**5. Ceramic–metal solid-state joining**

thermal cycling (200–800 °C in air) as shown in **Figure 14**. It showed the former 'cup and cone' break inside the ceramic after the first cycle, while the latter failed from the interface of the

The article of Walker and Hodges [52] is intended to familiarize the designer with brazing methods commonly used to join metals to ceramics, discussed the advantages and disadvan‐ tages of two methods, and show the relative tensile strengths obtained from samples fabricated using these methods. In most article cases discussed, 94% alumina ceramic (6% glassy phase) ASTM-F19 tensile button samples were joined to Fe-29Ni-17Co alloy using a gold-or silverbased braze filler metal (**Figure 15**). The article is limited to the two metallization methods most commonly used for joining metals to ceramics: the molybdenum-manganese/nickel

**Figure 15.** Commonly used ceramic metallization methods: (A) moly-manganese metallization process; (B) thin-film

At the point when ceramics production are to be brazed, especially in those conditions where the coupling part is a metal, the individual differential thermal coefficient of thermal expansion between the coupling parts is of foremost significance. When in doubt engineers have a tendency to expect that the coefficient of thermal extension of a metal will be a few times that of a ceramic. This is not generally genuine, be that as it may. For instance ceramic, for example, silicon nitride and silicon carbide do, in fact, have low coefficient of thermal extension, and issues emerging from stresses produced amid the cooling stage can be normal in conditions where such materials are brazed to stainless steels or copper both of which being metals that have a high coefficient of thermal expansion. However, titanium, titanium alloys and some exceptional materials, for example, invar and kovar each have a coefficient of thermal expan‐ sion which is near that of alumina, while expansion coefficients of molybdenum and tungsten are near those of both silicon carbide and silicon nitride. This implies while picking the 'active' filler material that will be utilized for a specific employment it is fundamental to decide the differential coefficient of expansion that exists between the materials that are to be joined.

plating method and physical-vapour deposition or thin-film method.

ceramic foam after more than 60 cycles.

178 Joining Technologies

metallization process [52].

Solid state joining is a gathering of joining procedures which produces cohesion at tempera‐ tures basically underneath the melting point of the base materials being joined, without the expansion of brazing filler metal. Pressure might possibly be utilized. These procedures are infrequently mistakenly called solid state bonding forms: this gathering of joining procedures incorporates friction welding, diffusion and laser welding. In all of these welding processes, the parameters such as temperature, time and pressure separately or together to produce ceramic metal joint without melting of the base metal. Solid-state bonding process contains some of very oldest processes and some of them new. The bonding processes provide certain advantages as the base metal do not melt and compose an interface. The materials that joined keep their original properties without the heat-affected zone problems included when there is no melting for base materials [53]. At the point when dissimilar metals are joined their thermal expansion and conductivity is of substantially less significance with solid state welding than with the arc welding processes. Time, temperature and pressure are included; nonetheless, in some processes the time component is to a great degree short, in the microsec‐ ond run or up to a few moments. In different cases, the time is reached out to a few hours. As temperature expands time is generally decreased. Since each of these processes is different each will be qualified.

#### **5.1. Ceramic-metal friction welding**

Friction welding is a solid state joining that produces a bond under the compressive force of one rotating workpiece to another stationary workpiece [54]. Heat is generated at the weld interface during the friction between two materials because of the non-stop rubbing for different contact surfaces, which is produced later in the softening of metal (**Figure 16**). Finally, the metal side at the weld interface begins to flow elastically and forms an axial shortening [55]. When a certain amount of forging had occurred, the rotation stops and the compressive force are maintained or slightly increased to consolidate the weld. Some of the important operational parameters in friction welding are friction time, friction pressure and rotation speed [56].

Friction welding, like any welding process, has its specific advantages and disadvantages. The following are some advantages of friction welding such as no filler metal is needed. Flux and shielding gas are not required. The process is environmentally clean, no arcs, sparks, smoke or fumes are generated by clean parts [58]. Surface cleanliness is not as significant, compared with other welding processes, since friction welding tends to disrupt and displace surface films. There are narrow heat affected zones. The process is suitable for welding most engi‐ neering materials and is well suited for joining many dissimilar material combinations. In some cases of weld, the strength of the joint is equal to or greater than the strength of the weaker of the two materials joined. The bond that is created by the mechanical intermixing and solidifi‐ cation of the two materials is strong and free from voids and porosity. It can be cost effective and offers design engineers many more options than other methods.

Weiss [63] studied the residual stresses and strength of friction welded ceramic/metal joints. From this article, based on friction welding results of different ceramic materials to an aluminium alloy, the effect of residual stresses on the strength of ceramic-metal joints was calculated numerically. Heat conduction process calculations to evaluate the temperature distribution have been conducted by the method of finite element (FEM), using hardware experimental data for input. From this chapter, the theoretical analyses clearly show that edge geometry of the joint in the area of the interface (flash) has a strong effect on the weld joint strength. Improvement of weld joint strength seems to be possible by optimization of the geometry in the area of the weld interface. The effect of joining parameters on ceramic metal joint strength through residual stresses is comparatively low. However, the welding parame‐ ters may have more effect on the joint strength by means of the bonding process, resulting in

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Rombaut et al. [65] summarized of the literature review performed during the master thesis on friction welding on dissimilar materials. Of main interest in this work is the welding of steel to a ceramic material such as alumina (Al2O3). Because of the difficulties involved in the production of welding sound for this material combination, and not a lot of literature is available on this topic. This work begins with a discussion about the basics of friction welding and typical problems encountered in steel welding with ceramic. There are three major reasons related to joining problems noted for these materials combination. First, there is an important variance in the type of atomic bonds between metal like steel and ceramic. The joining in the ceramic is mostly from the nature of the ionic or covalent (usually a hybrid of these), while metals have a metallic bonding character. Second, there is often a very large difference in the thermal expansion between these materials. Ceramic is usually lower coefficient of thermal expansion. When two parts of materials cool down after joining, the thermal stresses will be push in the weld interface, this may lead to cracking after that. Third, the brittle mode and porous of ceramics makes it very hard to absorb fabrication defects. The strength of a ceramic

Seli et al. [68–70] presented the evaluation of mechanical and interfacial properties of friction welded alumina-mild steel rods with use of A6061 sheet as an interlayer. A preliminary simulation was made to predict the deformation, stress, strain and temperature distribution during the joining operation using a fully coupled thermo-mechanical FE model. This paper also starts with a discussion on the basics of friction welding and typical problems encountered in welding the dissimilar materials. Problems related to friction welding of dissimilar materials are not only related with specific characteristics such as melting point and hardness, but also with the reactions that occur at the joint interface. Metals generally have a coefficient thermal expansion higher than ceramics. Therefore, when joining ceramics to metals using friction welding, it will be induced very large thermal stress and in many cases these large stresses cause joint failure In order to overcome this problem, the development of solid-phase bonding processes, which a metal or composite metal-ceramic layers are placed between the ceramic

Uday et al. [20] investigated the effect of welding speeds (630–2500 rpm) on the mechanical strength of friction welded joint of alumina-YSZ composite and 6061 aluminium alloy. From

is highly dependent of its grain size and surface roughness [66, 67].

higher or lower bonding strength [64].

and metal surfaces to be joined [62, 71, 72].

**Figure 16.** Basic steps of friction welding of ceramic and metal [57].

There are also some limitations of the process like; one workpiece must have an axis of symmetry and be capable of being rotated about that axis. Preparation and alignment of the workpieces may be critical for developing uniform rubbing and heating, particularly with diameters greater than 50 mm. Capital equipment and tooling costs are high. Dry bearing and non-forgeable materials cannot be welded. If both parts are longer than 1 m, special machines are required. Free-machining alloys are difficult to weld [59].

For a particular application, heating time is determined during the setup or from previous experience [57, 60]. Excessive heating limits productivity and increase wastes material. Similarly, uneven heating as well as entrapped oxides causing unbonded areas at the interface may be due to insufficient welding time. The ranges of effective pressure are not essentially slight for forging and heating, although the selected pressures should be reproducible for any specific process. Friction pressure has influence on the axial shortening distance and the temperature gradient in the weld zone [61]. The friction pressure depends on the materials being joined and the surface joint geometry [59]. Selected pressure must be high enough to hold the faying surfaces in intimate contact to avoid oxidation. Joint quality is improved in many metals, including steels, by applying a forging force at the end of the heating period.

On the other hand, the rotational speeds are related to the welding material and welding diameter in the interface. They may have different effects on the mechanical properties of the friction joint. Increase the rotational speed may lead to more frictional heat at the interface, thus leading to a greater amount of softening materials, recrystallization, or even increased intermetallic formation [62]. In addition, depending upon the type of materials joined or more accurately, the physical and mechanical properties involved the rotational speed of the production of the various effects on the quality of the joint Therefore, an appropriate rotation speed must be used to minimize any harmful effects and produce good quality of joints is an effective pressure range pressure.

Weiss [63] studied the residual stresses and strength of friction welded ceramic/metal joints. From this article, based on friction welding results of different ceramic materials to an aluminium alloy, the effect of residual stresses on the strength of ceramic-metal joints was calculated numerically. Heat conduction process calculations to evaluate the temperature distribution have been conducted by the method of finite element (FEM), using hardware experimental data for input. From this chapter, the theoretical analyses clearly show that edge geometry of the joint in the area of the interface (flash) has a strong effect on the weld joint strength. Improvement of weld joint strength seems to be possible by optimization of the geometry in the area of the weld interface. The effect of joining parameters on ceramic metal joint strength through residual stresses is comparatively low. However, the welding parame‐ ters may have more effect on the joint strength by means of the bonding process, resulting in higher or lower bonding strength [64].

Rombaut et al. [65] summarized of the literature review performed during the master thesis on friction welding on dissimilar materials. Of main interest in this work is the welding of steel to a ceramic material such as alumina (Al2O3). Because of the difficulties involved in the production of welding sound for this material combination, and not a lot of literature is available on this topic. This work begins with a discussion about the basics of friction welding and typical problems encountered in steel welding with ceramic. There are three major reasons related to joining problems noted for these materials combination. First, there is an important variance in the type of atomic bonds between metal like steel and ceramic. The joining in the ceramic is mostly from the nature of the ionic or covalent (usually a hybrid of these), while metals have a metallic bonding character. Second, there is often a very large difference in the thermal expansion between these materials. Ceramic is usually lower coefficient of thermal expansion. When two parts of materials cool down after joining, the thermal stresses will be push in the weld interface, this may lead to cracking after that. Third, the brittle mode and porous of ceramics makes it very hard to absorb fabrication defects. The strength of a ceramic is highly dependent of its grain size and surface roughness [66, 67].

**Figure 16.** Basic steps of friction welding of ceramic and metal [57].

180 Joining Technologies

effective pressure range pressure.

are required. Free-machining alloys are difficult to weld [59].

There are also some limitations of the process like; one workpiece must have an axis of symmetry and be capable of being rotated about that axis. Preparation and alignment of the workpieces may be critical for developing uniform rubbing and heating, particularly with diameters greater than 50 mm. Capital equipment and tooling costs are high. Dry bearing and non-forgeable materials cannot be welded. If both parts are longer than 1 m, special machines

For a particular application, heating time is determined during the setup or from previous experience [57, 60]. Excessive heating limits productivity and increase wastes material. Similarly, uneven heating as well as entrapped oxides causing unbonded areas at the interface may be due to insufficient welding time. The ranges of effective pressure are not essentially slight for forging and heating, although the selected pressures should be reproducible for any specific process. Friction pressure has influence on the axial shortening distance and the temperature gradient in the weld zone [61]. The friction pressure depends on the materials being joined and the surface joint geometry [59]. Selected pressure must be high enough to hold the faying surfaces in intimate contact to avoid oxidation. Joint quality is improved in many metals, including steels, by applying a forging force at the end of the heating period.

On the other hand, the rotational speeds are related to the welding material and welding diameter in the interface. They may have different effects on the mechanical properties of the friction joint. Increase the rotational speed may lead to more frictional heat at the interface, thus leading to a greater amount of softening materials, recrystallization, or even increased intermetallic formation [62]. In addition, depending upon the type of materials joined or more accurately, the physical and mechanical properties involved the rotational speed of the production of the various effects on the quality of the joint Therefore, an appropriate rotation speed must be used to minimize any harmful effects and produce good quality of joints is an

Seli et al. [68–70] presented the evaluation of mechanical and interfacial properties of friction welded alumina-mild steel rods with use of A6061 sheet as an interlayer. A preliminary simulation was made to predict the deformation, stress, strain and temperature distribution during the joining operation using a fully coupled thermo-mechanical FE model. This paper also starts with a discussion on the basics of friction welding and typical problems encountered in welding the dissimilar materials. Problems related to friction welding of dissimilar materials are not only related with specific characteristics such as melting point and hardness, but also with the reactions that occur at the joint interface. Metals generally have a coefficient thermal expansion higher than ceramics. Therefore, when joining ceramics to metals using friction welding, it will be induced very large thermal stress and in many cases these large stresses cause joint failure In order to overcome this problem, the development of solid-phase bonding processes, which a metal or composite metal-ceramic layers are placed between the ceramic and metal surfaces to be joined [62, 71, 72].

Uday et al. [20] investigated the effect of welding speeds (630–2500 rpm) on the mechanical strength of friction welded joint of alumina-YSZ composite and 6061 aluminium alloy. From this study, alumina-0, 25 and 50 wt.% YSZ composite with 6061 aluminium alloy joints were welded successfully by friction welding. The bending strength values of alumina-25 wt% YSZ composite joint obtained were greater at a rotational speed of 630 rpm than at 2500 rpm. The bending strength values at the joints were smaller in the pure alumina joint at a rotational speed of 1250 rpm than at 2500 rpm. The joint with large thermal expansion mismatch decreased the strength. However, it occasionally happens that some specimen is strong but the other is weak even if they are of the same kind. This depends on the presence and distri‐ bution of internal flaws induced by thermal stress during the joining process. The ceramic composite (Al2O3-25 wt.% YSZ) joints were welded productively at the low rotational speed (630 rpm) compared with pure alumina when joining with aluminium alloy by friction welding. The frictional heat at low rotational speeds (630 rpm) [73] produced lower tempera‐ ture gradients in the surface of friction, with temperature falling in the radial direction. Friction at high speed 2500 rpm produced more heating along the whole of the interface. The lower heating of the rod end-faces reduced stresses within the rod material [59, 74].

#### **5.2. Ceramic-metal diffusion bonding**

Diffusion bonding is a joining method where the principal mechanism for joint formation is diffusion solid-state. Coalescence of the faying diffusion surfaces is accomplished through the application of pressure at raised temperature. No melting and limited macroscopic deforma‐ tion or relative motion of different parts occurs during bonding. A filler metal (diffusion aid) can or cannot be used between the faying surfaces [75]. It has involved interest as a means of joining ceramic and successes have been realized by controlling the microstructure of the interfaces formed. The first condition for diffusion bonding is to create an intimate linking between two surfaces to be joined to the atomic species comes into intimate contact. Further‐ more, to a good connection, there must be enough diffused between the materials in a reasonable period of time. Pressure can be applied by hot press or hot isostatic press on a diffusion couple. **Figure 17** shows illustrations of events during metal/ceramic diffusion bonding in solid-state [76].

Diffusion bonding is primarily employed in the joining of dissimilar materials, i.e. dissimilar metals, metal-glass, metal-ceramic and ceramic-glass, either directly or through the use of interlayers [1, 30, 77, 78]. It offers numerous points of interest, mostly the strength of the bonding line, which is equivalent to the base metals. The microstructure at the bonded area is precisely the same as the origin materials. Otherwise, this point of interest joining process requires a few entirely controlled conditions: spotless and smooth contacting surfaces which are free from oxides, and so forth, high temperature condition to advance diffusion process [79–81]. Then again, diffusion holding requires a considerably more joining time. Also, the equipment expenses are high because of the mix of high temperature and pressure in vacuum situations. This frequently constrains the part measurements, which might be unfavourable from a financial viewpoint [79].

**Figure 17.** Sequence of events during metal-ceramic diffusion bonding [1].

are avoided in diffusion bonding process [76, 79, 85].

The main process parameters which control the diffusion bonding process are temperature, time and pressure [82]. Process temperature parameter is the most important because of the way in a thermally activated process, a slight change in temperature will lead to a significant change in the kinetics of the process compared with other parameters, and almost all the mechanisms, including plastic deformation and disseminate sensitive to temperature [83]. The temperature chosen is typically in the region of 0.5–0.8 of the absolute melting temperature of the component having the lower melting point [84]. Hence, melting and melting related defects

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Zhang et al. [86] have introduced the research and development of joining methods of ceramics to metals, especially brazing, diffusion bonding and partial transition liquid phase bonding.

**Figure 17.** Sequence of events during metal-ceramic diffusion bonding [1].

this study, alumina-0, 25 and 50 wt.% YSZ composite with 6061 aluminium alloy joints were welded successfully by friction welding. The bending strength values of alumina-25 wt% YSZ composite joint obtained were greater at a rotational speed of 630 rpm than at 2500 rpm. The bending strength values at the joints were smaller in the pure alumina joint at a rotational speed of 1250 rpm than at 2500 rpm. The joint with large thermal expansion mismatch decreased the strength. However, it occasionally happens that some specimen is strong but the other is weak even if they are of the same kind. This depends on the presence and distri‐ bution of internal flaws induced by thermal stress during the joining process. The ceramic composite (Al2O3-25 wt.% YSZ) joints were welded productively at the low rotational speed (630 rpm) compared with pure alumina when joining with aluminium alloy by friction welding. The frictional heat at low rotational speeds (630 rpm) [73] produced lower tempera‐ ture gradients in the surface of friction, with temperature falling in the radial direction. Friction at high speed 2500 rpm produced more heating along the whole of the interface. The lower

heating of the rod end-faces reduced stresses within the rod material [59, 74].

Diffusion bonding is a joining method where the principal mechanism for joint formation is diffusion solid-state. Coalescence of the faying diffusion surfaces is accomplished through the application of pressure at raised temperature. No melting and limited macroscopic deforma‐ tion or relative motion of different parts occurs during bonding. A filler metal (diffusion aid) can or cannot be used between the faying surfaces [75]. It has involved interest as a means of joining ceramic and successes have been realized by controlling the microstructure of the interfaces formed. The first condition for diffusion bonding is to create an intimate linking between two surfaces to be joined to the atomic species comes into intimate contact. Further‐ more, to a good connection, there must be enough diffused between the materials in a reasonable period of time. Pressure can be applied by hot press or hot isostatic press on a diffusion couple. **Figure 17** shows illustrations of events during metal/ceramic diffusion

Diffusion bonding is primarily employed in the joining of dissimilar materials, i.e. dissimilar metals, metal-glass, metal-ceramic and ceramic-glass, either directly or through the use of interlayers [1, 30, 77, 78]. It offers numerous points of interest, mostly the strength of the bonding line, which is equivalent to the base metals. The microstructure at the bonded area is precisely the same as the origin materials. Otherwise, this point of interest joining process requires a few entirely controlled conditions: spotless and smooth contacting surfaces which are free from oxides, and so forth, high temperature condition to advance diffusion process [79–81]. Then again, diffusion holding requires a considerably more joining time. Also, the equipment expenses are high because of the mix of high temperature and pressure in vacuum situations. This frequently constrains the part measurements, which might be unfavourable

**5.2. Ceramic-metal diffusion bonding**

182 Joining Technologies

bonding in solid-state [76].

from a financial viewpoint [79].

The main process parameters which control the diffusion bonding process are temperature, time and pressure [82]. Process temperature parameter is the most important because of the way in a thermally activated process, a slight change in temperature will lead to a significant change in the kinetics of the process compared with other parameters, and almost all the mechanisms, including plastic deformation and disseminate sensitive to temperature [83]. The temperature chosen is typically in the region of 0.5–0.8 of the absolute melting temperature of the component having the lower melting point [84]. Hence, melting and melting related defects are avoided in diffusion bonding process [76, 79, 85].

Zhang et al. [86] have introduced the research and development of joining methods of ceramics to metals, especially brazing, diffusion bonding and partial transition liquid phase bonding. From this article, the diffusion bonding is a technology to achieve compact joint by diffusion of atoms, even chemical reactions between materials or interlayer and materials. The diffusion of atoms in interface is carried out by several mechanisms, such as the replacement of near atoms, movement of clearance atoms and movement of vacancies, etc. The surface of the materials to be joined must be clean and flat (the roughness less than 0. 4 μm). Joining time can be a few hours at a mild temperature (0. 6*T*m , *T*<sup>m</sup> is melting point of metal to be joined), also can be several minutes at high temperature (0. 8*T*m, ). Diffusion bonding can be achieved with inserted interlayer [87–92] or without interlayer [93, 94]. The diffusion interlayer can reduce the cracking, relax the thermal residual stress and improve the joining strength. The diffusion interlayer is produce of different element active to ceramics, such as titanium, niobium and zirconium etc.

Burger and Ruhle [95] studied the material transport mechanisms during the diffusion bonding for niobium (Nb) to alumina (Al2O3). According to this chapter, the many different material transport phenomena may occur during the diffusion bonding process of a metal to a ceramic at high temperatures. The operating transport mechanisms depend on the selected combination of materials as well as on the bonding conditions. So from this work, the results were completed in which different faces in the niobium surface were bonded to a polished alumina surface. The niobium metal had either a very flat polished surface, or well-defined flaws of different shapes and dimensions and that were presented into the surface. The authors were found that the chemical reactions control the transport of materials and according to the conditions chosen for these experiments. As well as the interdependence of the diffusion joining of ceramics and metals requires that two couples have a near contact over the entire area of the joint interface. Even if all defects are detached, there may still be residual thermal stresses due to reaction layers, dislocations, facets, chemical gradients, dislocation arrange‐ ments, and precipitates formed during bonding. On Nb/Al2O3 interfaces, thermal stresses are expected to be rather small since the thermal expansion coefficients of both materials are very near. No reaction layer was perceived.

**Figure 18.** Schematic of the principle of laser welding process [99].

temperature [97].

collected by the indicators [103].

The development of more effective joining techniques for structural ceramics could also have a great impact on their use in mass-produced components. However, there are several challenges on component manufacturing by ceramic processing techniques and by the material themselves. Deformation densified ceramics to form complex shapes is practically impossible because most ceramic materials are brittle even at elevated temperatures. Moreover, ceramics are undesirable for mass production because of their high cost and machining difficulties. Effective ceramic joining techniques can play an important role in improving the reliability of ceramic structures as well [100]. Ceramics are very sensitive to flaws, due to the quality of raw materials used in their production and to the characteristics of various processing techniques, such as machining. Several techniques have been developed to join ceramics for structural application: brazing with filler metals; diffusion bonding; microwave joining; and the use of interface layers designed to form a thin transient liquid phase at a relatively low bonding

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Many studies have been previously conducted on laser interactions with various metals and semiconductors, but few have been done in the processing of ceramics with lasers [101]. The advanced ceramic composite technology has offered more opportunity to fabricate complex structures of composite ceramic lasers, due to the availability of perfect inherent interface characteristics. One of the main problems in fusion welding of ceramics is to control cracking because of the residual thermal stresses. The result has been to give extra heating in a more extensive region around the zone of weld so that the net thermal slope of the extra heating and the joining source is presently adequately low so that no residual thermal stresses sufficiently high to cause cracking when reached [102]. This extra heating also allows the part to be heated and cooled very slowly enough to avoid thermal shock. In order to avoid weld cracking, and heated ceramic samples with radiant energy formed by halogen lamps, which have been

Exner and Nagel [104] have investigated about the laser welding of functional and construc‐ tional ceramics for microelectronics. They presented successful method of a laser welding

#### **5.3. Ceramic-metal laser welding**

Laser welds bonding technique is a new kind of welding technology [96]. It has been developed as an alternative to adhesive bonding and laser welding. Laser welding has a small heat effect zone, which has little effect on the adhesive bonding area [97, 98]. The adhesive in the fusion zone decomposes during the laser welding process, which produces little effect on the properties of the joint. Thus, it can be assumed that laser welding and adhesive bonding hardly affect each other (**Figure 18**). The advantages of laser welding and adhesive bonding are both included in the laser weld bonding technique. The adhesive provides excellent stress distri‐ bution over large bonding areas and laser welding improves the peel resistance of adhesives. Thus, a laser weld bonding joint has better mechanical properties than either a laser welded or adhesive bonded joint alone. Laser welds bonding is a new hybrid technique that combines metallurgical joining, mechanical joining and chemical bonding [96].

**Figure 18.** Schematic of the principle of laser welding process [99].

From this article, the diffusion bonding is a technology to achieve compact joint by diffusion of atoms, even chemical reactions between materials or interlayer and materials. The diffusion of atoms in interface is carried out by several mechanisms, such as the replacement of near atoms, movement of clearance atoms and movement of vacancies, etc. The surface of the materials to be joined must be clean and flat (the roughness less than 0. 4 μm). Joining time can be a few hours at a mild temperature (0. 6*T*m , *T*<sup>m</sup> is melting point of metal to be joined), also can be several minutes at high temperature (0. 8*T*m, ). Diffusion bonding can be achieved with inserted interlayer [87–92] or without interlayer [93, 94]. The diffusion interlayer can reduce the cracking, relax the thermal residual stress and improve the joining strength. The diffusion interlayer is produce of different element active to ceramics, such as titanium,

Burger and Ruhle [95] studied the material transport mechanisms during the diffusion bonding for niobium (Nb) to alumina (Al2O3). According to this chapter, the many different material transport phenomena may occur during the diffusion bonding process of a metal to a ceramic at high temperatures. The operating transport mechanisms depend on the selected combination of materials as well as on the bonding conditions. So from this work, the results were completed in which different faces in the niobium surface were bonded to a polished alumina surface. The niobium metal had either a very flat polished surface, or well-defined flaws of different shapes and dimensions and that were presented into the surface. The authors were found that the chemical reactions control the transport of materials and according to the conditions chosen for these experiments. As well as the interdependence of the diffusion joining of ceramics and metals requires that two couples have a near contact over the entire area of the joint interface. Even if all defects are detached, there may still be residual thermal stresses due to reaction layers, dislocations, facets, chemical gradients, dislocation arrange‐ ments, and precipitates formed during bonding. On Nb/Al2O3 interfaces, thermal stresses are expected to be rather small since the thermal expansion coefficients of both materials are very

Laser welds bonding technique is a new kind of welding technology [96]. It has been developed as an alternative to adhesive bonding and laser welding. Laser welding has a small heat effect zone, which has little effect on the adhesive bonding area [97, 98]. The adhesive in the fusion zone decomposes during the laser welding process, which produces little effect on the properties of the joint. Thus, it can be assumed that laser welding and adhesive bonding hardly affect each other (**Figure 18**). The advantages of laser welding and adhesive bonding are both included in the laser weld bonding technique. The adhesive provides excellent stress distri‐ bution over large bonding areas and laser welding improves the peel resistance of adhesives. Thus, a laser weld bonding joint has better mechanical properties than either a laser welded or adhesive bonded joint alone. Laser welds bonding is a new hybrid technique that combines

metallurgical joining, mechanical joining and chemical bonding [96].

niobium and zirconium etc.

184 Joining Technologies

near. No reaction layer was perceived.

**5.3. Ceramic-metal laser welding**

The development of more effective joining techniques for structural ceramics could also have a great impact on their use in mass-produced components. However, there are several challenges on component manufacturing by ceramic processing techniques and by the material themselves. Deformation densified ceramics to form complex shapes is practically impossible because most ceramic materials are brittle even at elevated temperatures. Moreover, ceramics are undesirable for mass production because of their high cost and machining difficulties. Effective ceramic joining techniques can play an important role in improving the reliability of ceramic structures as well [100]. Ceramics are very sensitive to flaws, due to the quality of raw materials used in their production and to the characteristics of various processing techniques, such as machining. Several techniques have been developed to join ceramics for structural application: brazing with filler metals; diffusion bonding; microwave joining; and the use of interface layers designed to form a thin transient liquid phase at a relatively low bonding temperature [97].

Many studies have been previously conducted on laser interactions with various metals and semiconductors, but few have been done in the processing of ceramics with lasers [101]. The advanced ceramic composite technology has offered more opportunity to fabricate complex structures of composite ceramic lasers, due to the availability of perfect inherent interface characteristics. One of the main problems in fusion welding of ceramics is to control cracking because of the residual thermal stresses. The result has been to give extra heating in a more extensive region around the zone of weld so that the net thermal slope of the extra heating and the joining source is presently adequately low so that no residual thermal stresses sufficiently high to cause cracking when reached [102]. This extra heating also allows the part to be heated and cooled very slowly enough to avoid thermal shock. In order to avoid weld cracking, and heated ceramic samples with radiant energy formed by halogen lamps, which have been collected by the indicators [103].

Exner and Nagel [104] have investigated about the laser welding of functional and construc‐ tional ceramics for microelectronics. They presented successful method of a laser welding technology developed in the Laser Institute of Mittelsachsen (Germany). The investigations of alumina laser welding with a purity of 97% showed that in general the technology is suitable. Furthermore, it enables them to carry out the procedure without furnaces and in a natural atmosphere within only a few minutes. It was established, that the high quality of laser welding joints are achievable. Homogeneous structure and lead to no loss of power also, loss of tangible property is not known. The technology permits joining up to a thickness of 3.5 mm. Through using particular preheating it is conceivable to settle the material by metals. The shortest distance from the joining area is more than 25 mm. Implementation of the technology develops the application of ceramic dramatically. All the outstanding advantages of the laser material processing are useful: touchlessness, flexibility , precision and high velocity [104].

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