Reserch On Relastic-plastic Performance Of Ultrafine-grained Si₂N₂O-Si₃N₄Brittle Ceramic Materials At Room Temperature

Ceramic materials have been widely developed and attached more attention in recentyears. However, ceramic materials’ mechanical performance was difficultly tested bytraditional experimental method due to its inherent brittleness and high hardness.Nanoindentation technology has advantages of operating easily, sample preparing simplyand high test precision, has become the main method to test the properties of the highhardness and difficult machining brittle materials. Mechanical properties ofultrafine-grained Si₂N₂O-Si₃N₄ceramic sintered at different temperature were tested bynanoindentation and uniaxial compression test. The nanoindentation test process wassimulated using the finite element software. The deformation process and stress-straindistribution of the indentation were analyzed. A theoretical method is proposed forcalculating the stress–strain relationship of brittle ceramic materials based on theexperimental nanoindentation data. Several relevant coefficients in the theoreticalcalculation formula are determined by comparing the calculation and simulation results.Ultrafine-grained Si₂N₂O-Si₃N₄composites are fabricated by hot press sintering ofamorphous nanosized silicon nitride powders at1600,1650, and1700°C. The averagegrain sizes of the sintered materials increase with increasing sintering temperature, withcorresponding values of280,360, and480nm. The elastic modulus of Si₂N₂O-Si₃N₄ceramics were tested by nanoindentation and uniaxial compression test, and it decreaseswith increasing sintering temperature. Hardness of Si₂N₂O-Si₃N₄ceramic was measuredby nanoindentation instrument, microhardness tester and Vickers hardness tester. Theresults show that: macroscopic Vickers hardness value was maximal and nanoindentationhardness value was minimal for the material sintered at the same temperature. Thisbecause the Vickers hardness has maximum load and indentation size, but thenanoindentation load and size is smaller. The nanoindentation hardness is related to theratio of the indentation maximum contact cross-sectional area A and the average graincross-sectional area S. As the ratio of A and S decreases, the fine-grain strengthening effectbecomes less evident, and the gap between nanoindentation hardness and microhardnessor macroscopic Vickers hardness increases. The nanoindentation process was simulated by finite element software calledMSC.MARC. The tip spherical surface radius of Berkovich indenter and stress-strainrelationship curve were derived through comparative analyze simulation results andexperimental results. Displacement, strain and stress distribution were analyzed accordingto the simulation results.The indentation morphology was determined by analyzing the geometric feature ofBerkovich tip used in nanoindentation test. A theoretical formula to calculate thestress-strain relationship of hard brittle material directly obtained by nanoindentation testdata is derived. The theoretical formula was proved to be correct by comparing thecalculated results with the simulated results.