The Preparation And Properties Characterization Of ZrO₂/ZrW₂O₈ Ceramic Matrix Composites With Near-Zero Thermal Expansion

With the development of the aerospace technology, the requirement of ceramic materials for the structural stability and safe reliability has become increasingly demanding. Thermal stresses can be produced by large temperature gradients due to the high thermal expansion coefficient（CTE） of the structural ceramics, which will damage the structure and reduce the thermal shock resistance and antioxidant ablation of the materials. It has been an effective way to improve the thermal shock resistance of the structural ceramics by adding isotropic negative thermal expansion coefficent material ZrW₂O₈ to the structural ceramic matrix materials in order to reduce the CTE and thermal stresses of the structural ceramics. ZrO₂/ZrW₂O₈ ceramic matrix composite with near-zero thermal expansion coefficient will be prepared by compounding the ZrO₂ matrix material with excellent mechanical properties and the second phase material ZrW₂O₈ with isotropic negative thermal expansion coefficent. The appropriate preparation technology will be selected preferentially by the properties characterization and analysis of ZrO₂/ZrW₂O₈ composite, then the size effect that could influence the properties of materials also has be analyzed.In this dissertation, the phase decomposition and solid-phase synthesis of the second phase material ZrW₂O₈ are investigated firstly, in order to grasp the properties of isotropic negative thermal expansion coefficent material ZrW₂O₈. Thermodynamic calculations show that ZrW₂O₈ has the spontaneous decomposition trend at normal temperature. By heating the single phase ZrW₂O₈ samples to different temperature points and detecting the phase components of the cooled samples, the phase decomposition of ZrW₂O₈ can be judged. Combined DSC-TGA analysis, it can be known that the phase decomposition of ZrW₂O₈ occurs during the temperature range of 780℃¹108℃ with products of ZrO₂ and WO3. ZrW₂O₈ can exist stably below this temperature range, while the solid-phase synthesis of ZrW₂O₈ occurs quickly above the temperature range. ZrW₂O₈ would appear liquid phase more than 1180℃. The solid-phase synthesis of ZrW₂O₈ experiment in which ZrO₂ and WO3 as raw materials shows that the cooling ways and sintering temperatures have important effects on synthetic ratio of ZrW₂O₈. The optimum sintering temperature range is from 1170℃ to 1200℃ and the best cooling way is water cooling.Based on the study of the phase decomposition and solid-phase synthesis of ZrW₂O₈, three kinds of thermal expansion coefficient prediction models（ROM model, Turner model and Kerner model） are used to predict the CTE of ZrO₂/ZrW₂O₈ composite with different mass ratios. The hot pressed sintering method（HP） and pressureless sintering method（S） are adopted respectively to explore the mechanical mixing and in situ reaction synthesis ways to prepare ZrO₂/ZrW₂O₈ composite. The characteristics of different preparation methods are analyzed by comparison, then the improved hot pressed sintering technology and the WO3 powders covering samples with airtight crucible method are put forward to get higher quality samples. As for pressureless sintering method, ZrO₂/ZrW₂O₈ composite with near-zero CTE prepared by the mechanical mixing way can be gotten with a mass ratio range of 1.5:1¹.3:1. The CTE of the samples is from 1.69×10-6/K to-0.53×10-6/K and the actual volume fraction of ZrW₂O₈ is 37%⁴2% on this condition. The maximum compression strength of the samples can reach 150 MPa. On the other hand, ZrO₂/ZrW₂O₈ composite with near-zero CTE prepared by the in situ reaction synthesis way can be obtained by use of ZrO₂ and WO3 with a mass ratio range of 0.93:1⁰.81:1. The CTE of the samples is from 1.08×10-6/K to-1.67×10-6/K and the actual volume fraction of ZrW₂O₈ is 40%⁴4%. In this situation, the sintered density and compression strength of the samples reduce obviously. The reasons are presented by use of the microstructure analysis and the optimum preparation technology of ZrO₂/ZrW₂O₈ composite with near-zero CTE is acquired. In the end, the size effect of ZrO₂/ZrW₂O₈ ceramic matrix composite is investigated using the finite element modeling analysis. The relationship between size, CTE and thermal stress of the samples is built and the development direction about large size and high strength for ZrO₂/ZrW₂O₈ is proposed.