Development of TiAlN PVD Coating Systems for Machining of Compacted Graphite Iron (CGI)

Abstract

Compacted graphite iron, with its improved mechanical properties, is the ideal candidate to replace grey cast iron in the automotive industry. Engine blocks made from CGI are lighter, smaller, and show higher fuel efficiency. However, machining compacted graphite iron is extremely challenging compared to grey cast iron, especially in continuous cutting operations. Difficulties in dry turning CGI can be attributed to a lack of a protective tribolayer and its high tendency to stick to the cutting tool, which results in built-up edge formation and adhesion wear. This research attempts to address challenges associated with CGI turning by developing a TiAlN coating with a focus on thickness and low residual stress. The mentioned coating was developed using a new technology called super fine cathode (SFC) which enables better control over residual stress generation and, therefore, enables the deposition of a TiAlN coating with higher thickness compared to its commercial range. The properties and performance of this coating were comprehensively studied using X-Ray diffraction, scanning electron microscopy, micro and nanomechanical indentation, scratch test, optical 3D microscopy, and tool wear studies. The results show that residual stress is the main measured feature limiting coating thickness and affecting wear pattern during the machining of CGI. By using a low substrate bias voltage and a high nitrogen pressure, deposition of a thick TiAlN coating under low compressive residual stress was achieved, which significantly delayed substrate exposure, reduced built-up edge formation, lowered the cutting forces, and improved tool life by 35%. Moreover, to improve the coating quality along the cutting edge, wet blasting was applied before the deposition process, which resulted in better edge quality and consistency.