6.4 Ceramic-Based Composites
85
the high cost limiting their practical applications [28, 31]. As a result, the toughening
of alumina-based ceramics has been the subject of a large number of investigations,
most of which are mainly based on the incorporation of toughening agents. In this
regard, various agents and fabrication methods have been employed, leading to an
increase in the value of the fracture toughness for alumina composites. For instance,
the fracture toughness values of 5.4, 4.8 and 5.0 MPa m
1/2 can be achieved for alumina composites containing 5 wt% glass phase [32] 13 vol.% Zr + Ag [33] and 13
vol.% Ni [34], respectively, using cold isostatic pressing and pressureless sintering.
Hot pressing can lead to the fabrication of alumina composites containing 20 wt%
YSZ [35] and 20 vol.% Fe [36], with fracture toughness values of 6.8 MPa m
1/2 and
6.6–10.2 MPa m
1/2 , respectively.
Recently, graphene has been investigated as a toughening agent in alumina ceramics. The alumina–graphene ceramics were fabricated by means of spark plasma sintering and exhibited fracture toughness values in the range of 3.5–5.2 MPa m
1/2
[37–39]. The graphene materials employed in these studies were prepared by various techniques comprising Hummers-based methods followed by the hydrazine
reduction [37], thermal exfoliation of intercalated graphite [38] and the liquid-phase
exfoliation of graphite [39]. Apart from the limitations related to the fabrication of
the graphene materials employed in these studies, the fabrication methods used for
the preparation of the ceramic materials, i.e., hot pressing and spark plasma sintering, are costly and involve the use of expensive equipment. Hence, more economic
methods for the preparation of both graphene and alumina–graphene ceramics are
needed in order to explore the future potential of graphene in the alumina ceramic
industry. It is also worth mentioning that graphene materials produced by different
techniques exhibit different characteristics.
The effect of molten salt-produced graphene nanosheets on the morphology and
the mechanical properties of alumina ceramics fabricated by means of slip casting and
pressureless sintering was investigated [6]. Slip casting process was adopted to fabricate alumina and alumina–graphene green bodies. To this end, graphene nanosheets
produced in molten LiCl were added to deionized water and dispersed using ultrasonication for 30 min. Then, a sufficient amount of alumina powder was added to
make a slip of the desired amount of carbon. The mixture was then ultrasonicated
for 30 min, and the resulting slurry was ball milled for 12 h. Then, small amounts
of deionized water and dispersant were added, in order to decrease the slurry’s viscosity, enhancing its castability, followed by 30 min ball milling. The slurry made
was casted into plaster molds to form rectangular specimens upon loss of the water.
Figure 6.6a shows photographs of the alumina and alumina–graphene green bodies produced by the slip casting process. The darker appearance of the graphene
containing alumina is clear in this figure. The casts were sintered in a resistant furnace at 1650 °C under a protective Ar atmosphere. Figure 6.6b exhibits the SEM
micrographs of the sintered alumina, with no graphene addition. This sample can
be characterized by the presence of coarse alumina grains with a size of 10–20 μm.
The fracture toughness of this sample was measured to be 4.50 MPa m
1/2 , in agreement with the fracture toughness values measured on pressureless sintered alumina
ceramics [30]. By adding only 0.5wt% molten salt-produced graphene nanosheets to
85
the high cost limiting their practical applications [28, 31]. As a result, the toughening
of alumina-based ceramics has been the subject of a large number of investigations,
most of which are mainly based on the incorporation of toughening agents. In this
regard, various agents and fabrication methods have been employed, leading to an
increase in the value of the fracture toughness for alumina composites. For instance,
the fracture toughness values of 5.4, 4.8 and 5.0 MPa m
1/2 can be achieved for alumina composites containing 5 wt% glass phase [32] 13 vol.% Zr + Ag [33] and 13
vol.% Ni [34], respectively, using cold isostatic pressing and pressureless sintering.
Hot pressing can lead to the fabrication of alumina composites containing 20 wt%
YSZ [35] and 20 vol.% Fe [36], with fracture toughness values of 6.8 MPa m
1/2 and
6.6–10.2 MPa m
1/2 , respectively.
Recently, graphene has been investigated as a toughening agent in alumina ceramics. The alumina–graphene ceramics were fabricated by means of spark plasma sintering and exhibited fracture toughness values in the range of 3.5–5.2 MPa m
1/2
[37–39]. The graphene materials employed in these studies were prepared by various techniques comprising Hummers-based methods followed by the hydrazine
reduction [37], thermal exfoliation of intercalated graphite [38] and the liquid-phase
exfoliation of graphite [39]. Apart from the limitations related to the fabrication of
the graphene materials employed in these studies, the fabrication methods used for
the preparation of the ceramic materials, i.e., hot pressing and spark plasma sintering, are costly and involve the use of expensive equipment. Hence, more economic
methods for the preparation of both graphene and alumina–graphene ceramics are
needed in order to explore the future potential of graphene in the alumina ceramic
industry. It is also worth mentioning that graphene materials produced by different
techniques exhibit different characteristics.
The effect of molten salt-produced graphene nanosheets on the morphology and
the mechanical properties of alumina ceramics fabricated by means of slip casting and
pressureless sintering was investigated [6]. Slip casting process was adopted to fabricate alumina and alumina–graphene green bodies. To this end, graphene nanosheets
produced in molten LiCl were added to deionized water and dispersed using ultrasonication for 30 min. Then, a sufficient amount of alumina powder was added to
make a slip of the desired amount of carbon. The mixture was then ultrasonicated
for 30 min, and the resulting slurry was ball milled for 12 h. Then, small amounts
of deionized water and dispersant were added, in order to decrease the slurry’s viscosity, enhancing its castability, followed by 30 min ball milling. The slurry made
was casted into plaster molds to form rectangular specimens upon loss of the water.
Figure 6.6a shows photographs of the alumina and alumina–graphene green bodies produced by the slip casting process. The darker appearance of the graphene
containing alumina is clear in this figure. The casts were sintered in a resistant furnace at 1650 °C under a protective Ar atmosphere. Figure 6.6b exhibits the SEM
micrographs of the sintered alumina, with no graphene addition. This sample can
be characterized by the presence of coarse alumina grains with a size of 10–20 μm.
The fracture toughness of this sample was measured to be 4.50 MPa m
1/2 , in agreement with the fracture toughness values measured on pressureless sintered alumina
ceramics [30]. By adding only 0.5wt% molten salt-produced graphene nanosheets to