Wettability and interface microstructure were studied for Nb-containing melts in the contact with ceramic oxide materials of Al2O3-SiO2 system. Ni-40.5 at.%Nb and Au-(0-20) at.%Nb alloys were investigated as prospective alloys for high temperature brazes. Ceramic samples ranged in composition from pure Al2O3 to pure SiO2. The wettability was measured by the sessile drop method. Microstructure of the oxide/metal interface was investigated by scanning electron microscopy and X-ray diffraction methods. The results showed that contact angles decreased with the increase of SiO2 content in the solid substrate for all the melts. Al2O3 was dissolved insignificantly in the melts under study. SiO2 formed intermediate chemical compounds at the interface with the melts: NbO, Nb6Ni6O and Nb5Si3 with the Ni-based melt and NbO2, Nb2O5 with the Au-based melt. Obtained results allow consideration of Nb alloys under study as high-temperature brazes for the Al2O3-SiO2 materials.
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Manuscript submitted on 21-8-2011 |
Original Manuscript | Wetting and Interface Microstructure in the System of Al2O3-SiO2 Based Ceramics/Nb-Containing Melts |
Nb is a prospective metal for high temperature brazing of oxide ceramic materials. It possesses fairly high chemical affinity to oxygen and can be applied as an adhesion-active additive to inert brazing alloys. It should be noted that chemical affinity of Nb to oxygen is lower than that of Ti. This should prevent the formation of thick intermediate phase layers between filler metal and the ceramic and so should promote joint strength. Brazes, containing Nb as an adhesion-active element, have not yet been developed for oxide materials brazing.
Nowadays joining with Nb is performed most often by diffusion bonding. Ample research has been performed on investigation of mechanical properties and interface microstructures of diffusion-bonded alumina/niobium assemblies. Detailed review of the results on this subject can be found in [1-2]. In many cases, copper interlayer was applied to promote the Nb/Al2O3 joint strength [1-2]. No reaction products were observed in the works [1, 3-5] after joining of niobium with single crystal alumina or polycrystalline alumina of high purity. NbOx layer was detected at the single crystal alumina/niobium interface in the work [6]. However, niobium silicides were observed at the niobium interfaces with low purity polycrystalline alumina [1, 7-9].
Literature data on the wettability of oxide materials by Nb-containing melt are not available except works [10-11]. According to [10], contact angle of Сu-1.4ат.%Nb melt decreases to 20° on Al2O3 substrate after 5 hours of exposure at 1150°C. Though, such low value of the contact angle can be also referred to the melt evaporation during the long high-temperature test. According to [11], Nb improves wettability of Al2O3 by medium Mn steel at the temperature 1600°C. Contact angle of the steel decreases from ca. 90° to ca. 37° with the addition of ~1.15 wt. % Nb.
Adhesion mechanism between pure niobium and aluminum oxide was discussed in the theoretical works [12, 13]. The authors showed the dominating role of the Nb-O bonds comparatively to Nb-Al bonds at the Al2O3/Nb interface. These theoretical findings are supported by the experimental detection of the NbOx layer at the single crystal alumina/niobium interface [6]. The nature of the bond for the SiO2/Nb systems has not been considered in the literature.
The purpose of the present work was to investigate the wettability, adhesion and interface microstructure of chosen Au-Nb and Ni-Nb melts in contact with Al2O3-SiO2 oxide materials (Fig. 1) and finding prospective Nb-containing alloys for the brazing of the oxides. The alloys for the study were chosen after consideration of the corresponding binary phase diagrams [14] and preliminary experimental investigations of some alloys with good Nb solubility and relatively low melting temperatures. Eutectic composition was chosen on the Ni-Nb phase diagram (Ni-40.5 at.%Nb, melting point 1175 °C ). Several compositions were chosen on the Au-Nb phase diagram in the range 0-20 at.%Nb (Au-0.9 at.%Nb, melting temperature ~1070 °C; Au-4.5 at.%Nb, melting temperature ~1140 °C; Au-7.5 at.%Nb, melting temperature ~1150 °C; Au-10 at.%Nb, melting temperature ~1200 °C; Au-20 at.%Nb, melting temperature ~1320 °C).
Fig. (1) Al2O3-SiO2 binary phase diagram and the solid substrate compositions (1 - sapphire, 2 - ceramic VK-94-1, 3 - mullite, 4 - hard porcelain, 5 - quartz glass). |
Capillary and contact phenomena at the interaction of Ni-Nb and Au-Nb melts with Al2O3-SiO2 materials are of practical and theoretical interests due to wide applications of these oxides in industry (high temperature insulators, refractory materials etc.).
Modified equipment [15] was employed to perform wettability tests and surface tension measurements. Experiments were performed in vacuum ~1-3x10-3Pa. Reported contact angles were measured by the sessile drop method during ~30-60 min isothermal exposure at the temperatures 1100-1350 °C. Also, advancing and receding contact angles were observed for several systems during immersion of the oxide plate into the melt with subsequent its extraction at the experimental temperature. Size of the immersed plates was ~20x5x1.5 mm.
Interface microstructure was studied for some samples by optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction methods (XRD). Superprobe 733 and X-ray diffractometer of DRON type were used for the study. SEM images were taken from the cross-sections of solidified sessile drop samples or from a surface of the Au-7.5 at.%Nb/SiО2 sample. X-ray patterns were taken from the surface of the oxide for the Au-7.5 at.%Nb/SiО2 sample and from powder of reaction products for the Ni-40.5 at.%Nb/SiО2 sample. Detailed description of the sample preparations for the XRD analysis will be given in the chapters 3.1.2 and 3.2.2.
Oxide substrates ranged in composition from pure Al2O3 to pure SiO2 (Fig. 1): leucosapphire (Al2O3), ceramic VK-94-1 (92Al2O3 - 5SiO2, at.%), mullite (60Al2O3 - 40SiO2, at.%), hard porcelain (20Al2O3 - 77SiO2, at.%), quartz glass (SiO2). Ceramic VK-94-1, mullite and hard porcelain contained insignificant amounts of other oxides than Al2O3 and SiO2. The influence of these other oxides on the wettability was insignificant due to their low concentration. All substrates were polished up to Ra~0.01 μm prior to the tests and were cleaned ultrasonically with acetone and alcohol.
Vacuum melted niobium (99.9%), gold (99.99%) and electrolytic nickel (99.99%) were used for the study. Ni-40.5 ат%Nb alloy was prepared preliminary by co-melting the components. Au-Nb alloys (Au-0.9 at.%Nb, Au-4.5 at.%Nb, Au-7.5 at.%Nb, Au-10 at.%Nb, Au-20 at.%Nb) were co-melted in situ during the wettability tests. Drop masses were ~200-500 mg.
The work of adhesion was calculated using the equation:
where Wa- work of adhesion, σlv- surface tension of the melt, θ- contact angle.
Surface tension for the Ni-40.5 at.%Nb melt was measured at 1250-1300 °C by the large drop method.
Surface tension of the Au-Nb melts was calculated at 1300 °C using equation for an ideal solution [16]:
where σ is the surface tension of the solution, σi is the surface tension of a pure component i, R is the gas constant, T is temperature, ώi is the area occupied by a mole of the component i as a monomolecular layer, xi and xiώ are molar parts of the component i in a bulk of the solution and in the surface layer respectively. Surface tension data for pure liquid Au and Nb were taken from [17] (1137 mJ/m2 and 1840 mJ/m2 respectively).
Fig. (2) presents data on the wettability of the oxides by the melt depending on SiO2 content in the solid substrate (1250 °C). The wettability improves with the increase of SiO2 content in the oxide from ~90° for Al2O3 to ~9° for SiO2.
Fig. (2) Wettability of the Al2O3-SiO2 oxides by Nb-containing melts in dependence on SiO2 content in the oxide (● - Ni- 40.5at.%Nb, 1250 °C; ■ - Au-10at.%Nb, 1250 °C; ♦ - Au- 20at.%Nb, 1320 °C). |
Surface tension was measured for the Ni-40.5 at.%Nb melt at 1250-1300 °C. The 1610 mJ/m2 value was obtained.
The detailed study was performed for the Ni-40.5 аt.%Nb/Al2O3 and Ni-40.5 ат%Nb/SiO2 systems.
Contact angle of the melt on sapphire was 93±3° at the melting point (1175 °C). The further heating up to 1250 °C resulted in minor contact angle decrease to 90±3°.
Advancing and receding contact angles were observed at 1250 °C at the immersion and extraction of the vertical sapphire plate into the melt. Hysteresis of the advancing and receding contact angles was insignificant ~3-6° (Fig. 3).
Image of the sapphire surface subjected to 45 min contact with the melt at 1250 °C is shown on Fig. (4A). Optical Light Microscopy observations revealed the evidences of the slight dissolution of the oxide in the melt.
Temperature dependence of the contact angle was registered. The contact angle was 27±3° at the melting point (1175 °C) and decreased to 9±2° with temperature rise to 1250 °C.
Advancing and receding contact angles were observed at 1250 °C at the immersion and extraction of the vertical quartz plate into the melt. Advancing contact angle ~9° was formed within 3-5 min after the immersion (Fig. 5A). The formation of a layer of solid reaction products was observed at the perimeter of the wetting after ~5 min of the isothermal exposure (Fig. 5A, zones 3-4). The layer of the reaction products grew with time and enhanced significantly the roughness of the plate (Figs. 5B, C). Excessive plate roughness did not allow the measuring of the receding contact angle. Interface reaction products were formed in macro quantities and detached from the quartz surface after cooling. A typical detached part of the reaction products is presented on the Fig. (5D).
The detached reaction products were collected and powdered for further investigation by XRD analysis. The main components, which have been revealed in the powder, are listed below as well as their crystal lattice parameters:
It should be noted that Nb6Ni6O is an isostructural phase with lattice parameters similar to Nb3Ni2Si phase [18-19]. Structure Nb6Ni6O is more probable for the case under the study due to molar balance of interface reaction products. Namely, general molar ratio Si/O should be ~1/2 for all interaction products (similar to molar Si/O ratio for SiO2).
SEM investigations were performed for the cross-sections of two regions of the interaction layer: 1) melt/interaction products boundary; 2) quartz glass/interaction products boundary (areas 1 and 2 on the Fig. (5D) respectively).
Element distributions at the melt/interaction products boundary showed (Fig. 6) that interface possessed layered structure with alterations of Nb-rich and Ni-rich regions and includes Si-rich regions.
Fig. (6) SEI image of the interface microstructure for SiO2 in contact with Ni-40,5ат%Nb melt and corresponding elemental X-ray maps (the melt/interaction products boundary). |
Similar element distributions were observed at the quartz glass/interaction products boundary (Fig. 7). The difference from the above interface is a more uniform distribution of Nb and Ni and higher Si content.
Fig. (7) SEI image of the interface microstructure for SiO2 in contact with Ni-40,5ат%Nb melt and corresponding elemental Xray maps (SiO2/interaction products boundary). |
The wettability of the oxides by Au-10 at.%Nb and Au-20 at.%Nb melts is plotted in Fig. (2) as a function of the SiO2 content in the solid substrate. The wettability improves with the increase of SiO2 concentration in the oxide, which is similar to the data for the Ni-40.5 at.%Nb melt. Contact angles decrease from ~75° and ~71° on sapphire to ~30° and ~18° on quartz glass with additions of 10 at.%Nb and 20 at.%Nb respectively.
Values of the surface tension 1160 mJ/m2 and 1226 mJ/m2 at 1300 °C were obtained by calculations for Au-10 at.%Nb and Au-20 at.%Nb melts.
The detailed study was performed for the Au-Nb/Al2O3 and Au-Nb/SiO2 systems.
Table 1 presents concentration and temperature dependencies of the contact angles for the Au-Nb melts/sapphire systems. Wettability improves with the increase of Nb concentration in the melt. Contact angles decrease slightly with the temperature rise.
Insignificant movement of the drop of the Au-7.5 at.%Nb melt over sapphire surface was observed at 1250 °C when the time of the isothermal exposure exceeded 30 minutes. The movement led to the appearance of a drop trace on the oxide surface. Usually the dimension of the drop trace was ~0.2-0.3 mm. The drop trace was investigated with Optical Light Microscopy. Indications of sapphire dissolution were observed (Fig. 4B).
Data on the wettability of SiO2 by Au-Nb melts are also listed in the Table 1. Similar to Al2O3, contact angles decrease with the increase of Nb concentration in the melt.
The movement of the Au-7.5 at.%Nb melt over SiO2 surface was observed at 1250 °C and isothermal exposure exceeding 30 minutes. The movement of the drop on the SiO2 surface was more pronounced than on the Al2O3 surface. Macro-trace of the drop (~1 cm) was observed after ~45 minutes of the exposure (Fig. 8). Composition and structure of the drop trace were investigated in detail to define interaction products formed directly at the experimental temperature.
SEI image of the drop trace is presented on the Fig. (9). It demonstrates that the interaction products were formed in the shape of elongated crystals on SiO2 surface. X-ray elemental maps revealed the expected presence of Nb, Au and Si in the interaction layer.
Fig. (9) Top view (SEI image) of the Au-7.5at.%Nb drop trace at the boundary with quartz glass and corresponding elemental X-ray maps. |
X-ray phase analysis from the surface of the drop trace showed, that the main surface phase is NbO2 (tetragonal crystal system, a = 1,36483 nm, c = 0.59642 nm). Nb2O5 phase (a = 2.11676 nm, b = 0.38542 nm, c = 1,92306 nm, β = 120.2210 degrees) and Au traces were also observed in the region under study.
Elemental maps for the cross-section of the Au-7.5 at.%Nb/SiO2 sessile drop sample showed segregation of Nb at the melt/quartz interface (Fig. 10).
Fig. (10) SEI image of the interface microstructure for SiO2 in contact with Au-7.5at.%Nb melt and corresponding elemental X-ray maps. |
High temperature wettability of oxides by liquid alloys is usually accompanied by chemical interaction at the solid/liquid interface. Chemical interaction means dissolution of the oxide in the wetting melt, formation of solid reaction products at the interface, etc. Wettability level and work of adhesion mainly depend on the Gibbs energy decrease during such interaction. Though, they can be also promoted by the metallic type of chemical bond of solid interaction products adjacent to the interface [15, 20, 21].
Au, Ni/Al2O3 and Au, Ni/SiO2 systems are well known as non-reactive systems with contact angle exceeding ~120° [21]. Nb has been added to Au and Ni melts in the present work to promote chemical interaction of the melts with the substrates and thus to promote the wettability.
Experimental results showed that Nb improves the wettability of sapphire by Au and Ni melts (Fig. 2), though the contact angle values exceed 60º. It should be noted that Au-Nb and Ni-Nb melts similarly interact with Al2O3. Some dissolution of the oxide was evident (Fig. 4). New chemical compounds were not observed at the interfaces. Obviously, chemical reactions at the interface and so the formation of new compounds are hampered with high thermodynamic stability of the aluminum oxide. The expected interface reactions possess high positive values of the Gibbs energies and cannot proceed [22, 23]:
The following physico-chemical explanation can be proposed to clear positive influence of Nb on the wettability of Al2O3 by Au and Ni melts. Au-Nb and Ni-Nb melts dissolve Al2O3 (Fig. 4). The dissolution leads to transition of some oxygen from the substrate to the melts. This oxygen participates further in the formation of niobium-oxygen clusters in the melts [21]. Chemisorption of the clusters at the interfaces promotes the wettability. It can be supposed that Ni-based and Au-based alloys possess the similar work of adhesion to sapphire (Fig. 11) due to the similar interface interaction with it.
The work of adhesion increases with the increase of SiO2 content in the substrate (Fig. 11). That results from an intensification of the chemical reactivity at the interface due to lower thermodynamic stability of SiO2 comparatively to Al2O3. Data on the microstructure investigation revealed the formation of new chemical compounds at the Au-Nb and Ni-Nb interfaces with SiO2.
It is necessary to note that thermodynamic affinity of silicon to oxygen is still higher than that of niobium and the reaction below is not thermodynamically favorable [23]:
However, there are reactions with the simultaneous formation of niobium oxides and silicides, possessing negative values of the Gibbs energies [23]:
Thus, silica can react with Nb-containing melt with the simultaneous formation of niobium oxide and silicide. Such processes are thermodynamically favorable.
Indeed, the data of the SEM observations and X-ray phase analysis showed that both Au- and Ni-based melts react with the SiO2 substrate intensively. The reaction products, corresponding to the equations (7-9), were revealed at the interfaces. Namely, Ni-40.5 ат.%Nb/SiO2 interface contains NbO and Nb5Si3. Au-7.5 at.%Nb/SiO2 interface contains NbО2 and Nb2О5 (Nb silicides were not detected which can be attributed to their dissolution in the melt).
Additional research was performed on the wettability of Nb5Si3 substrates by Ni-40.5 at.%Nb melt to prove that the formation of the interface compound with metallic type of chemical bond promotes the wettability. Nb5Si3 for the tests was obtained by melting the components in electro-arc furnace in purified Ar atmosphere. The obtained slab was cut into plates ~10x13 mm. The plates were polished and annealed in vacuum 10-3Pa at 1250 °C prior to the experiments. The results showed that Nb5Si3 is wetted well by the melt. The contact angle was ~25° at melting temperature (1175 °C) and decreased to ~13±2° with the temperature rise to 1250 °C. So, the formation of the Nb5Si3 (compound with metallic type of chemical bond) at the interface assists the wettability.
Obviously, good wettability and high work of adhesion, observed for the (Ni-Nb)/SiO2 system, result from complex action of two factors: 1) the decrease of the Gibbs energy during interface interaction; 2) the formation of the metallic compounds Nb6Ni6O, NbO, Nb5Si3 at the interface that promote the wettability. In contrast to the Ni-based melt, Au-based melts form NbО2 and Nb2О5 oxides that are rather ionic compounds and so do not assist the wettability. The contact angle and the work of adhesion for the (Au-Nb)/SiO2 system are defined by the chemical reactivity (the Gibbs energy decrease) during the interface interaction.
The hypothetical explanation can be proposed to interpret the differences in the interface product compositions formed by the Au-based and Ni-based alloys. Au-based melts contained less Nb, than Ni-based melts. Also, the oxygen removal from the contact zone was more problematic for the Au-based melt than for the Ni-based melt as the solubility of oxygen in Ni exceeds significantly its solubility in Au. Lower solubility of oxygen in Au led to comparatively higher oxygen concentration at the contact zone Au-Nb/SiO2 than at the contact zone Ni-Nb/SiO2. Higher oxygen concentration in combination with lower concentration of niobium resulted in the formation of Nb oxides with higher oxygen content at the boundary with Au-based melt (NbО2 and Nb2О5) comparatively to oxides with lower oxygen content at the boundary with Ni-based melt (Nb6Ni6O and NbO).
The intensive movement of the Au-7.5 at.%Nb drop over SiO2 surface was observed during long isothermal exposure at 1250 °C (Fig. 8). Probably the movement results from the different wettability of lower and higher Nb-oxides by the melt. Namely, lower niobium oxides are formed at the initial stage of the wettability. These oxides are wetted well by the melt. Later, a saturation of the oxides with oxygen takes place and their wettability decreases [15, 20]. Any chaotic contact of the drop with “fresh” SiO2 surface leads to the formation of a new layer of the lower oxides, which is wetted better by the melt. Advancing contact angle onto the new-formed surface is lower than receding contact angle from previously formed interface, which initiates the drop displacement. Probably, the movement of the drop is hardly noticeable at short time of the isothermal exposure due to insufficient time for the saturation of the reaction products with oxygen.
Therefore, the work of adhesion for the systems under study depends on the type of chemical interaction at the interface and type of the solid interface interaction products. The formation of the different oxides at the interfaces Ni-40.5 at.%Nb/SiO2 and Au-7.5 at.%Nb/SiO2 results from the different solubility of oxygen and the different Nb content in the melts.
None declared.
None declared.