_applied_  
_sciences_  
_Article_  
**Advanced Tribological Characterization of DLC Coatings**  
**Produced by Ne-HiPIMS for the Application on the Piston**  
**Rings of Internal Combustion Engines**  
**[PERSON] \({}^{1}\)\({}^{\copyright}\), [PERSON] \({}^{1}\), [PERSON] \({}^{1}\)\({}^{\copyright}\), [PERSON] \({}^{1,2}\)\({}^{\copyright}\), [PERSON] \({}^{1}\)\({}^{\copyright}\) and [PERSON] \({}^{1,*}\)\({}^{\copyright}\)**  
\({}^{1}\) Department of Mechanical Engineering, CEMMPRE, University of Coimbra, [PERSON], 3030-788 Coimbra, Portugal; [EMAIL_ADDRESS] (A.V. ); [EMAIL_ADDRESS] (D.F. ); [EMAIL_ADDRESS] (J.O. ); [EMAIL_ADDRESS] (A.C.); [EMAIL_ADDRESS] (A.R.) \({}^{2}\) LEDMak-IPIN, Laboratorio de Ensaios Desgaste e Materiais, Instituto Pedro Nunes, Rua Pedro Nunes, 3030-199 Coimbra, Portugal  
\({}^{*}\) Correspondence: [EMAIL_ADDRESS]  
**Featured Application: This work represented the real conditions of the lubricated contact between the piston rings and cylinder liners of internal combustion engines to prove that Diamond-like Carbon film deposited by Ne-HiPIMS technology can provide a major efficiency improvement if it is applied for piston rings instead of coatings like Chromium Nitride that are widely used. **  
**Abstract:** Piston rings (PR) are known for almost a quarter of the friction losses in internal combustion engines. This research work aims to improve the tribological performance of PR by a recently developed variant of Diamond-like Carbon (DLC) coatings deposited in a mixture of Ar and Ne plasma atmosphere (Ne-DLC) by high-power impulse magnetron sputtering (HiPIMS). For the benchmark, the widely used Chromium Nitride (CrN) and DLCs deposited in pure Ar plasma atmosphere (Ar-DLC) were used. The tribological tests were performed on a block-on-ring configuration under different lubrication regimes by varying temperatures and sliding speeds. The analysis of the results was performed by Stribeck curves corresponding to each sample. An improvement of the tribological performance was observed for Ne-DLC films by up to 22.8% reduction in COF compared to CrN in the boundary lubrication regime, whereas, for the Ar-DLC film, this reduction was only 9.5%. Moreover, the Ne-DLC films achieved ultralow friction of less than 0.001 during the transition to a hydrodynamic lubrication regime due to better wettability (lower contact angle) and higher surface free energy. Increasing the Ne up to 50% in the discharge gas also leads to an increase of hardness of DLC films from 19 to 24 GPa. **Keywords:** piston rings; tribology; DLC; HiPIMS; neon; high temperature; Stribeck curves; hardness; contact angles  
**1. Introduction**  
The efficiency of an internal combustion engine (ICE) is an increasingly important concept nowadays, largely due to climate change and the need to reduce CO\({}_{2}\) emissions into the atmosphere, trying, at all costs, to minimize the irreversible losses on our planet. Moreover, the efficiency of an ICE is also extremely important for the automotive industry, because the customers are looking for cars with less fuel consumption. It is estimated that a piston ring (PR) is responsible for about 24% of the friction losses in an ICE [1]. In other words, half of the friction losses from power cylinders are generated by PRs [2]. In this way, inevitably, the performance of an ICE is directly linked to the coefficient of friction generated by the contact of the PR with the cylinder liner [3]. To reduce friction losses in PRs, studies have focused on the development of new materials,such as lubricants, surface textures or coatings to be applied to PRs, with the ability to reduce the coefficient of friction and improve the desired performance [3; 4]. As for coatings, developments have led to the frequent use of hard ceramic coatings deposited by Physical Vapor Deposited (PVD), such as CrN, as it combines satisfactory tribological properties with high hardness, and corrosion resistance at high temperatures, with low production costs, which is a determinant for large-scale applications [5].