Delamination wear of nano-diamond coated cutting tools in composite machining

Nanostructured diamond (nano-diamond) coated cutting tools have a potential to supplant costly polycrystalline diamond tools. However, coating delaminations remain the primary wear mode that often results in catastrophic tool failures. Studying tool wear will help to understand machining parameter effects on coating delaminations. Moreover, monitoring coating delamination events can prevent production loss and assist process planning. In this study, nano-diamond coated cutting tools were investigated in machining aluminium matrix composites. Outside-diameter turning with a wide range of cutting conditions was conducted. Tool flank wear-land was periodically measured by optical microscopy and worn tools were examined using scanning electron microscopy. A dynamometer and an acoustic emission (AE) sensor were also used to monitor tool conditions during machining operations. The results are summarized as follows. (1) Tool wear evolutions include a low wear rate followed by an abrupt increase of flank wear due to coating delaminations. Such behaviour is consistent in all machining conditions tested. (2) Once coating is delaminated, cutting forces increase sharply along cutting with a high level of dynamic forces, especially for the radial and axial components. (3) AE signals including raw data, root-mean-square (RMS) values, and frequency responses all show distinct features before and after coating delaminations. Significant conclusions drawn from this study include the following. (1) The feed has a more dominant effect on cutting durations prior to coating delaminations because of the increased mechanical load. (2) Nano-diamond coating tools have greater coating delamination wear resistance than conventional microcrystalline diamond-coated tools. (3) AE signals may be used to identify if coating delaminations occur, which are recognized by significant reductions of AE-RMS values.