Reactive high-power impulse magnetron sputtering was used to deposit HfO2 films on Si substrates using a voltage pulse duration, t1, from 100 to 200 mu s and an deposition-averaged target power density, , from 7.2 to 54 Wcm(-2). The effects of these processing parameters on the microstructure and properties of the films were studied by atomic force microscopy, nano-indentation, X-ray diffraction, electron diffraction and high-resolution transmission electron microscopy. Four HfO2 films were prepared with (1) t(1) = 100 mu s, = 7.2 Wcm(-2) (T100S7), (2) t1 = 200 mu s, = 7.3 Wcm(-2) (T200S7), (3) t(1) = 200 mu s, = 18Wcm(-2) (T200S18) and (4) t(1) = 200 mu s, = 54Wcm(-2) (T200S54). All films were found to be composed of an interlayer next to the Si interface followed by a nano-columnar structure layer. The interlayer structure of the films was found to contain a population of lower density nanoscale regions. A reduction in in films T200S54, T200S18, T200S7 and T100S7 caused an increase in the interlayer thickness and a decrease in the width of the nano-columnar structures from similar to 46 nm to similar to 21 nm. This microstructural change was accompanied by a concomitant change of the grain boundary structure from tight and interlocking in films T200S54 and T200S18, to rough and thicker (similar to 1 nm) boundaries in films T200S7 and T100S7. Films prepared with larger t1 = 200 mu s have a monoclinic HfO2 structure and that with smaller t1 = 100 mu s exhibits a mixture of monoclinic and orthorhombic HfO2. A high hardness of 17.0-17.6 GPa was shown for films with a monoclinic HfO2 structure. The films exhibited a refractive index of 2.02-2.11 and an extinction coefficient between 0.1 x 10(-3) and 1 x 10(-3) (both at a wavelength of 550 nm). High refractive index was achieved for films T200S54 and T200S18 owing to the presence of a dense microstructure with sharp and interlocking grain boundaries. (C) 2016 Elsevier B.V. All rights reserved.