Indentation of Metallic and Cermet Thermal Spray Coatings
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JTTEE5 18:58–64 DOI: 10.1007/s11666-008-9247-4 1059-9630/$19.00 Ó ASM International
Indentation of Metallic and Cermet Thermal Spray Coatings W.B. Choi, L. Prchlik, S. Sampath, and A. Gouldstone (Submitted August 29, 2007; in revised form August 5, 2008) Indentation methods are presented by which the elastic and inelastic stress-strain characteristics of metallic thermal spray (TS) coatings on substrates may be extracted. The methods are based on existing techniques for brittle solids, and adapted for the finite geometry associated with coatings. Basic assumptions and derivations are given, along with guidelines for experimental measurement. Using these, indentation inelastic stress-strain curves are generated for NiCrAlY and Ni-Al bondcoats, as well as WC-Co cermet coatings. Elastic moduli are extracted for CoNiCrAlY coatings. Results are briefly discussed in the context of the effect of feedstock material, process and post-process heat treatment on the intrinsic properties of splats as well as their in-coating cohesion. The methods presented are attractive, particularly for the TS industry, due to the minimal specimen preparation and lack of intricate equipment required for measurement.
Keywords
bondcoat, cohesion, indentation testing, inelastic, modulus
1. Introduction Structure and properties of thermal spray (TS) coatings are very sensitive to spray method, input parameters, powder morphology, and environment. For a long time, this complex relationship limited expansion of TS into prime-reliant applications, that is to say, in which coating failure leads to component failure. Progress in process control in the past decade has increased the potential for coating design, and insertion of TS into non-traditional applications. Along with such improvements, the TS community has begun using next-generation characterization techniques. For example, 3D imaging using tomography and scattering techniques provides microstructural information beyond that obtained with conventional cross-sectional image analysis (Ref 1-5). In situ substrate curvature experiments and diffraction techniques have illustrated the development of coating stresses during deposition and post-process annealing (Ref 6). Significant progress has been made in indentation testing as well; such measurements have evolved beyond conventional hardness testing, for example to extract coating elastic modulus. In the literature, three primary methods for modulus measurement have been reported. Instrumented indentation is being adopted rapidly, using the Oliver-Pharr method (Ref 7). Knoop indentation allows modulus W.B. Choi, L. Prchlik and S. Sampath, Center for Thermal Spray Research, State University of New York, Stony Brook, NY; and A. Gouldstone, Department of Mechanical and Industrial Engineering, Northeastern University, Boston, MA. Contact e-mail: [email protected].
58—Volume 18(1) March 2009
extraction via measurement of elastic recovery of the imprint minor diagonal (Ref 8). Scratch tests under spherical contact on a gold-coated surface reve
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