Tribological Evaluation of Sintered and Conventional Gear Materials

During gear shifting, the contact between teeth composes a tribological system of considerable importance regarding energy dissipation in road vehicles. Improvement of tribological system efficiency leads to costs and pollutant emission reductions. Powder metallurgy (PM) is a near-net-shape technique that allows the production of parts with complex geometry - such as gears, lower costs and larger range of material utilization compared to other manufacturing processes. Furthermore, the presence of pores in sintered material could be a beneficial factor for friction reduction and wear resistance, due to oil reservoir and debris trapping effects, and film thickness variation that could enhance lubricant load support capacity. This work aims to evaluate the lubricant effect on support capacity due to surface pores in sintered steel with different levels of porosity. Results were compared with standard gear material based on friction coefficient results. Tribological lubricated unidirectional tests were performed in a pin-on-disk tribometer with constant load and velocity - conformal contact. Samples were characterized prior and after tests by scanning electron microscopy (SEM), optical profilometry and hardness measurements (HV 10). Density analysis through Archimedes’ method allowed the estimation of the pore volume fraction. Pores features - dimension and shape parameters - were evaluated using quantitative metallography analysis. A deterministic mixed lubrication model was used to calculate hydrodynamic and asperity contact pressure in the tested samples with different topography. The high porosity steel samples showed large pores with an irregular shape, whereas the low porosity samples had smaller pores with a regular round shape. Specimens with low and regular shape porosity presented a friction reduction of 15% as compared to the standard wrought gear steel. Conversely, high porosity levels and big pores caused an increase in friction coefficient throughout the performed tests. Hydrodynamic pressure calculation confirmed that low porosity and regular shape pores enhance lubricant support capacity.