| Carbon fiber-reinforced aluminum matrix composites have very good application prospects in terms of lightweighting.However,carbon fibers are easily oxidized in a high-temperature oxygen-containing environment.The strong interfacial reactions with the matrix result in deterioration of their performance,which limits their use in carbon fiber-reinforced composites.Therefore,it is necessary to prepare a coating on the surface of carbon fibers to reduce the erosion of oxygen on carbon fibers and weaken the interfacial reaction between carbon fibers and the matrix.When composited with a metal matrix,a metal coating is often prepared on the surface of the carbon fiber.But the anti-oxidation performance of metal coating is inferior to ceramic coating’s.In this paper,a Si3N4protective coating which has high-temperature anti-oxidation performance is formed by constructing a polyacrylonitrile（PAN）layer and an organopolysilazane（OPSZ）layer on the surface of carbon fibers with curing at low-temperature and cracking at high-temperature.The effects of heat treatment temperature,concentration of PAN and OPSZ layer on the oxidation resistance and monofilament tensile properties of carbon fiber were studied.The structure of OPSZ transformed ceramics was characterized,and the law of its microstructure and oxidation resistance with heat treatment temperature was obtained.The main research content and results are as follows:（1）The composition change of OPSZ converted ceramics was studied.The ceramic is amorphous at 1200℃,and gradually changes from amorphous to crystalline at 1450℃.According to XRD analysis,the crystalline part isα-Si3N4.The crystalline part of the ceramic product at1600℃are Si3N4and Si C.（2）The effects of heat treatment temperature on the carbon fibers were studied.The higher the heat treatment temperature,the greater the decline in the tensile strength of carbon fibers.The tensile properties of uncoated carbon fibers after heat treatment at 1200℃,1450℃and1600℃decreased by 39.3%,44.1%and 53.8%compared with raw carbon fiber.The oxidation resistance of coated carbon fiber will increase with the increase of heat treatment temperature,from 1200℃to 1600℃,the weight loss rate of carbon fiber will drop from 42.3%to 7.18%,and the coated carbon fiber after heat treatment at 1450℃and 1600℃The difference in weight loss rate is only 1%.（3）The properties of the PAN layer on the carbon fiber was studied.It seems that the tensile strength of carbon fiber monofilament coated with only 3%OPSZ loses 14%compared with the uncoated carbon fiber at the same temperature,while the tensile strength of carbon fiber monofilament coated with 3%PAN-3%OPSZ only loses 2.08%.The interfacial bonding force of carbon fiber coated with OPSZ coating is weak,while PAN can promote the adhesion of OPSZ on the surface of carbon fiber.Increasing the concentration of PAN layer will increase the relative content of Si on the surface of carbon fiber.PAN can also effectively delay the oxidation weight loss of carbon fiber,which will reduce the maximum weight loss rate of coated carbon fibers.When the PAN concentration>3%,the coating is too thick and easy to crack,which will lead to the decline of mechanical properties.（4）The properties of OPSZ layer on the carbon fiber was studied.The monofilament tensile strength of coated carbon fiber will decrease with the increase of OPSZ concentration,and the oxidation resistance at 900℃will show a trend of first increase and then decrease with the increase of OPSZ concentration.However,the oxidation resistance performance of carbon fiber at 700℃with different concentrations of OPSZ is basically the same.The tensile strength of PAN-OPSZ/carbon fiber prepared with PAN and OPSZ concentration of 3%（wt%）was only 2.08%lower than that of uncoated carbon fiber,indicating that it did not cause too much damage of carbon fiber.The oxidation resistance of the PAN-OPSZ/carbon fibers was significantly improved,with a weight loss of less than 8%in an air atmosphere at 700℃,while that of uncoated carbon fibers was as high as 70%.After oxidizing for 30mins,most of the mechanical properties of3%PAN-3%OPSZ/CF were still maintained,and the tensile strength of single filaments only decreased by 14.83%.The above results show that the PAN-OPSZ coating can effectively improve the high temperature oxidation resistance of carbon fibers,and has a very broad application prospect in the field of reinforced aluminum alloy matrix composites. |
