Surface & Coatings Technology 426 (2021) 127757 Available online 25 September 2021 0257-8972/© 2021 Elsevier B.V. All rights reserved. Microstructure and mechanical properties of plasma transferred wire arc spray coating on aluminum cylinder bores J. Zhang , D.C. Saha , H. Jahed * Fatigue and Stress Analysis Laboratory (FATSLab), Department of Mechanical & Mechatronics Engineering, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada A R T I C L E INFO Keywords: PTWA coating AlSi cylinder bore Microstructure Residual stress Hardness Failure mechanisms ABSTRACT This study examines the microstructure and mechanical properties of plasma transferred wire arc (PTWA) coating of typical alloyed steel, deposited on diecast aluminum alloy cylinder bores. The coating surface and microstructure were characterized in terms of surface roughness, features (i.e., defects, splats formation mech- anisms, distribution of oxides, re-solidifed particles, and interfacial metallurgical bonding) using laser scanning confocal microscope, scanning and transmission electron microscope (SEM and TEM). Residual stress through the thickness of the coating was measured using X-ray diffraction (XRD) and hole-drilling method. In post-processed samples, compressive residual stress was measured throughout the coating with a value close to 100 MPa at the interface, resulting from the thermal mismatch between coating and substrate materials. In terms of mechanical properties, coating hardness was estimated at both the micro- and nanoscale and examined the infuence of microstructure inhomogeneity on the mechanical performance and failure modes. Three-point bending tests and consequent analysis using the equivalent section method yielded stress-strain properties for both the substrate and coating materials with a maximum coating strength averaging 1480 ± 108 MPa. SEM observation of fracture surfaces showed three modes of failure involving coating delamination and breakage, which is related to the deposition process and the various features within the coating. 1. Introduction The research around thermal sprayed aluminum‑silicon alloy (AlSi) cylinder bores is part of the ongoing initiative to improve fuel economy and reduce greenhouse gas (GHG) emissions through automotive light- weighting efforts [1]. In the engine block, the replacement of grey cast iron with cast AlSi and iron/steel cylinder bore liners resulted in up to 50% weight reduction [2,3]. These cylinder bore liners undergo defor- mation that may lead to the development of a heat pocket, increasing oil and fuel consumption [4]. However, these liners are necessary due to the low hardness and poor wear resistance properties of hypoeutectic AlSi alloys, which do not satisfy the contact tribological requirements be- tween the piston ring and the cylinder bore wall. In comparison to the traditional liners, thermal sprayed coatings are signifcantly thinner, which can increase cylinder volume, improve engine effciency and tribological properties, and could potentially resolve the issue of deformation [5]. The plasma transferred wire arc (PTWA) thermal spray process was frst developed by Flame Spray Industries and is a common method of thermal spray application to liner-less cylinders. The process involves propelling the molten coating material (in this case, alloyed steel) to the substrate (diecast aluminum alloy) using a stream of atomizing gas at extremely high temperatures (~15,000 ◦ C) and velocities (200 to 300 m/s) to form splats on a substrate surface [6]. In the typical plasma spray process, the cooling rate is approximately within the 10 7 to 10 8 K/s range [7]. When molten particles are accelerated in the air, they are mixed with oxygen to produce Wustite (FeO) which is 70% harder than the coating steel matrix and thereby increases the wear resistance of the cylinder bore [5,8]. It has been previously shown that the hardness and roughness of the coating have a direct correlation to the coating wear performance [9]. It has been found in previous studies that due to the high-velocity material deposition process, the coating microstructure is comprised of both amorphous and crystalline structures [7]. However, this also means that the coating splat morphology consists of pores and other features that act as locations of stress concentration leading to failure [5]. Oxidation of the sprayed coating can take place in the vapor phase surrounding the droplet or at the droplet surface in a vein-like * Corresponding author. E-mail address: hamid.jahed@uwaterloo.ca (H. Jahed). Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat https://doi.org/10.1016/j.surfcoat.2021.127757 Received 13 June 2021; Received in revised form 21 August 2021; Accepted 20 September 2021