Intercolumnar Gaps Research Articles

Numerical modelling of CMAS infiltration and dimensionless numbers of anti-infiltration designs of thermal barrier coatings

Resistance to calcium–magnesium–alumina–silicate (CMAS) infiltration and corrosion is a crucial problem for safely applying thermal barrier coatings (TBCs) in advanced aero engines. This work proposes a CMAS infiltration model based on the phase-field method to simulate the CMAS infiltration process in TBCs and analyse the influence factor. Further, a key dimensionless number called relative driving force K=rσcos(θc)tsμϛ2hTC2 was found to determine the CMAS infiltration depth using dimensional analysis. A criterion of K<2 was derived and was considered to be the evaluation standard of TBCs with excellent CMAS infiltration resistance. Based on K minimization, novelty TBCs with microstructure of S-shaped intercolumnar gaps and spherical pores are expected to significantly improve the resistance to CMAS infiltration and corrosion.

Study of CMAS wettability, penetration, and degradation behaviors on EB-PVD 8YSZ coatings laser micro-glazed (LMGed) by an ultraviolet picosecond ultrashort pulsed laser

CaO-MgO-Al2O3-SiO2 (CMAS) is a primary contributor to the failure of thermal barrier coatings, particularly those with columnar structures fabricated via electron beam physical vapor deposition (EB-PVD). This paper presents the laser micro glazing (LMGing) of EB-PVD using an ultraviolet pulsed ultrashort pulsed laser. With the disappearance of inter-columnar gaps, a smooth LMGed layer with extremely narrow cracks (<0.9 μm in width) were formed on the surface. Employing the sessile-drop method, heat treatment demonstrated that the LMGed layer effectively suppressed the wettability, penetration, and degradation behaviors of CMAS. Compared to the original, the spreading area of CMAS on the LMGed layer was decreased by 48 % (37.8 mm2 → 19.5 mm2), while the contact angle was increased by 116 % (6.2° → 13.4°). Meanwhile, a small amount of CMAS was found in the inter-columnar gaps, while no CMAS aggregated at the bottom. The depth of the corrosion layer at the CMAS-coating interface decreased by 68 % (9.9 μm → 3.2 μm), ensuring the blocking effect of the LMGed layer.

Plasma sprayed duplex ytterbium disilicate/monosilicate EBCs and the transformation from ytterbia to ytterbium monosilicate during burner rig testing

Environmental Barrier Coating systems (EBCs) were thermally cycled in a burner rig test facility. Yb2Si2O7 (YbDS) layer was deposited by air plasma spraying while two suspension plasma sprayed Yb2SiO5 (YbMS) microstructures were evaluated in duplex (YbDS/YbMS) systems: columnar and segmentation cracked. EBCs underwent 2000 cycles at a surface temperature of 1300 °C without signs of delamination failure. A porous YbMS layer formed at the base of intercolumnar gaps and segmentation cracks in the duplex systems, presumably due to reactions with entrapped water vapor. Furthermore, Yb2O3 depletion zones were evident at both the surface and YbDS/YbMS interface of the duplex EBCs.

SEM-Guided Finite Element Simulation of Thermal Stresses in Multilayered Suspension Plasma-Sprayed TBCs

This study presents novel insights into thermal stress development and crack propagation mechanisms in single- and multilayered suspension plasma-sprayed (SPS) coatings of gadolinium zirconate (GZ) and yttria-stabilized zirconia (YSZ), thermally treated at 1150 °C. By combining image processing with finite element simulation, we pinpointed sites of high-stress concentration in the coatings, leading to specific cracking patterns. Our findings reveal a dynamic shift in the location of stress concentration from intercolumnar gaps to pores near the top coat/thermally grown oxide (TGO) interface with TGO thickening at elevated temperatures, promoting horizontal crack development across the ceramic layers. Significantly, the interface between the ceramic layer and TGO was found to be a critical area, experiencing the highest levels of both normal and shear stresses. These stresses influence failure modes: in double-layer SPS structures, relatively higher shear stresses can result in mode II failure, while in single-layer systems, the predominant normal stresses tend to cause mode I failure. Understanding stress behavior and failure mechanisms is essential for enhancing the durability of thermal barrier coatings (TBCs) in high-temperature applications. Therefore, by controlling the interfaces’ roughness along with improving interfacial toughness, the initiation and propagation of cracks can be delayed along these interfaces. Moreover, efforts to optimize the level of microstructural discontinuities, such as intercolumnar gaps and pores, within the creaming layer and close to the TGO interface should be undertaken to reduce crack formation in the TBC system.

A New Al2 O3 -Protected EB-PVD TBC with Superior CMAS Resistance.

Calcium-magnesium-aluminium-silicate (CMAS) attack is a longstanding challenge for yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) particularly at higher engine operating temperature. Here, a novel microstructural design is reported for YSZ TBCs to mitigate CMAS attack. The design is based on a drip coating method that creates a thin layer of nanoporous Al2 O3 around YSZ columnar grains produced by electron beam physical vapor deposition (EB-PVD). The nanoporous Al2 O3 enables fast crystallization of CMAS melt close to the TBC surface, in the inter-columnar gaps, and on the column walls, thereby suppressing CMAS infiltration and preventing further degradation of the TBCs due to CMAS attack. Indentation and three-point beam bending tests indicate that the highly porous Al2 O3 only slightly stiffens the TBC but offers superior resistance against sintering in long-term thermal exposure by reducing the intercolumnar contact. This work offers a new pathway for designing novel TBC architecture with excellent CMAS resistance, strain tolerance, and sintering resistance, which also points out new insight for assembly nanoporous ceramic in traditional ceramic structure for integrated functions.

Ultraviolet picosecond ultrashort pulsed laser micro glazed EB-PVD YSZ coatings for improving the molten salt (Na2SO4 + V2O5) corrosion resistance

In this study, yttria-stabilized zirconia was glazed for the first time using an ultraviolet picosecond ultrashort pulse laser (laser micro glazing, LMG). A smooth LMGed layer (surface roughness: ∼1.4 μm) with extremely narrow cracks (width: < 0.6 μm) was fabricated on the surface to resist molten salt corrosion. The hot corrosion resistance of LMGed coatings was systematically investigated. Two models of corrosion behavior were observed from the results: (І) widespread but slow corrosion of columnar crystals, and (II) rapid but little penetration in defects (inter-columnar gaps, and cracks). The thermal stabilization of the LMGed layer was greatly improved due to the positive effects of grain refinement on elevating oxygen vacancies and more uniform redistribution of Y3+. The content of YVO4 and m-ZrO2 on the LMGed coatings was significantly reduced compared to the non-LMGed coatings. The self-healing of narrow cracks under prolonged high temperatures resulted in slow and slight penetration of the molten salt along the grain boundaries. No visible grey YVO4 was observed throughout the interior, demonstrating the positive role of LMG technology in enhancing hot corrosion resistance.

Microstructure Refinement of EB-PVD Gadolinium Zirconate Thermal Barrier Coatings to Improve Their CMAS Resistance

Rare-earth zirconates are proven to be very effective in restricting the CMAS attack against thermal barrier coatings (TBCs) by forming quick crystalline reaction products that seal the porosity against infiltration. The microstructural effects on the efficacy of Electron Beam-Physical Vapor Deposition gadolinium zirconate (EB-PVD GZO) against CMAS attack are explored in this study. Four distinct GZO microstructures were manufactured and the response of two selected GZO variants to different CMAS and volcanic ash melts was studied for annealing times between 10 min and 50 h at 1250 °C. A significant variation in the microstructural characteristics was achieved by altering substrate temperature and rotation speed. A refined microstructure with smaller intercolumnar gaps and long feather arms lowered the CMAS infiltration by 56%–72%. Garnet phase, which formed as a continuous layer on top of apatite and fluorite, is identified as a beneficial reaction product that improves the CMAS resistance.

Performance Evaluation and Thermal Shock Behavior of PS-PVD (Gd0.9Yb0.1)2Zr2O7/YSZ Thermal Barrier Coatings

In this study, (Gd0.9Yb0.1)2Zr2O7 (GYbZ)/yttria-stabilized zirconia (YSZ) double-ceramic-layer (DCL) thermal barrier coatings (TBCs) were prepared by plasma spray-physical vapor deposition (PS-PVD). The microstructure, mechanical performance, and thermal shock behavior of coatings prepared with spraying distances of 600, 800, and 1000 mm were investigated. The GYbZ coating prepared with a spraying distance of 600 mm showed a closely packed columnar structure. However, the GYbZ coatings prepared with spraying distances of 800 and 1000 mm showed a quasi-columnar structure. The GYbZ coating prepared with a spraying distance of 800 mm had the thickest columnar crystals with obvious inter-columnar gaps. In addition, this coating exhibited excellent mechanical performance and the best thermal shock resistance. The primary failure patterns appearing during thermal shocking on the surface of TBCs can be classified into the following five types: caves, exfoliation, delamination cracks, spalled areas, and radiate cracks. Furthermore, the failure behavior of these coatings in water-quenching tests is clarified.

Al2O3-TiO2 coatings deposition by intermixed and double injection SPS concepts

Abstract This work focuses on the study on the novel hybrid method of simultaneous spraying from two different materials (Al2O3 and TiO2) by means of suspension plasma spraying (SPS) using submicron powder and water suspension. The goal was to attempt the deposition of intermixed alumina-titania coatings, namely: Al2O3 + 3 wt.% TiO2, Al2O3 + 13 wt.% TiO2, and Al2O3 + 40 wt.% TiO2. Such compositions are already used but in the form of conventionally plasma sprayed coatings, with micrometer-sized powder. Meanwhile, the injection of feedstocks with submicron-sized particles has not been established yet. In particular, this paper uses two routes of feedstock injection, (i) with the use of an intermixed suspension and (ii) a double injection of separate suspensions. The attention was paid to the characterization of the feedstock materials, description of deposition parameters as well as the morphology, microstructure, and phase composition of the obtained coatings. Among all coatings, Al2O3 + 40 wt.% TiO2 sprayed with double injection contained the most homogeneously distributed and melted splats. The results from this work demonstrate the possibility of coating deposition both by intermixed and double injection concepts but also the need for the further application-relevant optimization, related to the presence of intercolumnar gaps in the microstructure of the coatings.

Thermal shock life and failure behaviors of La2Zr2O7/YSZ, La2Ce2O7/YSZ and Gd2Zr2O7/YSZ DCL TBCs by EB-PVD

Double layer ceramic (DCL) thermal barrier coatings (TBCs) have attracted extensive attention due to their high thermal stability under high temperature. The thermal shock life and failure behaviors of DCL TBCs still remain a challenge. This work focuses on crystal structure, thermal expansion coefficients, thermal conductivity, microstructure and failure behaviors of the La2Zr2O7/YSZ (LZ/YSZ), La2Ce2O7/YSZ (LC/YSZ) and Gd2Zr2O7/YSZ (GZ/YSZ) DCL TBCs. The advanced ceramic material exhibits low thermal diffusivity and thermal conductivity. Furthermore, the advanced DCL TBCs exhibit good thermal shock life at 1100 °C due to the feathery microstructure with inter-columnar gaps and inter-columnar pores. After thermal shock test, the broken regions occur on the cracks area within ceramic coatings, TGO and interface leading to the TBCs instabilities. The TBCs interface instabilities play a key role on the failure behaviors of DCL TBCs.

Magnetron Sputtered Silicon Coatings as Oxidation Protection for Mo‐Based Alloys

Mo‐based alloys with solidus temperatures around and above 2000 °C are attractive high‐temperature structural materials for future applications in the hot section of gas turbines. However, their oxidation behavior is poor due to pesting starting at 600 °C and nonprotective oxide growth at temperatures above 1000 °C. To ensure a sufficient oxidation resistance over a wide temperature range, protective coatings become inevitable. Herein, silicon coatings have been applied by magnetron sputtering on Mo‐9Si‐8B and on titan–zirconium–molybdenum alloy (TZM). The coating architecture is designed to minimize the intercolumnar gaps and porosity, thereby increasing the density. Specimens are tested at 800 and 1200 °C in air isothermally for up to 300 h. The focus is put on the chemical reactions at the coating–substrate interface, the phase formation, and the evolution of the thermally grown oxide. An initially globular SiO2 evolves into a uniform SiO2 layer providing excellent oxidation protection. The investigations reveal a rather slow interdiffusion between the coating and the alloys when tested in air. At the coating–substrate interface exclusively, the Mo3Si phase develops. Finally, the phase formation at the coating–substrate interface is studied in detail for various heat treatments in air and vacuum.

Study of the microstructural control of Ba(Mg1/3Ta2/3)O3 perovskite thermal barrier coating deposited by solution precursor plasma spray

Inductively-coupled plasma spraying was used to deposit Ba(Mg1/3Ta2/3)O3 (BMT) on a titanium alloy substrate. By selecting the appropriate spraying parameter, it was possible to tune the BMT coating microstructure from dense splat-like layers to columns and segmented vertical cracks. Of those parameters, a short spraying distance of 34 mm was the most crucial parameter to form a columnar microstructure. An atomization probe that fed a bimodal droplet size distribution was considered as an important prerequisite to obtain a high-quality columnar structure. Small atomized droplet sizes contribute to the formation of columns, while the large ones ensure a high deposition rate. This is further discussed as part of the column formation mechanism. The effects of substrate surface roughness, precursor concentration and feed rate were studied separately. The surface roughness should be controlled between Ra = 1.3 μm and Ra = 2.7 μm to reduce the intercolumnar gaps. The columnar structure evolves into a segmented vertical cracked one when spraying concentrated precursors (30 wt%). High feed rate (>6 ml/min) can satisfy a series of requirements such as high deposition rate, low intercolumnar gaps and high intracolumnar porosity. Finally, a desired columnar structure with an intracolumnar porosity of 4.3% and an intercolumnar porosity of 9.9% was obtained by spraying a 10 wt% concentration precursor on a finely sand-blasted substrate (Ra = 1.3 μm) at a feed rate of 8 ml/min.

On the Large Near-Field Enhancement on Nanocolumnar Gold Substrates

One of the most important and distinctive features of plasmonic nanostructures is their ability to confine large electromagnetic fields on nanometric volumes; i.e., the so-called hot spots. The generation, control and characterization of the hot spots are fundamental for several applications, like surface-enhanced spectroscopies. In this work, we characterize the near-field distribution and enhancement of nanostructured gold thin films fabricated by glancing angle deposition magnetron sputtering. These films are composed of columnar nanostructures with high roughness and high density of inter-columnar gaps, where the electromagnetic radiation can be confined, generating hot spots. As expected, the hot spots are localized in the gaps between adjacent nanocolumns and we use scattering-type scanning near-field optical microscopy to image their distribution over the surface of the samples. The experimental results are compared with finite-difference time-domain simulations, finding an excellent agreement between them. The spectral dependence of the field-enhancement is also studied with the simulations, together with surface-enhanced Raman spectroscopy at different excitation wavelengths in the visible-NIR range, proving a broad-band response of the substrates. These findings may result in interesting applications in the field of surface-enhanced optical spectroscopies or sensing.

Flow Kinetics of Molten Silicates through Thermal Barrier Coating: A Numerical Study

Infiltration of molten calcium–magnesium–alumina–silicates (CMAS) through thermal barrier coatings (TBCs) causes structural degradation of TBC layers. The infiltration kinetics can be altered by careful tailoring of the electron beam physical vapor deposition (EB-PVD) microstructure such as feather arm lengths and inter-columnar gaps, etc. Morphology of the feathery columns and their inherent porosities directly influences the infiltration kinetics of molten CMAS. To understand the influence of columnar morphology on the kinetics of the CAMS flow, a finite element based parametric model was developed for describing a variety of EB-PVD top coat microstructures. A detailed numerical study was performed considering fluid-solid interactions (FSI) between the CMAS and TBC top coat (TC). The CMAS flow characteristics through these microstructures were assessed quantitatively and qualitatively. Finally, correlations between the morphological parameters and CMAS flow kinetics were established. It was shown that the rate of longitudinal and lateral infiltration could be minimized by reducing the gap between columns and increasing the length of the feather arms. The results also show that the microstructures with long feather arms having a lower lateral inclination decrease the CMAS infiltration rate, therefore, reduce the CMAS infiltration depth. The analyses allow the identification of key morphological features that are important for mitigating the CMAS infiltration.

Zirconium silicate growth induced by the thermochemical interaction of yttria-stablized zirconia coatings with molten CMAS deposits

We report on the microstructure evolution of yttria-stablized zirconia (YSZ) corroded by molten deposits of Ca10Mg7Al13Si70 (CMAS). The CMAS corrosion induces significant zirconium silicate (ZrSiO4) growth across the entire YSZ coating and causes dramatic microstructure changes marked by the complete disappearance of the feather-like structure of the columnar grains in the upper region of the coating and disintegration of the parent columnar grains into an equiaxed grains near the YSZ/Al2O3 interface. CMAS infiltration along the intercolumnar gaps results in ZrSiO4 growth into YSZ grains. Curvature-driven infiltration of the molten CMAS along ZrSiO4/YSZ interfaces gives rise to dendritic-like ZrSiO4 growth.

Response of molten silicate infiltrated Gd2Zr2O7 thermal barrier coatings to temperature gradients

The delamination tendency for bilayer gadolinium zirconate/yttria-stabilized zirconia (GZO/YSZ) thermal barrier coatings (TBCs) was investigated using a continuous laser-based thermal gradient test and a computational model for the analysis of multilayer structures. TBCs with different architectures were exposed to molten silicate compositions, representative of aero-engine deposits, and subjected to thermal cycles prescribed to impart specified levels of strain energy in the coating. The exposed surface of the intercolumnar gaps in the outer part of the GZO layer was found to rapidly dissolve into the intruding molten silicate and precipitate reaction products that seal the flow paths, limiting the penetration depth and the ensuing stiffening of the TBC. Nevertheless, the stiffened layer magnifies the thermal stresses in the coating upon thermal cycling. The influence of the thermal history and multilayer structure on the driving force for delamination was modeled and compared with the experimental results. The effects that the substrate coefficient of thermal expansion, the temperature gradient, and the TBC thickness have on the driving force for delamination were analyzed and the critical amount of stored elastic strain energy for failure under different scenarios was assessed.

Infrared radiative properties of EB-PVD thermal barrier coatings

In the present paper the infrared radiative properties of electron beam physical deposition (EB-PVD) partially yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) have been studied in combination with experimental measurement and theoretical model. The method of Finite-Difference-Time-Domain (FDTD) is employed to investigate the infrared spectral reflectance and transmittance of TBCs with various microstructural parameters and validated by a following experiment. The infrared radiation transfer mechanisms inside the coatings are explored and examined in detail. The results indicate that distinctive micro-nanostructures of EB-PVD coatings have important effects on its radiative performance, and the infrared spectral reflectance and transmittance of coatings are highly affected by the primary columnar morphology and pore architectures, such as intercolumnar gaps, feathery striation and closed globular pores. Further research suggests that scattering effects caused by different microstructures can greatly affect radiative transfer across the coatings. By optimizing the microstructure of coatings properly to enhance its backscattering effects, it can effectively increase the spectral reflectance and reduce the spectral transmittance, thereby reducing the radiation heat flows and providing a better thermal protection for the metallic substrate.

Microstructure characteristics of EB-PVD YSZ thermal barrier coatings corroded by molten volcanic ash

Abstract Y 2 O 3 -stabilized ZrO 2 (YSZ) thermal barrier coatings (TBCs) grown on a polycrystalline alumina (Al 2 O 3 ) substrate are corroded by a natural volcanic ash (VA) at 1150 °C. The VA corrosion-induced microstructure development and composition evolution across the YSZ coating layer and at the VA/YSZ and YSZ/Al 2 O 3 interfaces are investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). It is found that the VA-corroded YSZ coating undergoes grain growth and becomes well-connected along the deposition direction. It is shown that VA-molten depositions penetrate through the entire YSZ layer and fully fill the intercolumnar gaps in the YSZ layer, which results in crystalline ZrSiO 4 precipitates between the intercolumnar gaps near the VA/YSZ interface and anorthite (CaAl 2 Si 2 O 8 ) precipitates at the YSZ/Al 2 O 3 interface with the micro-crack formation. The growth stresses associated with the anorthite precipitation is suggested as the driving force to initiate the degradation of TBCs at the YSZ/Al 2 O 3 interface region.

Protectiveness of Pt and Gd2Zr2O7 layers on EB-PVD YSZ thermal barrier coatings against calcium–magnesium–alumina–silicate (CMAS) attack

Thermal barrier coatings (TBCs) are susceptible to degradation caused by environmental deposits mainly composed of calcium–magnesium–aluminum–silicon (CMAS). In this work, the degradation of electron beam physical vapour deposition (EB-PVD) yttria stabilized zirconia (YSZ) TBC due to CMAS attack was investigated. To protect the YSZ coating, a Pt film and a Gd2Zr2O7 (GZO) layer were deposited onto the YSZ coating surface by electroplating and EB-PVD, respectively. The coatings were heat treated at 1250°C for 4h with CMAS deposits, and their microstructure and chemical composition after CMAS attack were investigated. Electroplating a dense and defect-free Pt film on YSZ coating could provide effective protection from CMAS attack, but EB-PVD GZO coating was unable to prevent molten CMAS infiltration, mainly due to the large inter-columnar gaps. The associated mechanisms were discussed in detail.

Degradation of EB-PVD thermal barrier coatings caused by CMAS deposits

In aero-turbine engines, thermal barrier coatings (TBCs) must be capable to withstand harsh environments, such as high-temperature oxidation and hot-corrosion. Recently, a new failure mode of TBCs caused by calcium–magnesium–alumina–silicate (CMAS) glass has attracted increasing attention. In this paper, yttria stabilized zirconia (YSZ) TBCs produced by electron beam physical vapor deposition (EB-PVD) were exposed to CMAS deposits at 1250°C. The microstructure evolution and failure mechanism of the coatings were investigated. It has been shown that CMAS glass penetrated into the YSZ ceramic layer along the inter-columnar gaps and interacted with YSZ. As a result, an interaction zone of about 20μm thickness, which was the mixture of CMAS and YSZ with equiaxial structure, was formed in the YSZ surface layer after 4h heat-treatment at 1250°C. Meanwhile, yttria in YSZ layer as a stabilizer was dissolved in CMAS glass and caused accelerated monoclinic phase transformation. After 8h heat-treatment, degradation of YSZ TBC occurred by delamination cracking of YSZ layer, which is quite different from the traditional failure caused by interfacial cracking at the YSZ/metallic bond coat. Physical models have been built to describe the failure mechanism of EB-PVD TBCs attacked by CMAS deposits.