Figure 1 shows the rheological behaviors of YTZP slurries added with different amounts of dispersants. For the slurries added with Darven 821A and Darven C, the rheological behaviors exhibited shear thinning. The slurry viscosity was higher at low shear rates, attributed to the network structure resulting from Van der Waal’s attraction between nano-sized YTZP particles. As the shear rate increased, the network structure was broken up by the shearing force, which led to a decrease in viscosity. Darven C exhibited the poorest dispersant performance. The viscosity of slurry added Darven C increased rapidly as the solid content was increased above 45vol%, and hence only the rheology behavior was measured for the slurry with a solid content of 40vol%. For the slurry added with Dolapix CE64, as the dispersant addition was increased to 0.2wt%, the dispersion was improved and the slurry rheological behavior became nearly Newtonian, indicating that the YTZP slurry was completely dispersed. As the dispersant addition was further increased to 0.3wt%, the slurry rheology increased slightly. Table 1 shows the slurry viscosity results compared with those reported in the literature [22, 24, 25]. It was found that zirconia slurries can be dispersed better under alkaline conditions. This is due to the isoelectric point (IEP) of zirconia being at about pH 7 [26]. Therefore, in alkaline conditions, the zirconia particles exhibit higher zeta potential, leading to greater electrostatic repulsion. Moreover, Dolapix CE64 can be completely dissociated at pH 10 and then adsorbed onto the powder surface, hence leading to greater electrostatic repulsion and lower viscosity [27]. Figure 2 shows the particle size distribution of YTZP slurry added with 0.2wt% Dolapix CE64. This indicates that YTZP powder in the slurry exhibited a narrow particle size distribution and the mean particle size (d50) was about 165 nm which is close to the primary particle size of 94 nm. This confirms that Dolapix CE64 exhibits the best dispersant performance and well-dispersed YTZP slurry can be obtained after adding 0.2wt% Dolapix CE64.
Table 1
Comparison of the slurry viscosity of the result with those reported in the literature.
particle size | dispersant | solid content | pH | viscosity | reference |
94 nm | Dolapix CE64 0.2 wt% | 45 vol% | 10 | 0.24 Pa.s (shear rate = 0.1 1/s) | this study |
100 nm | NH4PAA 1 wt% | 50 wt% ≈14.2 vol% | 8 | 4.5 Pa.s (shear rate = 0.1 1/s) | [24] |
100 nm | Dolapix CE64 0.5 wt% | 60 wt% ≈ 20 vol% | 8 | 7.5 Pa.s (shear rate = 0.1 1/s) | [22] |
95 nm | NH4PAA | 45 vol% | 7.1 | 7.5 Pa.s (shear rate = 5 1/s) | [25] |
Table II Relative green densities of the samples prepared by gel casting.
Sample
|
Relative density (%)
|
Relative density after binder burnout (%)
|
Without CIP
|
60.7
|
58.4
|
CIP
|
65.5
|
62.5
|
Figure 3 shows the solid content effect on the rheological behaviors of YTZP slurries with 0.2wt% Dolapix CE64. All YTZP slurries exhibited shear-thinning behavior and the viscosity at a low shear rate increased significantly as the YTZP powder content was increased to 50vol%. At a low shear rate, the dissipation is governed by the rupture network structure. As the YTZP powder solid content was increased to 50vol% the steric barriers built by the adsorbed Doplaix CE64 on the powder surface came into contact and interpenetrated, which resulted in network structure build-up and yield behavior. The strong network structure and high yield stress of the YTZP slurry do not satisfy the viscosity requirements for the gel-casting process and easily result in bridging flocculation. Therefore, suitable YTZP slurry solid content with 0.2wt% Dolapix CE64 is 45 vol%.
After preparing well-dispersed YTZP slurry, epoxy monomers (EGDGE), plasticizers (glycerol), and polymerization initiators (DPTA) for gel casting must be added subsequently. Figure 4 displays the effect of adding different organic vehicles on the YTZP slurry viscosity. The viscosity nearly remained unchanged at a low shear rate and decreased slightly at a high shear rate after adding glycerol and EGDGE, suggesting the slurry remained well-dispersed. However, when DPTA was added, the viscosity at a low shear rate increased nearly 10 times. This indicates that the well-dispersed slurry may react with DPTA simultaneously once adding DPTA, resulting in gel formation. This makes the slurry unable to be poured into the mold.
FTIR spectroscopy can be used to investigate the reaction time between the monomer, EGDGE, and initiator, DPTA, which is shown in Fig. 5. When EGDGE reacts with DPTA, the glycidyl groups and ether groups of EGDGE will decrease, but a new functional hydroxyl (-OH) group appears [28]. Compared with EGDGE, no significant intensity change in the peaks at 1250 cm− 1 and 1120 cm− 1 was observed and the OH functional group did not appear after EGDGE and DPTA mixing for 0–20 min. As the mixing time was prolonged to 30 min, the glycidyl group peak was significantly weakened and the hydroxyl group appeared, indicating that polymerization started. Based on the above results, the polymerization incubation time is longer than 20 min, which is sufficient for gel casting.
Figure 6 shows the Dolapix CE64 addition effect on the polymerization rate measured by FTIR spectroscopy. It can be seen that an obvious hydroxyl peak appeared and the glycidyl group peak intensity significantly decreased immediately after mixing EGDGE, DPTA, and Dolapix CE64. The FTIR spectrum of the sample mixed for 20 min is similar to that of the immediately mixed sample. It suggests that the polymerization reaction has been completed immediately after mixing due to the Dolapix CE64 addition. The dispersant Dolapix CE64 contains many carboxyl groups [22]. The glycidyl group base catalysis reaction with the carboxyl groups proceed at low temperature [29], which leads to the slurry exhibiting limited stability at room temperature.
Figure 7 shows the HQ addition effect on the polymerization rate measured by FTIR spectroscopy. When HQ was added into the EGDGE, DPTA, and Dolapix CE64 mixture for 30 min, no significant intensity change in the peaks at 1250 cm− 1 and 1120 cm− 1 was observed and the OH functional group did not appear; indicating that polymerization had not occurred. This means that HQ can improve the slurry stability and extend the gel casting working time to more than 30 minutes, which is helpful for the subsequent molding process. Figure 8 shows the effects of different curing temperatures and soaking time on the slurry viscosity (the mixture of EGDGE, DPTA, HQ, and Dolapix CE64). The slurry viscosity remained nearly unchanged for a long time at 30oC, indicating the slurry exhibited good stability. When the temperature was raised to 45°C and 60°C, a significant increase in viscosity was found for 200 s and 20 s, respectively, indicating that gelation occurred. Figure 9 shows the appearance of gel-casted samples after curing at 45°C and 60°C. After curing at 45°C, some cracks were observed due to the poor green strength resulting from insufficient gelation. On the contrary, the sample cured at 60°C exhibited high green strength that can withstand severe volume shrinkage during dehydration without cracks.
The main organic vehicle in the slurry is monomers and plasticizers. The role of the monomer, EGDGE, in the slurry is to increase the green strength through the polymerization to become a binder. The function of the plasticizer, glycerol, is to allow the green body to be plastic, so that plasticizer can be a great aid in avoiding cracking by promoting plastic deformation. Cracks are thereby reduced during drying. In addition, glycerol can also reduce the glass transition temperature of the epoxy resin, which is beneficial to the increase in green density by promoting plastic deformation during CIP. Figure 10 shows the appearance of the gel-casted samples with different B/P ratios (binder/plasticizer ratio). It can be seen that many cracks occurred for the green body with a B/P ratio of 5:5 due to insufficient green strength during drying. On the other hand, the green body with a B/P ratio of 6:4 shrank uniformly and can be demolded smoothly without cracks because the green body had enough strength to withstand the huge volume shrinkage during drying.
Table II shows the relative green densities of the samples prepared by gel casting. The green density of the sample prepared by gel casting can reach more than 60% and about 58% after binder burnout. The green density of the gel-casted sample after CIP can further reach a relative density of 65.5% and 62.5% after binder burnout. Figure 11 shows the SEM microstructure of the green body prepared by gel casting and the CIP process. It can be seen that no large inter-agglomerate pore was observed, indicating the green body exhibited a homogeneous green microstructure. It reveals that the green body with a high density can be obtained by gel casting of a well-dispersed slurry and the plasticizer, glycerol, promoted the plastic deformation during CIP, leading to a higher green density, which will be beneficial to the densification.
Figure 12 shows the variation in shrinkage and instantaneous relative density with temperature for the YTZP green body. The two-stage sintering method was used to control the grain growth. Based on the dilatometry results, the first stage sintering temperature was chosen at 1300°C, where the relative density reached 83%, indicating that YTZP has reached the final stage of sintering. In the second sintering stage, the temperature was lowered to 1240°C and soaked for 24 h to inhibit the grain growth and continue densification. Figure 13 shows the SEM microstructures of the YTZP ceramic fracture surface after two-stage sintering (first stage sintering temperature:1300°C, the second stage sintering: 1240°C for 24 h). For the sample after two-stage sintering, the relative sintered density was 99.4%, the mean grain size was 195 nm (Fig. 14) and nearly no inter-granular pores were found. This suggests that two-stage sintering can simultaneously inhibit grain growth, and promote densification.
Figure 15 shows the YTZP ceramic XRD pattern after two-stage sintering. It indicates that a large amount of metastable tetragonal phase remained after high-temperature sintering and cooling to room temperature. The crystalline phase that existed in YTZP ceramics after two-stage sintering was 92.3% tetragonal phase and 7.7% monoclinic phase.
The average flexural strength, Vickers hardness, and fracture toughness (KIC) of the YTZP ceramics after two-stage sintering can reach 771 ± 210 MPa, 15.2 ± 0.3GPa, and 7.80 ± 1.63 MPa.m1/2, respectively. The Vickers hardness and flexural strength strongly depended on porosity and grain size, obeying Hall-Petch law [30–32] for dense ceramics with high densities (> 99%). It has been reported that grain boundaries can serve as the main obstacles for dislocation motion during deformation, and hence increase the hardness with decreasing grain size as a consequence of increasing grain boundary density [33]. Therefore, the YTZP ceramics prepared by the gel casting and two-stage sintering process exhibited superior hardness and fracture strength due to the high sintered density, small grain size, and homogeneous microstructure.
It is well known that YTZ ceramics have little monoclinic phase, exhibiting higher fracture strength than that with a 100% tetragonal phase [34]. The presence of a small amount of monoclinic phase in YTZP ceramics indicates that the tetragonal phase is relatively unstable and tends to transform into a monoclinic phase. During crack propagation, the tetragonal phase can transform into the monoclinic form due to tetragonal phase destabilization resulting from the large tensile stresses around a crack. The high fracture toughness for the YTZP ceramics may be attributed to tetragonal-to-monoclinic phase transformation constraint and its easy release during crack propagation. These results suggest that the combination of gel casting technology and two-stage sintering process can be a good candidate for preparing YTZP ceramics with complex shapes and superior mechanical properties.