Synthesis and microstructure control of bride-nitride composite ceramics by solid-state displacement reaction

研究了B 4 C-Ti、BN-Ti和BN-Zr体系中的反应过程，确定了合成细晶TiB 2-TiC-C、TiB 2-TiN和ZrB 2-ZrN复合材料的最佳反应温度和保温时间。对B_4 C和Ti的混合粉末在2000℃下进行反应热压，通过改变Ti/B_4 C混合比x，得到不同的反应产物：0<x<2时的B_4 C-Ti B_2-C，x=2时的TiB_2-C，2<x<3时的TiB_2-TiC-C和x=3时的TiB_2-TiC。相对密度在x=2时有最小值97.0%。Ti与BN在1100℃反应时，先生成中间产物TiB，然后TiB与BN反应生成TiB_2。TiB_2-TiN复合材料中TiN_x的N/Ti比略低。由于这一现象，反应后总会残留少量BN。在1100℃下，Zr与BN反应144 h后，反应停止。氩气流中的合成优于真空或真空胶囊中的合成。ZrN_x的非化学计量比随反应时间的延长而减小。

Reaction processes in the B_4C-Ti, BN-Ti and BN-Zr systems were examined for determining the most suitable reaction temperature and holding time for the syntheses of TiB_2-TiC-C, TiB_2-TiN and ZrB_2-ZrN composites with fine microstructure. For the TiB_2-TiC-C composites, their densification was tried by reactive hot pressing.For reactive hot pressing of powder mixtures of B_4C and Ti at 2000℃, different reaction products were formed by varying the Ti/B_4C mixing ratio x : B_4C-TiB_2-C at 0<x<2, TiB_2-C at x=2, TiB_2-TiC-C at 2<x<3 and TiB_2-TiC at x=3. Relative density had the minimum value 97.0% at x=2. Except for this mixing ratio, every sintered body was highly dense and had a relative density more than 98.4%.For the reaction between Ti and BN at 1100℃, TiB was first formed as an intermediate product and then TiB_2 was formed by the reaction between TiB and BN.N/Ti ratios of TiN_x in TiB_2-TiN composites were a little lower than those estimated from the mixing ratios. Because of this phenomenon, a little amount of BN always remained after reaction. The synthesis of a perfect TiB_2-TiN composite will need excessive addition of Ti.For the reaction between Zr and BN at 1100℃, the reaction ceased after a reaction time of 144 h. Syntheses in an Ar stream prefer to those in vacutun or in vacuumized capsule. The nonstoichiometry of ZrN_x decreased with the reaction time.