Enhanced thick thermal barrier coatings that exhibit varying porosity (original) (raw)
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Materials and Corrosion, 2004
The microstructural change, crack initiation and spallation of a vacuum plasma sprayed (VPS) thermal barrier coating on an INCONEL-738 superalloy substrate were investigated after successive 300 h thermal cycles at 1050 8C. The coating was characterised using Raman spectroscopy, scanning electron microscopy (SEM) energy dispersive X-ray analysis (EDX) and Auger electron spectroscopy (AES). Localised micro-cracks at the yttrium (III) oxide stabilised zirconium (IV) oxide (YSZ) ceramic coating/thermally grown oxide (TGO) interface were observed after 8 cycles. Spallation of the YSZ coating occurred after approximately 21 cycles. Significant amounts of the elements titanium, tantalum and chromium were found within the TGO together with the formation of nickel, cobalt and chromium-rich oxides at this TGO/YSZ interface.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2005
The efficacy of ceramic thermal barrier coatings (TBCs) used to protect and insulate metal components in engines increases with the thickness of the TBCs. However, the durabilities of thick TBCs deposited using conventional ceramics-coating deposition methods have not been adequate. Here, we demonstrate the feasibility of depositing highly durable, 4 mm thick TBCs of ZrO 2 -7 wt.% Y 2 O 3 (7YSZ) on bond-coated superalloy substrates using the solution-precursor plasma spray (SPPS) method. It was found that the average thermal cycling life of the 4 mm thick SPPS TBCs is 820 cycles. While most of the conventional air plasma-sprayed (APS) coatings of the same composition and thickness deposited on identical bond-coated superalloy substrates were found to be detached partially from the substrates in the as-sprayed condition, the APS TBC that was intact failed after 40 thermal cycles. The dramatic improvement in the thermal cycling life in the SPPS TBCs can be attributed to: (i) the significantly higher in-plane indentation-fracture toughness (over five-fold) in the SPPS TBCs over APS TBCs and (ii) the presence of the vertical cracks in SPPS TBCs resulting in a high degree of strain tolerance. The large thickness of the SPPS coatings also allowed us to characterize the mechanical properties of the ceramic top-coat in some detail. To that end, we report here the results from indentation-toughness tests and uniaxial-compression tests on the SPPS TBCs and the reference APS TBCs.
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.
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.
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
Mechanical properties of solution-precursor plasma-sprayed thermal barrier coatings
Surface & Coatings Technology, 2008
The microstructure of thermal barrier coatings (TBCs) of 7 wt.% Y 2 O 3 stabilized ZrO 2 (7YSZ) deposited using the solution-precursor plasma spray (SPPS) method has: (i) controlled porosity, (ii) vertical cracks, and (iii) lack of large-scale "splat" boundaries. An unusual feature of such SPPS TBCs is that they are well-adherent in ultra-thick forms (~4 mm thickness), where most other types of ultra-thick ceramic coatings fail spontaneously. Here a quantitative explanation is provided as to why as-deposited ultra-thick SPPS TBCs are so well-adherent. The mode II toughness of thin (0.2 mm) SPPS TBCs has been measured using the "barb" shear test, which is found to be 66 J m − 2 . Residual stresses in SPPS TBCs of thickness 0.2, 1.5, and 4.0 mm have been estimated using a microstructure-based object-oriented finite element (OOF) method. These stresses are found to be low, as a result of the strain-tolerant microstructure of the SPPS TBCs. The corresponding strain energy release rates that drive mode II cracks in the three different thickness SPPS TBCs have been found to be less than the mode II toughness.
Journal of Thermal Spray Technology
High-enthalpy hybrid water/argon-stabilized plasma (WSP-H) torch may be used for efficient deposition of coatings from dry powders, suspensions, and solutions. WSP-H torch was used to deposit two complete thermal barrier coatings (TBCs) with multilayered topcoat. NiCrAlY was used as bond-coat and deposited on nickelbased superalloy substrates. Topcoat consisted of up to three sublayers: (i) yttria-stabilized zirconia (ZrO 2-8 wt.%Y 2 O 3-YSZ) deposited from solution, (ii) gadolinium zirconate (Gd 2 Zr 2 O 7-GZO) deposited from suspension, and (iii) optional yttrium aluminum garnet (Y 3 Al 5 O 12-YAG) overlayer deposited from suspension. Each of the sublayers was intended to provide different functionalities, namely improved fracture toughness, low thermal conductivity, and high erosion resistance, respectively. High-temperature performance and thermal shock resistance of the deposited coatings were tested by thermal cycling fatigue ''TCF'' test (maximum temperature 1100°C, 1 h dwell per cycle) and ''laser-rig'' test (maximum temperature * 1530°C, 5 min dwell per cycle) exposing samples to isothermal and gradient thermal conditions, respectively. In both tests, coatings endured around 800 test cycles which shows great potential for further development of these layers and their application in demanding thermal conditions. Analysis of the samples after the test showed microstructural changes and identified reason of ultimate coating failure. Keywords hybrid plasma torch Á solution precursor spraying Á suspension spraying Á thermal barrier coatings (TBCs) Á thermal cycling Á water stabilized plasma Á yttria stabilized zirconia (YSZ)
Structural Stability of Thermal Barrier Coatings Produced by Thermal Spraying
One type of functionally graded coating system is thermal barrier coating (TBC), where the combination of ceramic and metallic coating is used both to reduce the temperature and to increase oxidation and corrosion resistance of the substrate. TBCs usually consist of the top ceramic coating based on YSZ (ZrO2+Y2O3) and the metallic bond coating of M-CrAlY type, where M means Ni, Co or their appropriate combination. Electron beam physical vapour deposition (EB-PVD) or vacuum (VPS), low pressure (LPPS) or atmospheric (APS) plasma spraying techniques are most frequently used as a deposition method. Despite some requirements on the product shape simplicity, the air plasma spraying offers high productivity, sufficient quality and much lower production costs in comparison with the EB-PVD technology. The contribution deals with high temperature structural stability of TBCs of YSZ + NiCrAlY and YSZ + CoNiCrAlY types that were produced by air plasma spraying on the INCONEL 713LC polycrystalli...
Engineered thermal barrier coatings deposited by suspension plasma spray
Materials Letters, 2017
Yttria stabilized zirconia (YSZ) is susceptible to CMAS (Calcium Magnesium Alumino Silicates) attack at high temperatures (>1200°C) which limits its durability. New ceramic materials which can overcome these high temperature challenges are highly desirable. This work investigates the feasibility of depositing two variations of three ceramic layered thermal barrier coatings. The first variation comprised of yttria as the top ceramic layer with gadolinium zirconate (GZ) as the intermediate layer and YSZ as the base layer. The second variation comprised of Yttrium Aluminum Garnet (YAG) as the top layer with gadolinium zirconate as the intermediate layer and YSZ as the base layer. Microstructural analysis of the as sprayed three layered TBCs were performed by SEM/EDS. Columnar microstructures with a relatively dense top layer were obtained in both the variations. The porosity content of the TBCs was measured by water intrusion and image analysis methods. Phase composition of each layer of the as sprayed TBCs was analyzed using XRD. YAG showed an amorphous phase whereas GZ showed a cubic defect fluorite phase and tetragonal phase was observed in YSZ. In the case of yttria, monoclinic and cubic phases were observed.