The effects of thermal cycling on the physical and mechanical properties of [NZP] ceramics Page: 4 of 18
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0.375 and 0.50 and will be referred to as BSO, BS17, BS25, BS37.5 and BS50,
respectively. The CS series has x equal to 0.25, 0.375 and 0.50 and will be
referred to as CS25, CS37.5 and CS50, respectively. Ceramic slips of each
composition were prepared using water as the carrier with appropriate binders and
dispersants. After ball milling for 24 h, 63.5 mm x 63.5 mm x 9.5 mm tiles were
slip cast using plaster of paris molds and then allowed to solidify before removal.
After the green tiles were dried they were bisque fired at 1000*C to remove the
binders and dispersants. The tiles were then sintered in air with a 10 h isothermal
hold time. The sintering temperature was 1625*C for the BS series and 1575*C for
the CS series. Finally the tiles were surface ground and sliced into test specimens.
Three expansion test specimens were prepared for each composition. The
specimens were ground to a length of 25 mm to match the length of the reference
standard (NBS sapphire) used in a Theta dual push-rod dilatometer. The bulk
linear thermal expansion was measured from 20*C to 1250*C in the presence of
static room air. The heating and cooling rate was 3*C/min., except below 300*C
during the cooling cycle where the cooling rate is slowed by heat dissipation from
Flexural strength determinations were conducted at room temperature according to
ASTM standards (C1161-90) using an Instron model 6027 universal testing
machine. Test specimens were the prescribed 3 mm x 4 mm x 50 mm and a 4-point
test fixture was used. The inner and outer span was 20 mm and 40 mm,
respectively. Typically there were 8 to 10 test specimens for each composition and
each test condition. The BS25 and CS25 compositions had 35 to 40 specimens for
each test condition.
Thermal cycling of test specimens was achieved by placing the specimens into
crucibles made from the BS25 composition. The crucibles were loaded into a
furnace where they were heated to 1250*C using a ramp rate of 20*C/min. with an
isothermal hold at temperature for 30 minutes, allowing all the specimens to reach
thermal equilibrium. The specimens were cooled in the furnace to below 100*C
before the next cycle commenced. The furnace atmosphere was room air.
Periodically the cycling was interrupted to determine weight loss of selected
specimens and to remove a portion of the test specimens which had undergone their
prescribed number of thermal cycles.
Fracture surfaces from flexural test specimens were prepared to determine changes
in the microstructure of the various compositions, before and after thermal cycling.
A Hitachi S-800 Field Emission Scanning Electron Microscope was used for the
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Jackson, T.B.; Limaye, S.Y. & Porter, W.D. The effects of thermal cycling on the physical and mechanical properties of [NZP] ceramics, report, December 31, 1994; Tennessee. (digital.library.unt.edu/ark:/67531/metadc625384/m1/4/: accessed December 11, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.