| Bearing has an important position in modern machinery and equipments,which is the important basic components,named as"the joint of high-end equipment".The main failure forms of the bearing parts are friction and wear failure,fatigue failure and so on.The domestic demand for the high-properties bearing steel is very strong in our country,because the failure of bearing parts may be always leading to serious safety accidents and economic losses.The chromizing coatings?Cr coatings?and the chrome-vanadium co-infiltration coatings?Cr-V coatings?have high hardness,good wear resistance,excellent thermal stability and oxidation resistance,which are ideal coatings for enhancing the surface properties of bearing steel and improving its service life.Pack cementation is a chemical heat treatment technology,which has the advantages of low cost,easy operation,and has obvious advantages in industrial applications.In this paper,the Cr coatings and Cr-V coatings were fabricated on GCr15 bearing steel by pack cementation.The effects of pretreatment,chromizing holding time,the kinds and contents of rare earth oxides on the morphology,structure and mechanical properties of Cr coatings were studied.And the effects of vanadium doping content on the morphology,structure and mechanical properties of Cr-V coatings were also researched.The surface morphology and cross section structure of the coatings were characterized by scanning electron microscope.The phase of the coatings was analyzed by X-ray diffraction.The mechanical properties were investigated by micro-Vickers tester and scratch tester.The tribological properties were evaluated by ball-on-disk rotating friction and wear tester.The main condusions of the paper are as follows:?1?Ion nitriding and gas carbonitriding pretreatment can promote chromizing,and gas carbonitriding pretreatment has better effect.Compared with untreated chromizing,the thickness of chromizing is increased by 61.8%.The main phase of untreated chromizing is carbo chromium compounds(Cr3C2,Cr7C3,Cr23C6).While the main phases of pretreated chromizing coatings are Cr3C2,Cr7C3 and?Cr,Fe?2N1-x.Compared with untreated chromizing,the chromizing coating pretreated by gas carbonitriding has better mechanical properties.Its surface hardness reaches 1420 HV0.01,the micro gradient hardness decreases slowly,the binding force reaches 85 N,and the indentation grade is HF1.?2?The surface morphology tends to be compact with the prolongation of chromizing holding time.The grains will become coarse when the holding time is too long.The thickness of chromizing coatings increases parabolically with the extension of holding time and the holding time has a little effect on the phase of chromizing coatings.?3?Different kinds of rare earth doping have little effect on the phase structure of the coatings,which mainly consists of Cr3C2,Cr7C3 and?Cr,Fe?2N1-x phases.Cr-RE coatings have good ability of wear resistant and antifriction,among them,the Cr-La coatings have the best tribological properties.Its average friction coefficient is 0.4714,and the wear rate is only 1/5of that of GCr15 matrix.The main wear mechanisms are adhesion transfer and oxidation wear.?4?Doping appropriate amount of rare earth oxides can effectively improve the thickness of the coating.There is a critical value for added the amount of rare earth oxides.If the content exceeds the critical value,the thickness of the coating will decrease.When the doping amount of rare earth oxide is 6%,the mechanical properties are the best.The indentation grade is HF1,and the wear rate is only 4.4806×10-7 mm3N-1m-1.?5?Appropriate doping of vanadium in the penetrating agent can induce penetration,and the coating mainly consists of Cr7C3,Cr23C6,VCx and VN phases.The Cr-20%V infiltrated layer has good comprehensive properties,and the indentation grade is HF1.The average friction coefficient and wear rate are 0.5045 and 3.2459×10-7 mm3N-1m-1 respectively.The main wear mechanisms are oxidation wear,adhesion wear and slight abrasive wear.?6?With the increase of load,the friction coefficient between chromizing coatings and chromium-vanadizing coatings decreases gradually,but the wear rate of chromizing coatings increases significantly,and the wear rate of chromizing-vanadizing coating increases slightly.Under 19.6 N load,the wear rate of chromizing-vanadizing coatings is only 1/3 of that of chromizing coatings. |
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