Sliding and Rolling Wear Behavior of HVOF-Sprayed Coatings Derived from Conventional, Fine and Nanostructured WC-12Co Powders W. Tillmann, I. Baumann, P. S. Hollingsworth, and L. Hagen (Submitted June 6, 2013; in revised form October 17, 2013) Fine structured and nanostructured materials represent a promising class of feedstock for future appli- cations, which has also attracted increasing interest in the thermal spray technology. Within the field of wear protection, the application of fine structured or nanostructured WC-Co powders in the High Velocity Oxy-Fuel flame spraying technique (HVOF) provides novel possibilities for the manufacturing of cermet coatings with improved mechanical and tribological characteristics. In this study the tribo- logical behavior of HVOF sprayed coatings derived from conventional, fine and nanostructured WC-12Co powders under sliding and rolling wear are investigated and the results are compared to C45 steel (Mat.-No. 1.0503). In addition, sliding and rolling wear effects on a microscopic level are scruti- nized. It has been shown that under optimized spray conditions the corresponding fine and nanostruc- tured WC-12Co coatings are able to obtain higher wear resistances and lower friction coefficients than the conventional coatings. This can be attributed to several scaling effects of the microstructure and to the phase evolution of the coating, which are discussed. Keywords HVOF, Nanostructured coatings, Fine structured coatings, WC-Co, Wear effects, Wear resistant coatings, Wear testing, Scaling effects 1. Introduction It is well known that wear has a great impact on the performance, reliability, safety, and economy of manu- facturing processes because it does not only determine the lifetime of tools or other machine components but also decisively affects the quality of the products. Wear natu- rally arises when functional parts underlie tribological interactions with another counter body as well as inter- mediate and / or ambient media. Wear represents an important system property, which depends on the configuration of the tribological system and the way, the material is removed (wear mechanisms). During a manu- facturing process the successive wear-related material removal leads to a significant alteration of the geometrical, structural and the strength properties of the functional parts. These alterations and the intermediate medium thereby incurred are the reaction of the material against the load and are described by wear effects. For many years, thermally sprayed cermet coatings like WC-Co have been successfully developed and applied to protect surfaces against wear or other deleterious effects such as corrosion. By now, WC-Co has emerged as the predominant coating system, which is typically used for wear problems at temperatures up to 400 °C (Ref 1). If corrosion is simultaneously involved in the wear process, WC-CoCr is generally preferred over WC-Co, because WC-CoCr coatings feature a higher corrosion resistance (Ref 2-5). However, at operation temperatures >400 °C the risk of thermal phase reactions such as decarburization or WC dissolution in the Co(Cr) binder phase increases, which result in deterioration or in a loss of the functional properties of the WC-Co(Cr) coatings. In this case, Cr 3 C 2 - NiCr coatings are rather used as wear protective coating, because they feature a higher thermal stability up to 900°C (Ref 6). Cermet coatings are mostly produced by means of high velocity oxy-fuel flame spraying technique (HVOF). Due to the unique combination of high hardness and moderate toughness, WC-Co coatings generally show a high resistance against various types of wear such as sliding, erosive and fretting wear (Ref 7). Therefore many investigations have been performed to study the wear behavior of WC-Co coatings, to relate it to the mechanical properties or to find fundamental interrelationships be- tween the parameter settings of the spray process and the wear properties of the coatings. Some researchers also This article is an invited paper selected from presentations at the 2013 International Thermal Spray Conference, held May 13-15, 2013, in Busan, South Korea, and has been expanded from the original presentation. W. Tillmann, I. Baumann, P.S. Hollingsworth, and L. Hagen, Institute of Materials Engineering, Faculty of Mechanical Engineering, Technische Universita ¨t Dortmund, Leonhard- Euler-Str. 2, 44227 Dortmund, Germany. Contact e-mails: wolfgang.tillmann@udo.edu and peter.hollingsworth@udo.edu. JTTEE5 23:262–280 DOI: 10.1007/s11666-013-0038-1 1059-9630/$19.00 Ó ASM International 262—Volume 23(1-2) January 2014 Journal of Thermal Spray Technology Peer Reviewed