Modified Indentation Techniques to Probe Inelasticity in Ni-5%Al Coatings from Different Processes
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W.B. Choi, Y. Wu, S. Sampath, and A. Gouldstone (Submitted April 24, 2008; in revised form June 28, 2008) In this study, two types of indentation experiments were performed on metallic (Ni-5%Al) coatings prepared by cold spray, high velocity oxy-fuel and air plasma spray. In the first type, spherical tips were used with increasing loads, and subsurface deformation was observed using a modified bonded interface technique. In the second type, cyclic loading was imposed with a sharp tip, and tip displacement was continuously recorded. Results suggest that cold spray coatings are brittle under contact loads in their as-sprayed condition, and that they exhibit a size effect that is quite different from those of the other coatings. That is to say, heterogeneities in mechanical behavior exist not as much on the single particle level as expected, but on a much larger scale of order 100 microns. This is attributed to long unbonded regions between particles, in a coating of otherwise high density. Fracture mechanics arguments support this hypothesis.
Keywords
fracture, indentation, spray deposition
1. Introduction Cold sprayed (CS) coatings are fabricated via the acceleration and impact of powder particles onto a target substrate. CS deposition does not involve the in-flight melting of particles; bonding is achieved by dissipation of kinetic energy to localized melting. For this reason, CS has advantages over other thermal spray (TS) processes in that oxidation does not occur, and >99% dense coatings are formed (Ref 1-3). In addition, quenching and thermal stresses are minimal, so coatings may be fabricated to high (2-3 mm) thickness without debonding. CS materials have been demonstrated for a wide range of metals, ceramics, and cermets (e.g., Ref 4-11), and microstructures appear dense and homogeneous in all cases, making this a potential alternative for many thick film applications. The properties of CS materials relative to their bulk counterparts have not been fully characterized and are not predictable given process parameters as input. Further, it is not known how feedstock material properties affect coating behavior, and no general coating properties have been ascertained, for ÔmetalsÕ for example. This is due to the complex nature of interparticle bonding that is still poorly understood. Models and experiments have indicated that an adiabatic shear instability exists in local W.B. Choi, Y. Wu, and S. Sampath, Center for Thermal Spray Research, Stony Brook University, Stony Brook, NY 117942275; and A. Gouldstone, Department of Mechanical and Industrial Engineering, Northeastern University, Boston, MA 02115. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
regions of interparticle contact upon impact, where the high rates of plastic strain cause sufficient heating for melting, and thus bonding (Ref 12). In the models, a critical velocity (Vc) exists, below which particles will not bond, but simply land, and in some cases, rebound completely. Thus, although CS coatings may be close to fully d
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