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Aerospace and Power Generation Applications
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Proceedings Papers
ITSC 2017, Thermal Spray 2017: Proceedings from the International Thermal Spray Conference, 23-30, June 7–9, 2017,
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Suspension plasma spraying (SPS) process has attracted extensive effort and interest as a method to produce fine-structured and functional coatings. In particular, thermal barrier coating (TBC) applied by SPS process has gained increasing interest due to its potential for producing coatings that provide superior thermal protection of gas turbine hot-section components as compared to conventional APS-TBC and even EB-PVD TBC. The unique columnar architecture and nano- and submicron sized grains in a SPS-TBC coatings demonstrate some advantages in thermal shock durability, low thermal conductivity, and high-temperature sintering resistance. This work addresses some practical aspects of using the SPS process for TBC applications before it becomes a reliable industry method. The spray capability and applicability of SPS to achieve uniform thickness and microstructure on curved substrates was evaluated in designed spray trials to simulate industrial parts with complex configurations. The performance of SPS-TBCs in erosion, free falling ballistic impact, and indentation loading tests was evaluated to simulate SPS-TBC performance in turbine service conditions. The behaviors of SPS-TBCs in those tests were correlated to key test factors including grit incident angles, impact object sizes, indentation head shapes, and coating surface curvatures. Finally, a turbine blade was coated and sectioned to verify SPS sprayability in multiple critical sections. The SPS trials and test results demonstrate that SPS is promising for innovative TBCs, but some challenges need to be addressed before it becomes an economical and reliable industrial process, especially for gas turbine components.
Proceedings Papers
ITSC 2017, Thermal Spray 2017: Proceedings from the International Thermal Spray Conference, 31-35, June 7–9, 2017,
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Gadolinium zirconate (GZ) is considered as a promising top coat candidate for high temperature (>1200 °C) thermal barrier coating (TBC) applications. Suspension plasma spray (SPS) technique has shown the capability to generate a wide range of microstructures which includes the more desirable columnar microstructure. In this study, GZ single layer TBCs were deposited by axial SPS process. The variable parameters include the standoff distance, solid load content of the suspension and input power. The cross section and top surface of the as sprayed TBCs were analyzed by SEM. The phase content in the as sprayed TBCs was analyzed by XRD. The porosity content of the as sprayed TBCs was measured using image analysis. In the SEM analysis, it was observed that a lower solid load content in the suspension favored the formation of a columnar microstructure. Additionally, at lower solid load content, increase in standoff distance resulted in columnar microstructure with high porosity content in the TBC. However, with higher solid content suspension and alteration of input power, only a dense vertical cracked microstructure can be obtained.
Proceedings Papers
ITSC 2017, Thermal Spray 2017: Proceedings from the International Thermal Spray Conference, 36-41, June 7–9, 2017,
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In this paper, yttria-stabilized zirconia (YSZ) coatings were prepared by plasma spraying of ready-to-spray suspensions provided by three different manufacturers. High-enthalpy hybrid water-argon plasma torch WSPH 500 was successfully used for deposition of coatings with porous and columnar microstructure consisting of tetragonal non-transformable phase. Sensitivity of the deposition process to variation of deposition conditions was also evaluated by the change of suspension injection point position. Slight differences in the microstructures of the deposited coatings (in particular character of porosity and mutual bonding of the microsplats) were reflected in slight but measurable differences in hardness and wear resistance of the coatings indicating changes in the coating cohesion. Tensile adhesion/cohesion strength of the coatings was found to be in the range of 9 to 15 MPa. High coating porosity desirable for low thermal conductivity combined with high suspension feed rate (from about 100 to 120 ml/min in this study) makes the WSP-H coatings promising for further development for example in thermal barrier coatings applications.
Proceedings Papers
ITSC 2017, Thermal Spray 2017: Proceedings from the International Thermal Spray Conference, 296-301, June 7–9, 2017,
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A method measuring the thermal conductivity and the interfacial thermal resistance of thermal barrier coatings (TBCs) which consist of metallic bond-coats (BCs) and ceramics top-coats (TCs) on superalloys was newly developed. It was based on the areal heat diffusion time method analysing the heat diffusion across multilayers. The developed method was experimentally verified using the BC and the TBC specimens coated by APS. It was found that there were the interfacial thermal resistance not only between the TC and the BC but also between the BC and the substrate. Furthermore, the thermal conductivities of the BC and the TC obtained from the BC and the TBC specimens by this method considering the interfacial thermal resistance were in good agreement with those measured from the free-standing specimen of each coating. Thus, it was confirmed that the newly developed method is effective to evaluate the thermal conductivity and the interfacial thermal resistance of the TBC.
Proceedings Papers
ITSC 2017, Thermal Spray 2017: Proceedings from the International Thermal Spray Conference, 302-307, June 7–9, 2017,
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Cyclic oxidation failure of Atmospheric Plasma Sprayed Thermal Barrier Coatings systems (APS TBCs), commonly used to insulate hot sections in gas turbines, usually results from the spallation of the ceramic top coat. Consequently, in order to predict such spalling phenomena, understanding the mechanisms for cracks initiation and propagation in thermal barrier coatings is of utmost concern for engine-makers. Failure of the TBC is strongly related to the thermal and mechanical properties of each component of the multi-materials system (substrate, bond coat and ceramic) but also to the response of the TBC as a whole. The purpose of the work is to assess the mechanical behaviour of thick TBC using experimental approach for TBC standard lamellar, porous and microcracked microstructure (classically obtained through APS coatings). The experimental characterisation of the mechanical behaviour of the ceramic top coat of the TBC is addressed on specifically designed and prepared free-standing specimens using three points bending (3PB) tests and Small Punch Testing (SPT). The tests are performed on free-standing top coats made of YSZ in the as deposited states and for specimens that undergone isothermal aging at 1100°C for various durations (1h, 10h and 100h). The results of test performed at room temperature using both mechanical testing techniques are compared. This allows to show the evolution of mechanical properties after thermal aging. Tests performed at 850°C in the SPT ring show that the evolution of properties resulting from this aging may be different at room temperature as compare to 850°C.
Proceedings Papers
ITSC 2017, Thermal Spray 2017: Proceedings from the International Thermal Spray Conference, 380-381, June 7–9, 2017,
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Environmental barrier coatings (EBC) are currently being investigated to protect ceramic matrix composite (CMC) turbine engine components in water-vapor rich combustion environments. Dense, crack-free, uniform and well-adhered coatings are demanded for this purpose. This paper represents an assessment of different thermal spray techniques for deposition of Yb 2 Si 2 O 7 and silicon (Si) EBC layers. Plasma spraying of refractory silicates is known to be complicated by undesired glass transition due to rapid solidification as well as evaporation of Si-bearing species during spraying. Plasma spraying of low-density Si also requires careful optimizations as it tends to oxidize during spraying, particularly at atmospheric conditions. Bearing these problems in mind, the Yb 2 Si 2 O 7 coatings were deposited by atmospheric plasma spraying (APS), high-velocity oxygen-fuel spraying (HVOF), and plasma-spray physical vapor deposition (PS-PVD) techniques. As-sprayed microstructure, amorphous content and phase composition of the coatings were analyzed. Based on the findings, the advantages and disadvantages of each method over other techniques are discussed with respect to process parameters and material properties.
Proceedings Papers
ITSC 2017, Thermal Spray 2017: Proceedings from the International Thermal Spray Conference, 382-387, June 7–9, 2017,
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Improvement in the performance of thermal barrier coating systems (TBCs) is one of the key objectives for further development of gas turbine applications. The material most commonly used as TBC topcoat is yttria stabilised zirconia (YSZ). However, the usage of YSZ is limited by the operating temperature range which in turn restricts the engine efficiency. Materials such as pyrochlores, perovskites, rare earth garnets, etc. are suitable candidates which could replace YSZ as they exhibit lower thermal conductivity and higher phase stability at elevated temperatures. The objective of this work was to investigate different multi-layered TBCs consisting of advanced topcoat materials fabricated by Suspension Plasma Spraying (SPS). The investigated topcoat materials were YSZ, dysprosia stabilised zirconia, gadolinium zirconiate, cerium doped YSZ and yttria fully stabilised zirconia. All topcoats were deposited with TriplexPro-210 plasma spray gun and radial injection of suspension. Lifetime of these samples was examined by thermal cyclic fatigue and thermal shock testing. Microstructure analysis of as-sprayed and failed specimens was performed with scanning electron microscope. The failure mechanisms in each case have been discussed in this article. The results show that SPS could be a promising route to produce multilayered TBCs for high temperature applications.
Proceedings Papers
ITSC 2017, Thermal Spray 2017: Proceedings from the International Thermal Spray Conference, 388-393, June 7–9, 2017,
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ZrO 2 -Y 2 O 3 (YSZ) thermal barrier coatings (TBCs) were manufactured via conventional Air Plasma Spray (APS), Suspension Plasma Spray (SPS) and an additional technology hereby termed Finely-dispersed-particle Air Plasma Spray (FAPS). The FAPS processing employs the exact same classification of finely dispersed particles as used in SPS; however, whereas SPS uses a liquid medium, in the case of FAPS the particles are fed conventionally via a carrier gas into the plasma spray torch by using a newly developed powder feeder for fine (suspension-like) particles (NRC patented technology). These finely dispersed YSZ particles consist of irregularly shaped (fluffy-like) agglomerates made from individual nano-sized particles. The conventional APS YSZ TBC was sprayed via a Metco 3MB torch, whereas, both SPS and FAPS YSZ TBCs were sprayed using the Mettech Axial III torch (using the same set of spray parameters). Both SPS and FAPS YSZ TBCs exhibited porous and vertically-cracked microstructures. The conventional APS YSZ TBC microstructure exhibited the traditional lamellar morphology. Elastic modulus, hardness and thermal conductivity values were evaluated for all YSZ TBCs. Microstructures and phase analysis were investigated via SEM and XRD.
Proceedings Papers
ITSC 2017, Thermal Spray 2017: Proceedings from the International Thermal Spray Conference, 456-461, June 7–9, 2017,
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NiCoCrAlY coatings are widely used as bond coats for ceramic thermal barrier coatings (TBCs) and oxidation and corrosion protective overlay coatings in industrial gas turbines. High temperature oxidation behaviour of NiCoCrAlYs has a great influence on the coating performance and lifetime of TBCs. A promising route to decrease the oxidation rate of such coatings is post-coating surface modification which can facilitate formation of a uniform alumina scale with a considerably slower growth rate compared to the as-sprayed coatings. In this work, the effect of surface treatment by means of shot peening and laser surface melting (LSM) on the oxidation resistance of high velocity air-fuel (HVAF) sprayed NiCoCrAlY coatings was studied. Isothermal oxidation was carried out at 1000 °C for 1000h. Results showed that the rough surface of as-sprayed HVAF sprayed coatings was significantly changed after shot peening and LSM treatment, with a compact and smooth appearance. After the exposure, the oxide scales formed on surface-treated NiCoCrAlY coatings showed different morphology and growth rate compared to those formed on as-sprayed coating surface. The oxidation behaviour of surface treated HVAF-sprayed NiCoCrAlY coatings were revealed and discussed.
Proceedings Papers
ITSC 2017, Thermal Spray 2017: Proceedings from the International Thermal Spray Conference, 462-466, June 7–9, 2017,
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Depending on the size and type defects of nickel-based alloy turbine blades two procedures are used mainly: cladding and high temperature brazing. The repair brazing of turbine blades is used to regenerate cracks and surface defects and is the focus of this work. In this contribution a two stage hybrid repair brazing process is presented which allows reducing the current process chain for repair brazing turbine blades. In the first stage of this process the filler metal (NiCrSi) then the hot gas corrosion protective coating (NiCoCrAlY) and finally the aluminium are applied in this order by atmospheric plasma spraying. In the second stage of this hybrid technology the applied coating system undergoes a heat treatment in which brazing and aluminising are combined. The temperature-time regime has an influence on the microstructure of the coating which is investigated in this work.
Proceedings Papers
ITSC 2017, Thermal Spray 2017: Proceedings from the International Thermal Spray Conference, 467-472, June 7–9, 2017,
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In this work, interdiffusion between different nickel-based superalloys and two MCrAlY bond coats, containing different chemical compositions, is investigated. To determine the influence of the coating deposition process, the MCrAlY bond coats were applied using two different spraying processes, high velocity oxygen fuel spraying (HVOF) and low-pressure plasma spraying (LPPS). Of primary interest is the evolution of Kirkendall porosity, which can form at the interface of substrate and bond coat and depends largely on the chemical compositions of the coating and substrate. Experimental evidence suggested also a dependence on the coating deposition process. Formation of porosity at the interface causes a degradation of the bonding strength between substrate and coating, with functional breakdown of the coating system as a worst result. After coating deposition, the samples were annealed at 1050 °C for varying test periods up to 2000 hours. Microstructural and compositional analyses were performed to determine and to evaluate the Kirkendall porosity. The results reveal a strong influence of both the spraying process and the chemical composition. The amount of Kirkendall porosity formed, as well as the location of appearance and the shape, is largely influenced by the coating deposition process. In general, samples with bond coats applied by means of HVOF show accelerated element diffusion. It is hypothesized that recrystallization of the substrate material, as a consequence of the surface treatment prior to coating deposition, is the main root cause for these observations.
Proceedings Papers
ITSC 2017, Thermal Spray 2017: Proceedings from the International Thermal Spray Conference, 525-531, June 7–9, 2017,
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Cr 3+ Photoluminescence piezo-spectroscopy (CPLPS) has been developed as a non-destructive inspection technique for the measurement of residual stresses within the thermally grown oxide (TGO) in thermal barrier coatings (TBCs). In this study, plasma spray - physical vapor deposition (PS-PVD) process was used to deposit yttria stabilized zirconia (YSZ) topcoat with quasi-columnar structures. Evolution of the microstructures and residual stress distribution in such kind structured TBCs before and after thermal cycle test on burner rigs were investigated. The accumulated tensile stress in the as-sprayed ceramic topcoat changed to compressive state after 100 cycles, and then gradually increased. In addition, the mapping compressive stresses in the TGO measured through the ceramic topcoat surface decreased rapidly firstly and then essentially maintain at a relatively stable value with further testing. Moreover, the pre-oxidation of the bondcoat could significantly affect the stress distribution in the TGO, in contrast, no obviously influence on the stresses in the YSZ topcoat.
Proceedings Papers
ITSC 2017, Thermal Spray 2017: Proceedings from the International Thermal Spray Conference, 532-536, June 7–9, 2017,
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NiCrAlYSi-BaF 2 /CaF 2 -Polyester material was developed for abradable seal of high-pressure compressor in gas turbine. The NiCrAlYSi-BaF 2 /CaF 2 -Polyester coatings were deposited on Ni-based superalloy substrates with an air plasma spray system. Friction and wear properties of the coatings were investigated in view of friction coefficient and wear morphologies of the coatings. The morphologies of the coatings were evaluated by field emission scanning electron microscopy. The results showed that NiCrAlYSi-7%BaF 2 /CaF 2 -10%Polyester coating exhibits lower macrohardness, lower friction coefficient and excellent abradability.
Proceedings Papers
ITSC 2017, Thermal Spray 2017: Proceedings from the International Thermal Spray Conference, 697-702, June 7–9, 2017,
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The microstructure of thermal barrier coatings (TBC) plays an important role in the thermal cycling behavior of TBCs. In this study, ceramic coatings with different pore structures were prepared by atmospheric plasma spraying (APS). Graphite with different morphology was used as pore former to adjust the pore structure of the coatings. Then, the thermal cycling behavior of TBCs with different structure was characterized. By depositing a porous 8YSZ layer on the conventional 8YSZ layer, the thermal cycling life of TBCs can be improved. However, when the porosity of the porous layer increased to about 30%, the porous layer detached from the under layer after several cycles. An elastic energy model was applied to explain the thermal cycling behavior of TBCs
Proceedings Papers
ITSC 2017, Thermal Spray 2017: Proceedings from the International Thermal Spray Conference, 703-708, June 7–9, 2017,
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As a new processing technology, plasma spray physical vapor deposition (PS-PVD) is capable to deposit coatings out of the vapor phase with high deposition rate. Moreover, the resulting quasi-columnar coatings were unique, hardly deposited by other process. Due to its low thermal conductivity and excellent superior strain tolerance, quasi-columnar coatings attract much attention in the thermal barrier coatings (TBCs) field. In this paper, the morphology variation of the quasi-columnar yttria stabilized zirconia (YSZ) coatings deposited under different conditions was investigated. Combined the morphology of the initial deposits during short time spraying, the deposition mechanism of quasi-column microstructure was concluded. It can be found that the quasi-column coating was formed by co-deposition of vapor phases and solid particles. The vapor phase can develop into column, while the solid particles not only intensified the shadowing effect but also destroyed the nucleus during vapor deposition.
Proceedings Papers
ITSC 2017, Thermal Spray 2017: Proceedings from the International Thermal Spray Conference, 709-713, June 7–9, 2017,
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The growth behavior of thermally grown oxide (TGO) on the bond coats of thermal barrier coatings (TBCs) has a significant impact on the TBCs lifetime. The splashed particles on the thermally sprayed MCrAlY bond coat surface have weak bonding with the underlying bulk coating, which will lead to the formation of mixed oxides and promote the failure of TBCs. In this study, the interface between the splashed particles and underlying bond coating was healed by vacuum pre-treatment (pre-diffusion). The bond coats were divided into three groups by the ways of pre-treatments before deposition of the ceramic top coating. Two groups of bond coats were subjected to two different pre-treatments (pre-diffusion-oxidation and pre-oxidation) prior to cyclic oxidation tests. The third bond coats were directly subjected to the same cyclic oxidation tests. Results show that the different TBCs present different cyclic oxidation behavior. The TBCs with pre-diffusion-oxidation exhibited the best cyclic oxidation resistance, and the ones without pre-treatment had the worst cyclic oxidation resistance. It is revealed that the healing of interface between the splashed particles and underlying bulk coating can effectively improve the cyclic oxidation resistance. This study will benefit the development of advanced TBCs with long lifetime and high reliability.