Next Generation Thermal Barrier Coatings for the Gas Turbine Industry (original) (raw)
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Coatings, 2016
Axial suspension plasma spraying (ASPS) is an advanced thermal spraying technique, which enables the creation of specific microstructures in thermal barrier coatings (TBCs) used for gas turbine applications. However, the widely varying dimensional scale of pores, ranging from a few nanometers to a few tenths of micrometers, makes it difficult to experimentally measure and analyze porosity in SPS coatings and correlate it with thermal conductivity or other functional characteristics of the TBCs. In this work, an image analysis technique carried out at two distinct magnifications, i.e., low (500×) and high (10,000×), was adopted to analyze the wide range of porosity. Isothermal heat treatment of five different coatings was performed at 1150 • C for 200 h under a controlled atmosphere. Significant microstructural changes, such as inter-columnar spacing widening or coalescence of pores (pore coarsening), closure or densification of pores (sintering) and crystallite size growth, were noticed in all the coatings. The noted changes in thermal conductivity of the coatings following isothermal heat treatment are attributable to sintering, crystallite size growth and pore coarsening.
Optimization of High Porosity Thermal Barrier Coatings Generated with a Porosity Former
Journal of Thermal Spray Technology, 2015
Yttria-stabilized zirconia thermal barrier coatings are extensively used in turbine industry; however, increasing performance requirements have begun to make conventional air plasma sprayed coatings insufficient for future needs. Since the thermal conductivity of bulk material cannot be lowered easily; the design of highly porous coatings may be the most efficient way to achieve coatings with low thermal conductivity. Thus the approach of fabrication of coatings with a high porosity level based on plasma spraying of ceramic particles of dysprosia-stabilized zirconia mixed with polymer particles, has been tested. Both polymer and ceramic particles melt in plasma and after impact onto a substrate they form a coating. When the coating is subjected to heat treatment, polymer burns out and a complex structure of pores and cracks is formed. In order to obtain desired porosity level and microstructural features in coatings; a design of experiments, based on changes in spray distance, powder feeding rate, and plasmaforming atmosphere, was performed. Acquired coatings were evaluated for thermal conductivity and thermo-cyclic fatigue, and their morphology was assessed using scanning electron microscopy. It was shown that porosity level can be controlled by appropriate changes in spraying parameters.
Coatings
High-porosity thermal barrier coatings are utilized on gas turbine components where maximizing the coating thermal insulation capability is the primary design criteria. Though such coatings have been in industrial use for some time, manufacturing high-porosity coatings quickly and efficiently has proven challenging. With the industry demand to increase productivity and reduce waste generation, there is a drive to look at improved coating manufacturing methods. This article looks at high-porosity coatings manufactured using a high-power plasma system in comparison with a current industrial coating. A commercial spray powder is compared with an experimental Low-Density powder developed to maximize coating porosity without sacrificing coating deposition efficiency. The resultant coatings have been assessed for their microstructure, adhesion strength, furnace cyclic lifetime, thermal conductivity and sintering behavior. Finally, the impact of spray processing on coating economics is dis...
Journal of Thermal Spray Technology, 2012
The properties and performance of plasma-sprayed thermal barrier coatings (TBCs) are strongly dependent on the microstructural defects, which are affected by starting powder morphology and processing conditions. Of particular interest is the use of hollow powders which not only allow for efficient melting of zirconia ceramics but also produce lower conductivity and more compliant coatings. Typical industrial hollow spray powders have an assortment of densities resulting in masking potential advantages of the hollow morphology. In this study, we have conducted process mapping strategies using a novel uniform shell thickness hollow powder to control the defect microstructure and properties. Correlations among coating properties, microstructure, and processing reveal feasibility to produce highly compliant and low conductivity TBC through a combination of optimized feedstock and processing conditions. The results are presented through the framework of process maps establishing correlations among process, microstructure, and properties and providing opportunities for optimization of TBCs.
Surface and Coatings Technology, 2015
To enhance efficiency of gas turbines, new thermal barrier coatings (TBCs) must be designed which improve upon the thermal stability limit of 7wt% yttria stabilized zirconia (7YSZ), ~1200°C. This tenant has lead led to the development of new TBC materials and microstructures capable of improved high temperature performance. This study focused on increasing the erosion durability of cubic zirconia based TBCs, traditionally less durable than the metastable t" zirconia based TBCs. Composite TBC microstructures composed of a low thermal conductivity/high temperature stable cubic Low-k matrix phase and a durable t" Low-k secondary phase were deposited via APS. Monolithic coatings composed of cubic Low-k and t" Low-k were also deposited, in addition to a 7YSZ benchmark. The thermal conductivity and erosion resistance durability were then measured and it was found that both of the Low-k materials have significantly reduced thermal conductivities, with monolithic t" Low-k and cubic Low-k improving upon 7YSZ by ~13% and ~25%, respectively. The 40 wt% t" Low-k composite (40 wt% t" Low-k-60 wt% cubic Low-k) showed a ~22% reduction in thermal conductivity over 7YSZ, indicating even at high levels, the t" Low-k secondary phase had a minimal impact on thermal in the composite coating. It was observed that a mere 20 wt% t"
Journal of Thermal Spray Technology, 2009
A review is presented of how heat transfer takes place in plasma-sprayed (zirconia-based) thermal barrier coatings (TBCs) during operation of gas turbines. These characteristics of TBCs are naturally of central importance to their function. Current state-of-the-art TBCs have relatively high levels of porosity (~15%) and the pore architecture (i.e., its morphology, connectivity, and scale) has a strong influence on the heat flow. Contributions from convective, conductive and radiative heat transfer are considered, under a range of operating conditions, and the characteristics are illustrated with experimental data and modeling predictions. In fact, convective heat flow within TBCs usually makes a negligible contribution to the overall heat transfer through the coating, although what might be described as convection can be important if there are gross through-thickness defects such as segmentation cracks. Radiative heat transfer, on the other hand, can be significant within TBCs, depending on temperature and radiation scattering lengths, which in turn are sensitive to the grain structure and the pore architecture. Under most conditions of current interest, conductive heat transfer is largely predominant. However, it is not only conduction through solid ceramic that is important. Depending on the pore architecture, conduction through gas in the pores can play a significant role, particularly at the high gas pressures typically acting in gas turbines (although rarely applied in laboratory measurements of conductivity). The durability of the pore structure under service conditions is also of importance, and this review covers some recent work on how the pore architecture, and hence the conductivity, is affected by sintering phenomena. Some information is presented concerning the areas in which research and development work needs to be focussed if improvements in coating performance are to be achieved.
Int. J. Electrochem. …, 2012
Advanced ceramic multilayered coatings are commonly used as protective coatings for engine metal components, where, aerospace gas turbine engines are now designed such that the heat resistant super alloys operate at temperature very close to their melting, so current strategies for performance improvement are centered on thermal barrier coatings. The main focus of this work is to study the effect of different parameters of air plasma spraying technique for various thermal barrier coatings comprised of zirconia stabilized with magnesia top coat and nickel-aluminum bond coat as well as their properties with those obtained using reference techniques. The deviations of the parameters from the optimum conditions are discussed. The investigation shows that: the deviation of the plasma spray parameters from the optimum conditions led to create a poor contact between the bond coat and the Nibase-super alloy substrate, increase of the cracks resulting from the relaxation of residual stresses in the planer direction (open porosity), increase of the voids resulting from poor deformation of partially melted particle (few micrometer void), and present of the un-melted particles. The conclusions of this experimental study are in good agreement with theoretical predictions resulting from a sensitivity analysis reported in a previous study.
Recent developments in the designing of deposition of thermal barrier coatings – A review
Materials Today: Proceedings, 2020
The thermal barrier coatings (TBCs) provide insulation to the components of gas turbine by providing higher amount of thermal energy and also reduce the cooling requirement of the top ceramic surface. The top ceramic coat plays a significant role in surging the efficiency of the TBCs. The 8 wt% Yttria partially stabilized zirconia (YSZ) is the top coat material is widely used because of its ability to withstand aggressive conditions of high temperature and pressure. Air plasma Spraying (APS) technique frequently used in gas turbine industry to deposit 8 wt%YSZ TBCs. There are some important spray parameters which play very important role in the deposition of TBCs such as powder feed rate (g/min), current (A), voltage (V), Power (KW) and standoff/spray distance (mm), and these parameters help to regulate the phase composition, isothermal thermal stability, improved microstructure, corrosion resistance and mechanical properties like residual stresses and bonding strength. Recently new surface modification techniques have been developed such as plasma-enhanced chemical vapour deposition (PECVD), electrostatic spray assisted vapour deposition (ESAVD), and solution precursor plasma spray (SPPS), sol-gel and suspension plasma spraying (SPS). These processes enhance the features of developed coatings which include the formation of vertical cracks by utilizing segmentation process and so on. Furthermore, the effect of varying the deposition process parameters like substrate temperature low plasma gun speed, high passage thickness, contact area between splats have been discussed. This review gives a good understanding about the TBCs and inspires researchers to find new ideas for the improvement in this field.
Thermal Stability of Air Plasma Spray and Solution Precursor Plasma Spray Thermal Barrier Coatings
Journal of The American Ceramic Society, 2007
Yttria-stabilized zirconia (7YSZ) thermal barrier coatings (TBCs) were produced by conventional air plasma spray (APS) and solution precursor plasma spray (SPPS) processes. Both TBCs were isothermally heat treated from 1200° to 1500°C for 100 h. Changes in the phase content, microstructure, and hardness were investigated. The nontransformable tetragonal (t′) phase is the predominant phase in both the as-sprayed APS and SPPS TBCs. APS and SPPS coatings exhibit similar thermal stability behavior such as densification rate, hardness increase, and grain coarsening rate. Both the as-received and heat-treated APS and SPPS TBCs show a bimodal pore size distribution with nano- and micro-size pores. After 1400°C/100 h heat treatment, equiaxed grains replace the columnar structure in APS TBCs and the splat structure disappears. Vertical cracks remain after the 1500°C/100 h exposure in SPPS TBCs. The monoclinic phase appears in APS TBCs after a 1400°C/100 h exposure and in SPPS coatings after a 1500°C/100 h exposure.
Improving the lifetime of suspension plasma sprayed thermal barrier coatings
Surface & Coatings Technology, 2017
Development of thermal barrier coating systems (TBCs) for gas turbine applications allowing higher combustion temperatures is of high interest since it results in higher fuel efficiency and lower emissions. TBCs produced by suspension plasma spraying (SPS) have been shown to exhibit significantly lower thermal conductivity as compared to conventional systems due to their very fine porosity microstructure. However they have not been commercialised yet due to low reliability and life expectancy of the coatings. In addition to the initial topcoat microstructure and its sintering resistance, lifetime of a TBC system is highly dependent on bondcoat chemistry as it influences the growth rate of thermally grown oxide (TGO) layer. To enhance the lifetime of SPS TBCs, fundamental understanding of relationships between topcoat microstructure and its evolution with time, bondcoat chemistry, TGO growth rate, and lifetime is essential. The objective of this work was to study the effect of topcoat microstructure evolution and TGO growth rate on lifetime in SPS TBC systems. Experimental MCrAlY bondcoat powders with different aluminium activities were investigated and compared to a commercial bondcoat powder. High velocity air fuel spraying was used for bondcoat deposition while axial-SPS was used for yttria stabilised zirconia topcoat deposition. Lifetime was examined by thermal cyclic fatigue testing. Isothermal heat treatment was performed to study TGO evolution with time. The