Research On Nano-composite Materials Of Ti(C,N)-based Cermets

In this dissertation, the relationship among composition, manufacturing process, microstructure and properties of nano-composite Ti(C,N)-based cermets has been systematically investigated by thermal analysis (TG), X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), Field emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis, and Nitrogen/Oxygen determinator. In the first part of the dissertation, the influence of composition, additive, grain refinement on the microstructures and properties of Ti(C,N)-based cermets has been critically overviewed, which includes the research development on grain refinement of WC-Co cemented carbides; Briquettability of nano-powder, pressing techniques, the relevant pressing theories, and nano-sintering characteristics and nano-sintering techniques have been summarized. Based on above work, the purpose and significance of the dissertation have been pointed out. The influence of chemical composition, particle size and pressing process parameters on briquettability of Ti(C,N) mixed powders has been studied. The calculation relation representing briquettability of component powder and Ti(C,N) mixed powders was established, and the calculated values was consistent with experiment results. Micro-mechanics mechanism on pressing process of metal-ceramic mixed powders was analysed theoretically. Four organic plasticizers of (PVA), PVA+glycerine, PVA+paraffin, PVA+polyethylene glycol (PEG) was designed and used in the die pressing process of Ti(C,N) mixed powders. The influence of the four organic plasticizers on powder briquettability, green strength, mechanical properties and porosity of sintered cermets has been investigated. The conclusions were obtained as follows: compared with other plasticizers, the thermal decomposition characteristic of plasticizer PVA+paraffin was much more in favor of dewaxing process of Ti(C,N)-based cermets, and sintered cermets behaved lower porosity and the best mechanical properties. The preparation of nano-composite Ti(C,N)-based cermets by hot-press sintering (HP) was attempted and the densification process, phase transformation and microstructure characteristics conferred by HP were systemically investigated. It was found that cermet density was decided by hot pressure and phase transformation of cermet was mainly determined by sintering temperature.Cermet sintered at 1400oC for 20min behaved the best mechanical properties. The influence of VC addition on grain growth inhibition of nano-composite Ti(C,N)-based cermets during HP sintering process has been studied. In the earlier stage of sintering, VC didn't inhibit hard phase grain growth. When grain size was over 1μm, as sintering process continued, VC restrained grain growth obviously. However, the area proportion of abnormally growth grain in cermets increased due to VC addition after grain growed up obviously and grain size was over 5μm. The relation of porosity and grain size of nano-composite Ti(C,N)-base cermets during sintering process has been studies. The experiment results showed that pore, especially for open pores, could inhibit grain growth effectively. The change of oxygen content of nano-composite Ti(C,N)-based cermets during sintering process has been studied. Some conclusions were drawn as follows: when nano-composite Ti(C,N)-based cermets sintered at 1200oC for 30min, 2.68wt% oxygen was wiped off; after holding in 720min at 1200oC, there were still 0.21wt% oxygen keeping in nano-composite Ti(C,N)-based cermets. The preparation of nano-composite Ti(C,N)-based cermets by two-stage vacuum sintering has been attempted. It was found that grain growth of nano-composite Ti(C,N)-based cermets could be inhibited effectively by using two-stage sintering and that 400nm grain size of cermets fabricated by the technique could be obtained, and TRS got 1913MPa, and the hardness reached the level of HRA89. The influence of nano-hard phase powder addition on nano-composite Ti(C,N)-basedcermets has been investigated. As the amount of nano TiC,TiN addition increased, linear shrinkage ratio of cermets increased, and mechanical properties also increased. Sintered cermets containing ball milled nano-hard phase powder addition showed better mechanical properties of that TRS was 2016MPa and hardness was HRA89.2. The addition of nano-WC powder inhibited grain growth of nano-composite Ti(C,N)-based cermets during sintering process.