Influence of Environment and Load-Controlled Tribo-chemistry on Wear Performance of Ni-Cr-B-Si Hardfaced Coating
摘要
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, Cr5B3 and Cr3 B4), carbides (Cr23C6, Fe3C and Cr7C3) 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.