Fabrication and Properties of Thermal Sprayed AlSi-Based Coatings from Nanocomposite Powders A. Limpichaipanit, C. Banjongprasert, P. Jaiban, and S. Jiansirisomboon (Submitted April 17, 2012; in revised form September 29, 2012) AlSi-based nanocomposite powders (where nanoparticles were TiO 2 , ZrO 2 , and Al 2 O 3 and the amount of reinforcement was 2.5, 5, and 10 wt.%) were made by ball milling and then thermal sprayed using low velocity oxy-fuel technique. The AlSi-based nanocomposite powders had nanosized ceramic reinforce- ment adhered to the surface of the powders after ball milling. The AlSi-based coatings had the typical thermal spray microstructure where lamellae, oxide layers, unmelted particles, and pores could be seen. Submicron second phase in the form of agglomerates, molten splats, or unmelted particles between AlSi lamellae could be observed as well. Hardness and porosity of the coatings increased when more ceramic second phase particles (harder than AlSi) were added. Sliding wear tests were carried out in pin-on-disk geometry. The wear tracks of AlSi and AlSi-based coatings show plastic deformation as the main material removal mechanism during the sliding wear test. The sliding wear rate of the coatings decreased as more second phase ceramic particles were added. It was due to an increase in the hardness and a decrease in the friction coefficient of the coatings. Keywords AlSi coatings, hardness, low velocity oxy-fuel (LVOF), nanocomposite powders, wear resistance 1. Introduction Aluminum-silicon (AlSi) alloys are widely used in industrial applications for engine parts (e.g., piston liners) which require high wear resistance because of their rea- sonably high hardness from Si phase, their castability and their low density compared to many other alloys. There- fore, AlSi alloy coatings are one of the possible solutions for industrial maintenance where the coating is applied to enhance the wear resistance of the surface. For example, the AlSi coatings are applied to cylindrical liners in engine blocks to prolong their lifespan (Ref 1). AlSi coatings are applied to substrate by thermal spray because this tech- nique is considered to be convenient to produce coatings for metals, ceramics, polymers, and composites (Ref 2). The other advantage is that the mechanical properties and the microstructure of the substrate have minimal or no change after thermal spray. Thermal spray is used to produce AlSi coatings with a thickness in the range of a few hundreds micron. The process involves heating feedstock powders or wires to molten or semi-molten state. Then the molten particles are accelerated to reach a substrate and rapidly solidified to form coatings. AlSi coatings can be fabricated by sev- eral thermal spray techniques: namely, low velocity oxy- fuel (LVOF) or flame spray (FS) (Ref 3), high velocity oxy-fuel (HVOF) (Ref 4) and arc spray (AS) (Ref 1, 5). Flame spray is the most basic operation in thermal spray technology because the cost of production is low and it is less time-consuming. The temperature during thermal spray is ~2000-3000 °C, depending on the feed rate of oxygen and fuel gas, which is suitable for melting AlSi alloy feedstock powder. However, the AlSi coatings pro- duced by flame spray have relatively high porosity and this affects the microstructure and mechanical properties of the coatings such as hardness and wear resistance. In order to improve mechanical properties of AlSi coatings, second phase ceramic particles or reinforcements are added to the alloy to make the so-called metal matrix composites (MMCs). Addition of second phase ceramic particles results in higher hardness compared to mono- lithic AlSi. The reinforcements added to AlSi are, for example, SiC, Al 2 O 3 , or B 4 C (Ref 2, 6-8). The mechanical properties could be further enhanced by adding second phase reinforcements which have the particle size in sub- micron range to produce ‘‘nanocomposites’’. The term nanocomposite was first coined in the 1980s by Roy et al. (Ref 9) and the previous research showed that nanocom- posites had superior mechanical properties compared to monolithic materials, such as strength, fracture toughness, and wear resistance. However, the main problem of nanocomposite fabrication is an uneven distribution of nanosized reinforcement particles. In this work, the AlSi-based coatings were made by flame spray of nanocomposite powders. The second phase A. Limpichaipanit, C. Banjongprasert, P. Jaiban, and S. Jiansirisomboon, Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand. Contact e-mail: sukanda@chiangmai.ac.th. JTTEE5 22:18–26 DOI: 10.1007/s11666-012-9844-0 1059-9630/$19.00 Ó ASM International 18—Volume 22(1) February 2013 Journal of Thermal Spray Technology Peer Reviewed