The corrosion resistance of AlN thin film deposited on Mg-Li alloy by Radio Frequency Magnetron Sputtering Technique
Date Issued
2010
Date
2010
Author(s)
Wu, Tso-Feng
Abstract
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.
Subjects
Mg-Li alloy
AlN thin film
corrosion resistance property
r.f. magnetron sputtering
AlN/Al dual thin film
Type
thesis
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