The effects of process parameters on the microstructural morphology of aluminium oxide (Al2O3) coatings on Ir have been studied. Al2O3 coatings were deposited on Ir-coated isotropic graphite (IG) substrates at substrate temperatures of room temperature (RT)-1073 K, RF power of 200-600 W in an Ar, or Ar + 1-10% O-2, sputtering gas atmosphere by RF magnetron sputtering. AI,O, coatings which were deposited at high substrate temperatures and high RF powers in an Ar, or an Ar + O-2, sputtering gas atmosphere were found to contain a dense, fine columnar structure with a gamma-Al2O3 phase, low Ar content and a relatively high hardness value of ca. 1050 H-v. Furthermore, high resolution transmission electron microscopy (HRTEM) results revealed the epitaxial growth of Al2O3 coatings on Ir-coated IG substrate. It was found that the interface between Al2O3 and Ir coatings was sharp and Al2O3 coatings remained intact with the Ir-coated IG substrate.