EP0186266A1

EP0186266A1 - Erosion-resistant coating system

Info

Publication number: EP0186266A1
Application number: EP85307105A
Authority: EP
Inventors: Louis J. Fiedler; Subhash K. Naik
Original assignee: Avco Corp
Current assignee: Avco Corp
Priority date: 1984-11-19
Filing date: 1985-10-03
Publication date: 1986-07-02
Also published as: JPS61127873A

Abstract

Erosion resistance is imparted to a metallic substrate without an attendant loss of fatigue life in the substrate by applying to the substrate a first layer comprising palladium, platinum or nickel in direct contact with the substrate and then applying a second layer which overcoats the first layer, the second layer being comprised of a tungsten-carbon alloy.

Description

This invention relates, in general, to coatings for steel and titanium substrates and more particularly to novel two-layered erosion-resistant coatings which may be applied to steel and titanium gas turbine engine compressor blades without an attendant loss in fatigue life.
Gas turbine engine compressor blades are conventionally fabricated from steel or titanium alloys. The blades are subjected to severe erosion when operated in sand and dust environments. Blade erosion reduces compressor efficiency, requiring premature blade replacement.
There are presently available a wide variety of erosion resistant coatings such as tungsten and carbon (U.S. 4,147,820), platinum metals (U.S. 3,309,292) and boron (U.S. 2,822,302). However, these coatings which have been identified by the art for imparting erosion resistance to substrates such as tungsten and steel alloy compressor blades, promote sharp drops in fatigue properties of the substrates creating the initiation of cracks and fractures with an attendant reduction in the service life of the substrate. This effect of the fatigue life of the substrate is believed due to the fact that the erosion-resistant coatings of the prior art are hard materials which produce residual stress and accompanying strains in the substrate thereby accelerating a reduction in the fatigue strength of the substrate.
Therefore, there exists a need in the art for erosion-resistant coating systems which do not deleteriously affect the fatigue life of the substrate to which they are applied.
It is, therefore, an object of the present invention to provide a novel coating system which is devoid of the above-noted disadvantages.
It is another object of, the present invention to provide a two-layered coating which has good erosion resistance which does not deleteriously affect the fatigue life of the substrate material upon which it is applied.
It is a further object of this invention to minimize residual stress and accompanying strains in the erosion-resistant coating system to ameliorate the deleterious effect of the fatigue life of the coated substrate.
It is still another object of this invention to provide a coating system which may be used in the hot, corrosive atmospheres of the type in which gas turbine compressor components operate.
It is still another object of this invention to provide a coating system having broad application in providing erosion-resistance to a wide variety of gas turbine compressor components without degrading the fatigue life of the components.
The foregoing objects and other objects of the present invention are accomplished by an erosion-resistant coating system comprising two successively applied layers of different respective materials. The second-applied coating layer is formed of a tungsten-carbon alloy erosion-resistant material. The first-applied layer or interlayer, which is applied directly to the titanium or steel substrate, is formed of a ductile material, such as platinum, palladium or nickel which is capable of preventing diffusion of material from the second-applied layer into or completely through the first applied layer and thus into the substrate. The substrate is thereby protected from degradation of material or engineering properties. Residual stress and accompanying tensile strains in the coating system are minimized by applying the second layer on the first layer at a relatively low temperature, i.e. about 500°_F to about 1400°F which allows for a fine grain and/or a columnar grain structured coating.
By the practice of the present invention there is provided an erosion resistant tungsten-carbon alloy coated titanium or steel alloy substrate in which the deleterious effect on the fatigue life of the substrate previously encountered with such erosion resistant coatings is substantially eliminated.
In the coating systems of the present invention, the first layer of ductile metal applied directly adjacent to the titanium or steel alloy substrate is believed to provide a diffusion barrier, preventing the second layer from diffusing into and degrading the substrate material, and does not itself degrade the substrate material properties when applied thereto. Most erosion-resistant coatings of the tungsten-carbon alloy type are brittle and certain components of these coating materials, e.g. carbon, boron, nitrogen and oxygen will, at the temperatures normally used for coating application, embrittle the substrate alloy. Thus, it has been previously determined in work on titanium carbide/nitride coatings on titanium, that an embrittling alpha case layer is created on the titanium substrate. In the practice of the present invention, it is believed that the ductile first layer applied to the substrate acts as a barrier to the diffusion of embrittling components of the second tungsten-carbon alloy layer onto the substrate layer as well as acting as a crack arrestor, which by the retardation of the crack propagation rate results in improved fatigue life performance by the substrate.
With respect to the second, erosion resistant coating layer, the coating is applied under conditions whereby residual stress and tensile strain in the coating is minimized to promote retention of fatigue life in the substrate, any strains in the coating system tending to induce cracks in the substrate which deleteriously affect the fatigue life thereof. Specifically, stress in the coating system is a function of the difference in the coefficients of thermal expansion between coating (Δoc) and the difference in temperature between the substrate (room temperature) and the coating deposition temperature (ΔT). Thus stress (0) in the coating system can be represented by the formula
σ =Δ α x Δ T
In view of the formula, stress in the coating can be reduced by either reducing the Δα by using a coating material having a coefficient of expansion closely corresponding to that of the substrate or reducingΔT by using a lower temperature at which the coating is deposited. Tungsten-carbon alloy erosion-resistant coatings are conventionally applied at 1800°-2000°F. In a preferred embodiment of the present invention, the tungsten-carbon alloy erosion-resistant coating is applied at a temperature between about 500°F and about 1400°F whereby improved fatigue life of the substrate is achieved.
Any suitable substrate material may be used with the two-layered coatings of the present invention. Typical substrate materials include steel alloys, titanium alloys, nickel base and cobalt base super-alloys, dispersion-strengthened alloys, composites, single crystal and directional eutectics. While any suitable substrate material may be used, particularly good results are obtained when stainless steel or titanium alloys are used with the novel two-layer coatings disclosed herein.
The first layer of the coating of this invention contains palladium, platinum or nickel. While any suitable palladium, platinum or nickel-containing metal may be used, nickel or palladium is preferred, especially when stainless steel is the substrate being coated, and platinum or nickel is preferred when a titanium alloy is used as the substrate material being coated. This palladium, platinum or nickel-containing layer as already discussed, acts as a diffusion barrier and protects the substrate during further coating with the hard tungsten-carbon alloy overlayer.
The layers comprising the coating of this invention may be of any suitable thickness. Particularly good results are obtained with the first palladium, platinum or nickel-containing layer being between about 0.0001 inches and about 0.002 inches and the second tungsten-carbon alloy layer being between about 0.0005 inches and about 0.003 inches.
Any suitable coating technique may be used to apply the first layer of the coating to the substrate material. Typical methods include electroplating, sputtering, ion-plating, electrocladding, pack coating, and chemical vapor deposition, among others. While any suitable technique may be used, it is preferred to employ an electroplating, sputtering, chemical vapor deposition, or ion-plating process. Any suitable technique, likewise, may be used to apply the erosion-resistant tungsten-carbon layer to the palladium, platinum or nickel interlayer. Preferred methods of achieving this low temperature deposition include chemical vapor deposition/controlled nucleation thermochemical deposition, sputtering, physical vapor deposition and electroplating processes.
In practicing the coating procedure of the present invention, the surface of the substrate to be coated is first shot peened to provide compressive stresses therein. The shot peened surface is then thoroughly cleaned with a detergent, chlorinated solvent, or acidic or alkaline cleaning reagents to remove any remaining oil or light metal oxides, scale or other contaminants.
To insure good adherence of the first layer of platinum, palladium or nickel, the cleaned substrate is activated to effect final removal of adsorbed oxygen. As already indicated, the first layer is applied to the surface of the substrate by such conventional coating techniques as electroplating, chemical vapor deposition (C_VD), sputtering or ion plating. If electroplating is the coating method chosen, then activation of the substrate surface is conveniently accomplished by anodic or cathodic electrocleaning in an alkaline or acidic cleaning bath by the passage therethrough of the required electrical current. Plating is then accomplished using conventional plating baths such as a Watts nickel sulfate-chloride bath or a platinum/palladium amino nitrite/diamino nitrite bath. If CVD is elected for the coating application, then activation is accomplished by the passage of a hydrogen gas over the substrate surface. CVD is then accomplished using the volatilizable halide salt of the metal to be deposited and reacting these gases with hydrogen or other gases at the appropriate temperature, e.g. below 1400°_F to effect deposition of the metallic layer.
If sputtering is chosen as the method of coating application, bias sputtering can be used to activate the substrate. Deposition of the first metallic interlayer is accomplished with sputtering or ion-vapor plating using high purity targents of the metals chosen to form the interlayer.
Coating application of the second layer of tungsten-carbon alloy over the first metallic layer as already discussed is accomplished at a temperature not exceeding 1400°F by CVD, sputtering or other conventional coating processes.
If CVD is chosen for the deposition of the tungsten-carbon alloy, a gaseous mixture of WF₆, H₂, a suitable organic compound containing carbon, oxygen and hydrogen, and an inert gaseous diluent such as argon is flowed into a reaction chamber containing the first layer coated substrate heated to a temperature of about 800 to about 1200°F and the gaseous mixture is allowed to react and deposit on the heated substrate.
If sputtering is chosen for the deposition of the tungsten-carbon alloy, high purity targets of the alloy are fabricated and sputter coating equipment is used to coat the first layer coated substrate with the target material.
Several of the above described coating techniques have been utilized in connection with this invention which are described in the following example which further illustrates the present invention.

EXAMPLE

The surfaces of individual C 450 stainless steel were first thoroughly cleaned free of all dirt, grease and other objectionable matter followed by conditioning by means of shot peening. The cleaned surface of the substrate was then electroplated with a 0.2 to 0.8 mil thick coating of nickel or palladium using a Watts nickel sufamate or palladium amino nitrate plating bath, respectively. A second coating consisting of a tungsten-carbon alloy containing 93.88 to 97.8% tungsten and 2.12 to 6.12% carbon was deposited over the first coating using a CVD coating process. In this process, coating was achieved by vapor deposition by reacting a gaseous mixture of WF₆, H₂, an organic compound containing carbon, oxygen and hydrogen with tungsten. The substrate was preheated to 1000°F for 30 - 60 minutes before deposition was initiated and this temperature was maintained throughout the coating operation. Deposition time was controlled to obtain a coating thickness of about 1-3 mils. The hardness of the tungsten-carbon alloy coating was 2050 kg/mm2.

I. Erosion Resistance of Coated Specimens

Coated substrate specimens were tested for erosion resistance using S.S. White erosion testing equipment. When using this equipment, the coated specimen is subjected to a pressurized blast of sand which is impinged on the specimen at selected impingement angles from a 1/2 inch diameter nozzle spaced from the specimen. The conditions under which the erosion testing using sand impingement were performed are as follows:
^*Setting on S.S. White equipment, powder chamber is vibrated 60 times per second to produce desired powder flow rate.
The specimens were blasted with sand at 30° and 90° sand impingement angles for 5 minutes.
The erosive wear of the specimen was measured as the volume of coating material lost per minute of sand impingement. The results of the erosive wear tests are recorded in Table I below.
For purposes of comparison, the procedure of the Example was repeated with the exception that the C450 stainless steel substrate was not coated. The results of this comparative erosive wear test are also recorded in Tabl e I.
By reference to Table I, it is immediately apparent that the uncoated specimens exhibited an erosion rate which was at least 14-23 times greater than the coated specimens.

II_. Fatigue Life of Coated Specimens

Fatigue bend plate (modified Krause) test specimens were coated in accordance with the Example were then subjected to fatigue testing in a bend plate testing machine by clamping both ends of the specimen. An uncoated C 450 stainless steel substrate was used as a control for baseline determination. Each specimen was tested at room temperature with an A ratio (sa/sm) ratio = 1 and were mechanically vibrated to failure at a frequency f=₃₀ Hz. The stress level was varied from 55 to 60 ksi. Failure was indicated by breakage of the test specimen.
The results of the fatigue testing are given below in
By reference to the data recorded in Table II, it is immediately apparent that the coated C-450 stainless steel specimens prepared in accordance with the present invention exhibited no degradation in fatigue life when compared to baseline (uncoated) C 450 steel.

III. Fatigue Life of Coated First stage Compressor Blades

First stage compressor blades fabricated from AM 350 stainless steel were coated with a Ni/W-C coating system in accordance with the Example. The total coating thickness was 2-3 mils with a coating hardness of 1950-2050 kg/mm², The coated blades were evaluated for fatigue life using a Beehive tester in which the blades were air-jet excited at their fundamental bending mode frequency while rigidly clamped at the dovetail root. The test was conducted at room temperature. The conditions of the test were as follows:
The failure point was indicated by the loss of natural frequency at the rate of 10 cycles/second. In this beehive test, an acceptable fatigue life is 300,000 cycles. The first coated blade was determined to have a fatigue life of 430,000 cycles and the second coated blade had a fatigue life of 385,000 cycles whereby the coated blades exceeded the fatigue life specification for the blades thereby confirming the fact that the erosion resistant coating system does not degrade the fatigue life of the substrate to which it is applied.
Some of the many advantages of the present invention should now be readily apparent by reference to the foregoing Example. For example, a novel coating system has been provided which is capable of preventing or reducing the erosion of metals such as steel and alloys thereof, particularly in an operating environment such as a gas turbine engine. This is accomplished without substantial degradation of material properties of the structure to which the coating system is applied.
while specific components of the present system are defined above, many other variables may be introduced which may in any way affect, enhance or otherwise improve the coating systems of the present invention. These are intended to be included herein.
Although variations are shown in the present application, many modifications and ramifications will occur to those skilled in the art upon a reading of the present disclosure. These, too, are intended to be included herein.

Claims

1. A two-layered coating which comprises a first layer comprising palladium, platinum or nickel and a second layer overcoating said first layer comprising a tungsten-carbon alloy.

2. The coating of claim 1 wherein said second layer is deposited upon said first layer at temperature of from about 260°C (500°F) to about 760⁰C (1400⁰F).

3. The coating of claim 1 or 2, wherein the thickness of said first layer is from about 0.0001 inches to about 0.002 inches and the thickness of said second layer is from about 0.0005 inches to about 0.004 inches.

4. An article of manufacture comprising a substrate overcoated with the coating of claim 1, 2 or 3, wherein said first layer of said coating is in direct contact with said substrate whereby the coating imparts erosion resistance to the substrate without an attendant loss in fatigue life.

5. The article of claim 4 wherein said substrate is a steel or titanium alloy.

6. A method of preparing a two-layered coating comprising depositing a second layer comprising tungsten and carbon upon a first layer comprising palladium, platinum or nickel, the deposition taking place at a temperature of from about 260⁰C (500°F) to about 760⁰C (1400°F).

7. The method of claim 6, wherein low temperature deposition of said second layer upon said first layer is achieved by CVD/CNTD, sputtering, physical vapor deposition or electroless plating processes.

8. The method of claim 6 or 7 wherein said first layer is deposited on a substrate material prior to being overcoated with said second layer.

9. The method of claim 8 wherein said first layer is deposited upon said substrate by an electroplating, sputtering or ion-plating process.

10. A method for imparting erosion resistance to a substrate without an attendant loss in the fatigue life of the substrate which comprises applying to the substrate a first layer comprising palladium, platinum or nickel in direct contact with the substrate and then applying a second layer which overcoats the first layer, the second layer being comprised of a tungsten-carbon alloy.

11. The method of claim 10 wherein the second layer is deposited upon the first layer at a temperature of from about 260°C (500°F) to about 760°C (1400°F).

12. The method of claim 8, 10 or 11, wherein the substrate is comprised of a steel or titanium alloy.