Wu Weitao

Corrosion performance of ion-plated titanium and yttrium-modified TiN coatings

Abstract The corrosion behaviour of vacuum-evaporated titanium, ion-plated titanium, ion-plated TiN and yttrium-modified TiN coatings was studied in three corrosive media. Ion-plated titanium coatings exhibited better corrosion resistance than vacuum-evaporated coatings, which may be related to the effect of negative bias which enabled the former to become more compact and less rough. There is no distinguishable difference in the chemical composition and phase constituents of the top surface between TiN and Ti(Y)N coatings, but study of their pole figures revealed that there is a strong (110) texture for TiN coatings, and (112) texture for Ti(Y)N coatings. The latter lattice plane is closer to TiN(111) with the highest degree of microcompactness and may be responsible for the better corrosion resistance of Ti(Y)N coatings.

Hot corrosion of yttrium-modified aluminide coatings

Abstract The beneficial effect of yttrium on the oxidation and hot corrosion resistance of electron beam-physical vapour deposition and low pressure plasma spraying overlay coatings has been confirmed by many studies. However, these coating processes have two disadvantages: (1) a restriction on the application of the coatings to complex geometries and (2) high equipment investment. In this study, a very simple slurry process was used to introduce yttrium into an aluminide coating. Its hot corrosion was tested in 25%NaCl + 75%Na2SO4 at 1123 K and the result showed that the hot corrosion resistance of the aluminide coating is greatly improved by the addition of yttrium. The internal oxidation and sulphidation of yttrium-free coating is more severe than that of the coating containing yttrium. It is observed that yttrium is enriched in the outer Al2O3 layer which is very adherent to the coating. However, oxide spalling occurred on the surface of the yttrium-free aluminide coating.

Hot corrosion behaviour of nickel-based superalloy irradiated with a laser beam

Abstract The structure and hot corrosion behaviour of cast nickel-based superalloy M38 irradiated with a CO 2 laser beam of 1.3 kW output have been studied using optical microscopy, X-ray diffraction, scanning electron microscopy and electron probe microanalysis and compared with the structure and hot corrosion behaviour of the unirradiated alloy. The hot corrosion tests were carried out in a crucible filled with 75 mol.% Na 2 SO 4 plus 25 mol.% NaCl at 900°C for 15, 35, 55, 80 and 100 h. The test results showed that the irradiated alloy exhibited a hot corrosion resistance which was a factor of 2–3 better than that of the original unirradiated cast alloy. In contrast with the significant internal oxidation and sulphidation along grain boundaries in the unirradiated cast alloy, it was noted that, beneath a dense protective oxide scale consisting of mainly Cr 2 O 3 , there was no sign of internal oxidation and sulphidation in the irradiated alloy. The irradiated surface layer may be regarded as an effective coating which belongs to the superalloy itself.

Laser cladding of oxide-ceramic powder with enamel frit on a steel substrate

Laser cladding of oxide ceramic powder mixed with ground enamel frit containing Co and Ni on a steel substrate was performed by use of a 2Kw CO2 laser. Accordingly, a uniform, dence, adhesive and crack-free coating of Al2O3/ Al2O3-SiO2 based oxide ceramic may be prepared onto steel surface. The microstructural characteristics of the formed coatings was characterized by SEM, EDXA, EPM ect. with their morphology, microstructure and composition. Finally, it’s charactaristics were discussed in terms of so-called “thermal barrier”, “higher Re ”and “visco -elastic”.Laser cladding of oxide ceramic powder mixed with ground enamel frit containing Co and Ni on a steel substrate was performed by use of a 2Kw CO2 laser. Accordingly, a uniform, dence, adhesive and crack-free coating of Al2O3/ Al2O3-SiO2 based oxide ceramic may be prepared onto steel surface. The microstructural characteristics of the formed coatings was characterized by SEM, EDXA, EPM ect. with their morphology, microstructure and composition. Finally, it’s charactaristics were discussed in terms of so-called “thermal barrier”, “higher Re ”and “visco -elastic”.

Laser clad glass coating on carbon steel

Abstract The feasibility of laser clad glass coating on carbon steel substrate is investigated. With a suitable selection of laser processing variables, thin, dense and crack-free glass coating has been obtained with excellent adhesion and good surface integrity. Cracking-inhibition of glass coating during laser cladding is attributed to the visco-elastic deformation that the glass phase undergoes in the course of solidification above the softening temperature by relieving thermal stress from the heterogeneous contraction. Crystallization occurs in the glass melt during solidification, demonstrating the heterogeneous nucleation at substrate surface and the orientational growth of nuclei due to the directional heat conduction. The present investigation identifies laser power, travel speed and preheating as the key variables for optimisation.