Turning of CFRC composites using Si3N4 and thin CVD diamond coated Si3N4 tools

Carbon fibre reinforced carbon composites were turned using uncoated and CVD diamond coated Si3N4 cutting tools. Si3N4 inserts were fabricated by pressureless sintering and some of them were coated with a 2.5mum thick diamond film produced by hot filament chemical vapour deposition. The cutting forces and the tool wear behaviour were investigated as a function of the cutting speed (v = from 2.5 to 10 m(.)s(-1)). The cutting forces were recorded and the final tool damage was evaluated by optical and scanning electron microscopy observations. The turning tests performed with the uncoated tips show that the cutting forces notably increase with the cutting speed and the cutting length. Severe wear of the tool flank face takes place due to rubbing by the abrasive carbon powder generated during the turning operation, the tool wear increasing with the cutting speed. Conversely, CVD diamond coated Si3N4 tips exhibit low and constant cutting forces at the different cutting parameters, due to the formation of a carbon lubricant layer. The existence of this layer and the outstanding diamond hardness anticipates the use of CVD diamond coated materials for the high speed machining of CFRC composites.