氫氧基磷灰石(HA)具有優良的生物相容性，是最適合用於生醫臨床植入的生物活性陶瓷材料，尤其以電漿熔射被覆HA塗層(HACs)於Ti-6Al-4V合金植入物更被廣泛用於骨科之人工關節的取代。然而電漿熔射製程經常造成HA的相分解與低結晶度等問題，當長期處在與體液接觸的環境時，將容易導致HA塗層的溶解與剝落，而利用後熱處理改善HA塗層之結晶化特性則是有效的解決方案。因此本實驗即有系統的改變後熱處理參數，綜合檢討水熱法(100~200ºC)、真空與大氣後熱處理(400~800ºC)的實驗結果，研究主題包括HA塗層後熱處理結晶化效應的微觀組織解析，尤其著重於探討水熱法的低溫結晶化機制。此外也將釐清後熱處理結晶化對HA塗層結合強度變動之影響，並藉由韋伯分佈函數檢討HA塗層之可靠度。
　　實驗結果顯示不論是水熱法、真空或大氣後熱處理，HA塗層結晶度皆會隨著加熱溫度升高而提升，後熱處理有助於促進HA塗層的結晶化。值得注意的實驗結果是水熱法只需採取100~200ºC之相對低溫即可使HA塗層結晶度提升至與真空或大氣之高溫後熱處理相當的效果。此外，水熱法的飽和水蒸氣加熱氛圍將有助於完全消除因電漿熔射過程所導致的TCP、TP與CaO等雜相，獲得高相純度的HA塗層。低溫水熱法具有顯著優於高溫真空、大氣熱處理的結晶化效果。
　　藉由Arrhenius反應動力學理論探討電漿熔射HA塗層的後熱處理結晶化速率促進機制得知，加熱溫度是促進真空與大氣熱處理結晶化反應進行的要因，並且至少須在500ºC以上才開始有顯著結晶化情形。水熱法在相對低溫即可完成高溫後熱處理結晶化反應的理由點在於飽和水蒸氣壓的加熱氛圍降低了HA塗層結晶化所需之結晶活化能，水熱法的加壓水合效應乃是主導結晶化速率加速進行的要因。水熱法也使得HA結構因電漿熔射所損失的OH基再度獲得鍵結形成接近理論Ca/P計量比之HA結晶，並具有缺陷修復效應而促進HA塗層的緻密化。
　　根據所有後熱處理條件試片進行塗層結合強度試驗之結果顯示，造成HA塗層結合強度變動的要因是受到後熱處理結晶化效應以及塗層微結構劣化效應的影響，並具有使結合強度最佳化的後熱處理溫度，分別為大氣與真空熱處理的600ºC，以及水熱法的150ºC。藉由HA塗層楊氏係數的測定結果發現，楊氏係數隨著後熱處理結晶化而提升，其意義表示結晶化效應將可改善HA塗層的破裂韌性，此乃後熱處理結晶化促進HA塗層結合強度提高的原因。此外水熱法的缺陷修復效應更有助於促進塗層的緻密化而減緩塗層發生劣化，因此水熱法HA塗層結合強度普遍高於大氣或真空熱處理塗層。從韋伯可靠度函數的解析結果顯示，經過後熱處理而獲得強化之HA塗層，其破壞型態皆屬於韋伯模數m > 1類型之損耗破壞模式，代表後熱處理將促進HA塗層更具有生醫應用上的可靠性。
　　然而一旦加熱溫度提高至600ºC以上進行後熱處理結晶化卻反致HA塗層結合強度劣化，此現象可藉由熱分析之結果加以解釋：真空與大氣高溫熱處理將伴隨顯著的塗層結晶化收縮的現象，並且因結晶化HA塗層與Ti-6Al-4V熱膨脹係數的差異而造成HA塗層受到熱應力而誘發結晶化收縮裂縫，導致HA塗層結構嚴重破裂。如此一來，塗層微結構的劣化除了造成結合強度下降之外，也將導致結合強度值變動程度增大，甚至造成韋伯模數與HA塗層可靠度的降低。因此HA塗層利用低溫水熱法的加壓水合結晶化以及缺陷修復效應，將可以避免高溫後熱處理結晶化收縮所導致的HA塗層結構破壞以及結合強度劣化等不良後果。

　　Hydroxyapatite (HA) is a bioactive calcium phosphate ceramic with high potential as an implant material for clinical applications owing to its excellent biocompatibility with a human bone. Plasma-sprayed HA coatings (HACs) on the Ti-6Al-4V alloy is especially a widely applied process of designing a biological fixation for orthopedic joint implant. However, it has been recognized that plasma-sprayed HACs with a fair amount of impurity phases and amorphous calcium phosphate will induce the dissolution and the dissociation for a period of implantation. For the biological stability, a significant crystallization of plasma-sprayed HACs with an appropriate post-heat treatment is an effective method to improve the phase purity and the crystallinity of HACs. Therefore, the objective of this investigation is to systematically study the crystallization effect of post-heat treatments, including hydrothermal treatment at 100-200ºC, vacuum and atmospheric heat treatments at 400-800ºC, on the evolution of HACs microstructural features, especially focuses on clarifying the effect of hydrothermal treatment with lower crystallization temperatures. Moreover, this study will also clarify the crystallization effect of post-heat treatments on the bonding strength fluctuation and assess the reliability of the HACs with an analysis of the Weibull distribution function.
　　The experimental results show that the crystallinity of HACs increases with increasing heating temperatures not only by both the atmospheric and the vacuum heat treatments, but also by the hydrothermal treatment. Post-heat treatments can improve the crystallization of the HACs. It is worth noting that just lower heating temperatures 100-200ºC are required to see a significant rise in crystallinity for the hydrothermally-treated HACs compared with the vacuum and atmospheric heat treatments. Furthermore, the ambient saturated steam atmosphere of the hydrothermal treatment is more favorable to eliminate the TCP, TP and CaO impurity phases of plasma-sprayed HACs. Low-temperature hydrothermal treatment shows a more superior crystallization effect than high-temperature post-heat treatments in a vacuum or in an atmospheric environment.
　　According to the reaction kinetics of Arrhenius equation, the heating temperature, especially higher than 500ºC, is a control factor for promoting the HA crystallization of vacuum and atmospheric post-heat treatments. It should be worth noting that the ambient saturated steam pressure of hydrothermal treatment is thought of as an important factor that not only reduces the activation energy of HA crystallization calculated from the Arrhenius equation, but also plays an important role in lowering crystallization temperatures. Experimental evidence confirmed that the saturated steam pressure of hydrothermal treatment can promote and accelerate the HA crystallization. The finely-crystallized HA crystals, with a Ca/P ratio of 1.67, can be recognized as a nucleation and grain growth phenomenon by the replenishment of OH groups into the hydroxyl-deficient HA structure. A defect-healing effect can be recognized to diminish the spraying defects with these crystalline HA.
　　Referring to the bonding strength testing results, it can be recognized that the HA crystallization and the microstructural degradation are two effects to dominate the bonding strength evolution and data fluctuation. These effects result in optimal heating conditions, which can be acquired at 600C for both vacuum and atmospheric heat treatments, and a lower temperature of 150C for hydrothermal treatment. The Young’s modulus test is used to determine the dependence of post-heat treatment on the bonding strength. The improved Young’s modulus of the heat-treated HACs with increasing HA crystallinity implies that the fracture toughness of the HACs increased with crystallization. Therefore, the bonding strength of the heat-treated HACs is further enhanced owing to the improvement of Young’s modulus from crystallization. And a higher bonding strength of hydrothermally-treated HACs than vacuum and atmospheric heat-treated ones can be recognized for its much denser microstructure from the defect-healing effect to reduce the coating degradation. From the analysis of Weibull distribution function, the strengthening HACs are generally reliable materials with a wear-out failure model. It means that post-heat treatments promote plasma-sprayed HACs to have a better reliability for further biological applications.
　　According to the results of thermal dilatometry, the bonding strength degradation of vacuum and atmospheric heat-treated HACs occurred at higher heating temperatures than 600C is mainly due to the effect of its significant crystallization-induced contraction. This effect generates a thermal stress within the HACs because the mismatch of the thermal expansion coefficient between the crystallized HACs and Ti-6Al-4V substrate, and the HACs will be cracked in the form of contraction-induced cracks after vacuum and atmospheric heat treatments. The cracked microstructural feature will not only induce a bonding strength degradation of heat-treated HACs, but also increase the bonding strength data fluctuation. Moreover, this phenomenon may even induce a low Weibull modulus and a decreased reliability of the HACs. Therefore, the hydrothermal treatment of HACs with low-temperature crystallization and a defect-healing effect will be effective to prevent the above-mentioned microstructural defect and bonding strength degradation from the high-temperature crystallization process.

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