**2. Sintering techniques**

Sintering is the process of consolidating powder compact by a thermal treatment to obtain materials with special properties. It is one of the widely used techniques in the powder metallurgy and ceramic processing. These techniques enable heating through interaction between electromagnetic field and materials.

#### **2.1 Conventional sintering**

Powder forming and, in particular, pressing are among the best ways to build flat objects. At first, the metal powder is pressed into a die, having a geometry close to the one of the final part, together with binders (to increase compact ability) at room temperature to form the so-called green part, which is typically strong enough to be handled gently. The sintering of the green part can be divided into three stages: (i) preheating; (ii) sintering, at a temperature maintained for a time depending on the strength of the bond which needs to be obtained; and (iii) cooling. Usually all the three stages are performed in a controlled atmosphere furnace, in order to prevent phenomena such as oxidation or unwanted chemical reactions. Sintering of stainless steel and refractories is usually performed under vacuum conditions.

As the name might suggest, in liquid sintering a portion of the material is in the liquid phase. This procedure is typically used for cermet, metal, and ceramic sintering. During liquid phase sintering, a liquid phase coexists with a particulate solid at sintering temperature. The goodness of sintering in this case depends strongly on the wetting properties of the liquid part [10].

#### **2.2 Electric current-aided sintering**

Heating induced by electric current has some interesting advantages with respect to conventional heating sources such as the lower sintering temperature, which allows to process nanometric powders, shorter time duration, and better material properties [11–13].

Pulsed electric current sintering (PECS) is characterized by the simultaneous action of a current-induced heating and a uniaxial pressure. PECS allows to reach high heating rates and influences mass transport. Typically, a pulsed DC current

**83**

into.

*Prologue: The New Era of Sintering*

**2.3 Microwave sintering**

instead of outside-in [22].

**2.4 Hot isostatic pressing**

applied [9].

*DOI: http://dx.doi.org/10.5772/intechopen.85338*

nanocomposites, avoiding considerable grain growth [8].

sintering is now employed also for all metal powders [17–21].

is applied with a relatively low voltage (below 10 V), and the current is applied by pulsing patterns. The pulsing pattern is made up of a sequence of pulses (3.3 ms each) followed by an interruption of current, i.e., a pulse pattern of 12-2 means that 12 pulses are applied, followed by a duration of 2 pulses where the current is not

The same simultaneous application of heating and uniaxial pressure is performed during spark plasma sintering (SPS) high densification at temperatures which are typically around 200°C lower than in conventional sintering. Thus, SPS is a new powder consolidation technique used to fabricate bulk shapes and nanostructured materials. This method uses uniaxial pressure and pulsed direct electric current to consolidate the powders at short sintering time and a relatively low sintering temperature compared to conventional hot pressing sintering techniques. The short sintering time is suitable for preserving the amorphous structure without undesirable phase transformations. During SPS heating is due to a pulsed DC current passing through the die which contains the powder, while pressure is applied on it. The characteristics, therefore, include the high heating rate, the application of a pressure, and the effect of the current. While similar for some features to conventional sintering, the SPS process is typically characterized by a higher heating rate such as 300°C/min compared to a maximum of 10°C/min reachable during conventional sintering. Therefore, high relative densities can be obtained in a very short time, allowing to sinter nanometric powders, nanostructured ceramics, or

Microwave sintering, which includes heating and sintering, is a powerful process

for sintering ceramic, ceramic composites, cermets, and metals. In microwave sintering the heating is obtained through a noncontact method which results in enhancement of the process in terms of reaction and diffusion kinetics, shorter cycle time, finer microstructures, and other unique features leading to considerable improvement in the mechanical properties and further energy savings [14–16]. While developed for ceramic, inorganic, and polymeric materials, microwave

During microwave sintering, heating takes place via absorption/coupling. of the microwave field followed by the so-called volumetric heating (viz., heating of the material as a whole) due to the conversion of the electromagnetic energy

thermal energy. In this particular case, heating is generated within the material in an instantaneous way, which depends strongly on the material properties. Therefore, with respect to conventional sintering, the heating profile is inside-out

Hot isostatic pressing (HIP) is a manufacturing technique based on the simultaneous application of temperature and pressure to materials (usually powders) for a definite time to increase the density of materials. This process was invented in 1955 to improve aircraft systems and nuclear industry. Today it became an emerging technology in the processing of high-density powders that is used in aerospace, automotive, medical defense, etc. [23]. This technique has several advantages and involves highly complex shapes of finished parts, the powders are consolidated at lower temperatures achieving higher densities, the finished parts have homogenous density, the high gas density results in rapid heating and shorter time, and the

#### *Prologue: The New Era of Sintering DOI: http://dx.doi.org/10.5772/intechopen.85338*

is applied with a relatively low voltage (below 10 V), and the current is applied by pulsing patterns. The pulsing pattern is made up of a sequence of pulses (3.3 ms each) followed by an interruption of current, i.e., a pulse pattern of 12-2 means that 12 pulses are applied, followed by a duration of 2 pulses where the current is not applied [9].

The same simultaneous application of heating and uniaxial pressure is performed during spark plasma sintering (SPS) high densification at temperatures which are typically around 200°C lower than in conventional sintering. Thus, SPS is a new powder consolidation technique used to fabricate bulk shapes and nanostructured materials. This method uses uniaxial pressure and pulsed direct electric current to consolidate the powders at short sintering time and a relatively low sintering temperature compared to conventional hot pressing sintering techniques. The short sintering time is suitable for preserving the amorphous structure without undesirable phase transformations. During SPS heating is due to a pulsed DC current passing through the die which contains the powder, while pressure is applied on it. The characteristics, therefore, include the high heating rate, the application of a pressure, and the effect of the current. While similar for some features to conventional sintering, the SPS process is typically characterized by a higher heating rate such as 300°C/min compared to a maximum of 10°C/min reachable during conventional sintering. Therefore, high relative densities can be obtained in a very short time, allowing to sinter nanometric powders, nanostructured ceramics, or nanocomposites, avoiding considerable grain growth [8].

#### **2.3 Microwave sintering**

Microwave sintering, which includes heating and sintering, is a powerful process for sintering ceramic, ceramic composites, cermets, and metals. In microwave sintering the heating is obtained through a noncontact method which results in enhancement of the process in terms of reaction and diffusion kinetics, shorter cycle time, finer microstructures, and other unique features leading to considerable improvement in the mechanical properties and further energy savings [14–16]. While developed for ceramic, inorganic, and polymeric materials, microwave sintering is now employed also for all metal powders [17–21].

During microwave sintering, heating takes place via absorption/coupling.

of the microwave field followed by the so-called volumetric heating (viz., heating of the material as a whole) due to the conversion of the electromagnetic energy into.

thermal energy. In this particular case, heating is generated within the material in an instantaneous way, which depends strongly on the material properties. Therefore, with respect to conventional sintering, the heating profile is inside-out instead of outside-in [22].

#### **2.4 Hot isostatic pressing**

Hot isostatic pressing (HIP) is a manufacturing technique based on the simultaneous application of temperature and pressure to materials (usually powders) for a definite time to increase the density of materials. This process was invented in 1955 to improve aircraft systems and nuclear industry. Today it became an emerging technology in the processing of high-density powders that is used in aerospace, automotive, medical defense, etc. [23]. This technique has several advantages and involves highly complex shapes of finished parts, the powders are consolidated at lower temperatures achieving higher densities, the finished parts have homogenous density, the high gas density results in rapid heating and shorter time, and the

brittle materials can be processed because of the more uniform heating [24]. Thus, HIP involves the simultaneous application of high temperature and pressure cycles [25]. In the first cycle (cold loading cycle), the temperature is increased some time after the pressure reaching their peak at the same time to give good geometric control in sheet metal encapsulation. In the second cycle (hot loading cycle), the pressure is applied after the temperature has reached its desired value. In the third cycle, the temperature is raised only after pressure reaches its desired value. In the final cycle, the pressure and the temperature are increased simultaneously to reduce the processing time.

The advantage of HIP is the reduction of production times and the variation of properties in the solid, obtaining almost finished parts and parts with complex shapes or small sizes due to its isostatic processing. It is noteworthy in this instance that the model used will be validated over similar powder forming processes where the application of temperature and pressure is performed simultaneously [26–28].
