The basic mechanical properties of advanced ceramics are crucial for the design, preparation, and safe application of ceramic components. Although there are various standards at home and abroad to specify the testing of these properties, young researchers and designers of ceramics often have some problems in terms of operation or sample preparation during the performance testing process. In order to facilitate accurate understanding and acquisition of the basic mechanical properties of advanced ceramics, the main testing principles and standard testing methods by bending for elastic modulus, bending strength, and fracture toughness of advanced ceramics were summarized in this paper. The most commonly used standard testing methods, as well as related sample requirements, testing devices, testing procedures, and calculation methods are discussed. It is helpful and for accurate characterization and evaluation of the mechanical properties of advanced ceramic materials.

The research of prestressed ceramics which fabricated by introducing the prestress in the surface layer of ceramics is a hot topic in the ceramics field. In this work, the research progresses of preparation method, reinforcing effects and the evaluation of residual stress of prestressed ceramics are summarized. Results indicated that prestressed coating reinforcement method was a simple, low-cost and effective way to strengthen ceramics. By introducing residual compressive stress in the surface layer of ceramics, the fracture energy and impact resistance of brittle materials were improved effectively. That was attributed to the compressive stresses can inhibit crack initiation. Thus the service life of ceramic components was extended. Up to now, this method was applied on architectural ceramics, domestic ceramics, structural ceramics and functional ceramics. According to the reported works, the flexural strength of prestressed Al₂O₃ was increased by about 37%~39% compared to the uncoated Al₂O₃ ceramics. And the pre-stressed ZrO₂ ceramic achieved a flexural strength of 32%~45% higher than that of the common ZrO₂ ceramic. While the prestressed porcelain tiles possess a 50%~102% higher flexural strength than that of uncoated counterpart. For the functional ceramics, prestress strengthening method was applied on the preparation of solid electrolyte successfully. Due to the sufficiently high compressive stress in the surface layer of solid electrolyte, cracks and dendrite penetration were restrained. Hence, a controllable, high-performance and mechanically stable solid electrolyte was obtained. Above all, the reinforcing effects of residual stress on ceramics were significantly effective.

In order to illuminate the strengthening mechanism of prestressed ceramics, indentation deformation was used to understand the effect of residual stress on crack propagation. By comparing the length and the expanded direction of crack in ceramics with and without coating, the form of residual stress was cleared. In addition, the residual stress can also be determined by using relative method.

In general, prestressed coating reinforcement method is a novel and effective preparation technology, which can also be applied in the concrete and the glass fields. The pre-stressing design was universal and has great application prospects.

It is well known that the compressive stress in surface layer can significantly improve the flexural strength and impact resistance of brittle materials, such as prestressed concrete and tempered glass. For the past century, many prestressed concrete and tempered glass have been manufactured successfully and widely used in various fields. However, little progress has been made for ceramic materials because of the high melting temperature and the intrinsic brittleness of ceramics. How to achieve the preset compressive stress on the ceramic surface is the key issue for prestressed ceramic research. This work introduces two ways for the formation of compressive stress on ceramic surface, namely surface coating method and surface extrusion method. Both advanced ceramics and traditional ceramics can use prestressing reinforcement to significantly improve the strength and durability of ceramic components. For advanced oxide ceramics, prestressing can increase the flexural strength by 40%–50%, while for traditional ceramics, the strength can be doubled. In addition, the structural-functional integration of the alumina ceramic substrate was achieved, i.e. both the strength and thermal conductivity of alumina sample were improved through the mixed coating. Besides, prestress strengthening method was applied to prepare the high-performance and mechanically stable solid electrolyte. Due to the sufficiently high compressive stress in the surface layer of solid electrolyte, cracks and dendrite penetration were restrained. Therefore, a controllable and long-life solid electrolyte was obtained. In general, by introducing compressive stress in the surface layer of ceramics, the fracture energy and impact resistance of ceramics were improved effectively.

The main factors affecting surface prestressing by coating method are the ratio of expansion coefficient and the ratio of elastic modulus between the coating and the substrate material, as well as the ratio of cross-sectional area. It was found that, to generate the compressive stress on ceramic surface by coating method, it is required that the coating material has a lower coefficient of thermal expansion (CTE) and similar sintering temperature relative to the substrate. The optimal CTE condition for a pre-stressed ceramic component was described as α_c/α_s＜0.83. Besides, both theoretical analysis and experimental results show that the compressive stress and crack resistance in the surface layer increase with the increasing ratio of the cross-sectional area of substrate to coating. While the main influencing factors of surface extrusion method are the temperature and time of ion exchange. As the temperature and the time of ion exchange increasing, the effect of prestress reinforcement improved firstly and then remained unchanged.

Furthermore, inspired by the design of pre-stressed ceramics, the sintering deformation of Al₂O₃ components would be controlled by taking advantage of the difference in shrinkage between substrate and coating. By adjusting the composition and the position of coating, the Al₂O₃ ceramics with different shapes can be fabricated successfully. Besides, according to the deformation level, this method can be used to select the suitable coating raw materials. Generally, the way to fabricate the shape controlled ceramic components by the gradient shrinkage is a novel and effective method, which can be used for the plate and shell products with complex shapes.

It is well known that the residual stress is not a constant. It is related to the position, the ratio of CTE of substrate to coating, interface bonding, and so on. The form and the magnitude of residual stress affect the mechanical properties of specimens. Hence, the evaluation of residual stress in prestressed ceramics is very important for their engineering applications. To evaluate the residual stress in prestressed ceramics, indentation deformation was used to illuminate the effect of residual stress on crack propagation. By comparing the length and the expanded direction of crack in ceramics with and without coating, the form of residual stress was cleared. Results show that the compressive stress could hinder the crack extension, while the tensile stress could promote the crack extension. Besides, the residual stress could be calculated via relative method. The formula derivation and experimental results show that the residual stress is determined by the elasticity modulus, the coefficient of thermal expansion, the cross-sectional ratio of the coating to the substrate and the temperature and the temperature. And it decreases with increasing temperature because of the stress relaxation.

Summary and prospects By introducing preset compressive stress on the ceramic surface through the surface coating method or the surface extrusion method, the flexural strength and impact resistance of pre-stressed ceramics could be improved significantly. The relative research shows that the flexural strength of structural ceramics, architectural ceramics and domestic ceramics prepared by this new method could increase by 50%, 70% and 100%, respectively. The simple and economical pre-stressing design has shown great application prospects in the architectural ceramics, domestic ceramics, structural ceramics and structural–functional integration ceramics. Moreover, there is no limitation of size and shape in the fabrication of pre-stressed ceramic components, which is suitable for production of industrialization.

The prestressing reinforcement has been proved to be effective for ceramics, but how this surface prestressing affects other mechanical properties is still unclear. The influence of surface residual compressive stress on the mechanical properties of composite members, including bending strength, elastic modulus, fracture toughness, hardness and damage tolerance are investigated in this work. In order to achieve suitable surface compressive stress and the appropriate interface shear stress, mullite and aluminum oxide are mixed and ground into a slurry with a mass fraction ratio of 1∶1 and coated on the pre-sintered alumina substrate. The coated samples were prepared by pressureless sintering. With the temperature drop, different shrinkage rates and interface constraints of the two materials form the residual compressive stress in the surface layer. Mechanical properties of the coated alumina were tested. The experimental results show that the prestressed design can effectively improve the bending strength of alumina by 38.9%; The fracture toughness is increased by 36.5%, and the elastic modulus and hardness are slightly decreased. The damage tolerance of the prestressed ceramic is increased from 0.3894 m^1/2 to 0.4527 m^1/2, which is 16.26% higher.

A pre-stressing design and a simple fabrication technology to substantially improve the strength of ceramic components are presented. Residual surface compressive stress is generated in ceramic components by pressureless sintering of a green bulk coated with a thin layer of low coefficient of thermal expansion (CTE). The stress level can be controlled by changing the cross-section area ratio, Young's modulus ratio and CTE ratio of the coating. Pre-stressed ZrO₂ ceramics coated with Al₂O₃ can achieve a flexural strength of 1330 ± 52 MPa, 45% higher than their uncoated counterpart. Similarly, the flexural strength of building porcelain tiles is increased by 70%, from 67 ± 3 MPa to 114 ± 5 MPa. The damage tolerance of pre-stressed ZrO₂ ceramics is excellent with a high residual strength of ~1200 MPa in a thermal shock test at 325 ℃. This simple technique can improve the mechanical performance of ceramic components with no limitation of size and shape.

The accurate evaluation of the elastic modulus of ceramic coatings at high temperature (HT) is of high significance for industrial application, yet it is not easy to get the practical modulus at HT due to the difficulty of the deformation measurement and coating separation from the composite samples. This work presented a simple approach in which relative method was used twice to solve this problem indirectly. Given a single-face or double-face coated beam sample, the relative method was firstly used to determine the real mid-span deflection of the three-point bending piece at HT, and secondly to derive the analytical relation among the HT moduli of the coating, the coated and uncoated samples. Thus the HT modulus of the coatings on beam samples is determined uniquely via the measured HT moduli of the samples with and without coatings. For a ring sample (from tube with outer-side, inner-side, and double-side coating), the relative method was used firstly to determine the real compression deformation of a split ring sample at HT, secondly to derive the relationship among the slope of load-deformation curve of the coated ring, the HT modulus of the coating and substrate. Thus, the HT modulus of ceramic coatings can be evaluated by the substrate modulus and the load-deformation data of coated rings. Mathematic expressions of those calculations were derived for the beam and ring samples. CVD-SiC coatings on graphite substrate were selected as the testing samples, of which the measured modulus ranging from room temperature to 2100 ℃ demonstrated the validity and convenience of the relative method.