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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.

43

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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