Improvement of the temperature resistance of aluminium-matrix composites using an acid phosphate binder

Abstract

The use of phosphate binders instead of the widely used silica binder resulted in improved temperature resistance, increased tensile strength and decreased coefficient of thermal expansion. The effects were largest for the phosphate binder which contained the largest amount of phosphoric acid (P/Al atom ratio = 24 in the liquid binder). These effects were probably due to the protection of the SiC whiskers by the binder phases (aluminium metaphosphate or aluminium orthophosphate), the binder-SiC reaction product (SiP₂O₇) and the binder-aluminium reaction product (AIP) from further reaction between the SiC and aluminium. The tensile strength of the composite containing the SiC whisker preform made with the phosphate binder (P/Al atom ratio = 6 or 24 in the liquid binder) was increased after heating at up to 600 °C for 240 h. The silicon phosphate (SiP₂O₇) acted as an in situ binder and was primarily responsible for increasing the compressive strength of the preform and increasing the temperature resistance of the composite. The carbon fibre composite containing the preform made by using the phosphate binder (P/Al atom ratio = 24 in the liquid binder) with either water or acetone as the liquid carrier during wet forming of the preform had a higher tensile strength than the carbon fibre composite made by using the silica binder. After composite heat exposure to 600 °C for 14 h, the carbon fibre composite made by using this phosphate binder with acetone as the liquid carrier during wet forming of the preform showed the best temperature resistance, while the carbon fibre composites made by using this phosphate binder with water as the carrier showed the second best temperature resistance, and that made by using silica binder was the worst. The reason for the better effect of the phosphate binder than the silica binder is probably due to the ability of the phosphate binder and the binder-aluminium reaction product (AIP) to protect the carbon fibres from the undesirable reaction between the carbon fibres and aluminium. The lack of a binder-fibre reaction contributed to making the carbon fibre composites less temperature resistant than the SiC whisker composites. The use of a higher binder concentration is attractive for increasing the temperature resistance of the composites. The binder concentration in the preform can be increased by increasing the binder concentration in the slurry used in the wet forming of the preform.