Ceramic Coatings Grown In-Situ On Al Alloy And Their Friction And Wear And Corrosion Resistance

In order to solve the poor wear and corrosion properties, the oxide ceramic coatings were formed in-situ on aluminium alloys using the bi-pulsed micro-arc oxidation (MAO) power. The composition, structure and performance of the oxide coatings formed on different aluminium alloys were studied. The study was also conducted on the composition, structure and mechanical performance of black ceramic coating consisting of alumina and zirconia, respectively. The effect of alloy components on the structure, color and properties was discussed. The in-situ growth mechanism of oxide ceramic coatings was discussed in different electrolytes. The model of ceramic coatings'resistance to heat shock was established and used to analyze the variation of heat stress during the cycles of heat shock. The corrosion resistance, the friction behavior and wear mechanism of three kinds of coatings were estimated and discussed.The X-ray diffraction (XRD), scanning electron microscope (SEM), electrode probe microscope analysis (EPMA), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscope (XPS) were used to study the composition, morphology, the element distribution and variation of element valence. The corrosion resistance was studied by electrochemical methods. The coating hardness was measured by micro-hardness instrument. The wear lost and friction coefficient of aluminium alloys and ceramic coatings were studied by a ball-on-disk tester under the dry sliding conditions.It was shown that the MAO coating formed on pure aluminium and LC9 consisted ofα-Al₂O₃ andγ-Al₂O₃, respectively. The coating formed on LY12 was composed of large numbers ofγ-Al₂O₃ and littleα-Al₂O₃. The heat shock resistance was increased by the followed orders: LC9﹤LY12﹤Al, and the corrosion resistance was increased by the subsequence of Al, LC9 and LY12. Coatings'maximal hardness of Al, LY12 and LC9 were 33.4 MPa, 22.15 MPa and 16.8 MPa, respectively. The black Al₂O₃ ceramic coating was prepared on LY12 aluminium alloys in solution of Na5P3O10-CrO3. The chromium was not crystal and had the valence of 0, +3, +6. CrO3 benefited to the transformation ofγ-Al₂O₃ toα-Al₂O₃ and increased the growth, hardness, thickness and corrosion resistance, and improved the compactness.The coating consisting of m-ZrO2,t-ZrO2,γ-Al₂O₃ was prepared on LY12 in K₂ZrF₆-NaH₂PO₂. The proper increase of negative current density was in favor of the thickness increase of the coating and eliminated KZr2(PO3)3. The maximum of Knoop hardness could reach 16.75Gpa. The adherent strength of coating to substrate was beyond 17.5Mpa. Increasing the concentration of NaH₂PO₂ could improve the dot-corrosion resistance of the coating. The coating was mainly composed ofγ-Al₂O₃,α-Al₂O₃ and a little c-ZrO2. A small quantity of K₂ZrF₆ accelerated the coating growth and increased the compactness, promoted the transformation fromγ-Al₂O₃ toα-Al₂O₃ and consequently improved the corrosion resistance. The maximum Knoop hardness could reach 23.41Gpa.The heat shock cycle experiment includes three stages: heat uprising, heat preservation and heat decreasing. During the heat shock process, the stress prepared in K₂ZrF₆-NaH₂PO₂ was increased quickly to 575Mpa, and then up to 780 MPa slowly, and there was no strip peered off after 38 cycles of heat shock. The heat stress of coating prepared in NaAlO2-K₂ZrF₆ was quickly increased to 558Mpa, and then up to 994 MPa slowly, and there was no strip peered off after 29 cycles of heat shock. The heat shock resistance of coating prepared in K₂ZrF₆-NaH₂PO₂ was better.The LY12 aluminium alloys had the most rate of wear and tear and grinding abrasion occurred against Si3N4 ball on ball-on-disk tester with the rate of 1000 r/min under dry friction conditions. However, the wear resistance of the ceramic coatings was enhanced remarkably. The ceramic coating prepared in the solution of K₂ZrF₆-NaH₂PO₂ had the friction coefficient between 0.35 and 1.367 and minimum wear rate was 1.61×10-6g/min under a 150g load. The wear mechanism gradually converted from abrasion wear and adhesive abrasion to fatigue wear. The ceramic coating prepared in the NaAlO2-K₂ZrF₆ solution had the minimum friction coefficient of 0.214 and wear rate of 4.33×10-6g/min under a 300g load. The wear mechanism changed from abrasion wear to fatigue wear and abrasion wear. The high-wear-resistance friction pairs Si3N4 were badly worn, and Si from friction pairs was found in the wear scar on the ceramic coating. The ceramic coating prepared in the solution of Na5P3O10-CrO3had the friction coefficient of 0.306 and wear rate of 2.1×10-5g/min under a 100g load. Plastic deformation occurred to some extent, which induced smooth abrasion and brittle rupture of the coating.