Effects of anisotropy of lignum vitae wood on its tribological performances

The oriented growth causes obvious anisotropy of lignum vitae wood, and results in different mechanical properties under different cell fiber directions, which lead to different wear behaviors. Investigating the influences of lignum vitae's anisotropy on its tribology properties is useful for exhibiting the best tribological performances of the wood or bionic wood composites to extend their service life. In this study, the wear processes obviously presented two stages due to the anisotropy of lignum vitae cell fiber. The cell fibers were first subjected to the separation process, then the impact and bending processes when the friction forces were perpendicular to the cell fiber direction. Owing to the low mechanical property of lignum vitae under the vertical fiber direction, the fibers and vessel cells were easy to be destroyed and broken, and resulted in severe wear process, larger coefficients of friction (COF) and wear rates. The cell fibers experienced tensile stress caused along the cell fiber direction and were difficult to be broken because of good tensile strength, and the surface damages on the lignum vitae wear surface were at a low level. In particular, the COF was stable around 0.05, and presented excellent self-lubricating properties and wear resistance. This study will provide a powerful guide for the tribological design or application of wood materials and bionic wood frictional composites.