Surface Characterization of Plasma Sprayed Hydroxyapatite Coatings

Abstract

Plasma sprayed hydroxyapatite (HA) coatings have been widely used for dental and orthopedic applications for fast fixation between the implant and the human anatomy. In this study, the HA coatings were atmospherically plasma sprayed (APs) using different spray power and stand-off distance (SOD), and the coating surfaces were characterized using various methods. X-ray diffraction (XRD) revealed the presence of crystalline HA, amorphous calcium phosphate (ACP) and some other crystalline phases including tricalcium phosphate (a-TCP and P-TCP), tetracalcium phosphate ('ITCP) and calcium oxide (CaO). Fourier transform idtared (FTIR) spectroscopy showed that both OH and P04343 bands of HA changed after spraying and the HA phase identified by XRD was actually the solid solution of oxyapatite (OAp) in HA, i.e., oxyhydroxyapatite (OHA). The phase composition of the HA coatings varied with respect to the spray power and SOD. The Surface morphology and roughness were analyzed via scanning electron microscopy (SEM) and profilometry respectively, and the results related to the phase composition INTRODUCTION Plasma sprayed hydroxyapatite [Ca10(P04)6(OH)2, i.e., HA] coatings on metallic implants have attracted great interest in the biomedical field due to the good strength and ductility of the metal and the increased biocompatibility of the HA as well as the fast cementless fixation.ls2 HA coating also enhances the bone ingrowth3 and protects the surrounding bone agamst metal-ion release from the metallic implant It has been indicated that the composition and the structure of the plasma sprayed HA coating are different from those of the original HA feedstock due to the high plasma temperature and rapid cooling rate2 Some amorphous calcium phosphate (ACP), tricalcium phosphate (a-TCP and P-TCP), tetracalcium phosphate (TTCP) and calcium oxide (CaO) were detected using X-ray diffraction (XRD) in previous ~tudies.'~~~' In addition, solid solution of oxyapatite (OAp) in HA, i.e., oxyhydroxyapatite (OHA), also formed in HA coatings due to the dehydroxylation of HA, which was usually examined using Fourier Transform Infrared @TIR) ~pectroscopy.~*~~ Hydroxyapatite is very stable phase in body fluids, but the dissolution rates of all new phases were found much higher than HA, which is in the order of ACP >> TTCP > a-TCP > OHA > P-TCP >> HA and will possibly lead to degradation of the HA coating.80 Calcium oxide has no biocompatiability and dissolves sigmficantly faster than TCP and, thus, it is detrimental phase for the overall implant structure. The phase composition of HA coatings generally varied as result of different spray parameters and led to different dissolution rates of HA coatings in physiological solutions. The coating surface is especially of importance, since, once implanted, it is directly in contact with the bone and body fluid, and the dissolution rate of the surface will be deciding factor for both the fixation period and the fixation strength between the coating and bone.12 In addition to the phase composition of the coating surface, some microstructural evolution characteristics such as surface morphology and surface roughness also affect the dissolution rate of the HA coating. The present work aims to investigate the surface characteristics of the HA coatings sprayed using different spray power and SOD. These technological details of thermal spray are expected to manifest themselves with regards to the in vitro and in vivo behavior of the HA coatings. EXPERIMENTAL Plasma Spraying The feedstock were fully crystalline pure HA powders with particle size distribution in the range of 10 120 pn. They were produced by spray drying followed by heat treatment. A Metco 3MB plasma torch with GH nozzle (Sulzer Metco, Westbury, NY) was used for the atmospheric plasma spray (APS) process. Argon was used as the primary gas (at 50 slpm) and the carrier gas (at 3.65 slpm). Hydrogen was used as the s e c o n m gas while its volume was adjusted to obtain different spray voltages. The powders were sprayed at 14 ghin and three types of spray power and two stand-off distances were used. The mild steel substrates were grit-blasted using A1203 grit and cleaned with acetone before spraying. All coatings were sprayed for the same number of passes and the thickness varied fkom 70 to 120 pn owing to the different deposition efficiencies. Surface Characterization The HA coating surfaces were scanned using computer controlled Philips PW 1729 X-ray diffractometer with CuKoll radiation at 40 kV and 30 rnA. The goniometer was set at scan rate of 0.005 Olsec over 28 range of 20-60°. The acquired X-ray diffraction (XRD) pattern were identified by comparing with the JCPDS (Joint Committee on Powder Diffraction Standard) standards.' The crystallinity of the HA coating was calculated using the following equation: ' J Where Ac is the total integrated intensity of all HA peaks within 25-37O (All angular terms are expressed in terms of 20). It is calculated by multiplying the area of the most intensive (21 1) peak of HA by 3.23, which is the ratio of the total intensity of all HA peaks within 25-37O in JCPDS card (9-432) to the intensity of the (211) peak. The term AA'' is the integrated intensity of the ACP phase, which was evaluated using the area of the amorphous hump between 25Oand 37O. All peak area calculations were performed using curve fitting and the error was estimated within five percent of the mean value. A Nicolet MAGNA-IR 760 spectrometer was used to record the in t iad spectra of the HA powders and coatings. Feedstock powders or powders scmped from the coating surface were mixed with KBr at weight ratio of around 1:20 and pressed into pellets. The spectra were acquired aver the range of 400 4000 mi1 with resolution of 4 mi1. Each spectrum was scanned 4 times to increase the signal-to-noise ratio. The HA coating surfaces were coated with thin layer of carbon and then examined using Philips ISI-SX-30 SEM to ascertain the morphology. The surface roughness (Ra) of the HA coating was measured using Hommel Tester TlOOO Profilometer and each coating was measured 15 times to obtain average value. RESULTS AND DISCUSSION Phase Analysis XRD analysis: Figure 1 shows the XRD patterns of the HA coatings sprayed at different powers and SODS. At the same SOD, when the spray power inmxsed, the overall intensity of HA peaks decreased and the amorphous hump became more obvious. Meanwhile, the peaks of all impurity phases (a-TCP, P-TCP, TTCP and CaO) also increased. On the other hand, at the same spray power, the intensity of all HA peaks decreased appreciably and the amorphous hump became sigruficant when the SOD increased from 80 mm to 160 mm. The peaks of a-TCP, P-TCP and TTCP did not exhibit obvious changes while the CaO peaks increased significantly with the SOD at higher spray power. 20 25 30 35 40 45 50 55 60 20 25 30 35 40 45 50 55 60 2theta (degree) 2theta (degree) SOD = 80 mm SOD = 160 mm Figure 1. XRD patterns of HA coatings sprayed at different spray power and SOD. a is a-TCP, fl is P-TCP, T' is l T P , * is CaO. All other peaks belong to HA. Figure 2. Crystallinity of HA coatings sprayed at different power and SOD