CMAS corrosion and thermal cycle of Al-modified PS-PVD environmental barrier coating

doi:10.1016/j.ceramint.2018.06.019

Ceramics International

Volume 44, Issue 13, September 2018, Pages 15959-15964

https://doi.org/10.1016/j.ceramint.2018.06.019 Get rights and content

Abstract

Environmental barrier coatings (EBCs) with a tri-layer structure consisting of silicon, mulltite and Yb₂SiO₅ were deposited by plasma spray-physical vapor deposition (PS-PVD) on SiC/SiC ceramic matrix composite (CMC). To improve the calcium-aluminum-magnesium-silicates (CMAS) corrosion resistance of EBCs, a novel method called “Al-modification of EBCs surface” was proposed, wherein an Al film was first deposited on surface of Yb₂SiO₅ top coating by magnetron sputtering, and afterwards the vacuum heat treatment was carried out on the Al-deposited EBCs samples. The thermal cycle performance of both as-sprayed and Al-modified EBCs was characterized from 1300 °C to the temperature of environmental water. Moreover, the CMAS corrosion property of both types of EBCs at 1300 °C has been compared with each other. And the corrosion mechanism between CMAS and Al-modified EBCs has been investigated. The results show that the Al-modified EBCs have a better thermal cycle and CMAS corrosion performance than that of as-sprayed EBCs.

Introduction

The improvement of gas turbine inlet temperature is a key for increasing the fuel efficiency and reducing the carbon emission of a gas turbine. This improvement is dependent on the development of advanced materials. The SiC/SiC ceramic matrix composites (CMCs), due to its high limit point of temperature capability, have been investigated extensively as a potential structural material for hot gas parts for next-generation gas turbine [1], [2], [3], [4], [5]. However, a large number of investigations have implied that when used in gas turbines the disadvantage factor of SiC/SiC CMC is the observable weight loss in a combustion environment. Owing to the presence of oxygen and high-temperature water vapor in a combustion environment, SiC changes into Si(OH)₄ via reactions (1) and (2) with a recession rate close to 1 µm/h when rising temperature to 1300 °C [6], [7]. Thus, the environmental barrier coatings (EBCs) are essential for SiC/SiC CMCs before operation.SiC + 2O₂ = SiO₂ + CO₂ (g)SiO₂(s) + 2H₂O (g) = Si(OH)₄ (g)

During the turbine engine operation, apart from water-oxygen corrosion of EBCs, CMAS (calcium-magnesium aluminosilicate) corrosion resulting from many molten silica-containing sand dust and volcano ash materials deposition on EBCs surface is another major cause of performance degradation of hot-section components [6]. This molten CMAS interacts chemically and mechanically with the EBCs, causing premature coating failure. Although, a large amount of work has been done to investigate the mechanism by which CAMS can cause failure in Yb₂SiO₅ EBCs [8], [9], [10], the CMAS corrosion resistance of EBCs still needs to be improved.

The objective of this paper is to provide an alternative method to protect Yb₂SiO₅ coating from CMAS corrosion. In this investigation, a process called “Al-modification of EBCs surface” that can be an alternative approach to improve the CMAS resistance of EBCs was applied and carefully controlled, where an Al film was firstly deposited on the surface of EBCs samples, and then as-deposited EBCs samples were vacuum heat treated. The emphasis is placed on the corrosion behaviors of as-sprayed and Al-modified EBCs in contact with CMAS at high temperature.

Section snippets

Experimental materials and methods

Tri-layers of EBCs (Si/mullite/Yb₂SiO₅) were produced by plasma spray-physical vapor deposition on SiC/SiC CMC substrate (Precursor Infiltration and Pyrolysis，PIP). The EBCs system consisted of a ～50 µm thick Yb₂SiO₅ (the powders were self-prepared by authors, powders size distribution: 10–50 µm) top coating and a ～50 µm thick mullite (powders size distribution: 3–60 µm) oxidation resistant layer, both sprayed on a substrate heated to 900 °C by spray torch. And a ～50 µm thick silicon layer

Misconstruction evolution before and after Al-modification

The EBCs (Si/mullite/Yb₂SiO₅) were prepared by PS-PVD as shown in Fig. 2(a). The Fig. 2(a) shows that no pores or cracks were observed in the Si and Yb₂SiO₅ coating. On the contrary, many closed pores located in mullite coating. As the melting point of Si material is about 1410 °C, Si powders can thus be melted sufficiently under the high energy density and high velocity of plasma jet in O3CP gun. The molten Si splats will have a close packing when deposited on substrate. The size of original

Conclusions

The tri-layer environmental barrier coatings (EBCs) comprised of a silicon bond coating protected by two layers of mullite and Yb₂SiO₅ as the top coatings was prepared by plasma spray-physical vapor deposition (PS-PVD). Al-modification for EBCs as an alternative method to improve the CMAS resistance was provided. The thermal cycle and CMAS resistance performances of as-sprayed and Al-modified EBCs were compared. Several conclusions can be drawn from this work as follows.

(1)
After Al-modification,

Acknowledgements

We would like to acknowledge financial supports from National Key Research Program (2017YFB0306100), Guangdong Academy of Sciences Program (No. 2017GDASCX-0843, 2017GDASCX-0202), Guangdong Technical Research Program (No. 2014B070706026), Guangdong Natural Science Foundation (No. 2016A030312015, 2017A030310315), and Guangzhou Technical Research Program (No. 201605131008557, 201707010385, 201510010095).

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CMAS corrosion and thermal cycle of Al-modified PS-PVD environmental barrier coating

Abstract

Introduction

Section snippets

Experimental materials and methods

Misconstruction evolution before and after Al-modification

Conclusions

Acknowledgements

Acta Mater.

Acta Mater.

J. Eur. Ceram. Soc.

J. Adv. Ceram.

Ceram. Int.

J. Eur. Ceram. Soc.

Ceram. Int.

Surf. Coat. Technol.

Surf. Coat. Technol.

Appl. Surf. Sci.