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To improve the tribological properties of sulfur-containing Ni-based self-lubricating composites, FGH97-WS2-Cr-CeO2 (FWCC) were prepared by spark plasma sintering technique. All composites prepared obtained new phases CrxSy and M6C, originating from the decomposition of WS2. Homogenization and fine microstructure resulted in the best mechanical properties of the FWCC composites, with the microhardness of 542 ± 12 HV, compressive yield strength of 1910 ± 52 MPa, and flexural strength of 954 ± 34 MPa. When the composites exposed to 600 °C, the oxidation of CrxSy dominated the oxidative weight gain. The coefficient of friction and wear rate of FWCC composites both tend to decrease with increasing temperature. The optimum tribological properties of FWCC composites are obtained at 600 °C with the coefficient of friction of 0.23 and the wear rate of 0.22 × 10-5 mm3 N-1 m-1. The wear rate is two orders of magnitude lower than that of FGH97-WS2 composites (FW). This is mainly due to the synergistic effect of Cr and CeO2. The addition of Cr promotes the formation of enough Cr2O3 oxide particles during friction, while the addition of CeO2 not only enhances the adhesion of the oxide enamel layer, but also improves the high-temperature stability of the sulfide. [ABSTRACT FROM AUTHOR]

Metal matrix self-lubricating composites are required to possess well-balanced mechanical and lubricating properties when they are applied as mechanical parts. However, the decomposition of disulfide lubricants usually occurs during the sintering process and unfortunately reduces their mechanical properties. In this work, CeO2 was added to a model Cu/WS2 composite fabricated by spark plasma sintering (SPS). The effect of CeO2 doping on the interfacial microstructure and mechanical and tribological properties was investigated. The results indicated that CeO2 nanoparticles preferentially adsorbed onto the lubricant disulfide, which segmented the Cu/WS2 diffusion layer and reduced Joule heat at the interface while competing with WS2 to intensify the heat dissipation. As a consequence, interfacial overheating was largely suppressed, and WS2 decomposition was successfully prevented. The CeO2-doped Cu/WS2 composite breaks through a trade-off between mechanical and lubricating properties, exhibiting a high bending strength of 295.1 MPa, a hardness of 98.5 HV, a low friction coefficient of 0.19, and a wear rate as low as 4.12 × 10−5 mm3/N m. [ABSTRACT FROM AUTHOR]

The effect of CoO and NiO on the adherence between enamel coating and steel is investigated experimentally in this study. Enamel coatings are applied to the steel using a wet process with slurries. A total of five enamels were prepared including three single layer enamels: blank enamel ( BL), NiO bearing enamel ( MTNi3), CoO bearing enamel ( MTCo3), and two double layer enamels: BL- MTNi3, BL- MTCo3, which consisted an inner layer of BL and an outer layer of MTNi3 or MTCo3. Falling-weights tests were performed and the cross-sections at the enamel/steel interface were analyzed with a scanning electron microscopy ( SEM) coupled with an energy dispersive spectroscopy ( EDS) and electron probe microanalysis ( EPMA). Results show that the increases of adherence for MTCo3 and MTNi3 coated steel are attributed to the micro-alloys precipitation at the enamel/steel interface. These micro-alloy precipitations grow parallel to the steel/enamel interface for MTNi3, while they are dendritic and perpendicular for MTCo3. Compared with MTCo3, the negative reaction energy between NiO and FeO for MTNi3 causes micro-alloy precipitations formed far away from the enamel/steel interface, resulting in different morphology and adherence. [ABSTRACT FROM AUTHOR]

Enamel coating, which is of low thermal conductivity and high chemical stability, has been employed for the protection of many hot parts against oxidation. It is composed of multiple oxides, and each oxide component plays a certain role on thermophysical properties. In this paper, a type of silicate enamel coating with different contents of oxide component Al 2 O 3 was prepared on K444 superalloy. Its microstructural evolution and cyclic oxidation behavior at 1000 °C were investigated. Results indicated that the A0 enamel coating, which was free of Al 2 O 3 in composition, suffered from serious crystallization at high temperature. The formation of quartz, cristobalite and SrSiO 3 crystals resulted in significant mass loss during cyclic oxidation. However, precipitation of these crystals was almost completely prohibited in the A10 and A15 coatings with 10 and 15 wt% Al 2 O 3 , respectively. The A10 coating had no mass loss after cyclic oxidation. But much high content of Al 2 O 3 resulted in the formation of abundant long strip crystals of SrAl 2 Si 2 O 8 in the A15 coating, which led to minor spallation. Furthermore, enamel coatings with Al 2 O 3 (above 6 wt%) promoted the formation of a continuous alumina scale at the enamel/alloy interface, which improved oxidation resistance and increased bonding strength. The well grown alumina scale at interface as well as the mild crystallization ensured high spallation resistance of the enamel coating A10. [ABSTRACT FROM AUTHOR]

The steam oxidation behavior of three heterogeneous HR3C alloys was investigated at 650 °C comparatively. After a long-term oxidation process for 1000 h, the results demonstrated that the commercial HR3C alloy already exhibited a high oxidation resistance. However, the spallation resistance of the oxide scale was low during the initial oxidation period. The addition of a moderate amount of Nb into the alloy (1#HR3C) increased the oxidation resistance of the alloy. In addition, the improvement of the microstructural stability was substantially influenced by solid solution strengthening and fine grain strengthening. However, the addition of excessive Nb could significantly affect the growth model of the oxide scale and negatively affect the oxidation performance and microstructural evolution of the alloy (2#HR3C). [ABSTRACT FROM AUTHOR]

Corrosion of three SiO 2 –B 2 O 3 –Al 2 O 3 –CaF 2 -R 2 O (R = Na and K) based enamels with different ZrO 2 contents in H 2 SO 4 and NaOH solutions was studied. The addition of ZrO 2 reduced acid resistance of enamels. Especially for enamel with 5 wt% zirconia, the weight loss reached 4.04 mg/cm2 after 30-day corrosion. On the contrary, substituting ZrO 2 for SiO 2 greatly improved the alkali resistance of enamel, since the weight loss of enamel with ZrO 2 was only 0.3 mg/cm2 after 20-h corrosion, which decreased to less than one third of that of enamel without ZrO 2. Corrosion mechanism was comparatively discussed in these two solutions. [ABSTRACT FROM AUTHOR]

Corrosion of three bulk enamels with different silica contents in sulfuric acid was studied. Catastrophic corrosion happened for enamel with 54.6 wt% silica. Its weight loss reached 36.90 mg/cm 2 after seven-day corrosion. A porous leached layer with tremendous cracks formed at surface during corrosion, which favored acid invading and further corrosion. On the contrary, the durability of enamels with silica content over 60 wt% was guaranteed by their highly connected silicate network and a developed dense gel layer that clogged corrosion pits. Their weight losses were less than 0.2 mg/cm 2 after seven-day corrosion. [ABSTRACT FROM AUTHOR]