Study On Plasma Nitriding And Erosion Mechanism Of Marine TC4 Titanium Alloy

With the development of the country’s great marine engineering strategy,research on materials for marine transportation engineering has attracted much attention.Titanium alloys,known as the"ocean metal",have been widely used in structural components of marine transportation equipment due to its high specific strength,good weldability,and excellent corrosion resistance.Ti-6Al-4V（TC4）titanium alloy is mainly used in pressure hulls,pressure equalizing containers,and shell skeletons of marine transportation equipment.Nevertheless,their use in practical applications is limited due to their poor tribological properties.The application of nitriding technologies to titanium and titanium alloys has been applied to the Ti–6Al–4V alloy.Plasma nitriding（PN）has been proven to be one of the most effective methods that improved the tribological properties of titanium and its alloy,by which a hard compound layer with excellent wear resistance can be produced on the top of the matrix.However,the temperature of plasma nitriding technology is generally from 600℃to 1000℃.Treatment at high temperature for long time will lead to a series of problems:firstly,the high temperature and the long duration would give rise to the compound layer growing thicker;secondly,the grain growth of titanium alloy matrix is caused by too high temperature,which causes the decrease of the strength of matrix and fatigue resistance.The former promotes the premature initiation of cracks from the surface,while the latter reduces the ability of the matrix to resist crack propagation.Although the surface properties of the specimen are improved,the properties of the substrate are significantly degraded,resulting the decrease of final comprehensive properties.Therefore,lowering plasma nitriding temperature is of importance and has been a huge challenge.In this study,a pretreatment was conducted to modifiy the plasma nitriding of TC4 alloy under lower temperature.The material used in the present investigation was Ti-6Al-4V sheet.FMRR was applied to fabricate a nanostructured layer in the surface of Ti-6Al-4V alloy.And then the FMRR samples were plasma nitrided at a lower temperature.A growth model of nitrided layer has been established based on Fick’s Law and Brakman research.The effect of FMRR treatment on the plasma nitriding of TC4 alloy under low temperature was also discussed.HRTEM,TEM,SEM and XRD were utilized to characterize the microstructure of the top surface of FMRR sample and the nitrided layer.（1）The results indicated that the grains in the top surface layer of substract were refined due to the severe plastic deformation which was induced by FMRR treatment.The thickness of the severe plastic deformation was 30-60um approximately.High density dislocations,twins,and low angle grain boundaries were observed in the top surface layer of FMRR sample.A nanostructured layer was obtained in the top surface layer with the average grain size of 65～90nm,45～70nm and 20～40nm,when the duration was 15 min,30 min and 45 min respectively.No new phase was induced in the top surface of Ti-6Al-4V by FMRR treatment.The diffraction peaks of samples were broader compared with the original samples.（2）The results indicated that the plasma nitriding of TC4 alloy at low temperature was improved by FMRR treatment.The refinement of grains,creation of large amounts of grain boundaries,high density dislocations,twins,and low angle grain boundaries observed in the top surface layer of FMRR sample provided additional energy and structures for the subsequent nitriding at lower temperature.The nitrided layer in the top surface of FMRR sample after plasma nitriding included a bright compound layer and a transition layer.The average thickness of nitrided layer increased with the duration of FMRR treatment,which was 19.8μm,26.0μm and 33.3μm,increased by 1.77,2.32 and 2.97 times compared with the original samples when nitrided at 480℃;27.9μm,33.1μm and 37.3μm,increased by1.67,1.97 and 2.20 times compared with the original samples when nitrided at 500℃;40.3μm,86.8μm and 102.1μm,increased by 2.00,4.32 and 5.08 times in comparison with the original samples when nitrided at 550℃.The nitrided layer involved Ti N、Ti₂N and Ti₄N_3-X compounds and strengthed the substrate significantly.Average size of spherical particles in the top surface layer decreased with the duration of FMRR treatment,which was 6.4μm,4.5μm,6.2μm and2.3μm at 480℃,2.1μm,1.8μm,1.7μm and 1.5μm at 500℃,2.2μm,1.9μm,1.7μm and1.6μm at 550℃,when the duration was 0min,15 min,30 min and 45 min respectively.（3）The results of electrochemical test indicated that a thicker and more continuous nitrided layer was obtained by plasma nitriding assisted by FMRR treatment on the surface of TC4 titanium alloy,which prevent the breakdown of TC4 titanium alloy due to a thinner nitrided layer on the surface when it is used in an environment containing a high concentration of Cl^-.The mechanism of the nitrided layer on the surface of TC4 was found to be pitting corrosion type.The corrosion resistance of TC4 alloy was improved.The highest₍（8）-0.164V and the lowest I₍（8）1.241×10^-7A/cm² were measured in the nitrided layer when the samples were FMRR treated for 30min and nitrided at 480℃.The highest₍（8）and the lowest I₍（8）were measured in the nitrided layer when the samples were FMRR treated for 45min and nitrided at 500℃and 550℃.（4）The results of friction and wear tests indicated that the tribological properties of TC4 alloy was improved by FMRR treatment.The friction coefficient of samples decreased with the increase of duration of FMRR treatment when nitrided at the same temperature.The friction coefficient of samples first increased with the increase of sliding time and then decreased with the increase of sliding time and finally stabilized.The friction coefficient and wear loss of samples increased as the load increase.The worn surface of the nitrided layer of FMRR samples for 45min was characterized by step morphology and spalling of the nitrided layer.And the worn surface was relatively smooth with some evidence of ploughing.It was expected that the grain-abrasion and adhesion-abrasion were the main wear mechanisms.