Investigation On The Formation Mechanism And Properties Of In-situ NbC/Fe Surface Composites

The NbC/Fe surface composites were prepared by in situ synthesis which combines casting with the heat treatment technique.And,micro-nano NbC compound layer was formed on the surface of HT300 substrate.The physical and chemical properties,microstructure of the composites were characterized using scanning electron microscope(SEM),transmission electron microscope(TEM)and energy dispersive spectral(EDS)techniques.Growth kinetics of NbC compound layer was investigated by the classical parabolic theory and atomic diffusion mechanism.The wear behavior of NbC/Fe surface composites was studied in dry sliding friction conditions through the tribological theory.Adhesion strength of NbC layer/substrate interface,and failure behaviour were explored use a single scratch experiment.The research results show that:(1)Niobium carbide(NbC)compound layers on gray cast iron(HT300)substrate are prepared by in situ synthesis at 1070,1100,or 1130? for 15,30,45,or 60 min.The compound layer consists of NbC and ?-Fe phases.The average grain size of NbC is approximately 400nm at 1100? for 60 min.The growth kinetics of NbC compound layer by in situ synthesis is examined by measuring the depth of the NbC layer as a function of time and temperature.The estimated growth rate constant varies from 1.84×10-9cm2/s to 2.57×10-9 cm2/s,and activation energy of the NbC layer is calculated as 86.01 kJ/mol.(2)The gradually decreased distribution of the NbC volume fraction with the increase of grain size along the depth direction of the composite profile leads to the different microhardness and elastic modulus in compound layer,diffusion zone and substrate.The tribological properties are evaluated under dry sliding,conditions using a.pin-on-disc sliding wear test system.The friction coefficient of the surface compound layer is lower than that of the diffusion zone and iron substrate.Also,the NbC compound layer improves the wear resistance of substrate surface.The improvement in the friction and wear properties is owing to enhancement of surface hardness,due to refined and stiff NbC grains.The worn surfaces observed using SEM reveal the dominant adhesive nature of wear and mild abrasive wear in compound layer.The abrasive wear mainly occurs in diffusion zone.(3)NbC compound layer(thick 21.3 ?m)on the surface of iron substrate is produced by in situ synthesis at 1070? for 10 min.The analyses reveal that the layer consists of dense NbC grains and ?-Fe phase.The grain size of NbC is 350 nm on average.Owing to the ultrafine-grained NbC particles improve the microstructure,the hardness and elastic modulus of compound layer are 21.1 GPa and 376.1 GPa,respectively.The critical load(LC)in the micro-scratch test is used as a measure of scratch adhesion.The adhesion strength at NbC layer/substrate interface is 95 N in which NbC compound layer was delaminated from the substrate.The result indicates that the compound coating exhibits excellent adherence to the substrate.The deformation behavior by micro-scratch demonstrates that the layer is brittle fracture at high load,cracks propagate along the grain boundaries,and the failure of the compound layer is caused by the interaction of radial and lateral cracks.