Cutting performance of time-modulated chemical vapour deposited diamond coated tool inserts during machining graphite

In this investigation, two types of diamond coatings were deposited onto commercially available cemented tungsten carbide (WC-Co) tool inserts using two distinctive processes: i) conventional hot filament chemical vapour deposition (HFCVD) and ii) our recently developed time-modulated chemical vapour deposition process (TMCVD). The TMCVD process enabled the production of smaller sized diamond grains by the promotion of secondary nucleation processes occurring during the larger flow methane modulations. With the conventional HFCVD process, the methane concentration in the gas phase was kept constant during the entire diamond chemical vapour deposition (CVD) process and we labelled the resulting coating as “conventional diamond”, whereas the coating was referred to as “TMCVD diamond” when the methane content in the gas phase was time-modulated by changing its flow rate during CVD. Inserts coated employing both conventional HFCVD and TMCVD deposition process were characterized by scanning electron microscopy (SEM) and Raman spectroscopy and then were tested for turning performance using graphite as working material. For sake of comparison, polycrystalline diamond (PCD) and bare WC-Co inserts were also used for graphite turning. The different wear mechanisms have been compared and discussed in terms of diamond coating microstructure. Repeated turning tests showed that the TMCVD diamond coated inserts exhibit a better wear resistance with respect to inserts coated with conventional diamond coatings, PCD and bare WC-Co inserts.