Alina Corina DumitrașCu

Cr₂O₃-based ceramic coatings are widely used in wear-critical applications; however, their tribological performance under dry sliding conditions can be limited by brittleness and frictional instability. In heavy-duty vehicles, the king pin–bushing contact operates under severe dry sliding conditions, motivating the investigation of composite Cr₂O₃–nTiO₂ coatings as a potential surface engineering solution. In this study, Cr₂O₃–TiO₂ coatings containing 0, 10, 20, 30, and 40 wt% TiO₂ were deposited by atmospheric plasma spraying (APS) from mechanically mixed powders. Phase composition was analyzed by X-ray diffraction using an X’Pert PRO MRD diffractometer, while microstructure and elemental distribution were examined by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) on a FEG Quattro C microscope. Mechanical properties were evaluated by Vickers microhardness, instrumented indentation and scratch testing, while dry sliding wear behavior was assessed by pin-on-disc tests performed on a CETR UMT-2 tribometer against a bronze counterbody, with continuous monitoring of the coefficient of friction (COF). The results show that plasma spraying produces lamellar composite coatings with intrinsic porosity and locally modified phase composition. Cr₂O₃-rich coatings exhibit higher hardness (1198 HV2 compared with 877 HV2 for Cr₂O₃–40TiO₂ corresponding to an increase of approximately 36%) and improved resistance to indentation, reflected by lower penetration depths and higher elastic modulus values (134 GPa for S0 compared with 77 GPa for S2). These coatings also exhibit a more stable friction response and reduced material transfer from the bronze counterbody, as confirmed by the lower mass loss of the pins (0.0295 g for S0 compared with 0.0473 g for S4, corresponding to a reduction of about 38%). Increasing TiO₂ content leads to changes in friction stability and wear behavior associated with microstructural heterogeneity. These findings indicate that the sliding wear performance of Cr₂O₃–nTiO₂ coatings is governed by elastic–plastic stability under localized contact loading and support their applicability for dry sliding king pin–bushing systems in heavy-duty vehicles.

In this paper, the authors have analytically and experimentally investigated the friction torque in an angular contact ball bearing (ACBB) considering the following configurations: (i) a modified ball bearing with three equidistantly positioned balls without a cage and (ii) three modified ball bearings having four, six and eight balls with a phenolic cage. The experimental tests were realised at a low axial load and a rotational speed between 100 and 500 rpm under lubricated conditions. The test results for the ACBB with three balls without a cage showed that the friction torque is generated only by the rolling contacts between the balls and the two raceways in lubricated conditions. Considering the low axial load, the influence of the hydrodynamic rolling resistance was the dominant source of the frictional torque. The cage presence added supplementary friction, causing the friction torque to increase up to 60–65% compared to the cage-less configuration. Also, an additional increase in the frictional torque by 35–40% was observed for every two additional balls. The friction torque component that is generated by the cage presence does not depend on the number of balls added but rather on the rotational speed. All the tests were performed using the spin-down methodology, and the resulting friction torques (T_exp) were determined by integrating the dynamic equation during the deceleration process using Python-based software (Python version 3.10). The input and output parameters are presented for each test. Logarithmic diagrams revealing the dependence between the experimental friction torque and angular speed were obtained and fitted, and the relations have been determined for all tests. The analytical results of the friction torque were obtained based on Houpert’s model for all modified ball bearing configurations.