Alumina (Al2O3) (industrial pure grade), zirconia (ZrO2) (≥95% purity), and pseudo-boehmite (AlOOH) (industrial pure grade), were purchased from Nuoda chemical Co., Ltd (Zibo, China). Ammonium citrate ((NH4)3C6H5O7) (≥95% purity) was purchased from Bodi chemical Co., Ltd (Tianjin, China). Concentrated nitric acid (HNO3) (analytical pure grade) was purchased from Shijiazhuang reagent factory (Shijiazhuang, China). Silicon Carbide Whisker (SiCw) (industrial pure grade) was purchased from Jie Chuang materials technology Co., Ltd. (Xuzhou, China). All materials were used as received. Distilled water was generated from Key Laboratory of Inorganic Nonmetallic Materials of Hebei Province.

Methods

This experiment was designed by orthogonal method. The solid content was set at 37% based on former experimental results [13]. Density of each raw material was calibrated by pycnometer method before the mixture calculation [14]. The orthogonal design was shown in Table 1.

A mixture of distilled water, pseudo-boehmite, and ammonium citrate, with the mass ratio of 100: 9: 0.3 was added into a planetary ball mill and milled for 80 min at a speed of 190r/min. Then certain amount of Al2O3, ZrO2 (0.02 Y) and SiCw were added, followed by 80 min to get slurry. The pH of this slurry was adjusted to 3-4 by diluted nitric acid. Another 60-min mill was performed, followed by screening out mill beads, removing bubbles, and gel formation.

Gel was injected into molds coated with silicone grease and dried in air dry oven. During the drying process, temperature was increased from 35℃ (hold on 12 h) to 60℃. hold on 24 h). Sintering process was carried out as orthogonal test method. The sintered product was cut into ceramic sample with size 30mm ×5mm × 3.5mm.

Characterization

Bending strength of sample was measured by three-point bending test method with span of 25 mm, pressure head loading speed of 1 mm/min (GB/T 1445-1993). Water absorption was measured by Archimedes Drainage method [15]. X-ray diffraction pattern of samples was recorded from 20° to 80° at a scanning rate of 10°/min by D/MAX2500PC diffract meter. Microstructure of sample was investigated by ZEISS LEO Supra35 field emission Scanning Electron Microscope (SEM) (Carl Zeiss NTS Ltd., UK).

Results and Discussion

The Orthogonal Test Analysis

Orthogonal test results were listed in Table 1. The experiment was designed according to four factors and three levels orthogonal table. Four factors included ZrO2 content, temperature, time and SiCw content. Each factor set up three levels. The parameters involved in the calculation based on following equations

      (1)

Ki was defined as sum of bending strength at the level "i" corresponding to a factor

  (2)

Based on the value of bending strength was plotted against ZrO2 content, temperature, holding time, and SiCw content respectively as shown in Figure1.The results showed that bending strength increased as ZrO2 content, it reached to the maximum value with ZrO2 content of 35%. Among the three-temperature tested in this study, sample sintered at 1560℃ showed the best bending strength. Bending strength also increased along with holding time, it reached to maximum with holding time of 6h. While, the bending strength decreased with increase of SiCw content. Sample showed maximal bending strength, when SiCw content was 3%. Among the above four factors, SiCw content has the largest R, so it was the main factor that affected the bending strength of samples.

Based on above analysis, the best preparation process was summarized as following: ZrO2 content of 35%, sintering temperature of 1560℃, holding time for 6 h, and SiCw content of 3%. Among the four factors which could affect bending strength, SiCw content took the main role, followed by ZrO2 content, holding time and sintering temperature. And the R value of SiCw content was great higher than that of the other three factors. Because the optimum process was beyond the scope of the nine orthogonal tests, we need to do confirmatory test.

Confirmatory Test

SEM photo of samples from NO.5 orthogonal test and confirmatory test were shown in Figure 2. In orthogonal test, sample made by the No.5 test performed best in all characterization. Compared with NO.5 orthogonal test, sample of confirmatory test had better density on both surface and structure. And the particle size of confirmatory sample was more uniform. The bending strength of confirmatory sample was 267.80MPa, which was bigger than that of the NO.5 orthogonal test (250.61MPa). The result of confirmatory test demonstrated that the optimal process proposed in this study was correct.

Effect of SiCw Content on Samples

Orthogonal test results showed that among the four factors which affected bending strength, SiCw content took the main role. And bending strength was proportional to SiCw content within the content range measured in former orthogonal test. The effect of SiCw content on bending strength should be further researched. A larger SiCw content range with smaller gradient (set as 1.5%, 2%, 2.5%, 3%) was adopted in following experiment.

The bending strength of samples prepared with different SiCw content according to modified formula was shown in Figure 3. This figure showed that the ceramic samples had excellent bending strength, which increased as SiCw content fist and then decreased. The maximum of bending strength was 274.14 MPa with 2.5% SiCw content.

For ceramic dental bracket materials, water absorption should within the range of 0~2% to satisfy the application requirement [16]. Generally, there was certain amount of porosity in green body. Particles in ceramic body were supported by pseudo-boehmite network structure. After sintering, the touch surface between particles was increased, which leaded to particles accumulation. The center distance between particles decreased continuously as the result of particle volume shrinkage. Grain boundary was formed consequently. During sintering, pores in ceramic body were deformed, shrank, separated and removed. Finally, dense ceramic body was obtained. Water absorption testing results were shown in Table 2. Results showed that water absorption increased as the content of SiCw. When content of SiCw was higher than 2.5%, the water absorption was beyond the desired range of 0~2%. The small water absorption is corresponding small porosity to a certain extent in Archimedes method. Thus, among all SiCw contents tested in this study, sample with 2.0% SiCw content had the lowest porosity and highest density.

In the previous discussions, as SiCw content increased, sample has a better particle distribution and better bending strength, as shown in Figure 3. However, as SiCw content increased, water absorption of sample increased. Considering bending strength and water absorption, the maximum bending strength of 274.14MPa and qualified water absorption were obtained at 2.5% SiCw content. So, the best content of SiCw was 2.5%.

Toughening Mechanism

Toughening Mechanism of ZrO2

XRD pattern of the ceramic sample was shown in Figure 4. We can find that amount of non-room temperature phase tetragonal zirconia (t-ZrO2) was remained [17-19]. In this study, ZrO2 and Al2O3 were distributed evenly in electric double layers through the gelation function of pseudo-boehmite. An independent network structure was formed which could prevent the precipitation and stratification of ZrO2 and Al2O3 with different diameters. Thus, fine ZrO2 particles stacked homogenously in the matrix of Al2O3. ZrO2 and Al2O3 had different thermal expansion coefficient. So, during sintering and cooling, the expansion of Al2O3 particle had a compressive stress on ZrO2, which limited transformation of ZrO2 from t-ZrO2 to monoclinic zirconia (m-ZrO2). It leaded to the residues of t-ZrO2, as shown in Figure 4. When ceramic was under external shock and cracked, t-ZrO2 could change the conduction direction of crack, shield crack tip, and prevent the expansion of crack [20,21]. Thus, the brittleness of ceramic sample was decreased and bending strength was increased. In sum, ZTA ceramic prepared by pseudo-boehmite gel could not only increase ceramic density but also take full advantage of toughening effect of ZrO2.

Toughening Mechanism of SiCw

SEM images of fracture surface were shown in Figure 5. Whisker pull-out phenomenon could be observed in Figure 5 (a). When crack propagation encountered high strength whisker, near the crack tip, the interfacial shear stress between whisker and matrix reached to shear yield strength of matrix. The whisker would not break due to high tensile strength. Instead, it would be pulled out from the matrix to consume external load energy and generate fine crack to absorb more energy. In this way, ceramic was toughened by whisker [22].

The parts of Figure 5 (b) marked with white box showed the crack deflection mechanism. Crack deflection happened when the binding force between whisker and matrix interface was weak. When the cracks extended from matrix to whisker, crack would grow along grain due to the dissociation of whisker and matrix. Crack deflection could not only increase new surface area, but also prevent the crack grow beyond critical size, thus the strength has been increased [22].

In sum, toughening effect of SiCw was shown as whisker pull-out phenomenon and crack deflection in this experiment. Besides, the addition of SiCw had inhibition effect on particles growth in Al2O3 matrix. Thus, pores and cracks could be avoided due to coarse grain and quick grain boundary migration. All of these will benefit strength of ceramic materials

 Synergistic Toughening Mechanism

The toughness of ZTA-SiCw composite ceramics should be determined by the combination of whisker pull-out phenomenon, crack deflection, and t→m-ZrO2 (tetragonal zirconia -to-monoclinic zirconia) phase transformation toughening mechanisms.

Although SiCw had inhibition effect on particles growth in Al2O3 matrix, the residual content of t-ZrO2 was reduced as the addition of SiCw. As a consequence, the t→m-ZrO2 phase transformation toughening effect will decreased as the increase of SiCw content. While the toughening effect from crack deflection and whisker pull-out phenomenon were increase. The experiment results showed that, the synergistic toughening effect of the three-mechanism obtained maximum value when SiCw content was 2.5%. As the SiCw content continue to increase, the synergistic toughening effect decreased. In addition, quick furnace cooling rate in the experiment would lead to thermal expansion coefficient mismatch. And this mismatch could generate tensile stress to induce t→m-ZrO2 phase transformation. Consequently, t-ZrO2 content will be decreased, as well as its toughening effect.

In sum, the premise of synergistic toughening mechanism for ZTA-SiCw composite ceramic was that: (a) volume expansion generated from t→m-ZrO2 phase transition should not impede whisker pulling-out and separation from matrix interface [23]. (b) The addition of SiCw should not reduce residues of t-ZrO2. (c) A proper furnace cooling rate was necessary to minimize the phase transition of t→m-ZrO2 in advance.

Conclusions

• In this study, a structure stable ceramic green body was prepared with ZrO2, Al2O3, and SiCw evenly distributed in electric double layer network through gelation function of pseudo-boehmite.

• The addition of SiCw had obviously toughening and reinforcement effect on ZTA composite ceramic. As the content of SiCw increased, the strength of samples increased first then decreased. The maximum strength of 274.14MPa was obtained at 2.5% SiCw content. There was a synergistic toughening effect between phase transformation and whisker toughening.

• The best preparation technology of ZTA-SiCw dental brackets ceramics by pseudo-boehmite was summarized as following: ZrO2 content of 35%; SiCw content of 2.5%; holding time for 6h; and sintering temperature of 1560℃.