Investigation of Dry Sliding Friction, Wear and Mechanical Behavior of the Ti-6Al-7Nb Alloy after Thermal Oxidation

Abstract

The mechanical and tribological characteristics of the Ti-6Al-7Nb alloy were investigated within a wide range of temperature and time parameters of thermal oxidation. The hardness, HIT, and indentation modulus, EIT, of the alloy in question, with and without an anti-wear oxide layer, were determined. The tribological properties of sliding couples were studied under technically dry friction conditions, using a ball-on-disc tribometer. The test pieces were non-oxidized and oxidized Ti-6Al-7Nb alloy discs, and Al2O3, ZrO2, and 100Cr6 balls were used as counter specimens. After thermal oxidation, the surface of the titanium alloy was characterized by a significantly higher hardness, HIT (8–10 GPa), compared to the surface not covered with oxide layers (3.6 GPa). The study showed that the curvature of the loading segments increased with an increasing oxidation temperature, indicating a strong positive dependence of hardness on the thermal oxidation temperature. The value of the indentation modulus, EIT, was also found to increase with the increasing oxidation temperature. The intensity of the tribological processes was strictly dependent on the oxidation parameters and the couple’s material (Al2O3, ZrO2, 100Cr6). It has been shown that the thermal oxidation process makes it possible to control, within a wide range, the friction-wear characteristics of the Ti-6Al-7Nb alloy.

Effect of thermal oxidation on the dry sliding friction and wear behaviour of CP-Ti on CP-Ti tribopairs

“…Most of the reported work were conducted between a TO treated titanium specimen and a counter-body made of other materials, such as a ceramic, a steel and a polymer counter-body. All the reported results demonstrate that TO is effective in improving the wear resistance of titanium and its alloys by as little as 25% up to as high as 4 orders of magnitude, depending on the wear test and TO treatment conditions [12,21,27]. Some of reported tests were conducted under relatively large loads, resulting in the wearing-through of the oxide layer and even the oxygen diffusion zone [12,25,28], where the measured wear properties were not characteristic of the oxide layer and the oxygen diffusion zone, but of the composite system.…”

“…All the reported results demonstrate that TO is effective in improving the wear resistance of titanium and its alloys by as little as 25% up to as high as 4 orders of magnitude, depending on the wear test and TO treatment conditions [12,21,27]. Some of reported tests were conducted under relatively large loads, resulting in the wearing-through of the oxide layer and even the oxygen diffusion zone [12,25,28], where the measured wear properties were not characteristic of the oxide layer and the oxygen diffusion zone, but of the composite system. Under light loading conditions where the oxide layer and oxygen diffusion zone were not worn-through, it has been shown that the oxide layer has extremely good wear resistance, which could be more than 2 orders of magnitude better than untreated titanium [17,21,29].…”

“…Among many surface engineering techniques, thermal oxidation (TO) has proven a cost-effective and efficient technique to engineer the surfaces of titanium and its alloys to improve their surface properties [5,8,9], including tribological properties [5,[9][10][11][12], corrosion resistance [11,13,14], tribocorrosion properties [15,16], and biotribological properties [17,18]. TO treatments are normally carried out at temperatures between 550 ℃ and 900 ℃ for various durations, ranging from less than 1 h to 100 h. Due to titanium's high affinity with oxygen and its ability to dissolve a large amount of oxygen in its crystal lattice, TO results in the formation of a TiO 2 oxide layer at the surface and an oxygen diffusion zone at the subsurface [19,20].…”

“…The tribological properties of TO treated Ti and its alloys have been studied by many investigators under various environmental conditions, including dry [10,12,21,25], oil lubrication [9,11], corrosive solutions [15][16][17][18], and high temperatures [22,26,27]. Most of the reported work were conducted between a TO treated titanium specimen and a counter-body made of other materials, such as a ceramic, a steel and a polymer counter-body.…”

“…Regarding the effect of TO on the coefficient of friction (COF) of titanium, inconsistent results have been reported by different investigators, probably due to the complex nature of friction and different counter-bodies and testing conditions employed. There are reports that TO can reduce COF [17,18], does not influence COF and even increases COF of titanium sliding against other counter-bodies [10,12,32]. But when tested under light loading conditions, it has been clearly shown that the oxide layer can offer relatively low and stable friction, and when the oxide layer was worn-through, the TO treated specimen lost its friction-reducing ability [21].…”