Wenbin Xue

Growth regularity of ceramic coatings formed by microarc oxidation on Al–Cu–Mg alloy

Abstract Growth regularity of ceramic coatings formed on Al–Cu–Mg alloy by microarc oxidation was investigated, and the formation mechanism of coatings was discussed. After oxidation of several hours with linear growth, the growth rate of coatings decreases gradually. During the early stage, the sample geometrical dimension increases with oxidation. However, when the ceramic oxide coating reaches a certain thickness, the sample geometrical dimension will no longer increase although the total coating thickness keeps increasing. The oxide coatings contain a loose layer and a compact layer. The loose layer is formed first. After several hours, the compact layer begins to grow towards the Al substrate while the thickness of the loose layer changes little. The thickness of the compact layer can finally reach over 75% of the total coating thickness. After the surface loose layer is ground off, the sample dimension is approximately the same size as the sample before the treatment.

Structure and properties characterization of ceramic coatings produced on Ti–6Al–4V alloy by microarc oxidation in aluminate solution

Microstructure and phase composition of the ceramic coatings formed on Ti–6Al–4V alloy by alternating-current microarc oxidation in aluminate solution were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Distributions of nanohardness, H, and elastic modulus, E, along the coating thickness were determined using a nanoindentation method. After microarc oxidation treatment, the microstructure in the Ti–6Al–4V substrate near the coating/metal interface has not been changed; meanwhile, the titanium substrate has hardly absorbed oxygen atoms. The oxide coating is mainly composed of TiO2 rutile and TiAl2O5 compounds. The rutile fraction in the inner layer coating is much higher than in the outer layer. On the contrary, the TiAl2O5 phase has a higher fraction in the outer layer. On the other hand, the nanoindentation test shows from the coating surface to the interior of the coating that the H and E gradually increase. The maximum H and E in the coating can reach 13 and 230 GPa, respectively. The changes of TiO2 and TiAl2O5 contents along the coating thickness determine the H and E profiles in the coating.

Effect of microarc discharge surface treatment on the tensile properties of Al–Cu–Mg alloy

Abstract A thick ceramic coating was prepared on Al–Cu–Mg alloy by microarc discharge in aqueous solution. The tensile properties of the alloy before and after microarc oxidation (MAO) surface treatment were tested, then the fractography and morphology of ceramic oxide coatings were investigated using scanning electron microscope (SEM). It is showed that the tensile properties of aluminum alloy have smaller change after the alloy has undergone microarc discharge treatment. For all specimens with different thickness coatings, the decreases of yield strength, tensile strength and elastic modulus are less than 5%, and the contraction of area rises while the elongation slightly decreases. After the coatings are polished, the tensile properties of the alloy are improved rather small. The surface of tensile specimens uniformly remains a large quantity of tiny fragments of alumina coatings. That implies that the ceramic coating has good adhesion with aluminum alloy substrate.

Microstructure and properties of ceramic coatings produced on 2024 aluminum alloy by microarc oxidation

The microstructures of the microarc oxidation coatings and 2024 aluminum alloy substrate were observed using the scanning electron microscope (SEM) and the phase composition of the coatings was analyzed by X-ray diffraction (XRD). Furthermore, the profiles of the nanohardness, H, and elastic modulus, E, along the coating depth were first determined using the mechanical properties microprobe. The microarc oxidation coatings consist of two layers—a loose layer and a compact layer. The H and E in the compact layer are about 18–32 GPa, 280–390 GPa, respectively. The H and E profiles are similar, and both of them exhibit a maximum value at a same depth of the coatings. The distribution of α-Al2O3 phase content determines the H and E profiles in the coatings. The changes of α-Al2O3 and γ-Al2O3 contents result from the different cooling rates of the molten alumina in the microarc discharge channel at the different depths of the coatings. After the microarc oxidation treatment, the microstructure of the alloy substrate, even near the Al/Al2O3 interface, has not been changed.

Evaluation of the mechanical properties of microarc oxidation coatings and 2024 aluminium alloy substrate

A determination of the phase constituents of ceramic coatings produced on Al–Cu–Mg alloy by microarc discharge in alkaline solution was performed using x-ray diffraction. The profiles of the hardness, H, and elastic modulus, E, across the ceramic coating were determined by means of nanoindentation. In addition, a study of the influence of microarc oxidation coatings on the tensile properties of the aluminium alloy was also carried out. The results show that the H-and E-profiles are similar, and both of them exhibit a maximum value at the same depth of coating. The distribution of the α-Al2O3 phase content determines the H- and E-profiles of the coatings. The tensile properties of 2024 aluminium alloy show less change after the alloy has undergone microarc discharge surface treatment.