INTRODUCTION

The manufacture of materials for electronics and materials for functional purposes and tools demands structures with gradient properties. The development of such structures makes it possible to effectively change the optical, mechanical, and electric properties of near-surface layers of different materials with respect to volume layers. One of the trends in the development of gradient structures is represented by radiation treatment techniques such as surface treatment with high-current pulsed beams of low-energy electrons [1–4], laser treatment [5, 6], and ion beam treatment [7–18]. Ion beams with the ion energy of 25–50 keV are widely used for the implantation of semiconductors [17–19]. Recently, there appeared new ion accelerators providing ion beams of different ions in a wide range of energies and power densities [20–24], which are used for the modification of metals and alloys [25–31]. Ion beams of different ions (both continuous and pulsed) with energies of several keV to hundreds of keV are used.

It is of interest to use the ion treatment for the modification of ceramic materials. Since they are characterized by increased strength, chemical resistance, and melting point, the ion modification can be used for the development of gradient ceramic structures, which is mostly impossible using the classic treatment techniques. Examples of the use of ion beams for modification near-surface layers of ceramic materials are given in [8, 14–16]. It is shown that the treatment with high-intensity ion beams leads to the change in the phase and chemical composition of near-surface layers and the development of a microstructure with a significantly smaller grain size than in the ceramic volume. Thus, ion beams can be used for the effective modification of near-surface properties of ceramic materials. The issue of the mechanisms of ion impact on the properties of materials is the subject of study of many researchers.

The issue of the mechanisms of ion impact on the properties of materials is widely explored. Apparently, the actualization of possible mechanisms is highly dependent on the type of ions, their energy, the treatment conditions, and the type of material. In [32–34], it is shown that the treatment of metals and alloys with argon ion beams with the energy of about 20–40 keV is accompanied by the modification of their mechanical properties, which is manifested by strengthening of treated tape metal materials much deeper than the depth of ion penetration during the treatment. Owing to the fact that, under such accelerating voltages and current densities, no noticeable surface erosion of treated material occurs, it is of interest from both the scientific and practical viewpoint to establish the regularities of impact of this type of ion treatment on ceramic materials widely used, e.g., in microelectronics.

In microelectronics, corundum ceramic is especially important, being predominantly used as a substrate material. Along with locating active elements on the ceramic surface, current conductive layers are formed, e.g., by thermal or ion-plasma spraying of metal targets in vacuum. Aluminum oxide ceramic has increased hardness, stable phase composition, and high dielectric characteristics. The development of methods for control of structure and properties of this type of ceramic by ion treatment can widen significantly the scope of this type of ceramic. It is important to know how the ion treatment affects its functional properties. In addition, since aluminum oxide ceramic is characterized by an increased hardness and the phase composition stability, this type of ceramic is of interest as a model material for the study of the processes of modification of ceramic and metal materials by ion treatment.

The aim of this work is to study the effect of the surface treatment with continuous ion beams with the average energy of 30 keV and the current density of 300 μA/cm2 on the near-surface properties of corundum ceramic.

METHOD OF EXPERIMENT

The object of study is aluminum oxide ceramic (corundum). Samples with the size of 1 cm2 and the thickness of 0.45–0.50 mm were cut out of standard polished ceramic substrates of 40 × 60 mm2. Samples were irradiated using an ILM-1 ion implanter equipped with an Pulsar-1M ion source based on a low-pressure glow discharge with a cold hollow cathode [35]. This source provides cylindrical ion beams with the cross-sectional area of about 100 cm2 with a constant ion current density in this region j of 50 to 500 μA/cm2 under accelerating voltages U of 5 to 40 keV. The irradiation was performed with argon ions with the energy of 30 keV and the ion current density j = 300 μA/cm2. Two irradiation modes with continuous beams with the fluences of 1016 cm–2 (mode A) and 1017 cm–2 (mode B) were implemented for 6 and 54 s, respectively. The polished surface of ceramic samples was subjected to irradiation.

The temperature of samples during the ion treatments was controlled in an automatic mode using a computer system for measuring digital signals on the basis of ADAM-4000 modules according to the EMF readings of a chromel–alumel thermocouple welded to a blank sample made of stainless steel with the thickness of 1 mm and the area of 1 cm2. The temperatures of heating of the blank sample at the final phase of the modes A and B were 390 and 1033 K, respectively. The measurement error was ±5 K.

The surface of the irradiated ceramic was analyzed by scanning electron microscopy using a Hitachi TM-3000 instrument equipped with a backscattered electron detector. The static electrical conductivity of ceramic samples was measured in the temperature range T = 25–300°C by the two-probe method of spreading resistance [36].

X-ray phase analysis was performed using an ARL X’tra X-ray diffractometer with a semiconductor Si(Li) Peltier detector (monochromatic CuKα radiation in the 2θ angle range of 20°–75°). The X-ray pictures were processed using the Powder Cell 2.4 software.

The projective range of Ar+ ions under the energy of 30 keV in the ceramic was calculated using the KB-03 software program specially developed in a laboratory at the Institute of Electrophysics, Ural Branch, Russian Academy of Sciences (Yekaterinburg, Russia). The software is designed for the calculation of alloying profiles and electron and nuclear energy losses along the target depth under braking of ions on the basis of solving the kinetic Boltzmann equations. The software was developed using the data from [37]. In addition, the projective range of ions was evaluated by the Monte Carlo method (TRIM) [38].

The mechanical properties of the corundum ceramic before and after the radiation treatment were studied by dynamic nanoindentation using a Nano Indenter G 200 nanohardnes tester, which provides indentation in a wide range of loads. A modified Berkovich pyramid (α = 65.27°) was used as an indenter; the load was 250 mN (25 g). About 8–10 indentations were made on the surfaces of samples. The distance between indentations was 50 μm. The statistical measurement error was within 10%.

EXPERIMENTAL RESULTS AND DISCUSSION

In Fig. 1, photomicrographs of the surface of the corundum ceramic sample before and after the ion treatment are shown. It can be seen from Fig. 1a that the sample surface in the initial state was a polished surface, on which one can notice a slight manifestation of granular microstructure and pores distributed randomly over the surface.

Fig. 1.
figure 1

Surface of ceramic: (a) in the initial state; (b) after ion treatment (mode B).

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It was established that the impact of a continuous beam of argon Ar+ ions in mode A on the surface of aluminum oxide ceramic does not change the structure of the sample with respect to the initial state. After the irradiation of the surface of the sample in mode B, a clear manifestation of granular microstructure can be seen (Fig. 1b). Thus, the ion treatment in modes A and B does not lead to structural changes in the near-surface layers of the ceramic but only to the manifestation of the initial granular structure, which can be explained by non-coincidence of etching rates of surface grains and intergrain boundaries.

The static electrical conductivity of modified layers of the ceramic was measured using a V7-21A universal voltmeter under constant voltage Umax = 5 V supplied to the electrodes. The maximum limit of sensitivity in the measurements was 1 × 10–11 A. During the measurements of electrical conductivity of corundum within the sensitivity range of the used instrument, no electrical conductivity was registered. It is known [16, 39] that the ion treatment of ceramic crystals is accompanied by the increase by several orders of magnitudes in the electrical conductivity of irradiated near-surface layers caused by the disturbance in the stoichiometry with respect to oxygen. For corundum, no noticeable changes in the conductivity were registered, which is explained by preservation of the initial stoichiometric composition.

In Fig. 2, the P–h diagram of the kinetic indentation of the corundum ceramic in the initial stated before the surface treatment under the maximum load applied on the indenter Pmax = 250 mN is shown. The appearance of the P–h diagram after the surface treatment does not change. The diagram has the following regions: the loading branch 1 characterizing the resistance of the material to plastic deformations; the horizontal region 2 corresponding to the exposure of the sample under the maximum load; and the region of unloading to the zero value (full unloading) 3 [40, 41]. The presence of these regions corresponds to the recommendations of the hardness measurement standards concerning the sequence of loading the sample [42, 43]. The time of loading and unloading in all cases was the same, namely, 75 s. From Fig. 2, it can be seen that the residual depth h0 of an indentation after unloading differs significantly from the maximum indenter penetration depth hmax. This indicates active processes of material recovery in the deformed region in the unloading mode. The process of change in the indentation size can be evaluated on the basis of the value of elastic deformation of an indentation:

$$\alpha = \frac{{({{h}_{{\max }}} - {{h}_{0}})}}{{{{h}_{{\max }}}}}.$$
((1))
Fig. 2.
figure 2

P–h indentation diagram for ceramic before ion treatment with the load on indenter Pmax = 250 mN.

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Under the maximum load on applied on the indenter Pmax = 250 mN, the value of elastic deformation α was 0.47 for the unirradiated ceramic and α = 0.51 for the irradiated sample in mode A.

The mechanical characteristics were determined on the basis of the developed curves of the P–h diagram using the Oliver and Pharr method [44].

In Table 1, the deformation characteristics of the studied ceramic before and after the Ar+ treatment registered under the maximum load applied on the indenter Pmax = 250 mN are given.

Table 1.   Mechanical properties of corundum ceramic before and after Ar+ treatment
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According to the nanoindentation data given in Table 1, after the treatment with the Ar+ ion beam, there is a change in the mechanical properties of near-surface layers of the corundum ceramic, namely, the increase in the modulus of elasticity by about 2 times and the increase in the nanohardness by 1.5 times compared to the initial values. The maximum indenter penetration depth after the treatment decreases by about Δhmax = 248 ± 20.4 nm. The change in the deformation characteristics after the irradiation with continuous beams using different fluences is ambiguous. The observed slight decrease in the HIT and EIT values in mode B compared to mode A can be explained by two simultaneous processes caused by the radiation impact. Along with the ion implantation, there is etching of thin near-surface layers under the ion treatment. Prevalence of the latter process over the former one with the increase in the fluence can eventually lead to the decrease in the modification efficiency. Note that, after the irradiation of the ceramic in modes A and B, the nanoindentation results lie within the mean-square deviation range of measurements (Table 1).

The resistance of material to plastic deformation according to [45] is evaluated according to the value of the parameter K = \({{H_{{IT}}^{3}} \mathord{\left/ {\vphantom {{H_{{IT}}^{3}} {E_{{IT}}^{2}.}}} \right. \kern-0em} {E_{{IT}}^{2}.}}\) After the ion treatment, the value of this parameter decreases (Table 1), thus indicating the increase in the plasticity of modified ceramic layers.

In Table 2, the results of the calculations of projective ranges of Ar+ ions using the algorithms presented in [37, 38] are demonstrated.

Table 2.   The projective ranges of Ar+ ions
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From the comparison of the results in Tables 1 and 2, it can be seen that the observed change in the deformation properties occurs at depths hmax which significantly exceed the depth Rp of the radiation penetration of implanted ions. This can be explained by the long-range effect model observed in different materials under beam impacts [46–48].

To determine the reasons that are responsible for the change in the deformation characteristics of the modified ceramic surface, the structural-phase analysis of near-surface layers was performed. The results of this analysis are given in Fig. 3. The analysis of the results using the Powder Cell 2.4 software shows that, in the initial state and after the radiation impact, the ceramic contains only the α-Al2O3 phase characterized by trigonal syngony.

Fig. 3.
figure 3

Diffraction patterns: (a) initial corundum ceramic, (b) after ion treatment (mode B).

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In the X-ray picture after the irradiation (Fig. 3b), along with corundum, additional reflexes typical of the phase can be clearly seen. The same phase is present in the initial state; because of lower intensity; the peaks are not so clearly seen (Fig. 3a). It is impossible to analyze the presence of the unknown phase because of the precise composition of substrate material used in the study is unknown.

As can be seen from the diffraction patterns shown in Fig. 3, the ion treatment of the corundum surface leads to the change in the intensities of the reflexes. This can be explained by the manifestation of the dislocation mechanism under the action of the ion treatment [15], which leads to strengthening or weakening of reflections of X-ray beams from plane systems. In Table 3, the results of the X-ray structural analysis are shown.

Table 3.   Results of X-ray diffraction analysis of corundum ceramic samples before and after the Ar+ treatment
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According to the X-ray structural studies, the size (L) of the coherent scattering region of the α phase of corundum and the crystal lattice microstrains (Δd/d) were evaluated (Table 3). In the ceramic layer modified with a continuous Ar+ ion beam, the L value was lower than in the initial state. The irradiation increased the (Δd/d) value. The cumulative experimental results confirm the assumption about the manifestation of the dislocation strengthening mechanism.

Hence, the modification of the near-surface layers of the ceramic under the action of a continuous Ar+ ion beam is of a purely radiation nature and can be explained within the shock wave mechanism model typical of metals and alloys [11, 46–48].

CONCLUSIONS

The effect of continuous Ar+ ion beams with energy of 30 keV and fluences of 1016 and 1017 cm–2 at the ion current density of 300 μA/cm2 on aluminum oxide ceramic (corundum) was studied.

The radiation treatment does not lead to structural changes of the ceramic in near-surface layers; only the initial granular structure is shown owing to the difference in the etching rates of surface grains and intergrain boundaries.

In the irradiated ceramic, no noticeable change in the electrical conductivity typical of ion crystals under ion irradiation is observed [39]. This means that, in aluminum oxide ceramic, in the process of ion treatment, disturbance in the stoichiometry with respect to oxygen is insignificant and its dielectric properties change only slightly.

The ion treatment does not lead to change in the phase composition. Simultaneously, a partial rotation of planes is observed, which is manifested by the change in the intensity of X-ray reflexes. The increase in the fluence causes the decrease in the coherent scattering region and the increase in the crystal lattice microstrains.

After the irradiation of the ceramic with Ar+ ions, the mechanical properties of the near-surface layers change, which is manifested by the increase in both hardness and modulus of elasticity of the modified surface. Simultaneously, a long-range effect is observed: the mechanical properties change at a depth significantly exceeding the radiation penetration depth of implanted ions.