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

Drilling performances of diamond-like carbon coatings incorporating W (W-DLC) deposited on high-speed steel tools were evaluated when drilling Ti-6Al-4V at 25 degrees C and under thermally assisted machining (TAM) conditions at 400 degrees C. Dry drilling using W-DLC coated drills caused immediate tool failure as a result of titanium adhesion. The tool lives improved for TAM drilling using W-DLC when the Ti-6Al-4V was placed (with dry surface) in a cooling bath at -80 degrees C and resulted in low and uniform drilling torques as well as good surface finish. The low coefficient of friction (COF) of W-DLC against Ti-6Al-4V 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 degrees C during drilling operations and also to prevent adhesion with minimal built-up edge (BUE) formation during drilling. The TAM performance of W-DLC coated drills was shown to be similar to that of WC-Co drills used in conventional flooded drilling.