Electro-spark coatings for enhanced performance of twist drills K.R.C. Soma Raju ⁎ , Nadimul Haque Faisal, D. Srinivasa Rao, S.V. Joshi, G. Sundararajan Surface Engineering Division, International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Balapur Post, Hyderabad-500 005, India Received 14 November 2006; accepted in revised form 16 July 2007 Available online 15 August 2007 Abstract Surface engineering approaches are being increasingly employed for enhancing the effective life of twist drills with a view to reduce machining costs. The electro-spark coating (ESC) technique provides a promising means of depositing wear resistant coatings that can potentially enhance the performance of these tools. However, it is often necessary to also optimize the machining conditions for coated tools to achieve an enhanced tool life. In the present investigation, varying spindle speeds were employed at a fixed vertical feed to evaluate the performance of WC–8Co ESC coated HSS drills in comparison to bare HSS drills. The number of holes drilled before reaching a preset average flank wear (0.5 mm), or catastrophic failure of the drill, was taken as the measure of tool life. The drill flank wear, monitored at regular intervals, as well as the cutting torque and thrust measured for all holes, were considered to be the key criteria for optimizing the cutting conditions. Results indicate that the WC–8Co coated drill tool life can be increased by a factor of more than 5, depending on the machining conditions selected. Furthermore, flank wear of the drill was found to increase rapidly at the end of drill life. Cutting torque data was also found to provide a useful indicator for predicting the end of tool life. © 2007 Elsevier B.V. All rights reserved. Keywords: Electro-spark coating; WC–Co; Twist drills; Cutting edge wear; Thrust force; Cutting torque; Tool life; Hole quality 1. Introduction Electro-spark coating (ESC), also known as electro-spark alloying (ESA) and pulsed electrode surfacing/deposition, is a promising surface engineering tool for imparting an enhanced wear resistance to engineering components prone to degrada- tion due to any of the wear phenomena. Its popularity can be mainly attributed to its simplicity, cost effectiveness and an ever-growing regime of applications. The ESC process is well suited to deposit a wide range of coatings on diverse component materials. Notwithstanding the advent of improved tool materials, such as cemented carbides and ceramics, high speed steel (HSS) continues to comprise roughly about 50% of all cutting tools used worldwide and this number could be even higher in developing countries. This is mainly because HSS is cheaper and tougher than other common cutting tool materials. However, the major drawback of HSS is its low hot hardness compared to other popular tool materials such as WC, TiC etc. and this limits the use of HSS tools to low cutting speeds. In addition to developments in the HSS material itself, it is widely acknowledged that only adopting a surface modification approach and coating the tools with hard wear resistant surface layers can achieve any significant future improvement in tool performance. Consequently, various coating materials such as TiN, AlN, TiCN, CrN, TiAlN and diamond and diamond like carbon coatings have been introduced for commercial applica- tions either as monolayers, multi layers or functionally graded coatings [1–10]. The potential advantages of a coated tool are generally three-fold: (a) enhanced tool life (b) improved product surface quality and (c) increased production rate [11–18]. Interest in the ESC technique for tool applications has been motivated by the fact that the proven PVD/CVD based hard coatings, which have clearly established the efficacy of employing wear resistant coatings on tools, are costly and involve infrastructure that demands huge investment. In contrast, the ESC technique is easily affordable to normally every tool-room and yet versatile enough to be applied on diverse parts ranging from kitchen knives to gas turbine components [19]. Notwithstanding Available online at www.sciencedirect.com Surface & Coatings Technology 202 (2008) 1636 – 1644 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. Tel.: +91 40 24441075; fax: +91 40 24442699. E-mail address: somaraju_k@yahoo.com (K.R.C. Soma Raju). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.07.084