Improving the adhesion of amorphous carbon coatings on cemented carbide through plasma cleaning

Diamond-like amorphous carbon coatings 1 μm thick were deposited onto cemented carbide substrates by magnetron sputtering of a graphite target in argon under different substrate bias powers and chamber pressures. Scratch testing was used to assess the coating adhesion. X-ray photoelectron spectroscopy depth profiling was employed to quantify cobalt loss at the substrate surface as a function of bias power during plasma cleaning. It was found that under the same deposition conditions, the scratch adhesion strength increased with the bias power during plasma cleaning and reached a maximum at about 200 W or -210 V in terms of induced voltage. After that, further increases in bias power led to a decrease in adhesion. The increase was attributed to better cleaning of the sample surface and removal of surface cobalt while the decrease in adhesion was linked to an increase in residual stress which resulted in a different failure mechanism. Thus, an increase in the deposition power density, and therefore more severe ion bombardment, led to higher residual stress and lower adhesion. Under constant bias and deposition power, however, it was established that below a certain minimum chamber pressure spontaneous coating detachment occurs.