| Extreme marine environment has very strict requirements on the performances of protective coatings on engineering equipment.It is required that the coating materials not only have good tribological properties,but also have excellent corrosion resistance in seawater medium.But unfortunately,the existing materials can not fully meet these requirements.The new type high entropy alloy coatings show excellent performances of wear resistance,corrosion resistance and corrosive wear resistance.Therefore,it is of great significance to develop high entropy alloy coatings for ocean engineering.This thesis chooses AlCrFeNiW-(Cu,Nb,Ti)as the alloy system,using similar element substitution and empirical parameters(atomic radius difference,Ω value,valence electron concentration,etc.)methods to optimize and design both excellent tribological properties and corrosion resistance high entropy alloy coating compositions.The laser cladding technology is used to prepare the designed coatings on the surface of low-cost structural steel materials(304 stainless steel and Q235 steel).The microstructures,wear resistance,corrosion resistance and corrosive wear resistance of the coatings in seawater are systematically analyzed by a variety of testing methods to reveal the wear,corrosion and corrosive wear mechanisms.(1)Based on the advantages of FCC,BCC and its ordered phases in corrosion resistance and wear resistance of high entropy alloy coating,using the similar element substitution method,the series of AlCrFe2Ni2W0.2Cux high entropy alloy coatings are designed and prepared.It is found that with the increase of Cu content,the volume fraction of soft FCC phase first decreases and then increases,resulting in the average hardness of coatings first increases and then decreases.Especially when x=0.25,the coating hardness reaches the highest value of 583 HV under the combined action of fine grain strengthening,solid solution strengthening and second phase strengthening.In accordance with the Archard law,the dry friction and wear properties of the coatings first increase and then decrease with the increase of Cu content,especially the wear rate of AlCrFe2Ni2W0.2Cu0.25 coating is 6×10-5 mm3/(N·m),its wear resistance is relatively better,and the wear mechanism is mainly the combined action of slight abrasive wear and oxidation wear.In addition,due to the transformation of the existing mode of Cu element in the coating from solid solution to segregation,the galvanic corrosion of the coating first being weakened and then being strengthened,and the corrosion resistance shows a trend of first being enhanced and then being weakened,that is,when x=0.25,the corrosion current density of the coating is the lowest(8.6 μA/cm2),the corrosion resistance is the best.Moreover,with the effective combination of the anti-wear property and"protective film",the sliding wear rate of AlCrFe2Ni2W0.2Cu0.25 coating in seawater environment is the lowest(6.65×10-7 mm3/(N·m)),and the corrosive wear resistance is the best.(2)In order to solve the problem of the deterioration of tribological properties caused by too much FCC phase in the Cu containing coating system,the design idea of replacing FCC phase with intermetallic compounds with excellent wear and corrosion resistance,and meanwhile retaining BCC phase is proposed in this paper.Thus the AlCrFeNi2W0.2Nbx high entropy alloy coatings with BCC and Laves phases was successfully obtained.The results show that with the increase of Nb content,the microstructures of coatings change from hypoeutectic to fully eutectic,and then to hypereutectic.The content of hard Laves phase increases gradually with the increase of Nb content,which leads to the increase of coatings hardness.Especially when x=2.0,the coating hardness reaches the highest(891 HV),which is the result of solution strengthening and dispersion strengthening.However,the existence of excessive Laves phase can easily lead to brittle fracture of the coating during sliding,thus the dry friction and wear properties of the coating first increase and then decrease slightly with the increase of Nb content.Therefore,when x=1.5,the dry friction and wear properties of the coating are the best,the wear rate is in the order of 10-6 mm3/(N·m),and the wear mechanism is mainly oxidation wear.And due to the increase of Laves phase content,the corrosion resistance of the coating also shows an increasing trend,that is,the corrosion resistance of AlCrFeNi2W0.2Nb2.0 coating is the most outstanding(corrosion current density is 5.2 μA/cm2).In addition,with the increase of Nb content,the wear resistance of the coating is improved,accompanied by the increase of brittleness,which leads to the trend that the wear resistance of the coating in seawater first increases and then decreases.Especially when x=0.75,the coating does not suffer from brittle fracture,the dense friction "protective film"formed on the surface of the coating effectively weakens the effect of friction and wear,wear rate is the smallest(8.8×10-7 mm3/(N·m)),showing excellent corrosive wear resistance.(3)In view of the phenomenon of "brittle fracture" in the sliding process caused by Laves phase in Nb containing coatings,the AlCrFeNiW0.2Tix high entropy alloy coatings with BCC and ordered B2 phase and excellent comprehensive properties are designed and prepared by replacing Laves phase with ordered B2 phase in this paper.It is found that with the addition of Ti,the coatings exhibit a typical "sunflower like" microstructure,accompanied by the precipitation of nano spherical particles in the "stamen" and "petal" areas.Thus,the hardness of AlCrFeNiW0.2Tix coatings increases gradually under the combined action of fine grain strengthening,second phase strengthening and solid solution strengthening,that is,when x=0.5,the hardness of the coating reaches the highest(693 HV).Accordingly,the dry friction and wear properties of the coatings increase with the increase of Ti content,especially for the AlCrFeNiW0.2Ti0.5 coating,the wear rate is(1.3×10-5 mm3/(N m))is the lowest,and the wear resistance is the best,which shows the main wear mechanism of oxidation wear.Moreover,Ti element also plays a positive role in improving the corrosion resistance and corrosive wear resistance of the coatings in seawater.Especially when x=0.5,the corrosion current density is 1.04 μA/cm2,the corrosion resistance of the coating is the best,and the corrosion mechanism is mainly slight pitting.In addition to the friction reducing effect of"protective film" of surface friction,the contact stress between AlCrFeNiW0.2Ti0.5 coating and coupled pair is also greatly reduced under the "polishing effect",resulting in a low wear rate of 2.3 ×10-7 mm3/N·m,showing excellent corrosive wear resistance. |
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