Cost-efficient metal-ceramic composites - novel foam-preforms, casting processes and characterisation

Because of their dissatisfactory cost-performance ratio MMCs (metal matrix composites) are still not established in industry, although they show improved properties compared to pure metals in some application fields. The present paper describes the development of enhanced MMCs based on silicon carbide (SiC) foams made by the Schwartzwalder process. Therefore, foams with cell sizes of 30, 45 and 60 ppi (pores per inch) based on pressure less SSiC (sintered SiC) were developed. They were coated with layers of coarse SiC particles, which form a rough strut surface. The ceramic content of the foams could be increased to values of 20-30 mass%. Additionally, foam preforms based on clay-bonded SiC (as they are known from molten metal infiltration) were tested. The preforms were infiltrated with aluminium alloys AlSi9Cu3 and AlSi7Mg(0.6) and cast iron EN-GJSA-XNiCr35-5-2 and EN-GJL-250. For aluminium alloys HPC (high pressure die casting) as well as gravity casting was applied, whereas iron was only infiltrated by gravity casting. For HPC an excellent interlocking of metal and preform was observed because of the microporosity of the rough surface of the SSiC foam struts. By the use of gravity casting preform cells up to 45 ppi could be well infiltrated. Microporosity in the ceramic coating and the typical hollow struts of the foams did not show metal infiltration. Even by use of moderate ceramic volume fractions pressure-infiltrated aluminium matrix composites showed a high specific stiffness of up to E/rho=42 GPa.cm3/g compared to conventional Al or Mg alloys (E/rho = 25-27 GPa.cm3/g). Ceramic foam based MMCs produced by pressure less casting showed no advantages in mechanical properties compared to pure metals. Nevertheless it can be expected that they can provide improved wear resistance and lower thermal expansion coefficients.