A strategy to improve tool life by controlling cohesive failure in thick TiAlN coating during turning of CGI

Abstract

Beside machining condition and parameters, coating properties is one of the main factors affecting tool performance and wear pattern. These properties include coating hardness and elastic modulus, adhesion to substrate, roughness, defects, and most importantly residual stress. In general, the properties of a physical vapor deposition (PVD) coating can be controlled by adjusting deposition parameters such as substrate bias voltage and nitrogen pressure. Therefore, a coating can be tailored to withstand a specific wear mode which is dominant in machining the workpiece material. In this work a strategy is suggested to decrease adhesion wear and formation of built-up edge during machining of compacted graphite iron (CGI). This strategy involves deposition of thick TiAlN coating with low residual stress in order to promote cohesive failure within the coating. For this purpose, monolayer Ti40Al60N PVD coatings with high thickness (5–10 μm) were deposited under various deposition parameters. A comprehensive coating characterization was done using scanning electron microscope (SEM), X-ray diffraction (XRD), nanoindentation, and scratch test. Tool life of selected coatings were then tested under dry finishing of compacted graphite iron, and wear pattern was investigated with optical and scanning electron microscope. The results showed that coating under low compressive residual stress is successful in reducing sticking and built-up edge formation. This phenomenon happens with gradual cohesive failure of the coating under adhesion wear, resulting in a higher tool life under adhesive wear.