Introduction
Aluminium Oxide Ceramics due primarily to its good heat conductivity (of about 30 W´m-1´K-1) and a relatively low cost is considered as an attractive material in designing of various components when “thermal management issues” are of concern. The high thermal conductivity of the Alumina combined with large surface area of the substrates result in the rapid transfer of thermal energy to the surrounding. This cools the heat sink and whatever it is in direct thermal contact with. Use of coolants (e.g. CO2) ensures good transfer of thermal energy to the heat sink.
On the first sight, a specification of the industrial problem is rather simple. First, a system of rectangular grooves (with dimensions of an order of 1 mm and distance in between of about 1 mm) can be machined at the surface of the alumina plate, and then two of such “surface patterned” ceramic plates is bonded (“face-to-face”) together. As a result, a system of “channels” inside a (bulk) ceramic substrate will be created.
In the present project, brazing was chosen as a bonding technique. In this MEMO we will discuss a number of brazing experiments involved metalized (with different conductor materials) alumina and AgCu28 alloy as well as preliminary results on “active” brazing of “bare” Al2O3-ceramics using AgCu26.5Ti3 filler metal
Experimental
Brazing of the Alumina metalized with three different conducting materials was performed in vacuum furnace at using AgCu28 filler metal. “As-received” metalized ceramic plates have dimensions 40 ´ 40 ´ 2 mm. The AgCu28 brazing alloy was used in the form of 100 mm foil.
“Active” brazing of “bare” Al2O3-ceramics (plates 20 ´ 20 ´ 2 mm) was carried out in the same vacuum furnace using 200 mm foil of AgCu26.5Ti3 filler alloy. In each case a “small load” was applied upon the brazing assembly as shown in Fig. 1.
a) b)
Fig. 1: General views of the brazing assembly (prior to placing in the furnace) based on: a) metalized (with different conductor materials) Al2O3-ceramics and AgCu28 filler alloy; b) “bare” Al2O3-ceramics and AgCu26.5Ti3 “active” filler metal (Sample MT 12-181). Note. In each case a “small load” of about is applied upon the brazing assembly.
After standard metallographic preparation all brazements were examined using Optical microscopy, Scanning Electron Microscopy (SEM) and Electron Probe Microanalysis (EPMA).
In order to facilitate further discussion, it seems worthwhile to provide at this point some general information about metallization techniques used in the present investigation.
A thick-film (“hybride”) metallization is constructed on a ceramic substrate by using fine metallic (Ag, Ag/Pd) particles mixed with binding materials (low-melting point glass or copper oxide) to promote adhesion to the substrate and to hold the metal particles in contact. The metal particles and binding material are mixed with an organic material to form a relatively thick mixture called an ink or paste. The majority of the commercially available inks are thixotropic, i.e. these pastes are thick (viscous) under normal conditions, but flow (become thin, less viscous) over time when shaken, agitated, or otherwise stressed.
Generally, the binding consists of fine particles with a low melting point, such as lead-borosilicate glass (“solder glass”). Most of the solder glasses have a lead oxide (PbO) content ranging from 70 to 85 wt.%, boron oxide (B2O3) from 10 to 20 wt.%, and silicon dioxide from 10 to 20 wt.%. Thermal expansion may range from 8 to 12 ´ 10-6 K-1, and “softening point” is between 350 and 500 °C. It is also relevant to mention that a small addition of ZnO and Al2O3 is frequently used to modify the desired properties and to improve the chemical stability of the glasses. The glass holds the metallic particles in contact and promotes adhesion to the ceramic by reacting with the substrate during the firing process (usually at ~850 °C). The glass has a lower “melting” point than the metallic particles (Ag or Ag/Pd) and, during firing, wets the
metallic particles, suspending them in contact with each other. Physico-chemical interaction is also takes place between the glass and the ceramic substrate.
Another type of binder material is copper oxide, which forms “a chemical bond” between the metallic phase and ceramic substrate and is used instead of glass particles.
The purpose of the organic binder is to hold the metallic and glass particles in suspension prior to firing and to provide the proper fluid properties for coating application or screen printing.
Conductor pastes can be classified into three types of their binding agent/adhesion mechanisms: fritted, fritless, or mixed binding systems. As to the metallization systems employed in the present study, a low cost, oxide-bond (fritless) pure Ag conductor material and screen printable pastes based on fritted conductor materials with the Ag:Pd ratio in the metallic phase 4:1 and 6:1, respectively.
It is important to emphasize that all these metallization systems have been developed to promote wetting of the Al2O3-substrates by “soft” solders (e.g. Sn62Pb36Ag2, etc.).
Perhaps, one more thing is to be mentioned here. Glass frits having a low softening temperature may be used by themselves to join ceramics to each other (and to metals) if they are being used in such a thin layers that they become “incorporated in the crystal lattice” on heating so that operation of the joint is then possible at substantially higher temperatures than suggested by the “bulk” softening temperature of the frit.
Results and Discussion
- Brazing of metalized Al2O3-ceramics
It appeared to be not possible to join Al2O3-plates metalized with the oxide-bond (fritless) pure Ag) conductor material. This can be appreciated simple by looking at the optical images shown in Fig. 2. Clearly, under the experimental conditions, no wetting of the ceramic substrates by the Ag28Cu brazing alloy took place. Most likely, during the brazing
Fig. 2: Optical image of the “abutting” surfaces of the Al2O3-ceramic parts initially metalized with the oxide-bond conductor material after an attempt of vacuum brazing with the AgCu28 alloy Note that virtually no wetting of the ceramic surfaces by the brazing alloy occurred under experimental conditions
cycle the metallization layer initially present at the ceramic surface was (“thermally”) disintegrated, and “bare” Al2O3-surface was exposed to the liquid AgCu28 alloy. This alloy is not an “active filler metal”, and no significant wetting of the alumina can be expected.
In contrast, Al2O3-ceramic plates initially metalized with fritted conductor materials, can be joined together using a conventional Ag28Cu brazing alloy. However, microscopic examination of the brazement cross-sections uncovered remarkable discontinuity of the brazing seam (Figs. 3 and 6).
Fig. 3: Cross-section (optical image) of the metalized with fritted conductor material Al2O3- ceramics joint brazed in vacuum using AgCu28 filler alloy. Sample MT 12-173. Note: Discontinuity of the brazing seam is readily apparent.
Close inspection of the brazing seam in the vicinity of the filler metal/ceramics interface revealed some “residues” of the metallization material not only in the areas adjacent to the Al2O3- ceramics, but also inside of the filler metal. (Figs. 4, 5, 7, 8). It is conceivable that during brazing cycle, when sample temperature exceeds softening temperature of the
Fig. 4: Microstructure of the brazing seam (optical micrograph, bright-field image) in the Al2O3 / Al2O3 joint shown in Fig. 3. Surfaces of the initial ceramic components were metalized with fritted conductor material and brazed in vacuum using AgCu28 filler alloy. The image is taken from the “sound” domain of the brazement cross-section. Some “residues” of the metallization material found in the filler metal after brazing are indicated by arrows. (See also Fig. 5.)
a) b)
Fig. 5: a). Back-scattered Electron Image (BEI) of the brazing seam in the Al2O3 / Al2O3 joint shown in Figs. 3 and 4 (sample MT 12-173; fritted metallization system) taken in the vicinity of the filler metal/ceramic interface; b). X-ray spectrum taken with the Energy Dispersive Spectrometer (EDS) from the area depicted as a rectangular in (a). Note the presence in the X-ray spectrum of the characteristic lines of Pb, Si and Zn
glass constituent of the metallization (350 -500 °C), disintegration of the surface (inter)layer occurs. Most likely, metallic phase (Ag/Pd) of a coating system is incorporated (dissolved) into
liquid brazing alloy, and some remnants of the glass constituent (“frit”) present in the metallization layer can be detached from the ceramic interface.
It is also important to notice from the X-ray spectra shown in the Figs. 5b and 8b that fritted conductor materials C2040 and C2060 used in the present investigation contain Zinc Oxide (ZnO). As already explained, small additions of ZnO (and Al2O3) is frequently used to modify the desired properties and to improve the chemical stability of the “solder” glasses.
Fig. 6: Cross-section (optical image) of the metalized with the fritted conductor material Al2O3- ceramics joint brazed in vacuum using AgCu28 filler alloy. Sample MT 12-172. Note: Discontinuity of the brazing seam is readily apparent.
Fig. 7: Microstructure of the brazing seam (optical micrograph, bright-field image) in the Al2O3 / Al2O3 joint shown in Fig. 6. Surfaces of the initial ceramic components were metalized with the fritted conductor material and brazed in vacuum using AgCu28 filler alloy. The image is taken from the “sound” domain of the brazement cross-section. Some “residues” of the metallization material found in the filler metal after brazing are indicated by arrows. (See also Fig. 8.)
a) b)
Fig. 8: a). Back-scattered Electron Image (BEI) of the brazing seam in the Al2O3 / Al2O3 joint shown in Figs. 6 and 6 (sample MT 12-173; fritted metallization system) taken in the vicinity of the filler metal/ceramic interface; b). X-ray spectrum taken with the Energy Dispersive Spectrometer (EDS) from the area depicted as a rectangular in (a). Note the presence in the X-ray spectrum of the characteristic lines of Pb(Bi?), Si and Zn
- Preliminary results on the active brazing of the “bare”Al2O3-ceramics
As expected, Al2O3-ceramics can be directly joined (without any prior metallization) by means of the “active brazing” (Fig. 9). In this work, AgCu26.5Ti3 “active” filler metal was employed. Titanium is an activator used in commercial brazes, reflecting the singular characteristic that it can form very wettable reaction products.
a) b)
Fig. 9: Optical Images of the “bare” Al2O3-ceramic components after brazing using AgCu26.5Ti3 “active” filler metal (Sample MT 12-181). The formation of the “fillet” during the brazing process is clear visible. Note some “dark coloration” of the filler metal at the “edges” of the brazing seam.
Although no sample cross-sections have been examined yet, the joint looks to be sound. Also, the formation of the “fillet” during the brazing process was observed, which underlines good wetting of the ceramic surfaces by brazing alloy. The puzzling question that still remains at this moment is a clear visible “dark coloration” of the filler metal at the “edges” of the brazing seam.
Concluding Remarks
1). It is clear now that in our case “high-temperature brazing” of the Al2O3-ceramics metalized with the oxide-bond (fritless) pure Ag conductor material as well as with the fritted screen printable pastes are not a viable option. It is necessary to emphasize once again that all these metallization systems have been developed to promote wetting of the Al2O3-substrates by “soft” solders.
2). As to the “active” brazing, it might be, in principle, a solution, although other filler metals, like for example, those containing Indium and/or Tin should also be taken into further consideration. From a practical point, the following question might cause uneasiness here. “In what way should an “active filler metal” be (pre-) placed in the brazing assembly in order to keep a system of “channels” inside a (bulk) ceramic substrate open?” It is important to realize that upon brazing “active” filler metal will wet (indiscriminately) all ceramic surfaces including those of the rectangular grooves machined at the surface of the alumina substrates.
Obviously, any bonding technique involved prior metallization of the ceramics is free of this problem. A system of rectangular grooves can be machined at the surface of the coated (metalized) alumina substrates, and during subsequent brazing with a conventional (not an “active”!) alloy, wetting will occur exclusively alone the remaining metalized surfaces.
3). Of direct relevance to the last issue is the use of unstable TiH2 coatings. This process consists of applying TiH2 to the alumina surface as fine mesh material suspended in volatile carrier or binding agents such as aqueous ethylene glycol or methyl methacrylate. The hydride dissociates when heated in a vacuum to 300-500 °C to leave a layer of “fresh” metallic titanium on the ceramic surface which can react to adhere. Thus the process need merely involve a dwell or slow ramp through the dissociation range as the ceramic workpieces are heated to the brazing temperature. Conventional, titanium-free (not “active”!) braze alloys will wet the alumina surfaces where the TiH2 has been applied successfully but not elsewhere, and hence their flow is controllable. Unfortunately, many of the details of this technique are not characterized or described in the open literature.
