The Effects of Compressive Residual Stress on Properties of Kyanite-Coated Zirconia Toughened Alumina Ceramics.

In this study, the prestressed coating reinforcement method was employed to create kyanite-coated zirconia toughened alumina (ZTA) prestressed ceramics. Due to the mismatch of the coefficient of thermal expansion (CTE) between the coating and substrate, compressive residual stress was introduced in the coating. The effects of compressive residual stress on the mechanical properties of ZTA have been demonstrated. Results show that the flexural strength of the kyanite-coated ZTA ceramics improved by 40% at room temperature compared to ZTA ceramics. In addition, the temperature dependence of mechanical properties has also been discussed. And the results show that the reinforcement gradually diminished with increasing temperature and eventually disappeared at 1000 °C. The modulus of elasticity of the material also exhibits a decreasing trend. Furthermore, the introduction of the prestressing coating enhanced the thermal shock resistance, but the strengthening effect diminished as the temperature increased and completely disappeared at 800 °C.

Thickness effect of an alumina-zirconia-mullite composite coating on the properties of zirconia.

It is predicted that the thickness of a coating has major effects on a substrate in terms of mechanical and thermal properties. In this study, an Al2O3-ZrO2-SiO2 slurry was prepared as a coating material, which formed an alumina-zirconia-mullite composite coating after sintering. The alumina-zirconia-mullite composite coating was coated on a zirconia substrate to generate compressive stress in the coating due to the mismatch of the coefficient of thermal expansion (CTE). A series of coated samples with different coating thicknesses from ∼10 µm to ∼200 µm were prepared to investigate the effects of coating thickness. The residual compressive stress, thermal conductivity, CTE, and Young's modulus of the coating material were determined by relative methods, and the flexural strength of the coated and uncoated samples was measured by 3-point bending. The strength of the coated samples was 1362.98 ± 30.29 MPa, which is a 54.07% enhancement compared to the uncoated samples. The thermal conductivity of the coated samples was also increased compared to that of the uncoated samples. For a given thickness of the substrate of 2 mm, there was an optimum thickness of the coating of 90 µm, which showed the greatest strength compared to the other samples. Coatings that were too thin or too thick did not show the best reinforcement. Moreover, the porosity of the coated samples was also determined and discussed in this study. Comparison samples without SiO2 were also manufactured, and their flexural strength and thermal conductivity were found to not be as good as the samples with SiO2.

Ultra-Fast Thermal Shock Evaluation of Ti2AlC Ceramic.

In this work, the rapid thermal shock behavior of Ti2AlC ceramics was studied using induction heating. The present evaluation method possesses the merits of very rapid heating within tens of seconds and fast quenching in water of less than 0.1 s, removing the shortcomings of traditional thermal shock. For comparison, the samples were also quenched in the air to investigate the thermal shock mechanisms. The results showed that the abnormal shock occurred in the samples when quenching in water, ascribed to the formed oxide layer on the surface of Ti2AlC ceramic inhibited the water penetration into the substrate. The quenched Ti2AlC samples still had a high residual flexural strength above 167 MPa up to 1150 °C, exhibiting promising applications in the high-temperature fields.

[Finite element analysis of total hip replacement].

Total hip replacement is a very effective method to cure many kinds of hip joint illnesses. About a century ago, it was first used in clinic. Since then, total hip replacement has been well developed. Hip joints sustain the most load of body, so people pay great attention to the hip prothesis' mechanics property. Especially after the finite element analysis was widely used in biomechanics investigation, the stress distribution of different designs can be easily compared with each other, and the relatively better parameters of the design could be decided. The stress distribution of different materials with the same design also can be valued. However, studies have indicated that total hip joint replacement still has some disadvantages. Loosening of the hip prothesis is still the most likely cause of the failure of surgery, and generally this is believed to stem from either mechanical failure of the fixation in response to over high density stresses, or osteolysis of the surrounding bone stock responsing to particular wear debris. Many researchers on computational studies have considered the potential for the former one, but only a few have attempted to tackle the latter. The process of osteolysis of the bone is not yet completely known. Nowadays, in order to solve the problems of loosening, investigators are trying to find different methods. Some of them are working on improving the geometry parameters and the shape of the hip prothesises, some are trying to find new suitable biomaterials, and, at the same time, the fixation methods are under deliberation.

[Analysis of the fracture processes in all-ceramic crowns by finite element analysis].

OBJECTIVE: To analyze the effects of core material and design on the fracture mechanism of veneered all-ceramic crowns. METHODS: The fracture process of 6 veneered alumina or zirconia crowns with different core design (well-distributed core, not well distributed core, and core with cervical ring) under load was analyzed by RFPA'2D finite element analysis software. RESULTS: All the six tested crowns fractured due to tension failure, and the crack started at the porcelain in the cusp and spread along the interface between core and porcelain. Under the conditions of this test, the break was only related to the porcelain and not the core, and the crack of porcelain took place earlier in zirconia crowns than in alumina crowns. Minimum stress distribution in cervical ring core design crown and maximum stress distribution in not well distributed core design crown could be seen at the neck area. CONCLUSIONS: Zirconia crowns presented greater stress at the interface between core and porcelain than alumina crowns. The not well distributed core design did not increase the rise of break. The neck area was the weak area with tensile stress concentration in the cervical ring core design.

[Development of hip joint simulators].

Currently, hip replacement is an effective treatment for some hip joint diseases. The friction and wear of a prothesis in the human body are the main causes for the failure of joint arthroplasty. It is, therefore, very important to simulate the working conditions of a hip prosthesis in order to get an optimal design and successful clinical applications. This article summarizes wear testing methods of hip prostheses and the developing status of their simulators. Three key aspects of the simulators, i.e., the structures, motions and lubrications, are analyzed in detail. At the end, the developing trend of the simulators are discussed.

[Effect of core: dentin thickness ratio on the flexure strength of IPS Empress II heat-pressed all-ceramic restorative material].

OBJECTIVE: To evaluate the effect of core:dentin thickness ratio on the flexure strength, fracture mode and origin of bilayered IPS Empress II ceramic composite specimens. METHODS: IPS Empress II core ceramic, dentin porcelain and bilayered composite specimens with core:dentin thickness ratio of 2:1 and 1:1 were tested in three-point flexure strength. Mean strengths and standard deviations were determined. The optical microscopy was employed for identification of the fracture mode and origin. RESULTS: The flexure strength of dentin porcelain was the smallest(62.7 MPa), and the strength of bilayered composite specimens was smaller than single-layered core ceramic(190.2 MPa). The core: dentin ratio did not influence the strength of bilayered composite specimens. The frequency of occurrence of bilayered specimen delaminations was higher in the group of core: dentin thickness ratio of 1:1 than in the group of 2:1. CONCLUSION: IPS Empress II core ceramic was significantly stronger than veneering dentin porcelain. Core:dentin thickness ratio could significantly influence the fracture mode and origin, and bilayered IPS Empress II ceramic composite specimens showed little influence in the fracture strength.