Improvement of platinum adhesion to carbon surfaces using PVD coatings

a r t i c l e i n f o The adhesion of Pt to carbon surfaces is an important technological consideration in proton exchange membrane fuel cells (PEMFCs). Thin films of Au, Ti and Cr were deposited on graphite, non-hydrogenated diamond-like carbon (NH-DLC) and hydrogenated DLC (H-DLC) coatings' surfaces using a physical vapour deposition (PVD) process. The friction force curves obtained from sliding a Pt pin against these surfaces were used to evaluate the adhesion of Pt to the coated carbon surfaces. Interface strength calculations for graphite and diamond surfaces were carried out using first principles simulations. The incorporation of the interfacial PVD films enhanced the adhesion between Pt and graphite; rather than interfacial separation taking place, the bonds in graphite were broken (graphite decohesion) as confirmed by the first principles calculations. The bond between Pt and NH-DLC was stronger than that between Pt and graphite, and the transfer of Pt to the uncoated NH-DLC, as well as the Ti-and Cr-coated NH-DLC surfaces occurred as a result of the breaking of Pt–Pt bonds (Pt decohesion). Au film was peeled off the NH-DLC surface by the Pt pin contact, consistent with the calculated work of separation for the Au/carbon interface, which was weaker than works of decohesion for both NH-DLC and Pt. In case of the H-DLC, the low adhesion of Pt to this surface was improved by the PVD coatings, but the improvement was less compared to the coated graphite surfaces, as all PVD films were peeled off the H-DLC surfaces by the sliding action of the Pt pin. A broad range of friction and adhesion properties are expected between carbon surfaces and metallic materials, as carbon forms different crystalline (e.g., graphite, diamond) and amorphous (e.g., diamond-like carbon (DLC)) structures, and these structures interact with the counterface and the surrounding environment in different ways. The early works of Bowden and Young [1] showed that the friction of graphite and diamond surfaces was affected by the surrounding atmosphere, and the presence of surface films significantly reduced the friction. Other researchers [2–4] reported that if graphite is transferred to a metal's surface during sliding in air a low friction between the metal and the graphite is obtained due to graphite layers (graphene planes) sliding over one-another. Recent studies that used first principles calculations [5–9] to model interfaces between metals and carbon structures proved to be useful for understanding the adhesion …