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Interfacial Binding at Tic Based Cermets from First Principles Investigations~*

Author: ShenYuFang
Tutor: ZouZhengGuang
School: Guangxi University
Course: Chemical processes
Keywords: First principles Cermets Interface Electronic structures Bonding properties
CLC: O485
Type: PhD thesis
Year: 2012
Downloads: 163
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Abstract


The comprehensive properties of cermets critically depend on the interficial binding properties of metals and ceramics within one device. Due to a large dissimilarity in physical and chemical properties of both phases, the interficial binding properties are influenced by such factors as wetting of metals to ceramics and interfacial stesses, especially the former. Therefore, the study on interfacial of metal/ceramics have been one of the hot topics in cermets field.Generally speaking, there are two aspects of studies on interfaces of cermets, i.e. experimental and theoretical. However, both of them give less microscopic view to wetting and bonding behavior of metal/ceramic interfaces. Especially enrichment of the third phase and defects at metal/ceramic interfaces would have great infulence on the interfacial behaviors and properties of materials. It is therefore very important to explore the microscopic mechanisms of the impact of additives and defects on interfacial properties of cermets.First principles studies of bulk metal Fe and ceramic TiC, pure TiC/Fe interfaces,TiC/Fe interfaces with additives and point defects, TiC(Al2O3)/Ti-Al(Fe-Al) interfaces were made. The interfacial atom structures, electronic structures and interfacial bonding/binding were thoroughly analysized. The thesis are mainly devided into four parts: Firstly, electronic structure and bonding of intrinsic structre of bulk TiC, Fe and their interfaces were studied. Results showed that two main bonding types were formed in bulk TiC, i.e. pdσ covalent bond between Cp-Tid and dda metallic bond between Tid-Tid, while, there are mainly dd metallic bond between Fed-Fed in bulk Fe. The surface energies of both phases show as follows:TiC(100)<TiC(210)<TiC(111-c)<TiC(110)<TiC(111-Ti)<TiC(211)Fe(100)<Fe(110)<Fe(310)<Fe(311)<Fe(111)Studies of adhesion at TiC(100)/Fe(100) interfaces showed that if Fe-C bond formed at the interface, adhesion would reach at a miximum value:2400mJ/m. Moreever, the interfacial bonding properties of TiC/Fe can also be influenced by the number of TiC layers. The interfacial adhesion varied from3.325J/m2,2.106J/m2to1.770J/m2with the number of TiC layers changing from1,2to3. Wetting and binding properties at TiC/Fe interfaces decreased with the increase of the layers of ceramic phase.Secondly, the effect of point defects types and distribution of electroc and bonding properties at interfaces were also studied, especially C vacancies(Vc). Results showed that the formation energy of Vc was1/10smaller than that of VTj. Vacancy formation energy in surfaces was smaller than that of inside bulk. Vc and VTi point defects are both favoriable to wetting and binding properties of TiC/Fe interface. Study on electronic structures and bonding in TiCx(x=0.75-1.0), wetting and adhesion at TiC/Fe interfaces showed that ordered Vc could change the bonding types in TiCx phases, forming new types of bond, i.e. VCp-Tid pdπ which was not found in Vc free TiC. Ti-C covalent bonds and Ti-Ti metallic bonds around Vc became stronger at TiCx/Fe interface compared to VC free TiC/Fe interface, while there showed a increase in ionic bonding in the internal of TiCx phases. Wetting properties at TiCx/Fe interface were improved after the changes in electronic structures and bond types caused by VC. TiC0.75/Fe interfaces showed better wetting and adhesion than that of TiC/Fe. This is mainly due to the fact that the bonding states under fermi level are increased, while the antibonding states up the fermi level are decreased. Morever, three vacancy peaks were found in beta-PDOS of TiC0.75/Fe interface. They are at-0.567eV,-3.985eV and-4.745eV, the former being of d character, and the latter two p-d.Thirdly, the effect of transition metal additives (Me=Mo, W, V, Nb, Co, Ni, Cd, Ag) on interfacial properties of TiC/Fe were analysized. It was found that Mo, W, V, Ni, Co can improve the wetting properties of TiC/Fe interfaces, while Nb, Cd, Ag can’t. The additives may either incorporate into the crystal structures to the metallic phase or the ceramic phase, while the degree of difficulty is different. Mo,W,Co were incorporated mainly into structure of TiC with less amount into Fe phase. However, Ni, V were incorporated mainly into structure of Fe, only lesser amount of them incorporated into TiC. However, it was the lesser part incorporations not the main part that plays a key role in improving the wettability of TiC/Fe interfaces. The key mechanisms in improving the wettablility and adhesion of TiC/Fe interfaces is that:p orbital of C interfacial atom hybridizated with d orbital of Me, forming strong C-Me polar covalent bond, with C-Fe and C-Ti covalent bond strengthened at the same time. Morever, d-d metallic bond were also formed between Me and Fe, Ti.Lastly, interfacial properties of TiC(Al2O3)/Ti-Al(Fe-Al) cermets were studies. Ti-Al/Al2O3cermets were prepared by in-situ SHS method. XRD and SEM eximinations were made to give a detailed information of phases and microscopic structures, which was confirmed to be TiAl-Ti3Al-Al2O3. First principles studies were made to the new structures and it was found that O-Al and O-Ti bonds were formed between Ti3Al-Al2O3interface. O-Al interfacial bond showed more covalent characters than that in bulk A12O3. Studies on TiC/Ti-Al(Fe-Al) interfaces showed that covalent bonding were formed on both TiC/FeAl and TiC/Ti3Al intefaces. They are C-Fe bonds at TiC/FeAl, C-Ti andC-Al bonds at TiC/Ti3Al. It was also found that interfacial bonding between intermetallic phases and TiC ceramics were more covalent characteric than that of bulk TiC. Morever, interfacial bonding in cermets showed more metallic characters than that in bulk ceramics, giving rise to the wettability and adhesion properties of TiC/Ti-Al(Fe-Al) interfaces.

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