Wetting Oxide and Non-Oxide Ceramics with Active Metals

Abstract

Brazing has been used to join structural ceramics to devices mostly manufactured using metal alloys. Direct brazing employs filler alloys containing an active metal, usually Ti, to wet the ceramic substrate. However, the cost of active filler alloys is usually around 10 times higher than that of active metal-free alloys. In addition, the concentration of the active metal is usually small, thus limiting the formation of a continuous reaction layer onto the entire ceramic surface. Alternatively, ceramic substrates can be previously metallized and fully coated with an active element to be wetted by conventional active metal free filler alloys. Ceramics can be metallized by different techniques, including mechanical metallization. It consists in frictioning an active metal bit, traditionally made of Ti, against the ceramic. Conventional tools can be used and the method is easily automated to large batches. Moreover, contrary to other techniques, mechanical metallization is carried out at room temperature and no hazardous fluxing agents are used. Although Ti is traditionally employed in mechanical metallization, the technique is not limited to it. Indeed, the exclusive use of Ti univocally determines the microstructure of the resulting ceramic/titanium and titanium/filler alloy interfaces. Although the formation of a reaction layer is beneficial to the mechanical strength and reliability of brazed components, precipitation zones and intermetallics embrittle the joints and affect their mechanical behavior. Therefore, the objective of the present study was to assess the potential use of alternative active metals in the mechanical metallization of structural oxide (alumina e zirconia) and non-oxide (silicon carbide and nitride) ceramics. Ceramic substrates were mechanically metallized using Ti, Ta, Nb and Zircaloy 2 (mainly 98.25 % Zr and 1.45% Sn). These metals are abundant in Brazil and therefore strategically important. The wettability of the metallized surfaces was evaluated using three commercially available active metal free filler alloys: VH 780 (Ag-28 Cu), VH 950 (Au – 18 Ni) and SCP 2 (Ag – 31.5 Cu – 10 Pd). The results showed that it was possible to mechanically metallize all ceramic surfaces with the active metals investigated. The wetting tests revealed limited potential for the use of Nb and Ta. On the other hand Zircaloy 2 was successfully employed as active metal for both oxide and non oxide ceramics.