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Eingeladener Vortrag
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The tribological behaviour under dry sliding conditions of Magneli phase Ti4O7 and e-phase Ti2Cr2O7 with α-alumina as a sliding partner has been characterised at room temperature by oscillating sliding. The mechanical properties as well as Vickers hardness measurements do not reveal big differences between the materials, and 4-point bending strengths are around 200 MPa for both materials. Although the friction coefficients are in the medium range of 0.6 and do not differ much between the materials, the wear behaviour is very different. At room temperature, the wear coefficient of Ti2O7 is strongly dependent on relative humidity, and wear is highest under dry conditions when relative humidity is low as it is typically found for many ceramic materials. Ti2Cr2O7 shows remarkably different wear behaviour with the lowest wear in dry conditions. The measured coefficients of wear at low relative humidity of 4% are more than 2 orders of magnitude lower than those for Ti2O7.
The friction and wear behaviour of SiC, Si3N4 and SiC/Si3N4 composite ceramics were investigated with oscillating sliding (gross slip fretting) at room temperature. The influence of counter body material and the humidity of the surrounding air was studied with a ball-on-disc configuration with different ball materials (1000Cr6, Al2O3 SiC and Si3N4). The effect of RH on friction is marginal with exception of SiC (low friction) as counter body material. The wear behaviour, however, is strongly affected by humidity, showing inverse trends for different counter body materials. Consequently, the wear behaviour of a tribo couple can be improved by selecting an adequate mating material. The results reveal the necessity to control RH in tribological tests. For estimation of the performance of tribo couples under varying environmental conditions, a variation of RH is required. In tribo couples with single phase SiC, either as ball or disc, the tribological behaviour of the system is dominated by SiC. The friction behaviour of the composite material is in between the behaviour of the two single phase materials, Si3N4 and SiC, whereas the wear behaviour is very similar to that of single phase Si3N4.
Analysis of oxidative processes in SiC-TiC-TiB2 ceramic composites during tribological loading
(2009)
The high temperature tribological performance of tetrahedral amorphous carbon coatings has been analyzed at elevated temperatures up to 250 °C in air against three different counterbody materials-steel 100Cr6, a-alumina and silicon nitride. The results show that the counterbody material influences the friction and wear behavior and therefore coating life time strongly. This effect is well known for these coatings at room temperature under dry environmental conditions, equivalent to conditions above 100 °C when water molecules desorb from the surface. However, the sharp difference in tribological performance between silicon nitride on the one hand and alumina and steel on the other hand cannot be understood in this context. Analyzing the friction behavior during the running-in phase, it is evident that only alumina and steel form a stable interface with constant low friction and relatively low wear rates. Silicon nitride forms an unstable interface with fluctuating COF and relatively high wear rates due to its own inherent tendency to tribo-oxidation.
Wear mechanisms, as adhesion, abrasion, fatigue and tribochemical wear, are complex in their physical and chemical nature. A theoretical description and prediction of wear are in most cases still far from reality. It is, therefore, important to use test rigs before a practical application of a given material combination under tribological loading.
On the other hand, model equations can be helpful for wear description, if a single wear mechanism is dominant.
Under oscillating and continuous sliding contact conditions, equations for wear calculation are presented to describe running-in and stationary wear behaviour of metals and ceramics. By using shear energy density, real area of contact, flash temperature, activation energy and numerical simulations with the method of movable cellular automata (MCA), wear data were calculated and compared to experimental laboratory results.