Performance of thermally sprayed Si/mullite/BSAS environmental barrier coatings exposed to thermal cycling in water vapor environment

Abstract

The ongoing development of environmental barrier coatings (EBCs) offers the prospect to implement the full potential of silicon-based ceramic matrix composites (CMCs) for high temperature structural applications, more specifically the hot zones of gas turbine engines. The current state-of-the-art EBC system comprises a Si bond coat, a mullite (Al₆Si₂O₁₃) interlayer and a barium–strontium aluminosilicate (BSAS) (Ba_1 − xSr_xAl₂Si₂O₈; 0 < x < 1) crack-resistant and water vapor attack resistant top coat. In this study, fully crystalline air plasma sprayed Si/mullite/BSAS-celsian EBCs were engineered under controlled conditions on SiC substrates. The influence of water vapor corrosion on the structural and mechanical properties of a Si/Mullite/BSAS EBC architecture was assessed by furnace thermal cycle testing (i.e.; 50 and 100 cycles, 2 h/cycles at 1300 °C in water vapor atmosphere). The elastic modulus values of the as-sprayed BSAS top coat (~ 75 ± 6 GPa as determined via indentation) did not exhibit major changes after thermal exposure (~ 78 ± 8 GPa). In addition, the BSAS layer exhibited crack healing at high temperatures, the density of cracks decreasing from 15 cracks/cm in the as-sprayed state to 2 cracks/cm after thermal cycling. These characteristics of the BSAS top coat were related to its glass-ceramic nature, the phase/chemical stabilities of the BSAS-celsian at high temperatures and the engineered deposition conditions at which it was deposited. The overall performance at high-temperature of this functionally graded EBC architecture is discussed and correlated to its microstructural characteristics.

Introduction

In the quest to replace metallic components in specific hot-sections of advanced gas turbine engines, multilayered environmental barrier coatings (EBCs) are used to protect silicon (Si) based (e.g., SiC and Si₃N₄) ceramic matrix composites (CMCs) and so to implement their full potential as high temperature structural materials [1], [2], [3], [4]. The thin protective SiO₂ silica layer that usually forms on the surface of a Si-based ceramic at high temperature and in dry atmosphere conditions becomes volatile in the presence of water vapor that is actually present in a combustion environment [5], [6], [7]. The volatilization of silicon hydroxide (Si(OH)₄) leads to severe surface recession that ultimately depends on the temperature of exposure and the velocity of the combustion gasses. Consequently, the multilayered coatings envisioned to improve the durability of ceramic engine components have been ‘labeled’ as EBCs. Following two decades of research and development into the topic, the requirements for an efficient EBC system were identified as follows: (i) good chemical compatibility, (ii) a proper match of their CTEs with the substrate and between the layers (ii) low thermal conductivity, (iii) high crack resistance offered by phase stability, low stress, (i.e.; low elastic modulus) and sintering resistance, and (iv) high temperature H₂O vapor stability.

Mullite (Al₆Si₂O₁₃), a low cost refractory oxide ceramic, has excellent high temperature properties such as high thermal shock and thermal stress resistance and most importantly a low coefficient of thermal expansion (CTE) (4.5–5.5 × 10^− 6/°C) that represents a close match with SiC (~ 4.02 × 10^− 6/°C) and melt infiltrated SiC CMC (~ 5.9 × 10^− 6/°C), and thus has attracted much interest as a protective coating [8], [9], [10]. However, mullite does not exhibit an optimal resistance to H₂O vapor attack [10], [11], [12], and so to surmount this limitation, complementary layered coatings on top of mullite have been used to create an ensemble that offers the sought functionality. In this context, in the early stages of the EBC development, yttria-stabilized zirconia (YSZ) was used as top coat candidate due to its track record as a successful thermal barrier coating (TBC) for metallic components in gas turbine engines, highlighted by its stability in water vapor [13]. The major weakness of YSZ was found to be its large CTE value (~ 10 × 10^− 6/°C) [8], i.e., almost twice that of SiC or mullite.

Even though these initial generations of EBCs provided protection from water vapor for a few hundred hours at ∼ 1300 °C, for longer exposures and under thermal cycling, the large CTE mismatch caused severe cracking and delamination, leading to a premature EBC failure. Thus the YSZ top coat was replaced with a barium–strontium aluminosilicate (BSAS) (Ba_1 − xSr_xAl₂Si₂O₈; 0 < x < 1), which exhibited improved crack resistance due to reduced tensile stress resulting from the low CTE, “low” elastic modulus of BSAS [14], [15] and a good resistance against water vapor attack [6]. In addition, a Si bond coat (similar CTE to SiC and Si₃N₄) [6] was added to improve the adhesion of the overall system [14], [15].

Although numerous efforts are undertaken in investigating different EBC architectures [16], [17], [18], the current commercial state-of-the-art EBC system remains the one based on a Si bond coat, a mullite interlayer and a BSAS water vapor-attack and crack-resistant top coat [1], [2], [14], [19], [20], [21]. In particular, studies on phase evolution of BSAS in EBCs represented a major point of interest. The monoclinic celsian phase was found to be the most advantageous due to its stability and low CTE value (i.e.; 4.0-4.5 × 10^− 6/°C) [8], and thus a close match with that of SiC.

In a work previously reported by the authors [22], Si/mullite/BSAS EBCs deposited via air plasma spray (APS) were engineered under controlled deposition conditions on SiC substrates. In that study, X-ray diffraction (XRD) analysis indicated the direct production of as-sprayed, fully crystalline, Si, mullite and BSAS coatings. No assisted substrate heating during spraying or post-spray annealing techniques was necessary. Specifically for BSAS, the desired stable celsian phase was achieved in the as-sprayed state without significant amount of amorphous content or initial crystallization of a metastable hexacelsian phase. The latter would be detrimental to the EBC performance due to the phase transformation into celsian at high temperatures (≥ 1200 °C) and its high CTE value (8.37 × 10^− 6/°C) [19], [20]. The as-sprayed EBCs were isothermally annealed at 1300 °C in water vapor environment up to 500 h. The as-sprayed BSAS did not exhibit any significant phase transformation after this thermal exposure both as-sprayed and annealed coatings exhibited mostly the celsian phase of BSAS.

In the present work, the effect of high-temperature exposure (i.e., 1300 °C), in a continuous flow of H₂O vapor (90%H₂O/10%air) and under thermal cycling conditions on the EBCs mechanical and overall performance was assessed. Two different experimental approaches i.e. instrumented indentation testing and laser ultrasonics were employed to determine the elastic properties of each individual coating comprised in the thermally sprayed EBC architecture.