Effects Of Yttria And Heating Processes On Microstructure And Tribological Properties Of Boronized TC4 Titanium Alloy

As a stable and reliable surface strengthening method,solid powder boronizing is highly preferred because of the advantages of excellent bonding between the boronizing layer and the substrate,simple process operation and low cost,etc.It can form a multifunctional boronizing layer with high hardness,wear resistance,corrosion resistance and oxidation resistance on the surface of titanium and titanium alloy to reduce the risk of wear loss,which has become one of the effective ways for surface strengthening of titanium and titanium alloy.However,the diffusion efficiency of B atoms and the organization and properties of the boronizing layer are influenced by various factors,such as the substrate type,the composition of the pack mixture and the heating process,especially the composition of the pack mixture and the heating process.In this paper,the solid powder boronizing method was used to boronize TC4 titanium alloy.The basic pack mixture of boronizing consists of B,NaF and Al₂O₃ powders.The effects of Y₂O₃ addition and heating process（constant temperature process,superimposed process and cyclic phase change diffusion（CPCD）process）on the microstructure,phase composition,surface hardness and friction coefficient of TC4 alloy boronizing layer were investigated.The effect of Y₂O₃ on the boron penetration process was analyzed by thermodynamics,and the growth rate constant of the boronizing layer under different heating processes were compared by growth kinetics.The main conclusions obtained from the paper are as follows:（1）When adding Y₂O₃into the pack mixture,the boronizing layer obtained on the TC4substrate is composed of the dense and continuous TiB₂layer and the TiB whisker diffusion layer,which is the same as the cross-sectional morphology of the boronizing sample with free Y₂O₃ addition.Adding 1～3 wt.%Y₂O₃ into the pack mixture increases the thickness of the boronizing layer by up to 40.24%compared with that of the boronizing layer prepared in the Y₂O₃ free pack mixture.The best catalytic diffusion effect of Y₂O₃ on the boronizing layer growth occurs in the pack mixture with 3 wt.%Y₂O₃ addition.Nevertheless,adding 5～7 wt.%Y₂O₃ into the pack mixture restrains the growth of the boronizing layer.By using the pack mixture with 3 wt.%Y₂O₃addition,the surface hardness of the boronizing sample increases by35.54%and the friction coefficient of the boronizing sample reduces by 28.57%compared with that of the boronizing sample prepared in the Y₂O₃ free pack mixture.（2）Thermodynamic analysis shows that BF₃ generated by the reaction in the pack mixture diffuses to the surface of TC4 alloy under the concentration gradient and reacts with Ti atoms to form TiB,TiB₂ and active B atoms to realize the infiltration of B atoms during the boronizing process.Meanwhile,BF₃ reacts with Y₂O₃ in the pack mixture to form B₂O₃ and YF₃,which will generate Y atoms under the strong reducing property of Na atoms.The Y atoms that are slightly solubled on the surface of the substrate lead to lattice distortion in some regions and promote the diffusion of the B atoms.（3）Compared with the thickness of boronizing layer prepared by 1070℃/8 h process,the thickness of boronizing layer prepared by superimposed process（a heating process including heating up to 990℃for 4 h,then heating up to 1070℃in 10 min and holding for 4 h）is 11.8%higher.The thickness of the boronizing layer is about 14.53μm obtained by the CPCD process（a heating process of 8 times cycles including heating up to 990℃for 30 min,then heating up to 1020℃in 10 min and holding for 30 min,finally cooling to 990℃in 10 min.）,which is near the thickness of the boronizing layer at 1070℃/8 h process.However,the CPCD process has the lower boronizing temperature.Therefore,it can be seen that both the superimposed process and the CPCD process demonstrate the higher efficientcy in depositing boron on TC4 alloy compare to the boronizing process at constant temperature.（4）The surface hardness of the TC4 alloy are improved by thermostatic boronizing process（990℃/8 h and 1070℃/8 h）,superimposed boronizing process and CPCD boronizing process.The surface hardness of TC4 alloy boronized in superimposed process is the highest,about1519.89 HV_0.1,and its friction coefficient is also the lowest,about 0.13.