## 1 Introduction  
The bearing support transfers the loads from the superstructure down to the substructure stably. Early bearing supports were made up of steel or reinforced concrete. With the advancement of technology, the types of bearing supports diversified with the appearance of pendulum bearing support, disk bearing support, elastic bearing support, spot bearing support, etc. [1, 2, 3, 4, 5, 6, 7]. Among them, conventional bearing supports have relatively high vertical load bearing capacity and medium-high to medium horizontal load bearing capacity. The traditional bearing support basically consists of an upper bearing plate, a friction material, and a lower bearing plate as shown in Figure 1.  
Figure 1: Configuration of bearing support: (**a**) Concept figure; (**b**) Schematic diagram. A friction material should have low friction coefficient to accommodate horizontal and rotational displacement caused by external forces such as vehicle, temperature, wind load, etc. [8]. If friction coefficient of the friction material is not maintained in a low state, it will cause serious damage to a super structure, which will lead to a decrease in the performance of structures. In general, the durability of a bridge is considered to be about 50 years, but the durability of an existing spherical bearing support is about 5 years, which is about 1/10 of the durability of the bridge. The durability period indicates the time of occurrence of defects, and the replacement period of the general bridge support is about 15 years. The biggest cause of defect occurrence is the amount of wear and deformation of a friction material accommodating displacements occurring in a bearing support. A commonly used bearing support was a high-strength brass spherical bearing supports by inserting graphite and molybdenum, which are non-fueled lubricants, into spherical high-power brass. However, changes in the price of a bearing support depending on changes in copper prices represent a problem, and the longer the period of use, the less function of a friction material. In particular, since this type of a support is composed of contact between the metal surface and the metal surface as shown in Figure 2, integration occurring due to rust generation is a problem. To overcome these problems, the current friction materials in most bearing supports are engineering plastic (EP), ultra-high molecular weight polyethylene (UHMWPE) and polytetrafluoroethylene (PTFE). Through the analysis of the high-speed railway PSC box bridge, [PERSON] et al. [9] selected a 40 m simple span and a two-span continuous bridge as representative types and reviewed the travel distance by live load by applying PTFE. The distance traveled by live load was considered as a major factor in EN1337-2 [10], but the accumulated distance by live load for one year was 145 m, significantly exceeding 10 m by temperature load. Recently, PTFE has been replaced by EP due to the excellent properties of EP and UHMWPE [11; 12; 13; 14]. Even if EP develops relatively higher friction coefficient than that of PTFE, it offers higher durability and allowable bearing resistance. Notably, a long-term friction test showed that the sliding distance of EP is 2 to 5 times longer than that of PTFE [15] with a 3 times higher friction coefficient [16; 17; 18; 19; 20]. Such types of friction materials are lubricated to make smoother behavior between upper and lower bearing plates. However, the longer the service period of structures, the lower the lubrication performance of friction materials due to the loss of lubricant or deformation of friction materials. Any damage of the friction material can degrade the structural performance of structures. In such case, the entire bearing support must be replaced since the friction material cannot be replaced separately, which translates into an increased maintenance cost and an increased life-cycle cost of the structures. In order to revise the problems of PTFE and EP, various studies on friction materials using new materials are being conducted. [PERSON] et al.