Segmented Thermal Barrier Coatings Research Articles

Optimization of Segmented Thermal Barrier Coatings (s-TBCs) for High-Temperature Applications

AbstractHot section components of stationary gas turbines, such as turbine blades and vanes, are coated with thermal barrier coatings (TBCs) to increase the component life. TBCs provide thermal insulation to the metallic components from hot gas in the gas turbines. The TBCs represent high-performance ceramics and are mainly composed of yttria-stabilized zirconia (YSZ) to fulfill the thermal insulation function. The microstructure of the TBCs should be porous to decrease heat conduction. Besides the porous TBCs, the subsequently developed vertically segmented thermal barrier coatings (s-TBCs) feature outstanding thermal durability. For the formation of this segmented coating microstructure, the YSZ should be deposited under high thermal tensile stress during the coating process. Therefore, substrates are heated just before the coating by plasma or in an oven in recent research. In this work, the development of process parameters for s-TBCs produced by atmospheric plasma spray (APS) without pre-heating is presented. Within the experiments, the relevant process parameters, such as plasma gases, powder feed rate, surface speed, and pathway strategy, have been optimized to achieve the segmented coating microstructure with high deposition efficiency by a conventional plasma torch. Furthermore, YSZ powders used in this study are characterized, and the effect of powder characteristics on the coating microstructure is investigated. The coating microstructure in this work aims to achieve the formation of a high number of vertical cracks with a combination of low internal residual stress and high adhesive tensile strength for the s-TBCs.

Developments in Thermodynamic Models of Deposit-Induced Corrosion of High-Temperature Coatings

Degradation caused by silicate deposits limits the lifetime of protective coatings in gas turbine engines. In order to design more durable coating materials and architectures it is important to understand the behavior of ingested debris leading to the formation of deposits on component surfaces, and the subsequent reaction of those deposits with the coating materials. This article reviews recent advances in thermodynamic models and databases capable of predicting these reactions. It also describes the development of modeling approaches to capture key features associated with the intrinsic behavior of debris and deposits, their interaction with dense environmental barrier coatings, and reactions occurring during infiltration of molten deposits into porous or segmented thermal barrier coatings.

Conjugate flow and heat transfer analysis between segmented thermal barrier coatings and cooling film

The segmented thermal barrier coatings (TBCs) with vertical cracks have similar thermal insulation properties to layered coatings prepared by atmosphere plasma spraying (APS), and have similar mechanical properties to columnar coatings prepared by electron beam physical deposition (EB-PVD), which are very suitable for gas turbines with frequent start-up and variable load. Vertical cracks in the segmented APS-TBCs change the surface morphologies and internal microstructures, so it is necessary to study the internal heat transfer performance of the coating and the conjugate heat transfer characteristics between the TBCs and the cooling film. In this work, the numerical reconstruction models of TBCs with different crack lengths and openings were obtained based on the optimized quartet structure generation set (QSGS) method. By using the double distribution function lattice Boltzmann method (DDF-LBM), the temperature distribution of the coating, the conjugate heat transfer between TBCs and the cooling film and the flow characteristics of the cooling film were studied. The results showed that the vertical cracks with diverse sizes have significant effects on the coating-film cooling system, on which the crack opening has a greater effect. The temperature non-uniformity coefficient shows bimodal distribution in TBCs and the highest temperature is located at the crack center on the coating surface, which can easily lead to local hot spots near crack tips and further result in an acceleration of coating structure's fatigue failure. Meanwhile, the presence of cracks strengthens the interfacial convection heat transfer characteristics. The convection heat transfer coefficient change rate goes up 10% with the crack opening widens. Correspondingly, the greater deformation of the cooling film streamline and value of the vertical velocity are generated under the pre-existing larger size cracks. The results can provide us a powerful guide for the design of advanced coating structures and a strong foundation for a better understanding of the failure behavior of segmented TBCs.

Empirical Modeling of Abrasive Waterjet Process for Controlled Depth Machining of Dense Segmented Ceramic Thermal Barrier Coatings

Abrasive waterjet (AWJ) controlled depth machining shows promise to be one of the most efficient non-conventional structuring techniques for dense segmented Thermal Barrier Coatings (STBC) on turbomachinery hot gas components made of Yttria-Stabilized Zirconia. Exemplary applications in the field of gas turbine technology are engraving of structures to optimize gas turbine performance and the stripping process of TBC within the repair process chain. As there are no comprehensive process models available, the development of an appropriate AWJ machining process is demanding. Thus, deeper process understanding and modeling need to be investigated.This paper shows an empirical modeling of AWJ process for controlled depth machining of dense segmented TBC material. The practical trials are based on a Design of Experiments (DoE). The investigated influencing parameters are water pressure, abrasive mass flow, feed rate, hatch distance and machining angle. The considered target variables are ablation depth and surface roughness. Furthermore, the process stability is investigated. The developed empirical model results in an acceleration of process parameter determinations.

Effect of Target Coating Thickness on the Abrasive Waterjet Machining Response of 8YSZ Segmented Thermal Barrier Coatings

Segmented thermal barrier coatings (STBC) are a more strain-tolerant and erosion resistant alternative of the conventional porous thermal barrier coatings for hot gas components in gas turbines. Due to their relatively higher thermal conductivity and the ever-increasing turbine inlet temperature, there is a need for thicker STBCs. This paper presents an investigation of the microstructures and properties of STBCs with respect to coating thicknesses. Coatings with two significantly different thicknesses were fabricated under identical conditions and evaluated using standard metallographic methods. It was found that the microstructures for thin and thick coatings had subtle differences and hardness values decreased with in increasing thickness. When subjected to an abrasive waterjet machining process, the thicker STBCs showed a higher wear rate. A hypothesis is proposed to explain the observed thickness dependent changes and the microstructures and properties of STBCs.

Segmented Thermal Barrier Coatings for ID and OD Components Using the SinplexPro Plasma Torch

Thermal barrier coatings (TBCs) with high thermal strain tolerance and erosion resistance are commonly applied onto the inner and outer diameters of hot sections of gas-turbine engine components. In this work, strain-tolerant, segmented TBCs were developed using the SinplexPro-cascaded torch. Design of experiments were carried out to study the effect of process variables such as plasma power, powder feeding rate, spraying distance and surface speed on the coating microstructure and properties. Optimized process parameters for the segmented coating microstructures at shorter spray distance ( 114mm) are achieved, which are targeted for spraying inner diameter (ID) and outer diameter (OD) engine components, respectively. The plasma torch hardware life was evaluated by torch cycle duration runs. Examples of highly strain-tolerant TBCs onto the ID and OD engine components were demonstrated, highlighting the wide versatility and process range of the SinplexPro.

Highly Segmented Thermal Barrier Coatings Deposited by Suspension Plasma Spray: Effects of Spray Process on Microstructure

Effects of the ceramic powder size used for suspension as well as several processing parameters in suspension plasma spraying of YSZ were investigated experimentally, aiming to fabricate highly segmented microstructures for thermal barrier coating (TBC) applications. Particle image velocimetry (PIV) was used to observe the atomization process and the velocity distribution of atomized droplets and ceramic particles travelling toward the substrates. The tested parameters included the secondary plasma gas (He versus H2), suspension injection flow rate, and substrate surface roughness. Results indicated that a plasma jet with a relatively higher content of He or H2 as the secondary plasma gas was critical to produce highly segmented YSZ TBCs with a crack density up to ~12 cracks/mm. The optimized suspension flow rate played an important role to realize coatings with a reduced porosity level and improved adhesion. An increased powder size and higher operation power level were beneficial for the formation of highly segmented coatings onto substrates with a wider range of surface roughness.

Suspension Plasma Spraying: Process Characteristics and Applications

Suspension plasma spraying (SPS) offers the manufacture of unique microstructures which are not possible with conventional powdery feedstock. Due to the considerably smaller size of the droplets and also the further fragmentation of these in the plasma jet, the attainable microstructural features like splat and pore sizes can be downsized to the nanometer range. Our present understanding of the deposition process including injection, suspension plasma plume interaction, and deposition will be outlined. The drawn conclusions are based on analysis of the coating microstructures in combination with particle temperature and velocity measurements as well as enthalpy probe investigations. The last measurements with the water cooled stagnation probe gives valuable information on the interaction of the carrier fluid with the plasma plume. Meanwhile, different areas of application of SPS coatings are known. In this paper, the focus will be on coatings for energy systems. Thermal barrier coatings (TBCs) for modern gas turbines are one important application field. SPS coatings offer the manufacture of strain-tolerant, segmented TBCs with low thermal conductivity. In addition, highly reflective coatings, which reduce the thermal load of the parts from radiation, can be produced. Further applications of SPS coatings as cathode layers in solid oxide fuel cells (SOFC) and for photovoltaic (PV) applications will be presented.

Thermal shock resistance and mechanical properties of La 2Ce 2O 7 thermal barrier coatings with segmented structure

La 2Ce 2O 7 (LCO) is a promising candidate material for thermal barrier coatings (TBCs) application because of its higher temperature capability and better thermal insulation property relative to yttria stabilized zirconia (YSZ). In this work, La 2Ce 2O 7 TBC with segmentation crack structure was produced by atmospheric plasma spray (APS). The mechanical properties of the sprayed coatings at room temperature including microhardness, Young's modulus, fracture toughness and tensile strength were evaluated. The Young's modulus and microhardness of the segmented coating were measured to be about 25 and 5 GPa, relatively higher than those of the non-segmented coating, respectively. The fracture toughness of the LCO coating is in a range of 1.3–1.5 MPa m 1/2, about 40% lower than that of the YSZ coating. The segmented TBC had a lifetime of more than 700 cycles, improving the lifetime by nearly two times as compared to the non-segmented TBC. The failure of the segmented coating occurred by chipping spallation and delamination cracking within the coating.

Thermal Cycling and Hot-Corrosion Behavior of Plasma Sprayed Segmented Thermal Barrier Coatings

Modified zirconia thermal barrier coatings (TBCs) with segmentation cracks were sprayed onto a TMS 82+ single crystalline substrate. The thermal cycling lifetime of the modified TBC was improved by 10 times compared to that of the traditional non-segmented TBC. Also, the modified coating showed much better resistance to high temperature cyclic hot-corrosion.

Microstructures and Properties of Plasma‐Sprayed Segmented Thermal Barrier Coatings

Thermal barrier coatings (TBCs) with different levels of segmentation crack density were produced by spraying two types of ZrO2–8Y2O3 powders. The fused and crushed powder has a greater capability of forming segmented coatings than the hollow sphere (HOSP) one. The highly segmented coatings reveal much lower porosity compared with traditionally sprayed coatings, thereby compromising the property of thermal insulation of TBCs. Microstructure and thermal conductivity of the HOSP coatings are more sensitive to the changes in spray conditions. Segmentation cracks had a strong influence in decreasing Young's modulus of coatings. Fifty hours heat treatment at 1250°C had little effect on the mechanical property of the highly segmented coatings.

Thermal Cycling Behavior of Plasma Sprayed Segmented Thermal Barrier Coatings

Thermal barrier coatings (TBCs) with different levels of segmentation crack densities (Ds) were sprayed at different substrate temperatures (Ts). Thermal cycling resistance of the TBC specimen was examined. The segmented coating significantly improved the thermal cycling resistance as compared to the traditional non-segmented coating. Maximum thermal cycling lifetimes were achieved in the coating with a crack density level of 2.2 mm � 1 . New segmentation cracks were hardly generated during thermal cycling testing. Spallation of segments within the segmented coating occurred, which is different from spallation of the whole coating from substrate in the case of traditional coating. Oxidation of bond coat and limited phase decomposition of YSZ topcoat were considered as not responsible for the failure of TBCs. Turbine engine components are subjected to high thermal and mechanical loads. Ceramic thermal barrier coatings (TBCs) are used to protect the components from thermal loads by decreasing the surface temperature of the metallic substrate. Generally, plasma sprayed (PS) thermal barrier coatings (TBCs) have exhibited shorter thermal cycling lifetimes than electron beam physical vapor deposition (EB- PVD) TBCs, which makes PS TBCs suitable only for less exacting applications in gas turbines. The failure of plasma sprayed TBCs is mainly caused by the sintering and the progressive destabilization of the zirconia topcoat, due to thermal mismatch stresses applied to the coating, and the formation of thermally grown oxide (TGO), as well as thermal shock. Recently, it was reported that macrocracks in TBC perpendicular to a substrate, namely segmentation cracks, are beneficial to increase the thermal cycling lifetime of plasma sprayed TBCs, by accommodating the stresses arising from the different thermal expansions between the coating and substrate. 1-3) Such macrocracks can be developed by post treatment such as laser glazing, 4,5) or directly introduced during the deposition by applying high heat input to the substrate and high lamellar thickness. 2,3,6) The formation mechanism of segmentation cracks and thermo-physical properties of the segmented coatings have been investigated. 7-9) However, the thermal cycling behavior and associated failure mechanism of the segmented TBCs are seldom reported. In this paper, thick TBCs with different levels of segmentation crack densities (Ds) are produced by controlling the spray parameters. The thermal cycling behavior of traditional non-segmented and modified seg- mented TBCs is studied. The failure mechanisms of these TBCs are also discussed. 2. Experimental

Segmented thermal barrier coatings produced by atmospheric plasma spraying hollow powders

Segmented thermal barrier coatings were produced by plasma spraying hollow and solid ZrO 2–8 wt.% Y 2O 3 powders. The solid powder (SP) had greater capability of producing segmentation cracks in coating compared with the hollow powder (HP). High substrate temperature ( T s) gave rise to an increased segmentation crack density ( D s). The segmentation crack network was still stable even after sintering at 1300 °C for 15 h. The microcracks that are mainly attributed to delaminations between splats had a negative effect on the origin and propagation of segmentation cracks.

Effects of Heat Treatment on Microstructures and Physical Properties of Segmented Thermal Barrier Coatings

Highly segmented thermal barrier coatings (TBCs) were produced by ‘‘hot’’ plasma spraying. The effects of heat treatment on the microstructures, mechanical and thermophysical properties were studied. The segmented coatings are denser than traditionally plasma-sprayed TBCs due to its good insplat bonding at high substrate temperature. The segmentation cracks and associating branching cracks hardly propagated or closed during sintering process, indicative of a good stability of crack network. Due to its low porosity, the segmented coatings compromised the property of thermal insulation of TBCs. For the coatings after 24 h heat treatment at 1523 K, the thermal conductivity was improved by around 35%. The segmentation cracks had a strong impact on decreasing the Young’s modulus. Heat treatment could not effectively promote the increase of the Young’s modulus, especially for the case of highly segmented coatings.