FE-EPMA measurements of compositional gradients in cemented tungsten carbides Jun Guo a , Mark Koopman a , Z. Zak Fang a, ⁎, Xu Wang a , Peng Fan a , Michael C. Rowe b a The Department of Metallurgical Engineering, The University of Utah, Salt Lake City, UT 84112, United States b The School of the Environment, Washington State University, Pullman, WA 99164-2812, United States abstract article info Article history: Received 6 September 2012 Accepted 7 October 2012 Keywords: Functionally graded WC–Co (FG WC–Co) Field emission electron probe microanalysis (FE-EPMA) Carbon analysis Carbon diffusion Liquid Co migration Previous studies have shown that carbon diffusion can play a key role in the formation of the Co gradient in functionally graded (FG) WC–Co. Analysis of the carbon composition in the Co binder phase of WC–Co alloys is challenging, however, not only due to the small volume of the Co phase in these materials, but also because of the difficulty in characterizing light elements like carbon within matrices of higher atomic weight. In this study, a method via field emission electron probe microanalysis (FE-EPMA) capable of very fine beam size was developed to measure the carbon composition in the Co phase of graded WC–Co materials. A plateau of 0.8 wt.% carbon was measured for approximately 130 μm from the surface into the sample, followed by a steep drop to 0.55 wt.% and a more gradual decline of carbon into the interior that was accompanied by a corresponding increase in the W gradient within Co pockets of the microstructure. These results are discussed in relation to theory and prior simulations. © 2012 Published by Elsevier Ltd. 1. Introduction Cemented tungsten carbide (WC–Co) is one of the most widely used industrial tool materials for a wide range of applications, including: metal cutting and forming, drilling for the gas and oil industries, and construction [1]. Typically, WC–Co is a composite consisting of WC par- ticles uniformly distributed in a Co binder phase. To improve the surface hardness and wear resistance of WC–Co, research efforts have been on- going to create functionally graded (FG) WC–Co materials with a gradi- ent in cobalt content (and, consequently WC content) and/or WC grain size [2–24]. An innovative method to obtain FG WC–Co has been devel- oped in which heat treating previously sintered fully-dense WC–Co in a carbon rich atmosphere results in a decreased Co content near the sur- face with a gradient into the bulk of the material [20–24]. The creation of such graded microstructures in the WC–Co system has sought to pro- duce a hard-surface tough-core structure. This microstructure combines high hardness and good fracture toughness in a single component, and thus, leads to significant gains in wear performance and durability. Previous studies have examined the mechanism and kinetics of this process [20–22], and have shown that the gradient forms due to phase transformations and liquid migration as a consequence of the inward diffusion of carbon. The carburizing atmosphere is intro- duced at temperatures where the WC–C–Co phase diagram indicates coexistence of WC with both liquid and solid Co, and the additional carbon at the surface leads to an initial conversion of solid Co to liquid Co in the near surface region. The gradient in liquid distribution be- tween the surface and the interior drives the inward liquid migration, and as a result, the Co gradient is developed in WC–Co with reduced Co content at the surface region. Although the proposed theory ex- plains the observed gradient formation during the carburization pro- cess, there still exists a lack of experimental validation concerning the carbon concentration through the gradient zone. Knowledge of the carbon content distribution in the near surface region of the FG WC–Co is important for a thorough understanding of the carbon dif- fusion process, and in further exploring the role C plays in establishing the Co gradient. To obtain the carbon distribution it is necessary to analyze the carbon compositional variations as a func- tion of depth from the surface. A number of spectroscopic techniques are available for quantitative analysis of carbon, or for measuring compositional depth profiles. These include: X-ray photoelectron spectroscopy (XPS), optical emis- sion spectrometry (OES) and electron probe microanalysis (EPMA). Obtaining the concentration depth profiles with XPS or OES, involves an orderly and progressive removal of the sample material from the sur- face by ion bombardment or ion sputtering, with compositional mea- surement at regular intervals [25,26]. Such sputtering techniques are limited to fairly shallow depths, however, due to the confounding ef- fects of preferential sputtering rates for elemental atoms of such dispa- rate size and mobility. Standard EPMA can also be used to yield composition depth profiles by performing a point by point line-sweep along the cross-sectioned surface of a sample, but the spatial resolution is larger than 1 μm [27]. The analysis of our samples required spatial resolution of at least 0.5 μm, since the carbon diffusion is mainly through the Co phase which is in small pockets between WC grains. In this study, we employed the high resolution EPMA, equipped with a field-emission (FE) gun capable of very fine probe sizes (down to ~50 nm), which opens the possibility for carbon analysis Int. Journal of Refractory Metals and Hard Materials 36 (2013) 265–270 ⁎ Corresponding author. Tel.: +1 801 581 8128 E-mail address: zak.fang@utah.edu (Z.Z. Fang). 0263-4368/$ – see front matter © 2012 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.ijrmhm.2012.10.003 Contents lists available at SciVerse ScienceDirect Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM