Papers by Author: Ye Dong He

Abstract: Improved structure design has been acknowledged as an effective approach to obtain thermal barrier coating (TBC) with excellent resistance to high-temperature oxidation and spallation. Since ceramic composites can effectively enhance the strength and durability of ceramic components and improve their fracture toughness, it is reasonable to propose that composite structure of TBC should possess improved mechanical properties than its traditional structure with single phase in bond coat and top coat. In this paper, the progress in novel composite TBC developed by the authors’ research group has been summarized, including TBCs with bond coat of ceramic/ceramic and ceramic/noble metal composite coatings which replace the traditional alloy bond coat, and also TBCs with novel composite structure have been studied. It has been investigated that these TBCs exhibit not only excellent high-temperature oxidation resistance, but also excellent resistance to cracking, spallation and buckling under thermal cycling. Such beneficial effects can be attributed to the sealing mechanism of these coatings on oxygen diffusion, the improvement of thermal expansion mismatch and the toughening effects of these composite structures. It would be a growing trend in introducing various composite modes into the traditional thermal barrier coating or developing new composite structure of thermal barrier coating.

Abstract: Cathodic plasma electrolysis (CPE) is an emerging surface engineering technology in the recent years. In present work CPE was successfully used for deposition Ni-P coatings on carbon steel. The deposition experimental results show that CPE possesses high coating deposition rate, faster than 4 μm/min. SEM observations unveil the bonding between Ni-P coating and substrate is metallurgical. XRD diagram analysis indicates that the phases of as-deposited Ni-P coating are Ni and Ni3P, transformed from amorphous to crystallized state. The corrosion resistances of CPE and electroless plating (EP) coating have also been discussed by polarization curves in 3.5% NaCl solution at room temperature. Scratch tester was applied to study film-substrate adhesion of CPE Ni-P coating comparing with EP Ni-P coating.

Abstract: Fe-40Cr-0.5Ce and Ni-40Cr-0.5Ce alloys prepared by vacuum melting have been studied as inert anodes in aluminium electrolysis. High temperature corrosion of these alloy anodes was conducted in cryolite-alumina electrolytes at anodic current density of 0.5 A/cm², at 800°C and 900°C for 10h respectively. It is found that a Cr₂O₃ scale was selectively formed on the surface of Ni-40Cr-0.5Ce anode at 900°C, corresponding to excellent corrosion resistance during aluminium electrolysis and higher purity of aluminium product. While a composite oxide film was developed on the surface of Fe-40Cr-0.5Ce anode at 800°C. The formation of Cr₂O₃ scale is controlled by both of the dissolving rate of Cr₂O₃ scale in electrolyte and the diffusing rate of Cr in alloys, so the M-Cr alloys require a high content of Cr for inert anodes application.

Abstract: A novel technique has been developed to prepare micro-/nano-crystal aluminized ODS coatings on stainless steel and nickel-base superalloy. In this technique, the pack aluminizing is combined with the repeated ball impact. Pure Al powder is mixed with 1wt% Y2O3 powder by ball milling. The ultrafine Y2O3 powder is well dispersed in Al particles. The modified Al particles are welded to the surface of metals by ball impact, causing the refinement of coarse grains and acceleration of atomic diffusion. Micro-/nano-crystal alloy layer with Y2O3 grows outward at a much low temperature (below 600°C) and in short treatment duration, compared with conventional pack aluminizing. The effects of processing temperature and duration on formation of the coatings have been analyzed. The microstructure of the coatings is studied using the methods of SEM, AMF, EDS, XRF and XRD. The results indicate that the aluminized ODS coatings appear to be dense, homogeneous, micro-/nano-crystal structure, and consist mainly of Al-rich phases, such as Fe2Al5, FeAl3 NiAl3 and7 CrAl5. High temperature oxidation tests show that the coatings enhance the oxidation resistance.

Abstract: It is shown from the oxidation results that, for Fe-Cr, Ni-Cr, Co-Cr binary alloys with and without internal Cr2O3 precipitations after exposure, continuous external Cr2O3 scales formed on their surfaces, especially at the initial stage. It is therefore suggested that the transitions of oxidation for these alloys take place from externally to internally. There are two types of transitions for alloys: (1) the transition from temporary external oxidation to internal oxidation; and (2) the transition from permanent external oxidation to internal oxidation. The thermodynamic conditions for these transitions have been analyzed, and the criterion for prediction of the minimum solute concentration of a binary solid solution alloy required for the second transition has been derived. It is also suggested that the external oxide scale is an important factor to determine the formation of pure solvent metal nodules on the surfaces.