Tribological behaviour of DP600 dual phase steel on uni- and bi- directional discontinuos sliding wear

Abstract

Most of the tribological studies within the frame of the mild oxidative wear mechanism have suggested that the friction and wear behaviour are contact and sliding speed dependent. The wear model proposed by Quinn (1962, 1983, 1992, 1994, 1998) does not distinguish between the contact conditions prevailing at the pin/ball and at the disc, which are completely dissimilar. At a macroscopic level, the pin/ball is in continuous contact with the disc. Conversely, each point of the wear track on the disc is only loaded when the pin/ball goes over it. Consequently, Garcia et al. (2003) proposed a modification to Quinn model for mild oxidative wear, which was particularly adapted for the disc material. This new model showed the significant role of contact frequency as an operational wear parameter. The objective of this study is to ascertain whether the tribological behaviour in mild oxidative wear conditions where discontinuous sliding contacts take centre stage can be influenced by different materials, sliding geometries (uni- and bi-directional sliding tests) and even distinct approaches to wear measurement. For that, a thorough design of experiments was conducted to cover a range of parameters based on the sliding speed, contact frequency, and wear track diameter or stroke length. Besides, a dual phase steel, namely DP600, was chosen as the model disc material. Initially, to independently evaluate the effects of the operational wear parameters, i.e. contact frequency and sliding speed, on the wear rate of DP600 steel disc sliding against corundum ball, a series of unidirectional wear tests was carried out at a given constant sliding speed but at different rotation speeds by modifying the wear track diameter; and vice versa. The coefficient of friction and wear rate exhibited not only a highly dependence on the sliding speed, but also on the contact frequency which appears to be the key factor determining the wear behaviour of the DP600 even at constant sliding speed. Furthermore, the validity of Garcia-Ramil-Celis model (Garcia et al., 2003) was confirmed for discontinuous sliding contact at contact frequencies below 7 Hz. Interestingly, above 7 Hz, the worn disc behaves as if it was subjected to a continuous sliding contact accordingly to the Quinn model. In a further step, analysis of the contact frequency effect on bi-directional sliding direction ¿i.e. reciprocating sliding- was evaluated. Moreover, it was also used the energy wear approach alongside the conventional Archard approach to study such contact frequency effect. For that, a similar set of tests were performed. The coefficient of friction and wear rate also showed a strong dependence on the sliding speed and the contact frequency. Once again, the contact frequency appears to be a key factor to define the wear behaviour of DP600 steel specimens. Besides, the wear volume showed a directly linear relationship with the dissipated energy. It was concluded that the heat generation at the interface could contribute to a modification in the rheology or composition of the oxidised layer. Finally, the role of the sliding direction was further analysed by performing a third wear test method that is scarcely used, i.e. ball-on-disc continuously reversed (bidirectional) wear test. This test allowed one to compare both unidirectional and bidirectional motion at given sliding speeds and contact frequencies but taking into account the reversing sliding direction effect. According to the definition of cyclic number, CN, as proposed by Tang et al. (2011a), such CN indicates how often the rotation direction of the disc is reversed. The unidirectional sliding showed more severe wear damage than tests conducted under bidirectional motion. In addition, it was demonstrated that the Bauschinger effect took place during the bidirectional sliding processes, when CN is above 5.