Fe- or Mn-powders were mixed with the diamonds and the mixtures were heat treated under different gas atmospheres like hydrogen or argon gas and at varying temperatures in order to roughen the surface of the diamond particles. Subsequently the reaction layers are removed by the addition of aqueous solutions of HCl. The modified surface structure after the etching process is investigated by electron microscopy showing increased roughening of the formerly flat faces of the diamond particles with increasing heat treatment temperature. After drying the diamonds, composites were prepared by gas pressure infiltration with pure Al. For treatments in the temperature range from 750 to 850°C the thermal conductivity can be improved by up to 20 percent compared to composites based on un-treated powders.