الفهرس 
INTRODUCTION AND OBJECT OF INVESTIGATIONOnly 14 pages are availabe for public view
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Abstract
Zirconia has recently attracted a significant attention in fabrication of various kinds of functional and structural ceramics that are widely used in high tech applications such as fuel cells, oxygen sensors, cutting blades, grinding media, knives, and ceramic bearing hip replacement. It is a polymorphic material that exists in three phases: monoclinic, tetragonal, and cubic allotropes. Upon cooling down from high-temperatures, the tetragonal-monoclinic martensitic transformation gives rise to a volume expansion of ~ 3-4%. Such large volume expansion causes large stresses, which could induce micro-cracking leading to mechanical failure. In this study commercial monoclinic zirconia was fully stabilized in the cubic form at room temperature through magnesium aluminate spinel (MA), magnesium aluminate spinel with excess magnesia (MR) and magnesium aluminate spinel with excess alumina (AR) addition to protect zirconia composite ceramics from failure.
Despite the good properties as well as compatibility of MA with zirconia ceramic systems, effect of MA addition on densification and stabilization of raw m-ZrO2 has not been well studied yet. In this context, this investigation was designed; firstly, to illustrate the effect of MA addition on the stabilization and densification properties of zirconia ceramics, and secondly, to develop stabilized and durable zirconia based composite ceramics. Besides, MA raw powders have been synthesized in this work from municipal industrial wastes, Mg and Al scraps, through a facile, inexpensive co-precipitation scheme. In addition, these studies not only help to avoid pollution by turning municipal wastes generated from industry into valuable spinel but also recommend utilization of these synthesized materials in stabilization and densification of m-ZrO2. The outcomes indicated that spinel addition has a promising effect on both densification and the stabilization behavior of m-ZrO2 where some designed batches gave rise to fully stabilized cubic zirconia with improved physico-mechanical properties. In this respect, this study offers huge potentials to develop highly stabilized cubic zirconia refractory ceramics for harsh-environment applications and makes it possible not only to overcome the environmental pollution ensuing from the accumulation of these industrial wastes, but also, to valorize and find value added to these residues.
This investigation includes three main sections:
Section (A): Magnesium Aluminate Spinel Synthesis and characterization
Three different spinel powder compositions MA, MR and AR were prepared through co-precipitation method with MgO: Al2O3 molar ratios of 1:1, 2:1 and 1:2 respectively. Based on the results obtained regarding this section, the following conclusions could be derived: chemical analysis of the synthesized powders by the XRF technique showed the major oxides of MA, MR and AR. Also, phase evaluation by X-ray diffraction confirmed the presence of the spinel (MA) phase and magnesia phase in MR sample powder. While alumina was not detected in (AR) powder due to the higher temperature needed to obtain crystalline alumina phases.
Section (B): Zirconia Spinel Composites Preparation
Six different sets were prepared by mixing MA, MR and AR powders with different proportions of commercial m-ZrO2 and Y2O3 where; the MA, MR and AR content ranged from 0 to 50 wt., % with the increment of 10 wt., %. The obtained powders were sintered using the spark plasma sintering technique at different temperatures with a heating rate of 100 ºC /min, applied pressure of 40 Mpa and holding time of 30 min under vacuum.
Section (C): Zirconia Spinel Composites characterization
C.1. Phase evolution
XRD analysis revealed that irrespective to sintering temperature, zirconia free spinel composites (Z0) showed the characteristic peaks of the t-zirconium yttrium oxide phase, in line with the corresponding peaks of m-ZrO2 phase. With the temperature increase, the m-ZrO2 content decreases slightly and this is attributed to the stabilization of m-ZrO2 in the tetragonal form by the diffusion of Y3+ into the zirconia lattice. Magnesium aluminate spinel addition proved to have a strong effect on zirconia stabilization through the consumption of magnesium and aluminum ions in the complete transformation of monoclinic zirconia to tetragonal and cubic zirconia. Excess magnesia in spinel phase restricts grain growth of spinel and thereby facilitates sintering. Moreover, the densification and zirconia stabilization behavior decrease gradually from the magnesia rich compositions to the alumina rich compositions.
C.2. Densification characteristics
The obtained results indicated that the ball milling of the prepared powder mixtures proved to be an effective way to induce densification process of zirconia ceramics by reducing particle size and increasing sintering activity, enabling an apparent porosity reduction and volume shrinkage enhancement. Also, the magnesium aluminate spinel addition showed an improvement in sintering rate and densification properties of zirconia ceramics as follows:
Z0 composite sintered at 1200 ºC showed a porosity of ~23.5 %, then decreased to ~8.4% for ZMR20.
ZMR10-50 composites sintered at 1300 ºC showed porosity content less than 1.6%.
ZMR50 composite sintered at 1200 ºC has a porosity content of 7.8 % compared to 19 % for ZMA50 and this confirm the great effect of excess magnesia on densification of zirconia composite ceramic.
ZMR10-50 and ZMA10-50 composites sintered at 1300 ºC have porosity less than 1.6 %, but with a large percent of m-ZrO2 due to the rapid densification property of SPS technique.
C.3. Microstructure investigation
FESEM images of sintered composites at 1400 ºC showed the presence of pores as well as agglomeration and non-uniform distribution of yttria particles in Z0 composite while; the porosity was greatly reduced and disappeared with the spinel addition. The EDS point analysis at the bright region of zirconia phase confirms the presence of yttrium, aluminum, and magnesium ions which suggest the stabilization of zirconia by all that elements. Also, the Mg2+, Al3+, Y3+ ions content was decreased by transferring from ZMR to ZMA and ZAR composites. Consequently, the m-zirconia content was decreased in ZMR composites compared to ZMA and ZAR composites with respect to the sintering temperature.
C.4. Mechanical investigation
Vickers indentation method was used for hardness and fracture toughness measurements. The shape of the obtained indentations was normal in type and has a similar shape like the metal materials and no cracks were initiated in composite Z0 and Z10 from the diagonal corners after indentation by a load of 1 Kg. Although Z20-Z40 composites showed some cracks, but not from all the diagonal corners, Z50 composite showed an ideal diagonal with clear cracks from all its corners. The results of mechanical investigation indicated that spinel addition has significantly improved the hardness properties of zirconia ceramics; and in contrast the fracture toughness has decreased with the increase of spinel content due to the low toughness of spinel compared to zirconia. Accordingly, hardness values were increased from ~ 8.7 GPa for Z0 composite to about 13, 13.8 and 14.1 GPa for ZMR50, ZMA50 sintered at 1400 ºC and ZAR50 composites sintered at 1500 ºC respectively. Also, fracture toughness was decreased from 5.6 MPa m0.5 for Z0 composite to about 3.4, 3.7 and 4 MPa m0.5 for ZMR50, ZMA50 sintered at 1400 ºC and ZAR50 composites sintered at 1500 ºC respectively.
The compression values of the sintered composites gradually increased with spinel addition reaching its maxima at 20 wt., % (1800 MPa as in ZMA case), then sharply decreased with the gradual increase of spinel content above 20 wt., %. Consequently, the optimum spinel content that could be utilized to stabilize m-ZrO2 into c-ZrO2, together with maintaining the superior mechanical properties of zirconia ceramics could be in the range from ~20 to 25 wt., %.
C.5. Low temperature degradation
Composites resistance to low temperature degradation, aging, test was performed. The change in m-ZrO2 content was evaluated by calculating the m-ZrO2 content before and after the aging test. The obtained results indicated that the aging kinetics of ZrO2-spinel composites have a passive behavior toward t-m-ZrO2 transformation. While Z0 composite shows a little increase in m-ZrO2 content. The results revealed that for Z0 composite, heating for five hours at 134 ºC and water vapor pressure of 2 bars is sufficient to convert nearly 2.8 % of t-ZrO2 to m-ZrO2 but it doesn`t affect the ZMR composites. So, the investigated composites showed high resistance to low temperature degradation in the moisture atmosphere, demonstrating its validity to be potentially applied for various medical and engineering applications.
It was found that stoichiometric composition and sintering temperature/time have a considerable effect on the microstructure of the prepared ceramic composites. ZMR composites showed moderate strength with improved sintering properties at relatively lower temperature. ZAR composites showed moderate strength, but with low sintering and densification properties. While, ZMA composites showed improved strength with moderate sintering and densification properties. The investigated composites showed high resistance to low temperature degradation in the moisture atmosphere, demonstrating its validity to be potentially applied for various medical and engineering applications.
Zirconia has recently attracted a significant attention in fabrication of various kinds of functional and structural ceramics that are widely used in high tech applications such as fuel cells, oxygen sensors, cutting blades, grinding media, knives, and ceramic bearing hip replacement. It is a polymorphic material that exists in three phases: monoclinic, tetragonal, and cubic allotropes. Upon cooling down from high-temperatures, the tetragonal-monoclinic martensitic transformation gives rise to a volume expansion of ~ 3-4%. Such large volume expansion causes large stresses, which could induce micro-cracking leading to mechanical failure. In this study commercial monoclinic zirconia was fully stabilized in the cubic form at room temperature through magnesium aluminate spinel (MA), magnesium aluminate spinel with excess magnesia (MR) and magnesium aluminate spinel with excess alumina (AR) addition to protect zirconia composite ceramics from failure.
Despite the good properties as well as compatibility of MA with zirconia ceramic systems, effect of MA addition on densification and stabilization of raw m-ZrO2 has not been well studied yet. In this context, this investigation was designed; firstly, to illustrate the effect of MA addition on the stabilization and densification properties of zirconia ceramics, and secondly, to develop stabilized and durable zirconia based composite ceramics. Besides, MA raw powders have been synthesized in this work from municipal industrial wastes, Mg and Al scraps, through a facile, inexpensive co-precipitation scheme. In addition, these studies not only help to avoid pollution by turning municipal wastes generated from industry into valuable spinel but also recommend utilization of these synthesized materials in stabilization and densification of m-ZrO2. The outcomes indicated that spinel addition has a promising effect on both densification and the stabilization behavior of m-ZrO2 where some designed batches gave rise to fully stabilized cubic zirconia with improved physico-mechanical properties. In this respect, this study offers huge potentials to develop highly stabilized cubic zirconia refractory ceramics for harsh-environment applications and makes it possible not only to overcome the environmental pollution ensuing from the accumulation of these industrial wastes, but also, to valorize and find value added to these residues.
This investigation includes three main sections:
Section (A): Magnesium Aluminate Spinel Synthesis and characterization
Three different spinel powder compositions MA, MR and AR were prepared through co-precipitation method with MgO: Al2O3 molar ratios of 1:1, 2:1 and 1:2 respectively. Based on the results obtained regarding this section, the following conclusions could be derived: chemical analysis of the synthesized powders by the XRF technique showed the major oxides of MA, MR and AR. Also, phase evaluation by X-ray diffraction confirmed the presence of the spinel (MA) phase and magnesia phase in MR sample powder. While alumina was not detected in (AR) powder due to the higher temperature needed to obtain crystalline alumina phases.
Section (B): Zirconia Spinel Composites Preparation
Six different sets were prepared by mixing MA, MR and AR powders with different proportions of commercial m-ZrO2 and Y2O3 where; the MA, MR and AR content ranged from 0 to 50 wt., % with the increment of 10 wt., %. The obtained powders were sintered using the spark plasma sintering technique at different temperatures with a heating rate of 100 ºC /min, applied pressure of 40 Mpa and holding time of 30 min under vacuum.
Section (C): Zirconia Spinel Composites characterization
C.1. Phase evolution
XRD analysis revealed that irrespective to sintering temperature, zirconia free spinel composites (Z0) showed the characteristic peaks of the t-zirconium yttrium oxide phase, in line with the corresponding peaks of m-ZrO2 phase. With the temperature increase, the m-ZrO2 content decreases slightly and this is attributed to the stabilization of m-ZrO2 in the tetragonal form by the diffusion of Y3+ into the zirconia lattice. Magnesium aluminate spinel addition proved to have a strong effect on zirconia stabilization through the consumption of magnesium and aluminum ions in the complete transformation of monoclinic zirconia to tetragonal and cubic zirconia. Excess magnesia in spinel phase restricts grain growth of spinel and thereby facilitates sintering. Moreover, the densification and zirconia stabilization behavior decrease gradually from the magnesia rich compositions to the alumina rich compositions.
C.2. Densification characteristics
The obtained results indicated that the ball milling of the prepared powder mixtures proved to be an effective way to induce densification process of zirconia ceramics by reducing particle size and increasing sintering activity, enabling an apparent porosity reduction and volume shrinkage enhancement. Also, the magnesium aluminate spinel addition showed an improvement in sintering rate and densification properties of zirconia ceramics as follows:
Z0 composite sintered at 1200 ºC showed a porosity of ~23.5 %, then decreased to ~8.4% for ZMR20.
ZMR10-50 composites sintered at 1300 ºC showed porosity content less than 1.6%.
ZMR50 composite sintered at 1200 ºC has a porosity content of 7.8 % compared to 19 % for ZMA50 and this confirm the great effect of excess magnesia on densification of zirconia composite ceramic.
ZMR10-50 and ZMA10-50 composites sintered at 1300 ºC have porosity less than 1.6 %, but with a large percent of m-ZrO2 due to the rapid densification property of SPS technique.
C.3. Microstructure investigation
FESEM images of sintered composites at 1400 ºC showed the presence of pores as well as agglomeration and non-uniform distribution of yttria particles in Z0 composite while; the porosity was greatly reduced and disappeared with the spinel addition. The EDS point analysis at the bright region of zirconia phase confirms the presence of yttrium, aluminum, and magnesium ions which suggest the stabilization of zirconia by all that elements. Also, the Mg2+, Al3+, Y3+ ions content was decreased by transferring from ZMR to ZMA and ZAR composites. Consequently, the m-zirconia content was decreased in ZMR composites compared to ZMA and ZAR composites with respect to the sintering temperature.
C.4. Mechanical investigation
Vickers indentation method was used for hardness and fracture toughness measurements. The shape of the obtained indentations was normal in type and has a similar shape like the metal materials and no cracks were initiated in composite Z0 and Z10 from the diagonal corners after indentation by a load of 1 Kg. Although Z20-Z40 composites showed some cracks, but not from all the diagonal corners, Z50 composite showed an ideal diagonal with clear cracks from all its corners. The results of mechanical investigation indicated that spinel addition has significantly improved the hardness properties of zirconia ceramics; and in contrast the fracture toughness has decreased with the increase of spinel content due to the low toughness of spinel compared to zirconia. Accordingly, hardness values were increased from ~ 8.7 GPa for Z0 composite to about 13, 13.8 and 14.1 GPa for ZMR50, ZMA50 sintered at 1400 ºC and ZAR50 composites sintered at 1500 ºC respectively. Also, fracture toughness was decreased from 5.6 MPa m0.5 for Z0 composite to about 3.4, 3.7 and 4 MPa m0.5 for ZMR50, ZMA50 sintered at 1400 ºC and ZAR50 composites sintered at 1500 ºC respectively.
The compression values of the sintered composites gradually increased with spinel addition reaching its maxima at 20 wt., % (1800 MPa as in ZMA case), then sharply decreased with the gradual increase of spinel content above 20 wt., %. Consequently, the optimum spinel content that could be utilized to stabilize m-ZrO2 into c-ZrO2, together with maintaining the superior mechanical properties of zirconia ceramics could be in the range from ~20 to 25 wt., %.
C.5. Low temperature degradation
Composites resistance to low temperature degradation, aging, test was performed. The change in m-ZrO2 content was evaluated by calculating the m-ZrO2 content before and after the aging test. The obtained results indicated that the aging kinetics of ZrO2-spinel composites have a passive behavior toward t-m-ZrO2 transformation. While Z0 composite shows a little increase in m-ZrO2 content. The results revealed that for Z0 composite, heating for five hours at 134 ºC and water vapor pressure of 2 bars is sufficient to convert nearly 2.8 % of t-ZrO2 to m-ZrO2 but it doesn`t affect the ZMR composites. So, the investigated composites showed high resistance to low temperature degradation in the moisture atmosphere, demonstrating its validity to be potentially applied for various medical and engineering applications.
It was found that stoichiometric composition and sintering temperature/time have a considerable effect on the microstructure of the prepared ceramic composites. ZMR composites showed moderate strength with improved sintering properties at relatively lower temperature. ZAR composites showed moderate strength, but with low sintering and densification properties. While, ZMA composites showed improved strength with moderate sintering and densification properties. The investigated composites showed high resistance to low temperature degradation in the moisture atmosphere, demonstrating its validity to be potentially applied for various medical and engineering applications.