Effect of roughness on the wear behavior of HVOF coatings dry sliding against a friction material

The deposition process of WC – Co and Cr3 C2 – NiCr cermet coatings through a high velocity oxyfuel (HVOF) spray process was examined using powders of different carbide particle sizes that ranged f...

Effect of solid carbide particle size on deposition behaviour, microstructure and wear performance of HVOF cermet coatings

Pin-on-disc study of a friction material dry sliding against HVOF coated discs at room temperature and 300 °C

Running-in is the initial stage during which a tribological system reaches a steady-state condition. In this study, the running-in behaviour of a commercial brake friction material, pin-on-disc tested under dry sliding conditions, has been investigated to understand the role of different surface finishing of hard coatings. These coatings were deposited onto cast iron counterface discs and then mechanically polished to achieve set values of average roughness, Ra, and roughness skewness, Rsk. The tribological data were modelled using an exponential relation for the wear rate, according to a literature approach. The model parameters were related to the above-mentioned disc surface roughness parameters. The results provided indications on the wear mechanisms acting during the sliding action. Furthermore, these mechanisms were correlated with the formation of the friction layers on the pins and on the disc wear tracks. Of particular relevance is the finding that a negatively skewed surface roughness is fundamental to achieve the best running-in performances. The beneficial effects, coming from this surface treatment, are derived from the reduction in abrasion and from the improved formation dynamics of the friction layer, with particular regard to those parts (secondary plateaus) made of compacted wear debris.

Sliding Behaviour of Friction Material Against Cermet Coatings: Pin-on-Disc Study of the Running-in Stage

The dry sliding behavior of three commercial friction materials (codenamed FM1, FM2, and FM3) tested against a Co-free cermet coating produced by high-velocity oxy-fuel (HVOF) on gray cast-iron discs is investigated. FM1 is a conventional low-metallic friction material, FM2 is developed for using against HVOF-coated discs, and FM3 is a Cu-free friction material with a low content of abrasives and a relatively high concentration of steel fibers. For the tribological evaluation, they are tested on a pin-on-disc (PoD) test rig against Co-free HVOF-coated discs, with particular attention to the running-in stage, which is fundamental for the establishment of a friction layer between the two mating surfaces, i.e., the pin and disc. The PoD tests are performed at room temperature (RT) and a high temperature (HT) of 300 °C. At RT, all materials exhibit a long running-in stage. At HT, no running-in is observed in FM1 and FM2, whereas a shorter running-in period, with respect to the RT case, is observed in FM3 followed by the attainment of a comparatively high coefficient of friction. At RT, the pin wear is mild in all cases but severe at HT. FM3 shows the lowest wear rate at both temperatures. Moreover, the coated disc shows no wear when sliding against the FM3 friction material. All the results are interpreted considering the microstructural characteristics of the friction layers formed on the sliding surfaces. The findings of the present study provide insights into reducing wear in braking system components and hence reducing environmental particulate matter emissions from their wear, through the use of disc coatings.

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Pin-on-disc study of dry sliding behavior of Co-free HVOF-coated disc tested against different friction materials

High Velocity Oxygen Fuel coatings were produced using powders with different chemical compositions, and the microstructure, hardness and sliding wear behavior studied with reference to the characteristics of the starting powders. The microstructures of the coatings and the powders were investigated using scanning electron microscopy and X-ray diffraction. Room temperature dry sliding wear tests were performed using the block-on-ring test. Wear behavior of the coatings is anderstood in relation to the applied load, sliding speed and wear surface damage.

D. Doni

Papers

Microstructure and sliding wear behavior of thermal spray carbide coatings