Abstract
In order to solve the oxidation and oxygen embitterment of the TC4 alloy under the high temperature service environment, a kind of thermal protection coating was fabricated by Micro-arc oxidation technique on TC4 surface. The Micro-arc oxidation test was carried with two sorts of Al-P and Al-P-Zr electrolyte systems,and the processing parameters of depositing the composite ceramic coatings were optimized. The phase composition、morphology of the coatings surface、elemental distribution and microstructure were investigated by XRD 、 SEM 、 EPMA and Raman, individually. The thermal shock resistance to high temperature was characterized by experiment, and the isothermal oxidation at different temperature and the cyclic oxidation behavior were also studied.
Taking the film growth rate, surface porosity, size of micro-pore and surface roughness to generate the indicator function, MAO experiments were carried according to the orthogonal design optimization method. The affection of the electrical parameters, electrolyte temperature and oxidation time to the surface of the MAO composite ceramic coating was discussed. Its best optimum parameters were concluded as the following: pulse frequency of 550Hz, voltage of 425V, the duty cycle of 4%, electrolyte temperature of 20℃ and the oxidation reaction time of 120 minutes. The coating growth rate and surface quality of the coatings were mainly controled by pulse energy of single-polar pulse, which was increased with the raising of pulse voltage, lower frequency and duty cycle. However, with the increasing energy, the micro-plasma discharge was also more intensive, which made the formation of the plasma discharge channel leave greater micro-pore diameter on TC4 surface by solidified material, so the film surface roughness would also increase.
Generally, the macro-performance and micro-structure the MAO composite ceramic coating were both dependent on the electrolyte system, so in this paper a new kind of electrolyte system was developed by doping ZrO2 micron particles into the Al-P system in order to deposit composite ceramic coating on TC4 surface.
The MAO composite ceramic coating with and without ZrO2 particles both showed a dense inner and porous outer surface with micro-pore structure feature. The surface of MAO composite ceramic coating with ZrO2 particles was more roughness than those of MAO composite ceramic coating without ZrO2 particles, which clearly indicated that the presence of doped ZrO2 particles was beneficial to the growth of the coatings, but not good for surface toughness. The main crystal phases of MAO composite ceramic coating with ZrO2 were Al2TiO5, rutile TiO2 and ZrO2, no corundum phase. The main crystal phases of MAO composite ceramic coating without ZrO2 were Al2TiO5, rutile and anatase TiO2,while rutile and anatase had the weight percentage of 61.5%. Zr element would inward migrate into the films by the effect of electrophoresis rather than diffusion,especially during the early and late oxidation process. Al elements migrate through the discharge channel of the "short path" to the coating-substate interface and Ti element had a higher concentration on the interface, which indicating that the behavior of oxide coatings formation occurs mainly in areas adjacent to coating-substrate interface zone.
Experimental characterization and numerical simulation were applied to study the thermal shock resistance of the MAO composite ceramic coating with and without ZrO2 on TC4 surface by micro-arc oxidation. The analysis results showed that: after 50 cycles under temperature drop of 980℃, no cracks seen by naked eyes occurred on the surface and no debonding phenomenon happened in the interface bonding zone. The simulation results showed that: the relationship between thermal stress and substrate thickness was linear, but the relationship between thermal stress and coating thickness was parabolic; in the coating-substrate interface zone, the radial tensile stress and axial compressive stress both reached the maximum value; while on free surface of MAO composite ceramic coating, the radial compressive stress and axial tensile stress both reached the maximum value. Axial compressive stress was conducive to the bond strength of coating-substrate interface zone, while the axial tensile stress determined the destruction of the coating. Excellent thermal shock property of MAO composite ceramic coating was up to the special microstructure and mechanical properties,which also indicated that there was a kind of good metallurgical bond strength of coating-substrate interface.
Isothermal oxidation behavior and cyclic oxidation behavior of depositing and post-sealing MAO composite ceramic coatings were studied. The experimental results indicated that: the MAO composite ceramic coating had excellent oxidation resistance to high temperature, after cyclic oxidation at 1000 ℃ and isothermal oxidation of 110 hours, the coating surface had no spalling, which verified that it could be used as the thermal protection coating of TC4 matrix alloy. After sealing,the MAO composite ceramic coatings took on excellent high temperature antioxidant capacity. Compared with the blank samples, the maximum weight gain of sealing MAO films decreased by about 90%, the average oxidation rate decreased 93.3%.The cyclic oxidation and isothermal oxidation curves generally followed the parabolic law.
The high temperature oxidation behavior of samples was up to the thermal stability of the main crystal of MAO composite ceramic coating with and without ZrO2. Among the temperature range of 860℃-1280℃, β -Al2TiO5 took on the feature of thermal dynamic of non-eqilibrium. It would decomposite into the parent phase oxides of corundum and rutile. Because the new-formed corundum was easy to be a dense layer on the MAO composite ceramic coating, it could prevent the contact between the oxygen and the TC4 substrate. So this kind of composite ceramic coating could not only improve the anti-oxidation property of TC4, but also could be used as heat barrier.
Keywords: TC4 alloy, micro-arc oxidation (MAO), composite ceramic coating, thermal shock resistance, high temperature oxidation
本站文章未经允许不得转载；如欲转载请注明出处，北京桑尧科技开发有限公司网址：http://en.sunspraying.com/
|
