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Driven by the search for an optimum combination of particle velocity and process temperature to achieve dense hard metal coatings at high deposition efficiencies and powder feed rates, the high-velocity air-fuel spraying process (HVAF) was developed. In terms of achievable particle velocities and temperatures, this process can be classified between high-velocity oxy-fuel spraying (HVOF) and cold gas spraying (CGS). The particular advantages of HVAF regarding moderate process temperatures, high particle velocities as well as high productivity and efficiency suggest that the application of HVAF should be also investigated for the manufacture of MCrAlY (M = Co and/or Ni) bond coats (BCs) in thermal barrier coating (TBC) systems. In this work, corresponding HVAF spray parameters were developed based on detailed process analyses. Different diagnostics were carried out to characterize the working gas jet and the particles in flight. The coatings were investigated with respect to their microstructure, surface roughness and oxygen content. The spray process was assessed for its effectiveness. Process diagnostics as well as calculations of the gas flow in the jet and the particle acceleration and heating were applied to explain the governing mechanisms on the coating characteristics. The results show that HVAF is a promising alternative manufacturing process. [ABSTRACT FROM AUTHOR]

The integrity and properties of ceramic coatings produced by atmospheric plasma spraying are highly controlled by the splat morphology and splat interconnection. In this study, the influence of selected parameters (spray angle, surface velocity of the spray gun, and substrate temperature) on splat morphology and coating microstructure was investigated. A favorite set of spray gun parameters, of which their effects on splat morphology and coating microstructure have been verified by previous experiments, were used to conduct the experiments for the present work. It was found that depositing fully molten particles on a hot substrate increases the fraction of disk-like splats by about 60% at the expense of the fraction of irregular splats. Preheating the substrate also increases the pore count and level of coating porosity, while it does not influence the density of segmentation cracks. In contrast, the surface velocity of the spray gun does not affect the splat morphology while a slow speed decreases the coating porosity and plays a significant role in generating segmentation cracks. Shifting the spray angle by 15° distorts up to 20% of disk-like splats and slightly decreases the porosity level. However, changing the spray angle does not affect the generation of segmentation cracks. [ABSTRACT FROM AUTHOR]

The high-temperature oxidation and solid particle erosion of thermal barrier coating (TBC) system which consists of a 8 wt.% yttria-partially stabilized zirconia (YSZ) top coat and CoNiCrAlY bond coat deposited on Inconel 718 substrate via air plasma spraying (APS) process are studied experimentally. Isothermal oxidation tests of the APS-TBCs are conducted at 1050, 1100 and 1150 °C in air for up to 1970 h. Solid particle erosion tests are also performed on both as-deposited and heat-treated APS-TBC specimens at selected particle impingement angles and velocities in room temperature. The scanning electron microscopy (SEM) analyses of the cross sections of the APS-TBC specimens after the oxidation tests show the formation of thermally grown oxide (TGO) scale due to the oxidation of CoNiCrAlY bond coat, and the oxidation kinetics of TGO growth is described by the parabolic rate equation. The failure of the APS-TBC system under isothermal oxidation is associated with the spallation of the top coat through propagation and coalescence of cracks along the coating interface, which is affected by the TGO growth. The solid particle erosion rate of the top coat is found to increase with impingement angle and reach the maximum erosion rate at normal impingement of particles. The erosion resistance of the APS-TBC is observed to increase after the APS-TBC specimen has been exposed at 1100 and 1150 °C for 72 h, probably due to the sintering effect on the top coat, which results in reduction of top coat porosity. [ABSTRACT FROM AUTHOR]

Ceramics coatings produced by atmospheric plasma spraying (APS) on steel alloys parts involve high residual stress level that can affect the coating properties and integrity. Most of the experimental studies were focused on the stress evaluation into APS-produced coatings onto static targets. However, these studies have not been extended to stresses induced on rotating samples. Indeed, the thermal background and the deposition way onto a static target is different from a rotating one, resulting in indifferent coating residual stresses. The aim of this work is to estimate and understand the real-time stress evolution from the sandblasting process to the coating formation while the substrate is fixed at a rotating support thanks to a new system developed in the laboratory. Three different sources of stresses are analyzed in this paper: stresses induced by sandblasting, substrate heating, and splat quenching followed by the cooling down of the coating/substrate composite. Resulting stresses measurements are compared with after test ones made by incremental hole drilling. [ABSTRACT FROM AUTHOR]

The mechanical properties of plasma-sprayed thermal barrier coatings (TBC) are of great scientific and technological significance for the design and fabrication of TBC systems. The ultrasonic method combined with a sing-around method for mechanical properties measurement of TBC is deduced and the elastic modulus can be determined in the spray, or longitudinal, direction, and the transverse direction. Tested specimens of plasma-sprayed TBC are detached from the substrate and treated with thermal exposure at 1400 °C. The elastic moduli along the longitudinal and transverse directions of the TBCs are measured by different types of ultrasonic waves combined with a sing-around method, while the Poisson's ratio is also obtained simultaneously. The experimental results indicate that the magnitude of longitudinal elastic modulus is larger than that of the transverse one, and thus the plasma-sprayed TBC has an anisotropic mechanical property. Moreover, the elastic moduli along both longitudinal and transverse directions change with high-temperature exposure time, which consists of a rapid increasing stage followed by a slow decreasing stage. In addition, the magnitude of Poisson's ratio increases slightly from 0.05 to 0.2 with the high-temperature exposure time. Generally, the microstructures in the plasma-sprayed coatings and their evolution in a high-temperature environment are the main causes of the varying anisotropic mechanical properties. [ABSTRACT FROM AUTHOR]

A computer-controlled laser test rig (using a CO laser) offers an interesting alternative to traditional flame-based thermal gradient rigs in evaluating thermal barrier coatings (TBCs). The temperature gradient between the top and back surfaces of a TBC system can be controlled based on the laser power and a forced air back-face cooling system, enabling the temperature history of complete aircraft missions to be simulated. An air plasma spray-deposited TBC was tested and, based on experimental data available in the literature, the temperature gradients across the TBC system (ZrO-YO YSZ top coat/CoNiCrAlY bond coat/Inconel 625 substrate) and their respective frequencies during air-to-air combat missions of fighter jets were replicated. The missions included (i) idle/taxi on the runway, (ii) take-off and climbing, (iii) cruise trajectory to rendezvous zone, (iv) air-to-air combat maneuvering, (v) cruise trajectory back to runway, and (vi) idle/taxi after landing. The results show that the TBC thermal gradient experimental data in turbine engines can be replicated in the laser gradient rig, leading to an important tool to better engineer TBCs. [ABSTRACT FROM AUTHOR]

The suspension plasma spraying (SPS) process has been developed to permit the feeding of sub-micrometer-sized powder into the plasma plume. In contrast to electron beam-physical vapor deposition and plasma spray-physical vapor deposition, SPS enables the cost-efficient deposition of columnar-structured coatings. Due to their strain tolerance, these coatings play an important role in the field of thermal barrier coatings (TBCs). In addition to the cost-efficient process, attention was turned to the TBC material. Nowadays, yttria partially stabilized zirconia (YSZ) is used as standard TBC material. However, its long-term application at temperatures higher than 1200 °C is problematic. At these high temperatures, phase transitions and sintering effects lead to the degradation of the TBC system. To overcome those deficits of YSZ, Mg-Al-spinel was chosen as TBC material. Even though it has a lower melting point (~2135 °C) and a higher thermal conductivity (~2.5 W/m/K) than YSZ, Mg-Al-spinel provides phase stability at high temperatures in contrast to YSZ. The Mg-Al-spinel deposition by SPS resulted in columnar-structured coatings, which have been tested for their thermal cycling lifetime. Furthermore, the influence of substrate cooling during the spraying process on thermal cycling behavior, phase composition, and stoichiometry of the Mg-Al-spinel has been investigated. [ABSTRACT FROM AUTHOR]

Ceramic functional coatings are frequently applied to structural materials, covering a wide range of thermomechanical and electrochemical applications. The main limiting feature is their reliability when subjected to cyclic transient thermal stresses. The study described in this article is a continuation of earlier research study focused on acoustic emission (AE) monitoring of the thermomechanical aging effects in ceramic coatings. Here, emphasis is placed on the usefulness of combining AE short-term monitoring with finite element modeling (FEM) to predict the performance of such coatings when subjected to cyclic thermal loads. The FEM study presented in this article is based on a three-dimensional, time-dependent approach, of the stress fields that developed within the coatings during the post-deposition cooling step and the thermal cycling. Experiments were conducted using yttrium-stabilized zirconia (YSZ) and Alumina (AlO) ceramic coatings combined with a NiCr-based intermetallic bond coat. [ABSTRACT FROM AUTHOR]

In an effort to improve the performance of heat engines at high temperatures, advanced surface coatings have been developed from complex perovskites. Materials of Ba(Mg1/3Ta2/3)O3 and La(Al1/4Mg1/2Ta1/4)O3 composition were synthesized and applied as ceramic topcoats of thermal barrier coating (TBC) systems by atmospheric plasma spraying (APS) in single layer and in double-layer combination with conventional yttria stabilized zirconia (YSZ). Microstructural and phase analyses reveal that plasma spraying of complex perovskites is accompanied with the formation of vertical crack networks and secondary oxide phases which influence the failure mechanism of the TBCs. The low value of fracture toughness for the complex perovskites and the thermally grown oxide at the topcoat-bondcoat interface of the TBCs are, however, the major factors which lead to the coating failure on thermal cycling at about 1250 °C. [ABSTRACT FROM AUTHOR]

The thermal-barrier coatings (TBC) sprayed on hot-section components of aircraft turbine engines commonly consist of a partially stabilized zirconia top-coat and an intermediate bond-coat applied on the metallic substrate. The bond-coat is made of an aluminide alloy that at high engine temperatures forms thermally grown oxides (TGO). Although formation of a thin layer of aluminum oxide at the interface between the ceramic top-coat and the bond-coat has the beneficial effect of protecting the metallic substrate from hot gases, oxide formation at splat boundaries or pores within the bond-coat is a source of weakness. In this study, plasma-sprayed TBC specimens are manufactured from two types of bond-coat powders and exposed to elevated temperatures to form oxides at the ceramic-bond-coat boundary and within the bond-coat. The specimens are then tested using nondestructive evaluation (NDE) and destructive metallography and compared with the as-manufactured samples. The objective is to determine if NDE can identify the oxidation within the bond-coat and give indication of its severity. While ultrasonic testing can provide some indication of the degree of bond-coat oxidation, the eddy current (EC) technique clearly identifies severe oxide formation within the bond-coat. Imaging of the EC signals as the function of probe location provides information on the spatial variations in the degree of oxidation, and thereby identifies which components or areas are prone to premature damage. [ABSTRACT FROM AUTHOR]

The stiffness of air plasma sprayed (APS) thermal barrier coatings (TBCs) was determined from bending experiments combining strain analysis on a microstructural level with macroscopic mechanical parameters. Tests were performed with freestanding and attached TBCs, the latter either loaded in tension or in compression. Relationships are derived, which describe the TBC stiffness in a multilayer composite (attached TBC) and for a bimodular material that possess a lower stiffness in tension than in compression (stand-alone TBC). The increase of in-plane stiffness with increasing compressive stress emphasizes the importance of the spraying defects for the elastic response of the coating. [ABSTRACT FROM AUTHOR]

Porosity is an integral part of thermal barrier coatings (TBC) and is required to provide thermal insulation and to accommodate operational thermal stresses. The effective use of TBC in hot-section components of aircraft engines requires nondestructive testing (NDT) methods to detect porosity variations and measure thickness changes to reduce the risk of damage to the coating due to such variations. The eddy current method has been used to measure the thickness of a plasma-sprayed TBC coating and either ultrasonic or capacitance techniques have been applied to assess porosity content based on thickness values obtained using the eddy current tests. The porosity values estimated by the NDT methods have been confirmed by destructive testing, which included metallography and vacuum volumetric measurement using nitrogen absorption. [ABSTRACT FROM AUTHOR]

A durable ceramic thermal barrier coating is applied directly to a smooth, highly oxidation resistant intermetallic alloy in two layers. The first layer of ceramic is applied by low pressure plasma spraying and the second layer is applied by conventional atmospheric pressure plasma spraying. This approach would allow the use of plasma sprayed ceramic coatings in applications where a metallic bond coat is not desirable. [ABSTRACT FROM AUTHOR]