Thermal barrier coatings (TBCs) consisting of a yttria-partially-stabilized zirconia (YSZ) topcoat and a metallic bond coat are used to protect components in high temperature environments such as the hot section of a gas turbine. In this study, the bond coats have been fabricated using different thermal spray processes, including vacuum arc spaying, high-velocity oxyfuel flame spraying, low-pressure plasma spraying, air plasma spraying, and detonation spraying. The microstructure of the resulting TBCs is characterized based on XRD, SEM, and TEM analysis and various properties are measured, including high-temperature oxidation, cohesion strength, thermal impact, and heat insulation temperature.

Reusable space vehicles, which must withstand re-entry into the Earth's atmosphere, require external protection systems (TPS) which are usually in the forms of rigid surface in areas of high or moderate working temperature. High heat fluxes and temperatures related to high performance hypervelocity flights also require the use of TPS materials having good oxidation and thermal shock resistance, dimensional stability, and ablation resistance. Components by these materials are usually fabricated, starting from either billets or plate stocks, by uniaxial hot pressing, and complex parts, such as low radius edges, are then obtained by electrical discharge machining technique. This article investigates an alternative fabrication technology, based on plasma spraying, to produce near net shape components. Results of experimental activities, such as optimization of plasma spraying parameters based on a DOE approach, are reported and discussed.

The isothermal and cyclic oxidation of freestanding Ni-20Cr-10Al-lY thick coatings has been investigated at 1200°C using TGA, SEM, XRD and XPS techniques. Coatings produced by HVOF are dense and remain crack free after thermal treatments. The protective oxide layer formed did not flake off upon cyclic oxidation as confirmed by SEM analysis. In addition, three oxidation regimes were identified after analyzing TGA data: two below 1000 °C and a third one at approximately 1200°C. The regimes below 1000°C correspond to the selective oxidation of elements on the surface and at the subsurface of the coatings whereas the third regime involves element diffusion from the bulk of the coating to the surface. The oxidation regime became asymptotic at 1200 °C as stable oxides formed. The presence of water vapor affects neither the thickness nor the orientation of oxide crystals formed on the surface as confirmed by the X-ray analysis. The XPS and X-ray results show an inter-diffusion between the coating and substrate with a slight increase in chromium concentration at the interface. Element distribution within the oxide layer was found to follow the order: Al-(oxide)Y-(oxide)/Cr-(oxide)/Ni-(oxide)/NiCrAlY from the outermost oxide layer to the bulk of the coating. These results show that HVOF dense Ni-20Cr-10Al-lY sprayed coatings can be used as anti-oxidant barriers in both isothermal and cyclic oxidation at 1200°C.

The isothermal oxidation of bond coats composed of vacuum plasma sprayed (VPS) MCrAlY (NiCoCrAlYTa and CoNiCrAlY) or palladium modified nickel aluminides (NiPd + APVS) was studied in several oxygen partial pressures (10 5 , 1 and 10 -5 Pa), with two heating rates (20 and 60K/min) at different temperatures (900, 1000 and 1100°C). For MCrAlY coatings, Arrhenius plots of the parabolic rate constants show a kinetic transition below 1000°C. This could be linked to a transition from Al 2 O 3 to Cr 2 O 3 scale growth. Lower oxygen partial pressures induce lower parabolic rate constants at 900°C. This leads to the assumption that scales grown at low oxygen partial pressures are still formed of alumina at 900°C. Nevertheless, these results could not be confirmed by chemical analysis (EDS, XPS). The two tested heating rates show no influence on the oxidation kinetics of both MCrAlY coatings. In the case of aluminide, for low oxygen partial pressures, the parabolic kinetics are reduced of one order of magnitude (for P O2 = 10 5 to 1 Pa) and correspond to a thinner scale of α-alumina. Also, the heating rate modifies the parabolic kinetics (i.e. after the transient stage) and the total weight gains for all oxidation temperatures, with higher parabolic rate constants after heating at slower rate.

Thermal barrier coating systems have been heat treated in order to study the oxidation kinetics of the bond coat. All the surfaces of Ni superalloy substrates were sprayed with ~100 μm of a NiCrAlY bond coat, with or without ~250 μm of a ZrO 2 top coat. Thermogravimetric analysis (TGA) was used to monitor continuously the mass change as a result of oxidation of the bond coat during heating at 1000°C for 100 hours in flowing air. In addition, some specimens were heated to 1000°C in static air, cooled to room temperature, weighed and re-heated cyclically. The total exposure time was 1000 hours. Rates of weight gain were found to be higher for the cycled specimens, despite the absence of air flow. This is attributed to damage to the oxide film, which was predominantly α-Al 2 O 3 , as a consequence of differential thermal contraction stresses. The changing residual stress state during heat treatment was predicted using a previously-developed numerical model. A thin (1 mm) substrate with ~100 μm bond coat and ~250 μm ZrO 2 top coat was used in these simulations, which incorporated creep of the bond coat and the lateral strain associated with oxidation. It is concluded from these computations that, while high stresses develop in the oxide layer, the associated driving forces for interfacial debonding remain relatively low, as do specimen curvature changes. It seems likely that coating spallation after extensive oxide layer formation arises because the interface is strongly embrittled as the layer thickens.