| As the aluminum and oxygen solid solution of silicon nitride (Si3N4), SiAlON ceramics retains the high hardness, high temperature strength and excellent thermal shock resistance as well as that of Si3N4ceramics materials. Furthermore, it possesses better toughness and chemical stability than that of Si3N4ceramics materials. Cubic boron nitride (cBN) is characterized by the highest hardness except for diamond and has greater thermal conductivity and thermal stability than that of diamond. No oxidation phenomenon could be observed until cBN is heated to900℃in air. As a new generation of super hard materials, cBN attracts great attentions of scholars at home and abroad in the research of cutting tool materials. Spark plasma sintering (SPS) is a new sintering technology characteristic of making the materials densification at a rapid heating rate in a shot time. SiAlON/cBN composite material prepared by SPS method is expected to combine the performance advantages of both SiAlON and cBN mentioned above, which could be used as a new generation of cutting tool material with an excellent performance. Therefore, in the present study, SiAlON/cBN ceramics composite materials were prepared by using SPS method, the phase composition, microstructures and mechanical properties of the SiAlON/cBN composite were investigated in detail considering the different sintering temperatures, cBN contents, sintering aids contents and atmosphere conditions.The densification of SiAlON could be obtained in a higher sintering temperature holding for a certain time, while cBN would transform into hBN phase at a higher temperature. Thus when the temperature is higher than1550℃, the deteriorated combination between SiAlON and cBN could embarrass the increase of the properties of the composite materials. In order to modify the densification of SiAlON/cBN composites, surface coating treatment was conducted to form a SiO2film on the surface of the cBN powders by using Sol-Gel method in this study. Using TEOS and ammonia as raw materials to obtain the coated cBN powders, the effects of the total solvent volume, ammonia content, the water bath time, water bath temperature, cBN size and coating time on the cBN surface coating were investigated in detail. Finally, the SiAlON/cBN composites using uncoated cBN and coated cBN powders with the optimum compositions under the optimum sintering process were prepared by SPS method. The properties of both specimens were compared with each other to investigate the effect of the surface coating on the properties of the SiAlON/cBN composites.The experimental results were shown as follows.(1) The optimum composition of the SiAlON/cBN composite was SiAlON/20wt%cBN with0.2wt%Y2O3, and the optimum SPS process was at1500℃under40MPa pressure with a heating rate of100℃/min.(2) XRD analyses showed that SiAlON and cBN were identified as the main phases and a small amount of intermediate phase was also identified.(3) Observed by SEM the cBN particles in black contrast distributed on the SiAlON matrix. Some phase transformation of cBN to hBN caused by the increasing temperature led to interfacial deterioration between the cBN and SiAlON matrix. The interfacial bonding between cBN and SiAlON matrix were significantly enhanced by using the coated cBN powders as raw materials.(4) The optimum experimental parameters and conditions of cBN surface modification by sol-gel method were shown as follows. Solution A (8ml TEOS+32ml ethanol) and solution B (6ml ammonia+4ml H2O+30ml ethanol) were dropped into dispersion C (70ml ethanol+1g cBN) at a rate of4ml/3min, holding at40℃water bath for6h. Observed by TEM the thickness of SiO2layer around cBN powders was20-30nm coating for single time. The thickness of the coating film of SiO2increased with increasing the coating times.(5) Using the cBN powders coated by three times, SiAlON/20wt%cBN composite prepared at1500℃showed the optimum properties. The bulk density, hardness and fracture toughness were3.01g/cm3,17.1GPa and4.8MPa-m1/2, respectively, which were better than that of SiAlON/20wt%cBN composite with uncoated cBN powders. |
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