薄膜与小尺度材料及力学性能研究组

Micron-Scale Friction and Sliding Wear of Polycrystalline Silicon Thin Structural Films in Ambient Air

Alsem, D. H.   Dugger, M. T.   Stach, E. A.   Ritchie, R. O.

Abstract

Micron-scale static friction and wear coefficients, surface roughness, and resulting wear debris have been studied for sliding wear in polycrystalline silicon in ambient air at micro-Newton normal loads using on-chip sidewall test specimens, fabricated with the Sandia SUMMiT V$^{rm TM}$ process. With increasing number of wear cycles friction coefficients increased by a factor of two up to a steady-state regime, concomitant with a decay (after an initial sharp increase) in the wear coefficients and roughness. Wear coefficients were orders of magnitude smaller than reported macroscale values, suggesting that the wear resistance is higher at micrometer dimensions. Based on our observations, a sequence of micron-scale wear mechanisms is proposed involving: 1) a short adhesive wear regime ( $≪ 10^{4}$ cycles), where the oxide is worn away and the first silicon debris particles form and 2) a regime dominated by abrasive wear, where silicon particles (50–100 nm) are created by fracture through the grains ( $sim$500 nm). These particles subsequently oxidize and agglomerate into larger debris clusters, while “ploughing” by this debris leads to abrasive grooves associated with local cracking events rather than plastic deformation.

Keywords: Friction   microelectromechanical systems (MEMS)   silicon   thin films   wear  

Article abstract

Nature Materials 7, 672 - 677 (2008)
Published online: 29 June 2008 | doi:10.1038/nmat2221

The true toughness of human cortical bone measured with realistically short cracks

K. J. Koester¹, J. W. Ager, III¹ & R. O. Ritchie^1,2

Abstract

Bone is more difficult to break than to split. Although this is well known, and many studies exist on the behaviour of long cracks in bone, there is a need for data on the orientation-dependent crack-growth resistance behaviour of human cortical bone that accurately assesses its toughness at appropriate size scales. Here, we use in situ mechanical testing to examine how physiologically pertinent short (<600 m) cracks propagate in both the transverse and longitudinal orientations in cortical bone, using both crack-deflection/twist mechanics and nonlinear-elastic fracture mechanics to determine crack-resistance curves. We find that after only 500 m of cracking, the driving force for crack propagation was more than five times higher in the transverse (breaking) direction than in the longitudinal (splitting) direction owing to major crack deflections/twists, principally at cement sheaths. Indeed, our results show that the true transverse toughness of cortical bone is far higher than previously reported. However, the toughness in the longitudinal orientation, where cracks tend to follow the cement lines, is quite low at these small crack sizes; it is only when cracks become several millimetres in length that bridging mechanisms can fully develop leading to the (larger-crack) toughnesses generally quoted for bone.