Calcium–magnesium aluminosilicate (CMAS) reactions and degradation mechanisms of advanced environmental barrier coatings

Abstract

The thermochemical reactions between calcium-magnesium-aluminosilicate-(CMAS-) based road sand and several advanced turbine engine environmnetal barrier coating (EBC) materials were studied. The phase stability, reaction kinetics and degradation mechanisms of rare earth (RE)-silicates Yb 2 SiO 5 , Y 2 Si 2 O 7 , and RE-oxide doped HfO 2 and ZrO 2 under the CMAS infiltration condition at 1500 C were investigated, and the microstructure and phase characteristics of CMAS-EBC specimens were examined using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). Experimental results showed that the CMAS dissolved RE-silicates to form crystalline, highly non-stoichiometric apatite phases, and in particular attacking the silicate grain boundaries. Cross-section images show that the CMAS reacted with specimens and deeply penetrated into the EBC grain boundaries and formed extensive low-melting eutectic phases, causing grain boundary recession with increasing testing time in the silicate materials. The preliminary results also showed that CMAS reactions also formed low melting grain boundary phases in the higher concentration RE-oxide doped HfO 2 systems. The effect of the test temperature on CMAS reactions of the EBC materials will also be discussed. The faster diffusion exhibited by apatite and RE-doped oxide phases and the formation of extensive grain boundary low-melting phases may limit the CMAS resistance of some of the environmental barrier coatings at high temperatures.

Reactions of molten silicate deposits with yttrium monosilicate

“…Reactions of molten silicate deposits with RE monosilicates and disilicates exhibit significant similarities and differences. Disilicates frequently react with the deposits to form apatite, nominally of composition Ca 2 RE 8 (SiO 4 ) 6 O 2 .…”

“…In many cases, the reaction processes are sensitive to deposit composition, making the problem even more challenging to address. 9,15,16 Reactions of molten silicate deposits with RE monosilicates [17][18][19][20] and disilicates 16,[19][20][21][22][23][24][25][26] exhibit significant similarities and differences. Disilicates frequently react with the deposits to form apatite, nominally of composition Ca 2 RE 8 (SiO 4 ) 6 O 2 .…”

Degradation of a SiC‐SiC composite in water vapor environments

“…Siliceous debris ingested into aero‐engines produces molten deposits of calcium‐magnesium‐aluminosilicate (CMAS) glass. CMAS reacts with candidate EBC materials to form non‐protective phases . Because of mismatch in the thermal expansion coefficients of these phases relative to the underlying composite, both the deposits and the reaction products may crack upon cooling.…”

“…CMAS reacts with candidate EBC materials to form non-protective phases. [14][15][16][17][18][19][20][21][22] Because of mismatch in the thermal expansion coefficients of these phases relative to the underlying composite, both the deposits and the reaction products may crack upon cooling. Cracks can compromise the efficacy of the coating or lead to coating spallation during thermal cycling.…”

Matrix Development for Water Vapor Resistant SiC-Based Ceramic Matrix Composites

“…Candidate matrix materials must also satisfy a number of other requirements such as resistance to calcium magnesium aluminosilicate (CMAS) [33][34][35][36][37][38] engine operating at 10 atm total pressure with 10% water vapor. This approach of testing was adopted for this dissertation.…”