In situ synthesized TiB–TiN reinforced Ti6Al4V alloy composite coatings: Microstructure, tribological and in-vitro biocompatibility

In situ synthesized TiB–TiN reinforced Ti6Al4V alloy composite coatings: Microstructure, tribological and in-vitro biocompatibility

Author's Accepted Manuscript In situ synthesized TiB-TiN reinforced Ti6Al4V alloy composite coatings: Microstructure, Tribological and In-vitro Bioco...

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Author's Accepted Manuscript

In situ synthesized TiB-TiN reinforced Ti6Al4V alloy composite coatings: Microstructure, Tribological and In-vitro Biocompatibility Mitun Das, Kaushik Bhattacharya, Stanley A. Dittrick, Chitra Mandal, Vamsi Krishna Balla, T.S. Sampath Kumar, Amit Bandyopadhyay, Indranil Manna www.elsevier.com/locate/jmbbm

PII: DOI: Reference:

S1751-6161(13)00307-X http://dx.doi.org/10.1016/j.jmbbm.2013.09.006 JMBBM964

To appear in: Journal of the Mechanical Behavior of Biomedical Materials

Received date:9 May 2013 Revised date: 20 August 2013 Accepted date: 2 September 2013 Cite this article as: Mitun Das, Kaushik Bhattacharya, Stanley A. Dittrick, Chitra Mandal, Vamsi Krishna Balla, T.S. Sampath Kumar, Amit Bandyopadhyay, Indranil Manna, In situ synthesized TiB-TiN reinforced Ti6Al4V alloy composite coatings: Microstructure, Tribological and Invitro Biocompatibility, Journal of the Mechanical Behavior of Biomedical Materials, http://dx.doi.org/10.1016/j.jmbbm.2013.09.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

In situ synthesized TiB-TiN reinforced Ti6Al4V alloy composite coatings: Microstructure, Tribological and In-vitro Biocompatibility Mitun Dasa,c , Kaushik Bhattacharyab, Stanley A. Dittrickd, Chitra Mandalb, Vamsi Krishna Ballaa, T. S. Sampath Kumarc,*, Amit Bandyopadhyaya,d and Indranil Manna e a

b

Bioceramics and Coating Division, CSIR-Central Glass & Ceramic Research Institute 196, Raja S. C. Mullick Road, Kolkata-700032, India.

Cancer Biology and Inflammatory Disorders Division, CSIR-Indian Institute of Chemical Biology 4, Raja S.C. Mullick Road, Kolkata 700032, India c

Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai-60036, India. d

W. M. Keck Biomedical Materials Research Laboratory, School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164, USA. e

*

Indian Institute of Technology Kanpur, India

Corresponding Author: (1) Prof. T. S. Sampath Kumar Medical Materials Laboratory Department of Metallurgical & Materials Engineering Indian Institute of Technology Madras (IITM) Chennai 600 036, India Fax: +91 44 22574752 (Dept) Tel. No.: +91 044 2257 4772/5765 Email: [email protected]

Abstract Wear resistant TiB-TiN reinforced Ti6Al4V alloy composite coatings were deposited on Ti substrate using laser based additive manufacturing technology. Ti6Al4V alloy powder premixed with 5 and 15 wt.% of boron nitride (BN) powder was used to synthesize TiB-TiN reinforcements in situ during laser deposition. Influences of laser power, scanning speed and concentration of BN on the microstructure, mechanical, in vitro tribological and biological properties of the coatings were investigated. Microstructural analysis of the composite coatings showed that the high temperature generated due to laser interaction with Ti6Al4V alloy and BN results in situ formation of TiB and TiN phases. With increasing BN concentration, from 5 to 15 wt.%, the Young’s modulus of the composite coatings, measured by nanoindentation, increased from 170 r 5 GPa to 204 r 14 GPa. In vitro tribological tests showed significant increase in the wear resistance with increasing BN concentration. Under identical test conditions TiB-TiN composite coatings with 15 wt.% BN exhibited an order of magnitude less wear rate than CoCrMo alloy - a common material for articulating surfaces of orthopedic implants. Average top surface hardness of the composite coatings increased from 543 ± 21 HV to 877 ± 75 HV with increase in the BN concentration. In vitro biocompatibility and flow cytometry study showed that these composite coatings were non-toxic, exhibit similar cell-materials interactions and biocompatibility as that of commercially pure titanium (CP-Ti) samples. In summary, excellent in vitro wear resistance, high stiffness and suitable biocompatibility make these composite coatings as a potential material for load-bearing articulating surfaces towards orthopaedic implants.

Key words: Laser processing; Titanium boride; Titanium nitride; Load-bearing implants; Wear.

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1. Introduction

Among different joint replacements, total hip arthroplasty (THA) is considered one of the most effective surgical interventions for the patients suffering from debilitating pain due to excessive joint degeneration (Kurtz et al., 2007; Learmonth et al., 2007). A recent report indicated that in the US, the demand for hip revision procedures is projected to double between the years 2005 and 2026 (Kurtz et al., 2007). In general, wear debris generated from the articulating surfaces contribute significantly towards the aseptic loosening of THA (Jasty, 1993; Sargeant and Goswami, 2006; Sonntag et al., 2012). These periprosthetic wear debris activate macrophages and stimulate the release of pro-inflammatory mediators, which activate osteoclasts, leading to bone resorption and subsequent loosening of the implant (Sargeant and Goswami, 2006). As a result, metal-on-metal THA has become popular over the last few decades due to its low in vitro wear over metal-on-polyethylene or ceramic-onpolyethylene bearing surfaces (Sonntag et al., 2012). However, the soft tissue reaction with metallic wear debris from bearing surface and release of potentially carcinogenic metal ions in the blood stream have become serious concerns for alloys (Messer and Lucas, 1999; Schmalz and Garhammer, 2002; Smith et al., 2012). High levels of Co and Cr ions are known to be toxic (Messer and Lucas, 1999). Moreover, in a recent report, based on National Joint Registry of England and Wales, it was found that hip replaced by large head metal-on-metal bearing surfaces raised Cr and Co levels in the blood, initiated soft tissue reactions around the hip, which resulted swelling and loss of movement causing significantly higher number of revision surgeries within five years than other options (Smith et al., 2012).

Among different metallic biomaterials, titanium (Ti) is extensively used in various loadbearing applications, however due to its poor wear resistance, applications of Ti is avoided

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towards any articulating surfaces. Therefore, several surface treatments were attempted to improve tribological performance of Ti and its alloys (Sonntag et al., 2012; Zhecheva et al., 2005). Despite continuous improvement in design and development of new bearing surfaces that have the ability to function under high level of loading for a long period, development of highly wear resistant, non-toxic bearing material is still one of the most critical challenges in orthopaedic implant research today.

In recent years, in situ synthesized discontinuous fiber reinforced titanium matrix composite (TMC) coatings processed via high-energy lasers have been studied extensively (Balla et al., 2012; Banerjee et al., 2003; Das et al., 2012, 2010; Kooi et al., 2003; Ocel´k et al., 2005; Samuel et al., 2008; Wang et al., 2008). These coatings found to exhibit superior mechanical properties such as high fracture toughness, excellent mechanical strength, and good fatigue and wear resistance compared to their monolithic counterparts (Balla et al., 2012; Das et al., 2012; Majumdar et al., 2012). Among many in situ synthesized reinforcements for TMC, TiB has been proven to be promising due to its high elastic modulus, similar thermal expansion coefficient that minimizes residual stress and excellent interfacial bonding with titanium matrixes (Banerjee et al., 2003; Kooi et al., 2003; Ocel´k et al., 2005; Samuel et al., 2008; Wang et al., 2008). The earlier studies on TiB reinforced Ti metal matrix composite (MMC) coating fabricated via laser cladding and laser melt injection method, using Ti, B or TiB2 as feedstock powders, showed considerable improvement in tribological properties (Banerjee et al., 2003; Kooi et al., 2003; Ocel´k et al., 2005; Samuel et al., 2008; Wang et al., 2008). Kooi et al. (2003) discussed in detail the structure of the TiB precipitates formed during laser deposition of TiB-Ti composite coatings. Banerjee et al. (2003) and Samuel et al. (2008) discussed microstructure and wear resistance of laser deposited boride reinforced composites using elemental Ti and B powders. The previous results showed that the laser processed

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TiB/Ti6Al4V MMC composites have refined TiB precipitates, which offer attractive properties including increased stiffness and excellent wear resistance (Ocel´k et al., 2005; Samuel et al., 2008; Wang et al., 2008). These laser deposited composites with refined microstructure showed superior properties than the TiB/Ti6Al4V MMC prepared by other conventional techniques (Majumdar et al., 2012, 2011).

It was also reported that TiB reinforcement improves hardness and tensile strength of Ti– 35Nb–5.7Ta–7.2Zr (TNZT) alloy and exhibits better cell adhesion and spreading than the control material (polystyrene) (Majumdar et al., 2012). Recently, Miklaszewski et al. (2011) observed good cytocompatibility of TiB dispersed -Ti matrix composite layer prepared using a microplasma with 10 wt.% of B as precursor powder, which makes them a potential candidate for biomedical applications. In a recent paper, Balla et al. (2012) demonstrated that laser processed Ti/TiN metal matrix composite coatings reinforced with 40 wt.% TiN exhibits excellent biocompatibility, superior fracture toughness and comparable wear resistance to laser gas nitrided Ti. In our previous work, we have reported laser processed in situ synthesized TiB-TiN reinforced Ti6Al4V alloy composite coating using boron nitride (BN) (Das et al., 2012). BN in the system helps to form titanium boride as well as acts as a solid source of nitrogen to eliminate infiltration-limited reactions with gaseous nitrogen. The presence of TiN in combination with TiB in Ti6Al4V alloy matrix is expected to improve stiffness (Bellosi et al., 2000; Rangaraj et al., 2004) as well as biological properties (Piscanec et al., 2004).

We have successfully demonstrated the feasibility of creating in situ formed TiB-TiN reinforced Ti6Al4V alloy matrix composite coatings, with unique microstructural features, on commercially pure (CP) Ti substrate using Laser Engineered Net Shaping (LENS™) process

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(Das et al., 2012). LENSTM – a laser based additive manufacturing technology can fabricate alloys or composites in a single step with unique advantage of creating functionally graded coatings as well as coatings on complex shaped parts under argon atmosphere, which are not possible using conventional laser processing/manufacturing. Therefore, in this work we have explored the potential for LENSTM processed discontinuously reinforced composite coatings prepared via feeding Ti6Al4V powder pre-mixed with 5 and 15 wt.% BN towards articulating surfaces of load-bearing implants. The influence of process parameters on microstructure of in situ formed TiB-TiN reinforced Ti6Al4V alloy matrix composite coatings have been correlated with mechanical and in vitro tribological and biological properties of these coatings.

2. Experimental

2.1 Composite Coatings Fabrication Ti6Al4V alloy powder (ATI Powder Metals, PA, USA) and hexagonal boron nitride (h-BN) powder (H.C. Stark GmbH, Germany) with particle size range between 50 to 150 Pm and 0.5 to 2 Pm, respectively, were used as feedstock powder. Ti6Al4V alloy powder with 5 and 15 wt.% BN were blended with dilute polyvinyl alcohol solution in a turbula mixer for 24 h to create premixed composite agglomerates suitable for laser processing. The premixed powder was dried in a vacuum oven and sieved using mesh 100 and 270 sieves to create suitable feedstock (50 to 150 Pm) for LENS™. The premixed powder was poured into the powder feeder of LENS™ and the composite coatings were deposited. A LENS™-MR7 (Optomec Inc. Albuquerque, NM) equipped with a 500W continuous wave ytterbium doped fiber laser, beam size of 0.5 mm, was used to deposit two layer thick coatings on CP-Ti substrate of 10mm x 10mm area. All the composite coatings were prepared using a constant powder feed

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rate of 6 g/min. The coating thickness aimed was between 0.5 and 1.5 mm. A detailed description of the operation and capabilities of LENS™ can be found elsewhere (Bandyopadhyay et al., 2009). A hatch distance of 0.305 mm was used to achieve at least 50% overlap between consecutive laser tracks/deposits. Laser power and scan speed were optimized based on several experiments carried out using different laser powers and scan speed. It was observed that a laser power ranged between 300 W to 400 W is good enough to achieve complete melting of the powders. Laser powers above 400 W and scan speeds below 10 mm/s resulted in severe burning and plasma plume formation leading to porosity on the coatings. Therefore in the present work, the composite coatings containing 5 and 15 wt.% BN were deposited at 300 W and 400 W laser powers with a scan speeds of 10 and 20 mm/s to study their influence on the coating microstructures and related properties. In LENS• processing, two key parameters namely laser power and travel/scan speed can be combined into total heat input per unit area (I) as

I

P QD

(1)

where P is the laser power, v is the laser scan speed, and D is the laser beam diameter on the substrate (Mazumder et al., 1999). The heat input directly controls the microstructural features. The precursor powder compositions, sample identification, LENSTM parameters and energy density used in the present work are listed in Table 1. All coatings were fabricated in a glove box containing an argon atmosphere with O2 content less than 10 ppm to limit the oxidation of the alloy during processing.

2.2 Microstructural and Mechanical Characterization Microstructural characterization of the coatings was performed using scanning electron microscope and field emission scanning electron microscope (FESEM, SupraVP35 Carl

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Zeiss, Germany) equipped with a energy-dispersive spectrometer (EDS). An X’Pert Pro MPD diffractometer (PANalytical) using X’Celerator operating at 45 kV and 40 mA with Ni filtered CuK radiation was used to determine constituent phases in the coating. In order to examine the light elements distribution in the coating, elemental mappings of 50 Pm u 50 Pm area in the coatings’ cross-section was carried out by wavelength dispersive electron-probe microanalysis (EPMA). EPMA was carried out in a CAMECA SX100 instrument with LaB6 filament and beam diameter of 1 m. Vicker’s microhardness was measured (Wolpert Wilson Instruments 402MVD) on these samples using a 1000 g load for 30 s, and an average value of 15 measurements was reported. The profile of hardness across the coating thickness was determined by applying 500 g load for 10 s, and an average of ten measurements for each sample were reported. The Youngs modulus of the composite coating was determined using a nano-indenter (Fischeroscope H100 XYp, Fischer, Switzerland) with a triangular pyramid (Berkovich) diamond indenter at an applied load of 1000 mN. The load was applied at the rate of 3.3 x 10-4 N/s. From the load-depth curve, Youngs modulus was analyzed using Oliver and Pharr method (Oliver and Pharr, 1992). An average of five measurements on each sample is reported.

2.3 In vitro Tribological Performance Tribological experiments were carried out using a linear reciprocating ball-on-disc tribometer (NANOVEA, Microphotonics Inc., CA, USA), according to ASTM G133, with 3 mm diameter hardened chrome steel ball (100Cr6, 58 to 63 HRC) rubbing against in situ synthesized TiB-TiN composite coatings. Laser processed CoCrMo alloy samples were used as control for comparison. The top surface containing the coating was ground using a series of SiC grinding papers followed by fine polishing with up to 1m suspension of Al2O3 powder on velvet cloth. All samples, before each test, were ultrasonically cleaned in acetone

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to assure a clean surface. A constant linear oscillatory motion of 10 mm in length (the full cycle represents 20 mm of travel) with a constant speed of 1200 mm/min was used for all tests. The tests were carried out with a constant normal load of 5 N for sliding distance of 1000 m. The initial mean Hertzian contact pressure for 5 N load was found to be 2.95 GPa to 3.15 GPa in the composite coatings containing 5 and 15 wt.% BN, respectively. Such contact pressure is significantly higher than the peak contact pressure found at hip joints for a normal person during climbing up stair (Yoshidaa et al., 2006). The average wear rate of each test samples (3 nos) was calculated as mm3/Nm for 1000 m of sliding distance. All tests were carried out in aseptic condition in freshly prepared simulated body fluid (SBF) at 37 °C. The ionic concentrations of the SBF used in present study are as follows: 2.5mM of Ca2+, 1.5mM of Mg2+, 142.0mM of Na+, 5.0 mM of K+, 147.8 mM of Cl, 4.2 mM of HCO3, 1.0 mM of HPO42, 0.5 mM of SO42 (Balla et al., 2012). Statistical analysis was performed using Student’s t-test and P < 0.05 was considered statistically significant.

2.4 In vitro cell – material interactions Cell culture Human osteoblast-like cells (MG63) (ATCC, USA) were used in this study. In vitro cytotoxicity (Balla et al., 2012; Majumdar et al., 2012) behavior was evaluated for composite coatings with different BN concentrations i.e., 5BN-400/10 and 15BN-400/10, for 4, 7 and 11 days culture periods. For comparison, similar study was done on CP-Ti and negative control polymer disc. All the samples were sterilized by autoclaving at 121°C for 30 min. MG63 cells were cultured in a Minimum Essential Medium (MEM; Invitrogen Corporation), supplemented with 10% of fetal calf serum (FCS), 1% antibiotic –antimycotic solution in a humidified atmosphere at 37 °C and with 5% CO2. The culture medium was changed every 2–3 days and confluent cells were subcultured through trypsinization (trypsin/EDTA;

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Invitrogen Corporation). After counting, cells (1x104) were seeded on each sample surfaces, placed in 24-well plates.

Cell morphology The morphology of cells were observed using SEM after 3 days incubation on test samples. The cells adhering to the samples were washed with 0.1 M phosphate buffered saline (PBS) and fixed with 2% paraformaldehyde/2% glutaraldehyde in 0.1 M phosphate buffered saline overnight at 4 °C. Following this, post-fixation was made with 2% Osmium tetroxide (OsO4) for 2 h at room temperature. Fixed samples were then dehydrated in an ethanol series 30%, 50%, 70%, 95% and 100% three times. Dried samples were then gold coated and observed under an FESEM (Balla et al., 2012; Miklaszewski et al., 2011).

Cell proliferation analysis by MTT assay The MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] assay was used to evaluate cell viability (Sarkar et al., 2013). A 5 mg /ml MTT (Sigma, St. Louis, MO) solution was prepared by dissolving MTT in phosphate-buffered saline. An aliquot of 1 ml of diluted MTT solution (100 l in 900 l of MEM) was added to each sample to form formazan crystals by the active mitochondrial dehydrogenase enzyme of the live cells. After 3 h incubation, the samples were removed from the well and 500 l of solubilization solution DMSO (Dimethyl sulfoxide, Sigma) was added in each well to dissolve the formazan crystals. The culture plate was rocked for 10 min. An aliquot of 100 l of the resulting supernatant was transferred into a 96-well plate, and optical density was measured using plate reader (Thermo Scientific) at 550 nm. CP-Ti was used as the control set for this experiment.

Flow cytometric analysis

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i) Cell membrane integrity The flow cytometry analysis was conducted using propidium iodide (PI, Sigma), an intercalating agent and a fluorescent molecule to stain DNA, to detect morphologically dead cell population and cell membrane characteristics (Bhattacharya et al., 2010). Normally live cells are impermeable to PI, and therefore PI uptake was used to quantify the population of cells in which membrane integrity were lost. MG63 cells were cultured on metallic sample surfaces (CP-Ti and composite coatings) for 7 days and then cells were harvested and single cell suspension was stained with PI (5 g/ml) per manufacturer’s instructions. PI fluorescence was detected in the appropriate channel (typically FL-2H) of FACScan flow cytometer (BD FACSCalibur) using 488 nm laser illumination. Flow cytometer analysis was performed on 5000 individual cells by CellQuestPro software.

ii) Mitochondrial membrane potential analysis The mitochondrial membrane potential (
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Statistical analysis Statistical analysis was performed using Student’s t-test, with P < 0.05 being considered statistically significant. Triplicate samples were used in MTT assay to ensure reproducibility. The data are presented as mean ± standard deviation of several independent experiments.

3. Results and Discussion

3.1 Microstructure and phase analysis Typical SEM micrograph of Ti6Al4V alloy and 5 wt.% h-BN premixed powders morphologies are shown in Fig. 1. The starting powder mixture consists of coarse spherical Ti6Al4V alloy powders between 50 to 150 Pm, which is coated with fine flake-shaped h-BN powders. As BN powder is very fine and light, it is difficult to free flow through LENS process. Therefore, BN powders are coated on Ti6Al4V alloy powders to overcome clogging of fine BN powders in the delivery line and excessive loss of light BN powders during powders delivery by Ar gas. Such arrangement also facilitates homogeneous dispersion of in situ synthesized reactant products in the coating microstructure. Fig. 2 shows cross-sectional microstructure of laser processed Ti6Al4V alloy composite coatings, reinforced with BN. The coating microstructure exhibited good interfacial bonding with the substrate because of complete melting of deposited powder and the substrate at the interface. Further, no interlayer cracks, gross defects such as porosity or lack of fusion were observed suggesting complete melting of deposited powders. Several other researchers also report good metallurgical bonding between the deposits and the substrate of the coatings processed via lasers due to complete melting (Banerjee et al., 2003; Das et al., 2012, 2010; Majumdar et al., 2012). As a result, it is plausible that these coatings exhibit superior adhesion/bond strength than those

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processed via other techniques such as plasma spraying where the bonding occurs primarily due to solid-state diffusion (Miklaszewski et al., 2011). In the present investigation, two crack free layers were deposited and the coating thickness measured using optical microscope. The minimum and maximum coating thickness of 467 ± 26 m and 1320 ± 38 m were observed in 5BN-300/20 and 15BN-400/10 coatings, respectively.

Fig. 3 shows X-ray diffraction (XRD) patterns of starting premixed powders and laser processed Ti6Al4V alloy composite coatings containing 5 and 15 wt.% BN, processed at similar laser power (400 W) and scan speed (10 mm/s). XRD result of the starting powder confirmed the presence of both hexagonal boron nitride (h-BN) and Ti6Al4V alloy powder. XRD analysis of laser-processed coatings, 5BN-400/10 and 15BN-400/10, showed major peaks corresponding to Ti3N1.29 (JCPDS card no. 84-1123) followed by TiB (JCPDS card no. 73-2148) and TiN (JCPDS card no. 87-0632), in all coatings. Unreacted BN was not observed in any of the laser-processed coatings due to very small size and exothermic reaction between BN and liquid Ti that completely consume BN to form in situ synthesized Ti3N1.29, TiB and TiN. Moreover, nitrogen released from BN diffuse/dissolve into the liquid Ti owing to its wide range of solubility and can form nitrides during solidification (Zhecheva et al., 2005). However, the nitridation is a diffusion controlled process. Therefore, incomplete nitridation would arise due to insufficient reaction time caused by rapid solidification associated with laser processing and / or inadequate nitrogen in the reaction system (Mazumder and Manna, 2003).

High magnification FESEM microstructures of laser processed Ti6Al4V composite coatings containing 5 wt.% BN are shown in Fig. 4. The microstructures consist of phases exhibiting two different morphologies. It can be clearly seen from Fig. 4a and b that very fine whisker/

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nano rods are locally concentrated in the matrix and formed quasi-continuous network architecture. From XRD and EPMA analysis, these nano rods were identified as TiB. Similar kind of 3D network architecture was also found in Ti-TiB composite fabricated by reaction hot pressing (Huang et al., 2009). The dendritic/ plate type phases inside the 3D network of TiB were identified using EDS analysis and found to be TiNx (0.3  x  0.5). Morphology and size of the TiNx varied with N2 concentration in the system as well as total energy input. The microstructural observations, as shown in Fig. 4, confirm that the reaction products in 5BN-300/20 coatings processed at low energy input (38 J/mm2) were finer than those observed in 5BN-400/10 coatings processed with high energy input (102 J/mm2). In the laser processed composite coatings, in situ formed TiB rod diameter varies from 50 nm to 70 nm, as shown in Fig. 4c and d. Further, these TiB nano rods were random in 5BN-400/10 (102 J/mm2) coatings than in 5BN-300/20 (38 J/mm2) coatings. The microstructure of the composite coatings obtained under higher boron nitride (i.e., 15 wt% BN) concentration, shown in Fig. 5, exhibited larger TiNx dendrites and TiB nano rods as well as coarse needlelike structures at TiNx grain boundaries. It was found that BN has high laser absorption coefficient leading to a preferential absorbance of high amount of incident laser energy (Das et al., 2012). In addition, the reaction of titanium with boron nitride is exothermic as the heats of formation of the products are higher than the heats of formation of the starting materials (Gordienko and Evtushok, 2001). Therefore, increasing the BN concentration in the composite coatings increases the peak melt pool temperatures which consequently results in the formation of coarser reaction products.

During laser deposition of composite coatings, BN coated Ti6Al4V alloy powders were injected into the molten metal pool where BN dissociate or reacts with Ti and forming TiB. At the same time, nitrogen atoms diffuse away from BN to react with Ti forming Ti(N) solid

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solution or TiNx compound, as shown schematically in Fig. 6. According to Ti–B and Ti–N phase diagrams (Yu et al., 1987) N has a wide range of solid solubility in Ti due to smaller atomic radius (0.75 Å) and owing to its larger atomic radius, B atom (1.17 Å) has very low solubility (< 0.02 wt. %) in Ti. The microstructural observations clearly indicate that BN is completely consumed and the following reaction occurs: 2Ti + BN  TiB + Ti(N)

(1)

In the present study, the B concentration was nearly 2.5 and 7.5 wt % in 5BN and 15BN composite coatings, respectively (assuming very fine BN powder distributed homogeneously). Such concentrations of B facilitate the formation of TiB phase which is thermodynamically more stable than TiB2 in presence of excess Ti (Huang et al., 2009). Therefore, TiB2 phase was absent in the present composite coatings. In general, laserprocessed coatings achieve rapid cooling rate (103 – 108 K s-1) due to very fast localized heating, high scan speed and self-quenching by solid substrate (Das et al., 2010; Mazumder et al., 1999). Low energy input at high scan speed increases the thermal gradients near the melt zone and provide high cooling rate. It is known from the solidification theory that the scale of the solidification microstructure is inversely proportional to the square root of the cooling rate. Therefore, all the composite coatings of similar composition processed at lower energy input exhibited relatively finer microstructural features/phases than the coatings processed at higher energy input. As anticipated, microstructure shown in Fig. 5b and d, confirm that the reaction products in 15BN-400/20 coatings processed at low energy input (51 J/mm2) were finer than in the 15BN-300/10 coatings processed with high energy input (76 J/mm2).

Secondary electron (SE) image and corresponding EPMA elemental mapping of Ti, B, N of 15BN-300/20 composite coating is shown in Fig. 7. Elemental mapping shows that Ti is uniformly distributed in the analysis area, whereas the concentration of B and N are localized

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in different regions. The localized distribution of B and N suggests that the N concentration in the matrix is high due to its high solubility (up to 7.6 wt.%) in Ti. The regions with high concentration of nitrogen indicate the presence of TiNx dendrites in the matrix. The formation of TiNx compound at localized N concentration more than 7.6 wt.% can be explained based on Ti-N equilibrium phase diagram (Yu et al., 1987). It may be noted that the areas with high nitrogen concentration also exhibited depleted boron concentration and the B containing phases surrounds N rich regions. In general, due to higher atomic radius of B than N atom, diffusivity of B in liquid titanium is relatively lower than N atom. During interaction with liquid Ti, BN dissociate and increases the B concentration in the surrounding areas. While the N diffuse away from BN and influence larger portion of molten titanium to form a solid solution (reaction mechanism shown in Fig. 6). Because of lower diffusivity of B atom, homogeneous distribution of B in the liquid is dependent on strong convection stirring forces (Marangoni force) generated due to temperature gradients and gradient of the surface tension in the melt pool. This Marangoni force creates a convective flow inside the melt pool (Mazumder and Manna, 2003). These convective flows found to be directly proportional to the melt pool temperature, i.e., higher the melt pool temperature higher will be convective flows. As a result, the Ti-TiB eutectic phases are much more homogeneously distributed in Ti6Al4V+15BN coatings than in Ti6Al4V+5BN coatings. During cooling, the TiN phase nucleate first, as TiN melting point (3600°C) is much higher than TiB (2200°C), the Ti-TiB eutectic phase surround the hard and brittle TiN phase. This novel microstructure with nano scale dispersion improves stiffness and tribological properties of the composite coating.

3.2 Mechanical properties The load-displacement curves from nanoindentation tests made on the laser processed 5BN400/10 and 15BN-400/10 composite coating cross sections are presented in Fig. 8. Inset in

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Fig. 8 indicates that the loading-unloading curves of nanoindentation exhibit pop-ins, which can be attributed to finely distributed TiB nano rods and solid solutions of N in titanium matrix. The number and size of pop-in discontinuities in the curves were relatively higher in 15BN-400/10 coating than in 5BN-400/10. This difference is presumably due to the differences in the volume fraction and distribution of in situ formed hard TiB and TiN phases in the matrix. Table 2 summarizes the Young’s modulus of laser processed TiB-TiN reinforced Ti6Al4V metal matrix composite coatings. The experimental data show that average Young’s modulus increased from 184 ± 4 GPa in 5BN-400/10 coating to 204 ± 14 GPa for 15BN-400/10 coating. This improvement is due to the formation of more TiB and TiN phases in the composite coating with increasing BN concentration corroborated by microstructural analysis. Moreover, the Young’s modulus of the present composite coatings were found to be higher than the TMC containing either TiB or TiN (Banerjee et al., 2004; Biswas et al., 2009; Samuel et al., 2008). Fig. 9 shows typical microhardness profile across the coating cross section. All composite coatings showed smooth hardness profiles without any abrupt drop or increase in the hardness, which suggests microstructural uniformity and absence of gross defects in the coatings.

Indentation crack propagation through the composite was studied using 10 kg load. The composite coatings, 15BN-300/20 and 15BN-400/10, were selected because of their distinctly difference in reinforcing phases size (38 J/mm2 and 102 J/mm2 respectively). The SEM micrographs of the hardness indentations on transverse section of 15BN-300/20 and 15BN400/10 composite coatings are shown in Fig. 10. The lower magnification images of the indentation of 15BN-300/20 and 15BN-400/10, shown respectively in Fig. 10a and c, clearly reveal indentation cracks at all corners. However, the crack length was relatively small in 15BN-300/20 composite coating (Fig. 10a) suggesting its higher fracture toughness than that

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of 15BN-400/10 composite coating. It is worthwhile to mention here that no such indentation cracks were observed in composite coatings with 5 wt.% BN and could be due to small volume fraction and discontinuous distribution of in situ formed TiB and TiN phases in the matrix. On the other hand, coarse and relatively closely placed TiN phases were observed in 15BN-400/10 composite coatings processed with high energy input (102 J/mm2), shown in Fig. 10b and d, than in 15BN-300/20 composite coating processed at lower energy input (38 J/mm2). Due to high amount of brittle TiN phase, long and straight transgranular cracks were observed in 15BN-400/10 composite coating. In the case of 15BN-300/20 composite coating, smaller TiN phases were separated by thick regions of Ti-TiB phase which could have helped in crack tip blunting, crack deflection and bridging (Chang et al. 2010). In ceramic matrix composite, crack deflection and crack bridging mechanism enhance the resistance to crack propagation inside the composite, leading to improvement in the fracture toughness (Chang et al. 2010; Toda and Kobayashi, 1997). Crack deflection was observed in 15BN-30/20 coating where TiN dendrites were small and surrounded by tougher Ti-TiB eutectic phase.

3.3 In vitro tribological properties Fig. 11 shows experimentally determined wear rate, in mm3/Nm, of laser processed Ti6Al4V alloy composite coatings reinforced with 5 and 15 wt.% BN and CoCrMo alloy disc. The wear rate was calculated using the measured width of wear track and known curvature of the ball and linear oscillatory stroke length. Average wear rate of 5 wt.% and 15 wt.% BN coatings was between 1.51 × 10-4 to 4.26 x 10-5 mm3/Nm and 6.20 × 10-6 to 1.90 × 10-6 mm3/Nm, respectively. Experimental data clearly indicates that the BN concentration had strong influence on wear resistance of the composite coatings. The wear resistance of the TiB-TiN reinforced Ti6Al4V alloy composite coating increased notably (p < 0.05) with increasing BN concentration from 5 to 15 wt.%, at similar laser energy input. The lowest

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wear rate of 1.90 × 10-6 mm3/Nm was observed in 15BN-400/10 (102 J/mm2) coating, which is an order of magnitude lower than 5BN-400/10 coating. The superior wear resistance of the composite coatings is attributed to the in situ formation of hard TiB and TiN phases and increase in their volume fraction with increasing BN concentration. Further, with increasing laser energy input, there is a decreasing trend in wear rate although the difference is not very significant. Importantly, under identical experimental conditions, the wear rate of all composite coatings containing 15 wt.% BN was found to be an order of magnitude lower (p < 0.05) than CoCrMo alloy (1.62 × 105 mm3/Nm), which is the most popular material used for femoral ball in metal-on-metal or metal-on-polymer type hip implants. Experimental data clearly demonstrate that TiB-TiN reinforced Ti6Al4V alloy composite coatings have superior wear resistance than CoCrMo alloy. The wear rate of the present coating is in the order of 106 mm3/Nm, which was also observed in laser gas nitrided Ti and laser processed composite coating containing 40% TiN (Majumdar et al., 2012; Smith et al., 2012; Zhecheva et al., 2005). In the present coating system, owing to in situ formation, the hard phases are well bonded to the matrix and very fast solidification favoured homogeneous distribution of fine microstructural features, which subsequently improved the mechanical properties as well as wear resistance of the coating.

Table 2 summaries the average top surface hardness and coefficient of friction (against harden 100Cr6 steel ball) of laser processed TiB-TiN reinforced Ti6Al4V alloy composite coatings. Wear rates of these materials correlate well with their respective top surface hardness values. Average top surface hardness of CoCrMo alloy was 358±17 HV, which is lower than the hardness of the composite coatings. Since, wear resistance of a material is directly related to its hardness, increasing hardness improves wear resistance. The experimental data indicates that laser parameters (laser energy input) had no significant

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influence on the surface hardness of the coatings. Irrespective of laser parameters, the average hardness of 5 wt.% BN coatings was between 530 and 580 HV. However, BN concentration found to have strong influence and the hardness increased with increasing BN concentration. The hardness of 15 wt.% BN coatings was in the range of 700 and 790 HV. These hardness values of the composite coatings are much higher (p < 0.05) than CP Ti substrate with a hardness of 147 ± 22 HV. Moreover, because of strong interface cohesion between in situ form TiB and titanium matrix, the TiB whiskers strengthen the composites remarkably via a load-transfer mechanism. Additionally, the in situ formed TiB nano rods confine excessive dendritic growth of brittle TiN phase leading to enhanced fracture toughness of the composite coating. Therefore, the present TiB-TiN reinforced Ti6Al4V composite coatings show high wear resistance and can be used as potential material for wear resistant load bearing implant applications for superior performance than CoCrMo alloy.

3.4 In vitro cell–materials interactions The MG63 cells morphology on different metallic surfaces after 3 days of culture are shown in Fig. 12. There was no significant difference between cell morphologies on titanium and composite coating surfaces. The cells were well spread on all the surfaces and have grown across the surface. The filopodia and lamellipodia were well-established, and the cells covered the entire surface of titanium and coating samples. The cells had a predominantly flattened morphology and followed a continuous multilayered conformation, which suggested that the cells on the titanium surface and coating surfaces had similar viability.

MTT assay was carried out to assess cytotoxicity of laser processed TiB-TiN composite coatings using MG63 cell viability. Commercially pure Ti (CP-Ti) was used as a control sample to compare the possible toxic effect of TiB-TiN composite coatings (5BN-400/10 and

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15BN-400/10). The number of living cells proliferated on the composite coatings, Ti control and polystyrene well surfaces were determined by MTT assay. In MTT assay, optical density of the solution was measured to quantify the cell viability/living cell count. A higher optical density represents a higher concentration of living cells. Fig. 13 compares the percentage of viable cells, normalized by considering the optical density of polystyrene surface as 100%, observed on different samples after 4, 7 and 11 days of culture. The MTT assay revealed that the number of cells present on the polystyrene was significantly higher than metal surfaces (titanium and composite coatings). However, there were no significant differences (p> 0.05) in cell proliferation between the Ti control and the 5BN-400/10 or 15BN-400/10 composite coatings. Therefore, it can be concluded that the cytotoxicity of the TiB-TiN composite coatings are similar to that of the CP-Ti, which have already been used for biomedical applications in humans.

Propidium iodide (PI) dye has been used by several researchers to identify cell membrane integrity (Rieger et al., 2011). The histogram shown in Fig. 14a illustrates the podium iodide (PI) fluorescence pattern. In general, live cells are impermeable to PI whereas necrotic cells and late apoptotic cells are stained red by PI due to lack of plasma membrane integrity. After 7 days of incubation, all the cells from different metallic surfaces showed very low and similar PI positivity (9-12 % of gated cells) indicating similar biocompatibility of all the surfaces towards MG63 cells.

To compare the cytotoxicity of the coatings (5BN-400/10 and 15BN-400/10) with respect to CP-Ti surface, JC-1 dye was used to detect mitochondrial membrane potential by flow cytometric analysis. A suitable gate was applied to exclude debris and cell aggregates. In apoptotic and necrotic cells, JC-1 exists in monomeric form and gives green fluoresce,

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typically collected in FL1 channel of flow cytometer. Fig. 14b showed typical FL-1H/FL-2H dot plots for JC-1 stain MG63 cells collected from different metallic surfaces. A suitable gate was applied to calculate the percentage of depolarized cells, measured by increase of green fluorescence and decrease of red fluorescence of JC-1. The percentages of depolarized cells in 5BN-400/10 and 15BN-400/10 coatings are not significantly different from cells incubated on Ti6Al4V which indicates that coatings have similar bioactivity as titanium.

In summary, present results show that both the composite coatings (5BN-400/10 and 15BN400/10) are non-toxic and exhibit similar cell materials interactions to that of CP-Ti. In addition, the excellent in vitro wear resistance of these composites shows their potential as wear resistant contact surfaces for load bearing implant applications such as ball or acetabular cup for hip prosthesis. However, further studies are required to understand the corrosion, tribocorrosion, metal ion release and in vivo response of these materials so that their full potential for load bearing implant applications can be realized.

4. Conclusions Ti6Al4V alloy composite coatings reinforced with in situ synthesized TiB-TiN were successfully deposited on CP Ti substrate using LENS™. In situ decomposition of BN formed novel microstructures with homogeneously distributed fine TiB and TiN reinforcements, which improved mechanical and tribological properties of these composites. Average wear rates of BN reinforced Ti6Al4V alloy composite coatings in simulated body fluids were found to be in the range between 1.51 × 10-4 and 1.90 × 10-6 mm3/Nm for 5BN300/20 and 15BN-400/10 coatings, respectively. Lowest wear rate (1.90 × 10-6 mm3/Nm) was exhibited by Ti6Al4V-15BN composite coatings processed at the highest laser energy input (102 J/mm2) which was lower than the wear rate of laser processed CoCrMo alloy (1.62 ×

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105 mm3/Nm). Further, high modulus, excellent coating-substrate interfacial characteristics and in vitro cell-materials interactions of present laser processed Ti6Al4V metal matrix composite coatings reinforced with in situ synthesized TiB-TiN show their potential as wear resistant contact surfaces for biomedical applications.

Acknowledgements Authors would like to express their sincere gratitude to Late Dr. Debabrata Basu for his technical inputs. We sincerely acknowledge Dr. Mohan Wani, NCCS Pune for providing us MG63 cell line. Authors would also like to acknowledge the financial support from the Council of Scientific and Industrial Research (CSIR), India.

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LIST OF TABLE CAPTIONS Table 1. Composition of the starting premixed powder, LENS™ parameters and corresponding energy density used in the present work Table 2. Young’s modulus, average top surface hardness (HV1.0) and average coefficient of friction of in situ synthesized TiB-TiN reinforced Ti6Al4V alloy composite coatings processed using LENS™.

LIST OF FIGURE CAPTIONS Fig. 1 – SEM images of starting Ti6Al4V alloy and h-BN pre-mixed powders Fig. 2 – Typical cross-sectional microstructures of BN reinforced Ti6Al4V alloy composite coatings (a) 5BN-400/10 (102 J/mm2), (b) 15BN-400/10 (102 J/mm2). Fig. 3 – XRD patterns of laser processed BN reinforced Ti6Al4V alloy composite coatings. Fig. 4 – FESEM microstructure of laser processed Ti6Al4V alloy composite coatings processed by 5 wt.% BN (a) 5BN-400/10 (102 J/mm2) coating having 3D network architecture of TiB nano rods around coarse TiN particles, (b) 5BN-300/20 (38 J/mm2) coating having finer microstructure with 3D network architecture of TiB nano rods, (c) nano-

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size TiB rods in 5BN-400/10 (102 J/mm2) coating, (d) nano-size TiB rods in 5BN-300/20 (38 J/mm2) coating. Fig. 5 – Typical high magnification SEM microstructures showing Ti-BN reactions products in laser processed BN reinforced Ti6Al4V alloy composite coatings (a) 15BN-300/10 (76 J/mm2), (b) 15BN-300/10 (76 J/mm2), (c) 15BN-400/20 (51 J/mm2), (d) 15BN-400/20 (51 J/mm2). Fig. 6 – Schematic representation of Ti6Al4V-BN reaction steps. Fig. 7 – EPMA mapping of 15BN-300/20 composite showing distribution of Ti, B and N in the coating.

Fig. 8 – Load-displacement curves of Ti6Al4V alloy composite coatings reinforced with 5 and 15 wt.% BN synthesized at 400W power and 10 mm/s laser scan speed at the peak load of 1000mN (all indentations were made on a cross section of the coating) Fig. 9 – Hardness variation across the laser processed Ti6Al4V alloy composite coatings reinforced with 5 and 15 wt.% BN on CP-Ti substrate Fig. 10 – SEM micrographs of the hardness indentations (a) smaller indentation cracks in 15BN-300/20 (38 J/mm2) coating (b) high magnification SEM microstructure showing crack deflection and crack bridging due to more interaction with Ti-TiB phase (c) sharp indentation cracks emanating from all corners in 15BN-400/10 (102 J/mm2) coating (d) high magnification SEM microstructures showing no crack deflection Fig. 11 – Wear rate of laser processed Ti6Al4V alloy composite coatings reinforced with 5 and 15 wt.% BN and CoCrMo alloy against hardened 100Cr6 steel ball Fig. 12 – SEM micrographs of MG63 cells after 3 days of culture (a) Ti control, (b) 5BN400/10 coating and (c) 15BN-400/10 coating

Fig. 13 – MTT assay of MG63 cells observed on Ti and composite coating surfaces as a function of culture time. Percentages are based on data from cultures on polystyrene well, which are set at 100%. Fig. 14 – Flow cytometry analysis of MG63 cells incubated on different metallic surfaces for 7 days to assess influence of metallic surface on basis of different cell parameters (a) cells stained with propidium iodide (PI) to identify cell membrane integrity (b) cells stained with JC-1 showing mitochondrial membrane depolarization

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`

Table 1. Composition of the starting premixed powder, LENS™ parameters and corresponding energy density used in the present work Nomenclature 5BN-400/10 5BN-300/10 5BN-400/20 5BN-300/20 15BN-400/10 15BN-300/10 15BN-400/20 15BN-300/20

Powder composition Ti6Al4V + 5 wt.% BN

Ti6Al4V + 15 wt.% BN

LENS™ parameters Power (W) Scan speed (mm/s) 400 10 300 10 400 20 300 20 400 10 300 10 400 20 300 20

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Energy density (J/mm2) 102 76 51 38 102 76 51 38

Table 2. Young’s modulus, average top surface hardness (HV1.0) and average coefficient of friction of in-situ synthesized TiB-TiN reinforced Ti6Al4V alloy composite coatings processed using LENS™. Sample ID

E (GPa)

Ti substrate 5BN-400/10 5BN-300/10 5BN-400/20 5BN-300/20 15BN-400/10 15BN-300/10 15BN-400/20 15BN-300/20

103 184 r 4 163 r 4 160 r 5 170 r 5 204 r 14 200 r 15 184 r 6 194 r 8

Top surface Hardness (HV1.0) 147 ± 22 604 ± 32 570 ± 19 568 ± 13 543 ± 21 877 ± 75 765 ± 66 816 ± 64 733 ± 31

Coefficient of friction 0.47 ± 0.04 0.48 ± 0.04 0.46 ± 0.04 0.44 ± 0.04 0.49 ± 0.03 0.45 ± 0.04 0.49 ± 0.05 0.46 ± 0.06

Research Highlights x

In situ synthesized TiB+TiN reinforced Ti6Al4V alloy composite coatings were laser deposited on Ti.

x

Young’s modulus of the coatings was between 170 GPa and 204 GPa.

x

In vitro wear resistance of the coatings containing 15 wt% BN are found superior than CoCrMo alloy.

x

Composite coatings exhibited similar cell–material interactions as titanium.

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5a&b

Figure 5c&d

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12a

Figure 12b

Figure 12c

Figure 13

Figure 14