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

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 >1 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) 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

19. 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. HIP=Hot 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) ΔH=-724 kJ/mole • Form α-Si3N4 @ 1200oC • Liquifies between 1200oC and 1400oC • Form β-Si3N4 @ 1400oC • 21.7% change in volume

25. Reaction Bonding =N2 =Si =Si3N4

26. 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)

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 life=lowering 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 • 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

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.