Rheological behaviour of borosilicate composites with metallic and non-metallic dispersions

Silicate matrix composites are potential candidates for high-temperature applications. In the present investigation, the effect of metallic (Cu) and non-metallic (SiC particulates, platelets, short fibres and whiskers) additions on the rheological behaviour of borosilicate matrix composites has been evaluated. The hot-pressed composites were tested both in compression and tension in the temperature range of 625–725°C. SiC reinforced composites tested in compression exhibited varying degree of strengthening and strain rate sensitivity depending on the volume fraction and morphology of reinforcements. The degree of strengthening and strain rate sensitivity depends on the volume fraction and morphology of reinforcements. Strengthening effect increased with the volume fraction and aspect ratio of reinforcements. The flow behaviour of composites changed from Newtonian to non-Newtonian with strain rate sensitivity index value changing from unity to 0.48. A similar trend was seen in the rate sensitivity of copper composites. However, copper additions decreased the strength of the composites at lower temperatures because of the softer copper phase. Pre-oxidation of copper particles had certain strengthening effect on the composite. The apparent viscosity of SiC reinforced composites increased with volume fraction and aspect ratio of reinforcements. However, in particulate composites, the viscosity found to increase with particle size. The mechanical/hydrodynamic interactions among the particulates appeared to be responsible for such a behaviour. With increasing strain rate, the viscosity decreased progressively confirming the shear thinning of the composites. The tensile ductility of the composites with 40 vol% reinforcements was evaluated at 700°C. While 400% elongation was observed in SiC particulate, platelet and copper composites, in short fibre/whisker composites, the tensile elongation values were only 150%. Further, the elongation of SiC platelet and copper composites improved by decreasing temperature and volume fraction of reinforcements, and also elongation values >500% were recorded. The tensile ductility of borosilicate composites was limited by onset and growth of cavities nucleated at the reinforcement/matrix interfaces.