A nanoindentation induced blistering method has been used to extract the GaN/diamond interfacial toughness (adhesion energy) from four types of GaN-on-diamond samples with varying SiN_x interlayer thicknesses. The mode I energy release rate (G_IC) was quantified and is presented. Additionally, transient thermoreflectance has been used to measure the thermal boundary resistance (TBR) between the GaN and the diamond substrate. It was found that a thin SiN_x interlayer resulted in a lower TBR (15 m² K GW^-1) whilst maintaining a reasonable interfacial toughness (1.4±0.5 J m^-2). For interlayers of a similar thickness, samples with a high interfacial toughness and high residual stresses in the GaN had a smaller TBR. This indicates that the intrinsic interfacial characteristics that enhanced the interfacial toughness could be beneficial in improving the TBR.