Transient-liquid-phase bonding of UHTCs using refractory-metal-based interlayers for high-efficiency energy-generation applications

Project leaders

Laura Esposito, Noritaka Saito

Agreement

GIAPPONE - JSPS - Japan Society for the Promotion of Science

Call

CNR/JSPS 2012-2013

Department

Advanced Manufacturing Systems

Thematic area

Engineering, ICT and technologies for energy and transportation

Status of the project

New

Research proposal

This project seeks to fill an emerging technological need by developing effective methods of joining high-performance ultrahigh temperature ceramics (UHTCs) using metal-based materials through a fundamental understanding and exploitation of phenomena occurring at the interfaces between joining media and the materials being joined. The collaboration leverages extensive experience in processing UHTCs at CNR-ISTEC, conducting fundamental studies on high-temperature liquid metal-UHTC interactions at Kyushu University (KU), and developing innovative methods of joining UHTCs (ISTEC, KU).
Recent processing innovations, i.e., the identification of effective sintering aids (ISTEC), now enable the fabrication of high-density UHTCs with outstanding thermo-mechanical properties at more accessible processing temperatures and pressures, and have sparked interest in using UHTCs in energy-generation and other applications with severe, high-temperature use environments. Fabricating large complex-shaped ceramic parts is inherently difficult. Thus, practical utilization of UHTCs requires rapid, reliable, manufacturing-friendly joining methods; these are currently lacking. Parallel joining innovations are needed to fill this void. The proposed work executes interrelated critical tasks that 1) exploit the unique capabilities of ISTEC and KU, 2) enhance the educational breadth of participants, particularly students, and 3) are designed to enable formation of joints for high-temperature applications.
ISTEC is a recognized world leader in UHTC processing, has successfully processed a wide range of UHTCs, and extensively characterized their microstructures/properties [1-2]. ISTEC will take the lead in processing optimized UHTCs based on TaC, HfC, and ZrC with and without sintering aids (e.g., MoSi2, TaSi2, SiC). The sintering aids will affect both sintering and liquid-UHTC interactions during joining.
Neither diffusion bonding nor brazing provide the required combination of high-temperature capability and processing ease [3-6]. Transient-liquid-phase bonding (TLPB) emerges as the technique of choice for rapidly obtaining reliable high-performance refractory joints at reduced temperatures [7-8]. Initial tests (ISTEC) have shown that TLPB can produce well-bonded TaC UHTCs in short times at reduced bonding temperatures/pressures, encouraging a more comprehensive study.
In TLPB a multilayer interlayer with a refractory metal core layer flanked by liquid-forming layers (A/B/A structure) is inserted between the parts being joined. A liquid layer forms at reduced temperature between the core layer and the ceramic. The liquid incorporates some of the core layer, and can also dissolve or react with the ceramic. Liquid-former diffusion into the core layer causes the liquid to disappear (be "absorbed") at the bonding temperature. Continued liquid-former redistribution further increases the joint's remelt temperature, in some cases to >1000°C above the joining temperature, allowing elevated-temperature use. The key to successful TLPB is proper interlayer design and understanding how the interlayer and especially the liquid will interact with the ceramic and core layer being joined. Prof. Glaeser (UC Berkeley) developed TLPB of ceramics, has extensive interlayer-design experience, has interactions with CNR-ISTEC and KU on TLPB of UHTCs, and will serve as project advisor.
Wetting experiments will help establish a fundamental understanding of the reactivity/compatibility among the materials being joined, and in optimizing interlayer constituents and designs. Alloys duplicating the liquids formed during joining will be made. Due to the metallic bonding of UHTCs, these liquid alloys are expected to form acute contact angles (q) on UHTC substrates. Chemical changes that further reduce q are generally desirable, promoting liquid flow into interfacial gaps during joining. In joining UHTCs the interactions between the liquid metal, the UHTC and sintering aids can affect q and the amount and nature of the liquid present. Wetting experiments not only characterize the contact angle, but post mortem analysis of the droplet/substrate interface using SEM/EDS can reveal the development of beneficial or deleterious reaction layers and other phenomena (e.g., liquid infiltration) that affect joint integrity. KU has a long tradition of and outstanding facilities for conducting high-temperature (up to 2000°C) wetting experiments in support of TLPB efforts and will take the lead role in this aspect of the proposed work [9-11].
Screening experiments will use interlayers (refractory metals such as Nb and/or Mo coated with Ni or Co) proposed by Prof. Glaeser to bond small pieces of UHTCs to identify promising interlayer-UHTC combinations. The promising interlayers will define the compositions of liquids for more detailed wetting experiments (KU); bulk UHTCs of greatest promise, selected from the carbides ZrC, TaC and HfC, will be fabricated (ISTEC). The failure sources in unsuccessful interlayer-UHTC combinations will be identified to guide remedial changes in the interlayer design, joining conditions, or UHTC phase content. Guided iterative refinement of interlayers, bonding parameters and UHTC formulations will seek to broaden the range of successfully joined UHTCs. Close cooperation, information/materials exchanges, and extensive discussions highlighted by short-term research visits at critical junctures of the project are planned (see work plan).
Both ISTEC and KU have facilities for and prior experience with TLPB (Al2¬O¬3 at KU; carbide UHTCs at ISTEC) and other ceramic bonding techniques allowing task sharing. The screening and wetting experiments will guide the specific interlayer-UHTC combinations explored. Bonding experiments will be conducted primarily in "clean" high-vacuum furnaces under minimal load. Room and elevated temperature (up to 1500°C) mechanical testing will quantify joint strength and toughness. Fractography of failures at the joint will identify failure origins.
Interlayer composition-depth profiles from EPMA, and SEM will be used to assess the extent of liquid-former incorporation, and provide information on process kinetics and reaction phases. Effects of processing temperature, bonding pressure, time, and interlayer design on evolution rates will be quantified. TEM-based examination will be performed at KU at length scales approaching atomic dimensions. Particular attention will be paid to grain boundaries within the interlayer, and at the interlayer/UHTC interface. In combination with the mechanical property data this information will establish necessary processing-microstructure-property relationships.
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Research goals

The proposed research will extend TLP bonding methods to allow the rapid, reliable fabrication of robust joints involving UHTCs, and thus, to enable the full use of their unique properties in a wide range of applications. Enabling the introduction of UHTCs in both energy-generating and energy-consuming systems would allow operation at higher temperatures, increasing conversion efficiencies and potentially decreasing emissions.
By establishing the relationship between the diffusivity-solubility product of the liquid former, the interlayer design, and the isothermal solidification time, the work will enhance understanding of TLP bonding and facilitate identification of viable interlayers for joining a wider range of UHTCs.
TLP bonding allows engineers to take full advantage of UHTCs, while minimizing overall production costs, in contrast to currently available conventional bonding methods for ceramics. Establishing a reliable joining method will promote wider use of UHTCs, provide an avenue for quickly improving existing designs, and for developing new designs with significantly better performance. Such improvements will play a vital role in increasing efficiencies in clean energy-generation applications

Last update: 23/04/2024