Research progress on SiMOC coatings prepared by polymer pyrolysis and chemical vapor deposition

ABSTRACT

The SiOC materials have shown great potential for a number of engineering applications owing to their excellent physical and chemical properties. Due to the unique microstructure of SiOC coatings, different variants of polymer pyrolysis and chemical vapor deposition (CVD) processes are used to prepare SiOC coatings with different properties. However, in terms of their performance and potential for preparing variety of SiOC materials, a comparative study on these two methods has not been reviewed in a single article. Therefore, this article attempts to investigate the development of SiOC materials in recent years by comparing the two preparation methods. Furthermore, the research results of different types of SiMOC coatings, developed in past 20 years, have been critically discussed from three aspects, i.e. (1) the selection of precursor, (2) types of metal cationic additives, and (3) coating preparation methods. The in-depth study of SiMOC coatings suggests that irrespective of preparation method used to prepare SiMOC coatings, the selection of precursors can be divided into two broad categories: (a) polymer precursors and (b) organic precursors. Furthermore, it was revealed that by adding some metal elements (such as Al, Zr, HF, Ce, Ti, and Ta) to the precursor, some properties of SiOC coatings can be further improved. Therefore, the main disadvantages of the two preparation methods are: (1) the porosity problem when the polymer is converted to ceramics in polymer pyrolysis method and (2) the low deposition rate of the CVD method.

KEYWORDS:

• SiMOC coatings
• chemical vapor deposition (CVD)
• polymer pyrolysis

Introduction

The SiOC materials, with unique crystal structure, have been widely studied during the past two decades. Owing to their excellent physical and chemical properties, the SiOC materials have shown great application prospects in many fields. For example, by adding nano silicon carbide particles, SiOC materials exhibit good electrical characteristics and stable cycling, which can be used to prepare electrode materials for high-performance lithium-ion batteries [1]. Similarly, high-temperature oxidation resistance of engineering alloys can also be improved by depositing SiOC coatings. For example, high-temperature oxidation resistance of TiAl alloy at 800°C can be improved by nearly 10 times by depositing a layer of SiOC coating (with a thickness of several 10s of microns) on its surface [2]. The SiOC coatings also show excellent corrosion resistance. Research shows that the filter element displays relatively high mechanical strength and pure water permeation flux if a layer of SiOC ceramic membrane, with adjustable porosity, is coated on the YSZ (Yttria-stabilized zirconia) hollow fiber skeleton. It was verified that the mass loss of SiOC coated filter element in 20 wt.% H₂SO₄ solution was only 0.07 wt.% [3], which is very conducive to industrial wastewater treatment plants. The conductivity of the carbon-rich SiOC materials can reach as high as 4.28 s/cm in the air environment when the temperature is less than 400°C [4]. Such materials with high thermal stability and good electrical conductivity can be used as protective coatings for high-temperature microelectronic integrated system devices. The a-SiOC:H thin films, prepared by low-temperature remote hydrogen microwave plasma chemical vapor deposition (RP-CVD), have unique characteristic of optical refractive index which changes with the substrate temperature. Hence, it can be used to prepare dielectric materials or coatings for various electronic packages [5]. By adding ZrO₂ and free carbon into the SiOC coatings, the wear resistance and self-lubricating property of the coating are improved, which enables it to be applied in a dry lubrication environment where lubricant cannot be used due to various reasons [6]. In addition, SiOC coatings also have certain radiation resistance [7], etc.

Through careful selection and regulation of the precursors, the dual control over elemental composition and microstructure of SiOC coatings can be achieved. The O atoms and C atoms in SiOC coatings are randomly combined with Si atoms in a three-dimensional covalent structure to form the following five structural units: (1) SiO₄, (2) SiO₃C, (3) SiO₂C₂, (4) SiOC₃, and (5) SiC₄ [8]. As free carbon often exists in SiOC coatings, another chemical expression of SiOC coatings is SiO_2–2xC_x+yC_free, where x + y is the total amount of carbon atoms in SiOC coatings [9]. However, when the pyrolysis temperature or deposition temperature reaches a certain value, the microstructure of SiOC coatings will change with the change in temperature. When the pyrolysis temperature or deposition temperature is lower than 1200°C, the SiOC coatings usually consist of amorphous Si–O–C networks with amorphous free carbon, which makes SiOC coatings very dense with many good properties. However, when the pyrolysis temperature or deposition temperature exceeds 1250°C, phase separation usually occurs in SiOC coatings through Equation (1(1) $2\mathrm{S}\mathrm{i}\mathrm{O}\mathrm{C}\to \mathrm{S}\mathrm{i}\mathrm{C}+\mathrm{S}\mathrm{i}{\mathrm{O}}_2+\mathrm{C}.$(1) ) [10]. (1) $2\mathrm{S}\mathrm{i}\mathrm{O}\mathrm{C}\to \mathrm{S}\mathrm{i}\mathrm{C}+\mathrm{S}\mathrm{i}{\mathrm{O}}_2+\mathrm{C}.$(1)

The product of phase separation is usually composed of the SiO₂ phase, β-SiC nanocrystalline phase and graphite phase [11]. As shown in Figure 1, when the temperature exceeds 1300°C, the SiOC coating and the Si substrate will form the β-SiC [12]. In addition, the carbon content will affect the initial temperature of β-SiC phase separation. The higher the carbon content, the higher the crystallization point of β-SiC [13].

Figure 1. (a) HRTEM (High Resolution Transmission Electron Microscope) of β-SiC nanocrystals in the reaction layer between SiOC coatings and Si matrix and (b) dot matrix plan – dark gray: Si matrix; light gray: β-SiC crystal; black: amorphous Si–O–C network [12].

In recent years, the preparation of modified SiOC materials (with new composition and microstructure) by mixing different additives to the precursor at the molecular level has become a research hot spot [14]. The additives are mainly metal organic alcohols, which do not react with the precursor, but can be fully mixed with the precursor at the macro and micro scales after pyrolysis. They can quickly react after reaching the substrate surface to form uniform SiOC coatings with different properties [15]. In addition, the thermal expansion coefficient of SiOC material is closer to that of the most metallic materials, therefore it is very suitable to be used as a protective coating or functional coating on the surface of metallic materials [15].

The SiOC coatings are mostly used to prepare protective coatings. There are two kinds of carbon bonds in SiOC coatings, namely, Si–C bond in structural carbon and C–C bond in free carbon. Due to the extremely high chemical stability of Si–C bond, the structural integrity can be maintained even in the presence of high temperature and oxygen element. Interestingly, this does not affect the high-temperature resistance of the coating. Because the Si–C bond exists in the Si–O–C amorphous network, it has more significant impact on the properties of the coating, such as improving the mechanical properties of SiOC coatings. When SiOC coatings are used as high-temperature oxidation resistant protective coatings, the high-temperature oxidation resistance of the C–C bond is poor, it is particularly necessary to control the free carbon in the high-temperature oxidation protective SiOC coatings [16]. The carbon content in SiOC coatings will significantly affect the porosity of the coatings, thus affecting the compactness and protective effect of the coatings [17]. When the free carbon content is too high, the combined action of high temperature and oxygen element causes the carbon in the coatings to be oxidized into gaseous carbon oxides. Consequently, the carbon position is replaced by pores [18]. In addition, the porosity of SiOC protective coatings is closely related to the preparation method. Polymer pyrolysis and CVD are the two main methods to prepare SiOC protective coatings. When the SiOC coatings are prepared by the method of polymer pyrolysis, their compactness is not ideal and pores are easily generated therein. This is because, on the one hand, it is difficult to completely discharge the gas generated during the coating formation process in a certain temperature range (400–800°C). The evolved gas is sealed inside the coating to form pores in the coating. The generation of pores can be reduced to a certain extent by controlling the heating time and adding expansion additives, but it cannot be completely avoided [19]. On the other hand, there is a density difference between the polymer (1.0–1.2 g/cm³) and the ceramic phase (2.0–3.0 g/cm³) [20]. When the polymer is converted to ceramic, it results in great volume shrinkage, and also inevitably generates pores. However, when CVD is used to prepare SiOC coatings, stacking process at the molecular level ensures that the coating is highly dense and uniform. Moreover, it has good bonding with the substrate.

The SiOC can be prepared by polymer pyrolysis and CVD methods. However, the above-mentioned processes have not been scarcely reviewed in a single paper. Therefore, this review attempts to investigate the development of SiOC materials in recent years by comparing the two preparation methods. Polymer pyrolysis is the most widely used method for preparing ceramic materials. The SiOC bulk materials are generally prepared by this method. However, it is inevitable to avoid residual pores when the polymer is converted to ceramics in the polymer pyrolysis method. This has been a major problem puzzling the researchers in the early years of using the polymer pyrolysis method. Therefore, some researchers started using CVD technology to prepare SiOC materials. Because of the low deposition rate of CVD, this method is basically used to prepare SiOC coatings. The SiOC coatings, obtained by CVD technology, are generally dense and have very low porosity, thus meeting the needs of researchers at that time. However, with gradual increase in the severity level of the service environment of various engineering materials and the in-depth study on SiOC materials, the researchers found that only relying on SiOC materials could not meet the industrial demand. Therefore, SiMOC materials, with some metal ions, entered the field of vision. In summary, the current research focus is how to prepare SiMOC materials with metal ions and how much it improves the service performance. At present, the main disadvantages of the two preparation methods are: (1) the porosity problem when the polymer is converted to ceramics in polymer pyrolysis method and (2) the low deposition rate of the CVD method.

Precursor selection

Whether SiOC coatings are prepared by polymer pyrolysis or various CVD methods [21], the selection of precursors is the first issue to be considered.

Polymer pyrolysis

There are two typical polymer precursors, namely polymer polyhydroxymethylsiloxane (PHMS) and polydimethylsiloxane (PDMS). When PHMS is used as a precursor to prepare SiOC materials, it is difficult to prepare dense and well-bonded protective coatings due to poor wettability with other materials and low curability. The wettability and curability of PHMS can be improved by modifying PHMS with hydroxyl, alkyl, or other polymer functional groups [22]. When PDMS pyrolysis is used to form the coating, it leads to a low degree of cross linking due to the low interaction between its molecules. Consequentially, defects such as pores and cracks appear in the coating. At this time, the PDMS can be modified by adding an activator, such as tetraethoxysilane and transition metal alkoxide. This modification can reduce the shrinkage of PDMS when it transforms into ceramics, and effectively prevent the coating from cracking [23].

CVD methods

When SiOC coatings are prepared by a CVD method, the selected precursor is no longer polymer, but organic matter. Using organic compounds as precursors can effectively avoid the problems of high porosity and poor interface bonding caused by volume shrinkage. The type of precursor can be selected based on required coating characteristics. If a single organic source is used as the precursor to prepare SiOC coatings, Si, O, and C atoms should be present in the precursor to provide a silicon source, oxygen source, and carbon source, respectively. The advantage of a single organic source as a precursor is that the process is simple, but the elemental composition of the coatings is not adjustable, hence the desired coatings performance cannot be achieved. If the mixture of two or more organic compounds is used as the precursor simultaneously, and each organic compound provides one or more Si, O, and C atoms, respectively, the proportion of Si, O, and C atoms in the precursor can be adjusted through the organic compound ratio. Hence, control over elemental composition and performance of the coatings can be achieved. For example, when the mixture of hexamethyldisiloxane and n-hexane (C₆H₁₄) is used as the precursor, the former provides Si, O, and C atoms, while the latter provides C atoms [24]. The content of C atom in the coatings can be adjusted by changing the proportion of n-hexane. The research shows that carbon-rich SiOC coatings have better conductivity or corrosion resistance [24,25].

Addition of cationic additives

Whether SiOC materials are prepared by polymer pyrolysis or CVD methods, metal alkoxides can be added into the precursor to achieve the modification in SiOC. The modified SiOC materials, containing metallic elements, usually have better high-temperature resistance, corrosion resistance, and fewer defects [26]. The selected metallic elements usually include Al [27], Zr [28], Hf [29], Ce [30], Ti [31], Ta [32], etc. [33]. The metal alkoxides formed in modification of SiOC materials usually include secondary butanol aluminum (C₄H₉OAl), dimethyl dibutyl alcohol zirconium (C₁₁H₂₆OZr), tetrabutanol hafnium (Hf(OR)₄), cerium nitrate (Ce(NO₃)₃), tetraisoethanol titanium (C₁₂H₂₈O₄Ti), and ethanol tantalum (C₁₀H₂₅O₅Ta).

SiAlOC materials

The addition of Al ions can significantly improve the thermal stability of SiOC materials. The Al cations enter the Si–O–C network center by replacing Si cations [34]. Xu et al. [35] mixed polymethyl(phenyl) siloxane resin and sec-butanol aluminum in a specific proportion at a certain temperature, and prepared SiAlOC materials with good thermal stability by sol–gel method. In order to verify the high-temperature oxidation resistance of the material, they put it into a muffle furnace at different temperatures, and measured the weight loss rate, element composition, and microstructure changes of the material after holding for 1 h, as shown in Table 1. It can be seen that the weight loss rate of this material at 1400°C is only 1.33%, and the elemental composition has no obvious change, while the microstructure of the material is stable [35]. When the temperature reaches 1600°C, the weight loss rate of the material increases to 4.31%, while the elemental composition of C changes greatly. This shows that the addition of Al ions improves the high-temperature resistance of SiOC materials in the air environment in the temperature range of 1200–1400°C.

Table 1. Material weight loss rate and change in elemental composition of SiAlOC sample after annealing at different temperatures for 1 h in air environment [35].

Temperature (°C)	Weight loss rate (%)	Elemental composition
		Si (wt.%)	Al (wt.%)	O (wt.%)	C (wt.%)
1000	0.84	31.18	15.41	46.96	6.45
1200	1.15	31.86	13.14	48.26	6.74
1400	1.33	31.45	14.91	47.58	6.06
1600	4.31	33.46	13.35	48.96	4.23
1700	7.36	34.46	15.12	50.26	0.16

The ²⁹Si MAS NMR (Magic-Angle-Spinning Nuclear-Magnetic-Resonance) spectra of SiAlOC materials at different temperatures in the air environment are shown in Figure 2(a) [35]. It can be seen that when the temperature rises from 1000°C to 1200°C, the content of SiO₃C structural units is significantly reduced, which indicates that the samples have experienced the redistribution of the Si–O bond and Si–C bond. When the temperature reaches 1400°C, Q⁴ (nAl) (1 ≤ n ≤ 4) structural units begin to appear and increase with the increase in temperature, which indicates that Al ions enter the SiO₄ network and form Si–O–Al bonds. In addition, ²⁷Al MAS NMR spectra at different temperatures in the air environment are shown in Figure2(b). The Al ions in the material combine with O ions to form three structural units: AlO₄, AlO₅, and AlO₆.

Figure 2. (a) ²⁹Si MAS NMR spectra and (b) ²⁷Al MAS NMR spectra at different temperatures in the air environment [35].

Bik et al. [27] used modified alkoxysilane and sec-butanol aluminum to prepare SiAlOC materials by sol–gel method. They [27] compared the Raman spectra of the prepared SiOC materials and SiAlOC materials, as shown in Figure 3. According to Ferrari and Robertson [36] and Ferrari et al. [37], it can be inferred that the order of carbon in SiAlOC materials is different from that in SiOC materials. In addition, the energy band of SiOC materials at 1507 cm⁻¹ may be related to the sp³ carbon or C–H bond in amorphous a-C:H carbon. However, the energy band at 1507 cm⁻¹ is not observed in SiAlOC materials. These two results show that the addition of Al ions into SiOC materials is conducive to the reduction of free carbon content, and may make the carbon in the materials more regionally ordered.

Figure 3. Raman spectrum of SiOC and SiAlOC materials [27].

In conclusion, the addition of Al ions into SiOC materials reduces the content of free carbon in the materials [27]. Moreover, Al ions and O ions form AlO₄, AlO₅, and AlO₆ structural units [38]. When the temperature reaches a certain value, these structural units react with SiO₄ structural units to form a mullite structure, thereby reducing the content of SiO₄ structural units. The main reason for the failure of SiOC materials at high temperatures is the carbothermal reduction reaction between free carbon in the materials and SiO₄ structural units. However, when Al ions are introduced into SiOC materials, the contents of free carbon and SiO₄ structural units in the materials are reduced, so the carbothermal reduction is effectively inhibited, thus improving the thermal stability of the materials. Ma and Xu [39] also mixed polymethyl(phenyl)siloxane resin and sec-butanol aluminum in a specific proportion at a certain temperature, and prepared SiAlOC materials by sol–gel method. In order to verify whether SiAlOC materials have stronger high-temperature resistance without oxygen, they put them in a high-purity argon inert gas environment and held them at different temperatures for 1 h to determine the weight loss rate, elemental composition, and microstructural changes in the samples, as shown in Table 2 [39]. It can be seen that when the temperature is lower than 1500°C, the weight loss rate of SiAlOC material is very low, while the weight loss rate of SiAlOC material is only less than 3 wt.% when the temperature reaches 1600°C. This shows that the high-temperature resistance of SiAlOC material in the inert atmosphere is much stronger than that in air environment. It can be seen from Table 1 that when the temperature is raised from 1500°C to 1700°C, the content of C decreases dramatically. The reason may be that C in SiAlOC materials can easily react with oxygen in the air. Therefore, we can infer that the content of C element has an important influence on the high-temperature oxidation resistance of SiAlOC materials in the air environment.

Table 2. Material weight loss rate and element composition changes of SiAlOC samples annealed at a given temperature for 1 h in an argon inert gas environment [39].

Temperature (°C)	Weight loss rate (%)	Elemental composition
		Si (wt.%)	Al (wt.%)	O (wt.%)	C (wt.%)
1000	0	31.26	15.53	46.89	6.32
1200	0.07	31.38	15.62	46.96	6.04
1400	0.74	31.87	15.15	46.53	6.45
1500	1.57	30.95	16.80	45.32	6.93
1600	2.88	30.34	16.10	47.89	5.67

The SiAlOC materials, derived from SiOC materials, have excellent high-temperature oxidation resistance, which makes them applicable to many high-temperature environments. Li et al. [40] designed a pressure sensor using SiAlOC materials derived from polymer, as shown in Figure 4. The pressure sensor can replace the traditional silicon-based sensor and be used in high temperature, high pressure, corrosive and radioactive environments such as nuclear reactors and aviation propulsion.

Figure 4. (a) Overall schematic diagram of the experimental device and (b) circuit diagram of the experimental device [40].

Bik et al. [41] used electrochemical technology to study room temperature corrosion resistance of SiAlOC coatings deposited on Crofer 22APU stainless steel. Samples were tested in a wet corrosion environment using 3.5 wt.% NaCl aqueous solution. Some results are shown in Figure 5. It can be seen that the impedance of the ground Crofer 22APU stainless steel sample coated with SiAlOC is many times higher than that of the uncoated Crofer 22APU stainless steel sample, indicating that the SiAlOC coatings on the Crofer 22APU stainless steel effectively prevent the invasion of electrolyte in the corrosion solution. Bik et al. studied whether the polymer pyrolysis process would affect the matrix and thus the corrosion resistance of the matrix using the ground samples of uncoated Crofer 22APU stainless steel after the pyrolysis process. It was demonstrated that the corrosion resistance of ground uncoated Crofer 22APU stainless steel samples may improve after the pyrolysis process. This is attributed to the formation of a layer of Cr₂O₃/MnCr₂O₄ mixed oxide scale on the uncoated surface after the pyrolysis process, promoting wet corrosion resistance of the alloy. Therefore, Bik et al. concluded that a layer of Cr₂O₃/MnCr₂O₄ mixed oxide skin, formed between the SiAlOC coatings and the substrate, has stronger corrosion resistance by prolonging the heat treatment time during the pyrolysis process. This SiAlOC protective coatings, with strong corrosion resistance and thermal stability, can be used in solid oxide fuel cells [41].

Figure 5. (a) Uncoated Crofer 22APU stainless steel, G (ground sample of uncoated Crofer 22APU stainless steel after pyrolysis), G5 (ground sample of Crofer 22APU stainless steel coated with SiAlOC with the best process parameters); (b) local enlargement in (a); (c–e) equivalent circuit of the above three samples, respectively [42].

SiZrOC materials

Ionescu et al. [28] prepared SiOC/ZrO₂ materials using two precursors, one is polymethylsiloxane (PMS) directly added with zirconia powder and the other is PMS chemically modified by zirconia alcohol. First of all, the study showed that the two precursors used could significantly affect the crosslinking and ceramic behavior of PMS itself. Compared with PMS without any additives, PMS samples modified with zirconia alcohol will induce the crosslinking process at a lower temperature, thereby affecting the ceramic process. The elemental composition and analysis of SiOC/ZrO₂ materials, obtained by pyrolysis of different precursors at 1100°C, are shown in Table 3 [28]. It can be seen that the carbon content decreases with the increase in volume fraction of ZrO₂. In addition, the free carbon content of SiOC/ZrO₂ materials, obtained by pyrolysis of PMS which is chemically modified with zirconia alkoxide, is significantly higher than that of SiOC/ZrO₂ materials obtained by pyrolysis of PMS which is directly added with zirconia powder.

Table 3. Element composition and analysis of SiOC/ZrO₂ materials obtained from pyrolysis of different precursors at 1100°C [28].

Sample	C (wt.%)	O (wt.%)	Si (wt.%)	Zr (wt.%)	Chemical formula	ZrO2 content (mol.%)
5ZrO2-PMS	9.56	32.62	47.67	8.44	Si1O1.10C0.47/0.05ZrO2	4.67
10ZrO2-PMS	7.64	31.41	29.28	30.42	Si1O1.25C0.61/0.31ZrO2	20.39
20ZrO2-PMS	4.67	28.75	25.02	38.98	Si1O1.06C0.44/0.48ZrO2	33.10
30ZrO2-PMS	3.27	28.86	17.54	48.72	Si1O1.16C0.43/0.85ZrO2	45.70
5ZrOR-PMS	12.01	36.91	46.92	2.85	Si1O1.34C0.60/0.02ZrO2	1.55
10ZrOR-PMS	10.79	37.14	44.54	5.45	Si1O1.36C0.57/0.04ZrO2	3.10
20ZrOR-PMS	9.81	35.14	45.52	7.06	Si1O1.25C0.50/0.05ZrO2	4.27
30ZrOR-PMS	9.74	39.24	37.27	12.16	Si1O1.65C0.61/0.10ZrO2	6.54

Ionescu et al. [28] analyzed the XRD results and found that the crystalline state of SiOC/ZrO₂ materials, obtained using the above two different precursors, is also different. The SiOC/ZrO₂ materials (prepared by PMS with zirconia powder added directly) appear ZrO₂ crystalline phase, while the SiOC/ZrO₂ materials (prepared by PMS chemically modified by zirconia alkoxide) are almost completely amorphous. Ionescu et al. [28] carried out further these results using TEM, as shown in Figure 6. It can be clearly seen in Figure 6 that the SiOC/ZrO₂ material, prepared by PMS with zirconia powder added directly, contains many ZrO₂ particles of different sizes. Among these particles, the diameter of the larger particles nearly reaches to 500 nm. However, SiOC/ZrO₂ materials, prepared by PMS chemically modified with zirconia alkoxide, are almost completely amorphous. It can be seen from HRTEM images that there are only a few ZrO₂ microcrystals with a diameter of no more than 5 nm in the materials.

Figure 6. (a) TEM (Transmission Electron Microscope) photos of SiOC/ZrO₂ materials prepared by PMS with zirconia powder added directly, (b) HRTEM photograph of an area in (a), (c) TEM photos of SiOC/ZrO₂ materials prepared by PMS chemically modified by zirconia alkoxide, and (d) HRTEM photograph of an area in (c) [28].

SiHfOC materials

In addition, Ionescu et al. [29] also chemically modified polyssesquioxane with hafnium tetroxide as a precursor, and then pyrolyzed it to obtain SiOC/HfO₂ ceramics. Figure 7 shows the TEM images of PMS doped with 30% Hf(OⁿBu)₄ at 1300°C. It can be seen that HfO₂ particles are clearly visible and unevenly distributed. At this temperature, the Hf element exists in the form of HfO₂ in SiOC ceramics. Figure 8 shows the TEM images of PMS doped with 30% Hf(OⁿBu)₄ and annealed at 1600°C. Obviously, when the temperature rises to 1600°C, the existing form of the Hf element in SiOC ceramics transforms into HfSiO₄ structure. Ionescu et al. believed that it is precisely because of the formation of this structure that SiOC ceramics doped with Hf element have stronger thermal stability.

Figure 7. (a) TEM photos of PMS doped with 30% Hf(OⁿBu)₄ and annealed at 1300°C and (b) magnified image of the marked area in (a) [29].

Figure 8. (a) TEM photos of PMS doped with 30% Hf(OⁿBu)₄ and annealed at 1600°C and (b) magnified image of the marked area in (a) [29].

SiTiOC materials

Anand et al. [31] prepared SiTiOC ceramics by polymer pyrolysis of polymethylphenyl sesquioxane modified with titanium isopropanol. Figure 9 shows HRTEM images of SiTiOC ceramics (Ti = 20 mol.%) pyrolyzed at 1200°C. It can be seen that at this pyrolysis temperature, SiTiOC ceramics contain β-SiC nanocrystals and TiC nanocrystals. Subsequently, Anand et al. exposed the pure SiOC ceramics and SiTiOC ceramics into the oxidation atmosphere at 1300°C, and measured the weight loss rates of the two materials by thermogravimetry, as shown in Figure 10. The results show that the high-temperature oxidation resistance of SiOC ceramics can also be greatly improved by introducing Ti ions.

Figure 9. The HRTEM images of SiTiOC ceramics (Ti = 20 mol.%) pyrolyzed at 1200°C: (a) general microstructure of SiTiOC, (b) measurement of lattice stripe width using inverse FFT interpolation, (c) β-distribution of SiC nanocrystals in SiOC matrix, and (d) TiC nanoparticles in the SiOC system [31].

Figure 10. Oxidation test of pure SiOC and 20 mol.% titanium doped SiOC samples under flowing oxygen atmosphere by thermogravimetry. All samples were pyrolyzed at 1500°C [31].

SiCeOC materials

In the field of biomaterials, SiOC coatings are also considered to have protective [43] and biological activity [44] in a physiological fluid environment. In addition, the presence of CeO₂ in biomaterials will have a positive impact on the proliferation, differentiation, and further mineralization of bone cells [45]. Moreover, Ce ions will interact with the cell membrane [46], thus promoting antibacterial properties [47] and exhibiting low toxicity [48]. Based on this, Gaweda et al. [30] prepared the precursor containing Ce ion through the sol–gel method, and then pyrolyzed it at 800°C to obtain the SiCeOC coatings. They proved that the addition of a small amount of CeO₂ may improve the corrosion resistance of the SiOC coatings, and offer protection against bacterial organisms.

Above discussion suggests that the properties of SiMOC materials can be improved through addition of Al, Zr, Hf, Ce, Ti, Ta, etc. It is worth noting that the cost of adding these metallic elements varies greatly. However, after addition of these metallic elements, the improvement in the performance of SiMOC materials may not be quite different. In other words, addition of some expensive metallic elements to SiMOC is questionable. The purpose of adding Hf or Al element is to improve the high-temperature resistance of SiMOC materials, however, Hf is more expensive than Al. According to the report, the addition of Al can make SiAlOC materials stable at 1400°C (in air environment) and 1500°C (in inert gas environment). The SiHfOC materials with Hf can be used at 1500–1600°C in the air environment. Therefore, we should consider whether it is worth adding such expensive metals to further improve the performance of SiMOC materials for practical applications.

Preparation methods of SiOC coatings

As mentioned above, there are mainly two ways to prepare SiOC coatings, namely polymer pyrolysis and CVD.

We have investigated a number of SCI (Science Citation Index) papers published on the preparation of SiOC materials (coatings) by polymer pyrolysis and CVD since 2000. It was found that the number of papers published on the two methods is almost the same, but it is worth noting that the papers on the preparation of SiOC materials by CVD are concentrated in the period from 2000 to 2010, while the papers on the preparation of SiOC materials prepared by polymer pyrolysis are concentrated in the period from 2010 to 2022. This indicates that to prepare SiOC materials, the polymer pyrolysis method has occupied the mainstream in recent years, which may be associated with the fact that polymer pyrolysis is very convenient to introduce metal ions into SiOC structure. Therefore, we have reason to believe that the future research direction of SiOC coating will be further towards the addition of metal ions in SiMOC coating.

Polymer pyrolysis

First of all, when preparing SiOC coatings by polymer pyrolysis, additional methods should be taken into account to fix the liquid polymer precursor on the substrate before the next step of pyrolysis. These methods mainly include dip coating, spin coating, etc. [49]. Mazo et al. [50] prepared SiOC coatings on SiC fiber using different dip coating cycles. The surface morphology of SiC fiber with and without SiOC coatings is shown in Figure 11. It can be seen that the SiOC coating, prepared by dip coating, is relatively uniform and smooth, while the thickness of coating increases with the increase in dip coating cycles, thus resulting in increased surface defects.

Figure 11. Surface morphology of silicon carbide fiber (a) and silicon carbide fiber coated with SiOC using different immersion cycles: (b) one cycle, (c and d) two cycles, (e and f) three cycles, and (g and h) four cycles [50].

The advantage of this method is that the operating temperature is low [51] and it is not affected by the complex geometry of the substrate [50]. However, the disadvantages of this polymer pyrolysis method are also obvious which are usually difficult to solve, i.e. the shrinkage phenomenon will inevitably occur when the polymer is converted to ceramics. At high temperatures, if the bubbles in the coating do not come out completely before the end of the cooling, the pores, even cracks and other defects will eventually form in the coating. For this reason, the thickness of SiOC coatings, prepared by this method, is usually not very thick [52]. At present, the thickness of the coating can be increased as much as possible by adding some fillers to the polymer of the front drive [53].

CVD methods

CVD is a chemical reaction process that involves the dissociation or chemical reaction of gas reactants in the activation environment (e.g. heat, light, plasma, etc.) to form stable solid products [54]. There are many varieties of CVD processes, and SiOC coatings or bulk materials prepared by different CVD methods have considerable differences in their microstructure and performance.

Inductively coupled plasma-CVD and capacitively coupled-CVD methods

For example, Oh [55] prepared SiOC coatings by capacitively coupled plasma (CCP)-CVD and inductively coupled plasma (ICP)-CVD and discussed different properties of the two SiOC coatings. It was found that the refractive index of SiOC coatings, prepared by ICP-CVD, is independent of the thickness of the coating, while the refractive index of SiOC coatings (prepared by CCP-CVD with weak dissociation energy) is affected by the thickness of coating deposition.

Hot wire chemical vapor deposition method

In addition, in order to obtain SiOC:H coatings with low dielectric constant (k value) in the future, Godavarthi et al. [56] used hot wire CVD technology to remove the pore-forming agents from the porous SiOC:H coatings, as shown in Figure 12. It can be seen that the deposited coating has good compositional uniformity across the depth. After heat treatment at 1500°C, the C and H elements in the coating are partially removed. At the same time, a certain amount of W element can be found on the surface of the coating. After 1800°C heat treatment, the C and H elements in the coating are almost uniformly removed from the depth direction of the coating. Near the interface between SiOC:H coating and Si matrix, it is found that the content of C and H elements increases slightly. For the coating treated at 2000°C, the C and H elements have been uniformly removed from the coating. On the other hand, the W element also diffuses into the SiOC:H coating, and the higher the hot wire temperature, the higher the W pollution degree.

Figure 12. SIMS (Secondary Ion Mass Spectrometry) distribution of Si, O, and C elements constituting SiOC:H coating, and distribution of W (from W wire) under four conditions: (a) deposition coating, (b) atomic H treatment at filament temperature of 1500°C, (C) atomic H treatment at wick temperature of 1800°C, and (d) atomic H treatment at filament temperature of 2000°C [56].

Laser chemical vapor deposition method

Laser chemical vapor deposition is a CVD method with high deposition efficiency. Yu et al. [57] used this technology to prepare SiOC coatings on graphite substrates. The TEM images and selected diffraction patterns of SiOC coatings deposited at 1775 K and SiC nanocrystals are shown in Figure 13. It can be seen that SiC nanocrystals (containing high-density stacking faults) are uniformly dispersed in amorphous SiOC coatings with a diameter of 100–200 nm.

Figure 13. Transmission electron micrograph and selected area diffraction pattern of SiC nanocrystals in laser chemical vapor deposited SiOC coatings at different magnifications [57].

Plasma enhanced chemical vapor deposition and neutral beam enhanced-CVD methods

Plasma enhanced chemical vapor deposition is a commonly used method to deposit SiOC coatings at low temperatures [58]. However, UV photons and electrons from the plasma may damage the substrate during the etching process. In order to avoid this situation, Yasuhara et al. [59] proposed neutral beam enhanced (NBE)-CVD as a nondestructive method to deposit a layer of SiOC coating on the substrate. This method can also further reduce the dielectric constant (k value) of the coating, as shown in Figure 14. The precursor directly enters the chamber in the device and is adsorbed on the surface of the substrate. It does not cause damage to the surface of the substrate, because the neutral beam Ar bombards the surface without charged particles and UV photons.

Figure 14. Schematic diagram of SiOC coating deposited by NBE-CVD [59].

Metal organic chemical vapor deposition method

Liu et al. [25] prepared SiOC coatings on Si (100) substrates at 1100°C by metal organic chemical vapor deposition (MOCVD). Since the thermal expansion coefficients of SiOC coatings and Si substrate are still different, Liu et al. [25] directly introduced a thin SiOC buffer layer between the thicker SiOC coating and Si substrate, as shown in Figure 15. In a separate study, Liu et al. [24] also verified the excellent corrosion resistance and mechanical properties of SiOC coatings prepared by the MOCVD method.

Figure 15. Transmission electron micrograph of SiOC coating and selected area diffraction pattern of Si substrate and SiOC coating: (a) SiOC-1/Si interface and (b) SiOC-2 coating [25].

RP-CVD method

In addition, Uznanski et al. [60] used tetramethyldisiloxane as raw material to deposit SiOC coatings on silicon substrates by RP-CVD. The schematic diagram of this process is shown in Figure 16. Since the prepared thin SiOC coatings have smaller surface roughness, lower refractive index, and higher hardness, therefore they are expected to be used for the protection or packaging of electronic equipment.

Figure 16. Plasma system diagram [60].

Summary and concluding remarks

This paper introduces the research and development of SiMOC coatings in recent years from three aspects, i.e. (1) the selection of precursors, (2) the addition of metal cations, and (3) the preparation of coatings. Following is the summary and the main conclusions drawn from this article.

1. The method of polymer pyrolysis inevitably involves the shrinkage of polymer ceramics, so the selection and further control of precursors are particularly important. The wettability and curability of PHMS can be improved by modifying PHMS with hydroxyl, alkyl, or other polymer functional groups. In addition, the PDMS can be modified by adding an activator, such as tetraethoxysilane and transition metal alkoxide. This modification can reduce the shrinkage of PDMS when it transforms into ceramics, thereby effectively preventing the coating from cracking. For the CVD method, if the mixture of two or more organic compounds is collectively used as the precursor, wherein each organic compound provides one or more Si, O, and C atoms respectively, the proportion of Si, O, and C atoms in the precursor can be adjusted through the organic compound ratio. Hence, control over elemental composition and performance of the coatings can be achieved.

2. Addition of Al ions, Hf ions, and Ti ions to traditional SiOC coatings significantly improves their thermal stability and high-temperature oxidation resistance of the modified SiMOC coatings. For SiAlOC materials, the doped Al element reduces the C_free in SiAlOC materials, so as to improve the high-temperature resistance of materials. In addition, the SiAlOC protective coatings, with strong corrosion resistance and thermal stability, can be used in solid oxide fuel cells. For SiHfOC materials, SiHfOC materials (doped with Hf elements) have stronger thermal stability, due to the formation of HfSiO₄ structure. Moreover, the addition of Ce ions has significantly improved the corrosion resistance and biological activity of the SiCeOC coatings. Moreover, we should consider whether it is worth adding such expensive metals to further improve the performance of SiOC materials for practical applications.

3. When SiMOC coatings are prepared by polymer pyrolysis, additional methods should be taken into consideration to fix the liquid polymer precursor on the substrate before the start of next pyrolysis process. The advantages of this method include simple operation, low cost, and high deposition efficiency, while the disadvantages are that the prepared coating inevitably develops defects, such as pores, resulting in poor compactness. It is worth noting that the important advantage of this method is that it can be used to prepare SiMOC materials. The CVD technology is usually used to prepare high-quality thin SiOC coatings, because of its low deposition efficiency, high cost, complex preparation process, high quality of the prepared coating, good coating density, and strong regulation of coatings structure and composition. Since the raw materials of CVD must be converted into gas, its biggest disadvantage is that it is difficult to prepare SiMOC materials doped with metallic elements (especially Zr, Hf, etc. with high atomic mass).

4. To sum up, SiMOC coatings have many excellent properties, perfect preparation methods, and great application potential in many fields. The SiMOC materials, doped with metallic elements, usually have better performance than those of un-doped SiOC materials. Therefore, it can be predicted that the future research in the field of SiOC will be carried out in the direction of SiMOC materials doped with metal elements. For practical applications, it is worth thinking about how to balance the cost of metal doping and the improved performance of metal doped SiMOC materials.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the National Key Research and Development Project [Grant Number 2019YFB2005302].