FOR 576: Micro Structuring of Thermomechanically High Stressed Surfaces
Final Report Abstract
The wide expertise of the research group is shown by the five sub-projects. In the first subproject numercial simulation tools (K0RI3D / PRO) were enhanced in order to calculate the tribological properties of microstructures. A new approach was made by using a Lattice-Boltzmann model to describe the physical properties of the engine oil inside the microstructures. Through the new developed simulation technique, it was possible to determine the ideal layout of the microstructures. This knowledge was used as an input for the manufacturing processes and for further layout developments within the tribological experiments. Two different approaches regarding the manufacturing of micro structures were made. In the first stage a machining process, that uses an axially parallel turn-milling strategy, was developed. By adjusting the feed rate and the rotational frequencies, various micro dimple arrangements can be machined continuously. Typical dimensions vary from 1 mm to 2 mm in length, 50 µm to 100 µm in width and 5 µm to 30 µm in depth. To investigate the basic correlations between process variables, quality characteristics and chip formation mechanisms, fly-cutting tests were carried out. Second, a thermal spray process that allows the manufacturing of coatings with a defined porosity was developed. The microstructures are formed by the porosity by non-melted or re-solidified particles between the spray lamellae and a subsequent honing procedure. In the next sub-project optical sensors were developed in order to characterize the technical surfaces. A white light interferometer was used to measure small surface excerpts with high resolution. A coordinate measuring machine was modified with a confocal chromatic sensor in order to allow in-cylinder-measurements. Also data processing methods and optimized image alignment techniques were developed to fuse high resolution images of the microstructures. The tribological mechanism of machined and thermal sprayed micro structures were analyzed by tribometer testings and by a test rig specially designed for the cylinder liners. During testing, with the creation of machined micro dimples the minimum friction coefficient could be reduced by up to 79 % and shified by 77 % towards lower relative speeds. The effects which were identified indicate that machined micro dimples support hydrodynamic pressure build-up between the friction partners under planar contact conditions. When testing thermal sprayed micro structures, it is shown through all experiments that the coatings exhibit enhanced frictional properties compared lo uncoaled steel or cast Iron surfaces. In comparison with the reference samples the coatings provide the potential of using the microstructures to build a hydrodynamic pressure and additionally use the microstructures as an oil retention capacity. Due to the lamellar structure of the coatings, a new microstructured surface is exposed with the same properties after wear. Experimental analysis of microstructured cylinder liner were carried out on a single cylinder heavy-duty research engine under fired conditions. The friction, in terms of the friction mean effective pressure (FMEP), was determined using the "indication method". Additionally, oil consumption measurements were carried oul using an advanced mass spectroscopy syslem. The surfaces of the investigated cylinder liner were plateau honed, fine honed with machined micro dimples and coaled by thermal spraying. Depending on the micro dimple arrangement, cylinder liner with machined micro structures show a decrease of FMEP by a maximum of 19 % and significantly less oil consumption at low and medium engine loads compared to an ordinary plateau honed liner.
