Temperature profile optimization for microwave sintering of bulk Ni–Al2O3 functionally graded materials

Journal of Materials Processing Technology 214 (2014) 210–216

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Temperature profile optimization for microwave sintering of bulk Ni–Al2 O3 functionally graded materials Yu. V. Bykov, S.V. Egorov, A.G. Eremeev, V.V. Holoptsev, I.V. Plotnikov, K.I. Rybakov ∗,1 , V.E. Semenov 1 , A.A. Sorokin Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia

a r t i c l e

i n f o

Article history: Received 7 December 2012 Received in revised form 21 August 2013 Accepted 1 September 2013 Available online 8 September 2013 Keywords: Functionally graded materials Microwave sintering Temperature distribution Gyrotron

a b s t r a c t Bulk multilayer graded Ni–Al2 O3 samples have been sintered under heating by millimeter-wave radiation using a gyrotron system for high-temperature materials processing. By using a purposely designed thermal insulation arrangement, the temperature profile has been adjusted along the concentration gradient to accommodate for different sintering temperatures of the components. The sintered samples have flat boundaries between layers, and their microstructure is free from cracks and delamination. In addition to metal-ceramic graded transitions, metal-ceramic-metal graded insulator structures have also been fabricated. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The manufacturing of a functionally graded material (FGM) consists of forming a spatially inhomogeneous structure and consolidating this structure into a bulk material. The techniques to fabricate a graded material are divided in two classes according to their capability of producing a stepwise or continuous change in the powder compact composition. Comprehensive reviews of these methods as well as of the methods for consolidation have been published by Kieback et al. (2003) and Birman and Byrd (2007). The powder metallurgy and/or ceramic technology is widely used in the fabrication of metal-ceramic FGMs. The main challenge in the FGM consolidation is to match the sintering behavior of the compositionally non-uniform material, which is affected by porosity, particle size and composition of the powder mixture, in order to minimize the processing-related stresses. To solve this problem, many different grading approaches have been utilized, based on volume fraction gradients of constitutive material phases, porosity or pore-size gradients. For example, it has been shown that

∗ Corresponding author at: Institute of Applied Physics, Russian Academy of Sciences, 46 Ulyanov Street, Nizhny Novgorod 603950, Russia. Tel.: +7 831 416 4831; fax: +7 831 416 0616. E-mail addresses: [email protected], [email protected], [email protected] (K.I. Rybakov). 1 Also at Lobachevsky State University of Nizhny Novgorod, Nizhny Novgorod, Russia. 0924-0136/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2013.09.001

the control of the powder particle size in addition to the composition gradients can effectively adjust the sintering shrinkage and virtually eliminate the differential sintering stresses in the fabrication of the partially stabilized zirconia (PSZ) – stainless steel FGM (Watanabe, 1995). If the temperature ranges for densification of constitutive material phases are very different, such as in the case of many metal-ceramic composites, a promising method for the FGM sintering is development of a temperature gradient during sintering. This method has been successfully employed in the fabrication of a PSZ–TiAl FGM by pulse electric discharge consolidation (Kimura and Satoh, 1997), PSZ–NiCrAlY coating by a field-assisted sintering technique (FAST) (Hur et al., 1998), and PSZ-stainless steel bulk FGM by spark plasma sintering (SPS) (Tokita, 1999). In these techniques a proper temperature gradient along the material composition gradient is obtained by using advantageously the thermal conductivity of the pushrods and profiling the geometry of the graphite die. A significant advance in the application of the temperature gradient method for FGM fabrication can be achieved if the microwave heating is used for sintering. In contrast to the above mentioned techniques in which an article is heated from the surface by the thermal flux coming from the graphite die, the volumetric microwave heating and superficial heat loss from the article undergoing processing are naturally separated. This separation of thermal sources and sinks, inherent in microwave heating, makes it possible to construct the desired temperature distribution in the article more easily. An additional feature of microwave sintering

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of compositionally graded materials comes from the selectivity of microwave absorption. When a composite is fabricated from the materials with very different dielectric properties as in the case of metal and ceramics, the microwave absorption depends strongly on the volumetric ratio of the constituents (Egorov et al., 2010). This peculiarity of microwave heating can be used advantageously to create a proper non-uniform temperature distribution in the article undergoing sintering. In this paper we report fabrication of multilayer, compositionally graded Ni–Al2 O3 samples prepared by powder stacking using millimeter-wave heating. The millimeter-wave radiation (at frequencies of at least one order of magnitude higher than the traditional industrial microwave frequency, 2.45 GHz) is attractive for thermal gradient sintering of metal-ceramic FGM due to a more pronounced selectivity of heating. Besides, a millimeterwave gyrotron system for high-temperature materials processing has a large-size workchamber (as compared to the radiation wavelength), which facilitates introducing a pressing device within it (e.g., Kimura and Yamazaki, 2006). The methods of generating temperature profiles suitable for the sintering of both metal-ceramic and metal-ceramic-metal types of FGM are described.

2. Experiment The composite Al2 O3 –Ni samples were prepared from alumina powder with a mean particle size of 2 ␮m and nickel powder with a mean particle size of 2.5 ␮m. The powder compositions with the components ratio (100 − m)% Al2 O3 + m wt.% Ni, m = 0, 5, 10, 20, 40, 80, were obtained by mixing the respective amounts of powders for 30 h in a biconic mixer. The multi-layer samples were fabricated by stacking the prepared compositions and pressing them together. This method makes it possible to avoid sharp boundaries between adjacent composite layers. Six-layer samples were pressed in a steel mold under a pressure of 400 MPa at room temperature. The prepared multi-layer samples were 10 mm in diameter and about 15 mm in height. The microwave sintering experiments were carried out using a gyrotron system for microwave processing of materials with an output microwave power up to 5 kW at a frequency of 24 GHz (Bykov et al., 2004). The system workchamber has a volume of about 0.1 m3 and represents an untuned cavity with dimensions, L, much greater than the microwave radiation wavelength ( = 1.25 cm). In such a cavity hundreds of oscillation eigenmodes are excited simultaneously, and the superposition of the electromagnetic fields of these modes provides highly homogeneous spatial distribution of the microwave energy. As shown by Kimrey and Janney (1988), in such a cavity with L/ ≈ 100 the inhomogeneity of microwave field amplitude does not exceed 4% in the entire volume. For thermal insulation the samples were positioned within a casing fabricated of highly porous alumina fiber-based material, Valox-1750 (Thermoceramica Ltd., Russia). The gap between the sample and the casing was about 1 mm to allow the air in the powder sample to be released during sintering. In order to avoid oxidation of the FGM metal component, the workchamber was evacuated down to 5 Pa and filled with nitrogen to atmospheric pressure. Due to brittleness of the green samples the sintering was carried out in two stages. At the first stage the samples were pressureless heated at a rate of 10 ◦ C/min up to 1000 ◦ C and then held for 30 min at this temperature. The as-heated samples acquired sufficient mechanical strength to withstand the uniaxial pressure of 0.9 MPa that was applied at the second stage of sintering. A schematic view of the multi-layer sample positioned within the pressing setup is shown in Fig. 1. The pressing setup was installed in the workchamber of the gyrotron system.

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Fig. 1. The multi-layer sample positioned within the pressing setup: 1 – sample; 2 – thermal insulation; 3 – load; 4 – metal support; 5 – ceramic pushrod; 6 – thermocouples; 7 – microcontroller; and 8–computer.

A multi-channel system for temperature measurement was used in the experiments. The Pt/Pt-Rh thermocouple tips touched the side surfaces of the layers of the following compositions: pure Al2 O3 , 80% Al2 O3 + 20% Ni, and pure Ni. The control over the heating process was based on the readings of the thermocouple that touched the 80% Al2 O3 + 20% Ni layer located in the middle of the sample. The microwave power input to the workchamber was regulated automatically by a computerized control system. The automatic control over the microwave power makes it possible to compensate for the varying dielectric and thermal properties of the material and implement the preset temperature-time schedule of processing. The heating rate at the second stage of sintering was chosen equal to 10 ◦ C/min based on the maximum microwave power available for heating. The samples were held at a temperature of 1250 ◦ C (measured at the surface of the middle layer) for 60 min. After the hold the microwave power was either shut off and the samples cooled down at a rate of about 200 ◦ C/min, or the samples were cooled down to 700 ◦ C at a rate of 10 ◦ C/min by controlled reduction of the microwave power. The microstructure of the sintered samples was investigated on the polished surfaces of diametric cross-sections using a scanning electron microscope (JEOL-6490LV, Japan). The phase composition was determined by the X-ray diffraction spectra (Rigaku Ultima IV, Japan) using the ICDD PDF-2 database. The Vickers microhardness was determined on the polished surfaces using the Struers Duramin-5 microhardness tester (USA).

3. Results and discussion The temperatures at which compositionally uniform pellets of different metal-ceramic compositions (100-m) Al2 O3 + m Ni, m = 0, 5, 10, 20, 40, 80 vol.%, achieve maximum shrinkage were determined in a previous study (Bykov et al., 2012) with the same initial powders. The temperatures needed to obtain maximum shrinkage varied from 1400 ◦ C to 1600 ◦ C for pure metal and pure ceramic pellets, respectively. It is clear that in order to reduce stresses during the sintering of compositionally graded materials it is desirable to create such a temperature profile along the composition gradient that the shrinkages in layers of different composition become maximally close to each other. The temperature distribution that is established during microwave heating is determined by the balance of the microwave power absorbed in the sample and the heat loss from the sample to the environment. Neglecting the heat losses through the thermally insulated side surface of the cylindrical sample and assuming the material properties constant within each layer, the heat transfer

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equation describing the temperature distribution within the layer can be written in the following form: ∂T ∂2 T = ki 2 + wi , ∂t ∂x

(1)

where ci is heat capacity, i is density, and ki is thermal conductivity of the ith layer of the sample, and wi is the density of microwave power absorbed in the layer. The boundary conditions at the interfaces between the layers imply continuity of the temperature, T, and the heat flux density, q = −k(∂T/∂x). The entire temperature distribution is influenced by the heat losses through the end faces of the samples that are in contact with the pushrods, where the heat flux is determined as Q = k(Ts − T0 ) S/L, where Ts is the temperature of the sample at the contact with the pushrods and T0 is the temperature of the environment. The heat losses can be varied by using the pushrods made of materials with a properly chosen thermal conductivity, k, and by changing their cross-section, S, and length, L. To determine the values of the necessary correction of the temperature profile, a multi-layer sample was microwave heated in a “symmetric” heat loss configuration, with identical pushrods (10 mm in diameter and 20 mm in height, made of dense alumina ceramics) positioned below and above the sample. The temperature vs. time plots in the pure Al2 O3 layer and the layer of composition 20% Al2 O3 + 80% Ni are shown in Fig. 2 along with the preset temperature-time schedule in the layer of composition 80% Al2 O3 + 20% Ni. As follows from Fig. 2, in the case of symmetric heat loss configuration the temperatures of the top and bottom layers compacted from pure Al2 O3 and 20% Al2 O3 + 80% Ni are approximately equal to each other, whereas the temperature of the middle layer, 80% Al2 O3 + 20% Ni (T3 ), is up to 300◦ higher. Obviously, such a temperature distribution makes it impossible to sinter metal-rich and ceramic-rich layers to high density simultaneously. In addition, while the temperature of the middle layer is close enough to the maximum admissible sintering temperature of the alumina–nickel FGM (nickel melting point, 1453 ◦ C), the temperature in the pure Al2 O3 layer is below its onset of sintering. In order to achieve a significant difference between the temperatures in the metal-rich and ceramic-rich layers, an asymmetric heat loss configuration was implemented. In this configuration the pushrod that contacted the nickel-rich layer was made of stainless steel, and its diameter was profiled along the length. In addition, 1400

80 % Al2O3 + 20 % Ni

1200

2.5 20 % Al2O3 + 80 % Ni

1000 Temperature, °C

3

2

800

100 % Al2O3 1.5

600 Power

Power, kW

ci i

the height of the thermal insulation layer surrounding the upper (nickel-rich) part of the sample was reduced. The conditions under which a strongly inhomogeneous temperature profile along the sample height arises were determined in a series of experiments. The data shown in Fig. 3 demonstrate that with the use of this optimized asymmetric heat loss configuration, the temperature in the pure Al2 O3 layer has increased by 400 ◦ C, the temperature of the middle layer (80% Al2 O3 + 20% Ni) remaining equal to 1250 ◦ C. At the same time the temperature in the 20% Al2 O3 + 80% Ni layer has decreased by 100 ◦ C. The total temperature difference between the pure Al2 O3 layer and the 20% Al2 O3 + 80% Ni layer is 500 ◦ C. From a comparison between Figs. 2 and 3 it can be seen that the introduced additional heat losses that are necessary for the implementation of the optimized inhomogeneous temperature profile have required a more than twofold increase in the microwave power input to the workchamber. Listed in Table 1 are the values of diametric shrinkage of some layers of the multi-layer sample sintered in the inhomogeneous temperature field. It should be noted that the shrinkage of layers in the sintered multilayer sample exceeds the shrinkage values achieved at the same temperature in uniform pellets of the same composition. As follows from the sintering studies with uniformcomposition pellets, the metal powder compact has the lowest sintering temperature. This means that during the heating of a multi-layer sample the layer compacted from metal powder densifies first. Due to the difference in the shrinkage of adjacent layers, the ceramic-rich layers experience the effect of compressive stresses during their sintering. The plastic deformation of metal particles facilitates, to a certain extent, relaxation of the stresses. As a result the bending of interfaces between adjacent composite layers is insignificant, as seen in the photograph of the polished cross-section surface of a multi-layer sample sintered in an inhomogeneous temperature field (Fig. 4). An important issue in the development of multi-layer graded composite materials is the structure of the interfaces between layers of different composition. The probability of cracking and delamination is the highest at these interfaces. Fig. 5 shows SEM images of the interfaces in a multi-layer Al2 O3 –Ni sample microwave sintered in the asymmetric heat loss configuration. It can be seen that the interfaces do not exhibit cracks and/or delamination. An analysis of the SEM microstructure images shows that the pure Al2 O3 layer constitutes a sintered ceramic carcass. Since the

1

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0 9000

Time, s Fig. 2. Temperature vs. time in the layers of composition 80% Al2 O3 + 20% Ni, the layer of composition 20% Al2 O3 + 80% Ni, and the pure Al2 O3 layer in the course of microwave heating of a multi-layer sample whose both end faces are in contact with ceramic pushrods (symmetric heat loss configuration).

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10

1600 100 % Al2O3

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8 80 % Al2O3 + 20 % Ni

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Time, s Fig. 3. Temperature vs. time in the pure Al2 O3 layer, the layer of composition 80% Al2 O3 + 20% Ni, and the layer of composition 20% Al2 O3 + 80% Ni in the course of microwave heating of a multi-layer sample in the asymmetric heat loss configuration. Table 1 Diametric shrinkage of layers in a multi-layer graded Al2 O3 –Ni sample. Composition Sintering temperature (◦ C) Diametric shrinkage of layers in the multi-layer sample

100% Al2 O3 1390 0.133

Fig. 4. A photograph of the polished cross-section surface of a multi-layer sample, 15 mm in height and 9 mm in diameter, sintered in an inhomogeneous temperature field.

temperature (1390 ◦ C) was insufficient for full sintering, the carcass has a porosity of about 15%. The characteristic pore size (2–3 ␮m) is close to the initial alumina particle size, which suggests that no significant grain growth has occurred at this temperature. In the layers containing 5–10 wt.% Ni nickel particles join together and form alumina-free areas up to 100 ␮m in size. Due to low wettability of alumina particles by nickel and absence of interaction between them, the ductile nickel particles are extruded from the sites of localized alumina densification, which is accompanied by development of larger pores in the alumina matrix. In the layers containing 20 wt.% Ni and more, the nickel-rich areas join together and form a nickel carcass. Meanwhile, the localized alumina-rich areas grow to 200 ␮m and more in size and acquire a quasi-elliptic shape elongated perpendicular to the uniaxial pressure applied during sintering. This evidences in favor of a significant plastic deformation of nickel particles, even at a temperature of 900 ◦ C. It should be noted that on the whole the microstructure of the microwave sintered samples is similar to that of conventionally

80% Al2 O3 + 20% Ni 1250 0.067

20% Al2 O3 + 80% Ni 900 0.094

sintered Al2 O3 –Ni FGM compositions fabricated by hot pressing (Leushake et al., 1999), hot isostatic pressing (Bruck and Rabin, 1999), and pressureless sintering (Pines and Bruck, 2006). In many applications there is a need in electric insulation of metal components forming an article. In the fabrication of such articles operating at high temperature the most widespread method is brazing and diffusion bonding of metal components to a ceramic insulator (e.g., Suganuma et al., 1988). The stresses that arise in the joint can be reduced considerably if the ceramic part is replaced by a compositionally graded metal-ceramic transition. In the experimental effort aimed on the development of such a transition, two multi-layer samples of composition (100-m) Al2 O3 + m Ni, m = 0, 20, 30, 70, 80, 100 vol.% were joined to each other, bringing their alumina ends in contact. Since the pairs of the samples undergoing joining are compositionally symmetric relative to the contact plane, microwave heating was carried out in a symmetric thermal insulation casing. The nickel ends of the joined samples were brought in contact with 20 mm long stainless steel pushrods. The pushrods diameter was profiled along their length. The temperature was measured by thermocouples whose tips touched the side surfaces of the pure Al2 O3 layer and both pure Ni layers. The samples were microwave heated at a rate of 10 ◦ C/min up to a temperature of 1550 ◦ C in the pure Al2 O3 layer and then held for 60 min at this temperature. The cooling was carried out at a rate of 5 ◦ C/min down to 800 ◦ C and then 10 ◦ C/min down to 400 ◦ C under a controlled reduction of microwave power. The temperature–time graphs of these experiments are shown in Fig. 6, and a photograph of the sintered compositionally symmetric metal-ceramic-metal sample is shown in Fig. 7. The temperatures of the bottom (T ≈ 1200 ◦ C) and top (T ≈ 1300 ◦ C) nickel layers differ from the temperature of the pure alumina layer by 350 and 250 ◦ C, respectively. The difference between the temperatures of nickel layers is apparently associated with the convective heat flow from the sample. It has been shown by Egorov et al. (2010) that the maximum microwave absorption in metal-ceramic composite materials is observed when the relative volumetric fraction of metal particles is about 0.3–0.4. In

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Fig. 5. SEM images of the boundary area between layers of different composition in a microwave sintered seven layer Al2 O3 –Ni sample: (a) Al2 O3 /Al2 O3 + 5% Ni; (b) Al2 O3 + 5% Ni/Al2 O3 + 10% Ni; (c) Al2 O3 + 10% Ni/Al2 O3 + 20% Ni; (d) Al2 O3 + 20% Ni/Al2 O3 + 40% Ni; (e) Al2 O3 + 40% Ni/Al2 O3 + 80% Ni. Nickel particles are seen as light areas. Magnification 170×.

the experiments described here the heating of multi-layer metalceramic samples relies predominantly on the absorption in the layers of composition 80 vol.% Al2 O3 + 20 vol.% Ni and 70 vol.% Al2 O3 + 30 vol.% Ni. Pure alumina has very low microwave absorption (its dielectric loss tangent tan ı is (1–5) × 10−4 ); however, the temperature of the alumina layer located in the middle of the sample between the strongly absorbing metal-ceramic layers is close to the temperature of the latter. The metal-rich layers at both ends of the sample reflect almost all microwave radiation from their surface, and their temperatures are the lowest. Overall, due to selective microwave absorption in the layers with optimal (in terms of absorption) metal-to-ceramic fraction ratio. an inhomogeneous temperature distribution that is necessary for the sintering of metal-ceramic-metal samples arises over the sample height. This is illustrated by a comparison between the levels of microwave power needed to heat the metal-ceramic (Fig. 3) and metal-ceramic-metal

(Fig. 6) samples. Although in the latter case the sample mass is two times larger, and the steel pushrods facilitate heat loss from the sample, the microwave power in the latter case is lower by a factor of approximately 1.2. This is explained by the fact that the total mass of the strongly absorbing layers in the composition used for the metal-ceramic-metal sample is larger. Shown in Fig. 8 are the results of Vickers microhardness measurements in the layers of different composition of a 6-layer metal-ceramic sample using a load of 4.9 N. The results of measurements on the samples sintered at different temperatures demonstrate that with an increase with temperature the hardness increases in alumina-rich layers. This is associated with a decrease in their porosity. Meanwhile, the hardness of the layers containing more than 20% nickel remains constant and virtually does not depend on the sintering temperature. As follows from the microstructure investigation results, this is explained

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1800 1600

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Time, s Fig. 6. Temperature vs. time in the pure Al2 O3 layer, bottom and top Ni layer, in the course of microwave heating of a multi-layer metal-ceramic-metal sample in the symmetric heat loss configuration.

Fig. 7. Photograph of the microwave sintered compositionally symmetric Al2 O3 –Ni metal-ceramic-metal sample, 26 mm in length.

16000

100 % Al2O3

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95 % Al2O3 + 5 % Ni

by the formation of a metal matrix in these layers. Therefore their hardness is determined by the hardness of nickel and virtually does not depend on the content of the ceramic component. The microhardness obtained in this work for the Al2 O3 layer (HV ≈ 1350) agrees with the known data for dense ceramics if the porosity is accounted for. The hardness of the materials that do not exhibit pronounced anisotropy is proportional to the elastic modulus E, which depends on the porosity, ˘, as E = E0 (1 − 1.9˘ + 0.9˘ 2 ), where E0 is the elastic modulus of the ceramic of full theoretical density (Coble and Kingery, 1956). From here it follows that given the porosity of the layer,  = 0.13, the hardness value in it should be HV = 0.75·HV0 = 1350 (where HV0 ≈ 1800 is the hardness of dense Al2 O3 ceramics (e.g., BCE Special Ceramics, 2012), which coincides with the results of the microhardness measurements in the Al2 O3 layer. The measured microhardness in the composite Al2 O3 –Ni layers decreases more sharply with an increase in the nickel content than it could have been expected according to a mixing law (Hyoung et al., 2001). It occurs that the reason for the rapid decrease in the microhardness may be a porosity increase in the nickel-rich layers

90 % Al2O3 + 10 % Ni

80 % Al2O3 + 20 % Ni

60 % Al2O3 + 40 % Ni

20 % Al2O3 + 80 % Ni

Microhardness, MPa

12000 10000 8000 6000 4000 2000 0 0

2175

4350

6525

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Coordinate, µm Fig. 8. Vickers microhardness in a 6-layer FGM sample. The sintering temperature was 1000 ◦ C in the layer of composition 20% Al2 O3 + 80% Ni, 1250 ◦ C in the layer of composition 80% Al2 O3 + 20% Ni, and 1350 ◦ C in the pure Al2 O3 layer.

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and the changes in their microstructure morphology – in particular, associated with the significant growth of nickel particles. 4. Conclusion Multilayer FGM samples composed of Ni and Al2 O3 have been fabricated by powder stacking and consolidation by millimeter-wave sintering. A new method for producing the microwave-sustained temperature gradient has been developed to provide optimal temperature profile for the sintering of metalceramic FGM. The idea of the method is based on tailoring the thermal conductivity of the components adjacent to the sample, such as pushrods, similar to a technique previously used in SPS and FAST. However, the proposed method differs advantageously due to the separation of the volumetric selective heating and surface heat loss, inherent in the microwave heating. For the sintering of 6-layer metal-ceramic FGM samples, graded compositionally from pure alumina to 20% Al2 O3 + 80% Ni, the temperature was 900 ◦ C in the nickel-rich layer and 1390 ◦ C at the ceramic end. For the sintering of metal-ceramic-metal FGM samples representing an electrical insulator with compositionally graded transitions from metal to metal parts through a ceramic interlayer, a -shaped temperature distribution has been created with a temperature of 1200–1300 ◦ C at the metal ends and 1550 ◦ C in the ceramic. According to an SEM microstructure study, the fabricated samples do not have cracks and delaminations. The developed method can be used not only for the sintering of metal-ceramic FGM but also for other compositions of dissimilar materials with different sintering temperatures. Role of the funding source The metal-ceramic FGM research described herein was supported in part by the Ministry of education and science of the Russian Federation under the federal program “Scientific and scientific-pedagogical personnel of the innovative Russia”, grant contract No. 715. The continuing development of the gyrotron system for high-temperature processing of materials was partially supported by Russian Foundation for Basic Research, grants Nos. 11-08-00820 and 13-03-12075.

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