Materials Science and Engineering, A 155 (1992) 1 - 17 1 A comparative overview of molybdenum disilicide composites A. K. Vasudfvan Office of Naval Research, Arlington, VA 22217 (USA) J. J. Petrovic Los Alamos National Laboratory, Los Alamos, NM 87545 (USA) Abstract MoSi2-based composites possess significant potential to meet the demands of advanced high temperature structural applications in the range 1200-1600°C, in oxidizing and aggressive environments. These materials constitute an important new class of "high temperature structural silicides'. The intermetallic compound MoSi 2 possesses properties which make it a very desirable matrix for high temperature composites, and these properties are described and compared with those of other high melting point silicides. The developmental history of composites based on MoSi 2 is traced from its beginnings to the present. Mechanical property improvements derived from SiC and ZrO 2 reinforcements, as well as matrix alloying, are described, and properties of current MoSi2-based composites compared with those of silicon-based structural ceramics. Finally, important research and development directions for the continued improvement of MoSi~- based composites and their use as high temperature structural components are discussed. 1. Introduction In the last decade, several engine companies have undertaken studies to evaluate the needs and payoffs of the use of advanced materials in 21st century propul- sion systems in civil and military aircraft. These studies have shown that the use of these candidate materials (such as intermetallics) can have significant payoffs in terms of fuel efficiency, performance, higher strength- to-weight ratios and reduced direct operational costs FUEL 5o 30 PERCENq SAVINGS 20 SUPERSONIC DOC THROUGHFLOW 25 ~FAN WITH ADVANCE CORE 20 VARIABLE CYCLE ENGINE WITH ADVANCE CORE / 10 1968 TECHNOLOGY 5/ o CONCORDE TECHNOLOGY BASELINE I I 1971 1988 Fig. 1. High speed civil transport technology [ 1 ]. I 2o10 TIME ~ (DOCs). It has been projected that revolutionary gains in excess of 20% in fuel economy and about 10% reduction in the DOCs can be envisaged, if these advanced high temperature materials can be brought to fruition [1]. Figure 1 demonstrates the gains in the fuel and DOCs with improvements in engine technology that can be achieved in high speed civil transport pro- pulsion technology [1 ]. Smith [2] has evaluated the cost benefits, for the GT 20/100 engine, of using various materials that can influence the engine specific fuel consumption (SFC), cost, and weight on the aircraft DOCs at a fuel cost of $1.50 per gallon. He observed that the need for low density structural materials is important for weight savings, when compared with other property improve- ments, such as strength and stiffness (Fig. 2(a)). How- ever, SFC plays a dominant role in the DOCs when compared with component weight reduction alone (Fig. 2(b)). The next DOC effect is reflected in engine maintenance costs, which are directly related to the selection of reliable high temperature materials. It should be mentioned that such studies are specific to the aircraft, engine, mission and fuel costs. Materials with high temperature capability in combination with considerable design ingenuity can be considered as important steps to these improvements. From the large body of literature data on high tem- perature structural materials, one can broadly classify 0921-5093/92/$5.00 © 1992 - Elsevier Sequoia. All rights reserved
`
`GE-1007.001
`
`

`
`2
`
`A. K. Vasudévan, J. J. Petrovic
`
`/ MoSi2-based composites
`
`tion and mechanical (strength, toughness and creep)
`properties,
`in addition to being cost effective. The
`drawback of the superalloys is that
`they are high
`density materials. Recently, a few aluminides (such as
`Ni3Al, Ti3Al) have been considered as replacements
`for the conventional superalloys because of their room
`temperature ductility and lighter weight. However,
`these aluminides have poor oxidation resistance above
`about 650 °C and require coatings (just like the super-
`alloys). COMPGLAS, SiC-reinforced lithium alu-
`minum silicate ceramic matrix composites, are also
`being evaluated as a candidate material for light weight
`and high temperature structural applications. At
`present, such advanced materials have not proven to be
`cost effective to manufacture.
`
`2 A.K. Vasud~van, J. J. Petrovic / MoSi:based composites 16 (a) / f ../ Weight 8 l_ I 0 5 10 15 20 Property Improvement, % 6 -- (b) [ Engine GT 20/100 / / 0 10 20 Parameter Reduction, % Fig. 2. Evaluation of (a) weight savings [2] and (b) DOC benefits for an ultrahigh bypass engine. TABLE 1. List of candidate materials for high and low temperature applications < 1000 °C > 1000 °C Ni-base superalloys MAR M-246 Ni aluminides Ti aluminides COMPGLAS '- Use temperature--' Si-based ceramics 1000 °C SiC, Si3N4, SiC-SiC NiAI, NbAI3, TaAI 3 MoSi 2 them into two application groups: use temperature is below 1000 °C, and use temperature is above 1000 °C (Table 1 ). For applications in the range 800-1000 °C, current nickel-base superalloys (such as MAR M-246) are being used. Although these alloys require cooling during operational periods, they have excellent oxida- tion and mechanical (strength, toughness and creep) properties, in addition to being cost effective. The drawback of the superalloys is that they are high density materials. Recently, a few aluminides (such as Ni3AI, Ti3AI ) have been considered as replacements for the conventional superalloys because of their room temperature ductility and lighter weight. However, these aluminides have poor oxidation resistance above about 650 °C and require coatings (just like the super- alloys). COMPGLAS, SiC-reinforced lithium alu- minum silicate ceramic matrix composites, are also being evaluated as a candidate material for light weight and high temperature structural applications. At present, such advanced materials have not proven to be cost effective to manufacture. At higher temperatures, between 1000 and 1600 °C, primary candidates are silicon-based ceramic materials. At operating temperatures greater than 1000°C, severe requirements for oxidation-resistant materials become important. Although the silicon-base materials (and their composites) have excellent oxida- tion resistance and lower density, their development is considered to be higher risk because of their brittleness over the entire temperature range. In addition, they are at present not cost effective, since the cost of the start- ing material, fabrication and machining are significant. As a result, alternative candidate materials are under investigation. These are based on aluminide (NiAI, NbA13, TaA13 ) and silicide (MoSi2, TisSi 3 ) matrix com- positions. Their lower densities, higher melting points and high thermal conductivities make them attractive for high temperature engine applications. The alumi- nides are brittle at room temperature, have low strengths (and creep) at the required high tempera- tures, and lack long-time oxidation resistance above 1200 °C. Recently, however, a new class of silicide matrix composite (SMC) materials has been identified, designed around MoSi 2 which provides an alternative to structural ceramics as shown in Fig. 3, a plot show- ing the operating temperature vs. strength-to-weight ratio of various generic classes of materials [3]. The use of these higher temperature materials can mimimize turbine air cooling (commonly required for the super- alloys) and also minimize the number of engine parts, thus simplifying the design to reduce the engine weight. In addition, with higher operating temperature capabilities, one can expect efficient fuel combustion that can result in beneficial environmental effects. Details of the materials selection and design analysis are given by Stephens [1] and Smith [2]. In this article we shall briefly describe the historical development of the silicide materials and their physical and mechanical properties in comparison with other high temperature materials, and, finally, list a few thoughts on future research directions in these areas.
`
`At higher temperatures, between 1000 and 1600 °C,
`primary
`candidates
`are
`silicon-based
`ceramic
`materials. At operating temperatures greater
`than
`1000 °C, severe requirements for oxidation-resistant
`materials become important. Although the silicon-base
`materials (and their composites) have excellent oxida-
`tion resistance and lower density, their development is
`considered to be higher risk because of their brittleness
`over the entire temperature range. In addition, they are
`at present not cost effective, since the cost of the start-
`ing material, fabrication and machining are significant.
`As a result, alternative candidate materials are under
`investigation. These are based on aluminide (NiAl,
`NbAl3, TaAl3) and silicide (MoSi2, Ti5 Si3) matrix com-
`positions. Their lower densities, higher melting points
`and high thermal conductivities make them attractive
`for high temperature engine applications. The alumi-
`nides are brittle at
`room temperature, have low
`strengths (and creep) at the required high tempera-
`tures, and lack long—time oxidation resistance above
`1200 °C. Recently, however, a new class of silicide
`matrix composite (SMC) materials has been identified,
`designed around MoSi2 which provides an alternative
`to structural ceramics as shown in Fig. 3, a plot show-
`ing the operating temperature vs. strength-to-weight
`ratio of various generic classes of materials [3]. The use
`of these higher temperature materials can mimimize
`turbine air cooling (commonly required for the super-
`alloys) and also minimize the number of engine parts,
`thus simplifying the design to reduce the engine weight.
`In addition, with higher operating temperature
`capabilities, one can expect efficient fuel combustion
`that can result
`in beneficial environmental effects.
`
`
`
`5000 NM Mission
`
`Engine GT 20/100
`Fuel Price - $1.50/gal
`
`DOC
`Reduction
`%
`
`Parameter Reduction, %
`
`Fig. 2. Evaluation of (a) weight savings [2] and (b) DOC benefits
`for an ultrahigh bypass engine.
`
`TABLE 1. List of candidate materials for high and low
`temperature applications
`
`
`
`< 1000 °C >1000 °C
`
`Ni-base superalloys
`MAR M-246
`
`‘- Use temperature -* Si-based ceramics
`1000 °C
`SiC, Si3N4, SiC-SiC
`
`Ni aluminides
`
`NiAl, NbAl3, TaAl3
`
`MoSi2
`Ti aluminides
`COMPGLAS
`
`them into two application groups: use temperature is
`below 1000 °C, and use temperature is above 1000 °C
`(Table 1).
`For applications in the range 800-1000 °C, current
`nickel-base
`superalloys
`(such as MAR M-246)
`are being used. Although these alloys require cooling
`during operational periods, they have excellent oxida-
`
`Details of the materials selection and design analysis
`are given by Stephens [1] and Smith [2].
`In this article we shall briefly describe the historical
`development of the silicide materials and their physical
`and mechanical properties in comparison with other
`high temperature materials, and,
`finally,
`list a few
`thoughts on future research directions in these areas.
`
`GE-1007.002
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`GE-1007.002
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`
`A. K. Vasudévan, J. J. Petrovic
`
`/ M0Si_,-based composites
`
`3
`
`(C C c)
`Carbon/Carbon
`Composites
`
`Slllcldo Matrix Cornposltoslsllc)
`
`Metal Matrix
`Composites
`
`Ceramic
`Composites
`
`applied for in 1953). Readers are referred to an article
`by Schlichting [11] in which early uses of MoSi2 are
`nicely summarized.
`W. A. Maxwell (National Advisory Committee for
`Aeronautics) appears to have been the first person to
`seriously suggest
`the use of M0Si2 as a structural
`material
`in the early 1950s [12-14]. He noted the
`excellent oxidation resistance of MoSi2 and determined
`some of its elevated temperature mechanical proper-
`ties, including strength, creep resistance and thermal
`shock behavior. He also performed an initial study of
`MoSi;,-A1203 composites. Maxwell noted that the low
`temperature brittleness of MoSi2 was its major draw-
`back for structural applications. Curiously, Maxwell’s
`thoughtful initial work on MoSi2 was not continued.
`This is probably because the high temperature struc-
`tural materials community, at that time, was not yet
`ready to deal with brittle materials. Serious considera-
`tions of brittle materials for structural applications only
`began with the advent of structural ceramics in the
`early 1970s. Briefly, until now there have been no
`developments in the identification of phases in these
`silicide compounds, although Fitzer had observed
`MoSi2 and Mo5Si3 compounds in his coatings work
`[15]. Notable early work was the study of Nowotny et
`al.
`[16] of the Mo—Si—C ternary phase diagram,
`in
`which various phases and their relations are identified.
`The next major step in MoSi2 composite develop-
`ment occurred as a result of the work of E. Fitzer in
`
`A. K. VasudOvan, J, J. Petrovic / MoSi:-based composites 3 4000 - @ (2205) o (~ Slllclde Matrix Composltes(SMC) 3000 ~ (1650) E ,~ 2000 c o~ (1095) ~~ e~ 0 1000 ~~ (54o) \. I 10 3 10 4 Strength/Weight Ratio (Inches or mm x 25.4) Fig. 3. Strength:weight ratio of several classes of high tempera- ture structural materials as a function of the operating tempera- ture, indicating the superiority of alternative materials over the conventional alloys [ 3]. 2. History The intermetallic compound MoSi 2 has been known since 1907 [4], when it was considered as a high tem- perature corrosion-protective coating material for ductile metals. Because of its excellent oxidation behavior and the fact that it was brittle in nature, it was first applied as a coating material [4]. The use of MoSi 2 as a heat conductor in oxidizing atmospheres at tem- peratures up to 1650 °C is probably the most impor- tant current commercial application [5, 6]. For the heating of an electrical resistance furnace above 1400 °C, Haegglund and Rehnquist [7] proposed ele- ments composed of Mo-Si alloys with the 50% Mo-50% Si composition. The first patent of MoSi 2 and Mo-Si-A1 alloys for heat conductors made by powder metallurgy production was by Kieffer et al. [8]. It was Fitzer [9] in 1955 who conducted the first long-time resistance tests in air at surface temperatures of 1700°C. For technological applications (in the late 1940s) it was necessary to solve a number of basic problems: (1) prevention of oxidation "PEST"; (2) deformation resulting from its own weight (creep) and embrittlement on cooling from high temperatures (toughness and brittle-to-ductile transition), and (3) technological fabrication methods for the starting materials and finished parts. The first commercial heating elements were patented by Kanthal [10] in 1956 (the patent was applied for in 1953). Readers are referred to an article by Schlichting [11] in which early uses of MoSi 2 are nicely summarized. W. A. Maxwell (National Advisory Committee for Aeronautics) appears to have been the first person to seriously suggest the use of MoSi 2 as a structural material in the early 1950s [12-14]. He noted the excellent oxidation resistance of MoSi 2 and determined some of its elevated temperature mechanical proper- ties, including strength, creep resistance and thermal shock behavior. He also performed an initial study of MoSi2-AI203 composites. Maxwell noted that the low temperature brittleness of MoSi2 was its major draw- back for structural applications. Curiously, Maxwell's thoughtful initial work on MoSi 2 was not continued. This is probably because the high temperature struc- tural materials community, at that time, was not yet ready to deal with brittle materials. Serious considera- tions of brittle materials for structural applications only began with the advent of structural ceramics in the early 1970s. Briefly, until now there have been no developments in the identification of phases in these silicide compounds, although Fitzer had observed MoSi 2 and MosSi 3 compounds in his coatings work [15]. Notable early work was the study of Nowotny et al. [16] of the Mo-Si-C ternary phase diagram, in which various phases and their relations are identified. The next major step in MoSi 2 composite develop- ment occurred as a result of the work of E. Fitzer in Germany. In 1971, Erdoes [17] considered silicides as coating materials for gas turbine engines. In 1973, Fitzer et al. published work on the addition of AI:O 3 and SiC to MoSi2, which revealed an improvement in elevated temperature strength [18]. Fitzer also studied MoSi 2 composites reinforced with niobium wires, and demonstrated an improvement in mechanical proper- ties. These developments led Fitzer's colleague, J. Schlichting, to publish a detailed review article in 1978 [11], suggesting that MoSi 2 was an important matrix material for high temperature structural composites. Unfortunately, Schlichting's article was published in German, in a relatively obscure journal, and as a result apparently did not receive widespread attention. The next important step occurred in 1985, with the publication of two key articles. The first was an article by Fitzer and Remmele [19], describing in detail their work on niobium wire-MoSi2 matrix composites. In this article, they showed that niobium wire reinforce- ment significantly improved the room temperature mechanical properties of the composite. The second key article was that of Gac and Petrovic [20]. In this article, they established the feasibility of SiC whisker-MoSi 2 matrix composites, demonstrating improvements in room temperature strength and fracture toughness.
`
`4000
`(2205)
`
`3000
`(1650)
`
`2000
`(1095)
`
`
`
`
`
`OperatingTemperature,°F(°C)
`
`1000 ‘
`
`Conventi
`Materials
`(Titanium and
`Superalloys)
`
`(540)
`
`(40
`
`_A G.
`
`104
`
`Strength/Weight Ratio
`(Inches or mm x 25.4)
`
`Fig. 3. Strengthzweight ratio of several classes of high tempera-
`ture structural materials as a function of the operating tempera-
`ture, indicating the superiority of alternative materials over the
`conventional alloys [3].
`
`2. History
`
`The intermetallic compound MoSi2 has been known
`since 1907 [4], when it was considered as a high tem-
`perature corrosion-protective coating material
`for
`ductile metals. Because of
`its excellent oxidation
`
`behavior and the fact that it was brittle in nature, it was
`first applied as a coating material [4]. The use of MoSi2
`as a heat conductor in oxidizing atmospheres at tem-
`peratures up to 1650 °C is probably the most impor-
`tant current commercial application [5, 6]. For the
`heating of an electrical
`resistance furnace above
`1400 °C, Haegglund and Rehnquist [7] proposed ele-
`ments composed of Mo—Si alloys with the 50%
`Mo—50°/o Si composition. The first patent of MoSi2 and
`Mo—Si—Al alloys for heat conductors made by powder
`metallurgy production was by Kieffer et al. [8]. It was
`Fitzer [9] in 1955 who conducted the first long-time
`resistance tests in air at surface temperatures of
`1700 °C. For technological applications (in the late
`1940s) it was necessary to solve a number of basic
`problems:
`(1) prevention of oxidation “PEST”;
`(2)
`deformation resulting from its own weight (creep) and
`embrittlement on cooling from high temperatures
`(toughness and brittle—to-ductile transition), and (3)
`technological
`fabrication methods for
`the starting
`materials and finished parts.
`The
`first
`commercial heating elements were
`patented by Kanthal
`[10]
`in 1956 (the patent was
`
`Germany. In 1971, Erdoes [17] considered silicides as
`coating materials for gas turbine engines. In 1973,
`Fitzer et al. published work on the addition of A1203
`and SiC to MoSi3, which revealed an improvement in
`elevated temperature strength [18]. Fitzer also studied
`MoSi2 composites reinforced with niobium wires, and
`demonstrated an improvement in mechanical proper-
`ties. These developments led Fitzer’s colleague, J.
`Schlichting, to publish a detailed review article in 1978
`[11], suggesting that MoSi2 was an important matrix
`material for high temperature structural composites.
`Unfortunately, Schlichting’s article was published in
`German, in a relatively obscure journal, and as a result
`apparently did not receive widespread attention.
`The next important step occurred in 1985, with the
`publication of two key articles. The first was an article
`by Fitzer and Remmele [19], describing in detail their
`work on niobium wire—MoSi2 matrix composites. In
`this article, they showed that niobium wire reinforce-
`ment significantly improved the room temperature
`mechanical properties of the composite. The second
`key article was that of Gac and Petrovic [20]. In this
`article,
`they
`established
`the
`feasibility
`of SiC
`whisker—MoSi2 matrix composites, demonstrating
`improvements
`in room temperature strength and
`fracture toughness.
`
`GE-1007.003
`
`GE-1007.003
`
`

`
`4 4O I I I A. K. Vasud~van, J. J. Petrovic I I I 36 CO 32 ILl > ~. 28 o < "1- 24 O EE < LU 2O 03 ILl EE LL 16 O LU m 12 Z 8 4' MOS .OS ALAMOS ~ PRATT & WHITNEY 0 ~ , McDONNELL DOUGLAS, 1985 1986 1987 1988 1989 1990 1991 1992 YEAR Fig. 4. Growth of molybdenum disilicide research activities since 1986. / MoSi2-based composites After 1985, Fitzer did not appear to continue his work on MoSi 2 composites, possibly because he was retiring from active research. However, the work at the Los Alamos National Laboratory (LANL) continued at a low level, but steady pace under Department of Energy, Energy Conversion and Utilization Tech- nologies (DOE)-ECUT support. Continual improve- ments were made in the mechanical properties of SiC-MoSi2 composites [21-23], through a number of generations of composite materials. In particular, in 1988 Carter et al. [22] established that submieron vapor-solid SiC whisker-MoSi2 matrix composites exhibited mechanical property levels within the range of high temperature engineering applications. At LANL, additional support for MoSi 2 composite research was obtained through the Office of Naval Research, in 1989. This LANL work attracted the attention of researchers in the aerospace industry. In particular, in about 1986, S. Bose at Pratt & Whitney began examining SiC-MoSi 2 composites for potential aircraft engine applications, through an informal col- laboration with LANL. Also, in about 1988, P. J. Meschter at McDonnell-Douglas (currently at the General Electric Company) began studying MoSi z composites for aerospace applications [24]. In a pivotal meeting in May 1989, the decision was made to factor TABLE 2. An abbreviated history of MoSi 2 depicting a number of important events and milestones HISTORY OF MoSi2 [ ONR First High Temperature Structural Silicide Conference ,IX Lo~ Alamos Labs. US - PATENTS (Petrovic) Los Alamos Labs. IR-100 Award } Los Alamos Labs / ONR MoSi~Composites developmental studles '| ~/ ARPA-P,A &W. E I.teroe I,oRoviewM.ting CARTER (LANL-MIT) Thesis on vapor solid whiskers in MoSi 2 ~ ~ • t'~ LOS Alamas LabsJDOE-ECUT ~, • "~k.~"1989 f ,[ M°Si2 devel°pmental Studies(Petr°vic) J ---'~ ~ 988 "~" t ~ ERDOS (USA) Silicide coatingsl for gas turbine appfications , 1991 UMAKOSHI & HIRANO (Japan) . . Single CrysEal MoSi 2 ,WSi 2 Studies KEY PAPERS ON MoSi 2: FITZER & REMMELE (Germany) GAC & PETROVIC (USA) F1TZER (Germany) High temperature study of MoSi 2 MAXWELL (USA)First ~l-"~ suggested the use of MoSi 2 1 as a Structural mater~'al 191~ " 1907 195111P" ~ ~ KANTHAL (Sweden) Patent on MoSi2 for Commercial 1950 -~" ~" ~ heating element "7~.~ v~ ~ NOWOTNY (Gernmny) Mo-Si-C Phase diagram 9 ~K~B EFI~R;R ~ (CAMt ~EPL~e~tu2~ ~'~pa°~t 2~ ~2~tM~ i ~r b[~sY ~Tn methods N-------------HAEGGLUND & REHNQUIST (Sweden) Suggested 50Mo/50Si Alloys for furnace heating elements HOENIGSCHMID "~ (Germany) Suggested high melting silicides for -- WEDEKIND & PINTSCH ~ use in heat conductor material HOENIGSCHMID (Germany) Protective coating for Ductile metal
`
`GE-1007.004
`
`

`
`A. K. Vasudévan, J. J. Petrovic
`
`/ MOSi_,-based composites
`
`5
`
`MoSi2 composite work into a Pratt & Whitney DARPA
`Program on high temperature interrnetallics. Following
`that meeting, many other research groups began initi-
`ating work on MoSi2 composites.
`In 1986, there was only one research activity on
`MoSi2 composites, at LANL. In 1988, there were three
`research activities, LANL, Pratt & Whitney, and
`McDonnell—Douglas. By 1990, research activities had
`grown to the 30 + level (Fig. 4) including significant
`MoSi2 single—crystal work in Japan. In 1990, LANL
`won an R&D—l0O award for
`its development of
`MoSi3—SiC composites. The above discussions have
`briefly covered the history of MoSi2 materials and this
`history is schematically represented in Table 2.
`
`3. Properties of MoSi2 in relation to other high
`temperature structural materials
`
`Aronsson et al. [25] have catalogued various compo-
`sitions of well—characterized silicides in the form of a
`
`periodic table (Table 3). The three common molyb-
`denum-base silicides have been listed in the table.
`
`While the table clearly shows quite a large number of
`silicide compounds, only a limited number of them
`have been considered for mechanical property applica-
`tions, probably because of poor thermal stability. Table
`
`4 shows the comparison of MoSi2 properties with those
`of silicon—base ceramics such as SiC and Si3N4. While
`the silicon-based ceramic materials have excellent oxi-
`
`dation resistance, high melting point and strength
`properties, they lack high temperature ductility, ability
`to alloy, and ease of machining.
`The comparisons given in Table 4 indicate that the
`MoSi2 materials can be more reliable and less expen-
`sive than the silicon-based materials. Silicides are also
`
`non—toxic and are environmentally benign.
`Another attractive feature of MoSi2 is that it can be
`metallurgically alloyed with other silicides to improve
`its properties. Table 5 lists various potential high tem-
`perature silicides that may be considered for alloying
`with MoSi2. Such alloying opportunities make MoSi2
`materials more attractive relative to silicon-based
`
`ceramics, thus allowing other components of strength-
`ening to be used in addition to the composite approach.
`The table indicates that there are a few higher density
`silicides, such as W5 Si and Ta5Si3, which may not be
`attractive for alloying.
`The next important attribute of MoSi2 is that many
`important ceramic reinforcements are thermodynamic-
`ally stable with MoSi2. Table 6 gives the reinforcement
`compatibility of silicides, aluminides and SiC ceramic.
`Most of these silicide data were calculated (SOLGAS
`g mix) and/or experimentally verified around 1600 °C
`
`TABLE 3. Compositions of known silicides in the form of a periodic table
`
`A. K. Vasudkvan, J. J. Petrovic / MoSi,_-based composites 5 MoSi 2 composite work into a Pratt & Whitney DARPA Program on high temperature intermetallics. Following that meeting, many other research groups began initi- ating work on MoSi 2 composites. In 1986, there was only one research activity on MoSi 2 composites, at LANL. In 1988, there were three research activities, LANL, Pratt & Whitney, and McDonnell-Douglas. By 1990, research activities had grown to the 30 + level (Fig. 4) including significant MoSi~ single-crystal work in Japan. In 1990, LANL won an R&D-100 award for its development of MoSi2-SiC composites. The above discussions have briefly covered the history of MoSi 2 materials and this history is schematically represented in Table 2. 3. Properties of MoSi 2 in relation to other high temperature structural materials Aronsson et al. [25] have catalogued various compo- sitions of well-characterized silicides in the form of a periodic table (Table 3). The three common molyb- denum-base silicides have been listed in the table. While the table clearly shows quite a large number of silicide compounds, only a limited number of them have been considered for mechanical property applica- tions, probably because of poor thermal stability. Table 4 shows the comparison of MoSi 2 properties with those of silicon-base ceramics such as SiC and Si3N 4. While the silicon-based ceramic materials have excellent oxi- dation resistance, high melting point and strength properties, they lack high temperature ductility, ability to alloy, and ease of machining. The comparisons given in Table 4 indicate that the MoSi 2 materials can be more reliable and less expen- sive than the silicon-based materials. Silicides are also non-toxic and are environmentally benign. Another attractive feature of MoSi 2 is that it can be metallurgically alloyed with other silicides to improve its properties. Table 5 lists various potential high tem- perature silicides that may be considered for alloying with MoSi 2. Such alloying opportunities make MoSi 2 materials more attractive relative to silicon-based ceramics, thus allowing other components of strength- ening to be used in addition to the composite approach. The table indicates that there are a few higher density silicides, such as WsSi and TasSi3, which may not be attractive for alloying. The next important attribute of MoSi2 is that many important ceramic reinforcements are thermodynamic- ally stable with MoSi 2. Table 6 gives the reinforcement compatibility of silicides, aluminides and SiC ceramic. Most of these silicide data were calculated (SOLGAS mix) and/or experimentally verified around 1600°C TABLE 3. Compositions of known silicides in the form of a periodic table KSi RbSI CsSl LaSi2 Ca2SI ScsSi 3 CaSi casi= Sr2Si ' Ys Si3 $rSl YSi srs~ ysr2 BaSI LaSl 2 BaSl* 2 Composition of Known Silicides TI3SI V3SI Cr3Si TI 5 Si3 V 5Sl3 Cr 5 Sl 3 TISI VSI 2 CrSl "risl 2 CrSl 2 z~ ua Z~ Slsl Nb_SI Mo3SI Z,3 2 Nb~Sl* 3 Mo s Sl 3 Zr s sl s NbSl 2 MoSI 2 ZrSI ZrS| 2 Ni .SI K(CuSi) Mn.SI Fe3SI C02Si Ni~Si2 ~CuSi) Mn~SI 3 FesSl 3 CoS] Ni2SI* 7-Cu s Si MnSI FeSi CoSI 2 Nil SI 2 Cu3-Sl 8 MnSll 7 FeSl~ NISI CUl;SI 4 • NlSl 2 Rh 2 SI Ru 2 Sl Rh s Sl 3 Pd 3 SI RuSi* R ~0S1"~2 3 Pd2Sl RU 2 Sl 3 R PdSi RhSI Ir 3 Sl Hf = SI Ta3S I W5SI 3 R~ Sl 3 OsSi Ir 2 Sl Pt 3 Sl" Hf3Sl2 Ta2Si 3 WSI 2 ReSl Os2S ~ it3 S12 pt2 SI* HI SI Ta5Sl. 3 ReSI 2 IrSI Pt b~l 5 I'ffSI2 TaSI 2 irSi 3 PtSi i.s, llo ;] Eos, . iod2s.3..s...oy2s.. r2s.31Tm.s.31Y. 31L, s.3 I...= I"os' l I *There Is more than one phase with or near this composition.
`
`Com osition of Known Silicides
`
`IrS
`
`Tb, SIS
`TbS|2
`
`no, sis
`Hosl 2
`
`Pl¢,Sl5
`PlSl
`
`
`
`Luzsis
`
`
`
`
`‘There Is more than one phase with or near this composltlon.
`
`
`
` u,sI
`
`GE-1007.005
`
`GE-1007.005
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`

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`6
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`A. K. Vasudévan, J. J. Petrovic
`
`/ M0Si_,-based composites
`
`TABLE 4. Comparison of MoSi2 with silicon-based ceramics
`
`
`
` M0Si2 Si-based ceramics
`
`Advantages
`High melting point, 2030 °C
`Excellent oxidation resistance
`
`Brittle-to-ductile transition temperature (BDTT ) around 900-
`1000 °C
`
`Thermodynamically stable with potential ceramic reinforcements
`High thermal conductivity
`Potential for alloying with other high melting point silicides
`Can be electrodischarge machined (EDM)
`Cheaper to buy raw materials
`Non-toxic and environmentally benign
`
`Advantages
`Lower density
`Decomposition above 2100 °C
`Excellent oxidation resistance
`
`Thermodynamically stable with several ceramic reinforcements
`Good creep resistance
`
`Disadvantages
`Disadvantages
`Higher density
`Low touglmess
`No brittle-to-ductile transition
`Low toughness compared with conventional Ni-base superalloys
`Adequate creep resistance
`Difficult to alloy with other elements
`Difficult to machine
`Relatively higher coefficient of thermal expansion (CTE)
`Existence of PEST oxidation behavior
`Expensive because of cost of raw materials and fabrication
`
`6 A.K. VasudOvan, J. J. Petrovic / MoSi2-based composites TABLE 4. Comparison of MoSi2 with silicon-based ceramics MoSi2 Si-based ceramics Advantages High melting point, 2030 °C Excellent oxidation resistance Brittle-to-ductile transition temperature (BDTT) around 900- 1000 °C Thermodynamically stable with potential ceramic reinforcements High thermal conductivity Potential for alloying with other high melting point silicides Can be electrodischarge machined (EDM) Cheaper to buy raw materials Non-toxic and environmentally benign Disadvantages Higher density Low toughness compared with conventional Ni-base superalloys Adequate creep resistance Relatively higher coefficient of thermal expansion (CTE) Existence of PEST oxidation behavior Advantages Lower density Decomposition above 2100 °C Excellent oxidation resistance Thermodynamically stable with several ceramic reinforcements Good creep resistance Disadvantages Low toughness No brittle-to-ductile transition Difficult to alloy with other elements Difficult to machine Expensive because of cost of raw materials and fabrication TABLE 5. Potential silicide alloying species for MoSi 2 Silicide Melting Crystal Density point (°C) structure (g cm-3) MoSi 2 2030 Tetragonal 6.24 WSi2 2160 Tetragonal 9.86 NbSi2 1930 Hexagonal 5.66 TaSi 2 2200 Hexagonal 9.10 TiSi z 1500 Rhombohedral 4.04 MosSi 3 2160 Tetragonal 8.24 W 5Si 3 2370 Tetragonai 14.50 NbsSi 3 2480 Tetragonal 7.16 TasSi