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Elastic, plastic, and cracking properties of ZrO2, a ZrO2-metal composite, and a Ni-NiO composite, all plasma-sprayed as coatings on metallic substrates, have been studied. Data comparisons were made with bulk ZrO2, bulk nickel, and single-crystal silicon as reference materials. A nanoindentation and scratching apparatus was used for the measurements. Three different indenter and scratching tool shapes were used: 200 μm radius sphere, Vickers four-sided pyramid, and a 45° wedge with a 0.5-mm radius curved edge. During indentation, over a load range of 10 mN to 1 N, continuous load versus depth data were obtained in each case to permit analysis for material hardness and elastic modulus. The loading curve data are used to calculate hardness, and the unloading curve data are used to calculate elastic modulus. The analytical models used are described and compared. An alternate method for deriving elastic modulus from the initial portion of the loading curve is described. Comparison among the results for different tool shapes will be discussed. During instrumented scratching over a similar load range, values for load, tool depth, and scratch-resisting force were continuously measured. The applied load was either held constant at one or more selected values, or linearly increased during scratching. This load variation permitted identification of critical loads for severe cracking damage of the brittle materials. Scanning electron microscope (SEM) studies were used to characterize the cracking damage. The critical load values depended on tool shape and material microstructure. The studies identified damage mechanisms in the hard coatings, and critical loads for damage initiation. Damage morphology typically involved local surface cracking, edge chipping, and plastic deformation. The relative proportions of cracking and plastic response seemed to vary with tool shape, material, and microstructure.
surface coatings, surface treatments, wear testing, ceramics, coatings, plasma-sprayed coatings, scratching, indentation, hardness, elastic modulus
Guest scientist, National Institute of Standards and Technology, Gaithersburg, MD