Continuously deposited duplex coatings consisting of plasma nitriding and a-C:H:Si deposition

For engineering applications, diamond-like carbon films (DLC) are the most suitable coatings if high wear resistance and low friction are needed. One problem is their very poor adhesion on steel substrates when no intermediate layer is supplied. The most common method to deposit DLC is radio frequency PACVD (RF, 13.56 MHz), which is technically difficult and expensive to scale up to industrial dimensions. We tested different methods for plasma exitation with a great potential for upscaling. In this paper, the deposition of DLC coatings by medium-frequency PACVD (MF-PACVD) at pressures of several 100 Pa is presented. A mixture of methane and tetramethylsilane was used for depositing the Si-DLC films. The parameters are very similar to those of the well-known and industrialized plasma-nitriding process. The adhesion was improved by an intermediate Si-C:H layer with tetramethylsilane as a precursor. Rockwell indentation classes of HF=2 and critical loads up to 20 N could be reached. The coefficient of friction was dependent on the Si content in the films. A minimum was measured at Si contents of about 15 at.%. The mechanical properties of the films such as hardness, Young's modulus, and wear resistance were studied as a function of the process parameters. The properties thus obtained are comparable to those by RF-PACVD.