Ni含量对NiTi合金涂层组织及摩擦学性能的影响OA

To improve the wear resistance of TC4 titanium alloy surface,different NiTi coatings with varying nickel contents(60wt.%,63wt.%,65wt.%,67wt.%,and 70wt.%)were fabricated on TC4 substrate by plasma arc cladding.The microstructure,phase constituent,and hardness of the coatings were characterized by scanning electron microscopy(SEM),optical microscopy(OM),X-ray diffraction(XRD)and Vickers hardness tester.Their tribological properties were evaluated under four different loads(5,10,15,and 20 N)with a friction and wear tester(MS-M9000).The results indicated that the coatings with Ni contents ranging from 60wt.%to 67wt.%exhibited dense microstructures without defects such as cracks or pores.However,some obvious cracks were found in the 70NiTi coating.Despite the varying Ni contents,the NiTi coatings shared similar microstructural characteristics,such as the fine planar and cellular crystals around the fusion line,which transformed to relatively coarse columnar and dendritic structure in the middle and top regions.As the Ni content increased,the coating microstructure became finer from the fusion line to the coating surface.The presence of cracks in the 70NiTi coating altered the cooling path and rate,resulting in a coarse microstructure.All of the coatings had an average thickness exceeding 1.2 mm and consisted of NiTi toughening phase and Ti2Ni strengthening phase with various Ni contents which had a significant effect on the dilution ratio.As the dilution ratio increased with the Ni content,the volume fraction of the Ti2Ni strengthening phase progressively increased up to 78.6%in the 67NiTi coating with a maximum dilution ratio of 36%.Consequently,this coating achieved the highest hardness of 677.41HV0.2,which was 2.05 times that of the substrate.The average friction coefficient of both the coatings and the TC4 substrate increased gradually with an increase of the applied load,but the coatings demonstrated a higher average friction coefficient than the substrate under the same load.Furthermore,the friction coefficient increased progressively with the increase of Ni content and the difference in the average friction coefficient between the 70NiTi coating and the substrate reached a maximum value of 0.231 under 20 N normal load.The average wear rate of the coatings was significantly lower than that of the substrate and showed an initial decrease followed by an increase with the increase of Ni content from 60%to 70%.The 67NiTi coating exhibited the lowest wear rate of 2.74×10-4 mm3/(N·m)under 20 N normal load and the most significant improvement in wear resistance was achieved by 65%in comparison with the substrate.At this load,the TC4 substrate exhibited the highest average wear rate and the most severe wear by the mechanism of severe abrasive and adhesive wear.Conversely,as the dilution rate increased,the 60-67NiTi coatings showed a gradual reduction in both wear rate and severity,and the corresponding wear mechanism has transmitted to mild abrasive and adhesive wear.Based on a comprehensive analysis of the microstructural,hardness,and tribological properties,the 67NiTi coating demonstrated the best overall performance among all the NiTi coatings.Its high hardness and excellent wear resistance effectively improved the wear resistance of the titanium alloy surface.