Nanocrystalline diamond-coated cutting tools: Characterizations, thermo-mechanical analysis, and machining performance

Cutting tools coated with diamond using chemical vapor deposition (CVD) processes are economically attractive alternatives to costly sintered polycrystalline diamond (PCD) tools for machining non-ferrous materials. Recently, a high-power microwave plasma-assisted CVD technology capable of producing ultrafine diamond grains in the order of 10 nm was developed, and has been employed to fabricate nanocrystalline diamond (NCD) coating tools.;The major findings are summarized as follows. (1) NCD coatings have the lowest elasticity, but the highest hardness among all three types of diamond tools. (2) A thicker coating prevents interface delamination, but reduces tensile cracking resistance. Additionally, a lower coating elasticity results in a higher critical load to coating tensile cracking, but is unfavorable to delamination resistance. (3) Deposition stresses due to thermal expansion mismatch can reach in the order of GPa in compression in diamond coatings. Moreover, deposition stresses are augmented around a cutting edge, with the radial normal component being largely tensile. Further machining loading causes the stress reversal pattern. The thermal load, which reduces deposition residual stresses, is more dominant to stress evolutions than the mechanical load. (4) NCD tools outperform the MCD tools in machining and the coating delamination is the major failure mechanism that leads to catastrophic tool wear. After coating delaminated, the tool emits relatively intense high-frequency AE signals, whereas the root-mean-squared values reduce noticeably.;The objectives of this research are: (1) to characterize NCD coating tools together with microcrystalline diamond (MCD) coating tools and PCD tools, (2) to analyze the stress states of diamond coated cutting tools after the deposition as well as during subsequent machining operations, and (3) to evaluate machining performance of NCD tools compared to MCD and PCD tools. The research approaches include: (1) scanning electron microscopy for microstructures and topography, nanoindentation for mechanical properties, Raman spectroscopy for carbon bond purity, (2) indentation-based simulations incorporating a cohesive zone model for coating and interface failure analysis, (3) thermo-mechanical modeling using finite element analysis for coated tool stress evolutions, and (4) machining of A390 alloy and Al composite, instrumented with force and acoustic emission (AE) sensors, at different conditions for tool wear evaluations.