Controlled Oxidation And Wettability Investigation Of Kovar Alloy

Oxidation of kovar alloy is one of the key technologies for good kovar-glass sealing. It is impossible to achieve glass-kovar sealing directly until kovar is oxided. Because oxide film of kovar, Fe3O4, which has the very close coefficient of thermal expansion with substrate, can react with molten glass and form a transition layer to achieve a strong chemical combination. Therefore, how to obtain a uniform, compact and controllable thickness of a single oxide of Fe3O4has become the focus of this thesis.In this thesis, kovar alloy was oxided under N2(H2O)/H2atmosphere, in which accurate low oxygen partial pressure was obtained by controlling partial pressure ratio of H2O/H2. There is an orthogonal experiment eatablished about flow rate ratio of N2(H2O)/H2, oxidation temperature and N2flow rate.In order to optimize the best oxidation process parameters including flow rate ratio of N2(H2)/H2, oxidation temperature and N2flow rate, the effects of oxidation temperature and N2(H2O)/H2flow ratio were analyzed respectively, by means of XRD, SEM and materials testing machines et al, on the composition, morphology and thickness of kovar oxide film, wettability and cycle stability of kovar-to-glass seals. Chemical elements distribution analysis of oxides obtained under the optimum conditions, along oxides thickness and surface direction,was carried on by AES and XPS technology. Finally, the kinetics growth law of oxide layer under optimal process conditions was also studied.The results show that as flow rate ratio of N2(H2O)/H2increased at700℃and750℃, single Fe3O4phase was generated only; when N2(H2O)/H2=140/10,170/10and200/10at800℃, the oxide was for single Fe3O4, while N2(H2O)/H2=250/10for Fe3O4+FeO and N2(H2O)/H2=300/10for nothing; When N2(H2O)/H2=140/10,170/10,200/10and250/10at850℃and850℃, Fe3O4+FeO were also obtained, while N2(H2O)/H2=300/10for nothing.The system oxygen partial pressure depended on N2(H2O)/H2flow rate ratio, under same oxidation time and same oxidation temperature conditions, while the actual oxygen partial pressure of substrate surface on N2flow rate. When N2flow rate was small, increasing flow rate ratio of N2(H2O)/H2appropriately could improve oxygen partial pressure of the system and promote size and uniform of oxide grain; when N2flow rate was bigger than certain critical value, the opposite effect happened.Then the number of oxygen molecules and the actual oxygen partial pressure of surface would be lower,thus,which made oxide grain smaller.Under constant oxidation time and same oxidation atmosphere conditions, the size and uniform of oxide grain were related on oxidation temperature. As the oxidation temperature increased, the diffusion coefficient of oxygen molecules also increased and the size and uniform of oxide grain densification and film thickness raised.The results of the composition, uniform, thickness of oxide and wetting angle, tensile shear strength and thermal cycle stability show that the oxide layer obtained under the conditions of N2(H2O)/H2=200/10at800℃only met requirements of single component, uniform and dense infiltration performance. The minimum of wetting angle and the maximum of tensile shear strength were of3.01°and7.16MPa; There was oxide layer composed of single Fe3O4, the grain size of13.50, and9μm thick. Cracks existed after0/300℃thermal cycles for about100times. Kinetics growth law followed the parabolic line.In addition, the stability of mechanical properties of Kovar alloy before and after oxidation should be considered necessarily. The treatment of burning hydrogen was carried on before oxidation.In order to investigate stability of mechanical properties of Kovar, phase, grain size, and tensile strength were analyzed. The results show thatthe grain of kovar replied, recrystallized and formed equiaxed grain in treatment of burning hydrogen, completing structural transformation of bcc→fcc, thus improved tensile strength and ductility of Kovar alloy, and could maintain its own stability of mechanical properties. Through secondary burning hydrogen, it also proved oxidation treatment before sealing did not affect the stability of mechanical properties.