|
As many main components of optical devices and measurement equipment, as lenses, mirror, and targets, are made of glasses and ceramic materials, the integrity of these components is an important issue for quality prediction and assessment. It is well known, that machining processes can greatly influence the quality of products, not only at surfaces but also directly beneath it. This problem is even more severe, when machining hard and brittle materials as ceramics and glasses, as the chip removal process can lead to intensive crack initiation and stresses at surfaces, and thereby to sudden failure when loading those components. But under certain machining conditions, a small range of plastic flow has been observed in some ceramics and glasses as well. This change in their behavior is known as the brittle-to-ductile transition phenomenon. Until now, the transition process itself is not understood sufficiently, there is still no conclusive description of the process. The prediction of the critical depth of cut or the maximum pressure before crack formation is one of the key issues for maximizing the material removal rate and assuring the reliability of components at the same time. Central to the scientific evaluation of the micro cracking in machining of brittle solids is the indentation test, today widely adopted as material `hardness' indicator. In engineering, the indentation test is also considered as a simplified model of the penetration of abrasives into work material during the machining process. Simulations for silicon have confirmed the dependency of crack initiation on the indenter tip radius, demonstrating its brittleness in addition to elastic response of the bulk and to deformation around the indenter. These first results represent an important step towards the simulation of the brittle-to-ductile transition, which will greatly enhance the understanding of machining of hard and brittle materials. The results will then in turn form the basis for further improvement in machining technology and higher reliability of ceramic components by controlled surface integrity.
|
