| Metallic glasses(MGs)possess the unique atomic arrangement of short-range order and long-range disorder.This distinctive structure endows MGs with outstanding mechanical,physical and chemical properties,and makes MGs find the applications in high-tech fields such as new energy,aerospace,military equipment,precision instrument manufacturing,as well as high-value-added areas like electronic chips and sports equipment.Compared with its low tensile plasticity,the high hardness of MGs provides opportunities for its application as surface contact and functional materials.If its surface hardness and wear resistance can be further improved,it is expected to enhance and expand its application.Previous studies have achieved a preliminary improvement in the surface hardness of Zr-based MG through laser nitriding.However,the issues such as unclear laser nitriding process and the hardening mechanism of the nitrided surface,poor surface quality of the nitrided surface and the lack of research on characteristics of the nitrided surface still exist.To address this,this paper has investigated the nanosecond laser nitriding process,achieving a significant improvement in surface hardness of Zr-based MG and revealing the hardening mechanism of the nitrided surface.Furthermore,the nanoindentation and scratch characteristics of the surfaces treated with different laser nitriding parameters were comparatively studied,and the influence of laser nitriding on the indentation and scratch characteristics of the Zr-based MG surface were revealed.To address the issue of reduced surface quality caused by laser nitriding,laser finishing experiments were performed on the nitrided surface within a nitrogen atmosphere using the same laser equipment,significantly improving the quality of the nitrided surface.Additionally,through simulation analysis,the morphology evolution of molten pool during laser finishing process was studied,and the mechanism of laser finishing on the nitrided surface was revealed.Finally,friction and wear,as well as electrochemical corrosion tests were performed on the laser-nitrided and finished surfaces to analyze the mechanism of enhancing surface wear and corrosion resistance through laser treatment.The specific research contents of the paper are as follows:(1)The nanosecond laser nitriding process and surface hardening of Zr-based MG were investigated.Firstly,nanosecond laser irradiation experiments were performed on the surface of Zr-based MG in nitrogen atmosphere,and the effects of laser process parameters on surface/subsurface morphology and surface phase composition of MG were explored.Based on this,the orthogonal experiments were designed and performed with the aim of improving surface hardness,the effects of laser process parameters(laser average power,laser scanning speed,overlap rate of the adjacent scanning tracks and laser irradiation times)on the hardness of nitrided surface were analyzed.Based on this,the laser process parameters were optimized,resulting in a nearly threefold increase in surface hardness compared to the as-cast MG.Combined with the cross-sectional characteristics analysis of the nitrided surface,the hardening mechanism of the nitrided surface was further analyzed and revealed.(2)The effects of laser nitriding on micro-scale indentation and scratch characteristics of MG were investigated.Firstly,nanoindentation tests were performed on laser-nitrided surfaces,and morphology of residual indentation,shear bands,and serrated flow were analyzed.Further,micro/nano scratch tests were performed on laser-nitrided surfaces.The coefficients of friction,morphologies of residual scratches,and morphologies of cuttings were analyzed,and the effect of laser nitriding on scratch resistance of MG surface was evaluated.Meanwhile,the effect of laser nitriding on indentation and scratch characteristics of MG surface was revealed.(3)To improve quality of the laser-nitrided surface,the laser finishing process and mechanism of the laser-nitrided surface were investigated.Firstly,laser finishing experiments were carried out on the laser-nitrided surfaces,and the effects of laser process parameters(laser average power,laser scanning speed,overlap rate of the adjacent scanning tracks,and laser irradiation times)on morphology,three-dimensional profile,and roughness of laser-nitrided surface were systematically analyzed.Based on this,laser process parameters were optimized to reduce the surface roughness(Sa)from 0.265μm to 0.063μm,which improves the quality of laser-nitrided surface while maintaining surface hardness.Secondly,finite element simulation model was established to analyze the morphology evolution and temperature distribution of the molten pool during the laser finishing process.And combined with the experimental results,the laser finishing mechanism of the nitrided surface was analyzed and revealed.(4)Tests and analysis on the wear and electrochemical corrosion characteristics of lasernitrided and finished surfaces were investigated.Firstly,the friction and wear tests under dry friction and oil lubrication conditions were performed on laser-nitrided and finished surfaces.The morphology of wear track,coefficient of friction,wear rate,and element distribution of wear track were obtained,and the wear characteristics of laser-nitrided and finished surfaces were analyzed.Secondly,electrochemical corrosion tests were performed on laser-nitrided and finished surfaces.The potentiodynamic polarization curve and electrochemical impedance spectra were analyzed,and the corrosion resistance characteristics of laser-nitrided and finished surfaces were evaluated.In summary,the above results are expected to promote understanding of process and mechanism of nanosecond laser nitriding of MGs,provide a research basis for enhancing the surface performance and quality of MGs,and expand the structural and functional applications of of MGs. |
