Original Article
Interfacial microstructure and mechanical properties of ZTA/ZTA joints brazed with Ni-Ti filler metal

https://doi.org/10.1016/j.jeurceramsoc.2020.10.073Get rights and content

Abstract

The reliable brazing of the ZTA ceramic joints was successfully obtained using Ni-Ti filler metal. The microstructure and mechanical properties of the joints brazed at different temperatures were investigated. During the process of brazing, both Al2O3 and ZrO2 in the ZTA reacted with the Ni-Ti filler, resulting in the formation of the AlNi2Ti + Ni2Ti4O reaction layer adjacent to the ZTA substrate when brazed at 1350 °C for 30 min. NiTi and Ni3Ti compounds precipitated at the center of brazing seam. When the brazing temperature increased from 1320 °C to 1380 °C, the thickness of AlNi2Ti + Ni2Ti4O layer increased gradually. As the brazing temperature varied from 1400 °C to 1450 °C, TiO was formed adjacent to the ZTA substrate, along with the reduction of Ni2Ti4O. AlNi2Ti distributed at the interface and center of brazing seam. The maximum shear strength of 152 MPa was obtained when brazed at 1420 °C for 30 min.

Introduction

Due to the increasing advancements in the thermal efficiency of power generating equipment, the development of materials that can withstand high temperatures and severe corrosive environments is essential [1]. Ceramics are desirable due to their high level of wear, corrosion resistance, and exceptional mechanical properties at high temperatures [2]. The zirconia toughened alumina (ZTA) ceramics are important structural ceramics that have gained attention due to the hardening property and chemical stability of alumina, and the toughening effect of zirconia [3,4]. However, the ZTA ceramics show poor machinability due to their hard and brittle physical properties. This limits the scope of potential application as the difficulty in the fabrication of complex–shaped and large–sized components increases [5,6].

Currently, the complex-shaped ceramic components are assembled by producing smaller and less complex parts that are joined together. Active brazing is one of the most suitable methods for joining ceramics due to its convenience, cost-efficiency and excellent adaptability of size and shape of the joints [[7], [8], [9], [10], [11]]. The active elements such as Ti, Cr, Zr and V are always added to the metal matrix for the preparation of an active filler [[12], [13], [14], [15]]. The interfacial reactions between the ceramics and filler metal effectively improves the wettability of molten metal on the ceramics, which is helpful to improve the interfacial bonding level of the joints. The brazing of a wide range of ceramics to themselves or metals has been extensively explored in the literature [[16], [17], [18], [19], [20], [21], [22], [23]].

The investigation of the brazing process of ZTA to themselves or to metals was hardly reported, with inaccuracy in the published studies. Torvund et al. [24] used the Ag–Cu–Ti filler metal to self–braze the ZTA. The results showed that the ZTA/ZTA joint could be successfully obtained with interfacial reactions between the ZTA ceramics and filler. The bright zone was found to extend into the ceramic, which was attributed to the selective reaction between ZTA and Ti. However, solely Al2O3 was consumed during the reaction. The continuous reaction layer mainly constituting Ti, Cu, and O was presented at the ceramic/filler metal interface, although the interfacial reaction products and mechanisms were unknown. Wang et al. [25] brazed the ZTA ceramic to a TC4 alloy using the Ag–Cu eutectic filler metal with Cu foam interlayer. The results revealed that the Ti in the TC4 alloy diffused into the brazing seam to form a Ti-Cu compound layer next to the TC4. Simultaneously, TiO + Ti3(Cu, Al)3O layer was formed next to the ZTA. The Ti was expected to react with both ZrO2 and Al2O3 in the ZTA substrate. However, there was no clear evidence to support the reaction mechanism. The Ag–based filler alloys are expensive, and thus increases the production cost. Besides, the coefficients of thermal expansion (CTEs) of Ag-based filler alloys and ceramics are ∼20 × 10−6 K−1 and ∼ (3–10) × 10−6 K−1, respectively, which are quite different. The residual stress generated due to the large thermal mismatch between the filler alloy and ceramics further deteriorates the joint [26]. Therefore, the development of a low–cost and suitable filler alloy for joining ZTA ceramics finds great significance in science and engineering.

The Ni-Ti master alloys are cheaper and have lower CTEs than Ag-based alloys. The brazing of ZTA ceramics with the Ni–Ti alloy is highly significant for the promotion of ceramic brazing components. In this paper, the Ni–Ti alloy with an atomic ratio of 50:50 was used as the filler to join the ZTA ceramics. The typical microstructure of the brazed joint was established. The effects of brazing temperature on the microstructure and mechanical properties of the joints were comprehensively investigated. The microstructure evolution and fracture mechanism of the joints were proposed.

Section snippets

Experimental procedures

The ZTA ceramic containing 20 wt.% ZrO2 was used in the present study. Fig. 1 shows the microstructure and XRD pattern of the experimental ZTA ceramic. The white particles in Fig. 1(a) are ZrO2, while the grey matrix shows the presence of Al2O3. The dimensions of the ZTA specimens used for microstructure characterization were 15 mm × 15 mm × 5 mm. For shear strength test, the specimens of dimensions 15 mm × 15 mm × 20 mm and 30 mm × 20 mm × 10 mm were employed. The Ni50Ti50 master alloy ingot

Microstructure of ZTA/Ni-Ti/ZTA joint

Fig. 5 shows the backscattered electron images (BEI) and corresponding elemental distribution of the ZTA/Ni–Ti/ZTA joint brazed at a temperature of 1350 °C held for 30 min. It is seen from Fig. 5(a) that the joint exhibits appropriate homogeneous bonding without any defects such as micro–cracks and voids. The brazing seam could be divided into the interfacial reaction zone (marked as Zone І) and the remaining alloy zone at the center of the brazing seam (marked as Zone П). The microstructure

Interfacial reaction mechanism during brazing

At the brazing temperature of 1350 °C, Ni and Ti reacted with Al2O3 and ZrO2 simultaneously. The possible reactions are indicated as follows:12[Ti] + 6[Ni] + Al2O3→3Ni2Ti4O + 2[Al]2[Ni] + [Ti] + [Al]→AlNi2TiZrO2 + 4[Ti] + 2[Ni]→ZrO2-x + Ni2Ti4O

The elements Ni and Ti reacted with Al2O3 to form Ni2Ti4O and Al. Subsequently, a small amount of Al reacted with Ni and Ti to form the AlNi2Ti phase. Besides, Al was found to be solid-solubilized in the Ni2Ti4O phase to form a solid solution. Equation

Conclusions

The ZTA ceramics were successfully brazed using the Ni–Ti filler metal. The interfacial microstructure of the joints was characterized accurately. The effects of brazing temperature on the microstructure and mechanical properties of the joints were investigated. According to the results of this work, the following conclusions were obtained.

  • (1)

    The interfacial microstructure of the ZTA/Ni-Ti joints brazed at 1350 °C for 30 min was found to be ZTA/AlNi2Ti/Ni2Ti4O/NiTi + Ni3Ti. During the brazing

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgments

The authors gratefully acknowledge the financial support from the GDAS' Project of Science and Technology Development (2019GDASYL-0104024 and 2019GDASYL-0501014). The authors also acknowledge the financial support from the Guangdong Special Support Program (2017TQ04C645) and Science and Technology Planning Project of Guangdong Province (2018dr013).

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