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Title: The effect of nitriding on the fretting wear of a high strength steel at ambient and elevated temperatures
Author: Pearson, Stephen R.
Awarding Body: University of Nottingham
Current Institution: University of Nottingham
Date of Award: 2013
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This work is an experimental evaluation of the wear and friction of a high strength alloy steel (super-CMV (SCMV» in its as heat treated and plasma nitrided states under fretting conditions in air, at both ambient and elevated temperatures. In order to conduct the experimental programme, a new test rig and associated data processing and assessment capability was developed. Wearing couples in homogeneous and heterogeneous arrangements were tested to assess the effectiveness of the nitriding process as a fretting palliative-the heterogeneous mating is particularly representative of configurations found in aerospace transmission assemblies. A range of test conditions were examined with a line contact, including normal loads from 25 to 65Nmm-1, displacement amplitudes from 10 to 100 µm and temperatures from 24 to 450°C. At ambient temperatures, the wear behaviour was assessed using an energy-wear analysis, whereupon it was demonstrated that the wear volume was a linear function of the dissipated frictional energy (over the range of loading conditions) with a significant energy threshold before the onset of wear. Accordingly, the fretting wear over the full range of loading conditions could be described by a single wear rate and threshold value. The wear rate for homogeneous couples of nitrided super-CMV (SCMVN) was found to be 12 % lower than that for comparable SCMV pairings, although the lower threshold energy for the SCMVN case would lead to them suffering greater wear at < 2.5 kJ of dissipated energy (in the configuration examined). In heterogeneous couples, the harder SCMVN specimen was found to wear preferentially; while the SCMV specimen suffered severe plastic damage of the surface, a protective oxide debris bed was seen to form, which protected the underlying SCMV from wear and abraded the SCMVN specimen. At elevated temperatures, the tribology (of both SCMV and SCMVN) was dominated by the formation of a glaze-layer. The progressive formation of the glaze, with increasing temperature, led to a critical transition temperature (TT) above which a significant reduction in both the wear and friction of the materials was observed. For SCMV, after only a modest increase in temperature to 85 °C, the overall loss of material from the contact had become negative. At temperatures greater than 85°C, negative wear was maintained, with the coefficient of friction dropping monotonically until a slight minimum at 300°C. The behaviour for the SCMVN couples was very similar; the transition to negative wear occurred at a higher temperature of 150°C but the friction minimum was also found at 300°C. It is proposed that the changes in wear rate and friction coefficient were due to changes in the way that the oxide particles sintered to form a protective debris bed, with sintering of the oxide debris particles at these low temperatures being promoted by the nano-scale at which the oxide debris is formed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TA Engineering (General). Civil engineering (General)