2.1. 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 shown in
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
0.90Mg
0.10O
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.
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
0.95Mg
0.05O
2, while the largest crystallite was observed in Zr
0.99Mg
0.01O
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].
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
Figure 3. Peaks in all of the Mg-PSZ showed identical principal peaks at 2θ of 30.40°, 34.49°, 35.40°, 50.25°, 50.74°, 59.36°, 60.20°, 62.86, and 74.63°, corresponding to the crystal planes (101), (002), (110), (112), (200), (103), (211), (202), and (220). Those peaks and crystal planes are all associated with
t-ZrO
2 (JCPDS PDF2 no. 791770). 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.
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.
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
0.90Mg
0.10O
2 and the lowest at 91.28% shown in Zr
0.99Mg
0.01O
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
0.95Mg
0.05O
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
0.85Mg
0.15O
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.
2.3. 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 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].