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The influence of the phosphorous content and heat treatment on the nano-micro-structure, thickness and micro-hardness of electroless Ni-P coatings on steel

Czagány, Máté

Baumli, PĂ©ter

Kaptay, György

Elsevier

2017-06-16

TN Mining engineering. Metallurgy / bányászat, kohászat, fémipar

Electroless Ni-P coatings were obtained on steel substrates using different bath compositions, which lead
to different phosphorous contents of the coatings. In this paper the effect of the P-content in the Ni-P
coatings was experimentally studied on the thickness of the coating, on its nano-micro-structure and on
its micro-hardness. The as-received samples were nano-crystalline (mostly amorphous according to XRD)
and their micro-hardness was found to decrease with increasing the P-content. Upon annealing at 400 â—¦C
a new Ni3P phase was formed and the nano-cystalline Ni-rich grains coarsened to micro-grains. In this
annealed state the micro-hardness was found to increase with increasing the P-content. A complex model
was built to explain the experimental results. It was supposed that the as-received Ni-P coating contains
almost pure Ni nano-grains surrounded by segregated P atoms; as the grain grows, its surface is covered
more and more by the P atoms. When the grain is fully covered by the P atoms, further grain growth is
inhibited and the coating can grow further only due to nucleation of a new grain. Thus, the size of the
grains was found inversely proportional to the P-content of the Ni-P alloy. The need for a larger number of
nucleation events with decreasing grain size explains why the coating has a smaller thickness for smaller
grain size, i.e. higher P-content. The inverse Hall-Petch rule was found for the grain size dependence of
micro-hardness of the as deposited samples due to the grain boundary sliding of relatively hard Ni-rich
nano-crystals along the soft P atoms (higher P-content lead to lower micro-hardness through smaller
grain size). After annealing the micro-hardness was found to increase with the volumetric phase fraction
of the harder Ni3P phase within a relatively soft Ni matrix, i.e. it was found to increase with the P-content
of the Ni-P coating. The extrapolated value for the micro-hardness of the Ni3P phase is found about
757 ± 20 HV0.01.

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Article

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info:eu-repo/semantics/article

text

http://real.mtak.hu/64183/1/Czagany-Baumli-Kaptay.pdf

Czagány, Máté and Baumli, Péter and Kaptay, György (2017) The influence of the phosphorous content and heat treatment on the nano-micro-structure, thickness and micro-hardness of electroless Ni-P coatings on steel. Applied Surface Science, 423. pp. 160-169. ISSN 0169-4332

http://real.mtak.hu/64183/

https://doi.org/10.1016/j.apsusc.2017.06.168