Sintered (SSN) and HIPED (HIPSN) silicon nitride

Question 1

Why are high surface area powders required as compared to hot pressing?

Answer 1

~ this increases the oxygen content of the powders, which results in more extensive liquid phase formation

Question 2

How are these parts prepared?

Answer 2

~ parts are fired at 1700-1800C in a N2 atmosphere

~ dissociation of Si3N4 during sintering becomes a problem; at 1700, relative density starts to decrease

Question 3

How do we impede volatilization from the part?

Answer 3

~ the use of powder beds in which the component to be sintered is surrounded by a mixture of coarse powder of its own composition and hexagonal boron nitride

~ alternatively, increasing the nitrogen pressure to 10MPa (1500psi) has demonstrated substantial improvements in properties via suppression of Si3N4 decomposition

Question 4

What densities can be achieved?

Answer 4

~ relative densities of 97-99%

~ however, with higher glass contents than HPSN, it can affect high temp creep resistance and fracture strength

Question 5

How is fracture toughness enhanced for SSN formed under high N2 pressure?

Answer 5

~ by a high Y2O3 to Al2O3 ratio

~ fosters cracks debonding the glass β-Si3N4 interface

~ increases tendency for a propagating crack to be diverted along β-Si3N4 glass interfaces with increasing Y2O3/Al2O3 ratio

Question 6

What is encapsulated HIPING?

Answer 6

~using argon or nitrogen

~ has been successful

~ during cooling, much of the glass encapsulant cracks off, and the remainder is removed by sand blasting

~ Si3N4 pressurelessly sintered to a closed porosity state can be post-HIPED without encapsulation to form pore-free parts

~ HIPED Si3N4 has superior properties, and facilitates near net-shaped parts
–> for larger parts it can be more expensive