Thermomechanical behaviour of hexagonal boron nitride at elevated temperatures

Abstract

Hexagonal boron nitride (hBN) is attractive for applications at elevated temperatures due to its excellent thermal properties. It is thus of significant importance the knowledge of its thermomechanical behaviour and related structural changes at high temperatures. Herein, we heat up hBN of thickness ranging from one to four layers and monitor its mechanical response with Raman spectroscopy upon cooling to room temperature under constrained conditions. The thermal expansion coefficient (TEC) of hBN is thus estimated and is found to be negative in all cases and to decrease with the increase of flake thickness. The strain induced to the hBN flakes due to the thermal shrinkage is estimated to be in the range of ∼0.065–0.15%. Topographic images taken using atomic force microscopy reveal structural changes such as wrinkling formation as a result of the induced compressive strain upon cooling. The suspended hBN fails almost instantaneously under compressive load. The supported parts sustain higher compressive strain prior to failure and form localized wrinkles with much smaller wavelength as compared to the suspended hBN. The wrinkles formed in the supported section are qualitatively similar to those observed in graphene, reflecting the hexagonal lattice symmetry of these crystals. Finally, knowing the biaxial shift of the Raman G-peak with strain, we can herein calculate the maximum compression strain the supported hBN can support as a function of number of layers (from 1 to 4).