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Microwave Sintering of Advanced Composites Materials: A Review

2018, Materials Today: Proceedings

In recent years microwave sintering has gained significant attention based on the improved mechanical properties as compared to the conventional material processing. Microwave sintering has found its applications for the processing of metal powder, metal matrix composites, ceramics and also in the processing of metal ores. This article reviews about basic processing aspects of microwaves, microwave sintering and some of its applications comparing it with the conventional processing.

Available online at www.sciencedirect.com ScienceDirect Materials Today: Proceedings 5 (2018) 24287–24298 www.materialstoday.com/proceedings IConAMMA_2017 Microwave Sintering of Advanced Composites Materials: A Review Chirag Singhala*, Qasim Murtazab, Parvejc a Student, Department of Mechanical Engineering, ZHCET, AMU, Aligarh-202001, Uttar Pradesh, India Associate Professor, Department of Mechanical Engineering, ZHCET, AMU, Aligarh-202001, Uttar Pradesh, India c Assistant Professor, Department of Mechanical Engineering, ZHCET, AMU, Aligarh-202001, Uttar Pradesh, India b Abstract In recent years microwave sintering has gained significant attention based on the improved mechanical properties as compared to the conventional material processing. Microwave sintering has found its applications for the processing of metal powder, metal matrix composites, ceramics and also in the processing of metal ores. This article reviews about basic processing aspects of microwaves, microwave sintering and some of its applications comparing it with the conventional processing. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Applications [IConAMMA 2017]. Keywords: Microwave sintering; composite materials; microstructure 1. Introduction Microwave energy is very widely used form of electromagnetic radiation. Microwave energy is a combination of a magnetic and an electric field orthogonal to each other. Microwave energy radiation frequency lies in the range of 300MHz-300GHz and corresponding wavelength ranging between 0.01m to 1m. Processing of materials from the microwaves, is cost effective, time saving and environmental friendly as compared to the conventional processing (fig. 1) [1-5]. In recent times, there is a requirement of cost effective, environment friendly, less time consuming products in the industries and to the users. Microwave processing has emerged out as a better alternative to the conventional one by imparting improved qualities to the processed materials in terms of physical, mechanical, microstructural properties and along with reduced energy consumption. In microwave sintering, heating takes place * Corresponding author. Tel.: +91-9456467778 E-mail address: [email protected]; [email protected]; 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Applications [IConAMMA 2017]. 24288 Chirag Singhal, et al./ Materials Today: Proceedings 5 (2018) 24287–24298 at the core of the specimen as the material absorbs the radiation and convert it into heat within in specimen. This heat transfers from the specimen core to entire specimen volumetrically. Conventional heating is a surface heating phenomena in which first the surface is heated and then transferring of heat takes place into the materials by conduction, convection and radiation [6-7]. Powder metallurgy (PM) is processing of metal powder in which first the material powder is compacted by applying high pressure into green compacts. These green compacted specimens are sintered to get desired material properties and bonding with in composite at high temperatures in between 0.6-0.8Tm. Presently, there is challenging demand from PM processing to get improved and new sintering process with enhanced material properties in terms of mechanical and chemical in nature [6]. Fig. 1.Schematic showing the wide spectrum of electromagnetic radiation with their corresponding frequencies and wavelengths [6]. 2. Basics of microwave sintering of materials Microwave heating behavior of metal-matrix composites is totally different from dielectric materials. In metalbased materials electric field is not generated internally because of high conductivity of metals but present in dielectric [8]. Microwave sintering also depends upon the behavior of material towards the electric field and magnetic field induced by the microwaves. Physical properties of the materials are the key factor for effective heating through microwave radiations. Not full part of the microwave radiations generated by the applicators is absorbed by the specimen, part of these radiations absorbed, some part is transmitted and some part is reflected back [9]. The conductors are opaque to microwaves, as they reflect back the microwaves radiation and cause plasma formation which leads to surface heating such as bulk materials [7, 9]. Microwave heating of the material depends upon the penetration depth of the material. The penetration depth or skin depth of the microwave field inside a specimen is defined as the distance from the surface of the material at which the magnitude of the field strength drops by a factor of 1/e. Mathematically, it can be expressed as: [10]  1  0.029(  e o ) 0.5 (f ) (1) Where f is the microwave frequency (2.45 GHz), electrical conductivity of material (S m-1), e is  is the magnetic permeability of material (H m-1),  is the electrical resistivity of material (Ω m), and o is the incident wavelength of microwave (m). It is evident from the Eq. (1) that higher the conductivity of the material lesser will be the skin depth of the material. So, the skin depth for the metal based materials is very low as the metals are good conductors of heat. But, skin depth of metals gets increased with an increase in processing temperature of metals [8]. The capability of a material to transform microwave radiation into heat is defined as dielectric loss factor. Polarizability of material is measured by the dielectric constant of the material. Due to the absorption microwaves temperature of the specimen increases which can be governed by Eq. (2) [1, 6, 7] Chirag Singhal, et al./ Materials Today: Proceedings 5 (2018) 24287–24298 T 2f o  E  C p t Where 24289 2  o electrical permittivity in space,   is the dielectric loss factor. (2) In microwave processing, higher heating rates are achieved due to the combination of heat generated by polarization process, conductive and radiative heat losses in the materials. The microwave interactions with the dielectric materials depends upon two factors: power dissipation of microwaves (P) and the depth of penetration (  ) of microwaves inside the specimen. From Eq. (1) depth of penetration is given for the interaction of microwaves with metallic powders. The uniformity in the heating characteristics rely upon following factors The power dissipation is explained by Eq. (3) [7] 1 P     0     E 2  V  e 2z 2 Where E is the electric field through the surface, V is the volume, z is the distance into specimen, *frequency and  is the attenuation constant. (3)  is the 2*  2.1. Microwave hybrid heating (two directional microwave heating) In recent years, microwave sintering is performed over conventional domestic low frequency microwaves whose frequency is 2.45GHz as reported by various studies [6-11]. By using these applicators microwave power can be used for green compacts sintering, however major challenge is poor microwave absorption in the metal-matrix compact. This makes initiation of heating of materials difficult [1, 6, and 7]. On the other hand, in direct microwave sintering there may be problems of thermal instabilities, which can lead temperature runaway into the processed materials and resulted in overheat specimen catastrophically. The intrinsic temperature gradient present within the material during volumetric heating causes temperature non-uniformities, which at high heat gradient may cause uneven properties and cracking of the specimen. In a conventional heating, first heating of surface of metals initiates and then this heat is being transferred to inside to the specimen (direction of heating is from outside to inside) [2, 10, 11]. Microwave sintering is a volumetric phenomenon (fig. 2), in which heating is directed from inside to outside of specimen causing in higher temperature of the specimen at the core of the specimen and then heat is transferred to the surface of the specimen [11, 12]. Whereas, conventional heating results in poor microstructure, physical and mechanical sintering characteristics as compared to the microwave sintering due to variation in temperature rates in different areas of the specimen [2,7]. Instabilities occurring due to direct microwave heating that are high temperature gradients led to researcher to develop hybrid sintering. Microwave hybrid heating is a two directional microwave heating which uses materials having higher dielectric losses at room temperatures. Materials having high dielectric loss factor are used in microwave hybrid heating as this type of heating is two directional heating. These materials are used as an infrared heating source and are known as susceptor. Susceptor absorbs microwaves at low temperatures and reaches high temperatures. This heat is transferred to the sample by conventional way [6, 8]. This heating is a classical example of two directional microwave heating (fig.2). This is a combined action of microwaves and microwave-coupled external heating source. In this heating there is rapid sintering from both outside and inside of the powder compact [6-8]. Fig. 2. Schematic diagram showing the concept of two directional sintering (adapted) [2]. 24290 Chirag Singhal, et al./ Materials Today: Proceedings 5 (2018) 24287–24298 2.2. Energy Saving in Microwave Processing Nowadays, almost every type of oxide, non-oxide ceramics and metal matrix composites can be easily sintered with the help of microwave hybrid sintering which leads to volumetric heating. Researchers have reported that microwave assisted sintering decreases the consumption of energy to 10-100 times [7, 13-16]. This energy efficiency is achieved because microwaves couple directly with the material. As a result the heating of other parts of microwave environment that are not involved. Thus avoiding the heating of other parts that are not directly involved in the processing i.e. air and the walls of oven [1-2, 7]. Even with such decrease in energy consumption, the sintered specimen often shows enhanced microstructure & mechanical properties as compared to the conventional since the heating through microwave is more uniform and volumetric in nature [2,14-16]. 2.3. Reduction in Sintering Time In microwave heating there is direct absorption of radiations by the materials. Direct heating of specimen decreases heat losses. But in the case of conventional sintering compacts are heated radiatively and this leads to high temperature gradient within the compact. To avoid these conditions, the furnaces are used with isothermal holding at intermittent temperatures. Its helps to maintain the specimen at low temperature gradients and as a result the processing time of the sample increases. Due to the higher heating rates and higher thermal gradients in specimens processed by the conventional processing techniques, it causes distortion and inhomogeneous microstructure [1,1516]. Microwave hybrid sintering is one of the better alternatives to conventional material sintering. Microwaves radiations directly interact with the metal particles present in the compacted specimen that leads to the volumetric heating of the specimen [17]. Processing through this technique is frontrunner due to the reduction of the sintering time (fig. 3) by factor of 10 or more in many cases [13, 14]. a b Fig. 3.(a) Temperature versus time profile of power consumption and sintering time for conventional and microwave sintering [13]; (b) Temperature versus time profile for 316L and 434L austenitic stainless steel microwave heating and conventional heating [14]. 2.4. More uniform grain growth and increased density Powder compact density can be measured by using the Archimedes apparent weight density principle and dimensional measurement method. sintered density green density Densification Parameter theoretical density green density The microstructure obtained after microwave processing are more uniform in nature. As in microwave sintering there is optimization of various parameters such as direct absorption of microwaves which leads to uniform volumetric heating, higher heating rates at core of the specimens and higher diffusion rate. The power of the field generated by the microwaves is very high (30 times more) on comparing with the external field, as a result ionization takes place at the surface of pellets. Due to this diffusion of ions between specimen’s particles is increased and resulted in accelerated densification. Accelerated densification improves in mechanical properties as Chirag Singhal, et al./ Materials Today: Proceedings 5 (2018) 24287–24298 24291 compared to conventional sintering achieved because of finer microstructure developments and low porosity inside the specimen (fig. 4) [13,14]. (a) (b) Fig. 4(a) Comparison of densification parameter at different composition under conventional and microwave sintering at 1400°C [14], (b) Schematic diagram of the experimental set-up normally used in microwave sintering [2]. 3. Application of Microwave Sintering Microwave energy is one of the cutting edge technologies to produce metal matrix composites (fig. 5). Due to low cost, light weight, ease of fabrication and higher strength to weight ratios. Metal matrix composites (MMCs) are very widely used in automobile, aerospace industries and in various other industrial requirements. These composites are widely used in automobile industry, tribological applications such as various implants due to their improved microstructure and reduction in wear and tear [18-23]. Microwave sintering is not only confined to the composites but it is being used for the sintering of semiconductor, metals, alloys and so on. Reinforcement (Ceramic/Metal/Polymer) Additives Matrix (as per requirement) (Pure Metal powder) Composite Powder Cold/Hot Compaction of composite powder Direct Heating Hybrid Heating (Direct MW exposure to green compact) (Indirect Microwave Exposure to Green Compact through susceptor) Sintered Compact with 1) Wastage of energy and time 2) Poor mechanical and metallurgical properties 3) Non-Uniform Sintering 4) Random Distribution of grains Sintered Compact with 1) Energy and time saving 2) Better mechanical and metallurgical properties 3) Uniform and rapid Sintering 4) Uniform Distribution of grains Fig. 5. Steps for microwave processing of pure metal, metal alloy and metal-matrix composite powders (adapted) [16]. 24292 Chirag Singhal, et al./ Materials Today: Proceedings 5 (2018) 24287–24298 3.1. Composites of Al-TiN Sahu et al. successfully processed specimen through microwave hybrid sintering (MWS) of metal: Al (86, 76, 66 % by weight), reinforcement: TiN (10, 20, 30% by weight) and rest is additive i.e. copper. SiC is used as susceptor. In this sintering of specimen is carried out at 1-4 min in Argon inert gas environment at 700°C. Average particle size of copper powder is 25µm. Sintering of 2 min by the microwave is sufficient and if sintering time is more than 2 min causes melting out of base metal (Al) from the composite specimen. As the amount of TiN increases, hardness and wear resistance of the sample increases as compared to hot pressed composite is observed. Densification of the pellets increases with the increase in sintering time resulting in the shrinkage of specimens. TiN is a covalent compound due to this reason wettability with metals is poor but with the addition of Cu wettability of TiN increases and therefore copper was added to the mixture. They reported, on observing at the higher magnification good interfacial metal bonds with the second phase TiN particle and Al base metal (fig. 6). As a whole processing of MMCs through microwave is easy and environmental friendly due to the less processing time and energy consumption when compared with the conventional sintering processes [20]. Fig. 6. (a) SEM images sample Al-10TiN-4Cu; (b) SEM images of Al-30TiN-4Cu; (c) shows improved bonding of TiN reinforcement with Al matrix for specimen of composition Al-20TiN-4Cu [20]. 3.2. Composites of B4C-Aluminum In literature, the sintering of composite specimen was performed using a microwave oven at temperature (650,750,850,950°C) [21]. Average particle size of powders was 5µm of B4C, Al and Co. it reported that on increasing the sintering temperature from 850°C and more, the interfacial reaction takes place between Al and B4C which resulted in formation of Al3BC. Increase in the cohesion between Al and reinforcement is the outcome of higher sintering temperature resulting in higher atomic diffusion. On observing SEM and EDS images (fig. 7.), it can be inferred that dark and light spots are of B4C and Al particles, respectively and Al3BC is formed at the interface of particles probably. Addition of 1 wt% cobalt improves microstructural and mechanical properties of the composites. Further, microhardness and compressive strength values increased with increasing the weight fraction of B4C. It is concluded, with the use of microwave sintering composite specimens were manufactured with saving in energy and time [21]. Fig. 7. Backscattered images of B and C composites sintered at 850°C : (a) Al–10 wt% B4C–1 wt% Co and (b) Al–15 wt%B4C–1.5 wt % Co ; (c)Backscattered image and EDS of Al–20% B4C–2% Co composite sintered at 850 [21]. Chirag Singhal, et al./ Materials Today: Proceedings 5 (2018) 24287–24298 24293 3.3. Al-Ni50Ti50 Composites Microwave sintering was carried out with aluminium (99.5% purity, 7-10 µm average particle size), nickel (99.5% purity, 145 µm average particle size) and titanium (99.5% purity, 110 µm average particle size) powders [18]. Reddy et al. prepared metallic glass particles of Ni50Ti50 (used as reinforcement) with the help of ball mill. After 55 h of ball milling, XRD analysis of these powders revealed that the powders were amorphous. Specimens of the Al- matrix composites were processed by microwave assisted sintering processing. The fractographic morphologies of the pure Al and its composites under compressive loading in (fig. 8) clearly showing that shear mode fracture in both pure Al-composites. The bonding between the Ni and the Al matrix was in appropriate condition as indicated by the dimples at the bottom. Homogenous distribution of amorphous particles with small porosity at some locations is observed by the microstructural characterization results [18]. Fig. 8. The fracture morphologies of (a) pure Al; (b) Al-10wt% Ni50Ti50 and (c) Al-20wt% Ni50Ti50 [18]. 3.4. Calcia-doped Zirconia Basu et al. successfully sintered Calcia-doped Zirconia for biomedical application by microwave [22]. The processing of Calcia-doped Zirconia is done having a composition of Ca-PSZ (8 mol% CaO) and Ca-FSZ (16 mol% CaO) in ZrO2. The average particle size of Calcined powder was used is approximately 3 µm after using planetary ball mill and ranges from 300 nm-7 µm. Specimens were sintered (at 60 % power level) for 20 min and (at 100 % power level) for 20 min to target temperature (1500,1550,1585 °C). With the increase of temperature both densification and hardness increases for both the configuration. The tribological experiments were also carried out on optimized Ca-PSZ (MW-1585°C) and Ca-FSZ (MW-1585°C) in air as well as in SBF environment. The fracture surface reveals that the intergranular fracture mechanism (fig. 9) is the dominant fracture mechanism [22]. Fig. 9. SEM images of worn surfaces of Ca-FSZ ceramics (MW, 1585°C) (a, c, and e) and that on Ca-PSZ ceramic (b, d, and f) after testing against steel in SBF. In (a) and (b) the white ellipses show the wear scar after fretting in SBF medium. The cracks are prominent in the tribolayer presented in (e) and (f). The double pointed arrow indicates fretting direction. Fretting conditions: 105 cycles, 10 Hz frequency, 10 N load and 80 mm stroke length. Counterbody: 6 mm diameter steel ball [22]. 24294 Chirag Singhal, et al./ Materials Today: Proceedings 5 (2018) 24287–24298 3.5. Al2O3/Ti(C,N) micro-nano-composite ceramic Yin et al. has successfully sintered Al2O3/Ti(C,N) micro-nano-composite material for tool having a composition of 61wt% α- Al2O3 (0.6 μm, purity:99.99%), 30wt% Ti(C,N) particles (0.2 μm, purity:99.99%), 9wt% Ni and Mo (3 μm, purity: 99.99%) [23]. Green compacts were sintered at temperatures (1500,1550,1600,1650°C) with holding time of 5–15 min in flowing N2 atmosphere in a 2.45GHz multi-mode microwave furnace in an energy-saving conditions as compared to the gas-pressure sintering, the sintering temperature and holding time were reduced by 14% and 89%, respectively. It was observed that the radial shrinkage was much larger than the lateral shrinkage. It is clearly seen from (fig. 10) that the crack was continuously deflected about 90° as indicated by the arrows. As the sintering was carried out at the low temperature and short holding time which limited grain growth. The intragranular nano-particles induced residual stress in the micro- Al2O3 grains, which increased the ratio of grain boundary toughness to grain toughness. So the grains with intro-structures are more inclined to transgranular fracture [23]. Fig. 10. Vickers indentation rack propagation path of microwave sintered Al2O3/Ti(C, N) [23]. 3.6. Copper-graphite composites Metal-matrix composites of copper- graphite composites were processed successfully and sintered through microwave from electrolytic copper powder (average grain size of 12 µm) and mixed with the graphite powder (50 µm size) in volume fraction of 5,10,15,20,25 and 30% through powder metallurgy [24]. The mixture was thoroughly mixed for 2 hrs using electric agate pestle mortar which was rotating at 20 rpm. The preheated powder at 150°C was compacted uniaxial. Sintering of compacts was carried out in an microwave furnace of 3.2kW (2.45 GHz) at sintering temperatures i.e.700,850,900°C and isothermal holding time i.e. 10,20,30 min. The graphite particles were uniformly distributed in the matrix. SEM micrographs (fig. 11) shows that there are no cracks or fissures and confirms the advantage of faster and homogenous heating. The finer microstructure with relatively smaller and round pores are observed, enhances the performance of composites. This is due to the processing of specimen through microwave sintering. It is observed that with the increase of % graphite porosity decreases due to the closure of pores and selective coupling of the microwave interaction with the graphite. Increase in the amount of graphite content to copper composite causes decreases in the hardness. Due to the fact that, graphite is a soft material and using it more as reinforcement decreases the hardness of the material [24]. Fig. 11. SEM images of copper–graphite composite at different composition and magnification, (B1) copper-10% graphite, (B2) copper-20% graphite, (B3) copper-30% graphite, (C1) copper-10% graphite, (C2) copper-20% graphite and (C3) copper-30% graphite [24]. Chirag Singhal, et al./ Materials Today: Proceedings 5 (2018) 24287–24298 24295 3.7. Biomedical hydroxyapatite/magnesium composites Xiong et al. has successfully microwave sintered the hydroxyapatite/magnesium composites for biomedical application i.e. for bones [25]. HAp powder was used as the reinforcement having average size of 40 µm (5,10,15 wt %) with Mg powder having average size of 40 µm as the as the base metal for the processing of the sample. The densification of the specimen was done at microwave sintering temperature 500°C in an argon environment for 10 min while in the case of conventional sintering was done for 1 hr. It was observed that with an increase of wt% of HAp, hardness and strength of the specimen increases. The elemental mapping of HAp and Mg showed the uniform distribution of reinforcement particles for Ca and P elements. SEM images of the specimen confirm the presence of HAp and the number of HAp clusters (fig. 12 d-f) increases at higher magnification. It is to be noted that the accumulation of large number HAp particles are harmful for the improvement in terms of corrosion resistance and mechanical properties of the composites. Peaks of pure Mg and HAp is confirmed by the XRD diffraction plots of HAp-10/Mg but, MgO (Magnesium Oxide) presence is also shown due to the slight oxidation which is caused by the milling of powders during handling (fig. 14 g). In the XRD plots of HAp-10/Mg, it has been shown that there are no other peaks, hence it can be inferred that there is no chemical reaction taking place between Mg and HAp. This is an advantage for maintaining the bioactivity of metal matrix composites of HAp/Mg. It has also been seen that with the help of microwave sintering specimen is sintered with less consumption of energy [25]. Fig. 12. Optical microscopy of specimen at different at composition and different magnification HAp-5/Mg (a and d), HAp-10/Mg (b and e), and HAp-15/Mg (c and f); (g) X-ray diffraction patterns of HAp and HAp-10/Mg after sintering at 500 °C for a time period of 10 min [25]. 3.8. Vanadium carbide reinforced aluminum matrix composite The microwave sintering of the Al metal-matrix composites with vanadium carbide as the reinforcement were successfully carried out at temperature of 600°C with the help of microwave furnace having 900 W power and 2.45 GHz frequency and compared with conventional sintering which was done at temperature of 600°C with 10°C/min heating rate in a graphite bed for 1 hour [26]. Vanadium carbide is a transition metal carbide having excellent hightemperature strength and good corrosion resistance that make it useful as high-temperature structural material. Aluminium (250 mesh 99% purity) with 10 wt% vanadium carbide (average particle size of 200 nm, 99% purity) composite microwave sintered specimen have demonstrated better hardness as compared to the conventionally processed sample. Microwave sintering leads to the formation of transition element aluminides i.e. Al3V. FESEM micrographs (fig.13 a-b) show the presence of VC reinforcements, Al3V phase and porosities. The creation of porosities is due to the presence of Al3V phase, besides selective heating of microwave processing [26]. 24296 Chirag Singhal, et al./ Materials Today: Proceedings 5 (2018) 24287–24298 Fig. 13. (a) FESEM micrograph of sample sintered conventionally at 600°C, (b) FESEM micrograph of sample sintered by microwave at 600°C, (c) XRD patterns of Al-10wt%VC composite prepared by spark plasma, microwave and conventional sintering [26]. 3.9. Al-SiC-TiC hybrid composites using pure and 1056 aluminum powders Al-SiC-TiC hybrid composites using pure and 1056 aluminum powders were successfully sintered [27]. In this composites matrix two different aluminum powders were in the composition of Al-15wt%SiC-7wt%TiC. This composite was sintered by conventional processing as well as by microwave processing at temperature 650°C and 750°C for both type of powders i.e. pure and 1056 aluminum. XRD results (fig.14 a) shows that there is no reaction between aluminum and SiC or TiC particles which leads to the formation of Al4C3 phase for both 1056 and pure aluminium composites. As this reaction is a harmful reaction, however there may be possibility of low level of reaction products below the detection limits of XRD analysis. SEM micrographs of both samples exhibit uniform particle distribution of the reinforcements in the matrix. As it is clearly seen from (fig. 14 b) that there is considerable amount of porosity in the pure aluminum composites while in the case of 1056 aluminum matrix composites porosity is less due to the absence of aluminum oxide layer on the surface of 1056 aluminum powder. The mechanical properties and densification is improved in both microwave processed composite but composites containing 1056 aluminum showed better mechanical properties and improved microstructure compared to pure aluminum matrix composites. Reduction in the sintering time as well as energy consumption is observed [27]. Fig. 14(a) XRD patterns of sintered samples with pure aluminium as matrix, (b) Backscattered and secondary SEM images of: 1056-mic-750 and pure-mic-750 specimens [27]. Chirag Singhal, et al./ Materials Today: Proceedings 5 (2018) 24287–24298 24297 4. Future Development Microwave processing has achieved too much worldwide acceptance because of the improved properties imparted to the specimens. Some research showing that microwave processing can also be used in the bio-medical field such as dental implants to fabricate composites with better mechanical, tribological, microstructural characteristics compared to the conventionally processed materials [2-4, 22]. Microwave drilling of metal materials is in developing phase. Microwaves can also be used for drilling of the fresh wet bone tissue. Microwave drilling in bone provided certain merits such as mechanical properties remains unchanged, holes produced were substantially smoother than conventional mechanical drilling [28]. Low-grade waste energy utilization with the help of thermoelectrics is also ongoing research. Thermoelectric ceramics can also be studied with the help of microwave sintering [29]. Microwave heating of metal ores and its processing is also one of the developing fields of research [30]. Sheet metal heating and its processing can also be scope of work by microwave [31]. Casting of alloys in-situ through microwave is a growing field of interest in microwave processing [32-34]. 5. Conclusions Microwave processing has gained very much popularity in the field of metal material processing. Microwave hybrid sintering has vast area of working and this processing is not confined only to the sintering of metals powders but it can be used for the processing of alloys, composites, and ceramics. Till date the use of microwave processing work is only confined to the laboratory level because the microwave heating process is highly specific materials based and required large number of optimization process parameters. Uncontrolled microwave material processing leads to thermal runaway which causes unstable & non-uniform heating and so on. Nevertheless, processing of metals through microwave sintering has certain merits as compared to the conventional sintering. A summary of these are:      Improved mechanical and physical properties can be obtained using microwave hybrid sintering. Normally better grain distribution and higher density achieved through microwave sintering. Reduction in processing time is achieved in the through microwave sintering due to higher heating rates. Microwave sintering is much more energy efficient processing technique as compare to the conventional sintering. 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