Coefficient of Friction Measured from Nano- to Macro-Normal Loads on Plasma Sprayed Nanostructured Cermet Coatings
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TRODUCTION
THERMAL sprayed nanostructured cermet coatings are attracting attention due to their enhanced tribological properties. So far, main attention has been given on the synthesis of nanostructured powder and processing of thermal sprayed nanostructured coatings[1–3] together with some investigations on the mechanical and tribological properties.[4,5] Recently, we have reported[4,5] on the superior tribological and mechanical behavior of such nanostructured cermet coatings consisting of a relatively soft nanostructured FeCuAl-matrix and dispersed nanostructured alumina particles, as wear resistant ones, in applications where hardness as well as toughness are essential. The metallic phase in the coating provides toughness, while the ceramic phase increases hardness. Thought the starting powders for plasma spraying consisted of metallic and ceramic nanostructured powders, the resulting coating structure exhibits four different phases of different compositions and hardness. In a preceding investigation on this type of coatings,[5] the focus was on their overall response in tribological behavior with minor attention on its origin. The aim of this work is to carry out a fundamental investigation on the nature of friction recorded on nanostructured coatings which are microstructurally A.K. BASAK, Duelbeam Engineer, formerly with the Department MTM, KU Leuven, 3001 Leuven, Belgium, is now with Adelaide Microscopy, The University of Adelaide, Adelaide, SA, Australia. Contact e-mail: [email protected] J.-P. CELIS, Professor, is with the Department MTM, KU Leuven. M. VARDAVOULIAS, Managing Director, is with the PyroGenesis SA, Technological Park of Lavrion, 19500, Lavrion, Greece. P. MATTEAZZI, Founder and President, is with the CSGI and MBN Nanomaterialia, Via Bortolan 42, Vascon di Carbonera, TV, Italy. Manuscript submitted July 1, 2013. Article published online October 4, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
non-homogeneous and chemically reactive, and to correlate the friction behavior of individual phases on the overall friction behavior at macro-normal loads. The investigation of friction at nano-normal loads were investigated by lateral force microscopy (LFM)[6], whereas a MUST tester (Falex Tribology N.V.),[7] commercially available tribometer, and a home-made reciprocating sliding tester[8] were used at micro and macro-normal loads. II.
EXPERIMENTAL
A. Materials Atmospheric plasma spraying (APS) of the coatings was performed by PyroGenesis, Greece, using nanostructured powders supplied by MBN Nanomaterialia, Italy. The nanostructured powders were produced by a Mechanomade process,[9] which is basically a highenergy ball-milling. These nanostructured powders were then agglomerated by using an organic binder (commercially available) to obtain an agglomeration of about 50 lm in diameter. That binder did not induce any physical changes rather than holding the nano-sized powders together as verified by detail characterization of the powder and coating with the help of XRD and TEM. Details of this cha
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