Mechanical properties of pyrolysed wood:: a nanoindentation study

The present work focuses on changes of mechanical properties in pyrolysed spruce wood as a function of temperature up to 2400 degrees 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 degrees C. The indentation modulus shows complex changes with a minimum of 5 GPa around 400 degrees C and a maximum of 40 GPa around 1000 degrees 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 degrees C. A low density and a strongly cross-linked carbon structure may explain the mechanical behaviour at these intermediate temperatures. A final decrease of the modulus and a slight decrease of ductility for temperatures above 2000 degrees 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.