Preparation Of β-SiAlON/h-BN Composite Ceramics By Gel Casting And Its Performance Research

β-SiAlON ceramics have many excellent properties but still suffer insufficient thermal shock and oxidation resistance in practical engineering applications.Moreover,the traditionalβ-SiAlON ceramic preparation process has the disadvantages of high cost and complicated process flow.Therefore,improving the thermal shock and oxidation resistance ofβ-SiAlON ceramics,and adopting a simple and low-cost preparation method is the focus of research onβ-SiAlON ceramics.In this research,β-SiAlON/h-BN composite ceramics were successfully prepared by the gel casting combined with reactive bonding using Isobam as the gelling agent,and the effects of h-BN on the conventional and high temperature properties ofβ-SiAlON/h-BN composite ceramics and its mechanism of action were discussed.To address the problem of poor mechanical properties ofβ-SiAlON/h-BN composite ceramics,infiltration-resintering treatment and high temperature treatment were used to strengthen theβ-SiAlON/h-BN composite ceramics by infiltration-resintering and high temperature treatment,respectively.Finally,a protective film was successfully formed on the surface of the silicon particles by coating and heat treatment,which avoided the hydration of silicon and reduced the viscosity of the slurry.The main results are as follows:（1）Theβ-SiAlON/h-BN composite ceramics were successfully prepared by gel casting combined with reactive bonding using Isobam as the gelling agent.Due to the low elastic modulus of h-BN,the thermal stress generated during thermal shock is reduced,and with the introduction of weak phase h-BN,the crack is more likely to undergo toughening behavior during propagation,which makes the thermal shock resistance ofβ-SiAlON/h-BN significantly improved.Meanwhile,the introduction of h-BN hinders the crystallization of Si O₂ generated during the high temperature oxidation ofβ-SiAlON/h-BN and promotes the formation of mullite and borosilicate glass phases,which enhances the oxidation resistance ofβ-SiAlON.The empirical double exponential model proposed can describe the oxidation behavior of the preparedβ-SiAlON/h-BN composite ceramics.（2）The flexural strength ofβ-SiAlON/h-BN composite ceramics with h-BN addition of 20 wt.%was significantly enhanced after infiltration-resintering strengthening treatment.After three times of alumina-silica sol infiltration and sintering at 1500°C,the flexural strength was increased by 61.8%.After the high temperature strengthening treatment,the porosity of the samples was decreased and the dense density was increased significantly,and the flexural strength was enhanced by 99.8%.The high temperature strengthenedβ-SiAlON/h-BN composite ceramic still has excellent thermal shock resistance.After nine thermal shock cycles with a temperature difference of 900°C,the residual strength retention ratio was almost the same as that of the untreated samples.（3）The coating treatment and heat treatment changed the surface properties of silicon particles and reduced the viscosity of theβ-SiAlON/h-BN ceramic slurry.The pretreatment formed a protective film on the surface of the silicon particles,which effectively avoided the direct contact between silicon and water and reduced the hydration reaction of silicon to a greater extent.Compared with theβ-SiAlON/h-BN samples prepared with untreated silicon powder,the samples prepared with silicon powder pretreatment possess lower porosity,higher bulk density,and higher flexural strength.Compared withβ-SiAlON/h-BN samples prepared by adjusting the slurry p H,the sample prepared by pretreating silicon powder has higher flexural strength because the degree of hydration reaction of pretreated silicon powder is smaller.