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For obtaining a thick ceramic coating with good film-forming and wear resistance, a new type of composite electrolyte was composed of sodium phosphate and sodium hexametaphosphate as the basic electrolytes, while sodium tungstate and sodium tetraborate were added as additives. In this composite electrolyte system, micro-arc oxidation treatment was performed on 1060 aluminum alloy. Using the coating thickness and surface porosity as optimization indicators, the electrolyte formula was optimized using the L9(34) orthogonal test. Meanwhile, the microstructure, surface porosity, phase composition, thickness, microhardness and wear resistance of ceramic coatings prepared under this electrolyte formula were studied systematically. Results showed that the ceramic coating prepared with a formula of Na3PO4 concentration of 32.0 g/L, Na2B4O7 concentration of 16.0 g/L, Na2WO4 concentration of 4.5 g/L and(NaPO3)6 concentration of 30.0 g/L had the structure with uniform micropores and dense film layer, the thickness of 22.8 μm, the porosity of 3.86%, and the microhardness of 475 HV. Furthermore, the wear resistance of the coating was significantly improved compared to the substrate, and the ceramic coating were mainly composed of γ-Al2O3 and a small amount of α-Al2O3 composition.

The mechanisms of ductile–brittle transition and surface/subsurface crack damage during the grinding of plasma–sprayed alumina ceramic coatings were investigated in an experiment and simulation on single diamond abrasive grain cutting. We observed that the brittle damage modes of alumina ceramic include boundary cracks, median cracks and lateral fractures. The normal force of the abrasive grain results in the initiation of median cracks, whereas the tangential force of the abrasive grain results in the propagation of median cracks in the direction of the abrasive grain cutting. Some cracks propagate downward to form machined surface cracks, whereas others propagate to the unmachined surface of the workpiece to produce brittle removal. Owing to the alternating tensile and compressive stresses, the material in contact with the top of the abrasive grain fractures continuously, forming the main morphology of the machined surface. The geometry and cutting depth of the abrasive grain have a significant influence on the ductile–brittle transition, whereas the cutting speed of the abrasive grain have no significant influence. On one hand, the stress concentration at the pore defects result in crack propagation to the deep layer; on the other hand, it reduces the local strength of the surface material, produces brittle fracturing, and interrupts crack propagation. The pores exposed on the machined surface and the broken morphology around them are important factors for reducing the surface roughness. Experimental observations show that the machined surface morphology of the alumina ceramic coating is composed of brittle fracturing, ductile cutting and plowing, cracks, original pores, and unmelted particles. [ABSTRACT FROM AUTHOR]

The thermal conductive behavior of Zirconium diboride (ZrB 2 ) coated with different proportions nanoalumina (Al 2 O 3 ) in epoxy composites was investigated by the laser flash experimental and finite element analysis (FEA) methods. The coated ZrB 2 composite particles were categorized into the 3-1, 3-2, 3-3 and 3-4 systems, which corresponded to four different mass ratios of ZrB 2 particles to Al 2 O 3 particles, respectively. It could be found that the coated ZrB 2 composite particles were effective for increasing the thermal conductivity of the filled epoxy composites due to more effective formation of the conductive chain structure in the composites compared to the single-phase particles. In comparison, the system of 3-3 showed the most positive effect on improving the thermal conductive performance of epoxy composites. For composites with a 7 vol% of ZrB 2 /Al 2 O 3 composite particles of 3-3 system, its thermal conductivity was 0.65 W m −1 K −1 , increased by 20% and 79% relative to single-filled composites of ZrB 2 and Al 2 O 3 with same filled content, respectively. The predicted thermal conductive results of Al 2 O 3 coated ZrB 2 particles in epoxy matrix obtained by finite element analysis were in reasonable agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Abstract: Alumina ceramic coatings were fabricated on 304 stainless steel by cathodic plasma electrolytic deposition (CPED). Influence of treating frequency of the power supply on the microstructure and properties of the coatings were studied. The results indicated that coatings obtained at various frequencies on 304 stainless steels were all composed of α-Al2O3 and γ-Al2O3, and α-Al2O3 was the dominant phase. The contents of α-Al2O3 decreased gradually in a very small rate with increasing the frequency and γ-Al2O3 gradually increased. The surface of alumina ceramic coating was porous. With increasing the frequency, the coating surface gradually became less rough and more compact, resulting in low surface roughness. The bonding strength of Al2O3 coating was higher than 22MPa and was not strongly affected by treating frequency. With increasing the frequency, the alumina coated steels showed better and gradually increasing corrosion resistance than the uncoated one in 3.5% NaCl solution. The coating steel with desirable corrosion resistance was obtained at 800Hz whose corrosion current potential and corrosion density were −0.237V and 7.367×10−8 A/cm2, respectively. [Copyright &y& Elsevier]

Over the past decade, alumina ceramic coatings have become promising candidates for anti-wear and anti-corrosion applications owing to their excellent mechanical properties, such as chemical inertness and high temperature resistance, high hardness, and high abrasion resistance. In this work, alumina ceramic coatings are prepared via high-enthalpy atmospheric plasma spraying. The effect of plasma spraying parameters should be paid to attention, such as spraying power, powder feeding rate, and initial substrate temperature, on the mechanical and wear characteristics of the obtained coatings. According to the results, the increase in the plasma spraying power allows one to substantially reduce the porosity of coatings and in turn improves the wear resistance. The use of the higher powder feeding rates enables one to achieve the lower density and adhesion of coatings, which is due to the rising amount of unmolten particles. Finally, an increase in the initial substrate temperature reduces to a large extent the defects of the coating by decreasing the quenching stress, which enhances their mechanical properties. A decrease in the porosity of the coating is beneficial to improve the wear resistance. • Alumina coatings were prepared by high-enthalpy atmospheric plasma spraying. • Studies on microstructure, porosity, phase composition, adhesion, microhardness and wear resistance • Porosity is crucial for improving mechanical properties and wear resistance. [ABSTRACT FROM AUTHOR]