EFFECT OF HEAT-TREATMENT IN VACUUM PRIOR TO BONDING ON DIFFUSION BONDING OF TITANIUM, COPPER AND SUS304 STAINLESS-STEEL

The relationship between changes in surface composition heating in a v acuum and subsequent bonding phenomena was studied using an ultrahigh vacuum diffusion bonding equipment with Auger spectroscopy. A variety of materials were used such as titanium, copper and SUS304 stainless s teel. These materials could be joined to produce goad bonds once oxide s and contaminants based on oxygen and carbon were removed from the su rface. Heat treatment in vacuum was found to cause surface oxide disso ciation due to the dissolution of oxygen into the base metal and oxide reduction by carbon. However, Auger analysis of the surface revealed the formation of a sulfur-rich monolayer. It is thought that sulfur (i n the bulk material) segregates to the surface and forms this sulfur-r ich layer. The layer has the effect of preventing the adsorption of im purity elements such as oxygen and carbon which interfere with the joi ning process, but also lowers the bonding temperature. This is because the bonding temperature is determined bg the temperature at which sur face oxides dissociate and since the sulfur-rich layer dissociates at lower temperature than surface oxides based on oxygen, lower bonding t emperatures can be employed. Therefore, a new, lower temperature diffu sion bonding process has been developed. Unlike the conventional bondi ng process where the joints region is heated after the bonding surface s are brought into contact, in this method specimens are treated in va cuum prior to bonding.