HARDNESS CHARACTERISATION OF THIN CARBON COATINGS

Abstract

Microhardness indentations nonnal to the surface of thin coatings usually result in deformation of bothcoating and substrate. Such measurelnents lead to a ‘composite’ hardness nUlnber which is a function of the properties of the coating and substrate, the ratio of the depth of indentation to the coating thickness, and the indenter geometry. In order to determine the hardness of the coating it is necessary to find some way of separating the contributions from the coating and substrate which comprise the composite hardness. In the current investigation an attempt was made to measure the hardness of thin diamondlike carbon (a-C :H) coatings deposited by plasma assisted chemical vapour deposition (PACVD) using the ‘area law of mixtures rule’ proposed by Jönsson and Hogmark, thus enabling the quantification of both contributions. Composite and substrate hardness values were obtained for indentation loads ranging from 100 to 1000 g. In all cases a linear relationship was found betweenhardness and the reciprocal of the indentation diagonal. These correlations enabled the load invariant hardness to be determined for all the carbon coatings. With the exception of one sample, the Vickers hardness of the carbon coatings was determined to reside within the range 1900–2300 kg lmm–2. The carbon coatings were also briefly studied using X-ray diffraction (XRD) and Raman spectroscopy. For all samples the XRD spectra obtained showed a complete absence of long range crystallographic structure. The Raman spectra obtained were the same for all samples and similar to the spectra obtained elsewhere for a-C: H coatings deposited by PACVD, and indicate that there is both graphitic (Sp2) and diamond (Sp3) bonding present in the coatings.