Spear Jessica C, Custer James P, Batteas James D
Department of Chemistry, Texas A&M University, College Station, TX 77843, USA.
Nanoscale. 2015 Jun 14;7(22):10021-9. doi: 10.1039/c5nr01478f. Epub 2015 Apr 22.
Nanoscale carbon lubricants such as graphene, have garnered increased interest as protective surface coatings for devices, but its tribological properties have been shown to depend on its interactions with the underlying substrate surface and its degree of surface conformity. This conformity is especially of interest as real interfaces exhibit roughness on the order of ∼10 nm that can dramatically impact the contact area between the graphene film and the substrate. To examine the combined effects of surface interaction strength and roughness on the frictional properties of graphene, a combination of Atomic Force Microscopy (AFM) and Raman microspectroscopy has been used to explore substrate interactions and the frictional properties of single and few-layer graphene as a coating on silica nanoparticle films, which yield surfaces that mimic the nanoscaled asperities found in realistic devices. The interactions between the graphene and the substrate have been controlled by comparing their binding to hydrophilic (silanol terminated) and hydrophobic (octadecyltrichlorosilane modified) silica surfaces. AFM measurements revealed that graphene only partially conforms to the rough surfaces, with decreasing conformity, as the number of layers increase. Under higher mechanical loading the graphene conformity could be reversibly increased, allowing for a local estimation of the out-of-plane bending modulus of the film. The frictional properties were also found to depend on the number of layers, with the largest friction observed on single layers, ultimately decreasing to that of bulk graphite. This trend however, was found to disappear, depending on the tip-sample contact area and interfacial shear strain of the graphene associated with its adhesion to the substrate.
纳米级碳润滑剂，如石墨烯，作为器件的保护表面涂层已引起越来越多的关注，但其摩擦学性能已被证明取决于其与下层基底表面的相互作用及其表面贴合程度。这种贴合性尤其值得关注，因为实际界面呈现出约10纳米量级的粗糙度，这会显著影响石墨烯薄膜与基底之间的接触面积。为了研究表面相互作用强度和粗糙度对石墨烯摩擦性能的综合影响，原子力显微镜（AFM）和拉曼光谱相结合的方法被用于探索基底相互作用以及单层和少数层石墨烯作为二氧化硅纳米颗粒薄膜涂层的摩擦性能，这些二氧化硅纳米颗粒薄膜产生的表面模拟了实际器件中发现的纳米级微凸体。通过比较石墨烯与亲水性（硅烷醇封端）和疏水性（十八烷基三氯硅烷改性）二氧化硅表面的结合情况，控制了石墨烯与基底之间的相互作用。AFM测量表明，石墨烯仅部分贴合粗糙表面，且随着层数增加，贴合性降低。在更高的机械载荷下，石墨烯的贴合性可以可逆地增加，从而可以局部估计薄膜的面外弯曲模量。还发现摩擦性能取决于层数，单层石墨烯的摩擦力最大，最终降至块状石墨的摩擦力。然而，根据石墨烯与基底粘附相关的针尖 - 样品接触面积和界面剪切应变，这一趋势会消失。
