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 Very good thermal shock resistance because of low thermal expansion coefficient. Silicon nitride does not melt, but decomposes at temperatures about 1900 o C. – Strongly covalently bonded

Overview Background  Processing  Applications  Tribology Background – Alpha and Beta Silicon Nitride – Molecular Structure – Mechanical Properties Toughness –Sintering aids(Y )

Overview Background  Processing  Applications  Tribology Processing – Liquid Phase Sintering – Sintering – Hot-pressing – HIP (Hot isostatic pressing) – Reaction-bonding – Sintered reaction bonding

Overview Background  Processing  Applications  Tribology Applications – Rocket Thrusters – Ceramic Hybrid Ball Bearing – Turbochargers

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

Background What types of Silicon Nitride are there? Alpha hexagonal basal plane stacked in ABCDABCD sequence Beta hexagonal basal plane an alternate sequence ABABAB

Background Both alpha and beta consists of corner-sharing SiN 4 tetrahedra

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

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 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 Toughness

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

Fracture Toughness Addition of Y 2 O 3 promoted the development of high aspect ratio beta Si 3 N 4 grains Higher aspect ratio gave a higher toughness

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

Sintering Silicon nitride powder compacts can be sintered to near full density, without the application of any pressure MgO, Al 2 O 3, Y 2 O 3, rare earth oxides But mechanical properties of sintered silicon nitrides are inferior to those processed by hot-pressing

Hot Pressing (Pressure Sintering) T dye =1/2 T M Similar to sintering -Pressure and temperature applied simultaneously Accelerates densification by: -Increasing contact stress between particles -Rearranging particle position and improving packing

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 Refractive punch Powder Hydraulic Press Plug PRESSMASTER!!

Hot Pressing Hot-pressed silicon nitride is usually made with MgO or Y 2 O 3 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 Main Constituents Compression chamber Pressurized gas of argon or helium Evacuated and gas-sealed preform

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

Reaction Bonding 3Si(s) + 2 N 2  Si 3 N 4 (s) ΔH=-724 kJ/mole Form α-Si 3 N 1200 o C Liquifies between 1200 o C and 1400 o C Form β-Si 3 N 1400 o C – 21.7% change in volume

Reaction Bonding =N 2 =Si =Si 3 N 4

Reaction Bonding Concerns High surface reaction on surface – Closes surface pores – Prevent internal reaction – Sintering/hot pressing needed to remove excess porosity Evaporation of N O C – Si 3 N 4  3 Si +2 N 2 (g) – Solution? Over pressurize N 2 (g)

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

Processing

Processing

Applications silicon nitride offers high strength, low density, and good thermal shock resistance Silicon nitride thruster Left: Mounted in test stand. Right: Being tested with H 2 /O 2 propellants

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

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

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

Application Less Friction, Less Heat  lower wear  needs less lubrication  less energy consumption  reduced sound level  extends material life=lowering your operating costs

Application Extended Operating Life -typically yield 5 to 10 times longer life than conventional steel- steel ball bearings

Turbochargers

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

Introduction What’s the purpose of this study? We know that... Si 3 N 4 + 3O 2 = 3SiO 2 + 2N 2 SiO 2 interacts with water The goal is to determine the effects of water on Silicon Nitride -For coefficient of friction and wear rate

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

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% H 2 0, 8% H 2 0, 34% H 2 0

Friction coefficient vs Temperature µ for Argon and air about 0.65 from room temperature to 1273K µ for 8% H 2 0 about 0.3 from K Higher µ after critical temperature at 973K 34% H 2 0 has higher critical temperature Critical temperature depends on partial pressure of H 2 0

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

Wear Grooves and Rolls Optical micrograph of wear groove with 8% H 2 O 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” with 34% H 2 O 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” with 34% H 2 O 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” Image of fractured roll with small debris particles

TEM “Rolls” TEM of midsection and end Surface non- homogenous Smaller pieces are constituents of roll

Friction and Wear vs Temperature 2 transition temperatures for friction and wear At the lower transition temperature, for H 2 O trials, µ reduces to about 1/2 the coefficient of friction at room temperature.

Friction and Wear vs Temperature At the higher transition temperature, for H 2 O trials, the µ increases to level of air and argon This higher transition temperature is dependent on the partial pressure of water.

Lower Transition Temperature What going on at the lower transition temperature? Formation of Oxide Si 3 N 4 + 3O 2 = 3SiO 2 + 2N 2 The increase in temperature allows: -significant oxide formation to reduce µ and wear -H 2 0 vapor to modify SiO 2 and lower it’s viscosity to form rolls -No rolls if SiO 2 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 H 2 O vapor pressure SiO 2 layer breaks down -Becomes too soft -Displaced and squeezed out of contact surface Therefore wear increases Higher Transition Temperature

Conclusion  Formation of rolls is a big factor in reducing µ and wear  Formation of rolls are dependent on H 2 0 vapor pressure and temperature  Therefore µ and wear rates of silicon nitride are dependent on temperature and humidity

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., Ring, Terry A. Fundamentals of Ceramic Powder Processing and Synthesis. San Diego: Academic Press, EMA6448/ cture%20Toughness%20of%206%20wt%25%20Yttria%202%20wt%25%2 0Alumina%20Silicon%20Nitride.pdf