Wetting and reactivity in SiC joining by Ta alloys
Valenza F, Gambaro S, Passerone A, Cacciamani G #, Muolo ML
Institute of Condensed Matter Chemistry and Technologies for Energy – ICMATE-CNR, 6, de Marini, 16149, Genova, Italy
#Department of Chemistry and Industrial Chemistry (DCCI) – University of Genova, Italy
Introduction
SiC based Ceramic Matrix Composites (CMCs) are materials of interest for the aerospace industry
because of their appealing combinations of properties including high thermal and mechanical stability and
low weight. Reliable and effective joining technologies are required to assemble these materials in complex
shapes or to integrate them into existing metallic parts forming hybrid structures. Liquid based processes
(e.g. brazing, transient liquid phase bonding) are often preferable to avoid surface preparation, high
ESA IXV
pressures or when the adjoining surfaces are not flat. In this context, understanding wetting and interfacial Intermediate eXperimental
reactivity of liquid brazing alloys in contact with the materials to be joined, is essential.
re-entry Vehicle
Flaps made of
SiC-based CMCs
Background
Using pure Ni or Co for brazing SiC is unfeasible because of the strong reactivity towards
the ceramic substrate with dissolution and precipitation of graphite and silicides.
Steps of liquid-solid interactions:
1 - Liquid M in contact with SiC
2 - Dissolution of SiC until conc. A
3 - Precipitation of graphite
4 - Dissolution to B through the red line
Materials and methods
Results
(TaC more
stable than
SiC)
 control reactivity
 obtain high-temperature performances
Alloy
Ta
(at.%)
Ta
(wt.%)
TL (°C)
Ta-Ni_1
Ta-Co_1
Ta-Ni_2
Ta-Co_2
14.00
12.00
38.00
50.00
33.42
29.51
65.39
75.43
1366
1290
1395
1655
Low Ta content
High Ta content
Tests were conducted by the sessile drop technique. The
experimental apparatus consists of a furnace made of two concentric,
horizontal alumina tubes connected to a high vacuum system. Angles
and dimensions were measured with the AstraView image analysis
software.
 Atmosphere: vacuum (≈10-4 Pa)
 Metal/ceramic couples introduced
very fast (≈ 30s) into the furnace
when T and Po2 reached equilibrium.
Fast cooling.
Ceramic substrates: CVD-SiC chemically vapor
deposited, fully dense, 99.9995 % pure.
Polished down to 1 µm diamond paste to reach a
roughness Sa = 4 nm, measured using an optical
confocal-interferometric profilometer (Sensofar Sneox) over a 1.75 x 1.32 mm2 surface
Ni + Ta
Co + Ta
Ta has been added to pure Ni and Co in order to:
Surface of the CVD-SiC before
tests (Interferometer)
Low Ta content
Thermodynamic description
Ta-Ni_1, XTa= 0.14, θf = 26° (apparent)
Phase diagrams calculated by Thermocalc
help in interpreting the interfacial behavior.
T = 1368°C
•Dissolution of
ceramic.
•Formation of TaC,
graphite and
silicides
Ta-Co_1, XTa= 0.12, θf = 42° (apparent)
For low Ta content the interfacial behavior is reactive-dissolutive
Ta-Ni_2, XTa= 0.38, θf < 10°
CoTa 12at%
Graphite and silicides are expected
as in the C-Co-Si ternary system
High Ta content
Ta-Co_2, XTa= 0.50,
θf < 10°
T = 1578°C
CoTa 50at%
Metal-ceramic interface:
TaC (2 µm)
Bulk: no silicides, low
amount of Si dissolved
Metallic character of TaC:
better and faster wetting
For high Ta content the dissolution is almost suppressed
No graphite and silicides are expected,
only TaC stopping further dissolution
Conclusions
• A good wetting was obtained for Ta-X (X=Ni, Co) alloys in contact with CVD-SiC substrates.
• The interfacial behavior was found to be the result of the interplay between dissolution of the ceramic phase and formation of interfacial TaC.
• At low contents of Ta, dissolution of the SiC substrate was found similar to the well known systems Ni-Si-C and Co-Si-C.
• High contents of Ta led to the suppression of dissolution and formation of a layer of TaC at the metal/ceramic interface that enhances wetting.
• Thermodynamic description and the calculation of phase diagrams by CALPHAD are useful tools to interpret the interfacial phenomena.
Acknowledgments