Ceramic particulate composites in the system SiC-TiC-TiB2 sliding against SiC and Al2O3 under water
(1999)
The tribological behaviour of SiC, SiC-TiC and SiC-TiC-TiB2 was determined in oscillating sliding against SiC and -Al2O3 in water at room temperature. The tribo-systems with the composite materials containing TiC and TiB2 differ significantly from the systems with the single phase SiC: The wear is reduced and the friction is increased. The wear reduction up to a factor of 10 is mainly due to the formation of an oxide film containing titanium oxides which is soft, stable in water and well adhering to the bulk material. This oxide film is transferred to the alumina ball but not to the silicon carbide ball.
Contact formation and development are the basis of friction and wear modelling and understanding. Unanimously topography formation and development in friction contacts are regarded of highest importance for understanding and modelling friction processes. The frequently found running in behaviour of sliding contacts is—aside from the build up of reaction and transfer layers-at least partly caused by the topography development due to friction processes until a stable equilibrium state is reached.
Experimental results of friction and topography measurements are presented which demonstrate the mutual modification of friction and contact topography.
A special experimental set up with an AFM allowed to correlate the measured friction forces with the contact position and the topography at this point. In this way, friction force transitions and changes can be assigned to topography changes due to abrasion, adhesion and wear particle agglomeration.
Contact surfaces with artificial regular structures have been prepared to avoid problems with topography and friction correlation due to the statistical nature of roughness on technical contact surfaces. The friction effects of roughness were simulated by etched ditches of defined width, depth and distance on silicon or metal surfaces. This allowed to explain the mutual influences of topography and friction. The effect of a single asperity and of the 'roughness structures' could be demonstrated.
Topography measurements with an AFM correlated with the friction force could help to understand friction changes without changing any parameter.
Diamantähnliche Kohlenstoffschichten (DLC) zeichnen sich durch ein sehr günstiges Reibungs- und Verschleißverhalten aus. Auf vielen Werkzeugen und hochbelasteten Bauteilen ist DLC daher bereits erfolgreich im Einsatz. In naher Zukunft wird die Bedeutung dieser Schichten als Oberflächenschutz in tribologischen Systemen weiter ansteigen.
Wie viele Applikationsbeispiele zeigen, ist die Leistungsfähigkeit von DLC als Schicht zur Verringerung von Reibung und Verschleiß zwar groß, weist aber auch Grenzen auf. Vor allem bei höheren Temperaturen (> 300 °C) treten Strukturveränderungen oder sogar Oxidationsvorgänge auf, die auch die tribologischen Eigenschaften der Schichten verschlechtern können.
Reibungs- und Verschleißuntersuchungen (Schwingungsverschleiß) von DLC-Schichten wurden im Temperaturbereich von 25 °C bis 250 °C durchgeführt und zeigen ein schnelleres Durchreiben der Schicht bei höheren Temperaturen. Die Lebensdauer der Schichten wird mit zunehmender Prüftemperatur reduziert. Einsatzgrenzen für DLC-Schichten können abgeleitet werden.
The friction and wear behaviour of thin hard coatings, such as TiN and the promising class of C-based coatings (a-C, a-C:H, and diamond for example), are compared under oscillating and reciprocating sliding conditions. The typical effects of test parameters, such as stroke, frequency, normal force, relative humidity and test duration, are described as a basis for the proper selection of test conditions or, conversely, for the selection of suitable coatings for particular practical applications.
Friction and wear data from over 1000 vibrating tests using thin hard coatings against 100Cr6 and against Al2O3 have been compiled in a database. This allows easy manipulation and comparison of test results. Using selection criteria and filter procedures (e. g., lifetime of coatings, friction limits, and critical wear rate), suitable coating systems for different test conditions can be chosen from the database. The effects of test parameters on friction and wear behaviour and changes have anyway to be known for meaningful tribotesting, as well as for the selection of coatings.
Fundamentals of Tribotesting
(2000)
Tribology is an experiment-oriented branch of a complex, interdisciplinary science, in which testing plays a major role in the solution of technical problems, in the field as well as in the development of tribomaterials. Various test methods are used for this purpose, and the results are sensitive to the choice of test method and test conditions.
In this paper, the effects of test parameters on friction and wear results are considered, some selected results of model tests with different types of materials are presented, and guidelines are derived, that help improve the reliability and level of confidence of tribological tests performed in laboratories.
Amorphous SiO2 and Si3N4 plasma-enhanced chemical vapour deposited (PECVD) coatings were deposited on two different substrate materials (fused silica and borosilicate glass), with three coating thicknesses (0.1, 0.5, and 1.0 µm). The mechanical properties (hardness and elastic modulus) were determined by depth-sensing indentation, with loads from 700 mN down to 0.1 mN. Tribological behaviour was studied in instrumented oscillating sliding tests at room temperature with a ball-on-flat arrangement, in which the coated disc was tested against an alumina ball, at a load of 1 N. Interpretation of the measurement of hardness and modulus of the coatings has to take into consideration the influence of layer thickness and the effect of the substrate. Tensile film stress and crack generation were only observed for Si3N4 on fused silica above a threshold thickness. Friction and wear measurements show that the coating has an effect on friction, while wear is affected by the thin coatings only for a short running-in phase. The morphology of the wear scars indicates that the coatings have good adhesion. Despite crack generation, delamination effects were not observed. Indentation patterns similarly showed excellent lateral homogeneity of the mechanical properties over the entire film surface, and indicated that load-displacement curves may be used to characterise the system.