Studies Of ZrO₂/ZrW₂O₈ Composites With Low Thermal Expansion

In the field of aerospace, the thermal expansion of material is an important property of high-performance materials. In the process of preparation of some high temperature materials, due to the differences of the thermal expansion coefficients of components, crack, stress concentration, damage of the mechanical properties and etc.are tend to be occurred on materials. The development the research on negative inflation or low expansion composites has a potential value in the application of materials.Zr W2O8 has isotropic negative expansion performance in a wide temperature range,moreover, the absolute value of thermal expansion coefficient of Zr W2O8 is close to that of Zr O2, and there is no reaction between them. Therefore, in this research, Zr W2O8,WO3 and Zr O2 were used as raw materials, and the ceramic matrix composites with high density and low expansion were prepared by the adjusting of preparation technology.In this work, the properties of the composites sintered at different sintering temperatures using hot-pressed sintering method were studied. The results showed that Zr O2/Zr W2O8 composites with high densities which was up to 5.55 g/cm3 could be prepared by using both Zr O2+Zr W2O8(direct mixing method) and Zr O2+WO3(in-situ reaction method) as raw materials via hot-pressed sintering at 1200 and 1215 °C. There were no obviously differences in densities between the composites sintered at 1200 °C for 3 h and 1215 °C for 6 h, both of which were nearly 5.20 g/cm3. However, compared with those of the direct mixing method, the advantages of "in-situ reaction and hot-pressed sintering" method were not shown sufficiently, because of the existence of residual WO3. Besides, due to the instability of Zr W2O8 at 1200 °C, the sintering process was conducted at 1215 °C.Furthermore, the influence of sintering pressure on the properties of the composites sintered by both direct mixing(Zr O2+Zr W2O8) and in-situ reaction method(Zr O2+WO3)was investigated. After shaping process, to enhance the relative density and promote the in-situ reaction between the raw materials, the green bodies were pressed under a pressure of 200 MPa, then were sintered with or without pressure using at 1215 °C.Densities of the obtained composites were approximatively equal to the theoretical density. Compared with those obtained through in-situ reaction method, the composites achieved through direct mixing method owned a lower decomposition rate of Zr W2O8 and a lower thermal expansion coefficient. In addition, for the composites prepared by both direct mixing and in-situ reaction method, densities of the composites sinteredusing hot-pressed sintering were higher than those sintered without pressure, moreover,decomposition rate of Zr W2O8 and thermal expansion coefficients of the composites were lower.Adjusting the preparation technology,studied the properties of composite materials by different cooling method.Both direct mixing method and in-situ reaction method,the density, the Zr W2O8 decomposition rate,the thermal expansion coefficient of the composite material in air cooling is higher,lower,smaller than that of the composite materials in water cooling.The density of the composite prepared by in-situ reaction is5.57 g/cm3,and its instantaneous thermal expansion as low as 2.0 × 10-6/K,and its bending strength is 112.96 MPa.The density of the composite prepared by directly mixing is 5.62 g/cm3,and its instantaneous thermal expansion as low as 2.0×10-6/K,and its bending strength is 195.99 MPa.