The Preparation And Properties Of Silicon Nitride-based Continuous Functionally Graded Materials

Functionally graded materials demonstrate advantages of meeting different requirements and enhancing the reliability of components under demanding operating conditions. Reduction of residual stresses across the gradient developed by traditional preparation methods is the main challenge to ensure the properties of the materials, especially the layer-by-layer method which would produce nucleation of stresses at interfaces between two layers due to sharp variation of components. And some of the methods are too complex to prepare mass productions. To overcome these disadvantages, in this context, a new approach, a homogeneous powder mixture as the raw material was sintered in a steady continuous gradient temperature field by controlling the placement of die in the Spark Plasma Sintering (SPS) equipment, was introduced to fabricate Si₃N₄-based continuous functionally graded materials with different components. The elemental distribution, microstructure and the mechanical properties including the hardness and fracture toughness variation of the prepared sample were investigated. The effects of sintering additives or components on mechanical properties and microstructure were discussed.A steady continuous gradient temperature field in the die of SPS apparatus was produced by asymmetric displacement of die leading to difference in contact area of two sides of die and spacer, and hence obviously distinction of current intensity. BAS/Si₃N₄ continuous functionally graded material with continuous gradients inα/βphase contents and grains sizes was successfully fabricated by spark plasma sintering, using the raw material of a homogeneous powder mixture containing 80 wt%α-Si₃N₄ and 20 wt% BAS. The relative content of elongatedβphase increases continuously from 12 wt % in the bottom region to 83 wt% in the top zone of the sample. The hardness decreases from 16.7GPa to 12.6GPa, but the toughness varies at a reverse trend, increases from 3.22 MPa·m1/2 to 5.72MPa·m1/2, as the relativeβphase content increased. SEM micrograph indicated that the bottom region of the compact exhibited equiaxed fine-grained microstructures while the top zone with elongated coarser-grained microstructures.The (RE₂O₃+MgO)/Si₃N₄ functionally graded materials were prepared using MgO and Lu2O3/Gd2O3 as composite additives. The highest Vickers hardness and lowest fracture toughness in the lower-temperature surface is 20GPa and 5.30 MPa·m1/2 respectively, while the lowest hardness and highest fracture toughness in the higher-temperature surface 15GPa and 7.12MPa·m1/2. Numerous equiaxed fine grains were found in the lower-temperature surface and the number of equiaxed fine grains decreased gradually but elongated grains increased as the distance from the lower-temperature surface increased along the thickness direction, on the other hand, the size of elongated grains get larger gradually. Comparing to Gd2O3, Lu2O3 as the sintering additive is more effective to promote transformation ofα-Si₃N₄â†'β-Si₃N₄ phase and obtain larger diameter, yet elongated, reinforcing grains. Gd2O3 as the sintering additive results in reinforcing grains of a higher aspect ratio. Friction coefficients and wear rates of the cross-section of the compacts are lower while the relative content ofβphase decreases.SiC/Si₃N₄ composite functionally graded material was fabricated by spark plasma sintering, using a homogeneous powder as the raw material. The results showed that elemental distribution, microstructure and the mechanical properties including the hardness and fracture toughness gradually varied among the lower-temperature surface and opposite surface. The exitedα-SiC grains provide a great quantity of nucleation location forβ-Si₃N₄ phase to formβ-Si₃N₄ nuclei, but the migration of a Si₃N₄ grain boundary is substantially inhibited by inter-granular SiC particles, leading to numerous elongated fine grains in the SiC/Si₃N₄ composite functionally graded material.