Citation: Jensen, R.; Farhat, Z.; Islam, M.A.; Jarjoura, G. Effect of Coating Thickness on Wear Behaviour of Monolithic Ni-P and Ni-P-NiTi Composite Coatings. Solids 2022, 3, 620–642. https://doi.org/10.3390/ solids3040039 Academic Editor: Francisco J. G. Silva Received: 13 September 2022 Accepted: 19 October 2022 Published: 1 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article Effect of Coating Thickness on Wear Behaviour of Monolithic Ni-P and Ni-P-NiTi Composite Coatings Rielle Jensen 1, *, Zoheir Farhat 1 , Md. Aminul Islam 2 and George Jarjoura 1 1 Department of Mechanical Engineering, Dalhousie University, 1360 Barrington Street, Halifax, NS B3J 2X4, Canada 2 National Research Council Canada, 4250 Wesbrook Mall, Vancouver, BC V6T 1W5, Canada * Correspondence: rielle.jensen@dal.ca Abstract: Protective coatings can prolong the lifespan of engineering components. Electroless Ni-P coating is a very hard coating with high corrosion resistance, but low toughness. The addition of NiTi nanoparticles into the coating has shown the potential to increase the toughness of electroless Ni-P and could expand its usability as a protective coating for more applications. However, the study of the tribological behaviour and wear mechanisms of Ni-P-NiTi composite coating has been minimal. Furthermore, there is no studies on the effect of coating thickness on monolithic and composite electroless Ni-P coating wear behaviour. The wear rates of each coating were found by measuring the volume loss form multi-pass wear tests. The wear tracks were examine using a confocal microscope to observe the wear mechanisms. Each sample was tested using a spherical indenter and sharp indenter. It was found that the NiTi nanoparticle addition displayed toughening mechanisms and did improve the coating’s wear resistance. The 9 μm thick Ni-P-NiTi coating had less cracking and more uniform wear than the 9 μm thick Ni-P coating. For both the monolithic and composite coatings, their thicker version had higher wear resistance than their thinner counterpart. This was explained by the often observed trend in coatings where it has higher tensile stress near the substrate interface, which decreases and becomes compressive as thickness increases. Overall, the 9 μm thick Ni-P-NiTi coating had the highest wear resistance out of all the coatings tested. Keywords: Ni-P-NiTi composite coating; electroless nickel phosphorous; coating thickness; toughening mechanisms; wear 1. Introduction Engineering components that are subjected to wear and corrosion during their per- formance are required to have frequent maintenance, which can be costly [1,2]. However, their lifespan can be prolonged by the application of a surface coating [3]. A protective coating protects the component against wear and corrosion while preserving the substrate material’s mechanical properties [4–6]. Electroless Ni-P coating is already widely used due to its high adhesion, excellent wear and corrosion resistance, and many other additional specific properties [1,7,8]. The addition of secondary particles further improves a specific existing property to cater to the intended use [7,9]. For example, Ni-P coating hardness and wear resistance can be improved with the addition of nanoparticles. In recent years, the wear resistance of Ni-P has been studied with the addition of various ceramic carbide or oxide particles [1,10–15]. However, a major disadvantage of Ni-P coating is its brittle- ness, and the hard particle additions do little to improve Ni-P’s toughness [2,3]. However, developing a composite that uses a ductile particle addition could enhance toughness to a significant degree. The enhanced toughness along with Ni-P’s high hardness and high corrosion resistance would allow the use of Ni-P-based protective coatings to be expanded to even more applications [2,16]. The super-elastic NiTi alloy has uniquely high toughness and ductility, allowing it to be a potential candidate for an addition to Ni-P coating. In fact, Solids 2022, 3, 620–642. https://doi.org/10.3390/solids3040039 https://www.mdpi.com/journal/solids