Most ceramic materials [1
] have high melting temperatures, high stiffness, and high thermal stability. However, they have low fracture toughness and strength, and they show low electrical and thermal conductivities. Therefore, extensive research work has been devoted to the enhancement of the mechanical and functional properties of, ceramics through appropriate microstructure design. This includes making composites [2
] and nanocomposites [3
]. Ceramic nanocomposites are multiphase materials where the matrix phase is reinforced with one or more nano-phases, such that unique properties are obtained to meet or exceed design expectations [4
]. As proposed by Niihara in 1991, “the concept of ceramic nanocomposites involves the adoption of the nanocomposite approach for the microstructural tailoring of structural ceramic composites” [5
]. The dispersion of the nano-scale phase within the matrix grains and on the grain boundaries leads to the improvement in the mechanical, physical, and tribological properties of the nanocomposites. Figure 1
shows the most common nanoreinforcements and nanocomposite structures for ceramics. The nanoreinforcement could be: (i) zero-dimensional (0-D) round nanomaterials, such as nanoparticles and nanospheres with diameter (2r
) less than 100 nm, (ii) one-dimensional (1-D) needlelike-shaped nanomaterials, such as fiber or tube having diameter (2r
) less than 100 nm and aspect ratio of more than 100, or (iii) two-dimensional (2-D) platelike shaped nanomaterials, which can be layered materials with typically thickness (t
) of the order of 1 nm and aspect ratio in the other two directions at least of 25. The 0D nanoreinforcement might be dispersed in the matrix grains, located at the grain boundaries, or occupy both inter- and intra-granular positions as shown in Figure 1
d. Similarly, the 1D and 2D nanomaterials may be embedded in a micronic matrix, as presented in Figure 2
e,f, respectively. Figure 2
g illustrates a micron-sized matrix that is reinforced with a mixture of 0D and 1D nanoscale phases [6
Aluminum oxide, commonly called alumina [7
], is a ceramic material that is widely used in cutting tool [8
] and dental applications. Moreover, it is a potential material for producing chemical and electrical insulators [9
], as well as armouries [10
]. However, its wide use in several structural and functional applications is limited by not only the low toughness and strength, but also the low electrical and thermal conductivities. Fortunately, reinforcing alumina with a 0D, 1D, or 2D nanoscale phase has been found to be an effective practice for developing strong and tough alumina composites with unique functionalities [11
]. In this regard, alumina was reinforced by many nanoscale phases, including SiC [12
], CNT [21
], and graphene [24
Comprehensive review papers on the synthesis, processing, microstructure, properties, and performance of ceramic matrix composites, including alumina nanocomposites reinforced by SiC, ZrO2
, CNTs, and graphene, were published [6
]. Palmero et al. addressed critical issues related to the processing and properties of nanocomposite ceramics, particularly Al2
based composites [29
]. In another review paper, Palmero [6
] classified nanostructured ceramics and discussed basic structure–property relations, including the dependence of the materials properties on the grain size of the matrix and the reinforcement. Additionally, the author highlighted the influence of powder synthesis methods on the microstructure and properties of the consolidated materials. Ahmad et al. summarized advances, difficulties, and potential applications of carbon nanotube and graphene reinforced ceramic nanocomposites [11
]. Galusek and Galusková [30
] evaluated the influence of synthesis methods and additives on the properties of alumina-based nanocomposites. Estili and Sakka critically review the role of carbon nanotubes (CNTs) along with strengthening and toughening mechanisms, in ceramic nanocomposites [31
Alumina hybrid nanocomposites with tailored nanostructures, improved mechanical and tribological properties, and enhanced functionalities were developed. Figure 2
presents a typical schematic of hybrid microstructure design of Al2
reinforced by CNTs and SiC [32
]. The role of SiC particles is to strengthen alumina grain boundaries and improve the toughness. Additional fiber toughening mechanisms are due to CNTs. Moreover, CNTs and SiC have direct influence on the electrical and thermal conductivities of the hybrid composite.
Uniform distribution of the reinforcements in hybrid nanocomposite powders is a prerequisite for obtaining materials that have the anticipated properties [33
]. Nanoparticles have the tendency to agglomerate and nanotubes form entangled bundles; therefore, they need to be uniformly distributed in the matrix to obtain homogenous powdered materials [30
]. This is specifically important if a nanopowder matrix is reinforced with nanoparticles, nanotubes, and nanosheets to prepare a hybrid nanocomposite powder. Methods, such as simple wet dispersion and probe sonication [35
], ball milling [24
], molecular level mixing [21
], sol-gel processing [44
], and colloidal processing [22
] were used to improve the dispersion of the reinforcements in alumina matrix.
Traditional sintering techniques, such as furnace sintering [36
] and hot pressing sintering [35
], were used to sinter alumina hybrid nanocomposites that were reinforced with SWNTs and MWCNTs [36
], MWCNTs and SiCw [38
], graphene nanoplatelets (GNPs) and CNTs [35
], SiCW and TiC [50
], Graphene and Carbon nanotube [51
], and ZrO2
and MWCNTs [52
]. Although these techniques are known for their limitation in preventing the growth of the alumina matrix, the reported values of measured properties were not far from those obtained by the spark plasma sintering method. It is believed that spark plasma sintering (SPS) will continue to be a process of choice for developing alumina hybrid nanocomposites that have preferred microstructures and novel properties [41
]. This is because of the advantages of SPS over other sintering methods, which include high heating rate, low sintering time and temperature, and enhanced densification due to the role of the electric current. In addition, the process is binder-less, direct, and cost-effective. In fact, the use of SPS allowed for the sintering of alumina hybrid nancomposites to high relative density, while retaining the small grain size of the matrix. This includes Al2
], Alumina-GNPs-SiC [39
], Alumina-Graphene-CNTs [53
], and Al2
-graphene based hybrid nanocomposites [58
]. The majority of the performed studies were dedicated to the evaluation of mechanical properties and few researchers considered the tribological, electrical, and thermal properties.
As compared to composites [2
] and nanocomposites [3
], the development of hybrid ceramic nanocomposites is in its early stage and only limited research work is available in the literature. So far, the reported results are promising, potential applications are amazing, and the race to develop materials that have better performance is never ending. The mechanical and functional properties of ceramic nanocomposites strongly depend on: (i) the attributes of the matrix and reinforcement including, the intrinsic mechanical and physical properties, size, and dimensionality, (ii) the matrix-reinforcement interface, (iii) the degree of dispersion of the nanoscale phases, and (iv) the level of densification. In the present paper, the progress made and key issues in the synthesis and consolidation of alumina hybrid nanocomposites while using the spark plasma sintering method are reviewed. In addition, current challenges and potential applications are highlighted. Finally, future research directions for developing nanocomposites that have enhanced comprehensive performance are set.
3. Consolidation of Alumina Hybrid Nanocomposite Powders
The fact that alumina has poor sinterability, from the one hand, and the addition of a reinforcement to produce a composite lowers the densification, from the other hand, indicates that obtaining fully dense alumina hybrid nanocomposites is challenging. Conventional consolidation techniques, such as furnace sintering [36
] and hot pressing sintering [35
], and non-traditional sintering techniques, such as SPS [32
] and high-frequency induction heat sintering (HFIHS) [48
], were used to sinter alumina hybrid nanocomposites.
Fully dense Al2
-TiC hybrid nanocomposites that have TiC particles and SiC whiskers on the grain boundaries, and TiC particles within the Al2
grains, were developed. The best combination of strength and toughness was obtained at 4 wt.% TiC. The maximum values of hardness, strength, and toughness were 23.9 GPa, 1200 MPa, and 7.5 MPa m1/2
, respectively [50
]. In another study [38
], the Al2
-MWCNTs composites showed a sintered density of at least 99% and 60% improvement in both fracture toughness and flexural strength. In addition, CNTs were reported to reduce the wear of the highly dense Al2
-MWCNTs composites. Other researchers obtained almost fully dense (98%) Al2
reinforced by GNPs and CNTs. The monolithic alumina and hybrid nanocomposites had fracture toughness values of around 3.5 and 5.7 MPa m1/2
, respectively. The flexural strength increased from 360 MPa (monolithic alumina) to 424 MPa (Al2
-0.5wt% GNPs-1 wt% CNTs) [35
]. In other study, it was reported that reinforcing Al2
by 0.3wt.% GNPs and 1 wt.% CNTs led to an 86% reduction in the wear rate [51
]. This was attributed to the formation of a tribofilm on the worn surface due to GNPs exfoliation. The fracture toughness also increased because of the presence of CNTs. The incorporation of 1 vol% of CNTs into monolithic Al2
] was found to increase the fracture toughness by 8% and 35%, respectively, in comparison to the monolithic Al2
. The electrical conductivity increased from 10−12
) to 2.7×10−1
S/m for the composites containing 2 vol% of CNTs [52
]. In another work, Al2
hybrid nanocomposites that were reinforced by MWCNTs and SWCNTs showed lower hardness and fracture toughness when compared to Al2
because of the inhomogeneous dispersion of CNTs in the matrix [35
]. High-frequency induction heat sintering developed dense Al2
-MWCNTs-SiC hybrid nanocomposites with fine microstructure and substantial increase in fracture toughness (110%) and hardness (30%) when compared to pure alumina [48
]. The superior hardness was attributed to the fine microstructure and the hard SiC nanoparticles, while the improvement in toughness was believed to be due to the toughening mechanisms that were imparted by the two reinforcements [48
7. Spark Plasma Sintered Alumina
Gurt Santanach and co-workers investigated the influence of various sintering process parameters i.e., dwell temperature, applied pressure, dwell time, and pulse pattern on spark plasma sintered monolithic Al2
]. The authors used alumina powder with an average particle size of 0.14 µm and heating rate of 100 °C/min. Investigation of density and grain size of spark plasma sintered alumina at various sintering parameters showed two regimes: densification without grain growth that occurred at low temperatures and grain growth without further densification that occurred at higher temperatures with threshold from 1100 °C to 1200 °C. Increasing the dwell temperature from 600 °C to 1100 °C using same pressure and time (100 MPa, 5 min.) increased the density from 54.6% to 94.5% without significant grain growth, but further increase in temperature from 1100 °C to 1500 °C, although, increased density from 94.5% to 99.2% it increased grain size from 0.2 to 7.6 µm. Increasing the pressure from 10 to 100 MPa while keeping temperature and time constant (1500 °C, 3 min.) favored the grain growth from 4.3 to 7.6 µm while the density almost remained same at 99.2%. This grain growth was attributed to the fact that high temperature and pressure favored grain growth through grain boundary diffusion. Increasing the dwell time from 0 to 60 min. while keeping temperature and pressure constant (1100 °C, 100 MPa) increased the density from 90.8% to 99.8% with small grain growth. Thus, dwell time affects the proportion of porosity in specimens and can be used to control it.
The effects of grain growth inhibitor and different sintering parameters i.e., temperature, holding time, heating rate, pressure on the densification, grain growth, hardness, and fracture toughness of spark plasma sintered alumina was also reported [113
]. Alumina powder, with an average particle size of 0.4 µm, was used and 0.1% MgO was used as a grain growth inhibitor in few samples. It was reported that increasing the temperature from 1250 °C while leaving heating rate, sintering time, and pressure unchanged at values of 150–200 °C/min., 5 min., 50 MPa, respectively, resulted in fully dense sample either with or without MgO, but the grain growth exponentially increased and it was more prominent in samples without MgO. The hardness started to decrease from 21 GPa at 1250 °C to 16 GPa at 1500 °C while the fracture toughness almost remained constant. Increasing time from 0 to 40 min. while leaving other parameters constant i.e., 120 °C/min., 1200 °C, 50 MPa led to the increase of density from 95.8% to 100% at 20 min. and above while grain growth increased at higher holding times reaching a maximum value of 2 µm at 40 min. Hardness and fracture toughness remained almost the same within the range of 20–21 GPa and 3.2 ± 0.5 MPa m1/2
. An increasing pressure resulted in higher grain growth in samples that were sintered at temperature greater than 1200 °C than those sintered at temperature lower than 1200 °C and fully dense samples were obtained at all pressures greater than 50 MPa when temperatures 1200 °C and above were used with other parameters i.e., 150–200 °C/min., 3 min. remaining unchanged. Increasing heating rate from 50 to 600 °C/min. while using two different temperatures 1300 and 1400 °C without holding time at 50 MPa resulted in dense compacts with relative density greater than 99.5%. Grain growth was found to be more prominent in samples that were sintered at 1400 °C than 1300 °C. Grain growth decreased, while harness increased and fracture toughness remained same as heating rate increased.
Wang and co-workers [114
] studied the effect of particle sizes of starting powder, sintering parameters, and thickness of sintered samples on microstructure and densification of alumina consolidated by SPS. The authors used alumina powders with four different particle sizes (0.33, 0.4, 3.46, and 21.4 µm). When these powders were sintered at a constant heating rate, temperature, and pressure (200 °C/min., 1550 °C, 30 MPa), but with varying holding time from 0 to 30 min., smaller initial particle size resulted in higher relative density at all of the holding times. The authors concluded that the driving force for the densification of fine initial powder was greater than coarse powder during spark plasma sintering. Therefore, they used the powder with a particle size of 3.46 µm to study the effect of all other parameters. They reported that increasing the holding time increased the relative density of the sintered samples. At smaller holding times, the edge part of sintered samples was found to be more dense than the inside part, but this microstructural inhomogeneity disappeared at higher holding times. When the thickness of sintered samples was increased from 3 mm to 8 mm using constant SPS parameters (200 °C/min., 1550 °C, 10 min., 30 MPa), relative density was found to decrease from 99.5% to 97%, respectively. When the samples were sintered at different heating rates from 20 to 300 °C/min. while keeping other conditions constant (1550 °C, 10 min., 30 MPa), it was found that samples that were sintered at heating rates higher than 50 °C/min. reached 99% of theoretical densities while sample having heating rate of 20 °C/min. reached 97.6% theoretical density due to the abnormal grain growth at lower heating rates, which reduced the driving force for densification. Increasing pressure from 20 MPa to 40 MPa while keeping other parameters constant (200 °C/min., 1550 °C, 10 min.) increased the relative density from 97% to 99%.
Jinling Liu and co-workers [115
] studied the influence of the particle size of the starting powder and sintering temperature on the grain refining of alumina produced by SPS. They used alumina powders with a particle size of 1µm and 3µm. They sintered the samples at different temperatures (1200 and 1300 °C), but kept other parameters unchanged at 50 MPa, 150 °C/min., 2 min. for all samples. They found that the density of the samples having smaller initial particle size i.e., 1 µm was higher and it reached 100% densification value at a temperature of 1300 °C than the sample with large initial particle size of 3 µm, which reached a value of 89%. The grain sizes of sintered samples with 1 µm initial powder size at 1200 and 1300 °C were reported as 300 nm and 2.2 µm, respectively, while for 3 µm initial powder, the grain sizes were 110 nm and 700 nm, respectively. In the case of 3 µm initial powder, regardless of sintering temperature, the final grain size of sintered samples was found to be smaller than initial powder particle size, revealing the grain refining effect that was not found in the case of 1 µm powder.
Dibyendu Chakravarty and co-workers [116
] studied the effect of the addition of different sizes (100 nm, 15 nm) and different percentages of MgO (0.0625, 0.125 and 0.25%) as grain growth inhibitor during spark plasma sintering of alumina and compared it with spark plasma sintering of pure alumina. They used alumina powder having particle size of 150 nm. When the samples of pure alumina and alumina with different percentages of MgO were sintered at 1150 °C, 175 °C/min., 5 min., and 50 MPa, it was found that values of density, hardness, and fracture toughness of all the samples containing different percentages of MgO were higher than pure alumina along with less grain growth than pure alumina. The sample containing 0.125% MgO showed maximum densification along with minimum grain growth, while the hardness and fracture toughness values were also higher than others. With the increase in MgO content, it was found that some MgO segregate and remained as clusters within alumina matrix, which reduced the effectiveness of MgO as the grain growth inhibitor. Samples that were sintered with smaller particle size (15 nm) of MgO showed much higher properties with minimum grain growth than those containing larger particle size (100 nm) MgO. This was due to the fact that, in a smaller size of MgO, the amount of MgO per unit area of alumina grain boundary was large, which pinned down the grain boundaries very efficiently and restricted the grain boundary migration during sintering, resulting in finer structure.
Guo-Dong Zhan and others investigated the effectiveness of SPS for the consolidation of alumina [117
]. They used an alumina nanopowder with an average particle size of 50 nm. Sintering was performed at heating rate of 200 °C/min., pressure of 63 MPa, temperature of 1150 °C, and dwell time of 3 min. A 99.8% relative density along with 349 nm average grain size of the sintered compact was obtained. This showed the effectiveness of spark plasma sintering method. A hardness value of 20.3 GPa and fracture toughness value of 3.3 MPa m1/2
Maryse Demuynck and others [118
] investigated the effectiveness of spark plasma sintering process by consolidating alumina powders and then compared the results with conventional hot pressing (HP). They used alumina powder with a particle size of 0.45 µm. The HP experiments were carried out under hot resistive heating conditions (HPR) and hot inductive heating conditions (HPI). Samples that were prepared form SPS and HPI were of 20 mm diameter, while those with HPR were of 30 mm diameter. It was found that the samples sintered by SPS reached much higher densities within short sintering cycles than HPR and HPI. The quick heating rates in SPS allowed for reaching the temperature range quickly where the densification process was favored and kinetically separated from grain growth. The influence of heating rate on average grain size was found to be dependent on the used technique. By increasing heating rate, no effect was observed on the average grain size of sintered samples in SPS; in HRP it led to decrease in grain size and in HPI it caused grain coarsening. In SPS, using parameters 1500 °C, 100 °C/min., 6 min. and increasing the pressure from 16 MPa to 48 MPa resulted in the increase in relative density from 65% to 92%, while the grain size also increased from 1.2 to 2.5 µm, respectively.
Alumina powder with a particle size of 200 nm was consolidated while using spark plasma sintering [119
]. The powder was processed before sintering, where a slurry of alumina of concentration 30 mg/mL was formed with DMF solvent and then ball milled at 350 rpm with zirconia balls (10 mm dia.) for 4 h using powder to ball ratio (PBR) 1:20. The slurry was then dried at 90 °C for 10 h and then ground and sieved while using 250 mesh. Subsequently, further drying was done in vacuum oven at 90 °C for two days. Finally, alumina was sintered at sintering parameters of 100 °C/min., 1350 °C, 50 MPa, and 5 min. A relative density of 99.8% was achieved along with final grain size of 529 nm. Hardness, elastic modulus, and fracture toughness values reported were 22.9 GPa, 380 GPa, and 2.9 MPa m1/2
Jian Liu and others [120
] consolidated alumina powder with an average particle size of 150 nm by spark plasma sintering and evaluated its mechanical properties. They used 100 °C/min., 50 MPa, 1500 °C, and 3 min. as the sintering parameters. They achieved a relative density of 100%. Hardness, flexural strength, and fracture toughness values of 18.04 GPa, 400 MPa, and 3.53 MPa m1/2
, respectively, were reported in their study.
Morales-Rodriguez et al. [121
] studied the nanograin cluster coalescence and coarsening kinetics that are involved alumina sintered by SPS. They used a nanopowder having particle size that ranges between 30–40 nm. When temperature was increased from 1200 °C to 1250 °C while keeping others parameters constant (75 MPa, 10 min.), the relative density increased suddenly from 79% to 93% along with exponential grain growth from 100 nm to 290 nm. Increasing the time from 5 to 10 min. while keeping other sintering perimeters unchanged (75 MPa, 1300 °C), the relative density increased from 98% to 100% while the grain size also increased from 670 nm to 840 nm. Even when the short sintering time and low temperature were used, the nanostructure of alumina powder was lost and resulted in submicron sized grains in sintered compacts. From the XRD diffraction pattern, differences in preferred orientation of initial powder and sintered compact and smaller amount of strain were observed, which were induced during sintering and could affect mechanical characteristics. Abnormal grain growth and nanometric multi grains clusters were observed in HRSEM images. It was also found that, under different temperatures, the number of grains that form clusters inside larger grains also changes. When the nature of grain boundaries inside these clusters were studied by TEM, the results showed arrays of dislocations, which indicated that clusters consisted of subgrains separated by low angle grain boundaries. The formation of these clusters suggested that densification and grain growth during spark plasma sintering process took place mainly by grain rotation and sliding mechanism.
Guo-Dong Zhan et al. [122
] used the spark plasma sintering technique to consolidate alumina powder having a particle size of 50 nm and evaluated its mechanical properties. The authors used a heating rate of 200 °C/min., pressure of 63 MPa, temperature of 1150 °C, and time of three min. A relative density of 99.8% along with the grain size of 349 nm was reported. Fracture toughness was found to be 3.3 MPa m1/2
The effect of improper selection of sintering parameters on the sintering of alumina powder having a particle size of 0.21 µm was investigated [123
]. When pressure was increased from a very low value of 5.5 MPa to higher pressure of 15 MPa while other parameters were kept constant (200 °C/min., 1250 °C, 5 min.), the relative density increased from 93% to 99% and the grain size decreased from 0.67 µm to 0.58 µm. It was found that the specimens compacted using lower pressure of 5.5 MPa contained inhomogeneous microstructure, while those that were sintered at high pressure of 15 MPa contained fine microstructure. An enhanced densification rate and homogeneous microstructure at higher pressure were thought to be due to the fact that under higher pressure; each particle was surrounded by increased number of particles. Hardness, elastic modulus, and strength values were also increased due to the decrease in grain growth reaching values of 19.5 GPa, 380 GPa and 741 MPa, respectively, at 15 MPa. Fracture toughness was found to decrease from 4.4 to 2.2 MPa m1/2
. When the heating rate was increased from 100 °C/min. to 200 °C/min. while other parameters were kept constant (1250 °C, 15 MPa, 5 min.), the relative density increased from 98% to 99% and the grain size decreased from 0.68 µm to 0.58 µm. A homogenous microstructure was observed in both cases. Higher hardness, strength, and modulus values were reported at higher heating rate of 200 °C/min. due to a decrease in grain size, but fracture toughness value was reduced.
You Zhou et al. [124
] studied the densification and grain growth behavior during spark plasma sintering of alumina powder. Alumina powder with a particle size of 0.15 µm was used. The holding time was kept at 0 min. and pressure at 47 MPa in all experiments. At 1000 °C, when the heating rate was increased from very low to very high i.e., 50 °C/min. to 300 °C/min., the relative density slightly increased from 61.2% to 63.5%, but no grain growth was observed. Different microstructures were observed in SEM images. In the case of very fast heated specimen at 300 °C/min., necking extensively occurred between the neighboring particles, while in the case of slow heated specimen at 50 °C/min., no such phenomenon was observed. No grain growth was observed up to the temperature of 1100 °C, irrespective of heating rate, while density increased as temperature and heating rate was increased. When the temperature increased above 1150 °C, grain growth started, along with increased densification. At very high temperature i.e., 1400 °C, when heating rate increased from very low at 50 °C/min. to very high 300 °C/min., the density slightly decreased from 99.4% to 99.3%, while grain size decreased from 7.67 µm to 2.37 µm. Three stages were identified during spark plasma sintering: densification without grain growth, densification along with grain growth, and grain growth without further densification.
Morales-Rodriguez and others [125
] used spark plasma sintering for the consolidation of alumina powder with a particle size 30–40 nm. A heating rate of 300 °C/min., pressure of 75 MPa, temperature of 1300 °C, and time of 5 min. were used. The authors reported a relative density of 97.7%, grain size of 700 nm, and hardness value of 19 GPa.
Shan Meng and co-workers investigated the effect of sintering temperature and sintering aid content on densification and grain growth of spark plasma sintered alumina powder having an average particle size of 150 nm [126
]. They used different percentages of MgO (0–0.4 wt%) as grain growth inhibitors. They found that the highest densification along with minimum grain growth was obtained with 0.05 wt% MgO. Accordingly, 0.05 wt% MgO doped alumina was used for further study. A highest density of 99.8% along with minimum grain size of 680 nm was obtained when sintering was performed at 1250 °C, 100 °C/min., 60 MPa, and 5 min. The hardness and fracture toughness values were found to be 20.75 GPa and 4.45 MPa m1/2
, respectively. When the temperature was increased from 1250 °C to 1500 °C, a slight increase in density was observed, but grain sizes increased exponentially to 4 µm. Hardness and fracture toughness values were also decreased due to huge increase in grain size.
Kasperski and co-workers [127
] used the spark plasma sintering process for the consolidation of alumina powder with particle size of 140 nm. When alumina was sintered at higher temperature and higher pressure (1350 °C, 150 MPa) at heating rate of 100 °C/min. and for 6 min., 99% relative density was obtained along with grain size of 1.2 µm. The hardness and fracture toughness values were found to be 22 GPa and 5 MPa m1/2
, respectively. When temperature, pressure and time were reduced to 1150 °C, 100 MPa and 5 min., relative density increased to 100% while the grain size was reduced to 320 nm. The hardness and fracture toughness values were found to be 21.3 GPa and 5.4 MPa m1/2
Ken Hirota et al. [128
] used spark plasma sintering to compact alumina powder with a particle size of 300 nm. They used a heating rate of 100 °C/min., pressure of 30 MPa, temperature of 1300 °C, and time of 5 min. They reported a relative density of 99.3% and grain size of 4.4 µm. In addition to hardness and fracture toughness values of 19.5 GPa and 4.7 MPa m1/2
Alvarez-Clemares and co-workers [129
] studied the effects of spark plasma sintering on microstructure, mechanical properties, and creep behavior of alumina ceramic. They used alumina powder with an average particle size of 153 nm. A heating rate of 50 °C/min., pressure 80 MPa, temperature 1300 °C and time 2 min. were used for sintering. A 99.9% relative density was achieved. It was found that some amount of strains were induced in samples during sintering, so, after sintering, some samples were annealed to 1000 °C for 5 h to eliminate the residual stresses. Creep deformation for as-sintered specimen was found to be 1.4%, while 22% for annealed specimen. This was due to the fact that, during the sintering process, dislocations were induced that blocked the grain boundary movement, which resulted in low deformation of as-sintered specimen. Fracture toughness and flexural strength for as-sintered specimen were found to be 4.3 MPa m1/2
and 430 MPa, respectively, and 3.4 MPa m1/2
and 275 MPa, respectively, for the annealed specimen. The authors concluded that SPS induced strains that concentrate at grain boundaries and inhibit the crack growth, resulting in improved fracture toughness and flexural strength of the as-sintered specimen. They confirmed that SPS process induces strains that affect the mechanical properties of the sintered specimen.
Nanostructured alumina was consolidated to full density by SPS at a sintering temperature of 1150 °C and sintering time of 3 min. [130
]. The authors reported a value of 27.5 W/mK for the room temperature thermal conductivity of the fully dense alumina. In addition, they found that the increase in temperature from 25 °C to 500 °C leads to a decrease in thermal diffusivity from 0.088 to 0.03 cm2
/sec. In another work, the thermal conductivity of nanostructured alumina (200 nm), spark plasma sintered at 1400 for 3 min. (50 MPa), was found to decrease from around 34 to 13 W/mK with the increase in temperature from 300 to 800 K [131
]. An almost fully dense monolithic Al2
(99.6% of the theoretical density), as can be seen in Figure 8
, was consolidated at 1400 °C for 10 min. The thermal conductivity and specific heat were about 34.44 W/mK and 1.22 J/gK, respectively, at room temperature. Thermal conductivity was found to decrease with the increase in temperature and it reached 18.3 W/mK at 250 °C, while specific heat increased and reached a value of 1.55 J/gK at 250 °C, as shown in Figure 9