Papers by Author: De Tian Wan

Abstract: Tensile bond strength is one of the most significant properties for structural silicone sealants used in the glass curtain walls. During the service process, aging of the silicone sealants shall be involved in comprehensive actions of environment factors, e.g. temperature, humidity, and ultraviolet light etc. In this study, artificial accelerated aging test was conducted to make clear the development of tensile bond strength, Shore hardness and elongation. The test results show that: (i) the specimens under damp-heat test have more degeneration than specimens under humidity-freeze test; (ii) the environment of high temperature and high humidity leads to the change of tensile bond, Shore hardness, elongation, and results in interfacial failure of samples.

Abstract: Alumina ceramics are widely used in the demanding high temperature applications in which the high temperature elastic moduli (E_HT) is a key property for their reliability and safety. In this paper, the elastic modulus of alumina was determined by dynamic method (impulse excitation technique) and static tests (three-point bending test and four-point bending test). For the static tests, the relative method was applied to determine the accurate deflection measurement in the heating furnace. The measured results revealed that the modulus of alumina slowly decreased from RT to 1000 °C and rapidly decreased with the increasing temperatures from 1000 °C to 1300°C. The E_HT evaluated by dynamic method were higher than that tested by static tests with the reason of that impulse excitation technique only applied small forces onto a sample such that defects activity is negligible. Also the resonant frequencies couldn’t be measured easily at high temperature, because the vibration signal emitted by the sample was weak. The static approaches combined with relative method were beyond the limit to high temperatures, and they can be also used to evaluate the ultra-high temperature modulus.

Abstract: In order to overcome the limitations of common methods for measuring elastic modulus of the ultra-thin glass，a novel method is proposed for measuring the elastic modulus of the ultra-thin glass by cantilever beam vibration method．The measurement principle，apparatus，and manipulation were introduced. In this method, the measured ultra-thin glass is generated in the form of a cantilever beam, and the cantilever beam can be vibrated for a short time when transient excitation is applied. The natural frequency of the cantilever beam can be used to calculate the elastic modulus of the ultra-thin glass material. A series of ultra-thin glass samples with different thickness were tested in this work, the measured values are consistent with that given . The effect of beam length/thickness ratio on determination of elastic modulus of materials by means of cantilever method was analyzed, the result shows that the influence of the ratio can be neglected if the beam length is equal to or more than 10 times of the beam thickness.

Abstract: A simple test approach named relative method is developed for determining the thermal expansion coefficient of ceramic coatings. Although ceramic coatings are hardly separated from the substrates, it was evaluated in this work simply by need only the measured thermal expansion coefficient of coated samples and substrates. This novel method was demonstrated to be valid for rectangular beam samples of two types of coating configurations: sandwich coating and around coating. The feasibility of this test method was confirmed by experimental results of SiC coating.

Abstract: The elastic modulus and fracture strength at ultrahigh temperatures over 1500°C is very important for Ultrahigh temperature ceramics (UHTCs), but no available method can be used so far because of the limitation of the testing equipments. In this work, a novel testing equipment was developed to determine the elastic modulus and fracture strength (bending, tensile and shear strength) of ceramics from 1500 °C to 2200 °C. To check the availability and reliability of this equipment, several typical UHTCs including C, C/C fiber woven composite and C/SiC ceramic composite, were used as the testing samples. The results indicate that this new designed testing equipment is a good and feasible for evaluating the ultrahigh temperature mechanical properties of ceramics over 1500 °C in vacuum or in air.

Abstract: Alumina is a typical ceramic material and possesses high strengthand stiffness at both room temperature and high temperature. The split ring methodhad been established to evaluate the elastic modulus and bending strength of aluminatube materials at ambient temperature. However, both equations for modulus andstrength became lightly inapplicable with the increased temperature. For theelastic modulus, it was lack of precise approaches and advices for deformationmeasurement in the heating furnace. For the bending strength, changes of sampledimensions due to thermal expansion would take an effect on the calculatingresults. In this work, several improvements have been taken into account tocalibrate the above deviations. Results revealed that the modulus and strengthregularly decreased from room temperature to 1300 °C and accorded well with other conventional testing methods.It proved the accuracy and reliability of this modified split ring method,which might be used to evaluate other ceramic tube materials at hightemperature.

Abstract: Evaluation of the mechanical properties at ultra-high temperatures for ceramic composites is necessary and important for the safety of designing the ceramic components. In this work, a new and novel test method named as local ultra-high temperature together with applied load method (LUHTAL), was developed to determine the tensile, compressive, bending strength and fracture toughness of ceramic composites. The four point bending load was conducted to measure the bending strength and fracture toughness of ceramic composites after the center of the sample was heated up to about 1500-2000°C by oxygen-assisted spray combustion. To check the availability and reliability for this method, typical ceramic materials including ZrB₂/SiC and C/SiC fiber reinforced composite coated with Si, were used as the testing samples. It is indicated that this method is good and feasible for evaluating the mechanical properties of the ceramic composite at ultra-high temperatures in air.

Abstract: In this work, a new and novel test method was developed to determine the impact bending strength of ceramic composites at ultra-high temperature from 1500-2000 °C in air. Three-point impact bending test was carried out through a SiC pressure head with a dynamic force sensor fixed on a slider and movable along a guide rail. The impact load was adjusted by different saving energy and the impact speed was lower than 0.5 m/s. The center of the sample was heated up to about 1500-2000°C by oxygen-assisted spray combustion. An impact load was put on the specimen and the impact force was recorded automatically. The impact bending strength can be calculated from the maximal load and the sample size. To check the availability and reliability for this method, several ceramics including SiC, ZrB₂/SiC and C/C fiber reinforced composite without coating, were used as the testing samples. The results indicate that this method is a good and feasible method for evaluating the mechanical properties of the ceramic composite at ultra-high temperatures.

Abstract: Ti₃SiC₂-Al₂O₃ joint with strong interface has potential high temperature applications because it combines with the merits of hard ceramics and soft ceramics. The safety is strongly dependent on the interfacial bonding strength between Ti₃SiC₂ and Al₂O₃. In this work, the cross-section method was suggested to evaluate the tensile and shear bonding strength for Ti₃SiC₂-Al₂O₃ joint from room temperature to 800 °C in air. A novel testing fixture made of SiC was designed and machined to avoid the bending stress at the bonding surface during the testing process. It is indicated that the measured shear bonding strength is usually higher than tensile bonding strength for Ti₃SiC₂-Al₂O₃ joint. Both the tensile and shear bonding strength are decreased with the increment of testing temperatures. At 800 °C, the tensile and shear bonding strength are declined to be about 43.15% and 45.02% compared with those at room temperature, relatively. The mechanism for the strong interface between Ti₃SiC₂ and Al₂O₃ is also discussed.

Abstract: Adhesion is one of the most important mechanical properties of ceramic coatings. Scratch testing is considered as a simple and effective method to evaluate adhesion of ceramic coatings. In this paper, the critical normal forces and scratch morphologies for different coating-substrate systems were studied by scratch testing. It is shown that the critical normal force obtained by acoustic emission (AE) signals decreases from 12 N to 7 N when the applied normal force rate increases from 20 N/min to 100 N/min for CVD SiC on C, and the failure area of this scratched sample increases with increasing maximum normal force; Based on scratch morphologies, spallation or delamination can be observed for hard-brittle coatings on glass or metal, while discontinuous or continuous ductile perforation can be observed for ductile coatings on metal; The critical normal force for hard-brittle coatings can be effectively obtained by AE signals.