| The metals with carbide forming ability in transition metals have wide applications in aerospace and biomedical industries.However,there are some problems such as low surface hardness,low wear resistance and low oxidation resi stance in actual working conditions,resulting in huge economic losses.Therefore,it is important to prepare a protective coating on its surface.Carburization has the advantages of solicd phase transformation,in-situ reaction,and inward growth,which is an important process for surface modification of materials.But the coarse grains and carbon deposition on the surface usually limit the improvement of mechanical properties.In this work,we propose a new technique called interstitial carburization for fabricating carbide coatings on carbide-forming metals.The method selects a solid containing interstitial carbon atoms as the carbon source,and carburizes the carbide forming metal by hot pressing.The coatings are dense,they grow inward,and the volume fraction of carbide phase is extremely hich.Gray cast iron or high carbon steel is chosen as the carbon source to carburize strong carbide-forming metal tantalum,weak carbide-forming metal molybdenum and carbide-forming metal based alloy Ti6A14V,respectively.The microstructure and crystal orientation of the coatings can be analyzed by advanced modern analytical testing methods(XRD,SEM,TEM,EDS,and EBSD).The hardness of the coating in different directions was tested by Vickers hardness tester and nanoindenter,and the bond strength between the coating and the substrate interface was studied The carbide coatings on tantalum,molybdenum and titanium alloys can be successfully prepared,respectively.It demenstrates that interstitial carburization is suitable for various carbide-forming metals and their alloys.And it provides a new way to prepare carbide coatings on the surface of such metals and their alloys.The main features of the carbide coating prepared by interstitial carburizing are summarized as follows:(1)The coating is dense,and the the volume fraction of carbide phase is extremely high.For example,the dense coating of metal tantalum on the surface mainly is TaC,and contains a little Ta2C and unreacted Ta.The volume fraction of TaC and Ta2C is close to 99%.And the Mo2C volume fraction of dense coating on molybdenum substrate is close to 100%,almost pure ceramic layer.Since Ti6A14V subsrtate contains other elements,the volume fraction of TiC in dense coating slightly decreased to about 80%.(2)The carbide grains in the coating are fine,and the overall coating has a gradient microstructure.For example,The TaC grain size increases from 280 nm to 330 nm with increasing depth,and its grain morphology changes from equiaxed to columnar,displaying a gradient microstructure.The molybdenum surface coating consists of a uniform Mo2C columnar crystal with a short axis of 100 nm and a long axis of 1 ?m.Similarly,the microstructure is gradient in TiC grain size which increases exponentially from about 100 nm to 1 mm with depth.(3)The carbide coating grows inward,resulting in a strong bonding with the matrix.No carbon deposition on the surface.The hardness is significantly improved compared to the substrate material.For example,the composite hardness of the carbide coating on the surface of tantalum reaches 1900 HV,which is much higher than the hardness of the substrate(137 HV).The Ti6A14V surface carbide coating composite hardness of about 2400HV is 6 times that of the sub strate(400HV). |
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