Wear 271 (2011) 1543–1551 Contents lists available at ScienceDirect Wear jou rnal h om epage: www.elsevier.com/locate/wear Fretting wear characterization by point contact of nickel superalloy interfaces M. Lavella ∗ , D. Botto Politecnico di Torino, Dipartimento di Meccanica, Corso Duca degli Abruzzi, 24 – 10129 Torino, Italy a r t i c l e i n f o Article history: Received 2 September 2010 Received in revised form 11 January 2011 Accepted 11 January 2011 Keywords: Fretting wear High temperature Hysteresis cycle Superalloy Coating a b s t r a c t Fretting wear is a complex phenomenon that occurs at component interfaces that are subjected to low amplitude oscillation under high contact pressure. In turbomachinery fretting also occurs at the blade tip interfaces where shrouds, are machined. To diminish the fretting damage coatings are applied to the blade tips. The aim of this study is to compare the fretting wear behaviour of single crystal CMSX 4 superalloy interfaces both with and without plasma sprayed T-800 coating. Experiments were conducted with a nominal spherical surface which was pressed against a flat surface of the same material at a temperature of 800 ◦ C. The mating surfaces were worn with an alternating relative displacement with an amplitude of 30 m, at a frequency of 100 Hz. The hysteresis cycles, namely the tangential contact force against the relative displacement, were measured throughout the experiment. The comparison of the hysteresis cycles showed that the tangential contact stiffness of the coated surfaces is greater than that of the uncoated surfaces while the friction coefficient of the coated surfaces was lower than that of the uncoated. At the end of the wear process, the mating surfaces were measured by three-dimensional optical interferometry and observed through a SEM analysis. After 10 × 10 6 wear cycles, the uncoated surfaces showed a large change in the contact parameters and fretting cracks on the flat surface. In contrast, the coated surfaces did not show a measurable change in the contact parameters, while the coating damage on the flat surface would lead us to predict incipient catastrophic wear. © 2011 Elsevier B.V. All rights reserved. 1. Introduction High-cycle fatigue of turbine blades is a frequent source of fail- ure for turbomachinery. Premature failure may result from fretting wear of mating surfaces as reported in [1,2], where salient points relating to wear as a product failure mechanism are illustrated, for the general case and for a gas turbine blade respectively. The fretting behaviour of aerospace alloys was investigated in [3,4]. The blade resonant vibration amplitude is reduced by increasing the structural damping. To increase the structural damping devices using dry friction, such as under-platform dampers and shrouds [5–7], are common in the aerospace industry. Both of these devices dissipate energy from dry friction as a result of the relative motion between the contact surfaces. Shrouds are plates usually placed at the blade tip. During the assembly of the bladed disk the blades are twisted, the so called interlocking angle, in such a way that an interference fit results between the shrouds: the right side of one shroud is pressed against the left side of the other shroud on the neighbouring blade. If severe wear occurs on the contact sur- faces a loss of interference takes place. The shroud is no longer able to dissipate energy and a catastrophic blade failure can occur. ∗ Corresponding author. Tel.: +39 011 0906935; fax: +39 011 0906999. E-mail address: mario.lavella@polito.it (M. Lavella). One usual mitigation approach to the fretting wear is to perform a plasma spray of Tribaloy T-800 coatings to the contact surface of the shrouds. T-800 is a cobalt–chromium–molybdenum alloy which inhibits galling between sliding surfaces where lubrication is difficult. The microstructure of alloy T-800 consists of about 50% of the hard intermetallic Laves phase dispersed in a softer cobalt alloy matrix. The objective of this work is to compare the fretting wear of single crystal CMSX 4 superalloy contact surfaces both with and without T-800 coating. The macro-wear behaviour of Tribaloy T-800 layers was studied in [8] by means of a ball-on-disk and a block-on-ring test rig in air at room temperature. In [9] the perfor- mance of the Tribaloy T-800 and T-900 were evaluated in terms of micro hardness and macro wear rate, by means of a pin on disk rig at room temperature. Fretting wear studies, integrated with hys- teresis cycle measurements, were carried out at room temperature [10,11], mid temperature, 260/450 ◦ C [12,13], for Ti6Al4V interfaces without coating and with different types of coatings [14,15]. In this work micro-wear experiments were conducted under typical, severe operating conditions of the blade tip for a low pres- sure turbine disk. These are (i) high temperature 800 ◦ C, (ii) normal contact load of 32 N, set in order to match the contact pressure between shrouds due to the interlocking angle, and (iii) amplitude oscillation of the relative motion of 30 m, evaluated from a sim- ulation of the dynamics of the bladed disk. Specimens were used with the same bulk material both uncoated and coated with T-800 for the test. The results of this work are not directly comparable 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.01.064