# Presentation: 004970

## Elizabeth Opila, NASA Glenn Research Center
Dwight Myers, East Central University, Ada, OK

- Dwight Myers, East Central University, Ada, OK

- Alumina Volatility in Water Vapor at Elevated Temperatures

- Environmental Barrier Coatings for Microturbine and Industrial Gas Turbine Ceramics Workshop
- November 18–19, 2003Nashville, TN

## Motivation

- Drive to increase operating temperatures of turbine engines for power generation and propulsion
- Need for material systems that can be used at temperatures of 1200 to 1650C in combustion environments
- Al**2**O**3** is possible component of high temperature material systems
- oxide/oxide composites
- high temperature alumina-containing coatings
- Understand chemical durability of Al**2**O**3** in water vapor-containing combustion environments

## Background

- Primary volatilization reaction for alumina in water vapor:
- 1/2 Al**2**O**3**(s) + 3/2 H**2**O(g) = Al(OH)**3**(g)
- Thermochemical data estimated for Al(OH)**3**(g) using partition functions and structures of similar molecules, e.g., AlF**3**(g), B(OH)**3**(g)
- L.V. Gurvich, I.V. Veyts, C.B. Alcock, Thermodynamic Properties of Individual Substances, Begell House, Inc., New York, 1996.
- Al(OH)**3**(g) identified as volatile species from a transpiration study of a mixture of CaAl**2**O**4**(s) and CaAl**4**O**7**(s)
- A. Hashimoto, Geochim. Cosmochim. Acta 56, 511-32 (1992).
- Al**2**O**3**(s) recession measured and quantified in combustion test rig.  Pressure and temperature dependence consistent with Al(OH)**3**(g) formation.
- I. Yuri, T. Hisamatsu, ASME Turbo Expo, paper GT2003-38886.

## Volatile Species in Al-O-H System

- Calculated using Gurvich data:  Al2O3 +  1 bar H2O(g) + 1 bar O2(g)

## Objectives

- Experimentally determine temperature and water vapor partial pressure dependence of alumina volatility directly from alumina
- Confirm identity of volatile aluminum hydroxide species
- Identify combustion conditions where alumina volatility limits useful component life

## Material Description

- sapphire coupons
- 2.5 x 1.25 x 0.2 cm
- flame fusion grown
- <100 ppm impurities
- (0001) basal plane orientation
- General Ruby and Sapphire Corp., New Port Richey, FL

## Experimental Procedure

- Thermogravimetric Analysis Apparatus
- coupon weight measured before and after exposure
- TGA only used to monitor weight anomalies during experiment
- volatiles condense on cool portion of sample hanger
- TGA apparatus allows laminar flow over coupon
- T=1250 to 1500C
- P(H**2**O) = 0.15 to 0.68 atm, balance O**2**
- Ptotal = 1 atm

## Schematic Drawing of TGA Apparatus

## Alumina Volatility:  Weight Loss Rates

- Al2O3, 0.5 atm H2O/0.5 atm O2

## Determination of Al(OH)3(g) Partial Pressure from Weight Change

- Determination of Al(OH)**3**(g) Partial Pressure from Weight Change

- Measure w, calculate P
- Assumptions:
- volatility is controlled by transport of volatile species through gas boundary layer
- laminar flow over flat plate
- D is interdiffusion of Al(OH)**3**(g) in H**2**O(g)
- use collision diameter and integral of AlF**3**(g) as approximation for Al(OH)**3**(g)

## Temperature Dependence of Al(OH)3(g) formation

- Al2O3, 0.5 atm H2O/0.5 atm O2

## Pressure Dependence of Al(OH)3(g) formation

- Al2O3, 1450C

## Surface Etching of Sapphire after Exposure in High Temperature Water Vapor

- typical surface, (0001) basal plane

- 1250C, 0.5 atm H2O, 240h

## Surface Pitting of Sapphire after Exposure in High Temperature Water Vapor

- Hole put in coupon by grit blasting, (0001) basal plane surface

- 1350C, 0.5 atm H2O, 94h

## Etching of Sapphire Coupon Edge after Exposure in High Temperature Water Vapor

- polished surface

- ground beveled edge

- as-received

- 1500C, 235h, O2

- 1450C, 72h,
- 0.68 atm H2O

## Comparison of Alumina and Silica Volatility in High Temperature Water Vapor

- Volatile species in 1 atm water vapor

- Si(OH)**4**

- SiO(OH)**2**

- Al(OH)**3**

## Recession Map for Al2O3 Use in Combustion Environments

- Calculated with data of Gurvich et al.

## Summary and Conclusions

- Alumina volatility in water vapor measured directly by weight loss and found to agree with literature values.
- Pressure dependence of volatility consistent with Al(OH)**3**(g) formation.
- Surface etching of sapphire coupons observed in high temperature water vapor.
- Recession  P v**1/2** exp[-(210 kJ/mol)/RT].
- Alumina volatility will limit lifetimes of components and coatings for long term applications in combustion environments, e.g.,
- 			250 m recession in 10,000 h
- 		    	T=1300C, P=10 atm, v=50 m/sec

## Possible Future Work

- Transpiration studies on Al**2**O**3** + H**2**O
- more precise thermochemical data possible
- complement Hashimoto’s study on mixed calcium aluminates
- requires fusion technique to dissolve condensed volatile species
- Free jet sampling mass spectrometry of Al**2**O**3** + H**2**O
- first mass spectrometric identification of Al(OH)**3**(g)
- complement study of Vasiliy Smirnov conducted at much higher temperatures for other Al-O-H species

## Acknowledgments

- 	Ralph Garlick, Nathan Jacobson, Serene Farmer, all from NASA Glenn Research Center
- 	This work was funded by the Aeropropulsion and Power Program at NASA Glenn Research Center