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Effect of SiC additions on microstructure, mechanical properties and thermal shock behaviour of alumina–mullite–zirconia composites
Materials Science and Engineering A 530 (2011) 585–590
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Effect of SiC additions on microstructure, mechanical properties and thermal shock behaviour of alumina–mullite–zirconia composites H. Majidian ∗ , T. Ebadzadeh, E. Salahi Ceramic Division, Materials and Energy Research Centre, P.O. Box 14155-4777, Alborz, Iran
a r t i c l e
i n f o
Article history: Received 27 April 2011 Received in revised form 12 September 2011 Accepted 6 October 2011 Available online 18 October 2011 Keywords: Slip casting Composites Grain size Mechanical properties Thermal shock resistance
a b s t r a c t This paper describes the effect of additions of silicon carbide (SiC) particles on density, microstructure, mechanical properties and thermal shock behaviour of slip-cast alumina–mullite–zirconia composites. Alumina, zircon and SiC powders were used to prepare highly concentrated (51 vol.% loading) and stable aqueous suspensions by using Dolapix dispersant (0.5 wt.%). The sintering process was carried out in two stages, first at 1600 ◦ C and then 1500 ◦ C, with 2 and 5 h holding, respectively. Densities and thermal shock properties as well as strength, toughness and hardnessof the prepared composites were evaluated. Twostage sintering process of alumina–mullite–zirconia–SiC (AMZS) composites led to a remarkable grain size reduction and an increase of density. The maximum value of fracture toughness and minimum value of grain size were obtained by using 20 vol.% silicon carbide. Results showed that the addition of SiC particles (20 vol.%) improved the thermal shock resistance (T = 1000 ◦ C) of alumina–mullite–zirconia composites even after 10 heating cycles. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Improvement of high temperature mechanical behaviour is an increasing interest in ceramic materials for structural applications [1]. Alumina–mullite–zirconia (AMZ) composites are an important category of refractory materials and widely used in forehearth, glass melting furnaces as plungers, tubes and orifice rings [2]. AMZ refractory materials show a high strength and fracture toughness at room temperature [3], but these properties decrease significantly at high temperatures because of the formation of a glassy phase in the grain boundaries. AMZ composites used in the glass industry are subject to thermal shock stresses during installation. Aksel [3] reported that the strength of AMZ composites decreased gradually as the quench temperature increased and showed approximately 50% loss of strength after the 1st cycle, whereas the 40 subsequent cycles did not significantly alter the retained strength. Improving the performance of AMZ composites such as thermal shock resistance is a current expectation [4]. Densification and mechanical properties of alumina and its composites have been widely studied [5,6]. The toughness values of 5.5–7 MPa m1/2 have been reported for alumina–10 vol.% ZrO2 hot-pressed composite [5]. Poowancum [6] reported that the highest fracture toughness (7.3 MPa m1/2 ) of AMZ composites was obtained with CeO2 additive due to the
∗ Corresponding author. Tel.: +98 912 5177988; fax: +98 21 8877335. E-mail addresses: [email protected] (H. Majidian), [email protected] (T. Ebadzadeh), [email protected] (E. Salahi). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.10.027
effect of crack deflection and grain bridging mechanism. Dong et al. [7] reported that the highest fracture toughness was found to be 7.6 MPa m1/2 in alumina composite containing 5 wt.% SiC particles. Aksel [1] showed that the crack length of AMZ composite was much smaller than the calculated critical value. This is because the crack tip blunting takes place as a result of a volume increase during sintering by means of absorbing energy from the crack, thereby preventing the crack length from extending to the critical crack length, which can be associated with the improvement in KIC (1.6 MPa m1/2 , for slip cast AMZ composite). Therefore, more fracture energy was required in AMZ refractory materials to connect the cracks for propagation. Mazzei [8] suggested that the zirconia tetragonal-monoclinic phase transformation occurred during the sintering cooling, producing microcracks in the matrix that may be responsible for the crack branching. Jang et al. [9] attained the fracture toughness of 2.25 up to 5.25 MPa m1/2 for AMZ composite by increasing zirconia content, and suggested that the tetragonal to monoclinic transformation toughening potentially contributes to the toughening of AMZ composites. As the total content of zirconia increases, the toughening by the microcrack nucleation becomes increasingly important. Mechanical and thermal shock properties of refractory materials can be enhanced by the addition of silicon carbide (SiC). SiC is a ceramic material for many engineering applications because of its high strength and hardness, stability at high temperatures, resistance to thermal shock and abrasion [10–12]. Because of the abovementioned reasons, the addition of SiC particles to AMZ
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refractories has been suggested to improve the thermal shock resistance [13]. Silicon carbide ceramics are usually difficult to densify; the covalent nature of Si–C bond implies low self-diffusion coefficients which limits the densification process [14]. The sintering of SiC can be performed by adding sintering aids at high temperatures after holding for a long time [15]. On the other hand, obtaining a ceramic material with a high relative density and homogeneous microstructure consisting of small grains is one of the main aims in sintering process [16,17]. Thermal shock behaviour of alumina and AMZ composites has been studied earlier [18,19]. Ainsworth and Moore [20] studied the thermal shock resistance of alumina (99.5%) cylinders by quenching the specimens from selected temperatures between 150 ◦ C and 500 ◦ C into ice water. They observed that a temperature drop of 175 ◦ C decreases the strength appreciably. A gradual decrease in average retained strength of alumina was also observed up to the critical temperature difference of 270 ◦ C [21]. On the other hand, Aksel [1] showed there was a marked decline in strength values of AMZ composite between 300 ◦ C and 600 ◦ C, because of the nucleation of microcracks and their propagation. However, strength decreased more slowly, presumably because the number of cracks increased only slightly, as the further quench temperature rose. In the present work, slip casting was used to prepare samples because the homogeneity of particle packing in the green bodies is better controlled when colloidal shaping techniques are used for ceramic compacts. In this case, the sintering behaviour and final properties of materials will be improved [22–24]. In order to produce high-performance ceramics a well dispersed and highly concentrated slurry of fine powders should be prepared [25]. Since the final properties of a material depend on the microstructure formed during sintering, the sintering of prepared composites was carried out using two-stage sintering process. The improvement in the mechanical properties and thermal shock behaviour of AMZ composites have been investigated with the addition of SiC particles. 2. Experimental procedures 2.1. Materials The ␣-alumina (MR70, Martinswerk, Germany, >99.8%), zircon (Zircosil, Johnson-Matthey, Italy, >98.5%) and SiC (Mirali Company, Iran, >98%) powders with mean particle sizes of 0.6, 1.4 and 3 m, respectively, were used as the starting materials. Dolapix CE-64 (Zschimmer & Schwarz, 0.5 wt.% based on solid weight) was used to disperse SiC particles (10, 20 and 30 vol.%) in alumina–zircon suspensions containing 51 vol.% of solid according to Ref. [8]. 2.2. Composite preparation The mass ratio of alumina to zircon in all mixtures was 85/15. Each slip was mixed by using a planetary mill for 20 min and consolidated through slip casting method using plaster mould. The cast parts were healed at room temperature for 24 h, dried at 110 ◦ C in an oven and finally sintered at 1600 ◦ C and then at 1500 ◦ C after holding for 2 and 5 h, respectively. Samples sintered at 1600 ◦ C and 1650 ◦ C for 2 h were also prepared for comparison. 2.3. Physical and mechanical properties Density and apparent porosity of sintered samples were determined using the standard water adsorption method. Crystalline phases in fired samples were characterized by XRD (Siemens, D500 system) using CuK␣ radiation working with 30 kV accelerating voltage. The microstructure of sintered samples was observed by
Table 1 Relative density and apparent porosity of AMZS composites. SiC (vol.%)
Sample code
Relative density (%)
0a 0b 0c 10a 10b 10c 20a 20b 20c 30a 30b 30c
AMZ0S–C AMZ0S–H AMZ0S AMZ10S–C AMZ10S–H AMZ10S AMZ20S–C AMZ20S–H AMZ20S AMZ30S–C AMZ30S–H AMZ30S
96.83 97.37 98.25 92.7 92.95 93.50 88.12 89.34 89.44 83.80 81.96 85.82
a b c
± ± ± ± ± ± ± ± ± ± ± ±
0.3 0.3 0.25 0.2 0.25 0.1 0.1 0.3 0.4 0.2 0.4 0.3
Apparent porosity (%) 0.49 0.62 0.11 6.91 5.9 5.76 11.33 11.47 10.28 17.76 18.94 14.45
± ± ± ± ± ± ± ± ± ± ± ±
0.1 0.1 0.05 0.5 0. 5 0.5 0.4 0.2 0.4 0.3 0.5 0.5
Sintered at 1600 ◦ C/2 h. Sintered at 1650 ◦ C/2 h. Sintered at 1600 ◦ C/2 h followed by sintering at 1500 ◦ C/5 h.
SEM (VEGA II, TESCAN) on polished surfaces which were thermally etched at 1420 ◦ C for 20 min. Thermal shock testing was carried out by air quenching bars from maximum temperature, at 300 ◦ C intervals. In each thermal shock test, a specimen (5 mm × 6 mm × 35 mm) was heated for 15 min in the furnace to assure temperature uniformity and then cooled in air. After cooling, the retained strength values of samples were measured by a universal testing machine (Instron 1196, England) using three-point bending fixture with a span of 15 mm and a crosshead of 0.5 mm/min. At least seven specimens were tested to obtain a mean value for each quench temperature. Fracture toughness (KIC ) was then measured using the single edge notched beam (SENB) technique [26] with a notch cut of 0.3 mm width and 2 mm depth, producing a notch-to-depth ratio of 0.5. KIC was calculated from the maximum load using equation reported in ASTM standard 1421-01b. Vicker’s hardness (Hv ) was also measured by the diagonal impression generated by a Vicker’s indenter at a load of 5 kg for 15 s (ASTMC 1327-03). MEASUREMENT software was used to determine the grain size from the SEM micrographs for alumina–mullite–zirconia–SiC (AMZS) composites. 3. Results and discussion 3.1. Density and porosity Table 1 shows the relative density and apparent porosity of AMZS composites. As this table shows, the composites processed without SiC (AMZ0S) have higher density values than those of any other composites processed with SiC. Under the same sintering conditions, the porosity increased in composites processed with SiC which can be attributed to the higher refractoriness of SiC in relation to Al2 O3 . It has been reported that the added SiC apparently decreases the sinterability of Al2 O3 through the blocking grain boundary movement [27]. Lower relative density of alumina–SiC composites than that of monolithic Al2 O3 are mainly attributed to the incorporation of SiC particles, that blocked grain boundary movement and hindered the densification of Al2 O3 [7]. Since SiC particles were immobile and did not react with alumina during the sintering, the movement of the grain boundary is prohibited by SiC particles which delayed the densification of alumina [28]. Therefore, the density decreased after adding SiC particles. In fact, in bimodal packing systems, a second phase of nonsinterable particles considerably decreases the sintering rate by introducing shear stress at the hard particle/matrix interface [29]. In this case, matrix regions of compression are developed around interactive SiC. These regions increase density, produce a non-deformable network and constrain the shrinkage of the adjacent porous matrix. This constraint causes desintering and crack-like void formation, enhancing
H. Majidian et al. / Materials Science and Engineering A 530 (2011) 585–590 Table 2 Fracture toughness and hardness of AMZS composites.
5000
4000
Intensity (a.u.)
587
AMZ30S
3000 AMZ20S 2000
Sample code
Hardness (kgf/mm2 )
AMZ0S AMZ10S AMZ20S AMZ30S
1262 1107 687 643
± ± ± ±
84 60 15 24
Toughness (MPa m1/2 ) 7.29 7.70 8.13 7.86
± ± ± ±
2.0 1.2 1.0 1.1
AMZ10S
1000
AMZ0S
0 20
30
40
Two teta
50
60
70
Fig. 1. XRD pattern of AMZS composites (: alumina, : mullite, •: monoclinic zirconia, : tetragonal zirconia, and : SiC).
the porosity and decreasing the final density of AMZS composites [27]. The magnitude of this effect depends on the nonsinterable particles content. Also, the movement of pores along the grain boundaries may be limited by the presence of SiC particles during the final sintering stage [30]. It can be seen that in all the composites, the sintering temperature increased from 1600 ◦ C to 1650 ◦ C resulted in higher densities. The same trend was also observed after increasing sintering step while two sintering temperatures were used and led to an increase in density. For example, the relative density of AMZ10S composite was 92.7, 92.95 and 93.5% after sintering at (a) 1600 ◦ C/2 h, (b) 1650 ◦ C/2 h, and (c) 1600 ◦ C/2 h followed by 1500 ◦ C/5 h, respectively. As Table 1 further reveals, the apparent porosity of AMZS composites increased by addition of SiC particles. Since, the densification rate of SiC particles is lower than that of alumina, therefore, after sintering there would be exist a gap between alumina and SiC in the microstructure [29]. 3.2. X-ray diffraction pattern Fig. 1 shows the X-ray diffraction pattern of AMZS composites. The resulting phase composition consists of corundum, mullite, zirconia and SiC. No peaks of zircon and crystalline form of silica (cristobalite and tridymite) could be identified in Fig. 1 which means that zircon completely dissociated into ZrO2 and SiO2 and subsequently mullite was formed by reaction of SiO2 with alumina particles. In Fig. 1, it seems that as the SiC content in composites increased, the intensity of mullite peaks was found to decrease. Since the ratio of alumina to zircon should be kept constant in all composites, the amount of alumina and zircon decreased by the addition of SiC, therefore, the formation of mullite was suppressed. Quantitative analysis of AMZ20S samples by internal standard method showed that the amount of SiC (22 vol.%) was a little higher than its theoretical value (20 vol.%). Since the sintered samples were packed into abed of graphite powder, it may be concluded that some silica formed from decomposition of zircon would reacted with graphite and SiC particles were formed during sintering. From the results mentioned above it can be concluded that the amount of silica and thus mullite was decreased (Fig. 1).
expansion mismatch between Al2 O3 and SiC particles. Phase transformation of ZrO2 dispersion in the Al2 O3 matrix from tetragonal (t) to monoclinic (m) has been found to be effective for improving the fracture toughness. In these cases, the strong strain field and some microcracks were easily formed during the sintering process, due to the difference in the coefficient of thermal expansion [31]. These strain fields and microcracks led to intergranular fracture with crack deflection and thus reduce the crack propagation energy [32]. In addition when cracks propagate transgranularly in m-ZrO2 the driving force of crack propagation is reduced due to the plastic deformation of m-ZrO2 [33,34]. Mazzei et al. [35] have been reported that zirconia inclusions in AMZ composites are responsible for the crack deflection mechanism. Moreover, the SiC particles can display reinforcement components since their thermal expansion coefficient difference with alumina can built up compressive stresses in the matrix and also promote decreases in the average grain size of the alumina phase (comparing AMZ0S with AMZ10S, a reduction in the average grain size was observed). The major improvement in KIC in the AMZ refractory material is also due to the presence of microcracking and crack-closure compressive strains as a result of transformation toughening. This is because of thermal contraction mismatch between the phases and volume expansion associated with zirconia transformation and mullite formation. Thermal expansion mismatch leads to large tensile hoop stresses and crack development between alumina, mullite and zirconia grains during cooling from the fabrication temperature of 1600 ◦ C due to the significant difference on the coefficients of thermal expansion, which are 8.2 × 10−6 K−1 for alumina, 5.1 × 10−6 K−1 for mullite and 7.6 × 10−6 K−1 for zirconia [36,37]. The volume change can also result in the development of microcracking in the matrix. Under certain conditions, particularly for thermal shock resistance, numerous microcracks are desirable. Microcracking is also a predominant toughening mechanism in AMZ refractory materials [7]. The fracture energy was calculated from the fracture toughness and Young’s modulus to be 103 and 122 J/m2 for the AMZ0S and AMZ20S, respectively. The Young’s modulus of AMZ0S and AMZ20S composites during the flexural strength was 253 and 224 GPa. The increased fracture energy of AMZ composites with SiC may be associated with the formation of fine grains. Decreased grain size is accompanied by the increase of the grain boundary area. The enhanced branch of main crack along the increased grain boundary area may contribute to the increase of the fracture energy [38]. The fracture energy of AMZ20S was 20% as high as that of AMZ0S. The average Vickers hardness of all AMZS composites is lower than that of AMZ0S composite. The Vickers hardness is known to be very sensitive to the density [39]. The decrease in hardness values with the increasing of SiC particles can be related to the increase of porosity.
3.3. Mechanical properties 3.4. Microstructure The variations in mechanical properties by using different amounts of SiC in AMZS composites are shown in Table 2. It can be observed that the fracture toughness values increased with increasing SiC content from 0 to 20 vol.%. Further increase in SiC content up to 30 vol.% resulted in decreasing of fracture toughness. The gradual increase of fracture toughness of AMZS composites may be due to the presence of internal residual stresses resulting from thermal
Fig. 2 depicts the microstructure of AMZ0S and AMZ20S composites sintered by one- and two-stage process. Most of the SiC particles are well dispersed in the composite, which are located not only in the grain boundaries or triple-grain junctions but also in the grain interiors. The microstructure of AMZ0S composite (Fig. 2a–c) reveals larger grains with a broad grain size distribution compared
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Fig. 2. Microstructure of (a) AMZ0S–C, (b) AMZ0S–H, (c) AMZ0S, (d) AMZ20S–C, (e) AMZ20S–H and (f) AMZ20S composites.
to those of AMZ20S composite (Fig. 2d–f). The effect of SiC particles on reducing the grain size of composites is that SiC particles pin alumina grain boundaries and can inhibit grain growth of alumina particles [40]. It should also be noted that the average size of alumina grains significantly decreases by using two-stage sintering (Fig. 2c and f) as compared to one-stage sintering (Fig. 2b and e). This result showed that SiC particles and two-stage sintering have a certain influence on diffusion mechanism and grain growth of alumina in the AMZS system.
process. An obvious grain growth was observed in AMZ0S–H composites. The average grain sizes of 2.43 and 2.93 m were measured for AMZ0S–C and AMZ0S–H composites, respectively. As Table 3 further reveals, sample AMZ20S containing 20 vol.% SiC particles sintered by two-stage process has the smallest average grain size. SiC particles in an alumina matrix have the ability to refine the microstructure and improve the mechanical properties of composites by restricting grain growth of the alumina matrix; because SiC particles decrease grain boundary mobility and diffusivity as well as
3.5. Grain size measurements
Table 3 Average grain sizes of AMZS composites.
It has been well known that grain size has a strong effect on the mechanical properties of ceramic materials, such as hardness, strength, wear resistance and toughness [41,42]. On the other hand, the final stage of sintering is always accompanied by rapid grain growth, resulting in structure coarsening [43]. Table 3 shows that the AMZS composites sintered by two-stage process have smaller grain size compared to those sintered by one-stage
Composite code AMZ0S–C AMZ0S–H AMZ0S AMZ20S–C AMZ20S–H AMZ20S
Grain size (m) 2.43 2.93 1.98 1.42 1.51 1.35
± ± ± ± ± ±
0.1 0.2 0.1 0.1 0.2 0.05
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the rate of densification [40,44]. Using data presented in Table 3, it is concluded that significant grain refinement has been occurred in the AMZ20S composite sintered by two-stage process. As reported [40] zirconia inclusions are quite mobile, so that they migrate with grain boundaries during sintering. Thus, it can be implied that the Al2 O3 or zirconia inclusions also tend to coalesce, so that the grain size became too larger in AMZ0S composites. On the contrary, SiC particles are immobile, so that grain growth of Al2 O3 as well as zirconia would be interrupted by SiC located at grain boundary in AMZS composites. In fact, the presence of SiC second phase inclusions at grain boundaries hinders grain growth by grain boundary pinning [45].
contributed to lowering of thermal shock resistance. The strength of AMZS samples at quench temperature of 1200 ◦ C was higher compared to AMZ0S sample (Fig. 3). AMZS composites appeared to be significantly resistant to crack propagation or extension as a result of the existence of microcracks and pores in these composites. These microcracks and pores led to intergranular fracture with crack deflection and thus reduce the crack propagation energy [31]. It should be noted that decline in strength was not significant for AMZS composites until the quench temperature of 1100 ◦ C; this implies that the thermal shock resistance increased due to the effects of microcracking. Thermal expansion of alumina is much higher than that of SiC; then there are tensile stresses in the surrounding matrix and compression in the second phase particles. These compressive stresses can be passed to the grain boundaries, resulting in toughening and increasing of thermal shock resistance [46]. The possible explanations for the increase in thermal shock resistance are that the thermal expansion is lower and the thermal conductivity is higher for the AMZS composites than that of AMZ0S. Since, it has been established that the addition of SiC to an alumina matrix increases the thermal conductivity of alumina [47]. For AMZ0S and AMZ20S composites, the thermal expansion coefficients were measured to be 7.79 and 6.87 × 10−6 K−1 , respectively. The increase of thermal conductivity may have some beneficial effect on the thermal shock behaviour. The improvement in the thermal shock resistance of AMZS composites is believed to be due to the higher fracture toughness as compared to AMZ. Other researchers have reported similar improvements in the thermal shock resistance as fracture toughness increased [47,48].
3.6. Thermal shock behaviour
4. Conclusion
Thermal shock resistance of AMZS composites is shown after one and 10 cycles in Fig. 3 and Table 4, respectively. As Fig. 3 reveals, the retained strength of AMZ0S composite decreased sharply at the thermal shock temperature difference of 300 ◦ C; and after that it decreased gradually up to temperature difference of 1200 ◦ C. The marked decline in strength values of sample AMZ0S between 0 and 300 ◦ C may be attributed to the nucleation of microcracks and their propagation. However, the slow decrease of strength after 300 ◦ C is presumably due to the slight increase of the number of cracks. As shown in Fig. 3, there is no remarkable change in the fracture strength of AMZS composites until a temperature difference of 1100 ◦ C is reached. As Fig. 3 further reveals, a gradual decrease in average retained strength is observed in the temperature difference range 1100–1200 ◦ C. The flexural strength of samples decreased significantly by increasing thermal shock cycle from 1 (Fig. 3) to 10 (Table 4). Under the same thermal shock cycles, the strength of AMZ20S and AMZ30S composites were decreased relatively more slowly (12% and 6%, respectively) after 10 cycles, whereas the flexural strength of AMZ0S composite was decreased relatively sharply (38%) just after one cycle. In spite of the low strength of AMZ30S composite, it showed relatively high thermal shock resistance (Table 4). The influence of porosity on mechanical properties and thermal shock behaviour of tested samples was also considered. The high porosity of the AMZS composites could have contributed to higher thermal shock resistance. In addition, AMZ0S composite associated with the risk of abnormal grain growth and might have
Thermal shock resistance of alumina–mullite–zirconia–silicon carbide (AMZS) composites has been investigated. Two-stage heat treatment improved density and reduced porosity of composites without and with SiC additions. Thermal shock testing results showed that the retained strength of composites containing 20 vol.% SiC particles after temperature difference of 1100 ◦ C was still high (154 and 145 MPa after one and 10 cycles, respectively). Although the composite containing 30 vol.% SiC had a relatively low strength (122 MPa), the residual strength of this composite after shock test was still high (117 MPa after 10 cycles). Fracture toughness increased from 7.29 to 8.13 MPa m1/2 in composites with 0 and 20 vol.% SiC, respectively. The fracture toughness value of 8.13 MPa m1/2 was obtained for AMZ20S composite which is higher than that reported for similar composites. The grain growth of alumina was retarded by the SiC because of a grain boundary pinning effect. AMZS composites were mainly composed of alumina, mullite, zirconia and SiC and exhibited high fracture toughness and low hardness.
Strength (MPa)
190 170 150 130 110 90 70 0
300
600
900
1200
ΔT ( oC) AMZ0S
AMZ10S
AMZ20S
AMZ30S
Fig. 3. Strength of AMZS composites after 1 cycle thermal shock test.
Table 4 Strength of AMZS composites after 10 cycles thermal shock heating at 1000 ◦ C. Number of cycles
AMZ0S
AMZ10S
AMZ20S
AMZ30S
0 1 10
178 ± 13 127 ± 8 115 ± 15
167 ± 15 168 ± 10 123 ± 12
165 ± 12 156 ± 11 146 ± 9
122 ± 17 107 ± 6 117 ± 8
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of SiC additions on microstructure, mechanical properties and thermal shock behaviour of alumina–mullite–zirconia composites H. Majidian ∗ , T. Ebadzadeh, E. Salahi Ceramic Division, Materials and Energy Research Centre, P.O. Box 14155-4777, Alborz, Iran
a r t i c l e
i n f o
Article history: Received 27 April 2011 Received in revised form 12 September 2011 Accepted 6 October 2011 Available online 18 October 2011 Keywords: Slip casting Composites Grain size Mechanical properties Thermal shock resistance
a b s t r a c t This paper describes the effect of additions of silicon carbide (SiC) particles on density, microstructure, mechanical properties and thermal shock behaviour of slip-cast alumina–mullite–zirconia composites. Alumina, zircon and SiC powders were used to prepare highly concentrated (51 vol.% loading) and stable aqueous suspensions by using Dolapix dispersant (0.5 wt.%). The sintering process was carried out in two stages, first at 1600 ◦ C and then 1500 ◦ C, with 2 and 5 h holding, respectively. Densities and thermal shock properties as well as strength, toughness and hardnessof the prepared composites were evaluated. Twostage sintering process of alumina–mullite–zirconia–SiC (AMZS) composites led to a remarkable grain size reduction and an increase of density. The maximum value of fracture toughness and minimum value of grain size were obtained by using 20 vol.% silicon carbide. Results showed that the addition of SiC particles (20 vol.%) improved the thermal shock resistance (T = 1000 ◦ C) of alumina–mullite–zirconia composites even after 10 heating cycles. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Improvement of high temperature mechanical behaviour is an increasing interest in ceramic materials for structural applications [1]. Alumina–mullite–zirconia (AMZ) composites are an important category of refractory materials and widely used in forehearth, glass melting furnaces as plungers, tubes and orifice rings [2]. AMZ refractory materials show a high strength and fracture toughness at room temperature [3], but these properties decrease significantly at high temperatures because of the formation of a glassy phase in the grain boundaries. AMZ composites used in the glass industry are subject to thermal shock stresses during installation. Aksel [3] reported that the strength of AMZ composites decreased gradually as the quench temperature increased and showed approximately 50% loss of strength after the 1st cycle, whereas the 40 subsequent cycles did not significantly alter the retained strength. Improving the performance of AMZ composites such as thermal shock resistance is a current expectation [4]. Densification and mechanical properties of alumina and its composites have been widely studied [5,6]. The toughness values of 5.5–7 MPa m1/2 have been reported for alumina–10 vol.% ZrO2 hot-pressed composite [5]. Poowancum [6] reported that the highest fracture toughness (7.3 MPa m1/2 ) of AMZ composites was obtained with CeO2 additive due to the
∗ Corresponding author. Tel.: +98 912 5177988; fax: +98 21 8877335. E-mail addresses: [email protected] (H. Majidian), [email protected] (T. Ebadzadeh), [email protected] (E. Salahi). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.10.027
effect of crack deflection and grain bridging mechanism. Dong et al. [7] reported that the highest fracture toughness was found to be 7.6 MPa m1/2 in alumina composite containing 5 wt.% SiC particles. Aksel [1] showed that the crack length of AMZ composite was much smaller than the calculated critical value. This is because the crack tip blunting takes place as a result of a volume increase during sintering by means of absorbing energy from the crack, thereby preventing the crack length from extending to the critical crack length, which can be associated with the improvement in KIC (1.6 MPa m1/2 , for slip cast AMZ composite). Therefore, more fracture energy was required in AMZ refractory materials to connect the cracks for propagation. Mazzei [8] suggested that the zirconia tetragonal-monoclinic phase transformation occurred during the sintering cooling, producing microcracks in the matrix that may be responsible for the crack branching. Jang et al. [9] attained the fracture toughness of 2.25 up to 5.25 MPa m1/2 for AMZ composite by increasing zirconia content, and suggested that the tetragonal to monoclinic transformation toughening potentially contributes to the toughening of AMZ composites. As the total content of zirconia increases, the toughening by the microcrack nucleation becomes increasingly important. Mechanical and thermal shock properties of refractory materials can be enhanced by the addition of silicon carbide (SiC). SiC is a ceramic material for many engineering applications because of its high strength and hardness, stability at high temperatures, resistance to thermal shock and abrasion [10–12]. Because of the abovementioned reasons, the addition of SiC particles to AMZ
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refractories has been suggested to improve the thermal shock resistance [13]. Silicon carbide ceramics are usually difficult to densify; the covalent nature of Si–C bond implies low self-diffusion coefficients which limits the densification process [14]. The sintering of SiC can be performed by adding sintering aids at high temperatures after holding for a long time [15]. On the other hand, obtaining a ceramic material with a high relative density and homogeneous microstructure consisting of small grains is one of the main aims in sintering process [16,17]. Thermal shock behaviour of alumina and AMZ composites has been studied earlier [18,19]. Ainsworth and Moore [20] studied the thermal shock resistance of alumina (99.5%) cylinders by quenching the specimens from selected temperatures between 150 ◦ C and 500 ◦ C into ice water. They observed that a temperature drop of 175 ◦ C decreases the strength appreciably. A gradual decrease in average retained strength of alumina was also observed up to the critical temperature difference of 270 ◦ C [21]. On the other hand, Aksel [1] showed there was a marked decline in strength values of AMZ composite between 300 ◦ C and 600 ◦ C, because of the nucleation of microcracks and their propagation. However, strength decreased more slowly, presumably because the number of cracks increased only slightly, as the further quench temperature rose. In the present work, slip casting was used to prepare samples because the homogeneity of particle packing in the green bodies is better controlled when colloidal shaping techniques are used for ceramic compacts. In this case, the sintering behaviour and final properties of materials will be improved [22–24]. In order to produce high-performance ceramics a well dispersed and highly concentrated slurry of fine powders should be prepared [25]. Since the final properties of a material depend on the microstructure formed during sintering, the sintering of prepared composites was carried out using two-stage sintering process. The improvement in the mechanical properties and thermal shock behaviour of AMZ composites have been investigated with the addition of SiC particles. 2. Experimental procedures 2.1. Materials The ␣-alumina (MR70, Martinswerk, Germany, >99.8%), zircon (Zircosil, Johnson-Matthey, Italy, >98.5%) and SiC (Mirali Company, Iran, >98%) powders with mean particle sizes of 0.6, 1.4 and 3 m, respectively, were used as the starting materials. Dolapix CE-64 (Zschimmer & Schwarz, 0.5 wt.% based on solid weight) was used to disperse SiC particles (10, 20 and 30 vol.%) in alumina–zircon suspensions containing 51 vol.% of solid according to Ref. [8]. 2.2. Composite preparation The mass ratio of alumina to zircon in all mixtures was 85/15. Each slip was mixed by using a planetary mill for 20 min and consolidated through slip casting method using plaster mould. The cast parts were healed at room temperature for 24 h, dried at 110 ◦ C in an oven and finally sintered at 1600 ◦ C and then at 1500 ◦ C after holding for 2 and 5 h, respectively. Samples sintered at 1600 ◦ C and 1650 ◦ C for 2 h were also prepared for comparison. 2.3. Physical and mechanical properties Density and apparent porosity of sintered samples were determined using the standard water adsorption method. Crystalline phases in fired samples were characterized by XRD (Siemens, D500 system) using CuK␣ radiation working with 30 kV accelerating voltage. The microstructure of sintered samples was observed by
Table 1 Relative density and apparent porosity of AMZS composites. SiC (vol.%)
Sample code
Relative density (%)
0a 0b 0c 10a 10b 10c 20a 20b 20c 30a 30b 30c
AMZ0S–C AMZ0S–H AMZ0S AMZ10S–C AMZ10S–H AMZ10S AMZ20S–C AMZ20S–H AMZ20S AMZ30S–C AMZ30S–H AMZ30S
96.83 97.37 98.25 92.7 92.95 93.50 88.12 89.34 89.44 83.80 81.96 85.82
a b c
± ± ± ± ± ± ± ± ± ± ± ±
0.3 0.3 0.25 0.2 0.25 0.1 0.1 0.3 0.4 0.2 0.4 0.3
Apparent porosity (%) 0.49 0.62 0.11 6.91 5.9 5.76 11.33 11.47 10.28 17.76 18.94 14.45
± ± ± ± ± ± ± ± ± ± ± ±
0.1 0.1 0.05 0.5 0. 5 0.5 0.4 0.2 0.4 0.3 0.5 0.5
Sintered at 1600 ◦ C/2 h. Sintered at 1650 ◦ C/2 h. Sintered at 1600 ◦ C/2 h followed by sintering at 1500 ◦ C/5 h.
SEM (VEGA II, TESCAN) on polished surfaces which were thermally etched at 1420 ◦ C for 20 min. Thermal shock testing was carried out by air quenching bars from maximum temperature, at 300 ◦ C intervals. In each thermal shock test, a specimen (5 mm × 6 mm × 35 mm) was heated for 15 min in the furnace to assure temperature uniformity and then cooled in air. After cooling, the retained strength values of samples were measured by a universal testing machine (Instron 1196, England) using three-point bending fixture with a span of 15 mm and a crosshead of 0.5 mm/min. At least seven specimens were tested to obtain a mean value for each quench temperature. Fracture toughness (KIC ) was then measured using the single edge notched beam (SENB) technique [26] with a notch cut of 0.3 mm width and 2 mm depth, producing a notch-to-depth ratio of 0.5. KIC was calculated from the maximum load using equation reported in ASTM standard 1421-01b. Vicker’s hardness (Hv ) was also measured by the diagonal impression generated by a Vicker’s indenter at a load of 5 kg for 15 s (ASTMC 1327-03). MEASUREMENT software was used to determine the grain size from the SEM micrographs for alumina–mullite–zirconia–SiC (AMZS) composites. 3. Results and discussion 3.1. Density and porosity Table 1 shows the relative density and apparent porosity of AMZS composites. As this table shows, the composites processed without SiC (AMZ0S) have higher density values than those of any other composites processed with SiC. Under the same sintering conditions, the porosity increased in composites processed with SiC which can be attributed to the higher refractoriness of SiC in relation to Al2 O3 . It has been reported that the added SiC apparently decreases the sinterability of Al2 O3 through the blocking grain boundary movement [27]. Lower relative density of alumina–SiC composites than that of monolithic Al2 O3 are mainly attributed to the incorporation of SiC particles, that blocked grain boundary movement and hindered the densification of Al2 O3 [7]. Since SiC particles were immobile and did not react with alumina during the sintering, the movement of the grain boundary is prohibited by SiC particles which delayed the densification of alumina [28]. Therefore, the density decreased after adding SiC particles. In fact, in bimodal packing systems, a second phase of nonsinterable particles considerably decreases the sintering rate by introducing shear stress at the hard particle/matrix interface [29]. In this case, matrix regions of compression are developed around interactive SiC. These regions increase density, produce a non-deformable network and constrain the shrinkage of the adjacent porous matrix. This constraint causes desintering and crack-like void formation, enhancing
H. Majidian et al. / Materials Science and Engineering A 530 (2011) 585–590 Table 2 Fracture toughness and hardness of AMZS composites.
5000
4000
Intensity (a.u.)
587
AMZ30S
3000 AMZ20S 2000
Sample code
Hardness (kgf/mm2 )
AMZ0S AMZ10S AMZ20S AMZ30S
1262 1107 687 643
± ± ± ±
84 60 15 24
Toughness (MPa m1/2 ) 7.29 7.70 8.13 7.86
± ± ± ±
2.0 1.2 1.0 1.1
AMZ10S
1000
AMZ0S
0 20
30
40
Two teta
50
60
70
Fig. 1. XRD pattern of AMZS composites (: alumina, : mullite, •: monoclinic zirconia, : tetragonal zirconia, and : SiC).
the porosity and decreasing the final density of AMZS composites [27]. The magnitude of this effect depends on the nonsinterable particles content. Also, the movement of pores along the grain boundaries may be limited by the presence of SiC particles during the final sintering stage [30]. It can be seen that in all the composites, the sintering temperature increased from 1600 ◦ C to 1650 ◦ C resulted in higher densities. The same trend was also observed after increasing sintering step while two sintering temperatures were used and led to an increase in density. For example, the relative density of AMZ10S composite was 92.7, 92.95 and 93.5% after sintering at (a) 1600 ◦ C/2 h, (b) 1650 ◦ C/2 h, and (c) 1600 ◦ C/2 h followed by 1500 ◦ C/5 h, respectively. As Table 1 further reveals, the apparent porosity of AMZS composites increased by addition of SiC particles. Since, the densification rate of SiC particles is lower than that of alumina, therefore, after sintering there would be exist a gap between alumina and SiC in the microstructure [29]. 3.2. X-ray diffraction pattern Fig. 1 shows the X-ray diffraction pattern of AMZS composites. The resulting phase composition consists of corundum, mullite, zirconia and SiC. No peaks of zircon and crystalline form of silica (cristobalite and tridymite) could be identified in Fig. 1 which means that zircon completely dissociated into ZrO2 and SiO2 and subsequently mullite was formed by reaction of SiO2 with alumina particles. In Fig. 1, it seems that as the SiC content in composites increased, the intensity of mullite peaks was found to decrease. Since the ratio of alumina to zircon should be kept constant in all composites, the amount of alumina and zircon decreased by the addition of SiC, therefore, the formation of mullite was suppressed. Quantitative analysis of AMZ20S samples by internal standard method showed that the amount of SiC (22 vol.%) was a little higher than its theoretical value (20 vol.%). Since the sintered samples were packed into abed of graphite powder, it may be concluded that some silica formed from decomposition of zircon would reacted with graphite and SiC particles were formed during sintering. From the results mentioned above it can be concluded that the amount of silica and thus mullite was decreased (Fig. 1).
expansion mismatch between Al2 O3 and SiC particles. Phase transformation of ZrO2 dispersion in the Al2 O3 matrix from tetragonal (t) to monoclinic (m) has been found to be effective for improving the fracture toughness. In these cases, the strong strain field and some microcracks were easily formed during the sintering process, due to the difference in the coefficient of thermal expansion [31]. These strain fields and microcracks led to intergranular fracture with crack deflection and thus reduce the crack propagation energy [32]. In addition when cracks propagate transgranularly in m-ZrO2 the driving force of crack propagation is reduced due to the plastic deformation of m-ZrO2 [33,34]. Mazzei et al. [35] have been reported that zirconia inclusions in AMZ composites are responsible for the crack deflection mechanism. Moreover, the SiC particles can display reinforcement components since their thermal expansion coefficient difference with alumina can built up compressive stresses in the matrix and also promote decreases in the average grain size of the alumina phase (comparing AMZ0S with AMZ10S, a reduction in the average grain size was observed). The major improvement in KIC in the AMZ refractory material is also due to the presence of microcracking and crack-closure compressive strains as a result of transformation toughening. This is because of thermal contraction mismatch between the phases and volume expansion associated with zirconia transformation and mullite formation. Thermal expansion mismatch leads to large tensile hoop stresses and crack development between alumina, mullite and zirconia grains during cooling from the fabrication temperature of 1600 ◦ C due to the significant difference on the coefficients of thermal expansion, which are 8.2 × 10−6 K−1 for alumina, 5.1 × 10−6 K−1 for mullite and 7.6 × 10−6 K−1 for zirconia [36,37]. The volume change can also result in the development of microcracking in the matrix. Under certain conditions, particularly for thermal shock resistance, numerous microcracks are desirable. Microcracking is also a predominant toughening mechanism in AMZ refractory materials [7]. The fracture energy was calculated from the fracture toughness and Young’s modulus to be 103 and 122 J/m2 for the AMZ0S and AMZ20S, respectively. The Young’s modulus of AMZ0S and AMZ20S composites during the flexural strength was 253 and 224 GPa. The increased fracture energy of AMZ composites with SiC may be associated with the formation of fine grains. Decreased grain size is accompanied by the increase of the grain boundary area. The enhanced branch of main crack along the increased grain boundary area may contribute to the increase of the fracture energy [38]. The fracture energy of AMZ20S was 20% as high as that of AMZ0S. The average Vickers hardness of all AMZS composites is lower than that of AMZ0S composite. The Vickers hardness is known to be very sensitive to the density [39]. The decrease in hardness values with the increasing of SiC particles can be related to the increase of porosity.
3.3. Mechanical properties 3.4. Microstructure The variations in mechanical properties by using different amounts of SiC in AMZS composites are shown in Table 2. It can be observed that the fracture toughness values increased with increasing SiC content from 0 to 20 vol.%. Further increase in SiC content up to 30 vol.% resulted in decreasing of fracture toughness. The gradual increase of fracture toughness of AMZS composites may be due to the presence of internal residual stresses resulting from thermal
Fig. 2 depicts the microstructure of AMZ0S and AMZ20S composites sintered by one- and two-stage process. Most of the SiC particles are well dispersed in the composite, which are located not only in the grain boundaries or triple-grain junctions but also in the grain interiors. The microstructure of AMZ0S composite (Fig. 2a–c) reveals larger grains with a broad grain size distribution compared
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Fig. 2. Microstructure of (a) AMZ0S–C, (b) AMZ0S–H, (c) AMZ0S, (d) AMZ20S–C, (e) AMZ20S–H and (f) AMZ20S composites.
to those of AMZ20S composite (Fig. 2d–f). The effect of SiC particles on reducing the grain size of composites is that SiC particles pin alumina grain boundaries and can inhibit grain growth of alumina particles [40]. It should also be noted that the average size of alumina grains significantly decreases by using two-stage sintering (Fig. 2c and f) as compared to one-stage sintering (Fig. 2b and e). This result showed that SiC particles and two-stage sintering have a certain influence on diffusion mechanism and grain growth of alumina in the AMZS system.
process. An obvious grain growth was observed in AMZ0S–H composites. The average grain sizes of 2.43 and 2.93 m were measured for AMZ0S–C and AMZ0S–H composites, respectively. As Table 3 further reveals, sample AMZ20S containing 20 vol.% SiC particles sintered by two-stage process has the smallest average grain size. SiC particles in an alumina matrix have the ability to refine the microstructure and improve the mechanical properties of composites by restricting grain growth of the alumina matrix; because SiC particles decrease grain boundary mobility and diffusivity as well as
3.5. Grain size measurements
Table 3 Average grain sizes of AMZS composites.
It has been well known that grain size has a strong effect on the mechanical properties of ceramic materials, such as hardness, strength, wear resistance and toughness [41,42]. On the other hand, the final stage of sintering is always accompanied by rapid grain growth, resulting in structure coarsening [43]. Table 3 shows that the AMZS composites sintered by two-stage process have smaller grain size compared to those sintered by one-stage
Composite code AMZ0S–C AMZ0S–H AMZ0S AMZ20S–C AMZ20S–H AMZ20S
Grain size (m) 2.43 2.93 1.98 1.42 1.51 1.35
± ± ± ± ± ±
0.1 0.2 0.1 0.1 0.2 0.05
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the rate of densification [40,44]. Using data presented in Table 3, it is concluded that significant grain refinement has been occurred in the AMZ20S composite sintered by two-stage process. As reported [40] zirconia inclusions are quite mobile, so that they migrate with grain boundaries during sintering. Thus, it can be implied that the Al2 O3 or zirconia inclusions also tend to coalesce, so that the grain size became too larger in AMZ0S composites. On the contrary, SiC particles are immobile, so that grain growth of Al2 O3 as well as zirconia would be interrupted by SiC located at grain boundary in AMZS composites. In fact, the presence of SiC second phase inclusions at grain boundaries hinders grain growth by grain boundary pinning [45].
contributed to lowering of thermal shock resistance. The strength of AMZS samples at quench temperature of 1200 ◦ C was higher compared to AMZ0S sample (Fig. 3). AMZS composites appeared to be significantly resistant to crack propagation or extension as a result of the existence of microcracks and pores in these composites. These microcracks and pores led to intergranular fracture with crack deflection and thus reduce the crack propagation energy [31]. It should be noted that decline in strength was not significant for AMZS composites until the quench temperature of 1100 ◦ C; this implies that the thermal shock resistance increased due to the effects of microcracking. Thermal expansion of alumina is much higher than that of SiC; then there are tensile stresses in the surrounding matrix and compression in the second phase particles. These compressive stresses can be passed to the grain boundaries, resulting in toughening and increasing of thermal shock resistance [46]. The possible explanations for the increase in thermal shock resistance are that the thermal expansion is lower and the thermal conductivity is higher for the AMZS composites than that of AMZ0S. Since, it has been established that the addition of SiC to an alumina matrix increases the thermal conductivity of alumina [47]. For AMZ0S and AMZ20S composites, the thermal expansion coefficients were measured to be 7.79 and 6.87 × 10−6 K−1 , respectively. The increase of thermal conductivity may have some beneficial effect on the thermal shock behaviour. The improvement in the thermal shock resistance of AMZS composites is believed to be due to the higher fracture toughness as compared to AMZ. Other researchers have reported similar improvements in the thermal shock resistance as fracture toughness increased [47,48].
3.6. Thermal shock behaviour
4. Conclusion
Thermal shock resistance of AMZS composites is shown after one and 10 cycles in Fig. 3 and Table 4, respectively. As Fig. 3 reveals, the retained strength of AMZ0S composite decreased sharply at the thermal shock temperature difference of 300 ◦ C; and after that it decreased gradually up to temperature difference of 1200 ◦ C. The marked decline in strength values of sample AMZ0S between 0 and 300 ◦ C may be attributed to the nucleation of microcracks and their propagation. However, the slow decrease of strength after 300 ◦ C is presumably due to the slight increase of the number of cracks. As shown in Fig. 3, there is no remarkable change in the fracture strength of AMZS composites until a temperature difference of 1100 ◦ C is reached. As Fig. 3 further reveals, a gradual decrease in average retained strength is observed in the temperature difference range 1100–1200 ◦ C. The flexural strength of samples decreased significantly by increasing thermal shock cycle from 1 (Fig. 3) to 10 (Table 4). Under the same thermal shock cycles, the strength of AMZ20S and AMZ30S composites were decreased relatively more slowly (12% and 6%, respectively) after 10 cycles, whereas the flexural strength of AMZ0S composite was decreased relatively sharply (38%) just after one cycle. In spite of the low strength of AMZ30S composite, it showed relatively high thermal shock resistance (Table 4). The influence of porosity on mechanical properties and thermal shock behaviour of tested samples was also considered. The high porosity of the AMZS composites could have contributed to higher thermal shock resistance. In addition, AMZ0S composite associated with the risk of abnormal grain growth and might have
Thermal shock resistance of alumina–mullite–zirconia–silicon carbide (AMZS) composites has been investigated. Two-stage heat treatment improved density and reduced porosity of composites without and with SiC additions. Thermal shock testing results showed that the retained strength of composites containing 20 vol.% SiC particles after temperature difference of 1100 ◦ C was still high (154 and 145 MPa after one and 10 cycles, respectively). Although the composite containing 30 vol.% SiC had a relatively low strength (122 MPa), the residual strength of this composite after shock test was still high (117 MPa after 10 cycles). Fracture toughness increased from 7.29 to 8.13 MPa m1/2 in composites with 0 and 20 vol.% SiC, respectively. The fracture toughness value of 8.13 MPa m1/2 was obtained for AMZ20S composite which is higher than that reported for similar composites. The grain growth of alumina was retarded by the SiC because of a grain boundary pinning effect. AMZS composites were mainly composed of alumina, mullite, zirconia and SiC and exhibited high fracture toughness and low hardness.
Strength (MPa)
190 170 150 130 110 90 70 0
300
600
900
1200
ΔT ( oC) AMZ0S
AMZ10S
AMZ20S
AMZ30S
Fig. 3. Strength of AMZS composites after 1 cycle thermal shock test.
Table 4 Strength of AMZS composites after 10 cycles thermal shock heating at 1000 ◦ C. Number of cycles
AMZ0S
AMZ10S
AMZ20S
AMZ30S
0 1 10
178 ± 13 127 ± 8 115 ± 15
167 ± 15 168 ± 10 123 ± 12
165 ± 12 156 ± 11 146 ± 9
122 ± 17 107 ± 6 117 ± 8
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