24th International Colloquium Tribology - January 2024 79 Combination of DLC Coatings and Dedicated Lubricants in order to Achieve Supralow Friction in Highly Loaded Sliding Contacts Johnny Dufils 1* , Etienne Macron 1 , Christophe Héau 1 1 Institut de Recherche En Ingénierie des Surfaces (IREIS) ICE-T platform, HEF GROUPE, Andrézieux-Bouthéon, France * Corresponding author: jdufils@hef.group 1. Introduction The on-going mutation of the automotive sector towards more environmentally compatible technologies has also an impact on automotive lubricants. Indeed, the development of new lubricants becomes mandatory to satisfy both growing performance standards and increasingly stringent environmental regulations. Diamond-like carbon coatings have become widespread in the automotive industry. These coatings are used mainly for their resistance to abrasive wear, scuffing and seizure. However, modern engine oils do not take fully advantage of having DLC coated components as oils are mainly designed for uncoated steel components. In the literature, DLC coatings combined with specific lubricants or additives are known to exhibit supralow friction [1][2]. However, most of these results were not taken up industrially probably for multiple reasons. One of these reasons may be a lack of tribological relevance of the tests performed at the laboratory scale. In the following, alternative lubricants to conventional engine oils were combined with DLC coatings and tested in highly loaded sliding contacts compliant with valvetrain applications. 2. Materials & Methods 2.1 Materials & Lubricants Three alternative lubricants, designed as Lub A, Lub B and Lub C, were studied in DLC/ DLC and steel/ DLC contacts. Lub A has lower viscosity than lub B and lub B has lower viscosity than lub C. The tribological performance of these alternative lubricants were compared to three engine oils: a 5W30 grade oil which does not contain friction modifiers, a 0W30 grade oil containing a MoDTC friction modifier and a 0W12 grade oil also containing a MoDTC friction modifier. These lubricants were tested on a ring-on-flat tribometer in three material configurations: • Grinded steel ring (Ra = 0.2 mm)/ DLC coated flat • Polished steel ring (Ra = 0.03 mm)/ DLC coated flat • DLC coated ring (Ra= 0.03 mm)/ DLC coated flat The DLC coatings were all a-C: H coatings. The roughness parameter Ra of the flat sample was 0.02 mm. 2.2 Tribological testing Various tribological tests were performed on the HEF homemade ring-on-flat tribometer. The ring is set in rotation thanks to an electric motor. The ring is 35 mm in diameter and 8 mm in height leading to a line contact of 8 mm in length when the ring is rubbed against the flat sample. The contact maximum Hertzian pressure is set to 500 MPa. A key feature of the tribometer is the contact pressure being kept constant during the test even if there is wear on the flat sample. This is achieved thanks to a slow reciprocating motion of the flat sample. The sliding speed is varied between 0.6 and 2.7 m/ s. The loading and sliding conditions are typical of that of valvetrain components. The tests were performed at various lubricant temperatures between room temperature and 100-°C. Lub A was also tested in a motored valvetrain with a DLC coated camshaft and DLC coated cam tappets. The 0W30 grade oil was used for comparison. 3. Results & Discussion 3.1 Ring-on-flat testing Figure 1 shows the evolution of the coefficient of friction as a function of the sliding speed for the three alternative lubricants in a DLC/ DLC contact. Coefficients of friction of 0.005 are achieved for the three alternative lubricants but the speed at which supralow friction is achieved, depends on the lubricant. The viscosity of Lub C was optimized in order to get supralow friction in the whole range of speed investigated here. In these tribological conditions, conventional engine oils show coefficients of friction of 0.01 at best (see Figure 3(a)). Figure 2 shows the evolution of the coefficient of friction as a function of the sliding speed for Lub B in the three material configurations tested. It is shown that it is not necessary to have both surfaces coated with DLC in order to get supralow friction. Indeed, the polished steel/ DLC contact with Lub B shows coefficients of friction between 0.01 and 0.006. The friction reduction obtained with Lub B compared to conventional engine oils is even higher in a polished steel/ DLC contact than that obtained in a DLC/ DLC contact (see Figure 3(b)). However, increasing the roughness of the ring leads to higher friction coefficients due to contacting asperities. Figure 1: Evolution of the coefficient of friction as a function of the sliding speed for the three alternative lubricants in a DLC/ DLC contact tested 80 24th International Colloquium Tribology - January 2024 Combination of DLC Coatings and Dedicated Lubricants in order to Achieve Supralow Friction in Highly Loaded Sliding Contacts Figure 2: Evolution of the coefficient of friction as a function of the sliding speed for Lub B in the three material configurations tested 3.2 Tribological performance assessment on a motored valvetrain Figure 4 presents the evolution of the power dissipated by friction as a function of the camshaft rotation speed on the motored valvetrain for three configurations. The addition of a DLC coating on the camshaft led to a relatively small decrease in the power dissipated by friction (approx. 5W) with the 0W30 grade oil. Replacing the 0W30 grade oil by lubricant A and adding a DLC coating on the camshaft led to a large reduction of the power dissipated by friction (up to 70%). Figure 3a: Evolution of the coefficient of friction as a function of the sliding speed for Lub B and conventional engine oils in (a) a DLC/ DLC contact Figure 3b: Evolution of the coefficient of friction as a function of the sliding speed for Lub B and conventional engine oils in a polished steel/ DLC contact Figure 4: Evolution of the power dissipated by friction as a function of the camshaft rotation speed on the motored valvetrain for the 3 tested configurations 4. Conclusions The tests performed on the ring-on-flat tribometer showed that it is possible to strongly reduce the coefficients of friction of highly loaded sliding contacts by using DLC coatings with alternative lubricants compared to conventional lubricants. The excellent tribometer results are here validated on a valvetrain test bench with real components. These results illustrate that the use of DLC coatings may pave the way to alternative lubricants to conventional oils. References [1] Bouchet, M.I.D.B. et al. Scientific Reports 7 (2017). [2] Björling, M. et al. Tribology Letters 67: 23 (2019).