Spark Plasma Sintering of Nano Silicon Carbide Reinforced Alumina Ceramic Composites

At this point, it is reported that the amount, size, morphology and distribution of SiC phase added into the alumina matrix has a great effect on the mechanical properties of Al 2 O 3 [11]. Razavi et al. [10] produced the Al 2 O 3 -SiC composite ceramics by using spark plasma sintering (SPS) method at temperatures of 1600 °C for 10 min holding time and they measured the hardness and fracture toughness values of the monolithic Al 2 O 3 as 1387 Hv and 250.4 MPa, respectively. Researchers showed that hardness of the composite containing 20 wt. % Abstract Although Al 2 O 3 has been commercially preferred material, it cannot be used in applications subject to variable and sudden loads due to its low fracture toughness. In this study, as the primary purpose, nano-SiC particles were used as reinforcement phase to improve the fracture toughness of the Al 2 O 3 ceramic. Ultrasonic dispersion was used to ensure good dispersion of the nano reinforcement phase in the matrix. Production was carried out by spark plasma sintering method at temperatures of 1325 °C for 5 min under 40 MPa pressure and vacuum atmosphere. Densification behavior, density, microstructure, phase analysis, hardness and fracture toughness of the sintered Al 2 O 3 /nano-SiC composites were investigated. The highest hardness and fracture toughness of 22.83 GPa and 6.09 MPa·m 1/2 were achieved, respectively.


INTRODUCTION
Alumina (Al 2 O 3 ), as an engineering ceramic, has high melting temperature (2072 °C), high hardness, low density (3.9 gr/cm 3 ) and low thermal conductivity. In addition, its corrosion resistance and biocompatibility are very good. Thanks to these features, it has a high potential to be used in a number of sectors such as aerospace and automotive industries, ballistic armor applications, high temperature refractory materials, cutting and abrasive tools and biomaterials. However, fracture toughness value of Al 2 O 3 , which is resistance to crack propagation, is quite low [1][2][3][4][5]. This negative feature limits the use of Al 2 O 3 . On the other hand, there are many studies to increase the fracture toughness of Al 2 O 3 based ceramics through microstructure control and reinforcement phase mechanisms [1,3,4,[6][7][8][9]. The reinforcement phase mechanism of Al 2 O 3 matrix is occurred by secondary phases in the forms of particle and/or fiber. Thus, a composite material is produced. As shown schematically in Figure 1, the main purpose of the reinforcement phase mechanism is to reduce the energy of the crack propagating in the matrix. In the crack deflection mechanism given in Figure 1 (a), as the crack moves through the material, when it intersects with the reinforcement particles, the direction of the crack propagation plane changes and its energy decreases. In the other mechanism called as crack branching (Figure 1 (b)), the crack intersecting with the reinforcement particles splits into two or more cracks and its energy decreases. In the case of crack bridging mechanism shown in Figure 1 (c), the reinforcement phase in the form of fiber acts like a spring trying to close the crack and prevents further spreading the crack. When silicon carbide (SiC) is selected as the ceramic reinforcement phase, significant improvements in mechanical properties of the Al 2 O 3 are possible [10]. At this point, it is reported that the amount, size, morphology and distribution of SiC phase added into the alumina matrix has a great effect on the mechanical properties of Al 2 O 3 [11]. Razavi et al. [10]  On the other hand, although there are other pressure and non-pressure sintering processes in ceramic composite production, SPS is one of the newest and technological methods in the production of engineering ceramics. In this method, heat is produced directly on the powders and the molds thanks to the electric current applied in the form of square wave pulses that can be controlled completely, and therefore, the sintering temperature can be reached in a very short time. During the process, pressure is applied to the powders and the microstructure can be controlled by high heating rate [10,11].
In this study, Al 2 O 3 /nano-SiC composites containing different ratio of nano-SiC particles (0-30 vol. %) were produced by using spark plasma sintering process. To achieve good dispersion, the nano-SiC particles were dispersed ultrasonically in the Al 2 O 3 matrix. The effects of nano-SiC ratio on densification behavior, microstructure, mechanical properties and phase structure of composites were investigated.

Powder Preparation
Firstly, commercial Al 2 O 3 (US Research Nano Materials, USA, an avarage particle size 500 nm, alpha, 99+%) and (US Research Nano Materials, USA, 45-65 nm, beta, cubic, 99+%) SiC powders were weighed in the proportions given in Table 1. Then, the nano SiC powders were added to the merck quality ethanol in separate beakers. An ultrasonic probe (Heiscer-UP200Ht) was placed in beakers containing ethanol and nano SiC powders, and dispersion of nano-sized powders in ethanol was provided for 15 minutes. The process was continuously controlled so that the ethanol in the beaker did not overheat, the ethanol was kept cold with the aqueous system placed under the beaker, and the temperature of the system was measured with a thermometer. After the distribution of nano-sized powders, Al 2 O 3 powders were added to the same beakers in appropriate proportions and ultrasonic dispersion was performed for another 15 minutes. Then, magnetic stirring bars were placed to the beakers and the ethanol evaporated on a magnetic stirrer having heater and vacuum fan. The beakers were placed in a drying-oven at 80 °C to dry the powders completely.

Spark Plasma Sintering Process
Prepared powders were sintered by using spark plasma sintering process. A SPS system (7.40 MK VII, SPS Syntex Inc.) with a capacity of 20000A was used. Thanks to this system, composite specimens with 50 mm diameter and 5 mm thickness were produced as shown in Figure 2. Graphite molds having 50 mm inner-diameter and graphite punches were used for composite production. The inner surface of the mold was covered with graphite paper, the lower punch was placed and graphite papers were placed both on the surfaces of upper and lower punches. The powder to be sintered was poured into mold cavity and upper punch placed. Then, the graphite mold was pre-compressed under 10 MPa pressure by using a hand press. In order to minimize heat loss, the graphite molding system was surrounded by a graphite blanket and sintering carried out in a vacuum environment. During SPS process, current was increased manually and shrinkage curves were controlled continuously. 40 MPa pressure was applied to the punches and pulsed direct current (12 ms/on, 2 ms/off ) was passed through the samples and graphite molds during sintering process. The process was completed by holding the specimens 5 minutes at temperature of 1325 °C. Temperature measurements were performed by a pyrometer from the outside of chamber. During sintering, a software was used to control instant parameters such as shrinkage, shrinkage rate, temperature, vacuum value, current, voltage and pressure, and a heating rate of 150 °C/min was applied.

Characterization Methods
The sintering behaviors of the specimens were evaluated by using shrinkage curves obtained from the SPS device. Density of the specimens was determined using Archimedes' principle. Theoretical density values of composites were determined according to the mixture rule and these theoretical values were proportioned to the measured density values. Thus, the relative density values of the composites were calculated (Equation 1). (1) In this equation, D R is the relative density (g/cm 3 here; k is the geometry constant (0.016 ± 0.004), P is the applied load, E is the modulus of elasticity of the composite, H Vickers hardness value and c is half of the average crack length.

Densification behavior
The shrinkage curves for the produced samples were given in Figure 3 (a-d). These graphs were plotted as displacement of graphite punches (mm) versus temperature (°C).
During the SPS process, the densification occurring in the powders with the increase of temperature at constant pressure is determined from the displacement amount of the graphite punches due to the shrinkage. After a certain temperature value that is different for each material and composition, the shrinkage is completed and remains constant [6,8]. Therefore, in order to obtain a dense structure, the sintering temperature should be equal or higher than the temperature at which shrinkage is completed. In this study, the starting and completion temperatures of the shrinkage for monolithic Al 2 O 3 and Al 2 O 3 -nano-SiC composite samples were determined (see Table 2).  As seen in Figure 3 and Table 2, during SPS process performed with a constant heating rate, the starting and completion temperatures of the shrinkage decreased thanks to the nano-SiC added to Al 2 O 3 . The reason for this decrease occurring in the starting and ending temperatures of sintering was thought to be due to the increase in the conductivity of the composite by SiC, whose electrical conductivity is higher than Al 2 O 3 . The mechanism of the increase in electrical conductivity of Al 2 O 3 by adding SiC has been explained by Saheb at al [2]. As a result, it is possible to conclude that an improvement took place in the sintering behavior of the Al 2 O 3 thanks to added nano-SiC particles.
The densities of the specimens produced by the SPS process were determined by Archimedes' Principle. According to the results, it was understood that the relative density values of the specimens varied among 99.7% and 95.2% (see Table  3). The monolithic Al 2 O 3 (99.7%) had the highest relative density value among all samples. The lowest relative density value (95.2%) belonged to the specimen of 70A30S sample containing 30% SiC by volume. It was determined that relative density values decreased with increasing nano-SiC ra-tio. This decrease in density values occurred due to the poor sintering property of the SiC. A similar behavior (decrease in density with increasing SiC ratio added to Al 2 O 3 ) was observed in the study of Alweendo et al. [12].

Microstructural Characterizations
FESEM images taken from the fracture surface of the produced specimens using secondary electron detector were given in Figure 4 (a-d). In Figure 4 (b-d), nano-sized SiC grains has been noticed. Especially in Figure 4 (b) and (c), it is seen that the nano-sized SiC grains were homogeneously distributed in the Al 2 O 3 matrix. Therefore, it can be said that the ultrasonic nano-SiC dispersion process was successful up to 20% by volume. However, the same homogeneous distribution could not be observed in 70A30S sample containing 30% SiC by volume (Figure 4 (d)). In Figure 4 (a), the microstructure of monolithic Al 2 O 3 , the coaxial grains were quite distinct and it is understood that there was intergranular fracture. The granular structure in the 70A30S sample was not evident (Figure 4 (d)). This was due to the transgranular fracture. The fracture mode started to transform from intragranular fracture to transgranular fracture with the increasing SiC ratio. On the other hand, it is clearly seen that the grain size of the Al 2 O 3 matrix decreased with the addition of nano-SiC to the structure. Nano sized SiC particles located around Al 2 O 3 grains and prevented the propagation of the grain boundary. Therefore, the grain size of the Al 2 O 3 matrix decreased with the nano-SiC addition.
The elemental distributions (by weight) of the EDS spectra taken from the general surface regions of the samples were given in Table 4. In this table, Al, Si, C and O were chemical symbols of aluminum, silicon, carbon and oxygen, respectively. It is understood that the initial powder compositions (by volume → weight) and the values obtained from the EDS results of the produced samples were consistent.

Phase Analysis
In order to understand the phases formed after sintering, XRD analysis were performed and the patterns were given in Figure 5. This process was carried out with an X-ray diffractometer using Cu-Kα radiation at a scanning speed of 2°/min at 2θ: 10-80°. As a result, it was understood that the monolithic Al 2 O 3 sample had the structure of α-Al 2 O 3 (JCPDS: 71-1683) after SPS process. In case of adding SiC up to 20%, the peaks of the SiC phase appeared and there was no change in the Al 2 O 3 structure. However, peaks of the Al 2 Si 2 O 5 phase were revealed in the sample containing 30% SiC by volume.

Mechanical Properties
Vickers microhardness and fracture toughness values of Al 2 O 3 -based composites sintered under 40 MPa pressure for 5 min. with the SPS method are given in Table 3. While the hardness value of monolithic Al 2 O 3 was 18.27 GPa, it increased to 21.07 GPa with the addition of 10% SiC by volume and 22.34 GPa with the addition of 20% SiC by volume. This was due to the nano-SiC particles preventing dislocation movement during plastic deformation caused by hardness measurement. In addition, increasing in hardness could be explained by the reduction of the grain size of Al 2 O 3 matrix by adding nano-SiC to the structure. The grain boundary areas increased with decrease in grain size and these grain boundary areas prevented the movement of dislocations. However, it was determined that the hardness value did not increase any more if 30% SiC by volume was added to the A 2 O 3 matrix (22.83 GPa).
As a result of fracture toughness measurements, it was understood that a significant improvement occurred in the fracture toughness value with nano-SiC addition to Al 2 O 3 ( Table 3). The fracture toughness value of monolithic Al 2 O 3 , calculated as 2.94 MPa·m 1/2 , increased to 5.81 and 6.09 MPa·m 1/2 with the addition of 10% and 20% SiC, respectively. This increase in fracture toughness can be explained by the "Crack Deflection" mechanism caused by nano-sized SiC particles, as Shi et al [14] mentioned. The directions of the cracks propagating in the structure changed when they intersected with the SiC grains and moved in a zigzag pattern (see Figure 6). In this way, the energy of the cracks was reduced and the fracture toughness values of the Al 2 O 3 -nano SiC composite were improved.
However, the fracture toughness value of Al 2 O 3 containing 30% nano-SiC by volume decreased to 4.37 MPa·m 1/2 . A similar decrease in fracture toughness after a certain SiC addition (30% SiC by weight) was observed in a study in the literature [15]. This situation was explained by three mechanisms: (I) high agglomeration of nano-SiC particles occurring at 30% SiC addition (see Figure 7), (II) low fracture toughness of the Al 2 Si 2 O 5 (1-1.8 MPa·m 1/2 [16]), which formed in the 70A30S sample, (III) excessive residual stresses in the structure as a result of thermal expansion mismatches of Al 2 O 3 and SiC. As seen in Figure 7, which is the high magnification SEM image of 70A30S, nano-SiC particles agglomerated in the Al 2 O 3 grain boundary regions when 30% nano-SiC by vol- ume was added.

CONCLUSIONS
As a result of the experimental studies, cylindrical shaped Al 2 O 3 /nano-SiC specimens with a diameter of 50 mm were produced by spark plasma sintering process and their sintering behavior, density, microstructure, phase structure, hardness and fracture toughness were analyzed. According to these analysis: 1. The temperatures at which shrinkage started and completed were decreased with the increasing SiC addition. This situation was explained by the increase in the conductivity of the composite with the increasing SiC addition.
2. The relative density of monolithic Al 2 O 3 was 99.7%. However, this value decreased up to 95.2% with the increasing SiC addition.
3. The grain size decreased and the fracture mode changed to transgranular fracture from intergranular fracture with the SiC addition.
4. The hardness increased from 18.27 GPa to 22.83 GPa.
5. While the fracture toughness of the monolithic Al 2 O 3 was 2.94 MPa·m 1/2 , a maximum value of 6.09 MPa·m 1/2 was obtained with the addition of nano-SiC. 6. Addition of 20 vol% SiC was the maximum point for mechanical properties, and over this addition amount the fracture toughness value decreased due to the agglomeration of nano-SiC, excessive residual stress and the formation of Al 2 Si 2 O 5 phase.