Study On Boronizing Process And Boronizing Performance Of Titanium And Titanium Alloy

Titanium and titanium alloys have been extensively used in automotive, Aerospace, Petro-chemical, bio-medical and sports industries owing to their excellent characteristics, including satisfactory biological safety, high stiffness, high strength-to-weight ratio and excellent corrosion and oxidation resistance.However, low hardness and poor resistance limite their application especially in tribology and contact application.Therefore,it is necessary to improve the hardness and wear resistance properties. Surface modification techniques are effective methods to imrove the properties, which could significantly expand the application areas and have been the focus.In this paper, the boriding layers were prepared by molten salt boring on TA2 and TC4 surface widely used in industry to research the influences of boriding on different substrates. Boriding agents were mainly composed of B, Na2B4O7 and Al.The cross-section morphology and structure, phase composition,distribution of nano-hardness,wear resisitance of boriding layer were studied by SEM,XRD,WDS, Nanomechanical test system and fiction and Wear testing machine.The experiments were carried out on TA2 and TC4 surface in the 950-1100 ℃ temperature range for 5h, 10h,20h and 30h, respectively. The phase analysis shows that the boride layers onTA2 are mainly composed of whiskers TiB and continuous TiB2, whereas on TC4 the phase are mainly composed by the TiB2 when the boriding temperature above 1050℃ and the phase are TiB and TiB2 when the boriding temperature under 1000℃. The growth kinetics of boride layer had been studied and the results revealed that in the same boriding temperature the thickness of boride layer and the time can be fitted in the equation: d2=Kt, the order of K is 10-15.The hardness of boriding layer is gradient distribution along the thickness of boriding layer, and the maximum value of the outermost layer hardness is up to 23 GPa. The wear resistance tests show that the friction coefficient of boriding layer are range in 0.36-0.38 which is less than that of TA2 substrate, whereas the minimum friction coefficient of boriding layer is 0.30, which is less than the coefficient of boriding layer on TA2. The adhesive strength of boriding layer and the substrate is preferably, and the adhesive force is up to 64N. Electrochemical test results show that corrosion resistance of borided samples in acid and salt solution has been enhanced than the substrate.In order to study the effect of CeO2 to diffusion B, adding CeO2 to boriding agents.The results show that rare earth atoms can reduce nucleation radius, and promote new phase to nucleate, thereby refining boriding layer. The radius of rare earth atoms could change, the aberration energy would increase when the B atoms diffuse into the substrate, while providing more channels for the diffusion of B atoms, thereby promoting TiB phase to generate. The test results of boriding layer indicate because of refinning the organization of boriding layer, the nano-hardness of the sub-layer TiB decrease slowly, and the wear resistance and the adhesive strength both increase. Simultaneously, the corrosion resistance has also been enhanced.