Elsevier

Tribology International

Volume 37, Issue 8, August 2004, Pages 633-643
Tribology International

Frictional behaviour of oxygen diffusion hardened titanium in dry sliding against Co–28Cr–5W–4Fe–3Ni–1Si cobalt alloy

https://doi.org/10.1016/j.triboint.2004.01.011Get rights and content

Abstract

The results of conformal pin-on-disc tribological tests of a hard layer of the solid solution of oxygen in α-titanium sliding against a Co–28Cr–5W–4Fe–3Ni–1Si cobalt alloy counterspecimen are presented. The α-Ti(O) layer was diffusely produced over 2–8 h of oxidising in the superficial zone of a technical quality titanium specimen.

The friction and wear responses of the system were recorded and the wear mechanisms were studied. Investigations of the material structure and chemical constitution in micro-areas of the titanium specimen, cobalt alloy counterspecimen and wear debris formed in dry sliding were performed with a Philips XL20 microscope equipped with an EDAX analyser. Crushing of the α-Ti(O) layer, lowering of the wear rate after comminution of the hard α-Ti(O) layer, local tack spots and fine powder wear particles, mostly Ti oxides, were detected at the beginning of each test. Gradual brittle fracture and decay by pulverising of the α-Ti(O) particles embedded in both mating surfaces, which occurred during the test, led to the increase of the wear rate of the couple and domination of microcutting and tack spots spalling after their partial oxidation. Finally, after the disappearance of the α-Ti(O) loose particles, adhesive junctions, metal transfer and smearing become leading wear mechanisms.

Introduction

The continuous increase of applications of titanium and its alloys in the aircraft and aerospace industry, as well as chemical industry, power engineering, construction of environment protection devices, bone surgery and others, arises from their specific properties, such as high melting temperature, high strength, low density, high fatigue resistance and high corrosion resistance in sea water, especially at room temperature [1], [2], [3], [4]. The restricting factors for a wider scale of applications are low frictional wear resistance and tendency to seizure. These two factors have been suggested as being responsible for the poor frictional properties of pure metallic titanium and its alloys [5], as also the low resistance to plastic shearing and low work hardening and low protection provided by the surface oxides, which can be formed when high flash temperature is induced by frictional heating.

However, the use of titanium alloys and the development of methods of surface engineering create new areas of potential applications for modified titanium through the increase of wear resistance with simultaneous maintenance of other specific material properties [6], [7]. The high temperature oxidisation of titanium resulting in a significant rise of hardness of a superficially oxidised layer is proposed to be one of such methods [8], [9]. The frictional properties of superficially oxidised titanium, especially the resistance to wear, and its mechanisms, were the subject of investigations in various mating material configurations. This method has been recently applied for modification and significant increase of the wear resistance. It was found to be effective in dry sliding against a carbon steel AISI 1045 counterbody tempered up to the hardness of ∼620 HV. In comparison with metallic titanium when the linear wear rate of the couple was equal to 4.83×10−3 μm/(m N), it appeared to be lower by a factor of 160, with a relevant value of 3.0×10−5 μm/(m N) [10]. Instead of a severe adhesive wear regime, mild tribooxidation of the steel counterbody and mild abrasive wear by polishing of the specimen were discovered for the above-mentioned sliding couple [11].

Section snippets

Materials

This study deals with technical quality titanium Grade 1 including 0.04% Fe, 0.08% O and rest Ti of hardness 180 HV0.1. The baseline set of specimens was in the initial state, whereas the modified specimens were subjected to 2, 4 and 8 h of oxidation at 900 °C in air, and removal of oxides by cooling in water. The effect of this process was a hard surface with a constitution of specific “rose petals” as shown in Fig. 1, and the microgeometry presented by profilogram in Fig. 2.

Cylindrical pins

Results and discussion

The recorded data obtained for the reference Ti/Co-alloy couple and for the α-Ti(O)/Co-alloy couples are presented in Fig. 7, Fig. 8, respectively. They provide the coefficient of friction and linear wear of the couple plotted as a function of sliding distance. As shown in Fig. 7, the friction coefficient and the wear curves consist of a running-in period transferred after 300 m of sliding distance into a steady-state zone with a stabilised wear rate and minor fluctuations of the friction

Conclusions

  • 1.

    High temperature oxidation of titanium creates a hard superficial zone of oxygen solid solution in α-titanium, α-Ti(O), the thickness of which depends on the durability of this thermo-chemical process.

  • 2.

    The α-Ti(O) continuous layer in dry sliding against Co–28Cr–5W–4Fe–3Ni–1Si cobalt alloy demonstrates substantially lower (about two to six times maximum) wear resistance than metallic titanium, but only over a limited (50–200 m) sliding distance proportional to the thickness of the hardened zone

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