CrN and Cr-Al-N coatings were deposited by reactive magnetron sputtering on the glass substrate,and their corrosion behavior was studied. The electrochemical tests using both DC(polarization curves) and AC techniques(EIS) were carried out on Potentiostat/Galvanstat(EG&G) in 3.5%(mass fraction) NaCl solution. After immersed into NaCl solution for 1 h,the mass of the CrN coating keeps constant with the time continuing. This can be explained by the passivation of the coating. The comparison between the corrosion potential(φcorr) of the Cr-Al-N coatings with different aluminum contents reveals that the corrosion potentials of the aluminum contain coatings are nobler than that of the CrN coatings. This means that the addition of aluminum shifts the corrosion potential to more positive potential value. Among these coatings,CrN in NaCl solution exhibits the worst corrosion resistance,while the corrosion resistance of Cr0.63Al0.37N in NaCl solution is the best. The polarization data and EIS data suggest that addition of aluminum can improve the corrosion resistance of CrN coating.
Nanocrystalline CoCrAlY overlay coating was prepared on M38G superalloy by magnetron sputtering deposition. To investigate the oxidation behavior and phase transformation of alumina during oxidation, the oxidation experiments were conducted at 1 050 ℃ for various time in the range of 5?180 min. The phase compositions of the oxide scales were investigated by using glancing angle X-ray diffraction(XRD). The microstructure analysis of oxide scales was carried out by means of scanning electron microscopy(SEM). The growth process of metastable alumina at the grain boundaries and transformation to stable alumina were discussed. The results show that at the initial oxidation stage the mixture of δ-Al2O3, γ-Al2O3 and α-Al2O3 is formed on the sample surface rapidly. Especially, δ-Al2O3 and γ-Al2O3 prefer growing at the grain boundaries of CoCrAlY coating. With increasing oxidation time, δ-Al2O3 and γ-Al2O3 transform to θ-Al2O3, afterwards θ-Al2O3 transforms to α-Al2O3 gradually. After 180 min oxidation, θ-Al2O3 transforms into α-Al2O3 completely.
