The effects of thermal cycling on the physical and mechanical properties of [NZP] ceramics Page: 5 of 18
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RESULTS AND DISCUSSION
As mentioned above, two compositional series were chosen for this study that have
varying bulk thermal expansions and degrees of axial thermal expansion
anisotropy. Figures 1 and 2 show the relative expansions of the members of the
two series. Included in the figures are both the heating and cooling portion of the
thermal expansion curve. It can be seen that there are three isotropic compositions,
BS 17, BS25 and CS50, where the cooling curve lies on top of the heating curve.
The other compositions have varying degrees of anisotropy, the magnitude of
which can be estimated by the amount of hysteresis. Three specimens of each
composition were measured to insure sample to sample consistency. Figures 3 and
4 show the results obtained for the anisotropic CS25 and the isotropic BS25
specimens. There is good sample to sample consistency for all compositions
Initially it was believed that the bulk thermal expansion of many [NZP] type
ceramics changed with repeated thermal cycles to temperatures above 500*C. This
conclusion was reached when inconsistent expansion measurements were obtained
between samples of the same composition or samples measured more than once.
Subsequently it was determined that specimens which had undergone identical
preparation exhibited identical expansion curves. Identical sample preparation in
this case was sintering followed by surface grinding and finally slicing the sintered
tiles into test specimens. The slicing and grinding operations saturate the specimen
with a water-based coolant which has been determined to promote crack opening in
the ceramic . Because of the greater initial crack opening the first expansion
curve is lower than subsequent measurements. After the first run in the dilatometer
the absorbed water is driven off, allowing microcracks to close. Only a small
amount is reabsorbed from the room air upon cooling, partially reopening the
microcracks. Thus, the specimen is not restored to its original saturated condition.
Subsequent runs in the dilatometer are usually very similar, as shown in Fig. 5.
The differences can be explained by differences in room air humidity and the
amount of time between heat treatments and dilatometer measurements, Fig. 6. For
the highly anisotropic and negative thermal expansion compositions there is
evidence that at room temperature continued crack opening occurs at a very slow
rate until an equilibrium is reached. Thus, for this class of ceramics extreme care is
needed in order to produce accurate and comparable thermal expansion
measurements, especially for anisotropic compositions which have a grain size
larger than the critical grain size for microcrack formation. The average grain size
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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/m1/5/: accessed April 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.