Shrouded Plasma Sprayed and Flame Sprayed Hydroxyapatite Coatings: Morphology and Mechanical Characterisation

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1 Shrouded Plasma Sprayed and Flame Sprayed Hydroxyapatite Coatings: Morphology and Mechanical Characterisation A. Manoj Mittal*, B. S K Nath, C. Satya Prakash A*. Mech. Engg. Dept., Adesh Institute of Engineering and Technology, Faridkot, Punjab, B & C. Metallurgical and Materials Engineering Department, Indian Institute of Technology Roorkee, Roorkee , Uttarakhand, India * Corresponding author: mobile: ; fax: address: manojmittal74@yahoo.com Abstract-- Hydroxyapatite is a bioactive, osteoconductive material capable of forming direct and rigid biological bond with bone tissues in contact. Thermal sprayed hydroxyapatite coatings are widely used for various orthopaedic and dental applications. Bioceramic coatings were thermally sprayed on titanium substrates by flame spraying and shrouded plasma spraying process. The deposited coatings were characterised for surface roughness, microhardness and porosity. The shear bond strength of coatings were evaluated by pull-off method (ASTM F1044) and compared for different spray techniques. Calcium phosphate phases were analysed using X-ray diffraction. The initial feedstock and coatings were analysed by scanning electron microscope. Plasma sprayed coatings were found to be better compared to flame sprayed coatings in respect of their coating morphology, porosity and mechanical properties. Keywords -- Flame spraying, Plasma spraying, Shear bond strength, Thermal spraying. H I. INTRODUCTION ydroxyapatite (HA: Ca 10 (PO 4 ) 6 (OH) 2 ) is being used as substitute material for damaged bone and teeth over the past several decades because of its crystallographic and chemical similarity with various calcified tissues of vertebrates [1-2]. HA is widely used as bone spacers and fillers in bulk and granular forms with dense and porous structures due to its biological and chemical similarity to the inorganic phases of bone and teeth. Thermal sprayed HA coating is widely employed to enhance the osteointegration of medical implants and prostheses in a number of applications. The HA coated implants have a capacity to shorten the healing process compared to uncoated metallic implants. Plasma spraying and high velocity oxygen-fuel (HVOF) spraying are well known deposition techniques for bioactive hydroxyapatite coatings [3-10]. Plasma spraying process for hydroxyapatite coatings is approved by Food and Drug Administration (FDA) USA. Flame spray process is comparatively new process for deposition of hydroxyapatite coatings. In present study, an attempt has been made to evaluate and compare the mechanical properties of hydroxyapatite coating developed by two different thermal spray processes (plasma spraying and flame spraying) using two different hydroxyapatite powders. II. MATERIALS AND METHODS 1) Development of the Coatings Commercially pure titanium, being bio-inert, was selected as substrate material in rolled sheet form. Specimens with approximate dimensions 20 mm x 10 mm x 5 mm and 60 mm x 10 mm x 5 mm were cut from metallic sheet. The specimens were polished with SiC paper down to 220 grit, blasted with alumina powder (Grit 45) and subsequently air blasted to remove residual grit. The specimens were cleaned ultrasonically for 20 min in acetone and alcohol prior to deposition of coating by shrouded plasma spraying and flame spraying processes. The surface roughness (Ra) of blasted specimens was in a range of µm. The Hydroxyapatite powder for plasma spraying was commercially obtained from Plasma Biotal Limited, UK. The powder was blend of finer and coarse particles (Captal 60 1 ) with an average size of 45 µm. It is observed from Fig. 1a that the HA powder particles have spherical and angular morphology with wide particle size range of µm consistent with nominal range provided by the manufacturer. For flame spraying the hydroxyapatite powder was commercially obtained from IFGL bio Ceramics Ltd. Kolkata (India). The powder has spherical morphology with size in the range 5 20 µm (Fig. 1b). The smaller particle size for flame sprayed was chosen due to lower heat contents of flame spray process.

2 Fig. 1 SEM micrograph showing powder morphology for plasma spraying; flame spraying A 40 kw Miller thermal plasma spray apparatus available at Anod Plasma Limited, Kanpur (India) was used for plasma spraying. The spray parameters employed were: current: 750A; voltage: 40V; powder flow rate: 5.5rpm; spraying distance: mm; shielding gas (argon) pressure: 60 psi (413685N/m2) and carrier gas (argon) pressure: 40 psi ( N/m2). 2) Characterization of the coatings Coated samples after wheel polishing were subjected to Scanning electron microscopy (SEM) analysis. A field emission scanning electron microscope (Quanta 200 F: Czech Republic) was used for FESEM analysis. X-ray diffraction analysis of feedstock and coated specimens was carried out using Bruker AXS D-8 system (Germany) with Cu Kα radiation in a scan range of X Pert High Score (version 1.0a: Philips, Netherlands) software was used to analyze the XRD patterns. The crystallinity of coatings was measured with respect to crystallinity of starting powders by following equation. Ac (I), Ap Where Ac and Ap are the area under the main HA peaks of coating and powder respectively between (2θ) in XRD pattern. In order to evaluate the extent of porosity, the coated specimens were polished using cloth polishing wheel machine with 1µm lavigated alumina powder suspension. A Spray Jet 88EL developed by Metallizing Equipment Company Private Limited, Jodhpur (India) fitted with flame spray system was utilized for flame spraying. The spray parameters employed were: oxygen flow rate: 62 standard litre per minute (slpm), pressure: N/m2; propane flow rate: 48 slpm, pressure: N/m2; powder flow rate: 12g/min and spraying distance: mm. Specimens were then washed and dried before being examined with help of image analyzer using Dewinter Materials Pus 1.01 software based on ASTM B276. Ten readings were taken and average value has been reported. Surface roughness (Ra) of coatings was determined with non-contact optical profilometer (Wyco NT1100, Veecco Instruments Inc., USA) with software Vision-32. An average of 20 readings has been reported. For metallographic examination, plasma sprayed samples were cut along their cross-section with diamond cutter (Buechler s Precision Diamond Saw, Model: ISOMET 1000, USA). Thereafter, cut sections were hot mounted in Buechler s transoptic powder ( ) to study crosssectional details. The surface of specimens were first made plane by specially designed motor-driven emery belt (180 grit) and then polished manually down to 1000 grit using SiC emery papers. Finally polishing was carried out using cloth polishing wheel machine with 1µm lavigated alumina powder suspension. The shear bond strength of the coatings was measured using the ASTM standard F Two specimens 60mm x 10mm x 5mm, one coated and other bare were joined with help of epoxy adhesive and procured for 12 h at room temperature in a bench vice. The contact area of the joint was 10mm x 10mm. The shear load was applied along the surface of coating and transverse speed of jaws of machine was kept 0.1mm/ min.

3 very fine Microcracks can be seen on the coating surface. Surface morphology of flame sprayed coating (Figure 2b) show more unmelted particles and porosity as compared to plasma sprayed coatings. This might be due to low temperature and particle velocity involved in flame spraying as compared to plasma spraying. III. RESULTS AND DISCUSSION The surface morphology of as-sprayed coatings is shown in Figure 2. Plasma sprayed coating (Figure 2a) show some unmelted/partially melted particle on the surface of completely molten splats. A small amount of porosity and Pores Fig. 2 SEM micrograph showing surface morphology of coatings plasma sprayed; flame sprayed Fig. 3 SEM micrograph showing cross-sectional morphology of coatings plasma sprayed; flame sprayed Due to low temperature involved in flame spraying (3000 C) the hydroxyapatite particles does melt completely. The coatings developed by plasma sprayed process have significant lower amount particles due to high temperature of plasma jet. The plasma sprayed coatings predominantly consists of well flattened pancake type lamellae. The cross-sectional morphology of coatings is shown in Figure 3. In Figure 3 porosity, micro-cracks and grinding damage can be seen in cross-sectional morphology of coatings developed by both techniques. The grinding damage is much higher in flame sprayed coatings (Figure 3b) as compared to plasma sprayed coatings (Figure 3a). This might be

4 due to loosely bonded unmelted HA particles in flame sprayed coatings which had been removed during grinding/polishing of cross-section. The XRD patterns of feedstock and coatings developed by plasma spraying and flame spraying process are shown in Figure 4a-b and Figure 4c-d respectively. It can be seen from XRD pattern that both the powders are crystalline as no amorphous phase peak is observed in XRD pattern (Figure 4a & c). Tricalcium phosphates and CaO are present in range of (2θ) and (2θ) respectively in coatings developed by both processes (figure 4b & d). The crystallinity of coatings was measured with help of equation I. The flame sprayed coatings were found to be more crystalline as compared to plasma sprayed coatings. The crystallinity of plasma sprayed coatings and flame sprayed coatings are 47% and 63% respectively. The higher value of crystallinity of flame sprayed coatings might be due to higher percentage of unmelted particles, as particle melting and re-solidification lead to amorphous phase. The content of amorphous phases are highest in plasma sprayed coatings (compared to all thermal spray processes) due to high temperature of plasma jet. The surface roughness of plasma sprayed coatings was slightly higher than flame spared coatings. This might be due to bigger particle size of powder used for plasma spraying. The porosity of flame sprayed coatings was higher than plasma sprayed coatings, which might be due to higher percentage of unmelted particles in flame sprayed coating. The molten particles form splats on the substrate and fine grained equiaxed lamellae, pore filling effect of molten splats produce coatings with lesser porosity. Relative intensity (a.u.) b a b α a c c a- αtcp b- βtcp c- CaO Unmarked- HA (d) (c) Diffraction angle (2θ) Fig. 4 XRD pattern of HA powder and coatings HA for plasma spraying; plasma sprayed coating; (c) HA for flame spraying and (d) flame sprayed coatings. The shear bond test was conducted on mechanical testing equipment as described earlier using fixture shown in Figure 5. The macrographs of specimens after shear testing are shown in Figure 6. It can be seen from Figure that flame sprayed coatings (Figure 6a) failed due to cohesive failure while plasma sprayed coatings (Figure 6b) failed mainly due to adhesive failure. The cohesive failure suggest lower inter lamellae bonding of coating whereas, adhesive failure suggest higher inter lamellae bonding of coating. The shear bond strength of plasma sprayed and flame coating was measured as 6.58 and 2.45 MPa respectively.

5 Specimen holder Universal joint for selfalignment of assembly Epoxy Adhesive Coated Substrate Bare Substrate Fig. 5 Fixture for shear testing macrograph; schematic illustration

6 6 NUiCONE 2010 Loading specimen Coated specimen Coated specimen Loading specimen Fig. 6 Macrographs of specimens after shear testing flame sprayed coating plasma sprayed coating: ad-adhesive; co-cohesive IV. CONCLUSIONS 1. The surface morphology of plasma sprayed coatings shows some partially melted particles on completely molten splats whereas, loosely bonded unmelted particles are seen surface morphology of flame sprayed coatings. 2. Surface roughness of plasma sprayed coatings is higher whereas, crystallinity and porosity are lower as compared to flame sprayed coatings. 3. Shear bond strength of plasma sprayed coating was much higher than that of flame sprayed coatings, which might be due to higher energy content of plasma and higher velocity of particles in plasma spray process. [9] Chou, B-Y., Chang, E. Interface investigation of plasma-sprayed hydroxyapatite coating on titanium alloy with zro 2 intermediate layer as a bond coat, Scripta Materialia, 2001, 45, [10] Balani, B., Anderson, R., Laha, T., Andara, M., Tercero, J., Crumpler, E., Agarwal, A. Plasma-sprayed carbon nanotube reinforced hydroxyapatite coatings and their interaction with human osteoblasts in vitro, Biomaterials, 2007, 28, V. REFERENCES [1] Hench, L.L., Bioceramics: from concept to clinic. J. Amer. Ceram. Soc., 1991, 74, [2] Suchanek, W., Yoshimura, M., Processing and Properties of hydroxyapatite-based biomaterials for use as hard tissue replacement implants, Journal of Materials Research, 1998, 13, [3] Chang, E., Chang, W.J., Wang, B.C., Yang, C.Y., Plasma spraying of zirconia-reinforced hydroxyapatite composite coatings on titanium, J. Mater. Sci. Mater. Med., 1997, 8, [4] Fu, L., Khor, K.A., Lim, J.P,. Effect of yttria stabilized zirconia on plasma sprayed hydroxyapatite/yttria stabilized zirconia composite coatings, J. Amer. Ceram. Soc., 2002, 85, [5] Khor, K.A., Gu, Y., Quek, W C. H., Cheang, P,. Plasma spraying of graded hydroxyapatite/ti-6al-4v coatings, Surf. Coat. Technol. 2003, 168, [6] Li, H., Khor, K.A., Cheang, P., Impact Formation and microstructure characterization of thermal sprayed hydroxyapatite/titania composite coatings, Biomaterials, 2003, 24, [7] Li, H., Khor, K.A., Cheang, P., Titanium dioxide reinforced hydroxyapatite coatings deposited by high velocity oxy-fuel (HVOF) spray, Biomaterials, 2002, 23, [8] Zheng, X.B., Huang, M.H., Ding, C.X., Bond strength of plasma sprayed hydroxyapatite/ti composite coatings, Biomaterials, 2000, 21,