Deconvolution of hardness from data obtained from nanoindentation of rough surfaces
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Deconvolution of hardness from data obtained from nanoindentation of rough surfaces M. S. Bobji and S. K. Biswasa) Department of Mechanical Engineering, Indian Institute of Science, Bangalore, 560 012, India (Received 2 October 1998; accepted 15 March 1999)
Variation of hardness with penetration in nanoindentation of a rough surface is a compound effect of variation in asperity geometry with penetration, designated geometric effect, and genuine property gradients with depth as may exist in a near-surface zone. We simulate indentation of a rough surface numerically to elucidate the geometric effects and validate it by some model “macro” experiments. Finally, we formulate a general framework to deconvolute genuine property variation by normalizing the measured hardness with the geometric effect.
I. INTRODUCTION
When there is a relative motion between bodies in contact, power is dissipated. For example, this occurs in manufacturing of parts as in machining and metal working as well as in machineries of locomotion, power generation, and machine tools. The regions of contact in this situation are severely stressed and give rise to acute gradients of strain, strain rate, and temperature. The large strains near the surface change the topography, through flow or fracture, while the combination of strain rate and temperature in the near surface zone may change microstructure to promote cracking or flow. What precisely is the material response depends on how the microstructure evolves. So phenomenologically the topography and microstructure and therefore the topography and surface mechanical properties stem from the same process, the response of a material to traction. To an engineer both the topography and surface mechanical properties are interesting as they contribute not only to the loss of power through friction but also to the life cycle of the active components. The measurement of topography is now possible over a wide range of length scales, down to atomic scales using atomic force microscopy (AFM). The indentation technique is generally used to probe mechanical properties of solids,1,2 and the nanoindenter is being used now to probe surface properties where the scale of probing is comparable to that of the topography. It has been found that at low penetration depths, the hardness is different from the bulk hardness and the scatter in the measurement is high.3 Pollock et al.4 have reviewed the relevant theory and the experimentation that describe the behavior of materials in the 10–1000 nm depth range.
a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 14, No. 6, Jun 1999
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The variation in hardness at low penetration depths may be attributed to surface chemical effects,3,5 material property variation with depth,3 and/or surface roughness of the specimen being indented. The variation is also influenced by the method of measurement: depth sensing or imaging and by
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