Tribology

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

• Andy Lin
• Engr 540x
• Tribology

2
Introduction

• Whats 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)

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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

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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 its viscosity
  to form rolls
• No rolls if SiO2 is too hard and brittle

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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