Silicon Nitride

1
Silicon Nitride

• Andy Lin
• MATE 320
• 6/6/01

2
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

3
Overview

• Background?Processing?Applications?Tribology
• Background
• Alpha and Beta Silicon Nitride
• Molecular Structure
• Mechanical Properties
• Toughness
• Sintering aids(Y203)

4
Overview

• Background?Processing?Applications?Tribology
• Processing
• Liquid Phase Sintering
• Sintering
• Hot-pressing
• HIP (Hot isostatic pressing)
• Reaction-bonding
• Sintered reaction bonding

5
Overview

• Background?Processing?Applications?Tribology
• Applications
• Rocket Thrusters
  
  
• Ceramic Hybrid Ball Bearing
• Turbochargers
  
  

6
Overview

• Background?Processing?Applications?Tribology
• Tribology What is it?
• Friction and Wear of Silicon Nitride Exposed to
  Moisture at High Temperatures

7
Background

• What types of Silicon Nitride are there?

• Alpha
• hexagonal
• basal plane stacked in ABCDABCD
• sequence
• Beta
• hexagonal
• basal plane
• an alternate sequence
• ABABAB

8
Background

• Both alpha and beta consists of
• corner-sharing SiN4 tetrahedra

9
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

10
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.

11
Fracture Toughness

• The long beta-silicon nitride grains gt1 micron
• provide a high resistance to crack growth.
• deflect the crack propagation
• Absorbs load at crack tip

12
Fracture Toughness
13
Fracture Toughness

• The grains can be encouraged to grow by
  increasing the hot pressing time
• This results in different fracture toughness

14
Fracture Toughness

• Addition of Y2O3 promoted the development of high
  aspect ratio beta Si3N4 grains
• Higher aspect ratio gave a higher toughness

15
Processing

• Liquid Phase Sintering
• Sintering
• Hot-Pressing
• HIP (Hot Isostatic Pressing)
• Reaction Bonding
• Sintered Reaction Bonding

16
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

17
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

18
Hot Pressing(Pressure Sintering)
Tdye1/2 TM

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

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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

20
Hot Pressing
Hydraulic Press
PRESSMASTER!!
Refractive punch
Powder
Plug
21
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

22
HIPHot Isostatic Pressing

• Main Constituents
• Compression chamber
• Pressurized gas of argon or helium
• Evacuated and gas-sealed preform

23
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

24
Reaction Bonding

• 3Si(s) 2 N2? Si3N4(s) ?H-724 kJ/mole
• Form a-Si3N4 _at_ 1200oC
• Liquifies between 1200oC and 1400oC
• Form ß-Si3N4 _at_ 1400oC
• 21.7 change in volume

25
Reaction Bonding
N2 Si Si3N4
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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) _at_ 1850 OC
• Si3N4?3 Si 2 N2 (g)
• Solution? Over pressurize N2 (g)

27
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

28
Processing
29
Processing
30
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

31
Hybrid Ceramic Bearings

• Advantages
• High Speed and Acceleration
• Increased stiffness
• Less Friction, Less Heat
• Reduced Lubrication Requirements
• Low Thermal Expansion
• Extended Operating Life

32
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      

33
Application

• Increased stiffness-50 higher modulus of
  elasticity than steel resistance to deformation
• 15 to 20 increase in rigidity

34
Application

• Less Friction, Less Heat?lower wear
• ?needs less lubrication
• ?less energy consumption
• ?reduced sound level
• ?extends material lifelowering your operating
  costs

35
Application

• Extended Operating Life-typically yield 5 to 10
  timeslonger life than conventional steel-steel
  ball bearings

36
Turbochargers
37
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

38
Tribology
Friction and Wear of Silicon Nitride Exposed to
Moisture at High Temperatures
39
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

40
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

41
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

42
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

43
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

44
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

45
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)

46
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

47
TEM Rolls

• Image of fractured roll with small debris
  particles

48
TEM Rolls

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

49
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.

50
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.

51
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

52
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

53
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

54
Bibliography
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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
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