R.F.射頻磁控濺鍍氮化鋁薄膜於鎂鋰合金之抗腐蝕性質研究

Abstract

本實驗中採用近幾年來廣泛應用於運輸工具及3C產品之鎂鋰合金(LZ101)當做基材，其中Li所添加的量多達10%。而當鎂合金中在添加多達5~11%的鋰含量時，在結構中會存在著富含鎂的α-hcp及富含鋰的β-bcc之雙相結構。由於α及β雙相結構的因素，此時鎂合金具有良好的成形性以及低密度特性。但加入活性較大鋰原子的緣故，使得腐蝕行為更為加劇，故使用鎂鋰合金時，更須要有更好的表面處理方法來解決其較差的抗蝕性質。 本研究中將於鎂鋰合金基板上濺鍍氮化鋁薄膜，探討改變不同之濺鍍參數，如射頻功率50W ~ 200W、工作壓力1mtorr ~ 5mtorr、反應氣體氮氣流量5 s.c.c.m ~ 15 s.c.c.m、工作距離5cm、14cm等，其對薄膜性質之影響。或利用雙層鋁/氮化鋁薄膜層之沉積方法，來測試鎂鋰合金表面之抗腐蝕能力。利用XRD、SEM、XPS、TEM、恆電位儀、鹽霧試驗等儀器分別觀察其鍍膜層晶體結構、表面形貌、成份分佈、結晶結構、抗腐蝕能力等。而經過氮化鋁鍍膜處理試片，使用X-ray繞射儀與穿透式電子顯微鏡來判別其鍍膜層之結晶結構。利用EPMA定量檢測，量測鍍膜層上之成份含量。並且分別採用場發射槍掃描式電子顯微鏡以及原子力顯微鏡觀察鍍膜層之表面形貌與薄膜表面粗糙度。另外於鹽霧試驗以及恆電位儀測量經氮化鋁薄膜處理過後之鎂鋰合金試片，其抗腐蝕能力。最後運用X光光電子能譜中束縛能之位移來定出其薄膜層組成成份。 濺鍍過程中，當射頻功率上升、工作壓力與氮氣流量降低時，氮化鋁薄膜層厚度會明顯增加。射頻功率參數調高時，從靶材表面上濺擊出之原子擁有更大之動能。且工作壓力會影響濺擊原子之平均自由路徑，在工作壓力愈小環境下，平均自由路徑愈長，原子抵達基材表面上時動能愈不易散失。除了射頻功率與工作壓力外，氮氣氣體流量也是影響薄膜因素之一，當流量值愈大時，N2離子相對於Ar離子其濺鍍率明顯緩慢，導致沉積速率降低。若在長工作距離下濺鍍，濺射原子較易散亂排列，薄膜層表現出非結晶結構。若減短工作距離後，濺射原子將擁有較大之動能，而氮化鋁薄膜將形成柱狀晶結構，在(002)面之從優取向進行成核成長。最後發現氮化鋁薄膜層中若存在著許多小缺陷以及太大的殘留應力時，會使氮化鋁薄膜不具有抗蝕佳之特性，且易使腐蝕反應加速。 另外於鎂鋰合金基板上濺鍍氮化鋁/鋁雙層薄膜，並採用150W射頻功率、1mtorr工作壓力、10s.c.c.m氬氣流量、14公分工作距離等參數下，濺鍍鋁薄膜層於鎂鋰合金基板與氮化鋁薄膜層間。在腐蝕測試後，氮化鋁/鋁雙層薄膜比只有單一氮化鋁薄膜層擁有更佳之抗腐蝕能力。因為鋁鍍膜層表現出質地較軟之特性，使其在腐蝕環境中，應力腐蝕現象較不顯著。並且鋁薄膜層位於氮化鋁薄膜層與基材間，將扮演良好附著力的角色於其內。故能防止較脆的氮化鋁薄膜層，其腐蝕裂縫擴張等效應。

In this experience we use Magnesium alloys that are considered as the potential candidates for many applications in transportation vehicles and 3C product as substrate. It is known that the Mg-Li alloys with Li contents ranging from 5 to 11mass% will exhibit a two-phase structure of Mg-rich (α-hcp) and Li-rich (β-bcc) phases. This two-phase structure has excellent formability and extra-low density. But the Mg-Li alloys still have disadvantage of poor corrosion and wear resistance. These poor properties limit the application of Mg-Li alloys and need to be resolved effectively. In this study, the aluminum nitride (AlN) films were deposited by R.F. magnetron sputtering method on Mg-Li substrate under different sputtering power of 50W~200W、working pressure of 5×10-3torr~1×10-3torr、nitrogen flow of 5s.c.c.m ~15s.c.c.m and working distance of 14cm and 5cm. Crystal orientation was studied by X-ray diffraction and transmission electron microscopy. The composition and concentration of aluminum and nitrogen were observed by EPMA. Surface morphology and Rms roughness were examined respectively by SEM and AFM. Corrosion resistance was measured by potentio-dynamic polarization and salt-spray techniques. And chemical shift of electron binding energy was observed by XPS. As the sputtering power increased, the working pressure and nitrogen flow decreased. The thin film thickness was changed to be thicker. When the sputtering power value increased, the sputtered atoms from the target will gain more momentum. The working pressure will affect the mean free path of atoms. At lower working pressure , the particles had a longer mean free path ,and so had higher kinetic energy when they arrived at the substrate surface. In addition to sputtering power and working pressure effect, when nitrogen flow increased, the sputtering yield of N2+ will be lower compared to Ar+. It is also a important factor resulting in decreasing of the deposition rate. And it was found that, when deposited at longer working distance , the AlN films tended to be randomly orientated. With decreasing working distance , the sputtered atom will get more mobility on substrate. After deposited at short distance, the AlN films are showed c-axis (002) preferred orientation. However, they were found that the AlN films exhibited small structure defect and high residual stress, may improve the corrosion speed. AlN/Al dual protective coating were prepared on Mg-Li substrate, and the Al interlayer was deposited by R.F. magnetron sputtering method at sputtering power of 150W、working pressure of 1×10-3torr、argon flow of 10s.c.c.m、working distance of 14cm. Comparing with AlN coating, AlN/Al coating have better corrosion resistance, because Al coating exhibit soft properties and reduce the stress-corrosion cracking in an aggressive environment. Furthermore, Al coating can let AlN coating and Mg-Li substrate combine well and prevent the propagation of the crack generated in a brittle ceramic layer.