FACTA UNIVERSITATIS Series: Mechanical Engineering Vol. 17, N o 3, 2019, pp. 445 - 454 https://doi.org/10.22190/FUME190510040S © 2019 by University of Niš, Serbia | Creative Commons License: CC BY-NC-ND Original scientific paper SURFACE MORPHOLOGY AND MICROHARDNESS BEHAVIOR OF 316L IN HAP-PMEDM Gurpreet Singh 1 , Yubraj Lamichhane 2 , Amandeep Singh Bhui 2 , Sarabjeet Singh Sidhu 1 , Preetkanwal Singh Bains 1 , Prabin Mukhiya 2 1 Mechanical Engineering Dept., Beant College of Engineering and Technology, India 2 Mechanical Engineering Dept., Amritsar College of Engineering and Technology, India Abstract. The development of biomaterials for implants nowadays requires materials with superior mechanical and physical properties for enhanced osseointegration and sustained longevity. This research work was conducted to investigate the influence of nano hydroxyapatite (HAp) powder mixed electrical discharge machining (PMEDM) on surface morphology and microhardness of modified 316L stainless steel surface. The chosen process parameters were discharge current, pulse on/off duration and gap voltage in order to analyze the selected output responses. HAp concentration (15 g/l) along with reverse polarity was kept constant for current experimentation. The experimental results testified that surface morphology of PMEDM surface was significantly improved along with augmentation of 79% in microhardness (HV) of HAp modified surface of medical grade stainless steel. Furthermore, XRD and SEM characterization confirmed the deposition of calcium, phosphorous and inter-metallic compounds on HA-PMEDMed surface. The surface thus produced is expected to facilitate better bone-implant adhesion and bioactivity. Key Words: PMEDM, 316L SS, Copper Tool, HAp Powder, Microhardness, Bioactivity 1. INTRODUCTION Biomaterials are artificial organs or implants preferably from the family of ceramics, polymeric and metallic biomaterials (titanium alloys, stainless steel 316L and Cr-Co alloys) for the substitution of the damaged human body organ. Prominent properties such as cell proliferation, bone-implant adhesion, osseointegration, corrosion and wear behavior in fluidic environment within the individual primarily affect the acceptance or rejection of the selected biomaterials [1, 2]. The demand for orthopedic implants is rising day by day; fascinating the researchers and engineers to improve its surface integrity alongside offering better resistance towards wear and corrosion [3-6]. As a result, implant surface must be modified with Received May 10, 2019 / Accepted August 08, 2019 Corresponding author: Gurpreet Singh Department of Mechanical Engineering, Beant College of Engineering & Technology, Gurdaspur – 143521, Punjab, India E-mail: singh.gurpreet191@gmail.com 446 G. SINGH, Y. LAMICHHANE, A.S. BHUI, S.S. SIDHU, P.S. BAINS, P. MUKHIYA bioactive coating, avoiding the release of harmful ions causing corrosion and loosening of joint due to wear [7, 8]. Among all the established coating techniques, such as sol-gel, dip coating, ion implantation, PVD, CVD, laser coating, etc., the execution of EDM as an alternative to other surface modification methods is still in its beginning era [9]. In this method, bioactive powder is mixed in dielectric medium keeping workpiece as cathode (-) and tool electrode as anode (+) or simply negative polarity [10]. During the process, the produced thermal energy heated up the working area and with the flow of charge from positive to negative, the powder particles strike the surface and solidify in pulse-off duration and thus modify the substrate surface [11]. Different experimentations and techniques were explored by researchers in order to modify the surface characteristics of metallic biomaterials. Hubler et al. [12] studied wear and corrosion resistance of the 316L SS femoral implants by depositing the ceramics thin films of the transition metals nitrides and Ti/N and Cr/V layers. It was found that the coatings significantly improved the surface integrity, microhardness and corrosion resistance of the surface. Kumar et al. [13] investigated the electrochemical behavior and in-vitro bioactivity of polypyrrole/TiO2 ceramic nano-composites on 316L stainless steel. It was found that addition of TiO2 exhibits improved corrosion resistance properties of the substrate and offered better biocompatibility. Microhardness and in-vitro wear behavior of TiO2 treated 316L stainless steel was examined by Singh et al. [14] via electro-discharge treatment. They found that material transfer rate of EDM is appropriate for the surface modification of bare metal. Addition of TiO2 exhibits improved microhardness of 233% and protective efficiency of 80% compared to the substrate material. Chang et al. [15] inspected microhardness, corrosion resistance and protein adsorption properties of CuAlO2 deposition on 316L stainless steel. With the use of nano-indentation, it was found that corrosion resistance and microhardness were significantly enhanced with an increment of 46% after the process. Optimal ED machining parameters were investigated by Bhui et al. [16] for the surface modification of Ti6Al4V with graphite tool. Deposition of bioactive layer and formation of intermetallic compounds were examined using SEM and XRD respectively. Additionally, apatite growth was observed on machined sample confirming the bioactivity through SEM and EDS analysis. Harun et al. [17] in their study deeply reviewed the application of hydroxyapatite coatings for metallic biomaterials. It was found that hydroxyapatite powder dominantly improves the surface adhesion strength, biocompatibility and corrosion resistance of the metallic biomaterials [18-20]. Based on the literature survey and previous studies, it was seen that the use of EDM for modifying the biomaterials surface is still in its early stages [21, 22]. As there is no need for vacuum chamber or any special arrangement, the use of ED machining for the surface modification is a better choice when compared to other pricy techniques [23-25]. In the present study, stainless steel 316L; a well-known dominating biomaterial in the field of orthopaedics and joint replacement was investigated for modified surface integrities and microhardness in hydroxyapatite (HAp) powder mixed EDM with copper tool. 2. EXPERIMENTAL WORK 2.1. Materials Metallic biomaterial 316L stainless steel was procured as workpiece material for the current experimentation having thermal conductivity 21.5 W/m.k at 500 °C; melting point 1371-1399 °C; and density 7.99 g/cc. Tables 1 and 2 show the elemental composition of 316L SS and hydroxyapatite powder, respectively. A pure copper circular electrode (of 10mm Surface Morphology and Microhardness Behavior of 316L in HAp-PMEDM 447 in diameter) was used to machine the workpiece, whereas the HAp was in nano-size with true density 3.219 g/cm 3 and average particle size of 20-45 nm. Table 1 Chemical composition of 316L SS Element Si C Mn N P Cr Ni Mo S Fe % 0.41 0.01 1.07 0.10 0.02 16.13 10.15 2.05 0.01 Balance Table 2 Composition of hydroxyapatite powder Element Ca5(OH)(PO4)3 Al2O3 Fe2O3 % >99.5 <0.06 <0.02 2.2. Method Design of experiments (DOE) was generated according to Taguchi’s L18 orthogonal array employing mixed level design (2 1 3 4 ) with the help of Minitab-17. Five machining parameters, i.e. dielectric medium (Dm), current (Ip), pulse-on time (P-on), pulse-off time (P- off) and voltage (V), were chosen to vary at three levels (Table 3) for the output responses. Based on the levels of input parameters and experimental design, each experiment was performed on two different plates and an average of both is plotted for the result analysis. Machining time of 30 minutes and reverse polarity were kept constant throughout the experimentation. The following Table 4 illustrates the experimental design based on L18 orthogonal array. Table 3 Input machining parameters Input parameters (Symbol) Units Level 1 (low) Level 2 (medium) Level 3 (high) Dielectric medium (Dm) - Hydrocarbon oil Hydrocarbon oil + HAp - Current (Ip) A 20 24 28 Pulse-on time (P-on) µs 60 90 120 Pulse-off time (P-off) µs 60 90 120 Voltage(V) V 40 60 80 Table 4 Experimental design according on L18 orthogonal array Exp. Run Dielectric medium (Dm) Current (Ip) Pulse-on time (P-on) Pulse-off time (P-off) Voltage (V) 1 Hydrocarbon oil 20 60 60 40 2 Hydrocarbon oil 20 90 90 60 3 Hydrocarbon oil 20 120 120 80 4 Hydrocarbon oil 24 60 60 60 5 Hydrocarbon oil 24 90 90 80 6 Hydrocarbon oil 24 120 120 40 7 Hydrocarbon oil 28 60 90 40 8 Hydrocarbon oil 28 90 120 60 9 Hydrocarbon oil 28 120 60 80 10 Hydrocarbon oil + HAp 20 60 120 80 11 Hydrocarbon oil + HAp 20 90 60 40 12 Hydrocarbon oil + HAp 20 120 90 60 13 Hydrocarbon oil + HAp 24 60 90 80 14 Hydrocarbon oil + HAp 24 90 120 40 15 Hydrocarbon oil + HAp 24 120 60 60 16 Hydrocarbon oil + HAp 28 60 120 60 17 Hydrocarbon oil + HAp 28 90 60 80 18 Hydrocarbon oil + HAp 28 120 90 40 448 G. SINGH, Y. LAMICHHANE, A.S. BHUI, S.S. SIDHU, P.S. BAINS, P. MUKHIYA 2.3. Experimentation Out of total 18 experimental runs, nine were performed in pure medium i.e. hydrocarbon oil in ZNC-EDM (OSCARMAX, S645) dielectric tank itself whereas the following powder mixed trials were conducted in an in-house fabricated dielectric tank of capacity 12 liters. HAp was mixed at 15 g/l to the hydrocarbon oil and continuously circulated using a stirrer and pump to avoid the settling down of powder particles as shown in Fig. 1. Fig. 1 (a) Schematic experimental setup and (b) indigenously developed dielectric tank 2.4. Investigation of machined samples As it is evident from the previous studies [26-28] that biomaterial surface must be porous and must possess bioactive compounds to portray the bioactivity within the individual, the machined samples were investigated for porous microstructure, powder deposition and formation of new compounds using SEM and XRD analysis respectively. Furthermore, Mitutoyo microhardness tester (Fig. 2) with diamond indenter was used to scrutinize the improved hardness of the ED machined specimens. A load of 0.98 N was Fig. 2 Microhardness testing of ED machined samples Surface Morphology and Microhardness Behavior of 316L in HAp-PMEDM 449 applied for a dwell time of 10 seconds to profile a pyramidal imprint on the specimen. Three readings were taken on each machined sample for both the plates and showed in Table 5 (Rep 1 for mean of plate 1 and Rep 2 for mean of plate 2). Prior to measurement, the microhardness of substrate was computed at three different points and an average value of 291.80 HV was noted. 3. RESULTS AND DISCUSSION Based upon the experimentation performed, the following Table 5 demonstrates the output response values and S/N ratios for both the workpiece plates. Further, the output responses were statistically analyzed through ANOVA to evaluate the percentage contribution of each input parameter and subsequently their rank. Table 5 Response table for EDMed 316L stainless steel Exp. Run MRR (mg/min.) S/N ratio (MRR) Microhardness (MH) S/N ratio (MH) Rep 1 Rep 2 Rep 1 Rep 2 1 2.81 2.53 8.4944 386.5 419.9 52.0881 2 4.65 4.01 12.6585 228.2 342.4 48.5804 3 4.84 6.50 14.7914 319.1 406.3 51.0022 4 10.02 10.63 20.2664 347.8 372.7 51.1165 5 5.21 6.12 14.9799 538.9 369.5 52.6890 6 6.32 5.68 15.5259 515.9 612.2 54.9313 7 11.50 11.61 21.2551 475.2 453.6 53.3308 8 13.16 8.84 20.3220 374.3 404.2 51.7853 9 6.44 8.21 17.1055 459.7 581.8 54.1531 10 6.23 6.42 16.0183 819.4 758.7 57.9228 11 4.15 7.19 14.1225 557.8 665.1 55.6268 12 4.36 4.97 13.3213 859.4 770.9 58.1863 13 10.64 8.98 19.7400 636.2 578.3 55.6377 14 10.86 7.13 18.5156 663.1 567.7 55.7048 15 5.31 4.03 13.1408 904.3 796.9 58.5425 16 17.49 20.52 25.4945 904.6 769.8 58.3720 17 6.98 5.67 15.8812 779.5 823.2 58.0668 18 11.80 12.86 21.7952 958.3 796.9 58.7556 (Rep 1 and Rep 2: repetitions of experimentations on two separate plates) Digital weighing machine (Wensar, model: PGB 200) having least count of 0.001g was utilized for measuring the change in workpiece weight after each experimental run for evaluating the material removal rate of 316L SS using the following equation: Initial weight - Final weight MRR= ×1000 mg/min. Machining Time (1) The hardness of biomaterial plays a key role during the cyclic loading on the implanted part particularly in the case of knee and hip joint. For that reason, the signal-to-noise (S/N ratios) analysis for microhardness as well as MRR was calculated according to equation (2) using Taguchi’s criteria for larger-is-better for the current experimentation. 450 G. SINGH, Y. LAMICHHANE, A.S. BHUI, S.S. SIDHU, P.S. BAINS, P. MUKHIYA 2 1 1 1 10 log R iLB i S N R y                 (2) where R is the repetition of responses and yi the value of response at i th trial. 3.1. Analysis of Material Removal Rate Evaluation of MRR is a primary output response during the ED machining of the workpiece material. Minitab-17 was used to analyze the output values from Table 5 for both the workpiece plates in terms of signal-to-noise ratios and percentage contribution of input parameters. Fig. 3 and Table 6 illustrate the main effects plot for S/N ratios and analysis of variance (ANOVA) for MRR of current experimentation. Superior material removal rate (19.01 mg/min.) was witnessed at higher value of peak current (28A) and pulse-on-time (120µs). Current depicts the highest percentage contribution of 52.66% followed by pulse-off (14.28%) and pulse-on (8.99%). Based on the responses, it is discovered that with an increase in the current intensity, the rate of material removal is sharply augmented and similar results can be observed from the S/N ratios plot. Table 6 Analysis of variance for S/N ratios of MRR Source DF Seq SS Adj MS F-value p-value % Contribution Dielectric medium (Dm) 1 8.862 8.862 1.27 0.292 3.10 Ip (A) 2 150.409 75.205 10.81 0.005 * 52.66 P-on (µs) 2 25.686 12.843 1.85 0.219 8.99 P-off (µs) 2 40.784 20.392 2.93 0.111 14.28 Voltage 2 4.241 2.120 0.30 0.746 1.48 Residual Error 8 55.673 6.959 19.49 Total 17 285.656 100.00 * Most significant at 95% confidence level; Rank 1: current; Rank 2: pulse-off; Rank 3: pulse-on Fig. 3 S/N ratios plot for Material Removal Rate Surface Morphology and Microhardness Behavior of 316L in HAp-PMEDM 451 3.2. Analysis of Microhardness Analysis of variance was performed to check the dominance of hydroxyapatite powder and other chosen parameters; associated results for microhardness of EDMed 316L stainless steel surface are shown in Table 7. Superior microhardness of 877.60 HV is illustrated at the utmost values of current intensity (28A) and pulse-on-time (120µs) in the presence of HAp mixed dielectric (trial 18) with an increment of 79% and 160% comparative to the samples machined in hydrocarbon oil and substrate material, respectively. At a higher value of discharge current and pulse-on, the spark generation between tool and workpiece acts more rapidly permitting the deposition of HA powder mixed in the dielectric medium. The breakdown of electrolyte (hydrocarbon oil) also formed intermetallic compounds reacting with substrate elements and facilitates improved hardness. Similar results can be observed from Fig. 4 and Table 7, dielectric medium (% contribution: 74.06%) portray as the most prominent factor directly influencing the microhardness of ED machined 316L SS surface. Table 7 Analysis of variance for S/N ratios of Microhardness Source DF Seq SS Adj MS F-value p-value % Contribution Dielectric medium (Dm) 1 123.447 123.447 59.38 0.000 * 74.06 Ip (A) 2 10.199 5.100 2.45 0.148 6.12 P-on (µs) 2 14.373 7.186 3.46 0.083 8.63 P-off (µs) 2 0.683 0.341 0.16 0.851 0.41 Voltage 2 1.341 0.670 0.32 0.733 0.80 Residual Error 8 16.633 2.079 9.98 Total 17 166.675 100.00 * Most significant at 95% confidence level; Rank 1: dielectric; Rank 2: pulse-on; Rank 3: current Fig. 4 S/N ratios plot for Microhardness 452 G. SINGH, Y. LAMICHHANE, A.S. BHUI, S.S. SIDHU, P.S. BAINS, P. MUKHIYA 3.3. Surface evaluation of HA-PMEDMed 316L stainless steel The machined surface with maximum value of microhardness (trial 18) was further examined for porous structure and deposition of powder particles through scanning electron microscopy. Fig. 6 (b) showed the microstructure of HA powder mixed dielectric depicting porosity in conjunction with surface modification of the substrate surface and deposition of powder particles. Apart from this, sample exhibiting maximum hardness (trial 8) in pure dielectric medium was also examined using SEM (Fig. 6a) and illustrates the cracks, craters on its surface. The modified 316L stainless steel surface in HA powder mixed EDM was then analyzed for the changed elemental composition using XRD technique. Fig. 5 demonstrating the XRD pattern with the existence of various bioactive (calcium, phosphorus, calcium carbonate) and intermetallic (manganese silicide, chromium carbide, manganese silicide carbide) compounds on the HA-PMEDMed surface. As a result, modified surface not only restrict the fluidic reactions but also promotes the bioactivity offering better cell proliferation, biological fixation, etc. [29, 30]. Fig. 5 XRD pattern for HA-PMEDMed surface (trial 18) Fig. 6 SEM for maximum microhardness (a) machined in pure dielectric (trial 8); (b) porous surface with white powder layer (trial 18) (b) (a) Surface Morphology and Microhardness Behavior of 316L in HAp-PMEDM 453 4. CONCLUSIONS The current research work is an investigation of medical grade 316L stainless steel for the surface modification with hydroxyapatite powder mixed dielectric using reverse polarity of EDM. 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