Evaluation of Structural Stability, Mechanical Properties, and Corrosion Resistance of Magnesia Partially Stabilized Zirconia (Mg-PSZ)

Nano Zirconia (ZrO2) has been used in dental implants due to having excellent mechanical properties and biocompatibility that match the requirements for the purpose. Zirconia undergoes phase transformation during heating: monoclinic (room temperature to 1170 °C), tetragonal (1170 °C to 2370 °C), and cubic (>2370 °C). Most useful mechanical properties can be obtained when zirconia is in a multiphase form or in partially stabilized zirconia (PSZ), which is achieved by adding small amounts of a metal oxide dopant, such as MgO (magnesia). This study aimed to synthesize nano Mg-PSZ from a local resource found in West Kalimantan, Indonesia, and examine its structural stability, biochemical stability, and mechanical properties. Nano Mg-PSZ was prepared from a zircon local to Indonesia, from West Kalimantan Province, MgSO4∙7H2O, and polyethylene glycol (PEG)-6000 was used as a template. The obtained t-ZrO2 after calcination at 800 °C was shown to be stable at room temperature. The highest percentage of the t-ZrO2 phase was obtained at Zr0.95Mg0.05O2 with a variation of 99.5%. The hardness of Mg-PSZ increased from 554 MPa for ZrO2 without MgO doping to 5266 MPa for ZrO2 with a doping of 10% MgO. An in vitro biodegradation test showed that the greater the concentration of MgO in doping the ZrO2, the greater the degradation resistance of Mg-PSZ in simulated body fluid (SBF) solution.


Introduction
Zirconia (zirconium dioxide, ZrO 2 ), also referred to as "ceramic steel", has optimal properties for the use of dental implants due to its superior toughness, strength and resistance, excellent wear properties, and biocompatibility [1][2][3]. Amongst the commonly used materials in dentistry, zirconia has the advantage of being compatible for osteoblasts to adhere and proliferate [4]. It is known that the material in dental implant applications must be biocompatible, have antibacterial activity and low toxicity, be stable and resistant to corrosion, and have high performance for survival in the complex mouth environment [5][6][7][8].
Zirconia is known to have three types of crystal: monoclinic (m-ZrO 2 ), tetragonal (t-ZrO 2 ), and cubic (c-ZrO 2 ). The ZrO 2 phase is unstable and can undergo structural transformations such as t-ZrO 2 to m-ZrO 2 [9]. Metastable tetragonal zirconia is able to transform stress-assisted into the monoclinic state at an ambient temperature if an external load is applied [10]. At room temperature, ZrO 2 is in the form of a monoclinic phase, whereas to obtain the tetragonal and cubic phases requires a sintering temperature [11].
However, in several syntheses using lower calcination temperatures, tetragonal and cubic phases of ZrO 2 were obtained [1,12].
Zirconia with tetragonal and cubic structures are generally obtained by adding a stabilizer to the ZrO 2 lattice structure [13]. Stabilized zirconia generally consist of two or more mixtures of different ZrO 2 phases and is usually obtained by adding metal oxide dopants such as yttria (Y 2 O 3 ) or magnesia (MgO) to the ZrO 2 lattice [3,14]. Stabilized zirconia is a promising material due to its great physical and chemical properties, and thermal stability [15]. Yttria stabilized zirconia (Y-PSZ) is a zirconia-based dental implant material that is often used. However, Y 2 O 3 as a ZrO 2 stabilizer is relatively expensive. Therefore, other metal oxides are needed as zirconia stabilizers. When compared with Y-PSZ, magnesium-stabilized zirconia (Mg-PSZ) shows promising characteristics in several aspects, including good mechanical and thermal properties, good stability in low temperature degradation (LTD), and the same coefficient of thermal expansion as YSZ [16][17][18]. Therefore, Mg-PSZ composite has the potential as a dental implant application material at a relatively low price [1].
As we know, there are three crystallographic phases of zirconia, but it is known that the tetragonal phase (t-ZrO 2 ) has better mechanical properties and corrosion resistance than the monoclinic and cubic phases [19,20]. In previous studies, t-ZrO 2 was successfully stabilized by adding MgO as a stabilizer and using a low calcination temperature of 800 • C to produce Mg-PSZ. The use of MgO as much as 1, 5, and 10% w/w succeeded in stabilizing ZrO 2 in tetragonal and cubic forms at room temperature [1]. In addition to stabilizing the tetragonal zirconia phase, the addition of 25% MgO concentration increased the hardness (Vickers) of ZrO 2 from 554 to 6350 MPa and fracture strength from 5.2 to 25 MPa. The increase in the mechanical properties of the sample was caused by the formation of the t-ZrO 2 phase due to the presence of MgO as a stabilizer, which prevents the reverse allotropic transformation of zirconia so as to maintain the t-ZrO 2 phase at room temperature [21]. The lower porosity of the t-ZrO 2 phase compared with the other phases causes the corrosion resistance of t-ZrO 2 to be better than the m-ZrO 2 and c-ZrO 2 phases [19]. The addition of MgO as a stabilizer increased the hardness of ZrO 2 and gave 50% better wear performance than ZrO 2 without the addition of MgO [22].
The template method utilizing polyethylene glycol (PEG) is expected to affect the characteristics of ZrO 2 , including changing the morphology of the Mg-PSZ particles. PEG long chains play a role in helping the distribution of metal ions homogeneously and not clumping or settling in the solution so that magnesia-stabilized zirconia is obtained in nano size [14,23]. PEG attaches to the ZrO(OH) 2 molecule through hydrogen bonds. The hydroxyl group of the ZrO 2 precursor is covered by a PEG molecule so that aggregation occurring between particles is reduced. During calcination, organic substances are burned as gas, leaving the particles in the nanostructures [14]. This phenomenon reduces the aggregation of synthesized Mg-PSZ particles, so that, in previous studies, Mg-PSZ was obtained with a nano-sized particle diameter of 9-44 nm [1].
In this paper, the mechanical properties and corrosion resistance of Mg-PSZ will be tested. Based on ISO 13356:2015 Third Edition, dental implant materials have a hardness level of 11.8 GPa and the number of monoclinic phases is limited to 20% mass fraction. The Mg-PSZ precursors used in this study were ZrO 2 derived from local zirconium silicate-based zirconium hydroxide, an MgO stabilizer from a MgSO 4 ·7H 2 O precursor, and polyethylene glycol (PEG) as a template. This research is expected to determine the mechanical properties, corrosion resistance, and antibacterial activity of Mg-PSZ before continuing to direct application as dental implants.

Crystal-Phase Structure Mg-PSZ
The chemical composition of the ZrO(OH) 2 precursor was first evaluated using the ARL QUANT'X EDXRF Analyzer. Semi-quantitative XRF analysis was carried out to determine the purity of the local zircon and its contents [24]. The results of the XRF analysis  Table 1. It showed that ZrO(OH) 2 contained 79.24 wt% ZrO 2 and 11.06 wt% MgO, which would be taken into account in the next process to determine each molar ratio of each specimen. The ZrO 2 was then prepared from ZrO(OH) 2 after washing with water [1]. The mechanism of Mg-PSZ synthesis is the same as in previous studies [1]. The absorption bands in the range of 400-4000 cm −1 show several vibrational modes of strain and chemical bonding in the Zr 0.90 Mg 0.10 O 2 sample and PEG-6000 functional group. Mg-O bonds appear at 617.72 cm −1 , associated with the stretching vibrations of Mg-O bonds, while the Zr-O bond appears at 439.8 cm −1 . As shown by the FT-IR analysis in Figure 1, it was observed that the precursor ZrO 2 reacted with PEG degraded at pH 3 during synthesis, producing Zr-(ethylene glycolate)n and releasing water molecules on heating. PEG hydrogel degradation can occur through hydrolysis due to the presence of strong acids in the form of H2SO4, where the ester bond in the PEG polymer chain will be broken and produce ethylene glycolate as a monomer [25]. The degradation of PEG-6000 at pH 3 was supported by the very low intensity and weak peaks of the CH 2 -strain vibration at 2921.17 cm −1 and 2870.738 cm −1 compared with the normal PEG which shows a high intensity and strong peaks of the CH 2 strain vibration around 2890 cm −1 [1,2,14,26,27]. Then, heating, which is carried out at a temperature of 120 • C, can cause PEG-6000 to be degraded through a thermal degradation mechanism in which the heat and steam provided will facilitate the PEG's decomposition [28]. Thermal degradation refers to the breakdown of the molecules of a substance due to overheating, generally related to polymers, in this case PEG-6000 [29].
After drying at 120 • C (Figure 2), the XRD analysis results showed a strong peak at 2θ of 20 • , indicating the presence of ZrSiO 4 , a commonly found Zr mineral [30]. This specimen was likely derived from the ZrO(OH) 2 , which contained SiO 2 and led to the formation of ZrSiO 4 . Zirconium silicate is produced from the mineral zircon, which is mined from sand deposits containing several percent zircon and separated by gravity, where it is known as powdered zirconium silicate or as zircon flour [31]. As explained previously, our ZrO 2 precursor comes from local Indonesian zircon and the purification process carried out in previous studies [1] has not 100% separated the zircon and silicate. Therefore, the presence of the silicates is very likely to occur. Interestingly, we observed the presence of t-ZrO 2 , as shown in Figure 2, at the 2θ 30 • region. This specimen was confirmed by JCPDS PDF2 no. 791770 and confirmed the previous research that found that the addition of MgO caused the formation of t-ZrO 2 . However, our study differs from previous studies on Mg-PSZ composites. We conducted this study using a doping mechanism to obtain t-ZrO 2 and observed changes in the mechanical properties and stability of MgPSZ. The addition of certain stabilizers to the zirconia alloy can help maintain the tetragonal structure at room temperature [32]. The stabilizer used in this research is MgO, which can control the transformation of the stress-inducing phase from t-ZrO 2 to other phases. Based on previous studies regarding the MgPSZ composite, t-ZrO 2 was obtained at 800 • C, but with the doping mechanism in this study at 120 • C, t-ZrO 2 could be formed. However, further testing is required to determine the stability of the t-ZrO 2 .
After drying at 120 °C ( Figure 2), the XRD analysis results showed a strong peak at 2θ of 20°, indicating the presence of ZrSiO4, a commonly found Zr mineral [30]. This specimen was likely derived from the ZrO(OH)2, which contained SiO2 and led to the formation of ZrSiO4. Zirconium silicate is produced from the mineral zircon, which is mined from sand deposits containing several percent zircon and separated by gravity, where it is known as powdered zirconium silicate or as zircon flour [31]. As explained previously, our ZrO2 precursor comes from local Indonesian zircon and the purification process carried out in previous studies [1] has not 100% separated the zircon and silicate. Therefore, the presence of the silicates is very likely to occur. Interestingly, we observed the presence of t-ZrO2, as shown in Figure 2, at the 2θ 30° region. This specimen was confirmed by JCPDS PDF2 no. 791770 and confirmed the previous research that found that the addition of MgO caused the formation of t-ZrO2. However, our study differs from previous studies on Mg-PSZ composites. We conducted this study using a doping mechanism to obtain t- As shown by the XRD analysis in Table 2, the drying at 120 • C only resulted in a low crystallinity of~50%. The size of the crystallites observed ranged from submicron to micron, with the smallest crystallite of t-ZrO 2 at 164 nm and ZrSiO 4 at 202 nm, shown in Zr 0.95 Mg 0.05 O 2 , while the largest crystallite was observed in Zr 0.99 Mg 0.01 O 2 , at 771 nm and 4003 nm for t-ZrO 2 and ZrSiO 4 , respectively. This is in line with previous findings that nanoparticle Mg-PSZ was only observed after calcination at a temperature of 600-1000 • C [1]. Magnesium oxide as a stabilizing agent in the preparation of zirconia nano-powders has been demonstrated to have an inhibitory effect on the growth of particle grains and lead to smaller size and more uniform distribution compared with non-stabilized zirconia [33,34].
crystallinity of ~50%. The size of the crystallites observed ranged from submicron to micron, with the smallest crystallite of t-ZrO2 at 164 nm and ZrSiO4 at 202 nm, shown in Zr0.95Mg0.05O2, while the largest crystallite was observed in Zr0.99Mg0.01O2, at 771 nm and 4003 nm for t-ZrO2 and ZrSiO4, respectively. This is in line with previous findings that nanoparticle Mg-PSZ was only observed after calcination at a temperature of 600-1000 °C [1]. Magnesium oxide as a stabilizing agent in the preparation of zirconia nano-powders has been demonstrated to have an inhibitory effect on the growth of particle grains and lead to smaller size and more uniform distribution compared with non-stabilized zirconia [33,34].    Next, we observed a structural transformation in ZrO 2 after calcination, as shown by the XRD analysis in Figure 3. After being calcined at 800 • C, only t-ZrO 2 were observed in all the specimens, as shown in ). This is in accordance with a previous study that reported that MgO doping in ZrO 2 resulted in a t-ZrO 2 structure after calcination at 800 • C [1]. In a former study of Mg-PSZ, a minimum 16% of MgO was required to stabilize ZrO 2 and form t-ZrO 2 , [35]. In another study, MgO at 10% was shown to be sufficient as a stabilizer in obtaining a tetragonal phase [36]. However, we observed that a smaller concentration of MgO at 1% and 5% may also lead to stabilized t-ZrO 2 .
Molecules 2023, 28, 6054 6 of 12 791770). This is in accordance with a previous study that reported that MgO doping in ZrO2 resulted in a t-ZrO2 structure after calcination at 800 °C [1]. In a former study of Mg-PSZ, a minimum 16% of MgO was required to stabilize ZrO2 and form t-ZrO2, [35]. In another study, MgO at 10% was shown to be sufficient as a stabilizer in obtaining a tetragonal phase [36]. However, we observed that a smaller concentration of MgO at 1% and 5% may also lead to stabilized t-ZrO2. The stabilization of the ZrO2 structure is caused by cations having a larger radius than Zr 4+ replacing some of the Zr 4+ lattice point positions in the ZrO2 lattice with doping oxides to become pure ZrO2 material [37]. Meanwhile, a substituted solid solution is formed in this ZrO2 material through doping, which maintains the stable phase structure of the doped ZrO2 material at room temperature, thereby achieving a toughening effect for pure ZrO2 materials and leading to the formation of partially stabilized zirconia materials (PSZ) [38,39]. The mechanism of MgO in stabilizing ZrO2 can be explained by the difference in charge between the Zr 4+ ion and the Mg 2+ . The stabilization is caused by a defect in the lattice of a crystal due to doping ions having a lower valence, which leads to oxygen vacancy, as explained in the following equation.
MgO + Zr 4+ Zr′ + ½ O2 → Mg 2+ Zr′ + Vo + ZrO2 (1) The stabilization of the ZrO 2 structure is caused by cations having a larger radius than Zr 4+ replacing some of the Zr 4+ lattice point positions in the ZrO 2 lattice with doping oxides to become pure ZrO 2 material [37]. Meanwhile, a substituted solid solution is formed in this ZrO 2 material through doping, which maintains the stable phase structure of the doped ZrO 2 material at room temperature, thereby achieving a toughening effect for pure ZrO 2 materials and leading to the formation of partially stabilized zirconia materials (PSZ) [38,39]. The mechanism of MgO in stabilizing ZrO 2 can be explained by the difference in charge between the Zr 4+ ion and the Mg 2+ . The stabilization is caused by a defect in the lattice of a crystal due to doping ions having a lower valence, which leads to oxygen vacancy, as explained in the following equation.
MgO + Zr 4+ Zr + 1 /2 O 2 → Mg 2+ Zr + Vo + ZrO 2 (1) The reduction of oxygen takes place to balance the positive charge, leading to a neutrally charged Mg-doped ZrO 2 without free electrons [40]. Oxygen vacancies in the zirconia lattice can reduce the transformation temperature of the transition or metastable phase, and stabilize and increase the concentration of the tetragonal phase in the Zr-ZrO 2 binary system region [12]. The amount of oxygen vacancies in the ZrO 2 lattice influences the formation of a different phase of ZrO 2, where the tetragonal phase is formed with a low oxygen vacancy, while the cubic phase is formed with a higher oxygen vacancy [18,41].
As shown by the XRD analysis in Table 3, all the specimens of Mg-PSZ calcined at 800 • C have a high crystallinity, with the highest crystallinity of 96.35% being shown in Zr 0.90 Mg 0.10 O 2 and the lowest at 91.28% shown in Zr 0.99 Mg 0.01 O 2 . The size of the crystallite in all the samples were found to be in a nanometer scale. However, there was an increase in size along with an increase in Mg content, which was likely contributed by Mg. The overall obtained crystal is t-ZrO 2 , as shown in Table 4. The sample Zr 0.95 Mg 0.05 O 2 has the largest tetragonal phase composition of 99.5%, with a monoclinic phase composition of 0.5% as impurities, while the variation with the lowest tetragonal phase composition is Zr 0.85 Mg 0.15 O 2 at 96.2% and the monoclinic phase as an impurity is 2.7%. When compared with dental implants with ceramic material yttria-stabilized tetragonal zirconia (Y-TZP) based on ISO 13356:2015 Third Edition, the synthesized partially stabilized magnesia zirconia (Mg-PSZ) meets one of the requirements, which is that the minimum composition of the monoclinic phase is below 20%. Mass fraction has been successfully obtained with very low monoclinic fraction compositions ranging from 0.5 to 2.7% for all synthesized Mg-PSZ.

Mechanical Properties of Mg-PSZ
Hardness is one of the most important parameters for comparing dental implant material properties, as it is used to find the suitability of the clinical use of biomaterials [42]. A study explained that the addition of 10% MgO concentration increased the hardness of Vickers ZrO 2 from 554 to 6350 MPa [20]. The increased mechanical properties in t-ZrO 2 was due to the presence of MgO, which prevents the reverse allotropic transformation of zirconia. In general, an increase in hardness that requires a decrease in porosity is known as the Duckworth-Knudsen model [43]. The results of the hardness test are shown in Table 5. The Vickers hardness test showed that doping MgO in ZrO 2 successfully increased the hardness of the ZrO 2 , as shown in Table 5. Zr 0.90 Mg 0.10 O 2 has shown the highest Vickers hardness with a value of 5266 MPa. This obtained result is almost in accordance with a previous study that reported an increase in hardness to 6350 MPa after an addition of 10% MgO in ZrO 2 [20]. However, we did not observe a consistent trend in the increase in hardness. This is most likely due to the different surface roughness of each specimen [44]. As shown in Figure 4, the Zr 0.95 Mg 0.05 O 2 sample showed a less flat surface, while in contrast, Zr 0.90 Mg 0.10 O 2 showed a flat surface. This led to different pressures in the indenter when the test was carried out. When a rough surface is stressed, the resulting pressure triangle will produce a very large distance, which will cause the calculation of the Vickers hardness value to be small. Meanwhile, lower roughness leads to a smaller triangle size and the Vickers hardness value being large.
ness. This is most likely due to the different surface roughness of each specimen [44]. As shown in Figure 4, the Zr0.95Mg0.05O2 sample showed a less flat surface, while in contrast, Zr0.90Mg0.10O2 showed a flat surface. This led to different pressures in the indenter when the test was carried out. When a rough surface is stressed, the resulting pressure triangle will produce a very large distance, which will cause the calculation of the Vickers hardness value to be small. Meanwhile, lower roughness leads to a smaller triangle size and the Vickers hardness value being large.

Stability of Mg-PSZ
The stability test of Mg-PSZ was carried out by a simple in vitro biodegradation test. Each specimen was immersed in SBF (Simulated Body Fluid) solution for 3 days. As shown in Figure 5, the pH of the SBF solution was changed after the ZrO2 and Mg-PSZ samples were soaked for 3 days at 37 °C.

Stability of Mg-PSZ
The stability test of Mg-PSZ was carried out by a simple in vitro biodegradation test. Each specimen was immersed in SBF (Simulated Body Fluid) solution for 3 days. As shown in Figure 5, the pH of the SBF solution was changed after the ZrO 2 and Mg-PSZ samples were soaked for 3 days at 37 • C. In general, the dissolution reaction of ZrO2 in an aqueous medium follows the equation [45]: ZrO2(c) + (4 − n) H + → Zr(OH)n 4−n + (2 − n) H2O (2) Soaking the sample for 3 days in SBF solution at 37 °C showed a change in pH, as shown in Figure 5A. After soaking for 3 days, a significant change in pH was found in the ZrO2, which was at 8.76, and gradually lower changes in pH were observed in samples with increasing MgO. Additionally, this changes in pH corresponded with mass loss, as shown in Figure 5B. Thus, the changes in pH of the SBF solution in the sample is due to the release of Zr 4+ ions from the ZrO2.
The immersion of the ZrO2 and the variation of the Mg-PSZ carried out for 3 days at In general, the dissolution reaction of ZrO 2 in an aqueous medium follows the equation [45]: Soaking the sample for 3 days in SBF solution at 37 • C showed a change in pH, as shown in Figure 5A. After soaking for 3 days, a significant change in pH was found in the ZrO 2 , which was at 8.76, and gradually lower changes in pH were observed in samples with increasing MgO. Additionally, this changes in pH corresponded with mass loss, as shown in Figure 5B. Thus, the changes in pH of the SBF solution in the sample is due to the release of Zr 4+ ions from the ZrO 2 .
The immersion of the ZrO 2 and the variation of the Mg-PSZ carried out for 3 days at 37 • C showed the largest mass change for the ZrO 2 without MgO doping with a mass loss of 3.2545 g. Testing the variation of the Mg-PSZ sample in Figure 5B shows that the degradation of the sample that occurred is strongly influenced by the concentration of the MgO used. Sample variation 4, with the addition of 15% of MgO, showed the best resistance of the material to SBF with a lost weight of 0.0069 g. These data show a correlation between changes in SBF pH and the amount of ZrO 2 sample dissolved in the SBF solution.
Based on the variation of MgO concentrations, we concluded that the greater the concentration of MgO in doping the ZrO 2 , the greater the degradation resistance of the SBF solution. However, another thing to note is that although the addition of MgO showed a significant effect on ZrO 2 resistance, further tests (in vivo tests) were needed to determine the time of osteoblast formation in the bone and the effect of pH on osteoblast cells. This is because changes in pH in the osteoblast cell environment can provide an inflammatory response so that the formation of osteoblast cells becomes slow [46].

Preparation of Crystals
Prior to the synthesis, zirconium oxyhydroxide was analyzed using XRF to determine the ZrO 2 content. Next, 60 g of ZrO(OH) 2 was dissolved in 25 mL of 17.74 M H 2 SO 4 . The pH was adjusted to 3.0 by adding appropriate amount of 1 M (NH 4 )HCO 3 . Subsequently, MgSO 4 ·7H 2 O was added in a molar ratio of 0.01, 0.05, 0.10, or 0.15 compared with Zr. Afterwards, 10% PEG-6000 in water was added to the mixture of Mg:Zr at a volume ratio of 1:15 (PEG-6000:Mg-Zr). Mixture was continuously stirred until homogeneous for 1 h, then heated at 120 • C for 3 h and rested overnight at RT. The next day, gel was filtered and followed by drying at 100 • C for 2 days to obtain dry solid material. Finally, solid material was rinsed with hot deionized water to remove impurities and then dried again at 100 • C for 1 day. After drying, powder was collected for FTIR, XRD, and further processing. Powder was then calcined at 800 • C and further characterized using XRD and TEM [1,2].

Mechanical Properties
To evaluate the hardness of Mg-PSZ, material was made into a 2 cm × 2 cm × 1 cm block for microvickers test. The load was set at 200 g.f for 10 s. Vickers hardness was then calculated using the following formula [47]: HV = Vickers Hardness, P = Load (kg.f), α = opposite angle of the indenter, and d = the average indentation diagonal.

Stability
To examine the stability in a biological setting, Mg-PSZ was soaked in a simulated body fluid, followed by measurement of pH in the solution and changes in specimen weight, according to previous study [48]. Briefly, prepared Mg-PSZ block was soaked in a simulated body fluid which has very similar ionic composition to human blood plasma. The ion contents are shown in Table 6. After soaked for 3 days at 37 • C, each Mg-PSZ was weighted to measure changes compared with initial weight before soaking. Furthermore, pH of SBF before and after the experiment was measured.

Conclusions
The introduction of MgO led to the creation of t-ZrO 2 . A heating process at 800 • C enhanced the structural crystallinity and further stabilized ZrO 2 . While the utmost t-ZrO 2 composition (99.5%) was attained with 5% MgO (