Abstract

The effect of detonation synthesis nanodiamond powders on the structure and properties of electroless deposition Ni-B coatings on steel was studied by cross-sectional depth-sensing indentation, X-ray diffraction (XRD), and scanning electron microscopy. As-deposited and annealed coatings with and without nanodiamond additives were investigated. Raman spectroscopy and XRD were unable to detect diamond particles in the coatings. The improved mechanical properties of coatings can be explained by improved deposition conditions and finer structure of the coatings upon addition of nanodiamond powders to the deposition bath

Keywords

Diamond, Detonation, Coating, Nickel Boride, Hardness

Introduction

Since the discovery of detonation synthesis for ultradispersed diamonds in the 1960s, the full potential of nanodiamonds remains to be explored. Detonation synthesis produces nanosized carbon particles with average diameters of ~5 nm, featuring a diamond core covered by graphitic layers and amorphous carbon. The surface of nanodiamond particles is rich in various functional groups and can be further functionalized. The superior properties of the diamond core combined with a large and chemically tunable surface allow them to be used in a wide variety of ways: initial research performed primarily in the former Soviet Union suggests a broad range of industrial applications. Such uses include chemical and electrochemical co-deposition with metals, ultradispersed diamond-polymer composites, polishing, lubrication, and biomedical applications. Commercially, electroplating and lubricant additives currently show the highest potential. Still there is no accepted opinion concerning the mechanism by which coating properties are improved upon the addition of nanodiamond particles to the deposition bath. Two alternatives are currently under consideration. One explains the improvement by inclusion of nanodiamond into the structure of coatings; the other suggests that nanodiamond improves the deposition conditions resulting in decreased columnar structure and porosity of the coating. Addition of nanodiamond to plated coatings is especially important for Ni-based coatings which are expected to replace the Cr-containing coatings currently in use, which require toxic chemicals for their production. Ni-B coatings have been shown to act particularly well. When coated with Ni-B, residual pores in stainless steel or other metals become filled, which results in improved mechanical properties of the interface. The coatings also decrease corrosion and increase wear resistance compared to the steel substrate. However, when compared to the Cr coatings, Ni-B coatings feature lower mechanical properties and wear resistance, both due to their columnar structure. Previous results have shown that nanodiamond additives can increase hardness of Ni-based coatings by a factor of 2-3. Dispersed nanodiamonds added into the deposition bath have been empirically shown to further increase the tribological properties of Ni-B coatings. But still, the role of nanodiamond in the observed phenomena remains unclear. The aim of this study is to investigate the structure and mechanical properties of Ni-B coatings plated with and without nanodiamond additives to the deposition bath to obtain better insight into the role of nanodiamond in the Ni-B deposition process.