Reaction bonded silicon nitride (RBSN)

Question 1

How does reaction bonding of silicon nitride occur?

Answer 1

~ a shaped Si powder compact (via pressing, slip casting, or injection molding) is reacted in a N2 atmosphere for 2-12 days at 1250-1450C

~ Si particles typically coated with 3nm SiO2 coatings. Addition of some H2 to the N2 gas helps strip the coating

Question 2

What happens if the exothermic rxn proceeds too quickly?

Answer 2

~the Si powder will melt and ball up into globules too large to nitride, or will exude onto the surface of the part

Question 3

Does complete transformation of Si to Si3N4 happen?

Answer 3

~ typically not

~ both α and β Si3N4 form (β is favored above the melting point of Si, 1414C), plus 12-30% porosity

Question 4

What is the thickness of components?

Answer 4

~ thickness is limited to ~2nm due to limited permeability of nitrogen gas into the compact

Question 5

What is the advantage of RBSN?

Answer 5

~ the original dimension of the silicon compact remain virtually unchanged during nitriding

~ RBSN contains no liquid phase sintering additives, and thus has a high resistance to chemical attack since little susceptible GB glass is present

Question 6

How are RBSN parts shaped?

Answer 6

~ a Si powder compact can be bisque-fired to form necks between the particles, cooled, machined using conventional tooling to a final shape and then fired in nitrogen to form a RBSN part

Question 7

What is the fracture strength of RBSN parts?

Answer 7

~ not as good at hot-pressed, but reaction bonding is an inexpensive fabrication route for complex shapes

Question 8

What are RBSN specific applications?

Answer 8

~ thermocouple sheaths, nozzles for gas and arc welding, kiln furniture

~ Si3N4 is protected from oxidation degradation by a thin protective surface layer of amorphous SiO2, which forms starting at 800C. If damaged, it will reform