Photo-stimulated Luminescence Piezo-spectroscopy Research Articles

The Effect of APS-HVOF Bond Coating Thickness Ratio on TBC Furnace Cycle Lifetime

Combinations of NiCoCrAlY APS and high-velocity oxygen fuel (HVOF) bond coatings were deposited on alloy 247 disk substrates with APS yttria-stabilized zirconia (YSZ) top coatings to assess the benefit of air plasma sprayed (APS) ‘flash’ bond coatings. Using 1-h cycles at 1100 ℃ in air with 10% H2O and HVOF-only and APS-only bond coatings as a baseline, it was found that APS flash coatings extended the average coating lifetime by 16% to 35% with the thicker flash coating performing best. Principal component analysis and energy dispersive spectroscopy (EDS) compositional mapping on coatings characterized after 0, 100, 300 and 500 cycles and after failure showed that the Al in the bond coatings was depleted due to both oxidation and back diffusion into the substrate. The APS-only bond coating had significant oxidation throughout the bond coating and was so depleted in Al, that Al diffused from the substrate to the coating. Residual stress maps of the thermally-grown alumina scale were obtained every 100 cycles using photo-stimulated luminescence piezospectroscopy (PLPS) revealing that the Thick Flash coating had the slowest rate of damage accumulation in the oxide scale.

Growth stress and interdiffusion analysis of NiCoCrAlYTa coating during oxidation

ABSTRACT The NiCoCrAlYTa coating deposited on Ni-based superalloy using arc ion plating (AIP) technology was subjected to isothermal oxidation in air at 1050°C for up to 200 h. The microstructure, oxides scale thickness, growth stress and interface diffusion behaviour were characterized by using scanning electron microscopy, X-ray diffraction, electron probe micro-analyser and photo-stimulated luminescence piezo-spectroscopy. The results revealed that the microstructure of oxides scale was predominantly composed of α-Al2O3 with (Y, Ta)-rich oxides and Cr2O3 irregularly distributed during the oxidation process. The oxides scale thickness of NiCoCrAlYTa coating conformed to parabolic law, which indicated the coating had good oxidation resistance. Also, the relationship between the thickness and growth stress, as well as the oxidation time, was studied based on the Clark oxidation theory and Wagner oxidation model, developing a methodology to measure the high-temperature stress in NiCoCrAlYTa coating. The theoretical calculation value of the stress was basically consistent with the value of the experiment, which could effectively predict the growth stress in the coating and help understand the failure mechanism of NiCoCrAlYTa coating during the service life. In the oxidation process, the interdiffusion behaviour occurred at the interface between the oxides scale and the NiCoCrAlYTa coating was also discussed. The diffusion coefficient of typical elements was calculated by using Boltzmann–Matano method, which was helpful to analyse the inner oxidation of the NiCoCrAlYTa coating, providing in-depth understanding of the oxidation behaviour.

Role of bond coat processing methods on the durability of plasma sprayed thermal barrier systems

Metallic alloy bond coats are used as an interlayer to promote adhesion of ceramic Thermal Barrier Coatings (TBCs) to the superalloy substrates. These bond coats can be applied through a variety of processing means such as Air Plasma Spray (APS), High-Velocity Oxy Fuel spray (HVOF), and Vacuum Plasma Spray (VPS). While widely adopted in the industry, some uncertainties remain as to the specific microstructural attributes which are influenced by the processing routes and their implications on durability. This paper seeks to elucidate these subtleties between bond coat deposition processing routes through an integrated study of processing to connect to furnace cycle durability on a single bond coat chemistry of NiCoCrAlYHfSi. A standard porous APS TBC top coating based on yttria stabilized zirconia (7YSZ) was consistently applied on these distinct bond coats and tested for durability in cyclic furnace testing at 1100 °C. The results point to substantial differences in durability which are not readily explained only by the variances in coating roughness. Detailed assessment of deposited bond coat properties (modulus, thermal expansion, chemistry) provided some explanation relative to durability. The results suggest a complex interplay among roughness, compliance, aluminum diffusion pathway, and thermal expansion mismatch. Photo-stimulated luminescence piezospectroscopy (PLPS) provided additional insights enabling a comprehensive assessment of processing-performance linkages.

The Effect of HVOF Bond Coating with APS Flash Coating on TBC Performance

To study the benefit of ~ 50-µm-thick air-plasma-sprayed (APS) ‘flash’ bond coatings, NiCoCrAlYHfSi high-velocity oxygen fuel (HVOF) bond coatings were deposited on alloy 247 disk substrates with APS yttria-stabilized zirconia topcoatings. Using 1-h cycles at 1100 °C in air with 10% H2O and HVOF-only bond coatings as a baseline, APS flash coatings extended the average coating lifetime by > 10% using NiCoCrAlYHfSi and > 70% using NiCoCrAlY powder. Coatings were characterized after 0, 100 and 300 cycles and after failure. Residual stress in the thermally grown alumina scale was mapped every 100 cycles using photo-stimulated luminescence piezospectroscopy. The flash coating created an interlocked metal–alumina layer that appeared to inhibit critical crack formation, and the underlying HVOF layer prevented further internal oxidation and appeared to supply the APS flash coating with Al. The addition of Hf and Y to the flash coating increased internal oxidation and accelerated Al consumption, thereby reducing the benefit of the flash coating.

3D Microscopy to Assess the Effect of High Temperature Cyclic Oxidation on the Deformation of Cast and ODS FeCrAlY Alloys

Specimen deformation, oxide scale growth, and scale cracking were monitored for cyclic oxidation exposures at 1200 °C of three cast FeCrAlY(Hf) alloys and two oxide dispersion strengthened (ODS) FeCrAl alloys. Evolution of both the specimen geometry and surface deformation was quantified using 3D optical microscopy. The stress in the alumina scale was also measured using photo-stimulated luminescence piezospectroscopy. The goal was to assess the effect of stress generation and stress relaxation on scale spallation for FeCrAl materials with drastic differences in terms of high temperature strengths. After 1000, 1-h cycles, the cast alloys exhibited significant deformation with a convoluted oxide scale, leading for the two Hf-free FeCrAlY alloys to undergo crack formation and spallation. For the Hf-containing FeCrAlY, limited spallation was observed due to the localization of spallation at grain boundaries, the large grain structure, and the beneficial effect of Hf on scale adhesion. In contrast, limited deformation was measured for the ODS FeCrAl alloys, with minimal cracking and scale spallation for alloy PM2000 after 1000 h. A higher spallation rate was observed for ODS MA956, which was attributed to a high defect concentration in the alumina scale that formed. The mechanisms leading to spallation during cyclic oxidation for the cast and ODS FeCrAlY alloys are described.

APS TBC performance on directionally-solidified superalloy substrates with HVOF NiCoCrAlYHfSi bond coatings

Abstract 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% H2O. 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-Al2O3 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.

Effect of substrate curvature on the evolution of microstructure and residual stresses in EBPVD-TBC

A set of Ni-based single crystal superalloy aero-blade sections with electron beam physical vapour deposited thermal barrier coating (EBPVD-TBC) were used to investigate the role of substrate curvature on the microstructure and the residual stresses in the coating system. The residual stresses generated in the thermal barrier coating (TBC) and the thermally grown oxide (TGO) have been evaluated non-destructively by Raman spectroscopy (RS) and photo-stimulated luminescence piezo-spectroscopy (PLPS), respectively. In addition, residual stresses were measured in free-standing individual columns extracted from the TBCs for further understanding of the large Raman shift towards the tension side in the ceramic layer. For the TGO, three areas with varied substrate curvature were mapped by PLPS. These residual stresses are analysed as a function of the thermal exposure and the substrate curvature. Discussion focuses on the implications on how to correctly evaluate the residual stresses in EBPVD-TBCs based on Raman shifts and subsequently the role of substrate curvature on the degradation of the microstructure and evolution of residual stresses in this type of TBCs.

The effect of cycle frequency, H2O and CO2 on TBC lifetime with NiCoCrAlYHfSi bond coatings

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 100h cycles were used to simulate base-load power generation operation, compared to the 1h aero-engine standard cycle. Longer exposure times for 100h cycles increased the interdiffusion resulting in no clear effects of H2O and CO2 additions on average TBC lifetime for the 1483 substrates, contrary to the results with 1hcycles where the addition of H2O 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.

Evolution of residual stress distribution and microstructure in a platinum-aluminide coating

In the present paper, we consider the evolution of residual stress and the corresponding microstructure in the thermally grown oxide (TGO) formed on a type of platinum-aluminide coating, RT22, on a single crystal superalloy CMSX4. Eight coated specimens were oxidised in a simulated gas turbine environment at 850, 950, 1000 and 1050°C for periods of time extended up to 10 000 h. Photo-stimulated luminescence piezo-spectroscopy (PLPS) was used to measure the residual stress in the TGO in order to provide a detailed evaluation of the stress developed at each temperature. The stress variation was evaluated using a random measurement approach, which demonstrated that up to 100 measurements provide data that allows the stress distributions to be determined with high confidence. Scanning electron microscope (SEM) and energy dispersive X-ray (EDX) microanalysis have been adopted to investigate microstructural and compositional changes in the TGO and the coating as a function of time and temperature. The results are discussed with respect to the correlation between the statistically analysed stress distribution, degradation of the TGO and coating overall integrity.

The role of beam dispersion in Raman and photo-stimulated luminescence piezo-spectroscopy of yttria-stabilized zirconia in multi-layered coatings

Laser beam dispersion affects the resolution of Raman and photo-stimulated luminescence piezo-spectroscopy measurements of transparent materials. In this paper, we investigate the lateral spreading of the laser beam and the axial sampling depth of Raman spectroscopy measurements within thermal sprayed yttria-stabilized zirconia (YSZ) thin coatings. The lateral diameters of the laser beams (λ=632.8nm and 514nm) reach approximately ∼160μm after travelling through a thickness of 200μm of air plasma sprayed (APS) YSZ and ∼80μm after travelling through 120μm of electron beam physical vapour deposited YSZ. The Raman spectroscopy sampling depth was found to be between 30 and 40μm in APS YSZ. The beam dispersions within these two coatings were simulated using the ray-tracing software ZEMAX to understand the observed scattering patterns. The results are discussed with respect to the application of these two spectroscopic techniques in multi-layered thermal barrier coating systems.

Effects of substrate curvature radius, deposition temperature and coating thickness on the residual stress field of cylindrical thermal barrier coatings

In a thermal barrier coating (TBC) system with cylindrical geometry, the position of coating plays an important role in the distribution of residual stress. In this paper, the residual stress field in three different types of TBCs with cylindrical geometry has been analyzed. The main focus is on the effects of substrate curvature radius, deposition temperature and coating thickness on the residual stress distribution during a deposition process. The results show that the substrate curvature radius significantly affects the distributions of radial and hoop residual stresses, which are in good agreement with experimental measurements by photo-stimulated luminescence piezospectroscopy (Wang et al., Acta Mater., 2009, 57(1):182–195). The maximum radial residual stress locates closely to the coating/thermal grown oxide interface. However, the maximum hoop residual stress lies in the thermal grown oxide layer, which is much more than other three layers and presents a strong stress singularity along the thickness direction.

Assessment of the Spent Life Fraction of Gas Turbine Blades by Coating Life Modeling and Photostimulated Luminescence Piezospectroscopy

This paper describes the application of a double-sided approach to assess the spent life fraction of thermal barrier coatings (TBCs) at local positions of ex-service blades with NiCoCrAlY-Re bond coat and EB-PVD TBC. A life prediction code implemented with inverse problem solution routines was applied to determine the mean local temperature. On the single blade, large differences (>150°C) of mean local temperature were estimated between regions just after the cooling holes and far from them. A self-developed portable photostimulated luminescence piezospectroscopy (PLPS) instrument was also applied and the coating spent life fraction was evaluated with a recently proposed PLPS diagnostic parameter. The spent life fractions evaluated from mean local temperature values were in good agreement with those estimated from PLPS measurements. Lower spent life fractions were found in well cooled regions of all blades, while higher spent life fractions were calculated at the hottest positions of the blades, particularly on components coming from a plant with a double number of starts. The results confirm the reliability of the PLPS technique and its diagnostic capability also for NiCoCrAlY-Re bond coats; both local differences on the single component and blade-to-blade variations in coating spent life can be evidenced in a nondestructive way.

Analysis of localized damage in EB-PVD/(Ni, Pt)Al thermal barrier coatings

The structural integrity of the thermally grown oxide (TGO) in the EB-PVD/(Ni, Pt)Al thermal barrier coatings was examined using photo-stimulated luminescence piezo-spectroscopy (PLPS). PLPS spectra exhibiting both a high stress component and a low stress (or stress-free) component were observed during thermal cycling (i.e. bimodal luminescence). The fraction of bimodal spectra increases initially, then decreases, and increases again when close to failure. It is shown that the bimodal luminescence originates from stress relaxation caused by localized damage. It is proposed that the initial increase of bimodal luminescence is related to cracking caused by the volume change associated with the phase transformation of TGO from θ to α-Al 2O 3; whereas, the increase at the final stage is due to TGO cracking and spallation. Area stress maps show the gradual accumulation of damage and indicate that PLPS is a useful tool for detection of the initiation and progression of TBC spallation.

Evolution of photo-stimulated luminescence of EB-PVD/(Ni, Pt)Al thermal barrier coatings

Experiments are described which were designed to assess the suitability of photo-stimulated luminescence piezo-spectroscopy (PLPS) measurements as a basis for non-destructive inspection (NDI) and determination of life remaining of thermal barrier coatings (TBCs). Thermal cyclic tests were conducted on 7wt.% Y2O3 stabilized ZrO2 (YSZ) electron beam physical vapor deposited (EB-PVD)/(Ni, Pt)Al/CMSX-4 TBCs at two temperatures 1151 and 1121°C. The evolution of PLPS spectral characteristics (peak frequency shift, peak width and area ratio of peaks) was studied as a function of thermal cycles. It was observed that the average thermally grown oxide (TGO) stress and its standard deviation, and the area ratio of peaks show systematic change with thermal cycling, indicating that these characteristics can be used for NDI and determination of life remaining. The average TGO stress increases initially and then decreases monotonically with thermal cycling. The rate of change in the stress can be related to specimen life: the shallower the slope, the higher the life. The peak area ratio also decreases monotonically with cycling. The average TGO stress changes in a systematic manner versus remaining life fraction independent of temperature. Remaining life predictions were made based on average stress versus life fraction, which resulted in life assessments within ±13% of actual values excluding one specimen with abnormal behavior.

Photoluminescence peizospectroscopy for non-destructive residual life assessment of thermal barrier coatings

A quantitative approach, based on measurement of oxide tress evolution as a function of thermal cycling is described for identifying thermal barrier coating (TBC) damage and assessing remaining life. Residual stress in the thermally grown oxide was measured using photostimulated luminescence piezospectroscopy for electron beam physical vapour deposited TBCs, which were thermally cycled at three temperatures and over two cycling periods. The observed monotonic change in the oxide stress level with TBC life fraction is systematic with cycle time and nearly independent of exposure temperature. The temperature independence of the measured stress with life fraction would greatly simplify the use of this technology for the non-destructive evaluation of TBCs.

Stress variation with thermal cycling in the thermally grown oxide of an EB-PVD thermal barrier coating

A measurement approach for detecting thermal barrier coating (TBC) damage and assessing remaining life is described in this study. This is based on an experimental study of the oxide stress evolution as a function of thermal cycling. Residual stress in the thermally grown oxide (TGO) has been measured using photo-stimulated luminescence piezo spectroscopy (PLPS) for electron beam physical vapor deposited TBCs that were thermally cycled at 1100, 1121 and 1151 °C using 1-h and 24-h cycling periods. The observed monotonic change in the oxide stress level with life fraction is systematic with cycle time and nearly independent of exposure temperatures. The temperature independence of the measured stress with life fraction greatly simplifies the use of this technology for non-destructive evaluation of this TBC coating.

The effect of bond coat grit blasting on the durability and thermally grown oxide stress in an electron beam physical vapor deposited thermal barrier coating

Photo-stimulated luminescence Piezo-spectroscopy (PLPS) is being developed as a non-destructive technique for thermal barrier coatings (TBC). In this study, the evolution of photo-stimulated luminescence with thermal cycling was systematically investigated from the thermally grown oxide (TGO) in a production TBC, which consists of an electron beam physical vapor deposited (EB-PVD) 7 wt.% Y 2O 3–ZrO 2 top coat, a grit blasted (Ni,Pt)Al bond coat and a CMSX-4 superalloy substrate. The change of compressive stress in the TGO layer on the bond coat with thermal cycling was calculated from the wavelength shift of the luminescence spectra. The compressive stress increased from 1.0–2.2 GPa in the as received state to 2.8–3.3 GPa at 10 cycles, then gradually decreased to 1.2–1.9 GPa until 500 cycles and remained at this level until TBC spallation. Other fluorescence spectra characteristics, such as the width of R1 and R2 peaks and their relative intensity, were also evaluated. These PLPS measurements on TBCs with grit blasted bond coats are compared with previous measurements on similar TBC system but with non-grit blasted bond coats. It is concluded that the initial increase in stress is associated with the formation of a continuous oxide layer. The lower stress of the specimens with the grit blasted bond coats compared to that of the as-coated bond coats is associated partly with the greater surface roughness. And the fast decline in compressive stress is the result of bond coat surface rumpling facilitated by the initially rougher surface. The lifetime of the TBCs with grit blasted bond coats varies over a narrow range, 600–750 cycles with an average of 675 cycles, which is related to their consistent bond coat surface roughness.

Microstructural characterization of thermal barrier coatings on high pressure turbine blades

Thermal barrier coated high pressure turbine blades were characterized before and after the service by microstructural analysis and Cr 3+ photostimulated luminescence piezo-spectroscopy. Thermal barrier coatings, in this study, consisted of electron beam physical vapor deposited yttria partially stabilized zirconia (YSZ; ZrO 2–8 wt.% Y 2O 3), vapor-deposited aluminide bond coat and Ni-base superalloy. Compressive residual stress in thermally grown oxide, measured by Cr 3+ photostimulated luminescence piezo-spectroscopy, was observed to be in the order of 2.5∼3.0 GPa and varied slightly as a function of substrate geometry. X-Ray diffraction and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy were utilized to investigate the microstructural development of thermal barrier coatings. The as-deposited non-equilibrium tetragonal ( t′) phase in the YSZ coatings was observed to decompose after the service, but the monoclinic ( m) phase was only found in the YSZ coatings with concave substrate curvature on the pressure side of the HPT blade. Also, a significant sintering of ZrO 2–8 wt.% Y 2O 3 coating after the service was observed in the microstructure. Localized spallation of YSZ occurred within the thermally grown oxide (mostly α-Al 2O 3) and within the ZrO 2–8 wt.% Y 2O 3 coating for pressure and suction sides of the serviced high pressure turbine blade near the tip, respectively.

Thermal cycling of EB-PVD/MCrAlY thermal barrier coatings: I. Microstructural development and spallation mechanisms

Microstructural changes, damage initiation and spallation of a production TBC, which consists of: an electron beam physical vapor deposited ZrO 2–7 wt.% Y 2O 3 (YSZ); thermally grown oxide (TGO); MCrAlY bond coat; and a polycrystalline IN-738 superalloy substrate, were examined as a function of thermal cycles at 1121°C. Thermal cycling for TBC specimens in air consisted of a 10-min heat-up, a 40-min hold at 1121°C and a 10-min quench. Microstructural characterization was carried out by: photo-stimulated luminescence piezo-spectroscopy (PLPS); X-ray diffraction (XRD); scanning electron microscopy (SEM); and energy dispersive spectroscopy (EDS). Development of microstructure in the YSZ coatings, growth of the thermally grown oxide (TGO) and its constituents, and phase transformations in the MCrAlY bond coat were examined as a function of thermal cycles. Instability of the TGO/bond coat interface (i.e. rumpling), leading to localized cracking at the YSZ/TGO interface and within the YSZ, was observed after as few as five thermal cycles. Spallation of YSZ coatings occurred after approximately 400 cycles. Significant void formation at the TGO/bond coat interface and formation of a Ni/Co rich oxides at the TGO/bond coat interface due to rapid internal oxidation of Al-depleted MCrAlY bond coat were strong contributors to spallation failure of the TBC. The final spallation of TBC results from a ‘link-up’ of damage at the TGO/bond coat interface with the rumpling-induced micro-cracking at the YSZ/TGO interface.