395 On the tribo-cryogenic characteristics of titanium alloys N S M El-Tayeb ∗ , T C Yap, and P V Brevern Faculty of Engineering and Technology, Multimedia University, Melaka, Malaysia The manuscript was received on 7 August 2009 and was accepted after revision for publication on 25 November 2009. DOI: 10.1243/13506501JET703 Abstract: Friction and wear behaviours of Ti-6Al-4V (Ti64) and Ti-5Al-4V-0.6Mo-0.4Fe (Ti54) alloys sliding against a tungsten carbide wheel under dry and cryogenic sliding conditions were investigated at different sliding speeds, loads, and distances. Mathematical models utilizing response surface methodology were developed as a function of sliding conditions for predict- ing the friction and wear characteristics of both titanium alloys. The controlling variables were determined by utilizing design of experiments. The developed models are able to predict the mea- sured friction coefficient and wear volume with reasonable degree of accuracy for both titanium alloys within the ranges of studied parameters. Results showed no substantial difference between tribo-characteristics of both titanium alloys. Under cryogenic sliding condition, tribo-characteristics were lower than those obtained under dry sliding, except at higher load, cryogenic sliding surprisingly gave higher friction coefficient. Analyses of worn surfaces and wear debris by using scanning electron microscopy and energy- dispersive spectroscopy revealed that the role of cryogenic conditions at the sliding interface was partially attributed to changing material properties and possible hydrodynamic effect. Under dry sliding, the main wear modes were adhesion and delamination, while under cryogenic sliding, in addition to delamination, abrasion wear mode dominated. Keywords: friction coefficient, wear volume, titanium alloys, cryogenic sliding, response surface methodology 1 INTRODUCTION Titanium and its alloys are well known by their lightweight, high strength, corrosion resistance, and biocompatibility [1–3]. They are widely used in many applications, such as aeronautical, military [1, 2], bio- material for joint replacement and dental appliances [3, 4], and many other applications [5]. However, in sliding applications, titanium alloys have poor tribo- logical properties. Low thermal conductivity of tita- nium alloys is one of the main reasons that was reported earlier [1, 2] for poor machinability of these alloys. This causes the temperatures in the cutting tool and work piece to rise dramatically during machining, resulting in short life of the cutting tool (tool wear) and poor quality of the machined surface. Therefore, cryogenic fluid such as liquid nitrogen was used to replace conventional cutting fluid in the machining ∗ Corresponding author: Faculty of Engineering and Technology, Multimedia University, Jalan Ayer Keroh, Melaka 75450, Malaysia. email: nabil.eltayeb@mmu.edu.my; nabil.eltayeb@gmail.com of titanium alloys, and positive results of tool wear and surface finishes were obtained [6–10]. The mech- anism of cryogenic sliding can be understood better through tribological tests. However, work on cryogenic friction and wear of titanium alloys is very rare in the literature. The nearest work available in the liter- ature is the friction and wear of high-purity titanium studied by Basu et al. [11]. They found that friction and wear of high-purity titanium decreased when slid against steel in liquid nitrogen. Other researchers from German’s Federal Institute for Materials Research and Testing (BAM) studied the tribological behaviours of various material pairs using special designed cryo- tribometers and investigated the wear and friction of several pairs of material at room temperature and low temperature [12]. They reported that the tribologi- cal behaviour of steel–steel and TiN coating–steel are unchanged at 77 K but the wear and friction behaviour of polymers against steel changed completely at low temperature; that is the wear decreased from room temperature to 77K. Similar results were reported by Theiler et al. [13] for PTFE against steel at room tem- perature, in liquid nitrogen (77 K) and in liquid helium JET703 Proc. IMechE Vol. 224 Part J: J. Engineering Tribology