Plasma-sprayed Thermal Barrier Coatings Research Articles

The Evaluation of Durability of Plasma-Sprayed Thermal Barrier Coatings with Double-layer Bond Coat

The durability of atmospheric plasma-sprayed thermal barrier coatings (APS TBCs) with a double-layer bond coat was evaluated via isothermal cycling tests under 1120 °C. The bond coat consisted of a porosity layer deposited on the substrate and an oxidation layer deposited on the porosity layer. Two types of double-layer bond coats with different thickness ratios of the porosity layer to the oxidation layer (type A: 1:2 and type B: 2:1, respectively) were prepared. The results show that the porosity layer was oxidation free, the oxidation layer included a fraction of well-distributed α-Al2O3. The coefficient of thermal expansion of the oxidation layer was about 11.2 × 10−6 K−1, which was rather lower than that of the porosity layer. Thus, the oxidation layer can be regards as a secondary bond coat between ceramic topcoat and traditional bond coat. The thermal cyclic lifetime of type A TBCs was about 60 cycles, which exceeded 1.2 times the durability of type B TBCs. The delamination cracks in both TBCs all propagated in the ceramic topcoat, which were all identical to those in traditional TBCs. Therefore, the design of the double-layer bond coat affected the stress level rather than the stress distribution in TBCs.

Effect of bond-coat surface roughness on failure mechanism and lifetime of air plasma spraying thermal barrier coatings

This work investigates the failure mechanism of an air plasma spraying (APS) thermal barrier coatings (TBCs) with various bond coat (BC) surface toughness under constant temperature loading condition. Results show that the increased BC surface roughness decreased the lifetime. Moreover, the relationship between the TGO depth and the lifetime can be well described by linear law. The failure mechanism of the TBCs samples was studied by cross-sectional morphologies observation and using the finite element method (FEM). Cracks in the TBCs samples with higher BC surface roughness are initiated from the defects on the BC caused by Al depletion in the BC layer. However, cracks in the samples of TBCs with lower BC surface roughness condition preferentially occur at the TC-TGO interface. This finding could be attributed to the high mismatch stresses in the TC-TGO interface.

Investigation on the performance of air plasma sprayed thermal barrier coating with Lu/Hf-doped NiAl bond coat

The durability of air plasma sprayed (APS) thermal barrier coatings (TBCs) with NiAl bond coats with/without Lu/Hf doping was investigated under thermal cycling at 1150 °C, and compared with the conventional NiCoCrAlY sample. The oxidation kinetics, microstructure evolution and surface roughness of the bond coats were characterized and the cycle lifetime measured. The TBC with the NiAl bond coat exhibited a lifetime of 456 h, which was comparable with the NiCoCrAlY sample (480 h). By contrast, the NiAl-Lu/Hf TBC showed a two or three fold enhanced lifetime, i.e., 840 h and 1248 h, respectively. It is proposed that the enhanced lifetime is a result of combination of decreasing of the oxidation rate and retention of the interfacial toughness. The APS NiAl bond coat with Lu/Hf doping has a great potential for TBCs designed for higher temperature applications.

Finite Element Modelling of TBC Failure Mechanisms by Using XFEM and CZM

Thermal Barrier Coatings have been widely used in modern turbine engines to protect the nickel based metal substrate from the high temperature service conditions, 1600-1800 K. In this study, failure mechanisms of typical Air Plasma Sprayed Thermal Barrier Coatings (TBC) used in after-burner structures composed of three major layers: Inconel 718 substrate, NiCrAlY based metallic bond coat (BC) and Yttria Stabilized Zirconia (YSZ) based ceramic top coat (TC) are investigated. Investigation of the cracking mechanism of TBC in terms of design and performance is very important because the behavior of TBCs on ductile metallic substrates is brittle. To this end, four-point bending experiments reported in Kütükoğlu (2015) are analyzed by using the Extended Finite Element Method (XFEM) and the Cohesive Zone Method (CZM). All the analyses are conducted with the commercial finite element software ABAQUS. Three different models with varying TC and BC thicknesses are studied. It is observed that multiple vertical cracks are initiated in the TC. Cracks initiate at the top of YSZ and propagate through the whole TC until they reach the interface between the TC and the BC. Then, delaminations at the interface between the TC and the BC start. It is observed that the average spacing of cracks in TC increases with the increasing thickness of the TC and the delamination becomes prominent with the increasing TC thickness. Numerical results are found to be consistent with the experimental results.

Thermo-mechanical evaluation of plasma sprayed YSZ-based multi-layered thermal barrier coatings

Thermo-mechanical evaluation of plasma sprayed YSZ-based multi-layered thermal barrier coatings

Thermo-mechanical evaluation of plasma sprayed YSZ-based multi-layered thermal barrier coatings

The multi-layered YSZ, NiCoCrAlY, Cr2O3 and Al2O3 coatings of varying thickness (200 μm-400 μm) have been investigated for thermal barrier coating (TBC) application. Plasma spray technique has been utilised on piston crown to optimise the thermal fatigue life in view to reduce the heat losses. Four types of TBC overlay each with a 100 μm NiCrAlY bond coat were deposited on the A336 aluminium alloy substrate cut out of the diesel engine piston. The phase composition of coatings before and after the thermal shock testing was analysed by XRD and the lattice strain analysis was performed by Williamson-Hall analysis. It was observed that among the considered multi-layered coating configurations, those with 300 μm YSZ and 200 μm Cr2O3 top coat exhibited acceptable thermal shock resistance as the specimens sustain up to 298 and 325 thermal cycles respectively. The microstructural analysis suggested against the formation of any major deformity or structural changes at the higher temperature as investigated by thermal shock experiment.

Thermal cycling behaviour of plasma-sprayed thermal barrier coatings with pre-oxidized NiCrAlY and NiCoCrAlY bond-coats

This contribution deals with thermal cycling of thermal barrier coatings (TBCs) with NiCrAlY and NiCoCrAlY bond-coats, and yttria stabilized zirconia (YSZ) top-coat that were prepared by atmospheric plasma spraying from commercial powders. The samples were pre-oxidized, based on isothermal oxidation of the two bond-coats, in order to diminish the influence of residual stresses after spraying on thermal cycling experiments. Surface topographies of both bond-coats were measured using optical profilometry and several surface parameters were evaluated to characterize them. Microstructural degradation caused by thermal cycling was examined by scanning electron microscopy with an emphasis on the role of bond-coat surface features on local damage mechanisms. An important micromechanism for TBCs with the NiCoCrAlY bond-coat was multiple parallel cracking of the TGO layer that was likely also responsible for lower endurance of these coatings.

Combined effect of internal and external factors on sintering kinetics of plasma-sprayed thermal barrier coatings

Understanding the sintering kinetics of plasma-sprayed thermal barrier coatings (PS-TBCs) is crucial to retard their performance degradation. However, under real service condition, the sintering kinetics is often affected by multiple factors. This study investigated the sintering kinetics, in a novel scale-progressive view, under the combined-effect of internal and external factors. Results show that the sintering kinetics of PS-TBCs was highly associated with their unique sintering process from nanoscale to microscale. Firstly, sintering leads to nanoscopic roughening of the pore surface. Subsequently, multiple contacts are formed between counter-surfaces. As a result, microscopic healing of pores can be finally observed. In terms of external factors, the temperature further affects the level and rate of nanoscopic roughening. This is responsible for the differences of the microscopic healing ratios, as well as the macroscopic elastic modulus.

Failure of Multilayer Suspension Plasma Sprayed Thermal Barrier Coatings in the Presence of Na2SO4 and NaCl at 900\xa0\xb0C

The current investigation focuses on understanding the influence of a columnar microstructure and a sealing layer on the corrosion behavior of suspension plasma sprayed thermal barrier coatings (TBCs). Two different TBC systems were studied in this work. First is a double layer made of a composite of gadolinium zirconate + yttria stabilized zirconia (YSZ) deposited on top of YSZ. Second is a triple layer made of dense gadolinium zirconate deposited on top of gadolinium zirconate + YSZ over YSZ. Cyclic corrosion tests were conducted between 25 and 900 °C with an exposure time of 8 h at 900 °C. 75 wt.% Na2SO4 + 25 wt.% NaCl were used as the corrosive salts at a concentration of 6 mg/cm2. Scanning electron microscopy analysis of the samples’ cross sections showed that severe bond coat degradation had taken place for both the TBC systems, and the extent of bond coat degradation was relatively higher in the triple-layer system. It is believed that the sealing layer in the triple-layer system reduced the number of infiltration channels for the molten salts which resulted in overflowing of the salts to the sample edges and caused damage to develop relatively more from the edge.

Studies on Plasma Sprayed Thermal Barrier Coating with Increase in Coating Thickness

Studies on Plasma Sprayed Thermal Barrier Coating with Increase in Coating Thickness

The Temperature Distribution in Plasma-Sprayed Thermal-Barrier Coatings During Crack Propagation and Coalescence

A Finite-Element Model (FEM) for thermal-barrier coatings was employed to elaborate the temperature distribution on yttria-stabilized zirconia (YSZ) free surface during cracks coalescing, then the influence of sintering of YSZ induced by heat-transfer overlapping on energy release rate was quantificationally evaluated. A three-dimensional model including three layers was fabricated. Two types of cracks, with and without depth variations in YSZ coating, were introduced into the model, respectively. The temperature rise of YSZ coating over the crack is independent of each other at the beginning of crack propagation. As crack distance shortens, the independent temperature-rise regions begin to overlap, while maximum temperature is still located at the crack center before crack coalescence. The critical distance that the regions of temperature rise, just overlapping, is the sum of half lengths of two coalescing cracks (i.e., a1 + a2), which is independent of cracking path. The maximum temperature in YSZ sharply increases once cracks coalesce. Compared with one delamination crack, the effective energy-release rate induced by heat-transfer overlapping increases in the range of 0.2%–15%, depending on crack length and crack distance, which is on some level comparable to that of deterioration of thermal expansion misfit induced by temperature jump between crack faces.

Investigation on interfacial fracture toughness of plasma-sprayed TBCs using a three-point bending method

To determine the interfacial properties of thermal barrier coatings (TBCs) is imperative for their durability evaluation and further improvements. In this work, a ceramic coating (topcoat) and a NiCoCrALY bondcoat were atmospheric-plasma-sprayed (APS) on a superalloy substrate. A three-point bending test was adopted to initiate and propagate the topcoat/bondcoat (TC/BC) interfacial cracking. The load-displacement curves indicate three stages in crack initiation and stable crack growth. After a complete delamination, the fracture surfaces were examined by an optical microscope and a scanning electron microscope, which shows that the cracking plane was merely on the TC/BC interface. The displacement and strain fields of the TC/BC cross-section were acquired using digital image correlation (DIC) method, and the crack length was obtained with an inverse finite element method based on the crack opening displacement measured by the DIC method. Finally, the critical strain energy release rate Gc for crack propagation can be reliably determined from a theoretical analysis using compliance methods. The results indicate that experimental data are consistent with the beam theory analysis.

Influence of interface morphology on erosion failure of thermal barrier coatings

The thermal barrier coating system (TBCs) has complex structure and works in severe service environment. Erosion is one of the main factors causing the failure of TBCs. In the present study, the particle erosion process of atmospheric plasma sprayed (APS) thermal barrier coatings at elevated temperature was simulated by the finite element method. The effects of interface morphology on the penetration depth, particle ricochet velocity and interface stress state were studied, and the key parameters such as particle size, initial velocity and erosion position were also considered. The cosine curve with constant wavelength and varying amplitude was used to represent different interface roughness of TBCs. The results show that the interface morphology has little effect on the penetration depth of top coat (TC) and the particle ricochet velocity. The influence of particle erosion position related to the interface morphology is obvious. Basically, the greater the interface roughness is, the more violent the interfacial stress fluctuation is. During the erosion process, the stress in the middle of the interface is significantly higher than that at other positions. These results facilitate understanding of the particle erosion failure mechanism of APS TBCs. The influence of interface morphology should be considered in erosion research.

Fabrication and characterization of novel powder reconstitution derived nanostructured spherical La2(Zr0.75Ce0.25)2O7 feedstock for plasma spraying

High-performance thermal spraying feedstock is of great significance for obtaining high quality plasma-sprayed thermal barrier coatings (TBCs). Rare-earth zirconates TBCs have attracted much more attention so far. In this paper, the nanostructured La2(Zr0.75Ce0.25)2O7 (LCZ) feedstock was fabricated by the nanopowder reconstitution method. The microstructures of the LCZ feedstock were characterized by transmission electron microscopy, X-ray diffraction, Raman spectroscopy and scanning electron microscopy. Moreover, the thermo-physical properties of the LCZ feedstock were also evaluated. The results indicate that the spherical LCZ feedstock with the median particle size (d50) of 37.40 μm consists of only pyrochlore phase. Compared with the nanostructured La2Zr2O7 (LZ) feedstock, the tap density and flowability of the LCZ feedstock are enhanced considerably. As for the LCZ coating, the thermal conductivity is much lower and the thermal expansion coefficient (TEC) is relatively higher than the LZ coating. Therefore, the nanostructured LCZ feedstock can be a promising advanced thermal spraying feedstock used for double ceramic layer TBCs in future.

Revisiting the Birth of 7YSZ Thermal Barrier Coatings: Stephan Stecura †

Thermal barrier coatings are widely used in all turbine engines, typically using a 7 wt.% Y2O3–ZrO2 formulation. Extensive research and development over many decades have refined the processing and structure of these coatings for increased durability and reliability. New compositions demonstrate some unique advantages and are gaining in application. However, the “7YSZ” (7 wt.% yttria stabilized zirconia) formulation predominates and is still in widespread use. This special composition has been universally found to produce nanoscale precipitates of metastable t’ tetragonal phase, giving rise to a unique toughening mechanism via ferro-elastic switching under stress. This note recalls the original study that identified superior properties of 6–8 wt.% yttria stabilized zirconia (YSZ) plasma sprayed thermal barrier coatings, published in 1978. The impact of this discovery, arguably, continues in some form to this day. At one point, 7YSZ thermal barrier coatings were used in every new aircraft and ground power turbine engine produced worldwide. 7YSZ is a tribute to its inventor, Dr. Stephan Stecura, NASA retiree.

Sintering resistance of advanced plasma-sprayed thermal barrier coatings with strain-tolerant microstructures

Sintering is one of the key issues in the high temperature service of thermal barrier coatings (TBCs), considering the continuously increasing operation temperature of gas-turbine for higher energy efficiency. Based on the conventional processing method of air plasma spraying (APS), suspension plasma spraying (SPS) technique has been developed recently, in order to improve the strain tolerance of TBCs. This strain tolerance of plasma-sprayed TBCs is largely effected by the sintering behavior, which is presently not fully understood. In this work, evolution of mechanical properties, in terms of Young’s modulus and viscosity, is systematically investigated by in-situ three-point bending test at 1200 °C on free-standing coatings, including micro-cracked APS, segmented APS, vertically cracked SPS and columnar structured SPS TBCs and correlated to the microstructural evolution. Based on experimental results, power law relations are proposed for the sintering induced mechanical evolution, which deepen the understanding of the sintering behavior of plasma-sprayed TBCs.

The Influence of Ceramic Topcoat Sintering on Stress Distribution in Plasma-sprayed Thermal Barrier Coatings

The Influence of Ceramic Topcoat Sintering on Stress Distribution in Plasma-sprayed Thermal Barrier Coatings

Modified Bond Coatings Improve Service Life of Plasma Sprayed Thermal Barrier Coatings and Protective Performance of Overlay Coatings

Abstract New thermal and cold spray technologies are being developed to extend the lifespan of thermal barrier coating (TBC) systems at elevated temperatures. This article describes the relevant attributes of coating technologies such as high velocity air fuel, high velocity air plasma spray, and high pressure cold spray for use in TBC systems.

Scale-progressive healing mechanism dominating the ultrafast initial sintering kinetics of plasma-sprayed thermal barrier coatings

The thermal insulating performance of plasma-sprayed thermal barrier coatings (PS-TBCs) depends dominantly on inter-splat pores, which would be inevitably healed during high temperature exposure. The sintering kinetics of TBCs appears to be highly stage-sensitive. However, the ultrafast sintering kinetics during the initial sintering stage is not yet well understood. In this study, the sintering behavior of PS-TBCs was investigated in a scale-progressive (from nano- to micro-scale) way. Moreover, a novel healing mechanism suitable for lamellar TBCs was proposed based on a combined-effect of material and pore structure. Regarding the changing behavior of material, nano-scale roughening can be found at the as-deposited smooth pore surface after thermal exposure. Regarding the 2D featured inter-splat pores, the roughening behavior facilitates multiple contacts between the counter-surfaces of inter-splat pores. As a result, micro-scale bridge-connection can be observed at the healed parts. This multiple contacts mechanism caused by scale-progressive healing behavior significantly accelerated the matter transfer, resulting in ultrafast sintering kinetics at the initial sintering stage.

Oxidation behavior of thermal barrier coating systems with Al interlayer under isothermal loading

In the present study, the phenomena related to the Thermally Grown Oxides (TGO) in atmospheric plasma sprayed Thermal Barrier Coatings (TBCs) are discussed. CoNiCrAlY bond coatings were sprayed on Inconel 600 substrates. Subsequently, thin Al layers were deposited by DC-Magnetron sputtering. Finally, yttria-stabilized zirconia (YSZ) top coatings were deposited to form a three-layered TBC system. The thus produced aluminum interlayer containing thermal barrier coatings (Al-TBC) were subjected to isothermal exposure with different holding times at 1150 °C and compared with reference TBCs of the same kind, but without Al interlayers (R-TBC). The oxide film formation in the interface between bond coating (BC) and top coating (TC) was investigated by scanning electron microscope (SEM) after 100 and 300 h of high temperature isothermal exposure. The growth of this oxide film as a function of the isothermal exposure time was studied. As a result, the designed Al-TBC system exhibited better oxidation resistance in the BC/TC interface than the two-layered R-TBC system. This was lead back to the Al enrichment, which slows down the formation rate of transition metal oxides during thermal loading.