A Study On The Interface Valence Electron Structure And Properties Of Ti(C, N)-based Cermets

Valence electron structure of ceramic phases in cermets with different carbides and the interface valence electron structure of (Ti,Me) (C,N)/Ni (Co,α-Fe,Fe3Al,FeAl and NiAl) cermets materials were studied systematically by using Empirical Electron Theory in Solid and Molecules (EET theory) and bond length difference (BLD) method in this paper, and the relationship between interface valence electron structure parameters and mechanical properties of Ti(C, N)-based cermets was also studied. Subsequently, nano-modified Ti(C, N)-based cermets samples were prepared by liquid sintering techniques and the microstructure, mechanical properties and thermal shock performance were investigated.Firstly, the overall development, preparation methods of Ti(C, N)-based cermets and the effect of composition on the microstructure and mechanical properties were introduced. And then basic principles, research progress and application status of Yu's EET theory were described in detail.Secondly, the author investigated the effect of different carbides on the valence electron structure of complex ceramic phases in Ti(C,N) based cermets and then analyzed the relationship among valence electron structure, wettability and mechanical properties (hardness, strength and toughness) of cermets materials. And the interface valence electron structure parameters including nA(covalent electron number on the strongest bond),p (valence electron density on a specific crystal plane) andΔρ(interface valence electron density difference) were calculated based on the former calculations. The author obtain following results:(1) There is a relatively better continuity of valence electron density of (Ti,Mo)(C,N)/Ni system in comparison with other (Ti,Me)(C,N)/Ni cermets (Me stands for WC,NbC,TaC,Cr3C2 and VC) systems. Virtually, corresponding experiments also prove that Mo2C can play a most effective role in improving the wettability of metal(Ni)on ceramic ((Ti,Mo)(C,N)) and the impact sequence of different carbides on the wettability is Mo2C>TaC>WC>VC>NbC when the content of carbides is 10wt.%. (2) There is a more stable and continuous interface for (Ti,Me) (C,N)(110) /Ni (110) or Co(110) system because the interface valence electron density difference is below 10%.Generally speaking, the impact sequence of different metals or intermetallics on the interface valence electron density difference is Ni(Co)>FeAl>α-Fe>Fe3Al>NiAl. (3) After studying the relationship among valence electron structure, interface valence electron structure and strength&toughness the author pointed out that Ti(C, N) based cermets composites will possess better comprehensive mechanical properties if interfaces such as (110)Ni/(110)C and (110)Co/(110)C can be found or obtained in composites as more as possible as far as in-situ synthesis or preparation technique is concerned.Subsequently, the author prepared nano-modified Ti(C, N) based cermets samples and investigated the microstructure, mechanical properties and thermal shock properties of the prepared samples based on the above calculations. The following points are noticeable:(1) X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies found that cermets are still composed of ceramic phases and metallic phases and the coarser ceramic particles exhibit so-called core/rim structures. (2) In addition, a more homogeneous microstructure especially the ceramic phase is obtained with the increase of Ni and/or Co contents and the ceramic grains are found to be finer when the content of Mo increases from 5 wt.% to 15 wt.%. Experimental results also show that the mechanical properties are relatively better than common cermets, that is, the hardness (HRA) is above 91.5 and the bend strength is above 1350MPa. (3) As far as the hardness is concerned, the above calculation results agree well with the experimental results. (4) Thermal shock resistance results reveal that nano-modified Ti(C, N) based cermets possesses a better thermal shock resistance in comparison with conventional cermets material. Experiments also reveal that the quantity and size of the voids as well as the size of micro-cracks in cermets increases with thermal shock cycles (N) and the bending, deflection and bridging of thermal cracks are very apparent when thermal cracks propagate in cermets.