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Silicon Nitride. Andy Lin MATE 320 6/6/01. Facts of Silicon Nitride. Silicon nitride is one of the strongest structural ceramics (B 4 C, TiC, Al 2 O 3 , ZrO 2 ) In air, silicon nitride rapidly forms a surface silicon oxide layer. Good protection against oxidation

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silicon nitride

Silicon Nitride

Andy Lin

MATE 320

6/6/01

facts of silicon nitride
Facts of Silicon Nitride
  • Silicon nitride is one of the strongest structural ceramics (B4C, TiC, Al2O3, ZrO2)
  • In air, silicon nitride rapidly forms a surface silicon oxide layer. Good protection against oxidation
  • Very good thermal shock resistance because of low thermal expansion coefficient.
  • Silicon nitride does not melt, but decomposes at temperatures about 1900 oC.
    • Strongly covalently bonded
overview
Overview

BackgroundProcessingApplicationsTribology

  • Background
    • Alpha and Beta Silicon Nitride
    • Molecular Structure
    • Mechanical Properties
      • Toughness
        • Sintering aids(Y203)
overview4
Overview

BackgroundProcessingApplicationsTribology

  • Processing
    • Liquid Phase Sintering
    • Sintering
    • Hot-pressing
    • HIP (Hot isostatic pressing)
    • Reaction-bonding
    • Sintered reaction bonding
overview5
Overview

BackgroundProcessingApplicationsTribology

  • Applications
    • Rocket Thrusters
    • Ceramic Hybrid Ball Bearing
    • Turbochargers
overview6
Overview

BackgroundProcessingApplicationsTribology

  • Tribology – What is it?
    • Friction and Wear of Silicon Nitride Exposed to Moisture at High Temperatures
background
Background
  • Alpha
  • hexagonal
  • basal plane stacked in ABCDABCD
  • sequence
  • Beta
  • hexagonal
  • basal plane
  • an alternate sequence
  • ABABAB
  • What types of Silicon Nitride are there?
background8
Background
  • Both alpha and beta consists of

corner-sharing SiN4 tetrahedra

background9
Background

How important are alpha and beta?

  • Alpha
    • Bigger
    • More complex
    • More unstable
    • Goal: To minimize alpha during processing
  • Beta
    • Goal: Maximize Beta during processing
background10
Background
  • What determines toughness in silicon nitride?
  • 1)grain size
  • 2)aspect ratio of the grains.
    • Long beta silicon nitride have high aspect ratios

Where the aspect ratio is the ratio of grain length to grain diameter.

fracture toughness
Fracture Toughness
  • The long beta-silicon nitride grains >1 micron
    • provide a high resistance to crack growth.
    • deflect the crack propagation
    • Absorbs load at crack tip
fracture toughness13
Fracture Toughness
  • The grains can be encouraged to grow by increasing the hot pressing time
  • This results in different fracture toughness
fracture toughness14
Fracture Toughness
  • Addition of Y2O3 promoted the development of high aspect ratio beta Si3N4 grains
  • Higher aspect ratio gave a higher toughness
processing
Processing
  • Liquid Phase Sintering
  • Sintering
  • Hot-Pressing
  • HIP (Hot Isostatic Pressing)
  • Reaction Bonding
  • Sintered Reaction Bonding
slide16

Liquid Phase Sintering

  • Liquid dissolves the Alpha, which then precipitates out the more stable Beta
  • This causes a volume reduction
  • Very small amounts of residual Alpha
sintering
Sintering
  • Silicon nitride powder compacts can be sintered to near full density, without the application of any pressure
  • MgO, Al2O3, Y2O3, rare earth oxides
  • But mechanical properties of sintered silicon nitrides are inferior to those processed by hot-pressing
slide18

Hot Pressing(Pressure Sintering)

Tdye=1/2 TM

  • Similar to sintering
    • -Pressure and temperature applied simultaneously
  • Accelerates densification by:
    • -Increasing contact stress between particles
    • -Rearranging particle position and improving packing
slide19

Hot Pressing

  • Advantages
  • Reduces densification time
  • Reduce densification temperature
    • Reduce grain growth increases hardness
    • Minimize porosity
  • Result? Higher strength!!
  • Good for easy shapes
  • Disadvantage? Bad for intricate shapes
hot pressing
Hot Pressing

Hydraulic Press

PRESSMASTER!!

Refractive

punch

Powder

Plug

hot pressing21
Hot Pressing
  • Hot-pressed silicon nitride is usually made with MgO or Y2O3 sintering aids.
  • Application of pressure during sintering is instrumental in achieving nearly full density, resulting in very good properties.
  • Disadvantage? High processing cost
hip hot isostatic pressing
HIP=Hot Isostatic Pressing
  • Main Constituents
    • Compression chamber
    • Pressurized gas of argon or helium
    • Evacuated and gas-sealed preform
slide23
HIP
  • Hot isostatic pressing (HIP) improves the properties of silicon nitride
  • Applying uniform pressure results in greater material uniformity
    • Eliminates die-wall friction effects
  • Disadvantage?
    • High processing cost
slide24

Reaction Bonding

  • 3Si(s) + 2 N2 Si3N4(s) ΔH=-724 kJ/mole
  • Form α-Si3N4 @ 1200oC
  • Liquifies between 1200oC and 1400oC
  • Form β-Si3N4 @ 1400oC
    • 21.7% change in volume
slide25

Reaction Bonding

=N2

=Si

=Si3N4

slide26

Reaction Bonding Concerns

  • High surface reaction on surface
    • Closes surface pores
    • Prevent internal reaction
    • Sintering/hot pressing needed to remove excess porosity
  • Evaporation of N2 (g) @ 1850 OC
    • Si3N43 Si +2 N2 (g)
    • Solution? Over pressurize N2 (g)
reaction bonding
Reaction Bonding
  • Final product
    • much less expensive than hot-pressed or sintered materials
    • But has a porosity greater than 10%, which results in poor mechanical properties
applications
Applications

Silicon nitride thruster

Left: Mounted in test stand. Right: Being tested with H2/O2 propellants

  • silicon nitride offers high strength, low density, and good thermal shock resistance
hybrid ceramic bearings
Hybrid Ceramic Bearings

Advantages

High Speed and Acceleration

Increased stiffness

Less Friction, Less Heat

Reduced Lubrication Requirements

Low Thermal Expansion

Extended Operating Life

application
Application
  • High Speed and Acceleration
  • 40% as dense as steel
    • reduced weight produces less centrifugal forces imparted on the ringsless friction
  • reducing friction, allowing 30 to 50% higher running speeds
    • Needs less lubrication/maintenance      
application33
Application
  • Increased stiffness-50 % higher modulus of elasticity than steel resistance to deformation
  • 15 to 20% increase in rigidity
application34
Application
  • Less Friction, Less Heatlower wear

needs less lubrication

less energy consumption

reduced sound level

extends material life=lowering your operating costs

application35
Application
  • Extended Operating Life-typically yield 5 to 10 timeslonger life than conventional steel-steel ball bearings
turbochargers37
Turbochargers
  • Why use Silicon Nitride in turbos?
  • Lighter lower inertia and improved response time
    • Silicon Nitride rotors are lighter
    • Silicon Nitride bearings produce less friction
tribology
Tribology

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

slide39

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
slide40

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
slide41

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

slide42

Friction coefficient vs Temperature

Friction coefficient vs Temperature

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
slide43

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
slide44

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
slide45

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)
slide46

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
slide47

TEM “Rolls”

  • Image of fractured roll with small debris particles
slide48

TEM “Rolls”

  • TEM of midsection and end
  • Surface non-homogenous
  • Smaller pieces are constituents of roll
slide49

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.
slide50

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.
slide51

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
slide52

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
slide53

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
bibliography
Bibliography

Reed, James S., Principles of Ceramic Processing. New York:

John Wiley & Sons, Inc., 1995

Richerson, David W., Modern Ceramic Engineering.

New York: Marcel Dekker, Inc., 1992.

Ring, Terry A. Fundamentals of Ceramic Powder Processing and Synthesis. San Diego: Academic Press, 1996

http://www.nittan.co.jp/english/tech/et01.htm

http://www.mse.stanford.edu/people/faculty/dauskardt/ajay/Si3N4.html

http://www.mse.ufl.edu/~wsigmund/EMA4645-EMA6448/http://www.jfcc.or.jp/katudo/md/sekkei_en.html

http://www.angelfire.com/home/hondaracerf2/sini/main.htm

http://msewww.engin.umich.edu:81/people/halloran/pdf/Mode%20I%20Fracture%20Toughness%20of%206%20wt%25%20Yttria%202%20wt%25%20Alumina%20Silicon%20Nitride.pdf

http://www.pns.anl.gov/ckl_science/Materials/Si3N4_Results.html