Influence of plasma nitriding and PAPVD Cr 1 − x N x coating on the cavitation erosion resistance of an AISI 1045 steel

Abstract

Carbon steels are often used in engineering components that are subjected to a cavitation erosion phenomenon. An attempt to improve the cavitation erosion resistance of an AISI 1045 carbon steel was made by carrying out two surface treatments: plasma nitriding and Plasma-Assisted Physical Vapour Deposition (PAPVD) coating. The cavitation erosion resistance of four systems was investigated, as follows: (i) AISI 1045 steel; (ii) plasma nitrided AISI 1045 steel; (iii) PAPVD Cr 1 − x N x coating on AISI 1045 steel (non-duplex) and (iv) PAPVD Cr 1 − x N x coating on plasma nitrided AISI 1045 steel (duplex). Cavitation erosion tests were performed according to the ASTM G 32-85 standard. Mass loss plots were obtained as a function of time and statistical analyses (piecewise linear regressions) were performed in order to evaluate the cavitation erosion wear rates. The plasma nitrided AISI 1045 steel exhibited three distinctive wear regimes, with an intermediate stage characterised by a low cavitation erosion rate (2.0 mg h − 1 ) between the incubation period (first regime) and accelerated stage (third regime); this intermediate regime could be associated with the wear of the nitrided layer. For the duplex system, an additional wear regime could be detected in comparison to those of the plasma nitrided steel. This additional regime, which exhibited a cavitation erosion rate of 1.3 mg h − 1 , followed the first regime (incubation period) and was associated with the intrinsic wear of the PAPVD Cr 1 − x N x coating. The previous results were corroborated by SEM examination of eroded surface areas. The plasma nitriding treatment, whether or not combined with PAPVD coating, significantly increased the incubation period and was able to provide lower cavitation erosion rates for longer times. The Cr 1 − x N x deposition onto the AISI 1045 steel (non-duplex coating) led to localised erosion and the substrate was reached in the early stages of the cavitation process. The duplex system exhibited the lowest erosion rates over extended cavitation times; its superior performance in vibratory cavitation tests indicates that a combination of plasma nitriding + PAPVD coating should be used in applications where cavitation erosion resistance is a property of prime importance.