Advanced Materials Research Vols. 652-654

Abstract: The TiC/Fe-based composite coating was fabricated by plasma transferred arc (PTA) weld-surfacing process on substrate of Q235 steel with the mixture of ferrotitanium, ferrochromium and ferrosilicium powders. The microstructure and wear properties of the coating were investigated by means of X-ray diffraction (XRD), scanning electron micrograph (SEM), energy dispersive X-ray analysis (EDS), microhardness test and wear test. The results show that the coating consists of TiC, (Cr,Fe)7C3 and austenite. The composite coating is metallurgically bonded to the Q235 steel substrate. TiC particles formed by PTA weld-surfacing process presented cubic or “dendrite flower-like” morphology. The wear-resisting property of the coating was greatly improved compared with the substrate. The coating has excellent adhesive wear and grinding abrasion resisting force.

Abstract: Si-Al-Y co-deposition coatings on γ-TiAl alloy were prepared by pack cementation process at 1050°C for 3 h, Scanning electron microscope (SEM), energy dispersive X-ray spectrometer (EDS), X-ray diffractometer(XRD) and microhardness tester were employed to investigate the surface morphologies, surface element distributions, microstructures, phase constitutions and microhardness of the coatings. Friction-wear tests of the γ-TiAl alloy substrate and the coatings were conducted in air at ambient temperature. The results show that the coating prepared by co-depositing Si-Al-Y had a multiple layer structure: the outer layer was composed of TiSi₂, TiSi, Ti₅Si₄, Ti₅Si₃, the middle layer was composed of TiAl₃, TiAl₂, and the inner layer was Al rich layer. The microhardness of the surface is up to 1538HK_0.49.The wear mass loss is only one half as much as that of γ-TiAl matrix, and the wear resistance increases significantly.

Abstract: Silicate-based hybrid coatings were deposited on aluminum alloy substrates following the methodology of sol-gel procedure and are applying to survey the corrosion protective behavior. The results show that the optimal intact and uniform film can be obtained by preparing with Si:Al mole ratio 48:1 and tempering at 140°C. It was noted that this modified hybrid coating reduced the corrosion current density from 1.532×10-4A/cm2 (bare substrate) to 2.782×10-8 A/cm2 in 3.5 wt. % NaCl solution. The coating represents an excellent stability, especially at higher temperature, therefore could offer a significant protection to 2024 aluminum alloy substrates.

Abstract: This study examined the performance of abrasive jet polishing (AJP) on SKD61 mold surfaces using wax-coated SiC abrasives fabricated by gas atomization and the comparison of surface quality achieved by revealing significant surface roughness reduced with wax-coated abrasives. The wax coating of SiC abrasives smooth the sharp surfaces of abrasive powders and provides a cushioning buffer. These two features contribute and improve the direct impact of heavy peening, thus preventing surface hardening, superficial embedment or deep scratches. Furthermore, compound additives including water wax and water were added to the wax-coated #3000 SiC abrasives at a mix proportion of 500: 1000: 1500 (water wax: wax-coated SiC: water) for polishing micro-channels surfaces. The experimental results show that the compound additives provide addition cushioning effect facilitate the sliding and grinding of abrasives, resulting in fine polishing and good surface quality. The reflection properties of both linear and curved micro-channels were much enhanced by AJP using wax-coated #3000 SiC abrasives with the compound additives.

Abstract: Ni60A coating is obtained by oxygen-acetylene spray welding and high-frequency induction remelting process. Coating Surface, interface between matrix and coating, microhardness and wear resistance are investigated. The results show that both of the processes can achieve coating metallurgically bonded to matrix. But microhardness and wear resistance of Ni60A coating obtained by high-frequency induction remelting process are superior to those of Ni60A coating obtained by oxygen-acetylene spray welding process. At the same time, coating surface by high-frequency induction remelting process is of more brightness owing to less oxide skin. In order to more effectively prevent the hard phase from oxidation, the quartz glass pipe and inert gas should be considered to use in high-frequency induction remelting process.

Abstract: The free standing NiCrAlY and NiCoCrAlY coating samples were prepared by HVOF. The cyclic oxidation resistance of the coatings was evaluated and the coating surface morphology was investigated by SEM. The MCrAlY coating CTE was tested. The results show that the NiCoCrAlY coating exhibits better oxidation resistance than the NiCrAlY coating and the difference in MCrAlY CTE curve shape is considered as the main reason.

Abstract: The coating was made of base coating (NiCrAlY) and superficial coating (ZrO2 + Y2O3) by plasma spray. The microstructure and phase structure of the coating were investigated by using the scanning electron microscope and the X-ray diffraction. The oxidation process can be divided into three stages: The main oxidation products is θ-Al2O3 but not continuous α-Al2O3 film in oxidation prophase; Continuous α-Al2O3 film generated quickly with the transition of θ-Al2O3 to α-Al2O3 in the oxidation reaction. The oxidation reaction becomes complicated when the content of Al decrease and Ni is also oxidized to NiO. Oxidation kinetics curves of 1200°C is basically accord with parabolic law.

Abstract: A porous titania (TiO₂) coating with vermiform slots was prepared on the Ti substrate through micro-arc oxidation (MAO) treatment using sodium tetraborate as electrolyte. Morphologies and phase structure were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Results show that the rutile phase increases and anatase decreases gradually with increasing MAO time. The electrolyte of sodium tetraborate has significant influence on the formation of vermiform coatings, which determine the corrosive patterning in the first stage during MAO processing. The evolution of vermiform morphology is proposed as followed: some corrosive pores appear on the surface before arcing; afterward, the adjacent micropores in the dense regions link each other due to the high temperature result from continuous arc action; then, the micropores grow up to big pits and combine with each other with increasing MAO treating time; finally, the vermiform morphology forms on the surface of Ti metal.

Abstract: NiPtAl coatings are widely used as bondcoats for thermal barrier coating (TBC) systems during high temperature exposure. Pt modified aluminide coatings on the CMSX-4 Ni-base alloy were oxidized at 1150°C in different atmospheres. Cross-section oxide layer morphologies on the NiPtAl coatings after TBC failure were similar in air with that in air+15%H₂O. The surface morphologies of as-received low and high-Pt bondcoats showed great effect on the oxide layer growth and morphologies due to the different compositions within the two bondcoats. The irregular alumina surface on the low-Pt bondcoat was showed due to the slipping of the NiPtAl grains. Raman spectroscopy illustrated that the alumina mainly consisted θ and α-Al₂O₃ during the Ar+20%O₂ exposure, however, only α-Al₂O₃ could be found in Ar+4%H₂+2%H₂O during short time exposure. Low oxygen partial pressure of Ar+4%H₂+2%H₂O perhaps is the reason that spinel and θ-Al₂O₃ can not formed, and directly promotes the α-Al₂O₃ formation.

Abstract: EB-PVD TBC with conventional MCrAlY bondcoats was widely used within the high temperature environments. The effect of oxidation frequency on the lifetime of TBC with NiCoCrAlYZr and the failure mechanism was investigated in this paper. The TBC systems with Zr-doped MCrAlY bondocats presented a longer lifetime after discontinuous oxidation than cyclic oxidation. Formation of thick TGO in the TBC-system with Zr-containing bondcoat did not result in an immediate failure. The crack propagation in the case of the Zr-doped NiCoCrAlY bondcoat at the TGO/bondcoat interface was hindered due to the inhomogeneous TGO morphology. The inner oxidation and pores hindered the above small cracks propagation and then result a longer lifetime. However, the lifetime of TBC-system with NiCoCrAlY+Zr bondcoat is significantly shorter in the cyclic than in the discontinuous test due to the rapid propagation of cracks under high frequency thermal cycling conditions.