INTEGRATED CHEMICAL TREATMENT AND MAGNETO-RHEOLOGICAL FINISHING OF TITANIUM ALLOY FOR BIOMEDICAL APPLICATIONS

Abstract

Nano-finishing processes have transformed industrial surface finishing by offering significant opportunities to enhance surface integrity, particularly in biomedical applications. With the rising demand for artificial implants, expectations and standards for surface roughness are continually increasing. Achieving a consistent nanoscale finish remains a critical challenge, especially for replacement implant components such as femoral, knee, elbow, and hip joints, which must comply with ISO 7206-2:2011/AMD 1:2016 standards. Ti-6Al-4V (Grade 5) alloy is widely used in biomedical implants due to its superior combination of strength, toughness, corrosion resistance, biocompatibility, and relatively low density. Its α+β microstructure further enhances adaptability for biomedical applications. However, conventional finishing methods are effective for macro- and micro-scale finishing but are insufficient for achieving nanoscale precision, often leading to surface flaws such as cracks. To overcome these limitations, advanced finishing techniques such as magneto-rheological finishing (MRF) and abrasive flow finishing (AFF) have been developed. While they improve precision, challenges persist, including low finishing rates, microcrack formation, surface degradation, and reliance on hazardous chemicals, which raise concerns about safety and the environment. Future processes must therefore be more efficient, sustainable, and capable of achieving nanoscale precision without compromising material integrity or biocompatibility. This thesis develops two advanced processes for surface modification of Ti64 alloy for biomedical implant applications by sustainable, eco-friendly chemicals, including citric acid, Hydrogen peroxide, and Sodium hydroxide. Firstly, a novel thermochemical process (THCP) was developed, followed by simulated mineralization in Hank's Balanced Salt Solution (HBSS) to enhance the mechanical properties, biocompatibility, and bioactivity of Ti64 alloy. The study investigates the effects of eco-friendly chemicals on surface characteristics by XIX evaluating their ability to modify surfaces and determining the optimal pH value. The findings demonstrated that the modification effectively enhanced the Ti64 alloy's mechanical properties, with a significant increase in average microhardness of approximately 71% and a reduction in wear rate of approximately 64.29%, compared to the untreated Ti64 alloy. The surface modification with pH (5, 7, and 9) revealed absorptive properties, as evidenced by a contact angle below 90 °, indicating a hydrophilic surface that enhances cell attachment to biomaterials. After soaking Ti64 alloy in HBSS, a uniform coating layer of approximately 20.5 μm formed, leading to increased bioactivity, as evidenced by a Ca/P ratio of 1.67, comparable to that of hydroxyapatite in human bone. The hemolysis ratio of 0.027% at pH 7 indicates minimal Red Blood Cell (RBC) lysis and increased biocompatibility. The corrosion rate was enhanced with pH (5,7 and 9) approximately (1.975 × 10−2, 1.078 × 10−2, and 1.615 × 10−2) mm/year, respectively. These findings indicate that the novel process at neutral pH (7) is optimal for surface modification, as it is the most effective at enhancing the biocompatibility and bioactivity of the Ti64 alloy, making it suitable for biomedical implants. Secondly, an advanced finishing process using a hybrid chemical process to oxidize and soften the Ti alloy surface, followed by a magnetorheological fluid (CH-MR) to achieve nanoscale roughness with reduced processing time and enhanced surface quality. To systematically evaluate and optimize the influence of CH-MR process parameters, a Central Composite Design (CCD) under Response Surface Methodology (RSM) was employed. CCD combines factorial or fractional factorial points, axial points, and center points to develop a quadratic model for predicting and optimizing process responses. In this study, CCD was used to investigate the effects of critical parameters of pH value, working gap (WG), rotational speed (RS), and current (C) on the surface roughness (Sa) of Ti64 alloy. Analysis of Variance (ANOVA) revealed pH as the most influential parameter (19.14% contribution), followed by WG (15.73%), RS (12.85%), and C (9.93%), while the remaining variation was attributed to parameter XX interactions. Optimization yielded a minimum Sa of 38.20 nm after 30 minutes of finishing under the optimal parameters of pH of 5, a WG of 0.5 mm, an RS of 150 rpm, and a C of 3.5 A. Surface morphology and integrity were assessed using Field Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM), confirming a significant reduction in surface roughness and improvements in quality, with fewer surface defects. X-ray Photoelectron Spectroscopy (XPS) further elucidated the finishing mechanism, linking surface oxidation states to enhanced chemical- mechanical synergy. These characterizations validated the CH-MR process's ability to improve surface roughness and confirmed the absence of contamination or subsurface damage, both of which are crucial for biomedical and aerospace applications.