Hot pressed silicon nitride (HPSN)

Question 1

How is HPSN made?

Answer 1

~ batch additives of 2-3 vol% of one or more MgO, Al2O3, or Y2O3 are used. These combine with silica (which coats Si3N4 particles) to form a low melting liquid phase

~ hot pressed at 1550-1800C for 1-4hours under 15-30MPa uniaxial pressure in graphite dies. Dies are coated with hexagonal BN (white graphite) powder to prevent rxn of graphite with Si3N4, and acts as a solid lubricant to facilitate removal of the part

Question 2

Why is the minimum liquid phase required for liquid phase sintering preferred?

Answer 2

~ so that high temp properties (creep resistance) are minimally attenuated

~ the glassy interphase separating Si3N4 grains begins to soften at 1000C

Question 3

What is the effect of MgO additions?

Answer 3

~ result in a lower softening temp glass than with Y2O3 additions

Question 4

How can liquid phase forming additives be made more effective?

Answer 4

~ if they are homogeneously distributed in the green body, Si3N4 particles can be coated with the additives via addition of soluble organometallic compounds in the spray drying step

~ it is advantageous if the starting particles are mostly α-Si3N4 (ex 90mol% α, 10mol% β)

Question 5

What happens to α during sintering heat treatment?

Answer 5

~ α is preferentially soluble in the silicate liquid, precipitating out as β on β particles, which grow in the form of prismatic hexagonal rod-like crystals that eventually impinge on each other, forming an interlocked microstructure

~ dissolution of α particles makes room for anisometrically growing β phase

Question 6

What’s the effect of the high aspect ratio grains?

Answer 6

~ act to steer a propagating fracture crack on a tortuous path along their boundaries, increases fracture toughness by crack bridging, interlocking, and deflection

~ this microstructure does not form when sintering from β-Si3N4 powder

Question 7

What’s the effect of increasing extent of α transforming to β?

Answer 7

~ fracture strength improves, but once this transformation is complete, subsequent β phase grain growth degrades fracture toughening

Question 8

What is the effect of uniaxial pressure from the hot press?

Answer 8

~ anisometric grains show some texturing (preferred orientation) in response

~ 20% variation in bend strength with orientation

Question 9

What is the result of pre-pressing the powder mix in a metal die?

Answer 9

~ pre-pressing the powder mix in a metal die to form a compact prior to its introduction into the graphite hot press die attenuates contamination

Question 10

What shapes can be achieved?

Answer 10

~ hot pressed silicon nitride is limited to simple shaped billits, which must be machined to final shape using expensive diamond grinding