Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/2022
標題: 鈦鋁介金屬化合物之抗氧化被覆研究
On Protective Coatings of Titanium Intermatallics by Pack Ementation
作者: 黎衍璋
關鍵字: Pack Cementation;包覆粉浴法;Diffusion Coating;High Temperature Oxidation Resistance;Titanium Aluminide Intermatallics;擴散被覆;抗高溫氧化;鈦鋁介金屬化合物
出版社: 機械工程研究所
摘要: 
本研究探討應用擴散被覆(Diffusion Coating)技術之包覆粉浴
(Pack Cementa tion)法,在Ti3A1-Nb介金屬化合物表面形成擴散被覆
層之反應製程及機構,以期在高溫氣化環境中生成安定之A12O3保護膜,
並提高Ti3A1-Nb介金屬化合物的實際應用溫度。
經實驗結果顯示,鋁化被覆可有效達到被覆目的,且有優異的
抗氧化性能,而鋁量的添加,有助於形成A12O3擴散阻礙層,提昇抗氣
化性能;900℃,24hr鋁化被覆較1000℃及1050℃,24hr鋁化被覆之抗
氣化性佳。鉻化被覆則未能形成鉻鋁合金屬,以致被覆後表皮嚴重剝
落。95Cr-5A1高鉻低鋁被覆層氧化則有剝離氣化(Breakaway Oxidatiion)
現象。
Ti3A1-Nb介金屬化合物鋁化被覆模式為鈦向外擴散型,與鋁生
成 TiA13富鋁,以及TiA1中間介層。而氧化性能的優劣,端視是否形
成長距離且均質的高鋁活性TiA13富鋁層。Ti3A1-Nb介金屬化合物鋁化
被覆後氧化機構為AI2O3及TiO2混合氧化機構:初始氧化為A12O3混合
著以TiO2為主的介面控制機構,後續氧化為A12O3擴散控制機構。

Titanium aluminide intermetallic compounds including Ti3Al and Tial were identified as the promising candidate areospace materials for the next generation aviation requirements. However, the disadvantages of their low temperature brittleness and high temperature oxidation still prohibit these advanced materials from commercialization. The purpose of this study is to investigate the reaction mechanism and the influence of pack aluminizing and/or chromizing coatings on the oxidation resistance of Ti3Al-Nb intermetallics.
For pack aluminizing processes, the aluminim enrichment has successfully converted the surface layer Ti3Al substrate into TiAl3, which has higher tendency to grow stable α-Al2O3 at elevated temperatures. Experimental results showed that the 900℃, 24hrs pack revealed better oxidation resistance than the 1000℃, 24hrs and 1050℃, 24hrs packs. Sifnificant reduction in oxidation kinetics was achieved by the pack aluminizing process. The chromizing pack failed to convert the titanium aluminide into chrome-aluminide coatings which resulted in severe spallation from substrate surface. Due to the ordered structure, chromium diffused very inefficiently in the Ti3Al lattice. The simultaneous chrome-aluminizing process showed similar results as a low concentratin aluminizing process. The anticipated effect of Cr as the secondary oxygen getter was never observed.
The reaction mechanism of pack aluminizing is featured in the outward diffusion of Ti to react with Al coming fromt he activated pack to form TiAl3 phase at the solid/gas interface. The Nb distribution served as the reference system. The oxidation of the aluminized substrate started with co-oxidation of Al2O3 and TiO2 via interface control. Subsequently, the growth of Al2O3 took the control and formed a dense barrier layer between the substrate and the transient oxides. Due to the high concentration of Al content at surface, scattered cracks were observed. Within the 120 hrs testing period, most cracks were arrested by lateral oxidation. However, to achieve the optimum mechanical performance, a reduced aluminum concentration by low activity packs or rare earth doping is recommended.
URI: http://hdl.handle.net/11455/2022
Appears in Collections:機械工程學系所

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