Characterization of Thermal Stability and High-Temperature Tribological Behavior of Electroless Ni-B Coating
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PROTECTION of a soft metallic substrate against surface-related damage, such as wear, corrosion, and oxidation, is one of the challenging tasks for the tribology community. Hard tribological coatings, which successfully act as a protection against wear and friction at room temperature, often fail when they are exposed to elevated temperatures. Liquid lubricants also SOUPITAK PAL and VIKRAM JAYARAM are with the Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India. Contact e-mail: [email protected] ROHIT SARKAR is with the Department of Metallurgical and Materials Engineering, National Institute of Technology Karnataka, Surathkal, Karnataka 575025, India. The current research was carried out at the Department of Materials Engineering, IISc, Bangalore. During that period, Dr. Rohit Sarkar was working at a summer intern at IISc, Bangalore. His current affiliation is with Intel, Hilsboro, Oregon. The corresponding author of the paper, Dr. Soupitak Pal is currently a Post Doctoral Fellow at the Department of Materials, UC Santa Barbara, Santa Barbara, California. Manuscript submitted October 18, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
dissociate at high temperature. Hence, there is a serious need for a high-temperature tribological coating that can act as a solid lubricant during service. Key issues for designing such a high-temperature tribological coating are its oxidation resistance, microstructural stability, and retention of mechanical properties at elevated temperature. It was observed that completely crystalline electroless Ni-B coating possessed remarkable room-temperature properties, a very low wear rate, and a low coefficient of friction (COF) when it was tested following the ASTM-D2714 method under mineral oil lubrication.[1] The completely crystalline coating contains boride precipitates (Ni3B, Ni2B), which have a melting temperature of ~ 1150 C.[2] Significant oxidation of the Ni-B coating starts to occur at 800 C with a parabolic rate constant of 6.8 9 1012 g2 cm4 s1.[3] Thermogravimetric analysis of the coating displays no significant weight gain up to 600 C.[4] Therefore, present knowledge regarding the properties of the electroless Ni-B coating suggests that this material might be used as a potential candidate for high-temperature tribological coating up to a temperature of ~ 0.5 Tm (~ 500 to 600 C). Very recently, Mukhopadhyay et al. studied the
high-temperature wear behavior of as-deposited electroless Ni-B coating from room temperature to 500 C using pin-on-disk test geometry, where the coated AISI-1040 medium carbon steel was used as a pin against an EN-31 steel disk.[5,6] Since this work appears to overlap with the present submission, we address some important differences. It is a well-accepted fact in the community that prior to practical applications of the coating, the as-deposited coating is subjected to heat treatment for microstructure optimization. Heat treatment is carried out because the as-plated electroless Ni-B coatings are amorphous and
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