Materials Science Forum Vols. 539-543

Abstract: Multifunctional coatings open new dimensions due to a combination of properties like high friction and wear resistance, electrical attributes, heat or corrosion protection in one system. In this study multifunctional coatings for in-situ temperature measurements on cutting inserts as well as multilayer coatings have been investigated. Corresponding metallurgical analyses together with mechanical tests are presented.

Abstract: In the present study, Al2O3, (ZrO2-3mol% Y2O3)-39.6mass% Al2O3, Si3N4 and MgO substrates coated with SrSO4 particles were prepared, and their high-temperature tribological properties were investigated. It was clarified that SrSO4 worked as solid lubricant efficiently against Al2O3 and (ZrO2-3mol% Y2O3)-39.6mass% Al2O3 substrates in the temperature range of room temperature to 1073K in air. In addition, thin SrSO4 films were observed on the wear scars formed on the Al2O3 and (ZrO2-3mol% Y2O3)-39.6mass% Al2O3 substrates.

Abstract: Thermal cyclic lifetime and microstructural degradation of thermal barrier coatings (TBCs) with (Ni,Pt)Al bond coat and Hf- and/or Y-modified CSMX-4 superalloy substrates were examined. Thermal cyclic lifetime of TBCs was measured using a furnace thermal cycle test that consisted of 10-minute heat-up, 50-minute dwell at 1135°C, and 10-minute forced-air-quench. TBC lifetime was observed to improve from 600 cycles to over 3200 cycles with appropriate Hf- and/or Y-alloying of CMSX-4 superalloys. This significant improvement in TBC lifetime is the highest reported lifetime in literature with similar testing parameters. Cross-sectional microstructure of TBC specimens were examined by scanning electron microscopy (SEM) after the spallation failure. While undulation of TGO/bond coat interface (e.g., rumpling and racheting) was observed to be main damage mechanisms for TBCs on baseline CMSX-4, the same interface remained relatively flat for durable TBCs on Hf- and/or Y-modified CSMX-4. The parabolic growth constant of the TGO scale was slightly lower for TBCs with Hfand/ or Y-modified CSMX-4.

Abstract: There is a growing interest in use of the nickel-based alloy Inconel 625 coatings due to its ability to improve base materials high temperature properties. Thermal spraying methods such as Air Plasma Spraying (APS) can be considered as a convenient method to deposit this material. The present work deals with APS deposited Inconel 625 structures consisting of huge number of individual splats formed by impacting molten droplets on substrates during spraying process. It is clear that the splat formation mechanism which dominates its size, cohesion, and boundaries highly influences the microstructure of the coating. This paper presents a developed numerical technique performed to simulate splat formation using a three dimensional model. In this method flow field is solved by Finite Volume Method (FVM) and free surfaces are determined from Youngs’ Volume of Fraction method (VOF). Finally, the model prediction is correlated with the actual splat geometries.

Abstract: Ceramic coatings were synthesized on pure magnesium, binary Mg-Al and Mg-Zn alloys by plasma electrolytic oxidation (PEO) technique, and their mechanical and electrochemical properties were investigated. The coatings showed porous microstructure and some volcano top-like pores on the surface. The coatings on Mg-Al alloys consisted of MgO, MgAl2O4 and Al2O3. In the coatings on Mg-Zn alloys, MgO and ZnO were detected. The hardness and wear resistance showed slight dependence on Al content but no tendency to increase with increasing Zn content. The coated Mg alloys with Al, Zn showed better corrosion resistance in 3.5%NaCl solution but poor corrosion resistance in methyl alcohol, compared to the coated pure Mg.

Abstract: We have synthesized the high-density Ga2O3 nanowires on gold (Au)-coated silicon substrates using metalorganic chemical vapor deposition. The nanowires exhibited one-dimensional structures having circular cross sections with diameters in the range of 30-200 nm. The energy dispersive x-ray spectroscopy revealed that the nanowires contained elements of Ga and O, without Au-related impurities. X-ray diffraction analysis and high-resolution transmission electron microscopy showed that the Ga2O3 nanowires were crystalline.

Abstract: An electrolytic sulfurization at room temperature has been introduced in the present work, which process parameters are optimized to the quenched Cr12 substrate. Morphology and microstructure of the sulfide layers are investigated deeply, and their formation mechanism is discussed in detailed. According to optimum parameter of sulfurizing process ( assistant current density of 1.8A/dm2, the saturated rare-earth-halide solution containing 300g Na2S2O3, 60g KHSO3 and 1000ml H2O, 298K/20~40min), the ferrous sulfide coating was easily obtained on the quenched Cr12 substrate with a uniform thickness of above 10μm, no matter about the treated parts with the complex geometrical shapes. The tribological experiment results of a block-on-ring tribometer indicate that the sulfide coatings on hard substrate have a very low friction coefficient and could obviously reduce the wear extent of its counterpart. Compared with the traditional low-temperature process, this sulfurizing technology has such virtues as simple process, heatless, low cost, little deformation, without pollution question, etc., which could be employed for industrial solid lubricant due to the good friction-reducing characteristic of lamellar-structure FeS phase.

Abstract: A model hot forging test was carried out using tools of matrix high-speed steel. Dynamic thermal loads were applied to tools subjected to ion nitriding on the surface layer of the tool, and the conditions of deterioration and thermal softening were examined by measuring Vickers hardness in the vicinity of the tool surface after the thermal load test. In addition, static tempering softening tests were carried out, and thermal softening equations are obtained using tempering parameter on the basis of the obtained master tempering curves. When the thermal load was applied under the same conditions to nitrided and heat-treated tools, the degree of surface deterioration and thermal softening was small for nitrided tools. Equations of softening for two cases, i.e., specimen subjected only to heat treatment and specimen subjected to both heat treatment and nitriding, were expressed using tempering parameter.

Abstract: Diffusion bonding of Ti and Ti-Al (Ti-10, 20 and 40 mol%Al) alloys to high carbon steel was carried out at 1073 and 1273 K for 3.6 ks in a vacuum. It has been found that the joint with Ti-20 mol%Al alloy is separated in the vicinity of the interface promptly after the bonding treatment at 1273 K. Such a phenomenon could not be observed in other Ti-Al alloy/steel joints, and the Ti-20 mol%Al alloy/steel joint bonded at 1073 K showed a high strength of about 170 MPa. Therefore, this phenomenon depends on the bonding temperature and the composition of the Ti-Al alloys. From the observation results of the interface, it is thought that the diffusion of constituent elements across the interface is part of the reason for the separation phenomenon. The separated surface of the Ti-20 mol%Al alloy side showed diffraction peaks of TiC phase by XRD. Its Vickers hardness was about 1200 and approximately 4 times higher than that before the bonding treatment. It is expected that the separation phenomenon at the Ti-20 mol%Al alloy/steel interface serves surface modification of titanium materials, which show poor wear resistance.