# Experimental Wear Characterization and Durability Enhancement of an Aeronautic Braking System
## 1 Introduction  
Electromagnetic brakes are commonly employed in many industrial, medical, aeronautical, and military applications and even in robotics [1]. In particular, aeronautic electromagnetic brakes are key components of the landing gear of aircraft, which is considered one of the most critical systems for flight safety [2], as almost 60% of failures occur during landing and take-off operations [3]. Catastrophic malfunctions can occur in landing gear and its components as they are subjected to high inertial, impulsive, and cyclic loads [4], and for this reason, the dynamic characterization, design, testing, and maintenance of aeronautic components and braking systems are crucial operations to guarantee flight safety [5; 6; 7]. Aeronautic brakes have various functions, such as, first of all, the role of generating the required stopping torque under various service conditions through proper friction pairs, but they also serve as heat sinks, maintaining a low temperature by absorbing kinetic energy, and as structural elements to transfer the torque to the fires [8]. Designing electromagnetic braking systems requires a multidisciplinary approach, as the mechanical aspects, including tribological issues, must be combined with electromagnetic [9], structural, and thermal issues [10; 11], but the literature includes few studiesfocused on their overall behavior and design issues [12]. In addition, most of the tribological studies about brakes have focused on braking systems that are not for aeronautical applications. Some works have aimed at analyzing the squealing of brakes, with the aim of reducing this effect by using theoretical and experimental approaches [13; 14; 15]. Some other works have specifically aimed at studying the contact mechanics and wear behavior of the brakes [16; 17; 18]. From a tribological point of view, electromagnetic brakes for aircraft must be developed with high-efficiency friction materials, which must meet several requirements to ensure stability and wear resistance, such as high strength, high thermal conductivity, low thermal expansion, and high thermal stability [19]. For this reason, composite materials are the most widely used friction materials because they guarantee high mechanical properties due to their multicomponent structure. In fact, the characteristics of the individual ingredients of composite materials, such as binders, fibers, reinforcements, and fillers, influence the mechanical and tribological properties of the friction interfaces, so researchers have tried various formulations to test their impact on the tribological performance of brakes [20]. For electromagnetic brakes, the most widely used friction pads are metallic or semi-metallic, non-asbestos organic (NAO), Metal Matrix Composite (MMC), and ceramic or carbon-carbon [21]. Non-asbestos organic (NAO) materials are widely used as friction materials because of their low cost and environmentally friendly nature. Although numerical simulations can greatly contribute to the study of the performance of a brake in terms of efficiency and stability [22; 23], the wear resistance and efficiency of the braking torque of an electromagnetic brake are two crucial aspects to investigate, mostly through experimental characterization during the design phase [24; 25; 26], as different friction pairs can be tested with different brake architectures. This experimental approach is often employed in tribological applications regarding wear resistance optimization [27; 28]. The work presented in [24] demonstrated the improved functionality and performance of an electromechanical brake for the main rotor of a helicopter, proposing an optimized design. The brake was hydraulically actuated and was arranged as a disk brake and clasping arm. A wear model was used and matched with experimental results to choose the right materials and design configuration to obtain the required specifications.