Crystal orientation effects in scratch testing with a spherical indenter
- PDF / 1,465,084 Bytes
- 6 Pages / 584.957 x 782.986 pts Page_size
- 24 Downloads / 238 Views
Heidi Bo¨gershausen, Benedikt Sander, and Dierk Raabe Max-Planck-Institute fu¨r Eisenforschung, 40237 Du¨sseldorf, Germany (Received 3 June 2009; accepted 26 August 2009)
Spherical scratch tests were conducted in individual grains of a randomly oriented polycrystalline body-centered-cubic (bcc) Ti–Nb alloy. For each grain, scratch tests were conducted at four different levels of normal load, which resulted in varying amounts of plastic strain during indentation. The results show a dependence of the horizontal load component on the crystallographic orientation and on the amount of plastic strain. The component of the horizontal force that resulted from plastic deformation was found to correlate with the active slip systems for the particular grain orientation. I. INTRODUCTION
Several studies over the last 55 years have reported variation of friction coefficient with crystallographic orientation.1–8 The subject is of current interest for the fields of tribology and abrasion, more specific for coatings, thin films, biomaterial surfaces, and monolayers.9–12 The primary reason for the variation of friction with test direction appears to be differences in plastic deformation in different directions during the friction or scratch test.6–9 For specimens where the test direction is aligned with easy slip directions, dislocation theory can be used to explain the results.8,9 However, for random orientations, the differences have been difficult to correlate quantitatively, because of the complicated deformation and limited knowledge of the active slip systems. To address the problem of friction measurements in grains or different orientations, scratch tests were conducted in individual grains of a randomly oriented polycrystalline body-centered-cubic (bcc) Ti–Nb alloy. A spherical indenter was used at various loads to control the amount of plastic deformation. A simple analysis is developed to determine the contribution from plastic deformation and relate the deformation to the relevant slip systems in each grain. II. MATERIALS AND METHODS
The specimen was a polycrystalline Ti-30at.% Nb (45.4 wt% Nb) alloy, which has a stable bcc structure (b-phase).13 It was melted in an electric arc furnace under Argon (300 mbar). The furnace was equipped with a water-cooled copper crucible. The melt was held at a a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0108 J. Mater. Res., Vol. 25, No. 5, May 2010
http://journals.cambridge.org
Downloaded: 31 Mar 2016
peak temperature of 1830 to 1850 C to assure complete dissolution of the Nb. The electric arc method provided an intense stirring effect. Melting required about 30–60 s. To obtain cast samples of optimal chemical and structural homogeneity, all specimens were remelted several times. Each sample was stirred completely after remelting in the crucible, and then turned about its horizontal axis by use of an in-furnace manipulator, and subsequently reheated above the melting point. This procedure (melting, stirring, solidification,
Data Loading...
