1. Tribology
Friction and Wear of Silicon Nitride Exposed to Moisture at High Temperatures

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 K
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