*2.2. Ultrasonic Speed of Sound Measurement Methodology for Thin Coating LEP Systems*

Ultrasonic testing was undertaken with a Dolphitec ultrasonic system [35] using a pulse echo mode (ISO 16810 [32], and ISO 16811 [33]). This technique is based on analyzing the propagation of ultrasonic wave through the tested material. At each interface of the material, there is a spike in the ultrasonic response. This allows for the measuring the speed of sound through the material by finding the distance between the front-wall echo (spike response of the front face) and the back-wall echo and matching this to the material's actual physical measured thickness.

Ultrasonic scanning was employed to determine the acoustic impedance of neat LEP coating and filler materials. The acoustic impedance, *Z* can be calculated by:

$$Z = \rho \mathbb{C} \tag{1}$$

where, ρ is the materials density and *c* is the speed of sound. These measurements were captured using both single crystal 2.5 and 5 MHz probes. This allowed for the measurement of material impedance at varying probe frequencies, providing information on viscoelastic response of the selected materials.
