Synthesis Conditions and Properties of SiAlCN Coatings Obtained by Reactive Evaporation of Al in a Hollow Cathode Arc Discharge in Hexamethyldisilazane Vapors

Abstract

SiAlCN coatings were first obtained by the method of reactive evaporation of aluminum and plasma chemical activation of an organosilicon precursor in a hollow cathode arc discharge. The spectrum of discharge plasma was studied by optical emission spectroscopy under conditions of evaporation of Al in an Ar+N₂+hexamethyldisilazane vapor/gas medium, and it was shown that in the presence of a metal component in the plasma, not only did intensive activation of various components of the media occur but also an increased ionic effect on the surface of the coating was provided, with a deposition rate of up to 10.1 µm/h. The films had a dense and homogeneous structure and had a hardness of up to 31 GPa and good adhesion on stainless steel. The results of SEM, FTIR, and XRD showed that their structure was a nanocomposite consisting of an amorphous matrix based on SiCN and AlN with inclusions of AlCN nanocrystals.

1. Introduction

Ceramics and silicon-based composites are among the promising materials for use as high-temperature structural elements of next-generation gas turbines due to their excellent thermomechanical properties. However, these materials quickly disintegrate when used in oxidizing environments at high temperatures, especially in the presence of aggressive substances such as alkaline elements or water vapor. One possible solution to this problem is the current use of environment barrier coatings (EBCs), which prevent direct contact of silicon-based materials with aggressive media existing, for example, in turbine engines. One of the promising compounds for use as a barrier coating in extreme conditions is SiAlCN. The key properties of this compound are abnormal oxidation resistance up to 1400 °C [1] and high temperature resistance [2], which allow it to maintain chemical and thermal stability in the atmosphere at high temperatures, as well as high hardness and wear resistance [3]. The addition of aluminum makes coatings based on silicon carbonitride more resistant to oxidation at high temperatures [4]. Due to the combination of these properties, such coatings are better suited to protect against wear of components undergoing oxidation at high temperatures in comparison with their predecessors—nitride and carbonitride coatings [5].

One of the most common methods for obtaining such coatings is the CVD method of thermal decomposition of liquid-phase polysilazane [6,7] and the PVD method of magnetron sputtering of two-component Si/Al targets in a mixture of nitrogen and acetylene [1]. Thermal decomposition of reactive components requires sufficiently high temperatures (over 1000 °C), and the use of a ready-made liquid precursor with a ready-made composition does not allow the composition of films to be adjusted during synthesis. The disadvantages of conventional magnetron sputtering include relatively low deposition rates, and the use of multicomponent targets complicates the process of optimizing the chemical composition of the resulting films. In addition, to obtain dense coatings with a good microstructure by the magnetron method, it is necessary to use additional plasma generators [8], since the degree of ionization of target particles is quite low [9]. An alternative to these methods may be the use of low-toxic organosilicon volatile compounds (OSC) activated in gas-discharge plasma at low pressures. Among modern plasma methods for obtaining promising metal/ceramic coatings using preceramic liquid precursors, the most common are reactive magnetron sputtering of a metal target in an OSC-containing medium [10], and reactive arc evaporation of a metal cathode in a mixture of plasma-forming gas with precursor vapor [11]. However, the deposition rate of coatings by this method is relatively low, and the coatings obtained by magnetron sputtering often have a porous structure due to the low degree of ionization of the sputtered target particles; therefore, additional plasma generators are used to increase the ion concentration in such systems. Vacuum arc methods provide high deposition rates, but it is hard to eliminate microparticles and porosity formed in the coatings; using microdrop vapor filtration systems results in a significant reduction in deposition rate and ion current density.

In this work, a hybrid PVD+PECVD method of reactive evaporation of metal and plasma chemical decomposition of hexamethyldisilazane (HMDS) vapors was used for the first time to obtain SiAlCN coatings, and it was successfully tested in the production of nanocomposite TiSiCN coatings [12]. Such coatings have not been obtained in this way before. The advantages of the proposed approach in comparison with traditional CVD and PVD methods are as follows: the possibility of optimizing the composition of the medium by independently adjusting the crucible current, the flow of organosilicon precursor and chemically active gases, and changing the degree of activation of reactive components by changing the current in the crucible circuit or the current of the main discharge gap to obtain coatings with optimal chemical composition and properties; (i) this method does not require high synthesis temperatures (below 400 °C), and it also allows avoiding the use of toxic chemical compounds, which makes this method safe and environmentally friendly; (ii) a sufficiently high concentration of ions in plasma ensures a high ratio of ions to neutral atoms on the surface of the growing coating, which plays a key role in the formation of dense and hard coatings of good quality; (iii) wide range adjustment. The proposed approach for the production of ceramic barrier coatings is promising, since the use of a discharge with a self-heating hollow cathode in combination with soft anodic evaporation of metal makes it possible to generate a dense plasma that does not contain a microdroplet cathode fraction, as in cathode arc sputtering. At the same time, a wide range of operating parameters of such a discharge, along with injection and activation of vapors of the liquid precursor, provide not only sufficiently high growth rates of coatings in comparison, for example, with magnetron sputtering, but also flexible control of many synthesis conditions. At the same time, the properties of the coatings obtained are determined mainly by the method of preparation and synthesis conditions. Thus, the proposed method will make it possible to obtain films on products at sufficiently low temperatures of the substrates when the metal component evaporates at a given rate, with the ability to vary the degree of decomposition and activation of the organosilicon precursor and the degree of ionization of reactive gases by changing the discharge current from 0 to 50 A, and will allow within the framework of a single discharge system to obtain films with a smoothly varying range of concentrations of all elements and, accordingly, with fundamentally different structures and properties.

The purpose of this work is to study the conditions of formation and properties of SiAlCN coatings by the method of anodic evaporation of Al and decomposition of vapors of an organosilicon precursor in a plasma of a low-pressure hollow cathode arc discharge with an evaporated anode.

2. Materials and Methods

The experimental facility includes a 0.05 m³ vacuum chamber with a pumping system based on dry spiral and turbomolecular pumps, a discharge system based on a discharge with a self-heating hollow TiN cathode (70 mm long and 6 mm inner diameter) and a segment anode consisting of a water-cooled section made of stainless steel grade 12X18H10T and an uncooled anode crucible of titanium grade VT1–0, nitrided at a temperature close to the melting point of titanium for 5 h. For better thermal insulation, the crucible was placed in a molybdenum shield and in a ceramic tube, so current was supplied only to the end surface of the crucible, thereby achieving rapid heating and effective evaporation of Al from the melt surface. Granular aluminum weighing 0.4–0.6 g was loaded into the crucible. The distance from the crucible to the substrates was 7–8 cm. Hexamethyldisilazane (HMDS) was chosen as an organosilicon precursor, which contains all the necessary elements for the formation of SiCN coatings. AISI304 stainless steel samples were used as a substrate, which, before the experiment, were cleaned in acetone in an ultrasonic bath for 5 min at a temperature of 50 °C, then dried and placed on a holder in a vacuum chamber, which was subsequently pumped to a pressure of 7·10⁻⁵ Torr. The plasma-forming gas was Ar, supplied to the cathode cavity. For more precise adjustment of the chemical composition of the coatings, the composition of the vapor/gas mixture also included N2, which was injected directly into the chamber. HMDS was fed into the chamber through an evaporator located at a distance of 7–8 cm from the samples, the precursor flow was regulated by a Mini Cori-Flow device (Bronkhorst). Before the deposition cycle, the samples were ionized for 10 min in Ar at a bias voltage of −500 V relative to the anode. After cleaning, the bias voltage was reduced to −100 V. The current in the crucible circuit was regulated in the range of 5–10 A, the current of the main discharge gap was 12 A, the temperature of the crucible was 1200–1400 °C, and the temperature of the samples was 300–400 °C. The selected range of precursor and nitrogen fluxes provides conditions under which the ratio of elements in films should ensure a change in silicon concentration in the range of 10–40%, nitrogen in the range of 5–50%, such as to find optimal conditions for obtaining solid, dense, thermostable films with good adhesion, and changing the discharge current in the range from 0 to 50 A should ensure an optimal degree of decomposition of the precursor, its full use in film synthesis, sufficient activation of the gaseous medium, and a sufficient degree of ion exposure to the surface. The total pressure of the gas mixture during the deposition process was regulated in the range of 4⋅10⁻⁴–2.5⋅10⁻³ Torr. The choice of this pressure range is due not only to the stability of arc discharge but also to the providing of high deposition rates of coatings.

The plasma composition was analyzed using an OceanOptics HR2000 optical spectrometer. The coating thickness was measured using a Calotest device (CSM Instruments), which allows one to determine the thickness of films with an error from 1 to 5%, depending on the quality of the coatings obtained and the diameter of the steel ball for surface abrasion. For SiAlCN coatings, a ball with a diameter of 1.5 cm was used, so that the measurement error did not exceed 3%, while the coating thickness was measured 10 times for samples with a smooth, homogeneous surface without defects in the form of cracks and flakes; the standard deviation in this case did not exceed 5%. The hardness of the obtained films was measured by micro-indentation on a SHIMADZU DUH-211S device. The topology of the coating surface and the elemental composition were studied using a Tescan VEGA II XMU scanning electron microscope. This method involves the use of a wave dispersion analyzer, which makes it possible to more accurately determine the chemical composition of coatings with an experimental error of less than 2%. The composition of the coatings was also studied by IR spectroscopy on the Vertex 70 FTIR device. The XRD analysis was performed on a D8 DICCOVER diffractometer (Cu Ka1.2 λ = 1.542 Å) with a graphite monochromator on a diffracted beam.

3. Results and Discussion

The use of a liquid organosilicon precursor is, first, one of the simplest and most convenient ways to introduce silicon into coatings. Second, the participation of whole fragments of the initial molecule containing Si-C and Si-N bonds, rather than individual atoms, facilitates the formation of the final structure of the SiCN-based coating. Hexamethyldisilazane ([((CH₃))₃Si]₂NH, HMDS) was used as a silicon-containing precursor. The scheme of the experimental gas discharge system is shown in Figure 1. Plasma generation, aluminum evaporation, and decomposition of precursor vapors took place in one discharge cell based on a hollow cathode arc discharge. At the same time, by distributing currents between the hot and cold anode sections, the concentrations of gas and metal ions in the plasma can be varied, and the separate and independent injection of various components into the vapor/gas mixture allows precise control of the elemental composition of the coatings obtained.

The discharge current of the I_d could be regulated in the range of 5–50 A, while the current to the I_cr crucible could be set by the power sources used up to 10 A. The crucible was made of titanium with an inner surface of titanium nitride obtained by nitriding at a temperature of 1500–1600 °C for 5 h in an argon/nitrogen gas mixture (Q_Ar:Q_N2 = 30:40 sccm, P = 1.5 mTorr). Experiments have shown that such a crucible functions stably for several deposition cycles and does not collapse when operating in both inert and chemically active media at temperatures up to 1400 °C, which made it possible to obtain evaporation rates of Al up to 1 g/h.

The composition and activation degree of the components of the medium in which the coatings are deposited determine their properties. Figure 2 shows the characteristic optical emission spectrum of the discharge plasma in a Ar (30 sccm) + N₂ (10 sccm) + Al (1 g/h) + HMDS (1 g/h) vapor/gas medium at I_d = 10 A and I_cr = 6 A. Under typical coating deposition conditions, lines of excited argon Ar* in the wavelength range of 700–900 nm, molecular nitrogen (490–610 nm), and characteristic lines of aluminum Al* (394.4 and 396.2 nm) are observed in the spectra. Evidence of the deep decomposition of precursor molecules in plasma is the presence of hydrogen atoms H (656.3 nm) and carbon C (484.2 nm) in the spectrum of intense lines. The presence of the atomic nitrogen line N* (746.83 nm) may indicate not only a high degree of dissociation of N₂ but also an intensive decomposition of the HMDS molecule. The ionic composition of the plasma is represented by argon ions Ar⁺ (419.8 nm), aluminum Al⁺ (390.1 nm), and molecular nitrogen N₂⁺ (391.4 nm). An increase in the discharge current leads to a monotonous increase in the intensities of atomic hydrogen and carbon lines, which, with a constant precursor flow, indicates an increase in the degree of decomposition of HMDS molecules. And an increase in the intensity of ion lines (N₂⁺, Al⁺, Ar⁺) with current indicates an increase in the concentration of these ions and, accordingly, an intensification of the ion effect on the growing coating. Molecular nitrogen ions N₂⁺ (391.4 nm), whose content in the ion stream reached 5–10%, may indirectly indicate a high content of nitrogen atoms in plasma [13], which play an important role in the synthesis of nitride films and coatings. The measured ion current density near the samples was 8 mA/cm² at a current per 10 A crucible, while the evaporation rate of aluminum reached ~1 g/h. At the same time, the proportion of metal ions in the ion flow to the substrate surface reached 50%. Thus, by changing it, it is possible to not only achieve high metal evaporation rates but also increase the degree of activation of vapors of the organosilicon precursor, as well as change the degree of ionic action on the treated surface.

Films based on silicon carbonitride in various synthesis modes were obtained on AISI304 stainless steel samples located at a distance of 6 cm from the crucible. Since SiAlCN ceramic coatings are non-conductive, a positive charge accumulates on the surface of the growing coating during plasma deposition, which creates a potential barrier for ions entering the surface. To eliminate this effect and conserve the energy of bombarding ions, an RF offset (40 kHz) was applied to the samples during processing, in which, in the pauses between negative voltage pulses, an electron stream from the plasma entered the surface of the samples, compensating for the positive ion charge accumulated during ion exposure [14]. The temperature of the samples heated by thermal radiation from the crucible and accelerated ions from the plasma during synthesis was 300–400 °C, depending on the discharge current mode.

An increase in the flow of aluminum vapors, as well as the vapor flow of the precursor, almost linearly affects the deposition rate of coatings, which in the studied configuration of the gas discharge system reached 10.1 µm/h at a current in the crucible circuit of 7 A and a vapor flow of HMDS of 1 g/h. In this case, a change in the fluxes of the reactive components of the vapor/gas mixture leads to a corresponding change in the chemical composition of the coatings (

Table 1. Chemical composition of coatings in various synthesis modes.

№	Us, V	Icr, A	QN2, sccm	QHMDS, g/h	Si, at. %	Al, at. %	C, at. %	N, at. %	V, µm/h
1	−100	4	10	0.5	17.8	32.8	9.4	34.2	5.6
2	−100	5	10	0.5	19.4	33.5	8.2	29.9	7.6
3	−200	5	10	0.5	17.9	28.4	9.2	32.8	6.8
4	−300	5	10	0.5	28.5	15.7	21.4	24.9	6.2
5	−100	8	0	1	22.3	43.7	6.1	5.8	9.4
6	−100	6	0	1	18.2	29	22.8	13.4	10.1
7	−100	0	0	1	37.1	0	13.2	6.3	2.2
8	0	4	10	0.5	18.8	30.6	11.4	31.93	5.5

).

In the absence of a current to the crucible, the chemical composition of the coatings is close to the composition of the hexamethyldisilazane molecule [15]. With an increase in the current I_cr in the crucible circuit, the proportion of Al in the coating increases significantly to 43%, and the proportions of Si, C, and N decrease accordingly. An increase in the Icr current also leads to an increase in the coating rate from 2.2 to 10.1 µm/h due to an increase in the evaporation rate of the metal, and probably due to an intensification of the activation processes of the reactive components of the gas mixture with an increase in plasma concentration. An increase in the flow of the organosilicon precursor from 0.5 to 1 g/h leads to an increase in the content of Si and C, while the deposition rate is 6 and ~10 µm/h, respectively. However, it should be noted that at high (more than 7 A) currents per crucible, the quality of the coatings is greatly reduced, microdrops appear on the surface, and the coatings themselves begin to crack and peel off from the substrate (Figure 3), which is probably due to an increase in the level of deformation stresses in the coating and a decrease in adhesion sprayable coating.

At increased bias potentials, the coatings have a smoother surface, without significant defects, which may be due to ion etching of the surface; however, in such modes, the probability of coating detachment increases due to the high level of microstress induced by intense ion flow, the deposition rate of coatings also decreases by about 20% with an increase in the bias voltage from 50 to 300 V. The films obtained without bias had defects in the form of pores and craters. Therefore, the optimal range of bias stresses was (50–100 V). In terms of the combination of coating quality, adhesion, and hardness, the microhardness of the obtained films in optimal modes was ~27–31 GPa, dropping to 16–19 GPa at high crucible currents and zero bias potential. The elastic modulus varied in the range of 150–320 GPa with an increase in the hardness of the coating from 16 to 31 GPa. There is no crack formation in the films when indenting coatings at loads of up to 20 g.

Figure 4 shows the results of IR spectroscopy of SiCN-based coatings obtained in a hexamethyldisilazane medium without heating the crucible (curve 0), and SiAlCN coatings obtained by reactive evaporation of Al in HMDS vapors at a flow of HMDS 1 g/h and a current per crucible of 6 A (curves 1–3) at bias potentials of –100, –200, and –300 V, respectively. The main difference between the SiCN and SiAlCN samples is the presence of Al-N bonds. In the IR spectrum of samples with SiAlCN, there is a band of 594 cm⁻¹ belonging to the Si-C valence vibration group, which, according to the source [1], can be in a wide range of 570–1130 cm⁻¹. The bands 995 cm⁻¹ and 1245 cm⁻¹ can be attributed to the valence vibrations of Si-N [1], formed, possibly, as a result of the decomposition of the organosilicon precursor in plasma and the cleavage of hydrogen (H) in a fragment of the Si-NH hexamethyldisilazane molecule or as a result of the reaction of Si with N₂.

The presence of Si-C and Si-N valence vibrations in the spectrum is also confirmed by the data from the source [16]; however, the range and position of the absorption bands differ by several tens of cm⁻¹. This difference can be explained, for example, by the presence of hydrogen bonds in the coating, as well as spatial effects or contamination. The peaks of 1346 and 1412 cm⁻¹ can be attributed to the C=C oscillation, as shown in [17]. As can be seen, with an increase in the modulus of the displacement potential, the intensity and width of the Al-N bands in the range 110–1300 cm⁻¹ and C=C (1350–1900 cm⁻¹) increase, which indicates the influence of negative displacement on the incorporation of Al into the structure of PDC ceramics. The appearance of C=C bands is extremely important, as this may indicate the presence of a free carbon phase in the coating, which favorably affects the piezoresistivity of the coating. The data on the bands are confirmed by the sources [17]. Absorption peaks of hydrogen-containing C-H fragments are found in the range of wave numbers 2500–3200 cm⁻¹ [17,18].

X-ray diffraction (XRD) analysis showed that the coatings obtained at I_cr = 5 A were practically X-ray amorphous (Figure 5); however, weak peaks were found in the region of small angles 25–43°, which may be due to the presence of aluminum carbonitride Al₇C₃N₃ with orthorhombic symmetry and lattice parameters a = 3.24 Å, b = 5.66 Å, and c = 31.68 Å with an average crystallite size of 3.7 nm. At low I_cr (lower 4 A), such peaks were not observed. Typically, coatings of this type are amorphous [19], and temperatures above 1000 °C are required under normal conditions to obtain nanocrystalline phases (AlCN, SiC, or SiN) [20].

However, under our conditions, an intense ion flux is present on the substrate surface, which, as is known [21], can contribute to the formation of nanocrystalline phases at low (~400 °C) temperatures, which may explain the presence of nanocrystallites in the coatings we have obtained. The intensity of the AlCN peaks in the XRD spectra is sufficiently low, which, with a high Al content in the coating and an intense AlN peak in the IR spectra, indicates that the proportion of the nanocrystalline phase in the coating is quite small, and AlN is in an amorphous state in it. X-ray images also show peaks (O) (Figure 4), which may be due to organic adsorbents on the coating surface, often observed in the analysis of thin films and nanopowders.

Thus, the resulting coating is a nanocomposite, which may explain the higher microhardness values compared to the known data for amorphous SiAlCN coatings, as well as promise higher indicators of other mechanical characteristics.

As can be seen in Figure 6, the resulting SiAlCN coatings with a thickness of 6.8 microns have a homogeneous dense structure. At the same time, there are no microdroplets on the surface, the presence of which is characteristic, in particular, of the high-performance vacuum arc method. The coatings are non-conductive, translucent, and have good adhesion to the substrate.

4. Conclusions

AlSiCN cermet coatings were obtained for the first time by reactive evaporation of aluminum and decomposition of an organosilicon precursor in a hollow cathode arc discharge. It is shown that reactive anodic evaporation of Al not only makes it possible to provide a high amount of metal vapor flow without a microdroplet fraction, and, accordingly, a high deposition rate of coatings but also significantly increases the plasma concentration in the discharge and the density of the ion current on the treated surface. This, in turn, contributes both to an increase in the degree of activation of the hexamethyldisilazane and to an increase in the degree of ionic action on the sample surface. Moreover, the determining factor affecting the properties and quality of the obtained films was the degree of ionic action on the surface of the growing coating. At low ion current and ion energy densities, coatings are relatively soft and with poor microstructure. At high currents, the hardness of the coatings increases, but the level of microstress in the coating also increases, which leads to their cracking and reduced adhesion. In addition, microdrops appear in the vapor stream at elevated currents per crucible, which also worsens the quality of coatings. Thus, there is a certain range of parameters for obtaining films with optimal properties. As a result, a dense coating with a nanocomposite structure is formed, consisting of an amorphous matrix based on SiCN and AlN with incorporated AlCN nanocrystals with a hardness of up to 31 GPa. The coatings have good adhesion and have a dense structure, without the inclusion of microdrops. Thus, the developed method, tested on the test substrate material, makes it possible to apply solid films of the original structure to small parts with an area of up to several tens of square centimeters at a substrate temperature of 300–400 °C with a deposition rate of up to 10 µm/h, which is an order of magnitude higher than the rate of magnetron deposition and comparable to the rates of thermal decomposition of liquid-phase precursors.