Fabrication of solid state sintered SiC

Question 1

What is the history of fabricating solid state SiC?

Answer 1

~ until 1970s, it was assumed because of the strong, directional nature of the covalent bonding, there’s a high activation energy for self diffusion, making pressureless sintering difficult

~ Prochazka, 1973 GE, demonstrated that sub-micron β-SiC powder could be sintered to high densities (inert or vacuum atmosphere, 1950-2100C) with small batch additions of boron and carbon (done with α-SiC as well)

Question 2

What is the role of C additions?

Answer 2

~ most favorably done as pyrolyzed phenolic resin

~ used to react away SiO2 coatings invariably on SiC particles: SiO2 + 3C –> SiC + 2CO

Question 3

What is the role of B4C additions?

Answer 3

~ not fully understood

~ may be partially soluble in SiC, its differing valence state forming vacancies which enhance bulk diffusion

~ OR it may concentrate at the GB, lowering the interfacial energy, decreasing the driving force for grain growth

Question 4

What happens with excessive sintering temp/time?

Answer 4

~ abnormal grain growth, forming anisometric large grains with pores trapped inside
–> better mechanical properties result from fine, equiaxed grains

Question 5

What are the benefits of solid state sintered SiC?

Answer 5

~ retains its strength to high temp and is highly creep resistant

~ lack of second phase makes it inert (corrosion resistant)

~ hot pressing permits the use of coarser (less expensive) α-SiC, and/or lower firing temperatures (with a lower propensity for abnormal grain growth)