Alumina Ceramics Reinforced By Mullite-alumina Prestressed Coating And Its Performance Evaluation

Alumina ceramic has excellent properties such as high hardness,high temperature resistance,corrosion resistance and wear resistance,and are widely used in aerospace,new energy vehicles,machinery,electronics and many other fields.However,Alumina ceramic is also a typical brittle material,which can resist large pressure but cannot resist large tensile force,resulting in sudden failure without warning in some cases.Using prestressed reinforcement technology,the strength of concrete and glass（tempered glass）has been greatly improved resulting in inestimable great value.Alumina ceramic,as a large-scale structural ceramic used in industry,will play an important role in more fields if its strength and reliability can be improved through prestressed design.The design idea of prestressed alumina ceramic is to coat a layer with low thermal expansion coefficient on the surface of alumina ceramic body,and then a coating with compressive stress is formed through co-sintering which can achieve the purpose of strengthening and toughening.In this paper,mullite-alumina with low expansion coefficient is selected as the coating material,the surface coating material is optimized and the appropriate preparation method is explored.The relative method was used to evaluate the properties of the coating,the residual stress distribution and reinforcement mechanism were analyzed,and then the high temperature strength and thermal shock resistance of the prestressed composites were discussed.Firstly,according to the design idea of prestressed coating,the mixed powder of alumina and quartz is used as the coating raw material.The mixed slurry of alumina and quartz powder was coated on the pre-sintered alumina body,then the mullite-alumina coating with lower thermal expansion coefficient was synthesized in-situ after pressureless co-sintering.The pre-stressed strengthening of alumina was achieved by the residual compressive stress formed in the coating during the cooling process.The results show that when the content of quartz in the coating raw material is15wt%,the flexural strength of the composite reaches 549.44±27.2 MPa,which is37.19%higher than that of alumina.Afterwards,with the increase of quartz content,the flexural strength decreases sharply.The main reason is that the coating shrinkage will gradually decrease with the increase of quartz content,resulting in sintering mismatch and material damage.Secondly,By measuring the thermal expansion coefficient and elastic modulus of the coating and the substrate,the theoretical relationship between the residual compressive stress in the coating and the content of quartz and the substrate/coating thickness ratio（h_s/h_c）is analyzed.The results show that:the residual compressive stress is related to the thermal expansion coefficient differenceΔαand the coating elastic modulus E_c.When the content of quartz exceeds 15wt%,the residual compressive stress in the coating tends to be stable;when h_s/h_c>10,the increase of residual compressive stress in the coating tends to be stable.At the same time,optimization shows that reducing the particle size of the coating raw material has a better effect than reducing the sintering heating rate.Under the optimized quartz ratio at 15wt%,the prestressed coating increase the critical temperature for drop-off in residual strength of alumina from 220℃to 280℃,showing better thermal shock resistance.Finally,for the alumina ceramic substrate with a sintering temperature of 1650°C,the use of fused mullite and alumina as coating materials has a better prestress enhancement effect.Due to the internal stress,the destruction of the substrate will be accompanied by energy release and a lot of debris appeared after the substrate was damaged.The mullite-alumina coating reduces the dielectric constant of alumina ceramics which has a good application prospect.