Modernization of friction machine for studying fretting wear of materials and coatings in the gross slip fretting regime

The paper presents the results of long-term operation, improvement and modernization of a specialized friction machine designed and manufactured to study fretting wear of materials and coatings in gross slip regime. Four different regimes of friction contact surfaces behavior during fretting are known: full stick (seizing) and partial slip (stick-slip) regimes, which are characterized by fatigue failure and cracking, and regimes of gross slip and its transition to reciprocating sliding, for which the main damage contacting surfaces is wear process. It is typical for friction units in which moving contacts of friction surfaces are limited or specially designed, inherent in a large number of plants, machines and mechanisms, and in particular, friction units of gas turbine engines (GTE) and power plants based on them. The work examines not only the stages of modernization of a specialized friction machine, but also presents the main results obtained from studies of fretting wear of various friction units at each stage. The advantages of this machine are determined not only by the ability to control one of the main wear fretting factors — the amplitude of sliding (relative cyclic motion), but also, thanks to the use of collet clamps for fastening experimental samples, to test various forms of friction contacts, such as sphere – plane, plane – plane, cylinder – cylinder, cylinder – plane and others. At all stages of modernization of a specialized friction machine, various coating designs were developed and tested for fretting resistance to protect against fretting wear the friction surfaces, in particularly, of fan blade locks and friction pairs of the mid-span shrouds of gas turbine engine blades operating in gross slip regime. A comparison of the linear and volumetric fretting wear values of various coating options and their components made it possible to establish that the developed and proposed coatings have a good fretting resistance under the operating conditions specified and implemented on the friction machine. Using an analysis of variants of the dependences of the tangential friction force on the displacement values (amplitudes) of gross slip, the values of dissipation energy (friction energy) were obtained and fretting hysteresis loops were constructed. They made it possible to estimate the service life of each of the coatings under consideration with its known thickness and a given displacement, and by comparing the energy coefficients of volumetric wear to determine the most effective coating in terms of fretting resistance and service life. Based on the results of testing materials and coatings under plane-to-plane friction conditions at temperatures of 175 – 180°C, recommendations were developed for the utilization of these materials and technologies for use in friction pairs of mid-span shrouds of gas turbine engine blades. Thus, the considered and implemented capabilities of the modernized friction machine make it possible, under laboratory evaluation conditions, to study various materials and coatings under fretting wear conditions in model friction units of various power plants, machines and mechanisms under given operating conditions at normal and elevated temperatures.