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2944 |
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Thursday, September 29, 2011, Saal A 4:00 PM Wear Protection 2 |
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Wear behavior of HVOF-sprayed nanostructured WC-CoCr coatings |
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Tommi Varis* / VTT Technical Research Centre of Finland, Finland G Bolelli / University of Modena and Reggio Emilia, Italy U Kanerva/ VTT Technical research centre of Finland, Finland T Suhonen/ VTT Technical research centre of Finland, Finland B Bonferroni/ University of Modena and Reggio Emilia, Italy L. Lusvarghi/ University of Modena and Reggio Emilia, Italy |
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Thermal-sprayed cemented carbides are widely used for wear protection purposes. The use of nanocarbides can potentially improve hardness, wear performance and friction properties. Commercially nanosized WC-CoCr powders, however, are only available down to carbide size of 0,4 ¼m, as finer carbides may be significantly altered by conventional HVOF processes. The CJS (Carbide Jet Spray)-thermal spray system (Thermico, Germany) produces colder spray conditions, which help retaining the structure of fine carbides. This process was therefore employed in order to spray two experimental WC10wt%Co4wt.%Cr nanostructured powders, manufactured by VTT by spray-drying and sintering. The two powders had different agglomerate size distributions but both contained nanostructured (~200 nm) carbides. Three reference coatings were deposited using commercially-available WC10wt%Co4wt.%Cr powders (Durum Gmbh, Germany) containing sub-micrometric carbide particles: two were sprayed using the same CJS torch and a third one was obtained using a H2-fuelled Diamond Jet 2700 Hybrid torch. Although decarburisation of the nanostructured coatings occurred during spraying (as revealed by XRD), SEM micrographs show they still contain very fine carbide particles, whereas those obtained from commercially-available powders exhibit a multi-modal carbide size distribution, including both sub-micrometric (d500 nm) and micrometric particles. Ball-on-disk wear tests performed at room temperature against sintered Al2O3 balls show that the nanostructured coatings mostly undergo brittle fracture wear, which can be accounted for by decarburisation. The reference CJS-sprayed coatings, by contrast, exhibit ductile behaviour (plastic deformation and pull-out of single carbides), whereas the Diamond Jet-sprayed one exhibits a mix of both ductile and brittle wear mechanisms. Depending on the extent of brittle fracture, the wear rate of some of the nanostructured coatings is comparable to that of the reference multi-modal coatings, whereas some undergo more severe wear. At 500 °C, the wear behaviour changes significantly and becomes dominated by abrasive grooving in most cases. |