Investigations On The Running And Damage Mechanisms Of Radial And Composite Fretting

Tangential, radial, rotational, and torsional fretting are four basic types of fretting running modes. Presently, the absolute majority of researches have been focused on tangential fretting, and the research concerning composite fretting compounded of two basic modes has not been reported so far. Studies on radial and composite fretting, showing a broad prospect of engineering application in many fields such as railway, automobile, airplane, nuclear reactor, telecom, electric power, and artificial implantation, etc., not only have science significance of exploring the unknown, but also have important guidance to palliate fretting damages in industry. The main research works and obtained conclusions in this dissertation are as follows:(1) A new radial fretting apparatus has been developed to really simulate the case of radial fretting, laying an experimental foundation to investigate radial fretting.The new hydraulic radial fretting test rig has been successfully developed by modifying the clamps and the control programs from the tangential fretting apparatus with high precision. Fretting tests can be done on mode of imposed load or of imposed displacement. The speed of relative movement between contact pairs can be controlled by changing the speed of loading or frequency. The apparatus can really simulate the case of radial fretting. Results showed a better comparable characteristics and repeatability.(2) The running and damage mechanisms of radial fretting have been studied systematically.Under the ball-on-flat contact, the dynamic behaviours of radial fretting have been investigated by varying loads applied to different materials including Fe-C alloys (industrial pure iron, 1045 steel and 52100 steel), 2091 aluminium-lithium alloy, TiN coating, MoS2 coating and TiN+ MoS2 composite coating. Variations of normal load vs displacement between two contact surfaces have been analyzed as a function of cycles. The tests results showed that two basic styles of load-displacement curve (F~D curve), i.e. open cycle and close cycle, appeared in radial fretting. Three parameters of deformation rigiditiy (R), dissipated energy (Ed), and open displacement (δ) can be introduced to describe the F~D curves of radial fretting. The process of radial fretting was dependent strongly upon the load levels and characteristics of materials, while the radial displacement decreased and trended to a limit with the increase of number of cycles. The radial deformation rigidities, dissipated energies and open displacements, which have different values for different materials, changed with the increase of number of cycles. The effects of different loading speeds and surface roughness have also been studied in the case of radial fretting. The results showed that the loading speed obviously affected the deformation processes of radial fretting. Decreasing of the loading speed resulted in the increase of radial displacement, accompanying with increasing of the values of deformation rigidities, dissipated energies and open displacements in F~D curves. As the increase of the surface roughness of specimens, the radial displacements, deformation rigidities and open displacements all increased obviously, but the dissipated energies were not changed evidently.On the basis of analyses of elastic mechanics, in combination with microscopic examinations by profilometer, optical microscope, scanning electrical microscope (SEM) and energy dispersive spectroscope (EDX), it can be deduced that: a) for the contact pairs made of same materials, it is impossible for the relative micro-slip between contact interfaces to happen under the oscillatory normal load because of the coordination of elastic deformation; b) for the different materials of contact pairs, the micro-slip inducing radial fretting failures took place on the contact interface between the maximum and minimum contact radii that varied with the oscillatory normal load. In other words, the damages of radial fretting occur between the surfaces of two contact pairs which consist of different materials.The investigations on damages of radial fretting showed that the damages generally occurred in the zones where the second phases or defaults of materials existed. Although different materials are damaged to different extents, the mechanism of failure mainly appears as contact fatigue of the contact surfaces, i.e. the particles detached by delamination.The running behaviours and damage mechanisms of three coatings have been studied. The results indicated that TiN coating with high hardness and elastic modulus clearly improved the deformation rigidity of its substrate material, and reduced the dissipated energy and lasting time of the open F~D curves. MoS2 coating, which is in possession of low friction coefficient and hardness, was obtained an inverse result. It illuminated that the TiN coating enhanced the load bearing capacity, and enlarged the using range of substrate material. MoS2 coating and TiN+ MoS2 composite coating both possessed the good capability for palliating radial fretting damage. The microslip increased in radial fretting when the solid lubrication coating was used on the contact interfaces. More system energy can be absorbed in this process in favor of reducing vibration. For TiN+MoS2 composite coating, a compound feature of two individual coatings can be observed.The damages of contact fatigue of hard coatings were mainly dependent upon the test parameters and characteristics of coatings. Therefore, the experimental method of radial fretting can be proposed as one of new and potential methods to evaluate life of contact fatigue of hard coatings. (3) The combination of two basic fretting modes of radial and tangential fretting was successfully fulfilled for the first time.On the basis of radial fretting, the combination of radial and tangential fretting was successfully fulfilled for the first time though a delicate and simple design, i.e. obliquely placing the flat specimens. The features of radial and tangential fretting modes have been strongly reflected in the F~D curves of composite fretting. It is a significantly important for studies of composite fretting to palliate complex fretting damages in actual engineering applications.(4) The running and damage mechanisms of composite fretting have been studied systemically.The composite fretting tests have been carried out for different materials, such as 1045 steel, 52100 steel, 2091 aluminium-lithium alloy, TiN coating and MoS2 coating, under two inclined angles of 45oand 60o. The results indicated that there were three basic types of F~D curves, i.e. qusi-trapezoid cycle, elliptic cycle and linear cycle. The variations of the F~D curves as function of number of cycles were strongly dependent upon the load levels, inclined angles, characteristics of materials and test procedure. According to the three kinds F~D curves, the processes of composite fretting can be divided into three stages. The features of running and damage behaviours were as follows:a) Satge I (qusi-trapezoidal F~D curves): The parameters of Fl,Fs andδc can be introduced to describe the qusi- trapezoidal F~D curves. During the early stage of testing, the F~D curves of qusi-trapezoid cycles obviously displayed the features of slip. With the increase of the number of cycles, the displacements dropped down quickly. Some phenomenon of abrasion and adhesion can be obviously observed in wear scars with an asymmetry shape of the comet-like. The white layers of fretting, cracks parallel to surface and debris (the third body) have been formed during this stage.b) Satge II (elliptic F~D curves): When the parameter of Fl increased to a certain critical value, the F~D curves transformed suddenly from qusi-trapezoid cycle to elliptic cycle. The deformation entered into the range of elasto-plasticity during this stage. When the parallel and vertical cracks encountered, the plate-like particles detached by delamination mechanism. The third bodies were accumulated between the contact surfaces during this stage, and subsequently the two-body contact transformed to the three-body contact.c) Satge III (elliptic F~D curves): There is no sliding between the interfaces. Only elastic deformation of contact pairs occurred under the imposed loads. Because of accommodation of the third-body bed, the ratio of wear greatly decreased. The formation of fatigue cracks became the most important features in this stage. Three fretting regimes (slip, mixed and partial slip regimes) can be observed in one test procedure under certain conditions. The formation of mixed regime was the results of repetitious processes, in which cracks formed, cracks propagated and particles detached. On the basis of the analyses concerning the fretting white layers, the third body behaviours and the competition between wear and fatigue, a physical model of composite fretting damages were proposed. A displacement accommodation mechanism was also proposed to explain the variations of the displacements and the contact conditions during the processes of composite fretting.(5) With the change of relative inclined angle between contact pairs,the variations of fretting damage mechanisms of tangential, composite and radial fretting have been studied.During the transition from tangential fretting to composite fretting, and to radial fretting (inclined angle changed from 0o to 90o), the decreasing of component of tangential fretting was accompanied with the increasing of component of radial fretting. In the competition relation between wear and fatigue, the features of sliding wear reduced gradually, the features of radial contact fatigue held the predominance step by step. The formula for the controlling parameters of white layer formation of fretting has been built up at the same time during this research, which strongly supported the plastic deformation mechanism of the white layer formation.For MoS2 coating, the oxidation effect became weakened gradually with the increase of number of cycles, while the flow layers of MoS2 coating were more and more difficult to form. When the condition of radial fretting achieved, the MoS2 coating hardly damaged and oxidized as well.