ICMCTF1998 Session D4/E5: Properties and Applications of Diamond, Diamondlike and c-BN Coatings

Cubic boron nitride (c-BN) layers were deposited on silicon by a plasma activated process in a hollow cathode arc deposition device. The plasma parameters were measured by Langmuir probe diagnostics and the growth process is analyzed by in situ polarized infrared reflection spectroscopy.

The s-polarized infrared reflectance spectra of a growing BN film shows the structure of the in-plane vibration of sp² bonded BN at 1370 cm^-1 after a time delay t₁. The out of plane vibration at 780 cm^-1 can be observed too with a time delay t₂. The signature of the cubic phase can be detected later on with a time delay t₃. The time delays t₁ - t₃ depend on deposition conditions with the bias voltage as the most important parameter.

Starting at deposition conditions where c-BN layer growths the shift of the c-BN peak and the ratio of the peak heights c-BN/h-BN were investigated in situ in dependence of deposition time and bias voltage (DC and RF). Applying DC bias to the substrate the h-BN peak appears at -275 V. Applying RF the corresponding bias voltage becomes lower.

By simulation of the spectra, the damping constant of the c-BN oscillator was estimated to be 170 cm^-1, nearly independent of the thickness. The transverse optical mode frequency of the c-BN phonon starts at 1090 cm^-1, decreases to a minimum of 1070 cm^-1 at a film thickness of 300 nm and increases at higher thicknesses. Up to a film thickness of 300 nm the oscillator strength increases from 5x10⁵ cm^-2 to 22x10⁵ cm^-2 and remains constant during further growth.

Tribological properties of diamond coatings depend on a lot of parameters especially on the elaboration processes, the nature of the counterface and the environment.

This paper presents a study of a diamond coating deposited by flame process on a tungsten carbide pin and rubbing on a steel disc under different environments : air, water vapour, oxygen and argon with pressure variations.

The deposition conditions were chosen to obtain the growth of the diamond crystals following the {111} direction.

Concerning the friction coefficient, it appears that it increases with the pressure of oxygen and water vapour to a limit value, whereas the wear rate of the steel disc continues to increase with pressure.

Analyses from the disc, the pin surface and the wear debris are presented to explain the interactions between diamond and steel in the machining conditions and highlight the transfer mechanisms.

This behaviour of the diamond coating / steel couple is explained together by the evolution of the oxygenated terminal bondings on the surface of the diamond, by the growth of the oxide layer on the steel disc surface and the role, the composition of the wear debris and the manner which these are evacuated from the interface.

Due to their high hardness and low friction coefficient amorphous hard carbon "HC", diamondlike carbon"DLC" and diamond coatings have great potential for dry cutting operations. It is however not always clear which coating should be chosen for which application. The main differences between the films can be found in their hardness (respectively 5000-8000HV, 2500HV and 10000HV), their morphology (HC: amorphous, DLC : amorphous and diamond crystalline) and their thermal stability (HC : 600°C, DLC : 250°C and diamond 700°C).

. The films have been characterized with Raman spectroscopy (determination sp² - sp³ content), Vickers hardness measurement and scanning electron microscopy "SEM"measurements (thickness and morphology). The three coatings have been deposited on end-mills (both hardmetal and HSS (no diamond)) and on turning inserts (hardmetal). The coatings have been evaluated for a turning application of a metal matrix composite AlSiC (20%SiC), for a turning application of graphite and for a milling application of AlMgSi0.5. Wear analyses have been performed with SEM. These lead to recommendations on the use of the three coatings for dry cutting applications.