A Tribological Study on the Ball-on-Disc (BOD) Method, Taking Steel and Galvanic Hard Gold as Examples

This work was motivated by utilisation of the widespread ball-on-disc (BOD) method for potentially characterizing galvanically deposited layers, especially gold/copper/cadmium alloy layers (“hard gold”) deposited on polished 100Cr6-steel discs (Φ 18 mm) using a special module. The friction balls consisted of smooth alumina and exhibited a diameter of 6 mm. For comparison, similar experiments were made on pure steel-discs. Besides of friction diagrams, profiles were detected by means of a separated profilometer allowing the volumetric determination of the abrasion (wear). To visualize their often irregular shapes, the shape of the friction ball was placed computationally onto the grooves. Due to the semi-chaotic process, the reproducibility was only minor. Nevertheless, some fundamental tribological relations could be found. In particular, the established hypothesis of Achard and the related assumption of a material-specific, time-independent wear coefficient appeared to be inappropriate and thus untenable. Rather, observation showed that wear decreases not linearly but over-proportionally over time, particularly implicating the number of cycles (laps). Furthermore, when the rotation rate increases, a higher load is required to ensure steady abrasion. Although this method allows numerical characterizations of galvanic layers by means of specific constants such as the friction impact constant KW, revealing an obvious qualitative difference between steel and hardgold-plated steel, it is probably not optimum for such systems since it deviates too much from the real conditions of operation. However, the hereby found tribological laws could be of eminent general interest.

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Phosphate coatings against frictional corrosion at shafthub press-fit connections and as transmission element for forces and moments in press-fit connections

Fig. 1: The active principle of the formation of conversion coatings

Phosphating baths are mostly developed for microcrystalline single-phase coatings which can be precipitated with high reproducibility. The titanium phosphate pretreatment of steel promotes the formation of hopeite. This reaction leads to a deceleration of the covering process of the free surface and consequently to an increase in the amount of iron containing phosphophyllite. The different alloys and structures of the steel types is the reason for different rates of the pickling attack. Multiphase phosphate coatings containing phosphophyllite show improved tribological properties compared to zinc calcium phosphate coatings. This can be seen especially on the significantly decreased stick-slip inclination. The use of manganese phosphate coatings is to be preferred for many press-fit connections because they can be reproducibly precipitated, guarantee a higher torque transmission und successfully prevent tribo-oxidation.