Many technological applications at high temperatures require the use of refractory ceramics that can operate safely in hostile environments, retaining sufficiently their room-temperature engineering properties. Ultra-high temperature ceramics (UHTCs) are eligible for high temperature applications in several industrial sector like foundries or refractory industries. However, leading applications are currently found in aerospace due to the great interest in the development of materials that tolerate very high temperatures, pressures higher than 50 kPa, strong localized stress (mechanical contact and wear), and chemically aggressive environments (corrosive gases). The highly thermal demanding trajectories foreseen for future space plane-like winged re-entry vehicles dictate the need for base materials able to sustain operating temperatures in the interval 1600-2400 °C, theses materials must resist evaporation, erosion, oxidation as well as maintain mechanical integrity in the harsh re-entry environment. Hence, the technical challenges are nowadays centred on the development of high temperature materials that allow the manufacture of light components that are more resistant to oxidation, damage by contact, wear, thermal shock, and creep.The most extensively investigated of ultra-high temperature ceramics is zirconium diboride (ZrB2), probably because it is the lightest and cheapest. There is absence of models of sintering and microstructure evolution prevents the optimization of the process of manufacture of these ceramics. It is remarkable that, as of to date, there has been no systematic investigation of the use of possible additives that would allow obtain fine-grained and dense materials with as low as possible temperature of sintering in order to minimize the grain growth. This would without doubt mean an authentic revolution in the field of processing of ultra-high temperature resistant ceramics. As exposed above, there remains a need for systematic investigations aimed at elucidating the role of the crystal size dependence on the sintering behavior of pure ZrB2 and ZrB2 with amounts of SiC as sintering aid. This type of studies is critical to extract guidelines for the design of dense UHTC materials. With this in mind, the present PhD thesis was undertaken with the objective of analyzing the effect of the high-energy ball milling on sintering behavior of ZrB2-based composites. In particular, the analysis included the sintering mechanism, the content sintering additives, and the nature of the crystallite, plus a first detailed investigation of oxidation for ZrB2 powders induced by high-energy ball-milling in air.