Hardness and Normal Indentation of Polymers

Abstract

The hardness measurement has wide applications in the characterisation of the mechanical and physical properties of materials. This method is frequently used for metals, polymers, ceramics and coatings1. It has been used to relate hardness with certain physical and mechanical properties of materials. It has also been used to monitor and predict the service lifetime of prosthetic thermoplastics against a simulated human body environment2. Hardness is generally defined as the resistance of a material against local surface deformation. In an indentation test, a softer material is indented upon by a rigid indenter of specified tip geometry (conical, spherical, pyramid etc.) and hardness is usually computed as the ratio of indentation load to the projected area of contact between the indenter and the material in the plane of deforming surface. The area of contact may be measured actually, or indirectly, from the image of the residual indent on the softer surface after the indenter is removed. In this case the hardness value is controlled by a plastic property of the material. As an alternative the contact compliance curve (load-displacement curve) can be used to extract both plastic and elasto-plastic properties of the material. The actual choice of the technique used for hardness measurement depends to a great extent upon the type of the material tested and the kind of information desired from the test. For elastomeric materials such as rubbers, the rebound hardness is commonly used. In a rebound hardness test a rigid indenter is made to fall onto the sample surface from a specified height and the height of the rebound is measured. The energy absorbed by the sample material on impact is then related to the product of a “dynamic yield pressure” and the volume of the indent.