Relative Ti2AlC Scale Volatility under 1300 °C Combustion Conditions

James L Smialek

doi:10.3390/coatings10020142

Abstract

Turbine environments may degrade high temperature ceramics because of volatile hydroxide reaction products formed in water vapor. Accordingly, the volatility of transient TiO2 and steady-state Al2O3 scales formed on the oxidation-resistant Ti2AlC MAX phase ceramic was examined in 1300 °C high velocity (Mach 0.3, 100 m/s) and high pressure (6 atm, 25 m/s) burner rig tests (BRT). Unlike metals, the ceramic was stable at 1300 °C. Unlike SiC and Si3N4, neither burner test produced a weight loss, unless heavily pre-oxidized. Lower mass gains were produced in the BRT compared to furnace tests. The commonly observed initial, fast TiO2 transient scale was preferentially removed in hot burner gas (~10% water vapor). A lesser degree of gradual Al2O3 volatilization occurred, indicated by grain boundary porosity and crystallographic etching. Modified cubic-linear (growth-volatility) kinetics are suggested. Gas velocity and water vapor pressure play specific roles for each scale. Furthermore, a 7YSZ TBC on Ti2AlC survived for 500 h in the Mach 0.3 burner test at 1300 °C with no indication of volatility or spalling.

Highlights

Turbine environments may degrade high temperature ceramics because of volatile hydroxide reaction products formed in water vapor
A 7YSZ thermal barrier coatings (TBC) on Ti2 AlC survived for 500 h in the Mach 0.3 burner test at 1300 ◦ C with no indication of volatility or spalling
Water vapor attack has become a new element of study because of volatile hydroxides that form by reaction with Al2 O3 substrates or with SiO2 scales that form on SiC and Si3 N4

Summary

Introduction

Turbine environments may degrade high temperature ceramics because of volatile hydroxide reaction products formed in water vapor. Al2 O3 scales formed on the oxidation-resistant Ti2 AlC MAX phase ceramic was examined in 1300 ◦ C high velocity (Mach 0.3, 100 m/s) and high pressure (6 atm, 25 m/s) burner rig tests (BRT). The commonly observed initial, fast TiO2 transient scale was preferentially removed in hot burner gas (~10% water vapor). Water vapor attack has become a new element of study because of volatile hydroxides that form by reaction with Al2 O3 substrates or with SiO2 scales that form on SiC and Si3 N4. Thermal barrier coatings (TBC) allow for higher gas temperatures, but protect the metal and thermally grown oxide (TGO) from both high temperature and high velocity. SiO2 scales are known to have significantly increased growth rates in water vapor, whereas Al2 O3 scales show more complex effects on metals than on MAX phases [6,7,8,9]

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