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