Investigation of fretting behaviors of sealing materials

Chao Wang

Research output: ThesisDoctoral Thesis

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Abstract

Fretting is a common failure, which can be found in many machines where vibration is present. Since the 1980s, a number of studies have investigated the fretting behavior of metals thoroughly. However, for polymers, especially sealing materials, their fretting behavior is still not completely understood. Due to their unique material properties, the methods, which were developed for metals, cannot be applied directly to polymers. In this thesis, a new test method with a corresponding analysis approach was developed for thermoplastic polyurethane (TPU). New calculations for the coefficient of friction (COF) in late cycles of the gross slip regime were introduced, which took the surface damage and the dynamic influence of the test system into consideration. Additionally, the influence of contact conditions on fretting behavior was examined. Five different TPUs were tested and their fretting behaviors were compared. To connect sliding performance with fretting behavior, sliding tests were conducted using a component-like set-up on a precision rotary tribometer. Due to the different test methods and wear mechanisms, the COF in the fretting and ring on disc tests cannot be compared; however, it can be concluded that the material with better tribological properties presents the improved fretting behavior. Because alternating tangential stress superimposed with normal stress can lead to the generation of microcracks on the surface of TPUs, fretting can cause a severe reduction in fatigue life. However, the question remains: how does the material’s fatigue properties correlate with its tribological performance? To understand this correlation, the fatigue and tribological properties of filled and unfilled TPUs were characterized through various test configurations. Two wear models were verified for the tribological tests. One model predicts wear volume based on adhesive and fatigue wear mechanism, while the other one calculates the abrasive and fatigue wear volume by taking the surface roughness of the counter surface and surface energy into account. The stick-slip effect, which is due to the contact transition between static friction and kinetic friction, was observed in fretting tests. The real contact area controls the tangential force release at the interface, and thus, dictates the alternating friction force. Therefore, it is necessary to obtain the contact area change for a better understanding of this physical process. In-situ techniques were employed to observe the real contact area during the stick-slip movement. In these tests, stick and slip regions were observed within the stick phase. By using machine learning, the movement of the stick region was analyzed. The correlation between friction force and the stick and slip regions in the stick phase was introduced using the mathematical model. Additionally, instead of using smooth samples, turned samples were used in this thesis, which represent the real surfaces of end products. The last thirty years has shown a growing interest in tribology, the study of surface and contact mechanics, among mechanical engineers. For example, applying coatings is becoming a more popular approach to improve the tribological properties of bulk materials. In this thesis, the application of composite coatings on sealing materials under emergency running was studied. Composites with varying ratios of diamond-like carbon (DLC) a hard coating, and molybdenum disulfide, a soft coating were investigated. Four sealing materials were coated using the magnetron sputtering method. Chemical and physical methods were applied to characterize the coatings. Tribological tests were performed at first under dry and starved lubrication conditions. Subsequently, the best and worst coating of each substrate was verified in component-like tests. There is no universal coating that is optimal for all substrates. The topography and rigidity of substra
Translated title of the contributionUntersuchung des Frettingverhaltens von Dichtungswerkstoffen
Original languageEnglish
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Pinter, Gerald, Assessor A (internal)
  • Grün, Florian, Assessor B (internal)
Publication statusPublished - 2020

Bibliographical note

embargoed until null

Keywords

  • Fretting
  • Sealing
  • Friction
  • TPU
  • Wear

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