Gurmider Singh ,Sunpreet Singh ,Chnder Prksh ,Seerm Rmkrishn

a SIMAP,University of Grenoble Alpes,France

b Department of Mechanical Engineering,National University of Singapore,Singapore

c School of Mechanical Engineering,Lovely Professional University,Phagwara,Punjab 144411,India

Abstract In the present study,a novel method of surface finis improvement is proposed using shot blasting of soda lime (SBSL) beads on the Mg-AZ31 alloy.The effect of the soda blasting process parameters,such as blast pressure,stand-off distance,and blast duration,have been studied in-response of material removal rate (MRR) and surface roughness (SR) and corresponding statistical models have been obtained.The multi-objective optimization has also been performed to obtain parameters for maximum MRR and minimum SR.The corrosion behavior of the treated specimens has been performed to study their in-vitro biodegradability in simulated body flui (SBF) for 1,3,7,10,15,and 21 days.The wettability study of the SBSL treated samples has been investigated using sessile drop methodology.Further,cell adhesion test has also been performed to study the biocompatibility characteristics of the SBSL treated samples using Huh7 liver cell lines.Based on obtained quantitative data as well as scanning electron microscopy analysis of treated samples,the SBSL treatment of the AZ31 alloy has been found highly useful in producing biocompatibility surfaces along with desirable morphological features.

Keywords: AZ31;Soda-lime;Surface roughness;Material removal rate;Corrosion;Wettability;Biocompatibility.

1.Introduction

From the last two decades,metallic implants have been extensively used for the replacement or the regeneration of the damages human hard tissue[1].Generally,bio-compatible metals,such as stainless steel,titanium,and cobalt-chromium alloys,have been extensively used as the metallic implants for various orthopedic surgeries.However,these metallic implants suffer from critical issues such as stress shielding,toxic metal ions,weak degradation,and non-compatible mechanical properties,when compared to natural bone [2].Indeed,an alternative class of bio-metals,which gets dissolved inside the human body,is known as biodegradable materials.Three families of biodegradable metals are generally used,namely:iron,zinc and magnesium-based pure metals and alloys.Iron and its alloys possess prolongedin-vitrodegradation rate and undesirably higher mechanical properties that cause localized stress-concentration [3].On the other side,pure zinc and its alloys possess reasonablein-vitrodegradation rate but inferior mechanical properties[4].Commercially pure magnesium(Mg) offers very high biocompatible characteristics and nearto-bone mechanical properties [5],whereas,its rapidin-vitrodegradation should be controlled through surface treatments[6].

Indeed,there exist numerous surface treatment technologies.However,the selection of the one best suitable should be made based on the finall produced surface morphology as defect,including cracks,porous holes,and poor surface roughness,can fail the implant.Commercially,there exist numerous technologies for developing orthopedic implants,for example,investment casting [7-9],powder metallurgy[10-12],machining [13-15],welding [16-19] and additive manufacturing[20-25].However,most of the time,it is essential to adopt a post-treatment to reduce the as-produced surface defects,mainly concerning the surface roughness,cracks and inclusion of foreign elements,and poor biocompatibility.Since any developed implant would be directly exposed to the human blood and bone tissues;hence,it is supposed not to cause any kind of toxic effect,abnormal degradation,localized stresses.It has been found that the undesirable features of the implant can change DNA of the genome,specificall genotoxicity,and damage of the host tissues [26,27].In particular,the surface properties influenc the development of good osseointegration,including surface tension,surface energy and surface roughness.The surface energy usually based on surface composition,surface topography and its roughness[25].

There are many modern surface treatment techniques have been developed to enhance the surface properties of the fabricated components such as heat treatment,laser treatment,plasma spraying,and hot isostatic processing [28-30].Particularly concerning the topographic features,the bombarding of micro-shot beads,ranging from 2 to 60μm,with controlled manually/automated process controls can also lead to a dramatic change in the improvement of surface properties [31].As referred to shot blasting or shot peening,these treatments are basically in use from the ancient times by blacksmiths and sword makes for the improvement in the toughness of the cutting edges for the different tools and weapons [32].The micro blasting process-induced compressive residual stresses on the surface of the fabricated parts,which prevents propagation of cracks due to internal tensile stresses and improve the fatigue life [33].Not only this technique is used to improve the fatigue properties,but it is also used for surface modification It can be used to increase the surface finis of the rough surface [34] and or to rough the surface by beads bombarding on a fla surface [35].Kennedy et al.[36] used the shot blasting treatment to improve the surface finish hardness,and tool life of the high-speed steel turning tooltip.Singh and Pandey [37] used glass beads to improve the surface finis of the copper parts and to reduce the surface porosity after the sintering.Persenot et al.[38]used bombarding surface treatment to enhance the surface finis and fatigue life of the titanium alloy fabricated by electron beam melting.They reported improvement in surface roughness more than 64% with surface treatment.Gillstrom and Jarl [39] improved the surface roughness of the steel rods with the help of shot blasting treatment.Almangour and Yang[40]reported surface roughness improvement of more than 25% in the fabricated stainless steel by additive manufacturing.However,the shot blasting process highly depends upon the beads in the form of cutting raw materials.It could be metal,ceramic,glass,and abrasives.Many successful studies are indicating significant improvements in the surface topography of magnesium(Mg) alloys through shot blasting approach,for instance[41].Most of these studies are focused on investigating the corrosion behavior,biodegradability,and fatigue-based characteristics [42,43].

In the light of available facts,it has been concluded that,although,Mg alloys are highly recommended for the fabrication of implant,yet,the surface treatment of finall produced such implants is essential.The benefit of the shot blasting method were acknowledged and investigated a novel shot blasting of soda lime (SBSL) technique,helpful not only improving the surface finis but also biological features.Soda-lime glass beads have been used as the blasting media owing to its established candidacy in the biomedical applications [44-46].In the present study,the effects of SBSL process parameters on the obtained topological features of Mg-based AZ31 alloy has been studied,and statistical models for the same have been developed.Furthermore,the biological features of the treated AZ31 alloy have also been characterized following the standardized simulated body fluid-base biodegradability,wettability,and cell adhesion tests.The SBSL treated and un-treated AZ31 samples have been compared,through obtained quantitative observations and scanning electron microscopy(SEM)based analysis,for establishing the facts which made observable characteristic differences.The overall results indicated that SBSL treatment of AZ31 alloy had induced beneficia differences which advocated their potential candidature for biomedical applications.

2.Materials and methods

The as-casted magnesium (Mg) AZ31 alloy ingot (procured from Sebaas Magnesium Alloys,India) has been employed as the workpiece material.The ingot was cut into 30×30×10mm3substrates for the SBSL(ref.Fig.1(a)).The XRD spectrum of the workpiece is shown in Fig.1(b).The initial surface roughness of the workpiece was calculated as 6.23±0.86μm.The micron-size soda-lime glass beads were used as the shot blasting material (ref.Fig.1(c) and (d)).The in-house (School of Mechanical Engineering.Lovely Professional University,Phagwara,India) fabricated shot blasting setup was used for the finishin treatment.The schematic diagram of the experimental setup is shown in Fig.2.The beads were placed in the hopper,and air compressor was used to carry the beads with air pressure from the hopper to nozzle gun by pipes.The workpiece was placed over the substrate and under the nozzle gun.The mixture of compressed air and beads were shot blasted over the rough surface of the workpiece.The control valve was used to adjust the compressor pressure.Manual adjustments varied the stand-off distance(distance between the workpiece and nozzle).The filte was used to filte the unused glass beads and to reuse the beads for the shot blasting.

Fig.1.(a) Magnesium ingot sample,(b) XRD of raw material,(c) soda lime beads and (d) SEM of soda-lime beads.

Fig.2.Schematic diagram of the experimental setup of SBSL.

The design of experiment (DOE),based on Taguchi L9 orthogonal array,was used to study the effect of shot blasting processing parameters (such as the pressure of blasting,stand-off distance and duration of the blast) on the material removal rate (MRR) and surface roughness (SR) of the treated substrates.The weight of the substrates,before and after treatment,were measured using a digital weighing balance (accuracy 0.001mg) for the calculation of MRR.The SR of SBSL treated AZ31 substrates have been measured using surface profilomete (model:NanoMAp 1000 WLI).Further,analysis of variance (ANOVA) was carried out to obtain the statistical regression model with significan parameters contributions.The multi-objective optimization based genetic algorithm was used to optimize the experimental parameters to minimize the surface roughness and maximize the material removal rate.Further,the optimized parameters were used for multi experiments to validate the prediction.The SEM equipped EDS mapping of the treated substrates has been performed to analyze the deposited residues of soda-lime after SBSL treatment.

The simulated body flui (SBF) at 37°C was used for the static immersion test on the treated and untreated sample as per the ASTM G31-72 standard.The SBF solution was prepared with 7.4pH value as per the following composition:7.996g/l NaCl,0.350g/l NaHCO3,0.224g/l KCl,0.228g/l K2HPO4·3H2O,0.350g/l MgCl2·6H2O,40g/l 1 kmol/m3HCL,0.278g/l CaCl2,0.071g/l Na2SO4and 6.057g/l (CH2OH)3CNH2[47].The weight of the samples was measure before and after the corrosion period using a weighing balance (Shimadzu,Japan).The treated and untreated samples were immersed in 150mlof the prepared SBF solution for 1,3,7,10,15 and 21 days to measures the weight changes and pH value variation.The SBF medium was replaced in every 3rd day.To check the weight significantl,the samples after corrosion was firs gently rinsed using distilled water and ethanol,and further,the samples were allowed to dry at room temperature for one hour.The removed corrosion product from the samples was obtained by immersing the corroded samples for 10min in the solution of 595g/l HCl and 3.5g/l hexamethylene tetraamine.The pH meter,with an accuracy of 0.001,was used to monitor the pH of the SBF continuously.The Dinolite camera was used to capture the behavior of the surface of the samples after the corrosion.Also,the EDX and SEM analysis were carried out to characterize the composition and morphology of the degradation products.The plasma mass spectrometry (ICP-MS) was used to analysis the ion release concentration in the SBF solution at different time points.

Fig.3.(a) Un-treated specimens and (b) SBSL treated specimens.

The contact angle measurement was used to test the wettability of the treated and untreated samples.The deionized water drop was exposed on the surface for the contact angle measurement using drop shape analyzer (DSA 100,Germany) apparatus based on the sessile drop method.The highresolution based CCD camera employed to capture the images and analyzed by apparatus software.The software was based on the tangent method to measure the contact angle after settling of the drop.

The different measurement was taken before and after corrosion periods.The measurement data was taken fi e times to calculate the average and standard deviation.The cell adhesion test was performed to check the effect of shot blasting treatment on the biocompatibility study of the samples.First,the samples were kept in the low adherence 48 walls plated and sterilized by the ultra-violet light for 45 min.The 1×105Huh7 cells (liver cell lines) were seeded on the surface of the treated and -untreated samples and allowed to adhere for 2 h with 5%CO2supply at 37°C and saturated humidity.After the cells adhered,500 μL of complete media containing DMEM-F12 with 10% fetal bovine serum and 1% antibiotics (Penicillin-Steptomycin) was added in each well carefully,and the plates were kept in 5% CO2incubator (Sanyo,Japan) at 37°C for another 48 h.Further,samples were fi ed and observed with SEM after the cells had attached and proliferated for 1,3,and 7 days.Morphology observations were conducted on scanning electron microscope apparatus (SEM,Zeiss,Germany) at an accelerating voltage of 15kV.

3.Results and discussion

3.1.Effect of input parameters on SR and MRR

The pictorial view of the SBSL treated and untreated substrates are shown in Fig.3.The shine patches can be visualized on the surfaces of the SBSL specimens.Taguchi L9 based design of experiments was used to design the set of experiments to study the effect of the pressure of blasting,stand-off distance and duration of blast on the MRR and SR.The parameters range selected for the experimentation is given in Table 1.

Fig.4.Response of input process parameters for SR.

The compressor valve controlled the pressure of blasting,and maximum pressure was selected as 0.3MPa.The surface roughness was increased beyond the maximum pressure value available due to the high-intensity bombarding of soda-lime beads that caused dimples on the work surface.Similarly,the maximum level of the stand-off distance selected is 15mm.The maximum MRR and minimum SR were observed at the maximum stand-off distance value selected,and any further increase in the distance produced insignifican effect during preliminary experiments.The experimental runs as per the Taguchi L9 and output responses are shown in Table 2,respectively.

Table 1 Process parameters ranges and levels.

Table 2 Set of experiments and responses value.

Table 3 ANOVA for SR.

Fig.4 shows the main effect plot of input process variables on SR.The SR of the treated substrates was reduced with an increase in the blasting pressure from 0.1 to 0.3MPa.This is mainly because as the pressure increased the resulting kinetic energy,carried by,of the soda-lime beads increased and,therefore,produced a more significan impact on the substrate’s surface [35,48].This means that the surface irregularities of the substrate surface have been efficientl reduced.Further from SEM morphologies and roughness profile (ref.Fig.5),it can be seen that the surface profil had become very fin when the pressure was 0.3MPa as compared to the treatment done at 0.1MPa.The high peaks were observed in the surface profil with low blast pressure as compared to high blast pressure.Singh and Pandey [37] also found similar improvement in the surface roughness in terms of optical surface profile on the copper samples due to shot blasting treatment.

Generally,the shot blasting treatment induced the compressive stresses on the surface of the workpiece and prevent the surface from cracks.The rough surface has dimples in terms of deeper notches from where the crack can be propagated due to tensile stress concentration at rough notch points.These cracks mainly reduce the cross-sectionof the material and fail to support the loads.Therefore,the shot blasting treatment induce the compressive stresses at the surface and improve the surface roughness,which directly enhances the fatigue of the material [36].Apart from removing the surface irregularities,the soda-lime beads have also been embedded on the specimens treated surface at the highest level of the available range of pressure,and thus changed the surface chemistry of AZ31 alloy.To visualize the same,SEM equipped EDS mapping (refer Fig.6) has been carried out,indicating the presence of Mg,Ca,and Na traces.

Fig.5.SEM morphology and surface profil of SBSL treated AZ31 substrate at (a) 0.3MPa and (b) 0.1MPa.

Fig.6.SEM-equipped EDS mapping of SBSL treated AZ31 alloy substrate at 0.3MPa.

Fig.7.SEM-equipped EDS mapping of SBSL treated AZ31 alloy substrate at 15mm of stand-off distance.

In the case of stand-off distance,the SR of the substrates was drastically reduced by increasing the stand-off distance from 5 to 15mm.There can be two possible reasons behind this trend:(i) the soda-lime beads,in the can of short standoff distance,were unable to obtain the required kinetic energy owing to the inculcation of limited inertia and (ii) the beads were packed tightly between the nozzle and exposed surface and inter-collision of the same has changed the trajectory of the fl w.In the case of stand-off distance of 15mm,the SEMequipped EDS mapping,refer Fig.7,the significan traces of Ca and Na were also observed.This means that when the stand of distance was 15mm,the tendency of the soda-lime beads was high to get embedded inside the AZ31 alloy.However,when compared with Fig.6,it can be seen that the distribution of Ca and Na particles was less dense.Further,the SR of the substrates was reduced with an increase in the duration of blasting from 30 to 90s.This is mainly because the surface exposed to the higher duration of the time.Therefore,the substrate’s surface was machined better than the others.Indeed,the beads were able to fla the surface more significantly at the low range of blast time,which resulted in a low decrement in the surface roughness value with increasing blast time.

Fig.8.Contribution of input parameters for SR.

The analysis of variance (ANOVA) was used to analyze the experimental data and to obtain the statistical model.An ANOVA result for the SR is shown in Table 3.It can be seen from Table 3 that the input process parameters,such as pressure and stand-off distance,and the regression model are statistically significan as their respectivePvalue<0.05.However,the blast duration is insignifican for the SR (P-value>0.05).The percentage contribution of the input parameters for SR is given in Fig.8.TheR2value (94.07%) of the response predicted less randomness in the experiments.Eq.(1) represents the statistical model obtained after the regression analysis.A few sets of confirmatio experiments validated the obtained statistical model,and responses were obtained to nearby the predicted response.

Table 4 ANOVA table for MRR.

In the case of MRR,Fig.9 shows the responses of selected input process parameters.The MRR of the AZ31 alloy was increased with an increase in the blasting pressure.This is mainly because at higher levels of blasting pressure,sodalime beads have hit the exposed surface with higher kinetic energy and resulted in higher MRR.Indeed,the pressure at which the soda-lime particles have been released is the critical parameter in this case.The bombarded beads have sufficien intensity to cause the fracture of the peaks at the surface.In the case of stand-off,a trend similar to SR can be seen.

The MRR of the AZ31 alloy has been improved when the stand-off distance between the nozzle and surface is 15mm.The main credit goes to the un-interrupted and streamlined fl w of the soda-lime beads along the define trajectory of the motion.Since the beads flyin within the 15mm of stand-off distance got sufficien room as well time to excel the inertia force suitable for causing the impactful fracture.At last,in case of blast duration,it can be depicted that MRR has increased with duration of blast,maximum at 90s.However,the steepness of the trend indicates that it is of little importance.

Fig.9.Response of input process parameters for MRR.

Fig.10.Contribution of input parameters for MRR.

The ANOVA result for MRR is shown in Table 4.Similar to SR,input process parameters,such as pressure and standoff distance,and a regression model is statistically significan as their respectivePvalue<0.05.However,the blast duration is insignifican for the MRR (P-value>0.05).The percentage contribution of the input parameters for MRR is given in Fig.10.TheR2value (88.20%) of the predicted response has low randomness in the experiments.Further,Eq.(2) represents the statistical model obtained after the regression analysis.

Table 5 Optimization parameters along with optimized response values.

3.1.1.Multi-objective optimization

The multi-objective optimization based genetic algorithm was used to minimize the SR and maximize the MRR using MATLAB optimization toolbox.For instance,as the toolbox minimizes the objective function,the reciprocal of the MRR was used for the maximization.The constraints for the ranges of the parameters are given below in the following equation:

Table 5 shows the optimized parametric levels and the resulting statistical and experimental values.This table represents the optimized parameters and correspondingly the statistically predicted value.Some experiments were performed at the optimized parameters for the validation of the results.The experimental results were found to be approximate near the statistical model predicted values.After SBSL treatment,the micro-hardness of AZ31 substrate was increased by 19.5%.The micro-hardness was measured before and after SBSL treatment 73.8 HV and 91.75 HV.The grain size of the top layer of AZ31 was refined which improved the mechanical properties of the surface.

3.2.In-vitro biodegradability

Generally,the immersion test in simulated body flui(SBF) is preferred for the evaluation ofin-vitrobiodegradable and bioactive characteristics of metallic implants.This characterization provides in-depth information on long-term corrosion resistance in respect of corrosion rate,pH value variation,and changes in surface morphology.Therefore,in-vitrocorrosion test was performed with SBF as the environment to examine the biodegradable behavior of SBSL treated samples at optimized parameters as well as the untreated samples.Usually,the corrosion reaction between the Mg alloy and the physiological condition,for example,SBF,leads to the mass loss of workpiece and pH variation of the SBF.The reactions act for the corrosion mechanism of Mg is given below:

The static immersion test results in the SBF environment shows that the SBSL untreated and treated samples exhibit different biodegradation behavior.The corrosion rate,variation in pH value and Mg ion release concentration after various periods of immersion is shown in Fig.11.Dinolite and SEM images (ref.Fig.12) were taken to determine the corrosion behavior on the morphology of the surface after different corrosion periods.The corrosion rate was obtained to be higher for un-treated as compared to shot blasting treated samples (reg.Fig.11(a)).The corrosion rate of un-treated sample gradually increased up to within the firs 3 days of immersion test.The scratches in the form of rough roughness were indicated from the surface of the un-treated sample at 0th day of immersion (ref.Fig.12(a)).The pitting corrosion in the form of holes at the rough surface was obtained at 1st day and 3rd day of immersion,which rapidly increase the corrosion rate.The white layer was observed on the untreated surface on 3rd day of immersion (ref.Fig.12(c)),which indicates the formation of Mg(OH)2layer.The EDX analysis was carried out at the white layer for the confirmatio (ref.Table 6).Therefore,the formed Mg(OH)2layer on the surface of the sample slows down the reaction.Correspondingly,it slowed down the degradation rate with the extension of the immersion time and started increasing gradually after the 10th day of immersion.It might be due to the breakage of Mg(OH)2layer in the form of cracks,which were observed on the 15th day of immersion (ref.Fig.12(e)).The overall surface of the sample was loosened and indicating the complete corrosion of the alloy at 21st day of immersion (ref.Fig.12(f)).

Table 6 Details of the EDX analysis of untreated and treated samples after different immersion days (ID).

Fig.11.Immersion test results for un-treated and treated sample:(a) corrosion rate,(b) pH variation after different time of immersion time,and (c) Mg ion released concentration during different immersion time.

The severe corrosion reaction in terms of attack was suffered by the sample surface in different directions and caused pits in the form of holes at the surface.Fig.11(a) depicts the corrosion rate for the shot blasted treated sample.The corrosion rate was found to increase at a prolonged rate and even constant up to 3rd day of immersion.In the shot blasted samples,there are different reasons for the slow carrion rate.The shot-blasted treatment was carried out at the optimized parameters to obtain minimum surface roughness.The fin surface roughness as compared to the untreated samples was one of the reasons for the slow degradation rate.Fig.8(g) indicates the fla and fin surface finishe samples Dinolite and SEM image.A similar image was observed for the treated sample at day 3 of immersion (ref.Fig.12(h)),which indicated the slow corrosion of the surface.The smooth surface resulted in no local corrosion in the form of pitting [6].The formation of Mg(OH)2is likely to be different with the fin surface finish The layer formation would be higher on the fla surface of the alloy as compared to the rough surface,which was also observed from the EDX analysis of a treated sample at day 3 of immersion (ref.Table 6).However,after the 10th day of immersion,the corrosion rate was increased up to the 21st day of immersion.The low-density pitting corrosion was observed for the 15th day of corrosion,which increases the corrosion rate.Alvarez et al.[49] also found some less dense pitting corrosion on the fin surface of the AZ31 alloy.Moreover,in the present study,the residue of the soda-lime beads such as calcium oxide might form calcium ions which can breaks down the passive layer on the Mg and can cause pitting corrosion [50].

Fig.12.Dinolite and SEM images of un-treated samples after different periods of corrosion (a) 0,(b) 1,(c) 3,(d) 7,(e) 15,(f) 21 days and shot blasting treated samples after different periods of corrosion (g) 0,(h) 1,(i) 3,(j) 7,(k) 15,(l) 21 days.

The low-density breakage of the magnesium hydroxide layer caused the pitting corrosion,which can be observed by SEM images in Fig.8(i) and (j).Hence,the corrosion rate increased slightly after the 15th day of immersion of shot basting treated sample.Also,the other reason for the low corrosion rate in case of shot blasting treated samples might be due to the hard surface as compared to untreated samples.The shot-blasted beads induced compressive residual stresses on the surface and hardened the surface[14].The hard surface undergoes less pitting corrosion and depicts a low corrosion rate.The results were in-line with the study by Hamu et al.[51].Fig.11(b) depicts the change in pH values of the SBF after a different period of immersion test.It was observed that there was not much difference change in the pH and maximum value of pH reached at 7.88 after 21 day of immersion.However,the pH value changes were less in the case of treated samples.It was due to less pitting corrosion on the surface of the treated sample during immersion test.Fig.7(c)also depicts the Mg ion centration release in the SBF flui during the immersion test.With a high corrosion rate,the untreated samples possessed high Mg ion concentration release as compared to treated sample for different periods of immersion.The ion concentration release for the untreated samples was found to increase gradually.However,owing to higher hardness by shot blasting and strong magnesium hydroxide layer formation due to fin surface could act as a barrier and prevent the fast corrosion of the Mg alloy.Hence,the less release of Mg ion concentration was observed for the shot blasting treated sample.

3.3.Wettability and cell culture

Fig.13 represents the wettability behavior of the untreated and the treated samples after the different period of immersion.The contact angle was measured at different three locations,and the average was taken with standard deviation.The variation in the form of standard deviation can be due to the slight variation or local variation in the form of surface roughness and pitting corrosion.The data was captured after the 5s of impingement of water droplets at the surface of the samples.Also,the samples were cleaned gently with hot water to remove the residues for the exact evaluation of the contact angle.The shot blasting treated sample possessed a higher contact angle as compared to the untreated sample before immersion (0th day of immersion).It was due to the higher surface finis of the treated sample.The fin surface finis does not allow to ultimately settle down the drop on the surface [52].

Fig.13.Wettability analysis of untreated and treated samples after the different immersion period.

Also,some residue might be attached to the sample surface after shot blasting of soda-lime beads in the form of sodium,calcium or their oxides.The sodium oxide content decreased the contact angle due to thin layer formation [53].However,as the immersion period increased,the contact angle for both untreated and treated samples was obtained to be decreased.The decrement in the contact angle in the case of untreated samples was higher as compared to the treated samples.The reason was directly related to the corrosion rate.With an increase in corrosion exposure due to rough surface finis in untreated samples,the uniformity of the droplet was gradually changed.The high corrosion rate specimen possessed higher pitting corrosion which directly increased the non-uniformity of the water droplet.Also,with higher pitting corrosion,the surface possessed rough surface finis and decreased the contact angle.The results were inline with the study carried out by Sajid and Kiran [54].Therefore,with no pitting corrosion,up to 3 days of immersion of treated samples,less change in the contact angle was observed.However,as discussed above,low density pitting corrosion was observed after the 10th day of immersion in SBSL treated sample.The low-density pitting corrosion increased the roughness of the sample and reduced the contact angle.

Fig.14.SEM images of cell adhesion analysis of 1,3,and 5 days for (a)-(c) un-treated and (d)-(f) treated samples.

Fig.14 depicts the cell adhesion behavior of the un-treated and SBSL treated samples for bio-compatibility analysis.The morphology depicts better cell adhesion in the case of untreated samples as compared to shot blasting treated sample after 1st day.It was due to the rough surface,as discussed above.The rough surface provides beneficia mechanical locking for the initial stage of cell adhesion [55].Therefore,with high surface roughness,un-treated samples possessed better cell grows.The cells were observed to proliferate for the next three and fi e days.It could also be due to the higher hydrophilic,as discussed above.With a lower contact angle,cells adhere and grow faster in the untreated sample.Similar results of cell adhesion concerning surface roughness were published by different authors [56,57].

However,it is worth to notify that the primary objective of the present study was to propose a new method for the finishin with soda lime shot blasting to improve surface roughness and corrosion rate.But,the biocompatibility of the surface must be retained.The cells were found to adhere on the surface of the shot blasting treated samples.The low cell adhesion might be due to a fin surface finish However,due to some residue of the soda-lime such as sodium,and calcium oxide,the cell adhesion growth was rapidly increased as observed on the 5th day after the cell adhesion(ref.Fig.14(f)).

4.Conclusion

The conclusion of the present study is drawn in the following points:

·It has been found that the proposed SBSL technique for the treatment of AZ31 alloy is highly beneficia in improvements to surface properties.The proposed method of shot blasting using soda lime glass was able to improve the surface roughness of the as-cast EZ31 alloy.The blast pressure and stand-off distance have been found as the dominating parameters for both MRR and SR.The optimized parameters with blast pressure -0.3MPa,stand-off distance -13.62mm and blast duration -61.04s was obtained using a multi-objective optimization technique for maximum MRR (0.132g/s) and minimum SR (1.98μm).

·The corrosion rate after 21st day of the shot blasting treated sample was found to be significantl less (2.9mg cm-1d-1) as compared to the untreated samples (6.1mg cm-1d-1) due to better surface finis and hardened surface.Due to SBSL treatment,the surface microhardness of the sample is increased by 19.5%,which prevents the surface from degradation/corrosion.Moreover,a Ca-rich layer was developed SBSL treatment having low surface roughness value 1.98μm,which prevented the surface also from degradation and improved the corrosion resistance.

·The contact angle has been found to decrease with increasing the corrosion period time.However,the shot blasting treated samples results in a higher contact angle as compared to untreated samples.

·The un-treated samples have been resulted initially better for cell adhesion on the surface due to rough surface.However,the shot blasting treated samples also possessed excellent cell adhesion and growth characteristics on 3rd and 5th day of cell implantation.

Hence,the shot blasting with soda lime could be used in the medical industry for the improvement of surface roughness,corrosion and other surface characteristics of the fabricated metal implants.

Declaration of Competing Interest

The authors declare that they have no known competing financia interests or personal relationships that could have appeared to influenc the work reported in this paper.