Reactivity between rare‐earth oxides based thermal barrier coatings and a silicate melt

Abstract

The reactivity between rare-earth (RE-) oxide stabilized ZrO 2 or HfO 2 thermal barrier coatings (TBCs) and a calcium-magnesium-aluminum-silicate (CMAS) melt was studied at 1310°C. These reactions are representative of the ingestion of siliceous materials by the intake air of gas turbines (e.g., in aircraft engines) at high temperatures (>1200°C). These materials can melt and react with coated components in the hot section, resulting in premature failure. The goal of this work was to probe the effect of various RE (RE = Y, Yb, Dy, Gd, Nd, and Sm) oxides in the melt phase equilibrium and stability of the top-coating system. Thermodynamic calculations of the phase assemblage of the (1Àx) ZrO 2 -xY 2 O 3 coating materials and CMAS melt are compared with the experimental findings.CMAS was found to penetrate the samples at the grain boundaries and dissolve the coating materials to form silicate phases containing the RE elements. Furthermore, apatite and garnet crystalline phases formed in the samples with total REoxide content higher than 16 mol% in the reaction zone for the ZrO 2 system. In general, samples with nominal compositions ZrO 2 -9Dy 2 O 3 , HfO 2 -7Dy 2 O 3 , ZrO 2 -8Y 2 O 3 , HfO 2 -6Er 2 O 3 , ZrO 2 -9.5Y 2 O 3 -2.25Gd 2 O 3 -2.25Yb 2 O 3 , and ZrO 2 -30Y 2 O 3 exhibited lower reactivity, or more resistance, to CMAS than the other coating compositions. K E Y W O R D S coatings, degradation, rare earths, thermal barrier coatings

Thermodynamics of reaction between gas‐turbine ceramic coatings and ingested CMAS corrodents

“…The zirconia‐based coatings were prepared by air plasma spray (APS) deposition. Details of sample preparation, chemical and structural characterization is given in our previous work …”

“…EBCs were first applied on CMC shrouds of the LEAP engine manufactured by GE and Safran in 2016 and are still a maturing technology. EBCs and TBCs are both susceptible to attack by CMAS corrosion, and therefore the various melt properties, mechanisms of CMAS‐coating reactions, and strategies to mitigate coating degradation have been investigated extensively . One of the strategies of mitigating CMAS melt penetration in protective coatings is to promote formation of reaction crystalline products to retard molten CMAS infiltration.…”

“…A crystalline phase known as apatite has been identified as a potential reaction product for CMAS glasses. Formation of apatite can retard CMAS penetration by fast precipitation of a Ca‐containing phase which can block further infiltration while also destabilizing the melt via removal of calcium leading to its crystallization above the melting point of the initial melt composition …”

“…One seeks to design coatings that are thermodynamically stable relative to their reaction products . A second, eg, apatite formation described above, involves the idea of maximizing the reaction between the coating and CMAS to produce other crystalline phases that are relatively stable in contact with the melt and thus minimize its penetration depth . A third strategy involves the concept of a sacrificial ceramic layer to neutralize the melt before it reaches the underlayers and other structural components .…”

“…The selected TBC coating compositions are currently being evaluated for use and the EBCs are current state‐of‐the art . The yttrium and ytterbium apatites (CaY 4 Si 3 O 13 and CaYb 4 Si 3 O 13 ) were selected as they are well‐documented corrosion products between TBCs or EBCs and CMAS …”