There is a substantial increase on carbon-carbon composites use for engineering applications,
considering its high temperature properties and low specific mass. However the machining costs are
relatively high, and new cutting tools, mainly ceramics, must be developed to overcome such difficulty,
aiming cost reductions. In this work, silicon nitride based ceramics has been prepared , by pressureless
sintering of silicon nitride powders and appropriate amounts of Al2O3,Ce2O3, Y2O3 and AlN. Cutting tools
were prepared from the sintered materials, with geometry according to ISO1832. Selected cutting tools were
also diamond coated by a hot filament-assisted Chemical Vapor Deposition (HFCVD) diamond coating
process. Carbon Fiber Reinforced Carbon (CFRP) composites machining was performed, to evaluate the
diamond coating influence on machining performance. After the tests, the uncoated tools presented severe
flank wear and shorter life than the diamond coated ceramic tools. This flank wear is caused by the abrasive
carbon powder generated during the facing operation. On CVD diamond coated α-SiAlON ceramic tools, no
flank wear was observed, and the cutting edge remained unmodified, even for severe test conditions, such as
high cutting length and speed. Carbon particles, originated from the machined composite, do not promotes
diamond film rupture, but instead, acts as lubricant film and reduces composite surface initial roughness.