Diamond-like carbon coatings fabricated by the ball impact process

• Mechanochemical fabrication of DLC coatings by the ball impact process.
• DLC structure in the fabricated coatings disappears with extended treatment time.
• The threshold of maximum vibration acceleration of the chamber is 39 m/s2.
• The threshold of contact stress acting on the substrate surface is 1500–2000 MPa.

Favorable physical properties of diamond-like carbon (DLC) coatings such as high hardness, low friction, and high wear resistance are responsible for their widespread use in many industrial applications. However, suboptimal high-energy and high-cost processes, e.g., plasma CVD, are currently utilized in the industry for coating DLC. In this study, the ball impact process—a method for low-energy and low-cost metal surface treatment—was employed for coating DLC directly. The process involves the initiation of a mechanochemical reaction on a substrate surface in a methane gas atmosphere over several minutes. Repeated ball collisions are expected to generate coatings cores at the crystal grain boundaries of the substrate surface, followed by subsequent growth into interference color coatings with a DLC structure. Additional ball collisions destruct the interference color coatings and cause pyrolysis by a mechanochemical reaction, which are subsequently transformed to black carbon coatings without DLC structures. Because these changes proceed with extended treatment times, an optimum treatment time to obtain an ideal substrate surface coating exists. Based on the comparison between the experimental results obtained by varying the vibration frequency and the results of numerical simulations of ball behaviors using the discrete element method, the frequency threshold for fabricating DLC coatings was found to be 5 Hz (maximum vibration acceleration: 39 m/s2). This corresponds to a contact stress in the range 1500–2000 MPa acting on the substrate surface.