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Abstract: This study compared the oxidation behaviors of γ-γ′ NiPtAl coatings and traditional β-phase NiPtAl bond coatings. Both types of NiPtAl coatings were fabricated on two different single-crystal Ni-base superalloys using the same source of electroplated Pt. Characterization of the coating composition and alumina scale morphology after cyclic oxidation in dry oxygen at 1150°C indicated that the addition of Ti in one superalloy had affected the scale microstructure and had degraded scale adhesion on the γ-γ′ coating. In general, the alumina scale formed was thicker on the γ-γ′-coated substrates, and those coatings contained a larger number of pores, thought likely to be primarily due to a Kirkendall-type effect. [Copyright &y& Elsevier]

Abstract: This study investigated the impact of Pt content on the coating composition and subsequent oxidation behavior of γ–γ′ NiPtAl diffusion coatings. Coatings were fabricated by vacuum annealing electroplated Pt on single-crystal and directionally-solidified superalloy substrates. Specimens with 7 and 12 µm thick layers of electroplated Pt were annealed at 1100 or 1175 °C. Coating compositions were characterized to evaluate the distribution of Pt and Al as a function of annealing temperature and Pt thickness. Selected coatings were evaluated in cyclic oxidation at 1100 and 1150 °C. Coatings with higher Pt contents showed improvements in spallation resistance, particularly at 1100 °C for a directionally-solidified superalloy substrate with higher Hf and S contents. [Copyright &y& Elsevier]

Abstract: The cyclic oxidation behavior of simple aluminide and platinum aluminide coatings on various superalloy substrates was investigated at three temperatures. Coatings of β-NiAl, β-NiPtAl and Pt-modified γ–γ′ were tested and compared in dry oxygen at 1050, 1100 and 1150 °C. There was a clear influence of substrate sulfur content on the oxidation behavior of β-NiAl coatings at all temperatures. The impact of substrate impurities, especially S content, was significantly reduced for β-NiPtAl coatings at all temperatures. Simple Pt-modified γ–γ′ coatings showed excellent oxidation behavior up to 1100 °C on some alloys, but were more sensitive to substrate composition, especially Hf content, than were β-NiPtAl coatings. [Copyright &y& Elsevier]

The effects of Ti and B additions to a vacuum plasma sprayed (VPS) NiCoCrAlYHfSi bond coating on thermal barrier coating (TBC) performance were studied at 1100 °C and 900 °C and compared to high-velocity oxy-fuel (HVOF) bond coatings. Using alloy 247 substrates and air plasma sprayed Y 2 O 3 -stabilized ZrO 2 top coatings, additions of B or Ti + B did not improve the average TBC lifetime in 1-h cycles at 1100 °C in air with 10% H 2 O. The addition of Ti resulted in a decrease in lifetime. Photo-stimulated luminescence spectroscopy was used to map residual stresses in the thermally-grown Al 2 O 3 scale. At 900 °C, closer to a typical land based turbine operating bond coating temperature, specimens were examined after ten 500-h cycles in laboratory air and air with 10%H 2 O to study the effect of H 2 O. The addition of water vapor had little effect on the measured parabolic rate constants at 900 °C and a comparison of the oxide microstructures in both environments is reported. [ABSTRACT FROM AUTHOR]

In order to assess the role of temperature on the lifetime of thermal barrier coatings (TBCs) and the development of residual stresses in Al 2 O 3 scales grown under yttria-stabilized zirconia (YSZ) top coatings, two vacuum plasma sprayed (VPS) NiCoCrAlYHf bond coating (BC) compositions, with and without Si, were deposited on Hf-rich directionally-solidified (DS) 247 substrates and then coated with air plasma sprayed (APS) YSZ. Samples were thermally-cycled at temperatures ranging from 1075 to 1150 °C with 1-h cycles in air with 10% H 2 O. Photo-stimulated luminescence spectroscopy (PSLS) was used to map residual stresses in the Al 2 O 3 scale at the YSZ/BC interface from the same region at regular cycling intervals. All samples exhibited similar stress distributions after 100 1-h cycles with the exception of the YHfSi BC cycled at 1150 °C which had a lower average compressive stress with more delaminations. This specimen also had the shortest TBC lifetime which shows that the PSLS measurements correlate well to interfacial damage accumulation in TBCs. The presence of Si had no effect on lifetime or residual stress but was found to reduce the amount of internal oxidation. The effect of sample curvature on the interfacial stress and TBC lifetime was also assessed by comparing TBC-coated rod specimens to flat buttons. Rod specimens failed much earlier than flat specimens and had lower Al 2 O 3 residual stress. [ABSTRACT FROM AUTHOR]

Abstract: With the goal of investigating the reported detrimental effect of water vapor on thermal barrier coating (TBC) performance, furnace cycle experiments were conducted in dry O2 and air with 10 and 50% water vapor at 1100°C. The TBC systems evaluated were air plasma-sprayed (APS), yttria-stabilized zirconia (YSZ) top coatings with high velocity oxy fuel (HVOF)-deposited NiCoCrAlY or NiCoCrAlYHfSi bond coating. Average TBC lifetime was reduced by ~30% in air with 10% water vapor compared to cycling in dry O2, using 1h cycle durations. Superalloy substrates with Y and La additions also were investigated but showed no statistical change in the average TBC lifetime compared to the base CMSX4 superalloy. In all cases, the bond coating with Hf and Si additions increased YSZ lifetime by 20% or more. Experiments that increased water vapor to 50% showed no further decrease in TBC lifetime. Increasing the cycle frequency to 100h resulted in a large increase in TBC lifetime, especially for the NiCoCrAlYHfSi bond coatings. Co-doping the NiCoCrAl bond coat with Y and Hf was beneficial to TBC lifetime, but did not mitigate the detrimental impact of water vapor. [Copyright &y& Elsevier]

Directionally-solidified (DS) superalloy components with advanced thermal barrier coatings (TBC) to lower the metal operating temperature have the potential to replace more expensive single crystal superalloys for large land-based turbines. In order to assess relative TBC performance, furnace cyclic testing was used with superalloys 1483, X4 and Hf-rich DS 247 substrates and high velocity oxygen fuel (HVOF)-NiCoCrAlYHfSi bond coatings at 1100 °C with 1-h cycles in air with 10% H 2 O. With these coating and test conditions, there was no statistically-significant effect of substrate alloy on the average lifetime of the air plasma sprayed (APS) yttria-stabilized zirconia (YSZ) top coatings on small coupons. Using photo-stimulated luminescence piezospectroscopy maps at regular cycling intervals, the residual compressive stress in the α-Al 2 O 3 scale underneath the YSZ top coating and on a bare bond coating was similar for all three substrates and delaminations occurred at roughly the same rate and frequency. X-ray fluorescence (XRF) measurements collected from the bare bond coating surface revealed higher Ti interdiffusion occurring with the 1483 substrate, which contained the highest Ti content. [ABSTRACT FROM AUTHOR]

Furnace cycle testing of superalloy 1483 and X4 substrates with high velocity oxygen fuel (HVOF) NiCoCrAlYHfSi bond coatings and air plasma sprayed (APS) yttria-stabilized zirconia (YSZ) top coatings was conducted at 1100 °C in various environments. Average thermal barrier coating (TBC) lifetimes were 5–6 times longer when 100 h cycles were used to simulate base-load power generation operation, compared to the 1 h aero-engine standard cycle. Longer exposure times for 100 h cycles increased the interdiffusion resulting in no clear effects of H 2 O and CO 2 additions on average TBC lifetime for the 1483 substrates, contrary to the results with 1 h cycles where the addition of H 2 O reduced TBC lifetime. The lower Al content and perhaps the higher Ti content in 1483 compared to X4 resulted in lower TBC lifetimes for 1483. Photo-stimulated luminescence piezospectroscopy (PLPS) and 3D microscopy were used to measure residual stress in the alumina scale and surface roughness, respectively, on specimens with and without a YSZ top coating. [ABSTRACT FROM AUTHOR]

Photo-stimulated luminescence spectroscopy (PSLS), 3D microscopy and focused ion beam cross-sections were used to study the effect of water vapor on the cyclic (1hcycle time) oxidation behavior of simple and Pt-modified aluminide bond coatings on several superalloy substrates at 1125° and 1150°C. By tracking the same region over time, the change in surface roughness with thermal cycling could be attributed to large and small bond coating grains rising and sinking, respectively, with thermal cycling. Oxidation in air with 10% H2O showed a 0–37% increase in surface roughness compared to oxidation in dry air. Reducing the cycle temperature from 1150° to 1125°C, increased the thermal barrier coating lifetime by >4X but did not prevent bond coating rumpling. Compared to oxidation in dry air, PSLS identified more θ-Al2O3 formation on the Pt-modified aluminide bond coatings after 1h at 1125° or 1150°C in air with 10% H2O but only minor differences in the residual stress after longer cyclic oxidation exposures. Stress histograms produced by PSLS mapping help to elucidate Al2O3 scale damage accumulation in each condition but did not clearly identify a mechanism for the effect of water vapor on coating performance. [ABSTRACT FROM AUTHOR]

Abstract: The performance of thermal barrier coating (TBC) specimens with superalloy 1483 and X4 substrates and high velocity oxy fuel (HVOF)-NiCoCrAlYHfSi bond coatings was evaluated in dry and wet air in 1h furnace cycles at 1100°C. Compared to a prior study of NiCoCrAlYHfSi bond coatings, the HVOF bond coatings were rougher Ra =8μm, closer to commercial HVOF coatings. For TBC coatings on the X4 substrates cycled in air with 10% water vapor, increasing the roughness from Ra =5 to 8μm had little effect on the average TBC lifetime. The presence of 10% water vapor reduced the average TBC lifetime for 1483 by 13%, however, in both wet and dry conditions, the lifetimes for this substrate were 30–50% below those observed for X4 substrates. This may be attributed to the lower Al content and higher Ti content in 1483. Roughness and residual stress in the alumina scale were measured on specimens without a ceramic top coating, but no effects were observed due to the change in substrate or the addition of water vapor. [Copyright &y& Elsevier]

The addition of an air plasma sprayed (APS) "flash" bond coating layer on top of a high velocity oxy-fuel (HVOF) bond coating significantly extended the lifetime of APS yttria stabilized zirconia (YSZ) thermal barrier coatings (TBC) on rod specimens of superalloy 247 tested using 100-h cycles in air+10%H 2 O at 1100 °C. The flash bond coatings of both NiCoCrAlY and NiCoCrAlYHfSi powder were compared to an HVOF-only and a vacuum plasma sprayed (VPS) NiCoCrAlYHfSi bond coating. The flash coatings appear to form a mixed oxide-metal zone, which likely inhibits crack formation and therefore extends lifetime compared to conventional bond coatings. The underlying dense HVOF layer appeared to act as a source of Al for this intermixed zone and prevented the oxide from penetrating deeper into the bond coating. Residual stress in the thermally-grown Al 2 O 3 scale was measured using photo-luminescence piezospectroscopy (PLPS) as a function of time for each coating variation, including a comparison of concave and convex surfaces on a specially designed rod. • The effect of an outer flash coat on a dense bond coat on rods was studied. • The flash bond coating significantly increased coating lifetime. • Porosity of the top-coat correlated with coating lifetime. [ABSTRACT FROM AUTHOR]