Characterizations of nano-crystalline diamond coating cutting tools

Abstract

Diamond coatings made by processes such as chemical vapor deposition (CVD) have been increasingly explored for cutting tools applications, in particular, for machining lightweight high-strength materials. However, most literature to date reported that the wear resistance of CVD diamond tools is still distant to the polycrystalline diamond (PCD) counterparts. Recently, a microwave plasma-assisted CVD technology was developed to increase the diamond growth rate, and by adding nitrogen gas, this process can produce nano-structured coatings which consist of nano-diamond crystals embedded into a hard amorphous diamond-like carbon matrix and have high hardness and low surface roughness. In this study, the nano-crystalline diamond (NCD) coating tools were characterized, compared to the conventional microcrystalline diamond coating (MCD) and PCD tools, in surface topography, grain sizes, carbon bonds, and mechanical properties. Moreover, the diamond tools were evaluated in machining Al-matrix composite and high-strength Al alloy. The results show that the NCD tools have smoother surfaces than the MCD tools, but rougher than the polished PCD tools. As to the diamond crystals, the NCD tools have ultrafine grains and the PCD tools have the largest grains. For the cutting edges, the PCD tools have the sharpest edge resulted from polishing and the NCD tools have larger edge rounding than the MCD counterparts. In addition to the nature-diamond peak identified in the Raman spectra, the NCD and MCD tools have additional Raman shifts associated with graphite and other disordered carbon bonds. The NCD tools have a much higher hardness, but a lower elasticity, than both the MCD and PCD tools. In machining testing, the NCD tools substantially outperform the MCD tools and are comparable to the PCD tools. For both the NCD and MCD tools, coating delamination is the major tool wear mechanism that leads to catastrophic tool failures.