Enhanced Wear Resistance by Compressive Strengthening a Novel Combination of Laser and Ion Implantation Technology
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ENHANCED WEAR RESISTANCE BY COMPRESSIVE STRENGTHENING A NOVEL COMBINATION OF LASER AND ION IMPLANTATION TECHNOLOGY H. DE BEURS AND J. Th. M. DE HOSSON Department of Applied Physics, Materials Science Centre, University of Groningen, Nijenborgh 18, NL-9747 AG Groningen, The Netherlands
ABSTRACT Despite the advantages of laser processing for the production of wear resistant materials, laser surface melting results in tensile stresses because the melted layer shrinks during resolidification. These tensile stresses may lead to severe cracking of the material and to deleterious effects on the wear behaviour. Our basic idea presented in this paper is to convert the high tensile stresses in the laser melted surface into a compressive state after implantation. In general, neon implantation is not very effective in the reduction of wear rates. However, neon implantation into laser melted steel turns out to reduce the wear rate substantially. INTRODUCTION Many successful wear resistant materials consist of particles of a hard phase dispersed in a more ductile matrix. Such dispersions can be prepared by powder metallurgical techniques or by solidification of an eutectic structure from a melt. However, in the former technique the coatings produced by spray processes remain separated from the substrate by a sharp interface which is always a potential source of weakness. In the latter type of technique materials are prepared from the melt and the proportion of hard phases is controlled by equilibrium thermodynamics. A different approach, as presented in this paper, is to modify the surface layer by using a 1.5 kWatt CW CO 2 laser beam. Among the available laser applications, laser surface melting turns out to be a powerful technique for the production of wear resistant layers since it combines the advantages of local hardening, the possibility of surface alloying and the use of high quench rates. The latter may result in new metastable phases with novel wear properties. Indeed, not only the hardness, but also the ductility and internal stresses play an important role in the wear process. These phenomena are also affected by a laser surface treatment. Despite the advantages of laser processing for the production of wear resistant materials, laser surface melting results in tensile stresses because the melted layer shrinks during resolidification. As a result high tensile stresses in the surface layer are generated which may lead to severe cracking of the material. Tensile stresses in the order of several hundreds MPa are possible, which detrimentally influence the wear behaviour. Our basic idea is to convert these tensile stresses produced by laser melting into compressive ones by ion implantation. When pressurized bubbles of implanted ions are nucleated in the surface layer, one can imagine that the corresponding compressive stress field could annihilate the tensile stress field of the laser melted material. Furthermore, the surface layer might also be strengthened during wear, due to the interaction between moving dislocations
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