JOURNAL OF MATERIALS SCIENCE 29 (1994) 2557-2571 Review Processing of molybdenum disilicide Y.-L. JENG, E. J. LAVERNIA Materials Science and Engineering, Department of Mechanical and Aerospace Engineering, University of Cafifornia, Irvine, CA 92717, USA Inspection of the scientific literature reveals that intermetallic compounds have, in recent years, attracted considerable interest as a result of their unique elevated temperature characteristics. Among the wide range of intermetallic compounds that are actively being studied, MoSi 2 has been singled out as a result of its unique combination of properties, which include an excellent oxidation resistance, a high modulus of elasticity, and an elevated melting point (2030 ~ In view of this interest, the present work was undertaken with the objective of providing the reader with a comprehensive review of the mechanical and oxidation behaviour of MoSi 2, paying particular attention to the synergism between processing and microstructure. Accordingly, synthesis techniques, including powder metallurgy, self-propagating high- temperature synthesis, spray processing, solid-state displacement reactions, and exothermic dispersion, are critically reviewed and discussed. In addition, recent efforts aimed at using MoSi2 as a matrix material in metal-matrix composites are also critically reviewed and discussed. 1. Introduction Refractory metal silicides (e.g. MoSi2, NbSi 2, TaSi 2 and WSi/) are actively being used in a wide variety of applications, including very large scale integrated (VLSI) devices (as gate materials), interconnectors, ohmic contacts, heating elements and Schottky bar- riers, partly due to their excellent chemical and ther- mal stability and low electrical resistivity [1-5,1. In recent years, molybdenum disilicide (MoSi2) has at- tracted considerable attention [6-13,1 as an elevated- temperature structural material, as a result of its unique combination of physical characteristics. These include a moderate density of 6.31 gcm -3, a high melting point of approximately 2030 ~ an excellent oxidation resistance, and a high modulus at elevated temperatures. In view of these attributes, it is not surprising that MoSi z is considered to be one of the most promising candidate materials to be used in gas turbine engines expected to operate at temperatures of up to 1600~ There are two critical requirements that must be satisfied in order for a structural material to be suc- cessfully utilized in elevated-temperature applications. Firstly, the material must possess attractive combina- tions of mechanical properties at the intended applica- tion temperature. Secondly, the oxidation resistance of the material must be sufficiently high to prevent envir- onmental degradation during elevated temperature exposure. In view of these requirements, the discussion that follows is divided into two parts. Firstly, recent find- ings on the deformation behaviour of MoSiz are succinctly reviewed, followed by a summary on the 0022-2461 (cid:14)9 1994 Chapman & Hall measurement of the elastic properties of MoSi 2. Sec- ondly, the environmental behaviour of MoSi 2 is dis- cussed. Although MoSi 2 is inherently brittle at low temperatures, it exhibits plasticity at temperatures above the brittle-ductile transition temperature (BDTT, ~ 1000 ~ Accordingly, when MoSi 2 is de- formed at temperatures below 1000 ~ fracture often occurs without significant plastic deformation [14-16]. Interestingly, however, despite this lack of plasticity, a significant amount of slip markings ({1 10} (33 1)) near the fracture surface have been observed [15-17]. Moreover, in these studies, disloca- tions and stacking faults were reported to be active during elevated temperature deformation [16-1, thus rendering the deformation to be relatively ductile in nature, in contrast to that exhibited by most ceramic materials. At temperatures above 1200~ (100)- and (1 1 0)-type dislocations have been reported to control the plastic deformation [15-20]. In studies on the deformation behaviour of SiCwhisko,-reinforced MoSi z metal-matrix composites (MMCs), it was re- ported that when tested at 1200~ no dislocation dissociation was observed in this material [17]. It was also suggested that the formation of stacking faults on {1 10} planes further improves the high-temperature ductility [16]. The formation of stacking faults in MoSi 2 has been closely related to the phase stability of the C 1 1 b tetragonal structure, relative to that of the C 4 0 hexagonal structure [16]. The C 1 1 b structure is a long-range ordered crystal structure made up by stacking three bcc lattices along the c axis, as depicted in Fig. 1. This particular crystal structure is thought to be responsible for the aniso- 2557
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`[OOll !F C) ,I ( ()//~ Si c o i ~-- [0101 Figure I C1 1b-type tetragonal unit cell structure of MoSi 2. a = 0.3205; c = 0.7845 nm [16]. tropy that is frequently associated with MoSi z. For example, compression tests on MoSi 2 single crystals at elevated temperatures showed that the dominant pri- mary slip planes are the { 1 1 0} planes although slip on the {01 3} planes was also observed 1-14-20]. At approximately 1900~ MoSi2 transforms from a tetragonal (C 1 1 b type, a = 0.3202, c = 0.7851 nm) to a hexagonal (C 4 0 type) crystal structure. Regarding the measurement of the elastic properties of MoSi=, some information is available. Nakamura and co-workers [18], for example, used a pulse echo method involving the measurement of elastic wave velocities in different orientations of MoSi 2 single crystals to determine the value of the elastic constants. Moreover, on the basis of this information, and using Voigt's, Reuss's, and Hill's approximations, Naka- mura and co-workers [18] determined the magnitude of the bulk modulus, K, (209.7 GPa), Young's modu- lus, E, (439.7 GPa), shear modulus, G, (191.1 GPa), and Poisson's ratio, v, (0.151) for polycrystalline MoSi2. Regarding environmental stability, MoSi2 exhibits an excellent resistance to oxidation, equivalent to that of SiC. This is evidenced by the successful utilization of MoSi2 as heating elements capable of operating in air, at temperatures in excess of 1500~ [21-25]. In particular, the best grade of MoSi= heating element, commercially known as "Kanthal Super", is capable of operating up to a temperature of 1800~ This material consists of a mixture of fine MoSiz particles bonded together with an aluminosilicate (xA1203 (cid:12)9 ySiOa) glass phase [25]. The excellent oxidation res- istance of MoSi2 is attributed in part to the formation of a self-healing, glassy silica (SiO2) layer, which pre- vents the MoSi z matrix from undergoing further oxi- dation. However, this low melting-point, glassy phase is detrimental to the elevated temperature strength. The formation of weak second phase or liquid phase along grain boundaries drastically enhances grain boundary sliding and mass transport during elevated temperature exposure, and thus reduces creep resist- ance. In addition, the relative low elevated temper- ature strength of MoSi 2 further diminishes the creep resistance. On the basis of the above findings, it is evident that despite its attractive combinations of elastic properties and oxidation resistance, MoSi2 is inherently brittle when deformed at temperatures below 1000~ and furthermore exhibits a poor creep resistance at tem- peratures above 1200-1300~ Therefore, investig- ators have sought to achieve further improvements in low-temperature ductility and elevated temperature strength by blending MoSi 2 with a soft metallic phase (e.g. ductile phase reinforcement) and a hard ceramic phase (metal matrix composites), respectively [6-13, 26-53]. Despite encouraging preliminary results ob- tained with these two approaches, there are various issues that must be addressed before MoSi z may be successfully utilized as an elevated temperature struc- tural material. In MMCs, for example, thermodyn- amic incompatibility between the MoSi a matrix and the ceramic reinforcements may ultimately degrade the oxidation resistance of MoSi2. To that effect, the thermodynamic stability of MoSi2 reinforced with various types of ceramic reinforcements has been stud- ied extensively [9, 11, 30, 37, 40, 41, 43, 54-62] and will be discussed in a later section. The objective of the present paper is to examine the various synthesis approaches that have been utilized in an attempt to improve the physical and mechanical behaviour of MoSi2 and MoSi2-based composites. Recent research results are emphasized, paying par- ticular attention to key fundamental issues derived from novel synthesis approaches. We begin with a discussion of processing techniques, followed by recent findings on oxidation behaviour and mechan- ical properties. The paper concludeswith a section on creep behaviour. 2. Processing techniques A wide variety of processing techniques have been successfully utilized to synthesize MoSi2 and the asso- ciated composites. In the following sections, these are critically discussed, paying particular attention to recent research findings. To discuss the results in a coherent manner, the present section has been sub- divided into five subsections: powder metallurgy (PM) techniques, self-propagating high temperature syn- thesis (SPS), spray processing, solid-state displace- ment reactions, and exothermic dispersion (XD TM) techniques. 2.1. Powder metallurgy techniques Powder metallurgy techniques typically involve the consolidation of discrete powders into a bulk form using temperature and/or pressure. Accordingly, the present discussion is divided into three sub-sections entitled: pressure-assisted sintering, reaction sintering, and mechanical alloying. 2558
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`2. 1.1. Pressure-assisted processing techniques The pressure-assisted processing technique, including hot pressing, sinter forging, hot isostatic pressing (HIP), and hot extrusion, is an operation in which powder preforms are consolidated by applying pres- sure at elevated temperatures [63, 64]. A specific vari- ation of this synthesis methodology, known as pressureless sintering, involves heating the powder preform in the absence of an external superimposed pressure. When applied to high-melting-point, high- strength materials, however, pressureless sintering is often time- and energy-intensive, and furthermore, often does not result in a fully dense product [63, 64]. In pressure-assisted processing, with the presence of a superimposed pressure, mechanisms such as grain rotation, grain boundary sliding, fragmentation, plas- tic deformation, and enhanced diffusion along grain boundaries all play a critical role in providing an enhanced densification rate which leads to reduced energy consumption, and often full densification [65-67]. Among the various pressure-assisted pro- cessing techniques, hot pressing and hot extrusion techniques typically involve heating the MoSi z pow- ders in a die held under uniaxial pressure to densify or deform the material to a predetermined condition. In the case of sinter forging, the procedure is similar to that of hot pressing, but no lateral constraint is re- quired. These three techniques, hot extrusion, hot pressing and sinter forging, are relatively easy and in- expensive to operate, but unfortunately they are sever- ely limited in terms of their ability to produce complex shapes. Hot isostatic pressing, although more elabor- ate and costly to operate, provides advantages, such as no density variations, almost unlimited aspect ratio, uniform grain structure and the ability of achieving very complicated shape products, over the other three techniques [.63,64]. The modelling of pressure- assisted techniques has been aggressively pursued for several decades. Unfortunately, factors such as the variations in powder packing, varying particle mor- phologies, the influence of friction on deformation, and environmental effects have hindered the successful development of accurate models that may ulti- mately be applied to real processing situations [66-69]. Among the various pressure-assisted processing techniques that are available, hot pressing is most widely used. It has been successfully utilized to process monolithic MoSi 2 [.7] and a wide variety of MoSiz- based composites, such as those containing Nb- [31], W- [,31], C- [,32], ZrO2- [.43, 45], A120 3- [,46], SiC- [.7, 34,42], and TiC- [37] reinforced MoSi2, with significant improvements in both flexure strength and fracture toughness. However, materials prepared by uniaxially hot-pressing a mixture of reinforcement and MoSi 2 powders are inherently simple in shape, and due to their poor machinability, often cannot be fur- ther processed into desired shapes. It is also evident, however, that further work is needed before pressure- assisted processing techniques may be commercially used to synthesize complex shapes of MoSi 2 and MoSi2-based composites. 2. 1.2. Reaction sintering Reaction sintering processing techniques generally in- volve the in situ reaction of constituent powders, generally pressed into a preform, to produce a bulk composite preform that is different from the initial reactants [70]. Reactions can be either very fast (even explosive) or very sluggish, and either exothermic or endothermic, depending on starting and final reactant compositions, reactant compact microstructures; pro- cessing environments, and thermal boundary condi- tions [70]. In the discussion that follows, reaction sintering is defined as 'reaction-assisted sintering', which is diffusion-controlled and non-ignitable, and therefore is relatively sluggish. The high reaction rate processing techniques will be discussed in the SPS section. Inspection of the available literature reveals that a wide variety of chemical reactions are generally in- volved in the preparation of structural materials using reaction techniques, ranging from powder synthesis (e.g., sol-gel technique, polymer pyrolysis, and de- composition of salts and organometallics) to direct production of a bulk material (e.g., chemical vapour deposition (CVD) and chemical vapour infiltration (CVI) techniques) [71]. However, as stated by Rice [70], although various reactions are capable of produ- cing composite materials, of relevance are "reactions that can produce composite powders for consolida- tion into composites, or more commonly reactions involving a powder compact to form a composite product with the aid of heat, possibly with pressure as well, such as hot pressing, or hot isostatic pressing". One well-known example, dating back to early 1980s, of the reaction synthesis approach involves the preparation of zirconia-toughened mullite (ZrO2 + 3A1203'2SIO2) by employing the following reac- tion [72]: 2ZrSiO4 -4- 3A120 3 --* 3AlzO3"2SiO 2 + 2ZrO 2 (1) to form mullite-zirconia composites, which exhibited outstanding combination of mechanical properties. Recently, Messner and Chiang [.73-75] have suc- cessfully fabricated liquid-phase reaction-bonded SiC- MoSi 2 composites by using alloyed Si-Mo melts on the basis of the following reaction: xC(graphite) + yMo + (x + 2y)Si ---, xSiC + yMoSi 2 (2) In this SiC-MoSi 2 composite, MoSi 2 is utilized as a reinforcement to toughen the SiC matrix. It is worth noting that the final SiC-MoSi2 composite did not contain residual Si, whose presence is highly deleteri- ous in elevated temperature applications. The liquid- phase reaction-bonding technique not only preserves the advantages of CVI, but also significantly reduces the time required for processing, and is capable of achieving a final density of 90%. However, it has been found [.73] that below the melting of MoSi 2, mixtures of Me and Si do not react completely, and a reaction layer develops, which inhibits further reaction. On the basis of the previous findings [73,743, Weiser et al. [.76] employed pressureless reaction 2559
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`sintering to fabricate MoSi2-based composites. In their work, MoSi2 matrices with either 0 or 20 vol % SiC particulate were mixed with 10 vol % of elemental Mo and Si (an atomic ratio of 1:2), and then sintered in Ar at various temperatures. The addition of ele- mental Mo and Si was found to greatly enhance the densification of monolithic MoSi2 at temperatures as low as 1400 ~ However, almost no densification was found in the composites containing SiC particulates, and this result was attributed to swelling and poor consolidation resulting from the undesirable reaction between SiC and Mo-Si elemental additives. On the basis of the above-mentioned findings, and despite encouraging preliminary results, it is evident that the optimization of process parameters during reaction sintering requires further attention. However, even if the properties that are achieved with reaction sintered materials were only comparable with those of materials processed by conventional means, reaction sintering should potentially be the preferred approach in terms of production costs. Furthermore, it may be beneficial to use pressure-assisted processing tech- niques, combined with reaction sintering and liquid- phase sintering, e.g. Mo-Si reaction to form MoSi 2 and melting of Si, because this approach should pro- vide a synergistic effect, not only to diminish the produc- tion costs, but also to improve mechanical behaviour. Si 2. 1.3. Mechanical alloying In recent years, the mechanical alloying (MA) tech- nique, an application of mechano-chemistry, has at- tracted considerable interest [77 99]. MA involves repeated welding, fracturing and rewelding of powders (elemental or alloyed) during high-energy milling under a controlled atmosphere, as shown in Fig. 2 [77]. This process can be divided into four stages [96]: (i) first stage: an intense cold welding period; (ii) intermediate stage: a rapid fracturing period, forming lamellae; (iii) final stage: a moderate cold welding period, producing finer and more convoluted lamellae; (iv) completion stage: a steady state period. Factors such as the amount of cold work, inter-layer spacing, and interstitial contamination critically influ- ence the formation of compounds during MA [77]. During the high-energy milling process, repeated par- ticle impact at contact points leads to a local concen- tration of energy, which under some circumstances may ignite a self-propagating reaction. This phenom- enon, originally described by Atzmon [79-81] as "hot-spotting', may lead to local melting and welding, enhanced inter-particle diffusion, chemical reaction, and ultimately to compound formation. The mechanical alloying technique has been exten- sively utilized to produce a wide variety of compounds with highly refined microstructures [77-88]. More- over, MA materials have been reported to exhibit non- equilibrium microstructural characteristics, such as the extension of solid solubilities [89, 90]. Not surpris- ingly, MA has been successfully used to combine alloying elements that would otherwise be unattain- able by conventional techniques. One notable ex- Hot spot Figure 2 Schematic of MoSi 2 compound formation by mechanical alloying [77]. ample is superconducting intermetallic Nb3Sn, which is difficult to prepare by conventional melting process, due to the significant melting point difference between Nb and Sn (~ 2240~ [91]. The large degree of microstructural refinement that is associated with MA materials often leads to dramatic improvements in mechanical behaviour. In 1966, for example, Seybolt [92] reported a 400% improvement in the rupture strength of MA A1203-FeA1 relative to that of the matrix alloy. Some current applications of the MA include the production of oxide-dispersion strengthened alloys and intermetallic compounds [93,94]; the devel- opment of oxidation and hot-corrosion resistant 2560
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`coatings [95]; the alloying of otherwise immiscible systems (e.g. Cu-Pb) [96]; the production of super- corroding and superconducting (Nb-Sn) materials [91]; and the preparation of amorphous compounds [78]. Recently, MA has been applied to the fabrication of silicides [77, 78, 86, 97, 98]. Accordingly, group V transition metal/Si silicides and amorphous Ti-Si alloy powders have been produced by mechanical alloying of elemental powders [78]. In group V trans- ition metal/silicon systems, amorphization was re- ported in the Ta-Si system, but not in the Nb-Si system [96]. Factors such as the highly exothermic release of heat of mixing, fast interdiffusion rates, local melting, and the presence of a high defect concentra- tion, have all been proposed to influence amorphiz- ation, although the precise mechanisms remain to be established [82]. The formation of MoSi 2 was first reported by Iwatomo & Uesaka [97], during MA of elemental Mo (5 l.tm) and Si (50 gm) powders. In this study, it was reported that m-MoSi 2, a low-temperature tetragonal phase, forms initially followed by the formation of a high-temperature hexagonal phase, normally present at temperatures in excess of 1900 ~ Following MA, the crystalline MoSi z particles were highly refined, exhibiting a grain size ranging from 5 to 10nm. Moreover, Iwatomo & Uesaka [97] also confirmed the presence of amorphous regions in the MA MoSi 2 powder during prolonged milling time. The sintered MA MoSi2 powders did not exhibit any significant differences in hardness or electrical conductivity when compared to equivalent material prepared using other techniques. However, the ultra-fine structure of the mechanically alloyed powders significantly reduced the sintering temperature (by nearly 400~ while yielding a final density in excess of 97% of theoretical [97]. Moreover, Ma et al. [98] reported that the formation of MoSi z by room-temperature high- energy ball milling of elemental powders is self-prop- agating. Very recently, Jayashankar & Kaufman [99] used MA to synthesize in-situ SiC-reinforced MoSi 2 com- posites. In this work, Mo and Si powders, combined to yield the desired stoichiometry, were mixed with 4 wt % of C, followed by high-energy mechanical attrition. The overall reaction: SiO 2 + 3C --~ SiO + O + 3C SiO + 2C + CO ---+ SiC + 2CO (3) was proposed to describe the formation of SiC, and the simultaneous elimination of SiO 2 during hot pres- sing of mechanically alloyed MoSi 2 composite pow- ders. The presence of C was found to significantly improve the overall homogeneity and cleanliness of the microstructure. Moreover, the significant weight loss noted in samples containing C was attributed to the formation of CO during MA, consistent with the results of Maloy et al. [32]. Schwarz et al. [100] recently reported a number of advantages that are available by using MA for the synthesis of MoSi2-based alloys starting from ele- mental constituents: (i) higher hot-pressed density; (ii) lower hot-pressing temperatures for consolidation; (iii) better chemical homogeneity; (iv) improved room temperature hardness with second-phase additions. It is worth noting, however, that the formation of amorphous SiO2 phase still occurred, although ex- treme care was taken during MA. In applications requiring creep resistance, it is imperative to produce MoSi 2 composites that are free of SiO 2. Hence, in principle, the MA technique may be successfully util- ized to process MoSi 2 and MoSi 2 composites, as long as careful control of impurity content and environ- mental control are duly exercised. 2.2. Self-propagating high-temperature synthesis Chemical reactions that are accompanied by the release of thermal energy are generally referred to as exothermic reactions. Self-propagating (or self- sustaining) high-temperature synthesis, so-called SPS (or SHS), is a technique involving the propagation of a high-temperature zone, driven by a highly exothermic reaction, through a compact of reactants. Although some of the reactions that were discussed are exo- thermic in nature, and hence in principle should be able to self-propagate, SPS generally proceeds at reac- tion rates that are substantially faster relative to those present during reaction sintering. The extent of reac- tion, q, is related to the temperature profile that is present during SPS, and may be represented by the following expression [101-103]: ST Cppv(T - To) - K 1 8X q(X) = ~,T (4) (K 2 -- ~:l)~x x + qpv where Cp and p are the heat capacity and density of the product, respectively; v the velocity of combustion wave; T the reaction temperature; To the initial tem- perature; ~c 1 and K 2 are the thermal conductivity of reactants and products, respectively; q is the heat of reaction; and x is the coordinate along which the combustion wave propagates. Inspection of the available literature shows that SPS and other related techniques have been suc- cessfully used to produce a variety of alloys and composites [70, 104-118]. Early studies on transition- metal silicides prepared by SPS techniques showed that the reaction product contains one or more inter- mediate phases [104, 105]. In related work, Tram- bukis & Munir [106], and Bhattacharya [107] dem- onstrated that it is possible to use SPS to synthesize TisSi 3 possessing attractive elevated temperature characteristics (e.g. strength and oxidation resistance) that are suitable for engineering applications. It has been found [106] that both heating rates and particle sizes critically influence the synthesis of silicides. In this study, SPS involved two reactions: a solid-state diffusion reaction for conditions of small particle size and low heating rates, and a liquid-state reaction for large particles and high heating rates. The time period 2561
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`during which diffusion was rate-controlling increased with decreasing heating rate and decreasing particle size, a trend that was also noted to result in the formation of more intermediate phases. Regarding mechanical behaviour, Bhattacharya [107] recently measured a fracture toughness of approximate 5 MPam 1/2 for SPS-processed 15 wt% SiC-TisSi 3 composite, and noted that the presence of SiC not only dramatically enhances fracture toughness, but also alters the kinetics and thermodynamics during SPS. Regarding the synthesis of MoSi 2 by SPS, very limited studies have been conducted [106-108]. In a recent study, Deevi [108] successfully synthesized single-phase MoSi 2 by SPS, which involves exother- mic diffusional reaction of solid Mo with liquid Si. It was found that product formation depends on the temperature and available diffusion reaction time. At low heating rates, M%Si 3 is the predominant phase prior to the melting of Si, while the amount ~of MoSi2 phase increases with increasing heating rate. No mechanical properties and microstructural details were provided in this study. In general, however, SPS processes typically involve relatively violent reaction rates, and thus it is often challenging to achieve a high degree of microstruc- tural control. Inspection of the recent scientific literat- ure, however, immediately reveals that this synthesis approach is receiving considerable attention [103, 113-118]. Some of the challenges that must be addressed if SPS processes are to live up to their potential include an accurate assessment of the rel- evant process economics; the minimization of residual porosity in SPS processed materials; and the de- velopment of control strategies in order to obtain reproducible microstructures and properties [103, 113-1183. 2.3. Plasma-spray processing Spray processes offer a unique opportunity to combine the benefits associated with fine particulate technology (e.g. microstructure refinement, alloy modifications, etc.) with in situ processing, and in some cases, near-net shape manufacturing. Spray-based technology has evolved over the past few decades, and as a result a variety of spray-based methods are currently available. These include low-pressure plasma deposition [28, 29], modified gas welding techniques [119], high-velocity oxyfuel thermal spraying E120] and spray atomization and deposition processing [121-126]. Spray processing generally in- volves highly non-equilibrium conditions, and as a result these processes offer the opportunity of modi- fying the properties of existing alloy systems, and developing novel alloy compositions. In principle, such an approach will inherently avoid the extreme thermal excursions, with concomitant microsegrega- tion, that are normally associated with casting pro- cesses. Furthermore, this approach also eliminates the need to handle fine reactive particulates, normally associated with powder metallurgical processes. Plasma technology has been researched extensively over the past four decades, and is similar to spray 2562 deposition in that it involves the deposition of discrete droplets onto a substrate or shaped container. Unlike spray deposition, however, in which the maximum attainable temperature is of the order of 1000-3000 ~ plasma torches are capable of achiev- ing temperatures in excess of 10 4 K. Accordingly, plasmas may be utilized to raise reactant temperatures high enough to make the chemical potential of the desired reaction negative, and to ensure high reaction rates. The plasma deposition technique has been success- fully utilized in a large number of commercial appli- cations, such as in corrosion, wear enhancement and surface treatment [28, 29, 127-1383. More recently, plasma-sprayed superconductors [134] and biomed- ical devices [135] are actively being pursued. A plasma spray;~g technique, known as low vacuum plasma deposition (LVPD), has also been successfully applied to fabricate highly dense bulk composite materials. A schematic diagram showing plasma de- position is shown in Fig. 3. Under a low-vacuum environment (especially with low oxygen content), highly dense composite materials may be readily syn- thesized. Similar to the gas-atomization and depos- ition process, the LVPD technique has advantages such as very small grain size, chemical homogeneity, non-equilibrium solubility and near final-shape pro- duction. In the LVPD process, an inert gas stream, such as Ar or Ni, is commonly used to accelerate the powder particles as they are being injected into the plasma gases. Due to the high speed of the plasma gases, exceeding Mach III in some cases, the time spent from injection to deposition is almost infinit- esimal (of the order of milliseconds). To obtain op- timal microstructural characteristics during plasma spraying, it is therefore critical to select appropriate gun-to-substrate distances by taking into account fac- tors such as alloy composition and particle size. If the particles are too large, for example, they will not melt completely, whereas if the particles are too small they may be vaporized by the plasma. Similarly, particle morphology has been shown to influence the final microstructure of plasma-sprayed materials and spherical particles are preferred over irregularly- shaped particles, due to the better fluidity from spher- ical particles which can provide a better powder feeding control and thus a better control on the final microstructure. Inert aas Powder il Ca inlet Figure 3 Schematic of plasma spray deposition processing [127].
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`Plasma processes may be readily utilized to prepare composite materials, simply by spraying either pre- blended composite particles or by simultaneously co- spraying metal and ceramic particles. In related work, Castro et al. [28, 29] successfully fabricated mono- lithic and composite MoSi 2 using the LVPD tech- nique. The density of both reinforced and monolithic plasma-sprayed MoSi2 was noted to be in the 95-98% range, and the materials exhibited a highly-refined microstructure. Moreover, the hardness and fracture toughness of the LVPD-processed materials were noted to be significantly improved relative to conven- tionally processed materials. An interesting, although not surprising, characteristic observed in the LVPD- processed material was the presence of a laminar, plate-like microstructure. When fracture tested in a direction parallel to the principal axis of the plates, preferential crack growth occurred along the prior splat-boundaries, suggesting less-than-optimum inter- particle bonding. In related work, Tiwari et al. [132, 136] used the vacuum plasma spraying (VPS) technique to prepare monolithic, TiB2-based and SiC- based MoSi2 composites. In this study, highly-dense monolithic and composite MoSi z were fabricated, with concomitant improvement in toughness re- sponse. Various reinforcement morphologies were re- ported, including splat-like TiB 2 and relatively equiaxed SiC. These morphologies were attributed to the concurrent melting of TiB 2 and the sublimation of SiC,