/ 

-c NASA CASE NO. LAR 14448-1 /L z- (, - 

PRINT FIG. 1 

NOTICE 

The invention disclosed in this document resulted from research in aeronautical and 

space activities performed under programs of the National Aeronautics and Space 

Administration. The invention is owned by NASA and is, therefore, available for licensing 

in accordance with the NASA Patent Licensing Regulation (14 Code of Federal 

Regulations 1 245.2). 

To encourage commercial utilization of NASA-owned inventions, it is NASA policy to grant 

licenses to commercial concerns. Although NASA encourages nonexclusive licensing to 

promote competition and achieve the widest possible utilization, NASA will provide the 

necessary incentive to the licensee to achieve early practical application of the invention. 

Address inquiries and all applications for license for this invention to NASA Patent 

Counsel, Langley Research Center, Code 143, Hampton, Vlrglnla 23665-5225. 

Serial No.: 07/91 2,960 

07-1 0-92 LaRC 

(VASA-Gasp-Lk3-1444s-1) N93-11912 

YVLTI-LAYFK IIGYT-WFIGHT PROTECTIVE 

CP4TIN" AY3 MFT~ILCI FUR APPLICATION 

Pat~nt Gpr~l icqtion (NASA) 17 p Uncl as 
AWARDS ABSTRACT 

MULTI-LAYER LIGHT-WEIGHT PROTECTIVE COATING 

AND METHOD FOR APPLICATION 

NASA Case No. LAR 14448-1 -SB 

Metals and their alloys have important structural applications in aircraft and 

aerospace vehicles. Their application under some conditions, however, is limited by their 

susceptibility to environmental attack. Titanium and its alloys are particularly attractive 

for aerospace applications because of their high strength and low density, but titanium 

reacts vigorously with oxygen and nitrogen at elevated temperatures resulting in serious 

degradation of mechanical properties. 

Previuos methods of protecting titanium alloys from environmental attack include 

the use of coatings applied by painting, physical vapor deposition, chemical vapor 

deposition, thermal plasma spraying and pack cementation. Limitations to these coatings 

include the interaction of titanium alloys with the coating materials and the relatively large 

thickness of coating required, e.g., 25 to 50 micrometers, to provide protection. 

Interactions of the coating with the alloy may have a deleterious effect on he mechanical 

properties of the alloy and thick coatings abrogate the advantages of the high strength 

to weight ratio of titanium altoys. 

The present invention is a multi-layer protective coating which overcomes the 

previous problems by depositing a thin reaction barrier on the metal substrate followed 

by application of a thin diffusion barrier upon the reaction barrier. A sealant layer may 

also be applied to the surface of the diffusion barrier. The total thickness of the multilayer coating is typically about 5 micrometers. An important aspect of the present 

invention is that the reaction barrier should be non-reactive or passivating with respect 

to the metal substrate and the diffusion barrier. The diffusion barrier should be nonreactive or passivating with respect to both the reaction barrier and the sealant layer. The 

sealant should be immiscible with the diffusion barrier. In addition, the sealant should 

have a softening point lower than the expected use temperature of the metal in order to 

permit it to flow and fill in any cracks or defects in the diffusion barrier and prevent 

exposure of the underlying reaction barrier and metal substrate to the environment. 

These characteristics of the three layers provide protection to the underlying metal 

without adding significant weight or degrading mechanical properties. 

Inventors: Karl E. Wiedemann 

Address: 1700 Yorktown Road 

Ronald K. Clark 

108 Marcella Road 

Tabb. VA 23602 Ham~ton. VA 23666 

Employer: 

ss: 

AS&M 

- ss: 

NASA LaRC 
Inventors: Patrick J. Taylor 

Address: 351 Level Greeb Court 

Harnpton, VA 23669 

ss: 

Employer: AS&M 

Initial Evaluator: W. Barry Lisagore 

Serial No. : 07/91 2,960 

Filed: 07-1 0-92 
Serial No. 071910,960 

07-1 0-92 

LAR 14448-1 -SB -1 - PATENT APPLICATION 

MULTI-LAY ER LIGHT-WEIGHT PROTECTIVE COATING 

AND METHOD FOR APPLICATION 

Origin of the lnvention 

5 

The invention described herein was jointly made in the performance of 

work under a NASA contract and an employee of the United States 

Government. In accordance with 35 U.S.C. 202, the contractor elected not to 

retain title. 

10 

Backqround of the lnvention, 

- 1. Field of the lnvention 

This invention relates generally to protective coatings and more 

specifically to a coating for the protection of reactive metals and their alloys 

from environmental attack at high temperatures. 

- 2. Description of the Related Art 

20 

Titanium alloys are critically important to aerospace applications because 

of their high strength and low density. Their application under some conditions, 

however, is limited by their susceptibility to environmental attack. Oxygen and 

nitrogen react vigorously with titanium alloys at elevated temperatures, forming 

25 surface oxides and causing solid-solution contamination of the alloy which 

reduces its ductility. Hydrogen, which may be used as a fuel or as a coolant 

in certain applications, also reacts with titanium alloys over a broad range of 

temperatures and pressures, forming hydrides which produce a severe loss in 

mechanical integrity. Because these reactions occur at elevated temperatures, 

30 the use of titanium alloys for long times has been limited to temperatures that 
LAR 1 4448-1 -SB -2- PATENT APPLICATION 

are several hundred degrees below the temperatures where strength properties 

are degraded. These limitations in high temperature applications prevent the 

realization of the maximum mechanical and structural efficiency of the 

materials. Light-weight protective coatings that shield the materials from the 

5 environment can significantly extend their maximum use temperature. 

Previous methods of protecting titanium alloys from attack by the 

environment include use of coatings applied by methods such as painting, 

physical vapor deposition, chemical vapor deposition, thermal plasma spraying 

and pack cementation. Many of these coatings, such as TiAI,, are heat treated 

10 after application, producing a reaction layer between the coating and the base 

material. Limitations to these coatings include the interaction of titanium alloys 

with the coating materials utilized and the relatively large thickness of coating 

required (about 25 to 50 micrometers) to provide a substantial degree of 

protection for structures using gauges of titanium alloy which may be less than 

15 .1 inch. Interactions of the coating with the alloy may have a deleterious effect 

on the mechanical properties of the alloy and thick coatings abrogate the 

advantages of the high strength to weight ratio of titanium alloys. 

Summarv of the Invention 

It is a primary object of the present invention to provide a thin, lightweight protective coating to protect titanium and other reactive metals and their 

alloys from attack by the environment when utilized in high-temperature 

applications. Additional objects and advantages of the present invention are 

25 apparent from the specification and drawings which follow. 

The foregoing objects are achieved by depositing a thin reaction barrier 

on the metal substrate followed by application of a thin diffusion barrier layer 

upon the reaction barrier. A sealant layer may also be applied to the surface 

of the diffusion barrier. 
LAR 1 4448-1 -SB -3- PATENT APPLICATION 

The reaction barrier should be either non-reactive or passivating with 

respect to the metal substrate and the diffusion barrier. The diffusion barrier 

should be non-reactive or passivating with respect to both the reaction barrier 

and the sealant layer. The sealant should be immiscible and non-reactive with 

5 the diffusion barrier. In addition, the sealant should have a softening point 

lower than the expected use temperature of the metal in order to permit it to 

flow and fill in any cracks or defects in the diffusion barrier and prevent 

exposure of the underlying reaction barrier and metal substrate to the 

environment. 

10 The reaction barrier may be a stable compound such as zirconium 

silicate, yttrium-stabilized cubic zirconia or barium-titanate. The solid-statediffusion barrier may be a high viscosity layer such as a silica-rich , i.e., 

containing more than 50% silica, glass matrix containing a lower softening point 

magnesium-phosphate glass droplet dispersion. This layer inhibits the solid15 state diffusion of oxygen to the metal substrate. The sealant may be a low 

viscosity layer such as a magnesium-phosphate glass to prevent direct 

exposure of the reaction barrier and metal substrate to the atmosphere through 

interconnected porosity or other flaws. 

To prepare and apply the reaction barrier, a solution with controlled 

20 drying and surface tension properties is prepared using alkoxides, hydroxides, 

salts and oxysalts. The metal alloy is immersed in the solution and then 

withdrawn at a controlled rate, applying a thin layer of solution on the surface 

of the metal alloy. The alloy is dried and the deposit is converted to an oxide 

by thermal decomposition in a furnace. 

25 The diffusion barrier layer is composed of compounds with low oxygen 

diffusivities such as a two-phase glass consisting of a silica-rich glass matrix 

plus a lower softening point magnesium-phosphate glass droplet dispersion. 

The droplet phase is soft at the use temperature so that it can promote selfhealing of defects and microcracks. To prepare and apply this layer, a solution 

30 with controlled drying and surface tension properties is prepared using 
LAR 14448-1 -SB -4- PATENT APPLICATION 

alkoxides, hydroxides, salts and esters. The metal alloy containing the reaction 

barrier is immersed in the solution and then withdrawn at a controlled rate, 

applying a thin layer of solution on the reaction barrier surface. The alloy is 

dried and the deposit is converted to a two-phase glass by thermal 

5 decomposition in a furnace. 

The sealant layer is composed of a compound such as a magnesiumphosphate glass which has a softening point below the use temperature of the 

alloy in order to permit flow of the compound to fill in any cracks or defects in 

the diffusion barrier. To prepare and apply this layer, a solution with controlled 

10 drying and surface tension properties is prepared using alkoxides, hydroxides, 

salts and esters. The metal alloy containing the reaction barrier and diffusion 

barrier is immersed in the solution and then withdrawn at a controlled rate, 

applying a thin layer of solution on the diffusion barrier surface. The alloy is 

dried and the deposit is converted to a glass by thermal decomposition in a 

15 furnace. 

Brief Description of the Drawinqs 

FIG. 1 is a schematic drawing of the light-weight protective coating. 

20 FIG. 2 is a photocopy of a scanning electron micrograph of a self-healed 

microcrack in the protective coating. 

FIG. 3 compares oxidation weight gain at 700°C for avariety of coatings. 

FIG. 4 compares oxidation weight gain at 982°C for a coated and 

uncoated titanium-aluminide alloy. 

25 FIG. 5 contains a photocopy of optical micrographs of coated and 

uncoated titanium alloys after oxidation at 982°C for one hour. 
LAR 1 4448-1 -SB -5- PATENT APPLICATION 

Description of the Preferred Embodiments 

Referring to FIG. 1, a reactive metal or alloy such as titanium 10 is 

coated first with a thin reaction barrier 20 composed of a compound such as 

5 zirconium silicate, yttrium-stabilized cubic zirconia or barium-titanate. A 

diffusion barrier 30, which consists of a two-phase glass such as a silica-rich 

glass matrix plus a lower softening point magnesium-phosphate glass droplet 

dispersion is then deposited on the reaction barrier 20. The thickness of the 

reaction barrier 20 is approximately 1 to 2 micrometers and the diffusion barrier 

10 layer 30 is approximately 2 to 3 micrometers thick. A further refinement 

comprises a triplex coating where a sealant layer 35 is applied to the diffusion 

barrier 30. The sealant is a low viscosity layer such as a magnesium-phosphate 

glass to prevent direct exposure of the reaction barrier 20 and the metal 

substrate 10 to the atmosphere through interconnected porosity or other flaws 

15 in the diffusion barrier 30. 

The reaction barrier 20 should be either non-reactive or passivating with 

respect to the metal substrate 10 and the diffusion barrier 30. The diffusion 

barrier 30 should be non-reactive or passivating with respect to both the 

reaction barrier 20 and the sealant layer 35. The sealant should be immiscible 

20 and non-reactive with the diffusion barrier 30. In addition, the sealant should 

have a softening point lower than the expected use temperature of the metal 

in order to permit it to flow and fill in any cracks or defects in the diffusion 

barrier 30 and prevent exposure of the underlying reaction barrier 20 and metal 

substrate 10 to the environment. 

25 The following are three illustrative examples of the method for preparing 

and applying a reaction barrier to the surface of titanium or a titanium alloy: 

Example R1. To prepare a zirconium silicate reaction barrier, a solution 

with controlled drying and surface tension properties was prepared using the 

following approximate quantities: 1.03 g. water, 0.53 g. HNO,, 5.05 g. EtOH 

30 and 2.09 g. tetra ethoxy silane. To this solution was added approximately 8.47 
LAR 1 4448-1 -SB -6- PATENT APPLICATION 

EtOH, 0.52 g. HNO, and 3.28 g. zirconium n-propoxide. The alloy to be coated 

was then immersed in the solution and withdrawn at a controlled rate in order 

to apply a thin layer of solution on the surface of the alloy. The alloy was then 

dried in the atmosphere, leaving the surfaces covered with a deposit of the 

5 prepared solution. The deposit was converted to a stable oxide by thermal 

decomposition in a furnace at approximately 900°C. This process was 

repeated as necessary to provide a total reaction barrier layer thickness of 

approximately 1 to 2 micrometers. 

Example R2. To prepare the yttria-stabilized cubic zirconia reaction 

10 barrier, a solution with controlled drying and surface tension properties was 

prepared using the following approximate quantities: 5.53 g. zirconium npropoxide, 1.15 g. yttrium nitrate, 18.75 g. MeOH and 5.40 g. HNO,. To this 

solution was added approximately 20 ml. MeOH and 20 ml. MEK. The alloy to 

be coated was then immersed in the solution and withdrawn at a controlled rate 

15 in order to apply a thin layer of solution on the surface of the alloy. The alloy 

was then dried in the atmosphere, leaving the surfaces covered with a deposit 

of the prepared solution. The deposit was converted to a stable oxide by 

thermal decomposition in a furnace at approximately 900°C. This process was 

repeated as necessary to provide a total reaction barrier layer thickness of 

20 approximately 1 to 2 micrometers. 

Example R3. To prepare a barium titanate reaction barrier, a solution 

with controlled drying and surface tension properties was prepared using the 

following approximate quantities: 9.76 g. barium perchlorate, 10.0 g. MeOH, 

0.75 g. acetic acid and 8.51 g. titanium butoxide. To this solution was added 

25 approximately 100 ml. MEK and 10 ml. MeOH. The alloy to be coated was 

then immersed in the solution and withdrawn at a controlled rate in order to 

apply a thin layer of solution on the surface of the alloy. The alloy was then 

dried in the atmosphere, leaving the surfaces covered with a deposit of the 

prepared solution. The deposit was converted to a stable oxide by thermal 

30 decomposition in a furnace at approximately 900°C. This process was 
LAR 1 4448-1 -SB -7- PATENT APPLICATION 

repeated as necessary to provide a total reaction barrier layer thickness of 

approximately 1 to 2 micrometers. 

The diffusion barrier layer is composed of oxides with low oxygen 

diffusivities such as a two-phase glass consisting of a silica-rich glass matrix 

5 plus a lower softening point magnesium-phosphate glass droplet dispersion. 

The diffusion barrier layer is tailored so that the droplet phase is soft at the use 

temperature so that it can promote self-healing of defects and microcracks 

within the diffusion barrier. 

The following are two illustrative examples of the method for preparing 

10 and applying a diffusion barrier for use on the preceding reaction barriers 

applied on titanium or a titanium alloy: 

Example Dl. To apply a two-phase glass diffusion barrier, a solution 

with controlled drying and surface tension properties was prepared using the 

following approximate quantities: 0.61 g. magnesium nitrate, 1.1 4 g. phosphoric 

15 acid, 17.00 g. EtOH and 5.76 g. tetra ethoxy silane. To this solution was then 

added approximately 2 g. water, 15 ml. MEK and 5 ml. MeOH. The alloy 

containing the previously applied reaction barrier was immersed in the solution 

and then withdrawn at a controlled rate, applying a thin layer of solution on the 

reaction barrier surface. The alloy was dried in the atmosphere, leaving the 

20 surfaces covered with a deposit of the prepared solution. The deposit was 

converted to a two-phase glass by thermal decomposition in a furnace at 

approximately 700°C. 

Example D2. The diffusivity of the diffusion barrier can be decreased by 

preparing a solution using aluminum nitrate. This solution was made using the 

25 following approximate quantities: 0.96 g. magnesium nitrate, 1.88 g. aluminum 

nitrate, 4.05 g. MeOH and 6.00 g. tetra ethoxy silane. To this solution was 

added approximately 9.85 g. MEK, 1.44 g. phosphoric acid and 0.50 g. MeOH. 

The alloy containing the previously applied reaction barrier was immersed in the 

solution and then withdrawn at a controlled rate, applying a thin layer of solution 

30 on the reaction barrier surface. The alloy was dried in the atmosphere, leaving 
LAR 1 4448-1 -SB -8- PATENT APPLICATION 

the surfaces covered with a deposit of the prepared solution. The deposit was 

converted to a two-phase glass by thermal decomposition in a furnace at 

approximately 700°C. 

To apply the sealant layer to either of the two preceding diffusion 

5 barriers, a solution was prepared using the following approximate quantities: 

0.94 g. aluminum nitrate, 2.69 g. magnesium nitrate, 8.21 g. MeOH and 2.83 

g. tetra ethoxy silane. To this solution was added approximately 2.71 g. 

phosphoric acid, 1.15 g. MeOH and 10.89 g. MEK. The alloy containing the 

previously applied reaction barrier and diffusion barrier was immersed in the 

10 solution and then withdrawn at a controlled rate, applying a thin layer of solution 

on the diffusion barrier surface. The alloy was dried in the atmosphere, leaving 

the surfaces covered with a deposit of the prepared solution. The deposit was 

converted to a glass by thermal decomposition in a furnace at approximately 

700°C. 

15 The presence of the magnesium-phosphate dispersion in the diffusion 

barrier layer 30 permits self-healing of microcracks that may form. FIG. 2 

shows a microcrack 40 in the diffusion barrier 30 applied according to Example 

D1 on a titanium sample that has been oxidized at 982°C for one hour. The 

microcrack 40 has been filled by the soft phosphorus-rich dispersion 50, 

20 thereby preventing attack of the metal substrate by oxygen and nitrogen in the 

environment. 

Tests of oxidation weight gain at 700°C and 982°C were conducted on 

a variety of coated and uncoated titanium samples to determine the relative 

effectiveness of the multi-layer protective coating. FIG. 3 shows that a 

25 commercial purity titanium alloy protected with diffusion barriers prepared 

according to Example Dl combined with yttrium-stabilized cubic zirconia and 

barium titanate reaction barriers showed significantly less weight increase due 

to attack by oxygen and nitrogen in the environment than the uncoated titanium 

alloy when tested at 700°C. Embrittlement of the metal occurs when weight 

30 gain due to oxidation reaches approximately .25 mg/cm2. A comparison of the 
LAR 1 4448-1 -SB -9- PATENT APPLICATION 

time required to obtain a weight change of .25 mg./cm2 indicates that titanium 

protected with the present invention takes over 100 times as long to become 

embrittled as uncoated specimens of titanium. 

FIG. 4 indicates that the 5 micrometer thick, multi-layer protective coating 

5 with a sealant layer, a diffusion barrier prepared according to Example 02 and 

a zirconium silicate reaction barrier applied on a Ti-1 4AI-21 Nb alloy provides 

improved protection from environmental attack when tested at 982OC compared 

to that obtained with a 40 micrometer thick coating of Sermalloy J. Sermalloy 

J is a trademark for a proprietary, aluminum-based coating available from 

10 Sermatech of Limerick, Pennsylvania. The triplex coating with the zirconium 

silicate reaction barrier and a diffusion barrier prepared according to Example 

02 also resulted in improved oxidation resistance when compared to a 5 

micrometer thick proprietary coating of AI-SIB, which is available from Lockheed 

Corporation. 

15 FIG. 5 shows the severe oxidation 60 that occurs on an uncoated 

specimen of commercial purity titanium alloy 10 that was oxidized one hour at 

982°C. In comparison, the alloy 10 protected with a multi-layer coating 70, 

consisting of an yttrium stabilized cubic zirconia reaction barrier and a diffusion 

barrier prepared according to Example Dl, and exposed to the same oxidizing 

20 environment as the uncoated specimen showed significantly reduced oxidation 

of the titanium alloy. 

Although the present invention has been described in detail with respect 

to certain preferred embodiments thereof, it is understood by those of skill in 

the art that variations and modifications in this detail may be made without any 

25 departure from the spirit and scope of the present invention, as defined in the 

hereto-appended claims. 

What is claimed is: 
LAR 14448-1 -SB 

/O 

-20- PATENT APPLICATION 

MULTI-LAY ER LIGHT-WEIGHT PROTECTIVE COATING 

AND METHOD FOR APPLICATION 

Abstract of the Disclosure 

5 

A thin, light-weight, multi-layer coating is provided for protecting metals 

and their alloys from environmental attack at high temperatures. A reaction 

barrier is applied to the metal substrate and a diffusion barrier is then applied 

to the reaction barrier. A sealant layer may also be applied to the diffusion 

10 barrier if desired. The reaction barrier is either non-reactive or passivating with 

respect to the metal substrate and the diffusion barrier. The diffusion barrier 

is either non-reactive or passivating with respect to the reaction barrier and the 

sealant layer. The sealant layer is immiscible with the diffusion barrier and has 

a softening point below the expected use temperature of the metal. 
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