E. Salgueiredo

Interfaces in nano-/microcrystalline multigrade CVD diamond coatings.

The interfaces of multilayered CVD diamond films grown by the hot-filament technique were characterized with high detail using HRTEM, STEM-EDX, and EELS. The results show that at the transition from micro- (MCD) to nanocrystalline diamond (NCD), a thin precursor graphitic film is formed, irrespectively of the NCD gas chemistry used (with or without argon). On the contrary, the transition of the NCD to MCD grade is free of carbon structures other than diamond, the result of a higher substrate temperature and more abundant atomic H in the gas chemistry. At those transitions WC nanoparticles could be found due to contamination from the filament, being also present at the first interface of the MCD layer with the silicon nitride substrate.

From Micro to Nanometric Grain Size CVD Diamond Tools

CVD diamond coated tools are developed for applications as different as turning of cemented carbides and bone drilling. The diamond films are deposited by Hot Filament Chemical Vapor Deposition (HFCVD), with grain sizes varying from conventional micrometric (12 urn) to nanometric (< 100 nm) and film thickness up to 50 jam. Silicon nitride (Si3N4) ceramics are chosen for the base material in order to guarantee maximal adhesion. Both the micrometric and nanometric CVD diamond grades endure the cemented carbide turning showing slight cratering, having flank wear as the main wear mode. However, nanocrystalline diamond present the best behavior regarding cutting forces (<150 N) and tool wear (KM=30 urn, KT=2 um and VB=110 urn) and workpiece surface finishing (Ra=0.2 um). In the case of the dental drilling experiments, a polymeric laminated test block is used to simulate the human mandible and maxilla. The temperature rise during drilling is monitored to prevent overheating above 42- 47 °C that is known to cause tissue death and implant failure. It is possible to drill with a CVD diamond SisN4 coated tool with significantly lower forces (fourfold smaller), lower rise in temperature (4°C less), lower spindle speeds (100 rpm) and higher infeed rates (30 mm/min), when compared to the commercial steel (AISI420) drill bits.