Patent Publication Number: US-2007099027-A1

Title: Wear resistant coatings

Description:
BACKGROUND  
      The present technique relates generally to wear resistant coatings. More particularly, embodiments of the present technique relate to low friction, wear resistant coatings that are stable in high temperature applications, such as gas turbine aircraft engines that may be subjected to temperatures from about −40 degree centigrade to about 900 degrees centigrade.  
      Wear resistant coatings are used extensively to improve the hardness and wear behavior of the base material in many structural applications including cutting tools, automotive parts, and turbine parts. Frequently, aircraft and helicopters equipped with gas turbine engines operate in high temperature, low humidity, and corrosive atmospheres. For example, in gas turbine aircraft engines, the variable stator vanes used for regulating airflow through the compressor are exposed to temperatures as high as 500 degree centigrade during take off. These variable stator vanes are also subjected to considerable loads between the vanes and bushings supporting the vanes. For example, the loads may include radial loads up to about 150 lbs and axial loads of up to about 20 lbs as a result of forces exerted by the airflow across the vanes. Commercial engines reach altitudes on the order of 40,000 feet, and at such levels, the availability of moisture is very low. In the absence of adequate amounts of moisture, conventional lubricants, such as graphite, lose their efficacy resulting in undesirable friction and wear between the stator vanes and bushings, among other components. Therefore, a technique is needed for lubricating and protecting these components from undesirable wear at a variety of operational conditions, including high temperatures, high altitudes, low humidity, and so forth.  
      Therefore, there is a need for improved wear resistant coatings that can operate under these severe conditions.  
     SUMMARY OF THE INVENTION  
      In one aspect, the embodiments of the invention provides a wear resistant coating including a hard backing including a metal alloy matrix dispersed with a plurality of hard particles; and a plurality of nano-layers disposed on the hard backing. The plurality of nano-layers has different characteristics from one another.  
      In another aspect, the embodiments of the invention provide a method of making a wear resistant coating. The method includes the steps of providing a substrate; disposing a hard backing; and disposing a plurality of nano-layers on the hard backing. The plurality of nano-layers has different characteristics from one another.  
      In another aspect, the embodiments of the invention provide an article. The article comprises a component; a wear resistant coating disposed on the component. The wear resistant coating includes a hard backing including a metal alloy matrix dispersed with a plurality of hard particles; and a plurality of nano-layers disposed on the hard backing.  
      In an exemplary embodiment, the invention provides an engine including a variable stator vane assembly consisting of a bushing or a pair of bushings, and a vane stem, referred to as a trunnion. The bushing has an inner surface in contact with an outside surface of the trunnion. The inner surface of the bushing and the outer surface of the variable stator vane are coated with a wear resistant coating including a hard backing including a metal alloy matrix dispersed with a plurality of hard particles; and a plurality of nano-layers disposed on the hard backing.  
      These and other aspects, advantages, and salient features of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims. 
    
    
     BRIEF DESCRIPTION OF FIGURES  
       FIG. 1  is a schematic illustration of a gas turbine engine in accordance with embodiments of the present technique;  
       FIG. 2  is a partial schematic view of a gas turbine engine compressor in accordance with embodiments of the present technique;  
       FIG. 3  is an exploded view of a typical variable stator vane assembly in accordance with embodiments of the present technique;  
       FIG. 4  is a schematic view of a system having a wear surface between a bushing and a shaft in accordance with embodiments of the present technique;  
       FIG. 5  is a schematic view of a system having a wear surface between a bushing and a shaft in accordance with embodiments of the present technique;  
       FIG. 6  is a cross sectional side view of a structure having a wear resistant coating in accordance with one embodiment of the present technique;  
       FIG. 7  is a cross sectional side view of a structure having a wear resistant coating in accordance with another embodiment of the present technique;  
       FIG. 8  is a flow diagram of the method of making a wear resistant coating in accordance with embodiments of the present technique;  
       FIG. 9  is a plot of wear on components coated with different wear resistant coatings in accordance with one embodiment of the present technique; and  
       FIG. 10  is a plot of wear on components coated with different wear resistant coatings in accordance with another embodiment of the present technique. 
    
    
     DETAILED DESCRIPTION  
      In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that terms such as “top,” “bottom,” “outward,” “inward,” “over,” and the like are words of convenience and are not to be construed as limiting terms. Furthermore, whenever a particular aspect of the invention is said to comprise or consist of at least one of a number of elements of a group and combinations thereof, it is understood that the aspect may comprise or consist of any of the elements of the group, either individually or in combination with any of the other elements of that group.  
      As used herein, a hard backing is a layer with a hardness value of at least about 700 Vickers Hardness. As used herein, a nano-layer is understood to be a layer, which has a layer thickness less than, for example, 500 nanometers. In certain embodiments, the nano-layer may have a thickness less than 400 nanometers, or less than 300 nanometers, or less than 200 nanometers, or less than 100 nanometers. In some embodiments, a number of such nano-layers may be disposed one over the other, thereby producing a conglomeration of multiple nano-layers with a potentially greater overall thickness.  
      In compressors, such as those used in turbine engines, variable stator vanes perform the function of regulating airflow through compressors by performing angular actuation. These variable stator vanes are actuated through control systems to ensure optimum operation of the high-pressure compressor without stalls. The following characteristics may improve the operational reliability and performance of these systems: a) Low fretting and sliding wear, b) Actuate at a friction that is low enough for the actuation system to operate, c) Not to undergo galling or seizure, d) be thermally stable, and e) operate at high altitudes where the humidity levels are very low. Some embodiments of the present invention are directed towards wear resistant coatings to protect such components. Moreover, the disclosed embodiments may be used in other stationary and moving components, such as stator vane stem-bushing assembly in centrifugal and reciprocating compressors, in vanes and stems in industrial gas turbine and aero-derivative engines, in fan blade-dovetail pin assemblies, piston rings, and cam-rocker arm etc.  
       FIG. 1  is a schematic illustration of a gas turbine engine  10  having one or more of wear resistant coatings of the invention disposed on the various components as discussed in detail below. For example, wear resistant coating may include a hard backing including a metal alloy matrix dispersed with a plurality of hard particles; and a plurality of nano-layers disposed on the hard backing. The illustrated gas turbine engine  10  includes a low-pressure compressor  12 , a high-pressure compressor  14 , and a combustor  16 . Engine  10  also includes a high-pressure turbine  18  and a low-pressure turbine  20 . Compressor  12  and turbine  20  are coupled by a first shaft  24 , and compressor  14  and turbine  18  are coupled by a second shaft  26 . During operation, air flows through low-pressure compressor  12  and compressed air is supplied from low-pressure compressor  12  to high-pressure compressor  14 . The highly compressed air is delivered to combustor  16 . Gas flow from combustor  16  drives turbines  18  and  20  before exiting gas turbine engine  10 . Again, the one or more of wear resistant coatings of the invention may be disposed on the various components to improve wear resistance under various operational conditions of the gas turbine engine  10 .  
       FIG. 2  is a partial schematic view of an embodiment of the gas turbine engine compressor  14  as illustrated in  FIG. 1 , wherein the various surfaces may be coated with one or more wear resistant coatings of the invention. For example, the wear resistant coating may include a hard backing including a metal alloy matrix dispersed with a plurality of hard particles; and a plurality of nano-layers disposed on the hard backing. Compressor  14  includes a plurality of stages, and each stage includes a row of rotor blades  40  and a row of variable vane assemblies  44 . In an exemplary embodiment, rotor blades  40  are supported by rotor disks  46  and are coupled to a rotor shaft  26 . Rotor shaft  26  is surrounded by a casing  50  that extends circumferentially around compressor  14  and supports variable vane assemblies  44 . Variable vane assemblies  44  each include a variable vane  52  and a vane stem  54  that extends substantially perpendicularly from a vane platform  56 . More specifically, vane platform  56  extends between variable vane  52  and vane stem  54 . Each vane stem  54  extends through a respective opening  58  defined in casing  50 . Casing  50  includes a plurality of openings  58 . Variable vane assemblies  44  also include a lever arm  60  that extends from each variable vane  52  and is utilized to selectively rotate variable vanes  52  for changing an orientation of vanes  52  relative to the flow path through compressor  14  to facilitate increased control of air flow through compressor  14 . Again, one or more of these components, among others, may include one or more wear resistant coatings as discussed in further detail below.  
       FIG. 3  is an exploded view of a variable vane assembly  44  that may include one or more wear resistant coatings in accordance with certain embodiments of the present technique. Vane airfoil  62  is shown as a cutaway. Integral vane stem  64  is located at a radially outer end of vane airfoil  62 . Vane stem  64  includes an attachment mechanism  66  depicted here as a threaded connection, although any other equivalent connection method such as a spline arrangement may be used. Vane stem  64  extends through opening  68  in casing  70 , again shown as a cutaway. Opening  68  includes a counterbore  72 , which receives an inner washer  74 . A bushing  76  slides into opening  68  and over upper vane stem  64 , filling the remaining space in opening  68  and improving the sliding engagement between casing  70  and upper vane stem  64 . This inner washer  74  may be replaced by the flange of a flanged rotating bushing, in which case a washer (not shown) would separate the lever arm  77  from the case. A first end  80  of lever arm  77  is assembled over vane stem  64  and is secured to vane stem  64  by a fastening mechanism  82 , here depicted as a locknut. The fastening mechanism  82  mates cooperatively with attachment mechanism  66 , depicted as a threaded end of upper vane stem  64 , to secure attachment mechanism  66  to vane stem  64 . Lever arm  77  includes a second end  84  that is integrally attached to the first end  80  by a web  86 . A projection  88  extends from second end  84  and is received by an aperture in an actuation ring. A second bushing  90  fits over projection  88  and into the aperture in actuation ring to improve the sliding engagement between actuation ring and projection  88 .  
      At the radially inner end of vane assembly  44 , an integral lower vane shaft  92  extends radially inward from vane airfoil  62 . Vane shaft  92  includes a first, large diameter shaft  94  and a second smaller diameter shaft  96 . A second bushing  98  is assembled over lower vane shaft  94 , which is received by an optional shroud  100 . A seal  102  is assembled radially inward of the shroud  100 , which seal  102  is contacted by teeth  104  positioned on the rotating apparatus of the engine. The teeth  104  wear into seal  102  to form a barrier to air leakage. An optional third fastening mechanism  106 , depicted as a locking pin, extends through at least one boundary of seal  102 , through shroud  100 , through second bushing  98  through aperture  108  in lower vane shaft  94 . When an optional fastening mechanism  106  is employed, any other mechanical fastening mechanism, such as for example a threaded bolt and locknut may be substituted for the lock pin. Optionally, a washer is placed between the axial faced large diameter  92  and shroud  100 .  
      An exploded view of a section of an airfoil  62  including inner bushing  110  and an outer bushing  112  along with trunnion  113  is shown in  FIG. 4 . In some embodiments, the airfoil  62  may include only one bushing  114  as shown in  FIG. 5 . The bushings and washers may be fabricated by a variety of techniques, such as by injection molding or by high temperature sintering of ceramics. The bushings are generally durable with good wear characteristics. In certain embodiments, the bushings are made of an inexpensive wear material, which is easily replaceable and designed as a consumable item. To determine the type of bushing for a particular design, physical properties, such as, for example, thermal expansion coefficient, operating temperature range, yield strength and elastic modulus of the mating materials, the forces exerted on the mating materials, wear per cycle, and the number of cycles over the expected life are used to determine the relative wear that will be experienced in an application. The wear resistant coating according to some embodiments of the present invention applied on the inner surface of the bushings  114 ,  110 , or  112  and on the outer surface of trunnion  113  generally prevents or substantially minimizes the wear loss and protects the components.  
      The foregoing systems and components may be coated with one or more wear resistant coatings, such as those illustrated with reference to  FIGS. 6 and 7 . Turning first to  FIG. 6 , one embodiment of a wear resistant coating  116  includes a hard backing  118  and a plurality of nano-layers  120 . Specifically, the hard backing  118  includes a metal alloy matrix dispersed with a plurality of hard particles, wherein the hard backing is disposed on a substrate  121 . In turn, the plurality of nano-layers  120  is disposed on the hard backing  118 . The hard backing  118  improves the wear resistance of the nano-layers  120  and substantially protects the substrate  121  from damage under friction. The substrate  121  may be any one of the components described in detail above with reference to  FIGS. 1-5 , or it may be a device or component for another application or system.  
      In one embodiment, the hard backing  118  has a hardness value of greater than about 700 Vickers Hardness, in another embodiment the hardness value is greater than about 800 Vickers Hardness, in another embodiment the hardness value is greater than about 900 Vickers Hardness, in another embodiment the hardness value is greater than about 1000 Vickers Hardness, in yet another embodiment the hardness value is greater than about 1200 Vickers Hardness. The hard backing  118  includes one or more materials selected to generally maintain or improve the corrosion resistance of the substrate material and it depends on the utility and application of the coated article. The hard backing  118  is characterized by adequate toughness and crushing strength, stability up to the operating temperature of the coated article, oxygen resistance, and compatibility with the top nano-layers  120 .  
      The hard backing  118  can include a variety of suitable metal alloy matrices, such as cobalt, cobalt-based superalloys, nickel-based superalloys, or ferrous-based superalloys. Such a superalloy may be formed of a nickel-based, ferrous-based, or a cobalt-based alloy, wherein nickel, iron, or cobalt is generally the greatest element in the superalloy by weight. Illustrative nickel-base superalloys include at least about 40 percent by weight of nickel, and some percentage of cobalt, or chromium, or aluminum, or tungsten, or molybdenum, or titanium, or iron, or any combination thereof. Examples of nickel-base superalloys are designated by the trade names Inconel®, Nimonic®, Rene® (e.g., Rene®80-, Rene®95, Rene®142, and Rene®N5 alloys), and Udimet®, and include directionally solidified and single crystal superalloys. (INCONEL®, Nimonic®, and Udimet® are registered trademarks of the Special Metals Corporation family of companies, Rene® is the registered trade mark of General Electric Company.) Illustrative cobalt-base superalloys include at least about 30 percent by weight of cobalt, and some percentage of nickel, or chromium, or aluminum, or tungsten, or molybdenum, or titanium, or iron, or any combination thereof. Examples of cobalt-based superalloys are designated by the trade names Haynes®, Nozzaloy®, Stellite®, and Ultimet® (Haynes® and Ultimet® are the registered trademarks of Haynes International, Inc, STELLITE® is a registered trademark of DELORO STELLITE COMPANY, INC). In some embodiments, the hard backing  118  includes nickel chromium aluminum alloy, or cobalt nickel chromium aluminum yttrium alloy, or nickel cobalt chromium tungsten alloy, or cobalt molybdenum chromium silicon alloy consisting of solid solution strengthened cobalt base matrix and an interpenetrating network of laves phase, consisting of about 25%or more of molybdenum, 6% or more of chromium and greater than 2% of silicon.  
      In the above embodiments, the metal matrix of the hard backing  118  is dispersed with a plurality of hard particles. The hard particles may include one or more of metal nitrides, or metal carbides, or metal borides, or combinations thereof. In certain embodiments, the metal matrix may be dispersed with particles of tungsten carbide, or titanium carbide, or chromium carbide, or silicon carbide, or diamond, or titanium nitride, or silicon nitride, or cubic boron nitride, or titanium boride, or chromium oxide, or aluminium oxide, or zirconium oxide, or silicon oxide, or zirconium oxide, or combinations thereof. The plurality of hard particles has an average particle size in the range of from about  100  nanometers to about 2 microns. In some embodiments, the average particle size is in a range of from about 100 nanometers to about 2000 nanometers with a significant portion of particles having particle size in the range of from about 100 nanometers to about 400 nanometers and the remaining particles having particle size less than about 2000 nanometers, and in other embodiments less than about 1000 nanometers. The particles which are sized between about 100 nanometers to about 400 nanometers may tend to strengthen the matrix and those less about 1000 nm may provide wear protection without making the coating abrasive with respect to the counterface material. The plurality of hard particles have a volume fraction in a range of from about 30 volume percent to about 70 volume percent of the total volume of the hard backing  118 . In some embodiments, the percentage of hard particles relative to the total volume of the hard backing  118  ranges between about 35 volume percent and about 65 volume percent, and in other embodiments between about 40 volume percent and about 60 volume percent, and in other embodiments between about 45 volume percent and about 55 volume percent. The spacing between the plurality of hard particles in one embodiment is in the range of from about 100 nm to about 700 nm, and in another embodiment it is from about 100 nanometers to about 500 nanometers. The hard backing  118  has a thickness in the range of from about 5 micrometers to about 500 micrometers. The thickness of the hard backing would depend on the loading exerted on the coating. In applications where the force exerted is higher, the hard backing would have to be thicker to support the nano-layer coating. In an exemplary embodiment, the hard backing  118  including a metallic matrix and a plurality of hard particles dispersed in the metallic matrix is deposited by either high velocity oxygen fuel thermal spraying, or high-velocity air-fuel spraying. Hard backing  118  deposited by the high velocity air fuel technique may provide nano-retention, lesser decarburization, finer spacing between hard particles, and lesser embrittlement of the binder used during deposition.  
      In one embodiment, the hard backing  118  includes nickel coatings with either phosphorous or boron additive. While such coatings have a hardness in the range of from about 700 Vickers Hardness to about 1000 Vickers Hardness, the wear resistance and hardness may be further improved by adding hard particles to the matrix by co-deposition. The hard particles added may include silicon carbide, silicon nitride, alumina, cubic boron nitride, or diamond.  
      Generally, the plurality of nano-layers  120  has different characteristics from one another. For example, the plurality of nano-layers  120  may include a plurality of alternating first layers  122  and second layers  124 . The first layer  122  comprises a first material and the second layer  124  comprises a second material different from the first material. The first material comprises a material including a metal nitride, or a metal boride, or a metal carbide, or a combination thereof. The second material includes a material including a metal, or a metal nitride, or a metal boride, or a metal carbide, or a combination thereof. In an exemplary embodiment, the nano-layers  120  include alternate layers of metal carbides such as TiC/ZrC. In another exemplary embodiment, the nano-layer  120  includes alternate layers of metal nitrides such as TiN/ZrN. In yet another exemplary embodiment, the nano-layer  116  includes alternate layers of a metal nitride and a metal carbide such as TiN/TiC. In certain embodiments, metal layers may be included between the nitride or carbide layers. In an exemplary embodiment, the nano-layers  120  include alternate layers of a metal and a metal nitride such as TiN/Ti/TiN.  
      The nano-layers  120 , components and associated methods of manufacture of the present invention, generally provide higher hardness and better wear resistance, impact resistance, erosion resistance and clearance control than would otherwise be predicted by the rule of mixtures. This is because when alternating layers of materials with different elastic moduli and crystal structures are brought together, the resistance to dislocation movement through the structure as a whole increases.  
      In certain embodiments, the thickness of each of the plurality of first layers  122  and second layers  124  is in the range of from about 3 nanometers to about 500 nanometers. In other embodiments the thickness is in the range of from about 10 nanometers to about 200 nanometers. In some embodiments, the thickness is in the range of from about 20 nanometers to about 100 nanometers. The thickness of the individual layers  122 ,  124  may be independently adjusted to control the hardness, strain tolerance and overall stability of the nano-layer coatings when subjected to thermo-mechanical stresses. The total thickness of the nano-layers  120  may range from about 1 micron to about 25 microns. In some embodiments, the total thickness is in the range of from about 1 micron to about 20 microns. In other embodiments, the total thickness is in the range of from about 5 microns to about 10 microns. The number of first layer  124  and second layers  124  utilized may vary, depending upon the thickness of each of the layers and the desired thickness of the nano-layers  120 . The total thickness of the coatings depends on the components that are to be operated and may be constrained by the total space available between the contacting parts.  
      In an exemplary embodiment, a wear resistant coating  116  comprises a hard backing  118  comprising cobalt chromium alloy dispersed with micron to submicron sized tungsten carbide particles and a plurality of alternate TiN and ZrN nano-layers  120  disposed on the hard backing  118 . In an exemplary embodiment, a wear resistant coating  116  includes a hard backing  118  including a metal alloy dispersed with a plurality of particles of metal oxide, wherein the metal alloy comprises a material including nickel or cobalt or iron; and a plurality of alternate metal nitride nano-layers  120  disposed on the hard backing  118 . Disposing nano-layers  120  on the wear resistant hard backing  118  introduces a gradual gradient in hardness, prevents premature buckling of the nano-layers  120  and hence increases the hardness and wear resistance of the entire coating  126 .  
      The wear resistant coating is suitable for moderately high operating temperatures. In one embodiment, the wear resistant coating  116  is thermally stable up to at least about temperature of at least about 500° C., in another embodiment at least about 600° C., in yet another embodiment at least about 700° C. The nano-layer coatings retain morphological stability at temperatures well above the anticipated operating temperature range. The nano-layers of the invention retain adequate oxidation resistance till about 650° C., which is well above the anticipated operating temperature range of variable stator vanes. The underlying hard backing has a thermal stability up to about 500° C. For higher temperature ranges the composition of the hard backing may be changed to a superalloy matrix with gamma prime formers reinforced with a more thermally stable hard particle such as chromium carbide or aluminum oxide.  
       FIG. 7  illustrates an exemplary embodiment of a wear resistant coating  126  including the plurality of nano-layers  120  disposed on the hard backing  118  as illustrated in  FIG. 6 , further illustrating a lubricating layer  128  disposed over the nano-layers  120 . The lubricant material of the lubricating layer  128  may be selected to be stable up to the operating temperature of the coated article, and under low moisture conditions. The lubricant material may include tungsten disulphide, or molybdenum disulfide, or hexagonal boron nitride, or tungsten telluride, or tungsten selenide, or molybdenum telluride, or molybdenum telluride, or combinations thereof. In an exemplary embodiment, the lubricant is tungsten disulphide. Tungsten disulphide provides low friction properties at temperatures up to about 550° C., even in the absence of moisture, which is typically the case at high altitudes (&gt;35,000 ft).  
      Embodiments of the present technique also include a method for making a wear resistant coating, such as described in detail above.  FIG. 8  is a flow chart illustrating a method according to one embodiment of the invention. The method  130  comprises the steps of providing a substrate  121  in step  132 ; disposing a hard backing  118  in step  134 ; and disposing a plurality of nano-layers  120  on the hard backing in step  136 .  
      The process of applying the coating begins by preparing the substrate surface to be coated. The first step is to remove debris and oxides from the substrate. Well known cleaning techniques such as degreasing, grit blasting, chemical cleaning, and/or electrochemical polishing may be used to obtain desired surface cleaning and finish.  
      Disposing the hard backing in step  134  comprises a method including electroless deposition, or high velocity oxygen fuel thermal spraying, or activated combustion high-velocity air-fuel spraying or electron beam physical vapor deposition. In some embodiments, the hard backing is metal alloy matrix deposited by either high velocity oxygen fuel thermal spraying, or activated combustion high-velocity air-fuel spraying. Hard backings deposited by these techniques advantageously provides nano-retention, lesser decarburization, finer spacing between hard particles and lesser embrittlement of the binder. Electroless deposition is also used when the hard backing includes nickel coating with boron or phosphorus doping. Hard particles may be dispersed in the electroless nickel matrix to further improve wear properties. Disposing the plurality of nano-layers in step  136  comprises physical vapor deposition.  
      Another aspect of the invention is to provide an article. The article includes a component; and a wear resistant coating disposed on the component. The wear resistant coating comprises a hard backing including a metal alloy matrix dispersed with a plurality of hard particles; and a plurality of nano-layers disposed on the hard backing. The hard backing includes any hard superalloy dispersed with a plurality of hard particles. The coating comprises a plurality of nano-layers disposed on the hard backing. The attributes of the hard backing and the nano-layers are described in the wear resistant coating embodiments above. The article includes a machine having components coated with wear resistant coatings of the invention that moves along one another. The article includes an engine having the component coated with the wear resistant coating of the invention. The engine includes a turbine engine having a variable stator vane, wherein the variable stator vane comprises the component. The engine includes a turbine engine having a variable stator vane including a trunnion and a bushing, wherein the bushing has an inner surface in contact with an outside surface of the trunnion, and wherein the inner surface of the bushing and the outer surface of the variable stator vane are coated with the wear resistant coating. The article includes a transportation vehicle having the component. In an exemplary embodiment, the transportation vehicle is an aircraft.  
      The use of hard backings as a buffer between the nano multilayered coating and a tough but ductile substrate helps in providing a tribo-system that is capable of sustaining high surface loads, but providing high wear resistance. Therefore, the wear-resistant coatings according to some embodiments of the present invention are suitable for any kind of substrate and are stable up to relatively high temperatures.  
      The embodiments of the invention described above, serve as a generic template to protect components from fretting wear, sliding wear, and friction under conditions where external lubrication is not possible. While the embodiments of the invention are described with respect to variable stator vanes and bushings in aircraft engines, the concepts described in this invention may be used in other application, where fretting wear under similar operating conditions remains an issue. In addition, the described invention may also be utilized under sliding wear conditions for similar material contact surfaces.  
      The following example serves to illustrate the features and advantages offered by the embodiments of the present invention, and are not intended to limit the invention thereto.  
      EXAMPLE 1  
      The outer surface of the stem of the variable stator vane was coated with various hard backing compositions by, activated combustion high velocity oxy fuel process, activated combustion high velocity air fuel process, and electroless Ni-diamond composite coating process. These coatings are named Hard Backing  1 ,  2  and  3  respectively. Hard backing  1  was a cobalt with a dispersion of tungsten carbide thermal spray through High Velocity Oxy Fuel process. Hard backing  2  was a cobalt chromium with a dispersion of tungsten carbide thermal spray through Activated combustion High Velocity Air Fuel process. Hard backing  3  was an electroless nickel-hard particle composite plating. The particle size of the tungsten carbide was between 0.2 to 5 micrometers. The coating thickness was 150 to 200 micrometers. The part with hard backing  3  was then further coated with alternate multilayers of titanium nitride (TiN) and zirconium nitride (ZrN) using PVD cathodic arc technique, wherein each layer was of thickness 120 nanometers. The total thickness of the TiN/ZrN multilayer system was 8 micrometers. The wear of the coated parts was measured using a customized test rig that simulates the operating conditions of the part in reciprocating motion. The coated stem was run against a bushing of Stellite 6. The test was run for 100 hours on the test rig and wear of both the bushing as well as the stem coating were measured.  
      Wear was plotted for parts coated with different hard backings and the plot  138  is shown in  FIG. 9 . Bars  140  and  142  indicate the wear of the stem and the bushing without any coating and form a base line. Bars  144  and  146  indicate the wear of the stem and the bushing with hard backing  1  respectively. Bars  148  and  150  indicate the wear of the stem and the bushing with hard backing  2  respectively. Bars  152  and  154  indicate the wear of the stem and the bushing with hard backing  3  along with the nano-layer coating of TiN/ZrN respectively. The plot  138  indicates that compared to baseline uncoated samples, the hard backings  1  and  2  show about 4-6 times lower wear on the Trunnion.  
      EXAMPLE 2  
      The outer surface of the stem of the variable stator vane was coated with nano-layers of TiN and ZrN to a thickness of 8 micrometers. This was run against Stellite 6 bushings in the simulated test in a test rig for 8 hours. Another stem was first coated with hard backing  4  which is a electroless Nickel diamond composite plating to a thickness of 200 microns on which an 8 micron PVD coating of TiN/ZrN nano-layers of total thickness of 8 microns was deposited. The layer thickness for each of TiN and ZrN was 120 nm. This was also run under the similar operating conditions as in example 1.  FIG. 10  shows plot  156  illustrating the wear comparison of the above 2 coatings compared to the uncoated specimen. Bars  158  and  160  indicate the wear of the stem and the bushing without any coating respectively. Bars  162  and  164  indicate the wear of the stem and the bushing with nano-layers of TiN/ZrN without any hard backing respectively. Bars  166  and  168  indicate the wear of the stem and the bushing with hard backing  4  along with TiN/ZrN nano-layers respectively. The wear of the TiN/ZrN coated stem  162  is 40% lower than the uncoated specimen  158 . However, the stem with TiN/ZrN on a hard backing  4  shows no wear in the 8 hour test at all (bar  166 ), indicating a tremendous improvement over the uncoated specimen.  
      While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. For example, though in the preceding paragraphs embodiments refer to an aircraft engine, it is valid for other components similarly exposed to mechanical, chemical and thermal stress. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.