Cold compaction and crushing of diamond powders during the sintering of polycrystalline diamond

[Objectives]  Polycrystalline diamond (PCDs) compacts are extensively used in industries such as oil drilling, geological exploration, and high-speed machining tools. To further enhance the density of PCDs and thus improve their performance, this research investigates the powder density, particle size distribution, and changes in their microstructure under different pressures for diamond micro-powders with varying particle sizes and proportions. The objective is to understand the fragmentation behavior and obtain diamond formulations with higher density.

[Methods] A set amount of diamond micro-powders was placed in a fixed-size niobium cup, shaken and compacted before sealing. The cup was then positioned in a synthetic pyrophyllite block, layered with metal plugs, molybdenum, carbon, zirconia, magnesia, and sodium chloride, replicating the structure used in synthesizing PCD tools. A cold isostatic press and a cubic press were employed to apply different pressure gradients. At 220 MPa, the sealed niobium cups underwent liquid pressurization in the cold isostatic press, while the cubic press facilitated pressures  of 20, 30, and 40 MPa. After compression, the samples’ mass and volume were measured to calculate their density. Scanning electron microscopy (SEM) was used to observe the morphology of the internal powders, and a laser particle size analyzer was used to observe particle size distribution. The crushing conditions and patterns of diamond micro-powders were then summarized.

[Results]  In summary, the density of diamond micro-powders increased with pressure. After compaction, density distribution ranges from 1.5 to 2.1 g/cm3. For samples of the same volume, coarser particles exhibited higher densities than finer ones, with mixed powder samples showing intermediate densities. This indicated that in the compacted state, there was a larger void ratio between fine particles, and in mixed powders, fine particles were not effectively dispersed among large particles. After cold isostatic pressing at 0.22 GPa, there was a significant increase in sample density, with the density of mixed powders increasing by nearly half, indicating a compact particle arrangement without crushing. Under pressure, particles re-arranged, reducing voids and allowing fine particles to fill the gaps between larger ones. High-pressure treatment in cubic pressing at 20, 30, and 40 MPa resulted in internal pressures of approximately 3.5, 5.0, and 7.0 GPa respectively. Diamond micro-powders underwent fragmentation under high external pressure, and sample densities continued to change, increasing with pressure. Particle size analysis of the crushed powders revealed significant changes in particle size distribution of large particles under high pressure, indicating severe crushing. The density of the mixed powder samples also increased. However, the addition of small diamond micro-powders significantly reduced the crushing of larger particles, with less changes in particle size distribution, which helped maintain the original formulation. SEM observation showed clear crushing of larger particles and validated the stacking model of different particle sizes in the mixed powders. Small particles were evenly distributed among the larger particles, acting as a buffer to increase contact points and reduce stress concentration, thus significantly reducing the occurrence of crushing and rapidly increasing density.

[Conclusions]  During the cold pressing stage of synthesis process for PCD compacts, diamond micro-powders of all types undergo varying degrees of crushing, with single-component large particles being most notably affected, while the inclusion of small diamond micro-powders significantly reduces this effect. Mixing large and small diamond particles effectively alleviates the crushing of diamond micro-powders and allows for higher density under the same pressure, resulting in PCDs with higher density and improved performance.