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Hybrid aging and nitriding treatment of nickel aluminum bronze by plasma-based ion implantation

Nickel-aluminum bronzes (NABs) comprise a versatile group of copper-based age hardenable quaternary alloys. Their composition usually comprises aluminum, nickel, iron, and manganese, besides copper. These alloys have a large range of applications, from architectural use to naval propellers. In the present work, a case study of a specific NAB alloy of technical importance was performed aiming to investigate the difference in wear resistance between samples that were aged and samples that received a novel hybrid aging + nitriding treatment carried out on a plasma immersion ion implantation (PIII) apparatus. Both regular aging and the hybrid treatment were carried out for solution-treated coin-shaped specimens, with martensitic matrix, in a PIII chamber under a nitrogen-based atmosphere. Three different voltage biases, 8, 10, and 12 kV, resulted in three distinct treatment temperatures, 370, 400 and 470 °C, for treatment times of 180 and 90 min, being the latter (90 min) used together with the smaller voltage (370 °C) for comparison purposes. From the coin-shaped sample, it was possible to study the two distinct treatments: i) the flat-surface-side exposed to plasma was subjected to the hybrid treatment; ii) the other flat-side (directly supported on the sample holder) was only aging-treated (hereafter termed ‘aged only’). Such methodology allowed to infer the bulk effects on the surface modification process. The XRD patterns of the hybrid-mode treated (hereafter termed ‘aged + nitrided’) surfaces showed formation of nitrogen-expanded phases, these surfaces showing hardness gains varying on the 11%–27% range when compared to the respective surfaces aged only. Scratch tests, in association with hardness and elastic modulus measurements, showed the role of the aging on the elasto-plastic response of both aged only and aged + nitrided surfaces. The nitriding resulted in significant gains of wear resistance for PIII carried out at the lowest temperatures: 43% and 41% less volume loss than the respective aged only surfaces for the treatments at 370 °C for 90 min and 180 min, respectively. Differently, the treatments at the highest temperature (450 °C) resulted in the worst wear performances, since the bulk over-aging surpassed any strength delivered by nitriding.