The ability to predict iron ore sinter metallurgical performance (eg TI, RDI and RI) is a key aim of microscopic sinter characterisation. This is because the physical properties of sinter, eg porosity, permeability and strength, are of critical importance if it is to perform optimally during iron making. Microindentation techniques are one established and ready means of characterising an individual sinter phases microhardness during microscopic sinter characterisation carried out to determine the modal abundance of sinter phases. Microhardness measurements therefore have the potential to improve understanding and prediction of sinter performance, especially when combined with phase abundance data.Microindentation testing was undertaken on a wide range of sinter phases from seven pot grate sinter samples using Vickers and Knoop indenters to determine the range and mean of sinter phase (eg SFCA) and morphology (eg prismatic or platy) microhardness. From these measurements, the mean fracture toughness of each sinter phase and phase type was calculated. A value for the composite microhardness (CH) and composite fracture toughness (CFT) was then calculated for each sinter sample, based upon the modal proportion of each sinter phase as determined by manual point counting.The composite microhardness and composite fracture toughness of each sinter sample, as well as for individual sinter phases (eg magnetite), was then compared to a range of metallurgical indices and major element chemistry for the same sinter samples to determine the extent and nature of any correlation. Moderate to strong negative correlations between sinter CH and sinter RDI (R2 = 0.81), RI (R2 = 0.76), TI (R2 = 0.75) and between sinter CFT and RDI (R2 = 0.53), RI (R2 = 0.77) and sinter TI (R2 = 0.62) were observed. This shows that trends in sinter microhardness can reflect trends in sinter macroscopic properties and therefore that the microindentation technique has the potential to be further developed as a practical tool to characterise and predict the metallurgical performance of sinter. CITATION:Peterson, M J, 2019. Quantifying the composite microhardness and fracture toughness of iron ore sinter and their relationship to sinter metallurgical indices, in Proceedings Iron Ore 2019, pp 401419 (The Australasian Institute of Mining and Metallurgy: Melbourne).