Preparation and Properties of Pre-Stressed Ceramic Plate

Ceramics are widely used because of their excellent mechanical properties, electrical insulation properties, high temperature resistance and corrosion resistance. Ceramic plate is one of important branch of ceramic components. However, brittle fracture caused by low toughness and high sensitivity to surface defects limits the reliability of ceramics. Therefore, there are a lot of research on strengthening and reducing defects of ceramic plate. In this work, a pre-stressed ceramic plate was obtained by a simple method, which was impregnated with a layer of alumina slurry on the surface of zirconia substrate and sintered at 1500°C. Because the expansion coefficient of alumina is lower than that of zirconia, a compressive stress is formed on the surface of plate, which can reduce the defect sensitivity and improve the bending strength of ceramic plate. Compared with the pure zirconia plate, the strength was improved with about 40%.


Introduction
Ceramic materials are widely used in the fields of aviation, aerospace, ship, weapon, electronics and nuclear industry because of their high insulation properties, excellent corrosion resistance and high mechanical strength. Ceramic plate, as an important branch of ceramic products, has important applications in armor protection, human body protection, mobile phone communication, integrated circuit and other fields [1][2]. However，brittle fracture caused by low toughness and high sensitivity to surface defects limits the reliability of ceramic plate. Therefore, there are a lot of research on strengthening and reducing defects of ceramic plate, such as Particle enhancement, fiber enhancement, grain refinement, surface enhancement, etc., have been proposed to enhance the mechanical properties of ceramics and obtained great achievements [3][4][5][6]. However, large-scale and economical fabrication technology has not been achieved.
In this paper, we apply a pre-stressed design model to realize the pre-stressed enhancement of ceramic plate by a simple process, which is expected to obtain industrial application.

Experimental procedure
ZrO 2 ceramic plate were made by the following steps: 1) gel injection molding to prepare green body; 2) binder removal at 800C; 3) the substrate samples pre-sintered at 1100C for 2 hours; 4) dip to the slurry to obtain Al 2 O 3 coating; 5) drying; 6) both Al 2 O 3 coated ZrO 2 and uncoated ZrO 2 samples were sintered at 1500°C for 2 hour.
The formulation gel injection molding and the alumina slurry are shown in table 1 and table 2 respectively. The preparation process of pre-stressedZrO 2 ceramic plate as shown in figure 1. The particle sizes of 3Y-ZrO 2 powders and Al 2 O 3 powders were 0.4 m and 0.5 m respectively. In the

Pre-stressed Design
To generate compressive stresses in the surface layer for ceramic plates, it is required that the coating material have a lower coefficient of thermal expansion (CTE) and similar sintering temperature relative to the base material. Since the CTE of the coating is lower than that of the base, the surface compression stress will be generated during the cooling process after high temperature sintering. Cross sections of square and circular substrates are shown in figure 2.
Due to the expansion coefficient of alumina is lower than that of zirconia, a compressive stress is formed on the surface of plate. The preliminary study shows that the pre-stressed can be pre-designed by adjusting the ratio of the coefficient of thermal expansion (CTE) and the ratio of the cross-sectional area of the base material and the coating material, so as to meet the specific engineering requirements. The pre-compressive stress can be calculated by equation (1) Where, is thepre-stressed; / is the ratio of the cross-sectional area of substrate to coating; / is the ratio of the elastic modulus of substrate to coating; / is the ratio of the CTE of substrate to coating; is theelastic modulus of substrate; is the CTE of substrate;  T c is the difference between sintering temperature and service temperature.

Effect of pre-stress on flexural strength
We have designed and fabricated pre-stressed ZrO 2 ceramic plates as shown in Figure 3. The measured flexural strength of ten pre-stressed ZrO 2 plate specimens is 1288 MPa (Figure 4(a)), which is 40% higher than its uncoated counterpart (920MPa).Tight bonding between the coating and the substrate indicates a good sintering compatibility as shown in Figure 4

Effect of cross-sectional area ratio of substrate to coating on flexural strength of the pre-stressed ceramic plate
We designed and manufactured pre-stressed ZrO 2 ceramic plates with different cross-sectional area ratio of the base material and the coating material, as shown in figure 5.The flexural strength of ceramic plates with different cross sectional area of the base material and the coating material was measured. The influence of cross sectional area on the flexural strength of ceramic plates was consistent with the predicted value of equation (1), as shown in Figure 6.