Chemomechanical effects of long-chain alcohols during nanoindentation
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Surface and chemomechanical effects are very important in tribology, wear and friction, but are difficult to quantify due to being confined in the near-surface region. Nanoindentation techniques have been successfully used to investigate environmental effects on mechanical response. In this work, nanoindentation tests have been performed on various materials (silicon, fused silica, and gold nanofilms on a glass substrate) immersed in long-chain alcohols (i.e., 1-hexanol, 1-heptanol, 1-octanol, and 1-nonanol). The results consistently show an increase in mechanical properties for silicon and gold nanofilms immersed in the alcohols at shallow nanoindentation depths. The results for fused silica show little effect of immersion. The changes in the observed mechanical properties are attributed to the ability of the long-chain organic molecules to sustain elastic strains when they are in confined geometry. These long organic chains also distribute the normal stress of the indenter over a larger area on the sample surface thereby causing a decrease in the perceived contact area (Ac). As a result the long-chain alcohols modify both the Ac and the elastic compliance of the contact. I. INTRODUCTION
It has been known for a long time that the surface and environmental effects play an important role in the plasticity of materials.1 They are also known to be vital in determining the tribological (friction and wear) properties of materials such as ceramics which show a strong dependence on sliding atmosphere.2,3 Components based on bulk ceramic materials and ceramic coatings have been observed to exhibit friction and wear behavior that is dependent on the environment in which they operate.4–6 Explanations for this focus on chemisorption affecting friction or near-surface mechanical properties being altered by the environment. A lubricating fluid between contacting surfaces can greatly reduce the frictional forces and wear. The lubricating fluid may play several roles such as forming a thin adsorption layer or giving a thin layer with a low shear stress compared to the stress needed to shear two contacting solid surfaces. The chemical reaction of the fluid with the surface may also change the materials mechanical properties. Indentation hardness is often used as a guide to a material’s ability to resist wear. The literature shows that indentation a)
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this focus issue during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr-policy DOI: 10.1557/jmr.2011.285 222
J. Mater. Res., Vol. 27, No. 1, Jan 14, 2012
has been used in several studies to understand the surface contact behavior of various materials in different types of environments. The earlier work using microindentation was often controversial7,8 in part due to the scale of the effects which only become pronounced in the near-surface region. Later a study by Czernuszka and Pag
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