Study on Dynamic Compaction of Diamond Powders

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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.