Influence of Environment and Load-Controlled Tribo-chemistry on Wear Performance of Ni-Cr-B-Si Hardfaced Coating

Abstract Ni-Cr-B-Si hardfaced coating is used in many mating components of Fast Breeder Reactors for improving their tribological properties. Selection of environment and load, plays a vital role in controlling wear loss of the hardfaced coating under rubbing conditions, in addition to its processing route. In the present study, a defect-free Ni-Cr-B-Si coating is made on 316L (N) stainless steel using Plasma Transferred Arc Welding (PTAW) process. Dominant phases dispersed in the γ -Ni matrix of the coating are identified to be borides (CrB, Cr 5 B 3 and Cr 3 B 4 ), carbides (Cr 23 C 6 , Fe 3 C and Cr 7 C 3 ) and nitride (BN). Average hardness of the coating is measured to be 536 ± 33 HV for 0.5 kg load. Tribological behavior of the coating is studied using a ball-on-disk tribometer at 2N and 5N loads in air (1 bar) and vacuum (10 −9 bar), in order to simulate its friction and wear performance at contact pressures relevant for various structural components placed in a sodium environment. Results revealed higher average friction coefficient in vacuum than that in air, which is due to variation in dominant wear mechanism from tribo-oxidation to micro-plowing. Specific wear rate increased with both increase in load from 2 to 5N and change in environment from air to vacuum. This is attributed to the increased delamination-type wear along with oxidation in air and micro-plowing in vacuum as revealed by the unique tribo-induced morphology and chemistry corresponding to each of the wear test conditions. Techniques such as scanning electron microscopy, energy-dispersive spectroscopy, and Raman spectroscopy are used for the examination of tribo-pair surfaces, the results of which helped to elucidate the possible wear mechanisms. Results from the present study necessitate the relevance of choosing a hardfaced coating with appropriate microstructural and tribological characteristics for an intended application in order to minimize the risk of wear.