Influence of tip defect and indenter shape on the mechanical properties determination by indentation of a TiB₂-60%B₄C ceramic composite

Instrumented indentation is very useful to determine the elastic modulus and hardness of materials. To obtain reliable values of the mechanical properties, the measurement of the indenter displacement into the material must be very precise. In practice, the frame compliance of the instrument and the indenter tip defect are among the most influential parameters. In microindentation, the frame compliance term must be calculated for each analysis. On the other hand, the tip defect can lead to a wrong estimation of the contact area, which is involved in both the elastic modulus and the hardness calculations. The present work is mainly focused on the influence of the tip defect on the determination of the mechanical properties of a composite TiB₂–60% B₄C hard material using Berkovich, Vickers, Knoop and spherical indenters. This composite system was obtained by sintering in vacuum, employing a pulsed electric current, without any sintering additives. It has been shown that the tip defect greatly affects the results related to the mechanical properties of the system under study, even for low defect sizes, which are usually neglected in practice. After correction, the elastic modulus of the TiB₂–60% B₄C is found to be equal to 490 GPa whatever the indenter shape used. This result corroborates the value of 500 GPa found for this system by employing the resonance frequency method according to the ASTM Standard e1876-99. For analyzing the hardness-load dependence, the representative parameter of the Proportional Specimen Resistance model becomes null when the tip defect is introduced into the hardness calculation. Additionally, this parameter is found to be directly proportional to the tip defect size, suggesting a special attention when characterizing the indentation size effect of materials. Finally, the Meyer hardness found for the TiB₂–60% B₄C is close to 20 GPa.status: publishe