Luiza Bonin

Electroless deposition of nickel-boron coatings using low frequency ultrasonic agitation: Effect of ultrasonic frequency on the coatings

&NA; The effect of ultrasound on the properties of Nickel‐Boron (NiB) coatings was investigated. NiB coatings were fabricated by electroless deposition using either ultrasonic or mechanical agitation. The deposition of Ni occurred in an aqueous bath containing a reducible metal salt (nickel chloride), reducing agent (sodium borohydride), complexing agent (ethylenediamine) and stabilizer (lead tungstate). Due to the instability of the borohydride in acidic, neutral and slightly alkaline media, pH was controlled at pH 12 ± 1 in order to avoid destabilizing the bath. Deposition was performed in three different configurations: one with a classical mechanical agitation at 300 rpm and the other two employing ultrasound at a frequency of either 20 or 35 kHz. The microstructures of the electroless coatings were characterized by a combination of optical Microscopy and Scanning Electron Microscope (SEM). The chemistry of the coatings was determined by ICP‐AES (Inductively Coupled Plasma ‐ Atomic Emission Spectrometry) after dissolution in aqua regia. The mechanical properties of the coatings were established by a combination of roughness measurements, Vickers microhardness and pin‐on‐disk tribology tests. Lastly, the corrosion properties were analysed by potentiodynamic polarization. The results showed that low frequency ultrasonic agitation could be used to produce coatings from an alkaline NiB bath and that the thickness of coatings obtained could be increased by over 50% compared to those produced using mechanical agitation. Although ultrasonic agitation produced a smoother coating and some alteration of the deposit morphology was observed, the mechanical and corrosion properties were very similar to those found when using mechanical agitation. HighlightsUltrasound did not bring significant modification of the hardness or wear behaviour.Ultrasonic agitation generates a significant increase of the plating rate.When ultrasonic agitation was employed the coating thickness increased by over 50%.Ultrasonic agitation generates a smoother and less porous morphology.

Chemical, morphological and structural characterisation of electroless duplex NiP/NiB coatings on steel

ABSTRACT Duplex electroless nickel coatings constituted of one layer of nickel-phosphorous and one of nickel-boron are a promising solution to provide combined wear and corrosion resistance to parts. Duplex systems were compared to systems of similar thickness constituted of only one material, in one or two layers. Duplex coatings present intermediate surface texture, but each layer keeps its typical cross-section morphology and structural features, even after heat treatment. The interfaces between the separate layers are sharp in the as-deposited state but not as much after heat treatment. When nickel-boron is deposited first, it influences slightly the grain growth of the subsequent nickel-phosphorous layer, but no influence can be observed when nickel-phosphorous is deposited first.

Mechanical properties of heat-treated duplex electroless nickel coatings

ABSTRACT Electroless nickel coatings present complementary properties: coatings formed using hypophosphite present usually better corrosion resistance while those made with borohydride present better mechanical and wear properties. Duplex coatings, combining both kinds of deposits, are designed to combine features of both nickel–phosphorus (ENP) and nickel–boron (ENB) coatings. In this study, monolayer and bilayer coatings were synthesised on mild steel substrates, in various combinations of ENP and ENB, with a total thickness of 20 µm and heat treated (400°C – 1 h) under non-reactive atmosphere (95% Ar–5% H2) to enhance their mechanical properties. The morphology, roughness, hardness, abrasive wear resistance and scratch behaviour of the coatings were investigated. Samples with ENP as the top layer showed a higher roughness than samples with ENB as the external layer. However, samples with ENB as the top layer provided better wear resistance, even if they presented a lower combined hardness. ENB monolayer showed the best resistance to scratch testing.