Synthesis, Characterization And Mechanics Properties Of Metal Ni-toughened MF/Al₂O₃ Composite

By introducing an appropriate amount of sintering additives including Nano-TiO2 and CMS, the traditional non-pressure sintering process was adopted along with optimal dispersion of fibers and reinforcement phase metal Ni to prepare the nickel- and mullite-toughened alumina ceramic matrix composites. The coatings of the fiber and the metal Ni were conducted to improve the toughness of alumina-based ceramic materials, respectively. Their mechanical properties were measured further, and the composite toughening mechanism was also investigated preliminarily.The influence factors on the mechanical properties of alumina ceramic matrix composites, such as milling time, milling medium, compressing pressure shaped and so on, were initially studied by orthogonal experiment. Through analysis of Webster hardness, the optimal parameters were found to be 24h milling time, 15 ml milling medium dosage, 100 MPa compressing pressure and 6% wt PVA content, resulting in the corresponding Webster hardness value being 10.8 GPa. The variation of alumina ceramic matrix composites were determined by DSC and TG. By analysing SEM images of the specimens treated at various temperatures, it was observed that the surface of the fiber became roughed along with the decrease of the mechanical property when the sintering temperature was higher than 1550℃. Hence, the sintering temperature should be limited below 1550℃.The effect of sintered method, Ni- or Al-doped and the coating of mullite fiber on the mechanical properties and thermal shock were aslo studied. The traditional process for the preparation of ceramics with and without buried graphite as atmosphere was adopted. It was found that the flexural strength, fracture toughness, relative density of the sample buried in graphite were more excellent than those of the sample sintered barely. Hence, the carbon sintering was preferred for this system. With increasing of Ni content, the flexural strength decreased but the fracture toughness increased, the relative density firstly increase, then decreased. The flexural strength, fracture toughness, Webster hardness value and relative density of Ni-MF/Al2O3 composite sintered at 1450℃were higher than those of the sample sintered at 1400℃. By investigating the varied volume of nickel in Ni-MF/Al2O3 composites sintered at 1450℃, it was found that the flexural strength of the composite decreased, the fracture toughness increased, the webster hardness value firstly increased and then decreased, the relative density firstly decreased, and then increase, finally decreased. The mechanical properties of the composite reinforced by the fibers without coated was higher than the ones coated with SiO2, but its flexural strength decreased on the whole. Fracture toughness was linear enhancement with increasing amount of nickel in the composite but decreased with SiO2-coated fibers. Aluminum was introduced as the sintering additive and binding agent to improve the combination between Ni and alumina ceramic matrix composites. Unfortunately, the flexure strength decreased; the fracture toughness, the webster hardness value and relative density firstly increased and then decreased. Thermal shock resistance was studied upon the alumina ceramic matrix composite, and cracks were found on the surface after sintering repeated five times. The bending strength of the alumina ceramic matrix composite became worse, but the fracture toughness and relative density increased, exhibiting better behavior in comparison to single alumina.In order to optimize the fiber content, the content of metallic Ni, sintering temperature and holding time, orthogonal experimental design method was used to study the toughening mechanism. Under the optimized conditions of 10% wt MF, 12% wt metallic Ni and sintering temperature at 1500℃, 60-minute hold, the obtained alumina ceramic matrix composite showed the best properties, where the relative density was 93.83 %, the bending strength was 730.427 Pa, the fracture toughness was 10.13 MPa?m1/2 and the Webster hardness was 10.096 GPa. The metal Ni and MF toughening mechanism was discussed based on the experimental result. It was found that the debonding and pullout of fiber, the bridging and plastic deformation of metal were the main reasons for the enhancement of toughness.