CMAS corrosion resistance,thermal shock resistance and numerical simulation of novel surface micromesh thermal barrier coatings

Thermal barrier coatings (TBCs) are widely used as insulating layers to protect the underlying metallic structure of gas turbine blades. However, the thermal cycling performance of TBCs is affected by their complex working environments, which may shorten their service life. Previous studies have shown that preparing a mesh structure in the bonding layer can relieve thermal stress and improve the bonding strength, thereby prolonging the service life of TBCs. In this paper, a micromesh structure was prepared on the surface of the bonding layer via wet etching. The microstructure and failure mechanism of the micromesh TBCs after CMAS (CaO-MgO-Al₂O₃-SiO₂) thermal erosion were investigated. Numerical simulation was combined with thermal shock experiments to study the stress distribution of the micromesh-structured TBCs. The results showed that the circular convex structure can effectively improve the CMAS corrosion resistance and thermal shock resistance of TBCs.     

Evaluation of hot corrosion behavior of plasma sprayed ceria and yttria stabilized zirconia thermal barrier coatings in the presence of Na2SO4+V2O5 molten salt

In this study, substrates of Inconel 738 LC superalloy coupons were first sprayed with a NiCoCrAlY bondcoat and then with a ceria and yttria stabilized zirconia (CYSZ; ZrO₂−25 wt%CeO₂−2.5 wt%Y₂O₃) topcoat by air plasma spraying (APS). Hot corrosion studies of plasma sprayed thermal barrier coatings (TBCs) were conducted in 45 wt%Na₂SO₄+55 wt%V₂O₅ molten salt at 1000 °C for 30 h. The results showed that the coating defects, such as pores and microcracks play important roles as effective paths for the salt penetration in hot corrosion. Based on the results, the reaction between molten salt and stabilizers of zirconia (Y₂O₃ and CeO₂), the formation of YVO₄, CeVO₄ and CeO₂ crystals, the detrimental phase transformation of zirconia from tetragonal to monoclinic due to the depletion of stabilizers and finally, the creation of stresses were recognized to be in the degradation mechanism of CYSZ ceramic coatings in the presence of molten sulfate–vanadate salt.     

Lanthanum magnesium hexaluminate thermal barrier coatings with pre-implanted vertical microcracks: Thermal cycling lifetime and CMAS corrosion behaviour

Atmospheric plasma-sprayed (APS) coatings have a layered structure as well as lower strain tolerance and a shorter lifetime than EB-PVD coatings. In this study, TBCs composed of a LaMgAl₁₁O₁₉ (LMA) top coat and a NiCrAlY bond coat were prepared by APS coupled with dry-ice blasting to implant vertical microcracks in the top coat. The thermal cycling lifetime and CMAS corrosion behaviour of LMA-TBCs with pre-implanted vertical microcracks were investigated in detail. The results show that the LMA top coat possesses an improved proportion of vertical microcracks and that the corresponding TBC has an improved thermal cycling lifetime. The vertical microcracks in the top coats, which not only reduce the thermal stress but also improve the strain tolerance of TBCs, dramatically contribute to the improvement in the thermal cycling lifetime. Surprisingly, the CMAS corrosion resistance of LMA-type TBCs with implanted vertical microcracks is better than that of conventional TBCs with a typical layered structure.     

Effect of TGO thickness on the thermal barrier coatings life under thermal shock and thermal cycle loading

Effect of thermally grown oxide (TGO) thickness on thermal shock resistance of thermal barrier coatings (TBCs) and also their behavior under a cyclic loading (including aging at maximum temperature) was evaluated experimentally. In order to form different thicknesses of TGO, coated samples experience isothermal loading at 1070?°C for various periods of times. Heat-treated samples were heated to 1000?°C and cooled down rapidly in water from the substrate side using a mechanical fixture. The life of samples was investigated as a function of TGO thickness. Furthermore, by performing an experiment the simultaneous effect of the TGO growth and thermal expansion mismatch– on the failure of thermal barrier coatings was evaluated. The results demonstrated that the presence of TGO with a thickness of 2–3?µm has a positive effect on the resistance against thermal shock.     

Thermal cycling and hot corrosion behavior of a novel LaPO4/YSZ double-ceramic-layer thermal barrier coating

LaPO₄ powders were produced by a chemical co-precipitation and calcination method. The ceramic exhibited a monazite structure, kept phase stability at 1400?°C for 100?h, and had low thermal conductivity (~ 1.41?W/m?K, 1000?°C). LaPO₄/Y₂O₃ partially stabilized ZrO₂ (LaPO₄/YSZ) double-ceramic-layer (DCL) thermal barrier coatings (TBCs) were fabricated by air plasma spray. The LaPO₄ coating contained many nanozones. Thermal cycling tests indicated that the spallation of LaPO₄/YSZ DCL TBCs initially occurred in the LaPO₄ coating. The failure mode was similar to those of many newly developed TBCs, probably due to the low toughness of the ceramics. LaPO₄/YSZ DCL TBCs were highly resistant to V₂O₅ corrosion. Exposed to V₂O₅ at 700–900?°C for 4?h, La(P,V)O₄ formed as the corrosion product, which had little detrimental effect on the coating microstructure. At 1000?°C for 4?h, a minor amount of LaVO₄ was generated.     

Tailoring the EB-PVD columnar microstructure to mitigate the infiltration of CMAS in 7YSZ thermal barrier coatings

The CMAS associated degradation of 7YSZ TBC layers is one of the serious problems in the aero engines that operate in dusty environments. CMAS infiltrates into TBC at high temperatures and stiffens the TBC which ultimately loses its strain tolerance and gets delaminated. The EB-PVD technique is used to coat TBCs exhibiting a columnar microstructure on parts such as blades and on vanes. By varying the EB-PVD process parameters, columnar morphology and porosity of the 7YSZ coating is changed and its effect on the CMAS infiltration behaviour is studied in detail. Two different TBC pore geometries were created and infiltration experiments were carried out at 1250 °C and 1225 °C for different time intervals. The 7YSZ coating with more ‘feathery’ features has resulted in higher CMAS resistance by at least by a factor of 2 than its less ‘feathery’ counterpart. These results are explained on the basis of a proposed physical model.     

Effects of Yb and Sc co-doping on the thermal properties and CMAS corrosion of garnet-type (Y3-xYbx)(Al5-xScx)O12 ceramics

Thermal barrier coatings with excellent thermal performance and corrosion resistance are essential for improving the performance of aero-engines. In this paper, (Y_3-xYb_x)(Al_5-xSc_x)O₁₂ (x = 0, 0.1, 0.2, 0.3) thermal barrier coating materials were synthesized by a combination of sol-gel method and ball milling refinement method. The thermal properties of the (Y_3-xYb_x)(Al_5-xSc_x)O₁₂ ceramics were significantly improved by increasing Yb and Sc doping content. Among designed ceramics, (Y_2.8Yb_0.2)(Al_4.8Sc_0.2)O₁₂ (YS-YAG) showed the lowest thermal conductivity (1.58 Wm^?1K^?1, at 800 °C) and the highest thermal expansion coefficient (10.7 × 10^?6 K^?1, at 1000 °C). In addition, calcium-magnesium- aluminum -silicate (CMAS) corrosion resistance of YS-YAG was further investigated. It was observed that YS-YAG ceramic effectively prevented CMAS corrosion due to its chemical inertness to CMAS as well as its unique and complex structure. Due to the excellent thermal properties and CMAS corrosion resistance, YS-YAG is considered to be prospective material for thermal barrier coatings.     

Laser surface modification of plasma sprayed CYSZ thermal barrier coatings

In this study, Inconel 738 LC superalloy coupons were first sprayed with a NiCoCrAlY bond coat and then with a ceria and yttria stabilized zirconia (CYSZ) top coat by air plasma spraying (APS). After that, the plasma sprayed CYSZ thermal barrier coatings (TBCs) were treated using a Nd:YAG pulsed laser. The effect of laser glazing on the microstructure of the coatings was investigated. The microstructures and surface topographies of both as-sprayed and laser glazed samples were investigated using field emission scanning electron microscope (FESEM) and atomic force microscope (AFM). The phases of the coatings were analyzed with X-ray diffractometry (XRD). The microstructural analysis results revealed that laser surface glazing of ceramic top coat reduced the surface roughness considerably, eliminated the surface porosities and produced a network of continuous cracks perpendicular to the surface. XRD patterns also showed that both as-sprayed and laser glazed top coats consisted of nonequibrium tetragonal (T′) phase.     

Comparison of thermal shock resistances of plasma-sprayed nanostructured and conventional yttria stabilized zirconia thermal barrier coatings

The main goal of the current study is evaluation and comparison of thermal shock behavior of plasma-sprayed nanostructured and conventional yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs). To this end, the nanostructured and conventional YSZ coatings were deposited by atmospheric plasma spraying (APS) on NiCoCrAlY-coated Inconel 738LC substrates. The thermal shock test was administered by quenching the samples in cold water of temperature 20–25 °C from 950 °C. In order to characterize elastic modulus of plasma-sprayed coatings, the Knoop indentation method was employed. Microstructural evaluation, elemental analysis, and phase analysis were performed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffractometry (XRD) respectively. The results revealed that failures of both nanostructured and conventional TBCs were due to the spallation of ceramic top coat. Thermal stresses caused by mismatch of thermal expansion coefficients between the ceramic top coat and the underlying metallic components were recognized as the major factor of TBC failure. However, the nanostructured TBC, due to bimodal unique microstructure, presented an average thermal cycling lifetime that was approximately 1.5 times higher than that of the conventional TBC.     

Plasma sprayed nanostructured GdPO4 thermal barrier coatings: Preparation microstructure and CMAS corrosion resistance

Nanostructured GdPO₄ thermal barrier coatings (TBCs) were prepared by air plasma spraying, and their phase structure evolution and microstructure variation due to calcium–magnesium–alumina–silicate (CMAS) attack have been investigated. The chemical composition of the coating is close to that of the agglomerated particles used for thermal spraying. Nanozones with porous structure are embedded in the coating microstructure, with a percentage of ~30%. CMAS corrosion tests indicated that nanostructured GdPO₄ coating is highly resistant to penetration by molten CMAS at 1250°C. Within 1 hour heat treatment duration, a continuous dense reaction layer forms on the coating surface, which are composed of P–Si apatite based on Ca_2+xGd_8?x(PO₄)_x(SiO₄)_6?xO₂, anorthite and spinel phases. This layer provides effective prevention against CMAS further infiltration into the coating. Prolonged heat treatment densifies the reaction layer but does not change its phase composition.     

Comparative study on thermal shock behavior of thick thermal barrier coatings fabricated with nano-based YSZ suspension and agglomerated particles

Two kinds of segmentation-crack structured YSZ thick thermal barrier coatings (TTBCs) were deposited by suspension plasma spraying (SPS) and atmospheric plasma spraying (APS) with nano-based suspension and agglomerated particles, respectively. The phase composition, microstructure evolution and failure behavior of both TBCs before and after thermal shock tests were systematically investigated. Microstructure of the APS coating exhibits typical segmentation-crack structure in the through-thickness direction, similar with the SPS coating. The densities of segmentation-crack in APS and SPS coatings were about 3 cracks mm⁻¹ and 4 cracks mm⁻¹_, respectively. The microstructure observation also showed that the columnar and equiaxed grains existed in the SPS coating. As for the thermal shock test, the spallation life of the APS TTBCs was 146 cycles, close to that of the SPS TTBCs (166 cycles). Failure of the APS coating is due to the spallation of fringe segments and splats.     

Thermo-mechanical properties and CMAS resistance of (Ho0.4Yb0.3Lu0.3)2SiO5 solid solution for environmental barrier coating applications

Rare earth monosilicate (RE₂SiO₅) is one of the most promising candidates as an environmental barrier coating (EBC) for SiC_f/SiC ceramic matrix composites. But single-component RE₂SiO₅ is hard to meet the multiple and harsh performance requirements of EBC which brings a significant challenge to their applications. Based on our previous research on single-component RE₂SiO₅ ceramics, (Ho_0.4Yb_0.3Lu_0.3)₂SiO₅ solid solution was designed and successfully fabricated in this work. Doping of multiple RE elements endows (Ho_0.4Yb_0.3Lu_0.3)₂SiO₅ with excellent thermal insulation properties and matched thermal expansion coefficient with SiC_f/SiC substrates. In addition, it exhibits lower elastic modulus and comparable hardness than that of single-component RE₂SiO₅. (Ho_0.4Yb_0.3Lu_0.3)₂SiO₅ also presents good resistance to calcium-magnesium alumino-silicates (CMAS) corrosion. Rational composition design allows (Ho_0.4Yb_0.3Lu_0.3)₂SiO₅ to retain the merits of single-component RE₂SiO₅ while taking advantage of the solid solution effect. The results of this work suggest (Ho_0.4Yb_0.3Lu_0.3)₂SiO₅ as a promising EBC candidate.     

Thermal shock resistance of air plasma sprayed thermal barrier coatings

The spallation resistance of an air plasma sprayed (APS) thermal barrier coating (TBC) to cool-down/reheat is evaluated for a pre-existing delamination crack. The delamination emanates from a vertical crack through the coating and resides at the interface between coating and underlying thermally grown oxide layer (TGO). The coating progressively sinters during engine operation, and this leads to a depth-dependent increase in modulus. Following high temperature exposure, the coating is subjected to a cooling/reheating cycle representative of engine shut-down and start-up. The interfacial stress intensity factors are calculated for the delamination crack over this thermal cycle and are compared with the mode-dependent fracture toughness of the interface between sintered APS and TGO. The study reveals the role played by microstructural evolution during sintering in dictating the spallation life of the thermal barrier coating, and also describes a test method for the measurement of delamination toughness of a thin coating.     

Effect of scandia content on the thermal shock behavior of SYSZ thermal sprayed barrier coatings

Nanostructured 4SYSZ (scandia (3.5 mol%) yttria (0.5 mol%) stabilized zirconia) and 5.5 SYSZ (5 mol% scandia and 0.5 mol% yttria) thermal barrier coatings (TBCs) were deposited on nickel-based superalloy using NiCrAlY as the bond coat by plasma spraying process. The thermal shock response of both as-sprayed TBCs was investigated at 1000 °C. Experimental results indicated that the nanostructured 5.5SYSZ TBCs have better thermal shock performance in contrast to 4SYSZ TBCs due to their higher tetragonal phase content and higher fracture toughness of this coating     

Preparation of plasma sprayed nanostructured GdPO4 thermal barrier coating and its hot corrosion behavior in molten salts

Nanostructured GdPO₄ coatings, designed as the outer layer of double-ceramic-layer thermal barrier coatings (DCL-TBCs), were produced by air plasma spraying (APS). The coatings have close chemical composition to that of the agglomerated particles used for thermal spray. Nanozones with porous structure are embedded in the coating microstructure, having a percentage of ~30%. Hot corrosion tests of the coatings were carried out in V₂O₅ and Na₂SO₄+V₂O₅ salts at 900 °C for 4 h. Results indicate that dense reaction layers, consisting of GdVO₄ and Gd₄(P₂O₇)₃, form on the coating surfaces, which could suppress further penetration of the molten salts. In the V₂O₅ molten salt, the reaction layer is thicker and less molten salt trace could be found beneath the layer.     

Microstructure and thermo-physical properties of yttria stabilized zirconia coatings with CMAS deposits

Yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) are used to protect hot-components in aero-engines from hot gases. In this paper, the microstructure and thermo-physical and mechanical properties of plasma sprayed YSZ coatings under the condition of calcium-magnesium-alumina-silicate (CMAS) deposits were investigated. Si and Ca in the CMAS rapidly penetrated the coating at 1250 °C and accelerated sintering of the coating. At the interface between the CMAS and YSZ coating, the YSZ coating was partially dissolved in the CMAS, inducing the phase transformation from tetragonal phase to monoclinic phase. Also, the porosity of the coating was reduced from ∼25% to 5%. As a result, the thermal diffusivity at 1200 °C increased from 0.3 mm²/s to 0.7 mm²/s, suggesting a significant degradation in the thermal barrier effect. Also, the coating showed a ∼40% increase in the microhardness. The degradation mechanism of TBC induced by CMAS was discussed.     

Evaluation of hot corrosion behavior of CSZ,CSZ/micro Al2O3 and CSZ/nano Al2O3 plasma sprayed thermal barrier coatings

Hot corrosion is one of the main destructive factors in thermal barrier coatings (TBCs) which come as a result of molten salt effect on the coating–gas interface. Hot corrosion behavior of three types of plasma sprayed TBCs was evaluated: usual CSZ, layer composite of CSZ/Micro Al₂O₃ and layer composite of CSZ/Nano Al₂O₃ in which Al₂O₃ was as a topcoat on CSZ layer. Hot corrosion studies of plasma sprayed thermal barrier coatings (TBCs) were conducted in 45 wt% Na₂SO₄+55 wt% V₂O₅ molten salt at 1050 °C for 40 h. The graded microstructure of the coatings was examined using scanning electron microscope (SEM) and X-ray diffractometer (XRD) before and after hot corrosion test. The results showed that no damage and hot corrosion products was found on the surface of CSZ/Nano Al₂O₃ coating and monoclinic ZrO₂ fraction was lower in CSZ/Micro Al₂O₃ coating in comparison with usual CSZ. reaction of molten salts with stabilizers of zirconia (Y₂O₃ and CeO₂) that accompanied by formation of monoclinic zirconia, irregular shape crystals of YVO₄, CeVO₄ and semi-cubic crystals of CeO₂ as hot corrosion products, caused the degradation of CSZ coating in usual CSZ and CSZ/Micro Al₂O₃ coating.     

Phase stability,thermo-physical properties and thermal cycling behavior of plasma-sprayed CTZ,CTZ/YSZ thermal barrier coatings

ZrO₂ co-stabilized by CeO₂ and TiO₂ with stable, nontransformable tetragonal phase has attracted much attention as a potential material for thermal barrier coatings (TBCs) applied at temperatures >?1200?°C. In this study, ZrO₂ co-stabilized by 15?mol% CeO₂ and 5?mol% TiO₂ (CTZ) and CTZ/YSZ (zirconia stabilized by 7.4?wt% Y₂O₃) double-ceramic-layer TBCs were respectively deposited by atmospheric plasma spraying. The microstructures, phase stability and thermo-physical properties of the CTZ coating were examined using scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric-differential scanning calorimeter (TG-DSC), laser pulses and dilatometry. Results showed that the CTZ coating with single tetragonal phase was more stable than the YSZ coating during isothermal heat-treatment at 1300?°C. The CTZ coating had a lower thermal conductivity than that of YSZ coating, decreasing from 0.89?W?m^?1 K^?1 to 0.76?W?m^?1 K^?1 with increasing temperature from room temperature to 1000?°C. The thermal expansion coefficients were in the range of 8.98?×?10^?6 K^?1 – 9.88 ×10^?6 K^?1. Samples were also thermally cycled at 1000?°C and 1100?°C. Failure of the TBCs was mainly a result of the thermal expansion mismatch between CTZ coating and superallloy substrate, the severe coating sintering and the reduction-oxidation of cerium oxide. The thermal durability of the TBCs at 1000?°C can be effectively enhanced by using a YSZ buffer layer, while the thermal cycling life of CTZ/YSZ double-ceramic-layer TBCs at 1100?°C was still unsatisfying. The thermal shock resistance of the CTZ coating should be improved; otherwise the promising properties of CTZ could not be transferred to a well-functioning coating.     

Hot corrosion behavior of multialkaline-earth aluminosilicate for environmental barrier coatings

Multialkaline-earth aluminosilicate Ba_1/3Sr_1/3Ca_1/3Al₂Si₂O₈ (BSCAS) were synthesized to serve as new environment barrier coatings. Their hot corrosion behavior in an Na₂SO₄ environment was studied in the temperature range of 900–1100 °C over a period of 100 h. The phase and cross-sectional morphology evolutions of the corroded samples were characterized via X-ray diffraction and scanning electron microscopy. Combined with the thermodynamic analysis of the possible reactions occurring during hot corrosion, the competitive out-diffusion of the alkaline-earth elements to react with Na₂SO₄ is believed to have a considerable influence on the hot corrosion behavior of BSCAS. The sluggish diffusion and the dense Ca₂Al₂SiO₇ layer, which originate from the competitive reactions of the multialkaline earth elements, lead to an improvement in the hot corrosion resistance of BSCAS. A model is proposed to describe the hot corrosion process.     

Hot corrosion resistance of air plasma sprayed ceramic Sm2SrAl2O7 (SSA) thermal barrier coatings in simulated gas turbine environments

Samarium strontium aluminate (Sm₂SrAl₂O₇-SSA) and Yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) were developed on NiCrAlY bond coated Inconel 718 superalloy substrate using air plasma spray process. The hot corrosion study was conducted in simulated gas turbine environments (molten mixtures of 50?wt% Na₂SO₄ + 50?wt% V₂O₅ and 90?wt% Na₂SO₄ + 5?wt% V₂O₅ + 5?wt% NaCl) for two different temperatures of 700 and 900?°C. A developed SSA TBCs showed about 8% and 22% lower lifetime at 700 and 900?°C, respectively than YSZ TBCs in 50?wt% Na₂SO₄ +?50?wt% V₂O₅ (vanadate). The hot corrosion life of SSA TBCs being found about 13% and 39% lower than YSZ TBCs in 90?wt% Na₂SO₄ +?5?wt% V₂O₅ +?5?wt% NaCl (chloride) at 700 and 900?°C, respectively. X-ray diffraction results showed the formation of SmVO₄, SrV₂O₆, and SrSO₄ as a major hot corrosion product in 50?wt% Na₂SO₄ +?50?wt% V₂O₅ and 90?wt% Na₂SO₄ +?5?wt% V₂O₅ +?5?wt% NaCl environments respectively for SSA TBCs. Similarly, YSZ TBCs also showed YVO₄ as hot corrosion product in vanadate and chloride environments. Both the TBCs suffer a more severe hot corrosion attack in chloride environment at 900?°C. The leaching of Sr²⁺ and Y³⁺ ions from SSA and YSZ respectively play a vital role in the destabilization of coating in vanadate and chloride environments at 700 and 900?°C. In both SSA and YSZ TBCs, the leaching of ion has significantly low influence as compared to attack by chloride ions at the bond coat-top coat interface in the presence of chloride environment. The hot corrosion resistance of SSA TBCs was improved three times higher in the presence of MgO and NiO inhibitor in vanadate environment at 900?°C mainly due to the formation of a stable Ni₃V₂O₈ phase at the surface.