Assessment of the Adhesion Performance of Diamond-Like Carbon Coatings at Elevated Temperature

Coatings of diamond-like carbon (DLC) are amorphous thin films with diamond-like properties and therefore excellently suited for the reduction of friction and wear in tribological systems. A reduced wear rate corresponds to a longer mean lifetime of the system. However, an adhesive failure of the DLC coating can lead to immediate failure of the application. Particularly, intrinsic compressive stresses within the DLC coatings are a driving force for delamination of these thin films. In order to assess the risk for an adhesive failure, it is common sense that the adhesion of the coating is the essential property of the coating system. However, it will be shown, that for DLC coatings, the delamination initiation and its propagation rely on different characteristic stress conditions. This is especially relevant for coating systems with extended adhesion layer systems. For a reliable assessment of the risk for adhesive failure, this interplay between initiation and propagation of delamination must be considered. Therefore the term adhesion performance is introduced as a systemic measure dependent on the application conditions and comprising delamination initiation and progression. For establishing the model for the adhesion performance, the Evans-&-Hutchinson-model based on buckling of an Euler column is extended by considering the initiation of buckling, as well as the re-initiation of buckling after break-away of a coating segment, where a characteristic residual crack remains and crack growth can be driven by corrosive means. Finally, thermal aging is investigated, motivated by elevated application temperatures, which can activate diffusion processes, altering the adhesion performance. A routine for assessing the adhesion performance is worked out based on the three aspects delamination initiation, buckling and re-initiation of delamination, complemented by the influence of thermal aging on the adhesion performance. This approach is performed on nine different adhesion layer designs as support for hydrogenated and H-free DLCs. The adhesion layer designs comprise metallic Cr- and Ti-adhesion layers with different kinds of carbide interlayers, such as CrxCy, TiC and SiC, partly with alterations in the deposition process. The coating systems are extensively analyzed via transmission electron microscopy, in order to correlate structure and phase compositions with the adhesion performance. A design concept for adequate adhesion performance with respect to automotive applications with steel substrates and elevated temperatures was furthermore derived based on the findings