Abstract
The need for the application of pressure during sintering is dictated also by certain characteristics of the sintering process of these materials. In particular, in the sintering of compacts from the matrix constituents of these materials (copper-tin [7], copper-aluminum [8]), the compacts are found to increase in volume. The volume growth of copper-tin and copper-aluminum powder alloys during sintering is linked with the diffusion processes of solid-solution formation and results from the inequality of the partial diffusion coefficients of the reacting components [9, 101. The magnitude of volume growth of a two-component system increases in the presence of components, such as graphite, which do not react with the elements present in the system [7, 11]. In view of this, the magnitude of volume growth of friction materials, which contain-as already noted above-a substantial amount of such components, markedly exceeds the magnitude of volume growth of the binary copper-tin or copper-aluminum system. The growth of compacts in the course of sintering increases their initial porosity. By applying a load to parts during sintering, it is possible both to suppress the volume growth of compacts and to obtain materials having a predetermined density. Nonmetallic constituents substantially lower the strength characteristics of the metallic matrix, as a result of which even nonporous friction materials have low strength and are sensitive to impact loads. Because of this, thin friction linings are generally mounted on a reinforcing steel support plate. It is usual for such linings to be bonded to the load-carrying plates (shells) during sintering by utilizing diffusion processes taking place between an electrolytic coating (copper, nickel, nickel-copper, copper-tin) on the support plate and the friction sintered layer. The adhesion (bond strength) of a sintered layer to a steel backing plate markedly increases when a load is applied to the part during the sintering process.
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