Cain, M. G. (Markys G.) (1990) Zirconia toughened ceramics. PhD thesis, University of Warwick.

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Abstract

The objectives for the thesis were to generate tough ceramics utiising the toughening
mechanisms inherent to zirconia materials. The aims have been realised with the
successful fabrication of hot pressed silicon nitride / zirconia composite ceramics.
The zirconia was prestabilised with two different types of dopant additives, yttria
and ceria, with the intention of understanding the chemical compatibility with the
silicon nitride matrix and the overall effect on the subsequent mechanical properties.
The volume fraction of added zirconia was also varied. The increased toughness over
silicon nitride materials alone was attributed to the toughening agents inherent to
zirconia which existed either in the form of the tetragonal polymorph or the monoclinic
variant. The toughening modes were dependent on initial chemistry of the
composite system. When the zirconia was prestabilised with yttria the tetragonal
polymorph was retained within the composite. The enhanced toughness was attributed
to a transformation toughening mechanism. However, when the zirconia
was prestabiised with ceria the depletion of Ce from solid solution with the zirconia
during processing resulted in the formation of the unstabiised monoclinic variant.
The enhanced toughness was attributed, in this case, to a microcrack type energy
absorption mechanism, similar to several ZTA composite ceramics.
Additionally, an experiment using ultrasound non-destructive testing, indicated
that Tetragonal Zirconia Polycrystals (TZP) is ferroelastic and, as such, can provide
a significant contribution to enhanced levels of fracture toughness in these materials
or composites containing the same. Further work has been conducted to actually
observe, as a function of applied unia.xial stress, the crystallographic changes occurring
within the bulk of a 3Y-TZP ceramic via neutron elastic scattering at the ILL,
Grenoble, France. This experiment has provided clear direct proof of the ferroelastic
nature of zirconia. A similar experiment will be carried out at the Rutherford
Laboratory, though with significantly improved statistics.
An approach to improve the high temperature properties of TZP via the chemical
alteration of the grain boundary phase was also considered. As a preliminary step
the grain boundary volume was increased through controlled additions of the grain
boundary composition in the form of both a premilled and a premelted glass. Poor
fired densities were attained, however, due to the solute additive partitioning from
the generation of an enhanced grain boundary phase to overstabilisation of the
zirconia resulting in the formation of cubic stabilised zirconia. Furthermore, the
incorporation of nitrogen within the grain boundary phase, via sintering TZP with
sole additions of A1N, resulted in the attainment of poor fired densities and hence
was not considered a suitable method for grain boundary modification.

Item Type:	Thesis or Dissertation (PhD)
Subjects:	Q Science > QC Physics
Library of Congress Subject Headings (LCSH):	Ceramics, Zirconium oxide
Official Date:	October 1990
Dates:	Date Event October 1990 Submitted
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Lewis, Mike H.
Extent:	iv, 169, [24] leaves
Language:	eng
URI:	http://wrap.warwick.ac.uk/id/eprint/35750

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