High density oxygen plasma ashing of CVD-diamond coating with minimum damage to WC (Co) tool substrates

Abstract

Huge amount of CVD-diamond coated milling tools were used for machining of CFRP and CFRTP sheets and blocks in the airplane and automotive industries. Because of chipping and tooth-tip damage in the diamond coatings during those dry machining processes, the tools must be exchanged with new ones to preserve the geometric accuracy in practice. WC (Co) substrate in these used tools had to be recycled to lower the production cost; reliable ashing process was necessary to remove only the used diamond coatings without significant damages even to the tooth tip of substrate. Furthermore, fast-rate ashing became a key to shorten the leading time for exchange of milling and drilling tools. High density oxygen plasma ashing method with use of the hollow cathode device was proposed to remove the used diamond coating with the film thickness of 10 μm. Both the emissive-light optical spectroscopy and the Langmuir probe method were employed to make quantitative diagnosis on the generated oxygen plasmas. The average ion density increased up to more than 1x10¹⁷ m^-3, higher than the conventional plasma states by one order. Activated oxygen atom had overwhelming population among the generated species in this high plasmas density. This high density oxygen flux was responsible for complete and fast-rate removal of CVD diamond coating; e.g. the average ashing rate turned to be more than 10 μm/hour. The short-shank, end-milling tools were employed to describe this ashing behavior with time. Corresponding to the variation of CO-peak intensity in the measured spectra by on-line spectroscopy, the diamond film thickness reduced monotonically with time up to 3.6 ks. The removal rate gradually decreased with time in the final stage. Fine tuning of oxygen plasma processing conditions was capable to reduce the damage depth of tool teeth tips down to 1 μm, significantly less than the standard tolerance of 5 μm.