Various HFCVD diamond coatings synergistically tuned using CH4 gas flow and working pressure and key merit evaluation of their coated tools

Abstract

Diamond-coated tools can meet the service requirements of high precision, high efficiency and long life with multi-element coupling, but the milling characteristics of graphite materials with high spindle speed, feed speed and high force may require tools with a high number of micro-edges and good adhesion strength. In the present study, microcrystalline diamond (MCD), nanocrystalline diamond (NCD) and gradient composite diamond (GCD) coatings were deposited on commercial end mills subjected to distinct CH4 gas flows and working pressure by hot filament chemical vapor deposition. The adhesion strength, erosion resistance and milling performance of the diamond coatings in milling graphite materials were evaluated. Besides, the effect of CH4 gas flows and working pressure on the growth model of diamond coatings and the mechanism of crack propagation when subjected to stress concentration were studied. The results show that the combination of high CH4 gas flows and low working pressure achieves the re-nucleation of diamond particles, the smaller grain, more graphite/amorphous carbon in the boundaries and larger residual stresses lead to continuous crack propagation, causing poor adhesion strength, erosion resistance and milling performance in NCD and GCD coatings. Conversely, the low CH4 gas flows to ensure the epitaxial growth required for MCD coating, the larger grain, higher diamond purity, columnar structure and smaller residual stresses allow the pinning effect between coating and substrate, altering and increasing the path of crack propagation, making it suitable for milling conditions with high speed and high force. This study provides significant insights into the links between diamond morphology, structure, grain size, boundaries, and phase composition and the mechanical properties of adhesion strength, erosion resistance, and milling properties by synergistically tuning or alternating the CH4 gas flows and working pressure.