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Development of a single-phase Ca-α-SiAlON ceramic from nanosized precursors using spark plasma sintering
Materials Science & Engineering A 673 (2016) 243–249
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Development of a single-phase Ca-α-SiAlON ceramic from nanosized precursors using spark plasma sintering Raja Muhammad Awais Khan a, Moath Mohammad Al Malki a, Abbas Saeed Hakeem b,n, Muhammad Ali Ehsan b, Tahar Laoui a,n a b
Mechanical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia Center of Excellence in Nanotechnology, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
art ic l e i nf o
a b s t r a c t
Article history: Received 30 May 2016 Received in revised form 15 July 2016 Accepted 17 July 2016 Available online 18 July 2016
Nitrogen-rich Ca-α-SiAlON ceramics were synthesized using nanosized precursors via spark plasma sintering process for various holding times (10, 20, 30 min) at relatively low temperatures of 1500 °C and 1600 °C. The effects of the experimental conditions, namely sintering time and temperature, on the densification and mechanical properties of the processed α-SiAlON were investigated. All of the conditions yielded nearly completely densified ceramics. A remarkable combination of hardness and toughness was obtained for the samples at either sintering temperature; and these values were greatest after 30 min sintering time, with Vickers hardness values of 21.6 GPa and 20.5 GPa and fracture toughness values of 7.3 MPa√m and 9.7 MPa√m for sintering temperatures of 1500 °C and 1600 °C, respectively. The differences in the mechanical properties of these samples were related to differences in the phases formed during the sintering process. & 2016 Elsevier B.V. All rights reserved.
Keywords: SiAlON Nano-ceramics Silicon nitride Spark plasma sintering Mechanical properties
1. Introduction Silicon nitride (Si3N4) ceramics exhibit exceptional thermal and mechanical properties, specifically hot hardness, thermal shock resistance and chemical inertness, which enable them to withstand severe working conditions [1]. However, full densification of Si3N4 is difficult to attain, due to the covalent bonds present between Si and N atoms. The conventional solid-state sintering of Si3N4, therefore, requires very high temperatures to provide sufficient atomic diffusion [2,3]. To overcome this temperature issue, metal oxide additives are incorporated as sintering aids in order to relax the sintering temperatures for such ceramics [1–4]. The resultant phases and mechanical properties of these ceramics are highly dependent on the added sintering aids. The most significant breakthrough in Si3N4 ceramics was the development of SiAlON ceramics, resulting from the utilization of alumina along with other oxide additives [5–7]. SiAlON ceramics are considered to be solid solutions of Si3N4, with Si and N partially replaced by Al and O, respectively [5,7–9]. Among different phases of SiAlON, α-SiAlON and β-SiAlON phases have been particularly paid attention to during the past two n
Corresponding authors. E-mail addresses: [email protected] (R.M.A. Khan), [email protected] (M.M. Al Malki), [email protected] (A.S. Hakeem), [email protected] (M.A. Ehsan), [email protected] (T. Laoui). http://dx.doi.org/10.1016/j.msea.2016.07.075 0921-5093/& 2016 Elsevier B.V. All rights reserved.
decades due to their notable mechanical properties, specifically the elevated hardness of α-SiAlON and the modest fracture toughness of β-SiAlON [10,11]. The general stoichiometric formula of α-SiAlON is Mx Si12 −(m + n)Al(m + n)OnN16 − n , in which ‘m’ and ‘n’ represent the number of Al-N and Al-O bonds respectively, ‘M’ stands for the added metallic cation, often alkaline and rare-earth elements, and ‘x’ is given by the formula x ¼m/v where v is the valence of the added cation [7]. α-SiAlON can be produced by reacting Si3N4, Al2O3, AlN and the added oxide of a particular metallic cation ‘M’, for instance Y2O3, CaO, etc. As the reaction proceeds, α-SiAlON precipitates out with a gradual decrease in the amount of the liquid phase until the reaction is completed with the formation of the α-SiAlON solid solution, leaving a small amount of un-reacted liquid in the grain boundary [7,12]. The mechanical properties of α- and β-SiAlONs have been generally explained on the basis of their microstructures. α-SiAlON grains exhibit an equiaxed morphology while β-SiAlON generally forms elongated grains. However, elongated α-SiAlON grains have also been produced in an attempt to improve the fracture toughness [13,14]. During the past few decades, lanthanide metal oxides, such as Yb2O3, La2O3, Nd2O3 and Dy2O3, have been the major research focus in the field of metal oxide sintering additives [15–18]. However, the relatively high cost of rare earth oxides [19] has prompted the use of alkaline earth metal oxides, including BaO,
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CaO, and MgO [20,21]. Due to the relatively high solubility of Ca2 þ in the α-SiAlON lattice and the limited resulting crystal distortion, CaO has been considered as a potential densification aid and as a good candidate for the reduction of grain boundary glass phase, which could otherwise be detrimental to the mechanical properties. Moreover, Ca compounds are relatively inexpensive, as they can be readily obtained from fly ash [22]. The use of spark plasma sintering (SPS), as a consolidation technique, has generated active attention in the synthesis of ceramic materials due to its higher heating rate and its novel pulsed-current-based heating [23,24]. Elongated forms of Ca αSiAlON with improved fracture toughness have been successfully produced using processing schemes involving high heating rates, such as SPS [25,26]. The present work aims to study the effect of nanosized starting powders on the synthesis of monolithic nitrogen rich α-SiAlON by the SPS technique, incorporating CaO as a densification additive. We believe that the combination of SPS with nano sized precursors for processing nitrogen-rich α-SiAlON ceramics doped with Ca additives, has not been investigated before. We anticipate a reduction in the sintering temperature and an enhancement in the mechanical properties.
2. Experimental procedure The nitrogen rich chemical composition was selected to be: Ca0.8Si9.2Al2.8O1.2N14.8 corresponding to m ¼1.6 and n ¼1.2 in the general formula of α-SiAlON. Table 1 displays the amount of each reactant used in weight percentage. These reactants include αSi3N4 (150 nm-Ube Industries SN-10, Japan), AlN (100 nm-Sigma Aldrich, Germany), SiO2 (10–20 nm-Sigma Aldrich, Germany) and CaO (o 160 nm-Sigma Aldrich, Germany).
3Si3N4 + 0.8CaO + 2.8AlN + 0.2SiO2 → Ca 0.8Si9.2Al2.8O1.2 N14.8
(1)
Powders precursor were carefully weighed in the appropriate amounts according to the chemical Eq. (1) to form samples each with a total mass of 5 g. The powders were homogenized using an ultrasonic probe sonicator (Model VC 750, Sonics, Connecticut, USA) for 30 min, utilizing ethanol as a mixing medium. Later, the mixtures were dried in a furnace at 80 °C for 12 h to remove ethanol. To carry out SPS, powder mixtures were first poured into 20 mm diameter graphite dies. Sintering was performed at two different temperatures of 1500 and 1600 °C for 10, 20 and 30 min holding time under a constant pressure of 50 MPa. A heating rate of 100 °C/min was adopted throughout this study to avoid formation of intermediate phases. Samples were then rapidly cooled down to ambient temperature. The cooling process can be divided into two stages, in which samples were firstly cooled down from the sintering temperature (1500 °C/1600 °C) to 400 °C at a rate of 100 °C/min, followed by switching off the furnace. SPS samples were cleaned of graphite using SiC grinding papers (120 grit size), followed by additional diamond disc grinding to remove the graphite paper and prepare clear surface to measure sample density. Later, automatic grinding and polishing machine (Automet 300 Buehler grinding machine) was utilized to prepare samples for subsequent microstructural and mechanical investigations. The grinding process included the use of diamond
wheels (74 mm, 40 mm, 20 mm,10 mm grit size), followed by the use of diamond suspensions (9 mm, 6 mm, 3 mm, 1 mm, 0.25 mm) in the polishing stage. To identify the phases present in the synthesized samples, a Rigaku MiniFlex X-ray diffractometer (Japan) was used with Cu Kα1 radiation (γ ¼ 0.15416 nm), tube current ¼10 mA and an accelerating voltage of 30 kV. Cross-section and fracture surface micrographs were obtained using a field emission scanning electron microscope (FESEM, Lyra 3, Tescan, Czech Republic) with accelerating voltages of 20–30 kV. Archimedes' principle was applied to evaluate the density of the sintered samples. A universal hardness tester (Zwick-Roell, ZHU250, Germany) was employed to evaluate the Vickers hardness of the sintered samples under a load of 98 N. The fracture toughness was evaluated based on the indentation method [27].
3. Results and discussion 3.1. Analysis of powder mixtures Fig. 1 displays an SEM micrograph of the investigated composition after mixing for 30 min in the probe sonicator. Particles size ranges from 100 nm to 1 mm, indicating powder agglomeration. The agglomeration of the nanosized powders resulted from physical interactions of their large surface areas. Rajesh et al. showed that although de-agglomeration is difficult to achieve, a probe sonicator, when compared to other mixing techniques, is capable of homogenizing powder mixtures, keeping the particles as small as theoretically possible [28]. However, and as revealed from our analysis, agglomeration can be restored easily in almost no time. Energy Dispersive X-Ray Spectroscopy (EDXS) mapping (Fig. 2) confirmed that a reasonable level of homogeneity was achieved after using the probe sonicator. 3.2. Sintering and densification The densities of samples sintered at 1500 °C and 1600 °C are listed in Table 2. The sample ID corresponds to the sintering parameters, for instance sample “1510” denotes a sintering temperature of 1500 °C carried out for 10 min. Samples densities fall in range of 3.15–3.20 g/cm3. The theoretical density for the selected composition was calculated based on the resultant phase to be
Table 1 The amount of each chemical powder reactant used in wt%. Composition
α-Si3N4
AlN
CaO
SiO2
Ca0.8Si9.2Al2.8O1.2N14.8
71.07
19.37
7.57
2.03 Fig. 1. SEM micrograph of the investigated composition after probe sonication.
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Fig. 2. EDXS mapping of the investigated composition after mixing revealing a homogenous distribution of the starting mixture.
Table 2 Density values for samples sintered at 1500 °C and 1600 °C. Sample ID 3
Density (g/cm )
1510
1520
1530
1610
1620
1630
3.15
3.15
3.19
3.18
3.18
3.20
3.21 g/cm3. The comparison of the experimental and theoretical densities suggests that the sintered samples reached approximately 98% densification. The pulsed-current heating of SPS aided the densification process at low sintering temperatures, along with higher surface areas associated with the nanosized precursors. CaO, as a densification additive, provided an extra amount of liquid phase at the first stages of sintering, which certainly facilitated densification. However, a slight decrease in the density was noticed in samples sintered for shorter durations at both temperatures, particularly for samples sintered at 1500 °C (sample 1510 and 1520). The large specific surface area associated with nanosized powders has been proven to contribute to the driving mechanism for sintering to achieve thermal equilibrium [29]. The equation developed by Hansen and his group estimates that the sintering rate would be enhanced four-fold if the average size of the reactant powders were reduced by one order of magnitude [30]. This relationship, however, assumes generally nanosized precursors, and that these precursors do not display notable agglomeration. The onset of the sintering process is affected by the use of nanosized precursors. Suryanarayana et al. has shown that sintering onset of nano starting materials is E0.3 Tm, while it starts at E0.65 Tm in the case of the conventional starting powders [31]. These factors and others are thought to contribute to the densification of the investigated composition by the solution-reprecipitation process at a relatively low peak temperature, taking into consideration the high nitrogen content of the investigated composition.
Fig. 3. XRD patterns of samples IDs 1510, 1520 and 1530, i.e., samples sintered at 1500 °C for 10, 20 and 30 min, respectively.
Fig. 4. XRD patterns of samples 1610, 1620 and 1630, i.e., samples sintered at 1600 °C for 10, 20 and 30 min, respectively.
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3.3. Phase analysis The XRD pattern of samples 1510, 1520 and 1530, i.e., those sintered at 1500 °C for 10, 20 and 30 min, respectively, are shown in Fig. 3. Similarly, Fig. 4 shows the XRD patterns of samples 1610, 1620 and 1630, i.e., those sintered at 1600 °C for 10, 20 and 30 min, respectively. Ca-α-SiAlON (Ca0.68Si9.96Al2.04O0.68N15.32) phase has been identified in all samples. However, samples sintered for shorter durations showed the presence of un-reacted AlN in the Ca-α-SiAlON matrix, but ultimately dissolved into a single (monolithic) α-SiAlON phase with m ¼1.36 and n ¼0.68 after 30 min of sintering at 1500 °C. The XRD patterns of the sintered
samples showed slightly stronger AlN peaks in samples sintered at 1500 °C than in those sintered at 1600 °C, which is attributed to the reduced dissolution of AlN at lower temperatures. As we were working with a single-phase system, a comparison was made between the designed and obtained compositions of αSiAlON. Normalizing the obtained Si content to its counterpart in the designed composition allowed the obtained α-SiAlON composition to be rewritten as Ca0.63Si9.2Al1.88O0.63N14.15, which revealed that the obtained product had proportionally less Ca, Al and O. It might be possible that these constituting elements were kept in the grain boundary, as the starting materials were liquidized early due the use of nano precursors [4].
Fig. 5. FESEM micrographs of the polished samples sintered at 1500 °C for (a) 10, (b) 20 and (c) 30 min and of samples sintered at 1600 °C for (d) 10, (e) 20 and (f) 30 min.
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3.4. Microstructural development FESEM micrographs of polished samples sintered at 1500 °C and 1600 °C are shown in Fig. 5. The micrographs showed reduced levels of porosity at increased time durations of sintering at both temperatures, as well as reduced porosity at the higher sintering temperature for a sintering duration of 10 min (compare Fig. 5a and d), which is consistent with the density data (Table 2). The reduced porosity at longer sintering duration is in agreement with the phase analysis, as it was shown earlier that some un-dissolved AlN was found in the samples sintered for 10 and 20 min at both temperatures. FESEM micrographs of etched samples 1530 and 1630 are shown in Fig. 6. Single-phase α-SiAlON grains can be seen in both micrographs, as confirmed by XRD analysis. The average grain size for sample 1530 (Fig. 6a) was determined to be three-times smaller than that of sample 1630 (Fig. 6b), a result of the temperature-dependent grain growth. Consolidation of SiAlONs ceramics takes place in liquid phase
247
sintering. Generally, the initial step observed with increasing temperature involves the reaction of CaO, SiO2 and surface oxides, such as Al2O3 in the case of AlN and SiO2 in the case of Si3N4, to form a Ca-containing liquid phase (Ca-Si-Al-O-N), known to be present above the eutectic temperature ( E1350 °C). This liquid phase firstly assists in arrangement of particles, and later in the solution-reprecipitation mechanism to form ultimately Ca-α-SiAlON. The overall chemical reaction, based on the XRD and FESEM observations, can be represented as follows:
α-Si3N4 ( surface SiO2 )+CaO + AlN+SiO2 → (1500 °C/1600 °C)Ca 0.68Si9.96Al2.04 O0.68 N15.32 + Ca-Si-Al-O-N ( liquid phase)
(2)
A pure α-SiAlON composition is nitrogen rich and the inherent amount of grain boundary liquid phase is scarce which, eventually, limits densification. However, in our work, because of the high heating rate (100 °C/min) associated with the SPS processing, few α-SiAlON nuclei formed at the start of sintering, and the remaining
Fig. 6. FESEM micrographs of etched α-SiAlON samples sintered at (a) 1500 °C and (b) 1600 °C for 30 min.
Fig. 7. FESEM images of fractured surfaces of samples sintered at (a) 1500 °C and (b) 1600 °C for 30 min, showing the elongation of α-SiAlON grains.
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Table 3 Mechanical properties of samples sintered at 1500 °C and 1600 °C. Sample ID
1510
1530
1610
1630
HV10 (GPa) KIC (MPa√m)
19.8 70.4 4.4 7 0.4
21.6 7 0.6 7.3 7 1.3
19.3 7 0.6 9.5 7 1.2
20.5 7 0.4 9.7 7 2.3
dissolved species formed a liquid phase, which helped to densify the ceramic at a low sintering temperature. The presence of a relatively large amount of liquid phase was shown to support mass transport and species diffusion to form elongated α-SiAlON grains [25,26]. Santos et al. [32] and Dong et al. [33] attributed their failure to grow elongated α-SiAlON grains to the low heating rate associated with the conventional pressure-assisted sintering techniques they used, namely GPS and HIP. However, sintering techniques with low heating rates were shown by Zhao et al. to be able to form such morphology if the starting composition contained a sufficient amount of liquid phase [34]. Fig. 7 shows the formation of elongated α-SiAlON grains in samples 1630 and 1530. Another factor that affects positively the elongation in α-SiAlON grains is the size of the precursors [13,35,36]. Formation of elongated α-SiAlON grains is considered to be an abnormal grain growth, and such growth occurs for SiAlON systems possessing αSi3N4 below 0.5 mm [13,35,36]. This type of grain growth was observed contributed to the elongation of α-SiAlON grains observed in our work (Fig. 7). 3.5. Mechanical properties Table 3 shows mechanical properties of the samples sintered at 1500 °C and 1600 °C for 10 and 30 min. At either temperature, hardness values increased with increasing durations of sintering. This trend was clearly attributed to the difference in the phases formed, as the samples sintered for short duration contained significant traces of un-dissolved AlN, as shown in the XRD analysis. The presence of dense monolithic Ca-α-SiAlON for the samples sintered for 30 min at 1500 °C and 1600 °C (samples 1530 and 1630, respectively) would explain their hardness values (21.6 GPa and 20.5 GPa, respectively). The relatively larger grain size of sample 1630 might be responsible for its hardness value, being slightly lower than that of 1530, as reported elsewhere [37]. The fracture toughness for the samples sintered at 1600 °C exhibit higher values than those sintered at 1500 °C due to, most probably, the larger grain size and higher aspect ratio of α-SiAlON grains (Figs. 6 and 7). Comparing our results with those found in literature under the same processing parameters (powder size, sintering technique, sintering parameters) within the same composition field is difficult. One of the closest systems we found in literature [38] was able to synthesize a similar composition (Ca-α-SiAlON) yielding Vickers hardness of 19.7 GPa with 5.1 MPa√m fracture toughness for samples sintered in a hot press at 1750 °C. The second reference [39] utilized a combination of cold-isostatic pressing and pressure-less sintering at 1800 °C, resulted in Vickers hardness of 17.9 GPa (fracture toughness was not reported).
4. Conclusions Densification and mechanical properties of a fixed nitrogen rich Ca-α-SiAlON composition were investigated. Samples were prepared from nanosized precursors utilizing spark plasma sintering (SPS) at 1500 °C and 1600 °C for 10, 20 and 30 min. The density of each product was found to be close to the theoretical density,
suggesting that rapid densification was achieved at relatively low temperatures because of the inherent capabilities provided by SPS. Single-phase nitrogen-rich α-Ca0.68Si9.96Al2.04O0.68N15.32 formed in samples that were sintered at 1500 °C and 1600 °C for 30 min, whereas a small amount of AlN remained un-reacted for samples sintered for shorter durations (10 and 20 min) at both temperatures. Vickers Hardness for both sets of samples sintered at 1500 °C and 1600 °C for 10, 20 and 30 min holding time showed an increasing trend with the holding time, a result of AlN traces present in samples sintered for shorter holding times. The larger grain size associated with sample 1630 is responsible for its relative lower hardness (20.5 GPa), as compared to sample 1530 (21.6 GPa). With regard to the fracture toughness, sintering for 30 min at either 1500 °C or 1600 °C yielded excellent fracture toughness values of 7.3 MPa√m and 9.7 MPa√m, respectively, as a consequence of the formation of elongated α-SiAlON grains.
Acknowledgment The authors would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project No. 12-ADV2411-04 as part of the National Science, Technology and Innovation Plan.
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Development of a single-phase Ca-α-SiAlON ceramic from nanosized precursors using spark plasma sintering Raja Muhammad Awais Khan a, Moath Mohammad Al Malki a, Abbas Saeed Hakeem b,n, Muhammad Ali Ehsan b, Tahar Laoui a,n a b
Mechanical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia Center of Excellence in Nanotechnology, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
art ic l e i nf o
a b s t r a c t
Article history: Received 30 May 2016 Received in revised form 15 July 2016 Accepted 17 July 2016 Available online 18 July 2016
Nitrogen-rich Ca-α-SiAlON ceramics were synthesized using nanosized precursors via spark plasma sintering process for various holding times (10, 20, 30 min) at relatively low temperatures of 1500 °C and 1600 °C. The effects of the experimental conditions, namely sintering time and temperature, on the densification and mechanical properties of the processed α-SiAlON were investigated. All of the conditions yielded nearly completely densified ceramics. A remarkable combination of hardness and toughness was obtained for the samples at either sintering temperature; and these values were greatest after 30 min sintering time, with Vickers hardness values of 21.6 GPa and 20.5 GPa and fracture toughness values of 7.3 MPa√m and 9.7 MPa√m for sintering temperatures of 1500 °C and 1600 °C, respectively. The differences in the mechanical properties of these samples were related to differences in the phases formed during the sintering process. & 2016 Elsevier B.V. All rights reserved.
Keywords: SiAlON Nano-ceramics Silicon nitride Spark plasma sintering Mechanical properties
1. Introduction Silicon nitride (Si3N4) ceramics exhibit exceptional thermal and mechanical properties, specifically hot hardness, thermal shock resistance and chemical inertness, which enable them to withstand severe working conditions [1]. However, full densification of Si3N4 is difficult to attain, due to the covalent bonds present between Si and N atoms. The conventional solid-state sintering of Si3N4, therefore, requires very high temperatures to provide sufficient atomic diffusion [2,3]. To overcome this temperature issue, metal oxide additives are incorporated as sintering aids in order to relax the sintering temperatures for such ceramics [1–4]. The resultant phases and mechanical properties of these ceramics are highly dependent on the added sintering aids. The most significant breakthrough in Si3N4 ceramics was the development of SiAlON ceramics, resulting from the utilization of alumina along with other oxide additives [5–7]. SiAlON ceramics are considered to be solid solutions of Si3N4, with Si and N partially replaced by Al and O, respectively [5,7–9]. Among different phases of SiAlON, α-SiAlON and β-SiAlON phases have been particularly paid attention to during the past two n
Corresponding authors. E-mail addresses: [email protected] (R.M.A. Khan), [email protected] (M.M. Al Malki), [email protected] (A.S. Hakeem), [email protected] (M.A. Ehsan), [email protected] (T. Laoui). http://dx.doi.org/10.1016/j.msea.2016.07.075 0921-5093/& 2016 Elsevier B.V. All rights reserved.
decades due to their notable mechanical properties, specifically the elevated hardness of α-SiAlON and the modest fracture toughness of β-SiAlON [10,11]. The general stoichiometric formula of α-SiAlON is Mx Si12 −(m + n)Al(m + n)OnN16 − n , in which ‘m’ and ‘n’ represent the number of Al-N and Al-O bonds respectively, ‘M’ stands for the added metallic cation, often alkaline and rare-earth elements, and ‘x’ is given by the formula x ¼m/v where v is the valence of the added cation [7]. α-SiAlON can be produced by reacting Si3N4, Al2O3, AlN and the added oxide of a particular metallic cation ‘M’, for instance Y2O3, CaO, etc. As the reaction proceeds, α-SiAlON precipitates out with a gradual decrease in the amount of the liquid phase until the reaction is completed with the formation of the α-SiAlON solid solution, leaving a small amount of un-reacted liquid in the grain boundary [7,12]. The mechanical properties of α- and β-SiAlONs have been generally explained on the basis of their microstructures. α-SiAlON grains exhibit an equiaxed morphology while β-SiAlON generally forms elongated grains. However, elongated α-SiAlON grains have also been produced in an attempt to improve the fracture toughness [13,14]. During the past few decades, lanthanide metal oxides, such as Yb2O3, La2O3, Nd2O3 and Dy2O3, have been the major research focus in the field of metal oxide sintering additives [15–18]. However, the relatively high cost of rare earth oxides [19] has prompted the use of alkaline earth metal oxides, including BaO,
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CaO, and MgO [20,21]. Due to the relatively high solubility of Ca2 þ in the α-SiAlON lattice and the limited resulting crystal distortion, CaO has been considered as a potential densification aid and as a good candidate for the reduction of grain boundary glass phase, which could otherwise be detrimental to the mechanical properties. Moreover, Ca compounds are relatively inexpensive, as they can be readily obtained from fly ash [22]. The use of spark plasma sintering (SPS), as a consolidation technique, has generated active attention in the synthesis of ceramic materials due to its higher heating rate and its novel pulsed-current-based heating [23,24]. Elongated forms of Ca αSiAlON with improved fracture toughness have been successfully produced using processing schemes involving high heating rates, such as SPS [25,26]. The present work aims to study the effect of nanosized starting powders on the synthesis of monolithic nitrogen rich α-SiAlON by the SPS technique, incorporating CaO as a densification additive. We believe that the combination of SPS with nano sized precursors for processing nitrogen-rich α-SiAlON ceramics doped with Ca additives, has not been investigated before. We anticipate a reduction in the sintering temperature and an enhancement in the mechanical properties.
2. Experimental procedure The nitrogen rich chemical composition was selected to be: Ca0.8Si9.2Al2.8O1.2N14.8 corresponding to m ¼1.6 and n ¼1.2 in the general formula of α-SiAlON. Table 1 displays the amount of each reactant used in weight percentage. These reactants include αSi3N4 (150 nm-Ube Industries SN-10, Japan), AlN (100 nm-Sigma Aldrich, Germany), SiO2 (10–20 nm-Sigma Aldrich, Germany) and CaO (o 160 nm-Sigma Aldrich, Germany).
3Si3N4 + 0.8CaO + 2.8AlN + 0.2SiO2 → Ca 0.8Si9.2Al2.8O1.2 N14.8
(1)
Powders precursor were carefully weighed in the appropriate amounts according to the chemical Eq. (1) to form samples each with a total mass of 5 g. The powders were homogenized using an ultrasonic probe sonicator (Model VC 750, Sonics, Connecticut, USA) for 30 min, utilizing ethanol as a mixing medium. Later, the mixtures were dried in a furnace at 80 °C for 12 h to remove ethanol. To carry out SPS, powder mixtures were first poured into 20 mm diameter graphite dies. Sintering was performed at two different temperatures of 1500 and 1600 °C for 10, 20 and 30 min holding time under a constant pressure of 50 MPa. A heating rate of 100 °C/min was adopted throughout this study to avoid formation of intermediate phases. Samples were then rapidly cooled down to ambient temperature. The cooling process can be divided into two stages, in which samples were firstly cooled down from the sintering temperature (1500 °C/1600 °C) to 400 °C at a rate of 100 °C/min, followed by switching off the furnace. SPS samples were cleaned of graphite using SiC grinding papers (120 grit size), followed by additional diamond disc grinding to remove the graphite paper and prepare clear surface to measure sample density. Later, automatic grinding and polishing machine (Automet 300 Buehler grinding machine) was utilized to prepare samples for subsequent microstructural and mechanical investigations. The grinding process included the use of diamond
wheels (74 mm, 40 mm, 20 mm,10 mm grit size), followed by the use of diamond suspensions (9 mm, 6 mm, 3 mm, 1 mm, 0.25 mm) in the polishing stage. To identify the phases present in the synthesized samples, a Rigaku MiniFlex X-ray diffractometer (Japan) was used with Cu Kα1 radiation (γ ¼ 0.15416 nm), tube current ¼10 mA and an accelerating voltage of 30 kV. Cross-section and fracture surface micrographs were obtained using a field emission scanning electron microscope (FESEM, Lyra 3, Tescan, Czech Republic) with accelerating voltages of 20–30 kV. Archimedes' principle was applied to evaluate the density of the sintered samples. A universal hardness tester (Zwick-Roell, ZHU250, Germany) was employed to evaluate the Vickers hardness of the sintered samples under a load of 98 N. The fracture toughness was evaluated based on the indentation method [27].
3. Results and discussion 3.1. Analysis of powder mixtures Fig. 1 displays an SEM micrograph of the investigated composition after mixing for 30 min in the probe sonicator. Particles size ranges from 100 nm to 1 mm, indicating powder agglomeration. The agglomeration of the nanosized powders resulted from physical interactions of their large surface areas. Rajesh et al. showed that although de-agglomeration is difficult to achieve, a probe sonicator, when compared to other mixing techniques, is capable of homogenizing powder mixtures, keeping the particles as small as theoretically possible [28]. However, and as revealed from our analysis, agglomeration can be restored easily in almost no time. Energy Dispersive X-Ray Spectroscopy (EDXS) mapping (Fig. 2) confirmed that a reasonable level of homogeneity was achieved after using the probe sonicator. 3.2. Sintering and densification The densities of samples sintered at 1500 °C and 1600 °C are listed in Table 2. The sample ID corresponds to the sintering parameters, for instance sample “1510” denotes a sintering temperature of 1500 °C carried out for 10 min. Samples densities fall in range of 3.15–3.20 g/cm3. The theoretical density for the selected composition was calculated based on the resultant phase to be
Table 1 The amount of each chemical powder reactant used in wt%. Composition
α-Si3N4
AlN
CaO
SiO2
Ca0.8Si9.2Al2.8O1.2N14.8
71.07
19.37
7.57
2.03 Fig. 1. SEM micrograph of the investigated composition after probe sonication.
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Fig. 2. EDXS mapping of the investigated composition after mixing revealing a homogenous distribution of the starting mixture.
Table 2 Density values for samples sintered at 1500 °C and 1600 °C. Sample ID 3
Density (g/cm )
1510
1520
1530
1610
1620
1630
3.15
3.15
3.19
3.18
3.18
3.20
3.21 g/cm3. The comparison of the experimental and theoretical densities suggests that the sintered samples reached approximately 98% densification. The pulsed-current heating of SPS aided the densification process at low sintering temperatures, along with higher surface areas associated with the nanosized precursors. CaO, as a densification additive, provided an extra amount of liquid phase at the first stages of sintering, which certainly facilitated densification. However, a slight decrease in the density was noticed in samples sintered for shorter durations at both temperatures, particularly for samples sintered at 1500 °C (sample 1510 and 1520). The large specific surface area associated with nanosized powders has been proven to contribute to the driving mechanism for sintering to achieve thermal equilibrium [29]. The equation developed by Hansen and his group estimates that the sintering rate would be enhanced four-fold if the average size of the reactant powders were reduced by one order of magnitude [30]. This relationship, however, assumes generally nanosized precursors, and that these precursors do not display notable agglomeration. The onset of the sintering process is affected by the use of nanosized precursors. Suryanarayana et al. has shown that sintering onset of nano starting materials is E0.3 Tm, while it starts at E0.65 Tm in the case of the conventional starting powders [31]. These factors and others are thought to contribute to the densification of the investigated composition by the solution-reprecipitation process at a relatively low peak temperature, taking into consideration the high nitrogen content of the investigated composition.
Fig. 3. XRD patterns of samples IDs 1510, 1520 and 1530, i.e., samples sintered at 1500 °C for 10, 20 and 30 min, respectively.
Fig. 4. XRD patterns of samples 1610, 1620 and 1630, i.e., samples sintered at 1600 °C for 10, 20 and 30 min, respectively.
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3.3. Phase analysis The XRD pattern of samples 1510, 1520 and 1530, i.e., those sintered at 1500 °C for 10, 20 and 30 min, respectively, are shown in Fig. 3. Similarly, Fig. 4 shows the XRD patterns of samples 1610, 1620 and 1630, i.e., those sintered at 1600 °C for 10, 20 and 30 min, respectively. Ca-α-SiAlON (Ca0.68Si9.96Al2.04O0.68N15.32) phase has been identified in all samples. However, samples sintered for shorter durations showed the presence of un-reacted AlN in the Ca-α-SiAlON matrix, but ultimately dissolved into a single (monolithic) α-SiAlON phase with m ¼1.36 and n ¼0.68 after 30 min of sintering at 1500 °C. The XRD patterns of the sintered
samples showed slightly stronger AlN peaks in samples sintered at 1500 °C than in those sintered at 1600 °C, which is attributed to the reduced dissolution of AlN at lower temperatures. As we were working with a single-phase system, a comparison was made between the designed and obtained compositions of αSiAlON. Normalizing the obtained Si content to its counterpart in the designed composition allowed the obtained α-SiAlON composition to be rewritten as Ca0.63Si9.2Al1.88O0.63N14.15, which revealed that the obtained product had proportionally less Ca, Al and O. It might be possible that these constituting elements were kept in the grain boundary, as the starting materials were liquidized early due the use of nano precursors [4].
Fig. 5. FESEM micrographs of the polished samples sintered at 1500 °C for (a) 10, (b) 20 and (c) 30 min and of samples sintered at 1600 °C for (d) 10, (e) 20 and (f) 30 min.
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3.4. Microstructural development FESEM micrographs of polished samples sintered at 1500 °C and 1600 °C are shown in Fig. 5. The micrographs showed reduced levels of porosity at increased time durations of sintering at both temperatures, as well as reduced porosity at the higher sintering temperature for a sintering duration of 10 min (compare Fig. 5a and d), which is consistent with the density data (Table 2). The reduced porosity at longer sintering duration is in agreement with the phase analysis, as it was shown earlier that some un-dissolved AlN was found in the samples sintered for 10 and 20 min at both temperatures. FESEM micrographs of etched samples 1530 and 1630 are shown in Fig. 6. Single-phase α-SiAlON grains can be seen in both micrographs, as confirmed by XRD analysis. The average grain size for sample 1530 (Fig. 6a) was determined to be three-times smaller than that of sample 1630 (Fig. 6b), a result of the temperature-dependent grain growth. Consolidation of SiAlONs ceramics takes place in liquid phase
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sintering. Generally, the initial step observed with increasing temperature involves the reaction of CaO, SiO2 and surface oxides, such as Al2O3 in the case of AlN and SiO2 in the case of Si3N4, to form a Ca-containing liquid phase (Ca-Si-Al-O-N), known to be present above the eutectic temperature ( E1350 °C). This liquid phase firstly assists in arrangement of particles, and later in the solution-reprecipitation mechanism to form ultimately Ca-α-SiAlON. The overall chemical reaction, based on the XRD and FESEM observations, can be represented as follows:
α-Si3N4 ( surface SiO2 )+CaO + AlN+SiO2 → (1500 °C/1600 °C)Ca 0.68Si9.96Al2.04 O0.68 N15.32 + Ca-Si-Al-O-N ( liquid phase)
(2)
A pure α-SiAlON composition is nitrogen rich and the inherent amount of grain boundary liquid phase is scarce which, eventually, limits densification. However, in our work, because of the high heating rate (100 °C/min) associated with the SPS processing, few α-SiAlON nuclei formed at the start of sintering, and the remaining
Fig. 6. FESEM micrographs of etched α-SiAlON samples sintered at (a) 1500 °C and (b) 1600 °C for 30 min.
Fig. 7. FESEM images of fractured surfaces of samples sintered at (a) 1500 °C and (b) 1600 °C for 30 min, showing the elongation of α-SiAlON grains.
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Table 3 Mechanical properties of samples sintered at 1500 °C and 1600 °C. Sample ID
1510
1530
1610
1630
HV10 (GPa) KIC (MPa√m)
19.8 70.4 4.4 7 0.4
21.6 7 0.6 7.3 7 1.3
19.3 7 0.6 9.5 7 1.2
20.5 7 0.4 9.7 7 2.3
dissolved species formed a liquid phase, which helped to densify the ceramic at a low sintering temperature. The presence of a relatively large amount of liquid phase was shown to support mass transport and species diffusion to form elongated α-SiAlON grains [25,26]. Santos et al. [32] and Dong et al. [33] attributed their failure to grow elongated α-SiAlON grains to the low heating rate associated with the conventional pressure-assisted sintering techniques they used, namely GPS and HIP. However, sintering techniques with low heating rates were shown by Zhao et al. to be able to form such morphology if the starting composition contained a sufficient amount of liquid phase [34]. Fig. 7 shows the formation of elongated α-SiAlON grains in samples 1630 and 1530. Another factor that affects positively the elongation in α-SiAlON grains is the size of the precursors [13,35,36]. Formation of elongated α-SiAlON grains is considered to be an abnormal grain growth, and such growth occurs for SiAlON systems possessing αSi3N4 below 0.5 mm [13,35,36]. This type of grain growth was observed contributed to the elongation of α-SiAlON grains observed in our work (Fig. 7). 3.5. Mechanical properties Table 3 shows mechanical properties of the samples sintered at 1500 °C and 1600 °C for 10 and 30 min. At either temperature, hardness values increased with increasing durations of sintering. This trend was clearly attributed to the difference in the phases formed, as the samples sintered for short duration contained significant traces of un-dissolved AlN, as shown in the XRD analysis. The presence of dense monolithic Ca-α-SiAlON for the samples sintered for 30 min at 1500 °C and 1600 °C (samples 1530 and 1630, respectively) would explain their hardness values (21.6 GPa and 20.5 GPa, respectively). The relatively larger grain size of sample 1630 might be responsible for its hardness value, being slightly lower than that of 1530, as reported elsewhere [37]. The fracture toughness for the samples sintered at 1600 °C exhibit higher values than those sintered at 1500 °C due to, most probably, the larger grain size and higher aspect ratio of α-SiAlON grains (Figs. 6 and 7). Comparing our results with those found in literature under the same processing parameters (powder size, sintering technique, sintering parameters) within the same composition field is difficult. One of the closest systems we found in literature [38] was able to synthesize a similar composition (Ca-α-SiAlON) yielding Vickers hardness of 19.7 GPa with 5.1 MPa√m fracture toughness for samples sintered in a hot press at 1750 °C. The second reference [39] utilized a combination of cold-isostatic pressing and pressure-less sintering at 1800 °C, resulted in Vickers hardness of 17.9 GPa (fracture toughness was not reported).
4. Conclusions Densification and mechanical properties of a fixed nitrogen rich Ca-α-SiAlON composition were investigated. Samples were prepared from nanosized precursors utilizing spark plasma sintering (SPS) at 1500 °C and 1600 °C for 10, 20 and 30 min. The density of each product was found to be close to the theoretical density,
suggesting that rapid densification was achieved at relatively low temperatures because of the inherent capabilities provided by SPS. Single-phase nitrogen-rich α-Ca0.68Si9.96Al2.04O0.68N15.32 formed in samples that were sintered at 1500 °C and 1600 °C for 30 min, whereas a small amount of AlN remained un-reacted for samples sintered for shorter durations (10 and 20 min) at both temperatures. Vickers Hardness for both sets of samples sintered at 1500 °C and 1600 °C for 10, 20 and 30 min holding time showed an increasing trend with the holding time, a result of AlN traces present in samples sintered for shorter holding times. The larger grain size associated with sample 1630 is responsible for its relative lower hardness (20.5 GPa), as compared to sample 1530 (21.6 GPa). With regard to the fracture toughness, sintering for 30 min at either 1500 °C or 1600 °C yielded excellent fracture toughness values of 7.3 MPa√m and 9.7 MPa√m, respectively, as a consequence of the formation of elongated α-SiAlON grains.
Acknowledgment The authors would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project No. 12-ADV2411-04 as part of the National Science, Technology and Innovation Plan.
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