Development of metallic materials for knee implant applications by micro-plasma arc additive manufacturing process

Abstract

This thesis reports on development of Co-Cr-Mo-xTi alloys and Ti-Ta-Nb-Mo-Zr high entropy alloy (HEA) by micro-plasma arc additive manufacturing (μ-PAAM) process and study of their invitro biocompatibility, microstructure, and mechanical properties for their possible knee implant applications. The experimental investigation was conducted in two-stages. Thirty pilot and 27 main experiments were conducted making single-track-single-layer depositions of Co-Cr-Mo-2Ti alloy and Ti-Ta-Nb-Mo-Zr HEA to identify optimum values of μ-PAAM process parameters for manufacturing their multi-track multi-layer depositions. In-vitro biocompatibility (in terms of cell viability using human cancerous (HeLa) cells, metallic ion release in PBS solution under pH value of 4.5, 5.5 and 7.5, and corrosion behaviour analysis under pH value of 7.4), density, porosity, microstructure, evolution of phases, microhardness, tensile and compression properties, and wear characteristic were studied for the samples of multi-layer deposition samples of Co-Cr-Mo-xTi alloys and Ti-Ta-Nb-Mo-Zr HEA. Additionally, bio-tribological characteristics of Ti-Ta-Nb-Mo-Zr HEA were studied in human body emulating biofluids by evaluating dry fretting wear characteristics, corrosion behaviour, and tribo-corrosion behaviour. Results indicated that avg. value of cell viability decreases with increase in incubation duration and increase in the prepared media concentration for all three variants of Co-Cr-Mo-xTi alloy. Overall avg. cell viability for Co-Cr-Mo-2Ti; Co-Cr-Mo- 4Ti; and Co-Cr-Mo-6Ti alloys is found to be 92%; 95%; and 85% respectively. Overall avg. released amounts of metallic ions from them are 126; 41;11, and 9 ppb for Co, Cr, Mo, and Ti ion respectively. Bulk density and relative density found to decrease with increase in Ti amount in the Co-Cr-Mo alloy due to smaller density of Ti than that Co-Cr-Mo alloy whereas porosity increased with Ti amount. Porous structure is beneficial for osseointegration of knee implant with the human bones. Microstructure and evolution of phase for Co-Cr-Mo-4Ti alloy indicated presence of α-Co phase having FCC crystal structure, ε-Co phase having HCP crystal structure, ß-titanium phase having BCC crystal structure, and inter-metallic phases of CoTi2 and lamellar chromium carbide (i.e., Cr23C6) at its grain boundaries.