A polycrystalline diamond (PCD) body with improved thermal stability is disclosed which comprises a PCD body which has had at least one of its previously empty pores infiltrated by a silicon containing alloy. According to the process of the invention, a porous PCD body is obtained, preferably by acid leaching a PCD body which was formed in the presence of a metal catalyst such as cobalt. The porous PCD body is then surrounded by either the desired silicon containing alloy, or by the constituents of that alloy, each preferably in powdered form. The PCD body with its surrounding material is then heated and pressed to temperatures sufficient to melt the surrounding material (thereby forming the silicon alloy if not already formed) and to cause it to infiltrate into the pores. After the infiltration, it is preferred to remove the excess silicon containing alloy from the external surfaces of the PCD body, such as by an acid bath. It is also preferred to include several porous PCD bodies per process cycle. The metal component of the alloy together with the relative amounts of silicon can be selected in order to achieve the lowest melting point of the alloy and the best performance characteristics of the infiltrated PCD bodies.