface)

• Sputtering - erosion of the deposition material by a plasma or ion beam (usually slow,

but high quality).

While the principal PVD methods are variants of thermal evaporation, arc evaporation, and

sputtering, there are many less widely used PVD methods, including

• Ion Beam Assisted (or Enhanced) Deposition (IBAD, IBED), which uses an ion beam

instead of plasma bombardment

• Jet Vapor Deposition (metal vapor entrained in a high velocity gas stream),

• laser ablation (evaporation by laser pulses), and

• exploding wire (evaporation by a large current pulse in a thin wire).

Table 23. Summary information - PVD

Process name Physical Vapor Deposition (PVD)

Description Vacuum deposition from a solid or liquid source - typically sputter,

arc, or evaporation

Coating materials Ta, nitrides and carbides (such as TiN, CrN)

Temperature Typically 150 - 450C. Without active cooling temperatures would

probably exceed 600C.

Minimum ID Sputtering - 2”, larger with blind holes. Plasma jet - less than 1⁄2”

Maximum depth None

Stage of development Sputtering - commercial development (in test at Boeing). Plasma jet

- research.

Primary advantages Cleanliness, range of chemistries for different applications

Primary disadvantages Vacuum process, cost, mostly thin coatings (thick coatings with

limited chemistry)
Page 57

Almost all PVD methods require ion bombardment, by a plasma or ion beam, both of the

surface, to clean it atomically prior to coating, and of the growing coating, which otherwise tends

to be weak and porous. In fact, inadequate or non-uniform ion bombardment is the most

common cause of failure in PVD coating.

Most PVD methods are not suitable for use in IDs because they are essentially line-of-sight, with

limited throwing power (i.e. ability to coat around corners and down holes).

10.2.1.ID sputtering

By its nature, PVD is inherently a line of

sight method. The most advanced ID PVD

method is the sputtering method developed

by Surface Solutions, Inc., of Colorado,

which is illustrated in Figure 24. Efficient

sputtering requires a high plasma density in

the vicinity of the material to be deposited

(the cathode), which is achieved by the use

of a magnetic field in a “magnetron”

configuration. In the magnetron electrons

follow corkscrew patterns around the

magnetic field lines, ionizing the gas, which

is usually held at a pressure of about 3 - 10

millitorr. The positive ions from the plasma

are attracted to the target, which they hit at

an energy of 500 - 1,000 eV, knocking (or

sputtering) the surface atoms from the

target.

Most sputter magnetrons have permanent

magnets to create the magnetic field. Since

permanent magnets are generally rather

bulky for ID coating, the approach adopted by Surface Solutions is to create the field by running

a very high current (1,000 - 2,000 A) down a water-cooled cathode in the form of a hollow tube

that is placed in the center of the hole. This current creates a circular magnetic field around the

rod and sputtering occurs down its entire length, depositing material along the entire ID.

10.2.2.Plasma jet

An alternative method is under development in Sweden21. It consists of a small gas jet made of

the material to be deposited, that can be biased to create a plasma that sputters material from

the jet onto the walls of the tube. An entire ID could be coated by moving the jet along the axis

of the hole. It would not be suitable for large areas, but is capable of coating very small

diameters. This method is in the research stage, and is far less developed than the Surface

Solutions method, and for that reason, we shall not consider it further here.

 

21 See, for example, H. Barankova, L. Bardos, and S. Berg, J. Electrochem. Soc., 142, 883

(1995); H. Barankova, L. Bardos, T. Nyberg, and S. Berg, J. Electrochem. Soc., 141, 375 (1994)

Magnetic

field power 

supply

1V

2,000A

-

Bias power 

supply

+

Figure 24. Schematic of Surface Solutions

sputtering system.
Page 58

10.2.3.Exploding wire

Another PVD process that would seem ideally suited to coating the insides of

tubes is exploding wire technology. The method consists of passing a fast

high-voltage (5-10 kV), high current pulse through a thin wire in the center of

a tube (usually by discharging a high-voltage capacitor through it). Over a

period of a few microseconds the wire melts and the vapor ionizes in the

electric field, forming an intense plasma and a rapidly-expanding sheath of

vapor and small droplets that expand to the wall and coat the tube ID.

Exploding wires have historically been used for fundamental plasma studies

or for igniting some of the older high-power lasers. In the 1980’s there was a

brief flurry of activity in evaluating the method for coating, but it disappeared

and has never surfaced as a coating process.

Grabatin et al22 actually used the process for deposition of Ta coatings onto

20mm ID, 70 mm long gun tubes to a thickness of about 0.004” by exploding

a single 1mm diameter wire. The coating performance appeared quite good.

Since this time, however, the process seems to have disappeared from the

literature, and is not being offered commercially.

10.2.4.High rate chrome deposition by PVD

Since the primary problem with chrome plate is not the chrome itself, but the hexavalent air

emissions, an alternative to chrome plating is PVD coating. (This does not, of course, address

the problem of stripping solution disposal.) A number of companies have developed PVD

chrome alternatives. For example, Implant Sciences of Wakefield, MA has developed a high

rate arc deposition process capable of depositing at rates >1μm/min (>0.002”/hr). Preliminary

data show that the material was about 25% softer than hard chrome (588 KnH, vs 766 KnH for

hard chrome), but performed much better in pin-on-disc wear tests in hydraulic lubricant. At this

point, however, the method uses a planar arc source that is capable only of deposition on

externals, and the deposition equipment would have to be re-engineered in order to use it for ID

coating.

There are methods for arc

deposition inside a tube. Wedtech

Corp. has a patent for arcevaporating material onto the inside

of a tube by spiraling the arc along

an internal electrode23. The method

is shown in Figure 26, which is

drawn from the patent. An arc

struck by a movable electrode at

one end of the depositing electrode

spirals along it, depositing coating

inside the tube. How well this works

in practice, however, is not known

since Wedtech Corp no longer

exists and the method is not in

commercial use.

 

22 H. Grabatin, V. Schlett, H. Stuke, and H. Weiss, J. Vac. Sci. Tech. A3, 2545 (1985).

23 US Patent # 4609564, “Method of and apparatus for the coating of a substrate with material

electrically transformed into a vapor phase”, Wedtech Corp., inventor Eduard Pinkhasov (1986).

Figure 25.

Exploding wire.

Figure 26. Wedtech spiral arc coating of tube ID - US

Patent #4,609,564.
Page 59

10.2.5.Ion beam assisted deposition (IBAD, IBED)

Ion beam methods of PVD use

an ion beam instead of a plasma

to clean the surface and provide

energy to the growing film.

These methods are not suitable

for ID coating because of the

difficulty of putting an ion beam

down an ID. Several people

(including ourselves) have

attempted to use an ion beam to

sputter a conical “slug” inside a tube to get around the line-of-sight restriction (Figure 27), but

have found that sputter rates are far too slow for cost-effective deposition.

Another proposed approach has been to use variants of the

plasma source (or plasma immersion) ion implantation

method (PSII, PI3

). This approach uses a plasma as the

source of ions, pulse-biasing the substrate (in this case, the

tube) to effect ion bombardment (Figure 28). This approach

can be thought of as a high voltage variation of the Surface

Solutions method outlined in Section 10.2.1, but without the

1,000A DC magnetron current. Proposed methods have

involved ideas such as alternately biasing a central rod

negative and positive to create alternating bombardment of

the rod (to sputter a coating) and the tube ID (to irradiate the

coating). This method will only work at quite high pressures

since the method relies on a plasma outside the dark field

(cathode fall region). This region around a high voltage electrode increases with voltage and

decreases with increasing pressure, posing a lower limit on the diameter of tube in which a

plasma can exist at a particular pressure and voltage. This method does not appear to offer

any substantial advantage over other (simpler and cheaper) sputter methods.

10.2.6.State of development

Of the PVD methods the only one close to commercial use for ID coating is the Surface

Solutions approach. The Surface Solutions method has been in existence for some time, but

has never become widely used. It is currently in the commercial development stage, with some

limited commercial use. In our discussions below we will assume the use of the Surface

Solutions ID coating method.

Praxair and Surface Solutions have been evaluating the method under a NIST ATP program,

which is nearing completion, for depositing traditional PVD coatings such as TiN. Because the

final report has not yet been submitted, Praxair has not been willing to discuss the results.

However, they say that they have been able to improve adhesion and coating quality,

presumably by increasing ion bombardment of the ID wall. The report for this program should be

released in the next few months.

In the Green Gun Barrel Project, the Army’s Benet Lab is developing its own version of the

Surface Solutions approach, which is in the development-demonstration stage. The Benet work

has been supported by a SERDP program, which has now ended. A vertical coating system has

been built, and has made a few test runs on gun barrels, but test firing results so far have not

been very good (albeit better than other alternatives tested). The primary problem appears to be

inadequate ion bombardment of the ID, both for plasma cleaning prior to coating, and for

Slug Ion beam

Coating

Figure 27. Possible proposed ion beam coating of ID.

Electrode

Dark space

Plasma

Figure 28. Possible proposed

PSII arrangement.
Page 60

establishing a high quality coating during the deposition process itself. The method is also very

slow - about 24 hours for a 3 mil coating.

Boeing Defense is just beginning tests of an ID coating addition made to an existing IVD coater.

While IVD has proved to be a viable commercial replacement for cadmium, the line of sight IVD

process is incapable of coating IDs. In mid-1998 Surface Solutions delivered to Boeing St Louis

equipment for depositing Al onto IDs, which is currently under test.

10.2.7.Coating materials and properties

For gun barrels, which are subject to extreme high temperature pulses and high velocity particle

erosion, there is evidence that electroplated tantalum is a great improvement over hard chrome.

Unfortunately Ta must be deposited by a salt bath method that is difficult and also has severe

environmental problems. Benet Labs also wanted a coating that would be smooth enough to

require no finishing after deposition. For these reasons they opted for sputtered Ta. It is

important to note that, while gun barrels and hydraulics are both chrome plated, their

wear mechanisms are entirely different, meaning that the optimum coating materials for

these two different applications will almost certainly be very different. Sputtered Ta may

or may not be comparable to chrome in wear resistance, as may Mo (which is thermal sprayed

on piston rings for wear), W, Zr, or, of course, vacuum Cr. Tests would be needed to evaluate

their relative wear properties.

• In general, the traditional PVD coatings such as TiN and CrN have been used for aerospace

wear applications24, and in some cases for chrome replacement, but not in IDs.

• More lubricious materials such as DLC have shown more promise for hydraulics25. In this

case the most critical issue is seal wear.

• The Japanese piston ring company, Taikuku, has developed, and sells commercially, a

piston ring with a 60μm thick CrN/O arc coating in place of chrome plate. The coating takes

several hours of deposition and must be highly polished after deposition. It is consequently

very expensive.

Clearly, from Table 24 and Table 25 far more is known at present about the properties of

traditional PVD coatings, and there is little solid information on PVD metals such as Cr and Ta.

There is almost no information on the properties of ID Cr and Ta, meaning that if they are to be

considered as chrome alternatives, a great deal of fundamental evaluation will be required. Of

course, the method is not limited to elements or simple compounds. It could be used to deposit

wear resistant alloys such as Hastelloys, Inconels, or Tribaloys. However, knowledge of their

properties is practically non-existent for PVD coatings, although there is a significant amount of

data for plasma sprayed coatings of these materials. Because the sputtering process takes the

material apart atom by atom and reassembles it at the substrate, it can reproduce the chemistry,

but in general it does not reproduce the microstructure of the bulk material. Therefore the

properties of PVD alloys may be quite different from those of the bulk materials.

 

24 GE Aircraft Engines, Sundstrand Aerospace, for example

25 “Evaluation of chrome rod alternative coatings”, Report #GTE0598 (1997).
Page 61

Table 24. Materials properties - typical wear-resistant PVD coatings (nitrides)

Property Value Notes

Materials TiN, ZrN, TiC,

TiAlN, TiCN,

BN, DLC, CrN,

Cr

Most materials are nitrides or carbides of transition

metals

Hardness 1200 - 2400

HVN

Depends on material and internal stress

Maximum thickness 15 μm Usually limited to about 5 μm

Corrosion resistance Moderate Limited by pinholes

Wear resistance Excellent Better than chrome, comparable to thermal sprays.

Note that wear life can be low because thickness is

small.

Lubricity A little less

lubricious than

chrome

DLC and BN are particularly lubricious

Porosity <1% Pinholes are primary problem

Stress and effect on

fatigue

Strongly

thicknessdependent

PVD coatings generally have very high compressive

stress. Fatigue is usually adversely affected at

coating thickness above a few microns.

Hydrogen

embrittlement

None

Table 25. Typical materials properties - PVD Cr and Ta coatings

Property Value - Cr Value - Ta Notes

Hardness 588 KnH Unknown Can be increased by N incorporation

Maximum thickness Unknown >0.003” Several mils are probably possible with both

materials

Corrosion resistance Similar to

hard EHC

Moderate NaCl solution tests

Wear resistance Pin-on-disc

better than

EHC (approx

1/10)

Unknown.

Hot erosion is

critical for

gun barrels

Wear rate is very sensitive to conditions.

Data on in-service wear not available

Lubricity Unknown Unknown

Porosity Unknown Unknown Depends on deposition method. Expect

pinhole corrosion

Stress and effect on

fatigue

Probably

compressive

Probably

compressive

Most thick PVD coatings reduce fatigue life

Hydrogen

embrittlement

None None
Page 62

10.3. Capabilities

Dimensions - IDs as small as 1” can be coated, and in a vertical system the item could be

several feet long.

Coating materials - Almost any metal, alloy, or simple compound (nitride or carbide) can be

sputter deposited, with varying rate and cost depending on the material.

Substrate materials - In principle the method is not limited by substrate. In practice, heat

sensitive alloys are likely to be tempered unless heat can be removed efficiently from the ID

surface through the wall.

Equipment - It appears to be possible for ID coating systems to be incorporated into existing

IVD units. However, they will require additional jigging, fixturing, and feedthroughs.

10.4. Limitations

Thickness - The primary limitation of PVD coatings is their thickness. The Ta gun barrel

coatings are unique in their thickness. In most cases (especially for the more wear resistant

ceramic coatings such as TiN and TiC) PVD coatings are deposited with quite a high

compressive stress as a result of ion bombardment during coating. This stress builds as the

thickness grows, until at some point it causes delamination. As a result PVD coatings are

typically about 3 μm thick, and seldom more than 15 μm (about 0.0005”), with Praxair being the

only aerospace PVD vendor we are aware of capable of offering these coatings in the 15 - 20 μm

range. The only PVD compound coating with a thickness of several mils is CrN/O used on piston

rings. As a result of thickness and cost limitations, PVD coatings are generally useless for

rebuilding worn components, which usually require thicknesses of 0.003-0.010”.

Because they are less brittle and can be deposited more easily at a higher rate, metals can be

realistically deposited at greater thicknesses than PVD ceramics. Ta coatings several mils thick

can be used in gun barrels because Ta, being a metal, has a high fracture toughness compared

with typical ceramic coatings, and we would expect that the lack of ion bombardment of the

substrate during coating limits the compressive stress. (This is also the situation for commercial

IVD aluminum, which can be deposited in thick layers.) We would expect a similar situation for

PVD Cr and for PVD Mo (which is presently arc or plasma sprayed for replacing chrome on

piston rings and some other automotive components).

Vacuum - The requirement for vacuum deposition imposes limitations on the size of items that

can be processed, as well as the types of items that can be coated. For example, complex parts

with slots and holes that act as reservoirs for oils and grease can be extremely difficult to coat

reliably. Most vacuum chambers are a meter or so in size, although Ivadizers are several feet in

diameter and long enough to accommodate even most landing gear parts. The need for vacuum

does tend to raise the cost of most PVD processes, and requires properly-trained technicians.

Deposition rate - The sputtering process is one of the slowest PVD coating methods, with a rate

of a few microns per hour. A gun barrel can be coated in about 24 hours. The deposition rate

for hard ceramic coatings is frequently much lower than that for metals (depending on how

process control is effected), and often drops to about a micron per hour. Combined with the high

capital cost of PVD equipment, these long deposition times lead to very high costs for thick PVD

ceramics, even where they can be deposited without flaking and delamination due to high

internal stress.

The deposition rate for metals can be far higher - according to Implant Sciences, Cr can be arc

deposited at more than a micron per minute.
Page 63

Temperature - Most hard coatings require a substrate temperature from 150C to 450C, and in

general the lower the deposition temperature, the less reliable the process. The lower range of

temperatures is not a problem for most chromed components, but the upper range would be

unacceptable for most aluminum alloys and high strength steels.

Blind holes - Because the method requires access to both ends of the cathode rod, the Surface

Solutions method cannot presently be used to coat blind holes. Surface Solutions is working on

a new design to overcome this limitation, but it is currently only in the concept stage. It will

doubtless be larger in diameter than the existing system because of the need to bring the current

down and back, presumably with magnetic shielding or a dipole arrangement to prevent the

opposing currents neutralizing the magnetic field.

10.5. Fit with OEMs and depots

Thickness - Because of the thickness limitation for hard PVD coatings, the fit with depot

maintenance is poor. PVD coatings can be (and are) used for chrome replacement by OEMs,

especially on components that are high tolerance and that never need to be rebuilt (e.g. engine

components that are typically replaced rather than rebuilt). However, the inability to rebuild

worn components makes PVD hard coating of no real value for hard chrome replacement

in the O&R environment.

Vacuum - The complexity of vacuum equipment does not generally fit well with depot

maintenance shops. It is very different from most other depot rebuilding and coating operations,

which are predominantly involved with machining, welding, thermal spray, and bath deposition.

However, with the replacement of cadmium plating by IVD aluminum, IVD (a vacuum PVD

process) is now in use at a number of depots, and the depots are building up experience with

operating this type of vacuum equipment. The equipment used for IVD aluminum is not typical

of that used for hard coatings, which is generally more complex. On the other hand, there have

been efforts in DoD to extend the capabilities of this type of equipment to hard coatings. In

principle, it may well be possible to incorporate ID PVD into existing IVD chambers, and

ensure a better fit with depot operations. This principle is being tested at Boeing-St. Louis for

depositing IVD aluminum onto component IDs. If it is successful at Boeing, the equipment may

be installed at Ogden ALC.

Material - One very favorable aspect of PVD is that the method can be used for rapid deposition

of chrome. At this point high rate chrome deposition has not been demonstrated for ID coating,

but there is no reason in principle that it should not be possible. It would presumably be easier

to obtain approval for PVD chrome because of its similarity to electroplated chrome. This

approach would eliminate air emissions, but would not eliminate chrome stripping and

grinding wastes (although grinding is likely to be minimal for smooth PVD coatings).

10.6. Typical costs and cost factors

In PVD coating, cost is directly related to deposition rate. Labor is the primary cost factor, with

capital equipment depreciation a close second in most cost models. Materials tend to be a

rather small fraction of the total cost. However, as the coating thickness grows, materials costs

become increasingly important. The Surface Solutions method requires a hollow tube of

tantalum, chromium, or other less-common metal. The cost of these types of materials can be

high and availability can be a problem. They should, however, be easier to obtain than the large

flat plates typically used in planar magnetrons. At present there are no data available on

production costs with ID magnetrons. However, there are various cost models for standard PVD

coatings.

For example, Implant Sciences estimates a production cost of $0.22 /in2

 for a 1 mil Cr coating on

simple cylinders, which works out to be about 20% greater than hard chrome plating. This

estimate takes into account normal business costs and actual coating costs, but does not include
Page 64

pretreatment (where we would expect PVD to cost a little more because of the need for more

careful cleaning), or post-treatment such as hydrogen embrittlement anneals (which raise the

cost of hard chrome) and grinding.

For work we have previously done under DARPA funding, we estimated costs for chrome plating

versus PVD and thermal spray methods. A 3μm CrN coating on piston rings (again a very

simple geometry with no hydrogen embrittlement relief needed even for chrome plate) had a

cost of about $0.55 /in2

 as against about $0.80 for chrome plate. In this case finishing (grinding)

was about 30% of the cost for hard chrome, but only 5% for PVD coating - an unusual situation.

When a 15μm CrN PVD coating was used (which Cummins Diesel considered would be

necessary to attain the same wear life as the 3 mil chrome plate), the cost was estimated at

about $1.70/in2

.

10.7. Developments needed

Thickness - At the OEM level, thin coatings are perfectly acceptable, provided that the wear life

(wear rate x thickness) is adequate. To be of value in the O&R environment, rebuild thicknesses

will be needed, requiring deposition rates of at least a mil per hour (about 10 times current hard

coating rates). At the same time methods must be devised to ensure that these very thick PVD

coatings have little internal stress.

Cleaning and ion bombardment - This is a critical issue. PVD coatings do not work unless the

surface is scrupulously clean. First, chemical cleaning is required using aqueous cleaners.

Present aqueous cleaning systems for PVD usually involve high pressure liquid jets, which

cannot be used effectively on tube IDs.

Secondly, the surface must be atomically cleaned by ion bombardment in vacuum immediately

prior to deposition. With the present geometry, it has proved very difficult to adequately ion

clean the substrate or ion-irradiate the growing coating, both of which are crucial to proper

adhesion and hardness. In standard PVD coating these issues are handled quite easily.

However, in a closed tube surface oxides must be removed without redepositing them

elsewhere, and there must be sufficient plasma density at the tube wall for effective ion

bombardment of the growing coating.

Watervliet has not yet found a way to do this, but Praxair seems to have come up with methods

that improve the coating quality, presumably involving some measure of solution to these ion

bombardment problems. The solution to this problem must be made generally available as part

of the deposition system.

Materials and performance - For OEM use, the standard (thin) nitrides and carbides are likely

to be the most useful. However, for O&R use, thicker coatings of metals such as Cr, Ta, or Mo

(which can be deposited much more rapidly with less internal stress) or perhaps of hard alloys

such as Hastelloy or Tribaloy, appear to be the only viable options. Extensive testing and

performance measurement will be needed to be sure of the adequacy of these coating materials.

Note that the mere fact that an alloy such as Hastelloy is used as the cathode merely ensures

that the coating will have similar chemistry. It does not ensure that it has similar properties,

since the properties of most alloys depend on their microstructure, which is likely to be

completely different in the coating from the bulk alloy. (PVD is a non-equilibrium process

whereas alloy manufacture is an equilibrium thermodynamic process.)

Equipment and process - Vacuum deposition equipment will need to be made simple to

operate, with built-in process menus. The most viable approach for O&R depots would seem to

be to install ID hard coating equipment into existing IVD chambers, which are large and which

the depots already know how to operate.

Surface preparation and finishing - Machining and cleaning of the tube ID are critical to proper

PVD coating adhesion - much more than any other type of coating. Unfortunately most cleaning

systems currently used for PVD processing are made for bath-cleaning and spraying ODs, but
Page 65

are not well-designed for ID treatment. Production PVD coating will require careful attention to

the design of new methods to ensure proper cleaning of IDs.

PVD coatings are generally considered to be very smooth and therefore to need no further

finishing. This is true for typical thin coatings. However, as coatings become thicker, differences

in grain growth rates tend to make some crystallographic orientations grow at the expense of

others, coarsening the surface. Proper grinding methods will need to be developed for PVD

coatings.

10.8. Recommendations

PVD coating technology appears to be a niche product for IDs, best suited to high-value OEM

products. Traditional PVD hard coatings are not suitable for O&R use since they cannot be

grown thick enough due to stress and cost limitations. Metallic PVD coatings, such as Cr, Ta,

Mo, and hard alloys, may be viable for rebuilding IDs. Unlike the nitrides the metals can be

deposited to higher thickness without excessive stress, and metal deposition is relatively simple.

However there are several issues that will need to be addressed:

• Deposition methods and equipment - Although the equipment exists to sputter deposit

metals on IDs, many of the details of the process require development, especially ion

cleaning and ion bombardment, both of which are critical to the PVD process.

Equipment will need to be redesigned for use in blind holes. We believe that the best

approach would be to adapt the Surface Solutions method to existing IVD chambers.

• Performance - Performance of the materials that can be used for rebuilding in typical ID

applications is unknown (and likely to be inferior to the traditional hard nitrides). In this

regard, Cr deposition holds the least risk, since it is basically the same material as that

which it replaces. However, microstructure is clearly going to be different, and details of

the chemistry may be changed as well. Furthermore, of course, PVD Cr does not

eliminate stripping and grinding wastes.

• Cleaning - this is a critical issue for IDs. While important for any coating method is

especially crucial for PVD coatings. Typical aqueous parts washing and precleaning,

which work well on ODs, will have to be adapted to IDs.

Overall, PVD coating could meet many of the OEM and O&R requirements for ID coating. While

the limitations of vacuum processing would make it difficult to do on the IDs of large

components, it may be a viable method for chrome replacement on medium to small component

IDs.
Page 66

11. Process Summary - Laser Processing
Page 67

11.1. Introduction

Laser processing breaks down into three main techniques:

• Laser cladding, in which a solid (powder, wire, etc.) is fused onto the surface

• Laser CVD (in which material is deposited from the gas phase)

• Laser alloying (in which material is integrated with the underlying surface)

• Laser surface modification (in which the laser beam is used to glaze or “peen” the

material without introducing additional elements).

While the latter two items may be of value to OEMs, in common with heat treatments, they are

not of value to O&R operations since they cannot be used for rebuild. In fact, however, the

methods are somewhat indistinct, since the difference between cladding and alloying, for

example, is primarily determined by the power and length of the laser pulse.

Laser methods are becoming more industrially viable as lasers have improved dramatically in

the past few years, with the production of high power Nd-YAG, CO2, and even diode lasers, that

are robust and designed for industrial use. Furthermore laser integrators can now design and

produce entire laser systems for cutting, engraving, and materials modification.

Table 26. Summary information - laser processing.

Process name Laser coating Laser alloying Laser modification

Description CVD or PVD

addition, primarily

welding

Surface melting and

alloying of material

on surface

Glazing or “peening”

by direct lasersurface interaction

Coating materials Metals and alloys Not a coating -

limited build-up

None

Temperature >500C Melt temperature Peening - low

Glazin - melt

Minimum ID Depends on method Depends on method Depends on method

Maximum depth Unlimited Unlimited Unlimited

Stage of development Niche production Development Development and

niche production

Primary advantages Simple heat source

Moderate capital cost

Excellent adhesion

Excellent adhesion

(not a coating)

Gradable alloys

Reduced porosity

(glazing)

Low temperature

(peening)

Primary disadvantages Easy to overheat

surface (fatigue

concern)

Not suitable for O&R

Not suitable for buildupNot suitable for buildup
Page 68

11.2. Process description

11.2.1. Laser cladding

Laser cladding is the general name for a method of

coating in which coating material is brought to the

surface (usually in powder or wire form) and melted (or

welded) onto the substrate using a high power laser such

as a Nd-YAG or CO2. Figure 29 shows a powder-fed

cladding system. The powder is fed from an angle

above, and the laser comes from directly above (hence

the heating of powder particles bounced or expelled

upwards into the beam.

Laser cladding requires sufficient surface heating to

weld the materials together, which means that the

surface heat treat is modified by the process, leaving a

heat-affected zone beneath the coating. The coating

tends to be very rough, with a “ploughed field” effect of

raised strips of weld material. Instability in the powder

feed can leave gaps in the coating with excessively

heated surface beneath.

Laser cladding tends to be used for high temperature

material. For example, GE Aircraft Engines uses the

method for coating the tips of some hot section turbine

blades. It is not easy to set up the correct processing

parameters, but once they are determined the method

can be very reliable.

11.2.2. Laser Induced Surface Improvement (LISI)

The University of Tennessee has

developed a surface alloying method

that they call Laser Induced Surface

Improvement (LISI). The material to

be alloyed is incorporated into a lightabsorbing slurry that is painted or

sprayed onto the surface. A high

power laser is then used to burn off

the organic material and create a

short-lived surface melt pool that

alloys the material into the substrate

to form a surface alloy.

While the method adds some

material to the surface, it is really an

alloying process rather than a coating

process, and as such cannot be used for rebuild.

IR laser beam

Powder

Figure 29. Powder-fed laser

cladding process.

Substrate

Laser

Alloy

Substrate

Slurry

Figure 30. LISI schematic. Slurry-coated surface

(left) is laser alloyed (right).
Page 69

11.2.3. QQC Diamond

QQC Diamond has been heavily marketed over the past two or three years. It is basically a

modified laser CVD technique involving the use of three laser beams26 that is proprietary to QQC

Corp. The details of the process are not understood, but it is believed that one laser aimed at

the substrate vaporizes carbon-containing material and redeposits it as diamond, while another

deposits carbon from the ambient hydrocarbon gas, and the diamond-like material produced

diffusion bonds to the substrate. The method is proprietary and would be complicated to use for

IDs. Furthermore, diamond coatings, while they may be very good for OEM use, cannot be used

for rebuild. Therefore we will not consider this approach further.

11.2.4. Laser glazing and peening

Laser glazing consists of using the beam to remelt the surface, which rapidly solidifies into a thin,

smooth “glaze”. This layer is usually hard and amorphous, so that it improves corrosion

resistance by eliminating through porosity. Both metals and ceramics can be glazed, and it is a

method that seems to be quite successful on thermal spray coatings.

A laser “peening” method has been introduced in which water is flowed across the surface and a

high power pulsed laser fired into it creates intense shock waves that peen the surface.

Neither of these methods is suitable as a chrome replacement since neither creates any build-up.

11.2.5. State of development

Laser cladding is used in production for niche applications such as turbine blades, but is not

available as a standard job shop process or as standard equipment.

The LISI process was developed at the University of Tennessee and is being offered by Surface

Treatment Technologies, Inc. of Tennessee, who are commercializing the process. It is still early

in the development cycle and is probably ready for niche applications. Limited data is available

one performance.

The QQC diamond process is being offered commercially by both QQC of Michigan and by QQC

West in Nevada, but its main application appears to be golf club heads, and for most other

applications it is still in the development stage.

11.2.6. Coating materials and properties

Laser cladding can be used for a wide variety of metal and alloy coatings. Since it is a weld

technique it cannot readily be used for ceramics, although it is perhaps possible that ceramic

particles might be melted in the surface.

11.3. Capabilities and advantages

Laser cladding - The method can be used to weld into the surface any compatible alloy that can

be delivered in wire or powder form. In general the method requires reasonable access for the

laser beam and the feed material, with the laser striking the surface at roughly normal incidence.

For IDs this would require the laser to be brought down a tube with a mirror at the end, or a

reflective light guide. The weld material would be most easily delivered as a wire, but could be

delivered in powder form just as it would be for ID thermal spray.

Because it is a weld, coating adhesion is excellent.

 

26 US Patent # 5,731,046: Fabrication of diamond and diamond-like carbon coatings. Also US

Patent #s 5,643,641, 5,635,243, 5,620,754.
Page 70

LISI - The process is intended to be a surface alloying method. However, it is possible to vary

the laser power and pulse duration to limit diffusion so as to form anything from a fully formed,

uniform alloy to an alloy with a high surface concentration of the alloying material. Enough

power must be supplied to ensure bonding at the interface, which means that essentially the

whole slurry must be melted.

Since the slurry is applied prior to laser processing, the difficulties of slurry delivery are largely

avoided. All that is required is that the slurry be uniformly distributed across the surface.

11.4. Limitations

Laser cladding:

• Because it is a weld process it involves substantial surface heating and the formation of

a heat-affected zone at the surface, raising serious concerns of overheating and fatigue.

• Laser cladding was tested during a previous DARPA contract27 that helped to decide the

direction of the HCAT. It was found to be too dangerous for general use because of the

likelihood of overheating, especially if the material feed was interrupted. The process

window between welding and alloying at the surface was too narrow for reliable operation

on diverse components. The weld material often cracked during multiple passes

because of stresses from repeated high heat and cool cycles. Therefore the method

was not recommended for depot use.

LISI:

• As an alloying process the method involves substantial surface heating, or even melting.

This raises serious concerns over the heat-affected surface layer and its effect upon

fatigue.

• Although some material is added, it alloys with the surface and does not form a build-up

layer. Substantial build-up might be possible by adding sufficient material, but that is not

the way the process is designed, and it would involve a great deal of surface heating.

• As with all laser systems, ID coating would require a mirror inside the ID close to the wall

where deposition or alloying is taking place. The system would need to be designed to

ensure that the mirror did not become contaminated either with metal or burned off

hydrocarbon. For IDs this is a much more serious problem than it is for ODs where the

laser can be placed further away from the surface being processed.

11.5. Fit with OEMs and depots

Laser cladding:

Laser cladding fits well with very specific applications and components in production. However,

in general these are best chosen to be high temperature materials, or components that can be

reheat-treated after cladding. The method is not suitable for O&R use since it is too easy to

damage components by overheating.

LISI:

The technique is similar to laser cladding, but with the primary differences that it is an alloying

method rather than a coating technique and that the alloying material is applied to the surface

prior to laser treating. This is an advantage in preventing heat damage, which can occur in laser

cladding when the powder or wire delivery is momentarily interrupted.

 

27 Final Report Grant #MDA972-93-1-0006 “Hard Chrome Coatings: Advanced Technology for

Waste Elimination”, PIs Keith Legg and George Nichols (1997).
Page 71

Since it does not add significant dimensions to the surface it is not suitable as an O&R method.

QQC Diamond:

As we have noted above, the QQC Diamond method is not suitable either for OEM or O&R

chrome replacement on IDs.

11.6. Developments needed

Laser coating methods are becoming increasingly viable as industrial processes, primarily

because of the widespread development of reliable laser equipment for the metalworking

industry - primarily for cutting and boring. However, at the present time laser methods cannot be

used as general coating methods since there are several basic issues that must be either

developed or evaluated if either of the above laser methods (or methods of similar type) are to

be used:

1. Measurements of the effect on component performance of the surface heat-affected

zone - It is assumed that disturbance of the existing heat treat will make this zone into a

poorly-defined weak point, possibly initiating cracks and early fatigue. This effect is

likely to vary from one material to another, and it is quite possible that in the high

strength steels used in most ID applications, the effect will be to improve fatigue

performance if compressive stress is created in the surface alloy .

2. Feedback control to prevent overheating - Modern lasers are stable, but heat input to the

surface can vary depending on the instantaneous particle of wire feed rate (cladding)

and the local slurry thickness (LISI). Sensing and feedback control would prevent

damage to heat-sensitive structural components. However, doing this for IDs will be

considerably more difficult than for ODs where access is relatively easy.

3. Understanding the limitations of multiple passes, which currently can result in cracking of

the coating, presumably due to differential thermal expansion in the rapid heat-cool

cycles.

4. Development of ID coating methods - There are some equipment problems peculiar to

ID coating that need to be solved, including introducing both the beam and the coating

material, and ensuring that internal optics do not become coated or damaged during the

process.

11.7. Recommendations

At this point we do not recommend laser processing for ID chrome replacement.

However, the future promise of laser processing is bright enough that it would be worth first

evaluating item 1 in Section 11.6 above to determine if there are inherent problems for these

methods on aircraft components, before proceeding to any further development.
Page 72

12. Process Summary - Weld Coating
Page 73

12.1. Introduction

Weld coating has been used for many years for hard-facing large items such as agricultural

equipment. In that case the weld is a very thick coating laid down from a weld rod, with

concomitant extreme surface heating. There are much finer methods now available to deposit

thin coatings with little substrate heating or to weld a surface layer directly onto the underlying

material. Of these there are two that are particularly useful for IDs:

• Electro-spark deposition, which is a form of micro-arc welding using a consumable

electrode

• Explosive bonding (cladding), in which a thin surface layer is attached across the entire

surface by use of an explosive charge.

Table 27. Summary information -

Process name Electrospark Deposition Explosive Cladding

Description Micro-arc weld facing Metallurgical joining by

explosively driving materials

together

Coating materials Metals (inc. W alloys, Cr, Ta

alloys, Stellite, Tribaloy,

MCrAlY); cermets with Ni or Co

(inc. WC, Cr3C2, TaC)

Formable metals; some ceramic

powders

Temperature < 100C bulk, local surface melt;

heat-affected zone < 1mil deep

<100 C bulk; unknown local

temperature during cladding

Minimum ID 0.3” ~1”

Maximum depth None None

Stage of development Limited commercial; validation

needed for aerospace

Early commercial; developmental

for aerospace

Primary advantages Low capital cost

Simplicity

Very small areas - no masking

Very thick repair

Low capital cost

Primary disadvantages Small areas only

WC-Co thickness limited

Operator training/safety - not

suitable for general use
Page 74

12.2. Process description

12.2.1. Micro-arc welding (Electrospark deposition, ESD)

The electrospark deposition method has been

in existence for more than 10 years. It is

essentially a miniature consumable-electrode

TIG arc welding method in which a small DC

arc transfers electrode material from the

cathodic electrode to the workpiece, laying

down 1μm or less on each pass. Almost any

electrically conductive material can be

deposited in this manner.

ESD is offered by several small companies,

including

1. Advanced Surfaces and Processes

(ASAP) in Oregon (PulsFusion

alloying)

2. Turbine Metal Technology in California

3. JIT Automation in Ontario, Canada

The primary non-commercial researcher in the technology is Roger Johnson of Battelle

Northwest.

Because the electrode can be made very small, the

method is capable of depositing coatings deeply into

very small IDs (as small as 1⁄4”). Any material that is

conductive and can be made into an electrode can be

deposited, but with differing efficiencies and

thicknesses.

Because it is a weld overlay, the coating creates a thin

heat-affected zone at the substrate surface (Figure 32).

The coating itself is fine-grained or essentially

amorphous because it is rapidly solidified from an

extremely small melt pool after each arc. Arcs occur at

a frequency of 10 - 1,000 Hz and each arc lasts about 1

- 10 μsec. The electrode remains essentially in contact

with the surface so that each arc is highly localized.

+

Power

Supply

-

Figure 31. Electrospark deposition (ESD).

Figure 32. Cross-sectional detail of

EDS coating (Technocoat, Japan).
Page 75

12.2.2. Explosive bonding (cladding)

Explosive cladding has been used for

many years. It consists of putting the

materials to be bonded in close

proximity and driving them together

with explosives (Figure 33). A high

bond strength is achieved kinetically,

rather than by heating, making it a

cold process. In fact the belief is that

as the explosive burns and drives the

two surfaces together, a powerful jet

of air is forced out from between

them, cleaning the surfaces to permit

a strong metallurgical bond (Figure

33, below).

For internals the method would

consist of making a thin “can”, which

would fit within the ID to be clad, and

driving it into the component with a

small explosive charge.

The advantage of the method is that, since it does not involve heating, it can be used to join

dissimilar materials such as aluminum and copper. The dynamics of the process usually creates

a wave structure at the interface that in cross-section looks like saw teeth. Sigmabond

Technologies, of Ontario, Canada claims to have found a way to make the interface smooth.

Obviously, with this process care must be taken to ensure that the component is capable of

withstanding the shock of the explosion.

12.2.3. State of development

ESD is a commercial process, and is used successfully for a number of applications, including

• sodium valves in sodium-cooled nuclear reactors

• nuclear reactor pump IDs for liquid lithium

• power turbine blade notch surfaces

• power turbine blade tip coatings for diffusion/oxidation/corrosion resistance

Although the process is developed, there is limited information on how it would affect the

properties of aerospace components. Therefore, for the purposes of applying it to the IDs of

aircraft components, such as hydraulics, we must consider the applications still to be in the

developmental stage.

Explosive cladding, in the form in which it could be applied to aircraft parts, is still very new.

While the process has been brought to limited commercial use there are many unknowns in the

process and its effect on components.

12.3. Coating materials and properties

Both of these methods are capable of depositing many different types of materials.

ESD - The coating and substrate materials must both be electrically conductive. Some alloy

coatings can be made very thick (especially Ni alloys), but some materials, such as WC-Co are

self-limited to about 0.001” because they begin to spark-erode equally on both electrode and

coating when this thickness is exceeded. One of the most successful wear-resistant ESD

Substrate

Air jet

Explosive

Explosive

Substrate

Cladding

Figure 33. Explosive bonding. Above - tube ID

cladding set-up. Below - explosion detail.
Page 76

coatings is chrome carbide in a Ni binder. Wear resistant alloys, such as Stellite, can also be

deposited.

Explosive bonding - The easiest materials to join are metals, including steels and wearresistant alloys. For ID use it must be possible to fabricate the metal “can” to the shape of the

ID. It is possible to use the method to clad with metal or ceramic powder. In this case the

explosion rapidly compresses and solidifies the powder into a solid. The porosity of the resulting

material is unknown.

12.4. Capabilities

ESD - The method can be used in very small, long IDs. The only requirement is that a small

electrode can be inserted into the component. Its advantages are:

• simplicity and low capital cost

• excellent adhesion - since it is a metallurgical weld

• low deposition temperature - since each arc is low power and short, heating is only at the

surface

• ability to coat very small diameters (~0.5”) and long lengths (10 feet)

• wide range of coating materials.

Explosive cladding - The method cannot be used for very small IDs, but could work well on any

simple ID, without complex shapes such as ring grooves. The cladding material must be

conformable to the shape of the surface being coated. Its advantages are:

• joining of any two materials, including thermodynamically immiscible metals

• good metallurgical bond

• very thick rebuild coating.

12.5. Limitations

ESD - Its disadvantages are:

• cannot be used for large areas (too slow and expensive)

Table 28. Typical materials properties - ESD and Explosive Cladding

Property ESD Explosive Cladding

Hard material examples Cr3C2-15Ni Stellite, tungsten

Maximum thickness 0.010” (WCCo < 0.001”)>.250”

Corrosion resistance Good -

Wear resistance Good -

Porosity Low ≤ bulk metal alloys;

ceramics - unknown

Stress and effect on fatigue Not known Not known

Hydrogen embrittlement None None
Page 77

• cannot be used for small ring grooves whose dimensions are similar to or smaller than

the electrodes

• possible fatigue debit resulting from heat-affected zone at the surface

• cracking of the coating has been observed, and there is likely to be tensile stress at the

surface due to coefficient of thermal expansion (CTE) mismatch - both potential fatigue

and corrosion problems

• less-conductive coating materials such as WC-Co are self-limiting to about 0.001” thick

• the surface of the coating is rough, which is a particular concern for replacing thin dense

chrome on IDs.

The most serious concern for aerospace companies who have examined the method (such as

Orenda) is cracking of the coating and a possible fatigue debit due to modification of the

microstructure of the component.

The small size of the electrode poses inherent component size and coating thickness limitations,

which are likely to severely restrict its application in O&R and in components such as landing

gear. We expect it only to be viable for small areas such as hydraulic actuator IDs, lugs, pins,

etc., but not for large areas such as landing gear IDs.

Explosive cladding - has the following disadvantages:

• only usable for simple shapes

• component must be able to withstand the mechanical shock

• safety issues inherent in explosives use - would probably have to be supplied as “repair

packs” with measured amount of explosive and clad preform.

The primary concerns are again with the effect of the process on fatigue. There is also likely to

be concern over any possible effects such as sub-surface crack growth, and institutional

concerns of operator training and safety.

12.6. Fit with OEMs and depots

ESD would fit easily into both the OEM and depot

environments. The equipment is small and simple to

operate. Figure 34 shows the power supply (rear),

and the electrode (left), with two steel roller guides to

which the coating is being applied. The electrode for

the unit shown in Figure 34 is hand-held, but for

consistency it would need to be robotic. This has

been done for coating nuclear reactor components.

The fit of explosive cladding is less easy to gauge. It

is best used for depositing layers some mils thick,

either of hard-facing material at the OEM or of buildup material at the O&R site. In fact, on simple

components it might be an excellent method of obtaining a very thick build layer. It could not be

used as a general repair technology, however, due to operator safety and liability issues.

Sigmabond has suggested that they could supply repair kits comprising a properly sized and

shaped “can” of cladding material together with the correct amount and form of explosive. At

this point, however, we would see it as a method to be applied by the vendor rather than by the

OEM or depot.

Figure 34. ESD unit (Technocoat,

Japan).
Page 78

12.7. Typical costs and cost factors

For ESD the primary cost factors are area (which translates directly into coating time) and

coating material (since different materials coat at different rates, with simple metals such as Ni

being fast and materials such as WC-Co being slow and/or self-limiting). A typical coating rate is

about 3 sq. in./minute for a 2 mil WC-Co coating, at a typical cost of about $5 - $7 per sq. in.

This means that a hydraulic actuator, for example, that is 1.25” ID and 8” long would take about

10 minutes to coat at a cost of about $200. On the other hand a landing gear 8” diameter and

48” long would take about 7 hours to coat at a cost of about $7,000 - far higher than either

chrome plating or thermal spray.

12.8. Developments needed

ESD - The primary developments that are needed are evaluating coating properties and the

performance of coated components. The primary issue is the nature and thickness of the heataffected layer and the effect it might have on fatigue and ultimate tensile strength. Other coating

issues such as CTE mismatch and cracking of the coating are also serious concerns.

For ID coating chrome replacement it would be ideal to be able to use WC-Co, just as is now

being done on ODs with HVOF. This will require the real cause of the limit to be determined,

and overcome if possible. If that cannot be done, a good alternative will need to be found that

offers similar performance (i.e. better than chrome).

For replacement of thin dense chrome on IDs, the ESD coating would need to be ground to

achieve the proper rms surface finish. A method of automatic deposition that would minimize or

eliminate this requirement would be very useful.

Explosive cladding - The effects of the process on both substrates and components will need to

be evaluated carefully. Since its most likely use is as a repair on specific components, or for

hard-facing specific OEM items, it will need to be evaluated and validated for those specific

niche applications.

12.9. Recommendations

The ESD method could be a good answer for ID coating of small components, such as

pins, hydraulic actuators, lugs, etc. Since there are serious questions as to the effect on

fatigue, cracking of the coatings, and surface stress on the component, these questions should

be addressed prior to any evaluation on components themselves. Methods for depositing

smooth coatings and for increasing the thickness of coatings such as WC-Co need to be found.

Explosive cladding is useful as a niche product for certain difficult OEM hard-facing or O&R thick

repair problems. As such it should be evaluated on a case-by-case basis only.
Page 79

13. Process Summary - Heat treating and Ion Implantation
Page 80

13.1. Introduction

Heart treating methods are currently being used to replace chrome plating on items such as

hydraulic piston ODs, and many could equally well be used on IDs. They are valid methods for

improving the wear and corrosion resistance of alloys, and as such make a great deal of sense

for OEMs. However, heat treating techniques are not suitable for depot rebuilding, since

they do not add any material to the substrate. We have included ion implantation in this

section since it has many similarities to traditional heat treating, especially in that it modifies the

existing material surface rather than adding a coating to it.

Table 29. Summary information - Heat treating and ion implantation

Process name Heat treating Ion implantation

Description Plasma or gas nitriding,

carburizing, nitrocarburizing, etc.

Injection of ions from a beam or,

in case of PSII, from a plasma

Coating materials None None

Temperature 500 - 800C < 200C

Minimum ID None for gas nitriding, 1” for

plasma nitriding

Probably >3” (PSII only)

Maximum depth Unlimited Unlimited (PSII)

Stage of development Commercial Research, development

Primary advantages Large scale commercial process

Can be done at OEM stage

In use for OD hydraulic chrome

replacement

Low temperature

Primary disadvantages Cannot be used for rebuild

High temperature

Cannot be used for rebuild
Page 81

13.2. Process description

13.2.1. Heat treating

There are a number of heat treating methods that might be considered potentially useful for ID

treatments at the OEM level only, including

• gas nitriding,

• gas carburizing,

• gas nitrocarburizing,

• plasma nitriding,

• plasma source ion implantation.

The gas nitriding, carburizing, and nitrocarburizing treatments

involve heat treating in an enclosed furnace at a temperature

of 500 - 1,000C for about 12 hours in an atmosphere

containing active gases (N2, NH4, CH4, H2, etc.). The result is

diffusion and formation of nitrides, carbides, etc. with nitrideand carbide-forming alloying elements to a case depth of up to

0.060”. Most steel gears, for example, are carburized.

Because they are gas processes these methods treat the

entire surface, including IDs. In fact, any surface that is not to

be treated must be copper plated and the copper stripped at

the end of the treatment.

The plasma processes utilize a combination of heat and plasma bombardment to carry out

similar heat treatment, and consequently can often be done at lower temperature. Plasma

nitriding, for example, is usually done at a temperature of 500 - 520C and a pressure of 1 - 10

Torr, giving a case depth of 0.005 - 0.025”. The plasma processes are generally cleaner and

better controlled than the gas methods (especially when pulsed rather than DC power is used).

Because plasma is easy to shield, the plasma methods do not require copper plate on areas that

are not to be nitrided. The plasma methods can be done in IDs provided care is taken to ensure

plasma penetration without forming excessive hollow cathode plasma densities, which can cause

extreme heating and erosion.

The gas and plasma heat treating processes are sometimes combined with gas or plasma

oxidation to produce an outer oxide layer that provides better corrosion protection. Several

companies offer this type of process commercially.

13.2.2. Ion implantation

Plasma source (or immersion) ion implantation (PSII or PI3) can be thought of as a high voltage

form of plasma nitriding, in which the temperature is kept very low. There has been some

debate, in fact, over the extent of surface heating and plasma nitriding that can take place in

PSII, since some workers have reported nitrogen diffusion profiles. Provided the temperature is

kept low, however, the method affects only the outermost few tenths of a micron.

Note that beam line implantation (which uses an ion beam instead of a plasma) is wholly

unsuitable for IDs since an ion beam is strictly line-of-sight and cannot be fired into the wall of an

ID except at grazing incidence.

13.2.3. State of development

The gas and plasma heat treating techniques are fully commercial processes that have been in

the market for many years (although the plasma processes are more heavily used in Europe

Figure 35. Plasma nitriding.

(Note bright plasma with dark

space next to surface.)
Page 82

than in the US). Full scale commercial equipment is readily available for both gas and plasma

processes.

Standard hydraulic rod heat treated by the Nitrotec nitrocarburizing process is available

commercially as a replacement for traditional chrome plated rod.

The PSII process has been under development for a number of years and is in limited

commercial use. One or two companies offer production equipment for the process28, and it has

been offered commercially29 as a way of producing more wear-resistant chrome.

13.3. Capabilities

The primary use of these heat treatments is for steels rather than for aluminum alloys, although

the method is now being tested for aluminum alloys30. The gas treating processes work very well

on both ODs and IDs. While the plasma processes work on IDs, care must be taken to ensure

that the ID is properly bathed in plasma. Depending on the pressure, the hole diameter, and

other factors the ID can have no ion bombardment for lack of plasma penetration, or it can be

excessively bombarded (with consequent etching and overheating) by formation of hollow

cathode plasmas.

13.4. Limitations

Materials - Because of the temperatures involved, these gas and plasma processes must be

properly integrated into the heat treatment for the original component. Because the surface of

the material is modified, the performance is strongly materials-dependent.

ID treatment - Beam line implantation cannot be used for ID treating. PSII could be used if the

ID were large enough to contain the anode and the dark space around it, with an adequate

amount of plasma for bombardment.

O&R use - Since these methods do not add material they are of no value for component

rebuilding. Furthermore, the treatments must often be followed by further heat treating to return

the component to its proper heat-treated state. This is not true for PSII, provided the process is

controlled to keep the part temperature low.

13.5. Fit with OEMs and depots

Gas or plasma surface nitriding, etc. is a relatively simple fit with OEM products, since OEM

components and steels are heat treated in any case, and the additional processing could well be

incorporated as part of that treatment. However, it is of no value for O&R since it cannot rebuild

worn components.

One should also consider carefully how the adoption of such a heat treat might affect subsequent

O&R processes. Coating processes that lock onto the surface physically, such as thermal

sprays, should not be affected by an underlying treatment. However, it is known that PVD

coatings adhere very poorly to some heat treated surfaces, especially to carburized surfaces,

and the same may be true of other processes (including electro- and electroless plates) that

depend on a chemical bond.

 

28 For example, North Star Research Corp

29 Empire Hard Chrome, Chicago. The company appears no longer to be offering the PSII

process (at least as a standard chrome treatment).

30 H.R. Stock, C. Jarms, F. Seidel, and J.E. Doring, Surface and Coatings Technology 94-95,

247-254 (1997).
Page 83

13.6. Typical costs and cost factors

Heat treating processes such as gas or plasma carburizing and nitriding are relatively

inexpensive since components can usually be treated in batches in very large furnaces (often 6’

or more in diameter, and 6 - 20’ high). For typical industrial products pricing is by the pound,

from $1.50/lb for small items to $0.60/lb for large items weighing several thousand pounds.

Following the heat treat the surface must usually be descaled and may require grinding to

remove the hard, brittle “white layer” and to bring the surface back to permissible Ra values.

Quality assurance is an additional major part of the cost for aerospace components, and usually

involves hardness testing and hardness depth profiling on coupons made during the heat

treatment.

After treatment, most materials must be further heat treated to bring the bulk properties back to

their original heat treating specifications.

13.7. Developments needed

With the exception of PSII, all the treatments are commercial and need only be adapted to each

material and component geometry.

Because there has been little use of these treatments on aluminum, work would be needed to

develop proper process parameters for aluminum alloys. In common with all other processes,

extensive data gathering will be needed to ensure that performance and producibility criteria are

met.

13.8. Recommendations

Gas or plasma nitriding, carburizing, or nitrocarburizing are useful technologies that could be

adopted for IDs. As we have already noted, nitriding, carburizing, etc. are only of value at the

OEM level because

• they cannot be used for rebuild

• standard vacuum or air heat treatments will usually be needed to bring the components back

to specified hardness and strength, which is relatively straightforward when part of the

manufacturing process, but more difficult to do at the O&R level.

However, if these treatments were to be adopted at the OEM level, it would be critical to ensure

that they do not make it difficult to overcoat at the O&R level. (Note that the heat treated

surfaces cannot be stripped, so that if the surface has to be removed for coating, it can only be

done by grinding.)

Since heat treating is fairly simple when done by the OEM, we do not believe that PSII offers any

significant improvement over traditional plasma nitriding for IDs, except in circumstances where

warpage is a significant problem. (Note that implanted surfaces are less likely to pose any

problem with subsequent coating at the O&R level. This is, in fact, one of the selling points of

ion implantation in plastics molds.)