4: Metal Matrix Composites

Chapter 4 Metal Matrix Composites Page

...... ***********n”*’” ...... 99 Current Applications and Market Opportunities ...... 99

Longer Term Applications...... ● ...... ● .. 99 Research and Development Priorities ...... 99

Introduction ● ● ● ● ● ● ● ● ● . ● . ● ● ● ● ● ● . ● ● ● ● ● ● ● ● ● . ● ● ● .-...... 100

Properties of Metal Matrix Composites ● ● ● ● . ● ● . . ● . . . ● ● ...... 100

● ● ...... 100 . Discontinuous Reinforcement ...... ’. . . . Continuous Reinforcement ● ...... 102 MMC Properties Compared to Other Structural Materials ...... 102 Design,● Processing, and Testing ...... ,....., .*...... 106 . Design . . . ● . ● ● ● ...... ● ● * ● ● * ● ● ● . . .**...... *. ... ●☛✎✎✎✎✎ . . 107

Processing. ● . . * . . . ● ● ...... ● ● . ● ● ● . . . . ● ...... ,.. ● . . . 107

Costs. ● . . . ● ● .. ● , ● ...... ● ● ● ● ● ● ...... 110 . Testing . ● ● . . . . . ● ● . . . . ● ...... ● ● ● ● ● ● ...... 111 Health and Safety ...... ● ...... 111

Applications...... ● ...... 112 Current Applications ...... ,.,...... 112 Future Applications ...... *.*...... 113

Markets . ● ... ● ... ● ...... 115 Research and Development Priories ...... 115

Very Important...... ● ☛✎✎☛✎✎ ✎ ✎ ✎ ✎ ...... ● 115. . Important ...... ✎ ✎ ✎ ✎ ✎ ✎ ✎ ✎ ✎ ✎ ✎ ...... 116 Desirable ...... ✎ ✎ ✎ ✎ ✎ ✎ ✎ ✎ ✎ ✎ ✎ ...... 117 Tables Table No. 4-1. Costs of a Representative Sample of MMC Reinforcements...... 101 4-2. Structural Properties of Representative MMCs, Compared to Other Materials ...... 103 4-3. Strength and Stiffness of Some Fiber-Reinforced MMCs...... 104 4-4. Properties of 6061 Aluminum Reinforced With Silicon Carbide Particulate .105 4-5. MMC Manufacturing Methods .. . * ...... 108 4-6. Selected MMC Processing Techniques and Their Characteristics ...... 109

i Chapter 4 Metal Matrix Composites

FINDINGS

Metal matrix composites (MMCs) usually con- 300,000 such pistons are produced and sold in sist of a low-density metal, such as aluminum or Japan annually. This development is very impor- magnesium, reinforced with particulate or fibers tant because it demonstrates that MMCs are at of a ceramic material, such as silicon carbide or least not prohibitively expensive for a very cost graphite. Compared with unreinforced metals, sensitive application. Other commercial applica- MMCs offer higher specific strength and stiffness, tions include cutting tools and circuit-breaker higher operating temperature, and greater wear contacts. resistance, as well as the opportunity to tailor these properties for a particular application. Longer Term Applications However, MMCs also have some disadvantages compared with metals. Chief among these are the Metal matrix composites with high specific stiff- higher cost of fabrication for high-performance ness and strength could be used in applications MMCs, and lower ductility and toughness. Pres- in which saving weight is an important factor. In- ently, MMCs tend to cluster around two extreme cluded in this category are robots, high-speed ma- types. One consists of very high performance chinery, and high-speed rotating shafts for ships composites reinforced with expensive continu- or land vehicles. Good wear resistance, along ous fibers and requiring expensive processing with high specific strength, also favors MMC use methods. The other consists of relatively low-cost in automotive engine and brake parts. Tailorable and low-performance composites reinforced with coefficient of thermal expansion and thermal con- relatively inexpensive particulate or fibers. The ductivity make them good candidates for lasers, cost of the first type is too high for any but mili- precision machinery, and electronic packaging. tary or space applications, whereas the cost/ben- However, the current level of development ef- efit advantages of the second type over unrein- fort appears to be inadequate to bring about com- forced metal alloys remain in doubt. mercialization of any of these in the next 5 years, with the possible exception of diesel engine Current Applications and Market pistons. Opportunities Based on information now in the public domain, the following military applications for Current markets for MMCs are primarily in mil- MMCs appear attractive: high-temperature fighter itary and aerospace applications. Experimental aircraft engines and structures; high-temperature MMC components have been developed for use missile structures; and spacecraft structures. Test- in aircraft, satellites, jet engines, missiles, and the ing of a National Aerospace Plane (NASP) pro- National Aeronautics and Space Administration totype is scheduled for the early to mid 1990s, (NASA) space shuttle. The first production appli- which might be too early to include MMCs. How- cation of a particulate-reinforced MMC in the ever, it may be possible to incorporate MMCs in United States is a set of covers for a missile guid- the structure or engines of the production vehicle. ance system. The most important commercial application to Research and Development Priorities date is the MMC diesel engine piston made by Toyota. This composite piston offers better wear MMCs are just beginning to be used in produc- resistance and high-temperature strength than the tion applications. In order to make present ma- cast iron piston it replaced. It is estimated that terials more commercially attractive, and to de-

99 100 . Advanced Materials by Design

velop better materials, the following research and cost fiber reinforcements is a major need. development priorities should receive attention: Continued development work on existing materials is important to lower costs as well. ● Cheaper Processes: To develop low-cost, ● Coatings: Research in the area of reinforce- highly reliable manufacturing processes, re- ment/matrix interface coatings is necessary. search should concentrate on optimizing These coatings can prevent deleterious chem- and evaluating processes such as plasma ical reactions between matrix and reinforce- spraying, powder metallurgy processes, ment which weaken the composite, particu- modified casting techniques, liquid metal in- larly at high temperature, and optimize the filtration and diffusion bonding. interracial fiber/matrix bond. ● Cheaper Materials: Development of Iower

INTRODUCTION

Metal matrix composites (MMCs) generally in dramatic improvements in MMC properties, consist of lightweight metal alloys of aluminum, but costs remain high. Continuously and discon- magnesium, or titanium, reinforced with ceramic tinuously reinforced MMCs have very different particulate, whiskers, or fibers.1 The reinforce- applications, and will be treated separately ment is very important because it determines the throughout this chapter. mechanical properties, cost, and performance of Tailorability is a key advantage of all types of a given composite. composites, but is particularly so in the case of Composites reinforced with particulate (dis- MMCs. MMCs can be designed to fulfill require- continuous types of reinforcement) can have ments that no other materials, including other ad- costs comparable to unreinforced metals, with vanced materials, can achieve. There are a num- significantly better hardness, and somewhat bet- ber of niche applications in aerospace structures ter stiffness and strength. Continuous reinforce- and electronics that capitalize on this advantage. ment (long fiber or wire reinforcement) can result

PROPERTIES OF METAL MATRIX COMPOSITES

There are considerable differences in published Property data given in this chapter are therefore property data for MMCs. This is partly due to the given as ranges rather than as single values. fact that there are no industry standards for Some MMC properties cannot be measured as MMCs, as there are for metals. Reinforcements they would be for monolithic metals. For in- and composites are typically made by proprie- stance, toughness is an important but hard-to- tary processes, and, as a consequence, the prop- define material property. Standard fracture me- erties of materials having the same nominal com- chanics tests and analytical methods for metals position can be radically different. The issue is are based on the assumption of self-similar crack further clouded by the fact that many reinforce- extension; i.e., a crack will simply lengthen with- ments and MMCs are still in the developmental out changing shape. Composites, however, are stage, and are continually being refined. Numer- non homogeneous materials with complex inter- ous test methods are used throughout the indus- nal damage patterns. As a result, the applicabil- try, and it is widely recognized that this is a ma- ity of conventional fracture mechanics to MMCs jor source of differences in reported properties.2 is controversial, especially for fiber-reinforced ma- 1 As used i n this chapter, the terms 1‘al u m inu m, ‘‘magnesium, ” terials. and “titanium” denote alloys of these materials used as matrix metals. ~Carl Zweben, “Metal Matrix Composites, ” contractor report for OTA, January 1987. —

Ch. 4—Metal Matrix Composites ● 101

Table 4-1 .—Costs of a Representative Sample of MMC Reinforcements Reinforcement Price ($/pound) Alumina-silica fiber...... 1 Silicon carbide particulate ...... 3 a Silicon carbide whisker ...... 95 Alumina fiber (FP) ...... 200 Boron fiber...... $262 Graphite fiber (P-100) ...... 950a Higher performance reinforcements (e.g., graphite and boron fibers) have significantly higher costs as well. aJoseph Dolowy, personal communication, DWA Composite Specialties, Inc., JuIy 1987. SOURCE: Carl Zweben, “Metal Matrix Composites, ” contractor report for OTA, January 1987.

In terms of tailorability, a very important advantage in MMC applications, particulate reinforcement offers various desirable properties. Boron carbide and silicon carbide, for instance, are widely used, inexpensive, commercial abrasives that can offer good wear resistance as well as high specific stiffness. Titanium carbide offers a high melting point and chemical inertness which are desirable properties for processing and stability in use. Tungsten carbide has high strength and hardness at high temperature. In composites, a general rule is that mechanical properties such as strength and stiffness tend to increase as reinforcement length increases.4 Particulate can be considered to be the limit of short fibers. Particulate-reinforced composites are Photo credit: DWA Composite Specialties, Inc. isotropic, having the same mechanical proper- Micrograph (75x magnification) of 20 percent volume ties in all directions. fraction silicon carbide particulate-reinforced 2124 aluminum. In principle, whiskers should confer superior properties because of their higher aspect ratio Discontinuous Reinforcement (length divided by diameter). However, whiskers There are two types of discontinuous reinforce- are ‘brittle and tend to break up into shorter ment for MMCs: particulate and whiskers. The lengths during processing. This reduces their rein- most common types of particulate are alumina, forcement efficiency, and makes the much higher boron carbide, silicon carbide, titanium carbide, cost of whisker reinforcement hard to justify. De- and tungsten carbide. The most common type velopment of improved processing techniques - of whisker is silicon carbide, but whiskers of alu- could produce whisker-reinforced MMCs with mina and silicon nitride have also been produced. mechanical properties superior to those made Whiskers generally cost more than particulate, from particuiates. as seen in table 4-1. For instance, silicon carbide Another disadvantage of using whisker rein- whiskers cost $95 per pound, whereas silicon car- forcement is that whiskers tend to become ori- bide particulate costs $3 per pound. Cost pro- ented by some processes, such as rolling and ex- jections show that although this difference will trusion, producing composites with different decrease as production volumes increase, par- properties in different directions (anisotropy).5 3 ticulate will always have a cost advantage. 41 bid. 5See the discussion on an isotropy in ch. 3 on Polymer Matrix Com- 3Ibid posites. .- —

102 ● Advanced Materials by Design

Anisotropy can be a desirable property, but it is when the composite is exposed to high temper- a disadvantage if it cannot be controlled precisely atures in processing or service. For example, bo- in the manufacture of the material. It is also more ron fiber can be coated with boron carbide and difficult to pack whiskers than particulate, and silicon carbide reaction barriers to prevent diffu- thus it is possible to obtain higher reinforce- sion and chemical reactions with the matrix that ment:matrix ratios (fiber volume fraction, v/o) decrease the strength of the composite. Alumina with particulate. Higher reinforcement percent- fibers can be given a surface coating of silica to ages lead to better mechanical properties such improve tensile strength. as higher strength. Coatings can also be used to tailor the bond strength between fiber and matrix. If adhesion Continuous Reinforcement between fiber and matrix is too good, cracks in In fiber reinforcement, by far the most com- the matrix propagate right through the fibers, and mon kind of continuous reinforcement, many the composite is brittle. By reducing the bond types of fibers are used; most of them are car- strength, coatings can enhance crack deflection bon or ceramic. Carbon types are referred to as at the interface, and lead to higher energy ab- graphite and are based on pitch or polyacryloni- sorption during fracture through fiber pullout trile (PAN) precursor. Ceramic types include alu- mechanisms. Sometimes a coating is needed to mina, silica, boron, alumina-silica, alumina-boria- promote wetting between the matrix and the fi- silica, zirconia, magnesia, mullite, boron nitride, ber, and thereby achieve a good bond. Graph- titanium diboride, silicon carbide, and boron car- ite can be coated with titanium diboride in or- bide. All of these fibers are brittle, flaw-sensitive der to promote wetting. materials. As such, they exhibit the phenomenon processing techniques, as well as coatings, can of size effect (see ch. 2); i.e., the strength of these be used to control deleterious fiber/matrix inter- fibers decreases as the length increases. actions. The application of pressure can be used Fiber/matrix interface coatings offer another to force intimate contact between fiber and ma- dimension of tailorability to MMCs. Coatings are trix and thus promote wetting; squeeze casting very important to the behavior of MMCs to pre- is one process that does this. vent undesirable reactions, improve the strength A less common type of continuous reinforce- of the fibers, and tailor the bond strength between ment is wire reinforcement. Wires are made of fiber and matrix. A reaction barrier is needed for such metals as titanium, tungsten, molybdenum, some fiber/matrix combinations, particularly beryllium, and stainless steel. Such wires offer some tailorability for certain niche applications; for example, tungsten wire offers good high-temperature creep resistance, which is an advantage . in fighter aircraft jet engines and other aerospace applications.

MMC Properties Compared to Other Structural Materials Table 4-2 compares the most important material properties of MMCs with those of other structural materials discussed in this assessment.

Strength and Stiffness Photo credit: DWA Composite Specialties, Inc. The stiffnesses and strengths of particulate-rein- Cross-section of continuous graphite fiber-reinforced forced aluminum MMCs are significantly better aluminum composite: 52 percent volume fraction fiber, uniaxially reinforced. 300x magnification than those of the aluminum matrix. For exam- .

Ch. 4—Metal Matrix Composites ● 103 pie, at a volume fraction of 40 percent silicon car- ing PMCs in structures subject to high transverse bide particulate reinforcement, the strength is stresses. about 65 percent greater than that of the 6061 6 High values of specific strength and specific aluminum matrix, and the stiffness is doubled. stiffness (strength and stiffness divided by den- Particulate-reinforced MMCs, which are isotropic sity) are desirable for high-strength, low-weight materials, have lower strength than the axial applications such as aircraft structures. Typically, strength (parallel to the direction of continuous particulate MMCs have somewhat better specific fiber reinforcement) of advanced polymer matrix strength and specific stiffness than the matrix composites (PMCs), see table 4-2. However, they metal, and fiber-reinforced MMCs have much have much better strength than the transverse better specific strength and specific stiffness than strength (perpendicular to the direction of con- the matrix metal. tinuous fiber reinforcement) of PMCs. The stiffness of particulate MMCs can be considered to Unfortunately, MMCs have a higher density be about the same as that of PMCs. than PMCs, making specific strength and specific stiffness lower than those for PMCs in the axial Unlike particulate-reinforced MMCs and mono- direction. Transverse specific strength and spe- lithic metals in general, fiber-reinforced MMCs cific stiffness of MMCs are still better than those can be highly an isotropic, having different strengths of PMCs. and stiffnesses in different directions. The high- est values of strength and stiffness are achieved High-Temperature Properties along the direction of fiber reinforcement. In this direction, strength and stiffness are much higher MMCs offer improved elevated-temperature than in the unreinforced metal, as shown in ta- strength and modulus over both PMCs and me- ble 4-2. In fact, the stiffness in the axial direction tals. Reinforcements make it possible to extend can be as high as six times that of the matrix ma- the useful temperature range of low density me- terial in a graphite fiber/aluminum matrix com- tals such as aluminum, which have limited high- posite: see table 4-3. However, in the transverse temperature capability (see table 4-2). MMCs typi- directions, strength values show no improvement cally have higher strength and stiffness than PMCs over the matrix metal. Transverse strengths and at 200 to 300° C (342 to 5720 F), although de- stiffnesses of continuous fiber-reinforced MMCs velopment of resins with higher temperature ca- compared to PMCs are very good, thereby giv- pabilities may be eroding this advantage. No ing MMCs an important advantage over the lead- other structural material, however, can compete with ceramics at very high temperature.

bCarl Zweben, “Metal ,Matrlx Composites, ” contractor report fot Fiber-reinforced MMCs experience matrix/rein- OTA, January 1987. forcement interface reactions at high tempera-

Table 4.2.—Structural Properties of Representative MMCs, Compared to Other Materials

Matrix(l)* Particulate(2)* Fiber(3)* Property metala MMCa MMCa PMCb Ceramic b Strength (M Pa) (axial) ...... -290 290-480 620-1,240 820-1,680 140-3,900 Stiffness (G Pa) (axial)...... -70 80-140 130-450 61-224 97-400 Specific strength (axial)...... -100 100-170 250-390 630-670 51-670 Specific gravity ...... 2.5-2.8 -2.8 2.5-3.2 1.3-2.5 2.7-5.8 Transverse strength (MPa)...... -290 290-480 30-170 11-56 140-3,900 Transverse stiffness (GPa) ...... Same as axial Same as axial 34-173 3-12 Same as axial Maximum use temperature (“C) ...... 180 300 300 260 1,200-1,600 Plane strain fracture toughness (MPa-m½) ...... 18-35 12-35 — — 3-9

PMC IS used to denote a range of materials including graphite/epoxy, graphite/polylmlde, boron/polyimide, and S-glass/epoxy Ceramic is used to denote a range of materials including zirconia, silicon carbide, and silicon nitride “NOTE: (1) 6061 aluminum (2) 6061 aluminum reinforced with 0-400/0 volume fractions of SiC particulate (3) 6061 aluminum reinforced with 50°/0 volume fractions of fibers of graphite, boron. silicon carbide, or alumina SOURCES a Carl Zweben, “Metal Matrix Composites, ” b contractor report for OTA, January 1987 "Guide to Selecting Engineered Materials. ” Advanced Materials and Processes, vol. 2, No 1, June 1987 104 ● Advanced Materials by Design

Table 4-3.—Strength and Stiffness of Some Fiber. Reinforced MMCs (Fiber V/O = 50%)

Tensile Tensile strength strength Stiffness Stiffness Material (axial) (transverse) (axial) (transverse) Matrix material MPa MPa GPa GPa Aluminum 6061-T6 ...., 290 290 70 70 Titanium Ti-6AI-4V 1170 1170 114 114 Composite Graphite a/aluminum . . . 690 30 450 34 Boron/aluminum. . . . . 1240 140 205 140 Silicon carbideb/ aluminum . . . . . 1240 100 205 140 Silicon carbidec/ aluminum ...... 1040 70 130 99 Alumina d/aluminum . . . . . 620 170 205 140 Silicon carbide8/ titanium ...... 1720 340 260 173 Graphitea/copper ...... 512 31 464 49 Properly Improvements of MMCs over unreinforced metals can be significant For example, the axial stiffness of graphite fiber-reinforced aluminum is roughly 6 times greater than that of the unreinforced aluminum The axial tensile strength of silicon carbide fiber-reinforced aluminum IS about 4 times greater than that of the unreinforced aluminum aP-120 b AVCO monofilament c"Nicalon" d Fibe r "FP" ‘AVCO monofilament automobiles demonstrated an 85 percent improve- SOURCES: For all composites except graophit/copper: Carl Zweben, “Metal Matrix Composites, ” ment in wear resistance over the cast iron pis- contractor report for OTA, January 1987. For graphite/copper only: C Kaufmann, Ameri- 9 can Cyanamid, personal communication, Oct. 19, 1987. ton with nickel insert used previously.

Fracture and Toughness tures. In addition, the transverse high-temperature strength of fiber-reinforced MMCs is only as There is a wide variation in fracture toughness good as that of the matrix metal, since mechani- among MMCs, although it is generally lower than cal properties in the transverse direction are dom- that of the monolithic metal. Fracture toughness inated by the matrix and the fiber/matrix inter- can vary between 65 and 100 percent of the frac- face. For example, at 320 C (608° F), the axial ture toughness of the monolithic metal alloy.10 tensile strength of boron fiber-reinforced alumi- Lower toughness is a trade-off for higher strength num is about 1.1 gigapascals (GPa) compared to and stiffness. Particulate-reinforced MMCs have only 0.07 GPa for monolithic 6061 aluminum, a lower ultimate tensile strain than the unrein- whereas the transverse strength is 0.08 GPa, forced metals (see table 4-4) which may be im- about the same as that for the monolithic 6061.7 portant in some applications. This brittleness can complicate the design process and make joining Wear Resistance more difficult as well. Comparison to PMCs is difficult, because the toughness of PMCs is very Wear resistance of MMCs is excellent compared temperature-dependent. to that of monolithic metals and PMCs, owing to the presence of the hard ceramic reinforcements. Thermal Properties For instance, in one test, the abrasive wear of 2024 aluminum under a 1 kilogram load was The introduction of silicon carbide particulate shown to be 6 times greater than the wear of the into aluminum results in materials having lower same alloy containing 20 percent volume frac- coefficients of thermal expansion, a desirable tion of silicon carbide whiskers.8 An alumina-silica property for some types of applications. By choos- fiber-reinforced aluminum piston used in Toyota ing an appropriate composition, the coefficient

7 Ibid. 9 lbid. 8 lbid. 10 lbid. Ch. 4—Metal Matrix Composites . 105

Table 4-4.—Properties of 6061 Aluminum Reinforced water damage than PMCs which can absorb With Silicon Carbide Particulate moisture, thereby reducing their high-temperature performance. Second, some MMCs, such as Tensile Stiffness strength Tensile reinforced titanium, can stand high-temperature Reinforcement (G Pa) (M Pa) strain corrosive environments, unlike PMCs. 0% 10 290 16.O% 10 83 7.0 Nevertheless, some MMCs are subject to envi- 15 86 3&l 6.5 ronmental degradation not found in PMCs. For 20 90 430 6.0 instance, graphite fibers undergo a galvanic re- 25 103 — 3.5 30 117 470 2.0 action with aluminum. This can be a problem 35 124 — 1.0 when the graphite/aluminum interface is exposed 40 140 480 0.6 to air or moisture. In addition, PMCs are resis- Note the sharp rise in stiffness in the 400/0 composite (140 GPa) compared to the unreinforced aluminum (10 GPa). Ultimate tensile strength nearly doubles tant to attack by many chemicals (e.g., acids) that and tensile strain decreases nearly a full order of magnitude. corrode aluminum, steel, and magnesium. SOURCE: Carl Zweben, “Metal Matrix Composites,” contractor report for OTA, January 1987. cost of thermal expansion can be near zero in some A major disadvantage of MMCs compared to MMCs. MMCs also tend to be good heat conduc- most other structural materials is that they are tors. Using high thermal conductivity graphite generally more expensive. Both constituent ma- fibers, aluminum-matrix or copper-matrix MMCs terial costs and processing costs are higher. Costs can have very high thermal conductivity, com- of higher performance reinforcements (mostly pared with other types of composites. fibers) are high, and lower cost reinforcements (mostly particulate) may not yield dramatic im- Environmental Behavior provements in performance. Cost/benefit ratios In terms of environmental stability, MMCs have of most MMCs dictate that they be used only in two advantages over PMCs. First, they suffer less high-performance applications. However, both

MICROWAVE CIRCUIT PACKAGING

KOVAR METAL MATRIX COMPOSITE

WEIGHT = 42 g WEIGHT = 15 g

THERMAL 9.6 BTU THERMAL 74 BTU = = CONDUCTIVITY HR-FT-°F CONDUCTIVITY HR-FT-°F

Photo credit: General Electric Co. Silicon carbide particle-reinforced aluminum microwave package 106 . Advanced Materials by Design

the material and production costs may drop as more experience is gained in MMC production.

Tailorability of Properties In some applications, MMCs offer unique combinations of properties that cannot be found in other materials. Electronics packaging for aircraft requires a hard-to-achieve combination of low coefficient of thermal expansion, high thermal conductivity, and low density. Certain MMCs can meet these requirements, replacing beryllium, which is scarce and presents toxicity problems. Several other examples can be cited to illus- trate the unique advantages of MMCs in specialty applications. A large heat transfer coefficient is desirable for space-based radiators; this property is offered by graphite fiber-reinforced copper, aluminum, and titanium (although this latter fiber/

matrix combination unfortunately has some in- Photo credit: Toyota Motor Corp. terface reaction problems). The higher transverse Aluminum diesel engine piston with local fiber strengths of MMCs compared to PMCs have led reinforcement in ring groove area to several MMC space applications, such as the space shuttle orbiter struts, made of boron fiber- reinforced aluminum. Use of local reinforcement to improve wear resistance makes possible the elimination of nickel One of the most important applications of and cast iron inserts. This reduces piston weight MMCs is the Toyota truck diesel engine piston, and increases thermal conductivity, improving produced at a rate of about 300,000 pistons per 11 engine performance and reducing vibration. Year. This consists of aluminum selectively reinforced in the critical region of the top ring groove This approach minimizes cost and thereby makes with a ring-shaped ceramic fiber preform. (A pre- MMCs more competitive with cast iron. This de- form is an assemblage of reinforcements in the sign not only offers better wear resistance and shape of the final product that can be infiltrated better high-temperature strength but also elimi- with the matrix to form a composite.) nates one type of part failure associated with the design it replaced. It is considered by some to Two types of fibers are used: alumina and be a development of historic proportions because alumina-silica. Both are relatively low-cost ma- it demonstrates that MMCs can be reliably mass terials originally developed for furnace insulation. produced. It also shows that at least one type of aluminum matrix MMC can perform reliably in 11 Ibid. — a very severe environment.

DESIGN, PROCESSING, AND TESTING

In ceramics and polymer matrix composites billets or sheets and later machined to a final manufacturing, the material is formed to its final shape. Viable, inexpensive near-net-shape tech- shape as the microstructure of the material is niques for forming MMCs have not been success- formed. MMCs can be produced in this way, or, fully developed as yet, and there is still a debate as is traditionally done with metals, formed into about on the advantages of producing standard Ch. 4—Metal Matrix Composites . 107

MMC production processes can be divided into primary and secondary processing methods, though these categories are not as distinct as is the case with monolithic metals. Primary processes (those processes used first to form the material) can be broken down into combining and consolidation operations. Secondary processes can be either shaping or joining operations. Ta- ble 4-5 shows the manufacturing methods discussed here and notes which types of operations are included in each method. As with ceramics, net-shape methods are very important manufacturing processes. The machin- Photo credit: AVCO Corp. ing of MMCs is very difficult and costly in that Centrifugal casting of silicon carbide-reinforced MMCs are very abrasive, and diamond tools are aluminum. needed. In addition, it is desirable to reduce the amount of scrap left from the machining process because the materials themselves are very ex- shapes to be machined by the purchaser to final pensive. form, compared to the custom production of near-net-shape parts. Forming of net-shape MMC Continuous Reinforcement12 parts necessitates close integration of design and manufacture, where production of standard MMC The following discussion covers the primary structural shapes does not. and secondary processes involved in the manufacture of MMCs with continuous reinforcement. Design Primary Processes.– Basic methods of combining and consolidating MMCs include liquid metal The variability of metal matrix composite prop- infiltration, modified casting processes, and depo- erties is a handicap to designing with these ma- sition methods such as plasma spraying. Hot terials. There are currently no design aids such pressing consolidates and shapes MMCs. Diffu- as property databases, performance standards, sion bonding consolidates, shapes and joins and standard test methods. In addition, most MMCs. See table 4-6 for selected MMC process- designers are much more familiar with monolithic ing methods and their characteristics. metals. The lack of experience with MMCs increases the design and manufacturing costs asso- There is considerably more disagreement on ciated with the development of a new product. what is the most promising method for process- As with ceramic structures, brittleness is a new ing fiber-reinforced MMCs than there is for proc- and difficult concept to most designers. MMCs essing of particulate-reinforced MMCs. The Toyota are brittle materials, and new methods of design diesel engine piston has been heralded as an ex- will become important in the development of ample of the promise of modified casting tech- MMC applications. niques for keeping costs down. Critics charge that these types of casting techniques have not proven Processing adequate for manufacturing MMCs with desirable mechanical properties. (The Toyota piston Because MMC technology is hardly beyond the stage of R&D at present, costs for all methods of producing MMCs are still high. Manufacturing methods must ensure good bonding between ma- 12 As examples in the following discussion of processing, two common types of particulate- and fiber-reinforced composites will be trix and reinforcement, and must not result in un- referred to as needed: silicon carbide particulate-reinforced alu- desirable matrix/fiber interracial reactions. minum, and boron fiber-reinforced alum inure. 108 ● Advanced Materials by Design

Table 4-5.—MMC Manufacturing Methods

Combines Consolidates Shapes Joins Primary methods: Casting ...... x x x Squeeze casting, compocasting, gravity casting, low-pressure casting Diffusion bonding ...... x x x Liquid infiltration ...... x x Gravity, inert gas pressure, vacuum infiltration Deposition ...... X x Chemical coating, plasma spraying, chemical vapor deposition, physical vapor deposition, electrochemical plating Powder processing ...... X x x Hot pressing, ball mill mixing, vacuum pressing, extrusion,— rolling Secondary methods: Shaping ...... x x Forging, extruding, rolling, bending, shearing, spinning, machining Machining ...... x Turning, boring, drilling, milling, sawing, grinding, routing, electrical discharge machining chemical milling, electrochemical milling Forming...... x x x Press brake, superplastic, creep forming Bonding...... x Adhesive, diffusion Fastening ...... x Soldering, brazing, welding ...... x SOURCE: Carl Zweben, “Metal Matrix Composites, ” contractor report for OTA, January 1987. uses fiber reinforcement mostly for wear resis- expensive than for monolithic structural metals. tance and less for more universally important As a rule, diamond tools are required because properties such as strength.) Some industry MMC carbide and other tools wear out too quickly. De- advocates suggest that processes such as diffu- spite this limitation, all of the basic mechanical sion bonding and plasma spraying are more prom- methods, such as drilling, sawing, milling, and ising for achieving high performance, and that turning have been proven to be effective with lower costs will come only with more produc- MMCs. Electrical discharge machining also has tion experience with these processes. been shown to be effective.

secondary Processes.–Machining processes Mechanical fastening methods, such as rivet- for MMCs reinforced with ceramic materials, which ing and bolting have been found to work for are hard and abrasive, generally are much more continuously-reinforced MMCs. The same is true Ch. 4—Metal Matrix Composites ● 109

Table 4-6.—Selected MMC Processing Techniques and Their Characteristics

Techniques Characteristics For fiber-reinforced MMCs: Liquid metal infiltration near-net-shape parts (low pressure) economical high porosity oxidation of matrix and fiber not reliable as yet Liquid metal infiltration (intert gas less porosity and oxidation than low pressure pressure, vacuum) techniques Low pressure casting near-net-shape parts low cost expensive preforms required three-dimensional preforms are difficult to make Squeeze casting good fiber wetting lower porosity expensive molds needed large capacity presses needed Plasma spraying potential for lower processing costs Diffusion bonding lower temperatures than hot pressing reduces fiber/matrix interactions not capable of net-shape parts except simple shapes slow, expensive fiber damage can occur Hot Pressing heats matrix above melt temperature, which can degrade reinforcement For particulate-reinforced MMCs: Powder metallurgy high volume fractions of particulate are possible (better properties) powders are expensive not for near-net-shape parts Liquid metal infiltration net-shape parts can use ingots rather than powders lower volume fraction of particulate (means lower mechanical properties) Squeeze casting may offer cost advantage; however molds and presses may be expensive SOURCE Carl Zweben, “Metal Matrix Composites, ” contractor report for OTA, January 1987

for adhesive bonding. Boron fiber-reinforced alu- forced MMCs are powder processing techniques. minum pieces can be metallurgically joined by There has not been much success at producing soldering, brazing, resistance welding and diffu- near-net-shape structures as yet. 13 (See discussion sion bonding. in chapter 2 on the desirabiIity of near-net-shape processing.) Other processes for discontinuously- Discontinuous Reinforcement reinforced MMCs are Iiquid metal infiltration and casting techniques (see table 4-6). The following discussion covers the primary and secondary processes involved in the manufac- At this time, there is no one MMC manufacture of MMCs with discontinuous reinforcement. turing method that holds great promise for reduc- Primary Processes.–The most common methods for producing particulate- and whisker-rein- 110 . Advanced Materials by Design

Photo credit: AVCO Specialty Materials Photo credit: AVCO Specialty Materials Centrifugal casting around braided tube of SiC Silicon carbide fiber-reinforced aluminum shaped by fibers and yarn. electrical discharge machining.

A wide variety of joining methods can be used including mechanical fastening techniques, met- allurgical methods employed with monolithic metals, and adhesive bonding as used for PMCs.

costs MMC costs are currently very high, and for them to come down, there must be some standardization and a reliable compilation of materials properties (in a form such as a design data- base). This can be achieved only through greater production experience with MMC materials. Photo credit: AVCO Specialty Materials Some experts argue that entirely new materials Diffusion bonded SiC fiber-reinforced titanium: and processes must be found that are cheaper cross section of a hollow turbine blade. than the present ones. Opponents of this view hold that development of new processes and ma- ing costs, although there seems to be some agree- terials will delay a decrease in costs of existing ment that for particulate-reinforced MMCs, liquid materials, because it will take much longer to gain metal infiltration and powder metallurgy techniques the production experience necessary. are likely candidates for future development. Most present users buy preformed shapes, such Secondary Processes.–One of the major ad- as billets, plates, bars and tubes, and then ma- vantages of particulate- and whisker-reinforced chine these shapes to specification. MMCs are MMCs is that most of the conventional metal- not currently sold in standard sizes; users can or- working processes can be used with minor mod- der any size, manufactured to order. This lack ifications (see table 4-5), Methods demonstrated of standardization keeps prices for shapes high. include forging, extruding, rolling, bending, Some users produce their own MMCs for use in- shearing, and machining. Although they are ex- house. For example, Lockheed Georgia produces pensive, all conventional machining methods silicon carbide fiber-reinforced aluminum matrix have been found to work for these materials, in- composites for designing, manufacturing and test- cluding turning, milling, and grinding. ing fighter aircraft fins. Ch. 4—Metal Matrix Composites . 111

The cost of an MMC part depends on many composites are complex, heterogeneous mate- factors including shape, type of matrix and rein- rials that are susceptible to more kinds of flaws, forcement, reinforcement volume fraction, rein- including delamination, fiber misalignment, and forcement orientation, primary and secondary fiber fracture. fabrication methods, tooling costs, and number of parts in the production run. At present, there Failure in monolithic metals occurs primarily is little information available on production costs. by crack propagation. The analytical tool called The only items produced in any quantity are the linear elastic fracture mechanics (LEFM) is used Toyota engine pistons. Costs for this application to predict the stress levels at which cracks in are proprietary information, but the costs of these metals will propagate unstably, causing failure. MMCs are at least not prohibitive in a very cost- Failure modes in composites, especially those sensitive product. reinforced with fibers or whiskers, are far more complex, and there are no verified analytical Costs are very volume sensitive; high costs keep methods to predict failure stress levels associated volumes low, and low volumes mean high costs. with observed defects. As a consequence, em- Of course, some materials are inherently expen- pirical methods have to be used to evaluate how sive and will never be cheap enough for wide- critical a given flaw is in an MMC. spread commercial use, regardless of volume. Two basic methods have been established as With the exception of alumina-silica discontinu- reliable flaw-detection techniques for MMCs: ous reinforcement fibers, reinforcement prices radiography and ultrasonic C-scan. Radiography, are orders of magnitude higher than those of me- useful only for thin panels, detects fiber misalign- tals used in mass production items such as au- ment and fractures. C-scan identifies delamina- tomobiles. Unfortunately, the stiffness of alumina- tion and voids. There are no existing nondestruc- silica fibers is not substantially greater than that tive evaluation (N DE) methods for reinforcement of aluminum. The room-temperature strength of degradation in MMCs. aluminum reinforced with these fibers also shows little or no improvement over monolithic alumi- The costs of NDE methods for MMCs should num, although wear resistance and elevated- be no greater than for monolithic metals. How- temperature strength are enhanced. The signifi- ever, the reliability of manufacturing processes cance of this is that the fibers that provide major for these materials has not been established and improvements in material properties are quite ex- MMCs cannot be reliably or repeatably produced pensive, while use of low-cost alumina-silica as yet. Because fabrication processes are not de- fibers provides only modest gains in strength and pendable at present, it is generally necessary to stiffness. use NDE for MMCs in cases where testing would Testing not be employed at all, or as extensively, for monolithic metals, This additional cost factor Unlike monolithic metals, in which the main should decrease as experience and confidence types of flaws are cracks, porosity, and inclusions, are gained with MMCs.

HEALTH AND SAFETY

There is little documentation as yet of safe han- Health hazards associated with MMCs are sim- dling and machining practices for MMCs. Mate- ilar to those found in the production of ceram- rials handling practices are given by materials ics and PMCs. As with ceramics the most serious safety data sheets, and few yet exist for MMCs. However, there is one materials safety data sheet that applies to the MMC production process of lqRObefl BUffenbarger, I rlternational Assoclatlon of Machinists and plasma spray ing.14 Aerospace Workers, personal communication, March 23, 1987. 112 . Advanced Materials by Design

health hazard is associated with the possible car- boria-silica. The effects of ceramic fibers on hu- cinogenic effects of ceramic fibers due to their mans are largely unknown. Until such data be- size and shape. MMC reinforcement fibers fall- come available, the indications discussed in chap- ing in this category include alumina, alumina- ter 2 for ceramics also apply to the manufacture silica, graphite, boron carbide, and alumina- of MMCs reinforced with these fibers.

APPLICATIONS

There are very few commercial applications of Current Applications MMCs at the present time. Industry observers believe that the level of development effort in the Current MMC markets in the United States are United States is not large enough to lead to sig- primarily military. MMCs have potential for use nificant near-term commercial use of MMCs. in aircraft structures, aircraft engines, and naval However, the Toyota truck diesel engine piston weapons systems. Experimental MMC compo- has spurred the major U.S. automakers to under- nents were first developed for applications in air- take preliminary activity in MMCs. Although con- craft, jet engines, rockets, and the space shuttle. siderable interest in these materials is being gen- There has been little government funding of erated in the United States, it will be decades MMCs for commercial applications in the United before they are likely to have an appreciable im- States, and only a small number of specialized pact on production levels of competing materials. applications have been developed by private industry. MMCs are not competing solely with monolithic metals; they are also competing with the Military/Space whole range of advanced materials, including The high stiffness and compression strength and PMCs, ceramics, and other new metal alloys. It low density of boron fiber-reinforced aluminum is not yet clear whether, compared to PMCs, ce- led to its use in production space shuttle orbiter ramics, and monolithic metals, MMCs will have struts. Its high cost has prevented wider use. big enough performance advantages to warrant use in a wide variety of applications. In fact, The National Aeronautics and Space Adminis- MMCs may be limited to niche applications in tration (NASA) Hubble Space Telescope uses two which the combinations of properties required antenna masts made of aluminum reinforced with cannot be satisfied by other structural materials. P-100 pitch-based graphite fibers. This material was selected because of its high specific stiffness The properties that make MMCs attractive are 16 and low coefficient of thermal expansion. This high strength and stiffness, good wear resistance, material replaced a dimensionally stable telescope and tailorable coefficient of thermal expansion mount and an aluminum waveguide, resulting in and thermal conductivity. Furthermore, develop- a 70 percent weight savings. 17 ment of high-temperature MMCs has been cited as a way to help reduce U.S. dependence on crit- Two companies are producing silicon carbide ical and strategic materials, such as manganese particulate-reinforced aluminum instrument cov- and cobalt. ’ 5 The following sections describe the ers for a missile guidance system. This is the first current and future applications for which these production application of particulate-reinforced properties are of value. MMCsin the United States.

161bid.

I Twilllam C, Riley, Research Opportu nites, Inc., personal com- IJzweben, op. cit., January 1987 munication, October 27, 1987. Ch. 4—Metal Matrix Composites ● 113

AIRFOIL CROSS SECTION

X75 X100 Photo credit: NASA Lewis Research Center Tungsten wire-reinforced super alloy gas turbine engine blade

Commercial The Toyota piston has stimulated a considerable interest in MMCs for pistons and other au- Photo credit: Advanced Composite Materials tomotive parts on a worldwide basis. Other com- Missile guidance system covers of silicon carbide ponents now being evaluated by automakers particle-reinforced aluminum. world-wide include connecting rods, cam fol- lowers, cylinder liners, brake parts, and drive shafts. Military/Space Experimental MMC bearings have been tested Based on information in the public domain, the on railroad cars in the United Kingdom. An ex- following applications appear likely to be devel- perimental material for electronic applications has oped in the United States: high-temperature been developed in Japan, consisting of copper fighter aircraft engines and structures, high-tem- reinforced with graphite fiber, Another electrical perature missile structures, spacecraft structures, use of MMCs is in circuit breakers. Graphite- high-speed mechanical systems, and electronic reinforced copper used as a “compliant layer” packaging. minimizes thermal stresses in an experimental Development of applications such as hyper- magnetohydrodynamic (MHD) generator ceramic sonic aircraft, which will require efficient high- channel in Japan. An aluminum tennis racket temperature structural materials, will undoubt- selectively reinforced with boron fiber was sold edly lead to increasing interest in MMCs. commercially in the United States for a brief time during the 1970s, Commercial Future Applications Commercial applications in the next 5 years are likely to be limited to diesel engine pistons, and Future applications include those which could perhaps sporting goods such as golf clubs, tennis be possible in the next 5 to 15 years. rackets, skis, and fishing poles. There are a num- .—— .

114 . Advanced Materials by Design ber of potential applications that are technically possible but for which the current level of development effort is inadequate to bring about com- mercialization in the next 5 years; these include: brake components, push rods, rocker arms, machinery and robot components, computer equipment, prosthetics, and electronic packaging. High Specific Stiffness and Strength.–MMC materials with high specific stiffness and strength are likely to have special merit in applications in which weight will be a critical factor. Included in this category are land-based vehicles, aircraft, ships, machinery in which parts experience high accelerations and decelerations, high-speed shafts and rotating devices subject to strong centrifugal forces. The high value of weight in aircraft makes use of MMCs a strong possibility for structural applications, as well as for engine and other mechan- nents, use of materials with high specific prop- ical system components. Mechanical properties erties - MMCs - will be a way of achieving future of MMCs could be important for a number of performance goals. medical applications, including replacement joints, bone splices, prosthetics, and wheelchairs. For high-speed rotating shafts, such as automo- bile and truck drive shafts, a major design con- There are numerous industrial machinery com- sideration is that the rotational speed at which ponents that could benefit from the superior spe- the shaft starts to vibrate unstably, called the crit- cific strength and stiffness of MMCs. For exam- ical speed, must be higher than the operating ple, high-speed packaging machines typically speed. As critical speed depends on the ratio of have reciprocating parts. Some types of high- stiffness to density, the high specific stiffness of speed machine tools have been developed to the MMCs makes them attractive candidates for this point where the limiting factor is the mass of the application. assemblies holding the cutting or grinding tools, which experience rapid accelerations and de- Attractive Thermal Properties. -There are a celerations. Further productivity improvements number of potential future applications for which will require materials with higher specific me- the unique combinations of physical properties chanical properties. Computer peripheral equip- of MMCs will be advantageous. For example, the ment, such as printers, tape drives, and magnetic special needs of electronic components present disk devices commonly have components that particularly attractive opportunities. must move rapidly. MMCs are well suited for Electronic devices use many ceramic and ce- such parts. ramic-like materials, that are brittle and have low The excellent mechanical properties of MMCs coefficients of thermal expansion (CTEs). Exam- make them prime candidates for future applica- ples are ceramic substrates such as alumina and tion in robots, in which the weight and inertia beryllia, and semiconductors such as silicon and of the components have a major effect on per- gallium arsenide. These components frequently formance and load capacity. Centrifugal forces are housed in small, metallic packages. MMCs are a major design consideration in high-speed can help prevent fracture of the components or rotating equipment, such as centrifuges, gener- failure of the solder or adhesive used to mount ators, and turbines. As these forces are directly these components, since the CTE of the package proportional to the mass of the rotating compo- can be tailored to match that of the device. Ch. 4—Metal Matrix Composites ● 115

Another desirable feature of packaging mate- Markets rials is high thermal conductivity to dissipate heat generated in the system, since the reliability of Market projections for MMCs vary widely. One electronic chips decreases as operating temper- market forecast by C. H. Kline is that MMCs will play no significant role in the current advanced ature increases. Two of the more common me- 18 tals now used in packaging are molybdenum and composite markets until after 1995. A second Kovar, a nickel-iron alloy. The thermal conduc- forecast (Technomic Consultants) is that U.S. non- tivity of Kovar is quite low, only about 5 percent military uses of MMCs will reach $100 million per year by 1994, and world-wide commercial uses of that of copper, and it cannot be used in appli- 19 cations in which large amounts of heat must be will reach $2 billion per year. dissipated. Molybdenum, which is more expen- There does seem to be agreement though, that sive and difficuIt to machine, is normally used in in the United States, MMC materials are likely to such cases. MMCs (e. g., with a high thermal con- be used primarily in military and space applica- ductivity copper matrix) could be used instead tions, and, to a much smaller degree, in electronic of these two materials. Examples of potential fu- and automotive applications. Because MMC ma- ture applications include heat sinks, power semi- terials are currently used mainly for research pur- conductor electrodes, and microwave carriers. poses, the markets for them are now only a few 20 The low CTE that can be achieved with some thousand pounds per year. Materials costs should MMCs makes them attractive for use in precision decrease with time, as production volumes in- machinery that undergoes significant temperature crease. However, the actual downward trend has change. For example, machines that assemble been slower than most predictions, and the cost/ precision devices are commonly aligned when performance benefits of MMCs have yet to be cold. After the machines warm up, thermal ex- demonstrated over alternative materials in large pansion frequently causes them to go out of align- volume applications. ment. This results in defective parts and down time required for realignment. Laser devices, —.. — which require extremely stable cavity lengths, are lgjacques shou~ens, Metal Matrix Composites Information Anal- ysis Center (MMCIAC), personal communication, March 27, 1987. another potential application for which low CTE lglbid. is an advantage. ZOI bid.

RESEARCH AND DEVELOPMENT PRIORITIES

Federal funding of MMC research and devel- The following section describes R&D priorities opment (R&D) comes mainly from the military. for MMCs in three broad categories of descend- Combined totals for the three services plus the ing priority. These categories reflect a consensus Defense Advanced Research Projects Agency as determined by OTA of research that needs to (DARPA) for 6.1,6.2, and 6.3A money were $29.7 be done in order to promote the development million for fiscal year 1987.21 There are a num- of these materials. ber of classified projects, and projects in other categories of funds, that also involve MMCs. Al- though the Department of Defense is the major Very Important government funding source, NASA also funds Cheaper Processes MMC research. NASA plans to provide $8.6 million for MMCs in fiscal year 1988, up from $5.6 First in importance is the need for low-cost, million in fiscal year 1987.22 highly-reliable manufacturing processes. Several industry experts advocate emphasis on modified ~ 1 Jerome” Persh, Department of Defense, personal comm u nlca- casting processes; others suggest diffusion bond- tlon, August 1987. ~~Brlan Qulgley, NatlOndl Aeronautics and Space Administration, ing and liquid metal infiltration as likely candi- personal communication, December 1987. dates for production of fiber-reinforced MMCs. 116 ● Advanced Materials by Design

There is some agreement that plasma spraying Important is also a promising method. For particulate-reinforced MMCs, powder metallurgy and liquid in- Environmental Behavior filtration techniques are considered most prom- It is critical to understand how reinforcements ising. To develop near-term applications, research and matrices interact, particularly at high tem- should concentrate on optimizing and evaluat- peratures, both during fabrication and in service. ing these processes, including development of A thorough understanding of material behavior low-cost preforms. is necessary to ensure reliable use and to provide guidelines for development of materials with improved properties. To date, study of the behavior Cheaper Materials of MMCs in deleterious environments has been Development of high-strength fiber reinforce- much more limited than for PMCs. Research is ments of significantly lower cost is a major need, needed in the areas of stress/temperature/defor- as there are no high-performance, low-cost fibers mation relations, strength, mechanical and ther- available at present. There are two schools of mal fatigue, impact, fracture, creep rupture, and thought as to the best approach to reducing costs. wear. Some analysts believe that the range of useful- ness can be expanded by the development of Fracture new fibers with better all-purpose design prop- The subject of fracture behavior in MMCs erties, such as higher strength, higher tempera- deserves special attention. The applicability of ture, and lower cost; they also see new matrix traditional analytical techniques is controversial alloys as important to facilitate processing and to because MMCs are strongly heterogeneous ma- optimize MMC performance. However, some terials. It seems reasonable that these traditional critics charge that this search for all-purpose fibers techniques should be valid at least for particulate- and better matrices is likely to be unrewarding reinforced MMCs, because they do not appear and that increased production experience with to have the complex internal failure modes asso- present fibers and matrices is the best route to ciated with fiber-reinforced MMCs. In view of the lower costs, particularly for potential near-term lack of agreement on how to characterize frac- applicat ions. ture behavior of MMCs, though, it will be necessary to rely on empirical methods until a clearer Interphase picture emerges. Control over the fiber/matrix interphase in Nondestructive Evaluation MMCs is critical to both the cost and performance of these materials. For example, new fiber Reliable NDE techniques must be developed coatings are needed to permit a single fiber to for MMCs. They should include techniques for be used with a variety of matrices. This would detecting flaws and analytical methods to evalu- be cheaper than developing a new fiber for each ate the significance of these flaws. In MMCs as matrix. in PMCs, there is the possibility of many kinds of flaws, including delamination, fiber misalign- At high temperatures, fiber/matrix interactions ment, and fracture. There are no N DE procedures can seriously degrade MMC strength. Research presently available for measuring the extent of in the area of coatings is desirable, not only to undesirable reinforcement/matrix interaction and prevent these deleterious reactions but also to resultant property degradation. promote the proper degree of wetting to form a good fiber/matrix bond. Coatings add to the ma- Machining terial and processing costs of MMCs, so that research into cheaper coatings and processes is es- The machining of MMCs is currently a very ex- sential. pensive process that requires diamond tools be- .-

Ch. 4—Metal Matrix Composites ● 117 cause of the materials’ very hard and abrasive ce- fabricating MMCs. What are needed are design ramic reinforcements. As machining is a major methods that take into account plasticity effects cost factor, development of improved methods and provide for development of efficient, selec- tailored to the unique properties of MMCs would tively reinforced structures and mechanical com- help to make these materials more competitive. ponents. Research on modeling of processing behavior is necessary, together with the eventual Desirable development of databases of properties and process parameters. Modeling Analytical modeling methods would be of value in helping to develop and optimize processes for