Densified wood was fabricated by the hot-pressing densification method. The microstructure and chemical composition of densified wood were characterized by scanning electron microscopy (SEM) and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy respectively. The influence of densification modification on the mechanical properties of wood was analyzed by characterizing the the density, surface hardness and mechanical properties of densified wood. The tribological properties of densified wood were investigated by the friction and wear tests which were carried out under different normal loads and rotational speeds. Then the wear area was characterized and analyzed to explore the friction and wear mechanism, and also compared with the natural wood.
SEM results showed that the natural wood in the untreated stage had a round and neatly arranged tracheid. However, the densified wood had completely collapsed wood cell wall and was closely interwoven in cross section after pretreatment and hot compaction stage. The ATR-FTIR spectroscopy showed that the characteristic peaks of hemicellulose and lignin in densified wood was changed obviously as compared with the infrared spectra of natural wood. SEM results also showed that the microstructure and cell wall chemical composition of natural wood were significantly changed by densification modification. The density, hardness and mechanical properties of natural wood and densified wood were characterized. The density and hardness of densified wood were 2.3 and 2.5 times those of natural wood respectively, and the mechanical strength of densified wood was 1.63 times that of natural wood. The densification modification has significantly improved the mechanical properties of natural wood.
As the normal load increased from 3 N to 15 N, and the average friction coefficient of densified wood increased from 0.163 to 0.242. The average friction coefficient of densified wood was 87.5% of that of natural wood as the load was greater than 3 N. The wear volume and volumetric wear rate of densified wood both increased with the increase of normal load. The wear volume was increased from 8.94×103 μm3 to 2.42×105 μm3 and the volumetric wear rate was increased from 2.97×10−4 mm3/(N·m) to 1.43×10−3 mm3/(N·m). At the normal load of 3 N, the difference of wear volume and volumetric wear rate between densified wood and natural wood was not obvious. With the increase of normal load, the wear volume and volumetric wear rate of densified wood were decreased significantly, which was about 50% of that of natural wood. According to the analysis of the wear area, the plastic deformation of the wear area was intensified with the increase of normal load, combined with adhesive wear were the two dominant wear mechanism.
As the rotational speed was increased from 30 r/min to 300 r/min, the average friction coefficient of densified wood was increased from 0.164 to 0.315. At low rotational speed, the friction coefficient changed gently over time. With the rotational speed increasing, the friction coefficient increased with time, which was more obvious with the larger rotational speed. At the same conditions, the average friction coefficient of densified wood was lower than that of natural wood with consistent variation trend. The wear volume of dense wood was increased from 5.0×104 μm3 to 1.57×105 μm3 with the increase of rotational speed. However, the volumetric wear rate was decreased from 1.47×10−3 mm3/(N·m) to 4.63×10−4 mm3/(N·m). At high rotational speed, the wear volume and volumetric wear rate of densified wood were only 50% of those of natural wood. As the rotational speed increased, scratches and scraps appeared in the wear area, indicating the abrasive wear was the main wear mechanism. Compared with natural wood, the wear width and depth of densified wood were smaller than those of natural wood. This work could provide a reference for the application of densified wood in many areas.
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