Thermal barrier coatings can protect hot end components from high temperature, high pressure, and high stresses, which are extensively applied in gas turbine engine blades, combustion chamber, and ducting and nozzle guide vanes. However, many impurities (sodium, sulfur) exist in the operating gases and fuels, condensation of which leads to the formation of molten corrosive salts during the long-time service process, which may lead to the serious hot corrosion failure and reduce the lifetime of thermal barrier coatings. Plasma-sprayed thermal barrier coatings possess the typical characteristic of pores and laminar structure, which may provide penetration paths for molten corrosive salts into the coating. To improve the hot corrosion resistance, it is necessary to produce a denser layer to prevent molten salts from penetrating into porous coatings. Laser alloying technology has the advantages of high energy density, short action time, and reliable processing quality, and thus can change the loose porous structure of the plasma-sprayed thermal barrier coatings. Meanwhile, the oxidation reaction of self-healing materials at high temperature can produce some oxidation products, which can further fill the pores and cracks in the porous coatings. Therefore, the combination of laser alloying technology and self-healing materials is an alternative method to improve the hot corrosion resistance of thermal barrier coatings, which is few reported. The purpose of this paper is to study the effect of laser alloying on the microstructure, phase composition, and hot corrosion properties of thermal barrier coatings.
In this study, the double-layer thermal barrier coatings of NiCrAlY/8YSZ were deposited onto superalloy substrate via air plasma spraying. The mixture powders of TiAl3 particles with 10% mass fraction and Ceria and Yttria-stabilized Zirconia (CYSZ) ceramic were pre-placed on the plasma-sprayed thermal barrier coatings, and then processed using a fiber-coupled semiconductor laser. The mixture of 25% NaCl and 75% Na2SO4 as the corrosive salts was spread on the surface of the plasma-sprayed and laser-alloyed thermal barrier coatings with deposition content of 10 mg/cm2. The hot corrosion test was conducted in a furnace at 900 ℃ for 4 h. Finally, the microstructure, phase composition and hot corrosion behaviors of the plasma-sprayed and laser-alloyed thermal barrier coatings were systematically investigated.
After the laser alloying treatment, the porous and laminar microstructures in the plasma-sprayed thermal barrier coatings were eliminated. As a result, dense columnar crystal structure and some segmented microcracks were formed in the laser-alloyed thermal barrier coatings (Fig. 2). The detrimental monoclinic zirconia (m-ZrO2) disappears after the air plasma spraying and the laser alloying treatment, and all phases in the 8YSZ powder turn to non-equilibrium tetragonal zirconia (t'-ZrO2) and cubic zirconia (c-ZrO2) (Fig. 3). Because of the rapid cooling and solidification rate of air plasma spraying and laser alloying treatment, the phase transformation of t'-ZrO2 to m-ZrO2 is restrained. After the hot corrosion at 900 ℃ for 4 h in molten salts (25% NaCl+75% Na2SO4), the corrosion products of Y2(SO4)3 and m-ZrO2 were found in the plasma-sprayed thermal barrier coatings, and Y2(SO4)3 and Al2O3 were detected in the laser-alloyed thermal barrier coatings [Figs. 5(a) and (b)]. Compared with the plasma-sprayed thermal barrier coatings, there are less corrosion products on the surface of the laser-alloyed thermal barrier coatings [Fig. 4(b) and Fig. 6(b)]. Therefore, the hot corrosion resistance of the laser-alloyed thermal barrier coatings is superior to that of the plasma-sprayed thermal barrier coatings. On the one hand, the dense columnar structure in the laser-alloyed thermal barrier coatings can inhibit the penetration of molten salts; on the other hand, the self-healing agent TiAl3 undergoes oxidation reaction at high temperature, and the formed Al2O3 and a small amount of TiO2 can fill the cracks, which can further reduce the hot corrosion reaction between molten salts and yttria stabilizer [Figs. 9(c), (d), and (e)].
In this study, a typical double-layer 8YSZ/NiCrAlY thermal barrier coating was prepared via air plasma spraying technology, and then the self-healing agent TiAl3 was introduced into the thermal barrier coatings through laser alloying technology. Microstructure, phase composition, and hot corrosion properties of the plasma-sprayed and the laser-alloyed thermal barrier coatings were investigated. The surface of the plasma-sprayed thermal barrier coatings is relatively rough, and there are many microcracks and pores within it. While the surface of the laser-alloyed thermal barrier coatings is smooth, and some fine segmented microcracks and dense columnar crystals are formed. After the hot corrosion in the 25% NaCl+75% Na2SO4 molten salt at 900 ℃ for 4 h, it was found that there were corrosion products Y2(SO4)3 and harmful m-ZrO2 in the plasma-sprayed and the laser-alloyed thermal barrier coatings; however, less corrosion products existed in the latter. The hot corrosion resistance of the laser-alloyed thermal barrier coatings is much better than that of the plasma-sprayed thermal barrier coatings. On the one hand, the oxidation reaction of self-healing agent TiAl3 during the process of high-temperature hot corrosion produces Al2O3 and less TiO2, and they can fill some cracks in the coating and reduce the penetration paths of the corrosion salts. On the other hand, the laser-alloyed layer with dense columnar structure can inhibit the penetration of molten salt and reduce the occurrence of thermal corrosion reaction. Finally, the hot corrosion resistance of laser-alloyed thermal barrier coatings is greatly improved.
