Mechanical properties of pyrolysed wood: a nanoindentation study

Abstract\n The present work is focused on changes of mechanical properties in pyrolysed spruce wood as a function of temperature up to 2400?C. Nanoindentation tests are used for the determination of mechanical properties at the scale of single wood cell walls. Hardness, indentation modulus and elasto-plastic/brittle behaviour of the carbonaceous residues are derived as function of pyrolysis temperature. Hardness values increase continuously by more than one order of magnitude to 4.5 GPa at 700?C. The indentation modulus shows complex changes with a minimum of 5 GPa around 400?C, and a maximum of 40 GPa around 1000?C. The deformation induced by the indenter is largely (visco-) plastic in native wood, but it is almost purely elastic in the carbonaceous residue with particular low values of the indentation ductility index around 700?C. A low density, strongly cross-linked carbon structure may explain the mechanical behaviour at these intermediate temperatures. A final decrease of the modulus and a slight recovery of ductility for temperatures above 2000?C can be attributed to a continuous structural transition of the material towards graphite-like stacking of carbon sheets and to preferred carbon orientation along the wood cell axis.