Characterizations of nano-crystalline diamond coating cutting tools J. Hu a , Y.K. Chou a, ⁎ , R.G. Thompson b , J. Burgess c , S. Street c a Mechanical Engineering Department, The University of Alabama, Tuscaloosa, Alabama, United States b Vista Engineering, Inc., Birmingham, Alabama, United States c Chemistry Department, The University of Alabama, Tuscaloosa, Alabama, United States Available online 2 August 2007 Abstract Diamond coatings made by processes such as chemical vapor deposition (CVD) have been increasingly explored for cutting tools applications, in particular, for machining lightweight high-strength materials. However, most literature to date reported that the wear resistance of CVD diamond tools is still distant to the polycrystalline diamond (PCD) counterparts. Recently, a microwave plasma-assisted CVD technology was developed to increase the diamond growth rate, and by adding nitrogen gas, this process can produce nano-structured coatings which consist of nano-diamond crystals embedded into a hard amorphous diamond-like carbon matrix and have high hardness and low surface roughness. In this study, the nano-crystalline diamond (NCD) coating tools were characterized, compared to the conventional microcrystalline diamond coating (MCD) and PCD tools, in surface topography, grain sizes, carbon bonds, and mechanical properties. Moreover, the diamond tools were evaluated in machining Al-matrix composite and high-strength Al alloy. The results show that the NCD tools have smoother surfaces than the MCD tools, but rougher than the polished PCD tools. As to the diamond crystals, the NCD tools have ultrafine grains and the PCD tools have the largest grains. For the cutting edges, the PCD tools have the sharpest edge resulted from polishing and the NCD tools have larger edge rounding than the MCD counterparts. In addition to the nature-diamond peak identified in the Raman spectra, the NCD and MCD tools have additional Raman shifts associated with graphite and other disordered carbon bonds. The NCD tools have a much higher hardness, but a lower elasticity, than both the MCD and PCD tools. In machining testing, the NCD tools substantially outperform the MCD tools and are comparable to the PCD tools. For both the NCD and MCD tools, coating delamination is the major tool wear mechanism that leads to catastrophic tool failures. © 2007 Elsevier B.V. All rights reserved. Keywords: Diamond coating; Nano-crystalline diamond; Machining; Material characterization; Tool wear 1. Introduction Diamond coatings produced by different CVD processes can have a range of properties due to the diamond purity, micro/ nano-structures, and the interface with the substrate, etc. Diamond purity and bond characteristics are generally evalu- ated using Raman spectroscopy, which relates molecular vibration frequencies to the inter-atomic bonds [1,2]. Diamond crystalline sizes and qualities have been evaluated by X-ray diffractometry (XRD) [3]. Nano-indentation has been particu- larly used for elasticity and hardness measurements [4]. Despite of their exceptional tribological properties for wear resistance, coating delamination is the major failure mode of CVD diamond coatings [2]. The adhesion problem at the interface combined with interfacial stresses lead to premature failures during machining by debonding. Various techniques have been proposed to enhance the adhesion strength [5–8]. Diamond coating adhesion has been qualitatively evaluated using a Rockwell hardness tester [5,9–11] or sliding/scratch test [12]. Diamond coatings, again owing to their extreme properties, have been investigated for cutting tool applications. Studies of CVD diamond coating tools, mostly tungsten carbide (WC) substrates, have been frequently reported. There are, however, mixed results of the CVD diamond tool performance. In a few applications, CVD diamond shows tool life comparable to PCD tools in machining high-Si Al alloys [6,13,14]. However, Shen cited large variations of coating performance [14]. On the other hand, several studies reported that wear resistance of CVD diamond tools is still distant to PCD counterparts [15–21], though some argued that CVD diamond tools have potential economical benefit because of multiple cutting tips per insert Available online at www.sciencedirect.com Surface & Coatings Technology 202 (2007) 1113 – 1117 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. The University of Alabama, Mechanical Engineering Department, 290 Hardaway Hall, 7th Ave., Tuscaloosa, Al 45487-0076, United States. Tel.: +1 205 348 0044; fax: +1 205 348 6419. E-mail address: kchou@eng.ua.edu (Y.K. Chou). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.07.050