Preparation And High-temperature Properties Of Reactive Hot-pressed Boron Nitride-alumina Ceramics

The in-situ reaction conditions and process between boron oxide and aluminum nitride have been thoroughly studied.The reactive hot-pressing process is also successfully conducted to fabricate the BN-Al₂O₃ composites with different BN addition.The effect of BN content on the microstructure,room temperature mechanical properties,and high temperature properties of the BN-Al₂O₃ ceramics are investigated.Furthermore,the weight loss mechanism at high temperature of the BN-Al₂O₃ ceramics is proposed.The study of the thermodynamics and kinetics about in-situ reaction reveals that the reaction process begins above 750? and accompanied by the formation of aluminum borate?2Al₂O₃·B₂O₃ and 9Al₂O₃·2B₂O₃?.With the temperature increasing,2Al₂O₃·B₂O₃ decomposes to 9Al₂O₃·2B₂O₃ and B₂O₃ liquid.At 1800?,the reaction between 9Al₂O₃·2B₂O₃ and AlN is completely finished and the in-situ BN and Al₂O₃ are formed.During the sintering process,it illustrates that the low-temperature densification can improve the sintering performance of the composites attributed to the low-temperature point of B₂O₃.In this paper,BN-Al₂O₃ composite ceramics with different BN content are prepared by adding h-BN into the reactants of in-situ reaction.It is proved that the bending strength,elastic modulus,and fracture toughness of the composites are significantly reduced with the increasing of BN content.When the addition amount of BN is 30 wt.%,the composites with distinct mechanical properties are obtained and the properties are as follows:flexural strength 94±1.97 MPa,elastic modulus63±2.1 GPa and fracture toughness 1.88±0.06 MPa·m^1/2.In addition,the bending strength of the reactive hot-pressed composites show slight anisotropy when the content of in-situ BN lower than 50%of the total BN content duo to the unique sheet structure of BN.The thermal diffusivity and thermal conductivity of BN-Al₂O₃ composite ceramics decrease with the increasing of temperature.At the same testing temperature,thermal diffusivity and thermal conductivity increase as the addition amount of BN increases.In addition,with the increasing of testing temperature,the thermal expansion coefficient of the BN-Al₂O₃ composite ceramics rises continuously.And the higher the BN content of the composites,the smaller the thermal expansion coefficient of the composites.The high temperature properties of the BN-Al₂O₃ composite ceramics are tested at different temperature.The results indicate that the high temperature flexural strength decrease firstly and then increase as the temperature rises.The high temperature flexural strength at 1800?of the composites with 40 wt.%BN addition deduce to a minimum value of 33.1 MPa.When the testing temperature rises to 2000?,the high temperature flexural strength increases to 50.06 MPa.Moreover,the high temperature flexural strength rises with the BN content increasing.For example,the high temperature flexural strength of the composites with 30 wt.%BN addition at 2000?reach a maximum value of 84.7 MPa,close to the flexural strength at room temperature.Furthermore,the high-temperature weight loss mechanism of BN-Al₂O₃ composite ceramics is also proposed.When soaking at1700¹900?,results reveals that the weight loss is mainly dominated by the the decomposition of BN at high-temperature negative pressure environment and the reverse reaction between the boron nitride and alumina.The weight loss of composites above 1800?is attributed to the volatilization of B₂O₃.