Structural and Thermal Behavior of Fe-Cr-Mo-P-B-C-Si Amorphous and Nanocrystalline HVOF Coatings
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. Movahedi, M.H. Enayati, and C.C. Wong (Submitted January 4, 2010; in revised form March 31, 2010) The microstructure, thermal behavior, and mechanical properties of amorphous/nanocrystalline 70Fe-15Cr-4Mo-5P-4B-1C-1Si (wt.%) coatings produced by high velocity oxy fuel (HVOF) spraying of mechanically alloyed powders were investigated by x-ray diffractometry (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Thermal stability of samples was investigated using differential scanning calorimetry (DSC). The results show that by adjusting the HVOF parameters especially fuel/oxygen ratio and proper selection of powder composition, the desired microstructure with different amount of amorphous and nanocrystalline phases and therefore with different mechanical properties could be obtained.
Keywords
amorphous, Fe-base, HVOF, mechanical alloying, nanocrystalline
1. Introduction Synthesizing amorphous and/or nanocrystalline layers on metal substrates can be utilized to improve surface performance such as wear and corrosion resistance (Ref 1). Greer et al. reported that amorphous alloys can have very good resistance to sliding and abrasive wear and the coatings can have low friction coefficient (Ref 2). Thermal spraying process is one of the techniques to deposit amorphous coatings on surfaces, where the amorphous structure is retained due to the sufficiently rapid cooling that inhibits long-range diffusion and crystallization. On impact with the substrate, droplet spreading occurs to give lamellar morphologies with cooling rates of 107-108 K/s (Ref 3). A number of researchers have investigated the use of air plasma spraying (APS), low pressure plasma spraying (LPPS) and vacuum plasma spraying (VPS) to deposit alloys, which are capable of solidifying as metallic glasses (Ref 4, 5). Kishitake et al. reported that mixed amorphous and crystalline structures B. Movahedi, Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156 83111, Iran and School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore; M.H. Enayati, Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156 83111, Iran and Poudrafshan Company (PACO), Isfahan Science and Technology Town (ISTT), Isfahan, Iran; and C.C. Wong, School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore. Contact e-mails: [email protected] and movahedi@ma. iut.ac.ir.
Journal of Thermal Spray Technology
are obtained in APS and LPPS coatings (Ref 5, 6). In recent years, there has been an increasing interest in the use of high velocity oxy-fuel (HVOF) thermal spraying for depositing protective coatings. In the HVOF process, the flame temperature is moderate (3000 °C) while the velocity of the powder particles is very high (850 m/s). The favorable properties given by HVOF process include low porosity (less 1%), high hardness (40-70 HRC), low oxide contents, high adhesion (
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