The Performance of Diamond Like Carbon Coated Drills in Thermally Assisted Drilling of Ti 6Al 4V

Abstract

Drilling performances of diamondlike carbon coatings incorporating W (WDLC) deposited on highspeed steel tools were evaluated when drilling Ti6Al4V at 25 آ°C and under thermally assisted machining (TAM) conditions at 400 آ°C. Dry drilling using WDLC coated drills caused immediate tool failure as a result of titanium adhesion. The tool lives improved for TAM drilling using WDLC when the Ti6Al4V was placed (with dry surface) in a cooling bath at −80 آ°C and resulted in low and uniform drilling torques as well as good surface finish. The low coefficient of friction (COF) of WDLC against Ti6Al4V observed under TAM conditions was attributed to the formation of W oxide layers at the tool surface, as determined by Raman spectroscopy. Introducing a cooling bath was necessary in order to restrict the temperature rise in the workpiece that caused rapid tool wear above 500 آ°C during drilling operations and also to prevent adhesion with minimal builtup edge (BUE) formation during drilling. The TAM performance of WDLC coated drills was shown to be similar to that of WCCo drills used in conventional flooded drilling.