Tribology

1. Tribology Friction and Wear of Silicon Nitride Exposed to Moisture at High Temperatures Andy Lin Engr 540x Tribology

2. Introduction • What’s the purpose of this study? We know that... • Si3N4 + 3O2 = 3SiO2 + 2N2 • SiO2 interacts with water • The goal is to determine the effects of water on Silicon Nitride -For coefficient of friction and wear rate

3. Purpose • Why is this Relevant? Applications… • Silicon nitride automobile applications exposed to water vapor • Bearing/components of gas turbine engines • Ceramic coating on metallic components

4. Experimental Procedure • Used sliding ball-on-flat apparatus in different environments containing water vapor at elevated temperature • Silicon nitride flats and isostatically pressed balls • 10,000 strokes (equivalent to 218 meters sliding distance) • Environments include: Argon, Air, 2% H20, 8% H20, 34% H20

5. Friction coefficient vs Temperature • µ for Argon and air • about 0.65 from room • temperature to 1273K • µ for 8% H20 about • 0.3 from 573-973K • Higher µ after critical • temperature at 973K • 34% H20 has higher • critical temperature • Critical temperature • depends on partial • pressure of H20

6. Wear Rate vs Temperature • Increased wear rate is • correlated with increased in µ • Transition to higher wear rate at 8% H20 also seen at 973K • Wear rate is lower in • presence of water as • compared with argon and air

7. Wear Grooves and Rolls • Optical micrograph of wear groove with 8% H2O vapor at 973K • Cylindrical rolls oriented perpendicular to sliding direction • Geometry of rolls dependent on temperature and water vapor content • Rolls provide mechanical support between surfaces and reduce actual surface area contact

8. SEM of “Rolls” • SEM of “rolls” with 34% H2O vapor at 873K • Rolls develop perpendicular to the sliding direction • Rolls are formed from smaller wear particles that adhere and form the cylinders (ie Playdoh)

9. SEM of “Rolls” • SEM of “rolls” with 34% H2O vapor at 873K • Surface shows delamination and resulting debris particles • Debris particles are flattened and curled into a roll • Many layers of debris can be seen on rolls

10. TEM “Rolls” • Image of fractured roll with small debris particles

11. TEM “Rolls” • TEM of midsection and end • Surface non-homogenous • Smaller pieces are constituents of roll

12. Friction and Wear vs Temperature • 2 transition temperatures for friction and wear • At the lower transition temperature, for H2O trials, µ reduces to about 1/2 the coefficient of friction at room temperature.

13. Friction and Wear vs Temperature • At the higher transition temperature, for H2O trials, the µ increases to level of air and argon • This higher transition temperature is dependent on the partial pressure of water.

14. Lower Transition Temperature • What going on at the lower transition temperature? • Formation of Oxide • Si3N4 + 3O2 = 3SiO2 + 2N2 • The increase in temperature allows: • significant oxide formation to reduce µ and wear • H20 vapor to modify SiO2 and lower it’s viscosity to form rolls • No rolls if SiO2 is too hard and brittle

15. Higher Transition Temperature • What going on at the higher transition temperature? • Rolls begin to break down • Bigger and thicker rolls last longer • Produced by higher H2O vapor pressure • SiO2 layer breaks down • Becomes too soft • Displaced and squeezed out of contact surface • Therefore wear increases

16. Conclusion • Formation of rolls is a big factor in reducing µ and wear • Formation of rolls are dependent on H20 vapor pressure and temperature • Therefore µ and wear rates of silicon nitride are dependent on temperature and humidity