Interface delamination of the thermal barrier coating subjected to local heating

To investigate the possible failure modes of the thermal barrier coating (TBC) used to protect the scramjet combustion chamber, the local heating via laser beam irradiation was utilized to simulate the service condition of high thermal flux and high temperature gradient. Firstly, the experimental method and process were described and the typical fracture morphology of the TBC under test were provided. Then, the theoretical and finite element modeling were carried out to study the temperature, deformation and stresses of the specimen when the top ceramic coat was subjected to local heating, and to demonstrate the mechanism on the failure of the TBC. It is revealed that the interface delamination shall appear and ultimately lead to the failure of the TBC under such thermal loading of local quick heating. According to the outcome of this study, the driving force of the interface delamination is influenced greatly by the key structural parameters and performance matching. Moreover, by utilizing the rules of the effects of these parameters on the fracture driving force, there is some possibility for the designer to optimize the performances of the TBC.

Hot-corrosion behaviors of overlay-clad yttria-stabilized zirconia coatings in contact with vanadate-sulfate salts

A dense clad overlay with chemical inertness was achieved on top of the plasma-sprayed YSZ thermal barrier coatings by laser in order to protect them from hot-corrosion attack. The Al2O3-clad YSZ coating exhibited good hot-corrosion behavior in contact with salt mixture of vanadium pentoxide (V2O5) and sodium sulfate (Na2SO4) for a longtime of 100 h at 1173 K. The LaPO4-clad YSZ coating showed corrosion resistance inferior to the Al2O3-clad one. Yttria was leached from YSZ by reaction between Y2O3 and V2O5, which caused progressive destabilization transformation of YSZ from tetragonal (t) to monoclinic (m) phase. The chemical inertness of the clad layers and the restrained infiltration of the molten corrosive salts by the dense clad layers were primary contributions to improvement of the hot-corrosion resistances.

Processing, microstructure and mechanical properties of air plasma-sprayed ceria-yttria co-stabilized zirconia coatings

Thermal barrier coatings (TBCs) are widely adopted to protect mechanical components in gas turbine engines operating at high temperature. Basically, the surface temperature of these components must be low enough to retain material properties within acceptable bounds and to extend component life. From this standpoint, air plasma-sprayed (APS) ceria and yttria co-stabilized zirconia (CYSZ) is particularly promising because it provides enhanced thermal insulation capabilities and resistance to hot corrosion. However, essential mechanical properties, such as hardness and Young's modulus, have been less thoroughly investigated. Knowledge of Young's modulus is of concern because it has a significant effect on strain tolerance and stress level and, hence, on durability. The focus of the present study was to determine the mechanical properties of APS CYSZ coatings. In particular, X-ray diffraction (XRD) is adopted for phase analysis of powders and as-sprayed coatings. In addition, scanning electron microscopy (SEM) and image analysis (IA) are employed to explore coating microstructure and porosity. Finally, the Young's modulus of the coating is determined using nanoindentation and a resonant method. The results obtained are then discussed and a cross-check on their consistency is carried out by resorting to a micromechanical model. © 2010 Blackwell Publishing Ltd.

Evaluation of WC-17Co and WC-10Co-4Cr thermal spray coatings by HVOF on the fatigue and corrosion strength of AISI 4340 steel

It is known that chromium electroplating is related to the reduction in the fatigue strength of base metal. However, chromium results in protection against wear and corrosion combined with chemical resistance and good lubricity. Environmental requirements are an important point to be considered in the search for possible alternatives to hard chrome plating. Aircraft landing gear manufactures are considering WC thermal spray coating applied by the high-velocity oxygen-fuel (HVOF) process an alternative candidate, which shows performance at least comparable to results, obtained for hard chrome plating. The aim of this study is to compare the influence of WC-17Co and WC-10Co-4Cr coatings applied by HVOF process and hard chromium electroplating on the fatigue strength of AISI 4340 steel, with and without shot peening. S-N curves were obtained in axial fatigue test for base material, chromium plated and tungsten carbide coated specimens. Tungsten carbide thermal spray coating results in higher fatigue strength when compared to hard chromium electroplated. Shot peening prior to thermal spraying showed to be an excellent alternative to increase fatigue strength of AISI 4340 steel. Experimental data showed higher axial fatigue and corrosion resistance in salt fog exposure for samples WC-10Co-4Cr HVOF coated when compared with WC-17Co. Fracture surface analysis by scanning electron microscopy (SEM) indicated the existence of a uniform coverage of nearly all substrates. (C) 2004 Elsevier B.V. All rights reserved.

Cr3C2-NiCr and WC-Ni thermal spray coatings as alternatives to hard chromium for erosion-corrosion resistance

Cr3C2-NiCr and WC-Ni coatings are widely used for wear applications at high and room temperature, respectively. Due to the high corrosion resistance of NiCr binder, Cr3C2-NiCr coatings are also used in corrosive environments. The application of WC-Ni coatings in corrosive media is 14 not recommended due to the poor corrosion resistance of the (pure Ni) metallic matrix. It is well known that the addition of Cr to the metallic binder improves the corrosion properties. Erosion-corrosion performance of thermal spray coatings is widely influenced by ceramic phase composition, the size of ceramic particles and also the composition of the metallic binder. In the present work, two types of HVOF thermal spray coatings (Cr3C2-NiCr and WC-Ni) obtained with different spray conditions were studied and compared with conventional micro-cracked hard chromium coatings. Both as-sprayed and polished samples were tested under two erosion-corrosion conditions with different erosivity. Tungsten carbide coatings showed better performance under the most erosive condition, while chromium carbide coatings were superior under less erosive conditions. Some of the tungsten carbide coatings and hard chromium showed similar erosion-corrosion behaviour under more and less erosive conditions. The erosion-corrosion and electrochemical results showed that surface polishing improved the erosion-corrosion properties of the thermally sprayed coatings. The corrosion behaviour of the different coatings has been compared using Electrochemical Impedance Spectroscopy (EIS) and polarization curves. Total material loss due to erosion-corrosion was determined by weight loss measurements. An estimation of the corrosion contribution to the total weight loss was also given. (c) 2007 Elsevier B.V. All rights reserved.

Corrosion studies on the plasma-sprayed nanostructured titanium dioxide coating

Purpose: The purpose of this paper is to report the resistance of plasma-sprayed titanium dioxide (TiO2) nanostructured coatings in a corrosive environment.----- Design/methodology/approach: Weight loss studies are performed according to ASTM G31 specifications in 3.5?wt% NaCl. Electrochemical polarization resistance measurements are made according to ASTM G59-91 specifications. Corrosion resistance in a humid and corrosive environment is determined by exposing the samples in a salt spray chamber for 100?h. Microstructural studies are carried out using an atomic force microscope and scanning electron microscope.----- Findings: The nanostructured TiO2 coatings offer good resistance to corrosion, as shown by the results of immersion, electrochemical and salt spray studies. The corrosion resistance of the coating is dictated primarily by the geometry of splat lamellae, density of unmelted nanoparticles, magnitude of porosity and surface homogeneity.----- Practical implications: The TiO2 nanostructured coatings show promising potential for use as abrasion, wear-resistant and thermal barrier coatings for service in harsh environments.----- Originality/value: The paper relates the corrosion resistance of nanostructured TiO2 coatings to their structure and surface morphology.

Hot corrosion of γ-Y2Si2O7 in strongly basic Na2CO3 molten salt environment

γ-Y2Si2O7 is a promising candidate both for high temperature structural applications and as thermal barrier coatings due to its unique combination of properties, such as high melting point, good machinability, high thermal stability, low linear thermal expansion coefficient (3.9 × 10-6 K-1, 25-1400 °C) and low thermal conductivity (<3 W/m K above 300 °C). In this work, the hot corrosion behavior of γ-Y2Si2O7 in strongly basic Na2CO3 molten salt at 850-1000 °C for 20 h in flowing air was investigated. In the employed conditions, multi-layer corrosion scales with total thickness less than 90 μm were formed. At 850-900 °C, the outmost layer of the scale was composed of the reprecipitation of Y2O3, the bottom of a Si-rich Na2O·xSiO2 (x > 3.65) melt layer, and the middle of a NaYSiO4 layer. At 1000 °C, the corrosion products turned out to be a mixture of NaY9Si6O26 and Si-rich Na2O·xSiO2 (x > 3.65). In all cases, a thin layer of protective SiO2 formed under the Na2O·xSiO2 melt and protected the bulk material from further corrosion.

Small-scale mechanical testing of materials

Small-scale mechanical testing of materials has gained prominence in the last decade or so due to the continuous miniaturization of components and devices in everyday application. This review describes the various micro-fabrication processes associated with the preparation of miniaturized specimens, geometries of test specimens and the small scale testing techniques used to determine the mechanical behaviour of materials at the length scales of a few hundred micro-meters and below. This is followed by illustrative examples in a selected class of materials. The choice of the case studies is based on the relevance of the materials used in today's world: evaluation of mechanical properties of thermal barrier coatings (TBCs), applied for enhanced high temperature protection of advanced gas turbine engine components, is essential since its failure by fracture leads to the collapse of the engine system. Si-based substrates, though brittle, are indispensible for MEMS/NEMS applications. Biological specimens, whose response to mechanical loads is important to ascertain their role in diseases and to mimic their structure for attaining high fracture toughness and impact resistance. An insight into the mechanisms behind the observed size effects in metallic systems can be exploited to achieve excellent strength at the nano-scale. A future outlook of where all this is heading is also presented.

Preliminary Investigations on Low-Pressure Laminar Plasma Spray Processing

The usual plasma spraying methods often involve entrainment of the surrounding air into the turbulent plasma core and result in coated materials having relatively high porosity and low adhesive strength. Therefore, exploration of new plasma spraying methods for fabricating high quality coatings to meet the requirement of special applications will be quite important. In this study, an alternative plasma spraying method, i.e. the low-pressure laminar plasma spraying process, is investigated and used in an attempt for spraying thermal barrier coatings (TBCs). Investigations on the characteristics of the laminar plasma jets, feeding methods for the ceramic powder and the formation process of the individual quenched splats have been carried out. The properties of the TBCs sprayed by laminar plasma jet process, such as the adhesive strength at the interface of the ceramic coating/bond coat, the surface roughness and microstructure, are examined by tensile tests and scanning electron microscope (SEM) observations.