PHASE TRANSFORMATIONS IN PLASMA SPRAYED HYDROXYAPATITE COATINGS

Transcription

1 Scripta mater. 42 (2000) PHASE TRANSFORMATIONS IN PLASMA SPRAYED HYDROXYAPATITE COATINGS C.F. Feng, K.A. Khor, E.J. Liu and P. Cheang* School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore , Singapore *School of Applied Science, Nanyang Technological University, Nanyang Avenue, Singapore , Singapore (Received August 30, 1999) (Accepted in revised form September 23, 1999) Keywords: Coating; Plasma spray; Differential scanning calorimetry (DSC); Hydroxyapatite; Phase transformation Introduction Hydroxyapatite (HA: Ca 10 (PO 4 ) 6 (OH) 2 ) is a bioactive material that has recently been receiving much attention mainly because of its ability to bond chemically with bone [1 3]. It is often applied clinically as a coating on an inert metallic implant such as Ti-6Al-4V. Plasma spraying is a widely used coating process to deposit HA powders onto the metallic implants. Amorphous calcium phosphate is normally produced in the coatings during the plasma spraying, not only due to the high cooling rate but also due to the relevant intrinsic properties of HA [4]. In addition, calcium phosphate phases other than crystalline HA and amorphous calcium phosphate are usually identified in the as-sprayed coatings. These phases include tri-calcium phosphate (TCP), tetra-calcium phosphate (TTCP) and/or CaO depending on the plasma conditions and the type of the HA material. A high crystallinity level is desirable in order for the materials to have good bioactive properties. Like other amorphous phases [5], the amorphous calcium phosphate in the as-sprayed coatings is thermodynamically metastable and an appropriate thermal treatment could induce a crystallization process to occur. That is why the as-sprayed coatings are usually subjected to a post heat-treating cycle. In this study, by means of differential scanning calorimetry (DSC) and X-ray diffraction (XRD), we aim to understand the phase transformations that could take place in an as-sprayed hydroxyapatite coating during a post heat-treatment process. We investigated the temperatures at which the phase transformations took place and elucidated fundamental understandings about the phase transformations (i.e. activation energy and enthalpy). Experimental The HA powder used in this study was produced in-house via a wet chemical reaction [6]. After being plasma-spheroidized and heat-treated at 800 C, the HA powder of m in size was deposited onto a Ti-6Al-4V substrate by plasma spraying. The plasma-spraying conditions for preparing the coating are given in Table 1. The DSC (Netzsch 404 C with TASC 414/3A controller) was used for getting information about the temperatures at which the phase transformations take place in the coating material during the thermal /00/$ see front matter Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S (99)

2 104 PLASMA SPRAYED HYDROXYAPATITE COATINGS TABLE 1 Plasma-Spraying Parameters for Preparing the Hydroxyapatite Coating Net energy: Primary gas: Auxiliary gas: Powder feedrate: Stand-off distance: 12 kw Argon (60 scfh) Helium (40 scfh) 10 g/min 10 cm process. The protective atmosphere used was nitrogen (flow rate: 150 ml/min) and the thermal cycle was as follows: heating up to 1000 C at 10 C/min followed by cooling down to room temperature at 20 C/min. Based on the DSC curve, intermediate products, which were formed as intermediate steps between the starting material and the final product, were also prepared in the DSC furnace: samples were heated up to predetermined temperatures at 10 C/min followed immediately by cooling at 20 C/min. The intermediate products along with the starting material and final product were analyzed by XRD analysis for phase determination (Philips vertical goniometer; generator: PW1830; Cu-K radiation). The enthalpies of the phase transformations were determined from the DSC data. On the other hand, in the DSC measurements when the heating rate varies the exothermic peak shifts. Based on the data of peak shifts versus heating rates the activation energies, being important kinetic parameters, for the phase transformations were calculated. Results and Discussion The DSC curve that corresponds to the formation of the final phase composition in the coating is presented in Fig. 1. It can be seen that there are three exothermic peaks during the heating process and Figure 1. DSC scan of the as-sprayed coating.

3 PLASMA SPRAYED HYDROXYAPATITE COATINGS 105 Figure 2. XRD patterns of the coating materials: the as-sprayed material (Pattern I), the intermediate products when the reactions were halted around point A (Pattern II) and point B (Pattern III) respectively, and the final product obtained after the thermal treatment up to 1000 C (Pattern IV) (see Fig. 1). the peak temperatures are 381 C, 601 C and 698 C respectively. The three peaks represent three intermediate steps between the starting material and the final product. That is, the phases of the final product are developed during heating and they are thermodynamically stable during cooling. The stability during cooling is confirmed because no thermal effects are observed during cooling (Fig. 1). Two points, i.e. A and B, are marked along the heating process in Fig. 1. For preparing the intermediate products, heating was halted at temperatures around these two points in the DSC furnace. In both DSC scans no thermal effects were found during cooling. In other words, like the final product, the intermediate products were developed during heating and were thermodynamically stable during cooling. The XRD patterns of the following samples are shown in Fig. 2: the as-sprayed coating material (Pattern I), the respective samples when the reactions were halted around points A (Pattern II) and Figure 3. A comparison of the (2 0 0) peak of CaO (refer to Fig 2 for the XRD pattern identifications).

4 106 PLASMA SPRAYED HYDROXYAPATITE COATINGS Figure 4. A comparison of the peak intensities among the four XRD patterns: three major TTCP peaks and a HA peak (refer to Fig 2 for the XRD pattern identifications). B (Pattern III), and the final product obtained after the thermal treatment up to 1000 C (Pattern IV). It appears that all the samples are composed of HA, CaO and TTCP. However, they differ in the crystallinity level and the relative amounts of the phases. Figs. 3 and 4 show the zoomed 2 ranges (i.e and ), which enables us to clearly compare the intensities of the relevant peaks of CaO/TTCP/HA among the different XRD patterns. These peaks are indicated in Table 2. They are chosen here because the intensity of each of them is contributed by a single phase. It can be seen from Figs. 3 and 4 that the intensities of the relevant major peaks of CaO and TTCP are not changed from Pattern I to Pattern II. That is, after the 1 st intermediate step the amounts of the two phases remain the same. As for HA, the (2 1 0) peak does not show up in Patterns I & II in Fig. 4 because the amount of this phase is not enough to give rise to a visible intensity of this minor peak. However, between these two XRD patterns there is an obvious difference in the intensity of the peak located around the 2 value of 31.8 (Fig. 2). In principle, the intensity of this peak is contributed by both HA and TTCP. Since the amount of TTCP is known not to vary after the 1 th intermediate step, we could conclude that the amount of HA increases after this step. Moreover, it is believed that the OH group in the newly produced amount of HA comes from the amorphous calcium phosphate. All things considered, the reaction taking place during the 1 st intermediate step can be described as follows: Amorphous calcium phosphate 3 HA (1) TABLE 2 Peaks Zoomed for a Clear Comparison about the Corresponding Intensities among Different XRD Patterns Phase CaO TTCP HA Peak identification (200) (032) (040) (221) (210) Standard peak from JCPDS card [7-9] Intensity

5 PLASMA SPRAYED HYDROXYAPATITE COATINGS 107 TABLE 3 The Peak Temperatures in the DSC Scans Corresponding to Different Heating Rates Peak temperature ( C) Heating rate ( C/min) Peak I Peak II Peak III Regarding the 2 nd intermediate step, Fig. 3 shows that the intensities of CaO in Patterns II & III are very comparable. This means that the 2 nd intermediate step leads to no change in the amount of CaO in the coating. However, the amount of TTCP increases after this intermediate step, as can be evidently seen in Fig. 4 by comparing the intensities of the three TTCP peaks between Patterns II & III. With respect to HA, the minor peak in Fig. 4, i.e. (2 1 0), starts to be present in Pattern III, which implies that the amount of HA increases from Pattern II to Pattern III. This point is more distinct if the (0 0 2) peak of HA is considered. In the JCPDS card for HA, the (0 0 2) peak occurs at (2 ) with the intensity of 40 [9]. By comparing the intensity of this peak between Patterns II & III it is clear that the 2 nd intermediate step results in an increment of the amount of HA. Furthermore, it is believed that during the thermal heating within this step HA and TTCP grow at the expense of the amorphous calcium phosphate. In short, the reaction formula during the 2 nd intermediate step can be written below: Amorphous calcium phosphate 3 HA TTCP (2) In connection with the 3 rd intermediate step, we noticed that the amount of CaO clearly increases after this step (Fig. 3). Similarly, the intensity of the HA peak presented in Fig. 4 is also observed to increase from Pattern III to Pattern IV. Logically, the growth of the HA and CaO also proceeds at the expense of amorphous calcium phosphate. The amount of TTCP practically remains unchanged after this step, which can be seen in Fig. 4 by comparing the intensities of the three TTCP peaks between Patterns III and IV. Accordingly, the reaction that takes place during this intermediate step is as follows: Amorphous calcium phosphate 3 HA CaO (3) In summary, when the as-sprayed coating is heated at 10 C/min up to 1000 C three intermediate reactive steps are involved, which correspond to three exothermic peaks whose peak temperatures are 381 C, 601 C and 698 C respectively. The reactions of the three intermediate steps are described in Formulae 1, 2 and 3 respectively. Furthermore, at a higher heating rate the exothermic peaks is observed to shift towards a higher temperature and vice versa (Table 3). As argued by Kissinger [10], the variation of peak temperature (T, Kelvin temperature) as a function of heating rate (, C/min) is given by: Ln T 2 / E c /RT C (4) where R is the gas constant ( J.K 1.Mole 1 ), E c the activation energy and C a constant. By plotting Ln(T 2 / ) versus 1/T linear regressions should be obtained for all the three intermediate steps (Fig. 5). The activation energies can then be calculated from the slopes and the results are included in Table 4. Alternatively, the activation energies can also be obtained by means of the Ozawa method [11]. Following this method, Ln is plotted against 1/T and similar values of the activation energies are

6 108 PLASMA SPRAYED HYDROXYAPATITE COATINGS Figure 5. Kissinger plots for determining the activation energies for the different intermediate reactive steps. accordingly determined (Table 4). In parallel, the heat produced by the reaction during each intermediate step can be known as well simply by determining the area under the exothermic peak. Conclusions When an as-sprayed hydroxyapatite coating is subjected to a thermal treatment in order to restore its crystallinity level, various phase transformations may take place. When the as-sprayed coating used in this study is heated up to 1000 C, three intermediate steps are involved between the starting material and the final product: I. Amorphous calcium phosphate 3 HA II. Amorphous calcium phosphate 3 HA TTCP III. Amorphous calcium phosphate 3 HA CaO The three intermediate steps correspond to three exothermic peaks during the heating process of a DSC scan. The peak temperatures are 381 C, 601 C and 698 C respectively if the heating rate is 10 C/min. The activation energies for the intermediate steps are 198kJ/mole, 262kJ/mole and 234kJ/mole respectively, and heats produced 8J/g, 49J/g and 2 J/g respectively. TABLE 4 The Activation Energies and Enthalpies for Different Intermediate Steps When the As-Sprayed Coating Is Heated Up to 1000 C Intermediate step identification Activation Kissinger method energy (kj/mole) Ozawa method Enthalpy (J/g)

7 PLASMA SPRAYED HYDROXYAPATITE COATINGS 109 Acknowledgment This research is financially assisted by NTU in the form of a research grant (JTARC4/96). The authors wish to thank Mr. W.K. Kweh for providing the as-sprayed material. References 1. S. F. Hulbert, C. Bokros, L. L. Hench, J. Wilson, and G. Heimke, in High Tech Ceramics, ed. P. Vincenzini, p. 189, Elsevier, Amsterdam, The Netherlands (1987). 2. L. L. Hench, J. Am. Ceram. Soc. 74, 1487 (1991). 3. M. Jarcho, Clin. Orthop. 157, 259 (1981). 4. K. A. Gross, C. C. Berndt, and H. Herman, J. Biomed. Mater. Res. 39, 407 (1998). 5. F. Q. Guo and K. Lu, Metall. Mater. Trans. 28A, 1123 (1997). 6. C. F. Feng, Y. H. Cao, W. K. Kweh, K.A. Khor, and P. Cheang, Presented at the 8th International Conference on Processing and Fabrication of Advanced Materials, Singaproe, 8 10 September JCPDS Card No (1994). 8. JCPDS Card No (1994). 9. JCPDS Card No (1994). 10. H. E. Kissinger, Anal. Chem. 29, 1702 (1957). 11. T. Ozawa, J. Therm. Anal. 2, 301 (1970).