亚微米硬材料的高压制备及强化机理探索

It is commonly acknowledged that nanocrystalline strengthening can increase the strength and toughness of known materials. Is there any other strengthening mechanism? This project proposes a high-pressure strengthening mechanism. The sample, which with the strength and toughness of nanomaterials, can be prepared by sintering at ultra-high pressure and high temperature with submicron hard materials. The hardness of it exceeds that of single crystal. For instance, by sintering at high pressure (8-10 GPa) and high temperature (2300 K), with 0.2-0.5um cBN single crystal starting material, under a load of 29.4 N, the sample reaches Hv of 75 GPa. Also, using 0.5um diamond powder as the starting material, sintered at high pressure (14 GPa) and high temperature (2200K), the sample has a hardness of Hv=125 GPa at a load of 49 N. It was found in the TEM analysis that nanoscale parallel stripe substructures exist in the submicron grains. Based on the experimental results, this project will prepare hard materials with better performance by relying on domestic cubic press with two-stage system developed by our laboratory, and at the same time, to explore the high-pressure strengthening mechanism in bulk hard materials. This strengthening mechanism can also be extended to WC materials, Al2O3, Si3N4 and other ceramic materials.

纳米强化是目前公认的既能提高材料强度又能提高材料韧性的强化方法，除此之外还存在其它的强化机制吗？本项目提出了一种高压强化机制。它采用亚微米硬材料在超高压、高温的条件下烧结，制备出的样品具有纳米材料的强度与韧性，其显微硬度超过了其单晶的硬度。例如采用0.2-0.5μm的cBN单晶材料，在（8-10GPa，2300k）的高压高温条件下烧结，其样品的显微硬度Hv=75GPa(3kg加载），同样采用0.5μm的金刚石单晶为初始材料，在（14GPa，2200K）高温高压下烧结，样品的显微硬度Hv=125GPa(5kg加载)。在TEM分析中发现在亚微米晶粒的内部存在纳米量级的平行条纹亚结构。本项目将在该实验结果的基础上，依靠本实验室自主研发的国产六面顶二级增压技术，制备出更好的硬材料，探索块体材料的异于纳米强化的高压强化机理。这一强化机理还可推广到WC材料，Al2O3,Si3N4等陶瓷材料领域。

通过改进六面顶压机一级组装，达到了12GPa、2100K的压力、温度条件，能够制备出纯相硬材料的块体样品；通过改进二级组装的加热原件，达到了20GPa、2500K的压力温度条件，制备出高性能纯相多晶立方氮化硼样品。采用亚微米cBN在11 GPa/1750℃，制备出了纯相PcBN烧结体，其显微硬度HV=72GPa（3KG加载），断裂韧性KIC=12.4MPaM1/2，耐热性1273℃；采用0.4~1微米cBN在14GPa/1700℃、14GPa/1800℃制备出了纯相透明PcBN烧结体，透光率为49%–57 % 、67 %–71 % (400 nm–800 nm)；同时1700℃样品其显微硬度HV=69GPa（3KG加载），断裂韧性KIC=11.6MPaM1/2。其显微硬度超过了cBN单晶的最高硬度45GPa。这是除了细晶强化的另外一种强化方法-即高压强化。本项目为研究开发新材料打开了一扇门。