|
1537 |
|
Monday, June 02, 2008, Auditorium 2 12:10 PM Automotive Coatings on cylinder liners and piston fire faces, testing of coatings and means of surface preparation for inner diameter coatings |
|
Characterization of nanocrystalline thermally sprayed coatings by means of 4-point bending fatigue tests |
|
Alfons Fischer / Universität Duisburg-Essen
Werkstofftechnik, Germany Mareike Hahn* / Universität Duisburg-Essen, Werkstofftechnik, Germany |
|
The future demands of diesel engines require new options for low-friction and wear-resistant materials in order to increase efficiency and achieve environmentally sound solutions. Thermally sprayed coatings might be such an option, because of the already existing variety of processes and materials, which could be extended in the future. Thus beside other means today nanocrystalline coatings are currently discussed, which should allow for the desired combination of structural, productional, and topographical properties. Beside sufficient tribological properties it is important that the composite (base material and coating) allows for an elongated endurance under cyclic mechanical and thermal stresses as well. Now many wear resistant coatings make use of hard phases which are either introduced by alloying (e.g. C and B for carbides and borides) or by the atmosphere (e.g. O for oxides) during spraying. The possible beneficial effect of all of these for the tribological properties is known. Now it is important to understand their influence on the integrity of such composites, which additionally contain pores as well, under fatigue. From other wear resistant materials it is known that crack initiation is accelerated and early unstable crack growth is likely. If these cracks are parallel to the surface they might lead to spalling and the delamination of large fractions the coating. Cracks perpendicular to the surface might propagate into the base material and provoke collateral damages by fracture or leakage. Counters to measure could be to restrict the volume fraction of hard phases and pores or to decrease their size. Thus nanocrystalline coatings generated by high energy spraying processes might allow for a high amount of wear resistant phases without the disadvantage of insufficient cyclic properties. There are three thermal spray processes in the focus of this project: TWA (Twin Wire Arc), HVOF (High Velocity Oxygen Fuel), and PTWA (Plasma Transferred Wire Arc). By means of these different commercial and newly developed Fe-base nanocrystalline coatings are sprayed onto bores and tested as to their productional (e.g. coating, honing), structural (e.g. friction, wear) and topographical (e.g. roughness) properties. This contribution focuses on the fatigue properties of these layered composites. Curved specimens are cut out of coated and honed cylinder bores and afterwards tested by means of four-point-bending stress controlled incremental step tests. Crack initiation and propagation are investigated and brought into relation to the relevant microstructure. |