Coating Development for High-Speed Turning of Stainless Steel Addressing Combined Wear Mechanism

Abstract

Austenitic stainless steel 304 (SS304) is one of the most widely used materials for aerospace, vehicle accessories, medical equipment, and ship parts. One of the reasons is that this material provides corrosion resistance and mechanical properties because of its chemical composition of predominantly chromium (Cr) and nickel (Ni). However, austenitic stainless steels have a high tendency to work-hardening and low thermal conductivity. This results in machining problems such as high cutting forces, unstable chip formation, and rapid tool failure. In addition, the work-hardening intensifies the frictional force at the tool-chip interface, which leads to a high localized temperature and causes diffusion and oxidation at the tool-workpiece interface. The aim of the current research was to develop novel compositions of adaptive PVD coatings for turning stainless steel, thereby cutting production costs, and improving product quality. The focus of this research was on a new generation of self-adaptive PVD coating designs and the use of these features to address machining issues during the cutting of stainless-steel material. It was found that during the high-speed turning of SS304, a complex combination of oxidation/diffusion wear mechanisms resulted in crater formation whereas abrasion/attrition led to flank wear. Therefore, the coating was designed and investigated with consideration of the dominant tool wear mechanisms, mechanical properties, and the tribological characteristics that were present in the cutting zone. In this research, different PVD and chemical vapour deposition (CVD) coating systems were studied for the semi-finish turning of SS304; monolayer AlTiN PVD coatings were investigated for the high-speed and ultra-high-speed turning of SS304 under various cooling conditions. Based on the research, a novel self-adaptive bilayer AlTiN PVD coating was developed and applied for the ultra-high-speed turning of SS304. The properties and performance of these coatings were comprehensively studied using X-Ray diffraction (XRD), scanning electron microscopy (SEM), nanomechanical indentation, scratch tests, optical 3D microscopy, tool wear studies, and tribological characterization. In addition, a chip cross-section analysis and surface integrity of the machined workpiece material were performed to study the machinability of SS304. The AlTiN PVD coating with the Al/Ti ratio of 60/40 was found to possess a favourable combination of tribological and micro-mechanical properties, and a novel self-adaptive bilayer AlTiN PVD coating was designed based on these findings. Tool life results have shown that the self-adaptive, bilayer AlTiN PVD coating improves tool life by over 1.3 - 4.6 times compared to the commercially available coatings.