Influence of Cr Interlayer Thickness on Residual Stress and Adhesion of Cr-DLC Multilayer Structure Films

To improve the bonding strength of the chromium-doped diamond-like carbon (Cr-DLC) films on the surface of the copper alloy, multilayer structure films of Cr/CrN/Cr-DLC with different thickness of Cr interlayer were designed and prepared on the copper alloy samples by magnetron sputtering and plasma enhanced chemical vapor deposition. The microstructure, residual stress, nanohardness, elastic modulus, bonding strength and impact toughness of the film were tested by high-resolution Raman spectrometer, film stress meter, nanoindenter, scratch tester and repetitive impact tester. The results show that the residual stress of Cr-DLC film on the surface of copper alloy decreases from -1.92 GPa to -0.47 GPa with increasing the thickness of Cr interlayer, reduced by 75.5%. When the thickness of Cr interlayer reaches 1.01 mu m, the bonding strength between the Cr-DLC film and the substrate is the best, which is 69% and 67% higher than the first (L-c1) and the second (L-c2) critical loads of Cr-DLC film without Cr adhesive layer, respectively. After 20 000 times of repetitive impact tests, there is no elastic recovery in the impact depth for all Cr-DLC coated samples, and exfoliation of a certain area for the film is observed at the center of the impact pit. Among all samples, Cr-DLC coated sample with Cr interlayer of 1.01 mu m in thickness has the smallest peeling area, and shows the best performance of repetitive impact resistance. Therefore, Cr interlayer with a certain thickness can significantly reduce the residual stress of the Cr-DLC film while improve the bonding strength and repetitive impact resistance.