OVf:'v'ew
Plasma Spray Forming Metals, Intermetallics, and Composites
Sanjay Sampath and Herbert Herman
Plasma spray processing is a droplet deposition method that combines the steps of melting, rapid solidification, and consolidation into a single step. The versatility of the technology enables the processing of freestanding bulk, near-net shapes of a wide range of alloys, intermetallics, ceramics, and composites, while still retaining the benefits of rapid solidification processing. In particular, it is possible to produce dense forms through vacuum plasma spraying.
INTRODUCTION
It is widely appreciated that rapid impact deposition of plasma-sprayed molten particles of a wide range of alloys and ceramics can yield metastable structures.1 Plasma spray processing is an upscaled version of droplet deposition from the melt, which combines the steps of melting, rapid solidification, and consolidation into a single operation. Thus, plasma spraying offers a novel method of producing bulk, near-net -shape forms, while retaining the benefits of rapid solidification processing (RSP)-homogeneity, grain size, metastable phases, etc .. Compared to other RSP approaches, plasma spraying offers the advantages of moderate to large throughputs (5-30 kg/h) and the ability to deposit objects of moderate complexity. Conventional plasma spraying in air (APS), although capable of producing rapidly solidified structures, is limited in its capability of spray forming metals and intermetallics. This is due to the presence of such defects as unmelted particles, oxide inclusions, and porosity. In addition, APS processing retains residual stresses produced during deposition, thus limiting the thickness of the deposit.
The advent of vacuum plasma spraying (VPS) or low-pressure plasma spraying has opened a new dimension in the plasma spray forming of materials. Using VPS, dense oxide- and pore-free deposits can be formed with nearly theoretical density. The deposits will have low levels of residual stresses, since the high temperature of the process leads to stress-relief annealing. Additionally, the high velocities that are attained in VPS can cause the particles to flatten to a much greater degree than for APS, contributing to increases in process efficiency and deposit density. In VPS, although the solidification is rapid, the deposit undergoes annealing due to the continu-
42
ous exposure of the deposit to high temperatures (>800°C). However, it is observed that this annealing is beneficial, since it provides stress relief as well as recrystallization and results in enhanced interparticle bonding.2 For these reasons, as well as others, VPS is viewed as an effective means for consolidating powders (e.g., reactive metals) and for the production of free-standing forms for high-performance applications.
It should be noted that there are a variety of non-plasma spray methods through which components can be spray formed. Such techniques were pioneered by Singer, and a current rendition is the Osprey process.3 Considerable research has been done on Osprey as a potential manufacturing technology for metals, alloys, and composites. Mathur et al. have reviewed developments in Osprey technology and its application for the near-net-shape manufacture of disks, billets, and tubes from a variety of metallic alloys and composites.4 In this method, molten material is directly atomized and spray formed /on a mandrel. While the throughput of such atomization devices is considerable (hundreds of kg/h), the melt particle velocity is not high,leading to a structure of deficient density, which generally requires post-deposition processing. In the case of plasma spray, due to the velocity afforded by the plasma flame, it is possible to achieve high-density deposits as well as interesting and potentially useful metastable states. Further, it is possible to plasma spray a wide range of materials, such as refractory metals, ceramics, and their composites.
A major advantage of plasma spray forming is the ability to produce dense, rapidly solidified free-forms without the need for any post-spray thermal or mechanical treatments. The process itself is described in the sidebar.
MATERIALS PROCESSING Metallic Alloys
In the late 1960s, Moss and coworkers at Sandia National Laboratories plasma sprayed AI-V alloys in an effort to form free-standing, dispersion-hardened parts.n.12 Plasma spraying in air yielded grains of a few thousand angstroms, which were stable up to 600°C. The goal was to form an intermetallic compound (AIll V) that could retard grain growth at high temperatures. Kellerer et al. and
Schuster et al. independently reported using APS to produce oxide dispersionstrengthened materials.13.14
Krishnanand and Cahn air plasma sprayed AI-Cu alloys onto cooled copper substrates.ls Considerable supersaturation was found, but both supersaturation and hardness dropped significantly when the deposit thickness exceeded 200 !lm. When the surface of the deposit was not cooled continuously during the deposition, the deposited alloy overaged, indicating excessive processing temperatures in the absence of cooling.
Bhat et al. air plasma sprayed an Fe-C alloy onto a cold steel substrate, and the resultant structure consisted of austenite and ferrite, both being supersaturated in carbon and exhibiting a conventional bainitic morphology.16 The highly metastable microstructures were obtained even in the absence of extraordinary cooling measures.
Recently, Murakami et al. evaluated the microstructures and mechanical properties of thick deposits of various Fe-C-Cr and Fe-C-Si alloys produced by VPS.17.18 In an effort to control the effects of self-annealing, they sprayed the powders onto both water-cooled and nonwater-cooled (normal) substrates. The normal samples showed greater density than the water-cooled samples, but with some loss in metastability .17 The normal samples showed higher tensile strengths and ductile behavior as compared to the water-cooled samples.18 Finally, the normal samples showed lower porosity and underwent considerable self-annealing, leading to improved interparticle bonding, consequently improving fracture strength.
In a program sponsored by the U.S. Air Force, General Electric sought to use VPS to fabricate aircraft engine components. I 9-2! The terminology rapid solidification plasma deposition (RSPD) depicts the philosophy of their approach. The overall goal was to develop an optimum manufacturing process to form fine-grained, free-standing superalloy specimens that would ideally not require post-spray densification. Various d.c. nontransferred arc plasma guns were evaluated in low-pressure atmospheres. A methodology was developed to use this process to fabricate air foils and combustor liners.19 The results of this
JOM • July 1993
Plasma spray processing is part of the generic materials processing technology of thermal spraying, in which powder, wire, or rod feedstock is melted in a hot flame and propelled to a prepared substrate, where it solidifies to form a deposit. Direct current (d.c.) and radio frequency (RF) thermal, high-pressure plasmas are extensively used in the material processing industries for extractive metallurgy, melting, deposition, and evaporation. The tradHlonal d.c. arc plasma gun, which was developed as a heat source; more than three decades ago, has been extenSively used toforlJl protective coatings, Thed.c. plasma is initIaled and stabifized between a tungsten catfiodeand a water-cooled copper anode by the ionizatio/1 of gases such as argon or N2 with additions of secondary gasessuch as helium or H2•
A schematic of a spray gun is shown in Figure A. The nature of the electrode geometry as well as the flowing ionized gas determines the enthalpy content and the lIuidcharactenstics of the hot, partially conductive plasma as it exits from the circular opening of the cylindrical anode. Due to the flame momentum, the arc closes the electrical circuit at the outer filce olthe anode. Assuming that the gas en/ers. the plasma at a rate that will enable' it to maintain stability \lnd that filtered d.c. etectrical power rs deli~eredi:onllnuously, the plasma Will form a cylindrical Hame.Ine enthalpy of this flame
THE PLA.SMA SPRAY PROCESS
will depend on the plasma gas type, gas flow rate, and input electrical power. It is into this exiting, high-temperature plasma flame that powdered feedstock (1 0-90 ~m in diameter} is introduced by the way of a carrier gas. The particles melt in transit and impinge on the substrafe where they flatten rapidly, solidify, and form a deposit througb successiVe impingement. The production of reproducible, dense deposits requires well-controlled power sources and powder feed systems as well as mass-~ow gas controllers and integrated automation for both gUfjandworkpiece.
A departure from gas stabllization Is the CzechdevelopeD ~ater-stabililed plasma (WSP) gun (Figure B). In lhis device, the worklng inert plasma gas is replaced by water, which swirls within the internal diameter of a cylindrical arc chamber. I A d.c. arc is created between a cathode and an external, internally watercooled. rotating anode positioned at a point outside tne gun. Ionic recombination outside the torch delivers considerable enthalpy. The WSP operates at 160 kW, enabling Ihegun 10 process considerably more than 100 kg/hof metal powder and more than 35 kgJh of refractory ceramiCs. In a jOint international program, the Czech Insti\Vte of Plasm" Physics and the StateUniverslty of ., NQW York atStony Brook are exploring the lise 01 the WSP spraying of metal-ceramiC graded dePosits to
Negative Electrical
Connection and Water
Outlet
Insulating Housing
Water-Cooled Copper Anode
\-
Tungsten Cathode Arc
Gas Inlet
Positive Electrical Connection '----- and Water Inlet
Figure A, A schematic of a d.c. plasma spray gun. In this configuratfon, two different powders can be injected into the plasma flame, thus enabling the formation of composite deposits. (Courtesy ofSandi~ National labOratories.)
Figure B. The plasma spray process using the water-stabilized plasma gun. (Courtesy of P. Chraska. Institute for Plasma Physics, Czechoslovakia.)
1993 July • JOM
enhance the thermal shock properties of hlgh'performance composite materials.
In contrast to d.c. plasmas. RF plasmas are generated and sustained by an electromagnetic field supplied through an inductive coil. The discharge occurs within a water-{;ooled quartz or ceramic tUbe. The RF plasma gun is an electrodeless process and, in principle, it is possible to employ reactive gases (e.g., 02 and N2) as well as oxygen-sensitJve metals without concern lor electrode degradation or materials contamination. Induction plasmas are characterized bylheir large Illaterjalthroughput, Jow energy density, and tow vet®ity,6 Unlike the radial powder injection in a d.c. plasma jet, the powder particles are axially injected through a water-cooled probe. The axial feeding, longer dWell ~me. and higher power capabilities make [(possible for RF plasmas to melt and deposit relatively large particles. Induction plasma spraying is currently being explored for a range of applications, including the production of continuous-fiber-reinforced composites.'
Due to the high temperature of fhe plasma lIame, virtually any refractory material can be melted and deposited. The only criterion is that the material nol decompose. Thus, the process enables the proceSSing of meMs, aUoys, and, in partiClllar, ~ramjes. Contmlled·atmosphere plasma spraylng, such as VPS, has ooDsiderAbly expanded the capabilities 01 the process. Vacuum plasma spraying yields supersonic pau[clll Velocltles to produce dense. oxIde-free deposits.
The high solIdification rates assoclated with plasma spraying (106 Kls)t.2 result in fine grain sizes and, in many systems, metastable phases. However, during the process, the deposit is exposed to the high-Iemperattlre flame as well as to adiabatic recalescence associated with successive solidification of one droplet upon another. These factors can lead topnase transfo rmatjons within the deposit. 2 The effect of the flamedeposit interaclt'on IS particuiarly pronounced Tn VPS, resulting in deposit-substrate temperatureS in excess of 800·C. This leads to self·annealing of the deposit. recovery, and recrystallization, procfuclng fine, equia)(ed micrOstructures. Additionally, the hIgh temperature, coupled with oxide-free particles, enhances particle sintering, leading to strong interparticle bonds. The reduction in residual stresses permits Ihe bulld-up of thjc~ deposits and, thus, free-standing forms_
HistoricaJ Background To out knowledge, the first use of thermal spray to
produce solid bodies was reported by Tumer and Ballard In 1924,8 who fabricated relatively large, solid bOdies of copper, zinc, tin, aluminum, and iron by combustion spraying into a steel mold. The authors stated that the as-spraYed specimens were machinable and, in fac~ "had the ring of wrought metal." As an extension of this work. these workers combustion-spray formed beRshaped (S.4 mm thick) parts of zinc, aluminum, and bronze onto a sleel mandrel. These bells "exhibited a definite ting."i It was reported thatlhese sprayed parts had ' reasonably high densities."
Since the early work described above, the plasma spray forming of free-standing shapes has been employed with Intle or no fanfare in sman and large spray shops. Some of those programs have been strictly R&D based, but others have given rise to actual parts, such as ceramic nose cones and rocket nozzles, for ex· arpple, it has been reported thai the FrencIJ jet-engine manufactorer SNECMA has plasma spray formed · a tungsten rocket nozzle having a wall thickness of 8 mm and an outer diameter of 2S0 mm.'~ Ukewise, PfasmaTechnikAG in Switzerland has reported spraying tungsten tubes 200 mm in diameter by 600 mm in length. with a 1 mm wall thickness. Forsome time, tungslen and tantalum crucibles have been plasma spray formed.1o
Molybdenum has been reported by Plasma Technlk AG to have been plasma sprayed into a variety of components, including rocket nozzle parts.
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program were indeed promising, pointing to the high potential of using plasma spraying to achieve bulk forms having enhanced mechanical, thermal, and corrosive properties. For example, thermalfatigue properties of the RSPD alloys were found to be excellent in both longitudinal and transverse directions. Further, the oxidation resistances of conventional and RSPD alloys were similar. Finally, relative to a combustor fabricated using RSPD, it was concluded that careful process design leads to significant life-cycle benefits. Some industrial groups have recently been exploring the use ofRF plasma techniques for the RSPD of superalloy gas turbine parts.
Vacuum plasma spray forming has been extensively examined as a potential method for fabricating rocket engine components. Ina jOinteffort,Rocketdyne and NASA have VPS formed NARloy-Z (a copper-based alloy) and IN 718 for
possible use in the space shuttle's main combustion chamber. 22.23 They observed that powder purity is critical to achieving desired properties in the sprayformed deposit. For alloy 718, the properties of the VPS deposit were superior to that of the cast alloy at cryogenic, room, and elevated temperatures.22 Further, the properties of VPS NARloy-Z were comparable to that of wrought alloy at room temperature (the properties are tied to the 02 level and powder purity). McKechnie et a1. have found that the properties of VPS-formed materials are dependent on the recrystallization and grain growth associated with the high-temperature processing, which eliminates the lamellar microstructure.23 In another program, researchers at Aerojet and Howmet have characterized VPSformed nickel-, iron-, and copper-based alloys for rocket-engine structural components.24 In this case, post-deposition
Figure 1. A VPS-formed tungsten cylinder. (Courtesy of Sandia National Laboratories.)
a 20 J.lm b 20 J.lm
Figure 2. (a) A cross-section micrograph of VPS-formed Ni3AI(Cr.Hf.B).33 (b) A cross-section micrograph of Ni3AI(Cr.Hf.B) produced by VPS and annealing (1,1 oeoc for two hours).
44
heat treatment was necessary to increase the strength of the spray-formed alloy to that of wrought alloys.
Plasma processing methods have been used extenSively for fabricating refractory metal components. SNECMA has formed tungsten rocket nozzles up to 8 mm in thickness using plasma spraying.1O Jackson et a1. spray formed nearnet shapes of several refractory metals and alloys using low-pressure RF methods. Molybdenum and niobium alloys of greater than 96% volume density were produced that showed limited oxidation.25 The tensile and rupture properties of the alloys, subsequent to heat treatment, were comparable to those of wrought alloys; however, the ductility was lower. Forming by VPS has been used extensively to produce tungsten crucibles and pipes for nuclear applications. Sickinger and Muehlburger have fabricated tubular shapes of titanium, tungsten, and molybdenum using VPS.26 Neiser et a1. investigated the thermal conductivity and steam reaction charac-. teristics of VPS-formed tungsten for fusion reactor applications.27 Figure 1 shows a spray-formed tungsten cylinder. The researchers observed that even though the spray-formed deposits were only 92% dense, the H2 evolution rates were similar to fully dense, hot-rolled tungsten. Prichard and coworkers processed cylindrical preforms of niobium and tantalum alloys and their composites using VPS.2B The as-sprayed deposits showed greater than 93% density with good mechanical properties.
Intermetallics
There has been considerable activity in recent years on a number of intermetallics (principally aluminides) for highstrength and high-temperature aerospace applications. The typical route of fabrication has been hot pressing, hot isostatic pressing, or hot extrusion. More recently, the rapid solidification of intermetallic compounds has been considered because of the desire for chemically homogeneous, fine-grained structures.29 Compared to other rapid solidification approaches, VPS offers the advantages of moderate to large throughput (kg/h), high denSity, and the ability to deposit objects of moderate complexity (line of sight) and produce near-net bulk forms. Vacuum plasma spraying has the additional advantages of low oxidation and stress-free deposits.
Table I. Tensile Properties of Processed Ni;,AI-B30
Yield Tensile Stress Stress Elong.
Process (MPa) (MPa) (%)
Melt-Spun 627 779 10 HIP* 455 1,311 45 VPS 731 1,413 26 It Hot isostatic pressing.
JOM • July 1993
Chang et al. compared the properties of melt-spun ribbons, hot isostatically pressed specimens, and plasma-sprayed Ni3AI-B alloys (Table I).3D The plasmasprayed specimens had excellent ductility (>30%) as well as the highest yield point and ultimate tensile strength. In a subsequent review, Taub and Jackson outlined the advantages of plasma spray forming aluminides and superalloys. The key benefit was the ability to produce a rapidly solidified form in the deposited state without a subsequent need for thermomechanical processing.31
In a recent study at the State University of New York at Stony Brook, the authors and coworkers examined two compositions of intermetallics: an L1 z-based Ni3A1 alloy (in wt.%: Ni-6.4Cr-10.8AI-IHf-O.05B-0.05C) and two-phase NiAI-Ni3AI (Ni-20 wt.% Al) .3Z For both alloys, which were approximately 3 mm thick, nearly theoretically dense test specimens were obtained by VPS processing. Figure 2a shows the microstructure of the as-deposited cross section, displaying very high density. The lamellar nature of the deposit is observed upon etching. Upon annealing at 1,l00°C for two hours, the lamellar structure is converted to equiaxed grains with some coarsening (Figure 2b). The mechanical properties of VPS-formed and annealed specimens are shown in Figure 3. It is clear that post-deposition annealing results in an increase in ductility, with a concurrent lowering of yield strength. The increase in ductility is attributed to the migration of boron to grain boundaries during annealing. In the as-sprayed alloy, the boron was distributed homogeneously due to rapid solidification. The reduction in yield strength is associated with an increase in grain size and obeys the Hall-Petch dependence.33
In the case of the NiAI-Ni3Al (Ni-34 at.% AI) two-phase intermetallic, a considerable increase in fracture strength was observed in the VPS-processed and annealed materials as compared to conventionally processed materials (Table lI).34 Neiser has used VPS to form a different two-phase Ni-AI composition (Ni-30 at. % Al).35 He observed considerable elongation (>75 %), suggesting superplastic behavior at 800°C in the VPSformed specimens that were pre-annealed at 1,100°C for two hours (Figure 4). It was further reported that the specimens tested at 600°C showed no ductility, and the specimen tested at 700°C showed limited elongation, suggesting
Table II. Tensile Properties of VPSFormed Two-Phase NiAI-Ni3AI34
Condition As-Sprayed 1,100°C/2 h 1,100°C/24 h
Density (glcm3)
6.29 6.36 6.41
1993 July • JOM
Fracture Total Strength Strain
(MPa) (%)
375 0.45 445 0.22 520 0.20
1000
As-Sprayed 800
.... Ann. llOOoC/2h · .. ! 600
Ann . 1 J OOoC/l4 h · · 400 '"
200
Room Temperature Test s
a 0 0.5 1.5 2 .5
Strain %
Figure 3. Tensile stress-strain behavior of VPS-formed and annealed Ni3AI(Cr,Hf,B).33
a threshold temperature for this superplastic behavior between 700-800°C.
Another intermetallic of current interest is MoSiz' which has a much higher melting point than the aluminides.36 In a recent investigation, Tiwari and the present authors evaluated the microstructures and properties ofVPS-formed MoSiz.
37 The deposits were 98.5% dense, chemically homogeneous, and fine grained. Figure 5 shows a transmission electron micrograph of a VPS-formed MoS4 deposit, revealing a bimodal distribution of grains. The grain size is in the range of O.1-D.6Ilm. As detailed in Table III, the room-temperature strength and fracture toughness show evidence of improvement over conventionally processed material.
As part of a Stony Brook study of difficult-to-form shape-memory alloys, equimolar NiTi was spray formed using both electric-arc sprayed wire38 and VPS. The VPS alloy showed a depreSSion in the transformation temperature due to disorder in the structure associated with rapid solidification.39 Annealing induced ordering and raised the transformation
temperature; the annealed samples exhibited shape-memory behavior.
Amorphous Alloys
The rapid solidification feature of plasma spraying enables the production of numerous amorphous alloys. Giessen and coworkers produced an amorphous sheet of a Zr-Cu alloy by plasma spraying onto a rapidly rotating disk within an inert gas chamber.40 One of the present authors has evaluated the microstructure and properties of plasma-sprayconsolidated nickel-based amorphous powders.41 An amorphous/microcrystalline dual-phase structure was observed in the APS deposit, while the VPS deposit was predominantly microcrystalline. However, the crystalline phase was a single, supersaturated solid solution. Excellent hardness and corrosion properties were observed in the freestanding forms.
Rare-Earth Magnets
Free-standing magnetic forms have been plasma sprayed for a number of years. Ferrite phase shifters and other magnetic devices were pioneered in the early 1970s by Harris and coworkers (see Reference 42). This group also plasma spray formed ferrite-dielectric composites. More recently, Kumar and Das plasma sprayed 3 mm thick Sm-Co intermetallic magnets, which have coercivities conSiderably higher than such magnets obtained through conventional sintering.43 These workers also found that the plasma spraying of binary SmZCo17 compositions could result in amorphous material, which, upon proper heat treatment, showed texturing.
Considerable unpublished work has been carried out on the plasma spray
10mm
Figure 4. A qualitative illustration of the high-temperature (800°C) tensile behavior of VPS-formed two-phase NiAI-Ni3AI.35 (Courtesy of R.A. Neiser, Universitat Bundeswahr, Germany.)
45
production of magnetic materials, but details are usually not forthcoming and the reports are typically anecdotal and not readily available in the open literature. Significant opportunities exist in the use of plasma spray technology to produce electronic and electromagnetic circuit components.
Ceramics
Ceramic shapes have been plasma spray formed for many years. For example, radomes and rocket nozzles were plasma spray formed onto removable mandrels some 30 years ago during the early days of the U.S. space program. In recent years, withimprovementsinprocessing and the availability of superior ceramic powders, it has become possible to fabricate a variety of high-performance components with plasma spraying. Examples include free-standing specimens of alumina-magnesia-spinel, which display excellent thermomechanical behavior and industrially relevant dielectric properties.44 Similar studies of sprayformed cordierite (2MgO-2AIP3-5Si02) indicate that plasma spraying can yield parts of this low-thermal-expansion ternary oxide with remarkable thermal shock behavior as well as interesting dielectric and electrical properties.4S
As mentioned in the sidebar, highpower, water-stabilized plasma guns have been used to form large sections of
ceramic oxides, including AlP3' partially stabilized zirconia, Crp3' and AIz TiOs·46
Schindler and Schultze utilized the water-stabilized plasma gun to produce large, rotationally symmetric, free-standing ceramic bodiesY A 5.5 m tube with a wall thickness of 1.5 cm and inside diameter of 2.5 cm was fabricated. A wide range of oxide ceramics, including AI203' Crp3' and Ti02, are produced by this process (Figure 6). The liqUid-stabilized gun enables throughputs approximately 20 times greater than that of conventional gas-stabilized guns.
A wide range of ceramic processing techniques has been employed to fabricate thin films and bulk forms of highcritical-temperature (high-T) superconductors. From the outset, plasma spraying was used to produce high-Te material; more recently, directional melt texturing (Figure 7) has been employed to enhance the critical currents of plasmasprayed free-standing forms to more than 5,000 AI cm2-levels that are exceeded only by single-crystal thin films.48 Wilber et al. formed a free-standing cylinder of superconducting Y-Ba-Cu by spraying on a carbon mandrel. The free-form showed a superconducting transition temperature of 82 K.49 The surface resistivity of the sample was lower. However, it was envisioned that with improved depOSition to prevent cracks and pores and with the use of a mandrel
Table III. Room-Temperature Mechanical Properties of VPS-Formed MoSI237
Condition
As-Sprayed 1,l00°C/2 h 1,100°C/24 h • Three-point bending.
Hardness (VHN)
1,201 ±45 1,203 ±60 1,093 ±25
Fracture Toughness (MParm)
4.7 4.8 5.9
Flexural Strength (MPa)·
280 310 364
0.5 ).lm
Figure 5. A transmission electron micrograph of a VPS-formed MoSi2 deposit showing a finegrained microstructure.
46
Figure 6. Various free-standing ceramic forms produced by water-stabilized plasma-spray forming. (Courtesy of P. Chraska, Institute of Plasma Physics, Czechoslovakia.)
other than carbon, surface resistivity could be significantly improved. Bulk parts, induding cables and motor parts, are expected to be produced by plasma spray forming.
COMPOSITES
Metal- and ceramic-matrix composites can be spray formed. The principal deficiency of traditional spray forming (e.g., Osprey) is the low velocity attained by the molten droplets, which results in low theoretical density and necessitates post-forming steps. High-velocity, onestep plasma spray processing, however, is well suited to the production of composite structures. Indeed, many studies demonstrate that the plasma spray forming of composites represents a synthesis ofrapid solidification and composite materials technologies. Additionally, this is a crucible-free process that offers the capability to produce such high-temperature materials as refractory metals and ceramics composites.
Metal-Matrix Composites
Both continuous and discontinuous composites can be produced by plasma spray, and a variety of matrices and reinforcements can be introduced into the plasma flame. With the ability to control the extent of melting, plasma spraying offers a unique method for composite fabrication. Using precise, computer-controlled, dual-feed VPS, Gruner et al. produced composite coatings of a Co-Ni-Cr-AI-Y matrix with an AIP3 reinforcement.50 This technique can be used to produce a functionally graded composite material (e.g., metal on one side, ceramic on the other, and a continuously graded ceramic-metal composite in between). Jager et al. used VPS to produce a free-standing cylinder of a continuously graded metal-ceramic composite of a nickel-based alloy and AIPi Crp3.51 They observed that the use of metal grading reduced the tendency of the material to crack.
Alcoa has used RF plasma to produce sheets of disperSion-strengthened and particulate-reinforced composites of AIFe-Ce alloys.52,s3 The sprayed form was
1993 July • JOM
z
v- / x
200 J,lm
Figure 7. The microstructure of a plasmasprayed and melt-textured high-Te superconductor.48 (Courtesy of H.G. Wang. Brookhaven National Laboratory.)
subsequently hot-rolled and annealed to achieve full density. The consolidated materials showed higher hardness and strength but had slightly lower ductilities than their powder metallurgy processed counterparts.
Jackson et al. exemplified the versatility of the process by producing a variety of metal-metal, metal-carbide, and metaloxide composites.54 Using a multiple powder feed system, particulate and laminated composites with a finegrained matrix and improved yield strength were produced, although ductility was decreased. In another approach, Siemers et al. used two plasma guns to form continuous and discontinuous laminated composites of metalmetal (superalloy-molybdenum), metalcarbide (superalloy-Cr3C2), and metaloxide (superalloy-AIP3) systems.55Itwas shown in both studies that the plasma spray process can be used to tailor composite structures of desired specific stiffness and strength.
Berndt and Yi investigated the possibility of incorporating large volumes of short fibers of yttria-partially stabilized zirconia and SiC into a metallic or ceramic matrix.56 In seeking to improve strength and fracture toughness, they examined a variety of blending and composite powder approaches to codeposit the fibers and the matrix material. In another investigation, Stanek and Castro cosprayed various ratios of copper and tungsten powders in an effort to produce a Cu/W laminated composite.57
1993 July. JOM
Intermetallic-Matrix Composites
As part of the Stony Brook Study, Tiwari et al. dual-fed TiB2 particles with Ni3AI powder to obtain a TiB2-reinforced Ni3AI composite.58 By varying particle size and feed rate, it was possible to obtain the desired volume fraction of the reinforcement with or without melting the second phase. Figure 8 shows a micrograph of a 10 vol.% TiB2-reinforced Ni3AI(Cr,Hf,B) composite obtained by dual-feed VPS. The micrograph shows unmelted TiB2 particulates uniformly distributed in the matrix. Table IV presents the room-temperature mechanical properties of such composites. Significant strengthening is observed through the addition of diboride particles, indicating good particle-matrix load transfer. This was attributed to the strong particle-matrix interface obtained by the VPS process.
In another study, Tiwari et al. VPS processed a comparably sized blended powder of MoSi2 and either SiC or TiB2 reinforcements. In the case of TiB2, the researchers observed melting of both powder constituents.59 Considerable enhancements in strength and fracture toughness were observed in the composites as compared to the unreinforced MoSi2 (Table V).
Using VPS, Castro et al. codeposited 10wt.% and20wt.% tantalum with MoSiz to produce a ductile-phase toughened composite.60 The addition of tantalum improved the fracture toughness; however, the fracture toughness was anisotropic, with the value being 20% higher parallel to the substrate rather than in the spray direction. This anisotropy was attributed to the lamellar nature of the deposit. Alman et al. plasma spray formed MoSizj AIP3 laminates by the
Table IV. Room-Temperature Mechanical PropertIes of VPS-Formed
NI3AI(Cr,Hf,B) and TiB2 ParticleReinforced Composites58
Material
Ni3Al(Cr,Hf,B) Matrix + 8 vol. % TiB2 Matrix + 11 vol.% TiB2
Flexural Strength
(MPa)
1,200 1,820
Tensile Strength
(MPa)
700 900
1,200 Note: Deposits were annealed in argon for two hours at
1,100'C.
sequential deposition of powders.61
These deposits were produced in an inert gas-filled chamber under atmospheric pressure. They observed an increase in the deposit's hardness with an increase in arc current. Additionally, the plasmasprayed laminate densified fully upon annealing.
Texas Instruments and Textron have developed a manufacturing process to produce titanium alloys and titanium aluminide foils using the RF plasma spray process.62 Here, a titanium alloy powder is RF plasma sprayed onto a mandrel to produce a preform. The plasma-sprayed preform is then cold rolled to produce a dense titanium alloy foil (Figure 9). These foils are subsequently used to form fiber-reinforced composites using the foil-fiber-foil approach. This technique appears to be economically viable for the production of titanium alloy and aluminide foils. Several other researchers have also examined the plasma spray process to directly form SiC-reinforced titanium-matrix composites.63
Ceramic-Matrix Composites
Plasma spraying offers a uniquely effective means for producing ceramicmatrix composites. The high tempera-
20 11m
Figure 8. A cross-section micrograph of a Ni3AlmB2 composite formed by VPS and annealed (1 .1 DOcC for two hours).58
47
hue and versatility of particle-injection techniques remove some barriers in the production of composites. Lapierre et a1. produced Alp3-matrix SiC composites using APS onto sacrificial graphite substrates.64Theplasma-sprayedA~03component had a predominantly metastable y phase, which transformed to a up on annealing. It was observed that the assprayed ceramic matrix was weak due to the lamellar nature of the deposit; however, this resulted in pseudoplastic behavior and strain tolerance. Annealing and the addition of the SiC reinforcement resulted in significant property improvements (Figure 10).
Reactive Spray Forming
A novel modification of plasma spray forming is reactive spraying. Here, the plasma spray nozzle is extended to include a reaction zone where reactive gases can be introduced into the plasma stream. The precursor gases are dissociated in the plasma and react with the molten particles to form compounds. With the proper selection of gases and particulates, it is possible to produce carbide-, nitride-, and silicide-reinforced metal-matrix composites.65 TsantrizQs has reacted TiCl4 and aluminum powders in the flame of a d.c. plasma, pro-
ducing a range of titanium aluminides with varying titanium content. 66 However, very limited homogeneity was obtained. While the process is still in its infancy, it offers promise for materials synthesis.
SHAPE DEPOSITION
Plasma spray forming has been successfully used to produce axisymmetric components, such as cylinders, tubes, and rods. In order for the process to be technically and economically acceptable to manufacturers, it is important to develop methods to produce complexly shaped near-net components. Toward this end, General Electric's RSPD program demonstrated a processing sequence for fabricating complexly shaped alloy turbine components.19
More recently, Weiss et a1. have utilized a sequential mask-and-deposit technique as a rapid prototyping approach for the spray forming of complexly shaped components (Figure 11).67 In this method, free-forms are spray deposited by the successive spraying of cross-sectional layers. The cross-sectional description is generated by slicing a three-dimensional computer representation. For each layer, a disposable mask is made that exposes the area where
Table V. Room-Temperature Mechanical Properties of MoSi2 Composites59
Hardness Fracture Toughness Flexural Strength Material Condition (VHN) (MPa.Jm) (MPa)* ---VPSMoSi2 As-Sprayed 1,201 4.7 280 MoSiz + 4 vol. % SiC As-Sprayed 1,228 5.4 300 MoSi2 + 4 vol. % SiC Annealed 1,160 7.9 410
1,100°C/24 h MoSiz + 20 vol. % TiBz As-Sprayed 1,057 6.1 380 • Three·point bending.
L---.J
25~m
Figure 9. A titanium alloy foil produced by RF induction plasma spraying and post-spray processing.62 (Courtesy of S. Jha, Texas Instruments.)
48
100
80
i f
60
1 40
£
20
a
0 Iil~OI
'" AlIO,.5YOMSJCCompoatt.
As-SP"',ft lOOO'"c.4 h 1300-C.lJl 1300-C-15b
Annealed
12. r---...,..-----,----,.-----,
I ••
I 8.
J ~ 6.
J 4.
2.
b
o AliOs
a A1I1Os -5 .om SiC Compl)l:lte
M-Sprayed lOOOt4 h 1300-C-lh lS00'"C-Uib
Annealed
Figure 10. The flexural strength and modulus of a plasma-spray-formed AlP/SiC ceramicmatrix composite.64
that material occurs. The mask is placed upon the top layer of the developing shape, and the exposed areas are sprayed. Masks are typically cut out of paper. Although the process is unique, it has several limitations, including the need for high-temperature masking devices to enable the plasma spray forming of high-temperature materials.
CONCLUSIONS
The versatility of plasma spraying enables the high-velocity processing of dense, strong parts from a wide range of materials. High-deposition rate plasma guns, coupled with advanced systems integration and real-time thickness-measurement devices, introduce the capability to spray form complex shapes. Clearly RF plasma will play an increased role in spray forming, but its velocity, as with Osprey, remains limited. A limiting factor will be that plasma spraying is a line-of-sight process; however, in principle, it is possible to use multiple computer-controlled guns to form shapes having complex geometries. One can envision computer-aided design, linked with a spray-control system, entering a future manufacturing environment.
The high-temperature plasma flame offers a novel route for the one-step processing of refractory intermetallics and ceramics. Additionally, the unique attributes of plasma spraying offer the potential for processing continuous, discontinuous, and laminated composites. Plasma spray-as an efficient individual gun or synthesized with other spray systems (e.g., as an atomizer for an electricarc wire gun)-should gain increased
1993 July. JOM
Figure 11 . A sprayed artifact produced by the mask-and-deposit technique. (Courtesy of L. Weiss, Carnegie Mellon University.)
recognition in the materials community as an important means of processing advanced materials.
ACKNOWLEDGEMENTS The authors thank Dr. R.A. Neiser (Sandia
National Laboratories) and Dr. R. Tiwari (Cleveland State University) for their assistance in preparing this overview. Thanks are also due to Dr. Lee Weiss (Carnegie Mellon University), Dr. Sunil Jha (Texas Instruments), and Dr. Hougong Wang (Sherritt Gordon) for supplying the photomicrographs.
References 1. s. Safai and H. Herman, Plas,,", Sprayed Materials, Treat ise Mtls. Sci. & Tech., ed. H. Herman (New York: Academic Press, 1981), pp. 183-214. 2. S. Sampalh, "Rapid Solidification During Plasma Spraying," Ph.D. Ihesis, State University o/New York, Stony Brook (]989). 3. AR.E. Singer, "The Principles of Spray Rolling of Metals," Metall. Maler. , 4 (1970), pp. 246-250. 4. P.C Mathur et aI., "Process Control, Modeling and Applications of Spray Casting," JOM, 41 (l0) (1989), pp. 23-28. 5. P. Chraska and M. Hrabovsky, "An Overview of Water Stabilized Plasma Guns and Their Applications," Proe. In lem. Thermal Spray Calif. , ed. CC Berndt (Materials Park, OH: ASM, 1992), pp. 81- 85. 6. M.1. Boulos, "R.F. Induction Plasma Spraying: Stale of the Art Review," f. of Thermal Spray Technology, 1 (l) (l992), pp. 33-40. 7. H.P. Wang et aI., "Elements of Intelligent Process Control for Plasma Deposition," 10M, 43 (I) (1991), pp. 22- 25. 8. T. Turner and W.E. Ballard, "Metal Spraying and Sprayed Meta!," I, Insl, Mel., 32 (1924), pp. 291-312, 9, T. Turner and W.E. Ballard, "Metal Spraying and Sprayed Meta!," f.lnsl. Mel., 32 (1924), pp. 291-312; and reported in W.E. Ballard,Melal Spraying (London: Griffin, 1963), pp. 393-394. . 10, "Plasma Spraying Technique, Basic Principles and Applications," Plasma Technik Information Bulletin (Wohlen, Switzerland: Plasma Technik, 1974). 11. M. Moss, "Dispersion Hardening in AI-V by Plasma-Jet Spray Quenching," Acla Mel., 16 (1968), pp. 321-326. 12. M. Moss and D.M. Shuster, "Mechanical Properties of Dispersion Strengthened Spray Quenched Al-V Alloys," Trans. ASM, 62 (1969), pp. 201-205. 13. H. Kellerer and B. Looman, "Production of AI-AlP, Dispersion Hardened Alloys by Plasma Spraying," Melall., 22 (l968), pp. 212-215. 14. D.M. Shuster and M. Moss, "Dispersion Strengthened AIAlP, by Plasma Spraying," I. Melals, 20 (10) (l968), pp, 63-66. 15. K.D. Krishnanand and R.W. Cahn, "Properties 01 Plasma Sprayed AI-Cu Alloys," Rapidly Quenched Melals, ed, N,J. Grant and B.C Giessen (Cambridge, MA: MIT Press, 1976), pp.67-75. 16. H. Herman and H. Bhat, "Metastable Phases Produced by Plasma Spraying," Synthesis and Properties of Melastable Phases, ed. E.S. Machlin and T.j. Rowland (Warrendale, PA: TMS, 1980), pp. 115-137. 17. K. Murakami et aI., "Rapid Solidification and Self-An-
nealing ofF-C-Si Alloys by Low Pressure Plasma Spraying," Maler. Sci. & Eng. , A117 (1989), pp. 207-214. 18. K. Murakami et al., "Microstructure and Mechanical Properties of Rapidly Solidified Deposited LayersofFe-C-Cr Alloys Produced by Low Pressure Plasma Spraying," Mater. Sci. & Eng., Al23 (1990), pp. 261-270. 19. AM. Johnson et aI., "Aircraft Engine Gas Turbine Component Fabrication Concepts Using RSPD," Proceedings of Rllpid Solidification Processing-III, ed . R, Mehrabian (Washington, D.C: NBS, 1982), pp. 6~1. 20. M.R. Jackson et aI., "Production of Metallurgical Structures by Rapid Solidification Plasma Deposition," J. Metals, 33 (II) (1981), pp. 23-27, 21. M.R. Jackson, R.W. Smashey, and L.G. Peterson, "The Mechanical Behavior of RSPD Materials," in Ref. 19, pp.198-208. 22. R.R. Holmes, D.H. Burnes, and T,N. MeKechnie, "VPS Forming of NARloy-Z and lnconel 718 Components for Liquid Rocket Engines Thermal Spray Research and Applications," Proceedings of Third Nalional Thermal Spray Conference, ed. T. Bernecki (Materials Park. OH: ASM, 1990), pp. 363-367, 23. T.N. MeKechnie et aI., "Metallurgy and Properties of Plasma Spray Formed Materials," Proceedings of Illterna/ional Thermal Spray Conferellee, ed. c.c. Berndt (Materials Park, OH: ASM, 1992), pp. 839-M5. 24. T. Nguyentat, KT. Dommer, and K.T. Bowen, "Metallurgical Evaluation of Plasma Sprayed Structural Materials for Rocket Engines," in Ref. 23, PP, 321-325, 25. M.R. Jackson et aI., "Refractory Melal Structures Produced by Low Pressure Plasma Deposition," General Electric Corporate Research, Technical Report 88CRD096. 26. A Sickinger and E. Muehlburger, "Advanced Low Pressure Plasma Application in Powder Metallurgy," Powder Metallurgy International, 24 (2) (1992), pp. 91-94. 27. R.A. Neiser et aI., "An Evaluation of Plasma Sprayed Tungsten for Fusion Reactors," Proc. of National Thermal Spray Conf. (Materials Park, OH: ASM, to be published). 28. PD, Prichard et al., "Vacuum Plasma Spray of Cb and Ta Matrix Composites," Metal and Ceramic Ma/rix Composites: Processing, Modelling and Mechanical Behavior, ed. R.B. Bhagat et a1. (Warrendale, PA: TMS, 1990), pp. 561-568. 29. c.c. Koch, "Rapid Solidification o/ Intermetallic Compounds," In/ernational Malerials Review, 33 (4) (1988), pp. 201-219. 30. KM. Chang, AI. Taub, and S.C Huang, "Property Comparison of Melt-Spun Ribbons and Consolidated Powders of N~AI-B," High-T;mpera/ure Ordered In/ermelallic Alloys, ed. Cc. Koch, CT. LlU, and N.S. Stoloff (Plltsburgh, PA: MRS, 1985), pp. 33S-342. 31. A.1. Taub and M.R. Jackson, "Mechanical Properties of Rapidly Solidified Nickel Base Superalloys and lntermetallics," in Ref. 30, pp. 389-403. 32. S. Sam path et aI., "Plasma Spray Consolidation of Ni-AI lntermetallics," in Ref. 22, pp. 357- 361. 33. R. Tiwari et aI., "Microstructure and Properties of Plasma Spray Formed L1, Based Ni,AI Alloy," unpublished work. 34. S. Sampath el aI., "Microstructure and Properties of Plasma Spray Consolidated Two-Phase Nickel Aluminides," Scripta Met. , 25 (1991), pp. 1425-1430. 35. R.A Neiser, unpublished work. 36. AX Vasudevan and J.J. Petrovic, "A ComparativeOverview of MaSi, Composites," Mat. Sci. and Eng., A155 (1992), pp.I-17. 37. R. Tiwari, H. Herman, and S. Sam path, "Spray Forming of MoSi, and MoSi,-Based Com posites," High Temperature Ordered Intermetallie Alloys IV, ed. L. johnson, D.P. Pope, and J.O. Stiegler (Pittsburgh, PA: MRS, 1991), pp. 807-813. 38. A.P. Jardine et aI., Scripla Met .. 24 (1990), p. 2391. 39. Y. Horan, AP. Jardine, and H, Herman, "VPS Forming of NiTi Shape Memory Alloy," Proc. 2nd Plasma Technik Symposium (Wahlen, Switzerland: Plasma Technik, 1991), pp. 99-103. 40. B.C Giessen el aI., "Sheet Production of an Amorphous Zr-Cu Alloy by Plasma Spray QuenChing:' Melal. Trans" 8A (1977), pp. 364-366. 41. S. Sampath, "Microstructural Characteristics of Plasma Spray Consolidated Amorphous Powders," Ma/.Sci.and Eng. (accepted for publication), 42. D. Harris et al.. "Polycrystalline Ferrite Films for Microwave Applications Deposited by Arc-Plasma," I. Appl. Phys., 41 (1970), pp. 1348-1349. 43. K. Kumar and D. D.s, "Equ ilibrium and Metastable Samarium-Cobalt Deposits Produced by Arc-Plasma Spraying," Thin Solid Films, 54 (1978), pp, 263-269. 44. H,G. Wang and H. Herman, Ceramic Bulletin, 68 (]989), p. 97, 45. H.G, Wang and H. Herman, "Plasma Sprayed Cordierite: Dielectric and Electrical Properties," Surf. Coat. and Tech., 37 (1989), pp. 297-303, 46. J. Ilavsky, K. Neufuss, and P. Chraska, "Influence of Spraying Parameters on the Properties of A~03 Deposits," Proc, 5th Conf. on Aluminum Oxide (Prague, Czechoslovakia:
Prague Inst. of Chern. Tech., 1990), pp. 6S-70. 47. S. Schindler and W. Schultz, "Plasma Generated Oxide Ceramic Components," Proc. 151 Plasma Technik Symposium (Wohlen, Switzerland: Plasma Technik, 1988), pp. 181-191. 48. H .G. Wang et aI., "Texture Growth Processing of Plasma Sprayed Y-Ba-Cu-OSuperconducting Deposits," Appl. Phys , Lelf .. 57 (23) (1990), pp. 249S-2497. 49. W.o. Wilber,JD. Reardon, and S. Rangaswamy, "Development of Plasma Sprayed Superconductors for Microwave Applications," Proc. of NatiOllal Thermal Spray COllferenee, ed. D.L. Houck (Materials Park, OH: ASM, 1988), pp. 227- 231. SO. H. Gruner, Proc. International Thermal Spray Conf. (New York: Pergamon, 1986), pp. 73-S2. 51. D.A. Jager, D. Stover, and H.G. Schutz, "Plasma Spraying of Graded Composites," in Proceedings of Nalional Thermal Spraying Conference, ed, T. Bernecki (Materials Park, OH: ASM, 199]), pp. 323-327. 52. S.A jones, j.R, AuhJ, and T,N, Meyer, "Particulate Reinforced AI-Based Composites Produced by Low Pressure RF Plasma Spraying," in Ref. 51, pp. 329-336. 53. L.M . Angers, J.R. Auhl, and T.N. Meyer, "Characterization of Dispersion Strengthened Aluminum Alloys Produced by Plasma Deposition," in Ref. 51, pp. 337- 344, 54. M.R. Jackson et aI. , "Composite Structures Produced by Low Pressure Plasma Deposition," Processing & Properties for Powder Metallurgy Composites, ed. p, Kumar, K Vedula, and A RittedWarrendale, PA: TMS, 1988), pp, 4S-57. 55. P.A. Siemers et aI., "Production of Composite Structures by Low Pressure Plasma Deposition," Cer. Eng. and Sci. Proc" 6 (1985), pp . 896-907, 56, CC Berndt and J.H. Yi, "The Manufacture and Microstructure of Fiber-Reinforced Thermally Sprayed Coatings," Surf. Coat . & Tech" 37 (1989), pp. 89-110. 57. P.W. Stanek and R G. Castro, "Microlaminate Composite Structures by Low Pressure Plasma Spray Deposition," Proc. of Int. Powder Metallurgy Calif. (Princeton, NJ: MPIF, 1988). 58. R. Tiwari et aI., "Plasma Spray Consolidation of High Temperature CompoSites," Mat, Sci, and Eng., AI44 (1991), pp.127-1 31. 59. R. Tiwari, H. Herman, and S. Sampath, "Vacuum Plasma Spraying of MoSi, and Its Composites," in Ref. 36, pp. 9S-100. 60. R.G. Castroet aI., "Ductile Phase Toughening olMoSi, by Low Pressure Plasma Spraying," in Ref. 36, pp. 101-107. 61. D.E. Alman et aI., "Fabrication, Structure and Properties of MoSi,-Based Composites," in Ref. 36, pp. 8S-93. 62, S.C jha et aI., "Titanium Aluminide Foils," Advanced Malerials alld Processes, 4 (1991), pp. 87-90. 63, R.A MacKay, PX Brindley, and F.H. Froes, "Continuous Fiber Reinforced Titanium Alumnide Composites," 10M, 43 (5) (1991), pp. 23-29. 64. K. Lapierre, H. Herman, and AG. Tobin, "The Microstructure and Properties of Plasma Sprayed Ceramic Composites;' Cer. Eng. Sci. Proc., 12 (7-8) (1991), pp. 1201-1221. 65. R.W. Smith, "Reactive Plasma Spray Forming for Advanced Material Synthesis," Powder Metallurgy Inlernational. 25 (1) (1993), p. 9-16. 66, P,G. Tzantrizos, "The Reactive Spray Forming Production of Ti-AJuminide in the Tail Flame of a D.C Plasma Torch," in Ref, 23, pp. 19S-199. 67. L.E. Weiss, F, Prinz, and D. Adams, "Solid Free-Form Fabrication by Thermal Spray Shape Deposition, in Ref. 23, pp. 847-1352,
ABOUT THE AUTHORS ____ _
SanJay Sampath earned his Ph_D_ in materials science and engineering at the State University of New York at Stony Brook in 1989. He is currently a senior research engineer at Osram Sylvania in Towanda, Pennsylvania. Dr. Sam path is also a member of TMS.
Herbert Herman earned his Ph.D. in materials science and engineering at Northwestern University in 1961 . He is currently a professor in the Department of Materials Science and Engineering at the State University of New York at Stony Brook. Dr. Herman is also a member of TMS.
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