| Ceramic materials, especially alumina ceramic, have series of superior properties such as high-temperature resistance, etching resistance, wearing resistance, lightweight, and so on. However, their fatal weakness-brittleness, seriously limit their practical usages. In such a case, the toughening to ceramics, as a result, has become into one of the key studies of materials science. So far, several toughening methods have been explored out and have produced significant effectiveness. Recently with the development of nano-technology, nanometer-sized particles toughening ceramic-based nanocomposites have been widely investigated.Nanocrystalline y-Ni-xFe alloys with x being in the range of 10~65wt%, so called Permalloy, are important soft magnetic materials. Especially y-Ni-22Fe, which have been widely used in industry things such as magnetic record heads, transformers or magnetic shielding materials due to its high permeability, low coercive force and relatively high saturation magnetization. r-Ni-Fe/Al2O3 nanocomposite, in which nanometer-sized y-Ni-Fe particles are incorporated into alumina matrix, not only is expected to improve brittleness of alumina, but also may introduced superior magnetic properties into alumina, realizing combination of superior mechanical properties with magnetic properties.This article consists of five chapters. Chapter one is an introduction, which simply introduced the development of ceramic-based composites and their toughening mechanisms. In chapter two, we fabricated r-Ni-Fe/Al2O3 nanocomposites successfully by using ball-milling mixing method plus hot-pressing process. Meanwhile, their microstructures are characterized by X-ray diffraction (XRD) analyser, transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM) and Brunauer-Emmett-Teller (BET). The results indicate that Ni-Fe particles are homogenously dispersed in the matrix in the composites. Apart from few large particles with 100nm in size, most of the particles sizes ranged from 10nm to 60nm for the composite powder. There are only two phases in the hot-pressed composite samples, one is r-Ni-Fe and the other is (X-Al2O3. Almost all Ni-Fe particles are dispersed at Al2O3 grains boundaries, indicating that the r-Ni-Fe/Al2O3 nanocomposite we fabricated belongs to intergrannular nanocomposites.Chapter three investigated the relationship between mechanical properties andNi-Fe contents such as densities, Vickers hardness, fracture toughness and fracture strength. Besides, we discussed the toughening mechanism of the material based on surface crack extension and fracture micro structure investigations. The densities of the bulk samples are all high with relative densities more than 98%, as the results shows. The Vickers hardness decreases monotonously from ~20GPa (AI2O3) to ~14GPa (19vol.%Ni-Fe/Al2O3) with increasing Ni-Fe content. In contrast, the fracture toughness increases monotonously from 4.7 MPa.m1/2 to 6.2 MPa.m1/2 as Ni-Fe content increased from 0 to 19vol.%. However, Fracture strength varies non-monotonously: it rises firstly and then decreases with increasing Ni-Fe content, and thus exhibits a maximum value of about 600MPa at 5vol.%Ni-Fe. We believe, according to discussion, that the increase of toughness comes from crack deflection and refinement of particle grains, and the fracture strength increases as toughness increasing.In chapter four, we measured the variation of the young's modulus for the composites with different Ni-Fe content during heating process by using film internal instrument. As a result, the young's modulus of the composites decreased a little during the heating process, which indicated that ceramics were thermal stable. On the other hand, it decreased from ~280GPa to ~220GPa as Ni-Fe content was increased from 0 to 19vol.%. This is reasonable considering that the young's modulus of Ni-Fe is much lower than the matrix.Chapter four characterized the magnetic properties of the composites with vibrating sample magnetometer. The composites exhibit h...
|
PDF Full Text Request