| Titanium alloys are widely used in aerospace manufacturing and the biomedical and petrochemical industries because of their high strength-to-weight ratio,good biocompatibility,and excellent corrosion resistance.With constant advances in science and technology,titanium alloy performance must meet increasingly high demands.Therefore,to improve the practical performance of titanium alloys and extend their potential for application in a wider range of fields,surface modification is often performed before actual use.Low hardness values and poor wear resistance have severely restricted the application of titanium alloys in some engineering fields.It is well known that wear fatigue originates from the titanium surface,but the service life of titanium plate can be improved by hardening the surface.Previous studies have shown that mechanical properties such as hardness,wear resistance,and corrosion resistance can be significantly enhanced by nitriding or boriding of titanium alloys.However,to obtain a certain thickness of the nitrided or borided layer,a sample must be subjected to a high processing temperature for a long duration.The high processing temperature and long processing duration increases the cost of production,and the high energy consumption indirectly causes increased environmental pollution.In addition,the nitrided or borided layers formed during this process usually show substantially lower surface toughness,which degrades their performance when they are exposed to severe service environments involving high shear and compressive and/or impact loading conditions.Therefore,finding a way to decrease the processing temperature while improving the toughness of nitrided or borided layers is currently a major challenge.It is generally known that diffusion along grain boundaries and defects is higher than diffusion through interior of grains.Hence,it is reasonable to expect that introducing a fine-grain layer on the surface of a material would significantly reduce the activation energy for the diffusion of nitrogen or boron and would enhance the nitriding or boriding kinetics.However,the various methods,such as mechanical rolling attrition(MRA),that have been developed for this purpose have still only been applied to small specimens;these methods have not been applied in the actual production of Ti gears or plates.To solve this problem,we have developed a new mechanical rolling attrition(MRA)method and a mechanical shot peening(MSP)method to produce a nanocrystalline layer on Ti gears and large Ti plates.In the subsequent process,nanocrystalline nitrided and borided layers,respectively,were successfully prepared on the surfaces of Ti specimens treated with MRA and MSP through a low-temperature chemical heat treatment.Due to the advantages that it is non-toxic,chemically stable,and non-polluting,TiO2 has been widely studied in relation to its application in the photocatalytic treatment of wastewater.However,its wider band gap induces a narrow light absorption wavelength(without ability to utilise the visible spectrum),and its higher electron-hole recombination rate induces lower photocatalytic efficiency.In addition,TiO2 has been used as a catalyst for the degradation organic pollutants in wastewater for decades,but powder TiO2 is difficult to disperse and recycle,and easily poisoning,and this limits its further application.Therefore,the immobilisation of a TiO2 photocatalyst that is highly responsive to visible light is essential for practical application.Self-doped black TiO2 has attracted great interest in recent years because of its novel TiO2 core and amorphous shell microstructure.The amorphous shell contains a large number of Ti3+ and Vo2+ atoms,which can increase the sunlight absorption tate to 85%.The organic dye degradation rate of black TiO2 is four times higher than that of general white TiO2 powder.Therefore,black TiO2 shows efficient utilisation of solar energy.However,the preparation of black TiO2 is commonly performed by hydrogenation,which requires processing of white TiO2 at high temperature and pressure for several days.Due to low yield and high cost,such processing cannot meet the actual production requirements.In this work,through a simple one-step SMA technology,we utilised TiO2 powder and TA2 sheets as the surface alloying coating material and substrate material,respectively,to fabricate a black nano-TiO2 coating on TA2 sheets showing high photocatalytic activity.In an experiment involving the photocatalytic degradation of organic wastewater,the black nano-TiO2 coating showed good photocatalytic activity in the photocatalytic degradation of rhodamine B-solution.The main results of this paper are summarised as follows:(1)Ti6A14V was processed using MRA treatment to produce a nanocrystalline Ti surface.A severe plastic deformation(SPD)layer of approximately 15 ?m thickness could be observed on the Ti alloy sheet surface(which included the nanocrystalline layer above the SPD layer and the lower part of the deformation layer).The thickness of the nanocrystalline surface layer increased with increasing MRA treatment duration but stabilised after 90 min.The grains in the nanocrystalline layer were refined to approximately 10 nm and presented an equiaxed and ultra-fine morphology with a random crystallographic orientation.The structural transformation of a-Ti into ?-Ti during MRA treatment was confirmed by XRD.This phase transformation is assumed to originate from an increase in the surface Gibbs energy during MRA treatment.Finally,the thermal stability of the MRA-treated samples was investigated by subjecting them to isothermal annealing at various temperatures.The ?-Ti nanocrystalline structures were stable at temperatures up to 600?,compared to 500? for the ?-Ti nanocrystalline structures.(2)Gaseous nitriding of the Ti6A14V alloy could be carried out at a relatively low temperature(600?)with nanocrystalline layer assistance.By gaseous nitriding for 5 h,a nanocrystalline nitrided layer of 15 ?m thickness was fabricated on the titanium alloy plate.The nanocrystalline nitrided layer was composed of nanoscale nitrides of TiN,Ti2N,and Ti with high nitrogen supersaturation.The hardness of the nanocrystalline nitrided layer was higher than that of the coarse-grained nitrided layer,and the toughness was evidently improved.The topmost surface hardness was 1080 HV,the wear mechanism was abrasive wear,and brittle assessment for I level.(3)Powder boriding of the Ti6A14V alloy could be carried out at a relatively low temperature(600?)with nanocrystalline layer assistance.The nanocrystalline borided layer was composed of nanoscale borides of TiB,Ti3B4,TiB2,and Ti with high boron supersaturation.The hardness of the nanocrystalline borided layer was higher than that of the coarse-grained borided layer,and the toughness was evidently improved.The excellent toughness of the nanocrystalline borided layer can be attributed to the formation of nanoscale borides.Moreover,through rigorous thermodynamic calculations,it was found that B4C can provide active boron atoms for the boriding process and can effectively prevent the oxidation of nanocrystalline Ti.(4)A severe plastic deformation layer of approximately 230 ?m thickness could be observed on the commercial Ti plate surface after MSP treatment.The thickness of the severe plastic deformation layer increased with increasing MPS treatment duration but stabilised after 30 min.The grains in the severe plastic deformation layer were refined to approximately 20 nm,and they presented an equiaxed and ultra-fine morphology with a random crystallographic orientation.In the subsequent nitriding process,gaseous nitriding of the commercial titanium plate could be carried out at a relatively low temperature(550?)with nanocrystalline layer assistance.The nanocrystalline nitrided layer was composed of nanoscale(10-50 nm)TiN,Ti2N,and Ti with high nitrogen supersaturation.The depth of the hardened layer of the nitrided MSP specimen was about 65 ?m,and the wear loss was 20%to 30%in comparison to a coarse-grained specimen treated using the same nitriding conditions.The nitrided layer showed high hardness and wear resistance,and the toughness was greatly improved by the formation of the nanocrystalline nitrided layer.(5)A nano-TiO2 coating of about 25 ?m was fabricated on a TA2 sheet after 90 min SMA treatment.The internal grains size was about 10 nm with an amorphous shell and rutile core structure.The results of XPS and EPR showed that there were many Ti3+ and oxygen defects on the surface of the TiO2 particle layer.The results of photocatalytic experiments showed that the degradation rate of 10 mg/L of rhodamine-B solution by using 20 mg of black TiO2 powder was 14.8%per hour,which is about twice that achieved with raw white TiO2 powder.The degradation rate of the nano-TiO2 coating plate was 3.4%per hour,which was the same as that of 20 mg of white TiO2 power under the same irradiation condition. |
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