James A Haynes

Influence of Sulfur, Platinum, and Hafnium on the Oxidation Behavior of CVD NiAl Bond Coatings

The influences of S, Pt, and Hf on the oxidation behavior of chemical vapor deposition (CVD) NiAl bond coatings on high-S and low-S superalloys were investigated. Cyclic and isothermal-oxidation testing at 1150°C revealed that alumina-scale adherence to NiAl coatings was very sensitive to substrate S impurities. Scale spallation, as well as the growth of voids at the oxide–metal interface, increased as S increased. However, Pt-modified coatings were not sensitive to S, and did not form voids at the oxide–metal interface. Transmission-electron microscopy (TEM) revealed that alumina scales formed on (Ni,Pt)Al had Hf ions (from the superalloy) segregated to grain boundaries, whereas Hf was not detected in the alumina scale formed on NiAl coatings. Results suggested that the detrimental influence of S on scale adherence to NiAl is primarily related to void growth, which is eliminated or significantly reduced in Pt-modified coatings. A simple model relating void growth to excess vacancies and sulfur segregation is proposed.

Martensitic transformation in CVD NiAl and (Ni,Pt)Al bond coatings

Abstract The martensitic phase transformation in single-phase β-NiAl and (Ni,Pt)Al coatings was investigated. After isothermal exposure to 1150 °C for 100 h, the β phase in both types of coatings was transformed to a martensite phase during cooling to room temperature. Martensitic transformation was also observed in the (Ni,Pt)Al bond coat with and without a YSZ top layer after thermal cycling at 1150 °C (700 1-h cycles). The transformation took place due to Al depletion in the coating from the formation of the Al2O3 scale and interdiffusion between the coating and superalloy substrate. The effects of the martensitic transformation on coating surface stability (‘rumpling’) via volume changes during the phase transformation are discussed with regard to TBC failure.

Thermal cycling behavior of plasma-sprayed thermal barrier coatings with various MCrAlX bond coats

The influence of bond coat composition on the spallation resistance of plasma-sprayed thermal barrier coatings (TBCs) on single-crystal René N5 substrates was assessed by furnace thermal cycle testing of TBCs with various vacuum plasma spray (VPS) or air plasma-spray (APS) MCrAlX (M=Ni and/or Co; and X=Y, Hf, and/or Si) bond coats. The TBC specimens with VPS bond coats were fabricated using identical parameters, with the exception of bond coat composition. The TBC lifetimes were compared with respect to MCrAlX composition (before and after oxidation testing) and MCrAlX properties (surface roughness, thermal expansion, hardness, and Young’s modulus). The average TBC spallation lifetimes varied significantly (from 174 to 344 1 h cycles at 1150 °C) as a function of bond coat composition. Results suggested a relationship between TBC durability and bond coat thermal expansion behavior below 900 °C. Although there were only slight differences in their relative rates of cyclic oxidation weight gain, VPS MCrAlX bond coats with better oxide scale adhesion provided superior TBC lifetimes.

Comparison of thermal expansion and oxidation behavior of various high-temperature coating materials and superalloys

Abstract The thermal expansion mismatch between a metallic substrate and its external oxide scale generates a strain on cooling that is a primary cause of spallation of protective oxide scales. This study compares thermal expansion behavior and cyclic oxidation performance of the two major composition classes of high-temperature commercial coatings for protection of single-crystal superalloys. The thermal expansion of cast MCrAlY (M = Ni and/or Co) alloys and cast aluminides (NiAl, (Ni,Pt)Al and Ni3Al) was measured at temperatures up to 1300°C and compared to that of a single-crystal Ni-base superalloy. The tendency for scale spallation from each alloy was evaluated by cyclic oxidation testing at 1150°C. The coefficients of thermal expansion for the aluminides were lower than those of the MCrAlY-based alloys at all temperatures and scale adherence to the Hf-doped aluminides was generally superior. Scale adherence to the various compositions of MCrAlY-type alloys did not directly correlate to their thermal expansion behavior or substrate strength. For both types of materials, the presence of a reactive element (Y,Hf, etc.) had no detectable effect on thermal expansion but a major effect on scale adherence. There was no obvious influence of Al content on the thermal expansion of β phase Ni–Al compositions. The addition of Pt resulted in a lower average thermal expansion for hyperstoichiometric (Ni,Pt)Al at temperatures above 930°C, but this effect was not observed in hypostoichiometric (Ni,Pt)Al.

Characterization of alumina scales formed during isothermal and cyclic oxidation of plasma-sprayed TBC systems at 1150°C

The isothermal- and cyclic-oxidation behavior ofthermal barrier coating (TBC) systems consisting ofvacuum plasma-sprayed (VPS) Ni-22Cr-10Al-1Y (wt.%) bondcoatings and air plasma-sprayed (APS)Y2O3-stabilized ZrO2 (YSZ) top coatings (onsingle-crystal superalloys) was investigated. Themicrostructures, flaw contents, and fracture behavior ofthe Al2O3 scales formed duringoxidation testing at 1150°C were characterized (by analysis of coating andscale fracture surfaces and metallographic crosssections). Significant localized fracture and bucklingof the Al2O3 scales that formedalong the bond-coat-top-coat interfaces were observed after cyclic oxidationof TBCs. However, substantial amounts of localized scaledamage did not induce rapid TBC failure. Decohesion ofthe columnar alumina scales on the rough bond-coat surfaces occurred by both internalAl2O3 fracture (parallel to themetal surface) and oxide-metal delamination. There weremicrostructural indications ofAl2O3 scale crack healing bysintering into planar arrays of voids. Alumina scales that formed onconvex NiCrAlY surfaces (with radii of 50 μm or less)often contained significant amounts of internal voids(along grain boundaries) after cyclic oxidation, whereas scales formed by isothermal oxidationcontained few visible voids. Accelerated void growth inAl2O3 scales on the irregularNiCrAlY surfaces appeared to be creep-related and wasattributed to the synergistic effects of geometric and thermalstresses.

Characterization of commercial EB-PVD TBC systems with CVD (Ni,Pt)Al bond coatings

Abstract Failure of electron-beam physical vapor deposition (EB-PVD) thermal barrier coatings (TBCs) with aluminide bond coats is strongly influenced by bond coat oxidation behavior. This study investigated oxide (Al2O3) formation during EB-PVD processing of TBCs with (Ni,Pt)Al bond coats. The effects of substrate composition, coating impurities and bond-coat grit-blasting on the oxide phases, residual stress and microstructure were evaluated. As-deposited, high-purity commercial bond coats contained high concentrations of sulfur and other impurities at their surfaces. Numerous small voids formed at the oxide–metal interface when as-deposited bond coats were oxidized during EB-PVD processing. Grit-blasting of (Ni,Pt)Al had a significant impact, since the formation of α-Al2O3 during EB-PVD processing was significantly enhanced and voids did not form beneath the scale. Preliminary cyclic oxidation testing suggested an influence of superalloy sulfur content on TBC durability.

Evaluation of iron-aluminide CVD coatings for high temperature corrosion protection

Abstract Chemical vapor deposited (CVD) Fe-Al coatings are being investigated to address fundamental issues concerning aluminide coating performance and lifetime. By using a well-controlled laboratory CVD procedure, the coatings are uniform in composition, purity and microstructure. A typical ferritic steel, Fe-9Cr-1Mo, and an austenitic stainless steel, 304L (nominally Fe-18Cr-9Ni), were coated to examine differences in the two types of substrates. For both substrates, the as-deposited coating consisted of a thin (<5 μm), Al-rich layer above a thicker (30–50 μm), lower Al content layer. To follow-up on initial results, which showed good coating performance in air+10vol.%H2O and H2S-H2O-H2-Ar, cyclic tests were performed in both environments at 800°C and more detailed characterization of the isothermally exposed coatings was conducted. During 2–5, 25h cycles at 800°C in H2S-H2O-H2-Ar, CVD coatings on both substrates showed progressively more attack during each cycle. However, in 1h cycles at 800°C in air + 10vol.%H2O, the coatings showed excellent performance, similar to cast Fe-(15–20at.%)Al specimens. The uncoated alloys were significantly attacked during all of these tests. Thermal expansion measurements show Al additions up to 20at% have little effect on the mean expansion of ferritic alloys but the higher thermal expansion of austenitic steels may be a better match with Fe3Al coatings.

Metastable tetragonal zirconia formation and transformation in reactively sputter deposited zirconia coatings

Zirconia coatings were produced by reactive d.c. magnetron sputter deposition using a system with multiple sputter sources and a biased substrate stage. Crystal structure and phase stability of the coatings were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Tetragonal zirconia with either a random orientation or a highly (111) preferred orientation was deposited when a substrate bias was applied, whereas coatings grown with no substrate bias had the equilibrium monoclinic structure. It was revealed that bias sputtering effectively decreased crystallite size in the as-deposited coatings, which resulted in room temperature stabilization of the metastable tetragonal phase. XRD analysis of annealed coatings showed that the volume fraction and stability of the tetragonal phase was strongly dependent on substrate bias and annealing temperature.

Monoclinic Zirconia Distributions in Plasma-Sprayed Thermal Barrier Coatings

Phase composition in an air plasma-sprayed Y2O3-stabilized ZrO2 (YSZ) top coating of a thermal barrier coating (TBC) system was characterized. Both the bulk phase content and localized pockets of monoclinic zirconia were measured with Raman spectroscopy. The starting powder consisted of ∼15 vol.% monoclinic zirconia, which decreased to ∼2 vol.% in the as-sprayed coating. Monoclinic zirconia was concentrated in porous pockets that were evenly distributed throughout the TBC. The pockets resulted from the presence of unmelted granules in the starting powder. The potential effect of the distributed monoclinic pockets on TBC performance is discussed.

Application of Infrared Imaging to the Study of Controlled Failure of Thermal Barrier Coatings

A technique that uses high resolution infrared (IR) imaging was developed to track and analyze damage evolution of thermal barrier coatings (TBCs) during controlled mechanical testing of a TBC specimen. Coating debonding and spallation were examined during a monotonic load-to-TBC-failure test. The infrared imaging, in concert with a controlled thermal gradient in the specimen, was particularly effective in identifying and tracking localized damage evolution because the damage in the TBC was always associated with a measurable surface-temperature change. It is demonstrated that the combined use of high-resolution infrared imaging and controlled mechanical testing of TBCs is an effective method to characterize the evolution of their failure.