The Instrumented Indentation Study of HVOF-Sprayed Hardmetal Coatings
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ˇa´rka Houdkova´, Olga Bla´hova´, Frantisˇek Zaha´lka, and Michaela Kasˇparova´ (Submitted April 27, 2011; in revised form June 21, 2011) Elastic-plastic properties, namely, hardness and YoungÕs modulus, of four HVOF-sprayed hardmetal coatings were measured by instrumented indentation using Oliver-Pharr method Nanoindenter XP MTS with a continuous stiffness measurement (CSM) module. The results show that with sufficient number of CSM measurements, one can distinguish between indents made in the hard particles and indents made in the binder material. This can be accomplished by analyzing the plots of hardness and YoungÕs modulus versus load (or versus indentation depth). Further development of the dependence curves enables the load (or indentation depth) to be set to correspond to the point of transition from a single structure component to the composite material and to determine the properties of both. Comparison of results of CSM measurement with the results of single indentation measurement at a defined load reveals a new perspective on the origin of the indentation size effect in hardmetal coatings. The measurements show that the increase in both the hardness and YoungÕs modulus with decreasing load is caused mainly by the predominant influence of hard particles in the coatings.
Keywords
coating, hardmetal, hardness, HVOF, mechanical properties, nanoindentation, thermal spraying, YoungÕs modulus
1. Introduction For evaluation of mechanical properties of thermally sprayed coatings, it is necessary to take into consideration their unique, heterogeneous lamellar microstructure. In addition to hardness, YoungÕs modulus, fracture toughness, the coatingsÕ porosity, cohesive strength, the content of oxides, and other microstructural defects also play their role (Ref 1, 2). While the mechanisms of formation of such coatings have been investigated in depth, detailed information on mechanical properties still remains difficult to obtain because of coatingsÕ heterogeneity. Macro-scale methods, such as three- (Ref 1, 3) or four-point bending test (Ref 4-6), tensile test (Ref 5) or surface hardness, and microhardness measurement do not capture all the effects of the strongly heterogeneous structure containing features resulting from the spraying process. In the case of sprayed hardmetals, the coatingsÕ microstructure is even more complicated, containing numerous hard particles in soft binding matrix in each individual splat.
Sˇa´rka Houdkova´ and Olga Bla´hova´, New Technologies Research Centre, University of West Bohemia, Univerzitnı´ 22, 306 14 Plzenˇ, Czech Republic; and Frantisˇek Zaha´lka and Michaela ´ Plzenˇ s.r.o., Tylova 46, 316 00 Plzenˇ, Czech Kasˇparova´, VZU Republic. Contact e-mails: [email protected], blahova@ntc. zcu.cz, [email protected] and [email protected].
Journal of Thermal Spray Technology
Indentation methods, particularly the instrumented indentation, enable hardness, viscoelastic properties, and YoungÕs modulus to be determined within very small material volumes. They also have the potent
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