Development and Characterization of Electroless Ni-P Composite Coatings with Superelastic nano-NiTi Additions
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Many types of protective coatings have been developed to protect oil and gas pipelines from erosion and corrosion damage. Such damage is caused by erosive particles and corrosive media flowing through the pipes. Electroless Ni-P coatings, which are known for their excellent wear and corrosion resistance, have the potential to protect low carbon steel pipelines against a multitude of detrimental conditions. However, low toughness upon deposition limits their current use. Therefore, the objective of this study is to incorporate superelastic nano-NiTi particles into the electroless Ni-P matrix to enhance toughness. Upon applied mechanical stress, these particles will undergo a reversible martensitic transformation that is responsible for superelastic nature. The ability to absorb larger amounts of strain than typical materials can can aid in absorbing crack propagation energy and deflect or arrest cracks in composites. Ni-P-nano-NiTi coatings were successfully plated on API X100 steel substrates and characterized using SEM, XRD, OM, and micro-Vickers hardness. In order to assess the effect of superelastic NiTi on toughness, several tests were done on composite coatings containing 5.14 wt.%, 6.07 wt.%, and 7.02 wt.% NiTi. Scratch and indentation testing using spherical indenters was coupled with acoustic emission to monitor cracking and fracture behaviour. Single particle erosion was done in order to gain perspective on the micro-mechanisms driving erosion in the composite coatings. Bend testing, along with acoustic emission was used to assess bend strength, elastic modulus, and cracking behaviour under bending conditions. It was found that the addition of superelastic nano-NiTi particles resulted in toughening of the monolithic Ni-P matrix under scratch, indentation and single particle conditions. Transformation toughening, micro-cracking, crack bridging and deflection were observed in the coatings.