An investigation on microstructural and mechanical

Anti-Corrosion Methods and Materials An investigation on microstructural and mechanical properties of porous zirconia-alumina nanocomposite prepared by solid state sintering method Ebrahim Yousefi, Morteza Adineh, Mohammad Bagher Askari,

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To cite this document: Ebrahim Yousefi, Morteza Adineh, Mohammad Bagher Askari, (2018) "An investigation on microstructural and mechanical properties of porous zirconia-alumina nanocomposite prepared by solid state sintering method", Anti-Corrosion Methods and Materials, https://doi.org/10.1108/ACMM-03-2017-1773 Permanent link to this document: https://doi.org/10.1108/ACMM-03-2017-1773 Downloaded on: 12 February 2018, At: 06:53 (PT) References: this document contains references to 37 other documents. To copy this document: [email protected] Access to this document was granted through an Emerald subscription provided by Token:Eprints:A5PMZTBGTQQ44VM7BJQ5:

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An investigation on microstructural and mechanical properties of porous zirconia-alumina nanocomposite prepared by solid state sintering method Ebrahim Yousefi Department of Nano technology, Mineral Industries Research Center (MIRC), Shahid Bahonar University of Kerman, Kerman, Iran

Morteza Adineh Department of Metallurgy and Materials Science, School of Engineering, Shahid Bahonar University of Kerman, Kerman, Iran, and

Mohammad Bagher Askari Downloaded by Doctor Mohammad bagher Askari At 06:53 12 February 2018 (PT)

Department of Physics, University of Guilan, Rasht, Iran and Department of Physics, Payame Noor University, Tehran, Iran Abstract Purpose – The purpose of this paper is to fabricate zirconia-nano alumina porous nanocomposites with different amount of alumina (0-30 Wt.%). Specimens were prepared by solid state sintering method at different temperature (1,400-1,700°C). Design/methodology/approach – Effects of processing temperature and amount of alumina on microstructure, distribution of nanoparticles, flexural and compressive strengths, micro-hardness and densification were investigated. Findings – Results indicated that interpenetration of particles and their contacts increased by increasing sintering temperature. As a consequence of better particles contacts and microstructure coarsening, the porosity decreased. As alumina nanoparticles content increased, the amount of porosity decreased conversely and distribution of pores become more uniform. Simultaneous enhancement of temperature and alumina nanoparticles content caused an improvement of flexural and compressive strengths because of an improvement of sintering process resulted from porosity reduction. Increase in hardness and density were observed as porosity values diminished and alumina nanoparticles were distributed well at micro zirconia grain boundaries as a result of increasing the process temperature. Originality/value – This article contains original research. Keywords Alumina, porous nanocomposites, solid state sintering, zirconia Paper type Research paper

1. Introduction Today, ceramics have attained wide applications in fabricating industrial parts because of special properties including high hardness, low density, excellent strength and corrosion resistance in chemical environment (Perrut, 2000; Scheffler and Paolo, 2006; Youssef et al., 2015; Chen et al., 2007; Kim et al., 2007; Phillips and Zabinski, 2004). Zirconia becomes a well-known ceramic with properties containing high melting temperature, good corrosion resistance, high strength and fracture toughness, especially in applications such as structural materials, cutting tools and etc. (Manicone et al., 2007; Chamberlain et al., 2004; Sun et al., 2009; Beuer et al., 2009; Guazzato et al., 2004; Kelly and Denry, 2008; Kumar et al., 2003). Garvie and Nicholson indicated that zirconia could play an important role in ceramics toughness via tetragonal to

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monoclinic transformation occurred in zirconia during cooling. Zirconia also could be used as a bioceramic in medical and dental applications because of high biocompatibility and beautiful appearance after polishing treatment (Garvie and Nicholson, 1972; Noheda et al., 2000; Choi et al., 2011; Vagkopoulou et al., 2009). However, monoclinic zirconia does not apply much in structural applications lonely as a result of micro crack creation in microstructure resulted by tetragonal to monoclinic phase transformation (Pontin et al., 2002). Zirconia base nanocomposite fabrication has been introduced as a most reliable solution for mechanical properties development (Manicone et al., 2007). Studies have shown that with alumina addition to zirconia and mixing their properties, excellent ceramics composites with high applicability in industry could be achievable. Hardness, strength, abrasion resistance and thermal conductions of alumina are more considerable than zirconia (Berghaus et al., 2008; Opeka et al., 1999; Cao et al., 2006). Alumina is a bioceramic as well and so zirconia-alumina composites with better properties can be used in medical implants and tooth base (Li and Hastings, 2016; Received 10 March 2017 Revised 16 August 2017 Accepted 22 August 2017

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Ebrahim Yousefi, Morteza Adineh and Mohammad Bagher Askari

Ahn et al., 2001). Two groups of ceramic composites have been made according to binary ZrO2 Al2 O3 systems. The first group is zirconia toughened alumina composites that lots of studies have been fulfilled to investigate their properties and microstructures. The second one is alumina toughened zirconia composites that have been less discussed, and little information is available about them. The grain size is another parameter which has influenced in mechanical, physical, electrical and magnetic improvement of ceramics composites (Barnett, 2008; Yu and Ang, 2002). Use of fine grain size that traditionally used in ceramic and metallic structures has been an impressive factor in mechanical properties development (Richerson, 2005). In ceramics materials, the grain size has been effected by factors such as primary powders size, compressive loading used by pressing process and also temperature, time and environment of sintering process (Wang et al., 2004). Therefore, in alumina-toughened zirconia ceramics, the best condition achieved when alumina particles dispersed between zirconia matrix in nano sizes monotonically followed by applying pressure, temperature and time of sintering in such a way that best physical and mechanical property such as favorable density and microstructure attained. The presence of particles in nano sizes as a reinforced phase can increase grain boundaries and develop other effective factors on desirable properties. Nano alumina usage in zirconia matrix can behave as a barrier to cracks present in matrix and promote toughness and flexural strength (De Aza et al., 2002). Studies in this scope have been focused on condensed alumina-zirconia composites including development of functionally graded composites from them, using microwave sintering as a production method, producing hybrid composites and doped zirconia to attain better properties whether in ZTA or in ATZ composites. However, porous micro zirconia-nano alumina nanocomposites gained vast applications in catalystic nano-membranes, nanocatalysts, nanofiltration and refractory insulators because of unique properties including chemical and thermal stability and appropriate strength (Del Colle et al., 2007; Wu et al., 2005; Shimizu et al., 2013). The purpose of this study is the determination of appropriate conditions to producing porous micro zirconia-nano alumina nanocomposite ceramics using solid state sintering method and investigate the effect of sintering temperature and alumina nanoparticles on physical and mechanical properties.

2. Experimental procedures In the present study, micro zirconia particles with density of 5.75 g/cm3 , average particle size of 3 m m and purity of 99 per cent were used. The above characterization for alumina nanoparticles were 3.96 g/cm3 , 20 nm and 99 per cent. Both particles were prepared with spherical morphology from US Research Nanomaterials Inc. To improve the sintering behavior of particles at high temperatures, inconsiderable amount of SiO2 were used. Trivial amount of polyvinyl alcohol was added to 150 cc double distilled water to improve the formability and coherence of alumina nanoparticles in micro zirconia matrix. Precise composition of zirconia and alumina particles have been reported in Tables I and II, respectively. To investigate the effect of process temperature and various alumina amounts as a reinforcement phase, four different

Table I The monoclinic zirconia powder composition (Wt.%) ZRO2

AL2O3

SIO2

FE2O3

TIO2

CL

99%

0.026%

0.005%

0.06%

0.03%

0.05%

Table II The alpha alumina nanoparticles composition (Wt.%) AL2O3

NA2O

SIO2

FE2O3

TIO2

99%

0.65%

0.08%

0.08%

0.014%

mixtures were chosen. The amounts of zirconia and alumina of each mixture as well as the abbreviated codes are shown in Table III. After mixing the above composition, 3 Wt. per cent of SiO2 and 3 cc of adhesive were added to ceramics mixtures. The adhesive were made by adding 3-5gr polyvinyl alcohol (proportional to alumina nanoparticles amount) to 150 cc double distilled water. The ceramics mixtures were stirred ultimately using mechanical stirrer. Regarding that the alumina nanoparticles possess high surface energy and, thus, intense tendency to agglomeration, dry ball milling method used to prevent aggregation. For this purpose, each ceramic mixture was ball milled separately by planetary ball mill under rotational speed of 200 rpm for 50 minutes. Specimens’ fabrications were done by means of a steel die which had interior hole with 6  2  1 cm dimensions. The placed powder inside the hole pressed under 300 MPa uniaxial pressures. The green samples were sintered in various temperatures of 1,400, 1,500, 1,600 and 1,700°C for 1 hour, and the heating rate was about 8°C/min. The density of nanocomposite samples were measured by weights and dimensions measuring of samples followed by dividing mass to volume before and after sintering. The Archimedes method according to ASTM C373-88 was used for this purpose. Microstructural analysis and alumina nanoparticles distribution in nanocomposite samples investigated via scanning electron microscope (Philips XL-30-FEG) and using thin layer of gold (10nm) coated on polished samples by sputter deposition. Compressive and flexural strength of the sintered nanocomposites were determined by 4208 Instron universal testing machine at ambient temperature according to ASTM-C1424 and ASTM-C1161 with the chosen loading rate of 0.5 mm/min. Three point load bending test was used to obtain flexural strength of the samples prepared in 3  4  45 mm dimension. To investigate the effect of alumina nanoparticles on interface area hardness, Vickers microhardness testing with the aid of Duramim–Struers machine under loading of 1 Kg was performed. It is noticeable to declare that for each sample with specified weight per cent, and at different Table III The amounts of monoclinic zirconia, alumina and abbreviated code of each prepared mixture ZRO2 (Wt.%) 100 90 80 70

AL2O3 (Wt.%)

Mixture codes

0 10 20 30

Z100 Z90A10 Z80A20 Z70A30

Porous zirconia-alumina nanocomposite

Anti-Corrosion Methods and Materials

Ebrahim Yousefi, Morteza Adineh and Mohammad Bagher Askari

temperatures, every mechanical test contains compressive and flexural strength and microhardness were reported as an average value for three samples and the testing conditions were the same.

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3. Results and discussions 3.1 Study of the samples microstructures and reinforcement particles distribution Figure 1(a) shows the scanning electron microscopy image of Z70A30 micro zirconia-nano alumina sample mixed by mechanical stirrer and Figure 1(b) is related to the same sample mixed by planetary ball mill. More aggregation and agglomeration is visible in ceramic mixture in Figure 2(a) because of more alumina nanoparticles used for this sample. However, when the same sample prepared by means of a ball milling method, the agglomerates formation diminished and further monotonic distribution of particles were resulted [Figure 1(b)]. The existence of agglomerates could lead to nonuniformed densification. Moreover, Ball milling application Figure 1 SEM micrographs of zirconia/alumina (Z70A30) nanocomposite

Figure 2 Effect of sintering temperature on Z70A30 sample microstructure

caused improvement in alumina nanoparticles dispensation amongst the micro zirconia matrix. Polished fracture surface of samples were studied by SEM micrographs to study the temperature and nano alumina particles effects on particle size and their distribution as well as porosity amounts. Figure 2 shows the sintering process effect on Z70A30 sample microstructure at various temperatures. As shown in Figure 2, porosities exist in microstructure of all samples sintered at different temperatures. In temperature range of 1,400-1,500°C, porosities are completely observable on surface and inside the sample and their shapes and distributions are indistinctive and non-uniform. At 1,400°C, as the sintering temperature was not enough, the connectivity of particles was low and grains were fine. Therefore, there were large amounts of porosity of about 41.2 per cent in Z70A30. However, at 1,700°C, as a consequence of increasing in diffusion rate and interpenetration of particles and, thus, betterment in particles connectivity, coarsening occurred and porosity amounts decreased to about 23.6 per cent. Moreover, increasing in sintering process temperature caused porosities distributed monotonically in nanocomposite microstructures and alumina nanoparticles could distribute well at zirconia grain boundaries which is in agreement with other report (Srdic et al., 2000; Vasylkiv et al., 2003; Li and Ye, 2006; Biamino et al., 2006; Zhou et al., 2005). Effect of various amounts of alumina nanoparticles content on ceramics particles distribution and connectivity of nanocomposite samples at constant temperature of 1,600 and 1,700°C are shown in Figures 3 and 4, respectively. Sample Z70A30 shows less amount of porosity compared to Z90A10 and Z80A20 as sintering capability enhanced with increasing of alumina nanoparticles. Besides, the porosity amounts decreased and their distributions became homogenized. Furthermore, alumina nanoparticles could distribute well between zirconia micro particles and caused more condensed microstructure since their size was in the range of nano. However, sample Z100 sintered at 1,600°C Figure 3 Effect of various amounts of alumina on samples microstructure at constant temperature of 1,600°C

Figure 4 Effect of various amounts of alumina on samples microstructure at constant temperature of 1,700°C

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Ebrahim Yousefi, Morteza Adineh and Mohammad Bagher Askari

showed high amount of porosity of about 31.5 per cent as the result of less diffusion of monoclinic zirconia. Basically, fine sized particles can aid sintering process because of short diffusion distances and high amounts of specific surface area. To determine the chemical compositions of prepared nanocomposite samples and investigate their microstructures, Energy-Dispersive X-ray Spectroscopy (EDS) was performed. The chemical composition analysis of Z80A20 and Z70A30 samples are indicated in Figures 5 and 6, respectively. Because of the EDS analysis for sample Z70A30 and presented peaks at 1.45 and 2.3keV, presence of aluminum and zirconium in nanocomposite structures were verified. The above elements peaks for sample Z80A20 are shown in 1.5 and 2.15 keV regions. Figure 5 shows EDX analysis of Z80A20 ceramic nanocomposite and Figure 6 shows EDX analysis of Z70A30 ceramic nanocomposite. EDX mapping was used to obtain elemental distribution in the surface of the Z70A30 nanocomposite (Figure 7). It can be seen that alumina nanoparticles density and distribution are Figure 5 EDX analysis of Z80A20 ceramic nanocomposite

Figure 6 EDX analysis of Z70A30 ceramic nanocomposite

homogeneous all over the microstructure. This suggests that solid state sintering was a successful process in monotonous distribution of reinforced nanoparticles (better interconnection of zirconia and alumina nanoparticles and diminishing the alumina nanoparticles agglomeration). It is crucial to be said that low aluminum image points in micrographs are because of minor weight per cent of alumina nanoparticles with respect to zirconia micro particles. 3.2 Characterization of density, porosity and mechanical properties The results of density and porosity amounts obtained from nanocomposite samples pressed with various alumina nanoparticles content are shown in Table IV. Average density and porosity of three samples reported as a density and porosity of each composition. As it can be seen from Table IV, density reduced with increasing alumina content. This could be justified by low theoretical density of alumina. Increasing in alumina nanoparticles content caused increasing in porosity of green samples because of none-uniform dispensation of alumina and zirconia particles and so their none-uniform density at different parts of the samples resulted from low amount of pressing. Therefore, for fabricating nanocomposite samples with low porosity, high density and condensed structure, higher compressive loads are needed; however, as the main goal of this research is production of porous nanocomposite, this amount of pressing is sufficient. It is said that pressing step can play an important role in producing porous nanocomposites . As sintering temperature increased, the density of samples increased too. Because of low sintering capability of monoclinic zirconia, its density indicated the least amount of variation amongst other samples with increasing temperature. Figure 8 shows the effect of sintering temperature on density of sintered nanocomposite samples. Nanocomposites’ density contents traverse rapid ascending trend as temperature rises. This caused by decreasing porosity resulted from betterment in sintering capability and surface diffusion of particles at elevated temperatures with respect to monoclinic zirconia. At low sintering temperatures, density increased in almost steady trend (initial stage of sintering), but this trend became linear as sintering process developed through more decreasing in porosity and approaching the central of particles (intermediate stage of sintering). Increasing in sintering temperature resulted to porosity decreasing because of the necking formation between particles, grain coarsening and more condensation, independent of chemical composition. Porosity amount of nanocomposite Z70A30 was much more than other nanocomposite samples at low temperatures due to high primary porosity of green samples. At high temperatures, porosity of Z70A30 sample reached to lower amounts in comparison to other samples because of better sintering capability of alumina nanoparticles with respect to monoclinic zirconia. However, Z100 sample showed maximum amount of porosity at 1,700°C in comparison with other samples. This suggested that increasing in alumina nanoparticles percentage caused decreasing in porosity of sintered samples resulted from sintering improvement at high temperatures. On the other hand, finer alumina nanoparticles compared with zirconia micro particles aided sintering process via high specific

Porous zirconia-alumina nanocomposite

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Ebrahim Yousefi, Morteza Adineh and Mohammad Bagher Askari

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Figure 7 EDX mapping and elemental distribution from the surface of Z70A30 nanocomposite

Table IV Results of density and porosity amounts of nanocomposite samples before sintering Sample

Density (GR/CM3)

Porosity amount (%)

Z100 Z90A10 Z80A20 Z70A30

3.8 3.35 3.21 2.95

34.2 37.5 39.7 41.3

surface area and shorter diffusion distances. In fact, alumina nanoparticles dispersed between zirconia micro particles is the reason of compressing improvement outcome from sintering. At low temperatures, sintering process is in initial stage and connections between particles are low. Therefore, variation in porosity amount is not significant but from 1,600°C, porosity is reduced suddenly. This occurred by entering into intermediate stage of sintering, more lattice and grain boundary diffusion between particles and consequently more condensation in nanocomposites microstructures. Figure 9 shows the porosity-

Figure 8 Density-sintering temperature diagram of sintered nanocomposite samples

Porous zirconia-alumina nanocomposite

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Ebrahim Yousefi, Morteza Adineh and Mohammad Bagher Askari

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Figure 9 Porosity-sintering temperature diagram for sintered nanocomposite samples at various temperatures

sintering temperature diagram for sintered nanocomposite samples at various temperatures. Different amounts of contraction were observed in prepared samples after sintering. Figure 10 shows the effect of various sintering temperatures and alumina nanoparticles amounts on contraction percentage of sintered nanocomposite samples. Dimensional changes and contraction amounts increased in samples as sintering temperature and alumina nanoparticles amounts increased. This could be because of simultaneous effects of increasing in sintering capability and densification of nanocomposite samples. At 1,400°C almost no contraction was observed, but at 1,700°C, considerable amount of contraction has been shown. Contraction amount of Z70A30 sample reached to 10 volume percent. Maximum and minimum content of contraction amounts related to Z70A30 and Z100, respectively. Results indicated that increasing in contraction amounts occurred after 1,500°C as a consequence of

Figure 10 Contraction amount-sintering temperature diagram for sintered nanocomposite samples at various temperatures

condensing in microstructure related to sintering process advancing and better connectivity between alumina nanoparticles and zirconia micro particles (transition from initial stage of sintering to intermediate stage). Compression test for each nanocomposite sample indicated that compressive strength increased with increasing in sintering temperature (Figure 11). Because of simultaneous effect of temperature and alumina nanoparticles content, the compressive strength for Z70A30 sample increased with faster rate with respect to Z80A20, Z90A10 and Z100 samples. The above parameters caused decreasing in porosity and improvement in sintering process. The minimum amount for compressive strength obtained for Z100 sample because of low sintering capability of monoclinic zirconia ceramics and weak particles connection. Results of three point load bending test for a constant chemical composition showed rising in flexural strength as sintering temperature increased (Figure 12). This is related to diminish in porosity amount. Flexural strength of Z70A30 and Z80A20 nanocomposites are much more than that of Z100 sample showing development in sintering behavior with alumina nanoparticles addition. Vickers microhardness test was performed for all samples at various temperatures (Figure 13). The microhardness data versus sintering temperature diagram interpret that amount of pores diminished significantly and alumina nanoparticles could distribute well at micro zirconia grain boundaries; thus, microhardness and density of nanocomposite samples improved. The maximum amount of microhardness was 16.45 for Z70A30 at 1,700°C sintering temperature. High temperatures caused better pores elimination and structural continuity. From these results, we conclude that using fine particles of alumina could lead to more condensed and uniformed microstructure because of the greater specific surface area of these particles. Besides, alumina could hinder the grain growth of zirconia particles. Uniform distribution of particles and finegrained structure reduced the size of the existed cracks and defects as well as the crack population within the microstructure (Rittidech and Tunkasiri, 2009). Finally, nanocomposite with high mechanical properties could be obtained. Figure 11 Diagram of compression strength according to sintering temperature for different nanocomposite samples

Porous zirconia-alumina nanocomposite

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Ebrahim Yousefi, Morteza Adineh and Mohammad Bagher Askari

Figure 12 Diagram of flexural strength versus sintering temperature for various nanocomposite samples

4. Conclusion 





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Figure 13 Microhardness versus sintering temperature diagram for different samples

The effect of various properties including density (Figure 8), porosity (Figure 9), contraction amount (Figure 10), compressive strength (Figure 11), flexural strength (Figure 12) and microhardness (Figure 13) versus sintering temperatures were studied for all the samples (Z100, Z90A10, Z80A20 and Z70A30). However; the focus of the explanations was about Z70A30. This sample encompass two important features: 1 It contained sufficient amounts of pores with homogenized distribution within the microstructure to be considered as an appropriate porous nanocomposite for structural applications. 2 Z70A30 could provide the best mechanical properties because of the decreasing in porosity amounts obtained by using suitable percentage of alumina.

Pure monoclinic zirconia possess low compressive and flexural strength however, using special amount of zirconia and high sintering temperatures caused fabricating of micro zirconia-nano alumina nanocomposites with desirable strength. Simultaneous increasing of alumina nanoparticles and sintering process temperatures aided to diminish nanocomposites porosity via grain coarsening and more structural densification. Nanocomposite samples produced with high amounts of alumina nanoparticles at elevated temperatures possess denser microstructure with monotonous distribution of porosities. Produced nanocomposites could be used in nanocatalyst and nanofiltration applications because of having considerable amounts of pores and strength to weight ratio. They can also develop the monoclinic zirconia consumption in industry because of high sintering capability, compression and flexural strength and favorable microhardness.

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Ebrahim Yousefi, Morteza Adineh and Mohammad Bagher Askari

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Corresponding author Mohammad Bagher Askari can be contacted at: [email protected]

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