Mechanical engineering often uses fluid lubrication to limit friction. Even in the presence of a lubricant, metallic contact between the sliding surfaces may readily occur. In running-in, the oil film thickness can be so thin that contact arises at the summit of asperities, thus, increasing both the friction coefficient and wear. Such regimes are the so-called mixed or boundary lubrication. In these situations, the friction coefficient varies continuously and it is therefore, necessary to calculate the friction coefficient at any given moment.Given two rough parallel surfaces in a lubricated environment, a model is herein proposed whereby the friction coefficient is controlled by a single hydrodynamic parameter. This experimental analysis seeks to take into account a wide variety of factors influencing the conditions in which the contact operates: functional parameters (normal load, sliding speed, viscosity), the contact pair's mechanical properties (elastic modulus, Poisson's ratio) and surface microgeometry (as expressed by the standardised roughness parameters). This extended friction model predicts tribological behaviour in lubrication regimes: thus, by judicious choice of materials (according to the most appropriate mechanical properties and surface roughness) one can reduce the running-in period. To confirm the model, different material pairs have been tested. The tests have been conducted using a pin on disc apparatus in a 100 Neutral solvent (100 NS) oil at 20 • C. Experimental results totally confirmed the model.

Purpose The purpose of this paper is to describe the case of the Valorbio research project, in which students of different high-level programs were involved in the experimental work and in the dissemination of results in collaboration with the research team. Design/methodology/approach The inclusion in higher education curricula of content related to the sustainable development should be a preferred mechanism for the dissemination of good practices of sustainability. Another equally important way to achieve this is to involve students in research projects that seek solutions to the societal challenges related to sustainable growth. The Valorbio project aims to meet the needs for treating and reusing wastewater and solid waste. Its main goal was the development of modular systems for wastewater treatment based on constructed wetlands, exploring the possibility of the treatment systems being composed of solid waste and by-products from significant industrial sectors. Findings The students’ contribution to the research work was relevant and simultaneously allowed them to acquire skills on sustainable development. Additionally, the students contributed to the dissemination of the results. The Valorbio project can thus be considered a successful application of the concept of project-based learning (PBL), as a way to include sustainability issues content in the higher education curricula. Originality/value The applied experimental work had an original approach regarding the equipment design, the waste materials valuation, as well as the integration of waste treatment processes in the circular economy paradigm. This paper is the first reported PBL experience involving students of short-cycle technical–professional programs in partnership with first and second-level students and a research team.

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