Correlation of the Debye sheath thickness and (Cr,Al)N coating properties for HPPMS, dcMS, CAE and PCAE processes

Physical vapor deposited (PVD) coatings are widely-used in tool and component applications, where the predominant aims are the enhancement of the lifetime and an improvement of the economic efficiency. The achievement of these aims amongst others strongly depends on a homogeneous distribution of the coating properties with respect to the entire functional surface, in particular on complex-shaped tools. In order to obtain a sufficient coating homogeneity, pulsed high power plasma processes such as the high power pulsed magnetron sputtering (HPPMS) or the pulsed cathodic arc evaporation (PCAE) has to be chosen. In order to characterize the influence of both technologies on the coating homogeneity, the Debye sheath thickness sD at the substrate side was determined, which is expected to strongly correlate with the coating homogeneity on complex surfaces, since it is a measure for the shielding of charged coating atoms. Measurements with varying pulse parameters were performed. In addition, as reference a direct current magnetron sputtering (dcMS) and cathodic arc evaporation (CAE) process were conducted, respectively. As it is widely used as protective coating for tools in many applications, the coating system (Cr,Al)N was chosen. In the first step, the Debye sheath thickness was determined by means of Langmuir probe. A Debye sheath thickness from sD = 430 μm to sD = 48 μm was found for the different processes and parameters. In the second step, the (Cr,Al)N coatings were deposited with selected process parameters on structured tungsten carbide substrates. The coating morphology, coating thickness, surface roughness and chemical composition were observed to be more homogeneous for processes with a lower Debye sheath thickness. The best results were obtained for the HPPMS and PCAE technology. In summary, processes and parameters were identified which can be used for the coating of complex structured tools.