Among current ceramic-metal joining methods, brazing has been regarded as a main technology for its convenience, cost benefits and excellent joint properties. Given that poor wettability of most alloys on ceramic surfaces, the wettability, crucial to the mechanical performance of ceramic-metal joint, could be enhanced by the addition of active elements (Ti, Zr, Cr etc.) owing to inducing interfacial reactions between substrates. The reaction on liquid/solid interfaces and the products are inevitable for a reactive wetting system, which exert great influences on the wetting process and joint properties.
Low-temperature bonding could contribute substantially to the release of residual thermal stress of brazed joints. The relatively low melting temperature of SnAgCu (SAC) solder and its good solderability render it a promising candidate for the low-temperature bonding of ZrO2 and metal.
In the present study, the reactive wetting behavior of SAC alloys with varying additions of active element Ti on ZrO2 surface was investigated in detail. Subsequently, the ZrO2 surface was metallized with SAC-Ti alloys and then bonded with copper at 250 °C for 1 min in atmosphere. The effects of Ti content on the evolution of interfacial microstructure and shear strength of the joints were analysed.
2. Experimental procedure
The mixed Sn0.3Ag0.7Cu-x%Ti (SAC-x%Ti, wt. %, x=1-5) powders, whose melting temperature ranges from 219 ºC to 231 ºC, were compressed into metal cylinders by a tablet pressing machine. The purity of polycrystalline ZrO2 ceramics was 99.5%.
The wetting experiments were performed using the sessile drop method under a high vacuum of 3×10-3 Pa. The cleaned metal bulk was placed on zirconia substrate. For the heating process, the sample was pre-heated to 200 °C, and maintained for 10 min to heat the sample homogeneously. The sample was subsequently heated to 1050 °C, and thereafter furnace-cooled down to the room temperature.
During isothermal wetting experiments, the assembled sample was pre-heated to 200 °C, and then was heated to a testing temperature (700-1000 °C) for 1h. The outline of melt bulk in the wetting process was photographed by a camera with the frequency of 4 frames/min. The contact angles of SAC-Ti alloys on zirconia were calculated. The accuracy of the values is estimated to be ± 2º.
ZrO2 ceramics were covered with SAC-x%Ti powders and metallized at 900 °C for 30 min. The metallized zirconia, SAC solder paste, and copper substrate were assembled as a sandwich structure. Subsequently, the assembly structure, was heated to 250 °C and maintained for 1 min in atmosphere.
The wetting samples, metallization layers, and joints used for microstructural observation were cross-sectioned perpendicular to the SAC-Ti alloy/ZrO2 interface. The interfacial microstructures of SAC-Ti alloy/ZrO2 interfaces were characterised using SEM equipped with EDS. The reaction phases on the SAC-4%Ti droplet/ZrO2 surface of etched zirconia were analysed using XRD. Five shear samples were tested for the joints pre-metallized with different Ti contents to obtain the average values and deviations of shear strengths. After shear test, the fracture of the joints was inspected using SEM.
3. Results and discussions
The wettability of the SAC metal powder on zirconia was improved with the addition of Ti. The contact angle decreased from 119° to 8° as the Ti content was increased from 0 to 5 %, and the lowest contact angle was obtained at the Ti content of 4 %. The matrix of the droplet was β-Sn, and some Ti6Sn5 and Ti3Sn were distributed in it. A TiOx reaction layer was formed at the droplet/zirconia interface, consisting of Ti2O, Ti4O7, Ti7O13 and TiO2 phase characterized by TEM.
The dynamic contact angle of SAC-Ti/zirconia system decreased with the thermal holding time prolonged at different temperatures. During the isothermal wetting process, consisting of rapid-spreading stage and equilibrium stage, the equilibrium contact angle decreased as temperatures increased. The spreading kinetics could be controlled by interfacial reaction and the wetting activation energy was calculated to be 108.8 kJ/mol.
A Sn-based metallization layer was successfully prepared on zirconia surface at 900 °C for 30 min with SAC-x%Ti metal powders. A continuous and relatively flat metallization layer merged with zirconia for the Ti content of 4 %. With the increase of Ti content, more Ti-Sn IMCs were formed in the metallization layer.
The metallized zirconia was joined to copper at 250 °C for 1 min in atmosphere with an SAC solder paste. The typical microstructure of the copper/SAC/pre-metallized zirconia joint was copper/Cu6Sn5 layer/β-Sn layer containing Ti3Sn and Ti6Sn5 phases/Ti3Sn layer/TiOx layer/zirconia. The shear strength of the joints increased first and decreased with the increase of Ti content. The highest value of 19.1 MPa was achieved for SAC-4%Ti, and meanwhile the joints mainly cracked along the reaction layers adjacent to zirconia and expanded to the seam.
The wetting behaviour of Ti-containing SAC on the surface of zirconia was investigated. Zirconia metallized with SAC-Ti was bonded to copper using an SAC soldering paste at low temperature in atmosphere. The microstructure and shear strength of the copper/SAC/pre-metallized zirconia joints were studied. The conclusions are summarized as follows:
The Ti content had a significant influence on wetting behavior and droplet shape by controlling interfacial reactions. The wettability of the SAC-x%Ti/zirconia system was obviously improved with the increase of Ti content due to the advanced Ti-O interfacial reaction products.
The morphology and microstructure of the metallization layer varied significantly as the content of Ti increased. A continuous and relatively flat metallization layer was obtained when the Ti content was 4 %.
The typical microstructure of the copper/SAC/pre-metallized zirconia joint bonded at 250 °C for 1 min in atmosphere was copper/Cu6Sn5 layer/β-Sn layer containing Ti3Sn and Ti6Sn5 phases/Ti3Sn layer/TiOx layer/zirconia.
Keywords: Wettability; Contact angle; Interfacial microstructure; metallization; Ti; ZrO2.