Investigation and optimization of the tribo mechanical properties of CrAlCN coatings using Design of Experiments
Abstract The control of friction as well as its adaption is essential for forming operations. Thin hard coatings have a significant influence on the performance of production processes and the service life of tools, especially for Sheet Bulk Metal Forming (SBMF) processes with high contact normal stresses and issues concerned with the filling of filigree functional elements. To handle these challenges, the CrAlCN coating system is generated by means of bipolar pulsed reactive magnetron sputtering, using Design of Experiments. A Central Composite Design is selected to investigate the cathode power, bias voltage, as well as the reactive gas flow composition (nitrogen and acetylene) The aim is to evaluate the correlations and the interaction of the investigated process parameters on the mechanical as well as the tribological behavior of the CrAlCN coating, and to develop models to obtain the desired coating properties. The generated coatings show a clear dependency on the selected process parameters. An increased acetylene flow leads to a reduction of the mechanical properties (hardness and Young's modulus) as well as a decreased adhesion of the CrAlCN coating. In contrast to the influence of the acetylene flow, a higher negative bias voltage leads to improved mechanical properties in the context of wear resistant thin films. The tribological properties revealed that the coefficient of friction is related to the chemical composition of the coating which can, on the one hand, be adjusted by the acetylene flow and, on the other hand, by the cathode power. The optimized CrAlCN coating was deposited onto forming punches and the friction was evaluated using DC04 and DP600 specimens in an adapted ring compression test. In comparison to polished and heat treated ASP(r)2023 steel (62 HRC) and a CrAlN reference coating, the developed coating shows a significant reduction of friction due to the carbon incorporation.