Microstructure And Properties Of The ZrC/SiBCN Multiphase Ceramics By Hot Pressing

In this dissertation, nano-sized ZrC powders were pepared by sol-gel process and SiBCN powders were prepared by mechanical alloying. By introducing the armorphous Si-B-C-N powders into the ZrC sol, Then ZrC/SiBCN bulk ceramics were sintered by hot-pressing technology. During the formation process of ZrC powders, The microstructure of the materials was studied at each stage. The phase and structure of ZrC/SiBCN ceramics after hot-pressing have been also studied. The effect of ZrC contents on the mechanical properties of ZrC/SiBCN ceramics were also studied as well as oxidation behavior of ZrC/SiBCN ceramics under different temperature.The results show that in the material selection of ZrC powders prepared by sol-gel method, propylalcohol zirconium(Zr(PrO)4)、acetylacetone(C5H8O2)、furfuryl alcohol(C5H6O2) and hydrochloric acid(HCl) were chosen as raw materials, ethanol as solvent, P123 as surfactants and pore-forming agent. The mole ratio for ZrC preparation was determined as 1:1.8 of Zr to C sources(Zr(PrO)4 and C5H6O2, respectively). The mole ratio of the four kinds of raw materials of normal propylalcohol zirconium : acetylacetone : furfuryl alcohol : hydrochloric acid were 1:1:1.8:1 for the synthesis of ZrC powders. The optimized heat-treatment parameter for nanoscale ZrC powders insulation is 1500 oC for 0.5 hours. The size of the prepared ZrC powder is uniform with diameter between 0.5~1 μm. After high-energy ball milling to get 2Si-B-3C-N, there found some metallurgical combination between powder particles, which resulted in hard-reunion phenomenon.The main phases of ZrC/SiBCN ceramics after hot-pressing were ZrC、β-SiC、α-SiC and BCN as confirmed by XRD. TEM results showed that there were twins and dislocations with high density inside β-SiC phase while BCN phase with amorphous turbulent layer structure. With the increment of ZrC mass fraction in ZrC/SiBCN ceramics, the flexural strength and the Vickers hardness both increased, but the elastic modulus was on the decline. ZrC15%-SiBCN had the best comprehensive mechanical properties with bulk density, bending strength, fracture toughness, Vicker’s hardness and elastic modulus of 2.64 g/cm3, 229.5±11.6 MPa, 2.11±0.26 MPa?m1/2, 113.37±5.2 GPa and 3.36±0.57 GPa.When the ZrC/SiBCN ceramics were oxidated at different temperature, with the increment of ZrC mass fraction in ZrC/SiBCN ceramics, holes and cracks on the surface of the material were gradually increasing. On the one hand, ZrO2 that the oxidation product of ZrC can not get effective sintering during 1100~1500oC, so the material surface was not dense, with more holes; On the other hand, when ZrC convert to ZrO2, it generated a lot of free carbon, and carbon react with O2 at high temperature easily. So it generated a lot of holes on the surface of the material and aggravated O to the diffusion within the material. When ZrC/SiBCN ceremics oxidated at temperature lower than 1300 oC or oxidated at 1500 oC for short time, the surface can formed a dense oxide layer, The layer can protect material effectively. When material oxidated at 1500 oC for long time, the oxidation film was damaged more serious, it lead to oxidated quickly.