Synthesis of Ti-Nb-C-N based composite coating on AISI-4340 steel by modified cathodic cage plasma deposition

In this study, we introduced a modified cathodic cage plasma deposition (CCPD) system to synthesize a Ti-Nb-C-N-based composite coating on AISI-4340 steel. This composite coating is widely applied in orthopedic joint implants due to its effective bioactivity and wear resistance. In the existing CCPD system, a metallic cathodic cage of a specific material is used to deposit the coating of such material. Thus, it cannot be used to deposit the coating of a composite material. Here, we inserted composite material rings (80% TiO2 + 10% Nb2O5 + 10% graphite) on the lid of the cathodic cage, where the cathodic cage holds these rings and works as a counter electrode for plasma generation. The composite coating is synthesized in nitrogen plasma at various temperatures (300-400 degrees C) and various hydrogen contents (20, 50, and 80%). It is observed that the hardness of AISI-4340 steel (280 HV0.25) can be increased up to 1430 HV0.25 for coating deposited at 400 degrees C and 50% hydrogen, which is 5 times higher than the untreated sample. The XRD pattern shows that composite coating consists of TiN, Ti2N, NbN, NbC, and TiO2. The contents of oxide phase TiO2 are reduced by adding hydrogen gas to the processing environment; thus, the nitride-based composite coating can be synthesized. The wear volume is significantly reduced by composite coating, specifically while using higher hydrogen contents and higher temperatures. This study shows that this modified CCPD system can improve the hardness and wear resistance by synthesizing this composite coating. In contrast with conventional techniques, separate metallic targets for each element in composite coating (titanium, niobium, and carbon targets) are not required, simple equipment, rough vacuum level are required, and high-processing efficiency are advantages of this system.