Intensive R&D work of more than one decade has demonstrated many unique coating properties, particularly for oxide ceramic coatings fabricated by suspension thermal spraying technology. Suspension spraying allows producing yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBC) with columnar microstructure, similar to those produced by electron-beam physical vapor deposition (EB-PVD), and vertically cracked morphologies, with a generally low thermal conductivity. Therefore, suspension sprayed YSZ TBCs are seen as an alternative to EB-PVD coatings and those produced by conventional air plasma spray (APS) processes. Nonetheless, the microstructure of the YSZ topcoat is strongly influenced by the properties of the metallic bondcoat. In this work, direct laser interference patterning (DLIP) was applied to texture the surface topography of Ni-alloy based plasma sprayed bondcoat. Suspension plasma spraying (SPS) was applied to produce YSZ coatings on top of as-sprayed and laser-patterned bondcoat. The samples were characterized in terms of microstructure, phase composition and thermal cycling performance. The influence of the bondcoat topography on the properties of suspension sprayed YSZ coatings is presented and discussed.

In this study, a novel self-healing concept is considered in order to increase the lifetime of thermal barrier coatings (TBCs) in modern gas turbines. For that purpose, SiC healing particles were introduced to conventional 8YSZ topcoats by using various plasma spray concepts, i.e., composite or multilayered coatings. All topcoats were sprayed by SG-100 plasma torch on previously deposited NiCrAlY bondcoats produced by conventional atmospheric plasma spraying. Coatings were subjected to thermal conductivity measurements by laser flash method up to 1000°C, isothermal oxidation testing up to 200h at 1100°C and finally thermal cyclic fatigue (TCF) lifetime testing at 1100°C. Microstructural coating evaluation was performed by scanning electronic microscope (SEM), in the as-produced and post high-temperature tested states. This was done to analyze the self-healing phenomena and its influence on the high-temperature performance of the newly developed TBCs.

Acquisition of a new LVPS and APS coating system at Delta Air Lines necessitated optimization of the coating parameters on both systems, especially for application of bond coat (LVPS) and top coat (APS) for a TBC coating system. To expedite the coating optimization, it was determined that a design of experiments (DOE) approach would best enable the establishment of the operating window for the two systems. Samples prepared were primarily evaluated for their performance while exposed to a cyclic oxidation cycle. Samples were also evaluated for the microstructure and composition using energy dispersive spectroscopy (EDS) analysis. Samples from the ceramic coating DOE were also evaluated for their erosion characteristics. Results indicate a low correlation between the individual bond coat parameters evaluated to the furnace cycle life. However, the top coat spray parameters were found to have a greater correlation to furnace cycle life and erosion performance.

As a critical technology, thermal barrier coatings (TBC) have been used in both aero engines and industrial gas turbines for a few decades, however, the most commonly used MCrAlY bond coats which control air plasma sprayed (APS) TBC lifetime are still deposited by the powders developed in 1980s. This motivates a reconsideration of development of MCrAlY at a fundamental level to understand why the huge efforts in the past three decades has so little impact on industrial application of MCrAlY alloys. Detailed examination of crack trajectories of thermally cycled samples and statistic image analyses of fracture surface of APS TBCs confirmed that APS TBCs predominately fails in top coat. Cracks initiate and propagate along splat boundaries next to interface area. TBC lifetime can be increased by either increasing top coat fracture strength (strain tolerance) or reducing the tensile stress in top coat or both. By focusing on the reduction of tensile stress in top coats, three new bond coat alloys have been designed and developed, and the significant progress in TBC lifetime have been achieved by using new alloys. Extremely high thermal cycle lifetime is attributed to the unique properties of new alloys, such as remarkably lower coefficient of thermal expansion (CTE) and weight fraction of β phase, absence of mixed / spinel oxides, and TGO self repair ability, which cannot be achieved by the existed MCrAlY alloys.

In this study, two types of thermal barrier coatings (TBC); duplex and functionally graded coatings were deposited on superalloy Nimonic 263 substrates using air plasma spray process. The duplex coating consists of YSZ top coat and NiCrAlY bond coat. The functionally graded coating consists of five layers with GZ as top layer, GZ+YSZ and YSZ+NiCrAlY as intermediate layers. The TBC samples were subjected to isothermal heat treatment at 1100 °C for 100 hours before undergoing thermal cyclic tests at 1200 °C up to 20% spallation to evaluate the oxidation and thermal fatigue resistance of the coatings. Results indicate that the functionally graded GZ TBC has a better cyclic life than the duplex YSZ TBC after isothermal heat treatment. The isothermal heat treatment also improved the thermal cyclic lifetime of the functionally graded GZ TBC by more than threefold in comparison to the as-sprayed GZ TBC.

Thermal barrier coatings (TBCs) consisting of a MCrAlY bond coat and a YSZ topcoat were air plasma sprayed onto Hastelloy X substrates. Samples were thermally cycled between 100 °C and 1100 °C and thermal fatigue failures were investigated via microstructure analyses. Final fatigue failure was caused by the formation of interface-parallel cracks in the topcoat, which was found to strongly related to the oxidation behavior of the bond coat. The development of oxide layers was therefore studied in detail and a thermo-kinetic model was used to explore the role of elemental diffusion in oxide formation.

This study evaluates the thermal cycling performance of thick thermal barrier coatings (TTBCs). YSZ topcoats with segmentation cracks were deposited by suspension plasma spraying (SPS) on Ni-base superalloy substrates with the aid of a CoNiCrAlY bond coat applied by HVOF spraying. The as-sprayed SPS coatings were characterized based on surface morphology, cross-sectional microstructure, and phase composition. Thermal cycling tests were then carried out on a burner rig that heated the coating surface to 1523 K, followed by quenching to 423 K using compressed air. The SPS coatings exhibited longer thermal shock life than atmospheric plasma sprayed (APS) YSZ, which is attributable to improved strain tolerance due to the presence of vertically segmented cracks.

Thermal cycle resistance of Ni-20Cr, Ni-50Cr and CoNiCrAlY coatings manufactured by air plasma spraying was investigated according to a Japanese Industrial Standard "Testing method for thermal cycle resistance of oxidation resistant metallic coatings (JIS H 8452)” established in 2008. The specimens were exposed at 1000 °C and 1093 °C in air under cyclic heating and cooling condition up to 100 times. The thermal cycle resistance of oxidation-resistant metallic coatings was found to depend strongly on the testing temperature and the chemical composition of the coating materials. In the thermal cycle test at 1000 °C, the remarkable failure was not observed in any specimen. However, in the thermal cycle test at 1093 °C, although the Ni-20Cr coating caused the spalling on the whole surface of coating, the Ni-50Cr and the CoNiCrAlY coatings exhibited the excellent thermal cycle resistance even after applying the thermal cycles of 100 times, The CoNiCrAlY coating showed the mass gain with increasing the numbers of thermal cycle due to the preferential oxidation between the splats of the thermal spray particles. Furthermore, the failure behavior of specimens was investigated in detail by SEM, XRD and EPMA etc.

High temperature thermal fatigue causes the failure of Thermal Barrier Coating (TBC) systems. Due to the difference in thickness and microstructure between thick TBCs and traditional thin TBCs, they cannot be assumed a-priori to possess the same failure mechanisms. Thick TBCs, consisting of a CoNiCrAlY bond coat and Yttria Partially Stabilised Zirconia top coat with different values of porosity, were produced by Air Plasma Spray. Thermal fatigue resistance limit of TBCs was tested by Furnace Cycling Tests (FCT) according to the specifications of an Original Equipment Manufacturer (OEM). TBC systems were analyzed before and after FCT. The morphological and chemical evolution of CoNiCrAlY/TGO microstructure was studied. Sintering effect, residual stress, phase transformation and fracture toughness were evaluated in the ceramic Top Coat. All the tested samples passed FCT according to the specification of an important OEM. Thermal fatigue resistance increases with the amount of porosity in the top coat. The compressive in-plane stresses increase in the TBC systems after thermal cycling, nevertheless the increasing rate has a trend contrary to the porosity level of top coat. The data suggest that the spallation happens at the TGO/Top Coat interface. The failure mechanism of thick TBCs subjected to thermal fatigue was eventually found to be similar to the failure mechanism of thin TBC systems made by APS.

High temperature thermal fatigue causes the failure of Thermal Barrier Coating (TBC) systems. This paper addresses the development of thick TBCs, focusing attention on the microstructure and the porosity of the Yttria Partially Stabilized Zirconia (YPSZ) coating, in relation to its resistance to thermal cycling fatigue. Thick TBCs, with different grade of porosity, were produced by means of a CoNiCrAlY bond coat and Yttria Partially Stabilised Zirconia top coat, both sprayed by Air Plasma Spray. The thermal fatigue resistance of new TBC systems and the evolution of the coatings before and after thermal cycling were evaluated. The limit of thermal fatigue resistance increases with amount of porosity in the top coat. Raman analysis shows that the compressive in-plane stress increases in the TBC systems after thermal cycling, nevertheless the increasing rate has a trend contrary to the porosity level of top coat.

Functionally graded material (FGM) designs aim to combat challenges posed by harsh and conflicting operational requirements. There is a great demand for manufacturing technologies that can fabricate net-shape components incorporating the FGM design concepts. The focus of this paper is on the development of net-shape components incorporating functionally gradient material designs. This novel manufacturing scheme combines the use of re-useable mandrels to generate complex shapes with a unique thermal spray process called “hybrid spray” that can deposit functionally designed materials. This generic hybrid spray process combines arc spraying with either high velocity oxy-fuel (HVOF) or plasma spraying (APS). The resulting benefits are; ability to achieve continuous composition variation, high deposition rates combined with the high coating densities. The multi-material component designs aim to provide cost as well as performance advantages. The geometric resolution and dimensional accuracy of these functional components are presented. This paper also reports results on material compatibility and process parameter development tests. Functional properties such as high temperature resistance and thermal fatigue performance are also reported.

Plasma sprayed thermal barrier coatings often face the problems of spallation and cracking in service owing to their poor bond strength and high residual stresses. Functionally graded thermal barrier coatings with a gradual compositional variation from heat resistant ceramics to fracture resistant metals are proposed to mitigate these problems. In this paper, functionally graded Y2O3 stabilized ZrO2 (YSZ) / NiCoCrAlY composite coatings were prepared using pre-alloyed and spheroidized composite powders. The mechanical and thermal properties of graded YSZ/NiCoCrAlY composite coatings, such as elastic modulus, bond strength, coefficient of thermal expansion, thermal cycling and oxidation resistance were investigated. Results showed that the bond strength and thermal cycling resistance of FGM coatings were much better than that of the duplex coatings. The coefficient of thermal expansion and elastic modulus changed gradually through the 5-layer functionally graded coating.

Trends of turbine blades of advanced aircraft gas turbine engines are to increase output power of the engines, to increase engine efficiency, and to reduce environmental emission, and thus, higher operating temperatures of the engines are required. One of the technologies for increasing the operating temperature is a thermal barrier splayed coating. A research paper claims that molybdenum silicide in a splayed coating has a self-healing capability for cracks formed in the coating by embedding the cracks with silicon dioxide formed from molybdenum silicide at high temperatures. This article discusses the methods for the extension of life of thermal barrier sprayed coatings by incorporating molybdenum silicide. It discusses monitoring method for detecting cracking conditions in heating and cooling cycles by signals of acoustic emission. A possibility of estimating fatigue life by utilizing an X-ray method for measuring residual stress is also considered.

This paper focuses on the mechanism responsible for crack expansion and contraction: the behavior of cracks in the ceramic bed of zirconium dioxide membranes formed by plasma spraying, and the thermal stresses caused by differences in thermal expansion factors between the base material and sprayed bed, under a thermally cyclic environment. The test sample was prepared by spraying NiCrAlY on a stainless-steel base material to form a substrate membrane, then covering it with sprayed 8YSZ (zirconium dioxide-type ceramic). To clarify the self-recoverability of fatigue cracks caused during the thermal cycle, membrane cracking and crack expansion/contraction is studied. Paper includes a German-language abstract.

This paper investigates the microstructure, bond adhesion, and high temperature and thermal fatigue performance of plasma-sprayed chromium carbide, nickel-chromium coatings after continuous exposure at high temperature inside a combustion chamber that simulates the extreme conditions existing in different industrial applications. Experiments on high temperature behavior under oxidative and neutral atmospheres and thermal fatigue tests are carried out in a test combustion chamber. Finally, the adhesion between the substrate and the coating layer is assessed using tensile tests. The paper discusses the results with special consideration of the microstructure development through the heat treatment. Paper includes a German-language abstract.

CrNiAlTi, NiCrBSi and WC-Ni coatings have been thermal and plasma sprayed projected over a stainless steel surface in order to protect it against heat and erosion actions encountered in power plant boilers. Their microstructure, porosity and microhardness have been measured. High temperature oxidation under an atmosphere similar to service conditions in power plants and thermal fatigue tests have also been performed in our experimental combustion chamber and, finally, the adhesion between the substrate and the coating layer has been evaluated by means of tensile tests. The obtained results have been discussed paying especial attention to the microstructural materials evolution due to thermal effects and coating projection methods.