Ni-Cr-B-Si hardfaced coating is used in Fast Breeder Reactors to reduce wear of mating surfaces of structural components. In the present study, a two-layer Ni-Cr-B-Si coating of thickness ~ 3 mm is made on AISI 316L (N) stainless steel by using Plasma Transferred Arc Welding (PTAW) process.

Specified weld parameters were used for the PTAW hardfacing in such a way that the hardness of the coating shows not much variation, throughout its entire surface, which would have happened as a result of expected microstructural heterogeneity.

Accordingly, the prepared coating consisted of a eutectic structure with CrsBs, CrB, Cr Cr23C6, and FeзC as major phases distributed in the Ni/NiзSi matrix. The average micro-hardness of the coating was measured to be 409±13 HV for 0.5 kg load.

Wear tests were conducted for the coating in a high-vacuum environment,10 mbar against a standard SIC counter body-ball at two different loads, 2N and SN both at room temperature. The steady state values of friction coefficient for the coating increased from 0.57±0.05 to 0.71+0.05 with change of load from 2N to 5N, but with a significant reduction in the wear coefficient, from 2.99×10-14 to 0.47x10-14 (m3/Nm). Improvement in wear performance with increase in the load is attributed to the tribo-induced microstructural as well as microchemical changes in the worn as well as unworn regions of the coating.

These changes were mainly with respect to the presence of boron and nitrogen enriched tribo-layer at the contact region of mating surfaces along with a few of the other elements which got incorporated in the coating as the result of its dilution by the stainless-steel substrate.

Optical as well as Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), Optical Emission Spectroscopy (OES), Electron Back Scattered Diffraction (EBSD), Laser Raman Spectroscopy (LRS) and Thermo-Calc Simulation were used for the investigation of underlying tribo-mechanism along with micro-hardness and roughness measurements.

Results from present study necessitates the relevance of choosing the suitable deposition parameters in order to produce a coating with desired microstructure, depending on the conditions of load under which it is proposed to move in an actual application environment, for minimizing the risk of wear.

This article is shared by N. Sreevidya, Revati Rani, K. Ganesan, Tom Mathews, C. R. Das and M. Vasudevan