Volume 07 - Powder Metal Technologies and Applications Part 7 ppsx

Fig. 4 Major thermal spray processes Combustion spray burns mixtures of fuels such as acetylene and oxidizing agents (air or oxygen) that are used to subsequently heat and accelerate particles of material injected into the hot expanding gas jets. Combustion spraying has, during the last 15 years, been subdivided into two distinct categories: (a) Flame spray, which uses low pressure (70 to 140 kPa, or 10 to 20 psi) fuel/oxygen mixtures, burning unrestricted and external to the spray gun itself in ambient air, and (b) HVOF spraying, which utilizes internal combustion at higher gas flow rates and pressures (up to 1.5 MPa, or several hundred psi) generating supersonic jet exit velocities. The powder particles are also injected internally in HVOF, such that the heat and momentum transfer to the particles, and hence particle velocity, are substantially higher (>1000 m/s) than in conventional flame spray. Wire arc spray utilizes two electrically conducting wires as feedstock, rather than powders. A continuous direct current (dc) arc melts the tips of the advancing wires and a high flow (typically >1500 slm) gas jet atomizes the molten material and accelerates the resulting small droplets toward the substrate being coated. A typical wire arc spray gun consists of a nozzle that directs a cool, high-velocity air, nitrogen or, in special cases argon, gas jet at the wire tips and a feedback- controlled wire-feed mechanism that continuously positions and advances the wires. Melt rate and interelectrode voltage determine the arc gap and size distribution of the atomized droplets leaving the tips of the wires and hence affect the final coating microstructure. Because cool gas jets are used to atomize the molten tips of feed wires in wire arc spray, the components being coated are not heated as much as in other thermal spray processes, an advantage when thermally sensitive substrate materials are being coated. Even thin sheets of paper can be coated by the wire arc process. Wire arc spraying is, however, limited to electrically conductive materials (metals) as feedstock, but has the lowest processing temperatures (at the substrate) of all the processes. Particle velocities vary, but are generally higher than in conventional flame spray and lower than HVOF or plasma spray. Plasma spraying (see the section that follows) has the highest particle-heating potential, but its particle velocities, although higher than conventional flame and wire arc spray, are generally lower than those of HVOF spraying. In other cases, accelerating gas cap extensions have been added to conventional wire arc spray systems to increase particle velocity and produce finer droplets, both leading to increased deposit densities, and reduced differences between the microstructures and properties of wire arc sprayed deposits and those produced by other thermal spray processes. Plasma spray can be broken down into three major categories: (a) air plasma spray (APS), (b) "vacuum" plasma spray (VPS) or low-pressure plasma spray (LPPS), and (c) controlled atmosphere plasma spray (CAPS), which also includes inert gas shrouded plasma jets, depending on the environment used. These distinctions affect the level of interaction between the process jet and materials being sprayed with the surrounding atmosphere and thus control the microstructure and properties of the sprayed materials. All current commercially significant plasma spray processes use a nontransferred (i.e., no arc is established to the workpiece or substrate) dc electric arc to heat gases to peak temperatures in excess of 25 × 10 3 K, producing jets of ionized "plasma" with temperature ranges of 3 × 10 3 to 15 × 10 3 K. At these temperatures, the plasma gases (Ar, H 2 , He, or N 2 ) are dissociated and ionized into an equilibrium mixture of positive ions and electrons as energy is pumped into them by the confined arc discharge. The plasma state gives the process both its name and its ability to melt any material exhibiting a stable melting point. A typical plasma spray device consists of a cylindrical, thoriated tungsten cathode, which emits electrons when heated by an electric arc, located concentrically and coaxially inside a cylindrical water-cooled copper nozzle. Electrons emitted from the cathode flow to the nozzle anode wall under the influence of the applied electric field, typically 30 to 80 V. Gases injected into this interelectrode region are heated, dissociated, and ionized by the electric arc and subsequently expanded to atmosphere through the nozzle as a subsonic or supersonic plasma jet, depending on the nozzle design and pressure ratio. The gas velocity may, in some cases, exceed 1.5 × 10 3 m/s, depending on the power input, nozzle design, gas composition and flow rate. Powders are injected into the plasma jet as fluidized streams of "carrier" gas and powder, either externally, by means of a tube that directs the stream of powder particles radially into the hot gas jet immediately after it exits the nozzle, or internally through powder feed ports through the nozzle wall itself. Internal injection produces longer particle heating or "dwell" times and improves melting efficiency, enabling higher melting temperature materials to be sprayed. Plasma spray devices generally use argon or mixtures of argon plus helium, hydrogen (H 2 ), or sometimes nitrogen (N 2 ) to generate the hot plasma jets used for spraying. Coatings sprayed at low pressures (VPS), in inert atmospheres in an enclosed pressure vessel, or using an inert gas shroud, have very low oxide inclusions and porosity levels. This is achieved by eliminating interaction between the molten particles and oxygen being entrained into the jets from the surrounding air. The sprayed coatings generally exhibit low oxide content, such that in many cases VPS materials have the same, or better, properties than cast materials. This characteristic has enabled VPS processes to be used to spray form net shapes of "difficult-to-process" materials, including ceramics, refractory metals, intermetallics, and composites. Inert gas shrouded plasma spray has the potential of achieving vacuum plasma quality coatings under air plasma spray conditions, thereby lowering the total processing cost. Several effective inert gas shroud designs have been developed and reported, including porous metal nozzle inserts (Ref 11) and discrete, multijet, parallel flow designs located downstream of the nozzle exit on a conventional plasma spray gun (Ref 12 ). As stated above, the structures and properties of sprayed materials can vary widely, depending on the process used, classes, and the processing conditions. The different processes each result in different thermal and velocity histories for the injected particles. Particle melting and deformation are thus different in each thermal spray class, leading to the differences observed in the performance of spray deposited materials. Key features of the different processes are as follows: • Conventional flame spray has the lowest heating potentia l and lowest mean particle velocity of all thermal spray processes, but is nevertheless used in many applications. • High- velocity oxygen fuel processes typically have the highest particle velocity, but its gas jet temperatures limit the maximum attainable particle temperatures. • Wire arc spray has lower processing temperatures and produces intermediate particle velocities, but owing to its low gas temperatures can be used to coat thermally sensitive substrates. • Plasma spray, with the highest processing t emperatures, is most flexible with respect to materials selection, and because of its ability to operate under inert gas conditions, is commonly chosen for consolidating oxidation-sensitive and reactive materials. Thermal spray has been defined as a particulate/droplet consolidation mechanism capable of processing virtually any material and most materials combinations. Despite the widespread use of thermal spray to produce coatings, and, to a lesser extent for spray forming, thermal spray has not been widely used for the synthesis and production of advanced materials. Developments in thermal spray over the last 25 years, including controlled atmosphere plasma spraying, HVOF, reactive plasma spraying, improved wire arc spray methods, new material and powder production methods, and particularly improved process controls have, however, resulted in many new opportunities for the technology in the area of powder consolidation. The range of materials that can be sprayed, their possible structures, and the application of thermal spray processing are limited only by the imagination of the users and the economic viability of the application. Thermal spray can process many materials, in many forms, to produce deposits on many substrate materials, thus making thermal spray an ideal technique for processing many advanced materials and structures. Many challenges, however, still remain: • Cost/economics • Compositional and structural control • Deposition rate • Control of deposit properties Thermal spray is now, owing to recent advances in materials, powder production techniques, process understanding, and control, at the leading edge of novel powder consolidation methods. Some recent advances are reviewed below that present the current state of the technology for implementation as a viable powder consolidation process. References cited in this section 10. R.W. Smith, Equipment and Theory, Lesson from Thermal Spray Technology, Course 51, Materials Engineering Institute, ASM International, 1992 11. H.C. Chen, Z. Duan, J.V.R. Heberlein, and E. Pfender, Influence of Shroud Gas Flow and Swirl Magnitude on Arc Jet Stability and Coating Quality in Plasma Spray, Proc. Ninth National Thermal Spray Conf. (Cincinnati, OH), ASM International, Oct 1996, p 553-561 12. M. Mohanty, R.W. S mith, R. Knight, W.L.T. Chen, and J.V.R. Heberlein, Shrouded Air Plasma Processing of Lightweight Coatings, Proc. Ninth National Thermal Spray Conf. (Cincinnati, OH), ASM International, Oct 1996, p 967 Thermal Spray Forming of Materials Richard Knight and Ronald W. Smith, Center for the Plasma Processing of Materials (CPPM), Drexel University, Philadelphia, PA Materials for Thermal Spray Thermal spray is capable of processing materials with jet temperatures ranging from 500 to 15 × 10 3 K, which enables virtually any material, or combination of materials, to be processed. Because only small volume powder particles are heated in a given time and because, on solidification, only small volumes, typically 5 to 100 m in diameter, are being cooled, segregation is not a limiting factor. On the other hand, residual stress can have detrimental effects on the properties and performance of sprayed coatings, its significance varying from process to process and material to material. Also, because materials are deposited in relatively thin (12 to 50 m, or 0.0005 to 0.002 in.) layers, many unique combinations of materials can be produced. Three characteristic types of deposit can be sprayed: (a) Monolithic materials, such as metals, alloys, intermetallics, ceramics, and polymers; (b) composite particles such as cermets (WC/Co, Cr 3 C 2 /NiCr, NiCrAlY/Al 2 O 3 , etc.), reinforced metals, and reinforced polymers; and (c) layered or graded materials. Some examples of these, and their particular advantages and associated technical challenges, are described below. 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 m, with typical grain diameters of 0.25 to 0.5 m, due to the high cooling rates achieved ( 10 6 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 T m (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). 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 Al 2 O 3 , ZrO 2 (stabilized with MgO, CeO, Y 2 O 3 , etc.),TiO 2 , Cr 2 O 3 , and MgO; carbides such as Cr 3 C 2 , TiC, Mo 2 C and SiC (generally in a supporting metal matrix) and diamond; nitrides such as TiN and Si 3 N 4 , 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 (Al 2 O 3 , Cr 2 O 3 , TiO 2 , Cr 3 C 2 , TiC, Mo 2 C, TiN, and diamond), thermal protection (Al 2 O 3 , ZrO 2 , MgO), electrical insulation (Al 2 O 3 , TiO 2 , MgO), and corrosion resistance. With the exception of SiC (which sublimes), diamond, and SiC or Si 3 N 4 (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 (Cr 2 O 3 ), and for electrical insulators (Al 2 O 3 ). 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, Ti 3 Al, Ni 3 Al, NiAl, and MoSi 2 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 MoSi 2 (Ref 17) and NiAl/Ni 3 Al (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 MoSi 2 powder particles in flight actually reduce the residual SiO 2 content below that of commercial MoSi 2 powders and hence reduce the SiO 2 "pest" reaction that degrades the properties of pressed-and-sintered or hot isostatically pressed MoSi 2 . The as-sprayed microstructures of thermally sprayed intermetallics are very similar to those of metals. Figure 7 shows the microstructure of thermal spray consolidated MoSi 2 , 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 (MoSi 2 ) 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, Cr 3 C 2 /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. Powders for Sprayed Composites Discontinuously reinforced composites produced using thermal spray techniques use either composite powders or direct reactive synthesis approaches, as described below. Powders can be produced mechanically, chemically, thermomechanically, or by using high-temperature synthesis. "Engineered powders" defines powders in which different phases are incorporated to produce a "microcomposition" of the final desired structure. Typically these powders contain the desired sizes, size distributions, and morphologies of the equilibrium phases. These powders also permit the introduction of higher concentrations of phases than those normally achievable through conventional melt or reaction processing. Figure 10 shows two types of powder that could be produced in this way: short ceramic fibers within a metal matrix and an intermetallic-matrix material reinforced with more ductile phases. The latter has been found to be a viable approach for increasing the fracture toughness (K Ic ) of intermetallics. Varying reinforcing phase combinations, compositions, morphologies, and distributions can be produced. The rapid heating and cooling experienced by these powders during thermal spray forming limits dissolution and degradation of the phases, which remain relatively unchanged after consolidation, although some microstructural refinement and solutioning can take place. Generally, more significant changes occur during conventional powder consolidation processes because of the longer processing times. Fig. 10 Schematic representations of typical "composite" thermal spray powders Mechanical Blends. Blended powders are produced by mixing the required proportions of a binder phase (a metal, an intermetallic, or a ceramic) together with a reinforcing phase. The matrix and reinforcing phases may segregate over time during shipping, handling, and feeding into a thermal spray process and often yield poor "as-sprayed" deposit uniformity. Many mechanical blends can, however, be "agglomerated" and fixed using an organic binder, which reduces their sensitivity to handling. Spherical agglomerates are known to flow and feed better, and spray drying has been used to produce such materials. Agglomerated powders have enabled thermally sprayed deposits with improved uniformity to be produced, although the high viscosity, shear, and thermal forces acting on injected agglomerated particles tend to break the agglomerates, leading to segregation and direct exposure of the second phase to the thermal spray jet. Physically and thermally stable agglomerates are thus desirable. Agglomeration, Sintering, and Melt Densification. The mechanical stability of agglomerated composite particles can be improved by agglomeration and sintering, which is a solid or rapid "matrix" melting and cooling process. Metal- matrix/ceramic hard-phase-reinforced powders (e.g., WC/Co, Cr 3 C 2 /NiCr, NiCrAlY/Al 2 O 3 , etc.) can be sintered in the solid state by heating in a protective atmosphere furnace, sometimes using a fluidized bed, followed by a gentle milling to break any weak interagglomerate bonds. Agglomerated and sintered powders retain a level of porosity, typically 5 to 10% by volume, depending on the reinforcing phase/matrix combination, the sintering time, and temperature. Retained porosity, although not generally a source of phase segregation in thermally sprayed deposits, can lead to higher as-sprayed porosity and increases the spray jet melting enthalpies required to melt the powders because the thermal conductivity of the individual particles is lowered by the porosity. Melting of metallic binding phases and retention of spherical particle morphologies can be achieved by processing the powders through a thermal plasma jet. Known as "plasma densification," this process produces spherical powders exhibiting near 100% particle density and uniform distributions of reinforcing phases. Plasma-densified powders result in the most uniform thermal spray-consolidated deposits, but at significantly increased cost owing to the additional handling, processing steps, and lower process yields. Self-propagating high-temperature synthesis (SHS) is a composite powder production method now finding increasing application in thermal spray forming. The SHS process exploits the high heats of formation released from exothermic reactants which, when mixed and ignited, produce a self-sustaining combustion-type reaction that propagates until all reactants are consumed. Self-propagating high-temperature synthesis processing uses mixtures of reactant powders placed in a reactor vessel, generally in an argon atmosphere. The reactant mixture is then ignited using an incandescent filament or laser beam. Self-propagating high-temperature synthesis has been developed to the point where it is now capable of producing composite materials with well-controlled distributions of carbides or other reacted phases in metallic or intermetallic matrices (Ref 21, 22). Self-propagating high-temperature synthesis produces low-density, porous "ingots" that must then be further processed by crushing, milling, and screening to yield powders in size distributions compatible with thermal spraying. Researchers have successfully thermally sprayed SHS-produced MoSi 2 , SiC-reinforced MoSi 2 , FeCr/TiC, and NiCr/TiC materials. The sprayed deposits reportedly yielded equivalent, and in many cases superior, properties to deposits produced from powders produced by other synthesis routes. Some examples of the improved properties obtained include increased wear resistance in TiC/metal-matrix composites and increased fracture toughness in intermetallic composites such as MoSi 2 /SiC (Ref 23). Figure 11 illustrates the SHS powder production route, including powder synthesis and consolidation via plasma spray forming. Fig. 11 SHS powder synthesis and plasma spray consolidation process References cited in this section 13. R.W. Smith, P. Kangutkar, R. Drossman, and R. Krepski, A New Iron Base Thermal Spray Coating for Wear Resistance, Proc. 13th International Thermal Spray Conf. (Orlando, FL), ASM International, 1992, p 653-659 14. K. Tanaka et al., J. Mater. Sci., Vol 22, 1987, p 2192-2198 15. K. Ishizaki et al., J. Mater. Sci., Vol 24, 1989, p 3553-3559 16. S. Matsumoto, T. Kobyashi, and M. Hino, Proc. Eighth International Symposium on Plasma Chemistry, Vol 4, IUPAC, 1987, p 2458 17. R.G. Castro et al., Ductile Phase Toughening of Molybdenum Disilicide by Low Pressure Plasma Spraying, Mater. Sci. Eng., Vol A155, 1992, p 101-107 18. S. Sampath, H. Herman, and S. Rangaswamy, Ni-Al Re-evaluated, Proc. First National Thermal Spraying Conf. (Orlando, FL), ASM International, 1987, p 47-53 19. L.S. Schadler, K.O. Laul, R.W. Smith, and E. Petrovicova, Microstructure and Mechanical Properties of Thermally Sprayed Silica/Nylon Nanocomposites, J. Thermal Spray Technol., Vol 6 (No. 4), 1997, p 475- 485 20. L.J. Westfall, Composite Monolayer Fabrication by an Arc-Spray Process, Proc. First National Thermal Spray Conf. (Orlando, FL), ASM International, 1987, p 417-426 21. Z.A. Munir and V. Anselmi-Tamburini, Self-Propagating Exotherm Reactions: The Synthesis of High- Temperature Materials by Combustion, Mater. Sci. Rep., Vol 3 (No. 7/8), 1989, p 277 22. S. Sampath et al., Synthesis of Intermetallic Composite Powders via Self-Propagating Synthesis, Proc. 1993 Powder Metallurgy World Congress (Kyoto, Japan), Japan Societ y Powder and Powder Metallurgy, 1993, p 401-403 23. R.G. Castro, H. Kung, and P.W. Stanek, Reactive Plasma Spraying of MoSi 2 using an Ar-10% CH 4 Powder Carrier Gas, Mater. Sci. Eng., Vol A185, 1994, p 65-70 Thermal Spray Forming of Materials Richard Knight and Ronald W. Smith, Center for the Plasma Processing of Materials (CPPM), Drexel University, Philadelphia, PA Thermal Spray Forming While the thermal spray technology described above originated primarily as a coating or surfacing process, with materials being deposited as thin (<250 m), nonload-bearing coatings for dimensional restoration, wear, corrosion, and thermal protection, it has also evolved, during the last 50 years, into a process capable of spray forming free-standing materials onto mandrels at thicknesses of >100 mm. Early applications included zirconia -oxygen sensors and bulk ceramic materials for crucibles. Historically "difficult-to-form" materials such as the refractory metals tungsten, hafnium, molybdenum, and tantalum; superalloys; and intermetallics have now all been deposited with varying degrees of success, with the ultimate mechanical properties of the sprayed deposit being the main limitation. Process technology advances, such as HVOF, which can produce sprayed deposits with lower tensile stresses and degrees of thermal degradation than other processes and with increased deposit density, controlled-atmosphere plasma spray, and improved powder formulations and morphologies have all contributed to the successful development of thermal spray forming of monolithic and composite materials. Thermally sprayed coating deposits are usually <1000 m thick, more typically 400 m. Thermal spray forming, however, can yield deposit thicknesses in excess of 25 mm. Free-standing shapes can be produced by spraying onto sacrificial mandrels, which are mechanically or chemically removed after spraying. The incremental nature of thermal spray processes, using particulate-based feedstocks, and with deposited layers typically 15 to 25 m thick, enables graded and laminated structural materials to be formed. Also, because of the high processing temperatures and high localized forming energies (high particle kinetic energies at impact), traditionally difficult-to-form materials can be processed into near-net-shape components. Virtually all common and refractory metals, many intermetallics, ceramics, and combinations of these have been spray formed as "composite" materials. The properties (apart from the ductility of metals) of these deposited materials, after postdeposition heat treatment, have been reported to be close to, if not exceeding, those of cast or wrought materials. The ductility of metals is limited by contamination sources occurring either within the feedstock powders used, or in the spray process itself, due to oxidation of the material during heating under atmospheric conditions. Nickel-base superalloys and other heat-resistant alloys have all been thermally spray formed during the last 20 years, either as preforms or as a component repair technique. Plasma spray in [...]... Composite Powders via Self-Propagating Synthesis, Proc 1993 Powder Metallurgy World Congress (Kyoto, Japan), Japan Society Powder and Powder Metallurgy, 1993, p 40 1-4 03 23 R.G Castro, H Kung, and P.W Stanek, Reactive Plasma Spraying of MoSi2 using an Ar-10% CH4 Powder Carrier Gas, Mater Sci Eng., Vol A185, 1994, p 6 5 -7 0 24 R.W Smith, Reactive Plasma Spray Forming for Advanced Materials Synthesis, Powder Metall... Meeting Metal Powder Association, Vol 14, 1958, p 7 9-9 0 3 R Reichmann, "Method of Fabricating Sintered Bodies, Especially Hollow Shapes of High Refractory Metals," German Patent 6 27, 980, 26 March 1936 4 D.J Shaw, Colloid and Surface Chemistry, 4th ed., Butterworth-Heinemann, 1992 5 F.V Lenel, Powder Metallurgy: Principles and Applications, 1st ed., Metal Powder Industries Federation, 1980, p 30 9-3 12 6... Steel Powder Part 1, Precis Met Molding, Vol 14, (No 8), 1956, p 4 0-4 1, 83 14 W G Lidman and R.V Rubino, Slip Casting of Stainless Steel Powder Part 2, Precis Met Molding, Vol 14, (No 9), 1956, p 6 4-6 6, 98 15 P.E Rempes, B.C Weber, and M.A Schwartz, Slip Casting of Metals, Ceramics and Cermets, Ceram Bull., Vol 37 (No 7) , 1958 16 F.H Norton, Fine Ceramics: Technology and Applications, 2nd ed., McGraw-Hill,... 1994, p 47 7- 4 83 8 R.W Smith and R Novak, Advances and Applications in U.S Thermal Spray Technology, Powder Metallurgy International, Vol 2 3-3 and 2 3-4 , Springer Verlag, 1991 9 M Thorpe, Chem Eng., Nov 1991, p 5 4-5 7 10 R.W Smith, Equipment and Theory, Lesson from Thermal Spray Technology, Course 51, Materials Engineering Institute, ASM International, 1992 11 H.C Chen, Z Duan, J.V.R Heberlein, and E Pfender,... IUPAC, 19 87, p 2458 17 R.G Castro et al., Ductile Phase Toughening of Molybdenum Disilicide by Low Pressure Plasma Spraying, Mater Sci Eng., Vol A155, 1992, p 10 1- 1 07 18 S Sampath, H Herman, and S Rangaswamy, Ni-Al Re-evaluated, Proc First National Thermal Spraying Conf (Orlando, FL), ASM International, 19 87, p 4 7- 5 3 19 L.S Schadler, K.O Laul, R.W Smith, and E Petrovicova, Microstructure and Mechanical... 4), 19 97, p 475 485 20 L.J Westfall, Composite Monolayer Fabrication by an Arc-Spray Process, Proc First National Thermal Spray Conf (Orlando, FL), ASM International, 19 87, p 41 7- 4 26 21 Z.A Munir and V Anselmi-Tamburini, Self-Propagating Exotherm Reactions: The Synthesis of HighTemperature Materials by Combustion, Mater Sci Rep., Vol 3 (No 7/ 8), 1989, p 277 22 S Sampath et al., Synthesis of Intermetallic... nonuniform settling rate of the metal particles starts taking place The various sizes and shapes of the particles also affects the stability of the slip These settling forces can be balanced by using powders of small and fairly uniform particle size and by increasing the viscosity of the suspension The use of finer particles has a further advantage because their high surface-area-to -volume ratio causes them... Part 1, Precis Met Molding, Vol 14, (No 8), 1956, p 4 0-4 1, 83 14 W G Lidman and R.V Rubino, Slip Casting of Stainless Steel Powder Part 2, Precis Met Molding, Vol 14, (No 9), 1956, p 6 4-6 6, 98 15 P.E Rempes, B.C Weber, and M.A Schwartz, Slip Casting of Metals, Ceramics and Cermets, Ceram Bull., Vol 37 (No 7) , 1958 Slip Casting of Metals R.A Haber and C.A Paredes, Ceramic Casting Technology Program,... Processing and Use in Design, 1st ed., Marcel Dekker, 1992, p 44 4-4 60 2 H.H Hausner, Slip Casting of Metal Powders, Proc.: Annual Meeting Metal Powder Association, Vol 14, 1958, p 7 9-9 0 3 R Reichmann, "Method of Fabricating Sintered Bodies, Especially Hollow Shapes of High Refractory Metals," German Patent 6 27, 980, 26 March 1936 4 D.J Shaw, Colloid and Surface Chemistry, 4th ed., Butterworth-Heinemann,... Smith, and R.W Smith, J Metals, Vol 1, 1981, p 146 2 S Sampath and H Herman, Thermal Spray Technology, ASM International, 1988, p 1-8 9 M Thorpe, Chem Eng., Nov 1991, p 5 4-5 7 24 R.W Smith, Reactive Plasma Spray Forming for Advanced Materials Synthesis, Powder Metall Int., Vol 25 (No 1), 1993, p 9-1 6 25 R.W Smith and R Knight, Thermal Spraying I: Powder Consolidation From Coating to Forming, JOM, Vol 47 . 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. Herman, and S. Rangaswamy, Ni-Al Re-evaluated, Proc. First National Thermal Spraying Conf. (Orlando, FL), ASM International, 19 87, p 4 7- 5 3 19. L.S. Schadler, K.O. Laul, R.W. Smith, and E Fabrication by an Arc-Spray Process, Proc. First National Thermal Spray Conf. (Orlando, FL), ASM International, 19 87, p 41 7- 4 26 21. Z.A. Munir and V. Anselmi-Tamburini, Self-Propagating Exotherm