Paper PS2+BI-ThA6
Improvement of the Adhesion of PECVD-deposited DLC Films on Metals

Thursday, October 18, 2007, 3:40 pm, Room 607

The coating of a material with a wear and corrosion resistant coating is required in many realms. Concerning biomedical applications for example a coating of implants made of shape memory alloys such as NiTi is needed to protect the implant against wear and corrosion and the surrounding tissue against the release of metal ions. One candidate for such a coating is a thin film of diamond like carbon (DLC, hard configuration of a-C:H), since it is very hard, wear resistant, has a low friction coefficient, is chemically inert and biocompatible. But up to the present the coating of most metals with a DLC film has shown many difficulties. The bad adhesion between substrate and DLC is due to the poor chemical binding between most metals and carbon and/or due to high internal stresses occurring in DLC films. One promising technique to overcome the problem of poor adhesion of the coating to the substrate is the deposition of a very thin interface layer on the substrate before the coating is deposited on top. By a correct material choice such an interface can replace weak coating-to-substrate bonds with strong coating-to-interface layer bonds and interface layer-to-substrate bonds. Another effect of the interface layer can be the reduction of internal stresses in the interface region. By the use of at least two layers on top of each other the probability of pinholes or defects going from the surface to the substrate is decreased. This is especially important if toxic substrates are to be coated. Therefore, we use a thin amorphous hydrogenated silicon (a-Si:H) film as an intermediate layer between metal and DLC film to enable the coating of metals with DLC films. Due to the formation of strong silicide bonds the adhesion of DLC films on metal is increased significantly. The thin films under investigation are deposited in a capacitively coupled discharge with acetylene and silane as precursor gases for the DLC and a-Si:H films, respectively. The a-Si:H films have a thickness of only several 10 nm, whereas the DLC films can have a thickness of 100 nm or more. It will be shown that an intermediate layer of a-Si:H significantly increases the adhesion of DLC on metals and how the film properties of a-Si:H and DLC influence the strength of adhesion. The influence of different parameters like applied power, substrate temperature, hydrogen content in the films, and others on the adhesion of the films also under durability tests will be presented.