| 其他摘要 | Layered ternary carbide Ti3SiC2 processes unique properties combining excellent characteristics of metals and ceramics such as low density, high modulus and fracture toughness, high electrical and thermal conductivity, good machinability, damage tolerance at room temperature, and good resistance to thermal shock and oxidation below 1100 oC. However, the low hardness, poor wear and strength impairing contact damage resistance, and unsatisfied oxidation resistance above1100 oC limit the application of Ti3SiC2 as high-temperature structural components. In this dissertation, Ti3SiC2 was strengthened to wide its application field with a qualification of keeping its merits such as low density, high modulus, and high electrical and thermal conductivity. The methods used are by forming particulate reinforcement composite and sandwich composite to strengthen Ti3SiC2. Furthermore, because fracture toughness is one of the most important material properties in fracture mechanics for ceramic materials, we suggest a new, simple, and reliable method to evaluate the toughness of ceramic materials.
Ti3Si(Al)C2/SiC composites were fabricated by the in situ hot pressing/solid-liquid reaction synthesis process from elemental powders of Ti, Si, Al, and graphite. The effects of SiC on the microstructure, mechanical properties at room and high temperatures, and wear and oxidation resistance were systemically investigated. Because the grain growth of Ti3Si(Al)C2 is inhibited by the presence of SiC through a pinning mechanism, the grain size of Ti3Si(Al)C2 is decreased with increasing SiC content in the composites. Compared with monolithic Ti3Si(Al)C2, the Ti3Si(Al)C2/SiC composites exhibit higher elastic modulus, Vickers hardness, fracture toughness, shear strength, contact damage resistance, improved wear and oxidation resistance, but have a slight loss in flexural strength. The enhancement of the properties of Ti3Si(Al)C2 is mainly ascribed to the excellent properties of SiC particles, and the strength degradation is due to the tensile stress in the Ti3Si(Al)C2 matrix.
(TiZr)3(SiAl)C2/(ZrTi)C composites were in situ synthesized from elemental powders of Ti, Zr, Si, Al, and graphite. The grain size of (TiZr)3Si(Al)C2 is decreased with increasing (ZrTi)C content in the composites. The addition of (ZrTi)C particles increases the elastic modulus, hardness, and flexural strength. The fracture toughness reaches the highest value in (TiZr)3(SiAl)C2/10 vol.% (ZrTi)C composite. The strengthening and toughening mechanisms were discussed. Based on the measured high-temperature mechanical properties, the suitable temperature of the composites used in air should be below 600 oC. But in high vacuum, it is up to 1300 oC.
The Al2O3/Ti3SiC2/Al2O3 sandwich composites with strong interfaces were in situ fabricated to strengthen Ti3SiC2. The obtained maximum strength of the sandwich sample was 14.5% higher than that of monolithic Ti3SiC2 when the optimum ratio of the Al2O3 coating thickness to the substrate thickness was 0.10, which agreed well with the calculated (0.087). The hardness of the surface was greatly improved because of the high hardness of Al2O3. The mechanism for the strong interface between the Ti3SiC2 and Al2O3 layer is due to the fact that the Si and Ti-Si liquid phases are formed and penetrate into the Al2O3 layers during the fabrication process. Based on the concepts of equivalent stiffness and three-point bending model, a modified relative method was derived to calculate the elastic modulus and strength of the unsymmetrical coating on the substrate.
The effects of grain size, notch width, and testing temperature on the fracture toughness of Ti3Si(Al)C2 and Ti3AlC2, were investigated using the chevron notched beam (CNB) method. For a fixed notch width and testing temperature, the fracture toughness of coarse-grained samples is higher than that of fined-grained samples. The critical notch width for valid KIC measurements of this kind of quasi-plastic ceramic is about 250 m. The high-temperature toughness of Ti3Si(Al)C2 and Ti3AlC2 with different grain sizes are insensitive to the testing temperatures before the brittle-ductile transition temperature (DBTT) (about 1100 oC), but it declines fast over 1100 oC. The degradation of the fracture toughness is mainly imputed to the decline of elastic modulus at temperatures above DBTT.
A new and simple method named single gradient-notched beam (SGNB) method was introduced to determine the fracture toughness of ceramic materials. Fracture toughness KIC can be calculated from the maximum load and shape factor under the assumption that the derivative of the compliance for a specimen with a gradient notch with respect to the relative length is the same as that for a specimen with a straight through crack. Using Ti3Si(Al)C2 and Al2O3 as the testing samples, the agreement among the KIC values obtained from the SGNB method, from the chevron notched beam method (CNB), and those from the single edge notched beam method (SENB) for the same notch width samples, demonstrates that the SGNB method is a simple and reliable method for fracture toughness determination of ceramics. |
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