WO2006028941A2 - Fretting wear protection for spline couplings - Google Patents

Abstract

Diamond-like carbon coatings are applied to the « male » spline components of spline couplings used in gas turbine generators to eliminate fretting wear and to defeat the adhesive wear mechanism.

Description

FRETTING WEAR PROTECTION FOR SPLINE COUPLINGS, ESPECIALLY FOR USE ON GAS TURBINE GENERATORS

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional application number 60/606,587, filed September 2, 2004, entitled "Fretting Wear Protection For Spline Couplings, Especially For Use On Gas Turbine Generators", which is incorporated herein by reference.

BACKGROUND ART

The application relates to spline couplings, for example, such as are used in gas turbine generators, and in particular a coating for the coupling which reduces fretting wear of the coupling.

Components of the basic gas turbine engine (Figure 1 ) include a compressor, combustor, and turbine. Air enters the front of the engine and goes through a compressor that raises the air's pressure considerably. Fuel is then added to this high-pressure air, and when this mixture is ignited, it expands rapidly and leaves the rear of the jet engine at a much higher velocity than when it entered. After the air/fuel mixture has been ignited, it passes through the turbines before it leaves the engine. These turbines are connected to the compressor by means of a shaft so when the turbine rotates, it also rotates the compressor.

A centrifugal compressor uses a spinning impeller (Figure 2) to draw in intake air and accelerate it outward by means of centrifugal force into a diffuser. The impeller is driven by the turbine through a spline adapter coupling (SAC), such as is shown in Figure 3, which is comprised of a tight fitted joint between a spline fixed to the impeller and a working spline coupled to the spline adapter gear shaft. Engine vibration can vary with the alignment of the drive system, and produces microscopic movement between the coupling and impeller causing fretting wear or micro-welding to develop in the coupling. A splined coupling is the most common method of connecting elements in rotating machinery- Their performance is limited not only by torsional fatigue, but also by fretting wear arising from angular misalignment.[1] Fretting wear is especially common in mechanical transmissions that have lightweight and flexible casings, such as those in helicopters. [2,3]

When components are subjected to very small relative vibratory movements at high frequency, an interactive form of wear, called fretting, takes place that is initiated by adhesion, amplified by corrosion, and is manifested by abrasion. Adhesive wear in this aspect is a welding then breaking of asperities from contacting surfaces. The ability of the asperities to weld depends upon the loading, vibration frequency and amplitude, and chemical compatibility of the contacting materials. When the local temperature of the asperity contact is sufficiently large, a flow of material across the interface will occur, followed by a rapid solidification. In most cases, the bonds formed at the welded junctions exceed the strength of the material, and as the asperities try to separate, the tangential stress is sufficient to fracture the material. This forms an abrasive particle or debris. Corrosion not only assists in bonding the contacting asperities, but it also reduces the strength of the surface materials making them easier to fracture.

Fretting wear of components has been traditionally addressed by increasing the hardness of the component, which reduces the rate of abrasion in some cases. Anti-corrosion surface treatments have also been used on occasion to slow the rate of wear. The only path to eliminate fretting wear is to defeat the adhesive wear mechanism. This is best accomplished by eliminating the ability of contacting asperities to micro-weld. Sometimes, improving the surface finish of the component (specifically reducing the plasticity index or asperity slope), local asperity stresses, and therefore temperatures can be reduced, thereby decreasing the propensity of the material to flow across the interface. However, when the contacting surfaces become too smooth, fretting wear can actualize accelerated. Tribological coatings have been used for fretting wear applications. Silver-plating has been known to work in a number of applications. Although the silver forms junctions between mating parts, as the parts move relative to each other, shearing occurs in the silver, not in the part material. Silver also forms a barrier to oxidation, and mitigates the effects of corrosion on fretting wear. However, over time, the silver can become displaced on the contact surface, exposing the parent metal to the mating part. Once the silver is displaced, the root cause for fretting returns and the joint starts to wear.

A number of publications have appeared touting the benefits of diamond-like carbon thin film coatings for fretting wear protection. Diamond-like carbon (DLC) refers to a family of thin film amorphous hydrocarbons (a-C:H) that are formed by vapor phase deposition. Two embodiments of DLC that work well as wear-resistant coatings for mechanical components processes are: (a) silicon reinforced DLC (Si/DLC) and (b) metal carbide reinforced DLC (MC/DLC).

Silicon addition into DLC not only enhances the hardness and modulus, it also improves the friction and wear behavior.[4,5,6] Benefits from the incorporation of metal carbides in the DLC structure include control of residual stress and significantly improved coating adhesion and toughness over traditional DLC films. MC/DLC coatings are composite structures comprised of sub-micrometer sized metal carbide crystallites embedded in an a-C:H matrix. MC/DLC coatings are usually compositionally graded. That is, a MC/DLC coating structure could be comprised of (1) a metallic layer to provide adhesion to the substrate, and (2) the functional MC/DLC coating. In order to not have a sharp interface between layers 1 and 2, it is common practice to insert a third layer between them in which the chemical composition is smoothly varied to transition from that of the adhesion layer to that of the functional layer. Sometimes the functional layer can be comprised of many alternating metal-rich and metal-poor sub-layers. In practice, it is much easier to obtain the excellent adhesion to steel alloys required for - A - most mechanical system components with the MC/DLC than with the Si/DLC coatings.

SUMMARY OF THE INVENTION

The subject of this invention is to use DLC coatings to provide fretting wear resistance to spline couplings of turbine gas generators.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing the major components of a gas turbine engine;

FIG. 2 is a perspective view of a spinning impeller used in a gas turbine engine;

FlG. 3 is a perspective view of a spline adapter coupling of the gas turbine engine;

FIG. 4 is a graph comparing the results of cylinders treated in accordance with the present invention with untreated cylinders.

BEST MODES FOR CARRYING OUT THE INVENTION

The following detailed description illustrates the invention by way of example and not by way of limitation. This description will clearly enable one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives and uses of the invention, including what we presently believe is the best mode of carrying out the invention. Additionally, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

Specific Application Example: A gas turbine generator that has been produced since 1960 is commonly used in military and commercial light operation helicopters. Removal of the compressor sections from failed engines reveals that the spur adapter gear shafts are fractured at the forward splines from severe spline wear due to fretting. To date, there have been 16 in-flight shut downs, including 4 fatalities from this single issue. To reduce the dangers posed by a failure of the compressor adapter coupling, it is now specified that the adapter coupling should be inspected every 600 hours. This means that there is significant expense associated with the more frequent maintenance intervals and loss of helicopter service.

Miyoshi et al.[7] observed in laboratory testing that a TiC/DLC coating improved the dry fretting wear resistance of both AISI 440C stainless steel and Ti₆AI₄V surfaces by almost a factor of 30 using a ball/disc laboratory test apparatus.

We have designed a laboratory scale test to measure the adhesive wear due to microscopic motion between contacts. In one test, untreated steel cylinders and steel cylinders coated with a WC/DLC coating (about 2-3 micrometers thick) were mechanically vibrated against untreated steel planes to test the ability of the coating to inhibit fretting wear.

In the test, highly loaded cylinders were vibrated at about 25 Hz against the planes for about 400 hours. To duplicate fretting wear, no rotation of the bearing was allowed and the magnitude of motion between the cylinder and plane surfaces was less than 1 mm. Three uncoated cylinders and three coated cylinders were tested. The total wear volume of the grooves formed on the uncoated planes as a result of this high frequency, microscopic motion was measured for each test. Significant fretting-type wear of the untreated planes was observed when untreated cylinders were microscopically vibrated against them. However, when WC/DLC coated cylinders were vibrated against the untreated planes, no adhesive or fretting-type wear was observed. The untreated planes did show some evidence of mild abrasive wear from the polishing action of the coating, but the wear depth of the races stabilized and no further wear of any kind occurred.

The results of the test are shown in Figure 4 where the average wear depths and standard deviations in micro-inches are plotted. Wear grooves on the planes caused by uncoated cylinders displayed evidence for an adhesive wear mode and were about 3 times deeper than wear grooved formed by cylinders coated with WC/aC:H. Addition-ally, in the case of the WC/aC:H coated cylinders, the wear mode was abrasive rather than adhesive, indicating that the coating successfully defeated the root cause of fretting (adhesive wear).

The results from the cylinder test show that the coating, if applied to spline couplings for turbines, will be effective in inhibiting fretting wear of the spline coupling. In a proposed illustrative embodiment a diamond- like carbon (DLC) coatings will be applied to at least one of the male and female spline components to defeat the adhesive wear mechanism. The coating can be applied to only one of the spline coupling components, such as the male component of the spline. The diamond-like carbon coating can be comprised of Si/DLC and/or MC/DLC. The coating can be applied to the spline in any desired manner. For example, the coating can be applied via chemical, physical, or chemical/physical vapor deposition. The coating can be applied to the male component of the spline to a depth of about 3-4 micrometers.

In one embodiment, the coating can be a WC/aC:H coating. Such a coating is commercially available from The Timken Company of Canton, Ohio under the product name ES300. The coating is comprised of three layers: (1) an adhesion layer which binds to the substrate (i.e., the spline); (2) a transition layer, and (3) a coating layer. The adhesion layer is approximately 0.1 micrometer thick and is comprised of chromium. The coating layer is about 2 micrometers thick and is comprised of a WC/aC:H coating. The WC/aC:H comprises WC particles embedded in an amorphous hydrocarbon matrix (aka DLC). The WC particles are crystallites with characteristic dimensions less than about 10 nanometers. The transition layer is about 0.1 micrometer thick and transitions between the Chromium of the adhesion layer and the WC/aC:H coating. Hence, the composition of the transition layer changes through the depth of the transition layer, the transition layer comprising mostly chromium near the adhesion layer and comprising mostly the WC/aC:H near the outer coating. As can be appreciated, the transition layer has a chromium gradient which decreases towards the coating and a WC/aC:H gradient which decreases towards the adhesion layer.

These coating layers can be applied by reactive magnetron sputtering, a physical vapor deposition process. The coating can be applied to the finished spline. No additional steps are required prior to coating. However, the use of a high energy mass finishing process to strengthen the teeth of the spline to increase the bending fatigue strength may be beneficial.

References:

1 S. B. Leen, T. H. Hyde, C. H. H. Ratsimba, E. J. Williams, and I. R. McCoII; "An Investigation of the Failure and Fretting Performance of a Representative Aero-Engine Spline Coupling", J. Strain Analysis 37, 565, (2002).

2 R. A. Newley; "The Mechanisms of Fretting Wear of Misaligned Splines in the Presence of a Lubricant"; PhD Thesis, Imperial College, London, 1978.

3 P. M. Ku and M. L. Valtierra; "Spline Wear - Effects of Design and Lubrication"; Trans. ASME, J. Engng for Industry, November 1975, 97 (B4) 1257.

4 J. Meneve, E. Dekempener, J. Smeets, Diamond Films Technol. 4 (1994) 23. 5 M.-G. Kim, K.-R. Lee, K. Y. Eun, Surf. Coat. Technol. 112 (1999) 204.

6 K.-R. Lee, M.-G. Kim, S.-J. Cho, K. Y. Eun, T.-Y. Seong, Thin Solid Films 308 (1997) 263.

7 K. Miyoshi et al "Sliding Wear and Fretting Wear of DLC- Based, Functionally Graded Nanocomposite Coatings"; NASA/TM-1999- 209076.

Claims

1. A method to eliminate fretting wear of spline couplings used in gas turbine generators by applying diamond-like carbon (DLC) coatings to at least one of the male and female spline components to defeat the adhesive wear mechanism.

2. The method of claim 1 wherein the coating is applied only to the male component of the spline.

3. The method of claim 1 wherein the coating comprises three layers: (1 ) about a Cr adhesion layer; (2) a DLC coating layer; and (3) transition layer between the adhesion and coating layer.

4. The method of claim 3 wherein the diamond-like carbon coating is chosen from the group of Si/DLC and MC/DLC or combinations thereof.

5. The method of claim 3 wherein the DLC coating comprises a WC/aC:H coating.

6. The method of claim 5 wherein the WC/aC:H coating comprises WC particles embedded in an amorphous hydrocarbon matrix.

7. The method of claim 6 wherein the WC particles have characteristic dimensions less than about 10 nanometers.

8. The method of claim 1 including a step of using of a high energy mass finishing process to strengthen the teeth of the spline prior to applying the coating to the spline.

9. The method of claim 1 wherein the diamond-like carbon coatings are applied by chemical, physical, or chemical/physical vapor deposition.

10. The method of claim 1 wherein the coating applied to the male component of the spline is less than about 5 micrometers thick.

11. A spline coupling for use in a gas turbine, the spline coupling comprising spline teeth; the improvement comprising applying a wear coating to the spline coupling teeth; said coating comprising a Cr adhesion layer; (2) a DLC coating layer; and (3) transition layer between the adhesion and coating layer.

12. The spline coupling of claim 11 wherein said adhesion layer is about 0.1 micron thick; said transition layer is about 0.1 micrometer thick; and said DLC coating layer is about 2 micrometers thick.

14. The spline coupling of claim 12 wherein the diamond-like carbon coating is chosen from the group of Si/DLC and MC/DLC or combinations thereof.

15. The spline coupling of claim 14 wherein the DLC coating comprises a WC/aC:H coating.

16. The spline coupling of claim 15 wherein the WC/aC:H coating comprises WC particles embedded in an amorphous hydrocarbon matrix.

17. The spline coupling of claim 16 wherein the WC particles have characteristic dimensions less than about 10 nanometers.