الفهرس 
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Abstract
Alumina and zirconia toughened alumina ceramics are probably the most widely
used materials for preparation of structural ceramics. ZT A materials have been used for decades for wear parts and cutting tools due to their high bending strength, hardness, toughness and abrasion resistance. In this study, the densification behavior, microstructure and mechanical properties of a- Ab03 and ZTA composites; in which Zr02 stabilized with 5 mol.% Y203 and doped
with 0.25, 0.5 and 0.75 wt% of either Nb205 or Ta205; were investigated. The
batches were uniaxially pressed at 220 MPa into discs of 13 mm-diameter and
4mm-height (for physical and microstructural characterization) and rectangular bars of dimensions 6 X 6 X 60 mm (for mechanical evaluation), then the samples were pressureless sintered at temperatures ranging between 1500°
and 1650 ”C for 1 h. The physical properties of the sintered bodies in terms of bulk density and apparent porosity were evaluated and X-ray diffractometer was used to determine the sintered bodies phase composition. The microstructure was characterized by scanning electron microscope (SEM) coupled with energy dispersion spectroscopy (EDS). The mechanical properties in terms of Vickers
hardness (HV1), three point bending strength and fracture toughness were also measured. The results showed that the addition of either Nb205 or Ta205 has accelerated the densification parameters, reinforced and toughened the
obtained bodies. The density of the pentavalent oxides doped ceramic bodies
increased with increasing the sintering temperature from 15000 to 1650 DC. The
optimal sintering temperature was 1650°C. Addition of Nb205 or Ta205 improved
the densification of bulk ceramic bodies. The highest relative density of ZTA doped with 0.75 wt.% Nb205 was 99.77 % and 99.72 % for 0.75 wt.% Ta205
doping. On the other hand, the maximum mechanical properties of the Nb205
doped alumina based ceramics for bending strength, fracture toughness and
Vickers hardness were 34.4, 35.5 and 29.5% higher than those of the undoped
alumina. It was noticed that the improvement of the mechanical properties of doped ZTA composites was much higher than that of the alumina doped bodies.
The maximum bending strength, fracture toughness and Vickers hardness of