Controllable distribution of reinforcements for reducing the strain energy in dissimilar ceramic/metal joints

Highlights

• SiC Reinforcements are controlled to cluster together in the center of joint domain.

• Controllable spatial distribution of SiC realizes multi-layer architecture of joint.

• For ceramic/metal joint, multi-layer architecture relaxes strain energy in ceramic.

• For ceramic/metal joint, drop of strain energy in ceramic increases shear strength.

Abstract

A novel design of a multi-layered joint architecture, characterized by the controllable spatial distribution of ceramic reinforcements, was obtained, when brazing ZrB₂-SiC ceramics and Ti-6Al-4 V reinforced by TiB whiskers with AgCu filler. To induce SiC reinforcements to cluster together, within the center of the filler and especially away from the ZrB₂-SiC, SiC was added into joint, in the form of a SiC interlayer featuring a 3D-bridged network structure with continuous micro-channels infiltrated by capillary force by liquid filler during brazing. Different SiC particles were connected together, by micro-bridges being not destroyed during brazing. The layer rich in SiC could reduce thermal expansion mismatch between different substrates, the AgCu next to the ZrB₂-SiC could relax the strain energy by high plastic deformation. Compared to joint brazed with single AgCu, joint strain energy decreased from 10.8 × 10⁻⁶ J to 8.6 × 10⁻⁶ J, accordingly joint shear strength increased from ∼5.4 MPa to ∼41.2 MPa.

Introduction

Zirconium diboride-silicon carbide composite ceramic (ZrB₂-SiC, herein labeled ZS), featuring good mechanical and physical properties at high temperatures, is a promising candidate for applications in high performance thermo-mechanical structures [1,2]. For practical applications, ZS is needed to bond with other metallic parts. As an excellent choice, Ti-6Al-4 V reinforced by TiB whiskers having 3D quasi-continuous network distribution (TC4-TiB_w, herein labeled TTw), possesses superior mechanical properties than traditional Ti-6Al-4 V alloy [3,4]. A reliable bonding of ZS/TTw assemblies would be essential for an efficient thermal protection, which consists in a considerable challenge that needs a deep understanding of the associated physics of the process.

Till now, the most efficient method for joining ceramics and metals has been active brazing [[5], [6], [7]] that addressed the problem of interatomic bonding at ceramic interface. However, coefficient of thermal expansion (CTE) mismatch between ceramics and metals induced elastic strain energy within the brittle ceramic matrix (U_e,c) [8] during cooling from the joining temperature, which (i) would deteriorate the mechanical properties of a brazed joint and (ii) can be used to evaluate the propensity to fracture of a joint [8].

In active brazing of ceramics to metals, one of the most efficient methods for relieving the U_e,c has been composite fillers with reinforcements [9]. In composite fillers, reinforcements with lower CTE (such as SiC [10], TiC [11], TiB [12] and W [13]) than metal substrates to be braze have been added in the metal filler, to reduce the CTE of brazing seam, for achieving a gradual transition of CTE from metal to ceramic matrix. Therefore, the gradual transition of CTE favors a uniform distribution of elastic strain energy within the whole joint domain: some of the strain energy is transferred from the ceramic matrix to composite brazing seam, thus decreasing the U_e,c. Moreover, when U_e,c decreases, the joint mechanical properties improve.

However, reinforcements in composite fillers with low CTE usually possess high yield strength [8,14]. Moreover, the space distribution of reinforcements cannot been controlled: the reinforcements have been distributed randomly and evenly within the joint domain. Therefore, the addition of reinforcements leads to the increase of the yield strength of integral brazing seam, including the region next to ceramic matrix within the brazing seam. This is highly detrimental for reducing U_e,c [8,15], since less U_e,c could be relaxed by the plastic deformation. Consequently, traditional composite fillers needs to be improved.

We formulated a hypothesis that a control of the spatial distribution of reinforcements would lead to significant reduction in the U_e,c and hence to an improvement of joint integrity, by targeting a multilayered architecture of brazing seam. In detail, this specific multilayered architecture of brazing seam contained two characteristic layers listed in detail in Table 1. In this multilayered architecture, (i) layer-C-NR-LY tends to relax the U_e,c by its high capability of plastic deformation; and (ii) layer-A-HR-LC tends to relax the U_e,c by the gradual transition of the CTE of integral joint domain. This multilayered architecture can be adapted to obtain an optimal joint with suitable configuration for relaxing the residual stress, soft/hard alternate material sequence, which has been validated in the design of multiple interlayers for dissimilar ceramic/metal joints [8].

In this study, to decrease the CTE mismatch between the ZS and TTw matrix, SiC particles were added into the AgCu brazing filler, as reinforcements with low CTE. Especially, to obtain the multilayered architecture of joint with spatial distribution of reinforcements, SiC particles were incorporated in the form of an interlayer of SiC particles featuring a 3D-bridged network structure with continuous micro-channels (denoted as 3BC-SiC). In the 3BC-SiC interlayer, adjacent SiC particles were sintered at only few points on surface and formed necks. The necks acted as micro-bridges connecting adjacent SiC particles. Moreover, 3D continuous micro-channels formed around SiC particles. During the brazing process, the micro-channels within the 3BC-SiC would be infiltrated by the AgCu liquid filler activated by Ti from TTw dissolution. Thus, the SiC particles would cluster together in brazing seam, if the micro-bridges were not destroyed by the reaction with the liquid filler.

Moreover, since AgCu liquid filler would surround the 3BC-SiC, SiC particles were controlled to cluster together in the center area of brazed joint, which was denoted as CTC-SiC. Then, the CTC-SiC layer could serve as the layer-A-HR-LC (Table 1), and the AgCu between the ZS matrix and the CTC-SiC layer could constitute the layer-C-NR-LY (Table 1). The CTC-SiC layer also can be seemed as the AgCu metal reinforced by SiC particles or the SiC ceramic toughened by the AgCu, which possessed both high strength and toughness [16,17]. In addition, within the infiltrated 3BC-SiC domain, the contact area between the SiC particles and AgCu was very large, which could also increase the strength of CTC-SiC layer.

For comparison, a joint with single AgCu filler was also brazed under the same conditions. This allowed for the evaluation of the positive effects of the interlayer on the microstructure and strength of joints.