1. Introduction
Roughly 80% of biomedical implants are developed using metallic materials, including stainless steel, cobalt-chromium, Nitinol, and titanium alloys. This is mainly due to the fact that the metallic biomaterials play a remarkable role in the recovery of dysfunctional organs and improving the life of human beings [
1]. Further, the need of the Ti-based biomaterials is consistently growing due to the rapid increase in the population of the elderly population, road accidents, and sports injuries. Ti-alloy based biomaterials have been used for the development of organs due to their excellent bio-mechanical performance [
2]. It has been reported that the most popular class of Ti alloys, Ti6Al4V, suffers from poor tribological properties and is mainly used for the restricted non-tribological applications [
3]. Further, in [
4], it has been reported that the surface flaws can lead to the implant failure due to propagation of the cracks. The intrinsic characteristics of this alloy tend to release aluminum and vanadium ions, which results in their accumulations on the host tissues and causes toxic reactions [
5,
6]. Underlining such facts, many research interests are focused on the development of an effective alternative, β-phase Ti-Nb-Ta-Zr (β-TNTZ) biomedical alloy, for orthopedic implants, especially actabular cup, shoulder joint, and knee joint assemblies [
7,
8,
9,
10,
11,
12,
13,
14,
15,
16].
However, the poor finishability of the β-TNTZ alloy is one of critical barriers that limit the performance quality by attracting bacterial infection [
17] and vulnerability to attract plaques [
18], which entail inflammation around the implant surface [
19,
20,
21]. Indeed, a wide range of finishing processes, including grinding [
22], honing [
23], ball burnishing [
24], flexible abrasive tools [
25], etc., have been developed for processing the free-form surfaces of the developed implants. As per [
26], the manual finishing of the implant surfaces is non-effective, imprecise, and takes more time. Apart from the conventional finishing processes, the chemical mechanical polishing is effective in polishing Ti implants [
27] and to obtain mirror-polished surfaces without any contamination and reacted layers [
28]. Furthermore, polishing techniques such as electro-polishing, magneto-electro-polishing [
29], and electron beam radiation [
30] are useful to surface finishing in nano-scale.
The magneto-rheological fluid assisted abrasive finishing (MRF-AF) process has been successful in producing nano-finished precise components [
26]. Further, the finishing of β-phase Ti-Nb-Ta-Zr alloy is difficult because of low surface hardness as compared to other biomaterial [
31]. Therefore, in the present study, the in-house developed β-phase Ti-Nb-Ta-Zr alloy has been heat-treated to increase the surface hardness of the alloy and to make it suitable for abrasive finishing. The magneto-rheological (MR) fluid used as a polishing medium consisted of carbonyl-iron-particles (CIP) and hard-abrasive-powder particles in base medium of synthetic mineral oil and grease. The rheological characteristics and the yield strength of the developed MR fluid affects by the externally applied magnetic field [
32]. Ultimately, the CIP in the MR fluid develops an interlinked chain along the direction of magnetic field, resulting in a semisolid abrasive tool to process hard surfaces [
33]. Barman et al. studied the effect of magnetorheological polishing fluid compositions on the surface finish of Ti-alloy. Ultra-fine surface roughness, ranging 10–70 nm, has been achieved using different types of rheological fluids [
34]. Barman et al. studied the effect of tool paths such as spiral and raster on the surface finishing of Ti-based bio-medical alloy. It has been observed that at tool rotational speed of 1200 rpm, working gap of 1 mm, and finishing time of 6.30 h, using a raster path provided the best surface finish and surface topography [
35]. Parameswari et al. studied the effect of abrasive particle concentration on surface finishing. The finishing rate has been significantly affected by initial roughness and concentration of abrasive particles [
36]. Nagdeve et al. developed a rotational-magnetorheological abrasive flow finishing (R-MRAFF) process based special tool for nano-finishing of femoral component of knee joint and surface finish in the range of 78–89 nm was attained, by considering the effect of various input process parameters [
37]. Barman et al. studied the influence of magnetic field-assisted finishing (MFAF) process on the various surface finishing and the average surface roughness obtained was 11.32 nm. The roughness parameters have obtained the values in the range of nano-meters and rendered better surface topography [
38].
From the available literature, the nano-finishing of β-phase Ti-Nb-Ta-Zr biomedical alloy has not been reported yet. The novelty of research work is that the high-strength β-phase Ti-Nb-Ta-Zr is very tough to process using convectional finishing processes. The MR-fluid based abrasive-finishing set was developed in-house and the capability of nano-finishing on heat-treated β-TNTZ substrate has been investigated using single and multi-objective optimization. The material removal (MR) and percentage change in surface roughness (%ΔRa) of the implant surface have been studied in response to input process parameters, such as carbonyl iron particles (CIP) concentration, rotational speed (Nt), and working gap (Gp). Further, the surface morphology and rendered image analysis have been performed to obtain the characteristics of the processed surfaces. The simulated body fluid (SBF) test has also been carried out to identify the corrosion resistivity of the MRF-AF finished β-TNTZ substrate specimens. Further, the as-corroded surfaces have been characterized to observe the effect of surface roughness on the achieved corrosion characteristics.
2. Materials and Methods
High-strength β-phase Ti-Nb-Ta-Zr alloy has been developed using vacuum-arc melting process. The samples of size 10 × 5 mm for the finishing process were cut from the as-developed ingot through a wire-cut electric discharge machining process (Model Ecocut, Electronica, India). After that, the prepared specimens were subjected to a heat-treatment process to improve the mechanical properties of β-phase Ti-Nb-Ta-Zr as reported in previous study [
39]. The microstructure of the samples before and after heat-treatment were examined by field emission scanning electron micrograph (FE-SEM; JEOL 7600F; JEOL Inc., Peabody, MA, USA) and associated energy dispersive spectroscopy (EDS, FE-SEM; JEOL 7600F; JEOL Inc., Peabody, MA, USA). From the microstructure analysis of untreated samples, it has been observed that the material comprised majorly β-type phases with grain size 250 µm, as can be seen in
Figure 1a. The related EDS spectrum conform to the elemental composition and wt.% of each elements present in the material; refer to
Figure 1b. After heat treatment, microstructure is refined and grain size becomes finer in the range of 100–150 µm; refer to
Figure 1c. The heat-treated microstructure comprised α-type and ω-type phases, which further improved the mechanical properties of alloy. As a result, the ultimate compressive-strength and surface-hardness of the developed alloy was enhanced to 1195 MPa and 515 HV, respectively, as suitable for load-bearing implants requisites.
Figure 1d shows the EDS spectrum and elemental composition of alloy after heat-treatment. The observations are close with the previous research studies [
39,
40,
41,
42].
The heat-treated β-phase Ti-Nb-Ta-Zr alloy specimens were then finished using an in-house developed MRF-AF setup; refer to
Figure 2. The MRF-AF processing consisted of three stages, such as development of magnetorheological-fluid, preparation of customized finishing magnetic assisted tool, and finishing of the work surfaces. A permanent magnet tool of material neodymium-iron (Nd-Fe-B) with magnetic flux intensity ~0.45 Taxella Gauss was used as tool for experimentation to provide the required magnetic field in the finishing zone. Generally, the working gap between the abrasive tool and β-phase Ti-Nb-Ta-Zr alloy workpiece was filled with the abrasive media that acted as a ball-end polishing brush. The speed at which the tool rotates plays a crucial role in attaining the required cutting forces to chip out the small amount of material from the work surface.
Table 1 shows the process parameters and their levels.
Presently, the three most crucial input process parameters of MRF-AF process have been selected (such as CIP, Nt, and Gp) to identify their impact on achieved MR and Ra. The MR from the work surface has been calculated by using Equation (1):
where,
is the density of the workpiece and
is the total volume of material removed. Digital weighing balance (Scientech, Delhi, India) of accuracy 0.01 mg was used for the MR calculations. Further, the Ra values of the as-finished work surfaces have been calculated by using a non-contact three-dimensional (3D) Surface Profilometer (Talysurf CCI Lite, Leicester, UK) that uses a white light interferometer equipped with the TalyMap Platinum 6.0. The measurement of Ra was taken at three different locations. The design of experimentation technique, based on Taguchi L9 orthogonal array, was used to perform the statistical analysis on the observed output responses (such as MR and Ra), and to identify the statistical importance of the selected input process parametric levels on the observed responses using analysis of variance (ANOVA) [
43].
Table 2 illustrates the control log of experimentation. Furthermore, corrosion performance parameter, corrosion-current, of the as-finished β-phase Ti-Nb-Ta-Zr alloy specimens has been studied in SBF medium using potentiodynamic polarization-based electrochemical system-1000E (make: Gamry Instruments, Warminster, PA, USA). The concentration of the SBF medium (pH 7.2) consisted of 9, 0.24, 0.43, and 0.2 g/L of NaCl, CaCl
2, KCl, and NaHCO
3 [
44]. For this, potential rate and scan range has been selected as 1 mV/s and −250OCP to +250OCP mV, respectively. Tafel extrapolation technique was used to calculate the corrosion-current (ICOR). Before evaluating ICOR, the specimens were dipped in the SBF solution for about 24 h and the test was conducted at 37 ± 0.1 °C.
4. Conclusions
In the present study, an investigation has been made to analyze the influence of the input process parameters of MRF-AF process on the MR and Ra of the finished β-phase Ti-Nb-Ta-Zr alloy. Besides this, the influence of the input process parameters on corrosion-resistance of the finished samples has also been studied. Based on the key findings, the following conclusions can be drawn:
It has been found that the MR of the processed alloy specimens has been significantly affected by all the selected input process parameters of MRF-AF. Furthermore, the optimized parametric levels as regards to MR are: CIP—40%vol., Nt—900 rpm, and Gp—1.0 mm.
However, in the case of Ra, it has been found that except Nt, none of the input process parameters are statistically significant. In this case, the optimized parametric levels identified are: CIP—40%vol., Nt—900 rpm, and Gp—1.5 mm. The confirmatory experimentation results have been found in good correlation with the predicted responses.
The results of the corrosion analysis of the developed samples highlighted that the corrosion resistance of the finished samples depends on their surface topography. It has been found that the samples possessed high surface finish developed a uniform layer of apatite in SBF medium that performed as a corrosion barrier. On the other side, the rough sites on the implant surface acted as the nuclei to propagate the corrosion mechanics that later resulted in shredding, pitting, and galvanic corrosion.
Overall, the results highlight that the MRF-AF process is highly suitable for producing nano-scale finishing of the biomedical implants made of high-strength β-phase Ti-Nb-Ta-Zr alloy.