| The traditional cemented carbide is a composite material formed by the uniform distribution of the special composite of the uniformly structured carbide skeleton intertwined with carbide ceramic phase skeleton and the metal bondering phase.The uniform structure determines the contradiction between the wear resistance and the fracture toughness of the conventional hard alloy,i.e.,the improvement of wear resistance always means the sacrifice of the fracture toughness and vice versa.This microstructure of the conventional hard alloys-the limitation of macro properties-mechanical property dilemma has severely limited its application in the cutting and machining of hard and difficult-to-machine materials.Functionally graded cemented carbide(FGC)provides an effective way to solve the contradiction problem that the wear resistance and fracture toughness of the conventional hard alloy cemented carbide can not be improved at the same time.At present,carburizing and nitriding are the two main processes synthesizing functionally graded cemented carbide.The preparation method of gradient cemented carbide mainly includes two methods:surface carburizing and surface nitriding.Compared with surface carburizing technology,surface nitriding can produce FGCs with Ti(CN)(Face Centered Cubic Phase)riched surface prepare special structural gradient cemented carbides with a surface rich cubic phase(Ti(CN)),which can improve the cutting performance of the tool to a greater extent.At present,The current research on nitrided gradient cemented carbide is mainly focused on the solid phase nitriding category(i.e.,during solid phase sintering stage).The gradient processed surface obtained is dominated by WC-based cemented carbides,and its performance has not been significantly improved which can only provide limited performance improvement.The current research on the nitriding process during liquid phase sintering of cemented carbide,however,is quite limited at the early stage of trial and error.The research has not been carried out yet about the critical effects of materials design and processing parameters,such as metal binder content,type,WC grain size,nitriding process,especially the nitrogen pressure on the mechanisms of gradient structure formation for FGCs.The formation mechanisms of gradient structure,as the function of the metal binder content,type,WC grain size,nitriding process,and especially the nitrogen pressure,has not been investigated to a sufficient extent yet.In addition,there is almost no research made on the thermodynamic modeling of the gradient structure evolution during nitriding process.The direct outcome due to the research limitation mentioned above is that no technique has been developed yet to accurately manipulate the gradient structure,leaving the huge potential of the combination and optimization of wear resistance and fracture toughness far from realization.This study made a profound investigation on the gradient structure evolution mechanisms regarding to the materials system and nitriding process based on the current research status for nitriding of FGCs.This study is focused on the effect of Co content,different metal binders,WC grain size,and the nitrogen pressure during sintering process on the microstructure of FGCs.The research results reveal the intrinsic relationship among materials composition,sintering process,as well as the gradient microstructure.Furthermore,a thermodynamic model has been successfully developed for nitriding of FGCs.A unique bilayer structured FGC has been synthesized based on this model.The cutting and wearing performance have been investigated in detail to this special FGC.At the same time,the CVD diamond coating process on this bilayer structured FGC has been explored and the coating properties have been evaluated as well.This study shows that:(1)Bilayer structured functionally graded carbides(FGCs),featuring Ti(C,N)based cermet of surface layer and coarse WC based carbide of transition layer,has been synthesized successfully in WC-TiC-Co system using nitriding process at liquid phase sintering stage.The surface layer has higher hardness and wear resistance while the intermediate layer has higher toughness.Both the thickness and microstructure of this bilayer structure can be accurately controlled through the materials design and sintering-nitriding process.This bilayer structured FGC shows superior combination of high wear resistance and high toughness over the traditional cemented carbides.(2)WC grain size,TiC content,as well as the metal binder type plays critical roles on the formation and evolution of gradient structure for FGCs.The increase of both the WC grain size and the TiC content accelerates the formation and growth of bilayer structure.At the same time,the Ni binder always results in a thicker surface layer compared with the Co binder(3)Nitrogen pressure is critical to the thermodynamic behavior of the bilayer structure during nitriding process.The surface layer thickness increases noticeably with the increase of nitrogen pressure.(4)This study develops a systematic outward growth model for bilayer structure of FGCs based on the microstructure analysis and the investigation of thermodynamic behavior of the Ti atom in liquid binder.This model,which clarifies the correlation among carbide materials design,nitriding process,and gradient microstructure,accurately presents the surface layer growth behavior,the interface migration behavior between surface and transition layers,as well as the interface migration behavior between intermediate layer and inner portion.For the first time,this model demonstrates that the square of surface layer thickness is proportional to the square root of the N2 pressure and the nitriding time.(5)The average cutting life of the bilayer structured FGCs developed in this study is significantly increased compared with the traditional cemented carbide of homogeneous microstructure.(6)The diamond coating on the bilayer structured FGC substrate developed in this study shows superior comprehensive properties compared with the diamond coating on the traditional cemented carbide substrate of homogeneous microstructure.This study,which includes the research into gradient microstructure,nitriding process,macro mechanical behavior,cutting performance,and coating properties of a unique bilayer structured FGC,significantly expands and deepens the current research on micro-mechanism,mechanical behavior,and cutting performance of FGCs.Furthermore,this study provides a brand new cutting material system and microstructure based thermodynamic model,and offers the theoretical basement and technical support for high efficient machining of hard to cut materials. |
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