Microstructure and Sliding Wear Behavior of Fe-Based Coatings Manufactured with HVOF and HVAF Thermal Spray Processes A. Milanti, V. Matikainen, G. Bolelli, H. Koivuluoto, L. Lusvarghi, and P. Vuoristo (Submitted September 25, 2015; in revised form February 10, 2016) The microstructure and micromechanical behavior of thermally sprayed Fe-based coatings manufactured with high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) processes were investigated. Fe-Cr-Ni-Si-B-C and Fe-Cr-Ni-Mo-Si-B-C powders were used as the feedstock materials. The coatings showed a highly dense microstructure with near-zero oxidation. The microstructure of the feedstock powders was better retained when sprayed with HVAF process. Differential scanning calorimetry re- vealed two small exothermic peaks at about 600 °C for the HVOF-sprayed coatings, without any increase in weight in thermogravimetric analysis. It suggested the re-precipitation of carbides that were dissolved during spraying due to the higher particle temperature reported by spray diagnostics system during the HVOF process (1800 °C) compared to the HVAF one (1400 °C). Micro- and nano-indentations helped to show the difference in inter-lamellar cohesive strength and, in turn, in the particle deposition mechanism. Coatings sprayed with Fe-Cr-Ni-Mo-Si-B-C composition possessed higher sliding wear resistance than that of Fe-Cr-Ni-Si-B-C due to higher nano-hardness. More specifically, HVOF-sprayed Fe-Cr-Ni-Mo-Si-B-C coating showed the largest intra-lamellar hardness, the largest elasticity, and high quality of particle interfaces which resulted in lower sliding wear rate. Keywords iron alloys, protective coatings, wear resistant coatings 1. Introduction Global economic crisis together with more strict envi- ronmental restrictions on materials and industrial manu- facturing processes have been forcing the research community to find cheaper and environmentally friendly solutions to increase the properties of bulk materials employed in several industrial applications such as mechanical equipment manufacturing, hydraulic compo- nents, pulp and paper manufacturing, aerospace tech- nologies, electrical engineering, and off-shore structures. Among several approaches, surface engineering is widely acknowledged as the main viable way to increase mechanical and chemical properties of bulk materials at affordable costs by modification of surfaces when wear, corrosion, fatigue, or creep are involved (Ref 1). Modifi- cation of the surfaces can be attained without alteration of the chemistry (e.g., thermal or mechanical treatments), with the alteration of the chemistry (e.g., thermochemical diffusion, chemical treatments) or by adding layers of material (Ref 2). Thermal spray technology belongs to the last category and it is gaining more and more attention due to the high versatility and the relatively low cost. A thermal spray process consists of a feedstock material (powders, wires, or rods), a source of heat, a spray gun, and a jet of gases which accelerate particles towards a substrate, onto which they impact to build a coating (Ref 3). For wear and corrosion applications, Ni-, Co-based alloys and hardmetals have been extensively deposited by thermal spray processes because of their good mechanical and chemical properties (Ref 4-7). However, over the last 10 years the price of Ni- and Co-based alloys remarkably grew (up to 40 9 10 3 euro/ton and now at about 10 9 10 3 euro/ton) forcing the main producers to over- come the problem by finding alternative suitable solutions (Ref 8). Moreover, the International Agency for Research on Cancer recognized six metals and/or their compounds as human carcinogens (arsenic, beryllium, cadmium, hex- avalent chromium, cobalt, and nickel). Even though the mechanisms of action of carcinogenic metals such as Co and Ni are still far from being completely elucidated, several regulations limit the usage of Ni and Co and their compounds due to the potential risk (e.g., toxicity, car- cinogenicity, and dermal sensitization) in certain circum- stances (Ref 9-12). He ´ riaud-Kraemer and Montavon (Ref 13) also highlighted the potential and real risks that workers are exposed to in thermal spray industry. Cobalt and nickel were found to possess one of the lowest per- missible exposure limit (PEL) and short-term exposure limit (STEL) values that the workers can be exposed to A. Milanti, V. Matikainen, H. Koivuluoto, and P. Vuoristo, Department of Materials Science, Tampere University of Technology, Tampere, Finland; and G. Bolelli and L. Lusvarghi, Department of Engineering ‘‘Enzo Ferrari’’, University of Modena and Reggio Emilia, Modena, Italy. Contact e-mail: andrea.milanti@tut.fi. JTTEE5 DOI: 10.1007/s11666-016-0410-z 1059-9630/$19.00 Ó ASM International Journal of Thermal Spray Technology Peer Reviewed