J.M. González-Carmona¹, V.H. Tapia Ramírez², G.C. Mondragón-Rodríguez¹, M. Bravo-Sanchez³, F. Martínez-Olvera¹, L.A. Cáceres-Díaz⁴, C. Felix-Martinez^1
1Centro de Ingeniería y Desarrollo Industrial, CIDESI-Querétaro, Querétaro, México
²CIATEQ A.C. Centro de Tecnología Avanzada, Aguascalientes, México
³Universidad de Guadalajara, Guadalajara, Jalisco, México
⁴CIATEQ A.C. Centro de Tecnología Avanzada, San Luis Potosí, México
Coating deposition methods have continuously been optimized to improve hard coatings performance and to increase the lifetime and efficiency of coated parts. For instance, the interaction between the piston ring and the cylinder liner represents a significant energy loss ranging from 40 to 55 % for diesel engines and 38 % for gasoline engines. However, friction is still a challenge that has been tackled using fuel-efficient and ultra-low-viscosity lubricants, which are beneficial and simultaneously decrease the frequency of the asperity contact, which causes the engine to wear and lose durability. Nevertheless, the contact between the cylinder walls and the piston (and in the piston seals rings), cause damage to the engine when boundary layer lubrication fails due to lubricant overheating or cold start. Diamond Like Carbon (DLC) is an excellent candidate to reduce friction and improve wear resistance of components such as piston-ring and cylinder liners used in engines. The present work investigates the effect of nitrogen doping in hydrogen-free DLC (N-DLC) coatings on the adhesion, mechanical stability and solid lubrication capacity. The N-DLCs were obtained at an industrial scale by the PVD arc-ion plating process (AIP). Coating surface morphology, average roughness and the sp2/sp3 fractions were analyzed to ensure the reliability of the deposition strategy to achieve thick (> 20 μm) N-DLCs solid lubricant coatings, obtaining nitrogen percentages between 8 to 11 at%. Upon the N-raise a decrease in sp3 bonds was observed. The hardness and the elastic modulus decreased upon the change of the sp3-content, however the elastic strain to failure increased and the resistance to plastic deformation decreased. Scratch tests showed no modifications in the adhesion for N-doping using 20% N₂ in the Ar-N₂ gas mixture. In contrast, high N-doping with 40 and 60% N2 resulted in wedging-type spallations and coating failure at lower critical loads. The lowest coefficients of friction were obtained (~0.07) for DLC coatings deposited with <60% N₂, which is attributed to the graphitic species and the transformation of the DLC to graphite during friction. The wear coefficients (~10-7 mm3/Nm) increased with the N-raise, and the wear mechanisms were mainly related to plastic deformation, despite the scratching at the edges of the tracks indicating non-homogeneous wear. Hydrogen-free DLCs deposited with < 60% N₂ improved their solid lubricant behavior, without significantly affecting their adhesion and mechanical performance.