Effect of Titanium Addition on Thermal Stability of Hydroxyapatite /zirconia Nanocomposite

HA/ZrO2 composite nanpowder with various addition of titanium (0.2-2.4wt%) were synthesized via wet chemical method using ZrOCl2.8H2O, H3PO4, Ca(OH)2 and tetraethylorthotitanate as precursor materials. The composites were then sintered at 9001200oC for 2h, respectively. Effect of titanium addition to the composite on the phase composition and thermal stability during thermal treatment was investigated. The results suggested that as the sintering temperature increased from 900°C to 1200oC and with the addition of titanium to the composite below 0.8wt%, main phases were HA and ZrO2. As the sintering temperature increased to 1200°C and with the addition of titanium to the composite above 0.8wt%, main phases were HA, ZrO2 and a small amount of dicalcium phosphate (DCP). There were still no tricalcium phosphate (β-TCP) and CaZrO3 phases observed, which indicated that the addition of titanium restrained decomposition of HA and thermal reaction between HA and ZrO2. It also suggested that the addition of 0.8wt% titanium to the composite was optimum for producing HA/ZrO2 composite nanopowder with expected main phases. With the addition of 0.8wt% titanium to the composite, SEM images showed that the individual particles of the two materials were still visible, which also implied that the reaction between HA and ZrO2 did not occur.


Introduction
Hydroxyapatite (HA) has been used for the regeneration of damaged and defective hard tissues.HA is one of the most extensively used synthetic calcium phosphates for bone replacement because of its chemical similarities to the inorganic component of bone and tooth [1][2].Although HA has excellent biocompatibility properties, it is limited in use due to its brittle nature.Composites of HA and ZrO 2 can combine the biocompatibility of HA and the high strength of ZrO 2 to be used in orthopedic applications.This composite has shown promising improvements in strength, hardness and toughness as compared to HA itself [3][4].
However, calcium phosphates exist in different forms exhibiting different crystal structures and Ca/P ratios such as hydroxyapatite, octacalcium phosphate, tricalcium phosphate (TCP), dicalcium phosphate dehydrate and dicalcium phosphate (DCP) [5].Different phases of calcium phosphate ceramics are used by the modern health care industry, depending upon whether a resorbable or bioactive material is desired [6][7].Two phases that are stable at body temperature and in contact with body fluid are DCP (CaHPO 4 .2H 2 O) at pH<4.2, and HA (Ca 10 (PO 4 ) 6 (OH) 2 ) at pH>4.2.Clearly, HA is the stable phase in the pH range ~7.3 and also the ideal phase for applications inside the human body [8][9].It was found that in most of the sintered samples, HA decomposed completely into β-TCP and CaO.The presence of ZrO 2 had a greater influence on phase changes in HA at different temperatures than that observed with pure HA.At sintering temperatures higher than 1000ºC, calcium from the HA diffused into the ZrO 2 phase, and the HA decomposed to TCP.Above about 1200ºC, CaZrO 3 was formed [10][11][12][13][14].Meanwhile, phase transformation of partially stabilized ZrO 2 from tetragonal to cubic structure can diminish the mechanical properties of these composite.Decomposition of HA and phase transformation of partially stabilized ZrO 2 from tetragonal to cubic structure result in changes in the physical and chemical properties of the material and thus affects the performance of implant material in a living body [15][16][17][18].Restraining decomposition of HA, preventing phase transformation of ZrO 2 from tetragonal to cubic structure and thermal reaction between HA and ZrO 2 , are, therefore, important problems both from a scientific and a biomedical application viewpoint.
The purpose of this study was to fabricate HA/ZrO 2 composite nanopowder with expected main phases by various addition of titanium.Since titanium was added to HA/ZrO 2 composite, where tetraethylortho titanate was used for the source of titanium, decomposition of HA and thermal reaction between HA and ZrO 2 were restrained.

Experimental Procedure
Commercially available ZrOCl 2 .8H 2 O, H 3 PO 4 , Ca(OH) 2 and tetraethylortho titanate were used in the present study as raw materials to fabricate HA/ZrO 2 composite with the addition of 0, 0.2, 0.8, 1.6, and 2.4 wt% of titanium, respectively.H 3 PO 4 and Ca(OH) 2 solution were mixed together with the adding tetraethylortho titanate dropwise using continuous magnetic stirring for 4h at room temperature according that the Ca/(Ti + P) ratio was fixed at 1.667.ZrOCl 2 .8H 2 O was dissolved in distirlled water and pH was maintained about 10 by the addition of ammonia solution.Then ZrOCl 2 solution was added drop wise into each solution respectively according that HA: ZrO 2 weight ratio was kept at 4:1 to produce 20wt% ZrO 2 -HA composite with various addition of titanium.Each mixture was continuously magnetic stirred for 6h and put in polythene bottles using ZrO 2 balls as milling media for about 24h, respectively, to obtain homogeneous mixture of various constituents.The mixed slip was then dried and cold pressed uniaxially under 10MPa in a graphite mold.The compacted samples were sintered at 900-1200ºC for 2h.
The sintered HA/ZrO 2 composite with various addition of titanium were then crushed into powders and subsequently characterised by X-ray diffraction (DNAX-RB, RlGAKU Corporation, Japan) and scanning electron microscopy (Cambridge, UK) for the phases composition.

Results and discussion
X-ray diffraction patterns of the as-prepared HA/ZrO 2 composite nanopowder with various addition of titanium sintered at 1200ºC for 2h are presented in Fig. 1.When the amount of titanium increased in the composites from 0.2wt% to 0.8wt%, all the XRD peaks were corresponding to pure HA and ZrO 2 as seen in Fig. 1.The composites with the addition of 0.8wt% titanium had better crystallinity than the composites with the addition of 0.2wt% titanium.When the addition of titanium to the composite was 1.6wt%, a small amount of HA decomposed into DCP, which are stable at body temperature and in contact with body fluid.With the amount of titanium increased in the composites from 1.6wt% to 2.4wt%, HA decomposed into DCP even more.The addition of titanium to HA/ZrO 2 composite strongly reduced the tendency of HA to decompose to β-TCP during sintering.It also can be found from Fig. 1(b) that with the addition of titanium increased in the composites from 0.2wt% to 2.4wt%, all the peaks of HA shifted to small angle significantly, indicating that titanium atoms did enter the structure of HA and replaced the phosphorus atoms, which resulted in the change of HA crystal lattices.There was no calcium titanite peak detected, which showed that HA did not react with titanium.It can be concluded that the addition of 0.8wt% titanium to the composites was optimum for obtaining HA/ZrO 2 composite nanopowder with expected main phases.Fig. 2 shows X-ray diffraction spectra of HA/ZrO 2 composite nanopowder without and with the addition of 0.8wt% titanium sintered at various temperature for 2h, respectively.As the sintering temperature increased to 1000ºC and without the addition of titanium to the composite, HA started to decompose to a mixture of β-TCP and HA and then reacted with ZrO 2 to form CaZrO 3 as shown in Fig. 2 (a).At about 1200 ºC, this decomposition was nearly complete, the main phases were TCP and CaZrO 3 and there were almost no HA, which was in according with reference [11].This may be because the as-prepared HA without titanium was lack of calcium(Ca 10-x (PO 4 ) 6-x (HP0 4 ) x (OH) 2-x ), in which HPO 4 2-decomposed into P 2 O 7 4-and then reacted with OH -to obtain β-TCP further at high sintering temperature.The thermal reactions in this stage are as equation ( 1), ( 2), ( 3) and ( 4 (4) With the addition of 0.8wt% titanium to the composite and with the sintering temperature increased from 900-1200ºC, the main phases were always HA and ZrO 2 in the composite as shown in Fig. 2(b).There was no intermediate compound such as β-TCP and CaZrO 3 yielded.As a result, the addition of titanium to HA/ZrO 2 composite restrained HA decomposition to β-TCP and thermal reaction between HA and ZrO 2.
Fig. 3 shows SEM images of HA/ZrO 2 composite nanopowder without and with the addition of 0.8wt% titanium sintered at 1200ºC for 2h, respectively.Without the addition of titanium to the composite, the SEM images as shown in Fig. 3(a) seemed to be partially densified.While with the addition of 0.8wt% titanium to the composite, the individual particles of the two materials were still visible.If a reaction occured, the contact area available between the two phases may influence the reaction and amount of new phases formed, which implied that HA and ZrO 2 did not react with each other.(b) with the addition of 0.8wt% titanium sintered at 1200ºC for 2h.
Without the addition of titanium to the composite, gray sliced area represented the mixture of agglomerate HA and β-TCP, while in the white area, some CaO and CaZrO 3 mixed together with the small amount of spheric ZrO 2 .With the addition of 0.8wt% titanium to the composite, sliced HA converted into rod-like gradually and grain size became much smaller.With the addition of titanium to the composite, the titanium atoms replacing the phosphorus atoms with different radius, leads to the crystal lattice abnormal and subsequently prevents the grain growth when sintered, which in turn results in much smaller grain size of the composite.

Conclusions
In summary, with the addition of titanium to HA/ZrO 2 composite, titanium atom entering into HA structure restrained HA decomposition, which improved HA thermal stability.Increasing the sintering temperature from 900°C to 1200ºC and with the addition of titanium to the composites below 0.8wt%, main phases were HA and ZrO 2 .As the sintering temperature increased to 1200ºC and with the addition of titanium to the composite above 0.8wt%, main phases were HA, ZrO 2 and a small amount of DCP.There were still no β-TCP and CaZrO 3 phases observed, which indicated that the addition of titanium restrained decomposition of HA and thermal reaction between HA and ZrO 2 .HA/ZrO 2 composite nanopowder was successfully prepared with expected main phases as the addition of titanium to the composite, which has shown promising combination the biocompatibility of HA and the high strength of ZrO 2 to be used in orthopedic applications.

Fig. 2 .
Fig. 2. X-ray diffraction spectra of HA/ZrO 2 composite nanopowder (a) without the addition of titanium; (b) with the addition of 0.8wt% titanium sintered at various temperature for 2h.

Fig. 3 .
Fig. 3.SEM images of HA/ZrO 2 composite nanopowder (a) without the addition of titanium;(b) with the addition of 0.8wt% titanium sintered at 1200ºC for 2h.