Effects of seasalt and sulfate additions on the melting and crystallization behavior of CMAS glass

Abstract

Particulate silicates (sand, dust, ash) present in the environment, which are ingested by aircraft gas-turbine engines (GTEs), are known to melt and deposit as calcia‒mangesia‒aluminosilicate (CMAS) glass on GTE hot-section hardware. These deposits degrade the protective ceramic thermal barrier coatings (TBCs) and environmental barrier coatings (EBCs) in the hot-section. Other particulates present in the environment, such as seasalt and sulfates, are also ingested by GTEs. Although CMAS-induced degradation of TBCs and EBCs have been studied, the effects of these other corrodents on the CMAS deposits, and how CMAS + corrodent mixtures degrade TBCs and EBCs remain unclear. This work examines systematically the effect of the addition of corrodents (CaSO₄, seasalt, or Na₂SO₄) to the CMAS on its melting and crystallization behavior. Various amounts of the corrodents were mixed separately with CMAS glass, and heat treated at various temperatures in air. Mass loss and changes to the chemical compositions were also evaluated. Phase evolution was studied experimentally, and it was compared to prospective equilibrium phases computed using the calculation of phase diagrams (CALPHAD) method. It is found that CaSO₄ alters the CMAS most significantly, and it may pose the greatest threat for exacerbating deposit-induced degradation of TBCs and EBCs in GTEs.