Evaluation of the Fracture Toughness on the Surface Layer in HIP-Sintered Silicon Nitride

概要

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To clarify the validity of evaluation of the threshold value of fracture toughness Kth on the surface layer of ceramics by sphere indentation test, indenters of various diameters 2R were used for sphere indentation tests with using Si3N4 specimens made by HIP-sintering and numerical calculation of the stress intensity factor KI was performed for surface cracks under ball-plate contact loading. The crack length ci was estimated from experimental results using KI, where ci is the length of the crack leading to a ring crack and the conditions for ring crack initiation were assumed to be KI>Kth. The average values of ci increased with increasing 2R in the case of small 2R, but the averages of ci gradually approached a constant value in cases with large 2R. The constant value of ci was estimated as 7.9-8.6 μm using Kth=5.3 MPa·m12; and was almost equivalent to the grain size of the test material. The same results were obtained in the previous study with Si3N4 specimens made by gas-pressure-sintering. Therefore, sphere indentation tests can be used to evaluate Kth of ceramics using KI for surface cracks.