The friction of wood

Abstract

Earlier work, especially on metals, has shown that the friction of unlubricated solids is largely due to strong adhesion occurring at the regions of real contact; in addition part of the friction may arise from the ploughing of the softer surface by asperities on the harder. The following paper shows that the friction of wood may be explained in a similar way, that is, it arises from an adhesion term and a deformation or ploughing term. In particular, it is found that the moisture content may have a profound effect-not necessarily a collateral one-on each of these terms. The friction was studied between balsam wood (Abies balsamea) and surfaces of steel and polytetrafluoroethylene (PTFE). The most revealing results are obtained when steel or PTFE spheres are slid or rolled over the wooden surface. The rolling friction is found to be due to internal friction or hysteresis losses in the wood and not to surface interaction. It is essentially the same for both steel and PTFE spheres. As the moisture content is raised from 0 to 30% the rolling friction increases and then remains constant; this is apparently due to changes effected in the mechanical rather than the surface properties of the wood. In contrast, the sliding friction is found to depend both on the moisture content of the wood and on the chemical nature of the other surface. For a hard slider of a given material under specified moisture conditions the sliding friction, F$_{s}$, may be expressed as the sum of two terms F$_{s}$ = F$_{D}$ + F$_{A}$; F$_{D}$ is the tangential force involved in deforming the wood and is directly related to the internal friction or hysteresis losses in the wood itself. It corresponds to the rolling friction. F$_{A}$ is the tangential force required to overcome interfacial adhesion. This is specific to the surfaces. It is high for steel whatever the moisture content of the wood, showing that the adhesion, which is probably due to hydrogen bonding, is strong in both cases. For PTFE the friction is fairly high when the wood has a low moisture content, implying a relatively strong adhesion, but it is very low when the moisture content exceeds 30 to 40%. This emphasizes the importance of surface structure in the adhesion term of friction in contrast to the deformation term which is essentially a bulk phenomenon.