4.2. Effect of Electron-Beam Treatment on the Surface of the Al2O3 Ceramic
In its original, unirradiated state, the Al
2O
3 substrate has a white color, as is characteristic of alumina ceramics. No grain structure appears on the surface of polished samples. After processing, the surface has a greyish tint. The change in color may testify to a broken stoichiometry of the composition in the surface layer inclined towards a shortage of oxygen. Depending on the surface temperature, the removal of oxygen from alumina ceramics is known to occur in a two-step manner: first through oxygen desorption from the surface, and then its release from the crystal lattice. The equation describing the dissociation reaction is [
48,
49]:
The conductivity of the ceramic samples was measured before and after electron-beam treatment. In measuring the conductivity of non-irradiated samples, the current through the measuring probes was recorded to the instrumentation accuracy (1 nA). No current was detected in samples after electron beam irradiation, which can be explained by the fact that the stoichiometric change in oxygen is insignificant due to the low processing temperature compared to the melting point. Indeed, according to [
50,
51], at 2350 °C, the density of free aluminum atoms in 100%-oxide alumina reaches 2.5%, however, a decrease in temperature to 2200 °C leads to a decrease in the oxygen content by an order of magnitude to 0.3%. With further decreases in temperature, one should expect a lower percentage of oxygen content in the surface layer.
Figure 5 shows SEM images of the surface after electron-beam irradiation of the aluminum oxide ceramic at different isothermal holding temperatures.
One can conclude from these images that electron-beam irradiation at a temperature of 1300 °C does not result in a significant change in surface structure. However, when the temperature rises to 1500 °C and above, melting occurs followed by re-crystallization of the ceramic surface layer and formation of micro blocks 50–100 µm in size, indicating texturization of the ceramic microstructure. Along with the micro-block boundaries, there are observed pores with an average size of 2–5 µm. At a surface temperature of 1700 °C, the micro-blocks form into regular cubic grains of size 100–150 µm, which is indicative of the formation of a new phase—the Al
2O
3 γ-phase (
Figure 5d). The presence of pores in the surface layer is connected with the process of their creation and growth in a thin layer of the melt due to the presence of gas-forming elements, which can evaporate (mainly of oxygen, which is a ceramic constituent, and which is released during oxide dissociation). The presence of a melt layer on the surface of irradiated ceramics at a surface temperature below the melting point (2072 °C) is possible due to local melting of a thin layer under the focused electron beam. Due to peculiarities of the electron beam scanning and positioning system, the movement of the beam, though rather fast (the scanning frequency is 100 Hz), is yet discrete. During the short time when the beam stops, the surface may heat to a higher temperature than the temperature of the whole surface measured by the pyrometer. The small average size of the pores formed after electron-beam irradiation is indicative of the short time of existence of the liquid phase in the surface layer and of rapid movement of the molten and solidified areas over the substrate surface. The formation of pores on the surface after re-crystallization is an undesirable effect, since pores are normally stress concentrators and facilitate cracking [
52]. In our case, no cracking was detected, and only grain boundaries were more prominent.
Analysis of the elemental composition of irradiated Al
2O
3 ceramic samples shows that electron-beam irradiation in a helium atmosphere does not result in noticeable change in the surface layer elemental composition.
Figure 6 shows typical profilograms of the relief of the surface areas of Al
2O
3 ceramic in its original state and after irradiation at a temperature of 1300 °C, 1400 °C, 1500 °C, and 1600 °C. The horizontal axis in
Figure 6 indicates the length in mm over which the surface roughness was measured and the vertical axis indicates the roughness parameter
Ra which is an arithmetic mean of absolute values of profile deviations within the base length. Thus, the surface roughness parameter is
Ra0 = 0.0258 µm for the original sample,
Ra1300 = 0.150 µm for the sample processed at 1300 °C, and
Ra1500 = 2.93 µm for the sample processed at 1500 °C.
As can be seen, with an increase in surface temperature, the roughness increases by several factors. However, when the temperature reaches 1600 °C, the irregularities decrease and the substrate surface levels out. With further increases in temperature, the roughness parameter continues to grow and reaches a maximum, as is confirmed by the SEM images,
Figure 5d. The increase in roughness is primarily connected with the fact that the electron beam energy introduced into the sample is spent on melting the surface layer, which upon cessation of irradiation undergoes rapid re-crystallization, leading to grain growth with the formations of surface protrusions that negatively affect surface roughness (
Figure 6b). A further increase in processing temperature leads to significant growth of aluminum oxide grains in the form of elongated rectangles and cubes.
Change in the surface structure brings about change in the ceramic strength properties.
Table 3 shows the results of measurements of microhardness by the Vickers method, which is based on denting a tetrahedral pyramid with an angle of 136° at the apex between opposite faces under a load; here the load was 5 kg. The data are given in the HV5 scale.
For the sample processed at a temperature of 1700 °C, the hardness was not measured because it was not possible to find a flat area on the surface suitable for measurement. Omitting this sample, one can see that the hardness tends to decrease with increasing isothermal holding temperature.
4.3. Effect of Electron-Beam Treatment on the Surface of the AlN Ceramic
As already mentioned, AlN, being a highly thermally conducting and non-toxic ceramic material, is in great demand by the electronics industry. As for the majority of ceramic materials, the purity of the original powder, the sintering mode, and the additives used play decisive roles in fabricating ceramics with the required properties after sintering. Also, the thermal conductivity properties of aluminum nitride depend on the conditions under which the sintered material is processed [
53].
Figure 7 shows SEM images of the surfaces of AlN substrates processed at different temperatures. The white inclusions in
Figure 7a–c are particles of yttrium aluminate, which is formed during the sintering of aluminum nitride ceramic. The covalent nature of the Al–N bond requires high sintering temperatures. Due to the presence of oxygen impurity in the original powder, oxygen penetrates into the nitride aluminum lattice during sintering and creates aluminum vacancies, which results in the scattering of phonons, and therefore in a decrease in thermal conductivity of the material. When an yttrium oxide additive is used, it interacts with Al
2O
3 particles located on the AlN surface; a liquid phase is formed, which intensifies sintering at lower temperature [
54].
After treatment at a temperature of 1300–1400 °C, changes in surface morphology are barely discernible; however, this temperature is not high enough for the formation of visible grain boundaries compared to the previously shown aluminum oxide. Further increase in processing temperature to 1500 °C is accompanied by the formation of a distinct grain structure on the irradiated surface, as well as shallow pores up to 10 µm in size evenly scattered over the AlN surface. The average grain size of aluminum oxide is 10 µm, and the bright inclusions corresponding to yttrium no longer appear in the surface structure. Instead, thin veins containing predominantly oxygen and yttrium are formed on the grain boundaries. After the temperature rises to 1600 °C (
Figure 7e), further transformation of the surface structure is observed, namely the absorption of small grains by larger ones, as evidenced by an increase in grain size as well by a decrease in the number of grains. The pore size increases to 15–20 µm. After treatment at a temperature of 1700 °C (
Figure 7f), significant irregularities appear on the ceramic surface and porosity continues to increase. Additionally, a well-formed aluminum–yttrium structure is seen on the surface, as in
Figure 8.
The elemental composition measured at different areas in
Figure 8 is summarized in
Table 4. The areas (I, II, III, and IV) are enumerated according to
Figure 8.
A histogram of the elemental distribution is shown in
Figure 9. As seen, regions I and II are characterized by low yttrium content. The content is minimum in region II, within the measurement accuracy. In region I, the yttrium content increases, and one can assume the formation of a third phase in the intergranular region. Region III has the highest yttrium content. The most probable mechanism responsible for the formation of high yttrium content is the high temperature in the processing region, which leads to the formation of an aluminum–yttrium structure through the reaction of yttrium with oxygen and nitrogen, yielding yttrium oxide Y
2O
3 and yttrium nitride YN, which, in turn interact with aluminum oxide Al
2O
3 to form this surface structure.
X-ray diffraction patterns of original and irradiated sample surfaces treated at temperatures of 1600 °C and 1700 °C are shown in
Figure 10.
X-ray diffraction analysis indicates that the AlN + Y
2O
3 samples consist predominantly of the AlN (P63mc (186)) phase. As seen from
Figure 10, the original AlN + Y
2O
3 sample also includes phases of Al
2O
3 (R-3ch (167)), Y
2O
3 (F2/m-3 (202)), and Y
4Al
2O
9 (P121/c1 (14)). At a temperature of 1600 °C, the YAlO
3 (P63/mmc (194)) phase is formed, a result of interaction of Y
4Al
2O
9 and Al
2O
3. As the temperature increases further to 1700 °C, it is likely to form the Y
3Al
5O
12 (Ia-3d (230)) phase of the yttrium–aluminum garnet as a result of reaction between YAlO
3 and Al
2O
3 [
55,
56]. Shown below are reactions for powders of the Al
2O
3–Y
2O
3 system synthesized using the nanospray drying method [
57]:
Thus, as the processing temperature increases, yttrium oxide first embeds in the grain boundaries as they are formed, and then rises to the surface to form an aluminum–yttrium structure, which may affect the strength and thermal properties.
Figure 11 shows the dependence of hardness, measured by the Vickers method, on processing temperature for aluminum nitride.
One can see from
Figure 11 that the sample microhardness decreases as the processing temperature increases to 1500 °C. This decrease can be associated with the formation of a region with a broken structure in the surface layer and increased porosity of the material, which is indirectly confirmed by SEM images of the surface shown in
Figure 7d–f. With further increase in temperature, along with an increase in the surface hardness due to the formation of the aluminum-yttrium structure, the hardness is improved due to the healing of small cracks and pores. However, the hardness increases but does not achieve the value of the original sample. A decrease in the hardness of high-density ceramics after irradiation by electrons and ions is a known effect [
58], which is associated with cracking of the surface layer after processing. An increase in microhardness on account of surface layer melting has been observed previously only for porous ceramics [
59], which were not a study subject of this work.
As with the aluminum oxide ceramic, roughness measurements for the aluminum nitride ceramic (
Table 5) show an increasing temperature dependence. As seen from
Table 5, this increase is not linear, but occurs in a stepwise manner.
At relatively low temperature of up to 1400 °C, the roughness varies within the measurement accuracy. However, at a temperature of 1500 °C, the roughness increases by more than a factor of four (the parameter
Ra increases from 0.68 to 2.66 µm). This roughness dependence correlates with the microscopic data (
Figure 7).
4.4. Thermal Conductivity Measurement of the AlN Ceramic after Electron-Beam Processing
The high thermal conductivity of the aluminum nitride ceramic, along with its electrical insulating and strength properties in comparison with other types of ceramics and its non-toxicity, makes this material important to the electronics industry. The theoretical value of thermal conductivity of AlN, according to [
60], can reach 320 W/(m·K); however, in practice, this value is limited for commercially available specimens by a great number of defects due to oxygen dissolved in the AlN lattice. According to [
61,
62,
63], the mechanism of defect formation can be as follows: oxygen dissolved in the AlN (O
N) lattice induces the formation of aluminum atom vacancies (V
Al), and due to the considerable mass mismatch between these vacancies and the surrounding lattice, a defect with very large phonon scattering cross-section is created. The presence of oxygen in AlN is connected with its content in the original powder used for sintering. This content may amount to 1%.
As noted above, since AlN is a material with a covalent bond, it has a low diffusion coefficient, and this requires high sintering temperatures (>1600 °C). Moreover, in the presence of protective oxide on the surface and oxygen in the crystal lattice, it is impossible to reach high thermal conductivity without using sintering additives. The role of additives is to form a liquid phase by interacting with the oxide layer (Al
2O
3) on the surface of compacted particles and to act as a “sink” to remove oxygen from the lattice [
64]. Other methods of achieving high thermal conductivity include sintering at temperatures above 1800 °C for several hours, irradiation by high-energy neutrons [
65], and heat treatment after sintering in various reducing atmospheres at high temperature [
66,
67,
68,
69]. In the work described here, for heat treatment we used electron-beam irradiation in residual atmosphere at a chamber pressure of 10 Pa.
Figure 12 shows the thermal conductivity of AlN ceramic after irradiation at different temperatures.
Increase in the isothermal holding temperature leads to an increase in thermal conductivity, with the largest change observed for samples sintered at 1500 °C and above, at which temperature an aluminum–yttrium structure is formed on the surface. Despite increase in surface roughness and decrease in strength, thermal conductivity increases with the formation of pores. This effect can be related to an increase in the size of ceramic grains with simultaneous decrease in their number. Decrease in phonon scattering on grain boundaries is apparently a factor that facilitates the increase in thermal conductivity.
4.5. Effect of Electron-Beam Processing on Ceramic Wettability
The contact wetting angle determines the degree of interaction between a liquid and a solid surface and characterizes its moisture resistance, anti-corrosion, anti-bacterial, and other properties. Depending on the purpose and conditions of applications, it may be advantageous to either increase or decrease the contact angle [
70,
71,
72]. The contact angle depends on interfacial surface energies of the liquid, solid, and gaseous states, as well as on the surface roughness and chemical composition. There are several methods of changing the contact angle, such as physical or chemical modification of the surface, application of coatings, creation of structures with various geometry and size, and others. [
73,
74]. In the present work we have explored the effect of electron-beam irradiation on surface properties and contact angle.
As noted above, the roughness increases after electron-beam processing, which in general should result in a decrease in the contact angle compared to the smooth surface, provided the same interfacial surface energies. This effect arises due to increase in contact area with the liquid and more possibilities for the formation of molecular bonds. However, in certain cases, the roughness can lead to an increase in contact angle due to the formation of air cavities between surface irregularities. This phenomenon is called composite wetting and can be used to create super hydrophobic surfaces with contact angle over 150° [
75].
Figure 13 and
Figure 14 show photographs of the original and processed substrate surfaces of aluminum oxide and aluminum nitride taken after processing at different temperatures. One can see that the droplets on the irradiated surface for both ceramics are larger than on the non-irradiated surface.
Table 6 shows the results of measurements of contact wetting angle for the substrates under study.
The data in
Table 6 are charted in
Figure 15. As can be seen, with increase in irradiation temperature, the contact angle varies differently for oxide and nitride ceramics. Thus, for Al
2O
3 samples treated at 1400 ℃ and above the surface wettability decreases. The ceramic surface exhibits hydrophobic properties, which is manifested by an increase in contact angle from 63° for the untreated surface to 90°–100° for the irradiated surface. The wettability dependence for AlN ceramic is nonmonotonic. At low processing temperature, the wettability decreases noticeably, then at a temperature of 1400 °C the contact angle abruptly decreases, and the surface acquires more hydrophobic properties: the contact angle decreases by more than a factor of two. With further increase in processing temperature, the contact angle increases, exceeding 100°.
The contact angle is determined by the interaction between molecules of the liquid and the solid surface. A lower contact angle indicates that the solid body has lower surface energy; the energy increases with increasing processing temperature. According to [
76], the observed increase in hydrophilicity can be related to increase in surface roughness, as in the case of irradiation by a femtosecond laser [
77]. Note also another possible mechanism is related to change in the chemical composition of the irradiated surface. After electron-beam processing the content of Al on the surface of AlN substrate increases (according to
Table 4 and
Figure 10). A thin layer of aluminum oxide is formed due to oxidation. However, a large number of Al
3+ and O
2− ions produced by electron-beam irradiation do not combine immediately, resulting in the formation of a Lewis acid and base pair, which expands the polar areas on the surface and improves the surface energy and hydrophilicity [
78,
79,
80].