The effects of thermal cycling on the physical and mechanical properties of [NZP] ceramics Page: 6 of 18
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for all the compositions tested in this study varied from 6 to 12 micrometers. The
anisotropic compositions (BSO, BS37.5, BS50, CS25 and CS37.5) have calculated
critical grain sizes that are less than 5 micrometers, thus they are subject to
Compositions which have an isotropic axial thermal expansion (BS 17, BS25 and
CS50) have little or no thermal expansion hysteresis. This is due to little or no
microcrack formation upon cooling from either the sintering temperature or the
thermal expansion measurement temperature, in this study 1250*C. The critical
grain size for microcrack formation in the isotropic compositions studied is greater
than 50 micrometers. These compositions also have bulk linear thermal expansion
curves that remain constant after many thermal cycles. Saturation with water from
the grinding operation does not affect the initial thermal expansion measurement,
Fig. 7 and 8. This is due to the fact that these compositions have little open
porosity due to the absence of microcracks and thus have very little absorbed water.
This is evident from examining the weight loss of the specimens as a function of the
number of thermal cycles, Fig. 9 and 10. The weight has been normalized to the
weight after 1 heating cycle to 1250*C. The isotropic compositions (BS25 and
CS50) have very little weight change, where as, the anisotropic and thus
microcracked compositions (BSO, BS37.5, BS50, CS25 and CS37.5) have large
initial weight losses.
The flexural strength of these ceramics is dependent on residual porosity, grain
size, number and severity of microcracks and processing flaws, with porosity and
microcracking being the predominant factors. The isotropic compositions have the
highest flexural strength of all the compositions tested due to the absence of
microcracking. In the CS series as the anisotropy increased the strength decreased,
Fig. 11. The room temperature flexural strength of the anisotropic compositions
was not improved due to any crack healing occurring during thermal cycling. It can
be assumed that the number of microcracks remained nearly constant and the
severity of the cracks, at least at the tensile surface was unchanged, Fig. 11 and 12.
Repeated thermal cycles did not have any beneficial or adverse effect on the flexural
strength of any of the compositions tested.
Photomicrographs reveal a similar microstructure for all compositions, with
generally equiaxed grains. The CS series has a slightly larger grain size than the
BS series. The micrographs of the isotropic compositions show transgranular
fracture and little evidence of microcracking as expected. After repeated thermal
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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. (https://digital.library.unt.edu/ark:/67531/metadc625384/m1/6/: accessed April 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.