TECHNICAL FIELD

The present invention is related to the indentation test methods for characterizing the various types of mechanical properties as well as the surface adhesion energy of elastic/elastoplastic/viscoelastic materials.

BACKGROUND OF THE ART

The conventional indentation test is a testing technique that evaluates various mechanical properties such as the hardness and the elastic modulus through measuring the size of residual impression that formed by pressing an indenter onto a surface of test specimen.

When an indenter is pressed onto the surface of a perfectly elastic body, its elastic modulus can be evaluated from the indentation-induced elastic deformation, while due to the plastic flow in elastoplastic indentation contact, the residual contact impression is formed after unloading. The size of this residual impression combined with the indentation load leads to the contact hardness. The elastic modulus and the contact hardness are the representative examples of several mechanical characteristics that are determined in indentation testing.

In ductile metals, the size of residual impression nearly coincides with the impression formed at the maximum indentation load due to the significant plastic flow. In ceramics and/or organic polymers with their significant elastoplastic and/or viscoelastic deformation and flow, on the other hand, their residual impressions after unloading are always smaller than those formed at the maximum indentation load due to the elastic recovery of impression induced in its unloading process.

Therefore, in order to quantitatively characterize the mechanical properties of a test specimen, it is necessary to make in-situ measurements of indentation contact, including the measurements of indentation load and penetration depth along with the indentation-induced contact area both in the loading and in the subsequent unloading processes.

In the conventional instrumented nanoindentation test method, it is only capable of measuring the penetration depth (h) under the applied indentation load (P). Outside the indentation contact area, the free-surface of an elastic body always sinks-in, while it piles-up for elastoplastic body. Without the information on these sink-in/pile-up contact profiles, therefore, it is by no means capable of conducting the quantitative characterization of mechanical properties of the test specimen indented. In order to circumvent this fatal issue of sink-in/pile-up contact profiles, undesirable approximations and assumptions have always been made in the conventional instrumented nanoindentation testing to estimate the indentation contact area (A) formed under the applied load (P). In other words, without the information on the indentation contact area A under the applied load P, no one determines in a quantitative manner the mechanical characteristics of any engineering materials.

The instrumented indentation microscope is the test system to overcome the difficulties included in the conventional instrumented nanoindentation test. The instrumented indentation microscope is capable of not only measuring the penetration depth (h), but also optically determining the projected contact area (A) in an in-situ manner under the applied load of indentation, leading to a quantitative characterization of mechanical characteristics without any of undesirable approximations/assumptions that are required in the conventional nanoindentation testing. The instrumented indentation microscope can therefore measure the projected contact area both for “sink-in” and “pile-up” contact surface profiles (refer to Patent Literature 1, Patent Literature 2, Patent Literature 3, Patent Literature 4, Patent Literature 5, Non Patent Literature 1, Non Patent Literature 2, Non Patent Literature 3, and Non Patent Literature 4).

CITATION LIST

Patent Literature

[Patent Literature 1] Japanese Patent Application Laid-Open No. 2005-195357 (U.S. Pat. No. 4,317,743)[Patent Literature 2] Utility Model Registration No. 3182252[Patent Literature 3] Japanese Patent Application Laid-Open No. 2015-175666 (U.S. Pat. No. 6,278,512)[Patent Literature 4] International Publication No. 2016/194985[Patent Literature 5] Japanese Patent Application Laid-Open No. 2017-146294

Non Patent Literature

[Non Patent Literature 1] T. Miyajima and M. Sakai, Optical indentation microscopy—a new family of instrumented indentation testing, Philosophical Magazine, Vol. 86, pp. 5729-5737 (2006)[Non Patent Literature 2] M. Sakai, N. Hakiri, and T. Miyajima, Instrumented indentation microscope: A powerful tool for the mechanical characterization in microscales, J. Mater. Res., Vol. 21, Issue No. 21, pp. 2298-2303 (2006)[Non Patent Literature 3] M. Sakai, S. Kawaguchi, and N. Hakiri, Contact-area-based FEA study on conical indentation problems for elastoplastic and viscoelastic-plastic bodies, J. Mater. Res., Vol. 27, No. 1, pp. 256-265 (2012)[Non Patent Literature 4] T. Mineta, S. Miura, K. Oka, and T. Miyajima, Plastic deformation behavior of Mg—Y alloy single crystals observed using in situ Brinell indentation, Materials Transactions, Vol. 59, No. 4, pp. 602-611 (2018).

SUMMARY OF INVENTION

Technical Problem

However, in characterizing the mechanical properties of a specimen by indentation test method, it is requisite to take into account the effect of surface adhesion of the specimen on the indentation contact behavior. In the cases of so-called the hard materials having rather large elastic modus (the plane strain Young's modulus, E′≥100 GPa) such as the engineering materials of metals and ceramics, due to their large elastic moduli, the surface adhesion, even if there exists, makes a rather minor effect on their contact behavior. In most cases, therefore, we can neglect the surface adhesion in their indentation contact testing.

On the other hand, the elastic moduli of soft materials including organic polymers, biomaterials, and microbiological mediums are rather small, and fall in the range of E′≈1 Pa-100 MPa, where the surface adhesion plays an essential role in their mechanical characteristics and functions, leading to critical difficulties in quantitatively characterizing the mechanical properties through using the conventional nanoindentation test systems.

The present invention has been made in view of such circumstances and aims to provide a test technique and analysis for simultaneously characterizing the adhesion energy as well as the various mechanical properties of soft material through taking into account the adhesion force at the contact interface between the material tested and the indenter.

Solution to Problem

The present invention provides the following technical method and procedure for solving the problems and issues described above.

A test method for characterizing the mechanical properties including the surface adhesion energy γ on the basis of the experimentally derived P-A relationship,

where P means the indentation load under the penetration depth h of an indenter pressed onto a test specimen with surface adhesion, and

A means the contact area of indentation at the contact radius a under the applied load of P.

Advantageous Effects of Invention

Based the present invention, it is possible to provide an indentation test method for simultaneously characterizing in a quantitative manner the adhesion energy as well as the various mechanical properties of soft materials with surface adhesion.

