The effects of thermal cycling on the physical and mechanical properties of [NZP] ceramics Page: 7 of 18
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cycles there is no apparent change in the microstructure, Fig. 13 and 14. Thermal
cycling to 1250*C does not cause any increase in the average grain size for these
compositions. The grain size and the degree of thermal expansion anisotropy
determine whether microcracking will occur in anisotropic compositions . In
all of the anisotropic compositions fabricated for this study the grain size was
sufficiently large so as to be greater than the critical grain size for microcracking to
occur. In all anisotropic compositions evidence of microcracking along grain
boundaries could be detected, Fig. 15-18. The micrographs of the anisotropic
compositions also show both transgranular and intergranular fracture modes.
The most striking feature shown in the micrographs is the presence of voids after
thermal cycling. The void formation looks much the same as the voids formed by
creep cavitation in other ceramics, but in this case no external stress was applied.
The voids or cavities are formed along grain boundaries and faces, Fig. 15-18. In
some cases voids could be detected in specimens after just one thermal cycle, but
were not found in any of the as sintered specimens. All of the anisotropic
compositions had some degree of void formation, with the more anisotropic and
negative expansion compositions having the greatest amount. Since the flexural
strength does not change with cycling this change in microstructure does not effect
the mechanical strength. Strength is still primarily a function of microcrack severity
Thermal cycling of BS and CS type [NZP] ceramics has little effect on their
physical and mechanical properties. The differences in the bulk thermal expansion
of microcracked anisotropic compositions are due to differences in the starting
conditions of the test specimens, i.e. the amount of microcrack opening due to
moisture absorption prior to performing the measurement. Mechanical strength is
unaffected and is generally related to the axial anisotropy of the composition and
thus the severity of the associated microcracking, for the cases where the grain size
is larger than the critical grain size for microcrack formation, as was the case in this
study. Thermal cycling to 1250*C does not produce any increase in strength
through microcrack healing. At temperatures up to 1250*C these [NZP]
compositions are very stable and should perform well in industrial applications.
Formation of voids in the anisotropic compositions was unexpected and merits
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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/m1/7/: accessed April 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.