Application of FIB in the preparation of TEM specimens of experimental supercoarse WC-Co(Ni) composites

Abstract

Advances in TEM's performance as well as in materials being investigated require the relevant improvement in sample preparation technology. For a long period of time TEM investigations of metals have been successfully performed using the electrochemical jet polishing method for the sample preparation, i.e. thinning. This classical approach has proved to be unsatisfactory in the case of electrically not conductive and/or multiphase materials – for instance various kinds of ceramics and composites. A milestone in solving this problem has been an elaboration of ion milling. Overcoming certain disadvantages of this technology, mainly sample damage, as well as providing site specificity of the sampling procedure, has been achieved owing to the onset and development of the focused ion beam (FIB) method. The current paper presents the authors' own experience in the preparation of TEM specimens of conventional WC-Co type and experimental WC-Ni based supercoarse sintered carbides. These metal matrix composites consist of large (ca. 10 μm) tungsten carbide particles and a binding metal phase (cobalt or nickel based). Because of the pronounced microstructural, mechanical and electrochemical inhomogeneity, as well as the size of hard WC particles, traditional (mechanical, jet polishing, electrochemical) procedures proved to be totally inadequate. Therefore multi stage lift-out procedure employing a focused beam of gallium ions of various energy (in situ lift-out of the sample plus H-bar geometry final thinning) and argon ion polishing aimed at the removal of the damaged (defected and amorphous) layer has been elaborated and successfully applied. A large number of defects in the WC particles (dislocations, stacking faults) and within the matrix areas (dislocations, microtwins) as well as high quality of the WC-metal matrix interfaces were found.