| Polymer matrix composites have been widely used in aerospace and medical fields due to the excellent specific strength and gradually replaced some traditional metals.Because of the characteristics of low density,high strength,high wear resistance and great thermal stability,Polyetheretherketone(PEEK)has become one of the most excellent matrix materials.Further,the addition of carbon fibers(CFs),especially continuous CFs,can significantly increase the mechanical properties,wear resistance and fatigue resistance of PEEK matrix composites.However,the wettability between the fibers and the matrix deteriorated as the CFs content increased to 50~60 wt.%.It is a huge challenge whether CFs reinforced PEEK matrix composites can overcome the decrease of comprehensive properties with the increase of CFs content.In addition,most of the current research on PEEK matrix composites is based on the room temperature,and there is little discussion on its high-temperature service performance and mechanism.Therefore,under the premise of maintaining a good fiber-matrix interface,it is of great significance and value to further increase the content of CFs,improve the comprehensive properties of the composites,and explore the high-temperature performance and mechanism of the composites for broadening their application fields.In this paper,66 wt.% unidirectional and 60 wt.% bidirectional braided continuous CFs reinforced PEEK matrix composites were prepared by compression molding and their service properties(the mechanical behavior at room and elevated temperature,the friction and wear behavior at elevated temperature,high speed and high pressure,the mechanical and ultrasonic ultrahigh frequency fatigue behavior at room temperature)were systematically studied.Compared with other materials,the two composites show high specific strength,excellent wear resistance and fatigue resistance.In addition,the room/high-temperature enhanced mechanism of mechanical properties,friction and wear resistance,the room-temperature fatigue resistance mechanism and the differences of the composites were also revealed.The main research contents and innovative achievements of this paper are as follows:1.The mechanical(tensile,bending)behaviors of the unidirectional and bidirectional braided CFs reinforced PEEK matrix composites at room temperature and above Tg were comprehensively considered,and their mechanical strengths and fracture morphologies were also analyzed.The high specific strength was highlighted by comparison with other polymers or metal matrix composites,and the mechanical properties enhancement mechanism at room or elevated temperature was revealed.The tensile/flexural strengths of the two composites at room temperature are 1053±23/1047±20 MPa and 820±31/931±14 MPa,respectively,while at 200°C,250°C and 300°C,the values become 810±10/521±12 MPa and 536±16/432±11 MPa,655±17/441±17 MPa and 476±24/327±9 MPa,458±22/ 290±18 MPa and 402±3/272±9 MPa,respectively.The slightly lower strength of bidirectional braided CFs reinforced PEEK matrix composites is attributed to the inherent woven fibers damage.Brittle fracture is the main reason for tensile or bending fracture at room temperature,and the high-quality CFs and great fiber-matrix interface bonding make the composites have higher mechanical strength.While at temperatures above Tg,the matrix softens,the interface separation between different sheets or between CFs and PEEK becomes more obvious with the increase of temperature,and the mechanical strength of the composites decreases.2.The influence of loads,temperatures and speeds on the friction and wear behavior of unidirectional and bidirectional braided CFs reinforced PEEK matrix composites were systematically studied.The coefficient of friction and wear rate of the two composites under various conditions were statistically calculated and compared with other metals or polymer matrix composites containing lubricating particles.Through in-depth analysis of the surface of the worn specimen and grinding discs,the friction and wear resistance enhancement mechanism at room or high temperature were revealed.The coefficient of friction,wear rate and surface damage degree of the two composites decreased with the increase of loads and increased with the increase of temperatures.The increase of speed at 200 °C has no obvious effect on the wear rate of the composites but increased the surface damage degree of the composites.The wear resistance of bidirectional braided CFs reinforced PEEK matrix composites is lower than that of unidirectional CFs reinforced PEEK matrix composites due to the inherent damage of the woven fibers,which makes the surface of composite easier to fall off.The room-temperature wear is dominated by the rolling friction of abrasive particles,which reduces the actual contact area of the composites.The greater the loads,the more uniform the falling composites is broken,and the smaller the damage to the surface of the composites.What's more,the increase of the speeds will accelerate the formation of the transfer film.While at 200 °C,the surface adhesion of the composite caused the exfoliation to increase,the damage continued to increase,and the wear was severe.3.The mechanical fatigue behaviors of unidirectional and bidirectional braided CFs reinforced PEEK matrix composites at R=-1 and 20 Hz were systematically studied,and the S-N curves,the differences in fracture morphologies and the effect of strain on the composites were analyzed.The fatigue resistance mechanism and differences of the two composites were revealed,and the ultrasonic ultrahigh frequency three-point bending fatigue behavior of unidirectional CFs reinforced PEEK matrix composites was further explored.The fatigue limit(10~7)of unidirectional and bidirectional braided CFs reinforced PEEK matrix composites is 360 MPa and 340 MPa,respectively.The former S-N curve is bilinear and has better adaptability to smaller stress amplitudes(below 380 MPa)and tensile strains,the microcracks are easy to initiate at the interfaces between different prepreg sheets and continue to accumulate and expand along the direction parallel or perpendicular to the loading,with interlayer separation as the main failure form;the latter has higher fatigue life when stress is higher(390~440 MPa)or compressive strain dominate,the pre-tightening force at the braided CFs increases the resistance to strain deformation as a residual stress release,but it is easy to generate stress concentration to initiate microcracks and expand,and the fracture mode is mainly fibers fracture.This paper provides an important experimental data and theoretical basis for the development and application of CFs reinforced polymers,especially PEEK matrix composites by analyzing the forming,mechanical behavior,and failure mechanisms of the two composite materials in service environment. |
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