Constructing highly aligned crystalline structure to enhance sliding wear performance of bulk polyamide 6

High performance wear-resistant polyamide 6 (PA6) not only needs low friction and wear, but also requires high thermal conductivity and outstanding mechanical strength. Introducing lubricants into polymers has proved to be a feasible method, however, the contribution of intrinsic structure of PA6 to tribological properties is often ignored. Herein, a novel approach to fabricate high performance wear-resistant bulk PA6 via constructing a highly aligned crystalline structure was proposed. Firstly, an intense flow field applied by force assembly elements (FAEs) was introduced into extrusion processing to prepare PA6 precursor sheets with initially aligned molecular chains. Next, the precursor sheets were uniaxially drawn at 120 degrees C to achieve highly aligned crystalline structure at low draw ratio. The X-ray diffraction results showed the crystal orientation of the precursor sheets is 42% higher than that of samples prepared without FAEs. Furthermore, tensile stress induced gamma-alpha phase transition and improved the crystallinity. As a result, the friction coefficient and wear rate of the bulk PA6 decreased 50.8% and 64.9%, respectively. Note that the maximum contact surface temperature reduced from 109.8 to 64.8 degrees C during high-speed friction because of the increasement of the thermal conductivity, effectively avoiding the aggravation of wear caused by surface softening. Moreover, the bulk PA6 also has excellent mechanical properties with a yield strength of 242 MPa and an elongation at break of 90%. Such remarkable enhancements in both mechanical and tribological properties are primarily attributed to the highly aligned crystalline structure with high alpha crystal form content. This work provides a way to design mechanically robust, wear-resistant polyamide applied in industry, transportation, etc.