Study On The Preparation And Properties Of Cubic Boron Nitride Compesites

Cubic boron nitride (cBN) is of outstanding mechanical and physical properties, including high hardness and thermal conductivity. Moreover, the thermal stability, chemical stability and resistance to oxidation of the cBN are better than diamond. These unique properties make cBN as an ideal cutting-tool candidate in processing of the hard-to-machine materials, such as alloy steel, quenched steel and cast iron. The application range and fields of cBN are restricted due to their bad processing property.In this study, TiO2 nanocrystals were coated on the surfaces of cBN microparticles via a heterogeneous precipitation. The different conditions of micro/nano cBN-TiO2 composite powders and micro/nano cBN-TiN composite powders were studied. The micro/nano cBN-TiN enhancing copper matrix composites were sintered by SPS. The effects of different contents of micro/nano cBN-TiN composite powders, the sintering temperature, pressure and holding time were studied. The TG-DTA technique was used to detect the thermo-dynamic behavior of the composites during the sintering processing. XRD, SEM and EDS techniques were used to characterize the samples. The relative density, microhardness and strength properties of composites were analyzed. The tribological properties were measured, too.The experimental results showed that it was relatively easily controlled to prepare micro-/nano cBN-TiO2 with butyl acetate titanate for titanium source by a sol-gel process. The optimal preparing conditions were as follows:the cBN microparticles were pretreated in a 14 mol/L HNO3 aqueous solution for 3 h; n [C2H5OH]:n [Ti(OC4H9)4]=10:1; pH=7, reaction tempreture was 20℃and n [H2O]:n [Ti(OC4H9)4]=10:1. Through TG-DTA analysis, the cBN-TiO2 precursor particles were calcined at 450℃for 2 h with a heating rate of 5℃/min to prepare TiO2/cBN composite particles. Nitridizing of TiO2-cBN composites in an NH3 flow to form TiN-cBN composites is under 950℃for 5 h.The relative densities of the composites are decreased with the content of cBN-TiN composite powder, and the micro hardness, flexural strength and resistivity are increased. The effects of sintering temperature on composites are very important. If the sintering temperature is too low, and the composites cannot get enough sintering. If the sintering temperature is too high, and the composites are excessive sintered. The performance of the composites is declined. The relative density increases with the increase in pressure. SPS has the superiority of short sintering time, so a long holding time is unfavorable for the physical properties of the samples. The optimal sintering conditions by SPS was as follows:the content of cBN-TiN composite powders is 15 vol.%, the sintering temperature was 700℃, the sintering presser was 30 MPa, and the holding time was for 5 min. The relative density, microhardness, flexural strength, resistivity and mass abrasion loss were respectively 96%,1273.6 MPa,423.3 MPa, 2.28×10-8Ω·m and 0.54 mg/m.