Formation of Layered Structure in Ceramics Based on Alumina Nanopowder Under Effect of Induction Heating

Abstract

The effect of induction heating on alumina ceramics and alumina ceramic composites based on α-Al₂O₃ nanopowders (additives: SiC, Si₃N₄, SiO₂, ZrO₂) has been examined. Various factors such as the structure, grain size, distribution of elements, hardness, fracture toughness, and wear rate of hot-pressed ceramic materials were assessed. Despite achieving improved densification of alumina ceramics at a higher temperature of 1720 °C, there is a consistent trend toward a decline in hardness and fracture toughness. Heating at lower temperatures of 1300–1500 °C results in the development of a strengthened surface layer with a fine-grained structure enriched with carbon. Therefore, the wear rate behavior of such ceramics differs from the behavior of samples made at higher temperatures of 1600–1720 °C. This fact indicates the presence of a non-thermal microwave effect of induction heating. The incorporation of additives to alumina leads to the formation of novel structures with altered crack propagation patterns. The optimal ceramic composite, containing 5 wt. % SiC, displayed superior hardness and the lowest wear rate when compared to pure alumina ceramics. Across all investigated composites, a short dwell time at 1700 °C results in an enhancement of the mechanical properties.

1. Introduction

Recently, the requirements for lightweight materials having high strength and high hardness have attracted much interest in the development of ceramic manufacturing processes, since ceramics are an attractive candidate for high-performance application, especially at high temperatures, mainly due to their unique combination of mechanical, chemical, and physical properties [1,2,3,4]. Alumina (Al₂O₃) ceramic possesses excellent merits, such as good wear and corrosion resistance, high hardness, and a low price, which make it one of the most intensively studied structural ceramics [5]. However, the inherent brittleness of ceramics limits their widespread application. Investigations have been carried out to determine ways of tailoring ceramic materials in order that one or more toughening mechanisms are activated in service. Microstructural manipulations, as well as composite formulations involving different phases, have been used with ceramic matrices [6]. Many works have been carried out to improve their mechanical properties thanks to nanotechnology, which makes it possible to overcome many shortcomings [4,7].

It is known that sintering pure Al₂O₃ nanoceramics with high relative density and nanograins is quite a complicated task [8]. To prepare the Al₂O₃ nanoceramics, many methods, such as hot-pressing, spark plasma sintering, pulse electric current sintering, etc., have been used [4,8]. Despite numerous technical difficulties, researchers have managed to develop techniques for the sintering of nanocrystalline ceramics which are both fully dense (or near fully dense) and still retain a nanograined structure. For example, hot-pressing techniques, when carried out under the best-chosen conditions, have all been shown to be capable of producing nanocrystalline ceramics. Microwave sintering has also produced near-nanocrystalline ceramics [9].

The composition of ceramic materials can greatly influence their mechanical properties [5]. The addition of particles of a second phase has been, by now, one of the most successful ways of improving the mechanical properties of polycrystalline alumina-based ceramic materials. An urgent demand has been imposed on the development of a new generation of ultra-high-temperature structural composite materials that can remain stable in aggressive environments at elevated temperatures [10]. The amount of additive introduced to the ceramics can determine such properties as fracture toughness, ambient- and high-temperature strengths, creep resistance, and oxidation resistance [11].

Engineering ceramics based on alumina (Al₂O₃) offer great potential since remarkably higher thermal and mechanical loading is possible [12]. However, the performance of these monolithic ceramics is still insufficient, since the principal parameters of strength, hardness, fracture toughness, and wear resistance are frequently uncorrelated with each other, and very often researchers working in the field of ceramics have failed to match the level of its mechanical properties. One of the ways for the improvement of performance is the laser-assisted surface modification process for application in dry friction systems [12]. Therefore, the formation of gradient structures and a modified layer on the surface of ceramics in the process of its manufacture can significantly simplify the method to obtain ceramics with predetermined or required properties.

Over the years, there has been growing evidence of unusual effects caused by the use of microwave energy. During microwave heating, not only an increase in sintering and grain growth, a decrease in the activation energy of diffusion, and an increase in ionic interdiffusion, but also the formation of a gradient structure in the sintered material is possible. The reason for its formation may be related to the difference between annealing and microwaving [13]. Unexpected magnetism in dielectric oxide nanostructured materials even at room temperature can also be the reason for the formation of a gradient structure [14,15]. The unique magnetism of nanomaterials is usually associated with various structural defects, such as cation vacancies, oxygen vacancies, and structural inhomogeneity, which is confirmed by numerous studies of highly defective magnetic oxides and nitrides [15]. It has also been shown that non-equilibrium processes occur during microwavingat temperatures > 1400 °C, which can lead to the appearance of unstable phases of beta- and gamma-aluminum oxide and even an amorphous phase on the surface from stable α-Al₂O₃, depending on time and temperature [13].

Therefore, the main goal of this research is to study the effect of induction heating and the influence of the sintering conditions, composition, and structure formation on the surface of ceramic materials and their properties.

2. Materials and Methods

Alumina powder (α-Al₂O₃) of high purity (99.9%) (, AKP-50, “Sumitomo Chemical”, Tokio, Japan), with a mean particle size of 0.18 µm, was used as the initial powder. A total of 5 wt. % of silicon carbide (SiC) (a-Sic, “Lonza group”, Ponte Vedra, FL, USA), silicon nitride (Si₃N₄) (a-Si₃N_4, “VIMATERIAL”, Laatzen, Germany), silica (SiO₂) (silicon dioxide, “Merck”, Darmstadt, Germany), and zirconia (ZrO₂) (R.Z.M. Premium B, “Crystal”, Henderson, Australia) powders of mean particle size < 1 µm were incorporated into the alumina powder.

Batches of powders based on alumina and additives were mixed in a barrel mixer in the methanol medium. The disks, 25 mm in diameter and 6 mm thick, were prepared by hot-pressing in a one-cavity graphite die under a pressure of 20 MPa applied along one axis using a high-frequency generator of 0.5 MHz at different temperatures (1300–1720 °C) and dwell times (2–211 min). After sintering, the apparent density of all dense samples was measured using a standard water immersion technique. Then, samples were cleaned, ground, and polished with a 1 µm diamond using the Automatic Grinder and Polisher machine (“Verder company”, Hope Valley, UK). Thermal etching of the surfaces of the samples based on alumina was carried out in air at 1470 °C for 1 h. The microstructures were examined using scanning electron microscopy JSM-6490LV (“Jeol”, Tokio, Japan). The mean grain size was determined by the standard linear intercept method using scanning electron micrographs of thermally etched materials. Hardness and fracture toughness (K_1c) were measured using the standard micro-indentation technique under a load of 98 N. The K_1c was calculated using the equation of Niihara [16].

Wet erosion of alumina discs was carried out in a modified high-torque attritor mill using 0.5 to 1 mm dimension crushed and fused alumina aggregate in water using a special technique [17,18]. A wear rate (R), measured in m · s⁻¹, was calculated after wearing for 2 h (t₁) and 4 h (t₂).

3. Results and Discussion

3.1. Hot-Pressed Alumina Ceramics

Hot-pressing of high-purity α-Al₂O₃ powder results in dense alumina ceramics (

Table 1. The densification conditions and properties of hot-pressed alumina ceramics.

Sample Number	Hot-Pressing Conditions		Total Porosity, %	Hardness HV, GPa	Fracture Toughness K1c, MPa·m1/2	Wear Rate, nm·s−1/2, W		W4h/W2h	Mean Grain Size, µm
	Temperature, °C	Time, min				t = 2 h	t = 4 h
1	1300	120	2.6	21.72	7.2	7.4	9.6	1.30	6.8
2	1350	211	0.7	21.46	6.36	8.7	16.5	1.90	3.5
3	1500	32	1.7	20.59	6.22	25.7	30.4	1.18	2.5
4	1550	60	1.7	17.21	6.00				6.6
5	1600	8	1.8	20.38	7.04	57.0	45.8	0.80	5
6	1700	30	0.9	17.57	5.83	68.0	56.3	0.83	9.2
7	1710	15	0.7	19.00	6.54	52.0	40.3	0.78	17.9
8	1720	15	1.2	15.95	5.96	62.9	39.6	0.63	21.9

). An SEM study of etched surfaces of ceramics demonstrated their features [19]. The mean grain size of aluminas logically increases with an increase in hot-pressing temperature, but its increase to 6.8 μm when the temperature of hot-pressing changes from 1300 °C to 1600 °C does not significantly affect the properties. However, at a hot-pressing temperature above 1600 °C, the average grain size of alumina ceramics increases to 9.2 μm, and all properties decrease (Table 1).

The SEM image of the fracture surface of ceramics produced at a temperature of 1550 °C (Table 1, sample 4) demonstrated predominantly intergranular fractures, since crystallite facets are clearly visible (Figure 1a). This material with a grain size of about 6.6 µm (Figure 1a) has fine intracrystalline closed porosity and coarse interparticle porosity inside. Increasing the hot-pressing temperature up to 1720 °C (Table 1, sample 8) results in an increase in grain size up to 21.9 µm in full-density aluminas with fine intracrystalline closed porosity inside the samples (Figure 1b). The presence of chipped areas on the fracture surface of such a material indicates transgranular failure and transgranular crack propagation (Figure 1b).

There is also a clear trend towards a decrease in the porosity for all hot-pressed samples with an increase in the temperature, despite a significant decrease in the dwell time (Figure 2a). Only at a low temperature of 1350 °C does doubling the dwell time lead to the lowest porosity of 0.7% and a grain size of 3.5 µm (Table 1, Sample 2). Despite the better densification of ceramics due to the reduction in porosity, a steady tendency of the deterioration of the hardness and fracture toughness with an increase in the hot-pressing temperature up to 1720 °C was observed (Figure 2b). In addition, it turned out that hardness is more sensitive to temperature growth than fracture toughness (Figure 2b). Consequently, it is possible to say that temperature is the most important parameter of hot-pressing. The dwell time and porosity show a negligible effect on hardness and fracture toughness under present conditions.

It is interesting to note that the wear rate of the samples obtained at low temperatures of 1300–1500 °C (Figure 3a) increases after 4 h of testing in contrast to the samples produced at higher temperatures (Figure 3b). The highest hot-pressing temperature of 1720 °C leads to a drop in the wear rate of the samples by approximately half after 4 h of testing (Table 1). Scanning electron microscope (SEM) analysis of the surface structure of the best alumina sample (Table 1, sample 1) has demonstrated the presence of nanosized grains of 0.5 μm (Figure 4a), enriched in carbon (Figure 5a). The grains of the bulk counterpart had a size of about 6.8 µm (Figure 4b) and stored less carbon (Figure 5b). This study confirms the positive effect of the fine-grained structure on the strengthening of the surface of alumina ceramics formed at low temperatures.

It is known that alumina is typically considered to be diamagnetic. When aluminum and oxygen bond to form Al₂O₃, the aluminum atoms lose three electrons to achieve a stable configuration, becoming positively charged ions (Al³⁺), and the oxygen atoms gain two electrons to form negatively charged ions (O²⁻). There are no unpaired electrons in this configuration. However, nanostructured powders of aluminum oxide (Al₂O₃) can exhibit different properties compared to their bulk counterparts due to their unique structural features and increased surface area-to-volume ratio. One of these properties can be the paramagnetic behavior, which arises due to several factors:

Defects and Surface Effects: Nanostructuring introduces a large number of defects, such as vacancies, dislocations, and grain boundaries, into the material. These defects can create localized electronic states within the band structure of Al₂O₃, which may lead to the presence of unpaired electrons and consequently paramagnetic behavior. Structural heterogeneity and defects in the structure, such as cationic and oxygen vacancies, can also be the cause for their phase transition to the paramagnetic state.

Surface States: The high surface area-to-volume ratio in nanostructured materials results in a larger fraction of atoms located at the surface. Surface atoms can have different electronic configurations compared to those in the bulk, leading to the presence of unpaired electrons and paramagnetic behavior.

Size Effects: At the nanoscale, quantum confinement effects become significant, altering the electronic and magnetic properties of the material. As the size of Al₂O₃ nanoparticles decreases, the electronic band structure may change, leading to the emergence of paramagnetic behavior.

Overall, the paramagnetic behavior of nanostructured Al₂O₃ is a result of a combination of factors, including defects, size effects, and surface states, which lead to the presence of unpaired electrons and magnetic moments within the material. Therefore, it can be concluded that the strengthened surface layer of alumina samples can be the result of the non-thermal effects of microwaving [20]. However, exposure at a higher temperature results in several changes: an enlargement of grain size, the calcination of low-melting grain-boundary phases, and the penetration of carbon through the surface. These alterations can disrupt the ordering of dipoles and degrade the surface properties of the alumina samples.

3.2. Alumina Ceramic Composites

Composite ceramics fabricated at a high temperature of 1700 °C with the addition of 5 wt. % SiC, Si₃N₄, SiO₂, and ZrO₂ demonstrated the formation of new structures (

Table 2. The densification conditions and properties of hot-pressed composites based on alumina.

Additives, 5 wt.%	Hot-Pressing Conditions		Total Porosity, %	Hardness HV, GPa	Fracture Toughness K1c MPa·m1/2	Wear Rate, nm·s−1/2		W4h/W2h	Mean Grain Size, µm
	Temp., °C	Time, min				t = 2 h	t = 4 h
SiC	1700	45	1.1	21.01	6.51	8.7	6.4	0.73	1.3
SiC	1700	10	2.3	22.16	6.04	8.1	4.0	0.49	1.7
Si3N4	1700	20	2.1	21.72	6.58	24.4	14.0	0.57	1.2
Si3N4	1700	2	1.5	20.80	7.64	20.5	9.9	0.48	1.0
SiO2	1700	15	2.1	19.90	5.81				1.8
SiO2	1700	15	2.5	20.80	6.01	21.1	11.0	0.52	2.3
SiO2	1700	5	3.3	19.07	6.18	20.3	9.6	0.47	0.9
ZrO2	1700	15	0.7	21.46	6.37	67.3	49.1	0.73	5.7
ZrO2	1700	2	1.0	19.37	6.17	34.0	32.3	0.95	6.9

, Figure 6). The fracture surface of the ceramic composite produced with the addition of 5 wt. % SiC has an overall smooth appearance and shows a transgranular crack propagation in the structure with a grain size of about 1.3–1.7 μm (Table 2, Figure 6a). The incorporation of 5 wt. % Si₃N₄ to the alumina contributes to the formation of a rougher, nonuniform structure on the surface of the fracture. A transgranular crack propagation takes place in a dense structure with a grain size of 1.0–1.2 μm, while some particularly elongated grains of 3 μm were observed due to the intergranular fracture (Table 2, Figure 6b). The transgranular fracture also occurs in the fully dense structure of the composite ceramics with grains of 0.9–2.3 μm and the addition of 5 wt. % SiO₂ (Table 2, Figure 6c). The propagation of an intergranular crack in the structure with a grain size of about 5.7–6.9 μm was typical only for the fracture surface of a ceramic composite obtained by the addition of 5 wt. % ZrO₂ (Table 2, Figure 6d).

An alumina composite with 5 wt. % SiC of the highest hardness was produced at a temperature of 1700 °C and a dwell time of 10 min (Table 2). The main hardness of alumina composites with the addition of 5 wt. % SiC and Si₃N₄ is slightly higher than for composites with the addition of SiO₂ and ZrO₂. The worst main hardness was displayed by the alumina composite with an addition of 5 wt. % SiO₂ (Table 2).

The highest and lowest values of fracture toughness of the considered ceramic composites, produced with the addition of 5 wt. % Si₃N₄ and SiO₂, respectively, are explained by the difference in their structures (Figure 6b,c). For the first structure, there are such hardening mechanisms as crack deflection, its branching, and its arrests due to the presence of elongated Si₃N₄ grains (Figure 6b), while for the second structure, these mechanisms do not work due to purely transgranular fractures (Figure 6c). Ceramic composites obtained by the addition of 5 wt. % ZrO₂ and SiC displayed the same fracture toughness, despite the significant difference in fracture surfaces and crack propagation in the structure (Table 2, Figure 6a,d).

Based on the data on hardness, fracture toughness, porosity, and mean grain size of the alumina composites, it can be concluded that, in general, the addition of 5 wt. % SiC, Si₃N_4, SiO₂, and ZrO₂ improves the basic properties of alumina ceramics by about 2–6% (Table 1 and Table 2). Only the wet erosive wear rate turned out to be the most sensitive to these additives. The formation of grain boundaries in the new compositions and structures in the alumina ceramics by the addition of 5 wt. % SiC, Si₃N₄, SiO₂, and ZrO₂ has sharply affected the wear rate (Table 1 and Table 2).

The worn surfaces of ceramic composites made with the addition of various additives vary [19]. The formation of new structures and, accordingly, their grain boundaries determines the fracture surface and the wear pattern of the ceramic’s surface. The highest value of the wear rate of ceramic composites prepared with the addition of 5 wt. % ZrO₂ is explained by a change in the mechanism of fracture that occurred due to changes in the composition of the grain boundaries (Table 2, Figure 6d). Incorporation of ZrO₂ changed the transgranular fracture of pure alumina ceramics produced at high temperatures to intergranular. Since the alumina grain boundaries were weakened by the new composition of Al-O-Zr, grain pullout and chipping became the dominant wear mechanisms. This composite material demonstrates the highest wear rate, close to the wear rate of pure alumina produced at higher temperatures (Table 1 and Table 2).

An alumina composite with 5 wt. % SiC, which has the lowest wear rate and an overall smooth appearance of a worn surface, was produced at a temperature of 1700 °C and a dwell time of 10 min (Table 2) [19]. Incorporation of SiC inhibited the growth of the alumina grain sizes and preserved the transgranular fracture of pure alumina ceramics prepared at high temperatures. The new Al-O-Si-C composition of the grain boundaries also promoted their strengthening. Therefore, the wear rate of the ceramic composite decreases as a result of the almost simultaneous wear of both the alumina grains and the grain boundary phase (Table 2).

Incorporation of Si₃N₄ into alumina results in a very rough, worn surface with areas where some separate, elongated grains of Si₃N₄ pulled out from the surface due to an intergranular fracture [19]. In general, the presence of elongated columnar grains in the matrix should provide additional strengthening of the alumina composite with 5 wt. % Si₃N₄. However, such microstructures did not increase the wear resistance of these nanocomposites compared to the wear resistance of an alumina composite with 5 wt. % SiC (Table 2). Consequently, the new Al-O-Si-N composition of the grain boundaries is not as strong as Al-O-Si-C.

A very smooth response to wear has been demonstrated by the nanostructured alumina composite with 5 wt% SiO₂ [19]. Strengthening of the grain boundaries by the Al-O-Si composition results in no fractures having occurred. However, in this case the wear rate was also higher than for the alumina nanocomposites with 5 wt.% SiC (Table 2). it looks like the main reason lies in the strength of the grain boundaries of the Al-O-Si-C composition.

Although all ceramic composites were obtained at the same temperature (1700 °C), it should be noted that those prepared with a short dwell time (2–10 min) exhibited a lower wear rate after 2 and 4 h of testing (Table 2). Moreover, hot-pressing at a dwell time of 15–45 min results in a smaller difference in the wear rate (W_4h/W_2h = 0.52–0.73) after 2 and 4 h of testing than at a dwell time of 2–10 min (W_4h/W_2h = 0.47–0.49) for composites made with the addition of 5 wt. % SiC, Si₃N₄ and SiO₂.

The formation of a strengthening surface layer at a short dwell time indirectly points to the nonthermal effect of the microwave electromagnetic field [20,21,22]. It means that materials based on Al₂O₃ nanopowders enriched by Si₃N₄, SiC, and SiO₂ display paramagnetic properties, which can be attributed to several factors, including the electronic structure, the interaction of additives with the host material (Al₂O₃), the formation of new grain boundaries with new properties and defects, and also the specific conditions of the ceramic material synthesis.

Additive Properties: Silicon nitride (Si₃N₄), silicon carbide (SiC), and silicon dioxide (SiO₂) promote a new grain boundary formation with new properties that can introduce defects and result in unpaired electrons within the host Al₂O₃ lattice and grain boundary phases, which leads to paramagnetic behavior. These additives may also contain their own defects that contribute to paramagnetism, such as crystal lattice defects.

Interaction with the Host Material: The specific interactions between the additives and the host Al₂O₃ material and the formation of new grain boundaries can influence the manifestation of paramagnetic properties. The additives may facilitate the formation of defect sites on the surface of alumina grains due to new grain boundary formation and within the Al₂O₃ lattice, thereby promoting paramagnetism. The nature and strength of these interactions can vary depending on the type and concentration of the additives.

Synthesis Conditions: The conditions under which the materials are synthesized can also play a crucial role in determining their magnetic properties. Factors such as temperature, pressure, and the duration of synthesis can affect the formation of defects in grain boundaries within the materials, which in turn can influence their magnetic behavior.

Overall, the presence of specific additives like Si₃N₄, SiC, and SiO₂, along with their interactions with the host Al₂O₃ material, which contribute to new grain boundary formation, may promote the formation of defect sites and unpaired electrons, leading to paramagnetic properties.

However, for composites made with the addition of 5 wt. % ZrO₂, the interaction with the microwave electromagnetic field is of a different nature. The difference in wear rates for a 2 min dwell time after 2 and 4 h of testing was 0.95, whereas it was 0.75 for a 15 min dwell time.

The difference in the nature of interactions between Al₂O₃ and ZrO₂ compared to Al₂O₃ and Si₃N₄, SiC, or SiO₂ can be attributed to several factors:

Chemical Composition: Zirconium oxide (ZrO₂) and silicon nitride (Si₃N₄), silicon carbide (SiC), or silicon dioxide (SiO₂) have different chemical compositions. ZrO₂ is an oxide of zirconium, while Si₃N₄, SiC, and SiO₂ are compounds of silicon and nitrogen or carbon. These differences in chemical composition result in different types of chemical bonds, interactions with the Al₂O₃ lattice, and the formation of grain boundaries of another composition.

Crystal Structure: Zirconium oxide (ZrO₂) and aluminum oxide (Al₂O₃) have different crystal structures. ZrO₂ typically adopts a cubic or tetragonal crystal structure at high temperatures, while Al₂O₃ has a hexagonal close-packed (hcp) crystal structure. Silicon nitride (Si₃N₄) and silicon carbide (SiC) also have different crystal structures. These differences in crystal structure can affect the types and strengths of interactions between the materials.

Thermal Stability: Zirconium oxide (ZrO₂) and aluminum oxide (Al₂O₃) exhibit different thermal stability characteristics. ZrO₂ has a higher melting point and thermal stability compared to Al₂O₃. These differences in thermal stability can affect the formation and stability of defect sites within the lattice, influencing the nature of interactions with the host material.

Overall, the differences in chemical composition, crystal structure, and thermal stability between ZrO₂ and Al₂O₃ compared to Si₃N₄, SiC, or SiO₂ can result in variations in the nature of interactions with the host Al₂O₃ lattice, ultimately affecting the manifestation of paramagnetic behavior.

Prolonged dwell time at high temperature, causing an increase in grain size, calcination of low-melting grain-boundary phases, and enrichment with carbon, leads to the ordering of lattice structure, the disappearing of unpaired electrons, and changes in the properties of the surface of the material.

4. Conclusions

The density and mechanical properties of ceramic materials based on high-purity α-Al₂O₃ are found to be primarily determined by the temperature of hot-pressing. Despite the better densification of alumina ceramics when the temperature of hot-pressing is high, there is a steady tendency towards the deterioration of its hardness and fracture toughness. Heating at low temperatures leads to the formation of a strengthened layer of fine-grained structure enriched with carbon on the surface of alumina ceramics under the effect of induction heating. Therefore, the wear rate of these ceramics increases with increasing test time, in contrast to samples made at higher temperatures.

The incorporation of 5 wt. % SiC, Si₃N₄, SiO₂, and ZrO₂ into alumina ceramics improves the basic properties by about 2–6%. The formation of new structures with new grain boundaries that determine the fracture surface and the wear surface of composites results in a decrease in the wear rate by up to 50% compared to pure alumina. The best ceramic composite with the highest hardness and lowest wear rate is made by the addition of 5 wt. % SiC to alumina. Reducing the dwell time of hot-pressing leads to an improvement in the mechanical properties of ceramic composites made with the addition of 5 wt. % SiC, Si₃N₄, and SiO₂ and to an increase in the difference in the wear rates at different test intervals. Therefore, based on the above studies of the surface properties of ceramic composites, it can be concluded that there is a non-thermal effect of the microwave on the formation of the modified surface layer at the beginning of the process of hot-pressing. For the ceramic composite made with the addition of 5 wt. % ZrO₂, the difference in wear rates at different test intervals is not so large. It is determined by intergranular fractures and the diamagnetic properties of its grain-boundary phases.