An investigation into the feasibility of combined diamond and diamond-like carbon coatings for effective dry turning of aluminium alloys

Abstract

The efficacy of combined diamond and diamond like carbon coatings, to allow for effective and efficient dry turning of aluminium alloy Al 6082, has been investigated. Optimised diamond and diamond-like carbon (DLC) coatings were combined and deposited onto a WC-Co insert using chemical vapour deposition (CVD) methods. DLC coatings were developed by testing the effects of bias voltage, deposition time and gas pressure. During the development of the DLC layer, the effects of substrate geometry and positioning in the deposition chamber were investigated. It was discovered that coating characteristics could vary significantly across the samples as a result of geometrical effects. This contradicted claims that, as plasma enhanced CVD is a non-line of sight deposition method, any variation in the coating due to geometry would be negligible. SEM analysis revealed coating thickness to increase by over 50%. AFM measurements showed coating roughness to increase by up to 30 times, whilst Raman spectroscopy highlighted a significant decrease in sp3 bonding. This variation in characteristics was seen, through the use of scratch testing, to translate into significantly reduced tribological performance. Friction was increased by 60% and critical load was only half of that of the coating applied to flat surface. The combined coatings were characterised and machining performance was evaluated. Coating characteristics were examined using SEM, AFM and Raman spectroscopy. Cutting trials designed to simulate the expected tool life were conducted. Micro and nano-crystalline diamond coatings, with and without an additional DLC layer were trialled along with a single layer DLC coating. Commercially available uncoated and TiN coating inserts of identical geometry were also trialled as a reference. The results showed that the addition of the DLC layer effectively reduced the roughness of the diamond, however, this did not translate into reduced adhesion of the aluminium to the cutting tip. It has been shown that for this particular machining scenario, a smoother coating effectively increased friction and adhesion of the workpiece material. The investigation has highlighted that due to the complex dynamics of material transfer effects in sliding, it cannot be assumed that a smoother surface layer will lead to improved tribological performance.