Enhancing the Oxidation Resistance of NiCrAlY Bond Coat by High-Current Pulsed Electron Beam Irradiation

The bond coat of a NiCrAlY thermal barrier coating plays an important role in solving the thermal expansion mismatch between a metal matrix and a ceramic layer and in improving the oxidation resistance of the whole thermal barrier coating. However, the NiCrAlY bond coat prepared by low-pressure plasma spraying is not conducive to its oxidation resistance because its lamellar structure is loose, porous and the surface is rough. To improve the oxidation resistance of the bond coat, the NiCrAlY bond coat prepared by plasma spraying was modified by high-current pulsed electron beam with different energy densities. Under the electron beam irradiation, the surface of the coating became smooth, and there was a 3–5 μm thick remelting layer on the surface. Under the irradiation, the thickness of the thermal growth oxide layer decreased, and the oxidation resistance was significantly improved, the oxidation product being mainly Al2O3.

Study on the Characteristics of a TBC System Containing a PVD-Al Interlayer under Isothermal Loading

In this work, the oxidation behavior of an atmospheric plasma-sprayed thermal barrier coating (TBC) system with a thin Al physical vapor deposition (PVD) film deposited over the bond coat is discussed. The TBC consisted of: (i) CoNiCrAlY bond coat sprayed on the Inconel 600 substrate; (ii) a thin Al interlayer deposited by direct current DC magnetron sputtering; and (iii) yttria-stabilized zirconia (YSZ) sprayed as the top coat. Such thermal barrier coatings (Al-TBC) were isothermally oxidized at 1150 °C with different holding times, and then they were compared with the reference TBC (R-TBC) systems without an Al interlayer (R-TBC). Scanning electron microscopy with energy-dispersive X-ray analysis was used to study the oxide formation along the bond coat (BC) and top coat (TC) interface, as well as crack formation in the yttria-stabilized zirconia top coat. Then, using Image Analysis, the oxide formation and crack formation were characterized in all specimens after a slow heating and cooling cycle, and after 100, 300, and 600 h of isothermal exposure. The results showed that the Al-TBC system proposed here exhibits higher oxidation resistance at the bond coat and top coat interface, less crack formation in the YSZ top coat, and enhanced mechanical stability compared to the conventional TBCs. It was found that enrichment of the bond coat and top coat interface with Al limited the formation of detrimental transition metal oxides during isothermal loading. Finally, the corresponding failure caused by thermally grown oxide (TGO) phenomena is “mixed failure mode” for both studied TBCs.

CHARACTERIZATION AND ELECTROCHEMICAL CORROSION ANALYSIS OF HEAT-TREATED REINFORCED HA COATINGS DEPOSITED BY VACUUM PLASMA SPRAY TECHNIQUE

Herein, a biomimetic coating of hydroxyapatite (HA)–Al2O3 and HA–ZrO2 was deposited on Ti–6Al–4V-alloy using vacuum plasma spray (VPS) technique. The bond-coat of ZrO2 has been introduced between the substrate and reinforced HA coatings to study the effect of bond-coat on structural, mechanical properties and electrochemical corrosion performance of the developed coatings. In addition, the impact of thermal treatment of coating was investigated on these properties too. Coating characteristics, such as morphology, porosity, surface roughness, and crystallinity were investigated. The corrosion performance of coatings was tested in Hank’s-based salt solution (HBSS). Significant enhancement in crystallinity and surface-hardness has been witnessed after heat treatment; nevertheless, porosity reduced. The electrochemical corrosion study revealed that the corrosion resistance of heat-treated samples was better than the as-sprayed coatings samples. The intensity of XRD peaks of all coatings increased after 24[Formula: see text]h immersion in HBSS for the electrochemical test in comparison to the intensity of peaks before the corrosion test.

Microstructure and Oxidation Resistance of Thermal Barrier Coatings with Different Ceramic Layer

Thermal barrier coatings are widely used for protection of gas turbine parts against high temperature oxidation and hot corrosion. In present work the microstructural assessment of TBCs produced by atmospheric plasma spray (APS) method was conducted. Three types of ceramic powders were used: magnesia- stabilized zirconia oxide (Metco 210), yttria stabilized zirconia oxide (YSZ -Metco 204) and fine-grained YSZ – Metco 6700. As a base material the Inconel 713 was used as well and CoNiCrAlY was plasma sprayed (APS) as a bond coat. The thickness of all ceramic layers was in range 80 – 110 μm. The elemental mapping of cross-section of magnesia-stabilized zirconia showed the presence of Mg, Zr and O in outer layer. In the YSZ ceramic layer the Y, Zr and O were observed during elemental mapping. The isothermal oxidation test was conducted at 1100 °C for 500 h in static laboratory air. On all samples the delamination and spallation of ceramic layers was observed. Chemical composition analysis of coatings showed the presence of two areas: the first one contained elements from bond coats: Ni, Cr, Al, Co and second area contained O, Cr Co and O that suggest the scale formation. The obtained results showed the total degradation of all ceramic layers as a result of internal stresses in bond-coat. Microscopic analysis showed the areas with complete degradation of bond coats and formation of thick oxides layer.