Abstract
wo low metal formulas (8% of iron powder) were prepared. Prototypes of brake pads were produced in order to compare the effect of both tin and antimony sulphides on both wear and the stability of the coefficient of friction under several conditions. Tin sulphides (both Sn(II) and Sn(IV)) were proposed as a replacement of antimony trisulphide and for this reason a comparison was needed in order to highlight the main differences at low loads of these materials, as usually are used. For this reason, the 5wt% level of sulphides in the friction formula was used as a reasonable amount for comparison. The substitution of antimony trisulphide (Sb2S3) with tin sulphides (SnSx) was the only variation between the two formulas. These two formulas were tested by means of dynotests performing SAE J2522, and VW TL110 wear tests ( both at 50 and 100 km h-1). The tribolayer formed on the rotor surface was studied by means of scanning electron microscopy (SEM) in both cases. The elemental analysis was carried out by means of energy dispersion spectroscopy (EDS). The comparison carried out through the SAE J2522 tests showed that in general the Sb2S3 provided a better friction stability, and a higher coefficient of friction. In the wear test at 100 kmh-1, the Sb2S3 exhibited a lower wear until 400°C, while at 500°C, the situation was inverted in favour of the SnSx material. Moreover, in the wear test at 50 kmh-1 the wear results were similar for both materials. However, disc thickness variation (DTV) seemed to be better for Sb2S3 for the considered formula. A clear and well-defined tribolayer was observed on the rotor surface rubbed against the material containing Sb2S3. On the contrary, SnSx generated a more irregular layer. The use of only a level of the studied lubricants was a limitation as well as a wear test with reduced braking conditions.
In a previous work, we showed that tins sulphides could interact with graphite increasing the effect of this lubricant, this is reflected in the lower wear at 500°C of SnSx in the test at 100 kmh-1. In this study, the formation of a steady composite tribolayer due to Sb2S3 indicated that the better friction stability is probably due to these characteristic.Sb2S3 and SnSx exhibited a different behaviour in the given formula. The better stability of the coefficient of friction as wells the lower DTV of Sb2S3 seemed to be correlated with the well-formed tribolayer on the rotor surface. However, wear at higher temperature is better for SnSx. Therefore, the substitution of Sb2S3 with SnSx implies that the formula must be revised and re-defined also for low contents (5 wt%) as those studied.