Silicon Nitride Andy Lin MATE 320 6/6/01. Facts of Silicon Nitride Silicon nitride is one of the...
Transcript of Silicon Nitride Andy Lin MATE 320 6/6/01. Facts of Silicon Nitride Silicon nitride is one of the...
Silicon NitrideSilicon Nitride
Andy Lin
MATE 320
6/6/01
Facts of Silicon NitrideFacts 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
OverviewOverview
BackgroundProcessingApplicationsTribology
Background– Alpha and Beta Silicon Nitride– Molecular Structure– Mechanical Properties
Toughness– Sintering aids(Y203)
OverviewOverview
BackgroundProcessingApplicationsTribology
Processing– Liquid Phase Sintering– Sintering– Hot-pressing– HIP (Hot isostatic pressing)– Reaction-bonding– Sintered reaction bonding
OverviewOverview
BackgroundProcessingApplicationsTribology
Applications– Rocket Thrusters
– Ceramic Hybrid Ball Bearing – Turbochargers
OverviewOverview
BackgroundProcessingApplicationsTribology
Tribology – What is it? – Friction and Wear of Silicon Nitride Exposed to
Moisture at High Temperatures
BackgroundBackgroundWhat types of Silicon Nitride are there?
•Alphahexagonalbasal plane stacked in ABCDABCDsequence
•Betahexagonalbasal planean alternate sequenceABABAB
BackgroundBackground
Both alpha and beta consists of
corner-sharing SiN4 tetrahedra
BackgroundBackgroundHow important are alpha and beta?– Alpha
Bigger More complex More unstable Goal: To minimize alpha during processing
– Beta Goal: Maximize Beta during processing
BackgroundBackground
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 ToughnessFracture 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 ToughnessFracture Toughness
Fracture ToughnessFracture Toughness
The grains can be encouraged to grow by increasing the hot pressing time
This results in different fracture toughness
Fracture ToughnessFracture Toughness
Addition of Y2O3 promoted the development of high aspect ratio beta Si3N4 grains
Higher aspect ratio gave a higher toughness
ProcessingProcessing
Liquid Phase SinteringSinteringHot-PressingHIP (Hot Isostatic Pressing)Reaction BondingSintered Reaction Bonding
•Liquid dissolves the Alpha, which then precipitates out the more stable Beta
•This causes a volume reduction
•Very small amounts of residual Alpha
Liquid Phase SinteringLiquid Phase Sintering
SinteringSintering
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
Hot PressingHot Pressing(Pressure Sintering)(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
Hot PressingHot Pressing
Advantages Reduces densification time Reduce densification temperature
– Reduce grain growth increases hardness– Minimize porosity
Result? Higher strength!! Good for easy shapesDisadvantage? Bad for intricate shapes
Hot PressingHot Pressing
Refractivepunch
Powder
Hydraulic Press
Plug
PRESSMASTER!!
Hot PressingHot 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 PressingHIP=Hot Isostatic Pressing
•Main Constituents•Compression chamber•Pressurized gas of argon or helium•Evacuated and gas-sealed preform
HIPHIP
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
Reaction BondingReaction Bonding
3Si(s) + 2 N2 Si3N4(s) ΔH=-724 kJ/mole
Form α-Si3N4 @ 1200oC
Liquifies between 1200oC and 1400oCForm β-Si3N4 @ 1400oC
– 21.7% change in volume
Reaction BondingReaction Bonding
=N2
=Si =Si3N4
Reaction Bonding ConcernsReaction Bonding Concerns
High surface reaction on surface– Closes surface pores– Prevent internal reaction– Sintering/hot pressing needed to remove excess
porosityEvaporation of N2 (g) @ 1850 OC
– Si3N43 Si +2 N2 (g)– Solution? Over pressurize N2 (g)
Reaction BondingReaction 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
ProcessingProcessing
ProcessingProcessing
ApplicationsApplications
silicon nitride offers high strength, low density, and good thermal shock resistance
Silicon nitride thruster
Left: Mounted in test stand. Right: Being tested with H2/O2 propellants
Hybrid Ceramic BearingsHybrid Ceramic Bearings
AdvantagesHigh Speed and Acceleration
Increased stiffness
Less Friction, Less Heat
Reduced Lubrication Requirements
Low Thermal Expansion
Extended Operating Life
ApplicationApplicationHigh Speed and Acceleration
40% as dense as steel– reduced weight produces less
centrifugal forces imparted on the ringsless friction
reducing friction, allowing 30 to 50% higher running speeds – Needs less
lubrication/maintenance
ApplicationApplication
Increased stiffness-50 % higher modulus of elasticity than steel
resistance to deformation
15 to 20% increase in rigidity
ApplicationApplication
Less Friction, Less Heatlower wear
needs less lubrication
less energy consumption
reduced sound level
extends material life=lowering your operating costs
ApplicationApplication
Extended Operating Life-typically yield 5 to 10 timeslonger life than conventional steel-steel ball bearings
TurbochargersTurbochargers
TurbochargersTurbochargers
Why use Silicon Nitride in turbos?• Lighter lower inertia and improved response time
•Silicon Nitride rotors are lighter•Silicon Nitride bearings produce less friction
TribologyTribology
Friction and Wear of Silicon Nitride Exposed to Moisture at High Temperatures
IntroductionIntroduction
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
PurposePurpose 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 ProcedureExperimental 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 TemperatureFriction coefficient vs TemperatureFriction coefficient vs TemperatureFriction coefficient vs TemperatureFriction coefficient vs TemperatureFriction coefficient vs Temperature•µ for Argon and air about 0.65 from room temperature to 1273K
•µ for 8% H20 about0.3 from 573-973K
•Higher µ after criticaltemperature at 973K
•34% H20 has higher critical temperature
•Critical temperaturedepends on partial pressure of H20
Wear Rate vs TemperatureWear 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 RollsWear 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”•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”•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”TEM “Rolls”
•Image of fractured roll with small debris particles
TEM “Rolls”TEM “Rolls”
• TEM of midsection and end
• Surface non-homogenous
• Smaller pieces are constituents of roll
Friction and Wear vs Friction and Wear vs TemperatureTemperature
•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 Friction and Wear vs TemperatureTemperature
• 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 TemperatureLower Transition Temperature
What going on at the lower transition temperature?
•Formation of OxideSi3N4 + 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
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
Higher Transition TemperatureHigher Transition Temperature
ConclusionConclusion
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
BibliographyBibliographyReed, James S., Principles of Ceramic Processing. New York:
John Wiley & Sons, Inc., 1995Richerson, 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.htmhttp://www.mse.stanford.edu/people/faculty/dauskardt/ajay/Si3N4.htmlhttp://www.mse.ufl.edu/~wsigmund/EMA4645-EMA6448/http://www.jfcc.or.jp/katudo/md/sekkei_en.htmlhttp://www.angelfire.com/home/hondaracerf2/sini/main.htmhttp://msewww.engin.umich.edu:81/people/halloran/pdf/Mode%20I%20Fracture%20Toughness%20of%206%20wt%25%20Yttria%202%20wt%25%20Alumina%20Silicon%20Nitride.pdfhttp://www.pns.anl.gov/ckl_science/Materials/Si3N4_Results.html