Study on Dynamic Compaction of Diamond Powders

金刚石粉末的冲击压缩可以引起金刚石颗粒之间的冶金结合。目前的研究人员发现，用2-4毫米颗粒的金刚石粉可以获得密度最高的强压实，但小于1毫米颗粒的金刚石粉不能获得良好的压实。在获得大型金刚石压坯时最困难的问题之一是裂纹的产生。这种产生是由冲击压缩状态下的压力迅速释放和金属胶囊内压力和温度的急剧分布引起的。裂纹的产生可以通过压块中应力的松弛过程来避免。我们开发了一种利用放热化学反应控制残余温度的新方法，作为该工艺的一项有用技术。在添加放热活性添加剂时，观察到两种显著的效果。一是添加剂的加入提高了压坯的显微硬度。另一个效果是修复后的密实中宏观和微观裂纹的减少。后一种效应在添加硅的2 ~ 4 mum级金刚石粉中表现得最为明显。在冲击压实过程中，粉末压实体内部的压力和温度梯度较大，导致其后表面裂纹数量大于前表面裂纹数量。金刚石粉末在金属胶囊内伴随放热化学反应的致密化过程尚不清楚。数值模拟是了解粉末冲击压缩过程的有效工具。通过与美国北卡罗莱纳州立大学堀江贵文教授的合作研究，提高了仿真的可靠性。

It is understood that shock compression of diamond powders can originate metallurgical bonding among diamond particles. The present investigators have found that strong compacts with the highest density were obtained with diamond powders of 2-4 mum grains, but good compacts were not gained with powders less than 1 mm grains. One of the most difficult problems in obtaining a large diamond compact is the generation of cracks. This generation is caused by a very rapid pressure release from the shock compression state and by steep distribution of pressure and temperature in the metal capsule. The crack generation can be avoided by relaxation process of stress produced in the compact. As a useful technique for the process, we have developed a new method of residual temperature control by means of exothermic chemical reaction. Two kinds of notable effects were observed, concerning the supplement of the exothermically reactive additives. The one was the increase in the microhardness of the compacts with the addition of the additives. The other effect was the decrease of macro- and micro-cracks in the recovered compacts. The latter effect was seen predominant in the compacted 2-4 mum grade diamond powders with silicon additives. The larger number of cracks found in the rear surfaces of the compacts with or without additives than in the front surfaces was apparently caused by the steep gradient of pressure and temperature within the powder compacts during the shock compaction process. Densification process of the diamond powder accompanying exothermic chemical reaction in the metal capsule is not clear. Numerical simulation is a useful tool to understand the process during shock compression of powders. Reliability of the simulation has been increased with cooperative research with Prof.Y.Horie of North Carolina State University.