**2. Metal-ceramic brazing**

In recent microjoining communities, ceramic-metal bonding is of prime importance due to its various applications in hybrid joints, such as ceramic packaging, ceramic sealants, thermoelectrics, and semiconductor transducers. The various types of microjoining processes are given in **Figure 1**.

However, due to the wide limitations of joining this couple owing to their wide gap in physical, mechanical, and thermodynamic behavior, they have always imposed a difficulty. Ceramics are lightweight, harder, and always inert toward the liquid metal and have a wide difference in thermal expansion behavior. Since metals are soft and malleable, for various functional applications, their joining is needed in semiconductors. There are various types of filler materials already available, such as Ag-Cu-Ti brazing system which is most suitable in metal-ceramic joining [7, 14].

The joining processes include various soldering, brazing, and welding processes depending upon the range of temperature of the filler. Soldering is a lowtemperature joining but weaker for most of the ceramics due to high-temperature

**225**

brazing processes.

*High-Entropy Alloys for Micro- and Nanojoining Applications*

requirements. A solder works below 450°C and may not work for high-temperature superalloy joining, while welding is a high-temperature process mostly used for steels [15–18]. In welding, the metal parts need to be melted at high temperatures using fillers known as welding rods which solidify and bond to the workpiece. However, this may not be suitable for the bonding of different metal parts where thermal damage is of concern. A brief detail of the various types of welding pro-

*Various types of welding processes according to the temperature of the application.*

Enormous amount of work has been done on the lightweight materials for automotive, defense, and aerospace applications in the last few decades, mainly on steel, ceramics, aluminum, and magnesium-based alloys. However, not all the materials have led to mass production for practical use. The studies reported are preliminary wherein most of the time, microjoining is ignored even if it is an essential part of

Therefore, a new structural material, for example, for automobiles, cannot be designed without any foundation for microjoining that is an essential component to maintain the structural integrity of the part in service. There are several important problems in microjoining, such as the presence of brittle IMCs at the interface, segregations, and casting defects (shrinkage porosity, gaseous inclusions, pores, blowholes, etc.). The situation becomes more serious for ceramics (ceramicmetal, ceramic-superalloy, ceramic-steel, etc.) [19]. For example, in various complex assembly applications (ceramic-metal plugs for igniting car engines), ceramics are difficult to join with metals due to their inherent poor wettability and difference in coefficient of thermal expansion (CTE). As shown in **Figure 3**, these two problems have limited the application of ceramics in combination with metals for many years. In this chapter we mainly focus on the brazing which is the most popular for ceramic-metal joints. However, first, we discuss a bit more about

the automobile, aerospace, consumer, and defense electronic industries.

*DOI: http://dx.doi.org/10.5772/intechopen.91166*

cesses is given in **Figure 2**.

**Figure 2.**

**3. Necessity of developing brazing fillers**

**Figure 1.** *Various types of metal-ceramic joining processes.*

*High-Entropy Alloys for Micro- and Nanojoining Applications DOI: http://dx.doi.org/10.5772/intechopen.91166*

#### **Figure 2.**

*Engineering Steels and High Entropy-Alloys*

**2. Metal-ceramic brazing**

metal-ceramic joining [7, 14].

where active elements (Ti, Zr, etc.) distribute randomly in solid solution. In addition, the alloy system should be free of any additional undesirable IMCs. Recently discovered high-entropy alloys have already shown that with proper design strategy, they might

In recent microjoining communities, ceramic-metal bonding is of prime importance due to its various applications in hybrid joints, such as ceramic packaging, ceramic sealants, thermoelectrics, and semiconductor transducers. The various

However, due to the wide limitations of joining this couple owing to their wide gap in physical, mechanical, and thermodynamic behavior, they have always imposed a difficulty. Ceramics are lightweight, harder, and always inert toward the liquid metal and have a wide difference in thermal expansion behavior. Since metals are soft and malleable, for various functional applications, their joining is needed in semiconductors. There are various types of filler materials already available, such as Ag-Cu-Ti brazing system which is most suitable in

The joining processes include various soldering, brazing, and welding processes depending upon the range of temperature of the filler. Soldering is a lowtemperature joining but weaker for most of the ceramics due to high-temperature

replace traditional materials for a variety of applications [13].

types of microjoining processes are given in **Figure 1**.

**224**

**Figure 1.**

*Various types of metal-ceramic joining processes.*

*Various types of welding processes according to the temperature of the application.*

requirements. A solder works below 450°C and may not work for high-temperature superalloy joining, while welding is a high-temperature process mostly used for steels [15–18]. In welding, the metal parts need to be melted at high temperatures using fillers known as welding rods which solidify and bond to the workpiece. However, this may not be suitable for the bonding of different metal parts where thermal damage is of concern. A brief detail of the various types of welding processes is given in **Figure 2**.

### **3. Necessity of developing brazing fillers**

Enormous amount of work has been done on the lightweight materials for automotive, defense, and aerospace applications in the last few decades, mainly on steel, ceramics, aluminum, and magnesium-based alloys. However, not all the materials have led to mass production for practical use. The studies reported are preliminary wherein most of the time, microjoining is ignored even if it is an essential part of the automobile, aerospace, consumer, and defense electronic industries.

Therefore, a new structural material, for example, for automobiles, cannot be designed without any foundation for microjoining that is an essential component to maintain the structural integrity of the part in service. There are several important problems in microjoining, such as the presence of brittle IMCs at the interface, segregations, and casting defects (shrinkage porosity, gaseous inclusions, pores, blowholes, etc.). The situation becomes more serious for ceramics (ceramicmetal, ceramic-superalloy, ceramic-steel, etc.) [19]. For example, in various complex assembly applications (ceramic-metal plugs for igniting car engines), ceramics are difficult to join with metals due to their inherent poor wettability and difference in coefficient of thermal expansion (CTE). As shown in **Figure 3**, these two problems have limited the application of ceramics in combination with metals for many years. In this chapter we mainly focus on the brazing which is the most popular for ceramic-metal joints. However, first, we discuss a bit more about brazing processes.

#### **Figure 3.**

*Metal-ceramic bonding issues. (a) Schematic of factors influencing on the reliability of ceramic-metal joint and (b) comparison of thermal coefficients of metals and ceramics. © 2016 Uday et al. [19] under CC BY 3.0 license.*

### **4. Active metal brazing**

There are various brazing processes, such as diffusion bonding, transient liquid phase bonding, reactive bonding, etc., out of which active metal brazing is more popular due to its suitability to ceramics. Active metal brazing enhances the surface wetting of the contacts due to the presence of active elements in the fillers. Recently authors have bonded the ZrO2 and Ti-6Al-4V using a composite filler of Ag-Cu-Ti to obtain a robust brazed joint [20]. Active metal brazing minimized the residual stresses and distortion in the joint poor strength and high-temperature stability are the issues in brazing industries. To understand brazing, we should have an idea of the wetting of the two contact surfaces. We describe the wetting theory in the following sections.

#### **4.1 Theory of wetting and spreading**

According to the wetting theory formulated by Thomas Young in 1805, the interfacial surface energies at the solid–liquid–vapor interface are related by an equation:

$$
\gamma\_{\rm sv} - \gamma\_{\rm sl=} \gamma\_{\rm lv} \cos \Theta \tag{1}
$$

Here, γsv, γsl, and γlv correspond to the interfacial energies of solid-vapor, solid–liquid and liquid–vapor interface (**Figure 4(a)**). According to the spreading parameter,

$$\mathbf{S} = \mathbf{\gamma}\_{\text{sv}} - \mathbf{\gamma}\_{\text{sl}} - \mathbf{\gamma}\_{\text{lv}} \tag{2}$$

**227**

substrate.

**Figure 5.**

**Figure 4.**

*(b) is adapted from [7]].*

**4.2 Factors influencing brazed joint**

the surface energy and enhance wetting [21].

*4.2.1 Issues in metal-ceramic wetting*

*Uday et al. [19] under CC BY 3.0 license.*

*4.2.2 Ag-Cu-Ti filler system*

*4.2.3 Surface condition*

*High-Entropy Alloys for Micro- and Nanojoining Applications*

in the wettability of the contact surface. The various situations of wetting are given in **Figure 5** depending on the contact angles between the metal and ceramic

*Liquid metal drop shape depending on the contact time: (a) initial contact or rigid solid surface, (b) formation of a ridge (vertical scale exaggerated), and (c) final equilibrium configuration on deformable solid. © 2016* 

*(a) Schematic of filler wetting at the substrate and (b) contact angle of metals on cBN substrate. [the data in* 

The wetting is influenced by the surface energy, reactivity of the contact materials to brazing filler, and type of filler used. The reaction between the contact surfaces and brazing filler is key to improve the wetting. In the active metal brazing, a higher wetting can be achieved using Ti, Sc, Cr, or Zr active metals that minimize

Various active fillers are developed in the past. The authors will focus mainly on Ag-Cu-Ti among conventional fillers. This alloy system has various features such as solid solution Ag(Cu) and intermetallic phase Ti2Cu and Ti2Ag compounds. The melting range is around 800–900°C and, therefore, is useful to bond ceramicsmetals for most of the systems. Ag promotes the activity of Ti and reaction proceeds

at a faster rate. There is no ternary intermetallic in Ag-Cu-Ti system [22].

Sharp edges and corners may raise the stress concentration and completely fail the ceramic-metal joint. There should be no surface cracks as they propagate

*DOI: http://dx.doi.org/10.5772/intechopen.91166*

γsv − γsl − γlv = 0 for minimum surface energy (high spreading) and S < 0 for poor wetting. **Figure 4(b)** gives the wetting angles of different metals at various temperatures [7].

We can see that the temperature has a great role in the wetting of metal fluid on the substrate. The increase of Ti and Sn in optimal proportion leads to an increment *High-Entropy Alloys for Micro- and Nanojoining Applications DOI: http://dx.doi.org/10.5772/intechopen.91166*

#### **Figure 4.**

*Engineering Steels and High Entropy-Alloys*

**4. Active metal brazing**

**Figure 3.**

*3.0 license.*

following sections.

**4.1 Theory of wetting and spreading**

There are various brazing processes, such as diffusion bonding, transient liquid phase bonding, reactive bonding, etc., out of which active metal brazing is more popular due to its suitability to ceramics. Active metal brazing enhances the surface wetting of the contacts due to the presence of active elements in the fillers. Recently authors have bonded the ZrO2 and Ti-6Al-4V using a composite filler of Ag-Cu-Ti to obtain a robust brazed joint [20]. Active metal brazing minimized the residual stresses and distortion in the joint poor strength and high-temperature stability are the issues in brazing industries. To understand brazing, we should have an idea of the wetting of the two contact surfaces. We describe the wetting theory in the

*Metal-ceramic bonding issues. (a) Schematic of factors influencing on the reliability of ceramic-metal joint and (b) comparison of thermal coefficients of metals and ceramics. © 2016 Uday et al. [19] under CC BY* 

According to the wetting theory formulated by Thomas Young in 1805, the interfacial surface energies at the solid–liquid–vapor interface are related by an equation:

Here, γsv, γsl, and γlv correspond to the interfacial energies of solid-vapor, solid–liquid and liquid–vapor interface (**Figure 4(a)**). According to the spreading

γsv − γsl − γlv = 0 for minimum surface energy (high spreading) and S < 0 for poor wetting. **Figure 4(b)** gives the wetting angles of different metals at various

We can see that the temperature has a great role in the wetting of metal fluid on the substrate. The increase of Ti and Sn in optimal proportion leads to an increment

γsv − γsl= γlv cos θ (1)

S = γsv − γsl − γlv (2)

**226**

parameter,

temperatures [7].

*(a) Schematic of filler wetting at the substrate and (b) contact angle of metals on cBN substrate. [the data in (b) is adapted from [7]].*

**Figure 5.**

*Liquid metal drop shape depending on the contact time: (a) initial contact or rigid solid surface, (b) formation of a ridge (vertical scale exaggerated), and (c) final equilibrium configuration on deformable solid. © 2016 Uday et al. [19] under CC BY 3.0 license.*

in the wettability of the contact surface. The various situations of wetting are given in **Figure 5** depending on the contact angles between the metal and ceramic substrate.

#### **4.2 Factors influencing brazed joint**

#### *4.2.1 Issues in metal-ceramic wetting*

The wetting is influenced by the surface energy, reactivity of the contact materials to brazing filler, and type of filler used. The reaction between the contact surfaces and brazing filler is key to improve the wetting. In the active metal brazing, a higher wetting can be achieved using Ti, Sc, Cr, or Zr active metals that minimize the surface energy and enhance wetting [21].

#### *4.2.2 Ag-Cu-Ti filler system*

Various active fillers are developed in the past. The authors will focus mainly on Ag-Cu-Ti among conventional fillers. This alloy system has various features such as solid solution Ag(Cu) and intermetallic phase Ti2Cu and Ti2Ag compounds. The melting range is around 800–900°C and, therefore, is useful to bond ceramicsmetals for most of the systems. Ag promotes the activity of Ti and reaction proceeds at a faster rate. There is no ternary intermetallic in Ag-Cu-Ti system [22].

#### *4.2.3 Surface condition*

Sharp edges and corners may raise the stress concentration and completely fail the ceramic-metal joint. There should be no surface cracks as they propagate to form a bigger crack which ultimately leads to joint failure. The damage can be repaired through re-sintering the part [19].

### *4.2.4 Thermal characteristics*

High-temperature gradients may cause the generation of thermal stresses and may not be desirable from the brazing point of view. In ceramic-metal bonding, if the ceramic has less CTE than metal, edge cracks will be formed, while core cracks are formed in the opposite case, as shown in **Figure 6** [19, 23, 24].

The remedy for minimizing these cracks is given by various researchers. Zhou [25] suggested the application of metal layer as an interlayer to release the stress by elastoplastic deformation, using composite layers, low-temperature joining, and proper heat treatment and joint design.

### **4.3 Interface characteristics**

The presence of chemical bonding also affects the brazing performance. For example, a thick interfacial layer causes more mismatch in thermal expansion and weakens the joint compared to a thin layer [26]. The operating temperature and time are key parameters to control the interfacial layer. The presence of pores and edges should be avoided before brazing to control the contact angle of the filler with the substrate.

## **4.4 Type of filler**

Filler type also affects the brazing operation. Researchers have also used nanotechnology to develop various brazing fillers, known as composite fillers [27–31]. These nanocomposite fillers refine the various IMCs present in the matrix and joint. However, the dispersion of nanoparticles in the composite matrix is cumbersome. For example, the active metal filler has been reinforced with metallic or ceramic additives to match the CTE and promote the brazing [20]. In the last few decades, the research community has found the solution to overcome the formation of IMCs in an alloy. Such a novel type of alloy design is known as high-entropy alloys as we describe in the following section.

#### **Figure 6.**

*Types of cracks in metal-ceramic joints. © 2016 Uday et al. [19] under CC BY 3.0 license. (a) Edge cracks in ceramic. (b) Core cracks in ceramic.*

**229**

stable bulk HEA.

*High-Entropy Alloys for Micro- and Nanojoining Applications*

**5. Advanced brazing fillers must be of high-entropy characteristics**

Most of the brazing fillers so far available for ceramic-metal joining are silver-based alloys mostly produced by casting approach, e.g., Ag-Cu, Ag-Cu-Ti, Ag-Cu-Sn-Ti, Ag-Cu-In-Ti, etc. [19–26]. Several elements are added to enhance the wetting of the contact surfaces which further produce unwanted IMCs. Moreover, conventional casting or melting techniques are not suitable for industrialization as they produce a number of defects (shrinkage porosity, gaseous inclusions, pores, segregation of constituents, impurities), and there are limitations on the shape/ size of the final product. In contrast, the advanced solid-state powder technology has rarely been used for HEA synthesis, and there is a lot more to explore. It offers a low-cost superior alternative over other expensive techniques (sputter deposition, evaporation, induction melting, arc and laser melting, etc.) Secondly, it is an environmentally benign and safe technique as compared to chemical routes (solution precipitation, plating, chemical vapor deposition) where toxic waste disposal and drainage costs are added up [17–19]. In addition, high-energy ball milling is a simple and effective way to produce novel nanostructured materials, homogeneous chemical distribution, and extension of solid solubility and widens the scope of HEA [34–40]. Further densification of milled powders by heat treatment, hotpressing, cold isostatic pressing, or spark plasma sintering (SPS) technique gives the

Conventional alloy design suggests the formation of various intermetallic compounds (IMCs) or complex phases with multiple alloying elements. These IMCs act as stress raiser points in alloys and composites which are strictly not desirable. Specially in microjoining applications, these IMCs can cause poor joint properties and catastrophic failure of the entire device. Yeh et al. broke this paradigm by suggesting high-entropy alloys (HEAs), composed of five or more elements in an equiatomic or near-equiatomic fraction varying from 5 to 35 at.% [32, 33]. Therefore, these HEAs contain usually simple solid solution phases rather than IMCs. Many HEAs with high strength, thermal stability, excellent corrosion and wear resistance have been reported. However, they are rarely explored for microjoining applications. Application of these novel HEAs in microjoining can provide several benefits. High entropy filler alloy realizes the active interfacial reaction with ceramic, and the solid solution forms the brazing seam during brazing. The interaction of additional IMCs in the brazing seam is prevented (unlike traditional brazing fillers). This further provides additional benefits like a reduced requirement of ceramic metallization partly with active metal (Ti, Zr), prevents stresses and distortion in joints, improves joint strength, depresses the brazing

*DOI: http://dx.doi.org/10.5772/intechopen.91166*

temperature, and saves energy.

**6. Fillers processed by powder technology**

**7. Research and development on HEA in microjoining**

HEAs are the novel and advanced alloys which are composed of 5–35 at.% where all the elements serve as principal elements. They are unique because of their attractive properties compared to their conventional alloys such as strength and ductility trade-off, high thermal stability, high wear, and corrosion resistance. HEAs are first discovered in 1995 by Yeh et al. and coined as multicomponent alloys in 2004 by Cantor et al. [32–33]. Previous studies have shown that most of the HEAs *Engineering Steels and High Entropy-Alloys*

*4.2.4 Thermal characteristics*

repaired through re-sintering the part [19].

proper heat treatment and joint design.

alloys as we describe in the following section.

**4.3 Interface characteristics**

the substrate.

**4.4 Type of filler**

to form a bigger crack which ultimately leads to joint failure. The damage can be

High-temperature gradients may cause the generation of thermal stresses and may not be desirable from the brazing point of view. In ceramic-metal bonding, if the ceramic has less CTE than metal, edge cracks will be formed, while core cracks

The remedy for minimizing these cracks is given by various researchers. Zhou [25] suggested the application of metal layer as an interlayer to release the stress by elastoplastic deformation, using composite layers, low-temperature joining, and

The presence of chemical bonding also affects the brazing performance. For example, a thick interfacial layer causes more mismatch in thermal expansion and weakens the joint compared to a thin layer [26]. The operating temperature and time are key parameters to control the interfacial layer. The presence of pores and edges should be avoided before brazing to control the contact angle of the filler with

Filler type also affects the brazing operation. Researchers have also used nanotechnology to develop various brazing fillers, known as composite fillers [27–31]. These nanocomposite fillers refine the various IMCs present in the matrix and joint. However, the dispersion of nanoparticles in the composite matrix is cumbersome. For example, the active metal filler has been reinforced with metallic or ceramic additives to match the CTE and promote the brazing [20]. In the last few decades, the research community has found the solution to overcome the formation of IMCs in an alloy. Such a novel type of alloy design is known as high-entropy

*Types of cracks in metal-ceramic joints. © 2016 Uday et al. [19] under CC BY 3.0 license. (a) Edge cracks in* 

are formed in the opposite case, as shown in **Figure 6** [19, 23, 24].

**228**

**Figure 6.**

*ceramic. (b) Core cracks in ceramic.*
