Multilayer structure dependent performance behaviour of CVD diamond thin film drilling tools during CFRP machining

Abstract

Abstract The potential of chemical vapor deposition (CVD) diamond coated tools in its application for machining lightweight materials, such as carbon fibre reinforced plastics (CFRPs), is currently limited to coating adhesion failure. Resulting spontaneous delamination and subsequent critical tool wear adversely affect the reproducibility of tool lifetime, process stability as well as machining quality. Although different morphologies and its variation of layer thicknesses on geometrically complex tools can be deposited, the interaction of coating and substrate determines the overall cutting tool strength. In addition, influencing factors on coating adhesion generated by the manufacturing process, such as tool substrate production and coating parameters, cause a high variance with regard to the cutting tool performance even within one tool batch. The scope of the presented investigation includes the application of varying CVD coated drilling tool specifications in order to analyse its performance regarding wear and process behaviour during machining of CFRP. Tool substrates ranging from tungsten carbide (WC) with varying cobalt content to silicon nitride (Si3N4) and silicon-aluminiumoxide nitride (SiALON) individually offer substrate material related tool failure conditions. Although Si3N4 substrates show an increase in averaged machined boreholes compared to WC-based substrate materials by 52 %, occasional tool shank breakage indicates a lack of mechanical strength within this tool specification. In contrast, WC-based substrate materials show typical wear on the rake face until reaching the wear criterion. Thus, process reliability is limited either by spontaneous coating delamination regarding WC-based cutting tools or cutting tool shank failure concerning silcion-based substrates respectively. The resulting workpiece integrity, i. e. surface quality, suffers from coating delamination during machining whereby fibre protrusion as well as workpiece delamination is occasionally observable.