Materials and Design 95 (2016) 510–517
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Tribological, electrochemical and in vitro biocompatibility properties of SiC reinforced composite coatings Mitun Das a, Vamsi Krishna Balla a, T.S. Sampath Kumar b,⁎, Amit Bandyopadhyay c, Indranil Manna a,⁎⁎,1 a b c
Bioceramics and Coating Division, CSIR-Central Glass & Ceramic Research Institute, 196 Raja S. C. Mullick Road, Kolkata 700032, India Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 60036, India W. M. Keck Biomedical Materials Research Laboratory, School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164, USA
a r t i c l e
i n f o
Article history: Received 7 August 2015 Received in revised form 28 January 2016 Accepted 29 January 2016 Available online xxxx Keywords: Laser processing Composite coatings Wear resistance Biocompatibility
a b s t r a c t In this work, laser engineered net shaping (LENS™) technique was used to create SiC reinforced titanium matrix composite (TMC) on titanium surface. These composite coatings developed by injecting SiC powder into titanium melt pool, created using a high-power laser. The laser parameters strongly inﬂuenced the dissolution of SiC particles leading to formation of TiSi2, Ti5Si3, and TiC phases along with residual SiC in the composite layers. A graded microstructure, with high concentration of secondary phases on top compared to the bottom region was formed. The reinforcing ceramic phases in the TMC coatings enhanced the wear resistance of titanium by about 100 times. The wear resistance of the composite coatings in physiological environment found to increase with increase in the laser power and concentration of SiC particles injected into the melt pool. These composite coatings also showed better corrosion resistance than Ti. In vitro biocompatibility studies performed using osteoblast (MG63) and ﬁbroblast cells (NIH3T3) demonstrated non-toxic nature of these composite coatings. It is envisioned that such coatings may ﬁnd applications in articulating surfaces of load-bearing implants to enhance their in vivo lifetime with reduced metal ion release. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Titanium (Ti) and its alloys are widely used as metallic biomaterials. However, they are not suitable for bearing surface of articulating implants such as hip, knee and shoulder implants [1–3]. In such applications, CoCrMo alloy is being used as the only metallic variant due to its high wear resistance. Since 2010, the use of CoCrMo alloy as metalon-metal articulating surface for hip implants declined rapidly due to adverse effects of metal wear debris and toxic corrosion products generated in vivo . A recent in vitro study showed that cobalt ions and wear debris have toxic effect on macrophages which is considered as an important factor for developing pseudotumors around the hip joint [5,6]. These concerns related to the use of CoCrMo alloy in load-bearing implants clearly demonstrate the need for alternative materials for articulating surfaces. The development efforts towards novel articulating surface materials can be classiﬁed as: (i) development of new bulk materials i.e., Si3N4, SiC, carbon-ﬁber reinforced polyetheretherketone; (ii) cushion bearings i.e., mimic the natural joint's tribology; (iii) surface ⁎ Correspondence to: T.S.S. Kumar, Department of Metallurgical & Materials Engineering, Indian Institute of Technology Madras (IITM), Chennai 600 036, India. ⁎⁎ Correspondence to: I. Manna, Indian Institute of Technology Kanpur (IITK), Kanpur 208016, India. E-mail addresses: [email protected]
(T.S.S. Kumar), [email protected]
(I. Manna). 1 Currently at: Indian Institute of Technology Kanpur, Kanpur 208016, India.
http://dx.doi.org/10.1016/j.matdes.2016.01.143 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
modiﬁcations such as surface oxidation of zirconium, titanium; and (iv) coatings such as diamond like carbon, nitrated and carbonated surfaces [3,7–9]. In the last few decades, coatings have been considered as useful alternatives because they can alter surface properties without modifying the bulk material properties [3,9]. Due to its high wear resistance and biocompatibility, TiN has been attempted as ﬁrst ceramic coating for load-bearing implants . Later, other coatings such as titanium carbide (TiC), titanium niobium nitride (TiNbN), titanium carbon nitride (TiCN), zirconium carbon nitride (ZrCN), and (Ti,Zr)CN were also studied using plasma, ion implantation, powder immersion reaction assisted physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques [11,12]. However, these hard coatings are thin and have limited bond strength with the substrate . Moreover, the long-term survivability of these coatings as articulating surface in the complex in vivo environment is unknown. In this work, we have used laser surface modiﬁcation to create wear resistance surfaces. Laser surface modiﬁcation can produce thick coatings on intricate surfaces with the strong metallurgical bond between the coating and the substrate [3,14–18]. Moreover, the coating thickness and composition can be tailored to suit application needs [18,19]. During the last few decades, ceramic reinforced titanium matrix composite (TMC) coatings are becoming popular because they combine unique properties of both ceramic and metal [3,14–20]. Laser melt injection (LMI) based surface modiﬁcation technique has been explored by various researchers to improve tribological properties of titanium by
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creating ceramic reinforced coatings. Ceramics like SiC, WC and TiC were used as reinforcing phases in titanium matrix [14–18]. Properties of TMC coatings depend on several factors namely concentration and scale of injected ceramic powder, reaction products, distribution and bonding of the particles with the matrix. The bonding of hard particles with matrix determines the tribological performance of such TMC coatings. Due to the high afﬁnity of Ti towards C, carbide ceramics can form strong bonding with the titanium matrix [14–18]. Among different ceramics, SiC particles are one of the most commonly used reinforcement for titanium. Kooi et al.  demonstrated SiC dispersed composite layers on Ti6Al4V alloy substrate using a high power Nd–YAG laser and observed the formation of TiC and Ti3SiC2 in the reaction layer around SiC particles. In our earlier studies, we have successfully deposited crack free, SiC reinforced TMC coating with functionally graded microstructure using laser engineered net shaping (LENS™) [14,16]. Due to ceramic reinforcement, these coatings showed excellent mechanical and tribological properties [14–16]. The chemical inertness and biocompatibility of SiC make it a potential material for bearing surface of implants [21,22]. Apart from bone implants, SiC coatings are currently being used on stents to enhance hemocompatibility . In our earlier work , we have reported preliminary understanding on the inﬂuence of laser parameters on microstructure of SiC reinforced TMC coatings and their dry sliding wear behavior. Similarly, we have reported  detailed microstructural analysis of these coatings. However, none of these investigations evaluated in vitro tribological properties, corrosion in physiological solution and biocompatibility of SiC reinforced TMC coatings. We hypothesize that addition of SiC in Ti can potentially improve in vitro electrochemical, tribological and biocompatibility properties. Therefore, in the present article, we have investigated the effect of process parameters such as laser power and scan speed on tribological behavior, electrochemical properties and in vitro biocompatibility of laser deposited SiC reinforced TMC coatings. 2. Experimental 2.1. Coating preparation A high-power laser equipped in laser engineered net shaping (LENS™) (MR7Optomec, Albuquerque, NM, USA) was used to create SiC reinforced composite surface on commercially pure titanium (CPTi). The wavelength of ytterbium doped ﬁber laser was 1070 nm and had 0.5 mm beam diameter. SiC powders with size between 50 and 150 μm were injected into the liquid metal pool, created using a laser, using argon carrier gas. The hatch distance, i.e. the distance between two consecutive laser tracks/deposits, was 0.305 mm, which ensured at least 50% overlapping of laser tracks during coating preparation. During laser scanning, the injected SiC particles interact with Ti at high temperature and rapidly solidify, creating a composite layer on top surface of CP-Ti. The coatings were prepared in a glove box with less than 10 ppm O2 and the powder feed rate was 13 g/min. Several deposits were prepared at varying laser powers (W) and scan speeds (mm/s) and visually defect free coatings were obtained at 400 W and 10 mm/s speed (SC400/10) and 300 W and 20 mm/s (SC300/20). These sound coatings were further characterized in terms of tribological, electrochemical and in vitro biocompatibility performance. 2.2. Tribological study Ball-on-disk wear tests were carried out, per ASTM G 99-95a, in simulated body ﬂuid (SBF) to evaluate tribological properties of SiC reinforced TMC coatings. The wear tests were performed under normal load of 5 N using hardened chrome steel ball (100Cr6, 58 to 63 HRC) and silicon nitride (Si3N4) ball with ⌀ 3 mm as counter bodies. A sliding speed of 20 mm/s was used, and Ti samples were used as control for comparison. The wear rate (mm3/Nm) was calculated using experimentally measured width of wear track and known curvature of the ball.
Scanning electron microscope (ProX, Phenom-World BV, Netherlands) was used to examine the worn surfaces of the samples. 2.3. Electrochemical study Potentiodynamic polarization was performed to determine the susceptibility of these coatings to localized corrosion in freshly prepared Hank's solution at pH 7.4. The tests were performed using a potentiostat/galvanostat (SP300, Bio-Logic SAS, France) with Pt mesh as a counter electrode and all potentials were measured against saturated calomel electrode (SCE) reference electrode. Before the test, all samples were polished up to 1 μm alumina suspension to mirror ﬁnish and ultrasonically cleaned in acetone. 2.4. In vitro cell culture The in vitro cytotoxicity of present coatings was determined using human osteoblast-like cells (MG63) and mouse embryonic ﬁbroblast cells line (NIH3T3). CP-Ti and polymer disks were used as control. The composite coating samples of identical dimensions were ground and polished to ensure identical surface roughness. These samples were cleaned thoroughly and sterilized in an autoclave at 121 °C for 30 min. The cells were cultured in a Minimum Essential Medium (MEM; Invitrogen Corporation) supplemented with 10% of fetal calf serum (FCS) and 1% antibiotic–antimycotic solution. The samples seeded with cells were incubated in a humidiﬁed atmosphere at 37 °C and with 5% CO2. After every 2–3 days, the culture medium was replaced with fresh medium. The conﬂuent cells were trypsinized (trypsin/ EDTA; Invitrogen Corporation) for subculture. A cell suspension containing 1 × 104 cells was seeded on each sample surfaces. The primary in vitro assessment of biocompatibility was carried out using MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] assay. A 5 mg/ml MTT (Sigma, St. Louis, MO) solution was added to each sample. The MTT enters the cells and passes into the mitochondria where mitochondrial succinate dehydrogenase reduced MTT to an insoluble, colored (dark purple) formazan product. After 3 h of incubation, the formazan crystals were dissolved in 500 μl of solubilization solution, DMSO (dimethyl sulfoxide, Sigma). An aliquot of 100 μl of the resulting supernatant was transferred into 96-well plate and optical density was measured using plate reader (Thermo Scientiﬁc) at 550 nm. Triplicate samples were used in MTT assay to ensure reproducibility. The MG63 cell morphology on the sample surface was observed using SEM. After three days of incubation, cells were ﬁxed as per the method indicated elsewhere . Statistical analysis was carried out using Student's t-test, with P b 0.05 being considered statistically signiﬁcant. 3. Result and discussion 3.1. Coating microstructures Typical cross-sectional microstructures of laser melt injected SiC reinforced composite coatings are shown in Fig. 1. These coatings exhibited SiC, TiC, Ti5Si3 and TiSi2 as major phases in Ti matrix. Variation of these phases with laser parameters has been reported in our earlier studies [14,16]. The microstructure close to the substrate region, shown in Fig. 1a, mainly consisted of lamellar eutectic microstructure with few islands of faceted Ti5Si3, dendritic TiC and large undissolved SiC particles . In general, the composite coatings showed graded microstructural features as shown in Fig. 1b. The formation of graded microstructure is attributable to the accumulation of injected SiC particles in the top region of the melt pool due to their lower density (3.21 g/cm3) than molten liquid Ti (4.14 g/cm3) [15,16]. Fig. 2a and b shows high magniﬁcation microstructures of top surface of SC400/10 and SC300/20 coatings and corresponding EDS analysis of constituent phases. The microstructures consisted of dendritic and elongated TiCx phases surrounded by Ti–Si intermetallic matrix phases. The presence
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Fig. 1. Cross-sectional microstructures of laser processed SiC reinforced composite coating. (a) Microstructure near the substrate; (b) Graded microstructure showing high concentration of reaction products in the top compared to middle portion of the composite coatings.
of TiCx and Ti–Si phases was conﬁrmed by EDS spot analysis. It is clear from the EDS analysis (Spot 1) that compositions of Ti–Si phases were different in both the coatings. The Si content in Ti–Si phase was relatively more in SC400/10 than in SC300/20 coating. Similarly, TiC phase was also found to be more in SC400/10 coatings than exhibited by SC300/20.
High concentration of TiC and other reaction products in SC400/10 coatings are primarily attributable to higher volume of SiC injected into the melt pool compared to SC300/20. In SC400/10 coating, the scan speed was low (10 mm/s) and power was high (400 W), that synergistically contributed to form larger melt pool volume and more interaction
Fig. 2. Cross-sectional microstructures with EDS analysis of coatings near the top surface. Microstructures mainly consist of non-stoichiometric phases of TiC and TiSix. (a) SC400/10 coating, (b) SC300/20 coating.
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time (between the SiC and liquid Ti) than SC300/20 coating. Therefore, the process parameters strongly inﬂuence both microstructure and compositional variations in these coatings. It is intuitive to expect that these microstructural and compositional variations have direct inﬂuence on mechanical and tribological behavior of these coatings. In our earlier study, we have observed that average top surface hardness of SC400/10 coating (1167 ± 194 HV) was more than that of SC300/20 coating (976 ± 71 HV) . 3.2. Tribological properties in simulated body ﬂuid (SBF) 3.2.1. Wear against hardened chrome steel (100Cr6) balls Table 1 shows the summary of ball-on-disk type rotating wear tests performed on these composite coatings using hardened chrome steel balls in simulated body ﬂuid at 37.4 °C. Wear was observed only on steel balls due to signiﬁcantly high hardness of laser processed SiC reinforced composite coatings. The wear rate of steel balls against SC300/20 and SC400/10 coatings was 4.94 × 10−6 and 8.64 × 10−6 mm3/Nm, respectively. The wear rate of the steel balls against SC400/10 coating is nearly two times higher than against SC300/20 coating. The SC400/10 coating contained more SiC particles and reaction products which contributed to its high hardness and consequent high wear of counter body (steel ball) during wear testing . However, the hardness of control Ti sample (HRC 36) was less than the hardened chrome steel ball, and therefore, it exhibited a wear rate of 1.44 × 10−3 mm3/Nm. No measurable wear was observed on steel ball when tested against Ti substrate. The average coefﬁcient of friction (COF) of titanium, SC300/ 20 and SC400/10 coatings against steel balls in SBF was 0.47, 0.56 and 0.50, respectively. Relatively high asperity contact area and less tribolayer formation are considered the key reasons for high COF in composite coatings. Fig. 3a shows the morphology of wear track on titanium against steel ball. The general wear track morphology appears to be plowing of material from the surface. These wear scars were characterized by the presence of deep grooves with plastically deformed edges . The EDS spectra of the wear track indicated the presence of Ti and O due to oxidation of Ti during wear process. However, the non-protective nature of this oxide triboﬁlm resulted in severe wear of Ti. No traces of iron were observed on the titanium wear track that indicates insigniﬁcant wear of steel ball against Ti substrate. Under present experimental conditions, the titanium transferred to the counter surface, work hardens and easily oxidize to form TiO2 . These oxide ﬁlms can break into pieces under the inﬂuence of rubbing and results in severe abrasive wear damage to the titanium surface. The deep scratches, Fig. 3a, indicate that the wear was predominantly abrasion type [24,25]. The BSE image, shown in Fig. 3b, represents the wear track of SC300/ 20 coating tested using steel ball in SBF. The SiC phase, marked as spot “1”, was found to be surrounded by Ti–TiSix composite matrix phase. Due to high hardness of the composite coating, the abrasive scratches were absent. However, distinct triboﬁlm was observed on the surface, shown as spot “2” in Fig. 3b. The EDS analysis showed the presence of Fe, O, Ca, P, Ti and Si in the ﬁlm. The peaks for Ti and Si are from the matrix, Ca and P come from precipitates of apatite in SBF. During wear process, the triboﬁlm ruptures and the steel ball wears off by hard reinforcing phases present in the coating. Similar surface morphology was observed on SC400/10 coatings. The reinforcing phases such as SiC
Table 1 Results of tribological testing performed in this investigation. Substrate
Wear rate of the steel ball (mm3/Nm)
Wear rate of substrate against steel ball (mm3/Nm)
Wear rate of substrate against Si3N4 ball (mm3/Nm)
SC300/20 SC400/10 Ti-substrate
4.94 × 10−6 7.19 × 10−6 Not measurable
Not measurable Not measurable 1.44 × 10−3
3.66 × 10−4 7.93 × 10−5 2.59 × 10−3
and the TiSix particles support the contact stresses, thus improving the wear resistance of these composite coatings . Similar results have also been reported by other researchers showing that metal matrix composites with hard ceramic particles improve wear resistance of the substrate [3,18,26]. 3.2.2. Wear against Si3N4 ball The summary of wear test results obtained using Si3N4 balls is presented in Table 1. The wear rates of titanium, SC300/20 and SC400/10 coatings were 2.59 × 10−3, 3.66 × 10−4 and 7.93 × 10−5 mm3/Nm, respectively. Among the composite coatings, SC400/10 coatings exhibited best wear performance against Si3N4 balls in SBF. This observation can be correlated with top surface microstructural changes among the composite coatings. Wear of the titanium matrix composite coatings decreased with increasing volume fraction of ceramic reinforcing particles, and it is about 100 times lower compared to the wear rate of titanium substrate. The average COFs of titanium, SC300/20 and SC400/10 coatings against Si3N4 balls in SBF are 0.49, 0.41 and 0.39, respectively. The reason for lower COF of these composite coatings than Ti substrate can be explained from wear track morphology. The wear track morphology, shown in Fig. 4a, with deep scratches indicates high wear of titanium against Si3N4 ball in SBF. The wear debris adhering to wear track consisted of Ti, O and trace amount of silicon, which suggests some tribochemical reaction between the Si3N4 ball and the Ti substrate in SBF. In the presence of SBF, a thin amorphous and hydrated SiO2 and Si(OH)4 transfer layer can form on Si3N4 ball and react with titanium . However, this triboﬁlms could not protect the substrate from wear. The wear track morphology of SiC reinforced titanium composite coating (SC400/10) tested using Si3N4 ball is shown in Fig. 4b. The worn surface was smooth with some embedded SiC particles. Apparently, there was no pull out of reinforcing SiC particles. However, observed cracking of the SiC particles may be due to fatigue fracture. The fractured SiC acted as abrasives between the contact surfaces and partially contributed to the overall wear process. Further, the surface ﬁlm observed on the surface, marked as spot “1” and its EDS analysis indicates that this tribolayer consists of Si, Ti, O, P and Ca. The elements like Ca and P came from SBF. This clearly indicates that the tribochemical reaction occurred between the coating, Si3N4 ball and simulated body ﬂuid. The tribochemical reactions of Si3N4 ball and SiC in the coating can produce amorphous hydrate Si(OH)4 according to the following reactions . Si3 N4 þ 12H2 O ¼ 3SiðOHÞ4 þ 4NH3 ΔG298 ¼ −1268:72 kJ=mol f
SiC þ 4H2 O ¼ SiðOHÞ4 þ CH4 ΔG298 ¼ −598:91 kJ=mol f
whereΔG298 is the reaction Gibbs free energy of formation at 298 K. It is f clear from the above reactions that hydration of silicon nitride is relatively easy than silicon carbide. These amorphous hydrated Si(OH)4 ﬁlms on the mating surfaces can reduce COF and subsequently the wear rate . On the other hand, these layers can also dissolve in water or break due to rubbing action and expose new surface to form fresh tribolayer. Therefore, it is believed that the tribochemical wear also contributed to the overall wear observed on the present composite coatings [28,29]. Li et al.  observed that tribochemical oxidation of SiC in water create a surface ﬁlm consisting of SiO2 and SiO2·nH2O. Further, the cracks in the present tribolayer, Fig. 4b, indicate that materials removal occurred by cracking as well as detachment of the tribolayer. Similar wear track morphology was also observed on SC300/20 coatings. Overall, the presence of hard counter surface (Si3N4) reduces the asperity contact area and the formation of tribolayer on the present coatings found to have some positive inﬂuence in reducing the COF and wear rate of present composite coatings [17,18].
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Fig. 3. Wear track morphology against hardened chrome steel ball in SBF. (a) Titanium surface; (b) SC300/20 coating.
3.3. Electrochemical behavior Electrochemical behavior of laser melt injected SiC reinforced titanium matrix composite coatings was studied in Hank's solution and compared with titanium substrate. Hank's solution was chosen, because of its lower Ca/P molar ratio than Kokubo's SBF solution. The low Ca/P ratio (1.823) of Hank's solution compared to SBF (2.5) takes signiﬁcantly longer time to precipitate apatite like calcium phosphate . Since these precipitations can inﬂuence the electrochemical test results, we have used Hank's solution to determine the corrosion behavior of present composite coatings. The corrosion current densities (Icorr), corrosion potential (Ecorr), and βa and βc slopes generated from Tafel plots are summarized in Table 2. In general, the corrosion potential (Ecorr) is the potential at which the rate of anodic and cathodic reactions are equal . The nobler Ecorr values of SiC reinforced coatings than untreated Ti indicate that they are less prone to corrosion. In addition, materials showed that lower Icorr values correspond to low corrosion rate, hence better corrosion resistance . Further, SC400/10 coatings exhibited more electropositive corrosion potential and lower Icorr than SC300/20 coatings indicating their better corrosion resistance. The increase in corrosion resistance of SC400/10 coatings compared to
SC300/20 could be due to higher coating thickness, less defects and most importantly high concentration of SiC and Ti–Si phases in these coatings. Higher concentrations of ceramics in the composite exposed less amount of Ti matrix to the electrolyte, which can reduce the electrochemical dissolution of Ti . Moreover, defect free coating with higher coating thickness act as effective barrier and improve the corrosion resistance. Further, the anodic Tafel slope of the bare titanium sample (βa = 356 mV/dec) was higher than that of laser melt injected SiC reinforced titanium matrix composite coatings (βa = 296.5 and 298 mV/dec), indicating better corrosion resistance of the composite coatings. This ﬁnding is in accord with the data reported in earlier investigations [12,18]. It is clear from the corrosion data that the SC400/10 coating exhibit superior corrosion behavior as compare to either SC300/20 coating or Ti substrate. 3.4. In vitro cell–material interactions Cytotoxicity of SiC reinforced TMC coatings was assessed using two cell lines namely mouse embryonic ﬁbroblast cells line (NIH3T3) and human osteoblast-like cells (MG63). In MTT assay study, violet formazan crystals are formed in the living cell. The amount of formazan
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Fig. 4. Wear track morphology against Si3N4 ball in SBF. (a) Titanium surface; (b) SC400/10 coating.
produced can be measured spectrophotometrically and the optical density is proportionally related to the number of living cells . Therefore, a higher optical density represents higher number of living cells. Fig. 5a shows the ﬁbroblast cells (NIH3T3) viability on different surfaces after 2 and 4 days of culture. The results indicate that the number of living cells on both SC300/20 and SC400/10 coatings were signiﬁcantly lower than those observed on titanium surface, after 2 and 4 days of incubation.
Table 2 Electrochemical parameters obtained from Tafel analysis of untreated Ti and SiC reinforced titanium matrix composites in Hank's solution. Sample
Ti SC400/10 SC300/20
−112.585 −49.745 −99.809
0.577 0.233 0.691
192.6 163.0 290.3
356.9 296.5 298.0
However, the percentage of viable cells on both composite coatings found to increase more than 20% with culture duration. Moreover, cell viability on SC300/20 and SC400/10 coatings found to have no statistical differences at identical culture durations. Since NIH3T3 cells are viable for more than 24 h and cell numbers are increasing with time, both coatings can be considered as biocompatible. However, low cell proliferation rate compare to Ti indicates that SC300/20 and SC400/10 coatings are relatively more bioinert than Ti. Further cytotoxicity evaluation of these materials was carried out using osteoblast cells (MG63). The MTT assay of MG63 cells for 4, 7 and 11 days is shown in Fig. 5b. The percentage of viable cells on titanium and composite coatings surfaces were signiﬁcantly lower than polystyrene surface. The MTT results showed that the MG63 cell proliferation on Ti sample is signiﬁcantly higher than SC300/20 or SC400/10 coatings. However, the number of viable cells increased with culture duration on SC300/20 and SC400/10 coatings. The number of viable cell
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surfaces such as Al2O3, CoCrMo, ultra-high molecular weight polyethylene (UHMWPE), etc. . A typical overview of MG63 cell distribution and morphologies on Ti, SC300/20 and SC400/10 surfaces for 3 days of culture is shown in Fig. 6. Cell morphologies such as outstretched or rounded, provide useful information about cell interactions with surfaces. On titanium surface, cells had grown covering the surface and cell layers were thick . The ﬁlopodia and lamellipodia were well-established and the cells covered the entire surface of titanium [3,9]. Cells use ﬁlopodia and lamellipodia for anchoring the surface, cell–cell interaction and
Fig. 5. MTT assay using (a) NIH3T3 cells and (b) MG63 cells observed on Ti and composite coatings surfaces as a function of culture time. Percentages are based on data from cultures on polystyrene well, which are set at 100%. (*) — represents statistically signiﬁcant (P b 0.05) data.
increased with culture time via cell growth and proliferation which indicates the non-toxic nature of these surfaces. The results of MG63 cells also demonstrate that the present composite surfaces are nontoxic but do not promote accelerated cell proliferation. It appears that the SC300/20 and SC400/10 coatings inhibit cell growth and multiplication, which indicates that these coatings are more bioinert compared to Ti surface. Pure SiC has no in vitro cytotoxicity and are currently being used in several in vivo applications such as sensors, heart stent coatings and scaffold for bone implant [21–23,33–35]. RF sputtered SiC coatings are found to be cytocompatible for human ﬁbroblast and osteoblast cells . Our result also demonstrated in vitro cyto-compatibility of laser deposited SiC reinforced TMC coatings. Based on the ﬁbroblast and osteoblast cells interactions with the SiC reinforced TMC coatings, it can be concluded that these coatings are biocompatible but have relatively less cell proliferation activity than that of Ti (control). It is envisioned that such coatings may ﬁnd applications in articulating surfaces, such as hip femoral head, where tissue ingrowth or adherence is not required. The articulating surfaces do not require any tissue ingrowth because these components must be free to move or articulate against each other, which provide natural movements of the hip and knee joints [7,21,22]. To achieve this mobility, commercially available hip or knee joint implants are ﬁtted with bioinert articulating
Fig. 6. SEM micrographs of MG63 cells morphology (after 3 days of culture) on different surfaces. (a) Ti control, (b) SC300/20 coating and (c) SC400/10 coating.
M. Das et al. / Materials and Design 95 (2016) 510–517
migration . In a previous work, it has been shown that the cell spreading was improved, and cell proliferation was increased with the SiC coatings . Similarly on SiC reinforced TMC coatings, cells had a predominantly ﬂattened morphology and followed a continuous layered conformation. No signiﬁcant difference in cell morphology was observed between the titanium and composite coatings (SC300/20 and SC400/10). Apparently, cell layers on coating surfaces, speciﬁcally on SC300/20 coating, were less compared to Ti surface. Flat morphologies of cell on all the surfaces demonstrate favorable cell–material interactions. 4. Conclusions The SiC reinforced titanium matrix composite coatings produced by laser melt injection process found to have graded microstructure with high concentration of reaction products and reinforcements on top compared to bottom region of the coatings. Laser processing parameters found to have measurable inﬂuence on dissolution of SiC and resultant ceramic phases in these coatings. The tribological and electrochemical properties of these coatings found to depend on their microstructural features, primarily the concentration of reinforcing or reaction products. Composite coatings deposited at 400 W laser power and 10 mm/s scan speed showed lowest wear rate against Si3N4 ball in SBF, which is 100 times lower than the wear rate of titanium substrate. The presence of reinforcing ceramic phases in Ti matrix enhanced the electrochemical properties of composite coatings. SC400/10 coatings exhibited highest corrosion potential (− 50 mV) compared to bare titanium surface (−115 mV). Present SiC reinforced TMC coatings found to be biocompatible and relatively more bioinert than Ti. However, no signiﬁcant difference was observed in cell–material interactions among the composite coatings. Acknowledgments Authors would like to acknowledge the ﬁnancial support from the Council of Scientiﬁc and Industrial Research (CSIR) (OLP248 and ESC0103) for establishing LENS™ facility at CSIR-CGCRI. References  K.G. Budinski, Tribological properties of titanium alloys, Wear 151 (1991) 203–217.  A. Sarmiento, G.A. Zych, L.L. Latta, R.R. Tarr, Clinical experiences with a titanium alloy total hip prosthesis: a posterior approach, Clin. Orthop. Relat. Res. 144 (1979) 166–173.  M. Das, K. Bhattacharya, S.A. Dittrick, C. Mandal, V.K. Balla, T.S.S. Kumar, A. Bandyopadhyay, I. Manna, In situ synthesized TiB–TiN reinforced Ti6Al4V alloy composite coatings: microstructure, tribological and in-vitro biocompatibility, J. Mech. Behav. Biomed. Mater. 29 (2014) 259–271.  Y. Liao, E. Hoffman, M. Wimmer, A. Fischer, J. Jacobs, L. Marks, CoCrMo metal-onmetal hip replacements, Phys. Chem. Chem. Phys. 15 (2013) 746–756.  Y.M. Kwon, Z. Xia, S. Glyn-Jones, D. Beard, H.S. Gill, D.W. Murray, Dose-dependent cytotoxicity of clinically relevant cobalt nanoparticles and ions on macrophages in vitro, Biomed. Mater. 4 (2009) 025018.  R.A.E. Clayton, I. Beggs, D.M. Salter, M.H. Grant, J.T. Patton, D.E. Porter, Inﬂammatory pseudotumor associated with femoral nerve palsy following metal-on-metal resurfacing of the hip, J. Bone Joint Surg. Am. 90A (2008) 1988–1993.  R. Sonntag, J. Reinders, J.P. Kretzer, What's next? Alternative materials for articulation in total joint replacement, Acta Biomater. 8 (2012) 2434–2441.  N. Pace, M. Marinelli, S. Spurio, Technical and histologic analysis of a retrieved carbon ﬁber-reinforced poly-ether-ether-ketone composite alumina-bearing liner 28 months after implantation, J. Arthroplast. 23 (2008) 151–155.  V.K. Balla, W. Xue, S. Bose, A. Bandyopadhyay, Laser-assisted Zr/ZrO2 coating on Ti for load-bearing implants, Acta Biomater. 5 (2009) 2800–2809.
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