由於鈦金屬的耐蝕性、質輕、強度高及延展性佳等特性，廣泛應用在航太、石化及生醫工業等方面上，但當鈦金屬應用在核能電廠或光電玻璃面板所需研磨漿料之輸送管路上，卻面臨嚴重的沖蝕磨耗破壞問題。因此本研究利用PTA(plasma transferred arc)被覆強化顆粒-NbC於商業純鈦表面，並藉由改變電流觀察其被覆層、被覆層與基材之界面間的凝固組織及硬度的變化情形，並探討被覆層之沖蝕磨耗機制。沖蝕試驗選用之顆粒為粒徑約295μm之SiO2，且沖蝕速度選擇為66 m/sec。
　　實驗結果顯示，高、低電流所得之被覆層為基地相固溶約10 at.% Nb及1 at.% C之α-Ti，NbC顆粒及NbC熔解後與Ti反應而重新晶出的TiC則散佈於基地相中，而由於NbC較鈦密度為高所導致的沉澱現象，因此被覆後以α-Ti為主之被覆層橫截面因碳化物種類及組織的不同，由表面端至基材端可大致分為：均為TiC的上被覆層、TiC＋NbC共存的下被覆層、界面區(界面層、部份熔解區)及基材(包括熱影響區)。另外因凝固由界面開始，因此界面區之TiC均較微細，同時因低被覆電流凝固速率較快，因此TiC晶出顆粒較高被覆電流微細，上被覆層的樹枝狀TiC晶出組織也有細化的傾向。同時由於異質成核效應，TiC晶出也見於NbC周圍。TiC與NbC中間則有一層可能因擴散反應而形成之βNb2C相，且高被覆電流所得的擴散層較低被覆電流者為厚。
　　被覆層由於固溶強化效應，所得之微硬度值較基材高，同時由於固溶C之強化效應較固溶Nb為高，因此高被覆電流固溶C較多情形下也顯示其微硬度較低被覆電流為高。另外由於固溶強化效應及第二相顆粒(NbC＋TiC)的強化，因此被覆層之體硬度顯著高於基材，而高被覆電流被覆層之體硬度值則隨著碳化物面積率增加(特別是TiC之硬度高，所以其強化效應較佳)及固溶效應增加較低被覆電流有所提升。
　　沖蝕磨耗試驗顯示低角度磨耗機構主要因微切削、剷犁造成塑性變形，進而進行破壞而有溝槽現象產生；高角度磨耗機構主要為TiC、NbC的誘發而造成之沖蝕脆性破壞。由於NbC、TiC在高角度情況下之脆性破壞主導磨耗機制(NbC脆裂情形較嚴重)，低角度則為α-Ti之延性破壞主導磨耗機制，同時在沖蝕磨耗特徵觀察上也發現被覆層於45o沖蝕條件有嚴重溝槽形貌，顯示材料容易被沖蝕移除。此外，當降低沖蝕角度(特別是15o以下)，沖蝕脆性龜裂現象則不明顯。所以在低角度可見被覆層有較佳的耐磨耗阻抗，高角度則以純鈦較佳。另外由於高被覆電流在15o之NbC破壞情形較低被覆電流略為嚴重，且固溶強化雖提昇硬度，但脆性相對增加，因此高被覆電流之磨耗阻抗較低被覆電流者差。最後也發現NbC外圍之擴散層有較NbC、TiC佳的磨耗阻抗(特別是低角度沖蝕情形下)，所以若能提高此層的厚度，也能提昇被覆層的磨耗阻抗。

　　Titanium has been widely used in aviation, petroleum, chemistry and biological medicine industry because of its good corrosion resistance, low density, high mechanical strength and good ductility. However, titanium will face erosion wear problem when it’s used in nuclear power plants or delivery pipe of grinding liquid used for photoelectrical glass plate. This study is to overlay reinforcing particle-NbC on the surface of commercially pure Ti with the technique of PTA (plasma transferred arc) to investigate not only the microstructural feature of overlayer, interface between overlayer and base metal but also the hardness profile by changing overlaying current. In addition, the purpose of this study will show the erosion wear mechanism of the overlayer. The SiO2 particle of 295mm mean diameter was selected as the erodent and the erosion speed is about 66 m/sec.
　　The results indicate that the matrix phase of the overlayer is α-Ti con- taining about 10 at. % Nb and 1 at. % C. NbC and precipitated TiC produced by dissolution of NbC reacting with Ti dispersed in the matrix. Higher density of NbC than Ti resulted in NbC deposited in the bottom of the overlayer. Therefore, by the differences of species of carbides and microstructure, the cross section of the overlayer (from surface to base metal) which is mainly made of α-Ti can be separated with four layers: upper overlayer with TiC, lower overlayer with TiC and NbC, interface region (interfacial layer and partially melted zone) and base metal (heat affected zone). Because solidification begins at the interface, the TiC in the interface was finer. Owing to faster solidification in the low-current condition, the TiC particle was finer than high-current one. Meanwhile, TiC precipitate due to heterogeneous nucleation can also be found around NbC. The diffusion layer between TiC and NbC may be βNb2C phase and the layer was thicker in the high current condition.
　　Owing to the solution hardening effect, the Vickers Hardness (H.V.) of the overlayer was higher than base metal. Besides, the overlayer of high current contained more C, the H.V. in all regions was higher than the low -current overlayer (the solution hardening of C is better than Nb). Due to the solution and second-phase particle (NbC＋TiC) hardening, the Rockwell Hardness (HRC.) of the overlayer enhanced a lot compared with base metal and the HRC. of high-current overlayer increased as the carbide area percentage increased (especially the harder TiC’s got better hardening effect).
　　Erosion wear test showed that under low impact angle, it’s microcutting and plowing lead to plastic deformation and grooving; under high impact angle, it’s brittle fracture of NbC and TiC lead to cracking. Because the fracture of NbC and TiC lead the high-angled erosion mechanism (the fracture of NbC is more serious), the ductile destruction of α-Ti lead the low-angled erosion mechanism and the grooving phenomenon was the most obvious at about 45o, it means the material can be easily removed. In addition, under low-angled impact (especially below the angle of 15o), the fracture of NbC was unobvious. Therefore, the overlayer’s got better erosion resistance under low angle impact while pure Ti’s got better one under high angle impact. Because the fracture of NbC of high current was more severe than low current one and solution hardening increased the hardness of the overlayer, though, the brittleness enhanced, high-current overlayer’s got worse erosion resistance than low-current one. The result also showed that the diffusion layer’s got better erosion resistance than NbC and TiC especially under oblique erosion, so if we can enhance the thickness of the layer, we can improve the erosion resistance of the overlayer.

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