1. Field of the Invention
The present invention relates to a composite material composed of a matrix of ceramics, metal, polymer, or the like and components dispersed in the matrix. The present invention also relates to a process for producing said composite material.
2. Description of the Related Art
A combination of two or more dissimilar materials improves the characteristic properties, such as wear and abrasion resistance, oxidation resistance, corrosion resistance, heat resistance, electrical and thermal conductivity, and mechanical strength, for which a monolithic material is not satisfactory. It also imparts new functional properties, such as magnetism, self-lubricity, and thermal and electrical conductivity, which a monolithic material does not possess. At present various materials are under study for new composite materials.
A composite material composed of a matrix and components (such as particles, whiskers, and fibers) of different materials from that of the matrix which are dispersed therein is noteworthy because the dispersed components exhibit the mechanical and functional properties. Therefore, it will find use for varied requirements which a monolithic material does not meet.
The composite material of conventional dispersion type is usually composed of a matrix and components randomly dispersed therein. It improves performance and functions owing to the dispersed component, but only insufficiently because the components are randomly dispersed.
To solve this problem, there was proposed a composite material in which the dispersed components constitute a continuous three-dimensional network structure. (Japanese Patent Laid-Open Nos. 243245/1985, 4750/1987, 119688/1989, 122066/1991, 174358/1991, and 37667/1992) it is claimed that the composite material permits the continuously dispersed component to fully exhibit its performance.
Japanese Patent Laid-open No. 243245/1985 discloses "a ceramic particles-reinforced metal composite material", which is composed of a porous ceramic skeleton (formed by sintering a mixture of ceramic material and ceramic whiskers) and a metal impregnated into pores in the ceramic skeleton. This composite material is claimed to have good heat shock resistance and crack resistance because of the continuous skeletal structure of the mixture of ceramic material and ceramic whiskers dispersed in the metal matrix.
Japanese Patent Laid-open No. 4750/1987 discloses "a positive temperature coefficient composition and a process for production thereof," said composition comprising a crystalline polymer and carbon whiskers of 0.05-1 mm in average length and 3-20 .mu.m in diameter. This composition is claimed to have carbon whiskers forming a continuous three-dimensional network microstructure in the polymer matrix, so that it permits the reduction of the amount of carbon whiskers to be used, which contributes to providing good PTC properties and cost saving.
Japanese Patent Laid-open No. 119688/1989 discloses "a resin-molded electrode and a process for production thereof," said electrode comprising a base material of thermosetting resin and electrically conductive metal particles (e.g., lead particles) continuously and reticulately dispersed therein. This electrode is claimed to have good corrosion resistance and mechanical properties and to be inexpensive.
Japanese Patent Laid-open No. 122066/1991 discloses "an aluminum-impregnated silicon carbide composite material and a process for production thereof", said composite material comprising a porous silicon carbide and aluminum impregnated into pores in the silicon carbide. This composite material is claimed to be light in weight and superior in strength, heat resistance, and wear and abrasion resistance on account of its unique combination of components--aluminum impregnated into continuous pores in the porous silicon carbide.
Japanese Patent Laid-open No. 174358/1991 discloses "a composite material composed of carbon (90 to 30 mol %) and silicon carbide (10 to 70 mol %) both forming the continuous phase". This composite material is claimed to have a great flexural strength and hence retain its shape even after the carbon component has disappeared due to oxidation.
Japanese Patent Laid-open No. 37667/1992 discloses "light-weight high-stiffness ceramics and application thereof", said ceramics having three-dimensional continuous network structure formed in a reaction-sintered matrix. This ceramics is said to have a high specific modulus of elasticity by virtue of its composite structure.
The six composite materials mentioned above suffer a disadvantage in common that their strength depends on the strength of the matrix or dispersed component whichever lower in strength or the density of the composite material and hence a desired strength will not be attained only by dispersing the components in the continuous three-dimensional network structure. Their additional disadvantage is that the composite material is continuously subjected to internal stress due to the difference in thermal expansion of the matrix and dispersed components, which lowers their mechanical properties such as impact resistance. Moreover, they need a special process of forming a matrix or dispersed components in a porous network structure and then impregnating one material into the other. This process is not suitable for production of dense composite materials, nor is it suitable for mass production because of its low efficiency. PA1 The second aspect of the present invention resides in a process for producing a composite material which is composed of a matrix and dispersed components which form a discontinuous three-dimensional network structure in the matrix, said process comprising the steps of preparing a raw material powder such that granules of desired shape for the matrix are discontinuously covered with components of desired shape for the dispersed phase, molding the raw material powder into a desired shape, and heating the molded article. PA1 It is not exactly known how the process of the present invention produces its outstanding effect. A probable reason is as follows: The process consists of three steps. In the first step, granules of desired shape for the matrix are discontinuously covered with components of desired shape for the dispersed phase. In the second step, the raw material powder is molded into a desired shape. The resulting molded article contains the discontinuously dispersed components between adjoining particles on one or both of the particles forming the matrix. In the third step, the molded article is heated for sintering so that adjoining particles not separated by the dispersed components fuse together and the molded article as a whole becomes dense. In this way there is obtained a composite material composed of a matrix and discontinuously dispersed components forming a three-dimensional network structure in the matrix.
Further, the sixth composite material mentioned above is a light-weight, high-stiffness ceramics in which the dispersed components are formed in a network structure by using atomized or pulverized metal particles coated with ceramic powder. It is poor in sinterability and hence is poor in strength because of the continuously dispersed component.
In order to solve the above-mentioned problems involved in prior art technology, the present inventors carried out a series of studies and experiments, which led to the present invention.