Growth, Microstructure And Performance Of TBC On Piston Aluminum Ally Formed By MAO

Aluminum alloy has the advantages of high specific strength and high specific modulus etc, so it's widely used in various industries such as aerospace, construction and transportation. However, it has poor heat-resistant quality, so the thermal barrier coatings (TBC) of aluminum alloy emerged in order to meet the demand of heat-resistant quality raised by technological development. Preparing TBC on aluminum alloy by micro-arc oxidation technology (MAO) has the advantages of simple operation, low cost, strong controllable etc. Besides, the coating has high hardness, good abrasion resistance, high corrosion resistance, good adhesion etc, so that MAO is applied in surface treatment of aluminum alloy. But, so far, making TBC on aluminum alloy by MAO is studied little, and there is no model that is able to well explain the mechanism of MAO.In this paper, MAO was used to make ceramic TBC in five different electrolytes (silicate system, silicate-phosphate system, silicate-composite rare earth salt system, zirconium salt system, zirconium salt-Y salt system), and the growth process, growth pattern, thickness and heat insulation temperature of the coatings in different electrolytes were investigated. The fitting formulas of coating growth were established according to coating thickness and time. The microstructure and elements distribution of the coatings were performed by SEM and EDS. Based on all of above, the growth mechanism, organization and performance of the coatings in different electrolytes were analyzed. Moreover, quantitative phase analysis of zirconium salt-Y salt system coating was conducted by XRD. the high temperature oxidation resistance was studied, and the porosity was detected through grease method and boiling method. What's more, the influence of sealing treatment on the organization and performance of the coating of zirconium salt-Y salt system was studied. At last, the phase stability of ZrO2 was studied.The results show that the growth processes of coatings in the five electrolytes are different. The coating thickness-time curves in silicate system and silicate-phosphate system exhibit parabolic relation. When adding phosphate to the silicate electrolyte system, the coating growth process stays the same, but the thickness doubles. The coatings have similar growth processes in silicate-composite rare earth salt system and zirconium salt-Y salt system, and their growth formulas are quadrinomial. Rare earth salt can improve the coating thickness, refine structure and promote outward growth of the layer in early oxidation phase that is it regulates the growth and microstructure of the coatings. The coating in the zirconium salt-Y salt system is the thickest (89.5μm) of the five coatings, that also has the highest insulation temperature (54.3℃).In the zirconium salt-Y salt electrolyte system, the layer has clear stage topography characteristics at different oxidation time, according to which it can be deduced that the process of MAO is divided into such stages as forming stage of the high impedance coating, micro-arc discharge stage, fusing, freezing, recrystallizing stage and damage ablation stage. The porosity of the coating is 47/cm2, and the apparent porosity is 11.12%. The sealing treatment makes the surface of the coating more uniform and less porous, but lower the heat insulation temperature of the coating. Weight gain curve of the sample with MAO coating follows the logarithmic law and the bi-logarithm law after oxygenating at 400℃for 24h and 124h, in addition, weight-increase rate of the samples with MAO coating is obvious less than the base which indicates its good high temperature oxidation resistance. After high-temperature oxidation, the compressive stresses appear in the coatings, and the surface microstructure of the layer isn't changed much, but the section of coating oxygenated for 124h suffers damage. The electrolyte ingredients participate in the film so that Y2O3 and Al2O3 stable the high temperature phase ZrO2 in the coating. When the sintering temperature of the mixed oxide powder is over 1200℃, Y2O3 can stable high temperature phase ZrO2. Besides, the content of Al2O3 can affects the type of the solid solution, when the content is more than 60%, Al2O3 can stable the high temperature ZrO2. Comparing the phases in the coating and in the sintering body of Al2O3, Y2O3 and ZrO2 shows that the coating composition is unevenly distributed.