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Microstructure and mechanical properties of SiC-SiC joints joined by spark plasma sintering
Author’s Accepted Manuscript Microstructure and mechanical properties of SiCSiC joints joined by spark plasma sintering Hongying Dong, Yadong Yu, Xilong Jin, Xin Tian, Weiyan He, Wen Ma www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)30880-X http://dx.doi.org/10.1016/j.ceramint.2016.06.049 CERI13065
To appear in: Ceramics International Received date: 22 March 2016 Revised date: 7 June 2016 Accepted date: 7 June 2016 Cite this article as: Hongying Dong, Yadong Yu, Xilong Jin, Xin Tian, Weiyan He and Wen Ma, Microstructure and mechanical properties of SiC-SiC joints joined by spark plasma sintering, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.06.049 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructure and mechanical properties of SiC-SiC joints joined by spark plasma sintering Hongying Donga, Yadong Yua, Xilong Jinb, Xin Tiana, Weiyan Hea, Wen Mab,* a
School of Chemical Engineering, Inner Mongolia University of Technology, Hohhot 010051, China b School of Materials Science and Engineering, Inner Mongolia University of Technology, Hohhot 010051, China * Corresponding author: School of Materials Science and Engineering, Inner Mongolia University of Technology, Aimin St., Hohhot 010051, China. Tel. /fax: +86 471 6575752. [email protected]
Abstract The development of reliable joining technology is of great importance for the full use of SiC. Ti3SiC2, which is used as a filler material for SiC joining, can meet the demands of neutron environment applications and can alleviate residual stress during the joining process. In this work, SiC was joined using different powders (Ti3SiC2 and 3Ti/1.2Si/2C/0.2Al) as filler materials and spark plasma sintering (SPS). The influence of the joining temperature on the flexural strength of the SiC joints at room temperature and at high temperatures was investigated. Based on X-ray diffraction and scanning electron microscopy analyses, SiC joints with 3Ti/1.2Si/2C/0.2Al powder as the filler material possess high flexural strengths of 133 MPa and 119 MPa at room temperature and at 1200°C, respectively. The superior flexural strength of the SiC joint at 1200°C is attributed to the phase transformation of TiO2 from anatase to rutile.
Keywords: SiC ceramic; Ti3SiC2; Spark plasma sintering; Ceramic joining
1. Introduction SiC is one of the most important structural ceramics with attractive properties such as high hardness, superior corrosion resistance, high thermal conductivity, and a low thermal expansion coefficient (TEC: 4.7×10-6 K-1). The low creep rate, good oxidation resistance, high strength, high stability, good thermal shock resistance and good corrosion resistance of SiC at high temperatures make it highly suitable for high-temperature applications [1]. Thus, SiC is commonly used in the burner components of aircraft and rockets, in the chambers and tubes of furnaces, and in heating elements in oxidizing atmospheres to temperatures up to 1400°C [2]. However, in most cases, SiC applications depend on its joining because the manufacture of SiC with a large size or a complicated shape is very difficult and expensive. In the field of nuclear energy production (fusion and fission), the requirements for SiC/SiC joining are extremely severe. A joining material must be compatible with the neutron environment, and the joining technique must comply with the fusion nuclear reactor design in which, SiC/SiC components that are several meters long and 3-mm thick must be joined in a reliable and feasible manner [3-11]. The development of joining technologies for SiC has been investigated intensively. Currently, SiC joining is normally realized by brazing and diffusion bonding using active metals/alloys and organic materials as fillers. However, the mismatch in the thermal expansion coefficients (TECs) of SiC and metals/alloys results in the generation of higher residual stress in the joints during the joining process. Additionally, the SiC joints using these metals/alloys as fillers exhibit low
oxidation and corrosion resistances and cannot be used in
challenging
high-temperature environments. The organic materials used as fillers for SiC joining can produce high porosities due to their pyrolysis during the joining process, decreasing the flexural strength of the SiC joints. Therefore, it is necessary to develop new inorganic materials with a high melting point and oxidation resistance for use as the filler in the joining process. Ti3SiC2 is a candidate filler material that possesses an unusual combination of metallic and ceramic properties, such as an especially high melting point, good oxidation resistance and high strength at elevated temperatures, which enable its use in high-temperature applications and as an oxidation-resistant filler for ceramic joining. In recent years, spark plasma sintering (SPS) has been widely used as a fast sintering method that results in a dense and fine-grained microstructure. SPS was proposed as a localized heating technique to obtain sound joints for SiC-based materials that are suitable for the nuclear industry and generally suitable for highly demanding applications [12-16]. Several studies on the use of SPS as a manufacturing technique for SiC materials have been published in the last decade [17-22]. In the field of materials joining, many studies have focused on metal-to-metal, ceramic-to-ceramic or ceramic-to-metal joints using conventional techniques [23-28]. However, there is still a dearth of available literature on the joining of materials by SPS. A mechanically reliable, low-activation and hermetic joint should be used in a nuclear environment. In this work, SPS was employed to join SiC, and the flexural
strength of the SiC joints joined using different powders as the filler at room temperature and at high temperatures was evaluated.
2. Experimental procedures The surfaces of SiC rods (Φ 10 mm × 25 mm，Astek Ceramic Co., Ltd, Shandong, China) with a bend strength of 360 MPa were polished with diamond paste and ultrasonically cleaned in ethanol for 30 min. Ti3SiC2 powder was used as a filler material after being synthesized, using a 3Ti/1.2Si/2C/0.2Al powder mixture as the starting material, by SPS at 1300°C for 5 min, followed by crushing and grinding into small particles with an average grain size of 15~20 μm and a purity of 99 wt.% (Fig. 1). A mixture of 3Ti/1.2Si/2C/0.2Al was also used as a filler material for SiC joining after it was mixed in ethanol and ground sufficiently using an agate mortar. A SiC rod, the filler and another SiC rod were loaded into a 10-mm diameter graphite die in a sandwich configuration and were sintered in an SPS system (Dr. SINTER, SPS-1050, Sumitomo Coal Mining Co. Ltd., Japan) under vacuum (< 6 Pa). The heating rate was set to 100 to 200°C/min, and the pressure was set to 30 MPa. The bend strengths of the joints (the flexural strength) at room temperature and at high temperatures were measured by the three-point bend test with a test span of 40 mm and a displacement rate of 0.5 mm/min, using a universal mechanical testing machine (Model WDW-200, Changchun New Test Instrument Co. Ltd., China). The schematic maps for the bend strength tests of the joints are shown in a previous work [29]. The microstructural observation of the joining zone was performed using a
scanning electron microscope (SEM, Model Hitachi S-3400, Hitachi Co., Ltd., Japan). The composition of this zone was determined using an energy dispersive X-ray spectrometer (EDX, Model Inca, Oxford Instruments, United Kingdom). The phase analyses of the fracture surfaces of the joints were conducted using X-ray diffraction (XRD, Model Bruker advance-D8, Bruker Co., Ltd., Germany).
3. Results and discussion 3.1 Joining of SiC using Ti3SiC2 powder as the filler material The flexural strengths of the SiC joints as a function of joining temperature are shown in Fig. 2. The flexural strength of the SiC joints increases remarkably as the joining temperature increases from 1300 to 1400°C with a maximum flexural strength of 66 MPa. The strength then decreases as the joining temperature increases. Photographs of a SiC joint obtained using Ti3SiC2 as the filler and joined at 1400°C before and after the bending test are shown in Fig. 3. Fracturing of the SiC joint occurred mainly at the SiC/Ti3SiC2 interface, as shown in Fig. 3b, and a small amount of SiC peeled off the SiC itself, indicating good bonding between SiC and Ti3SiC2. Fig. 4 shows the XRD pattern of the fracture surface of the SiC joint joined at 1400°C using a Ti3SiC2 filler. New phases of TiSi2 and TiC developed at the fracture surface of the SiC joint, implying that a chemical reaction occurred between SiC and Ti3SiC2. SiC diffraction peaks appear because the SiC base material was not completely removed from the analyzed fracture surface and because the fracture path
propagated at the SiC/Ti3SiC2 interface. Moreover, the graphite die and a high pressure were used during the actual joining process. Ti3SiC2 tends to decompose into TiCx (where x > 0.8) in the 1200~1350°C temperature range in a graphite-rich environment through Si out-diffusion and evaporation [30, 31]. At the SiC/Ti3SiC2 interface, it is easy to form a Si-enriched environment, and the following chemical reaction could occur [30]. Ti3SiC2+7Si→3TiSi2+2SiC
(1)
Furthermore, free Si normally remains in the SiC base material, enabling the chemical reaction between SiC and Ti3SiC2. Fig. 5 shows a SEM micrograph and line-scan of the polished fracture surface of a SiC joint joined at 1400°C using the Ti3SiC2 filler. The concentrations of Ti and Si changed gradually in a range of ~5 μm on both sides of the SiC/Ti3SiC2 interface (Fig. 5b). This implies that element diffusion occurred at the SiC/Ti3SiC2 interface. Moreover, SiC has some pores inside, enabling Ti3SiC2 to enter into the pores with increased contact area between the SiC and the Ti3SiC2, thus increasing the flexural strength of the SiC joint. However, the relatively low flexural strength of the SiC joint obtained using the Ti3SiC2 filler is due to the large particle size and irregular shape of the Ti3SiC2 powder, as well as the high porosity in the Ti3SiC2 interlayer. The three-point bending test for the SiC joint joined at 1400°C using the Ti3SiC2 filler was carried out at 500°C in air. The flexural strength of the SiC joint was 21 MPa at 500°C, which is a third of that for the SiC joint at room temperature. The SEM micrograph of the fracture surface of the SiC joint after the three-point bending
test at 500°C is shown in Fig. 6. The fracture surface of the SiC joint contained many small particles, which were confirmed as TiO2 by EDS analysis. The development of TiO2 particles was due to the oxidation of TiC and TiSi2, and resulted in the appearance of pores in the interlayer and at the SiC/Ti3SiC2 interface, as well as a decreased flexural strength of the SiC joint. Fig. 7 shows the XRD pattern of the fracture surface of the SiC joint joined at 1400°C using the Ti3SiC2 filler after the three-point bending test at 500°C. The TiO2 phase was newly developed, except for the Ti3SiC2, TiC and SiC phases, which is in good agreement with the SEM analysis.
3.2 Joining of SiC using the 3Ti/1.2Si/2C/0.2Al powder mixture as the filler material The SiC samples were successfully joined by SPS using the 3Ti/1.2Si/2C/0.2Al powder mixture as the filler material. The Ti3SiC2 interlayer was synthesized in situ during the joining process. The flexural strengths of the SiC joints at room temperature as a function of joining temperature are shown in Fig. 8. The changes in the flexural strengths of the SiC joints at room temperature can be divided into two stages. In the first stage, from 1200 to 1400°C, the flexural strength of the SiC joints increases almost linearly as the joining temperature increases. In the second stage, the flexural strengths of the SiC joints initially decrease as the joining temperature increases from 1400 to 1450°C and then increase with further increases in the joining temperature up to 1600°C, reaching a maximum value of 133 MPa. Fig. 9 shows photographs of a SiC joint obtained using the 3Ti/1.2Si/2C/0.2Al
filler joined at 1600°C before and after the bending test. The fracturing of the SiC joint occurred at the SiC/filler interface and inside the SiC close to the interface, as shown in Fig. 9b, indicating the presence of good bonding between the SiC and the filler. Fig. 10 shows an SEM micrograph and line-scan of the polished fracture surface of the SiC joint joined at 1600°C using the 3Ti/1.2Si/2C/0.2Al filler. The concentrations of Ti, Si and C changed drastically at the SiC/filler interface (Fig. 10b), indicating that no elemental diffusion occurred. As a result, the SiC joint was joined via the in situ reaction bonding mechanism. The new phase with small particles developed during the joining process, making the interlayer dense, thus increasing the mechanical properties. Moreover, considering that the SiC is not fully dense, it is possible for the 3Ti/1.2Si/2C/0.2Al filler to enter into the pores of the SiC with increased contact area between the SiC and 3Ti/1.2Si/2C/0.2Al filler, thus increasing the flexural strength of the SiC joint. SiC is used extensively in high-temperature environments due to its superior high-temperature properties; therefore, the flexural strength of the SiC joint at high temperatures is crucial. The flexural strengths of the SiC joints joined at 1600°C using the 3Ti/1.2Si/2C/0.2Al filler after the three-point bending test at different temperatures are shown in Fig. 11. The flexural strengths of the SiC joint tested at high temperatures are lower than those of the SiC joint tested at room temperature. The flexural strength of the SiC joint at room temperature was 133 MPa; this value decreased to 68 MPa in the test at 800°C and then increased to 119 MPa in the test at
1200°C (89% of the flexural strength at room temperature). The XRD patterns and SEM micrographs of the fracture surfaces of the SiC joints joined at 1600°C using the 3Ti/1.2Si/2C/0.2Al filler after the three-point bending tests at different temperatures are shown in Figs. 12 and 13, respectively. Small anatase TiO2 particles developed due to the oxidation of TiC at 500°C, as confirmed by the results shown in Fig. 12a and 13a, resulting in reduced flexural strength of the SiC joint. The TiC oxidation became significant at 800°C where more TiO2 particles developed (Fig. 12b), accompanied by the phase transformation from anatase to rutile for the TiO2 (Fig. 11). The TiO2 volume changed due to the phase transformation from anatase to rutile, producing higher porosity and resulting in the lowered flexural strength of the SiC joint. TiC was completely oxidized into rutile TiO2 at 1200°C as shown in Figs. 12c and 13c. The flexural strength of the SiC joint increased remarkably compared to that of the sample tested at 800°C, which is attributed to the densification of the rutile TiO2 as well as the high hardness of rutile TiO2 in contrast to anatase TiO2.
4. Conclusions The joining of the SiC ceramic was successfully realized via SPS using Ti3SiC2 and 3Ti/1.2Si/2C/0.2Al fillers. The room temperature flexural strengths of the SiC joints are strongly affected by the joining temperature, with the maximum values of 66 MPa and 133 MPa obtained using the Ti3SiC2 and 3Ti/1.2Si/2C/0.2Al fillers, respectively. The good sinterability of the 3Ti/1.2Si/2C/0.2Al filler and the chemical
reactions between the filler and SiC give rise to the high flexural strength of the SiC joint at room temperature. The high-temperature flexural strengths of the SiC joints are lower than the room temperature flexural strengths for both the Ti3SiC2 and 3Ti/1.2Si/2C/0.2Al fillers; this result is mainly attributed to the development of TiO2 due to the oxidation of TiC and/or TiSi2. However, the drastic increase in the flexural strength of the SiC joint obtained using the 3Ti/1.2Si/2C/0.2Al filler at 1200°C is attributed to the phase transformation of TiO2 from anatase to rutile.
Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. 51462026, 51062013), the Inner Mongolia Natural Science Foundation (No. 2014MS0509), and the Shanghai technical platform for testing and characterization of inorganic materials (No. 14DZ2292900).
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Fig. 1 XRD pattern of the synthesized bulk Ti3SiC2 by SPS at 1300°C for 5 min; the inset is the SEM microstructure of the synthesized bulk Ti3SiC2. Fig. 2 Flexural strengths of the SiC joints as a function of joining temperature. Fig. 3 Photographs of a SiC joint obtained using the Ti3SiC2 filler joined at 1400°C before (a) and after (b) the bending test. Fig. 4 XRD pattern of the fracture surface of the SiC joint joined at 1400°C using the Ti3SiC2 filler. Fig. 5 SEM micrograph (a) and line-scan of the polished fracture surface (b) of a SiC joint joined at 1400°C using the Ti3SiC2 filler. Fig. 6 SEM micrograph of the fracture surface of the SiC joint joined at 1400°C using the Ti3SiC2 filler after the three-point bending test at 500°C. Fig. 7 XRD pattern of the fracture surface of the SiC joint joined at 1400°C using the Ti3SiC2 filler after the three-point bending test at 500°C. Fig. 8 Flexural strengths of the SiC joints at room temperature as a function of joining temperature. Fig. 9 Photographs of a SiC joint obtained using the 3Ti/1.2Si/2C/0.2Al filler joined at 1600°C before (a) and after (b) the bending test. Fig. 10 SEM micrograph (a) and line-scan of the polished fracture surface (b) of the SiC joint joined at 1600°C using the 3Ti/1.2Si/2C/0.2Al filler. Fig. 11 Flexural strengths of the SiC joints joined at 1600°C using the 3Ti/1.2Si/2C/0.2Al filler after the three-point bending test at different temperatures. Fig. 12 XRD patterns of the fracture surfaces of the SiC joints joined at 1600°C using
the 3Ti/1.2Si/2C/0.2Al filler after the three-point bending tests at 500°C (a), 800°C (b) and 1200°C (c). Fig. 13 SEM micrographs of the fracture surfaces of the SiC joints joined at 1600°C using the 3Ti/1.2Si/2C/0.2Al filler after the three-point bending tests at 500°C (a), 800°C (b) and 1200°C (c).
PII: DOI: Reference:
S0272-8842(16)30880-X http://dx.doi.org/10.1016/j.ceramint.2016.06.049 CERI13065
To appear in: Ceramics International Received date: 22 March 2016 Revised date: 7 June 2016 Accepted date: 7 June 2016 Cite this article as: Hongying Dong, Yadong Yu, Xilong Jin, Xin Tian, Weiyan He and Wen Ma, Microstructure and mechanical properties of SiC-SiC joints joined by spark plasma sintering, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.06.049 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructure and mechanical properties of SiC-SiC joints joined by spark plasma sintering Hongying Donga, Yadong Yua, Xilong Jinb, Xin Tiana, Weiyan Hea, Wen Mab,* a
School of Chemical Engineering, Inner Mongolia University of Technology, Hohhot 010051, China b School of Materials Science and Engineering, Inner Mongolia University of Technology, Hohhot 010051, China * Corresponding author: School of Materials Science and Engineering, Inner Mongolia University of Technology, Aimin St., Hohhot 010051, China. Tel. /fax: +86 471 6575752. [email protected]
Abstract The development of reliable joining technology is of great importance for the full use of SiC. Ti3SiC2, which is used as a filler material for SiC joining, can meet the demands of neutron environment applications and can alleviate residual stress during the joining process. In this work, SiC was joined using different powders (Ti3SiC2 and 3Ti/1.2Si/2C/0.2Al) as filler materials and spark plasma sintering (SPS). The influence of the joining temperature on the flexural strength of the SiC joints at room temperature and at high temperatures was investigated. Based on X-ray diffraction and scanning electron microscopy analyses, SiC joints with 3Ti/1.2Si/2C/0.2Al powder as the filler material possess high flexural strengths of 133 MPa and 119 MPa at room temperature and at 1200°C, respectively. The superior flexural strength of the SiC joint at 1200°C is attributed to the phase transformation of TiO2 from anatase to rutile.
Keywords: SiC ceramic; Ti3SiC2; Spark plasma sintering; Ceramic joining
1. Introduction SiC is one of the most important structural ceramics with attractive properties such as high hardness, superior corrosion resistance, high thermal conductivity, and a low thermal expansion coefficient (TEC: 4.7×10-6 K-1). The low creep rate, good oxidation resistance, high strength, high stability, good thermal shock resistance and good corrosion resistance of SiC at high temperatures make it highly suitable for high-temperature applications [1]. Thus, SiC is commonly used in the burner components of aircraft and rockets, in the chambers and tubes of furnaces, and in heating elements in oxidizing atmospheres to temperatures up to 1400°C [2]. However, in most cases, SiC applications depend on its joining because the manufacture of SiC with a large size or a complicated shape is very difficult and expensive. In the field of nuclear energy production (fusion and fission), the requirements for SiC/SiC joining are extremely severe. A joining material must be compatible with the neutron environment, and the joining technique must comply with the fusion nuclear reactor design in which, SiC/SiC components that are several meters long and 3-mm thick must be joined in a reliable and feasible manner [3-11]. The development of joining technologies for SiC has been investigated intensively. Currently, SiC joining is normally realized by brazing and diffusion bonding using active metals/alloys and organic materials as fillers. However, the mismatch in the thermal expansion coefficients (TECs) of SiC and metals/alloys results in the generation of higher residual stress in the joints during the joining process. Additionally, the SiC joints using these metals/alloys as fillers exhibit low
oxidation and corrosion resistances and cannot be used in
challenging
high-temperature environments. The organic materials used as fillers for SiC joining can produce high porosities due to their pyrolysis during the joining process, decreasing the flexural strength of the SiC joints. Therefore, it is necessary to develop new inorganic materials with a high melting point and oxidation resistance for use as the filler in the joining process. Ti3SiC2 is a candidate filler material that possesses an unusual combination of metallic and ceramic properties, such as an especially high melting point, good oxidation resistance and high strength at elevated temperatures, which enable its use in high-temperature applications and as an oxidation-resistant filler for ceramic joining. In recent years, spark plasma sintering (SPS) has been widely used as a fast sintering method that results in a dense and fine-grained microstructure. SPS was proposed as a localized heating technique to obtain sound joints for SiC-based materials that are suitable for the nuclear industry and generally suitable for highly demanding applications [12-16]. Several studies on the use of SPS as a manufacturing technique for SiC materials have been published in the last decade [17-22]. In the field of materials joining, many studies have focused on metal-to-metal, ceramic-to-ceramic or ceramic-to-metal joints using conventional techniques [23-28]. However, there is still a dearth of available literature on the joining of materials by SPS. A mechanically reliable, low-activation and hermetic joint should be used in a nuclear environment. In this work, SPS was employed to join SiC, and the flexural
strength of the SiC joints joined using different powders as the filler at room temperature and at high temperatures was evaluated.
2. Experimental procedures The surfaces of SiC rods (Φ 10 mm × 25 mm，Astek Ceramic Co., Ltd, Shandong, China) with a bend strength of 360 MPa were polished with diamond paste and ultrasonically cleaned in ethanol for 30 min. Ti3SiC2 powder was used as a filler material after being synthesized, using a 3Ti/1.2Si/2C/0.2Al powder mixture as the starting material, by SPS at 1300°C for 5 min, followed by crushing and grinding into small particles with an average grain size of 15~20 μm and a purity of 99 wt.% (Fig. 1). A mixture of 3Ti/1.2Si/2C/0.2Al was also used as a filler material for SiC joining after it was mixed in ethanol and ground sufficiently using an agate mortar. A SiC rod, the filler and another SiC rod were loaded into a 10-mm diameter graphite die in a sandwich configuration and were sintered in an SPS system (Dr. SINTER, SPS-1050, Sumitomo Coal Mining Co. Ltd., Japan) under vacuum (< 6 Pa). The heating rate was set to 100 to 200°C/min, and the pressure was set to 30 MPa. The bend strengths of the joints (the flexural strength) at room temperature and at high temperatures were measured by the three-point bend test with a test span of 40 mm and a displacement rate of 0.5 mm/min, using a universal mechanical testing machine (Model WDW-200, Changchun New Test Instrument Co. Ltd., China). The schematic maps for the bend strength tests of the joints are shown in a previous work [29]. The microstructural observation of the joining zone was performed using a
scanning electron microscope (SEM, Model Hitachi S-3400, Hitachi Co., Ltd., Japan). The composition of this zone was determined using an energy dispersive X-ray spectrometer (EDX, Model Inca, Oxford Instruments, United Kingdom). The phase analyses of the fracture surfaces of the joints were conducted using X-ray diffraction (XRD, Model Bruker advance-D8, Bruker Co., Ltd., Germany).
3. Results and discussion 3.1 Joining of SiC using Ti3SiC2 powder as the filler material The flexural strengths of the SiC joints as a function of joining temperature are shown in Fig. 2. The flexural strength of the SiC joints increases remarkably as the joining temperature increases from 1300 to 1400°C with a maximum flexural strength of 66 MPa. The strength then decreases as the joining temperature increases. Photographs of a SiC joint obtained using Ti3SiC2 as the filler and joined at 1400°C before and after the bending test are shown in Fig. 3. Fracturing of the SiC joint occurred mainly at the SiC/Ti3SiC2 interface, as shown in Fig. 3b, and a small amount of SiC peeled off the SiC itself, indicating good bonding between SiC and Ti3SiC2. Fig. 4 shows the XRD pattern of the fracture surface of the SiC joint joined at 1400°C using a Ti3SiC2 filler. New phases of TiSi2 and TiC developed at the fracture surface of the SiC joint, implying that a chemical reaction occurred between SiC and Ti3SiC2. SiC diffraction peaks appear because the SiC base material was not completely removed from the analyzed fracture surface and because the fracture path
propagated at the SiC/Ti3SiC2 interface. Moreover, the graphite die and a high pressure were used during the actual joining process. Ti3SiC2 tends to decompose into TiCx (where x > 0.8) in the 1200~1350°C temperature range in a graphite-rich environment through Si out-diffusion and evaporation [30, 31]. At the SiC/Ti3SiC2 interface, it is easy to form a Si-enriched environment, and the following chemical reaction could occur [30]. Ti3SiC2+7Si→3TiSi2+2SiC
(1)
Furthermore, free Si normally remains in the SiC base material, enabling the chemical reaction between SiC and Ti3SiC2. Fig. 5 shows a SEM micrograph and line-scan of the polished fracture surface of a SiC joint joined at 1400°C using the Ti3SiC2 filler. The concentrations of Ti and Si changed gradually in a range of ~5 μm on both sides of the SiC/Ti3SiC2 interface (Fig. 5b). This implies that element diffusion occurred at the SiC/Ti3SiC2 interface. Moreover, SiC has some pores inside, enabling Ti3SiC2 to enter into the pores with increased contact area between the SiC and the Ti3SiC2, thus increasing the flexural strength of the SiC joint. However, the relatively low flexural strength of the SiC joint obtained using the Ti3SiC2 filler is due to the large particle size and irregular shape of the Ti3SiC2 powder, as well as the high porosity in the Ti3SiC2 interlayer. The three-point bending test for the SiC joint joined at 1400°C using the Ti3SiC2 filler was carried out at 500°C in air. The flexural strength of the SiC joint was 21 MPa at 500°C, which is a third of that for the SiC joint at room temperature. The SEM micrograph of the fracture surface of the SiC joint after the three-point bending
test at 500°C is shown in Fig. 6. The fracture surface of the SiC joint contained many small particles, which were confirmed as TiO2 by EDS analysis. The development of TiO2 particles was due to the oxidation of TiC and TiSi2, and resulted in the appearance of pores in the interlayer and at the SiC/Ti3SiC2 interface, as well as a decreased flexural strength of the SiC joint. Fig. 7 shows the XRD pattern of the fracture surface of the SiC joint joined at 1400°C using the Ti3SiC2 filler after the three-point bending test at 500°C. The TiO2 phase was newly developed, except for the Ti3SiC2, TiC and SiC phases, which is in good agreement with the SEM analysis.
3.2 Joining of SiC using the 3Ti/1.2Si/2C/0.2Al powder mixture as the filler material The SiC samples were successfully joined by SPS using the 3Ti/1.2Si/2C/0.2Al powder mixture as the filler material. The Ti3SiC2 interlayer was synthesized in situ during the joining process. The flexural strengths of the SiC joints at room temperature as a function of joining temperature are shown in Fig. 8. The changes in the flexural strengths of the SiC joints at room temperature can be divided into two stages. In the first stage, from 1200 to 1400°C, the flexural strength of the SiC joints increases almost linearly as the joining temperature increases. In the second stage, the flexural strengths of the SiC joints initially decrease as the joining temperature increases from 1400 to 1450°C and then increase with further increases in the joining temperature up to 1600°C, reaching a maximum value of 133 MPa. Fig. 9 shows photographs of a SiC joint obtained using the 3Ti/1.2Si/2C/0.2Al
filler joined at 1600°C before and after the bending test. The fracturing of the SiC joint occurred at the SiC/filler interface and inside the SiC close to the interface, as shown in Fig. 9b, indicating the presence of good bonding between the SiC and the filler. Fig. 10 shows an SEM micrograph and line-scan of the polished fracture surface of the SiC joint joined at 1600°C using the 3Ti/1.2Si/2C/0.2Al filler. The concentrations of Ti, Si and C changed drastically at the SiC/filler interface (Fig. 10b), indicating that no elemental diffusion occurred. As a result, the SiC joint was joined via the in situ reaction bonding mechanism. The new phase with small particles developed during the joining process, making the interlayer dense, thus increasing the mechanical properties. Moreover, considering that the SiC is not fully dense, it is possible for the 3Ti/1.2Si/2C/0.2Al filler to enter into the pores of the SiC with increased contact area between the SiC and 3Ti/1.2Si/2C/0.2Al filler, thus increasing the flexural strength of the SiC joint. SiC is used extensively in high-temperature environments due to its superior high-temperature properties; therefore, the flexural strength of the SiC joint at high temperatures is crucial. The flexural strengths of the SiC joints joined at 1600°C using the 3Ti/1.2Si/2C/0.2Al filler after the three-point bending test at different temperatures are shown in Fig. 11. The flexural strengths of the SiC joint tested at high temperatures are lower than those of the SiC joint tested at room temperature. The flexural strength of the SiC joint at room temperature was 133 MPa; this value decreased to 68 MPa in the test at 800°C and then increased to 119 MPa in the test at
1200°C (89% of the flexural strength at room temperature). The XRD patterns and SEM micrographs of the fracture surfaces of the SiC joints joined at 1600°C using the 3Ti/1.2Si/2C/0.2Al filler after the three-point bending tests at different temperatures are shown in Figs. 12 and 13, respectively. Small anatase TiO2 particles developed due to the oxidation of TiC at 500°C, as confirmed by the results shown in Fig. 12a and 13a, resulting in reduced flexural strength of the SiC joint. The TiC oxidation became significant at 800°C where more TiO2 particles developed (Fig. 12b), accompanied by the phase transformation from anatase to rutile for the TiO2 (Fig. 11). The TiO2 volume changed due to the phase transformation from anatase to rutile, producing higher porosity and resulting in the lowered flexural strength of the SiC joint. TiC was completely oxidized into rutile TiO2 at 1200°C as shown in Figs. 12c and 13c. The flexural strength of the SiC joint increased remarkably compared to that of the sample tested at 800°C, which is attributed to the densification of the rutile TiO2 as well as the high hardness of rutile TiO2 in contrast to anatase TiO2.
4. Conclusions The joining of the SiC ceramic was successfully realized via SPS using Ti3SiC2 and 3Ti/1.2Si/2C/0.2Al fillers. The room temperature flexural strengths of the SiC joints are strongly affected by the joining temperature, with the maximum values of 66 MPa and 133 MPa obtained using the Ti3SiC2 and 3Ti/1.2Si/2C/0.2Al fillers, respectively. The good sinterability of the 3Ti/1.2Si/2C/0.2Al filler and the chemical
reactions between the filler and SiC give rise to the high flexural strength of the SiC joint at room temperature. The high-temperature flexural strengths of the SiC joints are lower than the room temperature flexural strengths for both the Ti3SiC2 and 3Ti/1.2Si/2C/0.2Al fillers; this result is mainly attributed to the development of TiO2 due to the oxidation of TiC and/or TiSi2. However, the drastic increase in the flexural strength of the SiC joint obtained using the 3Ti/1.2Si/2C/0.2Al filler at 1200°C is attributed to the phase transformation of TiO2 from anatase to rutile.
Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. 51462026, 51062013), the Inner Mongolia Natural Science Foundation (No. 2014MS0509), and the Shanghai technical platform for testing and characterization of inorganic materials (No. 14DZ2292900).
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Fig. 1 XRD pattern of the synthesized bulk Ti3SiC2 by SPS at 1300°C for 5 min; the inset is the SEM microstructure of the synthesized bulk Ti3SiC2. Fig. 2 Flexural strengths of the SiC joints as a function of joining temperature. Fig. 3 Photographs of a SiC joint obtained using the Ti3SiC2 filler joined at 1400°C before (a) and after (b) the bending test. Fig. 4 XRD pattern of the fracture surface of the SiC joint joined at 1400°C using the Ti3SiC2 filler. Fig. 5 SEM micrograph (a) and line-scan of the polished fracture surface (b) of a SiC joint joined at 1400°C using the Ti3SiC2 filler. Fig. 6 SEM micrograph of the fracture surface of the SiC joint joined at 1400°C using the Ti3SiC2 filler after the three-point bending test at 500°C. Fig. 7 XRD pattern of the fracture surface of the SiC joint joined at 1400°C using the Ti3SiC2 filler after the three-point bending test at 500°C. Fig. 8 Flexural strengths of the SiC joints at room temperature as a function of joining temperature. Fig. 9 Photographs of a SiC joint obtained using the 3Ti/1.2Si/2C/0.2Al filler joined at 1600°C before (a) and after (b) the bending test. Fig. 10 SEM micrograph (a) and line-scan of the polished fracture surface (b) of the SiC joint joined at 1600°C using the 3Ti/1.2Si/2C/0.2Al filler. Fig. 11 Flexural strengths of the SiC joints joined at 1600°C using the 3Ti/1.2Si/2C/0.2Al filler after the three-point bending test at different temperatures. Fig. 12 XRD patterns of the fracture surfaces of the SiC joints joined at 1600°C using
the 3Ti/1.2Si/2C/0.2Al filler after the three-point bending tests at 500°C (a), 800°C (b) and 1200°C (c). Fig. 13 SEM micrographs of the fracture surfaces of the SiC joints joined at 1600°C using the 3Ti/1.2Si/2C/0.2Al filler after the three-point bending tests at 500°C (a), 800°C (b) and 1200°C (c).