metals Article Tribology and Airborne Particle Emission of Laser-Cladded Fe-Based Coatings versus Non-Asbestos Organic and Low-Metallic Brake Materials Yezhe Lyu 1,2 , Mara Leonardi 3 , Alessandro Mancini 3 , Jens Wahlström 1 and Ulf Olofsson 2, *   Citation: Lyu, Y.; Leonardi, M.; Mancini, A.; Wahlström, J.; Olofsson, U. Tribology and Airborne Particle Emission of Laser-Cladded Fe-Based Coatings versus Non-Asbestos Organic and Low-Metallic Brake Materials. Metals 2021, 11, 1703. https://doi.org/ 10.3390/met11111703 Academic Editor: Andre Paulo Tschiptschin Received: 27 September 2021 Accepted: 23 October 2021 Published: 26 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 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/). 1 Department of Mechanical Engineering Sciences, Lund University, SE-22100 Lund, Sweden; yezhe.lyu@mel.lth.se (Y.L.); jens.wahlstrom@mel.lth.se (J.W.) 2 Department of Machine Design, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden 3 Advanced R&D Department, Brembo S.p.A, 24040 Stezzano (BG), Italy; Mara_Leonardi@brembo.it (M.L.); Alessandro_Mancini@brembo.it (A.M.) * Correspondence: ulfo@md.kth.se; Tel.: +46-(0)-8-790-6304 Abstract: Laser cladding is a promising surface treatment for refurbishing worn-out cast-iron brake rotors. Previous studies on laser-cladded brake rotors have demonstrated their extensively higher wear and greater airborne particle emissions, compared with traditional cast iron rotors. In order to overcome this, a commercial non-asbestos organic (NAO) brake material is tested against Fe-based laser-cladded and traditional cast-iron brake rotors. Two low-metallic brake pad materials are also tested as references. The materials’ coefficients of friction, specific wear rates and particle number concentrations are evaluated. The results indicate that the NAO brake material showed lower wear and had fewer particle emissions than the low-metallic brake materials when deployed against both cast iron and laser-cladded brake rotors. The NAO/laser-cladding friction pairing showed wear, particle concentration and fraction of fine particles (sub 1 μm) equivalent to those of the low-metallic/cast-iron friction pairing, creating significant potential for application in refurbishing worn-out cast-iron brake rotors. Keywords: laser cladding; airborne particle emission; non-asbestos organic; low-metallic brake pad 1. Introduction A brake rotor is a crucial component in an automotive disc-brake system that slows a vehicle by friction with its brake pads. For decades, grey cast iron (GCI) has been the most popularly used material for brake rotors, owing to its excellent castability, high thermal conductivity, good damping capability and cost-effectiveness. However, GCI is not satisfying in terms of corrosion resistance, wear resistance or airborne particle emission [1]. Different attempts have been made to combat these deficiencies. One such strategy is the application of alternative materials to replace GCI in producing automotive brake rotors. Such alternative materials include metal matrix composites [2] and ceramic matrix composites [3], but these materials are more expensive than GCI, and, thus, are not suitable for commercial use in automotive brake rotors. Another strategy is to overlay a protective coating on the GCI substrate. This method is encouraging, since it is not only suitable for producing brand-new brake rotors, but also capable of refurbishing worn brake rotors. Aranke et al. carried out a thorough literature review on coating technologies and materials for GCI brake rotors [1]. Depending on their working temperatures, coating tech- nologies can be categorized by non-thermal spray processes and thermal spray processes. One of the thermal spray processes, HVOF (high-velocity oxy-fuel), has been shown to capably produce wear-resistant cermet coatings on GCI brake discs with reduced airborne particle emission [4–7]; the main disadvantages of HVOF are found in its very costly equipment and restrictive powder sizes (usually below 50 μm) [8]. Metals 2021, 11, 1703. https://doi.org/10.3390/met11111703 https://www.mdpi.com/journal/metals