Influence of Sintering on the Development of Alumina-Toughened …
165
occurrence of catastrophic failure during cooling and while undergoing phase transformation from tetragonal to monoclinic [15]. The tetragonal phase of zirconia in
the stable condition, without the use of dopants, can be obtained as fine particles that
are smaller than a critical size [16]. The zirconia produces the monoclinic phase at
room temperature and a transformation phase from monoclinic to tetragonal when
gradually heated up to a temperature of 1700 °C. Upon continued heating, a structural
change to the cubic phase occurs both at and near its melting point of 2716 °C [17].
During the last decade, the number of studies focused on the development of
alumina-toughened zirconia (ATZ) composites that offer a combination of enhanced
mechanical properties has engendered considerable scientific and technologyrelevant interest. Most noteworthy example is in the domain specific to dental applications, Zhang and co-workers [18] reinforced varying amounts of alumina (Al 2 O 3 )
in the range of 0.25–2.5 weight percent to a 2–3 mol% Yttria-stabilized zirconia
(tetragonal zirconia polycrystal TZP) and systematically investigated the degradation of its properties at low temperatures. Rittidech and co-workers [1] investigated
the influence of addition of 1 to 8 weight percent Y 2 O 3 on both phase formation
and fracture toughness of the developed Al 2 O 3 -xZrO 2 -Y 2 O 3 (AZY) ceramics. They
concluded from their detailed study that the addition of Y 2 O 3 does not appreciably
affect and/or influence shape of the grains in the AZY matrix.
The possible outcome of transformation toughening is that applied stresses can be
induced in the composite structure, which often reveals a combination of improved
mechanical properties and physical properties [19, 20]. Transformation toughening
of the zirconia from tetragonal (t) to monoclinic (m) almost always results in an
improved fracture toughness. This makes it highly suitable for selection and use in
applications such as the following: (i) as a hip implant, (ii) dental restoration, and (iii)
as thermal barrier coating [21, 22]. The properties of zirconia-based composites can
be improved by presence of a tetragonal phase, which upon the application of stress
results in transformation toughening [23, 24]. The tetragonal zirconia polycrystal
(TZP) ceramic was found to have a higher fracture toughness due essentially to
transformation toughening of zirconia phase. This also results in enhanced wear
performance during both spalling and micro-cracking [25]. The addition of 2 mol%
of Yttria to stabilize the tetragonal zirconia does limit the overall effectiveness of the
transformation toughening mechanism. The grain size of zirconium oxide (ZrO 2 )
was smaller than a critical size of 0.7 mm, which ensured an overall stability of the
tetragonal phase [of ZrO 2 ] at room temperature (25 °C) [26, 27].
After performing a critical review of the open literature, it was observed that
several studies were conducted on zirconia-based ceramic composites at the microscopoc level or scale on ceramic samples that were consolidated using the technique
of conventional sintering, while only a few studies were recorded at the nanoscale.
Recently, Oghbaei and co-workers, [28] reported the use of microwave sintering
(MW) technique or process as a novel sintering approach when compared one-onone with conventional sintering (CS), especially for those materials that require
sintering at an elevated temperature. Further, microwave sintering (MW) also overcomes few of the drawbacks, such as (i) poor and incomplete sintering [29], (ii)
165
occurrence of catastrophic failure during cooling and while undergoing phase transformation from tetragonal to monoclinic [15]. The tetragonal phase of zirconia in
the stable condition, without the use of dopants, can be obtained as fine particles that
are smaller than a critical size [16]. The zirconia produces the monoclinic phase at
room temperature and a transformation phase from monoclinic to tetragonal when
gradually heated up to a temperature of 1700 °C. Upon continued heating, a structural
change to the cubic phase occurs both at and near its melting point of 2716 °C [17].
During the last decade, the number of studies focused on the development of
alumina-toughened zirconia (ATZ) composites that offer a combination of enhanced
mechanical properties has engendered considerable scientific and technologyrelevant interest. Most noteworthy example is in the domain specific to dental applications, Zhang and co-workers [18] reinforced varying amounts of alumina (Al 2 O 3 )
in the range of 0.25–2.5 weight percent to a 2–3 mol% Yttria-stabilized zirconia
(tetragonal zirconia polycrystal TZP) and systematically investigated the degradation of its properties at low temperatures. Rittidech and co-workers [1] investigated
the influence of addition of 1 to 8 weight percent Y 2 O 3 on both phase formation
and fracture toughness of the developed Al 2 O 3 -xZrO 2 -Y 2 O 3 (AZY) ceramics. They
concluded from their detailed study that the addition of Y 2 O 3 does not appreciably
affect and/or influence shape of the grains in the AZY matrix.
The possible outcome of transformation toughening is that applied stresses can be
induced in the composite structure, which often reveals a combination of improved
mechanical properties and physical properties [19, 20]. Transformation toughening
of the zirconia from tetragonal (t) to monoclinic (m) almost always results in an
improved fracture toughness. This makes it highly suitable for selection and use in
applications such as the following: (i) as a hip implant, (ii) dental restoration, and (iii)
as thermal barrier coating [21, 22]. The properties of zirconia-based composites can
be improved by presence of a tetragonal phase, which upon the application of stress
results in transformation toughening [23, 24]. The tetragonal zirconia polycrystal
(TZP) ceramic was found to have a higher fracture toughness due essentially to
transformation toughening of zirconia phase. This also results in enhanced wear
performance during both spalling and micro-cracking [25]. The addition of 2 mol%
of Yttria to stabilize the tetragonal zirconia does limit the overall effectiveness of the
transformation toughening mechanism. The grain size of zirconium oxide (ZrO 2 )
was smaller than a critical size of 0.7 mm, which ensured an overall stability of the
tetragonal phase [of ZrO 2 ] at room temperature (25 °C) [26, 27].
After performing a critical review of the open literature, it was observed that
several studies were conducted on zirconia-based ceramic composites at the microscopoc level or scale on ceramic samples that were consolidated using the technique
of conventional sintering, while only a few studies were recorded at the nanoscale.
Recently, Oghbaei and co-workers, [28] reported the use of microwave sintering
(MW) technique or process as a novel sintering approach when compared one-onone with conventional sintering (CS), especially for those materials that require
sintering at an elevated temperature. Further, microwave sintering (MW) also overcomes few of the drawbacks, such as (i) poor and incomplete sintering [29], (ii)