Effect Of WC On Microstructure Evolution And Properties Of Ni Based Alloy Coating With Directional Structure

In this study,the flame spraying technology and induction remelting as well as forced cooling methods were used to prepare WC-strengthened Ni60/20%WC directional structure coating at different induction remelting temperatures(800?,900?,1000?,1100?,1200?).The influence of different induction remelting temperatures on the morphology,growth behavior,element distribution and properties of grain boundaries and grains in the directional structure coatings reinforced by WC were investigated,and the optimal remelting temperature was determined;After that,Ni60 prefabricated,remelted and directional coatings with different contents of WC(5%,20%,35%,50%)reinforced were prepared by different processes at the determined optimal remelting temperature.The effects of different contents of WC on the microstructure evolution,phase composition and mechanical properties of the composite coatings were studied.Secondly,the friction and wear properties of directional structure coatings with different WC content were studied.The influence of increasing WC content on the friction and wear properties of directional structure coatings was discussed.The wear mechanism and friction properties of directional structure coatings enhanced by WC were systematically analyzed.The results are as follows:(1)As the remelting temperature increases from 800°C to 1100°C,no obvious directional structure is formed in the coating,which is mainly characterized by a condensed fine structure,and the lamellar eutectic in the coating transforms to the structure of massive W compound and the embedded lath structure of massive W compound;When the remelting temperature further rises to 1200°C,the directional structure of the coating is more obvious.Because the alloy coating at this temperature has a large overheating state,the strong diffusion of remote W elements and the large temperature gradient during the rapid solidification process lead to change in the structure of the directional structure coating.At this time,the coating structure is dendrites,columnar crystals,and equiaxed crystals.In addition,WSi₂,W₂B and other phases formed jointly play a blocking and strengthening role on grain boundaries.(2)At different remelting temperatures,the edges of most WC particles in the coating were micro-melting,and the form of epitaxial growth precipitates whiskers,strips,and needles structure around the WC particles.After solidification,the WC particles show an elliptical form and are preserved at the interface;With the increase of remelting temperature,the lamellar eutectic in the coating gradually disappears at800??1100?,and the precipitated massive W compound gradually grows up,and finally presents a regular quadrilateral structure and is embedded in the edge of the lath grain boundary;Due to the different microstructure of the directional coating at different remelting temperatures,the hardness of the coating has also changed.At1200?,a large amount of hard strengthening phase is precipitated in the directional structure coating,which makes the microhardness the largest.The average value is A?about 699.8HV.(3)After induction remelting and induction remelting and forced cooling treatment of precast coatings with different WC content,as the WC content increases,the diffusion of W element in the coating and the increase of unmelted WC particles at the interface.It promotes the transformation of the coating structure and the growth of grains at the interface.The remelted coating structure is columnar crystals,equiaxed crystals and a small amount of randomly distributed dendrites.Directional structure coatings has columnar crystals,dendrites and a large number of equiaxed crystals grown in specific orientations along the direction of heat flow,and the coating structures prepared under the two processes grow up at the interface;The micro-melted WC at the interface of the directional structure coating will affect the growth of dendrites,making them show the characteristics of discontinuous,combined and preferential growth;In addition,the increase in WC content promotes the diffusion of W element and the formation of hard phases.The new phases such as WSi₂,W₂B,Ni W,Fe₆W₆C appearing in the directional structure coating are filled in the grain boundaries,evenly distributed,supporting and strengthening the grain boundaries,protecting the soft grains,and can greatly improve the coating's comprehensive performance.As a whole,the hardness of the induction remelting and directional structure coatings increases with the increase of WC content,but the difference between the two is not too large.(4)The friction coefficient and wear rate of the directional structure coating first decrease and then increase with the increase of the WC content in the coating;When the WC content is 20%,the friction coefficient and wear rate of the directional structure coating are the smallest,which are 0.274 and 1.21×10^-6 mm³/Nm,respectively.The wear resistance of the directional structure coating is the best under this composition;With the increase of WC content,the wear mode of the directional structure coating changes from fatigue wear and abrasive wear at 5%WC content to slight wear at 20%WC content.When the WC content is 35%,it is slight peeling and adhesive smearing.When the WC content increases to 50%,it is mainly adhesive wear and slight abrasive wear.