الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 252 |  |  |
| | | from | 252 | | |
Abstract
Current study was divided into two parts. In the first part, synthesis of
experimental non-stabilized monoclinic zirconia (NSZ), as the advantageous
tetragonal to monoclinic phase transformation of stabilized zirconia
responsible for toughening can also become counter-productive when the
material is exposed to humid environment at low temperatures, due to a
hydrothermal aging phenomenon known as low-temperature degradation
(LTD). Since there will be no transformation as the synthesized product will
be pure monoclinic zirconia so it is expected that there will be no effect of aging on the zirconia properties in comparison to partially (PSZ) and fully
(FSZ) stabilized zirconia.
Non-stabilized monoclinic zirconia powder was prepared under
hydrothermal condition using zirconium oxychloride solution as precursor
by optimization of the effect of PH of the reaction. The optimum preparation
parameter for obtaining pure nanocrystalline monoclinic zirconia was PH=2.
Powder characterization was consisted of x-ray diffraction (XRD), scanning
electron microscope (SEM) and transmission electron microscope (TEM), which
showed that pure monoclinic zirconia crystallites with narrow distribution of
nano-size was obtained with high crystallinity and spherical in shape.
After characterization, the ZrO2 monoclinic powder was pressed via
cold isostatic pressing under 300 MPa into disc specimens for further
comparison in the second part of the study. In the second part, evaluation of
the effect of shading and/or aging on the newly synthesized experimental
non-stabilized monoclinic zirconia (NSZ) was evaluated and compared to
commercially products of partially stabilized zirconia (PSZ) and fully
stabilized zirconia (FSZ) regarding some physico-mechanical properties
including biaxial flexural strength (BFS), fracture toughness, surface
roughness, microstructural analysis, amount of monoclinic transformation,
color stability (∆𝐸), contrast ratio (CR) and translucency parameters (TP).
A total of four hundred thirty-two disc shaped specimens were
prepared from the three types of zirconia. For PSZ and FSZ, discs were
milled from zirconia blanks via CAD/CAM milling machine. Half of the
specimens were shaded using aqueous based coloring liquid. All specimens
were sintered in muffle furnace; NSZ specimens were sintered using twostep sintering (TSS) technique which attain full densification without grain
growth. while PSZ and FSZ specimens were sintered according to the
manufacturer recommendations.
To evaluate the effect of low temperature degradation LTD;
accelerated aging for the shaded and non-shaded specimens of the three
types of studied zirconia, accelerated aging through autoclave at 134°C at a
water partial pressure of 2 bars for 5 hours has been achieved.
For each property, ten specimens were tested and compared to the
control group (non-shaded, non-aged). Testing procedures for biaxial
flexural strength BFS were tested according to according to the ISO
6872:2008 specifications “Dentistry – Ceramic materials, fracture analysis of
BFS specimens were carried out using SEM. Fracture toughness was tested
according to Japanese industrial standard (JIS R1607), surface roughness
was measured using confocal laser scanning microscope. Microstructural
analysis was evaluated with XRD, SEM and AFM. Optical properties wereChapter
evaluated according to the CIE 2007 L*a*b* color scale relative to the CIE
standard illuminant D65.