Monolithic Materials

Metals. Tungsten, molybdenum, rhenium, niobium, superalloys (nickel, iron, and cobalt base), zinc, aluminum, bronze, cast iron, mild and stainless steels, NiCr and NiCrAl alloys, cobalt-base Stellites, cobalt/nickel-base Tribaloys, and NiCrBSi "selffluxing" Colmonoy have all been successfully thermal spray consolidated either as coatings or structural deposits. Recently Tribolite (FeCrNiBSi) and AmaCor (amorphous) alloys have also been developed for spraying and exhibit excellent wear and corrosion resistance (Ref 13). Monolithic alloys have advantages due to their similarity to many base metals requiring repair, their high strength, and their corrosion, wear and/or oxidation resistance. Applications include automotive/diesel engine cylinder coatings; piston rings or valve stems; turbine engine blades, vanes, and combustors; protection of bridges and other corrosion-prone in frastructure; petrochemical pumps and valves; and mining and agricultural equipment. Except in the case of controlled-atmosphere spraying (VPS, inert chamber, and shrouded jets) thermally spraying these metals and alloys produces microreinforced composites of monolithic alloys due to their varying levels of oxide inclusions. Figure 2 shows a range of microstructures for thermally sprayed monolithic metals. These coatings exhibit characteristic lamellar microstructures with the long axis of impacted splats oriented parallel to the substrate surface, together with a distribution of similarly oriented oxides. Oxide content varies from relatively thick layers to finely distributed, particulate, intersplat phases, depending on whether the coatings are wire arc, plasma, or HVOF sprayed. The progressive increases in particle velocity of these processes leads to differing levels of oxide, and differing degrees of oxide breakup on impact at the surface. Oxides may increase coating hardness and may also provide lubricity. Conversely, excessive and continuous oxide networks can lead to cohesive failure of a coating and contribute to excessive wear debris. It is thus important, when selecting materials, coating processes, and processing parameters, that oxide content and structure be controlled to acceptable levels.

Thermal spray coatings may, depending on the spray process, particle velocity and size/size distribution, and spray distance, also contain varying levels of porosity and unmelted particles. High levels of porosity may lead to early deposit failure owing to poor intersplat cohesion. Conversely, low levels of porosity (<5%) may be beneficial in tribological applications through retention of lubricating oil films. Lamellar oxide layers can also lead to lower wear and friction due to the lubricity of the oxide. The porosity of thermal spray coatings is typically <5% by volume. The retention of some unmelted and/or resolidified particles can lead to lower deposit cohesive strength, especially in the case of "as-sprayed" materials with no postdeposition heat treatment or fusion.

Other key features of thermally sprayed deposits are their generally very fine grain structures and microcolumnar orientation. Thermally sprayed metals, for example, have reported grain sizes of <1 /'m prior to postdeposition heat treatment. Grain structure across an individual splat thus normally ranges from 10 to 50 i'm. with typical grain diameters of 0.25 to 0.5 i'in. due to the high cooling rates achieved (--1 ()' K/s) (Ref 1). Such rapid cooling rates, known to form fine-grained martensitic microstructures in steels, contribute to the high strengths exhibited by thermally sprayed materials. The "as-sprayed" microstructure of a typical metallic coating is shown schematically in Fig. 5.

Fig. 5 Schematic microstructure of an "as sprayed" thermally sprayed metal deposit

The tensile strengths of "as-sprayed" deposits can range from 10 to 60% of those of the fully cast or wrought material, depending on the spray process used. Spray conditions leading to higher oxide levels and lower deposit densities result in the lowest strengths. Controlled-atmosphere spraying leads to —60% strength, but requires postdeposition heat treatment to achieve near 100% values. Low "as-sprayed" strengths are related to limited intersplat diffusion and limited grain recrystallization during the rapid solidification characteristic of thermal spray processes. The microstructures of thermally sprayed metals are typically very uniform and exhibit excellent tensile properties. After heat treatment at >0.8 Tm (melt temperature), much of the characteristic lamellar structure recrystallizes, but depending on the alloy, grain growth may be limited to <100 /'m. The fine-grained structures, found in highly alloyed, grain-growth-stabilized superalloys, have been found to improve thermal fatigue properties, but to increase creep rates. After postdeposition heat treatment (vacuum or hot isostatic pressing), the high-temperature creep properties of these alloys have been found to be lower than cast alloys due to the fine grain sizes and the oxygen interstitials. In this reported work, final grain size in heat-treated microstructures was found to be substantially retained, due to very stable oxide and/or carbide networks formed during solidification after spraying, many times originating from the alloy feedstock powders. Final deposit ductilities were also lowered by the retention of relatively high oxygen contents originating from the surfaces of the original powders (Ref 14).

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Fig. 5 Schematic microstructure of an "as sprayed" thermally sprayed metal deposit

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On the beneficial side, the addition of nitrogen to steel matrices by promoting, rather than minimizing, atmospheric interactions during air plasma spraying has been shown to increase the strength of thermally sprayed steels and is a viable strengthening mechanism for selected monolithic alloys (Ref 15). Grain nucleation is suppressed in alloys containing high levels of silicon and boron, yielding so-called "amorphous" coatings (Ref 16). These "micrograined" or amorphous microstructures contribute to the high strengths and toughnesses of thermally sprayed metals at low to intermediate temperatures. Fatigue failure is also harder to propagate in such structures, except through coating defects such as oxide inclusions. The role of grain size effects on the wear performance of thermally sprayed coatings is understood, but has not been independently measured; hence it is hard, for example, to determine its contribution to sliding-wear performance.

Ceramics. Oxides such as Al2O3, ZrO2 (stabilized with MgO, CeO, Y2O3, etc.),TiO2, Cr2O3, and MgO; carbides such as Cr3C2, TiC, Mo2C and SiC (generally in a supporting metal matrix) and diamond; nitrides such as TiN and Si3N4, and spinels or perovskites such as mullite and superconducting oxides, have all been thermally spray deposited. Sprayed deposits of these materials are used to provide wear resistance (Al2O3, Cr2O3, TiO2, Cr3C2, TiC, Mo2C, TiN, and diamond), thermal protection (Al2O3, ZrO2, MgO), electrical insulation (Al2O3, TiO2, MgO), and corrosion resistance. With the exception of SiC (which sublimes), diamond, and SiC or Si3N4 (which must be in a metallic binder), ceramics are particularly suited to thermal spraying, with plasma spraying being most suitable due to its high jet temperatures.

The processing and materials flexibility and high temperatures gives plasma spraying a leading role in the spraying of thermal barrier coatings (TBCs), although the use of HVOF is also being investigated. Thermal barrier coatings are, after wear and corrosion coatings, likely to be one of the largest growth markets for thermal spray, with increased use in the automotive, metalworking, and chemical industries. Such broad usage, however, will require a more thorough understanding of the behavior of these materials during high-temperature service. Thermal spray processing of ceramic materials that melt, rather than decompose, is essentially the same as for metals; however, the higher mean melting temperatures and low thermal conductivity of ceramics limits the selections of thermal spray processes that can successfully melt these materials. Combustion spray methods generally have insufficient process jet enthalpies and/or particle dwell times to efficiently spray ceramic powders. Most HVOF systems are also limited in their efficient spraying of ceramics, where the powder sizes required are either too small or deposit efficiencies are too low for the processes to be economically viable. Wire arc spray is excluded because it needs conductive materials, leaving only plasma spraying as an economic method for spraying ceramic powders. There are, however, some specialized flame spray techniques such as the "Rokide" process (Norton Company), which uses a combustion jet to melt the tips off pressed-and-sintered ceramic rods and a secondary gas jet that atomizes the molten material from the melting rod tip. Air plasma spraying is, however, most widely used to deposit ceramics, finding applications in TBCs, wear coatings on printing rolls (Cr2O3), and for electrical insulators (Al2O3).

The microstructures of sprayed ceramics are similar to those of metals, with two important exceptions: grain orientation and microcracking. Rapid solidification of small droplets (generally <50 /'m) results in very fine grains as in metals; however, owing to the low thermal conductivity of ceramics, multiple grains may exist though a splat thickness, whereas in metals a single grain may cross an entire splat and even grow into an adjacent one. Intersplat and intrasplat microcracking is widespread in ceramic coatings, resulting from the accumulation of highly localized, residual cooling stresses. Figure 6 illustrates the typical microstructure of a thermally sprayed yttria-stabilized zirconia ceramic coating, showing fine grains, characteristic lamellar structure, and a network of porosity and microcracks.

Fig. 6 Typical microstructure of a thermally (plasma) sprayed ceramic (yttria-stabilized zirconia)

Splats normally exhibit through-thickness cracking owing to the very low ductility of most ceramics, but these cracks do not usually link up through the whole deposit thickness, at least not until an external stress is applied. Microcracking of splats is a major contributor to the effectiveness of TBCs, even under high-temperature gradients and moderate strains, conditions under which conventionally formed bulk ceramics would fail.

Intermetallics. Usually produced from powders due to their intrinsically low ductility, intermetallics are generally consolidated either by pressing and sintering or hot isostatic pressing. Over the last 15 years researchers have reported on the thermal spray consolidation and forming of intermetallic powders. The high heating and cooling rates of the thermal spray process reduce the segregation and residual stresses that ordinarily limit the formability of these brittle materials. Thermal spray processes are also able to deposit materials onto mandrels, building up thin layers of material and thus forming near-net shapes and providing the opportunity for "engineered microstructures" and functionally graded structures. Researchers have also reported on the plasma spraying of TiAl, Ti3Al, Ni3Al, NiAl, and MoSi2 with excellent deposit characteristics and properties. Improved ductilities have been obtained, with tensile strengths equal to, or better than, those of materials consolidated by other powder processing techniques.

Thermal spray processing of intermetallics, unlike ceramics, is generally not an application for plasma spraying. Plasma spray in controlled atmospheres is, however, the method of choice for the production of bulk deposits. High-velocity oxygen fuel and other combustion spray techniques are normally only used to spray compatible intermetallics, such as semiconducting, insulating, or corrosion- and wear-resistant coatings. Most intermetallics are very reactive at high temperatures and very sensitive to oxidation, hence the preference for using inert atmosphere plasma spray. Plasma spray forming is also well suited for the net-shape forming of brittle intermetallics. Investigators have found that plasma spray forming can actually increase the ductility of intermetallics such as MoSi2 (Ref 17) and NiAl/Ni3Al (Ref 18). These improvements were linked to a decrease in grain boundary and/or splat interface contaminants that limit localized plastic flow under an applied strain and lead to early crack linking, thus lowering the overall plastic flow, measured as ductility. It has been shown, for example, that reactive plasma species reacting at the surfaces of MoSi2 powder particles in flight actually reduce the residual SiO2 content below that of commercial MoSi2 powders and hence reduce the SiO2 "pest" reaction that degrades the properties of pressed-and-sintered or hot isostatically pressed MoSi2.

The as-sprayed microstructures of thermally sprayed intermetallics are very similar to those of metals. Figure 7 shows the microstructure of thermal spray consolidated MoSi2, showing the characteristic lamellar structure and fine grains. Intermetallics are generally somewhat more porous than sprayed metals owing to their limited ductility and low plasticity, which translates into a narrower "processing window" of plasma spray parameters for the production of high-density intermetallics, requiring tighter process control than for spraying metals. Higher-velocity plasma spray processes are preferred because the increased particle velocities result in more complete deformation of individual splats and denser deposits overall.

Fig. 7 Microstructure of a plasma spray consolidated intermetallic material (MoSi2)

Polymers. Polymeric materials can also be successfully thermally sprayed, provided they are available in particulate form. Thermal spraying of polymers has been commercially practiced for 20 years, and a growing number of thermoplastic polymers and copolymers have now been sprayed, including urethanes, ethylene vinyl alcohols (EVAs), nylon 11, polytetrafluoroethylene (PTFE), polyetheretherketone (PEEK), polymethylmethacrylate (PMMA), polyimide, and copolymers such as polyimide/polyamide, Surlyn, and Nucryl (Du Pont), and polyvinylidene fluoride (PVDF). Conventional flame spray and HVOF are the most widely used thermal spray consolidation methods used to date (Ref 19), although use of plasma spray has also been reported. Figure 8 shows a typical dense, well-bonded, HVOF-sprayed nylon coating. Consolidation of polymers to full densities and high cohesive strengths, unlike metals, relies to some extent on continued heat input to the polymer layers after deposition onto the substrate. This may be due to the more complex, long-range-ordered nature of the molecular bonding in polymers, compared to the simpler, short-range-order bonding in metals. Thermal spray, a high-temperature, rapid-heating/rapid-solidification process, has been found to decompose polymeric materials, lowering the molecular weight and producing "charring." In many cases, however, this does not impair the functionality of the sprayed coating, although it does change certain polymer properties compared to polymers consolidated by conventional means. Despite thermal spraying of polymers having been practiced for more than 20 years, little research into the material structures and properties has been carried out until recently. Commercial interest in this application is growing, for applications such as aqueous corrosion protection and as a solventless processing alternative to environmentally hazardous volatile organic compound (VOC)-based techniques.

Fig. 8 Microstructure of an HVOF-sprayed nylon coating Composite Materials

Thermal spraying, either as a coating or as a bulk structural consolidation process, has clearly demonstrated advantages for the production of composites. Difficult-to-process composites can be readily produced by thermal spray forming, with vacuum plasma spray being the process of choice for the most reactive matrix materials. Particulate-, fiber-, and whisker-reinforced composites have all been produced and used in various applications. Particulate-reinforced wear-resistant coatings such as WC/Co, Cr3C2/NiCr, and TiC/NiCr are the most common applications and comprise one of the largest single thermal spray application areas. Figure 9 shows schematically the diverse forms of composites that can be thermally spray formed.

Fig. 9 Schematic microstructures of possible thermally spray-formed composites. (a) Deposit with a particulate-reinforced second phase. (b) Deposit with a whisker-reinforced second phase. (c) Deposit with a continuous-fiber-reinforced second phase

Whiskers of particles can be incorporated using so-called "engineered" powders, mechanical blending, and by coinjecting different materials into a single spray jet. Mechanical blends and coinjection, although useful, have been found to result in segregation of the reinforcing phase and, in many cases, degradation of the second-phase whiskers or particles. Thermal spray composite materials can have reinforcing-phase contents ranging from 10 to 90% by volume, where the metal matrix acts as a binder, supporting the reinforcing phase. The ability to consolidate such fine-grained, high reinforcing phase content materials is a major advantage of thermal spray over other methods used to produce composites.

Thermal spraying of composite materials with discontinuous reinforcements, such as particulates or short fibers, is usually accomplished by spraying powders or powder blends. Investigators have developed techniques for the production of continuous fiber-reinforced materials that overcome the "line-of-sight" limitations of thermal spray processes. This includes "monotape" fabrication techniques, where continuous fibers are prewrapped around a mandrel and a thin layer of a metal, ceramic, or intermetallic matrix material is sprayed (Ref 20). Plasma, HVOF, and wire arc spray have been used, although in the cases of intermetallics and high-temperature alloys, controlled atmosphere plasma spray (VPS) has generally been used. The fibers are thus encapsulated within thin monolayer tapes and are subsequently removed for consolidation to full density by hot pressing with preferred fiber orientations, producing continuously reinforced bulk composites.

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