High Temperature Ceramic Coatings Research Articles

Residual stress effects of CMAS infiltration in high temperature jet engine ceramic coatings captured non-destructively with confocal Raman-based 3D rendering

Calcium-magnesium-aluminosilicates (CMAS) are ingested by jet engines and infiltrate into high temperature ceramic coatings during operation. This infiltration increases coating stiffness and promotes coating phase destabilization, encouraging micro-crack formation. Thermomechanical effects from CMAS infiltration were mapped over time with confocal Raman spectroscopy through residual stresses within EB-PVD 7YSZ coatings with microscale resolution. The results show an interplay between physical and chemical components influencing the residual stress. Physical mechanisms influence residual stress more after 1h of infiltration, inducing tensile loading up to 100 MPa on tetragonal ZrO2 Raman bands. Chemical mechanisms impart greater influence after 10h, inducing compressive loading up to 100 MPa. A monoclinic phase volume fraction of about 35% was observed as a transitional point for chemical mechanisms overtaking physical mechanisms in influencing residual stress. These results non-destructively elucidate changes within a coating’s residual stress during CMAS infiltration, aiding coating degradation monitoring during maintenance and towards implementing CMAS-mitigation strategies.

Emissivity of spinel and titanate structures aiming at the development of industrial high-temperature ceramic coatings

High-emissivity coatings have been pointed out as a promising energy-saving solution for industrial furnaces since their development in the aerospace area. However, due to a lack of in-depth knowledge of radiation heat transfer and thermal-optical interactions, as well as the struggle to obtain proper emissivity values, commercial versions of such coatings have not yet been widely established. In this work, the evaluation of some fundamentals on thermal-optical properties, and their potential applications, led to the development of engineered structures of usual inexpensive oxides, such as the titanate and spinel ones, which presented improved emissivity values, even at high temperatures (1500 °C), when compared to commercial references. The findings of these compounds indicated a considerable cost-effective saving potential for large industrial furnaces.

Investigation of thermal residual stresses during laser ablation of tantalum carbide coated graphite substrates using micro-Raman spectroscopy and COMSOL multiphysics

Laser ablation of high-temperature ceramic coatings results in thermal residual stresses due to which the coatings fail by cracking and debonding. Hence, the measurement of such residual stresses during laser ablation process holds utmost importance from the view of performance of coatings in extreme conditions. The present research aims at investigating the effect of laser parameters such as laser pulse energy, scanning speed and line spacing on thermal residual stresses induced in tantalum carbide-coated graphite substrates. Residual stresses were measured using micro-Raman spectroscopy and correlated with Raman peak shifts. Transient thermal analysis was performed using COMSOL Multiphysics to model the single ablated track and residual stresses were reported at low, moderate and high pulse energy regimes. The results showed that the initial laser conditions caused higher tensile residual stresses. Moderate pulse energy regime comprised higher compressive residual stresses due to off centre overlapping of the laser pulses. Higher pulse energy (250 μJ), higher scanning speed (1000 mm/s) and moderate line spacing (20 μm) caused accumulation of tensile residual stresses during the final stage of laser ablation. The deviation of experimental residual stresses from COMSOL numerical model was attributed to unaccounted additional stresses induced during thermal spraying process and deformation potentials in the numerical model.

Piezospectroscopic evaluation and damage identification for thermal barrier coatings subjected to simulated engine environments

The application of high temperature ceramic coatings has enabled aircraft and power generation turbines to run at higher inlet temperatures for greater efficiency. Their use extends the lifetime of the superalloy blades that bear thermal gradients and mechanical loads during operation. In this work, ex-situ photo-luminescence spectroscopy was conducted to investigate the stresses within the thermally grown oxide of a thermal barrier coated tubular sample following complex realistic conditions, such as induced thermal gradients, and long duration aging. The resulting high spatial resolution stress contour maps highlight the development of the thermally grown oxide in response to the complex conditions. The outcomes highlight both the role of the aging process and the oxide growth's influence on the stress profile which varies spatially across the specimen. The results further provide early detection of micro-damaged zones in the oxide layer nondestructively. Improving the understanding of the coating system's response to loading conditions will allow for more accurate system modeling and early detection and monitoring of damage zones, which is critical for improving efficiency and longevity of aircraft and power generation turbines.

In-situ synthesis of functional silica nanoparticles for enhancement the corrosion resistance of TBCs

Ultra high temperature ceramic coatings (UHTCs) have a potential use as thermal protection systems in rocket exhaust cones, insulating tiles for the space shuttle, and engine components. The porous ceramic coatings require a sealing strategy with ceramic nanoparticles to enhance the corrosion resistance and prevent the formation of thermally grown oxides (TGO) at the ceramic/metallic interface. In this context, we report a simple, yet highly versatile sol–gel process to seal the pores in alumina coating with silica nanoparticles. Tetraethylorthosilicate (TEOS) was used as sol–gel precursor. Polysilicate molecules are likely formed inside the coating pores during the sol–gel process. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests were performed to measure the corrosion resistance of alumina coatings before and after treatment with TEOS. A significant enhancement of the corrosion resistance of alumina coating was performed via post-treatment with sol–gel silica which perfectly sealed the open pores and voids in alumina coatings.

Effects of heat treatments on oxidation resistance and mechanical properties of ultra high temperature ceramic coatings

A critical point in the development of future generation space vehicles is the availability of materials able to withstand the extreme temperatures generated during orbital re-entry. UHTC (Ultra High Temperature Ceramics) materials, such as Zirconium diboride based ceramic composites, exhibit outstanding oxidation and thermal shock resistance, high emissivity and very high melting temperature: all these properties make ZrB 2 based materials possible candidates in designing thermal protection systems for the hottest structures of re-entry vehicles such as wing leading edges and nose-tip. An innovative, proprietary way to deposit ZrB 2 based coatings by plasma spraying has been developed. In the present work, the influence of high temperature exposure on oxidation resistance and mechanical properties has been investigated. Several coatings constituted by different amounts of SiC particles dispersed in a ZrB 2 matrix were produced; some samples were tested in as sprayed conditions, other ones were submitted to a high temperature heat treatment before testing. Oxidation resistance was determined by exposing samples in air at 2073 K for 180, 1800 and 2520 s. Oxidation kinetics were analyzed and modelled. Evaluation of flexural strength and elastic modulus by means of four-points bending tests were carried out on samples showing better oxidation behaviour; mechanical tests were performed also on samples exposed at a typical operative temperature (1800 s at 2073 K). Results showed that heat treatment does not affect oxidation resistance, whilst it significantly improves mechanical properties.

Ceramic to Metal Attachment Using Low Modulus BRUNSBOND® Pad

BRUNSBOND® Pad is a low modulus sintered fiber metal material developed for attaching high temperature ceramic coatings to metal substrates. The fiber metal pad acts as a compliant strain absorbing interlayer to accomodate the differences in thermal expansivity and operating temperature between the ceramic and metal substrate. Ceramic coatings, 0.060 – 0.125 in. thick, remain intact on metal substrates through severe multiple cycle thermal excursions. The fiber metal pad is comparable in insulating value to the ceramic coating. Applications include ceramic thermal barrier coatings for high temperature gas turbine engine seals and combustors, MHD electrodes, and internal combustion engine insulators.

Devices Developed for Accelerated Tests On High Temperature Ceramic Coatings—A Technical Note

Abstract Methods used by Solar Aircraft Company in accelerated testing of ceramic coatings for high temperature service are described. Testing cycles over extended periods are automatically handled by these devices reducing manpower and cost of making initial segregations of suitable materials for specific applications. Machines have been developed for cyclic vibration, wetting and drying and heating and cooling as well as combinations of these conditions. The company's experience indicates that slight modifications between coatings frequently determine whether they will satisfactorily resist the environment or not.