Partially stabilized zirconia

Question 1

How is the composition and sintering temperature chosen for PSZ? For Mg-PSZ?

Answer 1

~ selected close to the solvus curve (cubic SS/cubic SS + tetragonal) in the cubic SS single phase field

~ for Mg-PSZ, the higher MgO content, the lower the sintering and stabilizing temp can be

Question 2

What happens after sintering PSZ?

Answer 2

~ the sintered ceramic is quenched to retain cubic structure metastably at room temp (though very small 5-10nm tetragonal nuclei form within the cubic grains)

Question 3

What happens after quenching?

Answer 3

~ it is re-heated to an isothermal aging temperature (ex: 1400-1500C for Mg-PSZ)

~ tetragonal SS (MgO-deficient) lens-shaped crystals (oblate spheroids ~0.25 um) grow from the nuclei (and coarsen via ostwald ripening) within the 40-70um cubic SS grains

Question 4

What is the difference between MgO-deficient tetragonal SS and the matrix cubic SS phase?

Answer 4

~ more MgO deficient tetragonal SS is more metastable at room temp than matrix cubic SS phase

Question 5

What causes strengthening?

Answer 5

~ reduced sensitivity to stress intensification at crack tips ==> increased fracture toughness

Question 6

What is Kic?

Answer 6

~ fracture toughness/critical stress intensity factor, indicator of a brittle material’s resistance to fracture because of crack tip intensification

~ Kic = Bσfsqrt(c)
~ σf = stress at fracture
~ c = crack length
~ B: constant

Question 7

What is the relationship between fracture toughness and crack length?

Answer 7

~ larger the crack length for a given fracture strength, the higher the fracture toughness

Question 8

How is fracture toughness measured?

Answer 8

~ can be measured through fracture tests on specimens with intentionally-machined cracks, or calculated from cracks extending from the corners of harness indentations

Question 9

What is stress induced transformation toughening in PSZ?

Answer 9

~ transformation of the metastable tetragonal oblate spheroids to monoclinic, and its associated volume expansion, is inhibited during cooling from the aging temp via constraint from the surrounding cubic phase

Question 10

What happens with an external tensile stress?

Answer 10

~ if it causes an existing crack to extend, the intensified tensile stress field ahead of the crack RELAXES the compressive constraint of the matrix phase

~ this induces the oblate spheroids ahead of the crack tip to transform from tetragonal to MONOCLINIC

~ associated volume expansion puts the region ahead of the crack in compression (or less tension). Further, transformed particles in the wake of the crack tip act to squeeze the crack. Both impose a requirement for increased external stress to further propagate the crack

Question 11

What’s the relationship between thermodynamic driving force for t–>m transformation and oblate spheroid size?

Answer 11

~ the thermodynamic driving force for the tetragonal –> monoclinic transformation increases with increasing oblate spheroid size

~ thermodynamic driving force for t–>m transformation decreases with increasing temp (temp at which tetragonal is the stable phase is being approached), requiring higher local stress to induce the transformation
~ leads to smaller transformation zone width and a degraded transformation toughening contribution

Question 12

What ideally happens during aging?

Answer 12

~ the tetragonal oblate spheroids are coarsened to a size just below that necessary for spontaneous t–>m transformation during cooling, so the transformation is reserved to occur when tensile stress-induced

Question 13

What’s the result of over-aging?

Answer 13

~ results in spontaneous transformation during cooling from the aging temp, with a LOSS in fracture toughening

~ optimum aging times vary with stabilizer content

~ PSZ which has been aged to yield max fracture toughness at room temp shows a decreased fracture toughening with increasing temp

Question 14

What is the effect of Y2O3 addition to ZrO2?

Answer 14

~ LOWERS the equilibrium t–>m TRANSF TEMP; this lowers the thermodynamic driving force for the t–>m transformation of metastable tetragonal SS at room temp (as compared to Mg-PSZ), permitting larger oblate spheroids to remain metastable at room temp

~ requires a less stringent aging heat treatment schedule

Question 15

What’s the effect of CaO concentration in Ca-PSZ?

Answer 15

~ higher CaO concentrations degrade the max fracture strength, since tetragonal oblate spheroids of all sizes become too stable to transform to monoclinic under the tensile stress near a crack tip

Question 16

What’s relationship between fracture toughness of Ca-PSZ and temp?

Answer 16

~ decrease in fracture toughness of Ca-PSZ with increase temp

~ with increased temp, % of oblate spheroids on fracture surfaces transformed to monoclinic decreases

~ with increased temp, the distance perpendicularly away from the fracture surface that has demonstrated stress induced tetragonal to monoclinic transformation decreases