I have created a DEM model and I am now trying to work out the best values for the coefficient of restitution and friction coefficient to use in this.

I wondered if somebody could explain how these two parameters interact if I changed them both?

It’s hard to say much without knowing about your model. COR and friction are properties of the materials involved. COR describes elasticity/inelasticity of collisions (bouncing) and mu describes sliding behavior. You can try changing them and see what effect it has, but the starting values should be related to the materials in your model.

Here are a few literature references that might be of help:

Also:

" A comprehensive study of coefficients of restitution in dependence on material properties (elastic moduli, rheology), direction of impact, coefficient of friction and adhesive properties of impacting bodies can be found in Willert (2020)." This book is open access, but in German: Stoßprobleme in Physik, Technik und Medizin - Grundlagen und Anwendungen | Emanuel Willert | Springer