THESIS
2001
viii, 103 leaves : ill. ; 30 cm
Abstract
Plasma-sprayed hydroxyapatite (HA) coated Ti-6A1-4V is a promising material for biomedical implants. Successful applications of this implant material rely on fundamental understandings of the microstructure characteristics and mechanical behavior of the HA coating. The objectives of this M. Phil. study are (1) to characterize the heterogeneous features of the plasma sprayed HA coatings, and (2) to study the shear strength and shear fatigue of HA coatings by torsion testing....[
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Plasma-sprayed hydroxyapatite (HA) coated Ti-6A1-4V is a promising material for biomedical implants. Successful applications of this implant material rely on fundamental understandings of the microstructure characteristics and mechanical behavior of the HA coating. The objectives of this M. Phil. study are (1) to characterize the heterogeneous features of the plasma sprayed HA coatings, and (2) to study the shear strength and shear fatigue of HA coatings by torsion testing.
Although the XRD revealed multiphases in the as-sprayed coating and improvement of homogeneity in the coatings by post heat treatment, more advanced techniques, including micro Raman spectroscopy, ToF SIMS and XPS were employed to reveal the chemical and microstructure variation through the coating thickness direction by examining the cross-section of coatings. The results of micro Raman spectroscopy confirmed that chemical gradient exists along coating thickness direction. The coating crystallinity decreases with the distance from interface in the as-sprayed conditions. The Raman spectra also showed that the heat treatment effectively promote chemical homogeneity through the thickness direction. Two recently developed techniques were examined and their possible applications to the plasma sprayed HA coating were explored. The techniques include identifying calcium phosphates with PO
3-/PO
2- in ToF-SIMS and with the energy loss peaks of O 1s as well as Ca/P, O/Ca ratios in XPS. This study confirmed that such techniques can be used for identifying certain calcium phosphate phases in their pure power states. Their applications to the plasma sprayed coatings however have turned out to be unsatisfactory. This study also found out that it was difficult to use the information of energy loss peaks of O 1s in XPS, because the weak peaks make substantial errors in their measurement.
Torsion tests were employed to examine the coating bonding shear resistance, which eliminated the normal stress associated with other types of shear testing. The torsion load was applied by a frustum test device developed in this laboratory and generated pure shear stresses on plasma-sprayed HA coatings interface. Interfacial shear strength and fatigue strength were acquired for different coating thickness samples and for the as-sprayed and as-treated coating conditions. The interfacial shear strengths obtained by torsion loading were close to the values obtained by the frustum tension shearing in a previous study, e.g. 28 ~ 40 MPa. The thicker the coating is, the lower the interfacial shear strength. So-called staircase method was employed to approach coatings interfacial fatigue strength quickly, which resolved the uncertainty of fatigue failure of coatings during torsion. Also, higher fatigue resistance was found in thin coatings than in thick ones, consistent with the results of the frustum tension fatigue.
Keywords: plasma spray, calcium phosphate, hydroxyapatite, coating, inhomogeneity, XPS , ToF-SIMS, Raman, interfacial shear strength, tension, torsion, staircase, fatigue strength
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