BACKGROUND OF THE INVENTION Higher operating temperatures for gas turbines, aircraft engines and the like are continuously sought in order to improve their performance capability and increase their operating efficiency. As such operating temperatures increase, however, the high-temperature durability of the components of these gas turbines, aircraft engines and the like must correspondingly increase. Significant advances in high-temperature capabilities have been achieved through the formulation and development of nickel, cobalt and iron-based superalloys and the like. These superalloys have been designed to withstand temperatures in excess of about 800 degrees C and higher. Nonetheless, when used to form the components of a gas turbine, an aircraft engine or the like, the superalloys are susceptible to damage by oxidation and hot corrosion attack, and may not retain adequate mechanical properties. For this reason, such components are typically protected by an environmental or thermal insulation coating, typically referred to as a thermal barrier coating.

High-temperature wear resistance, erosion resistance, galling resistance and strength are important properties associated with the components and coating systems of gas turbines, aircraft engines and the like. In order to ensure adequate wear resistance, erosion resistance, galling resistance and strength at elevated temperatures, a coating system must retain its hardness, be adequately tough and be oxidation-resistant. The useful operating range of conventional high-temperature coating systems, such as carbides, triballoys and the like, is limited to about 800 degrees C to about 850 degrees C. Likewise, pure mono-phase ceramic coating systems have a relatively low toughness and do not perform well at elevated temperatures. Currently, the best available coating systems for wear resistance, erosion resistance, galling resistance and strength at elevated temperatures (in excess of about 1000 degrees C) include L605 bulk materials and yttria-stabilized zirconia (YSZ). MCrAlY-alumina micron- scale coating systems, including a plurality of relatively hard, brittle micron-sized ceramic particles, demonstrate properties comparable to those of WC-Co coating systems in room-temperature wear and erosion tests, however also do not perform well at elevated temperatures.

Thus, what is still needed is a coating system that provides enhanced wear resistance, erosion resistance, galling resistance and strength at elevated temperatures (in excess of about 1000 degrees C). This coating system must retain its hardness, be adequately tough and be oxidation-resistant.

Similarly, at relatively low temperatures, hydroelectric turbine components and the like undergo significant erosion when exposed to, for example, silt in rivers that exceeds about 1000 ppm. This problem may be particularly severe in South and Southeast Asia and South America, where silt content during the rainy season may exceed about 50000 ppm. The resulting erosion damage may cause decreased operating efficiency and necessitate costly maintenance-related shutdowns and the replacement of heavy components every few years. In order to avoid such problems, many power stations shut down their hydroelectric turbines when the silt content reaches a predetermined threshold, for example, about 5000 ppm.

These problems have been addressed by fabricating hydroelectric turbine components from 13-4 martensitic stainless steel, 16-5-1 stainless steel or the like in order to mitigate corrosion and improve the erosion resistance of the components. Ceramic coating systems, such as alumina, alumina-titania and chromia applied by an air plasma spray process, have also been utilized, with limited success. Likewise, NiCrBSi+WC-CoCr coating systems with micron-sized WC grains applied by a spray and fuse process and WC-CoCr coating systems with micron-sized WC grains applied by a hybrid DJ HVOF process have been utilized. None of these coating systems, however, have demonstrated adequate erosion resistance under conditions where the silt content is high and the water velocity is in the range of about 30 m/s to about 70 m/s.

Thus, what is still needed is a coating system that provides enhanced erosion resistance, corrosion resistance, solid particle impact damage resistance and cavitation resistance at relatively low temperatures.

BRIEF SUMMARY OF THE INVENTION In various embodiments, the present invention provides nano-structured coating systems, components and associated methods of manufacture demonstrating improved wear resistance, erosion resistance, galling resistance and strength at elevated temperatures (in excess of about 1000 degrees C). The coating systems include a plurality of relatively hard, brittle nano-sized ceramic particles disposed within a relatively ductile oxidation-resistant matrix. The oxidation-resistant matrix acts as a binder and includes a metallic alloy matrix with proven high-temperature capabilities.

For example, the metallic alloy matrix includes a nickel-based alloy (such as a Ni- based superalloy, NiCr, NiCrAlY or the like), a cobalt-based alloy (such as L605, HS188, CoCrAlY or the like), an intermetallic system (such as NiAl, Ni3Al or the like) or a shape memory alloy that absorbs impact energy through a martensitic transformation. The nano-sized ceramic particles may include aluminum oxide, zirconium oxide, yttrium oxide, a yttrium-based garnet, mullite, hafnia or a suitable combination thereof. Deleterious interactions between the metallic alloy matrix and the nano-sized ceramic particles are avoided by selecting metal-oxide combinations that are unlikely to result in run-away reactions. Preferably, the volume percent of the ceramic phase ranges from about 10% to about 95% and the size of the nano-sized ceramic particles ranges from about 5 nm to about 200 nm, providing a mean free spacing in the range of about 200 nm or less, more preferably in the range of about 100 nm or less. Optionally, a plurality of relatively hard, brittle micron-sized ceramic particles may also be disposed within the metallic alloy matrix, in a bimodal embodiment of the present invention.

The coating systems of the present invention demonstrate increased hardness at elevated temperatures with increased resistance to crack nucleation and propagation.

Specifically, the relatively hard, brittle nano-sized ceramic particles provide increased hardness at elevated temperatures by acting as an impediment to dislocation motion, thereby constraining the deformation of the metallic alloy matrix. Cracks are unlikely to nucleate because the relatively hard, brittle ceramic particles are nano-sized (or, alternatively, nano-sized and micron-sized) and are unlikely to propagate because the complex microstructure engineered makes propagation paths tortuous.

In various embodiments, the present invention also provides nano-structured coating systems, components and associated methods of manufacture demonstrating improved erosion resistance, corrosion resistance, solid particle impact damage resistance and cavitation resistance at relatively low temperatures. The coating systems include a plurality of nano-sized and micron-sized WC grains disposed within a corrosion- resistant CoCr binder. The use of nano-sized WC grains ensures that micro-cracking of the WC grains is avoided and that the mean distance between the WC grains is reduced, improving the erosion resistance of the coating systems. The use of micron- sized WC grains improves the erosion resistance of the coating systems at relatively shallow angles and makes propagation paths in the CoCr binder tortuous. The use of nano-sized WC grains with a relatively small mean free distance improves the overall toughness of the coating systems, allowing for a reduced CoCr content.

The coating systems of the present invention are deposited on a component via thermal spraying, composite plating (electro or electroless), brush composite plating, electron beam-physical vapor deposition, spray forming, mechanical alloying followed by powder compaction, blending with a braze alloy and applying by a braze process, spraying and fusing, laser remelting or any other conventional process.

In one embodiment of the present invention, a high-temperature coating system includes a substantially ductile binder matrix and a plurality of substantially hard nano-sized ceramic particles disposed within the substantially ductile binder matrix, wherein the mean free spacing between the plurality of nano-sized ceramic particles is on a nano-scale.

In another embodiment of the present invention, a high-temperature component includes a substrate material having a surface, a substantially ductile binder matrix disposed adjacent to the surface of the substrate material and a plurality of substantially hard nano-sized ceramic particles disposed within the substantially ductile binder matrix, wherein the mean free spacing between the plurality of nano- sized ceramic particles is on a nano-scale.

In a further embodiment of the present invention, a method for manufacturing a high- temperature coating system includes providing a substantially ductile binder matrix and disposing a plurality of substantially hard nano-sized ceramic particles within the substantially ductile binder matrix, wherein the mean free spacing between the plurality of nano-sized ceramic particles is on a nano-scale.

In a still further embodiment of the present invention, a low-temperature coating system includes a substantially corrosion-resistant binder matrix, wherein the substantially corrosion-resistant binder matrix includes at least one of cobalt, chromium, nickel, stainless steel, stainless steel alloyed with cobalt, an iron-based alloy, an amorphous material and a shape memory alloy. The low-temperature coating system also includes a plurality of substantially hard nano-sized grains disposed within the substantially corrosion-resistant binder matrix, wherein the plurality of substantially hard nano-sized grains include at least one of tungsten carbide, titanium carbide, titanium diboride, titanium alloy nitride, boron carbide, cubic boron nitride, silicon carbide, silicon nitride, diamond and an oxide, and wherein the mean free distance between the plurality of substantially hard nano-sized grains is on a nano-scale.

In a still further embodiment of the present invention, a low-temperature component includes a substrate material having a surface and a substantially corrosion-resistant binder matrix disposed adjacent to the surface of the substrate material, wherein the substantially corrosion-resistant binder matrix includes at least one of cobalt, chromium, nickel, stainless steel, stainless steel alloyed with cobalt, an iron-based alloy, an amorphous material and a shape memory alloy. The low-temperature component also includes a plurality of substantially hard nano-sized grains disposed within the substantially corrosion-resistant binder matrix, wherein the plurality of substantially hard nano-sized grains include at least one of tungsten carbide, titanium carbide, titanium diboride, titanium alloy nitride, boron carbide, cubic boron nitride, silicon carbide, silicon nitride diamond and an oxide, and wherein the mean free distance between the plurality of substantially hard nano-sized grains is on a nano- scale.

In a still further embodiment of the present invention, a method for manufacturing a low-temperature coating system includes providing a substantially corrosion-resistant binder matrix, wherein the substantially corrosion-resistant binder matrix includes at least one of cobalt, chromium, nickel, stainless steel, stainless steel alloyed with cobalt, an iron-based alloy, an amorphous material and a shape memory alloy. The method also includes disposing a plurality of substantially hard nano-sized grains within the substantially corrosion-resistant binder matrix, wherein the plurality of substantially hard nano-sized grains include at least one of tungsten carbide, titanium carbide, titanium diboride, titanium alloy nitride, boron carbide, cubic boron nitride, silicon carbide, silicon nitride, diamond and an oxide, and wherein the mean free distance between the plurality of substantially hard nano-sized grains is on a nano- scale.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a conceptual schematic diagram of one embodiment of a nano-structured coating system of the present invention, including a plurality of relatively hard, brittle nano-sized ceramic particles disposed within a relatively ductile oxidation-resistant matrix; Figure 2 is a graph illustrating the conceptual relationship between the grain size of ceramic particles and the mean free spacing between ceramic particles (d/X) in a cermet coating system and the fracture toughness (Kc) of the cermet coating system for both a nano regime and a micron regime for systems wherein the binder is ductile and does not contain a dissolved hard phase; Figure 3 is a conceptual schematic diagram of another embodiment of a nano- structured coating system of the present invention, including a plurality of relatively hard, brittle nano-sized ceramic particles and a plurality of relatively hard, brittle micron-sized ceramic particles disposed within a relatively ductile oxidation-resistant matrix; Figure 4 is a conceptual schematic diagram of the bimodal nano-structured coating system of Figure 3, highlighting a brittle mode and a ductile mode; and Figure 5 is a conceptual schematic diagram of a further embodiment of a nano- structured coating system of the present invention, including a plurality of nano-sized ceramic grains and a plurality of micron-sized ceramic grains disposed within a corrosion-resistant metallic binder.

DETAILED DESCRIPTION OF THE INVENTION As described above, in various embodiments, the present invention provides nano- structured coating systems, components and associated methods of manufacture demonstrating improved wear resistance, erosion resistance, galling resistance and strength at elevated temperatures (in excess of about 1000 degrees C). The nano- structured coating systems, components and associated methods of manufacture demonstrate these improved properties because they retain their hardness, are relatively tough and are oxidation-resistant. In various embodiments, the present invention also provides nano-structured coating systems, components and associated methods of manufacture demonstrating improved erosion resistance, corrosion resistance, solid particle impact damage resistance and cavitation resistance at relatively low temperatures.

Referring to Figure 1, the high-temperature coating system 10 of the present invention includes a plurality of relatively hard, brittle nano-sized ceramic particles 12 disposed within a relatively ductile oxidation-resistant matrix 14. As used herein,"relatively" or"substantially"hard means at least about 20% harder than quartz (greater than about 1200 Hv) and"relatively"or"substantially"ductile means having a ductility less than that of WC-Co. The oxidation-resistant matrix 14 acts as a binder and includes a metallic alloy matrix with proven high-temperature capabilities. For example, the metallic alloy matrix includes a nickel-based alloy (such as a Ni-based superalloy, NiCr, NiCrAlY or the like), a cobalt-based alloy (such as L605, HS188, CoCrAlY or the like), an intermetallic system (such as NiAl, Ni3Al or the like) or a shape memory alloy that absorbs impact energy through a martensitic transformation.

In general, the binder material includes, for example, a metal, an alloy, a superalloy, a braze alloy, a multiphase alloy, a low-temperature alloy, a high-temperature alloy (designed to be used at temperatures greater than about 700 degrees C), an intermetallic, a semiconductor metal or a ceramic material. The superalloy may include a nickel-based superalloy, a cobalt-based superalloy or an iron-based superalloy. The braze alloy may include a nickel alloy or a cobalt alloy and chromium, tungsten, boron or silicon. The multiphase alloy may include an alloy having the formula MCrAlY, where M is nickel, cobalt, iron or a suitable combination thereof. Suitable examples include, but are not limited to, NiCrAlY, CoNiCrAlY, CoCrAlY and FeCrAlY, with Cr content in the range of about 20% to about 35%, Al content in the range of about 8% to about 12%, Y content of less than about 2%, and Ni, Co and/or Fe comprising the balance. The low-temperature alloy includes, for example, an austenitic stainless steel, a ferritic stainless steel, an aluminum-based alloy, a cobalt-based alloy or a titanium-based alloy. The intermetallic includes, for example, nickel aluminide, tri-nickel aluminide, titanium aluminide, tri-titanium aluminide, penta-niobium trisilicide, niobium disilicide, or tri-niobium silicide. The semiconductor metal includes, for example, silicon. The ceramic material includes, for example, a ductile ceramic oxide, such as titanium dioxide. Optionally, the binder material includes trace amounts of a wetting agent, such as titanium, magnesium, oxygen, iron, nickel, chromium or the like. Preferably, the binder material remains stable in a temperature range of about 500 degrees C to about 1150 degrees C.

The nano-sized ceramic particles 12 may take the form of a plurality of nano- particles, nano-fibers, nano-tubes, nano-tetrapods or the like. The nano-sized ceramic particles 12 may include aluminum oxide, zirconium oxide, yttrium oxide, a yttrium- based garnet, mullite, hafnia or a suitable combination thereof. In general, the nano- sized ceramic particles 12 may include a ceramic oxide, a ceramic carbide, a ceramic nitride, a ceramic boride, a metal silicide, a ceramic oxycarbide, a ceramic oxynitride, carbon (such as diamond) or the like. The ceramic oxide includes, for example, a metal oxide, a semiconductor oxide or a mixed oxide. The metal oxide includes, for example, a rare earth metal oxide, a refractory metal oxide, a refractory oxide or a reactive metal oxide. The semiconductor oxide includes, for example, an oxide of silicon. The mixed oxide includes, for example, a yttrium aluminum oxide, a yttrium iron oxide, a zirconium silicate, a calcia-stabilized zirconia, a ceria-stabilized zirconia, a magnesia-stabilized zirconia, a yttria-stabilized zirconia, mullite, a garnet, a metal titanate, a metal lanthanate, a metal zirconate or a metal silicate. The metal of the metal titanate, metal lanthanate, metal zirconate or metal silicate may include aluminum, magnesium or zirconium. The rare earth metal oxide includes, for example, an oxide containing lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium or yttrium. The refractory metal oxide includes, for example, an oxide containing zirconium, hafnium, chromium, molybdenum, niobium, rhenium, tantalum, tungsten or vanadium. The refractory oxide includes, for example, an oxide containing aluminum, magnesium or calcium.

The reactive metal oxide includes, for example, an oxide containing titanium, nickel, cobalt or iron. The ceramic carbide includes, for example, a metal carbide or a semiconductor carbide. The metal carbide includes, for example, a carbide containing chromium, niobium, hafnium, tantalum, titanium, molybdenum, boron or tungsten.

The semiconductor carbide includes, for example, a carbide containing silicon. The ceramic nitride includes, for example, a nitride containing aluminum, chromium, niobium, silicon, boron, zirconium or titanium. The ceramic boride includes, for example, a boride containing titanium diboride or zirconium diboride. The metal silicide includes, for example, a silicide containing chromium, molybdenum, tantalum, titanium, tungsten or zirconium. The nano-sized ceramic particles 12 may also include a commercially available tungsten carbide material modified through the addition of chrome, enhancing its erosion resistance, as is described in greater detail herein below.

The relatively hard, brittle nano-sized ceramic particles 12 are disposed within the relatively ductile oxidation-resistant matrix 14 using, for example, a milling apparatus, such as a mechanical alloying apparatus, a high-energy ball milling apparatus, a reactive ball milling apparatus or a cryomilling apparatus. In general, the binder powder and the ceramic particles are uniformly blended to form a particulate material which is thermally sprayed or brazed. Alternatively, the ceramic particles are suspended in a plating bath and electrolytically entrapped to form a composite coating by a tank or brush-plating process. The ceramic particles may also be suspended in an electroless plating bath and entrapped to form the composite coating. Finally, as described above, the binder powder and the ceramic particles may be compacted together to form one or more composite ingots and evaporated in an electron-beam physical vapor deposition process to form nano-particles in a binder matrix.

Deleterious interactions between the metallic alloy matrix and the nano-sized ceramic particles 12 are avoided by selecting metal-oxide combinations that are unlikely to result in run-away reactions. The metal-oxide combination is also selected such that it is immiscible in the metallic alloy matrix. Unlike carbides and nitrides, well-selected metal-oxide combinations are thermodynamically more stable than their metallic alloy counterparts and will not dissolve into the metallic alloy matrix, retaining the ductility of the metallic alloy matrix.

The key microstructural factors in obtaining improved wear resistance, erosion resistance, galling resistance and strength at elevated temperatures include the volume fraction of the ceramic phase, the grain size of the ceramic particles and the mean free path of the metallic alloy matrix. Specifically, wear resistance is a function of hardness (H) and fracture toughness (Kc). Hardness is a function of the mean free spacing between the ceramic particles (X-1/2) and fracture toughness is a function of the grain size of the ceramic particles and the mean free spacing between the ceramic particles (d/X). The relationship between d/k and fracture toughness for both the nano regime and the micron regime is illustrated in Figure 2. As the grain size of the ceramic particles (d) decreases, the stress concentration associated with the ceramic particle embedded in the metallic alloy matrix decreases, and is more readily accommodated by the tough, ductile matrix. Preferably, the metallic alloy matrix is free of embrittling dissolved phases.

Referring again to Figure 1, the volume percent of the ceramic phase preferably ranges from about 10% to about 95% and the size or diameter 16 of the nano-sized ceramic particles 12 ranges from about 5 nm to about 250 nm. As a result, the mean free spacing 18 between the dispersed nano-sized ceramic particles 12 is on the order of about 200 nm or less, preferably on the order of about 100 nm or less. This ensures high-temperature hardness of the resulting coating system 10. Adequate toughness is ensured by the nano-scale of the relatively hard, brittle nano-sized ceramic particles 12, limiting starting flaw sizes to a nano-scale while retaining the inherent ductility of the metallic alloy matrix.

In order to provide improved oxidation resistance with the coating system 10 of the present invention above about 1000 degrees C, the metallic alloy matrix is chosen from MCrAlY, L605, HS188, aluminides (Ni or Ti) or the like. For intermediate temperatures (below about 850 degrees C), the metallic alloy matrix is selected from Triballoy 800, NiCrBSi, Ni20% Cr5% Al, Ni20% Cr or the like. It will be readily apparent to those of ordinary skill in the art, however, that other suitable materials may be used.

The coating system 10 demonstrates both high hardness and high toughness, ensuring improved wear resistance. Because the dispersed nano-sized ceramic particles 12 have a relatively low chemical affinity for metallic counterfaces, the coating system 10 demonstrates improved galling resistance as well. This is especially true under high-pressure contact conditions if a suitable Co-based metallic alloy matrix or the like is used, such as a triballoy or L605. The nano-scale of the nano-sized ceramic particles 12 ensures that, when a high-pressure contact with the counterface material occurs, the relatively hard, brittle nano-sized ceramic particles 12 are unable to penetrate into the counterface material to cause extensive scoring. Thus, the nano- scale of the nano-sized ceramic particles 12 minimizes counterface wear.

Under erosion conditions, even coating systems including predominantly hard, brittle constituents can be made to behave in a ductile manner if the microstructural feature sizes of the hard, brittle constituents are substantially smaller than the associated impact crater. The ductile response, however, is heavily constrained due to the presence of the hard, brittle constituents. The erosion rate under such conditions decreases with a decrease in the mean free spacing between the hard, brittle constituents. Because of the nano-scale of the nano-sized ceramic particles 12 and the mean free spacing 18 of the present invention, improved erosion resistance results.

Referring to Figures 3 and 4, a plurality of relatively hard, brittle micron-sized ceramic particles 20 may also be disposed within the metallic alloy matrix, in a bimodal embodiment of the present invention, to provide improved wear resistance, erosion resistance, galling resistance and strength at relatively high and relatively low temperatures. The bimodal coating system 30 includes a brittle mode 32, experiencing only moderate wear at elevated temperatures, and a ductile mode 34, experiencing only low wear at elevated temperatures. Preferably, the micron-sized ceramic particles 20 have a size or diameter not exceeding about 1 micron. With regard to the brittle mode 32, any cracks present in the relatively hard, brittle micron- sized ceramic particles 20 are blunted by the relatively ductile metallic alloy matrix.

As described above, with regard to the ductile mode 34, cracks in the relatively hard, brittle nano-sized ceramic particles 12 are prevented and ductile deformation of the metallic alloy matrix is heavily constrained, resulting in improved wear resistance.

Specifically, the nano-sized ceramic particles 12 provide increased hardness at elevated temperatures by acting as an impediment to dislocation motion, thereby constraining the deformation of the metallic alloy matrix. Cracks are unlikely to nucleate because the brittle ceramic particles are nano-sized and are unlikely to propagate because the complex microstructure engineered makes propagation paths tortuous.

The micron-sized ceramic particles 20 may include aluminum oxide, zirconium oxide, yttrium oxide, a yttrium-based garnet, mullite, hafnia or a suitable combination thereof. In general, the micron-sized ceramic particles 20 may include a ceramic oxide, a ceramic carbide, a ceramic nitride, a ceramic boride, a metal silicide, a ceramic oxycarbide, a ceramic oxynitride, carbon (such as diamond) or the like. The ceramic oxide includes, for example, a metal oxide, a semiconductor oxide or a mixed oxide. The metal oxide includes, for example, a rare earth metal oxide, a refractory metal oxide, a refractory oxide or a reactive metal oxide. The semiconductor oxide includes, for example, an oxide of silicon. The mixed oxide includes, for example, a yttrium aluminum oxide, a yttrium iron oxide, a zirconium silicate, a calcia-stabilized zirconia, a ceria-stabilized zirconia, a magnesia-stabilized zirconia, a yttria-stabilized zirconia, a mullite, a garnet, a metal titanate, a metal lanthanate, a metal zirconate or a metal silicate. The metal of the metal titanate, metal lanthanate, metal zirconate or metal silicate may include aluminum, magnesium or zirconium. The rare earth metal oxide includes, for example, an oxide containing lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium or yttrium. The refractory metal oxide includes, for example, an oxide containing zirconium, hafnium, chromium, molybdenum, niobium, rhenium, tantalum, tungsten or vanadium. The refractory oxide includes, for example, an oxide containing aluminum, magnesium or calcium. The reactive metal oxide includes, for example, an oxide containing titanium, nickel, cobalt or iron. The ceramic carbide includes, for example, a metal carbide or a semiconductor carbide. The metal carbide includes, for example, a carbide containing chromium, niobium, hafnium, tantalum, titanium, molybdenum, boron or tungsten. The semiconductor carbide includes, for example, a carbide containing silicon. The ceramic nitride includes, for example, a nitride containing aluminum, chromium, niobium, silicon, boron, zirconium or titanium. The ceramic boride includes, for example, a boride containing titanium diboride or zirconium diboride. The metal silicide includes, for example, a silicide containing chromium, molybdenum, tantalum, titanium, tungsten or zirconium.

The nano-structured coating systems 10,30 of the present invention are deposited on a component, consisting of a substrate material, via thermal spraying, composite plating (electro or electroless), brush composite plating, electron beam-physical vapor deposition, spray forming, mechanical alloying followed by powder compaction, blending with a braze alloy and applying by a braze process, spraying and fusing, laser remelting or any other conventional process. In the case that a very thick coating is required, the metal-oxide combination of the present invention is applied by a spray and fuse process and then subsequently reheated to fuse the metallic alloy matrix. The coating system of the present invention may also be synthesized into a thermally sprayable grade through agglomeration and sintering or mechanical alloying followed by agglomeration to desired sizes. The partial pressure of oxygen is adjusted during powder synthesis to engineer enhanced metal-oxide adhesion.

Suitable thermal spraying apparatuses/processes include, HVOF and HVAF processes, wire arc processes, air plasma processes, low-pressure plasma processes and the like.

Engineered components that utilize the nano-structured coating systems 10,30 of the present invention include those associated with gas turbines, hydroelectric turbines, aircraft engines, internal combustion engines and the like. For example, the engineered component may include a guide vane, a facing plate, a needle valve or seat, a shaft, a bearing seal, a compressor blade, a seal, a runner, an impeller vane in a centrifugal pump, an impeller vane in a centrifugal compressor, interstage seals, a seal in a piston rod, a piston ring, a fan blade, a compressor, abrasion tips in a steam turbine, abrasion tips in a gas turbine, a labyrinth seal, a gas path seal in a disc, an exhaust valve in an internal combustion engine, a brush seal, a turbocharge impeller in an internal combustion engine, a connecting rod in an internal combustion engine, an interstage lip seal, moving parts in a combustor assembly, an HPT rear stage blade coating, an LPT blade coating, a z-notch interlock, an after burner support, an exhaust flap, a cutting tool, and the like.

Referring to Figure 5, with regard to relatively low-temperature hydroelectric turbine applications, thermally-sprayed tungsten carbide coating systems are used to protect components from wear, erosion and abrasion. However, such damage typically occurs by a combination of solid particle erosion, corrosion, solid particle impact damage and cavitation. In order to combat the combined effects of erosion and corrosion, chromium is often added to a cobalt binder which contains a plurality of micron-scale WC grains. The low-temperature nano-structured coating system 40 of the present invention, however, improves the erosion resistance and toughness associated with these conventional low-temperature coating systems, while preserving their corrosion resistance.

As described above, the low-temperature nano-structured coating system 40 of the present invention includes a plurality of nano-sized WC grains 42 and, optionally, micron-sized WC grains 44 disposed within a corrosion-resistant CoCr binder or matrix 46. The use of the nano-sized WC grains 42 ensures that micro-cracking of the WC grains is avoided and that the mean distance 48 between the WC grains is reduced, improving the erosion resistance of the coating system 40. The use of the micron-sized WC grains 44 improves the erosion resistance of the coating system 40 at relatively shallow angles and makes propagation paths in the CoCr binder 46 tortuous. The use of the nano-sized WC grains 42 with a relatively small mean free distance 48 improves the overall toughness of the coating system 40, allowing for a reduced CoCr content. Using nano-sized WC grains 42 with a relatively small mean free distance 48 also improves the toughness of the CoCr binder 46 in situations where the CoCr binder 46 includes dissolved phases that make the CoCr binder 46 less ductile. Alternatively, the CoCr binder 46 may be replaced with other metallic elements, such as nickel, stainless steel, stainless steel alloyed with cobalt, an iron- based alloy, an amorphous material and/or a shape memory alloy that absorbs impact energy through a martensitic transformation. Likewise, the nano-sized WC grains 42 may be replaced with titanium carbide, titanium diboride, titanium alloy nitride, boron carbide, cubic boron nitride, silicon carbide, silicon nitride, diamond or an oxide, such as A1203 or the like. These alternative particles may be harder and/or tougher than WC, and may be lighter. Other possible combinations include matrix alloys that have nano-particles that precipitate from the bulk matrix alloy material, such as gamma prime or a carbide. Optionally, these nano-particles precipitate during the hot coating process from such materials as an amorphous alloy starting material. The amorphous material provides a strong matrix material due to its lack of dislocations and grain boundaries and may be used to create fine nano-sized grains for dislocation disruption and improved strength.

The typical CoCr content of commercially available WC-CoCr thermally-sprayed powders is about 1 Owt% Co and about 4wt% Cr. The CoCr content of the coating system 40 of the present invention is in the range of about 6wt% (about 4wt% Co and about 2 wt% Cr) to about l4wt% (up to about Swt% Cr, with the balance of up to about 9wt% Co). Typically, coating systems with a smaller metallic binder content have better erosion resistance at relatively shallow angles of impact, but are relatively brittle and demonstrate poor erosion resistance at relatively steep angles of impact, as well as poor fracture toughness. The use of the nano-sized WC grains 42, and a lower CoCr content, reduces the mean free distance between the WC grains, increasing the inherent toughness of the coating system 40 and improving its erosion resistance.

Preferably, the nano-sized WC grains 42 have a size or diameter 50 in the range of about 10 nm to about 250 nm.

Optionally, the use of the micron-sized WC grains 44, which preferably have a size or diameter in the range of about 0.5 microns to about 2 microns, allows the coating system 40 to resist erosion at relatively shallow angles of impact and, in the event that a microcrack is formed in the CoCr binder 46, its propagation is made tortuous as the crack must deflect around the micron-sized WC grains 44. Preferably, the mean distance 48 between WC grains is between about 50 nm and about 500 nm, more preferably between about 50 nm and about 250 mn. Preferably, the overall volume percent of WC grains is between about 5% and about 95%, more preferably between about 50% and about 95% (with about 70% of the overall volume percent including the nano-sized WC grains 42 and about 30% of the overall volume percent including the micron-sized WC grains 44). If nickel, stainless steel and/or stainless steel alloyed with cobalt are used, their overall volume percent is preferably less than about 20%.

The Cr in the CoCr binder 46 serves two important purposes. First, the Cr improves the overall corrosion resistance of the coating system 40. Second, the Cr limits the dissolution of primary WC during the spraying process and ensures the higher retention of the primary WC phase, enhancing the erosion resistance of the coating system 40.

The low-temperature nano-structured coating system 40 of the present invention is deposited on a component, consisting of a substrate material, using any conventional thermal spraying process, preferably an HVOF or HVAF process, or cold spray process. The substrate material includes, for example, an Mg, Al, Cu, Fe, Ni or Co- based alloy. A key consideration associated with the thermal spraying processes is the optimization of spraying conditions to limit the dissolution of WC during the deposition process, ensure adequate retention of the WC phase and reduce the formation of eta phase in the binder. The coating system 40 may also be deposited on a component by blending the WC-CoCr powder with a braze alloy matrix and brazing the blend onto a surface by a braze tape process or by slurry coating and firing. The WC-CoCr powder is blended with a composition such as NiCrBSi, deposited by a combustion-sprayed torch and subsequently fused, or it is blended with a lower melting-point flux or any other braze alloy and melted using a laser or the like. The ratio of the braze alloy and the WC-CoCr powder is optimized to ensure that the blends demonstrate adequate erosion resistance.

Although the present invention has been illustrated and described with reference to preferred embodiments and examples thereof, it will be readily apparent to those of ordinary skill in the art that other embodiments and examples may perform similar functions and/or achieve similar results. All such equivalent embodiments and examples are within the spirit and scope of the present invention and are intended to be covered by the following claims.