A Study On Processing And Microstructure Of Functionally Graded Cemented Carbide With FCC Phase Riched Surface Layer

Functionally graded cemented carbide?FGC?shows the advantage of improving both hardness and fracture toughness simultaneously,which is usually a technical hurdle for conventional cemented carbide with homogeneous microstructure.At the same time,FGC is a great candidate as the substrate for coating applications.FGCs with FCC phase riched surface layer have drawn wide attention recently.This type of FGC has the surface layer of face centered cubic phases such as Ti?CN?or TiN,which provides better wear resistance,oxidization resistance,and crater resistance for surface layer as compared with core portion,which enable its wide applications in high-speed machining.Currently,the main processing for FGCs with FCC phase riched surface layer includes solid state nitriding and low pressure nitriding,while the research on high pressure nitriding??4 MPa?or Ar-N₂ combined nitriding have rarely been reported internationally.The current material system of FGCs with FCC phase riched surface layer is limited to WC-TiC-Co,little research has been carried out on new material development,which hampers FGCs with FCC phase riched surface layer from fully exerting its technical advantages.This study made a new material development beyond the current WC-TiC-Co system for FGCs with FCC phase riched surface layer.The effects of binding phase type?Co or Ni?,source of Ti?from either TiC or Ti?,and high pressure nitriding processes on the microstructure of FGCs were investigated comprehensively.Furthermore,the CVD diamond coatings have been successfully made on the FGC substrate processed in this study without any pretreatment,which proves to be impossible for conventional carbide substrate.It was found in this study that:1.For WC-TiC-Co alloys,high pressure nitriding process results in a surface layer enriched with Ti?CN?.For WC-Ti-Co alloys,high pressure nitriding process results in a surface layer enriched with TiN and the surface layer is much thinner but denser than Ti?CN?riched surface layer of WC-TiC-Co alloys.2.Compared with Co binding phase,Ni binding phase results in significantly thicker surface layer enriched with FCC phase for WC-TiC/Ti-Co/Ni alloys.3.For WC-TiC/Ti-Co/Ni alloys,the study on two-step sintering of Ar presintering+high pressure nitriding shows that the Ti?CN?riched surface layer thickness increases with TiC content while the TiN riched surface layer thickness decreases with Ti content when the TiC or Ti content is over 5 wt.%.4.For WC-TiC-Co/Ni alloys,the Ti?CN?riched surface layer has lower hardness than intermediate transition layer and core of the alloys,while for WC-Ti-Co/Ni alloys,the TiN riched surface layer has higher hardness than intermediate transition layer and core of the alloys.5.Compared with direct nitriding process,the two-step sintering process with Ar pre-sintering+high pressure nitriding results in higher density,thicker FCC riched surface layer and lower surface roughness.6.Compared with two-step sintering process,the one-step sintering process shows the advantages of higher efficiency and lower cost.It is found that for WC-TiC-Ni/Co alloys,one-step sintering of Ar+N₂ combination?the partial pressure of Ar is 1.2 MPa and the partial pressure of N₂ is 4.8 MPa?results in the good gradient structure,and for WC-Ti-Ni/Co alloys,one-steop sintering of Ar+N₂ combination?the partial pressure of Ar is 4.8 MPa and the partial pressure of N₂ is 1.2 MPa?results in the good gradient structure.7.The CVD diamond coating can be successfully made on FGC substrates of WC-TiC/Ti-Ni/Co alloys with FCC phase riched surface layer without any complicated pretreatment on the surface of substrate while it proves difficult to make CVD diamond coating on conventional carbide substrate without any gradient structure.8.The diamond coating on FGC substrate with TiN riched surface layer has better overall properties than that on FGC substrate with Ti?CN?riched surface layer.