ICMCTF1998 Session E5/D4-2: Properties & Applications of Diamond, Diamondlike and c-BN Coatings (2)

CNx coating is expected as hard overcoat of magnetic rigi d disk with long life. In this research, carbon sputtering and Nitrogen ion implantation were done simultaneously for production of carbon nitride as io n beam assisted deposition. After producing several coating of carbon includ ing nitrogen, their tribological properties were investigated. Substrate was silicon wafer disks with the diameter of 50mm and the thickness of 0.07Μm . After Ar ion bombardment for cleaning disk, Ar ion beam sputtering of carb on and implantation of nitrogen ion by N ion beam were done simultaneously. The acceleration energy and electric current of N ion beam was 500 eV and 40 mA/cm², respectively. The thickness of coating deposited was about 100nm. According to ESCA analysis, carbon nitride coating contained 90 at% car bon and 10 at% nitrogen. No oxygen was found in the bulk material of the coa ting. Pin-on-disk type friction test was conducted for carbon nitride coating against,Si₃N₄ ball. During friction, surface temperature was controlled within the range from 20°C to 270°C by the electric heater from the back side of the substrate. Normal load and sliding speed we re 10 mN and 0.007m/s, respectively.

Surface temperature is high er than 18°C, friction coefficient gradually decreased with sliding distance and reached to a certain stable value. The relationship between the reached values of friction coefficient and surface temperature was investiga ted. Friction coefficient decreased with surface temperature of substrate. At surface temperature of 270°C, 0.026 of friction coefficient was obtained. > Regarding on wear property, no wear scar was observed with > optical microscope. Secondly, the effect of atmospheric air > on friction coefficient was investigated. in only high > vacuum, low friction was observed at high tem perature.

In this paper the optimum parameters of RF plasma deposition which enabled us to obtain DLC (Diamond Like Carbon) films with maximum ratio of the sp³/sp² bondings, suitable for electron and photoelectron devices, were studied. A ailed study of the nano- and microstructure as a function of thickness was performed using AFM , STM and SEM , optical microscope, respectively. The deposited films were of thickness( 10 ≤ d ≤ 1000 nm). The results obtained from this study have s n that a mixture of microcrystalline (c) and amorphous (a) DLC and graphite (G) was deposited on the substrate. The ratio of c-(DLC/G) and of a-(DLC/G) is a function of the parameters of deposition (RF plasma power, CH4 pressure and type of substrate). man spectroscopy and microhardness investigation have given additional results which enabled us to obtain the conditions of deposition for which a maximum value of the ratio of diamond (sp³/) to graphite (sp²/) bondings.

Special attention was paid to the influence of the type of substrate on the composition of the DLC films. Deposition on silicon and germanium single crystals, sapphire, Corning glass, and alumina were made. Substrates of average roughness less than 10 was observed, which are suitable for growing homogeneous amorphous films. Scratches or other defects on the substrates (e.g. sapphire) act as centers of crystallization. Images of AFM have shown the mechanism of increase of the DLC layers on the surfac f substrate between scratches.

Doping with iodine (I₂) was made by deposition the of the films from a mixture of CH₄ + 20 % I₂ . Measurements of dark and photocurrent have shown that I2 compensates the "dangling bonds" of the DLC films due to the disorder structur The decrease of dangling bonds which act as traps or recombination energetic levels, increased the life time of injected free carriers under light and the value of concentration of I₂ for which photocurrent appears at room temperature was determi .

We investigated the friction and wear properties of diamond coatings at high sliding speed (V=35 m/s).

Various types of diamond coatings were deposited on tungsten carbide pins.

These coatings are produced by combustion flame method under different deposition conditions.

The surface properties and morphology of the coatings have been characterised by surface analysis methods such as Scanning Electron Microscopy (SEM), and Raman Spectrometry (RS).

The coated pins slide against a steel XC30 rotating disc in ambient atmosphere.

The hardness of coated pin is 70 GPa and this one of disc after heat treatment is 5 GPa.

The experimental results show that at low sliding speed (V=1m/s) the diamond coatings exhibit very high friction coefficient, μ is about 0.8 to 1.

At high sliding speed (V=35 m/s) the friction coefficient of these films decreases to μ = 0.09 - 0.15, the wear of the diamond films was also decreased.

This tribological behaviour of diamond coatings can be explained by the transformation of sp3 to sp2 carbon sites on the rubbing surface during friction at high sliding speed, inducing their reconstruction or creation on them of double (π sp2) bonds.

A model of the friction is presented to explain this phenomenon.

Sputtered carbon films deposited by closed field unbalanced magnetron sputter ion plating up to 1 micrometer thick have shown improved tribological properties[1]. When the film thickness was increased the films’ performance in mechanical wear tests was reduced. This was thought to be a result of increased internal stresses within the film.

In order to produce low friction, wear resistant films of thicknesses greater than 1micrometer, the inclusion of oxygen, nitrogen and co-sputtered metal in the carbon films has been studied. Deposition parameters have been optimised to improve the performance in tribological tests in air, oil and water. The films have been characterised using micro hardness testing, pin on disc and reciprocating friction and wear tests and the results are presented. The structure of the coatings has been studied by optical microscopy and SEM of normal and taper sections produced by ball cratering techniques.

References.

[1]. ‘Novel DLC Coatings Deposited by Magnetron Sputtering of Carbon Targets’ D.G. Teer, A.H.S. Jones, V. Bellido Gonzalez, D. Camino. Metallurgical Coatings and Thin Films 1997, Proceedings of the 23rd International Conference on Metallurgical Coatings and Thin Films, San Diego, CA, USA, April 1997.