Introduction
Lightweight metal and ceramic matrix composites (MMCs and CMCs) reinforced by high-strength continuous ceramic fibers emerge as ideal structural materials in several applications mainly related to automotive, aircraft and aerospace industries because of their superior high-temperature strength, low density and improved damage tolerance (Ref
1). Both their lower specific weight and improved high-temperature stability combined with a better corrosion resistance of composites in comparison with monolithic metallic materials used over the past decades enable to make marketable energy-saving technologies capable to decrease the pollutants emissions. Indeed, in the case of MMCs, by combining lightweight materials such as Al-, Si-, Mg- or Ti-based alloys with high stiffness and high specific strength of carbon fibers, ceramic filaments or even dispersed particulates, it is possible to fabricate advanced materials with improved hardness, tensile strength, elastic modulus and other mechanical properties (Ref
2-
5). Few other properties, such as electrical and thermal conductivities, coefficient of thermal expansion, coefficient of friction, wear, corrosion and fatigue resistances, can also be addressed by a proper selection of the metal phase according to a specific application (Ref
6). However, the higher temperature capability makes CMCs more versatile as compared to MMCs in lightweight applications (Ref
7).
Today, the most extensively studied CMCs are fiber-reinforced SiC matrix composites, namely Cf/SiC, SiCf/SiC and C/C-SiC composites. Despite the manufacturing processes of CMCs have reached a high level of reproducibility, their use is currently limited to a great extent by the difficulties encountered in producing successfully large and complex CMCs shapes, and by their assembling and integration with dissimilar materials, i.e., metals, ceramics or other composites.
Reliable joining of the CMC to a metal phase is essential for preserving the overall composite properties and for saving weight of the overall structure. The joint reliability is ensured by the appropriate joint microstructure resulting from the interaction phenomena occurring at the metal/ceramic interfaces.
Among the techniques used for CMCs joining, the most user-friendly include mechanical joining (i.e., rivets) and adhesives, diffusion bonding, transient liquid phase bonding (solid state processes) and brazing (liquid-assisted process). Mechanical joining and adhesives are suitable only for low-temperature applications. Contrarily, for high-temperature applications, where high strength and corrosion resistance are the key requirements, the active metal brazing is more appropriate (Ref
8).
In general, aiming to preserve the starting SiC matrix thermo-mechanical properties in SiC-based composites, key requirements for a suitable joining technique include the chemical and physical compatibility with the SiC substrate (wettability, thermal expansion, weak SiC dissolution) and joining temperature below 1400 °C in order to avoid fiber degradation in the composites.
As reviewed by Liu et al. (Ref
8), a pressure-less process was developed to join SiC
f/SiC using carbonaceous mixtures as interlayers with a second step consisting of liquid Si infiltration by reaction forming/bonding mechanisms. In this context, several examples are reported in the literature on the use of Si-based alloys as brazing filler materials for promoting successfully joining processes of C
f/SiC and SiC
f/SiC composites by means of interaction mechanisms that control the manufacturing of tailored MMCs and CMCs via capillary and reactive melt infiltration (Ref
9-
12).
In this work, an improved joining technique of SiC-based materials by a one-step Si-based alloy infiltration reaction bonding is reported. As documented in Ref
9-
11, this method is suitable to produce joints with strong interfacial bonding. Moreover, compared with other joining methods, the reactive infiltration has the advantages to obtain cost-less nearly net shaped joints by reducing processing time and without applying any external pressure.
The feasibility study on a novel Si-16.2Ti (in at.%) eutectic alloy (hereafter indicated as Si-Ti eutectic alloy) used as a filler and the application of the above-mentioned joining/brazing technique to SiC
f/SiC at
T = 1350 °C under a vacuum was carried out. Both the constituents exhibit excellent overall mechanical and physical properties, such as low density, high-temperature oxidation resistance (due to the growth of a stable passivating and self-healing SiO
2 scale layer) and radiation resistance, making such assembling very promising in most advanced application fields involving aerospace, aviation, military and nuclear power (Ref
12-
14). In the latter case, the SiC
f/SiC composites are attracting as structural materials for fusion reactors because of their good mechanical properties at high temperature, low chemical sputtering, high oxygen gettering and very low activation at short and medium terms. Additionally, both Ti and Si elements and their alloys (Ref
15,
16) show very low equilibrium contact angles in short brazing time indicating perfect wetting and adherence to SiC. Few previous attempts to use Si-rich Si-Ti alloys as brazing materials are available in the literature. Indeed, the use of Si-Ti eutectic alloy to join monolithic SiC and SiC
f/SiC at the alloy melting temperature was reported in Ref
12. The rather low process temperature [
Tm = 1330 °C (Ref
17)] prevented any degradation of the fiber/matrix interfaces in the composite materials. Moreover, all the joints produced did not show any defects and debonding at the interface. In addition, the joint layer appeared well adherent to both the matrix and the fiber interphase, and the brazing alloy infiltration was successfully controlled. In addition, from room temperature to about 600 °C, the joints of SiC
f/SiC composites exhibit the same value of shear strength of 71 ± 10 MPa. However, the observed joint microstructure consists of two phases of TiSi
2 + Si and, due to the lower melting temperature of Si [
Tm = 1414 °C (Ref
17)], the nominal temperature of use in service is decreased.
Reliable C
f/C joints have been successfully obtained by a novel two-step brazing method (Ref
11) using Si-Ti eutectic alloy. In particular, effects of the infiltration and brazing operating parameters on the phases formed and resulting mechanical properties of the joints were investigated. The authors were mainly focused to produce Si-TiSi
2-type microstructure in the brazing seam by controlling the process parameters. A SiC phase was detected at the interface between Si-Ti alloy and C
f/C composites resulting in a CTE gradient bonding structure.
Dadras et al. (Ref
18) brazed C
f/C composites by using directly TiSi
2 as filler alloy. The joint obtained showed a maximum shear strength of 34.4 MPa at
T = 1164 °C, suggesting the use of TiSi
2 braze for applications at high temperatures. However, the lower melting point and better fluidity of Si-Ti eutectic alloy with respect to Ti silicide make it appropriate for the joining at lower temperature, with a good adhesion and limited SiC dissolution. Furthermore, the reduced operating temperature of the process allows better controlling the microstructure evolution and joint stability as compared to the infiltration process of Si-Ti melts into the SiC
f/SiC composites.
Preliminary investigations of thermophysical properties of the liquid phase, solid phase and the interfacial phenomena occurring between the liquid Si-based alloys in contact with C and SiC, in terms of wettability and reactivity (Ref
19-
23), are required in order to design liquid-assisted processes such as active brazing and reactive infiltration. Accordingly, to properly design the SiC
f/SiC joining by using liquid Si-Ti eutectics as braze material, the experimental studies on the interfacial properties of Si-16.2Ti/GC (Ref
24) and Si-16.2Ti/SiC (Ref
25) systems were performed as a function of temperature.
An overview of the results in terms of wettability, spreading kinetics and developed interfaces is given in the present work. The wettability studies provided evidences useful in properly addressing the CMC assembly and joining mainly by selecting the suitable process parameters.
The quality of the assembly has been evaluated through metallographic examinations on polished sections by optical microscopy and SEM/EDS techniques both on the whole assembly and at the joint interfaces developed between Si-Ti alloy, SiC matrix and SiC fibers.
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