A composite (MMC) with a metal matrix such as aluminum reinforced by SiC particles or fibers. It can raise stiffness and wear resistance and lower thermal expansion.
Japanese name: 金属基複合材料
Metal matrix composite appears in 1 practice question across 1 topic, including Composite materials.
Field: Composite materials
A metal matrix composite (MMC) uses a metal such as aluminum, magnesium, titanium or copper as the matrix, reinforced with particles, short fibers or continuous fibers of materials such as SiC and Al₂O₃. Particle-reinforced aluminum alloys are the most widely used, with particle contents ranging from about 10–20% by volume up to more than 60% in heat-dissipation materials. MMCs aim at properties between those of polymer matrix composites and ceramics: higher heat resistance than the former, and higher toughness and thermal conductivity than the latter. Cemented carbides, in which WC particles are bonded by Co, are sometimes regarded as MMCs in a broad sense.
The Young's modulus of a particle-reinforced composite lies between the upper bound of the rule of mixtures (equal strain in each phase) and the lower bound (equal stress). For aluminum (70 GPa) with 20 vol% SiC particles (410 GPa), the upper bound is about 138 GPa and the lower bound about 84 GPa, and actual values often fall in between, around 100 GPa. The linear expansion coefficient also falls with particle content, and with a large particle fraction it can be reduced from the matrix's roughly 23×10⁻⁶ K⁻¹ to values close to those of semiconductors and ceramic substrates. Being able to lower expansion alone while keeping thermal conductivity high is the advantage in heat-dissipation materials.
MMCs are made by stir casting, in which particles are mixed into molten metal and cast; powder metallurgy, in which metal and reinforcement powders are mixed and consolidated; and pressure infiltration, in which molten metal is forced into a preform of the reinforcement. Molten metals often wet particles poorly, so particles tend to cluster and distribute unevenly, and voids tend to remain. Also, prolonged contact at high temperature makes SiC react with aluminum to form brittle Al₄C₃, so the temperature, time and Si content of the matrix are controlled. Dislocations multiplied around particles by the expansion mismatch on cooling also affect the strength.
Aluminum alloy/SiC MMCs have been used for baseplates of power semiconductor modules, structural parts of precision equipment, brake components and bicycle frames. Their weaknesses, however, are that the hard particles wear cutting tools quickly so that diamond tools are needed, that ductility and toughness are lower than the matrix's, that they are expensive, and that the particles make recycling difficult. They are therefore generally reserved for parts where stiffness, wear resistance or low thermal expansion is particularly important and no other material can meet the requirements at a reasonable cost.
Answer: A. Compared with the matrix alloy, it has a higher Young's modulus and wear resistance and lower thermal expansion, but lower ductility and toughness
Adding SiC particles, which have a high Young's modulus of about 400 GPa and a low coefficient of thermal expansion of about 4×10⁻⁶ K⁻¹, makes the Young's modulus and wear resistance of the composite higher than those of the aluminum alloy matrix and its thermal expansion coefficient lower. Because the thermal expansion can be brought close to that of semiconductors and ceramic substrates, these composites are used for heat spreaders for power semiconductors and similar parts. On the other hand, cracks form easily around the hard particles, so ductility and toughness decrease. The statement that thermal expansion increases and ductility improves is backward on both counts. Also, SiC can react with hot molten aluminum to form brittle Al₄C₃, and the reaction is suppressed by controlling the processing temperature and the Si content of the matrix.
The matrix differs. FRP uses resin and MMCs use metal, so MMCs have higher heat resistance and conduct heat and electricity well. On the other hand, their density is higher and manufacturing tends to be more expensive.
Their Young's modulus and wear resistance rise greatly and their yield strength is often higher too, but elongation and toughness fall. If strength alone is the goal, a heat-treated aluminum alloy is often sufficient.
It is an MMC of aluminum with a high SiC content, used for heat-dissipating baseplates and packages of electronic components. It conducts heat well while bringing thermal expansion close to that of semiconductors and ceramic substrates, reducing thermal stress in solder joints.
Young's modulus Coefficient of linear thermal expansion Ceramic matrix composite CFRP (carbon-fiber-reinforced plastic) Critical fiber length Delamination GFRP (glass-fiber-reinforced plastic) Quasi-isotropic laminate Silane coupling agent