Improving the temperature resistance of aluminum-matrix composites by using an acid phosphate binder

The fabrication of metal-matrix composites by vacuum infiltration of a liquid metal (close to the liquidus temperature) into a SiC whisker preform under an inert gas pressure was shown to produce SiC whisker reinforced aluminum (12-37 vol.%) which was comparable in quality to those produced by powder metallurgy or squeeze casting.; The {dollar}alpha{dollar}-SiC whisker preform (22 vol.%) was made by wet forming using water and a binder. The use of phosphate binders (with Al(OH){dollar}sb3{dollar} and H{dollar}sb3{dollar}PO{dollar}sb4{dollar} as precursors) instead of the widely used silica (SiO{dollar}sb2{dollar}) binder resulted in increased tensile strength and temperature resistance of the SiC whisker composites. The excess H{dollar}sb3{dollar}PO{dollar}sb4{dollar} from the acid phosphate binder reacted with SiC to form SiP{dollar}sb2{dollar}O{dollar}sb7{dollar}, which strengthened the preform by acting as an in-situ binder. The excess H{dollar}sb3{dollar}PO{dollar}sb4{dollar} also reacted with Al to form AlP, which strengthened the Al/binder interface. The combination of SiP{dollar}sb2{dollar}O{dollar}sb7{dollar} and AlP caused the tensile strength of the SiC whisker composite to increase. The phases Al(PO{dollar}sb3)sb3{dollar}, SiP{dollar}sb2{dollar}O{dollar}sb7{dollar} and AlP all contributed to the strengthening of the composites.; Upon heating the SiC whisker composites (not the preforms) at 400 or 600{dollar}spcirc{dollar}C for 240 h, the tensile strength decreased for composites made with the silica binders, but increased for composites made with the phosphate binders. The latter effect increased with increasing P/Al ration in the phosphate binder. This is attributed mainly to the increased amount of SiP{dollar}sb2{dollar}O{dollar}sb7{dollar}, which was present in greater quantity than Al(PO{dollar}sb3)sb3{dollar} or AlP.; Al-matrix composites containing short (0.1 mm) pitch-based carbon fibers (55 vol.%) were similarly prepared, except that the carbon fiber preforms were heated at 200{dollar}spcirc{dollar}C prior to metal infiltration. Upon heating the composites (not the preforms) at 600{dollar}spcirc{dollar}C for 2-240 h, the tensile strength decreased for composites made with phosphate or silica binders. This is attributed to the absence of a binder/carbon reaction for both binders. The improvement of the fiber/matrix bonding due to the binder/Al reaction for the case of the phosphate binder was insufficient to overshadow the negative effect related to the absence of a binder/carbon reaction. Nevertheless, the strength decrease after heating at 600{dollar}spcirc{dollar}C for 14 h was less for composites made with the phosphate binder than those made with the silica binder. Moreover, the strength decreased less as the phosphate binder amount was increased.