Showing papers on "Thermal barrier coating published in 2016"

Current Thoughts on Reactive Element Effects in Alumina-Forming Systems: In Memory of John Stringer

Abstract: In memory of John Stringer (1934–2014), one of the leaders in studying the reactive element (RE) effects, this paper reviews the current status of understanding of the effect of RE dopants on high-temperature oxidation behavior, with an emphasis on recent research related to deploying alumina-forming alloys and coatings with optimal performance in commercial systems. In addition to the well-known interaction between indigenous sulfur and RE additions, effects have been observed with C, N, and O found in commercial alloys and coatings. While there are many similarities between alumina-forming alloys and coatings, the latter bring additional complicating factors such as the effects of O incorporation during thermal spraying MCrAlY coatings, coating roughness, and heat treatments that must be considered in optimizing the beneficial dopant addition. Analogies can be seen between RE effects in alloys and in the substrates beneath diffusion M–Al coatings. Recently, there has been more interest in the influence of mixed oxidant environments, since these may modify the manifestation of the RE effect. Finally, some thoughts are provided on optimizing the RE benefit and modeling oxidation of RE-doped alloys.

Theoretical and experimental determination of the major thermo-mechanical properties of RE2SiO5 (RE = Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y) for environmental and thermal barrier coating applications

Abstract: X2-RE 2 SiO 5 orthosilicates are promising candidate environmental/thermal barrier coating (ETBC) materials for silicon-based ceramics because of their excellent durability in high-temperature environments and potential low thermal conductivities. We herein present the mechanical and thermal properties of X2-RE 2 SiO 5 orthosilicates based on theoretical explorations of their elastic stiffness and thermal conductivity, and experimental evaluations of the macroscopic performances of dense specimens from room to high temperatures. Mechanical and thermal properties may be grouped into two: those that are sensitive to the rare-earth (RE) species, including flexural strength, elastic modulus, and thermal shock resistance, and those that are less sensitive to the RE species, including thermal conductivity, thermal expansion coefficient, and brittle-to-ductile transition temperature (BDTT). The orthosilicates show excellent elastic stiffness at high temperatures, high BDTTs, very low experimental thermal conductivities, and compatible thermal expansion coefficients. The reported information provides important material selection and optimization guidelines for X2-RE 2 SiO 5 as ETBC candidates.

Modeling of thermal properties and failure of thermal barrier coatings with the use of finite element methods: A review

Abstract: To understand the thermal insulation and failure problems of the thermal barrier coatings (TBCs) deeply is vital to evaluate the reliability and durability of the TBCs. Actually, experimental methods can not reflect the real case of the TBCs during its fabrication and service process. Finite element modeling (FEM) play an important role in studying these problems. Especially, FEM is very effective in calculating the thermal insulation and the fracture failure problems of the TBCs. In this paper, the research progress of the FEM on the study of the thermal insulation and associated failure problems of the TBCs has been reviewed. Firstly, from the aspect of the investigation of the heat insulation of the TBCs, the thermal analysis via FEM is widely used. The effective thermal conductivity, insulation temperature at different temperatures of the coating surface considering the thermal conduct, convection between the coating and the environment, heat radiation at high temperature and interfacial thermal resistance effect can be calculated by FEM. Secondly, the residual stress which is induced in the process of plasma spraying or caused by the thermal expansion coefficient mismatch between the coating and substrate and the temperature gradient variation under the actual service conditions can be also calculated via FEM. The solution method is based on the thermal–mechanical coupled technique. Thirdly, the failure problems of the TBCs under the actual service conditions can be calculated or simulated via FEM. The basic thought is using the fracture mechanic method. Previous investigation focused on the location of the maximum residual stress and try to find the possible failure positions of the TBCs, and to predict the possible failure modes of the TBCs. It belonged to static analysis. With the development of the FEM techniques, the virtual crack closure technique (VCCT), extended finite element method (XFEM) and cohesive zone model (CZM) have been used to simulate the crack propagation behavior of the TBCs. The failure patterns of the TBCs can be monitored timely and dynamically using these methods and the life prediction of the TBCs under the actual service conditions is expected to be realized eventually.

Finite element simulation of residual stress and failure mechanism in plasma sprayed thermal barrier coatings using actual microstructure as the representative volume

Abstract: The residual stress and failure mode of thermal barrier coating (TBC) containing metallic bond coat (BC) and ceramic top coat (TC) with and without thermally grown oxide (TGO) were predicted using a micromechanical-based finite element method (FEM). Actual microstructures of the TBC taken by a scanning electron microscope (SEM) were utilized as the representative volume elements (RVEs) in the computational model. Failure mode of the representative volume was numerically simulated as thermal stress localization during thermal cycle. Computations were done on the representative volume to quantitatively assess the effects of thermal and mechanical properties of the TBC constituents as well as the presence of TGO on the macroscopic mechanical response of the TBC. Comparisons of computed results with experiments verified that, the computational method can successfully predict residual stress and crack initiation mode of the studied thermal barrier coatings. Moreover, based on the computed results, both shear and normal failure mode occur in the thermal barrier coating which is in good agreement with experimental findings.

Columnar suspension plasma sprayed coating microstructural control for thermal barrier coating application

Abstract: Suspension plasma spraying (SPS) is used to perform enhanced YSZ coating with columnar microstructure for thermal barrier coating (TBC) applications. By combination of plasma flow, substrate preparation, suspension formulation and injection or coating kinematic management it is possible to tune SPS coating structure from widely-separated columns to a significantly more compact columnar structure. Among these parameters, substrate roughness control, combined with an adapted coating growth velocity, are identified as the most relevant. An analytical approach is presented to describe columns growth based on coating image analysis. It allows to give the expression of the lateral and normal growth speeds responsible of the columnar structure.

Gas-deposition mechanisms of 7YSZ coating based on plasma spray-physical vapor deposition

Abstract: Variable microstructures of ZrO2-7 wt.% Y2O3 (7YSZ) coating can be achieved by plasma spray-physical vapor deposition (PS-PVD). Based on gas-phase prepared 7YSZ coating, the deposition mechanisms and their dependency on process conditions were investigated. When the spray distance was 950 mm, heterogeneous nucleation occurred on the surface of substrate. Porous oriented columnar coating could be obtained on the nickel-based superalloy at a substrate temperature of 850 °C. As the substrate temperature decreases to room temperature, porous columnar coating tends to be dense coating that is mainly made of randomly oriented fine grains. But when the substrate was replaced by graphite, columnar coating was observed again. As the spray distance increases from 950 to 2200 mm, homogenous nucleation occurs at the end of plasma plume leading to formation of cluster agglomerated by nano-sized grains. Finally, fine grain constituted coating was obtained on substrate at room temperature regardless of substrate materials.

Effect of filler arrangement and networking of hexagonal boron nitride on the conductivity of new thermal management polymeric composites

Abstract: Polymeric composites have the potential to replace current electronic packaging and improve thermal management of electronics to open the way for further size shrinkage. Such composites need to provide high thermal conductivity and high electrical resistivity while possessing suitable mechanical properties. This paper reports the study of thermal behavior of arranged and random network of hexagonal boron nitride (hBN) in composite with different polymers. Thermal barrier factors and possible solutions to reduce the effect of these factors are thoroughly investigated. Thermal behavior of hBN filler network in composite with different polymers as well as the possibility of using green (bio-based) polymers as the matrix have been studied. Fully bio-based, partially bio-based and petroleum based composites are compared to investigate the effect of different polymer matrices on thermal properties of composites. Designed high thermally conductive and electrically insulative composites are expected to possess unique properties. This includes being lightweight, injection moldable, low coefficients of thermal expansion, eco-friendly, and with proper service temperature and mechanical properties. Such composites will result in more compact, lighter and cheaper electronics with higher performance, by avoiding the use of electrical fans, heat sinks and other methods of heat dissipation.

SiC/ZrB2–SiC–ZrC multilayer coating for carbon/carbon composites against ablation

Abstract: To improve the ablation resistance of carbon/carbon (C/C) composites at high temperature, a ZrB2–SiC–ZrC coating was prepared on the surface of SiC-coated C/C composites by supersonic atmosphere plasma spray (SAPS). Ablation resistance of the coated C/C composites was tested in an oxyacetylene torch environment with a heat flux of 2400 kW/m2 for 120, 150 and 200 s, respectively. After ablation for 120 s, the coating exhibited a three-layered structure consisting of a porous ZrO2 layer, a ZrO2–SiO2 thin layer and a SiC-depleted layer, whereas it transformed into a two-layered structure (ZrO2 recrystallized layer and ZrO2–SiO2 layer) after ablation for 150 s. With the increase of ablation time, the SiC inner coating was almost consumed and a gap emerged between the ZrO2 layer and the C/C matrix after ablation for 200 s. The good ablation resistance of the sprayed coating is mainly attributed to the layered structure acting as a thermal barrier and inhibiting inward diffusion of oxygen.

Production and characterisation of GZ/CYSZ alternative thermal barrier coatings with multilayered and functionally graded designs

Abstract: In this study multilayered and functionally graded Gd2Zr2O7/CYSZ thermal barrier coatings were designed and produced in 2,4,8, and 12 layers utilizing HVOF and APS processes to improve the thermal cycling performance of single layered Gd2Zr2O7. Microstructural, mechanical, and thermal properties of the coatings were investigated. The phase structure of the Gd2Zr2O7 was stable even after the coating and thermal cycling tests made at 1250 °C. The thermal conductivities and bonding strengths were between in 0.64–0.89 W/mK and 8.87–12.10 MPa, respectively. As far as thermal cycling performance of the designs are concerned while big spallation was observed (74%) in the single layered Gd2Zr2O7 coating after 160 cycles a significant improvement had been observed in cases of multilayered coatings and no failure was seen in functionally graded coatings even after 300 cycles. The results indicated that the thermal cycling performance of Gd2Zr2O7 coating was improved thanks to multilayered and especially functionally graded designs.

Process-structure-property relations for the erosion durability of plasma spray-physical vapor deposition (PS-PVD) thermal barrier coatings

Abstract: New thermal barrier coating (TBC) materials and microstructures are under development to increase gas turbine operating temperatures beyond the ~ 1200 °C threshold of standard 7 wt.% yttria stabilized zirconia (7YSZ). To deposit these advanced coatings, a new thermal spray deposition technique is used: Plasma Spray – Physical Vapor Deposition (PS-PVD). PS-PVD is capable of depositing from the vapor phase to yield strain tolerant columnar microstructures similar to Electron Beam – Physical Vapor Deposition (EB-PVD) or, alternatively, the traditional splat-like lamellar microstructure common to Air Plasma Spray (APS). This study investigates the process-structure relationships and resulting erosion response for plasma gas flow, amperage, and feed rate. It was found that in the selected design space, porosity and surface roughness vary from ~ 12–26% and ~ 5–10 μm, respectively. Erosion behavior is discussed and the mechanism is identified to be heavily dependent upon the intercolumnar spacing. The lowest erosion rates are similar to EB-PVD, while the highest erosion rates were closer to APS. This is attributed to the hybrid nature of the PS-PVD process and provides an opportunity to tailor coatings with a wide range of properties, and thus performance.

A novel low-thermal-conductivity plasma-sprayed thermal barrier coating controlled by large pores

Abstract: In order to reduce the thermal conductivity of thermal barrier coatings (TBCs) for land-based gas turbine applications, porous TBC (P-TBC) with a microstructure consisting of a distribution of large open pores was developed. A powder mixture of zirconia and polyester was used to form such open pores within the microstructure. The results showed that the thermal conductivity of the P-TBC monotonically decreased as the porosity increased. In addition, compared with conventional TBC with a thermal conductivity value of 1.2 W/(mK), the thermal conductivity of P-TBC with a polyester content 15 wt.% achieved a value as low as 0.3 W/(mK). This implies that the presence of large open pores leads to a dramatic reduction in the thermal conductivity, and the thermal conductivity can be freely controlled by adjusting the polyester content of the powder. A SEM-image-based finite element model was constructed. The FE analysis revealed that the presence of pores disturbed the heat flow. Thus, compared with the relatively straight lines that occurred for the TBC, which possessed few pores, it was considered that the presence of open pores within the P-TBC increased the length of the heat flow lines. This provides an explanation for the reduction in the apparent thermal conductivity of the P-TBC.

Thermal Cycling Behavior of Thermal Barrier Coatings with MCrAlY Bond Coat Irradiated by High-Current Pulsed Electron Beam.

Abstract: Microstructural modifications of a thermally sprayed MCrAlY bond coat subjected to high-current pulsed electron beam (HCPEB) and their relationships with thermal cycling behavior of thermal barrier coatings (TBCs) were investigated. Microstructural observations revealed that the rough surface of air plasma spraying (APS) samples was significantly remelted and replaced by many interconnected bulged nodules after HCPEB irradiation. Meanwhile, the parallel columnar grains with growth direction perpendicular to the coating surface were observed inside these bulged nodules. Substantial Y-rich Al2O3 bubbles and varieties of nanocrystallines were distributed evenly on the top of the modified layer. A physical model was proposed to describe the evaporation–condensation mechanism taking place at the irradiated surface for generating such surface morphologies. The results of thermal cycling test showed that HCPEB-TBCs presented higher thermal cycling resistance, the spalling area of which after 200 cycles accounted...

Influence of cracks in APS-TBCs on stress around TGO during thermal cycling: A numerical simulation study

Abstract: The failure of thermal barrier coatings (TBCs) fabricated by atmospheric plasma spraying (APS) during thermal cycling is often attributed to the accumulation of the thermal stress. However, in the end it comes down to the growth of thermally grown oxide (TGO) and propagation of the cracks around the TGO. Based on the 8 wt.% yttria stabilized zirconia (YSZ) TBCs fabricated by APS (APS-TBCs), the geometrical shape of the TGO layer which is located between the bond-coat (BC) and top-coat (TC) was simplified as sinusoidal curve with a certain amplitude and wavelength. The influence of the horizontal crack and vertical crack in the ceramic layer on the stress around the TGO layer during thermal cycling has been calculated using finite element method. The investigation results indicate that the stress concentration in the TBCs is dependent on the location of the vertical and horizontal crack. The vertical crack can partially release the stress concentration around the TGO layer compared with the horizontal crack if the crack is located above the peak of the TGO layer. When there is horizontal crack, the maximum tensile stress is located at the peak of the TGO/BC interface, while the maximum compressive stress is located at the spinodal position at the inner of the TGO. When there is vertical crack, the stress concentration tends to appear near to the crack tip, and the influences of horizontal crack and vertical crack on the stress around the TGO are distinctly different. In addition, the location of the vertical crack also has a certain influence on the distribution of the maximum stress. The vertical crack and the TGO have mutual effect on the stress concentration of each other. As for the horizontal crack, if it is located below the peak of the TGO layer (at the inner of the top-coat and near to the TGO layer), the crack tip has also exhibited stress concentration effect compared with the case that the horizontal crack is located above the peak of the TGO layer. The fracture mechanic parameters of the crack which may propagate, the propagation patterns of single crack and failure mechanisms of the APS-TBCs have also been calculated and discussed in this paper.

Isothermal Oxidation Behavior of Gd2Zr2O7/YSZ Multilayered Thermal Barrier Coatings

Abstract: Yttria stabilized zirconia (YSZ) is the state of the art ceramic top coat material used for TBC applications. The desire to achieve a higher engine efficiency of agas turbine engine by increasing the turbine inlet temperature has pushed YSZ toits upper limit. Above 1200°C, issues such as poor phase stability, high sinteringrates, and susceptibility to CMAS (calcium magnesium alumino silicates) degradation have been reported for YSZ based TBCs. Among the new materials,gadolinium zirconate (GZ) is an interesting alternative since it has shown attractive properties including resistance to CMAS attack. However, GZ has a poor thermo-chemical compatibility with the thermally grown oxide leading to poor thermal cyclic performance of GZ TBCs and that is why a multi-layered coating design seems feasible.This work presents a new approach of depositing GZ/YSZ multi-layered TBCs by the suspension plasma spray (SPS) process. Single layer YSZ TBCs were also deposited by SPS and used as a reference.The primary aim of the work was to compare the thermal conductivity and thermal cyclic life of the two coating designs. Thermal diffusivity of the YSZ single layer and GZ based multi-layered TBCs was measured using laser flash analysis (LFA). Thermal cyclic life of as sprayed coatings was evaluated at 1100°C, 1200°C and 1300°C respectively. It was shown that GZ based multi-layered TBCs had a lower thermal conductivity and higher thermal cyclic life compared to the single layer YSZ at all test temperatures. The second aim was to investigate the isothermal oxidation behaviour and erosion resistance of the two coating designs. The as sprayed TBCs were subjected toisothermal oxidation test at 1150°C. The GZ based multi-layered TBCs showed a lower weight gain than the single layer YSZ TBC. However, in the erosion test,the GZ based TBCs showed lower erosion resistance compared to the YSZ singlelayer TBC. In this work, it was shown that SPS is a promising production technique and that GZ is a promising material for TBCs.