| Titanium and titanium alloys are widely used in many areas such as aerospace,military,sport equipment and chemical engineering due to their high specific strength,low density,excellent corrosion resistance,stable moderate temperature properties,easy forming and good weldability.Especially for the aviation industry,with the improvement of the aircraft performance and the increasing of thrust-weight ratio of aircraft engines,titanium and titanium alloys show special attraction.However,low hardness,poor wear resistance and poor high temperature oxidation resistance of titanium and titanium alloys restrict their more applications as anti-wear and high-temperature parts.Since wear and oxidation predominantly occur at the materials surface,surface modification is a promising approach to improve surface properties of titanium and titanium alloys.Laser surface modification technique has received extensive application to fabricate wear resistance coating,corrosion resistance coating,bio-active coating and high temperature oxidation resistance coating on titanium and titanium alloys due to its advantages such as high energy density,controllable thickness of the coating and excellent metallurgical bonding between the coating and the substrate.Based on the basic requirement of high temperature protective coating and the mechanism of alloying elements on improving properties of titanium alloys,the paper fabricated Ti-Al-X multi-component alloyed coatings with different addition content of Nb or Si or both Nb and Si on Ti-6Al-4V alloy to improve surface properties,such as microhardness,wear resistance and high temperature oxidation resistance.Microstructure,microhardness,tribological behavior at different temperature and high temperature oxidation behavior at 800 ? of the coatings were analyzed.The mechanism of alloying elements was discussed.Ti-Al alloyed coating is mainly composed of TiAl and Ti3Al intermetallic compounds.The coating exhibits good metallurgical bonding with the substrate.The oxidation products of Ti-Al alloyed coating oxidized at 800 ? for 1000 h are R-TiO2 and ?-Al2O3,and the oxide scale has a multilayer microstructure consisted of TiO2/Al2O3/(Al2O3+Tio2)/oxygen diffusion zone from surface to the inside.This special microstructure of the oxide scale is caused by thermodynamics,such as the similar stability of TiO and AL2O3,and kinetic aspects,such as the high growth rate of TiO2 relative to Al2O3.The oxidation mass gain of Ti-Al alloyed coating oxidized at 800 ?for 1000 h is 42.2 mg/cm2,which is 1/6 of the substrate.Ti-Al alloyed coating exhibits good high temperature oxidation resistance.The improvement of high temperature oxidation resistance of Ti-Al alloyed coating is attributed to the higher Al content.The higher Al content,the higher proportion of Al2O3 formed in the oxide scale.High proportion of Al2O3 improves compactness and adhesion of the oxide scale on Ti-Al alloyed coating.Compared with Ti-6A1-4V alloy,Ti-Al alloyed coating has good wear resistance both at room temperature,600 ? and 800 ? due to higher microhardness.However,local disaster oxidation caused by cracks especially for penetrating cracks and spallation of the oxide scale hinder Ti-Al alloyed coating to provide a long-term protection against high temperature oxidation.Ti-Al-xNb alloyed coatings are mainly composed of TiAl and Ti3Al phases.Compared to Ti-Al coating,the amount of macro-cracks of Ti-AI-xNb coatings decreased due to the improvement of Nb addition on the ductility.No spallation of the oxide scales was observed on Ti-Al-xNb coatings throughout the oxidation cycle.The oxidation mass gains of Ti-Al-xNb alloyed coatings oxidized at 800 ? for 1000 h are 1/5.6-1/15 of Ti-6Al-4V alloy.With the increasing Nb addition content,high temperature oxidation resistance of Ti-Al-xNb alloyed coatings increases.However,excessive Nb addition in Ti-Al-xNb alloyed coatings reduces Al content greatly and decreases high temperature oxidation resistance of the coating.The synergistic effect mechanism of Nb and Al on improving high temperature oxidation resistance of Ti-Al-xNb alloyed coating includes decreasing the oxygen vacancies of TiO2 oxide,promoting the formation of Al2O3 in the oxide scale and improving adhesion of the oxide scale.Compared with Ti-6A1-4V alloy,Ti-Al-xNb alloyed coatings have higher wear resistance due to their higher microhardness.However,compared with Ti-Al alloyed coating,Ti-Al-xNb alloyed coatings have lower wear resistance due to their lower microhardness caused by Nb addition.Ti-Al-xSi alloyed coatings are composed of Ti-Al intermetallic compounds and Ti5Si3 reinforced phase.With the increasing Si addition content in the alloyed coatings,Ti-Al intermetallic compound changes from TiAl to TiAl2 and TiAl3.Due to high Young modulus of Ti5Si3,rare macro-cracks formed in Ti-Al-xSi coatings.The oxidation mass gains of Ti-Al-xSi coatings oxidized at 800 ? for 1000 h are 1/10-1/21 of the substrate.The oxidation mass gains of Ti-Al-xSi alloyed coatings are obviously lower than those of the substrate and Ti-Al alloyed coating.When the addition content of Nb and Si is the same in the coatings,Ti-Al-xSi alloyed coatings exhibit more excellent high temperature oxidation resistance than Ti-Al-xNb alloyed coatings.More Si addition in Ti-Al-xSi alloyed coatings,more excellent the high temperature oxidation resistance of the alloyed coatings have.The synergistic effect mechanism of Si and Al on improving high temperature oxidation resistance of Ti-Al-xSi coatings includes refining the oxide grains,promoting the formation of Al2O3 in the oxide scales,and forming Ti5Si3 and TiAl3 phases with excellent high temperature oxidation resistance.Ti-Al-xSi alloyed coatings have higher microhardness than the substrate.Compared with the substrate,Ti-Al alloyed coating and Ti-Al-xNb alloyed coatings,Ti-Al-xSi alloyed coatings exhibit excellent wear resistance under both at room and high temperature.With increasing Si addition content in the alloyed coatings,microhardness and wear resistance of Ti-Al-xSi alloyed coatings increase due to more hard Ti5Si3 phase formed.However,the adhesion of the oxide scale formed on Ti-Al-xSi coatings is not as good as Ti-Al-xNb alloyed coatings.Spallation of the oxide scale was observed on Ti-Al-xSi alloyed coatings with lower Si addition content.Moreover,some micro-cracks and more pores formed in Ti-Al-xSi alloyed coatings with higher Si addition content due to high brittleness of the coatings and the inhibition effect of Ti5Si3 on the gas escape and the liquid flow.Ti-Al-xNb-ySi alloyed coatings are composed of Ti-Al intermetallic compounds and Ti5Si3 reinforced phase.With increasing the Si addition content in the alloyed coatings,Ti-Al phases change from TiAl to TiAl3.Due to high Young modulus of Ti5Si3 and the improvement of Nb addition on ductility,no obvious cracks formed in the coatings.No spallation of the oxide scales on Ti-Al-xNb-ySi alloyed coatings was observed throughout the oxidation cycle,which illustrates that the oxide scales have excellent adhesion.The oxidation mass gain of Ti-Al-xNb-ySi alloyed coatings oxidized at 800 ? for 1000 h is only 1/26-1/57.1 of the substrate.The oxidation kinetic curves of Ti-Al-xNb-ySi alloyed coatings follow parabolic or parabolic-cubic law.Ti-Al-xNb-ySi alloyed coatings exhibit extremely excellent high temperature oxidation resistance.Compared with the oxidation data of Ti-Al-xNb,Ti-Al-xSi and Ti-Al-xNb-ySi alloyed coatings,the effect of combined adding Nb and Si on improving high temperature oxidation resistance of the coatings is better than the effect of single adding Nb or Si.Increasing the addition content of Nb and Si,especially Si,could obviously increase the proportion of Al2O3 in the oxide scales and reduce the oxidation rate.The synergistic effect mechanism of Nb,Si and Al on improving high temperature oxidation resistance of Ti-Al-xNb-ySi coatings includes refining the oxide grains obviously,promoting the formation of Al2O3 in the oxide scales obviously,increasing the adhesion of the oxide scales obviously and forming Ti5Si3 and TiAl3 phases with excellent high temperature oxidation resistance.Microhardness of Ti-Al-xNb-ySi alloyed coatings is higher than that of the substrate,Ti-Al alloyed coating and Ti-Al-xNb alloyed coatings due to the formation of hard phase Ti5Si3.Ti-Al-xNb-ySi alloyed coatings exhibit wear resistance under both at room temperature and high temperature compared with Ti-6A1-4V alloy,Ti-Al alloyed coating and Ti-Al-xNb alloyed coatings.With Si addition content increasing,microhardness and wear resistance of Ti-Al-xNb-ySi alloyed coatings increase due to more Ti5Si3 phase formation.Though microhardness of Ti-Al-xNb-ySi alloyed coatings is lower than that of Ti-Al-xSi alloyed coatings with the same Si addition content,Ti-Al-xNb-ySi alloyed coatings have more excellent wear resistance due to good ductility caused by Nb addition. |
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