Lessons learned from mapping diamond powder stresses can lead to improved compactness

Due to its extreme hardness, high-quality polycrystalline diamond is essential for a wide range of industrial applications such as drilling and cutting. The standard process for producing high-quality polycrystalline diamond has two main steps: the compaction of diamond powder at low temperature, and the high-pressure sintering of the compacted product at high temperature. The first step — the cold compaction — removes most of the space between crystals and is key to producing high-quality polycrystalline diamond.

Guan et al. analyzed the fragmentation of diamond powder during the cold compaction process through mapping the stresses in the diamond powder using in situ high-pressure synchrotron X-ray diffraction, and determined which conditions are more likely to produce graphite-phase carbon instead of diamond.

They studied how different levels of stress can affect the fragmentation of diamond. They used three types of diamond powder grains — to simulate coarse, medium and fine powders — and compressed them at ambient temperature using pressures ranging from 0.1 gigapascals to 8.0 gigapascals. They investigated the resulting products using probes including scanning electron microscopy and laser particle size analysis, and found different levels of fragmentation in all the diamonds. The authors characterized the process into three stages: first, the fracturing at the edges and corners; then, the fracturing along the crystal; and finally, the reordering of the fragmented particles into tighter formations.

The team concludes that a better understanding of the stresses in high-pressure and low-pressure regions, as well as how the external loading pressure is related to the phase diagram of carbon, can be used to fine-tune the temperature during compaction for producing higher-quality polycrystalline diamond.

Source: “Fragmentation and stress diversification in diamond powder under high pressure,” by Shixue Guan, Fang Peng, Hao Liang, Cong Fan, Lijie Tan, Zhiwei Wang, Yuanfen Zhang, Jiawei Zhang, Hong Yu, and Duanwei He, Journal of Applied Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5051749 .