Structure and mechanical/abrasive wear behavior of a purely natural composite: black-fiber palm wood

Abstract

After a short introduction to wood as a potential material for applications in which friction and wear are critical issues, the paper is focused on the special structure of black-fiber palm wood which resembles that of a quasi-unidirectional fiber composite. Further details of the wood’s morphology were investigated by a computer micro-tomography system, giving evidence of the porous, closed-cell foam structure of the ground tissue (“matrix”) in which the individual “black fibers” (which are actually vascular bundles (VB) consisting of longitudinal cells and vessels) are embedded. Thermogravimetric studies showed that the wood was temperature stable up to 272 °C above which a two-step thermal degradation took place. The mechanical properties of the palm wood were characterized by macro- and micro-hardness measurements and by compression tests in various directions. In both cases, the degree of deformation and the resulting hardness and compressive strength values were dependent on the fiber volume fraction and the fiber orientation relative to the loading direction. The fibers, respectively, the vascular bundles, were the load-bearing components in this natural composite system. Micro-hardness tests with a Vickers indenter on individual cells of the VBs resulted in more than six times higher values than the macro-hardness values of palm wood given in the literature. Wear tests against 160-µm-grain-sized SiC paper led to a higher wear rate when the fibers were in the contact plane (parallel or antiparallel), whereas the samples with fiber orientation normal to the contact plane were more abrasion resistant. Correlations showed that the specific abrasive wear rate was lower for higher values of compression strength and macro-hardness.