Characterization and High-Temperature Oxidation Behavior of Cold-Sprayed Ni-20Cr and Ni-50Cr Coatings on Boiler Steels
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INTRODUCTION
IN the conventional thermal spraying processes such as flame spraying, arc spraying, and plasma spraying, it is important that the material melts completely, whereas in high-velocity oxy-fuel (HVOF) spraying, a partial melting is sufficient to achieve a high-quality coating. The reason for this is that the particle velocities upon impact on the substrate are considerably higher in HVOF spraying. Subsequently, in cold spraying, this development toward higher particle velocities with simultaneously reduced and controlled heat input to the spray material was achieved. In this process, the gas temperature is well below the melting temperature of the material, so that the particles in the jet no longer melt.[1] The cold-spray process uses a high-pressure, highvelocity gas jet to impart the velocity for the coating particles.[2] A high-pressure jet, preheated to compensate for the adiabatic cooling due to expansion, is expanded through a converging/diverging nozzle to form a supersonic gas jet. Powder particles, transported by a carrier gas, are injected into this gas jet. Momentum transfer from the supersonic gas jet to the particles results in a high-velocity particle jet. These powder particles, on NIRAJ BALA, Assistant Professor, is with the Baba Banda Singh Bahadur Engineering College, Fatehgarh Sahib, Punjab, India. Contact e-mail: [email protected] HARPREET SINGH, Assistant Professor, is with the School of Mechanical, Materials and Energy Engineering, Indian Institute of Technology Ropar, Rupnagar, India. SATYA PRAKASH, Professor, is with the Metallurgical and Materials Engineering Department, Indian Institute of Technology Roorkee, Uttrakhand, India. Manuscript submitted September 21, 2010. Article published online June 25, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
impact onto the substrate surface, plastically deform and form interlinking splats, resulting in a coating.[2] In the cold-spray process, the temperature of the gas stream is always below the melting point of the particulate material, providing coatings developed primarily from particles in the solid state with very little oxidation.[3–7] No phase transformation occurs during the cold-spray process since it is a 100 pct solid-state process, implying no particle melting. Therefore, oxidation, nitriding, decarburizing, and any decomposition, in general, are avoided in this process. The as-sprayed coating phase composition should be the same as the powder phase composition.[8] Ni-based coatings are used in applications where wear resistance combined with oxidation or hot corrosion resistance is required.[9–11] Not much work has been published on cold-sprayed Ni-20Cr and Ni-50Cr coatings on boiler steels to the best of knowledge of the authors. Therefore, the main objective of the present research is to characterize the cold-sprayed Ni-20Cr and Ni-50Cr coatings on boiler steels. The study shall provide a metallurgical and mechanical properties database for these coatings. The outcome of the study can be useful to explore the possible appl
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