Principle and Practice to Achieve Improvements in TBC Thermal Cycle Lifetime

As a critical technology; thermal barrier coatings (TBC) have been used in both aero engines and industrial gas turbines for a few decades; however; the most commonly used MCrAlY bond coats which control air plasma sprayed (APS) TBC lifetime are still deposited by the powders developed in 1980s. This motivates a reconsideration of development of MCrAlY at a fundamental level to understand why the huge efforts in the past three decades has so little impact on industrial application of MCrAlY alloys. Detailed examination of crack trajectories of thermally cycled samples and statistic image analyses of fracture surface of APS TBCs confirmed that APS TBCs predominately fails in top coat. Cracks initiate and propagate along splat boundaries next to interface area. TBC lifetime can be increased by either increasing top coat fracture strength (strain tolerance) or reducing the tensile stress in top coat or both. By focusing on the reduction of tensile stress in top coats; three new bond coat alloys have been designed and developed; and the significant progress in TBC lifetime have been achieved by using new alloys. Extremely high thermal cycle lifetime is attributed to the unique properties of new alloys; such as remarkably lower coefficient of thermal expansion (CTE) and weight fraction of β phase; absence of mixed / spinel oxides; and TGO self repair ability; which cannot be achieved by the existed MCrAlY alloys.

The Effect of Coating Composition and Geometry on TBC Lifetime

Several factors are being investigated that affect the performance of thermal barrier coatings (TBC) for use in land-based gas turbines where coatings are mainly thermally sprayed. This study examined high velocity oxygen fuel (HVOF), air plasma sprayed (APS) and vacuum plasma sprayed (VPS) MCrAlYHfSi bond coatings with APS YSZ top coatings at 900°–1100°C. For superalloy 247 substrates and VPS coatings tested in 1-h cycles at 1100°C, removing 0.6wt.%Si had no effect on average lifetime in 1-h cycles at 1100°C, but adding 0.3%Ti had a negative effect. Rod specimens were coated with APS, HVOF and HVOF with an outer APS layer bond coating and tested in 100-h cycles in air+10%H2O at 1100°C. With an HVOF bond coating, initial results indicate that 12.5 mm diameter rod specimens have much shorter 100-h cycle lifetimes than disk specimens. Longer lifetimes were obtained when the bond coating had an inner HVOF layer and outer APS layer.

The Effect of Environment on TBC Lifetime

While the water vapor content of the combustion gas in natural gas-fired land based turbines is ∼10%, it can be 20–85% with coal-derived (syngas or H2) fuels or innovative turbine concepts for more efficient carbon capture. Additional concepts envisage working fluids with high CO2 contents to facilitate carbon capture and sequestration. To investigate the effects of changes in the gas composition on thermal barrier coating (TBC) lifetime, furnace cycling tests (1 and 100h cycles) were performed in air with 10, 50 and 90 vol.% water vapor and CO2-10%H2O and compared to prior results in dry air or O2. Two types of TBC’s were investigated: (1) diffusion bond coatings (Pt diffusion or Pt-modified aluminide) with commercial electron-beam physical vapor-deposited yttria-stabilized zirconia (YSZ) top coatings on second-generation superalloy N5 and N515 substrates and (2) high velocity oxygen fuel (HVOF) sprayed MCrAlYHfSi bond coatings with air-plasma sprayed YSZ top coatings on superalloys X4, 1483 or 247 substrates. For both types of coatings exposed in 1h cycles, the addition of water vapor resulted in a decrease in coating lifetime, except for Pt diffusion coatings which were unaffected by the environment. In 100h cycles, environment was less critical, perhaps because coating failure was chemical (i.e. due to interdiffusion) rather than mechanical. In both 1h and 100h cycles, CO2 did not appear to have any negative effect on coating lifetime.

Substrate Effect on the Lifetime of EB-PVD TBC Systems With 7YSZ and GDZ as Ceramic Top Coat Materials

Thermal barrier coatings (TBCs) consist of a multi-layered system where the different layers are deposited to improve the high temperature capability of the underlying substrate material. Substrate materials are usually made of Ni-based superalloys which have an outstanding combination of high-temperature strength, toughness and resistance to degradation in corrosive or oxidizing environments. In this study, the effect of different substrate materials on cyclic TBC lifetime has been studied by using two different superalloys, IN100 and CMSX-4, as the substrate materials. Two different ceramic topcoat materials, 7wt. % yttria partially stabilized zirconia (7YSZ) and Gadolinium Zirconate (GdZ), have been deposited on NiCoCrAlY bond coats. All depositions in this study have been carried out by Electron Beam Physical Vapor Deposition (EB-PVD). Lifetime measurements have been done by holding the systems at 1100°C for 50mins and then reducing the temperature to ambient level by forced air cooling for 10mins. The TBC lifetime in case of IN100 substrates is higher than on CMSX-4. The use of GdZ as topcoat material improves the lifetime for both IN100 and CMSX-4 based TBC systems, however, the lifetime for CMSX-4 based TBC system is still shorter than its IN100 counterpart. In this study, lifetime comparisons, changes in the microstructure and diffusion of different elements in the system are investigated.

Effect of Pt Content on TGO Growth in EB-PVD TBC Systems with Niptal Bondcoats

The oxidation behavior of Pt modified aluminide coating on the CMSX-4 Ni-base alloy plays major role to the EB-PVD TBC failure. The thermally growth oxide (TGO) is one of the most important factors to affect TBC lifetime. Two different Pt-content NiPtAl coatings in EB-PVD TBC systems were studied at 1100°C in air. The results indicated that cross-sections of oxide layer on the NiPtAl coatings within TBC in air were similar for the both bondcoats. The cracks could be found on the TBC/TGO/BC interfaces for the two bondcoats. The TGO morphologies of the low and high-Pt bondcoats on the side without TBC showed great different due to small PtAl particles size within high-Pt bondcoats. The irregular alumina on the both bondcoats was also showed on the sides with TBC compared to ones without TBC due to absence of the TBC. The TGO growth on the high-Pt bondcoats was faster than the low-Pt coatings during initial oxidation time. With the time increasing, the high-Pt content could suppress the TGO growth rate. Thinner TGO thickness could be obeserved on the both NiPtAl coatings due to the stress in TGO accumulation and oxide spallation.