Crystalline ytterbium disilicate environmental barrier coatings made by high velocity oxygen fuel spraying

Dense environmental barrier coatings (EBCs) are an essential prerequisite to exploit the advantages offered by SiC-based fiber reinforced ceramic matrix composites (CMCs) to increase efficiency in gas turbines. Today's state-of-the art materials for application as EBCs are rare-earth (RE) silicates which, however, form amorphous phases upon rapid quenching from the melt. This makes their processing by thermal spray a challenge. Recently, high velocity oxygen fuel (HVOF) spraying was proposed as potential solution since the melting degree of the feedstock can be controlled effectively. This work studies the deposition of ytterbium disilicate (YbDS) at short stand-off distances and variant total feed rates and oxy-fuel ratios of the working gas. It was found that the overall degree of crystallinity could be kept at high level above 90%. The kinetic energy transferred by impinging particles was found to be an effective parameter to control the densification of the coatings. Porosities well below 10% were achieved while fully dense coatings were impeded due to the progressive accumulation of stresses in the coatings.     

Electrochemical behaviour of thermally sprayed Cr3C2–NiCr coatings in 0.5 M H2SO4 media

The electrochemical behaviour of coated Cr₃C₂–NiCr steel in aerated 0.5 M H₂SO₄ solution was studied by means of electrochemical a.c. and d.c. measurements. A complete structural characterization of the coated steel before and after electrochemical tests was also carried out to access the corrosion mechanism of coated steel, electrolyte penetration through the coating, and to confirm the results obtained using electrochemical techniques. Two types of Cr₃C₂–NiCr coatings produced by a high velocity oxy-fuel spraying system (HVOF) were studied. Differences between coated steels are related to the spraying parameters reflecting their behaviour against corrosion phenomena. The electrochemical behaviour of the coated steel was strongly influenced by porosity and the presence of microcracks in the coating. Once the electrolyte reaches the steel substrate, it corrodes in a galvanic manner resulting in coating detachment from the steel.     

Electrochemical impedance studies of molten (0.9Na,0.1K)2SO4-induced hot corrosion of the Ni-based superalloy M38G at 900 °C in air

The hot corrosion behavior of the Ni-based superalloy M38G covered with a film of molten 0.9Na₂SO₄–0.1K₂SO₄ (mole fraction) has been studied by electrochemical impedance spectroscopy at 900 °C in air. For comparison, the corrosion of the alloy immersed in molten (Na,K)₂SO₄ was also examined. The electrochemical impedance spectra in deep molten salt during an initial stage consisted of a semicircle at high frequency and a line at low frequency indicating a diffusion-controlled reaction. With extended immersion the impedance spectra were composed of two capacitive loops, i.e., a small semicircle at high-frequency port, and a large one at low-frequency port. The change of the impedance spectra is related to the formation of a protective scale. Contrary to the corrosion in deep molten salt, the corrosion of the alloy in the presence of a film of fused salt presented the characteristics of two capacitive loops for all the duration of the experimental test. Equivalent circuits representing the corrosion of the alloy in both corrosion conditions are proposed to fit the impedance spectra and electrochemical parameters in the equivalent circuits are also calculated.     

Evolution of hot corrosion resistance of YSZ,Gd2Zr2O7, and Gd2Zr2O7 + YSZ composite thermal barrier coatings in Na2SO4 + V2O5 at 1050 °C

This paper compares the hot corrosion performance of yttria stabilized zirconia (YSZ), Gd₂Zr₂O₇, and YSZ + Gd₂Zr₂O₇ composite coatings in the presence of molten mixture of Na₂SO₄ + V₂O₅ at 1050 °C. These YSZ and rare earth zirconate coatings were prepared by atmospheric plasma spray (APS). Chemical interaction is found to be the major corrosive mechanism for the deterioration of these coatings. Characterizations using X-ray diffraction (XRD) and scanning electron microscope (SEM) indicate that in the case of YSZ, the reaction between NaVO₃ and Y₂O₃ produces YVO₄ and leads to the transformation of tetragonal ZrO₂ to monoclinic ZrO₂. For the Gd₂Zr₂O₇ + YSZ composite coating, by the formation of GdVO₄, the amount of YVO₄ formed on the YSZ + Gd₂Zr₂O₇ composite coating is significantly reduced. Molten salt also reacts with Gd₂Zr₂O₇ to form GdVO₄. Under a temperature of 1050 °C, Gd₂Zr₂O₇ based coatings are more stable, both thermally and chemically, than YSZ, and exhibit a better hot corrosion resistance.