Surface modification of biodegradable Mg alloy by adapting µEDM capabilities with cryogenically-treated tool electrodes

Abstract

Biomaterials are engineered to develop an interaction with living cells for therapeutic and diagnostic purposes. The last decade reported a tremendously rising shift in the requirement for miniaturized biomedical implants exhibiting high precision and comprising various biomaterials such as non-biodegradable titanium (Ti) alloys and biodegradable magnesium (Mg) alloys. The excellent mechanical properties and lightweight characteristics of Mg AZ91D alloy make it an emerging material for biomedical applications. In this regard, micro-electric discharge machining (µEDM) is an excellent method that can be used to make micro-components with high dimensional accuracy. In the present research, attempts were made to improve the µEDM capabilities by using cryogenically-treated copper (CTCTE) and brass tool electrodes (CTBTE) amid machining of biodegradable Mg AZ91D alloy, followed by their comparison with a pair of untreated copper (UCTE) and brass tool electrodes (UBTE) in terms of minimum machining-time and dimensional-irregularity. To investigate the possible modification on the surfaces achieved with minimum machining-time and dimensional-irregularity, the morphology, chemistry, micro-hardness, corrosion resistance, topography, and wettability of these surfaces were further examined. The surface produced by CTCTE exhibited the minimum surface micro-cracks and craters, acceptable recast layer thickness (2.6 µm), 17.45% improved micro-hardness, satisfactory corrosion resistance, adequate surface roughness (R_a: 1.08 µm), and suitable hydrophobic behavior (contact angle: 119°), confirming improved biodegradation rate. Additionally, a comparative analysis among the tool electrodes revealed that cryogenically-treated tool electrodes outperformed the untreated ones. CTCTE-induced modification on the Mg AZ91D alloy surface suggests its suitability in biodegradable medical implant applications.

Introduction

Untimely and repeated surgeries for implant removal and replacement are always painful and uncomfortable for the patient. Therefore, biodegradable implants are preferred wherever possible to reduce the number of such surgeries. Being lightweight materials, Mg and its alloys are used in structural applications, especially in the medical sector. Although Mg alloys are as structurally stable (density: 1.7 to 1.9 g/cm³) as human cortical bone (density: 1.75 g/cm³) and exhibit excellent biocompatibility for absorbable orthopedic implants, they are prone to quickly lose their structural strength when subjected to human body’s corrosive environment [1]. However, this property of biodegradation assists Mg and its alloy implants to get absorbed within the human body without acting as a toxic material and thus aids in avoiding secondary surgery. Since Mg automatically gets absorbed in the human body, this absorbability or biodegradation is desirable. However, if an implant biodegrades before recovering the diseased tissue, then this is recognized as the failure of that implant. In this regard, one of the possible approaches to handle this issue is by alloying Mg with adequate alloying elements [2]. Several Mg alloys are used in biomedical devices and implants, including Mg-Zn, Mg–Al-Zn, and Mg-WE [3]. For instance, one of the cast alloys of Mg designated as AZ91D is recognized for its high strength with a remarkable ability to resist corrosion. The tissue engineering scaffolds made of biodegradable and biocompatible Mg AZ91D alloy neither cause any damage to the adjacent bones or tissues nor any skin irritation. The timely biodegradation of biomedical implants based on this alloy is a major concern [4].

Besides this, the inadequate surface finish of the biomedical implants leads to a substantial deterioration in the exceedingly corrosive biological surroundings inside the human body. This suggests that the implants should have adequate surface properties compatible with the neighboring tissues [5]. Therefore, an extensive effort is made to upgrade the surface attributes of biomedical implants for better functional performance. In the case of Mg alloys implants, the credible approach to improve their early biodegradation, as well as the cell response towards them, is by improving their surface characteristics (at the macro- as well as micro-levels), displaying high aspect ratios, either via appropriate manufacturing [6] or surface modification technique [7].

Most Mg alloys are difficult to machine conventionally because the temperature generated at the tool-chip interface detrimentally affects the machined surface as these materials are flammable at a temperature above 650 °C [8, 9]. Therefore, the die sinker type’s electric discharge machining (EDM) was recommended as a potential solution over the conventional machining processes to achieve complex geometrical features with tight tolerances on Mg alloys [10]. In EDM, the discharge energy which causes temperature generation is accountable for the size of the crater formed during machining [11]. In this relation, the downscaled variant of EDM at the micro-level (µEDM) produces smaller discharge energy than macro-EDM amid the excess material subtraction, resulting in the expulsion of a relatively smaller amount of material towards higher precision of the machining [12]. Therefore, the latest literature reports extensive research on µEDM of biomaterials [13, 14].

Furthermore, the material of the tool electrode plays a crucial role during macro- and µEDM processes, which depends on the work-specimen’s characteristics to be machined and machining conditions. Therefore, EDM is run with tool electrodes made of varieties of materials such as copper, brass, graphite, molybdenum, and tungsten. The performance of a tool electrode is assessed in terms of its ability to produce improved (a) material subtraction rate, (b) dimensional accuracy, (c) degree of surface finish, (d) tool wear, and (e) white layer thickness, to name a few [15, 16]. In the past, macro-EDM of biomaterials was performed using different materials of tool electrodes namely copper, brass, graphite, silicon, and tungsten [10, 17]. A few comparative studies between copper (CTE) and brass tool electrodes (BTE) pointed to CTE as the superior tool electrode material over BTE, as CTE offered higher material removal coupled with a better surface finish [18, 19]. Additionally, a recent review outlined the incorporation of cryogenic treatment mainly to bring substantial improvements in EDM tool electrode wear, micro-structure, and mechanical properties. In this process, the specimen tool electrode is held at a temperature below − 130 °C for a specific duration [20]. An experimental study reported a considerable reduction in tool electrode wear compared with the untreated tool electrode due to the deep cryogenic treatment (DCT) of the copper-tungsten tool electrode [21]. The rate of material subtraction and grain refinement were also improved due to the cryogenic treatment of the tool electrode [22]. Another study displayed an overcut reduction of 9% during machining by a deep cryogenically-treated tool electrode [23]. A comparative analysis conducted on CTE and BTE demonstrated that cryogenic treatment could decrease the average crystallite size by 12 and 29% for CTE and BTE. Moreover, it could also decrease tool wear by 35 and 51% for CTE and BTE, respectively [24].

The surface modification or treatment of biomedical implants can enhance the biocompatibility of the host cell and fatigue life of the medical implant by forming a chemically modified nanostructured oxide layer on the material surface [25]. It was reported that ions liberated from the surface of the biodegradable implant get easily absorbed into the metabolic activity of the adjacent tissues without creating any cytotoxicity [26]. This oxide layer can also assist in enhancing the hardness of the surface depending on the number of carbides formed [27]. In the case of biodegradable Mg alloys, an inadequate oxide layer formation limits Mg and its alloys against corrosion during their biomedical application [18]. In this regard, EDM is often recommended as one of the efficient techniques to obtain a simultaneous noticeable surface modification on the EDMed surfaces [28]. The surface is modified by the EDM process when a significant portion of the total disintegrated material (from the workpiece and tool electrode surface) is carried away by the dielectric medium, and the leftover gets solidified on the work surface in the pattern of a white layer (also termed as “recast layer”) [13]. The formation of this solidified layer results in enhanced hardness of the work surface due to the existence of chemically formed compounds [14]. Such surface modification can assist in rapidly recovering the damaged tissue adjacent to the implant. However, the thickness of this layer is scaled down in the presence of body fluids and frictional forces [29].

Although EDM has several advantages, as stated above, the thermal impact of macro- and µEDM substantially alters the residual stress and crystalline structure, leading to the micro-crack formation on the Mg alloy surface. Therefore, a few studies have proposed the addition of conductive abrasive particles to EDM (AM-EDM) to substantially lessen the structuring of micro-cracks on the processed Mg alloys. For instance, as outlined by two recent experimental investigations, Zn-abrasive-assisted µEDM minimized the surface micro-cracks and thus achieved remarkable surface modification in terms of superior corrosion resistance on Mg alloys over the unmachined specimen [18, 30]. However, it can be argued that the studies on AM-EDM of Mg alloys suffered from certain weaknesses, such as excess thickness and inadequate uniformity of recast layer, which resulted in the poor surface quality of Mg alloys [31, 32]. Moreover, the overall manufacturing cost rises with the incorporation of different abrasives. Also, for implant applications, the abrasive needs to possess sufficient bioactivity. Therefore, alternative approaches are needed to achieve improved surface characteristics of Mg alloys. In this regard, despite the considerably promising properties of cryogenically-treated tool electrodes, their effect on surface characteristics, which notably vary the biodegradation rate, has been rarely investigated amid µEDM of Mg alloys. Therefore, the focus of this research was to advance the machinability of the Mg AZ91D alloy in terms of the minimum machining-time and dimensional-irregularity by tailoring µEDM’s efficiency with the inclusion of CTE and BTE in the untreated and cryogenically-treated state and to examine the impact of enhanced machining features on the machined surface characteristics for biomedical implant applications.

Experimental procedures

This section includes the details of the work-specimen specifications, input variables, characteristics of tool electrodes, experimental setup, experimentation approach, and performance measures with the assistance of three subsections that are as follows:

Work-specimen

This work aimed to investigate the µEDM performance of chosen work-specimen Mg AZ91D alloy. The work-specimen was in the plate form and possessed 1.83 g/cm³ density, 244 MPa tensile strength, 155 MPa yield strength, 63.6 HV micro-hardness, and 72.7 W/mK thermal conductivity. The elemental composition of the work-specimen is presented in Table 1. Several experimental trials were performed to achieve the machining depth of 2 mm.

Table 1.

Elemental composition of Mg AZ91D alloy (wt%)

Al	Mn	Zn	Mg
9.0	0.3	0.7	Balance

Experimental strategy and tool electrodes

The experimentation of this research aimed to arbitrate the optimal levels of the machining input variables amid the processing of the Mg work-specimen by applying the OVAT (one variable at a time) strategy. For this reason, some pilot experimental trials were carried out on the µEDM (Hyper-15, Sinergy Nano Systems) setup for a pre-decided machining depth using a pair of untreated CTE and BTE (diameter: 580 μm).

The OVAT approach targets to organize the trial runs within restricted settings, permitting the change in only one input variable at a time while keeping others at their steady values [33]. Hence, following the OVAT design in the present experimentation, each input variable was varied at a time and its impact on the machining-time and dimensional-irregularity was studied. The pilot runs comprised preestablished domain of µEDM input variables with straight polarity: voltage (100–170 V), tool electrode rotation speed (270–350 rpm), capacitance (50–150 µF), pulse on time (5–20 µs), discharge current (30–50 mA), and pulse interval time (35–55 µs). Besides, the previous study showed that Mg AZ91D alloy is prone to micro-crack formation during machining. The fractographic assessment demonstrated trans- and inter-granular micro-cracking and pitting of the surface, which resulted in decreased fatigue strength [34]. Copper and brass as tool electrode materials, widely used due to their low cost, easy availability, and good machinability characteristics, were chosen for this research. Therefore, this study attempted to reduce the micro-crack formation using CTE and BTE in their untreated and cryogenically-treated conditions process Mg AZ91D alloy work-specimens. In this regard, Table 2 shows the various properties of untreated tool electrodes.

Table 2.

Properties of untreated tool electrodes

Tool electrode	Melting point (°C)	Electrical resistivity (nΩm)	Thermal conductivity (W/mK)	Coefficient of thermal expansion (α(10−6 K−1))
CTE	1087	17	391	16.5
BTE	988	65	159	20

Furthermore, as a result of pilot runs, the indisputable combination of the optimal levels of input variables, achieving the most exceptional output variables, is displayed in Table 3. Another pair of CTE and BTE was cooled to − 196 °C in an LN2 container: cryocan (Cryogem, PS-34), containing liquid nitrogen (LN₂) at a temperature of − 196 °C for DCT (soaking time of 24 h). The temperature of the cryogenically-treated tool electrodes was slowly brought down to − 196 °C at a cooling rate of 1 °C/min and then brought back to room temperature at a heating rate of 1 °C/min.

Table 3.

Constant input variables

Parameters	Numeric value	Unit
Voltage	130	V
Speed	300	rpm
Capacitance	100	µF
Pulse on time	10	µs
Discharge current	40	mA
Pulse interval time	42	µs

Table 4 presents the nomenclature of all the tool electrodes (untreated and cryogenically-treated ones) used in the present research.

Table 4.

Nomenclature of untreated and cryogenically-treated tool electrodes

Tool electrode	Description	Nomenclature
Tool 1	Untreated copper tool electrode	UCTE
Tool 2	Cryogenically-treated copper tool electrode	CTCTE
Tool 3	Untreated brass tool electrode	UBTE
Tool 4	Cryogenically-treated brass tool electrode	CTBTE

Figure 1 shows the equipment and tool electrodes used in this experimental work. Figure 1(a) presents the cryocan used for cryogenic treatment and the LN₂ transfer device was used to receive the required quantity of LN₂ at the optimum pressure from the cryogenic storage dewar. Figure 1(b) presents the schematic diagram of the DCT cycle chosen for a set of CTE and BTE. Figure 1(c) and (d) show a set of UCTE and UBTE. Figure 1(e) shows the clamped work-specimen machined by an untreated tool electrode on the µEDM setup. Figure 1(f) presents a CAD model of the machining process for both tool electrodes. The CAD model was developed using Autodesk Fusion 360, showing the electric spark and debris removal during the µEDM process.

Fig. 1.

Equipment and tools used during experimentation. a Cryocan with LN₂ transfer device, b DCT cycle, c UCTE, d UBTE, e machining setup, and f CAD model of machining

Thus, the final OVAT-designed experimentation consisted of a total of 12 experimental runs (four trials with three reiterations with each tool electrode) for each output variable. Hence, each trial was replicated three times, and the mean numeric values of the machining-time and dimensional-irregularity were calculated and treated as the leading response values.

Analysis of the outcomes

The outcomes of this research were analyzed in the following systematic manner:

Calculation of machining-timeand dimensional-irregularity

A stopwatch (Model: EISCO-Digital Pro) was employed to measure the machining-time. The machined surfaces were viewed using Olympus BX51 optical microscope at a magnification of 100 × to locate and assess the dimensional-irregularity. The dimensional-irregularity was obtained by subtracting the tool electrode’s diameter from the produced hole’s diameter. The calculated values of these response variables were later analyzed graphically using the OriginPro 2022 software. Furthermore, the surfaces machined by each tool electrode in the minimum time and exhibiting the least dimensional-irregularity were termed as the “specimens or surfaces of interest-1” (SI-1) and were, therefore, captured by the Zeiss-GeminiSEM 500 field emission scanning electron microscope (FE-SEM) as well.

Morphological examination

FE-SEM was further used to conduct a detailed study on the SI-1. For this reason, metallography was performed on the SI-1. A CNC Wirecut electric discharge machining (Model: Electronica/Ecocut) facility was used to cut and slice (section) the samples to size (3 × 3 × 5 mm³). Furthermore, to abrade and polish the samples, SiC paper (grit size: 2000) and diamond paste (1 µm) with ethanol (as a lubricant), respectively, were used. The next step included sonication (ultrasonic bath in acetone) and drying (in warm air) of the samples. Additionally, samples were chemically etched using 4.2 gm picric acid, 10 ml acetic acid, 70 ml ethanol, and 10 ml distilled water, by immersion in the solution for 30–60 s [35] to reveal the lowest energy surfaces on the polished samples by chemical stripping of atomic layers. Furthermore, various FE-SEM images of the different views of the SI-1 were taken at higher magnifications to observe the surface defects and recast layers through visual inspection. Besides this, since the detailed investigation of surface crack density is beyond the scope of this research, a brief analysis of the same was performed in this work. For this reason, only the FE-SEM images of the machined surfaces exhibiting the minimum and maximum defects or cracks were considered. The visible cracks on the FE-SEM images were traced in red and an HSB (hue, saturation, and brightness) stack was created with the help of ImageJ software. HSB stacking sorted the image colors in Hue, Saturation, and Brightness planes to separate the cracks all over the images. Afterward, the centrelines (Skeleton) of the objects were created by the eroding of their surface using the “Skeletonize” Plugin. With this process, the traced cracks were transformed into a skeleton with a 1-pixel thickness. Skeleton length was measured using Plugin AnalyseSkeleton which tagged all the pixels in the branch created through the skeletonized image. The length of each branch was added to calculate the total length of the crack in the area in the image. The area was calculated by the dimensions of the image whose scale was set beforehand. A comparable value for the crack density was obtained by dividing the total crack length by the area of the image [36]. In this brief analysis, the calculated crack density did not include the width of the crack in the calculations. Therefore, the crack density comparison had a limitation of crack width. However, the same can be considered for calculations and more accurate results in subsequent studies. In continuation, the SI-1 were examined using energy-dispersive X-ray spectroscopy to assess the accumulated elemental compositions on them. Additionally, the crystalline plots and compositions of the SI-1 were obtained with the aid of the Rigaku-TTRAX III X-ray diffraction (XRD) setup. Crystalline phases were assessed by XRD with CuKα radiation (λ = 1.5406 Å). The plots were obtained at a step size of 0.01° and a continuous scan speed of 2°/min over a 2θ range of 20° to 95°.

The findings of the above two subsections assisted in funneling down this study to select the specimens or surfaces exhibiting the best morphological features, also indicating the most potential tool electrodes responsible to achieve the same. Therefore, such specimens or surfaces were addressed as SI-2 and were thus focused on further investigation in the later subsections.

Assessment of micro-hardness and corrosion behavior

It was reported that the machining with a cryogenically-treated tool electrode increased the micro-hardness of the work surface by 94.85% [37]. Thus, the current investigation also aimed to determine the micro-hardness variation resulting from carbon accumulation on the SI-2. Micro-indentation examinations were carried out using Vickers hardness tester (BSHT-FHV1-50, Blue Star Ltd.) to evaluate the micro-hardness of the SI-2.

Besides, an electrochemical reaction promotes the biodegradation of magnesium [38]. Therefore, the analysis of the corrosion resistance of the SI-2 was executed at room temperature by employing GAMRY Interface (1000 potentiostat/galvanostat/ZRA), which is a standard cell containing three electrodes. Graphite rod and saturated calomel electrode were engaged to act as the counter and reference electrode, respectively. At first, the polishing and cleaning of the SI-2 were performed using acetone and ethanol. In the next step, the SI-2 were air-dried, followed by their immersion in a 3.5 wt% NaCl solution towards the maintenance of the open-circuit potential (for 60 min). Furthermore, 5 mV amplitude was used (ranging from 100 kHz to 100 MHz) at 25 °C to conduct the electrochemical impedance spectroscopy. A chi-squared < 0.001 was ensured during the equivalent electric circuit fitting using ZSim software. Once again, the observations of this subsection helped to determine the surfaces(s) displaying the highest micro-hardness and corrosion resistance. Hence, such specimen(s) or surface(s) were designated as SI-3 in the next subsection of this study.

Evaluation of topographic and wetting attributes

The topographic or roughness features (heights, valleys, and projections) of a degradable implant surface notably influence the cell viability, biological reactivity, and corrosion behavior (ascertaining its degradation rate) and thus its clinical success [39]. For instance, roughness values ranging from 1 to 100 μm can considerably increase the osteoblast activity of an implant [40]. Therefore, this study further examines the topography of SI-3 by developing their three-dimensional image(s) and calculating their average roughness profile (R_a), root-mean-square roughness profile (R_q), the maximum-peak-to-valley height of roughness profile (R_t), and mean-peak-to-valley height of roughness profile (R_z) by using Alicona Mex software with Tescan Vega3 scanning electron microscope (SEM). R_a represents the arithmetical or integer average height of all the unadulterated roughness profile deflections from the centerline along the measurement length. R_q indicates the root-mean-square average of the profile peaks over the sampling dimension. R_t is the vertical length between the highest peak and lowest valley of the profile along the measured range. R_z displays the absolute height-to-depth mean of five consecutive sampling dimensions within the measuring length. Besides this, to investigate the wetting characteristics of the SI-3, the assessment of contact angle was executed using a DSA10, Krüss Drop-Shape Analyzer. The contact angle was determined with the aid of a deionized water droplet of 1 µL suspended from the syringe tip for a total of 60 s (with a time interval of 0.4 ms).

Characterization of the tool electrodes

After all the machining operations, the worn-out ends of all the electrodes were analyzed by FE-SEM and a Celestron Handheld Digital Microscope Pro. Furthermore, to experimentally assess the influence of cryogenic treatment on the electrical conductivity of both cryogenically-treated tool electrodes (CTCTE and CTBTE), their electrical resistivities were measured. The process of measuring the electrical resistivity included connecting one end of the banana cable with the front panel of Keithley 2450 SourceMeter and another end with the end face of the cryogenically-treated tool electrode to be tested. In this study, a 4-wire configuration was used due to the small diameter of the tool electrode. However, each test was conducted three times to achieve higher accuracy, and the mean resistivity was calculated for both CTCTE and CTBTE. The obtained electrical resistivity of each tool electrode was then converted into electrical conductivity.

Results and discussion

The outcomes of this research are presented and explained in the following subsections.

Effect of different tool electrodes on machining-time and dimensional-irregularity

Figure 2(a) and (b) show the comparisons of machining-time and dimensional-irregularity obtained using different tool electrodes. Copper is a better conductor of electricity; therefore, this was the reason why it could reduce the machining-time in both untreated (UCTE) and cryogenically-treated (CTCTE) conditions. Moreover, the superior performance of CTCTE over UCTE was possibly due to its improved properties, such as conductivity, as a result of cryogenic treatment. In comparison, the machining-time with CTBTE was longer than compared with UBTE. This is plausibly due to a decrease in the electrical resistivity in CTBTE because of DCT [24]. DCT adversely affected the performance of CTBTE, credibly due to a noticeable reduction in residual stress as a result of cryogenic treatment [41].

Fig. 2.

Comparison of a machining-time (min) and b dimensional-irregularity (µm)

CTCTE performed its task with the least machining-time and dimensional-irregularity. Copper has better electrical and thermal conductivities than brass; thus, the machining performance with CTE was superior to BTE under cryogenically-treated conditions [24].

Besides this, the geometrical aspect of the samples exhibiting the minimum value of the dimensional-irregularity (obtained by the different tool electrodes), also termed SI-1, has been further discussed with the help of Figs. 3 and 4. Figure 3 presents the FE-SEM images of the sectioned surfaces of SI-1 where the quadrant shape wall of the machined hole is evident (since the tool electrode was cylindrical). The marked locations in yellow (in Fig. 3) project the regions considered to obtain the FE-SEM images shown in Figs. 4, 5, and 6.

Fig. 3.

FE-SEM images (at 1000 × magnification) displaying the geometry of the machined regions through the sectioned surfaces

Fig. 4.

Optical microscopic and FE-SEM images of the surfaces machined by all four tool electrodes

Fig. 5.

FE-SEM images (at 5000 × magnification) of the surfaces machined by a UCTE, b CTCTE, c UBTE, and d CTBTE

Fig. 6.

HSB-stacked FE-SEM and skeletonized images of the surfaces machined by a CTCTE and b UBTE

Figure 4 comprises the combinations of optical microscopic (OMI) and FE-SEM images of SI-1 produced by UCTE (Fig. 4(a)), CTCTE (Fig. 4(b)), UBTE (Fig. 4(c)), and CTBTE (Fig. 4(d)). To show the dimensions of the machined surfaces, the OMIs (shown in Fig. 4) were captured at a magnification of 100 × , whereas to observe the top view of a specific location, the corresponding FE-SEM images were recorded at a magnification of 1000 × . In each OMI, the yellow circle represents the hole diameter obtained after machining, which is more than the diameter of the tool electrode. The OMI obtained by CTCTE presented the minimum dimensional-irregularity obtained during the experimentation.

Study of the morphology of SI-1

Figure 5(a), (b), (c), and (d) display the FE-SEM images of the SI-1 obtained with UCTE, CTCTE, UBTE, and CTBTE, respectively, demonstrating several surface flaws such as thermal micro-cracks and craters, which would possibly localize pitting corrosion. The least number of surface defects and the most uniformly distributed oxides, constituting a recast layer, were found on the SI-1 produced by CTCTE (Fig. 5(b)), followed by UCTE (Fig. 5(a)), CTBTE (Fig. 5(d)), and UBTE (Fig. 5(c)).

The oxide content in a recast layer acts as a passivating mean on the machined surface, which can prevent or minimize further corrosion by decreasing the migration of electrons for electrochemical reaction, and also promotes the immunomodulatory of the biomedical implant [42]. Figure 5(c) shows the maximum number of micro-cracks and craters on the µEDMed surface obtained by UBTE. The presence of micro-pits with a grey or frosted appearance might indicate fatigue failure [34]. This damage might give rise to local corrosion in the regions where micro-cracks pass through the impurities and debris inclusions. Figure 5(d) shows the moderate number of micro-cracks on the machined surface using CTBTE. In EDM, the formation of surface defects might be attributed to the fatigue failure of the machined surface at different locations as a result of continuous sparking via accumulated debris or inhomogeneity of the recast layer [21, 34]. The generated residual stresses during an EDM operation, when exceeding the ultimate tensile strength, lead to the generation of these defects. In addition, the displayed micro-structures of the recast layers exhibiting more micro-cracks and craters are likely to develop inferior passivity and key corrosion potentials [43]. Although AZ91D combines the mechanical properties with corrosion resistance in an outstanding manner, the EDM-induced surface defects or texture irregularities credibly demonstrated the effects of specific machining conditions, particularly the properties of the tool electrode, suggesting substantial alterations in the existing corrosion resistance of the workpiece. Corrosion resistance gets adversely affected by the rise in surface defects and vice versa. Also, a substantial increase in surface roughness can lead to crevice and pitting corrosion [44]. In an ideal EDM process, the workpiece is expected to absorb the maximum heat, while the tool electrode should ingest the least heat during sparking. However, a relatively lesser amount of heat was possibly ingested by UBTE due to its low thermal conductivity compared to other tool electrodes. Therefore, in this case, the maximum amount of heat was supposed to be absorbed by the work surface [13, 14], but credibly, the poor thermal conductivity of Mg AZ91D alloy (72.7 W/mK) did not permit the absorption of much heat. The thermal conductivity of untreated brass is approximately 2.2 times higher than that of this Mg alloy. Thus, the low thermal conductivity of the work surface failed to dissipate much heat from the heat-affected zone; hence, the remaining heat damaged the machined surface by inducing thermal micro-cracks and craters. Thus, the work surface machined by the tool electrodes possessing relatively low thermal conductivity was prone to more surface defects. However, CTCTE was able to produce a surface with the minimum surface defects due to its superior thermal conductivity. These outcomes are in sound agreement with previous research [45].

Furthermore, as mentioned in Sect. 3.2, for surface crack density analysis, only the FE-SEM images of the µEDMed surfaces observed with the minimum (Fig. 5(b)) and maximum cracks (Fig. 5(c)), produced by CTCTE and UBTE, respectively, were taken into consideration. In this regard, the HSB-stacked FE-SEM and skeletonized images of Fig. 5(b) and (c) are shown via Fig. 6(a) and (b), respectively.

Hence, with the help of Fig. 6(a) and (b), the following surface crack densities are obtained for the machined surfaces shown in Fig. 5(b) and (c):

Max branch length in Fig. 6(a) = 22.045 µm.

Dimensions of the image shown in Fig. 6(a) = L × B = 83.03 × 55.30 µm.

Surface crack density of Fig. 5(b): $\frac{22.045}{83.03\times 55.30}$ = 4.8012015 × 10⁻³ per µm.

Max branch length in Fig. 6(b) = 23.896 µm.

Dimensions of the image shown in Fig. 6(b) = L × B = 83.03 × 55.13 µm.

Surface crack density of Fig. 5(c): $\frac{23.896}{83.03\times 55.13}$ = 5.2203807 × 10⁻³ per µm.

The results of the calculations also confirm that the crack density of the surface machined by UTBE (shown in Fig. 5(c)) is greater than the surface machined by CTCTE (shown in Fig. 5(b)). Therefore, the severity of micro-cracking was once again confirmed to be more in Fig. 5(b).

In continuation, Fig. 7 presents the FE-SEM images showing the recast layers produced with UCTE, CTCTE, UBTE, and CTBTE on SI-1. The thickness of each layer was computed at different sites. The images were captured at different magnifications to obtain better clarity of the varying thickness of these layers.

Fig. 7.

FE-SEM images of the recast layers obtained by a UCTE and b CTCTE at 1.50 K × magnification and c UBTE and d CTBTE at 750 × magnification

The average thickness of the recast layer obtained with UCTE, CTCTE, UBTE, and CTBTE was 2.9, 2.6, 1.81, and 2.44 µm. The tool electrode with higher thermal conductivity and more refined grains could offer higher uniformity, thickness, and integrity of the oxide layer [45]. Also, as outlined previously, due to the difference between the thermal conductivities of UBTE and Mg AZ91D alloy, the surface machined with UBTE experienced more cracking and oxide content with poor uniformity, and less amount was observed on it. Hence, the recast layer with the least thickness was noticed with UBTE. Thus, the oxide layer thickness of 2.6 µm attained by using CTCTE was found by this study.

Besides this, machined SI-1 was detected with some precipitated compounds (Fig. 5), which were further confirmed by the energy-dispersive X-ray spectroscopy analysis (Fig. 8) to be metal oxides. Figure 8(a) presents the energy-dispersive X-ray spectroscopy of the unmachined surface of Mg AZ91D alloy with elements present before the machining operation. Figure 8(b)–(e) depict this spectroscopy of the surfaces machined by UCTE, CTCTE, UBTE, and CTBTE, respectively. These plots primarily demonstrated the variation in oxide content after machining with different tool electrodes. The value of the applied low voltage (130 V) during µEDM was noticed to be adequate to promote polarization and consequently form oxides [42]. In all four cases in Fig. 8(b)–(e), there appears to be a rise in the oxide content, which might indicate improved machinability and resistance to corrosion of these machined surfaces [18]. However, the cryogenically-treated tool electrodes achieved relatively lower oxide contents than the untreated tool electrodes. This suggests that the CTCTE and CTBTE produced thinner oxide layers on the machined surfaces compared to UCTE and UBTE, respectively, indicating less probability of recast layer defects such as micro-pores and cracks. Hence, Figs. 5(b), 7(b), and 8(c) suggest that machining with CTCTE resulted in extensive modification of the machined surface.

Fig. 8.

Energy-dispersive X-ray spectroscopy of the a unmachined surface, and surfaces machined by b UCTE, c CTCTE, d UBTE, and e CTBTE

The enhanced thermal conductivity of CTCTE due to cryogenic treatment plausibly occurred in quick heat transfer through the refined grains of the tool electrode, leading to the development of a comparatively smoother and more consistent oxide layer on the µEDMed surface. The higher material subtraction rate attained by cryogenically-treated tool electrodes was another plausible reason for this occurrence [45]. In addition, a study mentioned the possibility of a reduction in melting point with the reduction in hardness and vice versa [46], suggesting that cryogenic treatment can significantly vary the hardness, and consequently, may also alter the melting point of a material, depending on the grain size and arrangement of the molecules (molecule packing inside the material). Therefore, an improvement in the tool electrode’s hardness and grain refinement might be attributed to the enhancement in melting temperature and wear resistance as a result of cryogenic treatment [47]. Therefore, possibly due to improved melting points, the cryogenically-treated tool electrodes produced comparatively less damaged surfaces. Moreover, the improved melting point of the tool electrode would result in less melting and wear. Consequently, the cryogenically-treated tool electrodes would experience less melting and wear compared to the untreated ones, resulting in less or negligible deposition of tool electrode material into the recast layer, which would assist in reducing recast layer thickness and enhancement of uniformity (Fig. 7(b)). Hence, it can be once again witnessed that the tool electrode properties can significantly alter the surface chemistry of the workpiece [21]. However, the detailed study of the relationship between the tool electrode’s melting point and hardness is beyond the scope of the present research.

Furthermore, the crystallographic investigation of the unmachined Mg alloy surface and SI-1 was conducted using XRD analyses (Fig. 9). Crystalline phases ZnO and Mg alloy present the same hexagonal crystalline structure with similar lattice parameters. Thus, diffracted peaks of both phases overlap. ZnO was found with space group: P 63 m c (186), hexagonal crystal system, and cell parameters: a = 3.2049 Å and c = 5.1216 Å. Mg alloy was found with space group: P 63/m m c (194), hexagonal crystal system, and cell parameters: a = 3.2032 Å and c = 5.2010 Å. Furthermore, the unmachined surface presents only peaks of β-Mg₁₇Al₁₂. ZnO diffraction was more intense for the CTCTE surface, with peaks at ~ 37° and ~ 64°. However, the surface machined by UCTE presents intense diffraction peaks of MgO at ~ 48° and ~ 91°. The surface produced by UBTE showed mostly diffraction peaks of Mg alloy. ZnO was also seen in addition to two peaks of MgO at ~ 48° and ~ 71°. Apart from this, intense diffraction peaks of ZnO at ~ 37° and ~ 69° were evident on the surface machined by CTBTE. However, in this case, less intense peaks of Mg alloy and relatively smaller peaks of MgO at ~ 71° and ~ 82° were noticed. MgO detected in the XRD plot was formed as Mg is prone to oxidation under atmospheric conditions. MgO is also formed in the human body because of the reaction between Mg and body fluids which can foster osteoinductivity and osteoconductivity.

Fig. 9.

XRD analysis of unmachined surface and surfaces machined by UBTE, UCTE, CTBTE, and CTCTE

However, porous MgO layers can shield the implant surface against corrosion when the pH of body fluid is more than 11.5. The MgO layer becomes weak when pH is low and thus fails to serve the intended purpose [48]. Figure 9 shows the presence of a sufficient amount of ZnO on the surface obtained by CTCTE, suggesting improved corrosion resistance, anti-microbial, anti-bacterial, and biocompatible behavior of the µEDMed surface [49]. Moreover, zinc ions can encourage the cells’ response, resulting in speedy restoration of the diseased tissue. Additionally, during the current pandemic of COVID-19, in some of the most recent studies, zinc oxide is one of the most successful and potentially pharmacologically competent means to disrupt the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). With the help of its oxides, Zn can prevent the coronavirus from entering, mutating, and even spreading inside the body [50, 51]. Zn mainly guards the cell membrane (which largely hinders the virus entry) and also gets the impaired immune system restored [52]. In this regard, the presence of ZnO in the modified Mg AZ91D alloy surface strongly suggests its anti-viral property against coronavirus. The presence of β-Mg₁₇Al₁₂ coupled with oxide also can reduce the implant’s biodegradation [53]. Hence, the appearance of these layers can strongly suggest the most favorable modification on the surface produced by CTCTE. Also, as a result of this morphological examination, the surfaces machined by UCTE and CTCTE were noticed with the most preferable results and were therefore, labeled as SI-2 for the succeeding subsection.

Study of the micro-hardness and corrosion behavior of SI-2

A study reported a remarkably enhanced machining-induced micro-hardness and corrosion resistance on the machined Mg alloy work-specimen [54]. Hence, this research further investigated the micro-hardness followed by the corrosion resistance of SI-2. The micro-hardness of these machined surfaces is shown in Table 5.

Table 5.

Micro-hardness of the machined surface

Trials	Tool electrode
	UCTE	CTCTE
I	82.4	74.3
II	81.8	75.2
III	82.6	74.7
Average micro-hardness (HV)	82.2	74.7
Standard deviation (s)	0.416	0.450
Percentage increase in micro-hardness	29.24%	17.45%
Micro-hardness of the work-specimen before machining: 63.6 HV

The percentage increase in the micro-hardness of the µEDMed surface with UCTE was higher (29.24%) than the CTCTE (17.45%). A study found that micro-hardness was notably increased due to the presence of carbon on the machined surface [55]. The micro-hardness of the surface determines the performance of a biomedical device or implant. A considerable rise in micro-hardness, wear, and thermal stability can significantly advance the corrosion resistance of the surface [3]. However, the excess hardness may also result in micro-crack formation in the machined surface. Therefore, recast layer formation may act as a protective layer or may lead to material failure [18]. Hence, the hard surface produced by the UCTE displaying more micro-cracks might be undesirable for biomedical applications.

Furthermore, the electrochemical impedance spectroscopy Bode and Nyquist plots of the unmachined surface and surfaces machined with CTCTE and UCTE (SI-2) were achieved. The open-circuit potential (OCP), as shown in Fig. 10(a), was monitored for 60 min before electrochemical impedance spectroscopy.

Fig. 10.

a OCP vs. time and b potentiodynamic polarization curves of Mg AZ91D alloy surfaces in 3.5% NaCl

The OCP of the unmachined Mg alloy fluctuated throughout most of the test, only getting to a moderate stabilization in the last 10 min of monitoring. As observed by Chen et. al. [56], this OCP fluctuation may be ascribed to the poor protective properties of the loose oxide/corrosion product layer that is continuously formed and removed, exposing the substrate to further corrosion. Mg AZ91D alloy surfaces µEDMed with CTCTE and UCTE presented a stable OCP stabilized in values between − 1.57 and − 1.54 V_vs.Ag/AgCl. The potentiodynamic polarization curves of the unmachined and SI-2 are shown in Fig. 10(b). The cathodic branch of the polarization remains constant for all three surfaces, implying that the µEDM did not alter the kinetics of the cathodic reaction of the surface. However, surfaces that underwent µEDM, both with CTCTE and UCTE, presented smaller anodic currents, suggesting anodic corrosion protection. Also, the Tafel region of the potentiodynamic polarization curves indicates that these µEDMed surfaces exhibited a reduced corrosion current compared to the unmachined surface. Although, it should be pointed out that the determination of the corrosion potential and current values are only possible when both half-reactions are controlled by activation polarization, which is not the case of Mg alloys due to the anomalous hydrogen evolution when it is submitted to anodic polarization [57, 58].

The experimental electrochemical impedance spectroscopy data were fitted to an equivalent electric circuit composed of two time constants and one inductive loop, as presented in Fig. 11(a). Values of the fitted equivalent electric circuit elements are provided in Table 6, where a conventional capacitor on the second time constant was able to correctly represent the experimental data, as contrasted to the first time constant where a constant phase element was required. The equivalent electric circuit elements are defined as follows: R₁ stands for the solution resistance, R₂ for the interfacial charge transfer, R₃ for the diffusion resistance (associated with the formation of Mg(OH)₂), R₄ for the low-frequency resistance associated with corrosion cell reactions and adsorption/desorption of compounds on the electrode surface. Similarly, CPE stands for the constant phase element representing the double layer capacitance, C for the capacitance of the newly formed Mg(OH)₂, and L for the low-frequency inductance.

Fig. 11.

a Equivalent electric circuit used to fit the electrochemical impedance spectroscopy experimental data acquired in 3.5% NaCl and b Bode plot of the impedance modulus of unmachined and µEDMed Mg AZ91D alloy surface with CTCTE and UCTE

Table 6.

Fitting result of the electrochemical impedance spectra of unmachined and µEDMed Mg AZ91D alloy surface (produced with CTCTE and UCTE) in 3.5% NaCl

	R1	CPE	n	R2	C	R3	L	R4	Rp
	Ω.cm2	F.cm−2.s(n−1)		Ω.cm2	F.cm−2	Ω.cm2	H.cm−2	Ω.cm2	Ω.cm2
Unmachined Mg AZ91D alloy	5.60	2.32E − 4	0.73	11.2	4.15E − 3	9.13	2.27	22.39	10.65
Mg AZ91D alloy machined with CTCTE	18.34	1.10E − 4	0.65	674.7	–	–	220.50	1045.00	409.99
Mg AZ91D alloy machined with UCTE	11.69	7.36E − 4	0.44	250.9	–	–	78.78	344.10	145.10

The resistance to polarization (R_p) has been calculated considering R₂, R₃, and R₄. µEDMed Mg AZ91D alloy surfaces achieved with CTCTE and UCTE (SI-2) presented higher corrosion resistance values (or biodegradation rate) than the unmachined Mg AZ91D alloy surfaces due to increased charge transfer and low-frequency resistance, indicating substantial modification of these surfaces. Consequently, the polarization resistance of the SI-2 was considerably higher than the unmachined surface. The surface machined with CTCE exhibited the highest values of resistance to polarization which is in good compliance with the Bode plot of the impedance modulus (presented in Fig. 11(b)). On Nyquist electrochemical impedance spectroscopy spectra, all surfaces present a well-defined inductive loop in the low-frequency range, characteristic of Mg alloys immersed in chloride-containing media (Fig. 12) [59]. Despite the extensive literature description of the inductive behavior of Mg alloy under corrosion, its origin is still under debate, with authors ascribing it to the adsorption of corrosion products, hydrogen oxidation, hydrogen adsorption, and localized and micro-galvanic corrosion, among others [60, 61].

Fig. 12.

Nyquist plot of unmachined and µEDMed Mg AZ91D alloy surfaces (produced with CTCTE and UCTE)

The findings obtained from potentiodynamic polarization and electrochemical impedance spectroscopy demonstrated that the µEDM with CTCTE and UCTE improved the corrosion resistance and thus improved the biodegradation rate of the Mg AZ91D alloy surfaces compared to the unmachined one. Furthermore, CTCTE produced a surface noticed with higher polarization resistance compared to the surface achieved with UCTE. Hence, the specimen, evincing the most improved micro-hardness and corrosion resistance, was categorized at SI-3 and was, therefore, recommended to be considered for further analysis.

Study of the topography and wettability of SI-3

Based on the promising features, the surface produced by CTCTE (SI-3) was further investigated to examine its topography and wettability. The contact angle of a surface is used as a function of the surface roughness to characterize the wetting properties. One of the recent reviews reported that surface topographic characteristics, such as surface roughness, can largely determine the biodegradation rate, pitting, and in vitro behavior of Mg alloys. Improved surface roughness values point towards strong passivation and thus considerably reduced pitting [62]. Therefore, the study of surface topography holds great significance for biodegradable Mg alloy medical implants. For this reason, the two-dimensional (2-D) and three-dimensional (3-D) images of the modified Mg AZ91D alloy surface (SI-3) are displayed in Fig. 13 presenting Ra = 1.08 µm, Rq = 1.51 µm, Rt = 15.27 µm, and Rz = 9.56 µm. According to the obtained numerical value of Ra (1.0–2.0 μm), this modified surface can be concluded as moderately rough, suggesting it exhibits an optimized harmony between its degradation and cytocompatible characteristics [39].

Fig. 13.

2-D and 3-D view of the modified Mg AZ91D alloy surface

Moreover, the surface topography comprising nano- and micro-features further ascertains the surface wettability [63]. Hence, the next part of this series of surface examinations considered the wettability study. The wettability of the surface produced by CTCTE (SI-3) is evident at three distinct sites in Fig. 14.

Fig. 14.

Static water contact angles as a function of age for modified Mg AZ91D alloy surface

These images were recorded as soon as contact between the droplet (from the syringe tip) and the µEDMed surface gets established. This wettability determines adhesion characteristics, biocompatibility, and water repellency/absorption of a surface. The contact angle measurements were fit with the circle-fitting profile. The finally obtained average value of contact angle was 119° (> 90°), showing the hydrophobic nature of the surface. The possible reason for this increase in contact angle can be attributed to surface area/topography (with Ra = 1.08 µm) and surface chemistry (as a result of oxidation) [64]. Therefore, the dominant factor out of the two needs to be investigated in subsequent studies. Furthermore, Ra > 1 µm and the observed hydrophobicity (contact angle > 90°) on the modified surface possessing considerable water repellency might suggest a non-homogeneous wetting of this surface, which might result in poor cell attachment [63]. However, the same might also suggest substantially enhanced corrosion resistance [65], inhibition towards the adhesion of bacterial cells and blood platelet, less hemolysis ratio, and augmented cell toxicity [66]. Consequently, the same might exhibit considerably improved biocompatibility through the presence of amino groups for successive biomolecule grafting [67]. Thus, this surface might qualify to be adopted in clinical applications. However, at least a subsequent corrosion test would need to be conducted to confirm the actual water repellency of this surface in future studies.

Study of the used tool electrodes

The electrical conductivity investigation results showed an improvement of 6.4% and 2.8% for CTCTE and CTBTE, respectively. Uniform sparking was noticed when machining was performed using CTCTE and CTBTE. CTCTE was found to have better performance than the CTBTE as copper has relatively better thermal conductivity, melting point, and strength than brass. The electrical conductivity of the tool electrode is significantly enhanced through cryogenic treatment since it weakens the thermal vibration of the atoms in metal which in turn results in easy movement of the electrons [24]. This finding also agreed with the previous works that reported better performance of copper as a tool electrode material [13, 14, 18]. The high thermal conductivity of EDM tool electrodes resulted in less wear as it accelerates the heat transfer. Furthermore, the enhanced electrical conductivity resulted in improved thermal conductivity of the tool electrode, which reduced the generation of local temperature by accelerating the heat transfer [68]. Thus, the cryogenically-treated tool electrodes also have relatively high wear resistance. In this regard, Fig. 15(a)–(d) represent the FE-SEM and digital microscopic images (DMI) of UCTE, CTCTE, UBTE, and CTBTE. Since the tool wear and melting took place differently in each case (due to variation in the surrounding conditions), to obtain the best view of each of them (displaying the maximum deformation), these images were captured at different magnifications. The microscopic images confirm more wear to UCTE and UBTE than CTCTE and CTBTE. The higher melting temperatures of the cryogenically-treated tool electrodes render them to retain their shape during machining [24, 68]. Moreover, due to the pyrolysis of dielectric, more particles migrated from the UCTE and UBTE than CTCTE and CTBTE [27]. The microscopic images also suggested that shape retention of CTCTE and CTBTE was not significantly affected when compared with UCTE and UBTE. The accumulation of less carbon content on the µEDMed surface is a favorable condition obtained by the cryogenically-treated tool electrodes. Thus, the wear and damage to the CTCTE were less as compared to CTBTE. A large amount of carbon content increases the brittleness of the machined surface [45]. This promotes micro-crack development on the machined surface in contact with the host tissue when the machined part is used as a biomedical implant. This also results in damage to the host tissue, cytotoxicity, corrosion promotion, and implant failure [26, 29]. The dimensional accuracy of the machined hole was primarily affected because of tool wear. Moreover, as the machining depth increases, the elimination of debris particles is detrimentally affected, resulting in unstable spark and short-circuiting, followed by the building up of a nonuniform and thick recast layer. Hence, the overall machining performance deteriorated, increasing the machining-time and dimensional-inaccuracy.

Fig. 15.

FE-SEM and DMI of the different tool electrodes a UCTE (at 250 × magnification), b CTCTE (at 250 × magnification), c UBTE (at 300 × magnification), and d CTBTE (at 350 × magnification)

However, the main reason for achieving better dimensional accuracy with cryogenically-treated tool electrodes was its effective grain refinement [22, 24, 45, 47]. Therefore, FE-SEM analysis was further conducted to assess the variation in the particle size as (high magnification at 25kX) shown in Fig. 16. The smaller particles size in UCTE and CTCTE can be clearly visualized. The grain refinement caused by the cryogenic treatment can also be predicted. Each tool electrode was scanned using XRD. The Debye–Scherrer equation, as given in Eq. 1 [69], was used for measuring the average size of crystals of each tool electrode.

$$D=\frac{0.89\mathrm{\lambda }}{\beta \mathrm{Cos}\theta }$$ 1

Fig. 16.

High-magnification FE-SEM images of a UCTE, b CTCTE, c UTBE, and d CTBTE demonstrating the variation in crystal size at various locations

Here, λ represents the wavelength of the x-ray applied, β-FWHM represents radians at the scale of 2 $\theta$, $\theta$ represents the Bragg angle, and D represents the average size of the crystal (in nm). The measured values of D for each tool electrode and the percentage decrease are shown in Table 7.

Table 7.

Average crystallite size

Tool electrode	D (nm)	Percentage decrease (%)
UCTE	36	14
CTCTE	31
UBTE	37	30
CTBTE	26

The percentage decrease in the average size of the crystal was 14% and 30% in the case of CTE and BTE. These outcomes substantiate the finding of FE-SEM analysis as shown in Fig. 16. These findings also support the findings of previous work [24].

Conclusions and scope

The experimental investigation revealed the following major findings:

• The machining-time obtained with CTCTE was the least compared to other tool electrodes. Also, the hole obtained by CTCTE showed better dimensional-accuracy and surface integrity in comparison to the hole machined by UCTE.

• The machined surface obtained by UCTE showed the maximum micro-cracks compared to the surface machined by CTCTE, whereas the machined surface obtained by CTCTE displayed the least number of micro-cracks coupled with uniform deposition of oxide content.

• The recast layer achieved by CTCTE demonstrated favorable changes in surface micro-structure, composition, roughness, wettability, micro-hardness, and corrosion resistance.

• The micro-hardness of the µEDMed surface obtained by UCTE and CTCTE was improved by 29.24 and 17.45%, respectively.

• The improved micro-hardness of the machined surfaces obtained by UCTE and CTCTE indicated that the corrosion resistance or biodegradation of the machined surface was considerably improved, which was further confirmed by the subsequent corrosion test of these surfaces.

• Cryogenic treatment induced noticeable improvements in electrical, wear, and metallurgical properties of the cryogenically-treated tool electrodes.

• The presence of ZnO, detected in the X-ray diffraction of the modified surface, might indicate an anti-viral action against coronavirus (COVID-19). However, the same needs to be biologically and therapeutically validated.

• Although the modified surface displayed several potential findings to qualify for biomedical implant applications such as a medical stent, the subsequent future studies will need to conduct the necessary in vitro and in vivo cell viability, adhesion, and proliferation investigations to substantiate the claims.

Author contribution

Rahul Davis: experimental study, designing of methodology, original draft, and review.

Abhishek Singh: resources; supervision; validation; roles/writing—original draft; review.

Kishore Debnath: execution of experimental runs.

Anup Kumar Keshri: supervision, review, and validation.

Paulo Soares: conceptualization, formal analysis, investigation, methodology.

Luciane Sopchenski: analysis of corrosion behavior and writing.

Herman A. Terryn: characterization of the processed surfaces and review.

Ved Prakash: microscopic study and experimental work.

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All the authors declare that this paper has no available data or material.

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Competing interests

The authors declare no competing interests.