In this paper a new micromechanical formulation is proposed for modelling thermoelastic intergranular and transgranular damage and microcracking evolution in brittle polycrystalline materials. Polycrystalline microstructures are created through a Voronoi tessellation algorithm. Each crystal has an elastic orthotropic behaviour. Damage evolution along (inter- or trans-granular) interfaces is modelled using thermo-mechanical cohesive laws and, upon failure, non-linear frictional contact analysis is introduced to model separation, stick or slip. Numerical simulations are presented either to demonstrate the validity and study the physical implications of the proposed thermoelastic formulation, in comparison with other numerical methods as well as experimental observations and literature results.