YSZ Thermal Barrier Coatings Research Articles

Thermal shock resistance of thermal barrier coatings modified by selective laser remelting and alloying techniques

AbstractAiming to improve the thermal shock resistance of thermal barrier coatings (TBCs), the plasma‐sprayed 7YSZ TBCs were modified by selective laser remelting and selective laser alloying, respectively, in this study. A self‐healing agent TiAl3 was introduced into the 7YSZ TBCs by selective laser alloying to fill cracks during thermal cycling. The thermal shock experiments of the plasma‐sprayed, laser‐remelted, and laser‐alloyed TBCs were conducted by a means of heating and water‐quenching method. Results revealed that some segmented microcracks were distributed on the surface of the laser‐remelted and the laser‐alloyed zones, showing a dense columnar crystal structure. After thermal shock tests, the numbers of segmented microcracks on the laser‐remelted coating increased, whereas, in the laser‐alloyed condition, some irregular particles formed, leading to the decreased numbers of segmented microcracks. The laser‐alloyed coating exhibited the best thermal shock resistance, followed by the laser‐remelted condition, with the thermal shock lifetime 3.3 and 2.7 times higher than that of the as‐sprayed coating, respectively. On the one hand, both columnar grains and segmented microcracks in the laser‐treated zone could effectively improve the strain tolerance of coatings. On the other hand, the oxidation products of TiAl3 under high‐temperature condition could seal the microcracks to postpone the crack connection. Thus, the thermal shock resistance of the laser‐treated coatings was significantly improved.

Al-modification for PS-PVD 7YSZ TBCs to improve particle erosion and thermal cycle performances

Plasma spray-physical vapor deposition (PS-PVD) as a novel process was used to prepare feather-like columnar thermal barrier coatings (TBCs). This special microstructure shows good strain tolerance and non-line-of-sight (NLOS) deposition, giving great potential application in aero-engine. However, due to serious service environment of aero-engine, particle erosion performance is a weakness for PS-PVD 7YSZ TBCs. As a solution, an Al-modification approach was proposed in this investigation. Through in-situ reaction of Al and ZrO2, an α-Al2O3 overlay can be formed on the surface of 7YSZ columnar coating. The results demonstrate that this approach can improve particle erosion resistance since hardness improvement of Al-modified TBCs. Meanwhile, as another important performance of thermal cycle, it has a better optimization with 350-cycle water-quenching, compared with the as-sprayed TBCs.

The Effect of Bond Coat Roughness on the CMAS Hot Corrosion Resistance of EB-PVD Thermal Barrier Coatings

In a high-temperature, high-flame-velocity, and high-pressure gas corrosion environment, the intercolumnar pores and gaps of electron beam–physical vapor deposition (EB-PVD) thermal barrier coatings (TBCs) may serve as infiltration channels for molten calcium–magnesium–alumino–silicate (CMAS), leading to the severe degradation of TBCs. In order to clarify the relationship between the roughness of the bond coat and the CMAS corrosion resistance of the EB-PVD TBCs, 7 wt.% yttria-stabilized zirconia (7YSZ) TBCs were prepared on the surfaces of four different roughness-treated bond coats. The effect of the bond coat roughness on the columnar microstructure of the EB-PVD YSZ was investigated. The effect of the change of the bond coat’s microstructure on the CMAS corrosion resistance of the EB-PVD YSZ was studied in detail. The results showed that the reduction in the roughness of the bond coat contributes to the improved formation of the EB-PVD YSZ columns. The small and dense columns are similar to a lotus leaf-like structure, which could reduce the wettability of CMAS and minimize the spread area between the coating and the CMAS melt. Thus, the CMAS corrosion resistance of the coating can be greatly improved. This preparation process also provides a reference for the preparation of other TBC materials, improving the resistance to CMAS hot corrosion.

Effect of irregular microcracks on the hot corrosion behavior and thermal shock resistance of YSZ thermal barrier coatings

The durability and service performance of thermal barrier coatings (TBCs) is influenced by its microstructure. One type of yttria stabilized zirconia coating containing irregular microcracks (mc-YSZ) was prepared by atmospheric plasma spraying (APS) method in this study, and the differences in hot corrosion behavior and thermal shock resistance between the conventional YSZ coating and the mc-YSZ coating were investigated. Results showed that the presence of irregular microcracks facilitated the penetration of corrosive salt (V2O5 and Na2SO4) and accelerated the hot corrosion reaction. The deflection effect of irregular microcracks in the early stage of hot corrosion could suppress the propagation of cracks generated by corrosion; but irregular microcracks would merge with corrosion cracks in the later stages of corrosion and aggravate the failure of the coatings. Thermal shock tests showed no significant difference of the thermal shock lifetime between YSZ coating and mc-YSZ coating, which proved that this irregular microcrack could not effectively improve the strain tolerance. The present study provides a reference for the structure design of thermal barrier coatings: when designing the structure of the coating, the appearance of that irregular microcracks should be avoided.

Sc-doped Gd2Zr2O7 coating on YSZ thermal barrier coatings to resist CMAS + molten salt attack

Coupling corrosion resulting from calcium-magnesium-alumina-silicate (CMAS) and molten salt causes severe attack to YSZ thermal barrier coatings (TBCs). In this study, a Sc-doped Gd2Zr2O7 (GZO-Sc) coating is prepared on the YSZ coating to alleviate CMAS + NaVO3 (CN) corrosion. Results revealed that the GZO-Sc coating largely prevented CN (with a melting point of 1188 °C) attack to YSZ coatings at 1200 °C and 1250 °C. At the test temperatures, CN melted and penetrated the GZO-Sc coating, during which the melt and the coating reacted to form a dense reaction layer, consisting of Gd8Ca2Si6O26-based apatite and c-ZrO2 based fluorite phase independent on the temperature and time. With the increase of the dwell time, V was separated from CN melt during infiltration, causing many CaAl2Si2O8 crystals at the bottom of the reaction layer and a V-rich layer beneath the reaction layer, which were beneficial to the CN corrosion resistance of the coatings.

Fracture analysis of Pre-cracked 8YSZ TBCs with edge and internal cracks under Thermo-mechanical load: A numerical approach

The thermal barrier coatings (TBCs) are used for hot component sections of gas turbines and aero-engines. The paper presents the effect of thermo-mechanical on the pre-cracked TBCs to analyze fracture parameters using the FEM method. The TBC model is considered with edge and internal cracks with different crack configurations such as crack length and cracks position between ceramic top coat and bond coat to find the strain energy release rate (SERR) and stress intensity factor (SIF). The TBC mixed-mode I/II fracture criterion is established. The numerical results showed that the crack tip field is shear dominant under thermo-mechanical loading irrespective of different crack configurations. The edge crack's SERR and SIF mode I/II are more significant than the internal crack, resulting in delamination. The long edge crack showed crack driving force of 2.13 J/m2 at P1 position, whereas the shift in crack position till P7 reduces the SERR to 1.48 J/m2. The internal crack at P1 showed the SERR of 0.91 J/m2 which reduces to 0.85 J/m2 at position P7. The edge crack resulted in a high amount of shear stress which is caused due to large opening of crack faces. In all crack configurations, the increase in crack length showed an increase in crack driving force and large opening of crack faces due to high shear stress. Also, the crack positioned in the ceramic topcoat showed a high value of crack driving force, resulting in more probability of surface rupture followed by crack positioned at the interface of the topcoat/bond coat and crack positioned in the bond coat. The interface crack showed a high crack driving force compared to crack located near the interface due to bi-material and geometry discontinuity, resulting in stress intensification. The edge crack showed a high value of SIF mode I in the range of 0.1 to 0.35 MPa.m1/2 compared to internal crack which shows in the range of 0.05 to 0.18 MPa.m1/2. The edge crack shows high value of SIF mode II in the range of −0.05 to −0.22 MPa.m1/2 compared to internal cracks showing upto −0.1 MPa.m1/2. The crack length and position play an important role in crack initiation and propagation. The TBC structure with edge crack appears unstable and more likely to damage than internal crack, leading to an early failure mechanism.

Effect of Eu3+ Doping on Thermal Conductivity and Cyclic Thermal Shock Behaviour of Plasma-Sprayed Ysz Thermal Barrier Coatings

Effect of Eu3+ Doping on Thermal Conductivity and Cyclic Thermal Shock Behaviour of Plasma-Sprayed Ysz Thermal Barrier Coatings

Hot corrosion behaviors of as-sprayed and laser-remelted YSZ thermal barrier coatings at 950 °C

Hot corrosion behaviors of as-sprayed and laser-remelted YSZ thermal barrier coatings at 950 °C

Evolution of the Microstructure and Mechanisms of Performance Degradation in Eb-Pvd Ysz Thermal Barrier Coatings Corroded by Volcanic Ash at 1150°C

Evolution of the Microstructure and Mechanisms of Performance Degradation in Eb-Pvd Ysz Thermal Barrier Coatings Corroded by Volcanic Ash at 1150°C

Sintering governing the cracking behaviors of different La2Zr2O7/YSZ ceramic layer combination TBCs at 1150 °C

Sintering behavior is one of the most important indicators correlated to the service behaviors for thermal barrier coatings (TBCs). 5 different La2Zr2O7(LZO)/YSZ double ceramic layer (DCL) combination TBCs are subjected to gradient thermal cyclic tests under the surface temperature of 1150 °C, aiming to elucidate the service behaviors of the DCL TBCs under relative low temperature. The failure of DCL TBCs is the cracking of top coat along in-plane direction, the 8YSZ TBCs showed an vertical cracking after 2200 cycles, and the stress state transferred from tensile about 250 MPa to compressive of −60 MPa. The pure 8YSZ is superior to the DCL TBCs for the long-term application at relative low temperature. Sintering induced the competition of cracking driving force and resistance dominates the cracking of TBCs, the formation of vertical cracks released the stress of the 8Y TBCs system to a large extent and the lifetime was extended.

Analytical Model of the Process of Thermal Barrier Coating by the MO CVD Method

Integral regularities in the growth of 7YSZ thermal barrier coatings during MO CVD (Metal–Organic Chemical Vapor Deposition) are proposed. Within the framework of the model of the reacting boundary layer, the coating deposition process is considered as a process of independent global reactions of diffusion combustion of Zr(dpm)4 and Y(dpm)3 under convection conditions on a permeable surface. The rate of coating growth and the efficiency of using a precursor are analytically evaluated. The correctness of the proposed approach is confirmed by comparison with known experimental data. The considered model can be used to analyze the deposition of coatings from various mixtures of precursors, such as Nd(dpm)3, Hf(dpm)4, and Sm(dpm)3.

CMAS corrosion behavior of Sc doped Gd2Zr2O7 /YSZ thermal barrier coatings and their corrosion resistance mechanisms

YSZ thermal barrier coatings (TBCs) are vulnerable to calcium-magnesium-alumina-silicate (CMAS) attack, and a CMAS resistant TBC of Sc doped Gd2Zr2O7/YSZ are developed in this study. At 1250 ℃, corrosion products form a reaction layer at the CMAS/coating interface, which largely suppresses CMAS further infiltration. Compared with Gd2Zr2O7, Sc doped Gd2Zr2O7 has a higher ability to keep its lattice structure in the presence of CMAS, which is beneficial to maintain the coating microstructure under CMAS conditions, alleviating the coating degradation. First-principles calculation was conducted to investigate the corrosion products formation mechanisms, and the characteristics of CMAS/reaction layer and reaction layer/coating interfaces.

Microstructural and columnar growth characteristics of 7YSZ thermal barrier coatings fabricated by plasma spray physical vapor deposition

Thermal barrier coating (TBC) applications of 7 wt% yttria stabilized zirconia (7YSZ) must meet low thermal conductivity and excellent high temperature stability. However, the resulting morphology of 7YSZ TBCs fabricated by PS-PVD currently remains poorly studied. The present study addresses this issue by evaluating the microstructure, phase composition, grain distribution, and columnar crystal orientations of 7YSZ coatings fabricated by PS-PVD. The results demonstrate that the growth of YSZ columnar crystals exhibits an obvious preferred orientation [101] that is consistent throughout the entire coating cross-sectional area. The contents of tetragonal phase in the coating is 65.8 %, and the phase has a strongly preferred growth tendency toward <001> . The nano-indentation test shows that the microhardness of the coating increases gradually from bottom to top. In addition, the observed characteristics of 7YSZ particle are evaluated based on the results of numerical simulations of the temperature, velocity, and pressure fields in the plasma jet.

BEHAVIOR OF APSED TBCS SUBJECTED TO COOLING MEDIA IN THERMAL CYCLE SYSTEMS

In this study, first, yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) with CoNiCrAlY bond coats both were fabricated on IN718 superalloy substrate, using air plasma spraying (APS) techniques. In order to understand the actual microstructural morphological behavior, phase stability, and thermal cycling behaviors of 8YSZ TBCs were characterized by field emission spectroscopy equipped scanning electron microscopy (FESEM), differential scanning calorimetry (DSC) and thermal cycling test. DSC results confirmed the basic reason of selecting the high cycling temperature. The crack initiation and propagation under thermal stresses due to temperature differences from maximum temperature of 1100∘C to cooling medium; here, air cooling and water quenching are used. Arising from either edges or corners it was observed serious problem which is restraining thermal cycling lifetime of TBCs, indicating failure mechanism is independent from operating parameters of thermal cycling test. However, in comparison to water-cooled thermal fatigue life, air-cooled thermal fatigue lifetime was 2.36 times better.

Numerical and Experimental Research on Thermal Insulation Performance of Marine Diesel Engine Piston Based on YSZ Thermal Barrier Coating

Although YSZ ceramic coating has been used in the field of aeroengines for a long time to protect blades from high temperature erosion, its application on marine engines is still very rare. In this study, YSZ powder was sprayed onto the upper surface of the Al-Si alloy piston by atmospheric plasma spraying. The piston with or without ceramic coatings was applied to the diesel engine bench, and the ship propulsion characteristics test was carried out to study the effect of the coating on the performance of the diesel engine when the ship was sailing. The temperature field results show that under 25% load, the temperature of the top surface of the coated piston is about 30.91 °C higher than that of the conventional piston. The increase in the temperature of the combustion chamber is conducive to better combustion of the fuel in the cylinder of the diesel engine. Therefore, when the marine diesel engine is tested for propulsion characteristics, the thermal efficiency is increased by 5% under the condition of 25% load.

Effect of Spray Distance on the Microstructure and High Temperature Oxidation Resistance of Plasma Spray-Physical Vapor Deposition 7YSZ Thermal Barrier Coating

In order to study the effect of spray distance on the structure and high temperature oxidation resistance of feather-columnar thermal barrier coatings, the feather-columnar ZrO2-7wt. % Y2O3 (7YSZ) thermal barrier coatings were prepared at spray distances of 650 mm, 950 mm, 1100 mm, 1250 mm, and 1400 mm by plasma spray-physical vapor deposition (PS-PVD) technology. The surface roughness, micro morphology, and porosity of the sprayed 7YSZ coating were analyzed by 3D surface profiler, SEM, XRD, etc., and the impedance spectrum characteristics of the 7YSZ coating were characterized by electrochemical alternating current (AC) impedance technology. In addition, the high temperature oxidation resistance test of 7YSZ coating under different spray distances was carried out at a temperature of 1000 °C to study the influence of spray distance on the high temperature oxidation resistance of 7YSZ coating. The research results show that the surface roughness and porosity of feather-columnar 7YSZ coating increased sequentially with the increase of spray distance. At the same time, The YSZ grain boundary resistance value increased exponentially as the porosity of the coating increases. Where the spray distance was in the range of 650 mm and 1250 mm, the high temperature oxidation rate constant of the coating increased with the spray distance. However, the spray distance was greater than 1250 mm, and the spray distance had no significant effect on the high temperature oxidation resistance of the coating.

The effect of spray angle on the microstructural and mechanical properties of plasma sprayed8YSZ thermal barrier coatings

Thermal barrier coatings (TBCs) are favorable for better protection of gas turbines and aero engines at high temperatures. The TBCs were fabricated using NiCrAlY bond coat and 8% wt. yttria-stabilized zirconia (YSZ) topcoat onto the nickel-based superalloy Inconel 800 by atmospheric plasma spray. In this article, the investigation of microstructural and mechanical properties of 8YSZ TBCs with the effect of spray angle has been discussed. The microstructural and elemental analyses were conducted by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS). The porosity analysis was conducted based on SEM image analysis using a gray scale threshold. Mechanical properties such as coating hardness, surface roughness, and thickness are measured by indentation, surface profilometer, and optical microscopy. The result shows the effect of the spray angle over the coating surface in terms of pores and microcracks. The influence of the spray angle leads to different grain growth resulting in the shadow region. A large number of defects and a decrease in coating hardness were observed for 60 degrees pray angle compared to the 90 degrees pray angle. A large number of defects led to developing rough surfaces, resulting in low hardness and increased porosity. The experimental results showed that the plasma sprayed 8YSZ TBC with a 90 degree spray angle can improve the durability and performance of the TBCs, as it has better microstructural and mechanical properties.

Optimization of process parameters for thermal shock resistance and thermal barrier effects of 8YSZ thermal barrier coatings through orthogonal test

The thermal shock resistance and the thermal barrier effects are the key indices for thermal barrier coating (TBC) performances. The performance optimization of 7∼8 wt. % Y2O3 stabilized ZrO2 (7∼8YSZ) TBCs is a research focus at present. In this work, an orthogonal test was employed to study the relationship between parameters and performances and match process parameters for TBCs preparation. The scanning electron microscope was employed to analyze the microstructures of 8YSZ TBCs. The thermal shock resistance test and thermal barrier effect test were conducted to judge the performances of 8YSZ TBCs. It was found that the process factor had the greatest influences on porosity and elastic modulus was spraying distance and the factor had the greatest influence on bond strength was spraying power. The selected parameters were a spraying distance of 90 mm, an argon flow of 32 L/min, a hydrogen flow of 8 L/min, an electric current of 550 A, a spraying power of 35 kW, a feeding rate of 45 g/min, and a substrate preheating temperature of 373 K. The thermal shock life was ∼100 times under 1173∼298 K, and the thermal barrier temperature of optimized 8YSZ TBCs with thicknesses of 250 and 300 µm was ∼90 and ∼130 K under 1473 K, respectively. Through a comparative study, both thermal shock resistance and thermal barrier effects of 8YSZ TBCs were improved after process parameters’ optimization conducted by this method.

Model construction and effect of thermally grown oxide dynamic growth on distribution of thermal barrier coatings

A physical geometric model of the dynamic growth of thermally grown oxide (TGO) was established based on an analysis of the TGO growth of 8YSZ thermal barrier coatings during thermal cycling. Finite-element simulation was used to simulate the evolution law between the coating residual stress and thermal cycling, and the linear elasticity, creep effect, and stress accumulation in each thermal cycle were studied. The interface between the top coat (TC) and the bond coat (BC) was covered with a TGO layer that grew vertically and slowly in a layer-like manner. The stress in the TGO was distributed with a “layer” zonal gradient, and the TGO/BC boundaries were distributed uniformly with a large compressive stress, which decreased the TGO layer thickening. With the longitudinal rapid random TGO growth, the boundaries were subjected to a tensile stress, and a high tensile stress concentration area developed at the boundaries. The internal stress consisted of an alternating and mixed distribution of concentrated compressive and tensile stresses. The concentration area of the maximum equivalent stress was distributed in the one-layer TGO near the TC/TGO interface. When a microcrack formed at the TGO/BC boundaries, the crack was subjected to a tensile stress of different size, with a higher tensile stress at both ends, which facilitated crack expansion. Thus, the 8YSZ thermal barrier coating was prone to crack formation and expansion at the TGO/BC boundaries and in the TGO layer near the TC/TGO boundaries.

Investigation of calcium–magnesium-alumino-silicate (CMAS) resistance and hot corrosion behavior of YSZ and La2Zr2O7/YSZ thermal barrier coatings (TBCs) produced with CGDS method

Thermal barrier coatings (TBCs) commonly expose to oxidation, hot corrosion, and CaO-MgO-Al2O3-SiO2 (CMAS) attacks during the service condition. All of these attacks cause the spallation or delamination of TBCs from the bond coat. In this study, the effect of these attacks on YSZ and La2Zr2O7/YSZ TBC systems was investigated. YSZ and double layer La2Zr2O7/YSZ topcoats were deposited using electron beam physical technique (EB-PVD) on CoNiCrAlY bond coat produced by cold gas dynamic spray (CGDS) technique. CMAS tests at 1225 °C for 4, 8, 12, 16, 20, 24, 28, and 32 h and 5 h cyclic hot corrosion tests at 1000 °C were carried out on TBCs. Before and after the tests, TBCs were characterized using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), x-ray diffraction (XRD) and image analysis software program. At the end of the tests, the formed phases, microstructural changes, and general failure mechanisms were investigated in detail for each test. The general result shows that double-layered TBCs provide significant contributions and durability against high-temperature corrosive attacks of conventional YSZ TBC. In addition, in this study, hot corrosion damage mechanism and CMAS attack significantly damaged the column morphology of the EB-PVD method, reducing the thermal insulation properties and strain tolerance of TBCs. As a result of the tests, spallation and phase changes occurred in the damaged TBC systems.