Nano-Tribological Properties of Bio-Inspired Laminated Graphene Reinforced Aluminum Martrix Composite

Natural biological materials (such as bones, shells and spider silk, etc.) have evolved for a long time to obtain an ideal laminated structure composed of different materials arranged alternately, and consequently acquire excellent properties. As one of emerging two-dimensional nanomaterials, graphene has attracted more and more attentions, which is an ideal reinforcing component to construct bio-inspired laminated structure. Nevertheless, there are still several problems to restrict the use of graphene. For example, the structural integrity of graphene is often difficult to be guaranteed through ball milling during conventional powder metallurgy process. In addition, it is very challenging for graphene to reliably coat the contact points between the spherical powders. Moreover, currently, most of the relevant studies on biomimetic laminated graphene composites focus on the strengthening and toughening mechanism, and few studies focus on the microscopic friction, which can help reveal the intrinsic friction mechanism. In this work, nanoindentation and nanoscratch were used to study the mechanical properties and friction and wear properties of bio-inspired laminated aluminum matrix composite (BAMC). Herein, by assembling aluminum flakes cladded with graphene, BAMC inspired by natural biological materials was designed and prepared by means of improved powder metallurgy. The structural integrity of graphene can be ensured to the greatest extent by avoiding the structural destruction of graphene in the ball milling process. With the continuous update of material characterization technology, nano-testing technology has been widely used. Nanoscratch and nanoindentation testing techniques have mainly been used to characterize and analyze the mechanical properties of materials, critical depth of elastoplastic transition, microscopic deformation behavior, and friction and wear properties by single asperity. The complex surface interactions during macroscopic wear still remain to be a challenge to understand. Apparently, nanoscratch and nanoindentation testing techniques have the advantage to implement sliding asperity contact of a single point, from which the wear resistance of composites can be concluded. Especially, through contrasting the ramping code and the constant mode, nanoscratch can also be used to study the influence of micro-scale and nano-scale microstructure on friction properties. It can be inferred that, the nanoindentation and nanoscratch are in favor of establishing a relationship between the nature of microscopic friction and macroscopic experimental phenomena. As a consequence, mechanical properties and tribological behaviors of the pure Al and the BAMC were characterized by the nanoindentation test and the nanoscratch test. Based on the scanning electron microscopy and transmission electron microscopy results, the graphene was robustly bonded with the matrix, which possessed continuous, complete and clear layered stacking structure. Friction was composed of two components, i.e., adhesion and ploughing. Then, nanoindentation and nanoscratch were used to explore mechanical properties and the elastic-plastic transformation in the microscopic friction and wear process of BAMC and Al. Evidently, nanoscratch test was beneficial to analyze the contribution from the adhesion and ploughing, and revealed the microscopic friction and wear mechanism. The results showed that, compared with Al, the nanohardness of BAMC increased by about 24%, and the friction coefficien reduced by about 28%. The adhesion component and the ploughing component decreased by 32% and 16%, respectively. Upon nanoindentation on the biomimetic laminated structure, heterogeneous deformation induced strengthening occurred at the interface of graphene in the composites, which intensified the strain hardening ability and improved the hardness of BAMC. In the nanoscratch, the planar stacking of graphene can reduce both the adhesion and ploughing forces simultaneously, and the reduction of adhesion was dominant. The heterogeneous deformation induced strengthening and the adhesion reduction induced by graphene layers jointly improved the friction and wear resistance of BAMC. The reasons for the significant improvements of the mechanical properties, friction and wear properties of the BAMC were revealed from the micro-scale and nano-scale by using nano-testing technology, which might provide a theoretical basis for improving the friction and wear properties of the BAMC.