Key Engineering Materials Vols. 368-372

Abstract: Alumina-titania (60: 40 by weight) coatings were prepared by low power plasma thermal spraying with two types of plasma spray systems: internally-fed and externally-fed powder system. The coatings were investigated by many techniques, respectively X-ray diffraction (XRD), scanning electron microscopy (SEM) and micro-hardness tester. It was found that the thickness of coatings by low power plasma spray with an internally-fed powder system is higher than that of the coatings with an externally-fed powder system. The internally-fed powder system not only decreases the loss of powder but also increases the deposition efficiency for more than 80%. To better qualify the characteristics of these coatings prepared from spray-dried powders, microhardness was also studied.

Abstract: The chemical composition and the valence state of elements on the surface of ZrO2 thin films deposited on glass substrates have been studied by X-ray photoelectron spectroscopy. Results show that: elements of Na, Mg, Zr, Ca exist in the form of their respective stable state, such as Na2O, MgO, ZrO2, CaO, when heat treated at 500°C for 0.5h; but Si is unstable, and exhibit stoichiometrical disturbances. Results of chemical composition and their content by atom percent of ZrO2 thin films surface reveal that: Si, and Ca diffuse from glass to the thin films in scale; Na diffuses few and Mg collects to the thin films surface. The diffusion of Mg2+ and Ca2+ from glass to ZrO2 thin films is negative diffusion.

Abstract: Nanostructured coatings are experiencing a rapid development in recent years due to their potential applications in a wide variety of technological areas. In order to improve the properties of thermal barrier coatings, nanostructured 8 wt% yttria partial stabilized zirconia coatings were deposited by air plasma spraying with reconstituted nanoparticles on titanium alloy substrates. For comparison, the conventional counterparts were also fabricated. Their microstructures were investigated by using transmission electron micrograph and scanning electron micrograph. Their mechanical behaviors were evaluated via Vickers microhardness test and tensile test. The results indicate that the nanostructured zirconia coatings with original nanostructure and columnar grains have higher microhardness and adhesive strength than the conventional counterparts. Both the nanostructured coating and the conventional coating exhibit a bimodal distribution of microhardness values, which are analyzed via Weibull statistics. This phenomenon is attributed to the presence of molten and non-molten parts in the coatings.

Abstract: TiO2-Ni wear-resistant film on metal surface is manufactured by electroless composite plating. TiO2 content in the film depends on the concentration of TiO2 particle in the plating liquid. The optimal TiO2 content in the plating liquid is 15-20g/l. TiO2 content in the film is 5-8%. When TiO2-Ni composite film is treated at 200°C, its hardness increases with the prolonging of heat-treatment time. After 10 hours heat-treatment at 200°C, the hardness increases from HV 750 kg/mm2 to HV 1300 kg/mm2.

Abstract: Molybdate conversion coatings were prepared on the anodized surfaces of aluminum alloy substrates, and the corrosion resistance in 3.5% NaCl solution was also investigated. The effects of the main parameters on the quality of the coatings were discussed. The morphology and chemical composition of the conversion coatings was examined by SEM and EDS. SEM observation showed that the coatings were composed of small sheet grains with tiny cracks between them. EDS analysis showed that the main element of the coatings was molybdenum. The results of polarization curve showed that the coatings effectively inhabited the pitting corrosion of aluminum alloy substrates. In the end the possible conversion mechanism was discussed.

Abstract: Yttrium silicates coatings were deposited on SiC-C/C composites surface by a hydrothermal electrophoretic process using yttrium silicates nano-crystallites, isopropanol and siodine as source materials, solvent and charging agent, respectively. The yttrium silicates nano-crystallites were preprepared by a sonochemical process. The influence of deposition temperatures on the microstructures and crystallization of the yttrium silicates coatings were investigated. Surface and cross-section microstructures of the as-prepared multi-coatings were characterized by XRD and SEM. Results show that the coatings are composed by yttrium silicates crystallites with the main phase of Y2Si2O7 and a little Y2SiO5. The thickness and the density of the yttrium silicates coatings increases with the increase of deposition temperatures; and the coating process has little influence on the phase compositions of the coatings.

Abstract: Several kinds of ceramic powders including SiC, TiO2 and Al2O3 were selected as fillers blending with epoxy resin, curing agent and other additives to prepare a heavy-duty and anti-wear coating. The effects of filler sort, powder size, dispersion homogeneity on the coating properties were investigated. The results showed that the wear resistance and corrosion resistance of the coating could be improved significantly after meliorating the dispersion of the ceramic particles in epoxy resin. Other mechanical properties were also studied in order to support the view on the positive effect of the nano-particles in the epoxy coating.

Abstract: Silicon carbide coatings on graphite were prepared through polymer vapor pyrolysis deposition process (PVPD) under N2 atmosphere. During this process, some low molecular weight substances that polycarbosilane (PCS) pyrolyzed can be deposited on graphite, and they can convert into SiC in high temperature. The results of XRD showed that amorphous SiC coatings were formed on graphite when the pyrolysis temperature was 1000°C, andβ-SiC phase formed in the coatings when the temperature up to 1250°C. Effects of the coatings on the microstructure and properties were investigated. It was shown that the uniform dense SiC coatings could be obtained by carefully controlling the pyrolysis temperature and ramping rate when the number molecular weight of PCS was in the range of 1,000~1,500.

Abstract: The air plasma spray (APS) technology was used to prepare WC/NiCrAl and WC/NiCrBSi ceramic/metal composite wear preventive coating on cast iron substrate. The microstructure, microhardness, bond strength and slurry wear preventive performance of coating was measured. The results revealed that all of the coating prepared by APS has good qualities. The coating with NiCrBSi as its combining metal is better than that with NiCrAl on wear preventive performance, and coating containing more WC is supreme than that containing less WC. Finally, wear mechanics and the principle of the coating’s wear preventive property was concluded.

Abstract: A TiC coating was fabricated on a carbon steel surface by SHS/PHIP (combustion synthesis/pseudo heat isostatic pressing) process. Phases in the coating were determined by XRD. We employ SEM-EDS to analyze the microstructure of the coating and bonding interface between the substrate and the coating, and the distribution of elements adjacent to the interface. The microhardness of the coating was also measured. Our results showed that the coating was composed of TiC and Fe phases, with dense and pure microstructure. Metallurgical bonding was formed between the substrate and coating. The result of thermal shock resistance test showed that the coating has good bonding strength and hardness of the coating was four times higher than that of the substrate.