# Experimental Study on Properties of Ultrasonic Coupling Agent with Graphene in NDT
## 1 Introduction  
In recent years, ultrasonic nondestructive testing technology has been widely used in industrial testing, and has become an important means in the field of nondestructive testing. In ultrasonic testing, the selection of a coupling agent is very important [1]. These coupling agents improve detection accuracy in three main ways [2]. First, when the ultrasonic coupler is filled between the surface of the specimen and the surface in contact with the probe, the ultrasonic coupler removes air and fills uneven pits and gaps so that the transducer is in close contact with the surface of the specimen. Secondly, the ultrasonic coupling agent can reduce the acoustic impedance difference between the surface of the ultrasonic probe and the surface of the specimen, and reduce the ultrasonic energy reflection loss. Therefore, the energy transmission loss between the probe and the specimen is small. Third, the ultrasonic coupling agent as a \"lubricant\" can reduce the friction between the surface of the ultrasonic probe and the specimen, so that the ultrasonic probe flexible slides along the surface. Therefore, well-performing coupling agents can ensure that the ultrasonic wave propagates in the measured part more effectively, and enough sound intensity transmittance is obtained, so as to achieve the purpose of optimizing the detection. The coupling effect is also influenced by the properties of coupling agent materials. [PERSON] et al. [3] used a thin foil of annealed silver as a filler material/coupling agent at each component interface. Preliminary high temperature tests indicated that this coupling method was suitable for high temperature and achieved signal quality comparable to that at room temperature with ultrasonic gel. [PERSON] et al. [4] compared the coupling effects of several commonly used coupling agents, such as glycerin, petroleum jelly, and 'Kendon Laboratories CC' Ultra-Gel. [PERSON] and [PERSON] [5] argued that liquid coupling (grease, epoxy resin and water) was better than that of metal foil (aluminum), and analyzed the effect of coupling agent on ultrasonic propagation of longitudinal wave and shear wave. [PERSON] [6] analyzed the wave velocity and attenuation as functions of stress and coupling agent. Differences were more notable on attenuations, which were equally evaluated in time and frequency domains. Lead foil, grease, and honey scored the best among metal foils and visco-liquid compounds. [PERSON] [7] raised the selection of the best combination wedge/coupling, performed by searching separately for the best wedge material and the best coupling material. Three wedge materials and five coupling agents were tested. Among the three tested wedge materials, Teflon was found to be the best. The investigation on the coupling agent showed that the gel type materials were the best solutions. The 'thick' materials displaying higher viscosity were found as the worst. [PERSON] et al. [2] proposed a new method to improve the sound transmission performance of lipid-like coupling agent to form a new grease-like coupling agent by combining different metal ions such as Li\({}^{+}\), Na\({}^{+}\), and Ca\({}^{2+}\) with base grease. The experimental results showed that compared to other oil-based coupling agents, except for slightly delayed peak response time, lithium-based coupling agents had higher energy transfer efficiency of pulse signal and better pulse vibration waveform, which could improve energy transmission performance and greatly improve the sensitivity of ultrasonic flaw detection. Graphene is a single carbon layer consisting of sp\({}^{2}\) hybrid carbon atoms, and its thickness is the diameter of a single carbon atom. Carbons atoms are connected by \(\sigma\) and \(\pi\) bonds [8]. Due to its unique structure, graphene has excellent mechanical and electrical properties [9; 10]. As a two-dimensional carbon nanomaterial with a dense single-atom structure, graphene has attracted extensive attention due to its unique properties [11; 12].