Photothermal investigations on advanced ceramics

Abstract

Mechanical or thermal impacts on surfaces and near-surface layers during manufacturing processes often result in structural changes. They become detectable by photothermal techniques, when they correlate with alterations of thermal properties. We apply this approach on the detection of variations in ceramic surfaces. Vickers indentations model the mechanical load during grinding and a thermal surface treatment was performed by pulsed laser illumination. From the photothermal signals the depths and profiles of thermal conductivity variations depending on the different loading strengths could be reconstructed. 1. Introduction During the past few years advanced ceramic materials gathered great importance in manufacturing technologies [1]. This is mainly due to their good performance concerning hardness, ductility and wear resistance yielding increased manufacturing speeds. With respect to their further applicability, reliablility and functionality advanced ceramics have to fulfill high demands on shape, accuracy and surface finishing. For most applications grinding with diamond wheels is the final step during the manufacturing process of ceramic tools. During this process the diamond grains can induce micro-cracks and plastic deformations in the surface region of the ceramics. This may lead to a higher possibility of a damage when the ceramic tools are in use. The idea to heal these micro-cracks by laser heating of the surface after the grinding process is under investigation by various groups. The plastic deformation, resulting in micro-structural changes as well as the micro-cracks take influence on the thermal conductivity of the concerned region and therefore photothermal measurement techniques yield information about the thermal properties of the surface and near surface region [2]. To some extent mechanical or structural properties can be correlated with thermal ones which leads to an evaluation of technological properties like hardness etc. In order to clarify the metrological problem, figure 1a demonstrates the change in surface layers during a mechanical impact while grinding or polishing a ceramic surface [1]. Due to the mechanical and thermal load brittle fracture, surface defects, cracks and subsurface damages can be induced by the material removal and increased material compressions. The impacts of local pressures and heat in the contact zones during the machining process cause plastic deformations. Since all these processes significantly influence the mechanical strength and affect the later use of ceramics, their effects on materials' properties have to be determined nondestructively and contactless. As these changes happen in near-surface layers and are probably combined with variations of the thermal properties, they can be detected by photothermal means. Our special interest in this field is focused on the detection of altered zones caused by a manufacturing process and the thermal variations in near-surface layers due to mechanical or laser material processing. Basically, we assume that the varied mechanical properties, are caused by locally distributed inhomogeneities on and beneath the sample's surface (e.g. cracks, delaminations, alterations of microcristalline structures, varied material densifications, ,; porosities, etc.). In order to model the alterations of thermal properties (which are particularly correlated with mechanical parameters) by a slight mechanical load, we chose the Vickers