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

Binders are used in preforms from which metal-matrix composites are fabricated by liquid-metal infiltration. This paper (Part I) reports the preparation and characterization of various binders (by themselves). The characterization was in terms of the composition (by atomic absorption spectrometry and X-ray spectrometry), morphology, phases (by X-ray diffraction) and changes upon heating to 1200 °C (by thermogravimetric analysis and mass spectrometry). The binders studied include silica, phosphates (prepared from Al (OH)₃ and H₃PO₄) with the P/Al molar ratio ranging from 1-23, sodium silicate and acrylic emulsion. A phosphate binder was in the form of type B aluminium metaphosphate (Al(PO₃)₃) at 500 °C when the P/Al atom ratio = 2.2 in the liquid binder, and was in the form of type A aluminium metaphosphate (Al/(PO₃)₃) when the atom ratio ≥ 5.8 in the liquid binder. Most of the phosphate binders formed type A aluminium metaphosphate (Al(PO₃)₃) after heat treatment at 800 °C. The phosphate binder with P/Al atom ratio = 2.2 in the liquid binder formed cristobalite aluminium orthophosphate (AlPO₄), and the phosphate binder with P/Al atom ratio = 24 in the liquid binder formed a combination of cristobalite aluminium orthophosphate (AlPO₄) and metaphosphate glass after heat treatment at 1200 °C in argon.