Surface reinforcement of steel substrate by electroless nickel-boron.

Electroless nickel-boron coatings, obtained by chemical reduction, by sodium borohydride, of nickel ions in aqueous solution were synthesised on mild steel (ST 37) substrates and submitted to heat treatments in unreactive atmosphere (95% Ar - 5 % H2) at 400°C.
The structure modification induced by heat treatment was studied by X-ray diffraction and TEM. Transition from nearly amorphous to nanocrystazed structure was thus observed.
Mechanical reinforcement of the surface was assessed by micro- and nanohardness measurements (both on the free surface and on polished cross-sections), scratch tests and Taber abrasion tests. Hardness of the coatings was found in the order of 800 hk25 in the as-plated condition and increased up to 1200 hk25 after heat treatment. This lead to a improvement of the Taber Wear Index that decreased from 25 down to 13 after heat treatment. Scratch resistance was equally found acceptable.
Corrosion resistance of the system was also investigated in 3.5 wt. % NaCl. The behaviour of the coating was acceptable and the corrosion resistance increased after heat treatment.