Comparing the influence of different kinds of zirconia on properties and microstructure of Al₂O₃ ceramics

2.1 Starting materials and sample preparation

The starting materials used in the present work were fused zirconia-corundum with its chemical composition of 59.4% α-Al₂O₃-33.2% m-ZrO₂-7.4% t-ZrO₂ (denoted as AZ40, ≤7 μm), 3 mol% yttria-stabilized zirconia with its chemical composition of 76.2% t-ZrO₂-23.8% m-ZrO₂ (marked as 3Y-ZrO₂, ≤7 μm), monoclinic zirconia (ZrO₂ ≥99.0%, ≤7 μm), and white fused corundum (α-Al₂O₃ ≥98.5%, ≤44 μm). The mass ratio of α-Al₂O₃ to ZrO₂ was 85/15, ZrO₂ was introduced into Al₂O₃ ceramics in forms of AZ40, m-ZrO₂, and 3Y-ZrO₂, denoted as Z₁, Z₂ and Z₃, respectively, and the sample without ZrO₂ addition was marked as Z₀. These materials were weighed in terms of this mass ratio and mixed for 1 h in a polyurethane bottle with water and zirconia balls, then pressed at 150 MPa into samples with sizes of 180 mm×15 mm×15 mm and ∅ 36 mm×36 mm, and sintered in air. The thermal profile was a ramp of 7°C/min to 1100°C, 2°C/min to 1200°C, and followed by a ramp of 3°C /min to 1650°C, held for 3 h.

2.2 Sample characterization

Bulk density and apparent porosity were measured using Archimedes’ principle in water medium. Permanent linear change was examined by measuring the change in length of samples before and after firing. The modulus of rupture at room temperature and at 1400°C were determined by the standard three-point bending method. Cold crushing strength was measured using a hydraulic press machine in accordance with ISO 10059-1:1992. In thermal shock tests, the samples were heated to 1100°C and then quickly quenched in air. After three thermal shock cycles, the residual strength (σ_r) was measured and compared with the original strength (σ_f). Residual strength ratio (σ_r/σ_f) at ΔT=1100°C was a criterion for evaluating thermal shock resistance.

The phase contents were conducted using Philips X-ray diffractometer (XRD) with Cu Ka radiation (λ=1.5406 Å) in the 2θ range of 20–80°C for a period of 3°/min in the step scan mode. Microstructure and composition of samples were investigated by scanning electron microscopy (SEM, JEOL JSM-5610LV) equipped with an energy-dispersive spectrometry (EDS).

3.1 Physical properties at room temperature

The permanent linear change, apparent porosity, and bulk density of samples are listed in Table 1, showing that the addition of m-ZrO₂ or 3Y-ZrO₂ can promote the process of sintering densification, while AZ40 addition results in a volume expansion and reduces the densification of Al₂O₃ ceramics.

Figure 1 shows the microstructures of samples Z₀, Z₂, and Z₃. In contrast to samples Z₂ and Z₃, the SEM photograph of the Al₂O₃ sample shown in Figure 1A has a microstructure with coarse grains and large gaps, indicating a weak interface between crystals. For the sample with m-ZrO₂ or 3Y-ZrO₂ addition (see Figure 1B and C, respectively), the presence of the two distinct phases, Al₂O₃ (darker phase) and ZrO₂ (brighter phase), can clearly be observed. It is clear that ZrO₂ particles are homogeneously distributed in corundum skeleton structure inhibiting abnormal grain growth of Al₂O₃ and leading the dense structure.

Figure 1

SEM photographs of samples (A) Z₀, (B) Z₂, (C) Z₃, (D) and (E) EDS spectrum of scanned sites 1 and 2 in (B), respectively.

The micrograph of the AZ40 starting material shown in Figure 2A is a typical eutectic microstructure, which is made up of faceted colonies with a size of around 10–15 μm that consist of ordered ZrO₂ phases within the Al₂O₃ matrix. When it is introduced into Al₂O₃ ceramics and sintered at 1650°C for 3 h, the eutectic microstructure no longer exists, which is substituted by the microstructure that consists of coarse ZrO₂ particles within the Al₂O₃ matrix, as shown in Figure 2B. The grown ZrO₂ particles cannot be suppressed by Al₂O₃ matrix, t-phase has transformed spontaneously to m-phase during the cooling process. This is supported by the XRD pattern shown in Figure 4A; it can be seen that there is few t-ZrO₂ in the sample. The phase transformation of these grown t-ZrO₂ particles are accompanied by the volume expansion and crack formation, as shown in Figure 2C, which leads to a decrease in the density of Al₂O₃ ceramics.

Figure 2

SEM photographs of (A) AZ40 starting material, (B) AZ40 after 3 h at 1650°C, (C) the sample with AZ40 addition.

Cold crushing strength and modulus of rupture of samples are shown in Figure 3. As Figure 3 shows, m-ZrO₂ or 3Y-ZrO₂ addition is beneficial to improving the room temperature strength of Al₂O₃ ceramics, and the effect of the latter is more distinct, while those of the sample with AZ40 addition decrease.

Figure 3

Cold crushing strength and modulus of rupture of samples.

The improvement of the room temperature strength of samples is primarily attributed to the stress-induced phase transformation toughening and the enhancement of density. Through XRD analysis on the surfaces of samples, as shown in Figure 4, monoclinic peaks at the (111̅) and (111) planes and tetragonal peak at the (111) plane of ZrO₂ are detected; the amount of t-ZrO₂ in samples is evaluated using Eq. (1) and Eq. (2) below [16]:

Figure 4

XRD patterns of the surfaces of samples (A) Z₁, (B) Z₂, (C) Z₃, (D) Z₃ after the modulus of rupture test.

Here, X is the relative content in the sample, I is the absolute intensity, and the subscripts m and t refer to the monoclinic and the tetragonal phase, respectively.

For the sample with 3Y-ZrO₂ addition, the amount of t-ZrO₂ is calculated and found to be 12% and 5% on the fracture face before and after sample failure, respectively, indicating that a portion of t-ZrO₂ has transformed into m-phase during the modulus of rupture test, the crack propagation can be prevented by compressive stress due to this stress-induced phase transformation. In addition, the sample with 3Y-ZrO₂ addition has a dense structure. All these are important strengthening effects contributing to improvement of the room temperature strength of Al₂O₃ ceramics. But there is only a negligible amount of t-ZrO₂ in the sample with m-ZrO₂ addition; thus, the slight enhancement of its room temperature strength can be explained by the dense structure. As shown in the marked area by the arrows in Figure 2C, there is obvious defects in the sample with AZ40 addition, which lead to a decrease in room temperature strength of Al₂O₃ ceramics.

3.2 Hot modulus of rupture

Figure 5 shows the hot modulus of rupture of samples at 1400°C. It can be seen that the addition of m-ZrO₂ or 3Y-ZrO₂ can improve the hot modulus of rupture of Al₂O₃ ceramics, the values of them are 20 MPa and 22 MPa, respectively, while that of the sample with AZ40 addition decreases to 9 MPa.

Figure 5

Hot modulus of rupture of samples at 1400°C.

Some reports have pointed out that the principal factors influencing high temperature mechanical properties are the crystal effect and the glass effect [17, 18]. In this work, there is practically no glass phase in the samples; the controlling factor is the crystal effect (degree and mode of contact or bonding between crystals).

Figure 6 shows the fracture surfaces of samples Z₁, Z₂, and Z₃ after hot modulus of rupture test. In comparison with sample Z₁, there are fewer gaps and pores between crystals in the sample with m-ZrO₂ or 3Y-ZrO₂ addition (see Figure 6C and D, respectively), indicating a strong interface between crystals. Moreover, the study on the bonding between Al₂O₃-ZrO₂ by SEM and EDS shown in Figure 7 and Table 2 indicates that at the Al₂O₃-ZrO₂ interface, there is interdiffusion at grain boundaries between the two phases. All these are important strengthening effects contributing to improvement of the hot modulus of rupture of Al₂O₃ ceramics. The SEM photograph of the sample with AZ40 addition shown in Figure 6A has a microstructure with large gaps and defects, the AZ40 particles grow during heat treatment to form larger particles with a diameter ranging from 8 μm to 20 μm due to the martensitic transformation of ZrO₂, and even some ZrO₂ particles drop out from the Al₂O₃ matrix in AZ40 particles (see Figure 6B). These defects result in a decrease in the hot modulus of rupture of Al₂O₃ ceramics.

Figure 6

Fracture surfaces of samples (A) Z₁, (B) the magnification of the marked area in (A), (C) Z₂, and (D) Z₃.

Figure 7

SEM photograph showing bonding between zirconia and corundum (the sample Z₃).

3.3 Thermal shock resistance

The results of the thermal shock resistance of the samples are shown in Figure 8. As can be seen in Figure 8, the three kinds of ZrO₂ are beneficial to improving the thermal shock resistance of Al₂O₃ ceramics, the residual strength ratios of samples Z₁, Z₂, and Z₃ are 62%, 68%, and 71%, respectively, and the maximum residual strength of 44 MPa is achieved from the sample Z₃.

Figure 8

Residual strength ratio and residual strength of samples (ΔT=1100°C).

In order to further evaluate the thermal shock resistance behavior, the microstructures of samples Z₁, Z₂, and Z₃ after thermal shock tests are examined, as shown in Figure 9. Some microcracks are observed on the surfaces of all the samples due to the spontaneous transformation of ZrO₂, which can relax the stress concentration to enhance the thermal shock resistance of Al₂O₃ ceramics. In addition, for the sample with 3Y-ZrO₂ addition, t-ZrO₂ can prevent crack propagation by compressive stress due to the stress-induced phase transformation, which is beneficial to enhancing the residual strength of Al₂O₃ ceramics.

Figure 9

SEM photographs of samples (A) Z₁, (B) Z₂, (C) Z₃ after thermal shock tests at ΔT=1100°C.