Research On The Mechanical Properties,oxidation Resistance And Tribological Behavior Of WC-MgO-B₂O₃ Cemented Carbide

Due to the extremely high sintering temperature of WC-based cemented carbide without metallic binder at present,which makes it difficult to prepare.Therefore,it is urgent to further develop a WC-based cemented carbide without metallic binder which can be prepared at low temperature and has high fracture toughness.It has been studied that MgO-B₂O₃ system can form liquid phase at lower temperature.It is well known that the flow of liquid phase can effectively fill the gaps between WC grains.Therefore,using MgO-B₂O₃ to toughen WC can obtain dense bulk materials at relatively low temperature.In this paper,WC-MgO-B₂O₃composite powders was prepared by high-energy ball milling method with MgO and B₂O₃added to WC at the same time.In order to fully reveal the influence of MgO-B₂O₃ on the microstructure and mechanical properties of WC cemented carbide,the effects of MgO-B₂O₃addition and sintering temperature on the phase composition,microstructure evolution and mechanical properties of WC cemented carbide were emphatically studied.Then,W8MB(MgO-B₂O₃ content is 8 wt.%)with excellent comprehensive mechanical properties was selected to study the oxidation resistance and wear resistance,and compared with YG10cemented carbide to reveal the effect of MgO-B₂O₃ on the oxidation and wear behavior of WC cemented carbide.In this paper,WC composite powders with content of 2,4,6,8 and 10 wt.%of MgO-B₂O₃were prepared by high-energy ball milling method,and then the composite powders were sintered by SPS at a relative low temperature of 1400?.It is found that the morphology of part second phase in the microstructure changes significantly from fine needle to long rod and then to block when the content of the MgO-B₂O₃ increases from 2 wt.%to 10 wt.%.With the increase of content,the fracture toughness of cemented carbide increases first and then decreases.After sintered at 1300??1500?,it was found that the reaction product Mg₂B₂O₅decomposed into B₂O₃ liquid phase at?1312?and Mg₃B₂O₆ melted to form liquid phase at?1412?,which reduced the diffusion resistance of atoms,resulted in WC and the second phase particles grew up in stages.The particle size increased significantly at 1300?1350? and 1400?1450?,respectively.And the hardness and fracture toughness of cemented carbide first increase and then decrease as the increase of sintering temperature.After sintered at1400?,W8MB has better comprehensive mechanical properties,the hardness and fracture toughness reached 18.16±0.17 GPa and 9.45±0.37 MPa·m^1/2,respectively.The isothermal oxidation method was carried out to study the oxidation behavior of W8MB,and compared with YG10,the effect of the difference of the second phase on the oxidation resistance of two kinds of WC cemented carbide was discussed.The difference of oxidation weight gain between two materials was small below 700?.When the temperature further increased to 800?,oxidation weight gain of the two materials increased greatly.After oxidation at 900?for 3 hours,the weight gain per unit area of W8MB was only 86.70 mg/cm²,while that of YG10 was 96.18 mg/cm².Which indicates that W8MB has better oxidation resistance than YG10 at high temperature.On the one hand,the granular and long rod-shaped second phase on the surface of W8MB would not be oxidized,thus forming pits on the surface of the oxide film.The existence of the second phase reduces the surface area of W8MB in contact with oxygen and improves its oxidation resistance to certain extent.On the other hand,short rod shaped MgWO₄ formed after oxidation,which helps to cover the pores between WO₃grains,can hinder the inward diffusion of oxygen,thus reduce the oxidation of matrix,and improve the oxidation resistance of W8MB to a certain extent.For YG10,besides WO₃,a large amount of CoWO₄ is formed during oxidation at low temperature.The oxidation of Cois prior to that of WC,which reduces the oxidation resistance of WC.Similarly,YG10 was selected as the contrast material to study the effect of different loads on the friction coefficient and micro wear morphology of W8MB by fretting friction.It is found that the second phase of W8MB has higher volume fraction and is easier to wear due to its lower strength,which leads to higher stress on WC during friction.The main wear mechanism of W8MB is abrasive wear.And the main wear mechanism of YG10 is abrasive wear,oxidation wear and adhesive wear.