Materials Science Forum Vols. 546-549

Abstract: Based on double cermet layer structure, Mo-Al2O3 cermet solar selective coating was prepared on stainless steel substrate. A solar absorptance of 0.92 and normal emittance of 0.19 at room temperature have been achieved. Vacuum annealing treatment was done and its influences on the solar selective performance was discussed. Absorptance changed between 0.90 and 0.92 under the selected annealing temperature range of 350-800°C, and emittance varied from 0.19 to 0.23 when heated at 650°C. The microstructures before and after annealing process at different temperatures were investigated.

Abstract: The Hf doped NiAl coatings were co-evaporated and co-deposited onto the superalloy substrate by electron beam physical vapor deposition (EB-PVD). During heat-treatment, HfO2 was formed on the NiAl coatings. And, Hf enriched at the interface between the coating and the interdiffusion zone, which could prevent outward diffusion of elements in the substrate. The NiAl coating doped with 0.5% Hf effectively improved the high temperature oxidation resistance compared to the Hf free NiAl coating and the high Hf content coating. Also, the addition of Hf to the coating contributed to enhancing the adherence of TGO layer to coating.

Abstract: It has been found that under oxygen partial pressure of ~2×10-6 kPa, the high-temperature oxidation of thermal barrier coatings (TBCs) occurred during an electron beam physical vapor deposition (EB-PVD) process for producing the TBCs top ceramic coating. In the present investigation, two modified bond coats (BCs) of NiCrAlY with Si addition, and NiCrAlY with Co and Hf additions, were developed by Arc Ion-plating technique to study the effects of the EB-PVD process on thermally grown oxide (TGO) formation and growth. The isothermal and cyclic oxidation tests were conducted and the cross-sectional morphologies of the specimens were examined to compare the high-temperature oxidation behaviors of the two TBCs. It was found that a mixed oxide layer have been developed in the as-deposited TBCs with a NiCrAlYSi BC. The mixed oxide layer mainly included Cr2O3, NiO, Al2O3 and their spinel. With the mixed oxide layer, TBCs with the NiCrAlYSi BC showed a superior high-temperature resistance on later high-temperature exposure to TBCs with NiCoCrAlYHf BC, where no mixed oxide layer was observed. The pre-formed mixed oxide layer apparently shortened the time to fully develop a protective α-Al2O3 layer and therefore restrained the TGO growth in TBCs.

Abstract: Composite metastable Ti0.5Al0.5N, Ti0.49Al0.49Y0.02N and Ti0.48Al0.48Y0.04N coatings were deposited on a wrought martensite steel 1Cr11Ni2W2MoV for aero-engine compressor blades by arc ion plating respectively all coatings possessed a dense surface and the same B1NaCl phase structure and the lattice parameter increased with increase of the incorporated element Y. Oxidation-resistance of (Ti,Al,Y)N coatings at 800
for up to 500 hr was investigated and the results showed that the introduction of yttrium into the coatings dramastically improved the oxidation-resistance of the coatings in air. The presence of Y in the nitride coatings suppresses the outer diffusion of Fe through the nitride coating. And the oxide of yttrium may segregate to the grain boundaries of oxides, suppressing the outward diffusion of metallic elements in coating and the inward diffusion of oxygen. Besides, the existence of adequate Y in the coatings is in favor of suppression of the growth of oxide crystals, formation layered oxide scale and decrease of compressive stress of oxidation.

Abstract: Numerical method was used to simulate the stress state of thermal barrier coatings (TBCs) under thermal-mechanical coupled environment. The finite element (FE) model was built as hollow tube and boundary conditions including heating rate, cooling air convection and mechanical loadings were considered. The maximum stress locates where maximum temperature gradients is formed. This failure mode is consistent with the experimental results.

Abstract: Rare-earth modified coatings on 2024 aluminum alloy were prepared by sol-gel methods, aiming at improving the corrosion resistance with the environmental friendly features. The investigation put emphases on the effects of rare-earth additives on corrosion resistance and microstructure of sol-gel coatings. The results revealed that coated samples containing 2g·L-1 rare-earth not only delayed the incubation period of corrosion, but also hindered the development of it, and their corrosion rate decreased 80.3% compared with the bare ones and was 15.9% than the rare-earth free coated ones. The action of rare-earth on morphology and microstructure of sol-gel coatings for 2024 Al alloy was studied by SEM. High density good cohesion, and intact coverage were showed clearly ,which provided a reliable proof of the reformative effects of rare-earth additives.

Abstract: A tri-layer amorphous Ni-P alloy coating was prepared in the same bath by changing the temperatures and currents, followed by heat treament. We studied the behavior of corrosion-resistance of the tri-layer coating before and after heat-treatment at 360°C for different time in the 5% NaCl solution. The result showed that: the tri-layer amorphous Ni-P coating has the excellent performance of anti-corrosion compare with single electroless or electrodeposited Ni-P amorphous coating. The performance of anti-corrosion of the coating was improved by controlling the coating’s structure and the role of electrochemical protection. After being heat-treated at 360°C for 1 hour ,the coating’s structure changed from amorphous to crystalloid and the corrosion resistance of the heat-treated coatings was higher than as-plated tri-layer Ni-P coating.

Abstract: Tungsten has the highest melting point among all metal, which makes it withstand thermal shock and erosion in high temperature environments. In this study, Tungsten coatings were sprayed onto the oxygen-free copper substrates by plasma spraying using inert gases protection. XRD, SEM and EDS were used to identify the phases, morphologies and compositions of the coatings. Vickers micro-hardness and bonding strength of the tungsten coatings were also measured. The results revealed that the hardness distribution of the tungsten coatings was different along the thickness direction. The tungsten coating without any interlayer showed higher bonding strength than that of the other two coatings with NiCrAl and W/Cu interlayers, respectively.

Abstract: In the process of preparation of high silicon steel by EB-PVD, the effect of distance between ingot and substrate on the weight percent of silicon in the silicon steel was investigated through specially designed experiment. Energy dispersive spectroscopy (EDS), optical microscope (OM), scanning electron microscopy (SEM), x-ray diffraction (XRD) were used to characterize the weight percent of silicon, microstructure, crystal grain size and phase of the high silicon steel. The experimental results show that the weight percent of silicon in the silicon steel decreases with increasing of the distance between ingot and substrate, when the distance is about 415 mm, high silicon steel can be fabricated by EB-PVD. The microstructure of the high silicon steel was composed of columnar crystal grain, the size of the crystal is about 25-50 μm. The material back to the substrate side is composed of B2 and exhibits strong {400} texture.

Abstract: The melting temperature-pressure phase diagram [Tm(P)-P] for corundum (Al2O3), wustite (FeO) and magnesium oxide (MgO) are predicted through the Clapeyron equation where the pressure-dependent volume difference is modeled by introducing the effect of surface stress induced pressure. The model prediction is found to be consistent with the present experimental results.