Sintering Behavior and Property of Bioglass Modified HAAl 2 O 3 Composite

The bioglass modified HA-Al2O3 composites were successfully fabricated by mixing HA, synthesized by wet chemical method between precursor materials H3PO4 and Ca(OH)2, with 25wt% Al2O3 and different content of bioglass (5%, 25%, 45%, 65wt%) respectively, with a mole fraction of 53.9%SiO2, 22.6%Na2O, 21.8%CaO, and 1.7wt%P2O5, sintered in air at various temperatures (750-950°C) for 2h. when the content of bioglass is below 45wt% in the composite, HA decomposes completely and transforms to β-TCP. The main phase in this case are β-TCP, Al2O3 and Ca3(AlO3)2.When the content of bioglass is above 45wt% in the composite, the decomposition of HA to β-TCP is suppressed and the main phases in this case are Al2O3 and HA, DCP CaHPO4 and β-TCP, which almost have the same chemical composition, forming ternary-glass phase, and have better bioactive than pure HA. It can also be found that at the certain addition of bioglass, the higher sintered temperature, the bigger volume density and flexural strength of the composite are, but when the sintered temperature reaches 950°C, they decrease. This modified HA-Al2O3 composites by calcium silicate glass have a much lower sintering temperature and decrease the production cost much.


Introduction
Hydroxyapatite owing to its excellent biocompatibility, has been studied extensively as a bone replacement material.The use of synthetic HA is, however, limited to low load bearing applications due to its inferior mechanical properties (especially fracture toughness) compared to cortical bone [1][2][3].Hence, many attempts have been made to improve the mechanical properties through the incorporation of ceramic second phases, such as ZrO 2 [4], Al 2 O 3 [5], and TiO 2 [6].The addition of Al 2 O 3 in the form of particles [7][8][9] to HA matrix has drawn much attention due to its biocompatibility coupled with the tendency to enhance the mechanical properties of HA [10][11][12].They found that in most of the sintered samples, HA decomposes virtually completely into β-TCP and CaO and the latter in turn completely dissolves in Al 2 O 3 .The composites possess higher fracture toughness values than that of pure HA coupled with good biocompatibility behaviour.However, the addition of Al 2 O 3 results in the content of β-TCP increases and the biocompatibility of the composite decreases.Meanwhile, a appropriate content of β-TCP in HA is helpful for the growth of artificial bone and a higher content of β-TCP in HA have a serious influence on its mechanical property and chemical stability.Most importantly, the addition of Al 2 O 3 elevates the sintering temperate, so the application range of this composite is limited.Improving the mechanical property, decrease the sintering temperature and the production cost are becoming a hot topic.
In this research, calcium silicate glass was added to HA-Al 2 O 3 composite, sintered at 750-950°C for 2h, thermal reaction and crystallization occurred.Then HA TCP and DCP phase appear, forming ternary-glass phase, which makes the modified HA-Al 2 O 3 composite by calcium silicate glass have a much lower sintering temperature and decrease the production cost much.

Experimental
Commercially available SiO 2 P 2 O 5 NaH 2 PO 4 and CaCO 3 were used in the present study as raw materials to fabricate bioglass with a mole fraction of 53.9%SiO 2, 22.6%Na 2 O, 21.8%CaO, and 1.7%P 2 O 5 and the particle size less than 25µm.HA, synthesized by wet chemical method between precursor materials H 3 PO 4 and Ca(OH) 2 and sintered at 800°C for 1h., was mixed with different content of the as-prepared bioglass (5%,25%,45%, 65wt%) respectively.Then 25wt% Al 2 O 3 was added to each group of the mixture.Each mixed powders were put in deionised water using magnetic stirring for 40 min and then milled in polythene bottles using Al 2 O 3 balls as milling media for about 24h, respectively, to yield homogeneous mixing of the various constituents.The slip was then cast into bar shaped with dimensions of 5mm × 10mm × 10mm.The green specimens were dried at 150°C for 24h and then sintered in air at various temperature (750-950°C) for 2h to achieve bioglass modified HA-Al 2 O 3 composites.
The density of the as-prepared bioglass modified HA-Al 2 O 3 composites were measured by the Archimedes method.Three-point bending tests were used to evaluate the flexural strength.Phase identification was performed on X-ray diffraction (XRD, D/max-2200PC) using Cu Ka radiation.

Results and Discussion
Fig. 1shows the X-ray diffraction patterns of bioglass modified HA-Al 2 O 3 composites when the addition of bioglass is 45wt% sintered at 750-950°C for 2 h.It can be seen that when the sintering temperature is below 950°C, the main phase are HA and Al 2 O 3 with a small amount of DCP and β-TCP.After the sintering temperature reached 950°C, the content of β-TCP increases dramatically and the peaks of Ca 3 (AlO 3 ) 2 phase appear.The formation of Ca 3 (AlO 3 ) 2 by the solid solution of stoichioetric HA, thereby leading to the formation of an increased amount of β-TCP.It can be seen that strong β-TCP and Ca 3 (AlO 3 ) 2 peaks are observed, at the expense of HA peaks, which suggests an extensive decomposition reaction have occurred.The reactions in this stage can be shown as follows, which are in accordance with reference [13].
Ca 5 (PO 4 ) 3 OH→β-Ca 3 (PO 4 ) 2 +CaO+H 2 O (1) Al 2 O 3 +3CaO→Ca 3 (AlO 3 ) 2 (2) Fig. 2 shows XRD patterns of bioglass modified HA-Al 2 O 3 composites sintered at 950°C for 2h with various bioglass content.It shows that when the content of bioglass is below 45wt% in the composite, HA decomposes completely and transforms to β-TCP.The main phase in this case are β-TCP, Al 2 O 3 and Ca 3 (AlO 3 ) 2 .When the content of bioglass is above 45wt% in the composite, the decomposition of HA to β-TCP is suppressed since with the increasing of the content of bioglass, it has a block effect on sintering.The main phase in this case are HA, DCP, TCP and Al 2 O 3.  It indicates from Fig. 3 that with the increasing of bioglass content, the volume density of the composites decrease at the certain sintered temperature.When the content of bioglass is about 5wt%, the volume density reaches maximum.It shows from Fig. 4 that the flexural strength of the composites increase, with the increasing of sintering temperature and the maximum reaches 150MPa.The reason for this phenomenon is that the sintering system of the composite is solid sintering, bonds between the grains are loosen and so many pores exist without addition of the bioglass.With addition of the bioglass, the sintering system of the composite change to liquid sintering, which reduces the interface energy and elevates the sintering kinetics.When the content of bioglass is about 5wt%, with the liquid lubrication and tensile effect of capillary action, grains rearrange in some extent and parts of the pores as well as other defects are eliminated, which contributes to densification and grain size distribution is more uniform.However, when the content of bioglass is above 10wt%, excessive liquids distribute at the grain boundary and abnormal grain growth appears.Meanwhile, contraction stress produces during cooling and cracks occur along grain boundary, which contributes to reduce the dendification.It can also be found that at the certain addition of bioglass, the higher sintering temperature, the bigger volume density of the composite is, but when the sintering temperature reaches 950°C, it decreases.The reason for this phenomenon is that with the increasing of sintering temperature, the amount of molten state increase and pores decrease, which improves the densification.However, when the sintering temperature is at 950°C, the composite dehydrates, eliminating the hydrated water and turn to oxygen phosphorite.The reaction in this stage are as follows: 2OH→O+H 2 O↑ (3) Ca 10 (OH) 2 (PO 4 ) 6 →H 2 O+Ca 10 (PO 4 ) 6 O•ϒ (4) Where ϒ is vacancy and Ca 10 (PO 4 ) 6 O•□ is oxygen phosphorite.Since the decomposition reaction suppresses the sintering and decreases the sinterability, which reduces the dulk density of the composite at 950°C.

Conclusions
Bioglass modified HA-Al 2 O 3 composites were successfully fabricated by mixing different content of HA with 25wt% Al 2 O 3 and different content of bioglass, sintered in air at relatively low temperatures (750-950°C) for 2h.It can be concluded that when the content of bioglass is above 45wt% in the composite, the decomposition of HA to β-TCP is suppressed and the main phases in this case are Al 2 O 3 and HA, DCP and TCP, which almost have the same chemical composition, forming ternary-glass phase, and have better bioactive than pure HA.It can also be found that when the addition of bioglass keeps constant, the higher sintering temperature, the bigger volume density and bending strength of the composite are, but when the sintering temperature reaches 950°C, it decreases.This modified HA-Al 2 O 3 composite by calcium silicate glass has a much lower sintering temperature and decreases the production cost much.

Fig. 1 .
Fig. 1.X-ray diffraction patterns of bioglass modified HA-Al 2 O 3 composites as a function of sintering temperature when the addition of bioglass is 45wt%.

Fig. 2 .
Fig. 2. X-ray diffraction patterns of bioglass modified HA-AlG 2 O 3 composites as a function of bioglass content sintered at 950 o C.