Research Of Laser Remelting Of Plasma Sprayed Al₂O₃-13wt%TiO₂ Coatings On Titanium Alloy

In this paper, conventional Metco130 ceramic coating and nanostructured Al₂O₃-13wt%TiO₂ coating have been successfully fabricated by plasma spraying on a titanium alloy (TC4) substrate. A CO2 laser has been used to remelt the plasma sprayed coatings, with the purpose of homogenizing their microstructure, eliminating their porosity and enhancing their bonding strength. The influence of laser remelting on the microstructure and properties of the ceramic coatings was investigated.The microstructure, phase constitution and microhardness of the as-sprayed and laser remelted coatings were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive analysis of X-rays (EDAX) techniques and microhardness testing, respectively.Experimental results show that the as-sprayed coatings maintained good mechanical bonding to the substrate. The porosity of the nanostructured coating is less than that of the conventional Metco130 coating. After laser remelting, the remelted coatings with smooth surface, less crack were obtained. The remelted coatings possessed a more homogeneous microstructure, less porosity, and an excellent metallurgical bonding to the substrate. The defects and porosity of the as-sprayed coatings were eliminated.The microstructure analysis shows that the plasma sprayed coatings consisted ofα-Al₂O₃,γ-Al₂O₃ and TiO₂ phases. Amorphous phases were also found in the as-sprayed coatings. After laser remelting, the metastableγ-Al₂O₃ phase in the as-sprayed coatings was transferred to stableα-Al₂O₃. In addition, the reaction of Al₂O₃ and TiO₂ led to Al2TiO5. A bit of Ti of the substrate came into the molten pool during laser remelting. The average grain size in the as-sprayed coatings and laser remelted coatings was evaluated using Scherrer Equation. The results show that the plasma sprayed Al₂O₃-13wt%TiO₂ coatings were nanostructured. After laser remelting, the remelted coatings could still keep nanostructured. Two kinds of typical microstructures were found in the as-sprayed nanostructured coatings. One is the full-fused microstructure and the other is the three-dimensional net microstructure. In the laser remelted nanostructured coatings, columnar grain, cellular grain and their mixture microstructure were found in different zone. The different kinds of microstructure were due to different laser heating temperature, different cooling rate and nonuniform mass transfer process.The microhardness of the coatings before and after laser remelting was determined. It has been found that the microhardness of the as-sprayed coatings was about HV700¹050, which was 2³ times that of the microhardness of the substrate. The microhardness obtained after laser remelting was up to HV800¹300, which was higher than that of the microhardness of the as-sprayed coatings.During plasma spraying and laser remelting, the nanostructured coatings possessed microstructure heredity. The three-dimensional net microstructure could be found in the nanostructured feedstock. After plasma spraying, the net microstructure was kept in the as-sprayed coatings. At last, in the laser remelted coatings, the net microstructure feature could be also found in the ball-like micorstructure.