W-B-C coatings: microstructural nuances revealed by atomistic modelling
WBC materials show a combination of high hardness and high ductility which renders them a suitable coating e.g. for cutting tools. The material properties depend on the relative amount of W, B, and C atoms and on its inner structure (atomic arrangement) which can be influenced by deposition parameters.
A series of WBC materials of different W:B:C ratios has been produced by magnetron sputtering technique. Microstructural analysis and XPS measurements are presented. Given material is further characterized by the fractions of X-Y bonds (X, Y = W or B or C). The bonds are experimentally analyzed by deconvolution and fitting procedure applied to the XPS peaks.
The experimental bond analysis is complemented with bond analysis performed on structural models. An ab initio stochastic quenching method followed by atomic relaxation is applied to generate the structural WBC models. The structural models are generated to reproduce the chemical composition of the deposited WBC materials together with the assumption of fully amorphous structure.
We define a W-X dominant bond environment and use this definition to evaluate the fractions of W-X type environments in the structural models. A comparison of the experimentally evaluated W-X fractions with the W-X fraction obtained from the structural models shows a good agreement with the deposited materials which are fully amorphous. In cases when modelled WBC structures exhibit deviation from experiment, a subseqeunt analysis revealed partial crystallinity or grain boundary segregation, hence microstructural changes at nanoscale otherwise hard to be detected. Further insights are provided by advanced modelling of electronic structure, yielding predictive XPS spectra of these chemically complex materials.