On Fatigue Behavior of Cold Spray Coating
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On Fatigue Behavior of Cold Spray Coating A. Moridi1,2, S. M. Hassani-Gangaraj1,2 and M. Guagliano1 1 Politecnico di Milano, Via G. La Masa, 1, 20156, Milan, Italy. 2 Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA, 02139, U.S.A ABSTRACT Cold spray is a novel and promising technology to obtain surface coating. Notwithstanding the several technological advantages with respect to other processes, its diffusion is somewhat limited because of the limited knowledge on the mechanical properties of the cold sprayed materials and in particular, the fatigue behavior. Moreover, the existing data concerning fatigue behavior of coated specimens are controversial and different material system shows different behaviors. The aim of this study is to distinguish the involved parameters and their effect on fatigue behavior of cold sprayed systems. A critical discussion on four important parameters i.e. interface quality, material properties, deposition parameters and residual stress is given. The influential parameters are consolidated in one formula, which can predict the fatigue limit of cold spray system as a function of residual stress, coating hardness and stress gradient in the specimen. INTRODUCTION It is well-known that around 90% of mechanical failures are due to fatigue. This justifies the great importance of fully understanding the fatigue behavior of structural components for a reliable mechanical design. While the well-known mechanical surface treatments such as shot peening and nitriding can improve the fatigue life by means of developing compressive residual stress (RS) and/or surface work hardening [1]-[6], processes aimed at coating the surface most likely deteriorate the fatigue behavior. The magnitude of the compressive residual stress layer developed by a process such as shot peening depends on many factors and is at least as great as one-half the yield strength of the material [7]. Traditional thermal spray coatings, however, develop tensile RS within the substrate, which is detrimental for fatigue behavior [8]. One rather recent developed coating treatment, which is gaining increasing attention in several industrial applications, is cold spray (CS). Since the development of the process, wide range of materials from metals, ceramic and polymers to composite and nanostructure coatings has been CS-ed [9]. Adiabatic shear instability and the resulting plastic flow localization play the major role in the particle/substrate bonding [9],[11]. One of the most important processing parameters in CS is the critical velocity, which the particles should exceed in order to adhere to the substrate. Particle temperature, size distribution, oxygen content, nozzle shape and carrier gas type affect the critical velocity [12]-[15]. The influence of process conditions (gas pressure and temperature, substrate pre-heating, etc.) and the deposition strategy (single/multi pass deposited coatings, (thickness/pass ratio)) has also been shown to have a fundamental effect on the mechanical and microstructural prop
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