Evaluation of the kinetic friction performance of modified wood decking products

Sliding friction behavior plays a critical role in diverse systems to determine the serviceability of end-used wood products, such as safety. The kinetic or dynamic coefficient of friction (μk) is used to describe the relative motion between two objects, which is highly dependent on the material characteristics and surface topology. It has been discovered that the relationship between friction force and sliding time is a kind of regular/chaotic ‘stick–slip’ or random motion, i.e. nonlinear. However, studies on the nonlinear friction behavior of modified wood products are very limited. This study was aimed at providing a deep understanding of the kinetic friction behavior of wood decking products, including Chromated Copper Arsenate (CCA) treated spruce lumber (CCAS), heat-treated spruce lumber (HTS), wood plastic composite (WPC) lumber and untreated spruce lumber (UTS) as a control group. The authors extended their previous research on the evaluation of slip resistance of modified wood decking products in terms of static coefficient of friction (μs). The experimental design included four types of specimens (CCAS, HTS, UTS and WPC), two surface conditions (dry and wet), three sliding speeds (10, 50 and 250 mm/min), and two weights of sliding block (14 and 20 kg). The testing set up was in accordance with ASTM D2394 “Standard methods for simulated service testing of wood and wood-base finish flooring”. It was found that (1) ‘stick–slip’ motion only occurred when a 20 kg sliding block moved at a speed of 10 mm/min on the dry surface of CCAS, HTS, and UTS specimens, while under the rest conditions, a random motion was found. However, only the random motion was observed under all conditions for WPC specimens; (2) the average μk values of all types of specimens under wet surface condition were larger than those under dry surface condition. The μk values of WPC specimens were the lowest among four types of specimens, followed by UTS, and then HTS and CCAS; (3) the ratios of μk to μs of CCAS, HTS, and UTS specimens ranged from 0.70 to 0.85, while those of WPC specimens varied between 0.40 and 0.50; and (4) good linear correlations between μk and μs were found in modified wood products tested.

► Irregular stick-slip motion occurred in the dry wood based specimens tested by a heavy slider at a low sliding speed.
► Random motion was found in wood plastic composite (WPC) specimens at all conditions.
► The μk of WPC specimens was lower than that of modified/untreated solid wood specimens.
► The ratio of μs to μk of wood based specimens ranged from 0.70 to 0.85 but that of WPC specimens ranged from 0.40 to 0.50.