Effect of diamond surface orientation on the thermal boundary conductance between diamond and aluminum
 and  oriented diamond substrates were treated using Ar:H and Ar:O plasma treatments, and 1:1 HNO3:H2SO4 heated at 200 ̊C. Subsequent to these treatments, an aluminum layer was either evaporated or sputte- red on the substrates. The Thermal Boundary Conductance (TBC) as well as the interfacial acoustical reflection coefficient between this layer and the diamond substrate was then measured using a Time Domain ThermoReflec- tance (TDTR) experiment. For the Ar:H plasma treated surfaces the  oriented faces exhibited conductances 40 % lower than the  oriented ones, with the lowest measured TBC at 32±5 MWm−2K−1. The treatments that led to oxygen-terminated diamond surfaces (i.e. acid or Ar:O plasma treatments) showed no TBC anisotropy and the highest measured value was 230±25 MWm−2K−1 for samples treated with Ar:O plasma with a sputtered Al layer on top. Sputtered layers on oxygen-terminated surfaces showed sys- tematically higher TBC than their evaporated counterparts. The interfacial acoustic reflection coefficient correlated qualitatively with TBC when com- paring samples with the same type of surface terminations (O or H), but this correlation failed when comparing H and O terminated interfaces with each other.