Electron Microscopy Analysis on the Worn Surface of a High-Chromium White Iron During Dry Sliding Contact
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Electron Microscopy Analysis on the Worn Surface of a High-Chromium White Iron During Dry Sliding Contact A. Bedolla-Jacuinde1 and W.M. Rainforth2 1 Instituto de Investigaciones Metalúrgicas, Universidad Michoacana de San Nicolás de Hidalgo, Morelia, Michoacán. México. 2 Department of Engineering Materials, The University of Sheffield, Sheffield, UK.
ABSTRACT A series of microstructural phenomena within a thickness of material (tribolayer) below the worn surface, have been developed during dry sliding wear of a high-chromium cast iron. The overall wear behavior of the alloy is determined by the properties of this tribolayer. From the present work, a transmission electron microscopy analysis has been undertaken on the different features developing at different distances below the worn surface following wear sliding tests of a 17%Cr white cast iron alloy, whose microstructure is composed by 25% eutectic M7C3 carbides within an austenitic matrix. The observed phenomena is an increase in the dislocation density, plastic deformation by twinning followed by severe shear banding along with carbides fracture, a mechanical mixture formed by iron oxide and carbide particles produced from large carbides comminution, and finally a flat iron oxide layer. Wear debris was apparently created from the oxide film detaching from the outermost surface where equivalent strain is maximum. No evidence of strain induced martensite was observed from the present work, which has been reported in some austenitic materials. The implications of the microstructural evolution are discussed in terms of the wear theories and behavior of metals at high strains levels. INTRODUCTION Microstructural evolution during sliding contact between two surfaces is a very important factor in determining the wear resistance of a material, since sliding contact between the surfaces of ductile materials is always accompanied by severe plastic deformation localized to a small volume of material adjacent to the surface [1-7]. However, not too much is known about the work hardening behavior at the worn surface to be able to predict the ductility limit and model flow stress as a function of depth, both of which determine wear rate [2]. Currently, there is no definitive data to predict the deformation behavior at a worn surface in terms of slip systems, work hardening law, texture generated, strain distribution, flow stress, critical strain at which wear debris forms and depth below the surface at wear which debris forms [1]. Texture is one of these variables that have provided important evidence of the strain path at the worn surface. Rainforth [2] has underlined that crystallographic texture arising from the deformation can provide an important information on the strain path since some observations indicate that simple shear process occurs while some others suggest that pure shear is more appropriated. However, more data are required in this area. It is very difficult to predict work hardening behavior at a surface even for a single phase metal, where the only
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