Introduction: Biodegradable Mg-matrix composites with high specific strength, biocompatibility and biodegradability emerged as very potential candidates for implant applications. In the recent years, researchers paid a great attention on development of biodegradable Mg-matrix composites. However, the as-produced composites still exhibit limited mechanical properties[1],[2]. In this research, Spark Plasma Sintering (SPS) technique was employed for fabricating high mechanical property Mg-matrix in situ composites for temporary implant applications.
Materials and Methods: High purity Mg and ZnO powders were used for ball mixing with various mass ratios, Mg-5 mass% ZnO, Mg-10 mass% ZnO, Mg-20 mass% ZnO and Mg-50 mass% ZnO.
Mg-50 mass% ZnO powders were sintered by SPS at 450, 500 and 550 oC for 30 minutes in order to investigate the composition and microstructure evolution during in situ reactions.
Pure Mg, Mg-5 mass% ZnO, Mg-10 mass% ZnO and Mg-20 mass% ZnO powders were sintered at 550 oC for 30 minutes for mechanical property evaluation.
Results and Discussion: As shown in figure 1, XRD patterns of Mg-50 mass% ZnO composites sintered at 450, 500 and 550 oC indicated that there were several in situ reactions occurred during sintering process as following equations[3]:
From 450 to 500 oC,
Mg(s) + ZnO(s) → MgO(s) + Zn(l)
From 500 to 550oC,
Mg(s) + Zn(l) → Mg-Zn(l)
And during cooling,
Mg-Zn(l) → Mg(s) + MgxZny(s)
Where the subscripts s and l refer to solid and liquid phases respectively, while MgxZny refers to MgZn, Mg2Zn3 and Mg7Zn3.
Figure 2 shows compression stress-strain curves of pure Mg, Mg-5 mass% ZnO, Mg-10 mass% ZnO and Mg-20 mass% ZnO composites sintered at 550 oC. As could be seen that, Mg-matrix in situ composites exhibited not only much higher compression strengths but also higher strains at failure compared to those of pure Mg. Specifically, the highest strength at 380 MPa was observed in Mg-20 mass% ZnO composite, and the highest failure strain at 12.9% was achieved in Mg-5 mass% composite compared to 156 MPa strength and 10.2% failure strain of pure Mg 10.2%. These improvements in the strength and ductility of the composites are attributed as the positive effects of in situ reaction products, which act as reinforcements increasing both strength and ductility. As indicated from XRD analysis, there were liquid phases of Zn and Mg-Zn intermetallics formed from in situ reactions during sintering. These liquid phases created well bonding of both Mg particles-reinforcements and Mg-Mg particles resulting in improvement of both strength and failure strain of the composites. In other words, the formation of liquid in situ products strongly contributed to the enhancement of strength and ductility of the fabricated composites compared to pure Mg. Further comparison between as-produced composites and cortical bones indicated that the strength of the composites are much higher than that of cortical bones (100-230MPa)[4].
Conclusion: With the effect of in situ reactions, ultimate compression strengths and failure strains of the composites were significantly improved compared to those of pure Mg. The fabricated composites also have much higher compression strength than that of cortical bones. These make as-synthesized composites become very potential candidate for temporary implant applications.
References:
[1] Seong JW, Kim WJ. Acta Biomater 2015;11:531–42.
[2] Gu X, Zhou W, Zheng Y, Dong L, Xi Y, Chai D. Mater Sci Eng C 2010;30:827–32.
[3] Deng CJ, Wong ML, Ho MW, Yu P, Ng DHL. Compos Part A 2005;36:551–7.
[4] Kokubo T, Kim HM, Kawashita M. Biomaterials 2003;24:2161–75.