Structures And Properties Of New CPED Thermal Protective Coatings On Aluminum Alloy

Nano ZrO₂-Y₂O₃ ceramic layers based on cathode discharge induced by ZrO₂ sol, ZrO₂ nano-particles, and sol cooperate with nano-particles were coated on Al-Si alloys by Cathodic Plasma Electrolytic Deposition （CPED） techneque. The effect of the ZrO₂ sol, nano-particles and synergistic effect on the microstructure, morphology, growth rate, and the properties of heat insulation and thermal shock resistance of the ceramic layer were studied. Futhermore the evolution law of structure and properties, the film forming mechanism of the ZrO₂ sol and the particles participating in CPED was investigated in this paper. The surface and cross-section morphologies, and the phase composition of the ceramic layers were observed by scanning electron microscope （SEM） and X-ray diffraction （XRD）. The crystal structure and main growth crystal planes in ceramic layers were investigated by transmission electron microscope （TEM）.The thickness, heat insulation properties and thermal shock resistance were measured by eddy current thickness gauge, self-made heat insulation test device and thermal shock equipment, respectively.To compare with the traditional electrolyte, the addition of ZrO₂ sol and ZrO₂ nano-particles promoted the plasma micro-arc discharges, making more discharge channels existed on the surface of the ceramic layers, thus the growth rate was improved, and the growth rate of the ceramic layer in ZrO₂ sol system is significantly higher than that in ZrO₂ nano-particles. ZrO₂ colloidal particles participate in film formation process mainly through forming a gel layer on the coating suface to induce micro-arc discharge. However the ZrO₂ nano-particles through flowing into discharge channels when the micro-arc discharge occurs.The main phases in the ceramic layers of these two systems are α-Al2O3, SiO₂, t-ZrO₂ and high temperature phase —δ-Zr3Y4O12. Among them, with the concentration of ZrO₂ sol increases in the ZrO₂ sol system, degree of crystallizing of the high temperature phase Zr3Y4O12 and SiO₂ decreases gradually showed by the XRD diffraction peak. However, the XRD diffraction peaks of Zr3Y4O12 become stronger with the conte of ZrO₂ nano-particles increases in the ZrO₂ nano-particles system. So, the micro-area reaction temperatures of these two systems are all rise with the concentration of ZrO₂ sol/nano- particles increasing. From the result of TEM, nano-grains of 5nm-25nm were found in ceramic coating formed in 1.2g/L ZrO₂ sol system, with its mean particle size 20.7nm calculated by XRD. The selected area electron diffraction （SAED） pattern displayed that the coating has high crystallinity with very little amorphous component.While the ceramic coating in 3g/L ZrO₂ sol system has lower crystallinity with a mount of amorphous phase. Nano-grains, whose mean diameter is 24.3nm, can be seen in the ceramic layer coated in the system with ZrO₂ nano-particles. Higher crystallinity and a small amount of amorphous phase were shown in SAED pattern.The conclusion of the CPED induced by ZrO₂ sol cooperate with nano-particles showed that ceramic layer is thicker than that of the CPED induced by ZrO₂ sol or ZrO₂ nano-particles only when the consentration of ZrO₂ nano-particles is higher than ZrO₂ sol. The ceramic layer in this system not only contains the phases the former two systems does, but also generates another high temperature solid solution δ—Y6ZrO11. This product offers more oxygen vacancy to the ceramic layer, and suggesting that the synergistic effect of ZrO₂ sol and ZrO₂ nano-particles can further increase micro-area reaction temperature in CPED. Moreover,TEM result shows that ceramic coating in ZrO₂ sol cooperate with nano-particles system was composed of nano-grains with the average diameter of 22.2nm, and its crystallinity displayed in SAED pattern is pretty good. The growth rate of ceramic coating formed in ZrO₂ sol cooperate with nano-particles system is further higher than that in ZrO₂ sol system,and the slow growth period is brought forward. This indicates that the synergistic effect of ZrO₂ sol and nano-particles is also helpful for growth rate of the film.Ceramic layers in these three kinds of systems have good heat insulation performances.At the same concentration, ceramic layer in ZrO₂ sol cooperate with nano-particles system has the best heat insulation performance, followed by the ceramic layer in ZrO₂ nano-particles system. After thermal shock resistance test of 500,800 and 1000 times, the ceramic layers showed exellent binding performances, only discovered that the amount of micro-cracks increases with the test times rises. All in all, the ceramic coatings have good thermal shock resistance.