PUBLICATIONS

Nationa

(formerl

ERCE

and Technology

andards)

NIST Special Publication 754

Vapor Phase Deposition Studies of

Phosphate Esters on Metal and

Ceramic Surfaces

Douglas E. Deckman, Stephen M. Hsu, and E. Erwin Klaus

-QC

100

.1157

#754

1988

C2

DOE ECUT Tribology Program

Stephen M. Hsu, Project Leader

NZSTTC G>L too • US' 7 mo, 7±4 1983 <-,2-

NIST Special Publication 754

Vapor Phase Deposition Studies of

Phosphate Esters on Metal and

Ceramic Surfaces

Douglas E. Deckman and Stephen M. Hsu

Ceramics Division

Institute for Materials Science and Engineering

National Institute of Standards and Technology

(formerly National Bureau of Standards)

Gaithersburg, MD 20899

E. Erwin Klaus

Department of Chemical Engineering

The Pennsylvania State University

University Park, PA 16802

NOTE: As of 23 August 1988, the National Bureau of Standards (NBS) became the National Institute of

Standards and Technology (NIST) when President

Reagan signed into law the Omnibus Trade and

Competitiveness Act

September 1988

U.S. Department of Commerce G^i?5^

C. William Verity, Secretary

^

National Institute of Standards and Technology

(formerly National Bureau of Standards)

^ ^P"^,^

Ernest Ambler, Director ^^\<£^<& VV
Library of Congress

Catalog Card Number: 88-600578

National Institute of Standards

and Technology

Special Publication 754

154 pages (Sept. 1988)

CODEN: XNBSAV

U.S. Government Printing Office

Washington: 1988

For sale by the Superintendent

of Documents,

U.S. Government Printing Office,

Washington, DC 20402

Vot sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402
FOREWORD

This report is born out of a cooperative effort between the Chemicai

Engineering Department, at Pennsylvania State University (PSU) , the

Tribology Group of the National Bureau of Standards, and the partial

support of the DOE ECUT Tribology Program. For years, the NBS has

encouraged and sponsored graduate cooperative programs in which

promising young scientists (some of them NBS staff members) come to

NBS to conduct research, part of which often becomes the graduate's

thesis. While it is not the objective of NBS to teach and train

graduate students, such a program often furthers NBS programmatic

goals by attracting high caliber scientists working on areas where NBS

mission lies. Many excellent research papers have results and many

students, upon graduation, have chosen to stay at NBS to continue

their research careers. Such programs also draw many first rate

university professors to NBS through the participation of these

students in NBS research programs and projects, thus fostering NBSuniversity interactions and enhancing the scientific caliber of the

work and reputation at NBS and the participating university. At the same time, through the frequent contacts that NBS has with industries,

a natural university/government lab/industry relationship evolves,

bringing a team focus on many research projects of significant

economic and/technological impacts.

In 1984, primarily through the interactions of Dr. Stephen M. Hsu of

NBS and Professors Elmer Klaus and Larry Duda of PSU, a cooperative

program in Tribology was started. Three graduate students: Mr.

Richard Gates (a NBS staff member), Mr. Jeffrey Yellets and Mr.

Douglas Deckman were enrolled at the Chemical Engineering Department

at PSU. Three reports have been prepared to describe the fruit of

their relentless efforts in the last three years. These studies were conducted at NBS under the guidance of Dr. Stephen Hsu with the close

participation of Prof. Elmer Klaus who visited NBS frequently.

Dr. James Eberhardt , Mrs. Terry Levinson, and Mr. David Mello of the

DOE ECUT Tribology Program have at the same time sponsored a major

Tribology program at NBS. That program also benefited from the

studies conducted by Mr. Gates, Mr. Yellets, and Mr. Deckman. These

students, while not working on sponsored projects directly, enabled

NBS to explore some high risk, high pay off projects parallel to ECUT

projects. When appropriate, the students were supported by ECUT for some time. To this, we gratefully acknowledge the generous support of

DOE ECUT, without whose support many ideas would not be explored.

Stephen M. Hsu

Chief, Ceramics Division

ABSTRACT

This study focuses on a novel means of lubrication for ceramics

and metals at high temperatures; called vapor phase lubrication. The

deposition rate and mechanisms of tributyl phosphate ester and

tricresyl phosphate ester have been explored. A Thermalgravimetric

Analyzer (TGA) was modified to measure the rate of surface film

formation of vapor phase deposition of lubricants on metal surfaces at

elevated temperatures. Results from TGA studies showed that the vapor

deposition process was a complex function of substrate, time, and

temperature. Tricresyl phosphate ester (TCP) and tributyl phosphate

ester (TBP) were observed to produce similar amounts of film on

stainless steel substrates. The substrate had a substantial effect on

the amount and rate of film formation. The observed order of metal

activity was copper > stainless steel > nickel > platinum. Film

formation on nickel was found to be much more rapid at 600°C than

700°C. Also, the concentration of oxygen present during deposition

strongly influenced the amount of film formed. Scanning electron

microscopy was conducted on TBP films formed on Si3 NA , Al2 03

, and aSiC. The films produced contained globule and filamentous structures.

The films on each ceramic were unique, demonstrating that vapor

deposition is surface sensitive on ceramic substrates. High

temperature friction testing was conducted on a TCP coated Al2 03 substrate. The coating reduced the friction level for alumina, thus

demonstrating the potential of vapor lubrication for high temperature

lubrication of ceramics.

v

TABLE OF CONTENTS

Page

FOREWORD iii

ABSTRACT v

LIST OF FIGURES ix

LIST OF TABLES xiii

Chapter

1 INTRODUCTION 1

2 LITERATURE SURVEY 9

2.1 High Temperature Lubricants 9

2.1.1 Synthetic Lubricants, Solid Lubricants,

and Coatings 9 2.1.2 Metal Working and Metal Forming Lubricants ... 13

2.2 Previous Vapor Deposition Studies 16

2.3 Chemical Vapor Deposition Studies 24

3 EXPERIMENTAL APPROACH AND RESULTS 30

3.1 Experimental Approach 30

3.2 Kinetic Data on Metal Surfaces 31

3.2.1 TGA Vapor Deposition Apparatus 31

3.2.2 TGA Vapor Deposition Procedures 37

3.2.3 TGA Vapor Deposition Results 39

3.3 Ceramic Vapor Lubrication Studies 55

3.3.1 Design of Vapor Deposition Apparatus 55

3.3.2 Vapor Deposition Procedures 66

3.3.3 SEM Results 69

3.3.4 Performance Testing Equipment 80

3.3.5 Ball Bearing Holder Design for the Intermediate

Temperature Tribometer 86

3.3.6 Sample Preparation Development 89

3.3.7 Performance Testing Results 104

vii
I

TABLE OF CONTENTS (continued)

Page

4 CONCLUSIONS Ill

5 FUTURE WORK 114

REFERENCES 116

APPENDIX

A Physical Constants for TCP, TBP , and Argon

at Various Temperatures 123

B Physical Constants for Mixtures of 1% Phosphate

Ester in Argon at Various Temperatures 126

C Estimation of Diffusion Coefficients for Phosphate

Ester in Argon at Various Temperatures 129

D Calculation of Back Diffusion in the TGA 132

E Calculation of Mass Transfer Rates and Resistance

for the Vapor Deposition Process 138

F Calculation of Flow Regime During Vapor Deposition

Testing 141

viii
LIST OF FIGURES

Page

Figure

1 Energy Balance Comparison Among Alternative

Diesel Engines 4

2 Adiabatic Diesel Engine 5

3 Vapor Lubricated Four Ball Wear Tester 22

4 Effect of Load on Wear Scar Diameter for Several

Vapor Lubricants at 370°C on M50 Steel 23

5 TGA 32

6 TGA Vaporization Tube 33

7 Cutaway View of the TGA Furnace 35

8 Effect of Substrate on Deposition at 700°C Using

1.06% TBP 40

9 Effect of Lubricant on the Deposition of Stainless

Steel at 700°C 45

10 Effect of Temperature on the Deposition of Nickel

Using 1.06% TBP 46

11 Desorption of Deposit from Nickel Coated at 600°C

with 1.06% TBP 50

12 Arrhenius Plot for the Desorption of a TBP Film

on Nickel from 441 to 385°C 52

13 Arrhenius Plot for the Desorption of a TBP Film

on Nickel from 814 to 727°C 53

14 Vapor Deposition Equipment Designed by Lai 56

15 Vapor Deposition Equipment Designed by Min 57

16 Vapor Deposition Apparatus Designed by Gunsel .... 59

ix
LIST OF FIGURES (continued)

Page

Figure

17 Vapor Deposition Furnace and Quartz Tube Insert ... 61

18 Upper and Lower Baffles 62

19 Baffle Assembly 63

20 Quartz Tube and Baffle Assembly 65

21 Temperature Distribution in Deposition Portion of

Furnace 67

22 Wall Temperature and Center Line Temperature within

the Tubular Furnace 68

23 SEM Photographs of Si3 NA Coated with 0.73% TBP

at 700°C 71

24 EDAX Spectra of Filament Produced on Si 3 N4 Coated

with 0.73% TBP at 700°C 72

25 SEM Photographs of Al2 03 Coated with 0.73% TBP at

700°C 74

26 SEM Photographs of a-SiC Coated with 0.73% TBP at

700°C 75

27 Stages in the Growth of Filaments with a Catalyst

Particle in the Growing End 78

28 Ball on Three Flat Geometry 81

29 Side View of Ball on Three Flat Cup 82

30 Top and Side View of Ball on Three Flat Cup 83

31 Four Ball Wear Tester 84

32 Intermediate Temperature Tribometer 85

33 Alumina Specimen Holder Used in the Intermediate

Temperature Tribometer 87

x
LIST OF FIGURES (continued)

Page

Figure

34 Ball Bearing Holder for the Intermediate

Temperature Tribometer 88

35 a-SiC Ball in a Stainless Steel Ball Bearing

Holder at 25°C 91

36 a-SiC Ball in a Stainless Steel Ball Bearing

Holder at 1000°C 92

37 SEM Photographs of a Polished Al2 03 Specimen .... 94

38 Effect of Sputtering on Surface Contamination

Level 97

39 Effect of Cleaning Procedure on Surface

Contamination Level 102

40 Effect of Cleaning Procedure on Friction Level

- Dry A12 03 on Al2 03 , 150°C, 6 RPM 103

41 Effect of a TCP Vapor Coating on Al2 03 Tested

at 150°C, 6 RPM, and 10 kg 106

42 Transmission Optical Photograph of TCP Coated

A12 03

- Tested at 150°C 107

43 Effect of a TCP Vapor Coating on Al2 03 Tested

at 700°C, 1.5 cm/min, and 16 N 109

44 Modelled and Actual Flow Regimes within the TGA . . . 133

xi

LIST OF TABLES

Page

Table

1 Comparison of Operating Temperatures for an Adiabatic Diesel Engine and a Conventional

Diesel Engine 6

2 Operating Temperatures in the Automotive Gas

Turbine 7

3 Thermal and Oxidative Stability of Various

Lubricants 11

4 Typical Interfacial Temperatures for Various

Metalworking Processes 14

5 Normalized Film Formation Amounts for Various

Substrates at 700 °C 18

6 Normalized Initial Film Formation Rates for Various

Substrates at 700°C 19

7 SEM Vapor Deposition Film Characteristics 20

8 Gas Phase Reactions Involved in the Growth of

Amorphous Hydrogenated Silicon Films by

Plasma CVD 27

9 Effect of Temperature and Pressure on the

Microstructure of Fluoride CVD Tungsten Produced

at H2 /WF6 =6 28

10 TGA Temperature Calibration Using Paramagnetic

Standards 36

11 Initial Rate of Film Formation on Various Substrates

at 700°C 43

12 Effect of Oxygen on the Initial Rate of Film Formation

on Copper at 700°C 44

13 Previous Studies Showing Decreasing Amounts of Film

Formation with Increasing Temperature 47

14 Population of TPSR States for Carbon Deposited

on an Alumina- Supported Nickel Catalyst (G-56H)

Following Ethylene Exposure 51

xiii
LIST OF TABLES (continued)

Page

Table

15 Location of Catalyst Particle within Filaments .... 76

16 Activation Energies for Coke Formation on Various

Substrates 79

17 Coefficient of Thermal Expansion for Three Advanced

Ceramics and Stainless Steel 90

18 List of Possible Contaminants in the Sample

Preparation of Al2 03 Specimens 95

19 Solubility of Lapping Oil in Various Solvents .... 98

20 Solubility of cutting Fluid in Various Solvents ... 99

21 ESCA and Auger Determination of Carbon Residue

on A12 03 after Various Cleaning Procedures 100

22 Physical Constants of Argon at Various

Temperatures 123

23 Physical Constants of Tributyl Phosphate Ester

at Various Temperatures 123

24 Physical Constants of Tricresyl Phosphate Ester

at Various Temperatures 124

25 Physical Constants for a Mixture of 1% Tributyl

Phosphate Ester and 99% Argon 126

26 Physical Constants for a Mixture of 1% Tricresyl

Phosphate Ester and 99% Argon 126

27 Constants Needed to Calculate Diffusion

Coefficients 130

28 Diffusion Coefficients of Phosphate Esters in Argon

at Various Temperatures 131

xiv
1

Chapter 1

INTRODUCTION

There is a great technological need in our society to develop

better lubricants and more effective ways of applying lubricants. In

particular, there is a real need for improved lubrication for high

temperature applications. Low heat rejection engines (or so-called

adiabatic engines) and metal working and metal forming processes are

two examples where advanced lubrication technology is needed.

In any power plant, such as an engine, the overall cycle

efficiency is increased as operating temperature is increased. Metals

and alloys used in conventional engines have low strength and poor

oxidation resistance when operated at temperatures above existing

designs [1]. Ceramic materials have many desirable high temperature

properties needed for heat engine applications. These include high

strength at high temperature, excellent thermal shock resistance, low

thermal conductivity, high oxidation resistance, and good corrosion

resistance

.

The potential of ceramics as engine components has led to the

development of the adiabatic diesel and the automotive gas turbine.

The energy saving potential of these propulsion systems arises from

their high temperature operation. Secondary savings are realized

because ceramics typically have 40% of the density of metals. This

results in lower weight and inertia which translates into improved

engine performance. The efficiency improvements in these designs
2

could result in up to a 50% improvement over conventional diesel

engines [ 2 ]

.

The economic implications of these improvements are enormous. It

is estimated that the United States will ship 10 billion dollars worth

of advanced ceramics in the year 2000 and the ceramics industry will

create 250,000 new jobs. If the United States could become the

dominant producer of advanced ceramics a cumulative real GNP gain of

278.9 billion dollars (1981 dollars) could be realized between 1985

and 2005. But if foreign countries are able to achieve this dominance

a potential swing of 400 billion dollars could result.

For the case where the United States would dominate the advanced

ceramics market the following yearly savings are projected in the year

2000 [3]

Automotive imports reduced and partial

domestic replacement 8.3 billion

Truck imports reduced and partial domestic

replacement 2.5 billion

Fuel Savings 1.5 billion

Maintenance savings 0.3 billion

Also, reduced imports of engines and auto parts as well as increased

exports of motor vehicles, engines, and engine parts would have a

further impact on the trade balance. If foreign domination occurred,

oil imports would still be reduced and motor vehicle imports would be

unaffected if existing quotas remain.
3

Thus ceramics offer the United States manufacturers an

opportunity to regain the market share lost to fuel efficient imports

and improving the United States international competitiveness. But to

accomplish this the United States must win the international race to

develop a commercial heat engine.

In a combustion engine, the energy produced by fuel ignition is

divided between exhaust, the water cooling system, and useful work,

see Figure 1 [4] . The adiabatic engine removes the cooling system

thus reducing energy lost to this sink. This engine also combusts

fuel in an insulated cylinder at higher than conventional operating

temperatures thus enabling a more thermodynamically efficient

combustion process. Energy from the exhaust is further extracted by

turbocompounding. The resulting energy balance for an adiabatic

diesel engine is shown in Figure 1 . Figure 2 shows a schematic of

this engine

.

The elimination of the cooling system has numerous repercussions,

since cooling systems account for 50% of commercial and military field

engine failures. Improved reliability, simplified maintenance, and

reduced maintenance costs would result. For military vehicles the

loss of the cooling system means greater immunity to heat- seeking

weapons

.

Table 1 compares anticipated operating temperatures for the

adiabatic diesel to a conventional diesel engine. Table 2 shows

anticipated operating temperatures in an automotive gas turbine.

Clearly the development of ceramics for high temperature

applications such as adiabatic engines will have major economic
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Table 1 . Comparison of Operating Temperatures for an Adiabatic Diesel

Engine and a Conventional Diesel Engine

Engine Component

Operating Temperature °C

Conventional Engine Adiabatic Engine

Piston

Liner

Turbine Inlet

277

400-600(6)

600

1000(5)

600-1100(7)

1000(6)
Table 2. Operating Temperatures in the Automotive Gas Turbine

Engine Component Operating Temperature °C

Gas Turbine Rotor 1100(2)

Gas Turbine Stator 1510(2)

Plenums, Flow Controls

and Support Structures 980-1370(2)

Regenerator 980-1200(2)

Foil Bearing (Main Shaft) 650(7)
8

ramifications . A key barrier for high temperature operation of

advanced ceramics is the development of effective lubrication methods

for these temperatures.
9

Chapter 2

LITERATURE SURVEY

2 . 1 High Temperature Lubricants

2.1.1 Synthetic Lubricants. Solid Lubricants, and Coatings

The technological demands of advanced high temperature systems

poses two very fundamental questions. First, is lubrication needed

for these systems? And what type of lubrication will prove effective

in these operating regimes? The potential of unlubricated, air film

lubricated, synthetic liquid lubricated, solid lubricated and coated

systems are now examined.

The use of ceramics under dry conditions appears unfeasible for

heat engine applications. Breznak et al. [8] examined four different

advanced ceramics sliding with like and unlike pairs. In all cases

the kinetic coefficient of friction was between 0.25 and 0.5 which is

unacceptably high for engine applications. Also ceramics have been

observed to seize at very low load when tested under dry conditions

[9]. Thus, dry lubrication is unacceptable for heat engine and other

high temperature applications.

The use of a gas lubricated piston and cylinder liner has the

greatest potential for mechanical friction reduction in an adiabatic

diesel. But a severe barrier which must be overcome to make this

technology feasible is elimination of sidethrust transferred to the

piston from the connecting rod [10]. It is not likely that this

hurdle will be passed in the near future.
10

The use of liquid lubricants to lubricate advanced heat engines

requires withstanding high temperatures during continuous service in

oxidizing environments. It is generally accepted that the oxidative

stability of lubricants is less than the lubricant thermal stability

and oxidation is the predominant mechanism for lubricant degradation

[11]. Conventional mineral oils are being replaced by synthetic

lubricants because of improved additive response and improved

volatility characteristics. Improved additive response results in

greater oxidation stability for these synthetic fluids. Table 3

displays the thermal and oxidative stability limits for a number of

lubricants. The fluids with the highest thermal and oxidative

stability, the polyphenyl ethers, have a very limited life under

sustained high temperature operations. To circumvent these

limitations current research focuses on continuous lubricant delivery

methods to apply liquid lubricants.

One class of materials which have some potential for high

temperature lubrication is solid lubricants. There are three

requirements which must be met for solid lubricants to function [7].

First, they must have a high degree of plasticity for easy shear.

Second, they must possess the ability to coalesce their individual

particles under load and shear to form continuous coherent films.

Finally, they must adhere to the lubricated surface. Two general

categories of solid lubricants which will be discussed are coatings

and composites.

A coating can be lubricious by itself or it can react to form a

self - lubricating material. Sliney [21] has demonstrated that a PbO-
Table 3. Therma1 and Oxidative Stabilities of Various Lubricants

Lubricant Class Lubricant Lubricant Stability °C

Thermal Oxidative

Hydrocarbon

Mineral Oil

Octacosane

p -Quaterpheny

338(12)

350(12)

455(12)

150(13)

204(20)

Sebacate Esters

Neopentyl polyol

Esters

di-2-ethylhexyl sebacate

Trimethylol propane

Triheptanoate

277(14)

317(12)

177(13)

240(15)

Polypheny1 Ethers m- B i s ( m- phenoxy - phenoxy

benzene

m( m - PhenoxvnhpnnYv^ - nhpnu

p- phenoxy -phenyl ether

o-Bis(m- phenoxy - phenoxy

benzene

Bis(p-phenylphenyl) -ether

454(12)

450(12)

440(12)

460(12)

275-316(15-18)

275-316

275-316

275-316

Thio Ether

Thio -ether 390(16) 260(16)

Fluorinated Fluids

Fluorinated Polyether

Perfluorinated Polyether

triazine

Branched perfluoroalkyl

ether

Unbranched perfluoroalkyl

ether

389(18)

397(18)

390(16)

<316(18)

>316(18)

343(16)

316(16)

Phosphate Esters

t -butyl phenyl diphenyl

phosphate >270(19)

Silicone Fluids

tetra- alkyIs i lanes

tetra-p-biphenylyl-silane

370(20)

427(12)

340(20)
12

Si02 coating can lubricate effectively up to 650°C, but high sliding

rates are needed for good frictional characteristics at low

temperatures. Also, a CaF2

- BaF2 coating can provide lubrication

from 550 to 950°C. TiN and TiC are examples of coatings which react

to form lubricious films. TiC and TiN oxidize to form rutile (Ti02 )

at 540°C [22]. Rutile has a lamellar structure thus providing a low

shear film on top of a hard, high load carrying base. A similar

phenomenon occurs with Si3 NA which oxidizes to form SiO and Si02 [22].

Another class of materials being examined for high temperature

applications is composites. One class of composites which have been

widely studied is polyimide composites [23-27]. In these composites

an organic polymer, polyimide, is used as a matrix material. Solid

lubricants such as graphite fluoride (CF1 x ) n and MoS2 are used as

adjuvants to reduce friction at high temperatures. Graphite fibers

are added to reinforce the matrix and enhance wear properties at

elevated temperatures. Lubrication occurs during gradual wear through

the composite. Wear in these composites increases exponentially with

temperature. The maximum temperature of application for the polyimide

composite is controlled by their glass transition temperature.

Polyimide composites have been shown to be effective up to 370°C.

Sliney [28] demonstrated that a plasma sprayed coating containing 30%

nichrome , 30% Ag, 25% CaF2

, and 15% glass provided uniform wear and

low friction (0.2) from 0 to 870°C in nitrogen and air environments.

This composite has been used for an interstage seal in a small jet

engine. The operating temperature of the seal is 650°C. In general,

composites have a longer endurance than coatings, but are difficult
13

and time consuming to prepare. Also many composites lack required

high temperature strength properties needed.

In summary, effective lubrication is required for advanced high

temperature processes. Current high temperature lubricants (i.e.,

synthetic lubricants, solid lubricants, and coatings) are often

inadequate for system demands. One alternative is to continuously

supply lubricant in the vapor phase. This process is known as vapor

phase lubrication.

2.1.2 Metal Working and Metal Forming Lubricants

Another area where there is a large need for high temperature

lubrication is the area of metal working and metal forming processes.

Table 4 shows typical interfacial temperature for various metal

working and metal forming operations

In these metal working processes the lubricant provides a number

of functions. The ideal lubricant must reduce friction and wear

between the tool and the work piece. Also the lubricant must act as a

parting agent and prevent local welding, adhesion, pick-up, and

subsequent damage to the tool and the work piece surface. Parting

action can be achieved by lubricant vaporization. This produces a

balanced gas pressure to assist in quick release of the forging from

the die cavity. The lubricant must have low corrosion action and have

a low accumulation rate on the interacting surfaces. Also, the

lubricant should act as a cleaning agent preventing buildup of dirt or
14

Table 4. Typical Interfacial Temperatures for Various Metal

Working Processes

Application Temperature Regime, °C Material Processed

Hot Rolling 900-1050(29) Low Carbon Steel

Cold Die Forging 482-704(30) 1045 Steel

Isothermal Forging 899-982(31) Titanium Alloys

Metal Flow & Cie Filling 900°C(32) Steel

Drop Forging Die 900°C(dry) (33)

500°C(lubricated) (33)

0.4% Carbon Steel

Extrusion 816-1204(34") 4340 Steel

Metal Cutting 538(35) Titanium Alloy
15

scale. The lubricant must meet environmental, safety, and health

requirements. Finally, it must minimize processing costs.

Traditional methods of applying these lubricants for metal

working and metal forming operations include swabbing and spraying the

lubricant onto the tool surface [34]. This results in lubricant

vaporization, which can produce erosive wear, pitting of dies and

thermal fatigue [33]. In close die forging, lubricant vaporization

can produce a sufficiently high gas pressure to interfere with die

filling.

Conventional lubricants used for metal working and metal forming

processes include solvent based lubricants, water-based lubricants,

glass coatings, and graphite films. Solvent-based lubricants are used

because they require a very short coating time to apply a sufficient

lubricant film, also these lubricants are self cleaning. The

disadvantages of solvent-based lubricants include flammability and

toxicity. Because of these factors, legislation is restricting the

use of solvent-based lubricants [36] . Water-based lubricants use less

lubricant, thus lessening disposal problems. Also the carrier, water,

is nonhazardous . But there are numerous disadvantages associated with

water-based lubricants. First, they are not self-cleaning. Also,

longer times are required to put down a sufficient lubricant coating.

Because of the high heat capacity of water, these lubricants cool the

tool very quickly. This produces thermal shock which leads to heat

checking. Finally, these lubricants are not as effective as other

types and are more difficult and expensive to produce.
16

Glass coatings provide a high degree of lubricity and maintain a

continuous film for excellent protection. Drawbacks of the glass

coatings include poor releasing ability. Also they accumulate and

buildup in die cavities. Glass lubricants are corrosive toward some

alloys and they produce a rough surface finish and leave glass

stringers

.

A final type of lubricant used for metal working processes is

graphite films. These produce rough surface finishes. Also they have

poor removability from specimens and poor release from dies. Their

use also results in a less uniform metal flow. Clearly although there

are many lubricants available for high temperature metal working

processes there is much room for improvement.

The reason why these lubricants are effective at such high

temperatures is that their needed effective lifetime is very short.

Klaus and Lai [37] took advantage of this concept and demonstrated

that lubricants applied in the vapor phase are effective for die

casting operations. Consequently, because the lubricant can be

continuously supplied during vapor phase lubrication this technique

should be applicable to other continuous high temperature operations.

2 . 2 Previous Vapor Deposition Studies

One method of lubrication which has an enormous potential for

high temperature applications is vapor lubrication. Because the

lubricant is already vaporized, thermal fatigue of tool surfaces is

avoided. This is because the tool surface does not supply the latent
17

heat of vaporization to the contacting lubricant thus reducing thermal

shock and fatigue. Erosive wear and pitting are also prevented.

Previous vapor deposition studies have measured the amount of

vaporized film formation as a function of time using a Cahn

electrobalance . Wire specimens of iron, copper, stainless steel, and

nickel were used. Lubricants studied included polyol esters like

TMPTH, and phosphate esters including diphenyl ditertiarybutyl

phosphate (GT) , tricresyl phosphate (TCP) , and tributyl phosphate

(TBP)

A summary of film formation on various substrates at 700 °C is

presented in Table 5 [38-45]. The amount of film formed is normalized

per unit surface area and reported in /xg/cm2

. It is clear that film

formation is a function of substrate composition and lubricant type.

These conclusions are also reinforced by the initial film formation

rates displayed in Table 6. If a deposition test is allowed to

continue for long periods of time, similar rates are observed on all

substrates. This implies that once the catalytic metal surface

becomes deactivated, metal particles are no longer involved in the

deposition process; thus, rates look similar on all surfaces.

Scanning electron microscopy (SEM) studies [40,42-47] have found

that the nature of the film formed is a function of the substrate, the

substrate temperature, and the vaporized lubrication. Table 7

presents a summary the SEM studies. Both globule and filamentous

structures have been observed on metal surfaces, but no explanation

for their formation has been offered.
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Lubricant Substrate Temperature Film Characteristics Reference

TMPTH Stainless Steel 700°C Filamentous 42

Iron 700 8 C Filaments & Globules 42

Nickel 700°C Filamentous 42

PPE Stainless Steel 700°C Globules 42

Iron 700°C Globules 42

Nickel 700°C Globules 42

Copper 700 Q C Globules 42

Quartz 700 8 C

PAO Stainless Steel 600 °C Globules 43

Stainless Steel 700°C Filamentous 43

Iron 600°C Globules 43

Iron 700°C Filaments & Globules 43

Iron 800 8 C Filamentous 43

Nickel 600-800°C Filamentous 43

Copper 600-800°C Pitted Surface 43

GT Stainless Steel 600 8 C Globules 44

TCP Stainless Steel 500-700 8 C Globules 45 ,46

Iron 700 8 C Globules 40

Copper 700 8 C Globules 47

TBP Stainless Steel 600°C Globules 44
Performance testing of vapor delivered lubricants on M50 steel

surfaces was accomplished by using a modified four ball wear tester.

This tester is shown in Figure 3. In this system lubricant vapors

were introduced into the ball pot which was heated to 370 °C. Tests

were run for 30 minutes at a speed of 100 rpms under loads varying

from 6 to 60 legs . TBP, TCP, GT, TMPTH, and PPE were studied.

A summary of wear results is presented in Figure 4 [42,45,47].

The data representing each lubricant were those which provided

superior performance. Upon a detailed examination of these data one

finds that several general trends can be observed. The wear scar

diameter versus load plot parallels the Hertzian contact line for TCP,

TBP, GT, and TMPTH. This trend is commonly seen in literature

[48,49,50]. And three of these fluids are phosphate esters. The

polyphenyl ether and GT experienced seizure which is one measure of

load capacity for a lubricant. The polyphenyl ether had significantly

higher wear scars than the other lubricants particularly at low loads

.

Also , the TCP showed enhanced performance at high loads . Thus an

overall rating of decreasing wear resistance would give TCP > TBP —

TMPTH > GT > Polyphenyl ether. It is noteworthy that the TCP, TBP,

and GT tests were performed in nitrogen while TMPTH and polyphenyl

ether were tested in air. Also, vapor concentrations varied from

0.11% to 1%. This implies there are optimum concentrations and

environments for specific lubricants which give optimum performance.

The concentration levels used indicate that the vapor lubrication

delivers effective high temperature lubrication with minimal

quantities of lubricants.

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In summary, previous work has found that vapor -deposited films

provide lubricity in inert-to-air atmospheres. The formation of these

films depends on numerous variables, including:

Lubricant Type

Substrate Composition

Vapor Concentration

Carrier Gas Composition

Vapor Exposure Time

SEM studies have shown the presence of globular and filament- like

deposits, but no explanations were offered for their formation.

Another key issue is whether ceramic surfaces will behave like metal

surfaces in their film forming behavior.

2 . 3 Chemical Vapor Deposition Studies

A process closely related to vapor deposition is chemical vapor

deposition (CVD) . Many important materials are being prepared by CVD

for electronic and optic devices. They include films of silicon,

germanium, silicon nitride, silicon oxide, gallium arsenide, cadmium

sulfide, and zinc selenide [51]. In the CVD process a gaseous phase

chemically reacts at or near the specimen surface to produce one or

more condensed phases (deposit) plus gaseous product species. Various

types of CVD include thermal energy input CVD, plasma CVD, photochemical CVD, laser CVD, and acoustic CVD [52]. These techniques
25

differ in the manner which they produce gaseous reactant species. The

CVD process is influenced by numerous effects including surface

charges and gas phase reactions

Hitchman et al . found that electrostatic forces play a large role

in the deposition of films by CVD [52]. An example of this was found

in the growth of silicon films by thermal CVD. Key reactions are

shown below:

SiHA (g) : SiH.Ca)

SiH4 (a) : Si(s) + 2H2 (a)

H2 (a) : H2 (g)

The use of dopants as reactants greatly altered the growth rate and

the activation energy for deposition by influencing the electrostatic

properties of the surface. p-Dopants such as PH3 and AsH3 gave the

silicon substrate a partial positive charge, while n-dopants like B2 H6 give the surface a partial negative charge. The silane molecule,

SiHA

, contained a silicon atom which has a partial positive charge

while the hydrogen atoms had partial negative charges . Thus , as this

molecule approached the silicon surface adsorption was enhanced in the

presence of p-dopants while it was retarded in the presence of ndopants . Thus as the ratio of n-dopant to silane gas increased the

rate of deposition decreased and the activation energy for deposition

increased. The converse was true for p-dopants.

Gas phase reactions typically control deposition rate. An

example of this was reported by Scott et al . for the growth of
amorphous hydrogenated silicon films by plasma CVD [53] . Gas phase

reactions for this reaction are shown in Table 8. SiH2 diradicals are

intermediates for the process and lead to the formation of all primary

and secondary products of SiH^ pyrolysis. The rate determining step

for this process was homogeneous although film growth was

heterogeneous. The substrate temperature determined the film

composition, but the growth rate was determined by gas phase

reactions. Other authors [54] have observed that homogeneous

deposition was controlled by a combination of hot zone length, gas

flow rate, gas thermal diffusivity, and gas temperature.

The growth and formation mechanism for a CVD film was detailed by

Bryant [55]. Following adsorption, atoms diffused along the surface

of the substrate. Stable nuclei, consisting of a few atoms, formed at

preferred sites on the substrate surface. Growth proceeded by the

addition of individual atoms from the reactant gas stream and from

surface diffusion across the uncovered portion of the substrate. Atom

additions during the growth process occurred at positions of lowest

free energy like steps or kinks and proceeded to form crystallites.

The crystallites resembled islands in appearance and formed a thin

single crystal upon coalescence.

A number of factors have been shown to influence film deposition

rate, amount, structure, and uniformity. The nature of the substrate

surface can have a profound effect on the structure of deposits

formed. Also deposit microstructure is controlled by substrate

temperature [56], gas phase mass transport, and concentration of the

reactants. Table 9 shows the effect of temperature and pressure on
Table 8. Gas Phase Reactions Involved in Growth of Amorpho

Hydrogenated Silicon Films by Plasma CVD

SiH4 a SiH2 + H2 Si2 H6 » SiH2 + SiHA Si 3 H8 « SiH2 + Si2 H6 Si4 H10 = SiH2 + Si3 H8 etc

« SiH3 SiH + H2 Si3 H8 = Si2 H5 SiH + H2 Si 3 H8 * SiH3 SiH + SiHA SiA H10 = Si 3 H7 SiH + H2 SiA H10 « Si2 H5 SiH + SiHA SiA H10 « SiH3 SiH + Si2 H6 etc

Reprinted from Scott [53]
Table 9. Effect of Temperature and Pressure on the Microstructure

of Fluoride CVD Tungsten Produced at H2 /WF6 = 6

Temperature 760 torr 0.1 torr

(°C)

Microstructure Microstructure

250 No deposition fine grained (<1 ^m)

350 Less coarse grained

500 Coarse grained

600 Coarse grained

650 Coarse grained

700 Coarse grained

750 Botryoidal

800 Dendritic, powder

900 Dendritic

1000 Dendritic Whiskers , fine grained (<1 /xm)

1350 Epitaxial

Reprinted from Bryant [58]
29

deposit morphology as reported by Bryant [55] . Deposit morphology and

rate can be influenced by preferred adsorption of impurity gases.

While the minimum temperature for deposition depends upon substrate

material. Choice of substrate material influences the nucleation rate

and hence the growth rate. Film thickness uniformity is controlled by

the axial temperature profile, the pressure conditions, and the

reactant gas flow [57].

In many cases, the test parameters for CVD processes are similar

to those used in vapor deposition. But the reactant mix, the number

of intermediates, the number of adsorbed species, the number of

reactants , and consequently the products produced are much more

complex for vapor deposition studies. Still, this implies that

fundamental knowledge gained from CVD studies may advance research

efforts of vapor deposition studies.
Chapter 3

EXPERIMENTAL APPROACH AND RESULTS

3 . 1 Experimental Approach

The previous section has outlined the need for high temperature

lubricants for metal and ceramic systems. Vapor phase lubrication has

shown great promise for high temperature lubrication, but a number of

questions remain unanswered. Kinetic data collected during earlier

vapor deposition studies have been obtained by weighing samples after

a finite coating period. A more useful approach would be to generate

continuous kinetic data. This would provide true kinetic rate data

and save experimental time. Another key issue is whether advanced

ceramics will behave as metals have during vapor deposition.

Ceramics, often considered inert, may not react with vaporized

lubricants to form continuous films. If this is true, vapor

lubrication would not be a useful technology for the high temperature

lubrication of ceramics. The presence of sintering aids and dopant

may affect vapor deposition film formation behavior. And if films do

form on ceramics, what is their appearance and do they have lubricous

qualities? This thesis will address many of these questions. A TGA

apparatus is adapted to allow the collection of continuous kinetic

data on metal surfaces. The film forming behavior of TBP on Si3 NA ,

SiC, and Al2 03 is investigated. Finally the lubricating qualities of

vaporized lubricants on ceramics are investigated.
3 . 2 Kinetic Data on Metal Surfaces

3.2.1 TGA Vapor Deposition Apparatus

Vapor deposition studies were conducted to obtain continuous

kinetic data on metal specimens. The system used incorporated a

Thermalgravimetric Analyzer (TGA) into its design. A conventional TGA

is shown in Figure 5. Key features include a microbalance which

measures weight changes of a sample positioned with a furnace. A

purge flows through the microbalance to prevent reacted material from

contacting the delicate microbalance works. The furnace has a heating

capability of up to 1000°C. The furnace temperature was monitored by

a thermocouple located within the furnace just under the sample pan.

Also, the TGA has a side arm to introduce various atmospheres to the

furnace zone

.

A vaporization tube was constructed such that its nozzle would

fit in the side arm and would be positioned at the furnace mouth. The

TGA vaporization tube is shown in Figure 6 . The quartz tube had

separate carrier gas and air inlets. The air inlet was downstream

from the lubricant introduction point. This eliminated the

possibility of liquid phase oxidation of the lubricant. The lubricant

was injected through a drawn capillary tube into a plug of glass wool.

The glass wool provided a large surface area for the liquid lubricant

to interface with heated carrier gas, resulting in complete

vaporization. The drawn capillary tube minimized the amount of
Figure 5 TGA
33
34

lubricant present within the system at any time. The end of the

vaporization tube was fitted with a 12/1 female ball joint. A syringe

pump with a syringe epoxied within a 12/1 male ball joint was clamped

to the female ball joint of the TGA adapter using a pinch clamp.

The syringe pump provided a very controlled flow of lubricant through

the system.

Before any testing could be performed, the TGA furnace had to be

calibrated. Figure 7 displays a cutaway view of the furnace. During

calibration, a series of four paramagnetic standards were placed in a

gold pan within the furnace. A weight of zero was then entered into

the TGA data station. A magnet was placed outside the TGA tube such

that a maximum positive deflection occurred in the recorded sample

weight. The temperature of the furnace was then ramped at a rate of

10°C/min with the weight continuously monitored. Each paramagnetic

sample has a well defined Curie Point. This was the temperature

where, at and above, the sample loses its magnetic characteristics.

Thus, the weight measured was constant until the Curie Point was

reached and exceeded. At this point, a large decrease in weight was

observed. The indicated temperature of transition was compared to

known reference values and the controls were adjusted accordingly.

The process was repeated until the indicated Curie Point and actual

Curie Point were in accord for all standards. Table 10 compares the

actual values to the reference values. Clearly the TGA provides

controlled temperature over a very large temperature range.

The ideal specimen for TGA vapor deposition studies was one with

a large surface area to volume ratio. This maximizes the ratio of
Figure 7 . Cutaway View of the TGA Furnace
Table 10. TGA Temperature Calibration Using Paramagnetic Standards

Standard

Point, °C

Reference Curie

Point, °C

Measured Curie AT

Alumel 163 163.9 -0.9

Nickel 354 352.9 1.1

Perkalloy 596 596.0 0

Iron 780 776.2 3.8
37

deposited lubricant film to substrate weight thus providing maximum

sensitivity. Wires, 0.005" in diameter, were selected for the metals

studied in this work.

The wire samples were cut into 10" lengths. Then the specimens

were abraded with a 600 grit sand paper followed by steel wool. The

abraded samples were balled into spheres approximately 1/4" in

diameter. These spheres fit within the platinum basket in the TGA

furnace. The balls were then ultrasonically cleaned for 30 minutes in

acetone, hexane, and finally toluene. The specimens were then stored

in toluene until testing.

3.2.2 TGA Vapor Deposition Procedures

The test procedure consisted of removing a balled metal specimen

from the toluene and rinsing it off with ethanol. The sample was then

placed on the platinum basket in the TGA and the TGA was closed. The

purge through the microbalance was set at 40 cc/min. The vaporization

tube was positioned with its nozzle through the TGA side arm and

pointed at the balled sample in the TGA furnace. The syringe pump,

supported on a stand, was then clamped to the TGA vaporization tube.

A tube containing preheated carrier gas was then connected to the

carrier gas inlet. If argon studies were being performed, a plastic

cap was placed over the air inlet of the TGA vaporization tube. The

TGA vaporization tube was then wrapped with heating tape with a

thermocouple placed between the tape and the vaporization tube at the

point of lubricant introduction. A second heating tape was wrapped
38

around the TGA tube surrounding the furnace. The temperature of this

interface was also monitored by a thermocouple. The carrier gas flow

rate was set at 80 cc/min to flush out any residual gases from within

the furnace. Both heating tapes were turned on. The heating tape

wrapping the vaporization tube was set at 30°C above the saturation

temperature for the selected lubricant injection rate. This insured

complete vaporization. When both heating tapes had reached their

temperature set points, the purge rate was reduced to 25 cc/min, the

carrier gas flow rate was reduced to 40 cc/min, and the furnace was

ramped to the desired test temperature. The sample weight was

initially set at zero, then a weight suppression dial was adjusted so

that a sample weight of 0.15 mg was entered into the TGA data station.

This weight represented full scale (100%) on the TGA printout of

weight gain versus time. The weight suppression was further adjusted

to indicate only 10% sample (0.015 mg) was initially present. The

syringe pump was turned on and a weight gain versus time was

continuously recorded as vaporized lubricant deposited on the

specimens

.

The ability to continuously collect kinetic data is a major

advantage over previous systems. Data collected from earlier vapor

delivery system were obtained by measuring the weight gain at the end

of a given deposition time. Thus, many individual tests had to be

conducted to characterize a given substrate, lubricant, and deposition

temperature. This new technique allows a more complete

characterization by conducting only one test.
The possibility of back diffusion of vaporized lubricant reaching

the microbalance working is detailed in Appendix D. These

calculations demonstrate that no vaporized lubricant reaches the

microbalance

3.2.3 TGA Vapor Deposition Results

The TGA vapor deposition system was used to collect continuous

film formation data on copper, nickel, stainless steel, and platinum

substrates. Also, the effects of temperature and lubricant were

studied. Initial studies were conducted using purge rates of 25

cm^/min through the balance and 40 cm^/min through the vaporizing

tube

The effect of substrate on vapor deposition at 700°C using 1.06%

TBP is shown in Figure 8. Several observations are apparent from

this data. First, the vapor deposition of TBP is surface sensitive

with copper being more active than stainless steel, followed by nickel

and platinum. The same pattern of metal activity was observed for two

other phosphate esters, TCP and GT [47]. Also the rates are more

rapid during the first few minutes. This implies that the metal

surface is catalyzing the deposition process. As more deposit is

formed, the metal becomes encapsulated in deposit; thus, the rate is

reduced and approaches a steady state value for long test times. The

deposition rate on copper was still very rapid for long test times.

This indicates that copper remains active. This deposit forming rate

behavior has also been observed in coke formation studies. The rate

41

of coke formation is very rapid initially and decreases as time

progresses [58-60] . This is consistent with the scenario where

catalyst particles become deactivated and coke continues to form on

existing coke. This also agrees with previous vapor deposition

findings

.

Coke studies have also shown that the type of metal surface a gas

contacts at elevated temperatures has a major impact on the quantity

of coke deposit formed. The amount of coke produced on iron surfaces

decreases in the following order FeO > Fe ~ Fe2 03 > Fe 3 C [61]. While

iron produced substantial quantities of coke, LaCava et al. [62]

determined nickel forms even more. And a Ni-Fe surface produces more

coke than either of the parent materials [63] suggesting a synergetic

interaction between the two metals. In general, metals form more

deposit than nonmetallic materials. Holmen et al. [58] observed

deposits decrease in the following order: Co > steel > Cu > Mo ~

quartz. This inertness of silica based materials has been observed by

other authors [63,64]. Brown [64] found that silica coated HK 40

steel formed much less deposit than uncoated steel. While vycor

formed smaller amounts of deposit than an aluminized incoloy sample

which produced less deposit than unadulterated incoloy [59].

Nonmetallic oxide additives have been found to reduce coke formation

on a Ni-Fe surface by several methods according to Baker et al . [63].

Alumina and titania provide a physical barrier against hydrocarbon

adsorption and decomposition which results in coke formation. W03 ,

Mo03

, and Ta2 03 reduce the solubility of carbon in the metal while

silica reduces both solubility and diffusion of carbon through a
42

catalyst particle. Thus, the surface composition has a pronounced

effect on the coke formation which occurs.

Table 11 compares the initial rate of film formation on various

substrates at 700°C from previous studies [65] and this work. The

measured formation rates of this study are less than those of previous

studies. These differences can be explained by examining the effect

of using argon as a carrier gas. The argon carrier gas passed through

an oxygen scavenger which removed oxygen to a level of 1 PPM.

Commercial nitrogen used in previous studies contained substantially

more oxygen. Table 12 displays the effect oxygen on the initial film

formation rate on copper at 700°C. Clearly, the presence of oxygen

greatly enhances the rate of film formation. Thus, the lack of oxygen

may substantially reduce the amount of film formed on various

substrates

.

The effect of the lubricant on the deposition on stainless steel

at 700 °C is shown in Figure 9. An alkyl phosphate ester, TBP, is

compared to an aryl phosphate ester, TCP. Both fluids appear very

similar, showing an initially rapid deposition rate which is followed

by a slower rate at long test times. The similarity between the two

phosphate esters differs from the results observed by Lai [41], but

this is most likely related to the lack of oxygen within the TGA.

Figure 10 displays the effect of temperature on the deposition of

nickel using 1.06% TBP. The data shows that a much greater deposition

occurs on nickel at 600°C compared to 700°C. This inverse

relationship between temperature and amount of deposit has been

observed previously. Table 13 shows a compilation of other
Table 11. Initial Rate of Film Formation on Various Substrates

at 700°C

Initial Rate, /xg/cm2 • min

Substrate TBP 1 .06% TCP 1.55% [77] GT 1 75% [77]

Copper 19.6 210 280

Stainless

Steel 6.8 110 45

Nickel 3.8 15 10

Platinum 0.8
44

Table 12. Effect of Oxygen on the Initial Rate of Film Formation on Copper at 700 °C

Atmosphere

Lubricant N2 , A»g/cm2 • min Air, /ig/cm2• min

0.126% TCP [77] 29 651

0.106% GT [77] 89 1630
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researchers studying high temperature film formation who witnessed

decreasing amounts of film formation with increasing temperature. Lai

observed similar results for stainless steel coated with TBP [41].

This has also been observed for coke formation results on nickel

[58,59,62-64,66-77]. Also the film formation of GaAs has been shown

to decrease with increasing temperature starting as low as 600°C by

various authors [78-82]. Petzke et al. [78] and Leys et al . [79]

attributed this phenomena to homogeneous nucleation in the vapor

phase. Zaouk et al. [80] determined that dissociation of molecules in

the gas phase caused a reduction in deposition rates at elevated

temperatures. They also suggested increased rates of desorption as a

possible explanation [78]. Finally, Stringfellow et al. [81] and Shaw

[82] concluded that the rate of film growth at elevated temperatures

is limited by thermodynamic equilibrium. The occurrence of

homogeneous nucleation is not likely in this work. This is because

the low vapor concentrations used, 1.06% maximum, result in a very low

collision frequency thus, homogeneous nucleation is minimized. The

most likely explanation would be either thermodynamic equilibrium

limitation or desorption becoming dominant at elevated temperatures.

Because adsorption is exothermic and desorption is endothermic if

equilibrium of an adsorption-desorption reaction is achieved the

proportion of desorbed species would increase as the temperature is

raised, thus decreasing the film formation rate.

To gain a deeper understanding regarding the adsorptiondesorption characteristics of vapor deposited films, the following

experiment was conducted. A nickel sample that was coated at 600°C in
1.06% TBP for 80 minutes was placed in the platinum basket within the

TGA furnace. The furnace was then ramped at 10°C/min from 300 °C to

900°C in flowing argon. Figure 11 shows weight loss versus

temperature for the desorption of the surface film. At 900°C all of

the film had been desorbed.

The data in Figure 11 shows that the desorption of the vapor

deposited film is a staged process. A portion of the film is desorbed

between 350 and 550°C. A second part of the film is desorbed between

750 and 900°C. This implies the presence of different types of film

having different thermal stabilities. A similar phenomena has been

observed by researchers studying coke formation. Up to five different

types of coke have been observed to exist simultaneously. Table 14

provides a description of coke formed by reacting ethane with nickel

at various temperatures as reported by McCarty et al. [72].

Figure 12 shows an Arrhenius plot for the desorption of the low

temperature form of decomposed species from 441 to 385°C. A best line

fit is superimposed on the data. The regressed line had a correlation

coefficient of 0.98. The slope of the line is -15695.9. Thus the

activation energy for the desorption of the low temperature species is

31.2 kcal/mole. Figure 13 shows an enlarged view of the Arrhenius

plot for the desorption of the high temperature form of decomposed

species from 814 to 727°C. A regressed line is superimposed on the

data. The regressed line had a correlation coefficient of 0.998. The

slope of the line was - 20822.4. Thus, the activation energy for the

desorption of the high temperature form of the desorbed species is

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52

Figure 12. Arrhenius Plot for the Desorptlon of a TBP Film

from Nickel from 441 to 385°C
53

0.00092 0.00094 0.00096 0.00098 0.00100

1/T

Figure 13. Arrhenius Plot for the Desorption of a TBP Film

on Nickel from 814 to 727°C
54

41.4 kcal/mol. The large activation energies for the film indicate

that all the species are chemically bound, and not just physisorbed.

These data can now be used to provide further interpretation to

the rate data collected on nickel at 600°C and 700°C in Figure 10.

During vapor deposition adsorption and desorption occur

simultaneously. Therefore, the amount of film present at any one time

is a difference between the total amount of adsorption and desorption

which has occurred. From Figure 11, it is clear that desorption

occurs from 420 to 650°C. At about 650°C the desorption reaches a

near zero rate. Further desorption did not occur until 750°C. This

shows at 700°C the amount desorbed is controlled by system

thermodynamics which require higher activation energies for further

desorption. Therefore, for TBP deposited on nickel the thermodynamic

equilibrium of the system dictates a reduction in the amount of

deposit formed as temperature increases from 600 to 700°C, as seen in

Figure 10.

These results show that a TGA can be adapted for continuous vapor

deposition studies. A substantial substrate effect was observed for

the vapor deposition of TBP. The amount of film formed on various

substrates in decreasing order was Cu > SS > Ni > Pt. The absence of

oxygen greatly reduced the amount of film formed. Temperature studies

of TBP on nickel showed that more deposit was formed at 600°C than

700°C. This was caused by desorption becoming more dominant at 700°C.

A desorption experiment was conducted and two types of chemically

bound species with differing thermal stabilities were found. Overall,

it is very clear that the vapor deposition process is a very complex
55

phenomenon, but the versatile nature of the TGA provides many critical

insights by continuously measuring film deposit or film removal.

3 . 3 Ceramic Vapor Lubrication Studies

3.3.1 Design of Vapor Deposition Apparatus

During previous vapor deposition studies a number of vapor

deposition furnaces have been constructed. The first vapor deposition

apparatus was designed by Lai [41]. This unit is displayed in Figure

14. A close examination of the vapor deposition oven reveals several

drawbacks. First, large amounts of lubricants were required to fill

the stainless steel reservoir within oven C. The oven was maintained

at a high enough temperature to allow considerable degradation of the

lubricants. Also, wire and foil specimens were wrapped around a

stainless steel cylinder then contacted with the vaporized lubricant.

At the test temperatures, surface mobility of metal species has been

observed and may have been a factor in the observed deposition rates

[83-86] .

These factors led Min [87] to construct a modified vapor

deposition chamber shown in Figure 15. In this arrangement the

lubricant is placed within a stainless steel boat. The boat could

hold 4.6 ml of lubricant. Wire specimens were hung from a chromel

hook on the specimen holder. The boat was heated to the desired

vaporization temperature with carrier gas flowing from the deposition

target to the boat. When the target and the boat reached their

desired temperatures the flow direction was reversed and the specimen
56

was coated for a given length of time. The reversal of the carrier

gas allowed vapor above the boat to achieve a higher saturation level.

This resulted in an unsteady state vapor concentration in the carrier

gas. This may explain why Min [90] and Pinto [91] observed linear

deposition rates after an initial period of more rapid deposit

formation.

Gunsel [42] used the vapor deposit apparatus illustrated in

Figure 16. In this setup, the specimen was heated to the test

temperature in flowing nitrogen in oven 1. To perform deposition the

heated target was pushed along guides into a second oven in which

carrier gas and lubricant were flowing. Upon test completion, the

sample was removed from the deposition oven cooled and weighed on an

electrobalance

.

A controlled lubricant flow rate was achieved by injecting

lubricant with a syringe pump into a bed of packed glass helices.

This allowed intimate contact between the lubricant and heated carrier

gas, resulting in complete and controlled vaporization.

Design considerations for vapor deposition reactions has been

reviewed by Dapkus [89]. In general, there are two configurations

used for vapor phase deposition studies, vertical and horizontal

reactors. Vertical reactors position the substrate perpendicular to

the gas flow direction. Rotation of the substrate is required to

assure uniform depletion of gas constituents. The horizontal design

places the substrate parallel to or slightly inclined to the reactant

flow direction. This design results in a more uniform and controlled

gas flow over the substrate surface. The major disadvantage of the
i

59
60

horizontal design is increased interaction of reactant gases with

reactor walls in the deposition zone. For this study a horizontal

configuration was chosen because of control and simplicity.

A vapor deposition tubular furnace was designed to allow

controlled coating up to very high temperatures, 1200°C, and to coat

specimens up to 2 inches in diameter. This was accomplished by using

two tubular Mellen furnaces and joining them with a flange. A

dimensioned, scale drawing is shown in Figure 17. The function of the

first furnace was to vaporize the lubricant while deposition occurred

in the second. The temperature limits were 1000°C and 1200°C,

respectively, and each furnace was controlled by an independent

Eurotherm controller. A 2-1/4" diameter fused quartz tube was

inserted within the tubular furnace. Fused quartz was chosen because

of high thermal shock resistance, high temperature stability, and

relative inertness toward forming coke. Fused quartz can be heated to

1200°C and plunged into cold water without resulting in physical

damage [90]. Also, with a 1600°C softening point, fused quartz

maintains its structural integrity to very high temperatures. The

chemical inertness allows measurements which are indicative of

substrate being examined, and not a combination of substrate and

insert wall effects.

To aid in heat transfer within the vaporization portion of the

furnace, a series of baffles were designed for inside the fused quartz

tube. Figure 18 provides the dimensions of the baffles, and Figure 19

shows a scale drawing of the baffle assembly. The baffles were spaced

1/2" apart. This was consistent with the spacing criteria for baffles
61
9/32'

\1/4* Steel Shaft

U>per Baffles

tower Baffles

Figure 18. Upper and Lower Baffles

64

in the exchangers where the rule of thumb dictates a spacing greater

than 1/5 the tube diameter, but less than the tube diameter. The rest

position of the baffle assembly within the quartz tube was achieved by

making a dimple in the quartz tube which the baffle assembly would

butt up against. The baffles and support shaft were constructed of

1020 steel. A small clearance existed between the baffles and the

quartz tube. This accommodated the steel baffles greater thermal

expansion. The steel was chosen because of its high thermal

conductivity. The baffles contained a tube which allowed air to be

introduced down-stream of the lubricant vaporization if oxygen studies

were being conducted. The air tube ended in a loop which contained 10

holes evenly spaced around it. This feature provided a very uniform

gas phase distribution of oxygen or any other gas introduced through

the tube

.

Lubricant was injected onto glass wool and contacted counter

currently with the carrier gas. The glass wool provided a large

surface area to allow fluid and carrier gas to develop intimate

contact while counter current flow also provided more uniform

vaporization. Figure 20 is a scale drawing of the front part of the

quartz tube and baffle assembly. Controlled lubricant flow rates were

introduced to the system using a syringe pump. A syringe was epoxied

to a 12/1 male ball joint; thus, allowing the syringe pump to be

clamped to the lubricant inlet port. The lubricant flowed through a

1 mm diameter capillary tubing. The volume of the capillary within

the baffle assembly was 0.120 cm3. The quartz tube was fitted with a

66/60 female fused quartz ground joint. This joint was seated with a
65
66/60 male fused quartz ground joint. When the ground joints were

fitted together the lubricant and air supply tubes also seated. This

was accomplished by using compression spring loaded female ball joints

in the baffle assembly. These ball joints interfaced with the male

ball joints of the quartz tube cap. Air and carrier gas lines were

connected from flow meters to the indicated ports.

A typical temperature distribution for the deposition portion of

the quartz tube is shown in Figure 21. In this experiment, the

vaporization furnace was set at 300 °C and the set point on the

deposition furnace was 660°C. The use of a plug of quartz wool

improved the flat zone within the furnace. A uniform thermal flat

zone existed between 9 and 14 inches from the end of the furnace. All

deposition targets were placed in this region and a quartz wool plug

was used for all tests. Figure 22 shows a comparison of the wall

temperature to the center line temperature with the baffles not in

place. The center line temperature and the wall temperature are

nearly identical. This was a result of the slow carrier gas flow

rates used for all tests and the low heat capacity of the gaseous

mixture

.

3.3.2 Vapor Deposition Procedures

The coating procedure was as follows. A clean wad of glass wool

was put in the baffles where the lubricant would emerge from the

capillary tube. The baffles were sled into the quartz tube and

covered with the quartz tube cap. The syringe pump was connected to
o
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d

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00 CDm CN

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5-1
69

the quartz tube cap and the carrier gas line was connected. The air

line was sealed with a plastic cap if oxygen studies were not being

performed. Specimens were placed on a silica or alumina flat and slid

into the hot zone. A quartz wool plug was put in the end of the tube

and the tube was purged with carrier gas for 16 hours at a flow rate

of 3 ft3 /hr. The ovens were brought up to the desired temperatures

over the next five hours. The flow rate was reduced to the test

condition of 1.2 ft3 /hr (0.5 cm/s at STP) for 1 hour and deposition

was initiated. The sample was coated for a set time. The syringe

pump was turned off; then, 30 minutes later the furnace was turned off

to allow the samples to oven cool.

Vaporization temperature were chosen by using the vapor pressure

chart for hydrocarbons from Meyers [91]. This chart accurately

describes the vapor pressure-temperature relationship for phosphate

esters. This was verified by comparing chart values to empirical

equation results for tributyl phosphate [92]

.

3.3.3 SEM Results

The tubular furnace was used to study the vapor deposition of TBP

on different ceramics. Because ceramic materials are not available in

a wire form the TGA system could not be used to obtain kinetic data

for the ceramics

.

Two 1/4" diameter disks, 1/16" thick of Si3 NA , Al2 03

, and a-SiC

were placed on a steel plate 1-1/2" wide and placed within the tubular

furnace. The silicon nitride was Kyocera's type SN 220 from lot G211
70

number 8. The alumina flats were Coors type AD998. This material is

99.8 percent alumina. The silicon carbide was Carborundum's a-SiC

from batch 2. The specimens were coated for 14 minutes at 700°C using

0.73% TBP. The specimens were oven cooled and the films were examined

using an SEM. The micrographs were photographed with the specimens

tilted 70° to the surface normal. This provided an enhanced

perspective to the surface film characteristics.

Figure 23 shows an SEM of the lubricant film formed on silicon

nitride at a 10,000 and 20,000 fold magnification. The SEM photograph

shows a uniform and shallow covering of global structure and much

larger filamentous structures. EDAX spectra were taken in two areas

in Figure 23 and displayed in Figure 24. The upper spectra was taken

from the globular head of the filament in Figure 23. The second

spectra was taken from away from the filament head and was

representative of nonfilamentous background. The EDAX spectra reveal

that the filament head contains iron, while iron was not present in

other structures. The most likely source of this iron was from the

steel plate which supported the ceramic samples. Because the steel

plate was simultaneously coated during the deposition experiment, FeP

could have been transported in the vapor phase from the steel tray

into the ceramic substrate; thus, providing a source of iron atoms.

Filament structures have not been observed in vapor deposition studies

using aryl phosphate esters, but they have been observed when nonaromatic hydrocarbons are used. Thus, the alkyl chain of TBP may be

responsible for the filamentous growths. This hypothesis is supported

by liquid phase decomposition studies of phosphate esters. In the
a) 10,000X

Figure 23. SEM Photographs of Si3 NA Coated with 0.73% TBP

at 700 °C
72

T3

O

c/>C

o

0)o

ou

p-l

4Jc

a) u

oO

o

4-1

H

4-> <*> CJ CO

CD |-> (X • CO o

Q

42

4->

CN

3

60

•H
73

liquid phase Cho and Klaus [19] have shown that phosphate esters

decompose forming P-O-P bonds in inert atmospheres. If this happens

during vapor deposition, the alkyl substituent could be freed to react

with the iron to form the filamentous structures.

Figure 25 shows a AI2O3 substrate which was coated with TBP at

3,000X and 10,000X. Again filaments are formed, but they are

different in nature than those produced on Si3N4. The filaments are

shorter and narrower.

Figures 26 shows the coatings formed on a-SiC at 10,000X and

20,000X. This coating is distinctly different from the AI2O3 or

Si3N4. Both worms and globules are observed, but they are much

smaller than those previously observed.

These experiments show that iron can effect the nature of films

formed during vapor deposition. The fact that each ceramic formed

films which had distinctly different features indicates that the

deposition is not completely controlled by the iron. Thus the vapor

deposition process is substrate sensitive for ceramics.

The formation of filamentous structures has been observed in coke

formation studies. Carbon exhibiting growth as filaments can be

produced in several ways. Catalyst particles have been observed at

the filament top, filament base, or within the filament, depending on

the substrate [61,63,67-69]. Table 15 displays these findings.

The mechanism for formation of filament with catalysts particles

located at the growing tip has been detailed by Baker [61]. In

general, it involves the adsorption of a hydrocarbon molecule,

followed by decomposition. Carbon diffuses through the catalyst
b) 10,000X

Figure 25. SEM Photographs of Al2 03 Coated with 0.73% TBP at

700°C
a) 10.000X

Figure 26. SEM Photographs of a-SiC Coated with 0.73% TBP at

700°C
76

Table 15. Location of Catalytic Particle within Filaments

Catalyst Particle Position within Filaments

Base Middle Top

Fe FeO

Fe2 03 Ti02 Ni

Cr Co

Pt-Fe Ni-Fe
77

particle and precipitates at the rear of the particle. Growth is

terminated when diffusion is not rapid enough and the catalyst becomes

encapsulated in carbon. This is diagrammed in Figure 27. It is

suggested that the rate determining step in this process is diffusion

of carbon through the catalyst particle.

Table 16 shows activation energies observed for coke formation

under varying conditions. In many instances, the observed activation

energy agrees with the activation energy of diffusion of carbon

through the catalyst. But this is not always true. Some authors even

observed systems exhibiting a negative activation energy. This

phenomenon can occur when equilibrium is established before the rate

determining step and the reverse reaction has a larger activation

energy than the forward reaction. Clearly, Table 16 shows that the

rate controlling step in coke formation is specific to a combination

of substrate, temperature, and reacting gases. Thus, many strong

parallels exist between coke formation and vapor deposition.

This section has shown that vapor deposited films can be formed

on ceramics. Impurities which are present can have a substantial

influence on the type and amount of film which is formed. This also

suggests that sintering aids and dopants commonly used in the

processing of ceramics can be used to control vapor deposition film

characteristics. Thus, the vapor lubrication of ceramics can be

optimized by controlling the surface features of ceramic substrates to

reduce friction and wear of rubbing contacts. This also suggests that

on metal surfaces ion implantation can be used to achieve the desired
78

Figure 27. Stages in the Growth of Filaments with a Catalyst

Particle in the Growing End

Reprinted from Baker [58]
79

Table 16. Activation Energies for Coke Formation on Various Substrates

Catalyst Gas Activation Energy (kcal/mol) For Temperature

Range

°C

Reaction

Order

Carbon Growth Diffusion Observed

of Carbon

H2 Hydrocarbon

Cobalt Acetylene 33.0-33.3(68) 34.7 530-930

Vanadium 27.6(61) 27.8

Molybdenum 38.8(61) 41.0

Chromium Acetylene 27.1(68) 26.5 500-1000

Copper Benzene 50 (74) 800-1050 -0.29 1

Iron Acetylene

Benzene

Benzene

16.1(68)

16.6 (62)

24.3 (62)

10.5-16.5

650-1000

625-800

500-625

1 0 0 >0

Iron-Nickel Acetylene 33.6 34.0 480-925

Platinum- Iron Acetylene 19.0(69) 420-720

Incoloy 800

(20.5 Cr-32 Ni-46 Fe)

Ethane Hexane

27 (64) 800-950

Hk 40

(25 Cr-20 Ni-55 Fe)

Ethane Hexane

33.8 (64)

63.7

800-850

850-900

Hk 40 Silica Coated

Ethane Hexane76.2 (64) 825-900

Nickel C2 H2 C2 H» C3 H6 C*H8 C*H6 C2 H6

C3 H6 C6 H 14 C6 H1*

33

-3.0 - -70

50

47.5

16.3 18-65

33.3

-4720

33-34.8

475-450

525-625

>650

350-475

>625

>650

350-525

675-700

>700

0 0

0 12

±0 ±0

1

Benzene 48 475-600 0.9 -.36

Toluene 3618.5 475-600

>700

0 0
80

film formation characteristics of reduced friction and wear; thus,

optimizing lubrication.

3.3.4 Performance Testing Equipment

The preceding section has shown that vapor deposited films form

on ceramics. The next step was to determine if these films impart

lubricious qualities during performance testing. Performance testing

was done using two apparatuses. The first device was a ball on three

flat wear tester. Figure 28 shows the geometry of this system which

is very similar to a four ball tester. Flats, 1/16" thick and 1/4" in

diameter were held in circular recesses within the flat holder

displayed in Figures 29 and 30. The flat holder was screwed into a

heated four ball test cup. A 1/2" diameter ball was held in a chuck.

The chuck was driven by a rotating spindle at a given speed of

rotation. The system was loaded with a pneumatic piston and

frictional force was measured by monitoring the torque generated when

the rotating ball contacted the flats. A standard four ball wear

tester is displayed in Figure 31 illustrating many of these features.

The second performance tester used was a slow speed sliding

device which could operate at elevated temperatures. Figure 32

displays this unit. The components included a variable motor driven

slide capable of speeds up to 2 cm/min. The specimen holder was a

stainless steel ball bearing holder which was attached to the slide

via a frictional force transducer. The test sample was contained

within a brick insulated oven heated by SiC heating elements. A 1" x
81

Figure 28. Ball on Three Flat Geometry

1*995'

Top View

Figure 30. Top and Side View of Ball on Three Flat
Figure 31. Four Ball Wear Tester

86

1-3/4" x 0.060" alumina specimen was placed within the alumina

specimen holder shown in Figure 33. The ball bearing holder with the

ball bearing in place was connected to the friction transducer so that

the ball would contact the far end of the specimen flat. The ball

bearing holder and ball bearing supplied a 16 N force to the specimen.

With the specimen in place and loaded, the top of the oven was covered

with ceramic insulating brick. The furnace was turned on and when the

desired temperature was reached the motor was turned on, initiating

the test. Frictional force was recorded on a strip chart recorder.

3.3.5 Ball Bearing Holder Design for the Intermediate Temperature

Tribometer

Performance testing in the intermediate temperature tribometer

was done using a ball on flat configuration. This geometry was

selected because of simplicity and availability of flats and existing

supplies of 1/2" ball bearings. A ball bearing holder was designed

and constructed. This holder, shown in Figure 34, was designed so

that ceramic balls could be tested from room temperatures to 1000°C.

This was accomplished by constructing the holder with a 10° taper.

This follows the guidelines detailed in "Machinery's Handbook" [93],

where tapers must be less than 16° to be self holding. The rod

material was 304 stainless steel. This was chosen because of

availability and ease of construction. Because the steel has a higher

coefficient of expansion than most ceramics, a thermal expansion

calculation was performed detailing the bearing position in the holder
87

Figure 33. Alumina Specimen Holder Used in the Intermediate

Temperature Tribometer
88 ICM

COI

CO

<u u

CO •Hv

0)eu

<uc

a)

•uo u

<w a) 4J U CO

<u

B

T>O

*—I XI O >.-l 5C ^H C <0U

4->

0) >—I a

CO CD « H

CO

0)3

00

fx*
89

at 1000°C. Table 17 gives the thermal coefficient of expansion for

several ceramics and the 304 stainless steel. The coefficient of

expansion for a-SiC was chosen for design calculations because it had

the largest difference with the stainless steel. Because of the

tapered design a possibility existed that materials with a lower

coefficient of thermal expansion might completely slip into the

holder. Thus any design suitable for the a-SiC would be suitable for

materials with higher coefficients of thermal expansion. Figures 35

and 36 show the position of an a-SiC ball bearing within the holder at

25 and 1000°C. Clearly, this design accommodates ceramic ball

bearings from 25 to 1000°C.

3.3.6 Sample Preparation Development

Having established that vapor deposition coatings form on

ceramics, the next step was to conduct some performance tests on

A12 03. But before conducting ceramic performance testing a cleaning

procedure was developed for oxide ceramics. The purpose of this was

to minimize the affect of any contaminant films during performance

testing. The following procedure was developed to prepare Al2 03 disks

for wear testing in the previously described ball on three flat wear

tester. Alumina rod stock, 1/4" in diameter, was held in a vise which

was mounted on a magnetic stand. Specimens were cut to an approximate

thickness of 0.070" using a diamond cut off wheel. Each disk had a

small burr on one side. This was a remnant of the cutting process.

Batches of 37 disks were mounted with the burr side up on a steel
Table 17. Coefficient of Thermal Expansion for Three Advanced

Ceramics and Stainless Steel

Material Coefficient of Lineat Thermal Expansion,

cm/cm* 0 C

A12 03 a-SiC

Si3 N„

304 Stainless Steel

8.28 x 1(T 6

3.96 x 1CT 6

4.57 x 10-6

19.8 x 10" 6
Figure 35. a-SiC Ball in a Stainless Steel Ball Bearing

Holder at 25°C
Figure 36. a-SiC Ball in a Stainless Steel Ball Bearing

Holder at *1000°C
93

polishing mount. The disks were attached with a high temperature wax.

The polishing mount was then placed on the magnetic stand and the

diamond cut off wheel was replaced with a 3/4" wide diamond grinding

wheel. The burrs were ground off and the disks were surface ground to

a thickness of 0.065". The polishing mount was removed from the

magnetic stand and taken to an automatic polishing wheel. Automatic

polishing results in a nondirectional surface finish on the specimens.

First a 30 /i diamond impregnated grinding wheel was used followed by a

6 y. diamond impregnated grinding wheel. Specimens were then lapped

using 9, 6, then 3, and finally 1 n diamond powder. At the conclusion

of each lapping step, the specimens were examined using a

stereomicroscope . This was done to assure that all scratches from

previous polishing steps had been removed. The specimens were removed

from the polishing mount by heating the mount on a hot plate until the

mounting wax melted. Residual wax was removed by placing the

specimens in acetone. Uniformity of specimens was verified by

measuring the thickness of each disk with calipers and sizing the

disks. Figure 37 shows an SEM photograph of a cleaned and polished

specimen at 3 , 000X and 10.000X. The scale bar markers in lower left

corner of Figure 37 display a unit length for size comparison. The

left most mark represents the unit length. The number of marks, n,

after the first mark indicates how many microns the unit length

represents. The general formula is: unit length = 10n_1 microns.

Thus the first markers in Figure 37a and 37b represent 1 micron.

During the alumina sample preparation procedure, the specimens

are contacted with numerous contaminants. Table 18 shows a partial
a) 3,000X

0

b) 10.000X

Figure 37. SEM Photographs of a Polished Al2 03 Specimen
Table 18. List of Possible Contaminants in the Sample Preparation

of Al2 03 Specimens

Contaminants Comments

Cutting Fluid

Lapping Oil

Diamond Abrasive

Diamond Carrier Paste

Solvent Residuals

Adsorbed Gases

Oven Contaminants

Semi -synthetic borate amine soap

contains no S, CI, N, or phenolics

Contains paraffin oil, mineral seal oil,

and lard oil

Water and carbon dioxide
96

list of these materials. To ascertain if significant surface

contamination was occurring, an alumina sample disk rinsed with a

series of solvents was examined using ESCA. The prominent carbon peak

in Figure 38 reveals substantial carbon remained after the solvent

rinsing. Also the carbon was still prominent after 10 minutes at

sputtering at 3KeV indicating that the carbon contamination is not

elemental in nature. Thus the cutting fluid and the lapping oil were

the most likely source of carbon contamination. A solubility study

was conducted on these fluids in 5 different solvents. One ml of

solvent was placed in a 5 ml Erylmeyer flask. A 20 fi£ aliquot of oil

was added to the flask. The system was given 5 minutes to achieve

equilibrium, then the solution was examined visually to see if the

system contained only one phase. If only one phase was evident, oil

was added until 0.5 ml of oil had been added. Tables 19 and 20

display these results.

Table 19 shows that all fluids except methanol had an excellent

solubility for the lapping oil. Table 20 indicates that only pyridine

had adequate solubility for the cutting fluid. Therefore, pyridine

was chosen as the proper solvent to rinse the alumina specimens.

Several other cleaning techniques were explored and the carbon

remaining was determined using ESCA and Auger spectroscopy. Table 21

presents these results. Samples with high carbon contaminations

discolored when examined by the Auger beam. Sputtering the specimen

surface for 10 minutes removed approximately 30 A of material. A

majority of carbon resided in this top 30 A. Figure 38 demonstrates

the effect of sputtering. The carbon peak is reduced and oxygen and
97

Ai|sue)u|
Table 19. Solubility of Lapping Oil in Various Solvents

Solvent

Ratio of Volume Oil to Volume Solvents

0.02 0.04 0.06 0.08 0.1 0.2 0.3 0.4 0.5

MEK Yes Yes Yes Yes Yes Yes Yes Yes Yes

THF Yes Yes Yes Yes Yes Yes Yes Yes Yes

Hexane Yes Yes Yes Yes Yes Yes Yes Yes Yes

Pyridine Yes Yes Yes Yes Yes Yes Yes Yes Yes

Methanol No No No No No
99

Table 20. Solubility of Cutting Fluid in Various Solvents

Solvent

Ratio of Volume Oil to Volume Solvents

0.02 0.04 0.06 0.08 0.1 0.2 0.3 0.4 0.5

MEK No

THF No No No No No

Hexane No No No No No

Pyridine Yes Yes Yes Yes Yes

Methanol Yes* Yes* Yes* Yes* Yes* Yes* Yes* Yes* No

*Stable emulsion formed.
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aluminum relative levels are enhanced after sputtering. This is

apparent by examining the C/0 ratio before and after sputtering in

Table 21. The effectiveness of a high temperature bake out is readily

apparent in Table 21 after the surface has sputtered for 10 minutes.

Thus, after baking out specimens at elevated temperatures the carbon

is predominantly present on the surface and is most likely adsorbed

C02 and CO. Also the detergent cleaner Micro was a very effective

cleaning agent.

Thus, the developed cleaning procedure consisted of taking the

sized disks, in groups of 3, and placing them in 10 ml acid cleaned

vials. Then a 5 ml aliquot of pyridine was placed in each vial. The

vials were collectively covered by an acid cleaned petri dish and left

to soak in the pyridine overnight. The specimens were then sonicated

in an ultrasonic bath for 5 minutes in the pyridine. The pyridine was

decanted and residual was removed by placing the vials under flowing

de-ionized water. The de-ionized water had a specific resistance of

15 megohm-cm. The water was decanted and 5 ml aliquots of a 2%

detergent (Micro) solution was added to each vial. The specimens were

sonicated for 5 minutes, the Micro was decanted, the specimens were

rinsed with de- ionized water, and the water was drained. The vials

containing the specimens were placed in an oven to bake out at 580°C

overnight. Specimens were allowed to cool to the desired test

temperature before testing occurred. The marked improvement in

surface contamination level is displayed in Figure 39.

The influence of surface contaminants on performance testing is

evident in Figure 40. Alumina samples were tested in a ball on three

103

Figure 40. Effect of Cleaning Procedure on Friction Level

- Dry A12 03 on Al2 03 , 150°C, 6 RPM
104

flat configuration at 150°C, 6 rpm, and unlubricated. The load was

stepped from 5 to 10, 15, 20, 30, 40, 50, and 60 kilograms at 5 minute

intervals. The friction coefficient was monitored throughout the

test. One set of samples was cleaned by rinsing in acetone followed

by hexane then baking out at 150°C. The second set was cleaned using

the developed cleaning procedure. The specimens receiving the more

thorough cleaning exhibited a much higher friction level before

seizing at 22 minutes. Thus surface contaminants can have a

pronounced effect on the tribological properties of ceramics.

3.3.7 Performance Testing Results

The first attempt to demonstrate the effectiveness of vapor

lubrication for ceramics was done in the ball-on- three-flat

configuration at 150°C. Cleaned alumina disks were placed in the flat

holder and flats were marked to identify their relative position. The

flats were worn-in at 600 rpm at room temperature using 1.5 mis of deionized water as a lubricant. The load was stepped from 2 kgs to 10

kgs in 2 kg increments. Each load increment lasted 10 minutes. The

samples were removed and uniform 0.380 mm diameter wear scars were

measured on each specimen. The purpose of the wear in was to lessen

the load severity of the coated ball on three flat test by lowering

the mean contact pressure. The samples were sonicated in a 2% micro

solution to remove remaining alumina debris. The specimens were then

rinsed in de- ionized water and placed in the vapor deposition tubular

furnace to coat. TCP was chosen as the coating lubricant because it
105

exhibited superior performance results in previous studies as detailed

in the literature survey. The flats were coated at 700 °C for 30

minutes using 0.5% TCP. An approximate coating thickness is obtained

by assuming that Al2 03 produces similar deposits as quartz. A 375

/ig/cm2 deposit results when quartz is exposed to TCP at 700°C for 30

minutes [65]. This is equivalent to a 3.50 p thick coating. The

disks were transported from the oven into a covered petri dish on a

hot plate. The samples were then taken to the wear tester which was

preheated to 150°C. The specimens were reloaded with care taken to

relocate each specimen in its original position. The wear test was

conducted at 10 kgs, at 6 rpm for 2 hours. A baseline test was

conducted following the same procedure except specimens were not

coated. Figure 41 displays the results. The vapor coated specimen

showed no difference in performance from the uncoated samples. The

high initial friction of the vapor coated sample was an artifact

caused by the specimens not being precisely aligned. The steady state

friction of both tests are identical, within experimental error.

Figure 42 shows an optical transmission picture of one of the worn

coated disks. The jagged appearance of the vapor coating indicates

that the coating was brittle at 150°C and most likely failed in the

first seconds of testing. A second explanation for the film failure

is that because the substrate was coated at 700°C then cooled to 150°C

a large amount of residual stress could build up at the coatingsubstrate interface if a difference in linear thermal expansion

coefficients existed. Manasevit et al . [94] observed high residual
I 106
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108

stress levels when substrates and coatings had differing coefficients

of linear thermal expansion.

Vapor deposited coatings were also tested using the intermediate

temperature tribometer at 700°C. A cleaned alumina flat, 1 3/4" x 1"

x 0.060" was placed in the tubular furnace and coated with 0.5% TCP at

700 °C for 30 minutes. This produced a coating approximately 3.5 p

thick. The flat was a 96% Alumina from Kyocera. The coated flat was

transported in a petri dish on a hot plate and placed in the alumina

sample holder. The oven was heated to 700 °C and the test was

conducted in a ball on flat configuration under a 1.6 kg load. The

sliding speed was 1.5 cm/min. The resulting friction trace is shown

in Figure 43. The friction of an uncoated specimen tested at 700°C is

also shown in Figure 43. Clearly, the TCP coating reduced the

friction on the alumina flat. This is very significant because the

coating parameters chosen were not optimized to provide a lowest

coefficient of friction. Thus, an even greater reduction in friction

would be anticipated if a parametric study examining the effect of

lubricant type concentration, oxygen concentration, coating time, and

coating temperature was conducted. The fact that coefficient of

friction increased as the sliding distance increased could be caused

by two factors. First, the TCP coating could be entrapping wear

particles as the sliding proceeds. This would result in three body

wear which would lead to higher friction. An alternative explanation

is that as the exposure time to the 700°C atmosphere increased, the

lubricant desorbed. Coating desorption at high temperatures was

displayed in Figure 11. The result of this experiment indicate that

110

vapor lubricants have a great potential for lubricating ceramics at

elevated temperatures.
Ill

Chapter 4

CONCLUSIONS

Vapor deposition studies were conducted on metal and ceramic

systems. Films formed on these substrates when they were exposed to

lubricant vapors at elevated temperatures.

A TGA was adapted for vapor deposition studies. The TGA allowed

continuous kinetic data acquisition for the film formation process.

The TGA was used to study film formation on metals using TCP and TBP

.

These two fluids showed very similar deposition characteristics.

Substrate material strongly influenced the amount and rate of film

formation. The order of activity in decreasing order was copper >

stainless steel > nickel > platinum. These systems exhibited a rapid

initial rate followed by a reduced steady state rate. This implies

that the surface is initially active, but becomes passive as more

deposit forms. The passivation process may involve metal particle

encapsulation. The concentration of oxygen present was found to

strongly influence the amount of film formed. Nickel was shown to

form much more deposit at 600°C than 700°C. This occurrence of

decreasing amount of film formation with increasing temperature may be

related to increasing desorption rates at higher temperatures. A

desorption experiment on a coated nickel substrate determined that two

different types of material form during vapor deposition. The

activation energies of the desorbed species indicate that the species

were chemically bound and they have differing thermal stabilities.
112

Because ceramic substrates are not available in wire form,

deposition studies were conducted using circular disk 1/4" in

diameter. A tubular furnace was designed and constructed for this

portion of the study. The deposition of TBP on Si3 N4 , Al2 03

, and aSiC was investigated using SEM. The films produced were surface

sensitive. Globular and filamentous structures were observed. The

filaments contained iron particles in their growing tips. The iron

was transported from the steel tray which supported the flats. This

suggests that dopants and sintering aids can be selected to provide

optimum vapor deposition characteristics for ceramic surfaces.

Performance testing was conducted on vapor coated alumina using a

ball on three flat tester and an intermediate temperature tribometer.

Before performance testing was conducted on the ball on three flat

tester, a cleaning procedure was developed for oxide ceramics. ESCA

and Auger spectroscopy were used to determine the qualitative amount

of contaminants remaining after various procedures. The best cleaning

procedure consisted of soaking samples in pyridine overnight, followed

by soaking samples in a detergent solution and finally baking samples

overnight at 580°C before testing. Dry ball on three flat tests

showed that the recommended cleaning procedure resulted in a higher

coefficient of friction in dry sliding than a standard solvent rinse.

This demonstrates the importance that surface contaminants have in

tribology

.

A ball on flat sliding experiment was performed on a coated Al2 03

flat at 700°C using the intermediate temperature tribometer. TCP

coated flats displayed a reduced friction level compared to uncoated
113

flats; 0.160 to 0.230. This demonstrated that a vapor-deposited

coating, even under unoptimized conditions, provides tribological

benefits

.
1

Chapter 5

FUTURE WORK

This study indicates that vapor deposition coatings form on

ceramics and these coatings have lubricious qualities. Much more work

needs to be done in this area to demonstrate the effectiveness of

vapor lubrication for adiabatic diesel applications.

The fact that deposition is surface sensitive on ceramics

indicates that the effect of various solid solutions, dopants, and

other substrates needs to be investigated. Much physical information

could be gained by conducting in situ SEM experiments. Chemical

structural information could be obtained using a high temperature in

situ raman system. This experimental approach would allow the coating

to be more thoroughly characterized as it is formed.

There is a great need for in situ performance testing of vapor

lubricated ceramics at actual engine contact temperatures . The effect

of load, speed, temperature, atmosphere, with various vapor lubricants

needs to be explored.

The performance data coupled with chemical and physical data on

the vapor deposit would provide a pathway for designing better

lubricants for a given high temperature application.
115

ACKNOWLEDGEMENT

The authors wish to acknowledge the ECUT Tribology Program, Office of

Energy Conservation and Utilization Technology (ECUT) of the

Department of Energy for their partial support.
116

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90. Creyke, W.E.C., Sainsburg, I.E.J. , andMorrell, R. , "Design with

Nonductile Materials," Applied Science, 222 (1982).

91. Myers, Jr., H.S., Ph.D. Thesis, The Pennsylvania State

University, June (1952).

92. Schulz, W.W. , Narratil, J.D., and Talbot, A.E., "Science and

Technology of Tributyl Phosphate - Volume I, Synthesis,

Properties, Reactions and Analysis," CRC Press, Inc. (1984).

93. Oberg, E. and Jones, F.D., "Machinery's Handbook," 19th ed.

(1973).

94. Manasevit, H.M. and Simpson, W.I., J. Electrochem. Soc. 116 . No.

12, 1725-1732 (1969).

95. Arnold, J. Chem. Phys., 1, 170 (1933).

96. Bahlke and Kay, Ind. Eng. Chem., 21, 942 (1929).

97. Gambill, Chem. Eng., 121-124, August 25 (1958).
122

98. Nelson and Obert, Chem. Eng., 203-208, July (1954).

99. Herning and Zipperer, Gas-u. Wasserfach, 79, (49), 69 (1936).

100. Friend and Alder, "Transport Properties in Gases," Cambel and

Fenn (ed.), Northwestern University Press, Evans ton, IL, 128-131

(1958).

101. Wilke, C.R. and Lee , C.Y., Ind. Eng. Chem., 47, 1253 (1955).

102. LaBas , "The Molecular Volumes of Liquid Chemical Compounds,"

Longmans , London , (1915).

103. Hirschfelder , Bird, and Spotz, Trans. Am. Soc. Mech. Engrs .

, 71 . 921 (1949).

104. Weast, R.C., "Handbook of Chemistry and Physics," CRC Press,

56th ed. , (1975-1976)

.

105. Bird, R.B., Stewart, W.E., and Lightfoot, E.N. , "Transport

Phenomena," (1960).

106. Ranz, W.E. and Marshall, Jr., W.R. , Chem. Eng. Prog., 48, 141-

146 or 173-180 (1950).
APPENDIX A

Physical Constants for TCP, TBP, and Argon at Various Temperatures

Table 22. Physical Constants for Argon at Various Temperatures

T, °C Viscosity

,

Heat Capacity, Thermal Conductivity,

H Poise cal/g«°C cal/cm«s °C x 10" 7

200 310 0.1246

300 360 0.1246 4.23

400 440 0.1246 5.30

500 445 0.1246 6.35

600 490 0.1246 7.33

700 525 0.1246 8.14

800 560 0.1246 8.93

900 0.1246 9.67

1000 0.1246 10.50

Table 23. Physical Constants for Tributyl Phosphate Ester at

Various Temperatures

T, °C Viscosity

,

Heat Capacity, Thermal Conductivity,

H Poise cal/g»°C cal/cm'S °C x 10" 7

200 72.0 0.4984 4.17

300 89.1 0.5829 6.04

400 105.8 0.6674 8.21

500 122.1 0.7519 10.68

600 137.9 0.8364 13.41

700 153.3 0.9209 16.42

800 168.2 1.005 19.66

900 182.6 1.090 23. 15

1000 196.6 1.174 26.84
Table 24. Physical Constants for Tricresyl Phosphate Ester at

Various Temperatures

T, C Viscosity

,

/i Poise

Heat Capacity,

cal/g»°C

Thermal Conductivity,

cal/cm«s °C x 10" 7

200 51.2 0.4618 2.75

300 70.2 0.5401 4.41

400 84.0 0.6183 6.04

500 97.6 0.6966 7.90

600 110.8 0.7749 9.99

700 123.8 0.8532 12.28

800 136.5 0.9311 14.78

900 148.8 1.010 17.48

1000 160.9 1.088 20.36

viscosity of the phosphate esters was calculated using Arnolds

correlation [95]:

27.0 7M T

3/z

M = :

Vfa 2/3 (T + 1.47 Tb )

where n = viscosity, micropoise

M = molecular weight

Tb = normal boiling point, K

T = temperature, K

Vb = molar liquid volume at the normal boiling point, cm3 /mol

Heat capacity of the phosphate esters was estimated using the Bahlke

and Kay [96] equation for petroleum vapors.
C

P =

125

(4 - s) (t + 670)

6450

where C

p = heat capacity, cal/g °C

t = temperature, °F

s = specific gravity at 60°F

Thermal conductivity was obtained using the Gambill [97] group

averaged Prandtl numbers where for polar gases

C

p " = 0.86

Thus

k = 1.163 Cp n
126

APPENDIX B

Physical Constants for Mixtures of 1% Phosphate Ester

in Argon at Various Temperatures

Table 25. Physical Constants for a Mixture of 1% Tributyl Phosphate

Ester and 99% Argon

T, °C Density,

g/cm3 x 10" 4 Viscosity,\i Poise

Thermal Conductivity,

cal/cm»s»°C x 10" 6 Heat Capacity,

cal/g«°C

200 10.9 303.9 0.1459

300 8.96 353.1 1.54 0.1508

400 7.63 392.5 2.05 0.1558

500 6.65 436.8 2.62 0.1607

600 5.88 481.0 3.22 0.1657

700 5.28 515.5 3.86 0.1706

800 4.79 550.1 4.55 0.1755

900 5.27 0.1805

1000 6.04 0.1855

Table 26. Physical Constants for a Mixture of 1% Tricresyl Phosphate

Ester and 99% Argon

T, °C Density,

g/cm3 x 10" 4 Viscosity,

\i Poise

Thermal Conductivity,

cal/cm«s«°C x 10" 6 Heat Capacity,

cal/g«°C

200 11.1 303.6 0. 1534

300 9.18 352.9 1.33 0.1600

400 7.82 392.3 1.77 0.1667

500 6.81 436.6 2.26 0.1734

600 6.03 480.9 2.79 0.1801

700 5.41 515.4 3.35 0.1868

800 4.90 549.8 3.94 0.1934

900 4.57 0.2001

1000 5.25 0.2068
Density was calculated using the Nelson-Obert [98] compressibility

factor, z, in the following equation:

. Mw P

6 — RT z

where z = compressibility factor and was 1.0 for all temperatures

above 200 °C

Mw = average molecular weight of the gas

P = pressure, atmospheres

T = temperature , K

R = gas constant 82.057 atm»cm3 /K«mol

The viscosity of the gas mixture was calculated using the correlation

r 99

1

um = Iyi M*i) 1/2

where fxm = viscosity of the mixture, /x Poise

yi = mole fraction of component i

Hi = viscosity of component i, /x Poise

Mt = molecular weight of component i
128

Thermal conductivity of the mixture was calculated with the equation

[100]

K = I Yi MMj) 1/3

where km = thermal conductivity of the mixture, cal/g«°C ^ = thermal conductivity of component i, cal/g»°C

y± = mole fraction of component i

Mt = molecular weight of component i

The heat capacity of the mixture was calculated by using

Cpm = I x£ CPi

where Cpm = heat capacity of the mixture, cal/g»°C

C-p L = heat capacity of component i, cal/g«°C

yi i = mass fraction of component i
APPENDIX C

Estimation of Diffusion Coefficients for Phosphate Esters

in Argon at Various Temperatures

To calculate diffusion coefficients the Wikle and Lee modification

[101] of the Hirschfelder , Bird, and Spotz equation was used.

D =

B T

3 /

2

/I/Mi + 1/M2 P r

2 12 ID

where DG = gas diffusivity cm2 /s

B = (10.7 - 2.46 yi/Mi + 1/M2

) x 10" A

T = absolute temperature, K

Mx , M2 = molecular weights of components 1 and 2

P = absolute pressure, atm

r12 = collision diameter, angstroms

(rQ ) 1 + (rQ ) 2

r = 1.18(Vh ) 1/3

VK =

12

molal volume of liquid at normal boiling point, cm3 mol

ID = collision integral for diffusion, function of kB T/e

(0 (0

1 / 2

kB = Boltzmanns constant 1 . 38 x 10" 6 Erg/K

e 12 = energy of molecular interaction, ergs.
130

Table 27 shows constants needed to make this calculation and Table 28

displays the diffusion coefficients for tricresyl and tributyl

phosphate ester in argon at temperatures ranging from 200 to 800° C.

Table 27. Constants Needed to Calculate Diffusion Coefficients

Property

Fluid

Tributyl Phosphate

Ester

Tricresyl Phosphate

Ester

Argon

Density, g/cm3 0.9727 1.1955

Molecular Weight 266.32 368.37 39.9

e/kB> K 646.47 (103) 785.62 (102) 124.0(102)

rQ , angstroms 8.217 9.458 3.418(102)

Vb , cm3 /g mol 337.7 (102) 515 (101)

ri2*> angstrom 5.818 6.438

Boiling Point, °C 289 (104) 410 (104)

e 12 /kB *. K 283.13 312.12

I D

- 200°C 0.575 0.596

500 °C 0.488 0.50

600°C 0.47 0.484

700°C 0.459 0.47

800°C 0.447 0.46

*Property value of phosphate ester with argon.
Table 28. Diffusion Coefficients of Phosphate Esters in Argon

Various Temperatures

Temperature, °C

Fluid

Tributyl Phosphate Tricresyl Phosphate

200 0.0923 cm2 /s 0.0715 cm2 /s

500 0.2272 cm2 /s 0.1779 cm2 /s

600 0.2832 cm2 /s 0.2206 cm2 /s

700 0.3412 cm2 /s 0.2673 cm2 /s

800 0.4057 cm2 /s 0.3162 cm2 /s
APPENDIX D

Calculation of Back Diffusion in the TGA

The possibility of adapting a vapor delivery system to a TGA

system offers great promise. But there cannot by any possibiligy of

vaporized lubricant back diffusing into the TGA microbalance

mechanism. This appendix presents a mathematical model showing the

significance of back diffusion under test conditions.

The actual and idealized situations are detailed in Figure 44.

In the actual case the purge gas flow would be laminar in nature

.

Also the lubricant is introduced with its momentum directed away from

the balance. the idealized case assumes potential flow and a uniform

concentration of lubricant vapor at the lubricant introduction point.

The following assumptions are used in this model:

1. Potential flow

2. Constant dens'ity

3. Constant diffusion coefficient

4. Steady state

5. No radial gradients

6. No angular gradients

7. No reactions occurring

8. Isothermal conditions

9. V2 is constant

The equation of species continuity for constant density and diffus

coefficient is

at ar r ae az

>„ - 1 f!fi + i!^ + % tics,

\r 3r \dr

I

r

2 3 02 3z2 /

Assumptions 4, 5, 6, and 7 reduce the continuity equation to

az az2

v

letting z = flf yields

*

3CA =

a

2 cA az az^

letter /x = * means ___ = _ thus =

3z 3z 3z2 3z

Solving this yields

in fi = tfz + constant (a)

m = a exp (tfz)
135

but

dz

thus

t. -- a exp (#z)

dz

thus

CA = _ exp (^z) + constant ' (£)

Boundary conditions

at z = 0 CA = CAo

z = -oo CA = 0

when z = -oo

thus

fi = 0
at z = 0

C -

a

thus

a

- C

thus

CA = CAo eXP

A purge rate of 25 cm3 /min yields a linear velocity of 4.93 cm/min at

25 °C. The assumed average temperature upper bound from the lubricant

introduction point to the balance is 200°C.

Thus

Vz = 7.824 cm/min

From Appendix C the diffusion coefficient for tributyl phosphate

ester in argon at 200°C is 0.0923 cm2 /s. Note that this is the

lubricant with the highest coefficient of diffusion and the actual
137

diffusion temperature is probably less than 200°C thus this estimate

will be conservative.

V, 7 . 824 cm/min , . . _ _ , * = z = _ = 1.413 cm 1 DAB (0.0923 cm2 /s) 60 sec min

Thus

CAo = CAo exp(6.419Z)

A typical vapor phase concentraion is 1% or 7.6 Torr

thus

CA = 7.6 exp(1.413Z)

This distance from the point of lubricant introduction to the TGA

balance is 11.768 cm.

thus

CA = 7.6 exp[(1.413) - 11.68]

Cz - 5.168 x 10" 7 Torr = 0

Thus the vaporized lubricants will not affect the microbalance

.
138

APPENDIX E

Calculation of Mass Transfer Rates and Resistance

for the Vapor Deposition Process

Conditions

Temperature - 700 °C

Lubricant - TCP

Concentration - 1% (mol)

Linear Gas Flow Rate - 1.21 cm/s (at 700°C)

Flow around a circular disk sitting on a flat plate will be

approximated by forced convection around a submerged sphere for

laminar flow, one starts with the heat transfer equation

^ D = 2.0 + 0.6 [RJ 0 - 5 [Pr ] 1/3 [106]

by using mass transfer - heat transfer dimensionless group analogies

this equation is transformed to

j!"

D = 2.0 + 0.6 [R. 1° 5 r o il/3

A B

^ = Da£ 2.0 + 0.6

D

D V p

0 . 5

P PAB

1 / 3
139

where Re = Reynolds Number

S c = Schmidt Number

= mass transfer coefficient cm/s

D = sphere diameter; for this case use disk diameter, cm

V = linear gas flow rate, cm/s

All other quanities are previously defined note TCP was chosen because

it had the lower diffusion coefficient thus resulting in a more

conservative estimate.

0 . 5 0.2673 (0.6350) (1.21) (5.41 x 10" 4

) 2.0 + 0.6

0.6350 (5.15 x 10" 4

)

1 / 3 (5.15 x 10

- *)

(5.41 x 10

- *) (0.2673)

k,,, = 0.4209 [2.0 + 0.6 [0.8] 0 5 [3.56] 1/3

]

K = 1.188 cm/s

The rate of mass transfer to the substrate surface is given by

= (C'ab - c' AS )
where rm = mass transfer rate, g/s

a = surface area, cm2

C AB = bulk concentration, g/cm3

C AS = surface concentration, g/cm3

CAS = 0, assuming an instantaneous surface reaction

Cxox = L = 1-252 x 10" 5 ^

RT cm

CTCP = 1.252 x 10-7 cm3

C' Tcp = 4.612 x 1(T 5 S TCP

a = 0.6334 cm2

3

rm = [1.188 cm/s] [0.6334 cm2

] 4.612 x 10" 5 S TCP

cm3

= 3.47 x 10' 5 g TCP

The maximum initial deposition rate observed was 2.564 x 10" 7 g/s.

Because the mass transfer rate is much greater than the deposition

rate the vapor deposition process is not mass transfer controlled.
141

APPENDIX F

Calculation of Flow Regime During Vapor Deposition Testing

1) Tubular Furnace

Conditions

Temperature - 700 °C

Carrier Gas - Argon

Lubricant - Tributyl Phosphate Ester

Concentration - 1%

Volumetric Flow Rate - 1.2 ft3 /hr at 25 °C

Reynolds number

where D = tube diameter, 5.7 cm

Vz = linear velocity of gas

p = gas density

H = gas viscosity

Tube diameter = 5.7 cm

vz =

1.2 ft3 12 in 3 2.54 cm 3 1 hr 4

hr ft in 3600 s 11(5.7 cm) 2

= 0.370 cm/s at 25°C
142

V2 = (0.370)

973 ' 15 = 1.21 cra/s at 700°C

298.15

Using values from Table 24 in Appendix B

_ (5.7 cm) (1.21 cm/s) 5.28 x 10" A g/cm3 Re ~

5.155 x 10" * g/cm-s

Re = 7.05

therefore flow during vapor deposition testing in the tubular furnace

is laminar.

2) TGA System

Conditions

Temperature - 700 °C

Carrier Gas - Argon

Lubricant - Tributyl Phosphate Ester

Concentration - 1%

Volumetric Flow Rate - 40 cm3 /min at 25 °C

Reynolds number inside TGA Furnace

Re = 0.72

Reynolds number inside TGA vaporization tube nozzle

Re = 5.213

Assuming a nozzle temperature of 500°C.
NBS-114A irev. 2-8Q

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NIST/SP-754

2. Performing Organ. Report No 3. Publication Date

September 1988

4. TITLE AND SUBTITLE

Vapor Phase Deposition Studies of Phosphate Esters on Metal and Ceramic Surfaces

5. AUTHOR(S)

Douglas E. Deckman, Stephen M. Hsu, and E. Erwin Klaus

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

(formerly NATIONAL BUREAU OF STANDARDS)

U.S. DEPARTMENT OF COMMERCE

QAITHERSBURQ, MD 20889

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

SAME AS ITEM #6 ABOVE.

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 88-600578

[ | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

This study focuses on a novel means of lubrication for ceramics and metals at high

temperatures; called vapor phase lubrication. The deposition rate and mechanisms of

tributyl phosphate ester and tricresyl phosphate ester have been explored. A

Thermalgravimetric Analyzer (TGA) was modified to measure the rate of surface film

formation of vapor phase deposition of lubricants on metal surfaces at elevated

temperatures. Results from TGA studies showed that the vapor deposition process was a complex function of substrate, time, and temperature. Tricresyl phosphate ester (TCP)

and tributyl phosphate ester (TBP) were observed to produce similar amounts of film on stainless steel substrates. The observed order of metal activity was copper > stainless

steel > nickel > platinum. Film formation on nickel was found to be much more rapid at

600°C than 700°C. Also, the concentration of oxygen present during deposition strongly

influenced the amount of film formed. Scanning electron microscopy was conducted on TBP films formed on Si3N4, AI2O3, and a-SiC. The films produced contained globule and

filamentous structures. The films on each ceramic were unique, demonstrating that

vapor deposition is surface sensitive on ceramic substrates. High temperature friction

testing was conducted on a TCP coated AI2O3 substrate. The coating reduced the friction

level for alumina, thus demonstrating the potential of vapor lubrication for high

temperature lubrication of ceramics.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

high temperature lubrication; vapor phase lubrication

13. AVAILABILITY

Unlimited

I I For Official Distribution. Do Not Release to NTIS

|"~%| Order From Superintendent of Documents, U.S. Government Printing Office, Washington. D.C.

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