The total world's annual estimated direct cost of corrosion including wear damage is estimated up to 3-4% of developed countries’ national income (GDP) [1]. For several decades hard chromium coatings fabricated by plating has been the most used to protect components operating in high wear environment. However, hard chromium coatings are applied from electrolytic baths containing hexavalent chromium and they face many problems: health issues for the plating industry personnel, EU restrictions, functional defects of the coatings and low current efficiency [2]. Metal matrix composites reinforced by micro- and nano-particles, suitable for a large number of functional and structural applications, are under investigation worldwide. Different kinds of matrix metals have been coupled with several types of nanometric phases such as ceramic compounds, intermetallic materials and carbon allotropes. Composites can exhibit enhanced mechanical, electrical, magnetic, and/or optical properties compared with their conventional micron-scale (or larger) counterparts. Electrolytic composite coatings based on a Ni–P alloy matrix containing micro and nano particles of SiC, WC, B₄C, MWCNT, or Al₂O₃ have attracted attention as the one of alternatives to hard chromium due to their good mechanical and chemical properties including high hardness and enhanced wear resistance combined with a good corrosion resistance [3]. In the present study, Ni-P composite coatings were successfully electrodeposited on mild carbon steel from a sulphate based Ni-P electroplating baths reinforced with micro and nano B₄C. The effect of the electrolyte composition as well as the current density on the phosphorus co-deposition rate, particles incorporation rate, surface morphology and structure of the deposits has been investigated and evaluated. The effect of particles in solution is discussed in terms of nucleation of the composite coating. The effect of pulse plating is also evaluated, showing the benefit in terms of phosphorus content and uniformity, amount and distribution of co-deposited phase. A different behavior of the electrolyte containing nano and micro particles is observed under the Guglielmi model for the co-deposition mechanism. The B₄C metal matrix composites combined with pulse plating can offer improved micro hardness and wear resistance, together with good corrosion resistance. This behavior will be discussed in comparison to hard chromium coatings.

Acknowledgements: This work has partially received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement No. 606110 (HardAlt - " New generation of protective coatings alternative to hard chrome").

References

1. K.G. Budinski, M.K. Budinski, Engineering Materials: Properties and Selection (7th Edition), Prentice Hall, 2007.
2. B. Sonntag, V. Sundaram, “Substitution of Cr(VI)-containing coatings by the European automobile industry,” Atotech Deutschland GmbH report, 2005.
3. Z. Abdel Hamid, J. Metallurgical Engineering (ME) 3/1 (2014) 29.