Study On The Preparion And Properties Of Phosphate Composite Materials

Electro-magnetic wave penetrating composite materials is one of the key technologies of guided vehicle. Phosphate composite has similar performance with fused silica ceramic and quartz fibre reinforced silicon dioxide composite at lower than 800°C, it also has less cost and short production circle, so it becomes one of the preferred antenna cover materials of tactics missiles. In the present study, Si3N4 particles (Si₃N_4p) and aluminum borate whiskers - Si3N4 particles (AB_w-Si₃N_4p) were employed to prepare chromium aluminium phosphate matrix composites for antenna cover applications.In this paper, the chromium aluminium phosphate (CAP) solution was prepared from H3PO4, Al(OH)3, and CrO3 with a molar ratio of 12:3:1. Curing process, phase composition, mechanical properties, thermal stability and cement mechanisms of the solution were studied with Differential Theremal Analysis (DTA), X-ray Diffraction (XRD), Viscometer and Hygroscopic tests and so on. Based on above study and the temperature curing properties of the phosphate, Si₃N_4p/CAP and AB_w-Si₃N_4p/CAP were prepared by presureless sintering. The effects of reinforcement content, curing and sintering parameters on microstructure, mechanical and thermal properties of the composites were studied. The toughening mechanism was discussed.In the composites, Si₃N_4p, AB_w and large amount of pores were found to homogeniously distribute in the CAP matrix. No new phase and interface reaction was present when sintered below 900°C.When sintered at 900°C, Si₃N_4p/CAP has a higher flexural strength, but a lower fracture toughness than AB_w-Si₃N_4p/CAP, due to their relative density difference. The maximum flexural strength and facture toughness KIC of Si₃N_4p/CAP is 79.0MPa and 2.7 MPa·m^1/2 respectively, and these of AB_w-Si₃N_4p /CAP are 63.7 MPa and 3.1 MPa·m^1/2.The two composites have a similiar thermal expansion behavior, thermal expansion coefficient increased when the temperature increased. Different particles concentration and whisker content influenced thermal performance of the composite directly. The thermal expansion coefficients of the composites are very small, with the minimum values of 1.63×10^-6/K and 2.73×10^-6/K, repectively for Si₃N_4p/CAP and AB_w-Si₃N_4p/CAP. Both composites have high thermal shock resistance, though Si₃N_4p/CAP is a little bit superior to AB_w-Si₃N_4p/CAP.