Synthesis of nanostructured WC-12 pct Co coating using mechanical milling and high velocity oxygen fuel thermal spraying
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I. INTRODUCTION AS a result of their importance in industrial applications, usually with respect to wear resistance requirements, WCCo coatings have been widely studied.[1–20] The WC particles, with high hardness, provide a wear-resistant constituent, while the Co binder contributes mechanical support, and hence, toughness in the coatings. Hardness, wear resistance, and strength depend primarily on the size and volume fraction of WC particles. In a related, recent study, it was reported that a bulk nanostructured WC-Co material exhibited higher hardness and toughness in comparison with the conventional counterpart material.[21] In recent years, thermal spraying using nanostructured feedstock powders has yielded coatings with higher hardness, strength, and corrosion resistance than the corresponding conventional coatings.[22,23] It is, therefore, anticipated that nanostructured WC-12 pct Co coatings will possess better performance characteristics as well. The present article describes the synthesis and characterization of nanostructured WC-12 pct Co coating using mechanical milling and high velocity oxygen fuel (HVOF) thermal spraying. II. EXPERIMENTAL PROCEDURE A. Feedstock Powder Commercially available sintered WC-12 pct Co powder (Sulzer Metco, Diamalloy 2004) with a nominal powder size JIANHONG HE, Postdoctoral Scientist, MICHAEL ICE, Graduate Student, and ENRIQUE J. LAVERNIA, Professor and Chair, are with the Department of Chemical and Biochemical Engineering and Materials Science, University of California, Irvine, CA 92697-2575. STEVEN DALLEK, Research Chemist, is with the Carderock Division, Naval Surface Warfare Center, West Bethesda, MD 20817-5700. Manuscript submitted March 30, 1999.
METALLURGICAL AND MATERIALS TRANSACTIONS A
of (245 1 5.5) mm was chosen for this study. The powder was immersed in Hexane [H3C(CH2)4CH3] and mechanically milled with a modified Szegvari attritor Model B at a rate of 180 rpm for 20 hours in a stainless steel tank with WCCo cement balls. The ball-to-powder mass ratio was 20:1. Powder samples, extracted from the milling vessel every 2 hours, were used for scanning electron microscope (SEM) analysis performed on a PHILIPS* XL 30 FEG microscope. *PHILIPS is a trademark of Philips Electronics Instruments, Mahwah, NJ.
The X-ray diffraction measurements were carried out using a SIEMENS** D5000 diffractometer equipped with a graphite **SIEMENS is a trademark of Siemens Electrical Equipment, Toronto.
monochromator using Cu Ka (l 5 0.15406 nm) and Mo Ka (l 5 0.070923 nm) radiation scans with a step size of 0.01 deg and a step time of 1 second for phase identification. Detailed scans with a step size of 0.01 deg and a step time of 5 seconds were conducted on the powder for grain size measurements. For transmission electron microscope (TEM) analysis, the milled powders were dispersed in methanol and then deposited on carbon-grid substrates. Transmission electron microscope observations were performed using a PHILIPS CM20 microscope operated at 200 keV. On the basis of ASTM E1019 and AS
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