Composite Thermal Barrier Coatings Research Articles

Enhancement of Corrosion Resistance of Al2O3 + Sm2SrAl2O7 Composite Thermal Barrier Coatings by Laser Treatment

Enhancement of Corrosion Resistance of Al2O3 + Sm2SrAl2O7 Composite Thermal Barrier Coatings by Laser Treatment

Stiffening-resistant composite TBCs counteracting the substrate constraint effect during thermal exposure

Thermal barrier coatings (TBCs) endow metal components with an increased capacity to endure exposure to high temperatures. Plasma-sprayed TBCs exhibit excellent thermal insulation performance and are widely used in land-based thermal protection systems. However, plasma-sprayed TBCs are more prone to cracking during usage, significantly limiting their applications. The main issue is the sintering-induced stiffening that enhances the cracking driving force. In this work, stiffening-resistant TBCs were fabricated using a composite structure that counteracts the substrate constraint effect. First, the substrate constraint effect on the TBCs was studied based on the surface roughness. The results revealed that stiffening was the primary cause of substrate constraint-related failure. Two-dimensional (2D) interface gaps were spontaneously formed during thermal exposure, which could reduce stiffening by 20%. Moreover, the healing behavior of the 2D gaps was analyzed and used to explain the stiffening-resistant behavior. Finally, the effect of the newly formed 2D gaps on the lifespan of the TBCs was analyzed. Detailed simulations suggested that the newly formed 2D gaps were capable of reducing the strain energy release rate and led to the growth of large in-plane cracks. For future applications, it is necessary to fully take advantage of the newly-formed 2D gaps by avoiding their negative effects on TBC life span.

Investigation of the Radiation Resistance and Optical Properties of New Composite Thermal Barrier Coatings

Investigation of the Radiation Resistance and Optical Properties of New Composite Thermal Barrier Coatings

Correction to: Evaluation of CMAS Resistance and Failure Behavior for Phase Composite Thermal Barrier Coatings

Correction to: Evaluation of CMAS Resistance and Failure Behavior for Phase Composite Thermal Barrier Coatings

Novel Thermal Barrier Coatings with Phase Composite Structures for Extreme Environment Applications: Concept, Process, Evaluation and Performance

In this paper, a novel concept in the field of phase composite ceramics has been proposed and applied for creating the topcoats of durable thermal barrier coatings (TBCs), which is one of the most critical technologies for advanced high-efficiency gas turbine engines in extreme environments. The phase composite ceramic TBCs were designed to demonstrate superior and comprehensive performance-related merits, benefits, and advantages over conventional single-phase TBCs with a topcoat of 8YSZ or Gd2Zr2O7, including thermal phase stability, thermal shock durability, low thermal conductivity, and solid particle erosion resistance. In this paper, we review and summarize the development work conducted so far related to the phase composite ceramic concept, coatings processing, and experimental investigation into TBC behaviors at elevated temperatures (typically, ≥1250 °C) using different evaluation and characterization methods, including isothermal sintering, a burner rig test, a solid particle-impinging erosion test, and a CMAS corrosion test. Two-phase (t’+c) zirconia-based TBCs demonstrated improved thermal shock and erosion resistance in comparison to conventional single-phase (t’), 8YSZ TBC, and Gd2Zr2O7 TBC, when used separately. Additionally, a triple-phase (t’+c+YAG) TBC sample demonstrated superior CMAS resistance. The TBC’s damage modes and failure mechanisms for thermal phase stability, thermal cycling resistance, solid particle erosion behavior, and CMAS infiltration are also characterized and discussed in detail, in terms of microstructural characterization and performance evaluation.

Study of toughening behavior of SiC whiskers on 8YSZ thermal barrier coatings

The embrittlement of thermal barrier coatings (TBCs) in a high-temperature service environment inhibits the ability of the top-coat to resist crack initiation and propagation, in which the cracks are generated by erosion or wear of particles, limiting the service durability of TBCs. This study was based on the preparation of SiC whiskers (SiCw) / Y2O3 partially stabilized ZrO2 (YSZ) integrated composite powder, and the SiCw/YSZ composite TBCs was prepared through the atmospheric plasma spraying (APS) to improve the toughness of the top-coat. The effects of SiCw on the fracture toughness, tribological properties, erosion resistance and bending strength of the coatings were studied. The results showed that the fracture toughness of the composite coating with a SiCw length-to-diameter ratio (L/D ratio) of 250 was 24.6 % higher than the YSZ coating. Moreover, the composite coating presented excellent performance in wear, erosion, and resistance to crack propagation and spallation. The low amplitude of the Weibull modulus m of hardness distribution of the doped coating proved the stability effect of SiCw on the reinforcement and toughening of the coating.

Evaluation of CMAS Resistance and Failure Behavior for Phase Composite Thermal Barrier Coatings

Evaluation of CMAS Resistance and Failure Behavior for Phase Composite Thermal Barrier Coatings

Comparison of the hot corrosion behavior of the LZ, CSZ and LZ/CSZ composite thermal barrier coating

In the present study, the hot corrosion behavior of Air Plasma Spray (APS)-deposited Ceria Stabilized Zirconia-Lanthanum Zirconate (CSZ-LZ) composite thermal barrier coatings (TBCs) was investigated and compared with CSZ and LZ coatings. The hot corrosion tests were conducted at 950 °C in a cyclic mode in the presence of V2O5 + Na2SO4 molten salt mixture. The microstructural aspects were evaluated by energy dispersive spectroscopy (EDS) equipped scanning electron microscopy (SEM) and x-ray diffraction (XRD) methods. The results showed that CSZ, 75CSZ-25LZ, 50CSZ-50LZ, 25CSZ-75LZ and LZ coating systems lasted for 28–30 h, 24–26 h, 22–24 h, 20–22 h, and 18–20 h, respectively. The results revealed an improvement in the hot corrosion resistance of composite coating compared with LZ coating which can be attributed to better coefficient of thermal expansion (CTE) compatibility of composite coatings. The formation of hot corrosion products such as CeVO4, LaVO4, and also tetragonal to monoclinic transformation are the main reasons for the failure of these coatings.

Enhanced calcium–magnesium–aluminosilicate (CMAS) resistance of GdAlO3 (GAP) for composite thermal barrier coatings

AbstractThe impact of calcium–magnesium–alumino‐silicate (CMAS) degradation is a critical factor for development of new thermal and environmental barrier coatings. Several methods of preventing damage have been explored in the literature, with formation of an infiltration inhibiting reaction layer generally given the most attention. Gd2Zr2O7 (GZO) exemplifies this reaction with the rapid precipitation of apatite when in contact with CMAS. The present study compares the CMAS behavior of GZO to an alternative thermal barrier coating (TBC) material, GdAlO3 (GAP), which possesses high temperature phase stability through its melting point as well as a significantly higher toughness compared with GZO. The UCSB laboratory CMAS (35CaO–10MgO–7Al2O3–48SiO2) was utilized to explore equilibrium behavior with 50:50 mol% TBC:CMAS ratios at 1200, 1300, and 1400°C for various times. In addition, 8 and 35 mg/cm2 CMAS surface exposures were performed at 1425°C on dense pellets of each material to evaluate the infiltration and reaction in a more dynamic test. In the equilibrium tests, it was found that GAP appears to dissolve slower than GZO while producing an equivalent or higher amount of pore blocking apatite. In addition, GAP induces the intrinsic crystallization of the CMAS into a gehlenite phase, due in part to the participation of the Al2O3 from GAP. In surface exposures, GAP experienced a substantially thinner reaction zone compared with GZO after 10 h (87 ± 10 vs. 138 ± 4 μm) and a lack of strong sensitivity to CMAS loading when tested at 35 mg/cm2 after 10 h (85 ± 13 versus 246 ± 10 μm). The smaller reaction zone, loading agnostic behavior, and intrinsic crystallization of the glass suggest this material warrants further evaluation as a potential CMAS barrier and inclusion into composite TBCs.

Erosion behavior of Al2O3 + Sm2SrAl2O7 composite thermal barrier coatings

Thermal barrier coatings (TBC) are applied in gas turbines to protect the components from critical operating conditions at high temperatures. Foreign object damage is a major failure cause of turbine components due to erosion of the topcoat ceramic layer of the TBC system. In the present study, Samarium Strontium Aluminate (Sm2SrAl2O7) powders have been synthesized in the laboratory through molten salt synthesis, and thermal barrier coatings were developed using Atmospheric Plasma Spray (APS) technique on an Inconel substrate with NiCrAlY as a bond coat. The erosion behavior of the composite TBC system at temperatures of 200 °C and 800 °C at 30° and 90° impingement angles was analyzed using alumina erodent with a grit size of 50 µm. The erosion weight loss was measured in regular intervals and plotted against erodent mass. The samples exhibited a higher volumetric erosion at 90° and 800 °C. The morphological analysis of developed 70% Al2O3 + 30% Sm2SrAl2O7 composite coatings and eroded areas were performed using XRD, SEM, and surface profilometer. Erosion scars and surrounding deformed areas have been identified in the samples. The microstructure of the eroded areas in the samples showed micro-cutting and plowing as the major material removal mechanisms. Detailed phase analysis of the eroded locations in samples has been reported.

Control of the Pore Structure of Plasma-Sprayed Thermal Barrier Coatings through the Addition of Unmelted Porous YSZ Particles

In this study, a new pore structure control method for plasma-sprayed thermal barrier coatings (TBCs) through the addition of unmelted, porous yttria-stabilized zirconia (YSZ) particles was investigated. Through a unique way of feeding powder, two powder feeders were used simultaneously at different positions of the plasma flame to deposit a composite structure coating in which a conventional plasma-sprayed coating was used as a matrix and unmelted micro-agglomerated YSZ particles were dispersed in the dense conventional coating matrix as second-phase particles. The effects of the distribution and content of second-phase particles on the microstructure, mechanical properties, and lifetime were explored in a furnace cyclic test (24 h) of the composite coating. The mechanical properties and lifetime of the composite coating depend on the content and morphology of the particles embedded in the coating. The lifetime of the composite structure coatings is significantly higher than that of the conventional coatings. By adjusting the spraying parameters, the lifetime of the composite coating prepared under the optimum process is up to 145 days, which is about three times that of the conventional coating. The results of this study provide guidance for the preparation of high-performance composite structure TBCs.

Evaluation and Characterization of a Durable Composite Phase Thermal Barrier Coating in Solid Particle Erosion and Burner Rig Tests.

A new concept of composite phase ceramic had been proposed for the topcoat of a durable thermal barrier coating (TBC) system which is one of the critical technologies for advanced turbine engines. The composite phase TBCs showed promising performance related benefits over conventional single phase TBCs, including durability, material affordability, thermal stability and low thermal conductivity. The present work is to continue the effort to exploring the TBC behaviors of erosive wear by solid particle erosion test and thermal cyclic shock by special burner rig test. In the erosion test, the investigation was focused on the most important characteristics of erosion test results using different characterization methods, including the variations of erosion damages with impingement angle and finally the effect of high temperature sintering. In the burner rig test, the TBC was exposed to a rapidly thermal cycling condition introduced by a high heat flux and high-velocity combustion torch. The TBC damages and failure modes were identified and explained in terms of microstructural observation and mechanism discussion. The composite phase c + t′ TBC demonstrated improved erosion resistance relative to a Gd2Zr2O7 TBC and equivalent thermal shock resistance to a conventional 8YSZ TBC.

Infiltration Behavior of CMAS in LZ-YSZ Composite Thermal Barrier Coatings

Calcium magnesium alumina-silicate (CMAS) corrosion on thermal barrier coatings (TBCs) is one of the reasons for the early delamination of TBCs. In this study, the effects of CMAS on the microstructure evolution of TBCs were investigated for yttria-stabilized zirconia (YSZ), lanthanum zirconate (LZ), and composite coating with a 50:50 volume ratio of LZ and YSZ (LZ-YSZ). The LZ showed the mitigated penetration of CMAS by the formation of the reaction products produced by the high reactivity between the LZ and the molten CMAS. The LZ-YSZ showed a denser microstructure on the surface, resulting in retarding the penetration of CMAS. Therefore, the LZ-YSZ composite TBC is expected to prolong the lifetime performance and protect against further corrosive degradation of TBCs in high-temperature environments. The infiltration behavior and reaction mechanism of molten CMAS were discussed based on the microstructures after the corrosion test.

Adhesion behavior of calcia–magnesia–alumino–silicates on gadolinia-yttria-stabilized zirconia composite thermal barrier coatings

Abstract

Thermal resistance determination of Sm2Zr2O7 + 8YSZ composite type of TBC

The determination of thermal resistance related to splat boundaries in composite TBC (thermal barrier coating) systems and expressed as splat-to-splat heat transfer coefficient was analyzed. Sm2Zr2O7 and conventional 8YSZ powders were used for coatings deposition by atmospheric plasma spraying. Two types of internal morphology were deposited: monolayered type (Sm2Zr2O7, 8YSZ) and composite TBCs (Sm2Zr2O7 + 8YSZ with weight ratios of 75/25, 50/50 and 25/75). The primary investigation was related to microstructural characterization of ceramic layers (pores and cracks architecture description), thermal diffusivity and conductivity characterization by laser flash analysis and theoretical models. The thermal conductivity was initially calculated on the basis of thermal diffusivity, heat capacity and density measurement as a function of temperature. Those values were corrected due to the presence of pores (microstructural analysis) and effective vales of thermal conductivity were calculated. These values were used as the basis for additional calculations based on the Hasselman-Johnson model of thermal conductivity for composite materials. Based on quantitative data from image analysis and porosity corrected thermal conductivity for monolayered and composite TBC systems, the thermal boundary conductivity (splat-to-splat heat transfer coefficient) was calculated as a function of temperature for Sm2Zr2O7 + 8YSZ composite systems.

Experimental study of the microscopic characteristics of Ti-Based Laser Cladding Ti＋7YSZ composite thermal barrier coatings

Ti/7YSZ composite coating system was prepared on the surface of TC4 titanium alloy by laser cladding using YLS-3000 fiber laser system, The macroscopic feature, microstructure, element distribution and hardness distribution of Ti-Based Laser Cladding Ti＋7YSZ composite thermal barrier coatings were characterized by optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). The results showed that Ti-based composite thermal barrier coating system changes the microstructure from top to bottom at different laser scanning speeds. It is mainly composed of acicular α’ martensite, martensite core, α phase bundle and residual β phase, which has the characteristics of “Fine up and down, coarse in the middle”. Due to surface tension, buoyancy, molten pool agitation and self-density. Zr element is enriched in the binding zone, and its signal intensity level increases first and then decreases as the scanning speed increases. Due to the dislocation slip resistance between the martensite and the solid solution strengthening effect of the Zr forming replacement solid solution, the hardness of the cladding zone is significantly higher than that of the structure, which is 2.5 times that of the TC4 structure.

Composition, mechanical properties and thermal cycling performance of YSZ toughened La2Ce2O7 composite thermal barrier coatings

Poor fracture toughness of La2Ce2O7 (LC) seriously prevents its practical applications as thermal barrier coatings. In this study, 8 wt% yttria stabilized zirconia (YSZ) was introduced to toughen the plasma-sprayed LC coatings. The mechanical performance, thermophysical properties and thermal shock resistance of the coatings were improved significantly, although a solid solution reaction between LC and YSZ occurred during spraying. For instance, the toughness, adhesion and thermal conductivity at 1273 K for 20 mol% YSZ doped LC coating achieved ~1.25 MPa·m1/2, ~60 MPa and ~0.49 W/(m·K), respectively. Moreover, the thermal contraction was suppressed effectively. The enhanced toughness could be attributed to the increased fracture energy resulted from the reaction of LC with YSZ. The chipping spallation, caused by the high thermal stress as well as low fracture toughness, appeared within the ceramic top coat near the bond coat during cycling. These findings lay a theoretical foundation for the toughening of brittle coatings.

Durable aluminate toughened zirconate composite thermal barrier coating (TBC) materials for high temperature operation

AbstractResearch on advanced thermal barrier coating (TBC) materials capable of operating beyond 1200°C has primarily focused on the rare earth zirconate pyrochlores, particularly gadolinium zirconate (Gd2Zr2O7 – GZO). The drawback of this material is a significant reduction in durability due to a low fracture toughness. This study investigates utilization of a thermodynamically compatible gadolinia alumina perovskite (GdAlO3 – GAP) toughening phase to improve the durability of GZO. Dense pellets were fabricated to assess the material properties with minimal microstructural influence. Thermal stability, elastic modulus, hardness, indentation fracture resistance and erosion durability were evaluated for GZO, GAP, and composite pellets containing 10, 30, and 50 wt.% GAP. It was demonstrated that GAP and GZO are thermodynamically compatible through 1600°C and thus capable of operating well beyond the limits of traditional 7 wt.% yttria stabilized zirconia (YSZ). Grain sizes are maintained due to a lack of diffusion, and thus microstructural stability is enhanced. The GAP fracture toughness was shown to be over 2X that of GZO while exhibiting a lower elastic modulus and similar hardness. The 50:50 GZO‐GAP composite exhibited a 63% reduction in the absolute erosion rate, demonstrating the immense toughening capabilities of this system. The implications for composite TBCs utilizing this system are discussed, along with future work.

Mechanical properties of lanthanum zirconate-based composite thermal barrier coatings

ABSTRACTLanthanum zirconate is a promising candidate material for thermal barrier coating (TBC) applications due to its low thermal conductivity and high temperature phase stability. However, its application is limited by thermal durability caused by low fracture toughness and low coefficient of thermal expansion. We recently developed LZ/8YSZ composite TBC systems using blended LZ and 8YSZ powders, which have demonstrated excellent thermal cycling performance. In this study, the mechanical properties of the composite TBCs were characterised using both nanoindentation and Vicker’s microhardness tests. The nanoindentation results show that both Young’s modulus and nanohardness increase with increasing 8YSZ content, suggesting the mechanical properties can be tailored by changing the volume ratio of 8YSZ. The ratios of Young’s modulus to nanohardness remain constant, ∼18, irrespective to the coating’s composition. The microhardness results show the same dependence with 8YSZ content, which is confirmed by the analytic models based on composite theory.

Tailored microstructures of gadolinium zirconate/YSZ multi-layered thermal barrier coatings produced by suspension plasma spray: Durability and erosion testing

This work employed an axial suspension plasma spray (SPS) process to deposit two different gadolinium zirconate (GZ) based triple layered thermal barrier coatings (TBCs). The first was a ‘layered’ TBC (GZ dense/GZ/YSZ) where the base layer was YSZ, intermediate layer was a relatively porous GZ and the top layer was a relatively dense GZ. The second triple layered TBC was a ‘composite’ TBC (GZ dense/GZ + YSZ/YSZ) comprising of an YSZ base layer, a GZ + YSZ intermediate layer and a dense GZ top layer. The as sprayed TBCs (layered and composite) were characterized using SEM/EDS and XRD. Two different methods (water intrusion and image analysis) were used to measure the porosity content of the as sprayed TBCs. Fracture toughness measurements were made on the intermediate layers (GZ + YSZ layer of the composite TBC and porous GZ layer of the layered TBC respectively) using micro indentation tests. The GZ + YSZ layer in the composite TBC was shown to have a slightly higher fracture toughness than the relatively porous GZ layer in the layered TBC. Erosion performance of the as sprayed TBCs was evaluated at room temperature where the composite TBC showed higher erosion resistance than the layered TBC. However, in the burner rig test conducted at 1400 °C, the layered TBC showed higher thermal cyclic lifetime than the composite TBC. Failure analysis of the thermally cycled and eroded TBCs was performed using SEM and XRD.