| Tungsten and its alloys are widely used in the aerospace,military and nuclear industries due to its high melting point,high density,excellent high-temperature strength,high elastic modulus,and small thermal expansion coefficient,known as the "teeth" of industry.However,in some practical applications,either the performance or the lifetime of tungsten and its alloys are limited by low surface hardness and low abrasion resistance.The fabricate a coating to protective the surface is an effective approach for addressing this problem.We fabricated WC coatings on tungsten using the interstitial carburization technique.High-carbon steel was selected as the carbon source to supply interstitial carbon atoms,the interstitial carbon atoms diffused into tungsten,generating a carbide coating.The studied microstructure,phase transformation,hardness,adhesion of the coating to the substrate,friction and wear properties of the WC coating.The results indicated that:(1)The fabrication WC coating at 1100? is completely compact(nonporous),and the volume fraction of the ceramic phase is close to 100%.It consists of 96%WC and 4%W2C locating at the coating-substrate interface.This microstructure is formed by a two-step solid-state phase transition from W to W2C to WC,with the crystallographic orientation relationships of[111](101)w//[1120](0001)w2c//[1120](0001)wc.The WC grains interior contain a high density of(1010)stacking faults generated via vector 1/2[1211]collective shifts.(2)By comparing the microstructures of the coating fabricated by one-step hot-pressing at 1100? and 1150?,it is found that increasing the temperature by 50? can remarkably increase the growth rate of the coating as desired but generates undesired ? carbide(Fe6W6C)at the coating/substrate interface.Inspiron by the inference that Fe atoms are diffusible in tungsten but indiffusible in WC at 1150?,we proposed a two-step hot-pressing process to fabricate coating at 1150? without introducing ? carbide.The WC layer prepared at 1100? was utilized as a "semipermeable layer" to barrier Fe atoms and allow the diffusion of carbon atoms at 1150?.The two-step hot-pressing worked as without ? carbide,it is completely composed of WC and W2C.The obtained coating exhibits a distinct gradient microstructure with columnar WC grains.The thickness of the coating is proportional to the square root of the carburization time in each step(?=Kt1/2),the proportionality constant K=1.76 ?m·h-1/2 at 1100? and K=3.81?m-h-1/2 at 1150?.(3)The surface microhardness and cross-sectional nanohardness of the coating reach 2400 HV and 40 GPa,respectively,showing remarkable improvements compared to tungsten substrate(500 HV and 8 GPa).According to the indentation method,the average fracture toughness of the WC coating is 2.5 MPa·m1/2.The scratch results indicate that the coating and the substrate have strong adhesion strength at different loading rates(greater than 100N).Compared to the one-step hot-pressing at 1100?,the two-step interstitial carburization with the main carburization at 1150? has higher carburizing efficiency,and the total carburizing time for obtaining a coating with saturated hardness of 2400 HV is shortened from 8 h to 6 h.(4)The wear scar depth between WC coating and YG6 hard alloy ball under normal load of 20 N gradually increases with the time increase.The wear scar depth of the WC coating is 300 nm when wearing for 10 h and the tungsten substrate is 23.936?m,so fabricating the WC coating by the interstitial carburizing greatly improves the wear resistance of the tungsten.The wear between the WC coating and the YG6 cemented carbide ball is dominated by abrasive wear and adhesive wear. |
PDF Full Text Request