Papers by Keyword: Al₂O₃ Coating

Abstract: By adding sintering additives (CMS and nanoTiO₂), Ni-MF/Al₂O₃ composites were prepared by pressureless sintering. In order to ameliorate the combination between mullite fiber and alumina matrix, mullite fiber surface was coated with Al₂O₃ coating. To improve the property of alumina ceramic composite, Ni particles were chosen as toughening phase. For the preparation of composite, the optimum process was 10% MF, 13% Ni, and sintering temperature in 1400°C. The material performance can be effectively improved.

Abstract: The pristine LiNi_0.5Co_0.2Mn_0.3O₂ cathode material particles were successfully coated with Al₂O₃ via a heterogeneous nucleation process and subsequent heat-treatment. The structure and electrochemical properties of Al₂O₃-coated LiNi_0.5Co_0.2Mn_0.3O₂ were characterized by XRD and constant-current charge/discharge cycling tests. It was found that the Al₂O₃ coating did not alter the crystal structure of the cathode material. The Al₂O₃ coating layer had a negative influence on the dicharge specific capacity and a positive effect on the cycling performance. The decreased discharge capacity of the coated sample can be attributed to the poor clectronic and ionic conductivity of Al₂O₃ layer, which interfered the Li⁺ intercaltion/deintercaltion into/from the cathode material. The enhanced cycling performance can be ascribed to the suppression of the dissolution of transition metal ions by the HF acid in the eletrolyte and the side reations between electrode and electrolyte by the Al₂O₃ coating layer. The 2wt.% Al₂O₃-coated sample exhibits the optimal overall electrochemical performance, with discharge specific capacities of 157.0, 137.1 mAh/g at 0.2C and 3C rate, and capacity cycling retention rates of 97.34, 93.53% after 20 & 50 cycles, respectively.

Abstract: Microarc oxidation coatings on biomedical NiTi alloy were prepared in aluminate electrolytes with and without hypophosphate addition. The effect of hypophosphate concentrations on the characteristics of micro-arc oxidation coatings has been studied. The compositions and surface morphologies of the coatings prepared in different hypophosphate concentrations were determined by energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Surface roughness (Ra) and bonding strength of the coatings were measured by surface roughmeter and a direct pull-off test, respectively. The corrosion resistance of the coatings was evaluated in Hank’s solution using potentiodynamic polarization tests. The results show that all of the coatings exhibit a typical porous surface structure and mainly consist of γ-Al2O3 crystal phase. With increasing the hypophosphate concentrations, the elemental contents of Ni, Ti and P increase while Al decreases; the pore sizes and surface roughness of the coatings decrease firstly, reaching a minimum value at 0.01mol/L, and then increase; at the same time, the bonding strength increases up to 60MPa, and then decreases. The corrosion resistance of the coatings decreases with the increase of the hypophosphate concentrations, but all of the coated samples is better than that of the uncoated NiTi alloy.

Abstract: The surface of Al2O3 coating sprayed on 40Cr substrate was re-melted with high power continuous CO2 laser, and its micro-hardness and residual stresses were measured, respectively. The strengthening mechanism of Al2O3 coating by laser re-melting was analyzed and discussed. The experimental results shown that the surface of Al2O3 coating by laser re-melting is neat and smooth, and its compositions are even, its structures are compact, and Al2O3 coating is evenly distributed in its surface with grain forms, and its micro-hardness increases about 200%; Residual stress of Al2O3 coating by laser re-melting is changed into compressive stress from tensile stress, which is benefit to improving bonding strength of coating-substrate interface.

Abstract: Mullite film was formed on ß-SiAlON (Si6-zAlzOzN8-z, z = 3) ceramics by reaction of Al2O3 films with silica formed on oxidation to improve a high temperature water vapor corrosion resistance. Sintered ß-SiAlON was fabricated by hot pressing of ß-sialon (z = 3) powder without additive at 1900°C and at 24 MPa for 4 h in N2. The Al2O3 film was deposited on polished ß-SiAlON ceramics by a dip coating process of Al2O3 precursor sols prepared from the alkoxide solution and aluminum sol. The Al2O3 coated sample was heated in Ar/O2 (95/5 vol.%) at 1300°C for 2 h, resulting in the formation of mullite by reaction with silica produced from sialon. The resulting mullite-coated sample was corroded in H2O/Ar/O2 (90/8/2 vol.%) at 1200°C for 2 ~ 100 h. The corroded samples were characterized by XRD and SEM-EDS for the evaluation of the corrosion resistance of mullite-coated sialon.

Abstract: The corrosion resistance of MOCVD Al2O3 coating system was investigated to protect a TA6V Alloy under hot salt corrosion conditions: This coating was corroded with a salt deposit without mechanical loading at 480°C during 100 h. Corrosion products formed in salted areas were studied by Energy Dispersive Spectroscopy (EDS). Although all coated specimens were damaged with corrosion products presence in salted area, Al2O3 coatings showed the lowest salt damage on titanium substrate after a metallographic cross section observation compared to uncoated ones. As well as these interesting experimental results, coated specimens exhibit a good adherence on titanium substrate

Abstract: Al2O3 coatings were fabricated by multi-functional micro-plasma spray through axial powder feed. The flight particle velocity and temperature were measured by SprayWatch-2i. With the rising of Ar flow, the velocity of Al2O3 particles increases, but the temperature of particles decreases gradually. Al2O3 coatings were analyzed by SEM and XRD. The microstructure of coatings is density, low porosity and uniform morphology. X-ray diffraction results show that α-Al2O3 is the main phase in the original Al2O3 powders, but Al2O3 coatings consist of γ-Al2O3 and a small amount of α-Al2O3. The microhardness and bonding strength of Al2O3 coatings were also measured, which is HV0.11183~1387 and 28.6MPa, respectively.