Tribology2.ppt

7/31/2019 Tribology2.ppt

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Tribology

Friction and Wear of Silicon NitrideExposed to Moisture at High Temperatures


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Introduction

Whats the purpose of this study?

We know that...

Si3N

4+ 3O

2= 3SiO

2+ 2N

2 SiO2 interacts with water

The goal is to determine the effects of water onSilicon Nitride

-For coefficient of friction and wear

rate


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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


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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% H2

0, 8% H2

0, 34% H2

0


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Wear Rate vs Temperature

Increased wear rate iscorrelated with

increased in

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


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Wear Grooves and RollsOptical micrograph of wear

groove with 8% H2O vapor at973K

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


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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 thecylinders (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


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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


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Friction and Wear vs Temperature

2 transition temperatures for

friction and wear

At the lower transition

temperature, for H2O trials, reduces to about 1/2 the

coefficient of friction at room

temperature.


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Friction and Wear vs Temperature

At the higher transition

temperature, for H2O

trials, the increases to

level of air and argon

This higher transition

temperature isdependent on the partial

pressure of water.


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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 and wear

H20 vapor to modify SiO2and 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 softDisplaced and squeezed out of contact surface

Therefore wear increases


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Conclusion

Formation of rolls is a big factor in reducing and wear

Formation of rolls are dependent on H20vapor pressure and temperature

Therefore and wear rates of silicon nitrideare dependent on temperature and humidity

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