Reserch On Elastoplastic Performance And Size Effect Of Ceramic Materials By Microhardness Testing

Application scope of ceramic materials in engineering is more and more extensive with the progress and development of science and technology.Ceramic materials'mechanical performance was difficultly to plastic process and tested by traditional experimental method due to its inherent brittleness and high hardness.Microhardness test has advantages of operating easily,sample preparing simply and low cost,has become the main method to test the properties of the high hardness and difficult machining brittle materials.However,the hardness test cannot be used to directly test the elastoplastic property of the material,and the size effect will be generated when the hardness test is conducted,which will affect the accuracy of the hardness test.In this paper,ultrafine-grained Si₂N₂O-Si₃N₄ composites are fabricated by hot press sintering of amorphous nanosized silicon nitride powders at 1600,1650,and 1700°C.The hardness values of the three kinds of material were measured by different loads,the elasticity modulus of ceramic materials were tested by uniaxial compression test,the results show that hardness and elastic modulus of the ceramics decreased with the increase of sintering temperature.With the increase of load,the measured hardness value tends to decrease,showing obvious size effect.It is found that the generation and expansion of cracks have great influence on the indentation size effect.The microhardness testing process was simulated by finite element software called ABAQUS.The stress-strain relationship curve were derived through repeat comparative analyze simulation results and experimental results.Displacement,strain and stress distribution were analyzed according to the simulation results.According to the geometrical characteristics of Vickers diamond indenter in microhardness test,combined with the theory of hardness test and elastoplastic theory,a theoretical formula to calculate the stress-strain relationship of hard brittle material directly obtained by analyze simulation results and microhardness test data is derived.The theoretical formula was proved to be correct by comparing the calculated results with the simulated results.According to the size effect of microhardness test and the relation between the generation and expansion of indentation area crack,the model of indentation size effect of hard brittle materials in microhardness test is proposed.The length and opening angle of crack are the main factors influencing the size effect,the longer the crack length,the larger the crack opening angle,the more obvious the size effect.The generation and expansion of the crack in the indentation region were simulated by finite element method.Displacement,strain field and stress field in the indentation region with crack is analyzed.Influence of crack growth on elastic recovery of indentation area is discussed.The size effect crack growth model is proved to be correct.