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

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

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

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

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

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

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

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)

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

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

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

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.

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.

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

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

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