Functionally graded tungsten–epoxy particulate composites to control wave propagation in adhesive interlayers / matching layers

Many high-value sectors must control acoustic/shock waves through a material, component, or assembly. These waves often require transmission across dissimilar materials through a bonding ‘interlayer’ or ‘matching layer’. Epoxies are commonly used for this purpose, but they are ill-suited with low acoustic impedance. Particulate composites can improve many bulk material properties compared to pure epoxies, including their acoustic impedance, and can be functionally graded to alter properties through the composite thickness. In this study, tungsten–epoxy particulate composites with various tungsten volume fractions (≤35%) were fabricated under either atmospheric or autoclave conditions and subsequently characterised to assess their potential as interlayer materials for wave propagation control. A novel approach to functional grading through gravitational segregation was also explored, with success in lower tungsten volume fractions (≤10% and ≤20% for samples formed under atmospheric or autoclave conditions, respectively). Porosity, density, longitudinal sound speed, acoustic impedance, attenuation, and adhesive bond strength were measured, finding that autoclave samples offer reduced porosity and better functional grading control. Various desirable material property values/ranges were produced through controlling different processing parameters and composite compositions. This suggests particulate composites could be used to optimise desirable properties in bonding interlayers that require wave propagation control.