Microstructure And Properties Of In Situ Synthesized Ti₃SiC₂/SiC Ceramics By Hot Isostatic Pressing

In this dissertation, Ti3SiC2/mSiC multiphase ceramics with high SiC contents and high relative density which could be used to make high-temperature structural parts were in-situ fabricated with TiH2, SiC and graphite powders by hot isostatic pressing (HIP). Optical microscopy (OM), X-ray diffraction (XRD), thermo-gravimetric analysis (TG), in-situ observation of microstructure at high temperature, scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy(EDS) were used to systematically investigate characteristics of high temeperature oxidation resistance and oxidation mechanism of Ti3SiC2/mSiC ceramics. Densification mechanism, microstructure, mechanical properties at room and high temperature were intensively analyzed. The main conclusions are as follows:1. Dense Ti3SiC2/mSiC ceramics were fabricated with SiC, TiH2 and graphite powders by using an in-situ HIP under 1500-1600 ℃ with a pressure of 100MPa for 2-4h, in which the volume fraction of SiC reached 70%(here maximum m was 8mol) and its relative density was more than 98%. All the samples showed the main phases were Ti3SiC2, SiC and a little TiC. Their microstructures were fine and uniform. Dense Ti3SiC2/mSiC ceramics can be cut by electric spark cutting because of their good conductivities. The resistivity is lower than 150μΩ·cm when the volume fraction of SiC is less than 67%.2. Effects of SiC content, powder size and hot isostatic pressing parameters upon Ti3SiC2/mSiC ceramics’ relative density were analysed. Rigid sphere packing model was used to explain densification process of Ti3SiC2/mSiC ceramics. Hard SiC particles were stocked uniformly in space, and the plastic phase, Ti3SiC2, could fill into their clearances under high temperature. Thus SiC content had a significant influence on the density of Ti3SiC2/mSiC ceramics.3. The results of Vickers’ hardness and fracture toughness showed that the hardness of dense Ti3SiC2/mSiC ceramics increased with increasing of SiC content while the fracture toughness decreased correspondingly. The path of crack extension in Ti3SiC2 matrix was influenced by the grain orientation deflection, small SiC particles can cause the crack deflection, while the large SiC particles lead to crack through. The fracture toughness of Ti3SiC2/mSiC ceramics was enhanced with crack deflection.4. The results of three points bending strength tests showed that the flexural strength of Ti3SiC2/mSiC ceramics firstly increased with the increase of temperature below about 1000℃ because of stress concentration elimination caused by surface oxidation, and then decreased with the increase of temperature. The flexural strength of Ti3SiC2/mSiC ceramics at 1200℃ was reduced by half comparing with that of the room temperature.5. Results of tensile tests showed that low temperature fracture of Ti3SiC2/mSiC ceramics was a typical brittle fracture. It showed certain ability of plastic deformation at high temperature over 1000℃. Young’s modulus (E) of Ti3SiC2/mSiC ceramics decreased with the increase of temperature and E of Ti3SiC2/7SiC were 100GPa,20.6GPa,9.8GPa, respectively, when the temperature were 900 ℃,1000℃ and 1100 ℃.6. High temperature lasting and creep performance were investigated under tensile loading mode. Results showed that at the test condition the creep strain rate was in the order of 10-8-10-7S-1. The lasting lifetimes of Ti3SiC2/7SiC can be fitted to the follow equation: t=exp(17.6)σ-2.5 (min). High temperature creep performance of Ti3SiC2/7SiC ceramics with no accelerated deformation stage was different from that of the single-phase Ti3SiC2. The creep activation energy of Ti3SiC2/7SiC ceramics was 691.7KJ/mol. The creep rate, loading and temperature were in accordance with the following relation: ε=dε/dt=ε0exp(28.1)(σ/σ0)exp(-691700/RT) Here ε0=1S-1,σ0=1MPa,R=8.314J·mol-1·K-1.7. Kinetic curves of the weight gain of Ti3SiC2/mSiC ceramics vs oxidating time were drawn by TG testing at 800-1450℃. Results showed that the oxidation resistance was excellent at temperatures upto 1400℃. The oxidation resistance performance of Ti3SiC2/mSiC ceramics increased with increasing of SiC content. High temperature oxidation kinetics of Ti3SiC2/mSiC ceramics conformed to the parabolic law below 1200℃. It was found that a special phenomenon of long time antioxidant activity of Ti3SiC2/7SiC ceramics at 1300-1400℃ was higher than that of 1200℃.8. Based on in-situ observation of microstructure at high temperature of the oxidation process, results showed that obvious oxidation of TiC in Ti3SiC2/mSiC ceramics could be observed at 650℃ firstly, obvious oxidation of Ti3SiC2 could be seen at 800 ℃, obvious oxidation of SiC could be found at 1200℃. The composition and morphology of oxidized layers of Ti3SiC2/mSiC ceramics were analysed by XRD and SEM+EDS. Results showed that the oxidation products of Ti3SiC2/mSiC ceramics were amorphous SiO2 (oxidation of SiC), rutile TiO2 (oxidation of TiC and TisSiC2) and crystalline SiO2 (oxidation of Ti3SiC2).9. Based on results of in-situ observation of microstructure at high temperature, oxide composition analysis and characteristics of oxidation kinetics curve, the oxidation mechanism model of Ti3SiC2/7SiC ceramics was proposed. The oxidation resistance performance of Ti3SiC2/7SiC ceramics was controlled by the growth process and the morphology of surface oxides which were influenced by amorphous SiO2 formed by SiC oxidation. Rutile TiO2 grew in stepped growth form and {211} preferred orientation could formed in the free growth conditions, and grains’ shape were regularly in that there were gaps between the grains. The morphology and growth speed of TiO2 were restricted, which could lead to change the grain morphology TiO2 to form compact protective film, due to infiltration and fusion effect of amorphous SiO2 by the formation of SiC oxidation under high temperature. In this way it could effectively improve the high temperature oxidation resistance of Ti3SiC2/7SiC ceramics. The special phenomenon of long time high temperature oxidation performance of Ti3SiC2/7SiC multiphase ceramics could be explained by this model reasonably.