Experimentelle Evaluierung der Fresstragfähigkeit von Kolbenringen mit verschiedenen chrombasierten Schichten

Translated title of the contribution: Experimental evaluation of the scuffing limit of piston rings with different chromium-based coatings

Research output: ThesisMaster's Thesis

Abstract

Piston engines are utilized in various sectors of modern life, from the automotive industry to generators, using different fuels such as diesel, gasoline, and gas. To optimize these machines for sustainability, it is necessary to reduce piston machine losses and fuel consumption. An important area for improvement is the tribological optimization of the piston group, as a significant amount of energy is used to overcome friction in this area. By enhancing the contact properties between the piston ring and cylinder wall, a considerable amount of energy can be saved. This can be achieved by improving the topology of the friction partners, using low-viscosity lubricants, and applying enhanced coatings to the rings. Furthermore, increasing the power density of the piston machine typically results in higher pressures, leading to more demanding working conditions for the piston rings and their coatings, thus increasing the risk of scuffing. This study closely examines different coatings of piston rings for large gas engines in generators, determining and comparing scuffing limits. The coatings that were tested included chromium with aluminum oxide particles, chromium with nanodiamonds, and chromium-nitrogen-carbon coatings created through physical vapor deposition. A cast iron cylinder with spheroidal graphite was utilized for the cylinder samples. To facilitate a comparison before and after the tests, an optical microscope was employed to provide a closer examination of the sample surfaces and cross-sections. Additionally, analyses were conducted using a scanning electron microscope to allow for a more precise analysis of the material composition through energy-dispersive X-ray spectroscopy. The tests were conducted using a linear tribometer. Following an extended running-in period, the force was incrementally increased by 50 N every 45 minutes. Exceeding the scuffing limit was identified by a sudden, sharp rise in the coefficient of friction. A maximum coefficient of friction of 0.25 was established to halt the tests upon failure. Multiple measurements were taken during the tests and subsequently analyzed in depth. Due to variations in ring geometries, the scuffing limit needed to be adjusted based on the projected surfaces to ensure result comparability. The calculated pressure also more accurately reflects real conditions within the piston machine, where the piston rings are pressed against the cylinder wall under applied pressure. The determined nominal pressure was highest for rings with a coating created by physical vapor deposition. The difference compared to rings with a chromium layer with aluminum oxide was 49 %. Rings with nanodiamonds instead of aluminum oxide particles in the chromium layer had a 15 % higher nominal pressure. Rings with PVD coating had higher peaks in the coefficient of friction when the force was increased compared to the other rings tested. Furthermore, the presence of tribofilms and material transfer due to adhesive wear between the cylinder and ring could be observed under an optical microscope and a scanning electron microscope. This adhesive wear was not as prominent in rings with PVD coating as in the other rings tested, which may be attributed to the physical vapor deposition process and the nitrogen present in the coating. In future studies, a more detailed analysis could be conducted on the adhesive wear and sudden increase in the coefficient of friction of pistons with PVD coatings. It would also be interesting to conduct wear tests on rings with different coatings. Additionally, testing the effect of alternative fuels on the failure of these rings could provide valuable insights.
Translated title of the contributionExperimental evaluation of the scuffing limit of piston rings with different chromium-based coatings
Original languageGerman
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Grün, Florian, Supervisor (internal)
Award date28 Jun 2024
Publication statusPublished - 2024

Bibliographical note

embargoed until 03-05-2029

Keywords

  • tribology
  • piston ring
  • chromium-based coating
  • CKS
  • GDC
  • PVD
  • TE77
  • scuffing limit

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