17-05-2012, 01:11 PM
Coatings for Si3N4 and SiC Ceramics: an Historical Perspective
[attachment=22240]
Advanced Gas Turbines Provide a Material Challenges
• High Temperatures: 1900 to 2500F
• High and Cyclical Stresses: thermal and
mechanical
• High Velocity Gases: approaching (exceeding)
Mach 1
• High Pressures: ~ 4 to 10 atmospheres (for
small turbines)
• Exposure to combustion products
• Combined Temperature, Pressure, Velocity, &
Atmosphere which accelerate the detrimental
effect, i.e., slow crack growth, corrosion,
Providing Environmental Resistance is Challenging
• Self healing SiO2 not effective against water
vapor
• Transport in SiO2 is affected more by low level
impurities than Al2O3 and Cr2O3 [Pareek et al, 1991, Zheng
et al, 1992]
• Additives or impurities in surface layers increase
oxidation rates, oxygen diffusion rate, change
the rate controlling mechanism, and alter the
oxide scale structure [Opila et al, 2000]
• Even with low oxygen diffusion coatings, silica
will form under coatings
• Si3N4 & SiO2 have low CTE: CTE mismatch leads
to interface cracks & debonding
• Surface treatments to enhance adherence may
degrade substrate
Cyclic Testing has not been adequately Addressed
• Thermally grown oxides often develop at
substrate-coating interface during high
temperature exposure
• Growth strains may relax at high temperatures.
• Upon cool down, other components of the
coating may yield with temperature dependent
yield strength
• Tensile stresses increase with cyclic exposures
• This effect is not evaluated in static durability
testing
[attachment=22240]
Advanced Gas Turbines Provide a Material Challenges
• High Temperatures: 1900 to 2500F
• High and Cyclical Stresses: thermal and
mechanical
• High Velocity Gases: approaching (exceeding)
Mach 1
• High Pressures: ~ 4 to 10 atmospheres (for
small turbines)
• Exposure to combustion products
• Combined Temperature, Pressure, Velocity, &
Atmosphere which accelerate the detrimental
effect, i.e., slow crack growth, corrosion,
Providing Environmental Resistance is Challenging
• Self healing SiO2 not effective against water
vapor
• Transport in SiO2 is affected more by low level
impurities than Al2O3 and Cr2O3 [Pareek et al, 1991, Zheng
et al, 1992]
• Additives or impurities in surface layers increase
oxidation rates, oxygen diffusion rate, change
the rate controlling mechanism, and alter the
oxide scale structure [Opila et al, 2000]
• Even with low oxygen diffusion coatings, silica
will form under coatings
• Si3N4 & SiO2 have low CTE: CTE mismatch leads
to interface cracks & debonding
• Surface treatments to enhance adherence may
degrade substrate
Cyclic Testing has not been adequately Addressed
• Thermally grown oxides often develop at
substrate-coating interface during high
temperature exposure
• Growth strains may relax at high temperatures.
• Upon cool down, other components of the
coating may yield with temperature dependent
yield strength
• Tensile stresses increase with cyclic exposures
• This effect is not evaluated in static durability
testing