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A Study on the Evaluation of Surface Hardness by Using the Minimum Reflection

Contact-type methods that are widely uses to evaluate the surface hardness of materials include the indentation press and the rebound and scratch hardness methods and can be used to measured quantitatively the hardness of a materials surface. However, these contact hardness measurements are limited by specimen damage due to indentation and by the shape (curved shape) of the material. A disadvantage is that various measurement loads and methods are needed depending on the hardness of the material. When ultrasonic waves are incident at a specific angle (Rayleigh angle) on a liquid-solid interface, Rayleigh surface waves are generated, and a rapid decrease in reflectance, called the minimum reflection, occurs at the interface. The hardness of a material mainly depends on the change in the hardness of surface region, which can be evaluated from the propagation characteristics of a surface wave by using the minimum reflection phenomenon of ultrasonic waves. In these study, the propagation characteristics of a Rayleigh surface waves for hard specimens were evaluated by using the minimum reflection and the hard surface region was analyzed using modified perturbation theory. A non-contact hardness measurement method using the minimum reflection was proposed and the reliability of the proposed in this study, method was demonstrated by applications to standard hardness specimens.