Research Article Ni-Ti Shape Memory Alloy Coatings for Structural Applications: Optimization of HVOF Spraying Parameters CarmenDeCrescenzo , 1 Despina Karatza, 1 DinoMusmarra , 1 SimeoneChianese , 1 Theocharis Baxevanis, 2 PanagiotaT.Dalla, 3 DimitriosA.Exarchos, 3 KonstantinosG.Dassios , 3 andTheodoreE.Matikas 3 1 Department of Engineering, University of Campania Luigi Vanvitelli, 81031 Aversa, Italy 2 Department of Mechanical Engineering, University of Houston, Houston, TX 77204, USA 3 Department of Materials Science and Engineering, University of Ioannina, Ioannina 45110, Greece Correspondence should be addressed to Dino Musmarra; dino.musmarra@unicampania.it Received 9 February 2018; Revised 21 April 2018; Accepted 6 May 2018; Published 28 June 2018 Academic Editor: Shuo Yin Copyright©2018CarmenDeCrescenzoetal.isisanopenaccessarticledistributedundertheCreativeCommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. isworkaimsatcontributingtothedevelopmentofarevolutionarytechnologybasedonshapememoryalloy(SMA)coatings deposited on-site to large-scale metallic structural elements, which operate in extreme environmental conditions, such as steel bridges and buildings. e proposed technology will contribute to improve the integrity of metallic civil structures, to alter and controltheirmechanicalpropertiesbyexternalstimuli,tocontributetothestiffnessandrigidityofanelasticmetallicstructure,to safely withstand the expected loading conditions, and to provide corrosion protection. To prove the feasibility of the concept, investigationswerecarriedoutbydepositingcommercialNiTinolNi50.8Ti(at.%)powder,ontostainlesssteelsubstratesbyusing high-velocityoxygen-fuelthermalspraytechnology.WhiletheNiTinolhasbeenknownsincedecades,thisintermetallicalloy,as wellasnootheralloy,waseverusedastheSMA-coatingmaterial.Duetotheinfluenceofdynamicsofsprayingandtheimpact energyofthepowderparticlesonthepropertiesofthermallysprayedcoatings,theeffectsofthemainsprayparameters,namely, spray distance, fuel-to-oxygen feed rate ratio, and coating thickness, on the quality and properties of the coating, in terms of hardness, adhesion, roughness, and microstructure, were investigated. 1.Introduction Ni-Ti shape memory alloys are extremely interesting ma- terials both for their ability of showing the shape memory effect(SME)andfortheirelevatedstrengthandductility[1]. An attractive Ni-Ti-based SMA is NiTinol, a nearly equiatomic intermetallics of nickel and titanium. NiTinol shape memory properties were first discovered by Buehler and Wiley at the Naval Ordnance Laboratory, Maryland, United States [2] (the name NiTinol is derived from the chemicalsymbol“NiTi”followedby“NOL,”theacronymfor Naval Ordnance Laboratory). IncommonwithotherSMAs,NiTinolshowstwoimportant mechanical features: shape memory effect and pseudoelasticity. e former is related to fatigue and fracture resistances of an alloy and consists in its ability to return to its initial shape upon heating to the austenite phase (high-temperature phase havingB2cubicstructure)afterhavingbeendeformedinthe martensite phase (low-temperature monoclinic phase) [3–5]; the latter is due to the stress-induced martensitic transforma- tionuponloadingandthesubsequentstrainrecoveryupon unloadingattemperaturesabovetheaustenitetemperature A f [1, 6]. NiTinol’s ability to undergo a thermal- or stress- inducedmartensiticphasetransformationanditsrecoverable strains that are much greater than those in traditional alloys, specificallybetween8%and10%[3,7],makeitthemost popular shape memory alloy [3]. NiTinol offers additional advantage for the targeted ap- plication since there is a good understanding of its thermo- mechanical response and the crystallography, as well as of the Hindawi Advances in Materials Science and Engineering Volume 2018, Article ID 7867302, 10 pages https://doi.org/10.1155/2018/7867302