Effect of layer thickness on thermal properties of multilayer thin films produced by PVD

Cr/CrN/CrAlN, CrN/CrAlN and Cr/CrN thin layers were deposited by PVD (Physical Vapor Deposition). The multilayers were obtained from the combined deposition of different layers Cr, CrN and CrAlN thick films on on AISI4140 steel and silicon substrates at 200 °C, and evaluated with respect to fundamental properties such as structure and thermal properties. Cr, CrN and CrAlN single layers were also prepared for comparison purposes. The structural and morphological properties of PVD layers were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with EDS + WDS microanalyses, stresses were determined by the Newton’s rings methods using the Stoney’s equation and surface hardening and hardness profiles were evaluated by micro hardness measurements. The XRD data and HRTEM showed that both the Cr/CrN, CrN/CrAlN and Cr/CrN/CrAlN multilayer coatings exhibited B1NaCl structure with a prominent reflection along (200) plane, and CrAlN sub-layer microstructures composed of nanocrystallites uniformly embedded in an amorphous matrix. The innovation of this work was to use the thickness of three different coating types to determine the thermal properties. Furthermore, an empirical equation was developed for the thermal properties variations with temperature of AISI4140 steel coated with different multilayer coatings. The thermal conductivity of CrAlN single layered was lower than the multilayer and the bulk material AISI4140. Moreover, the influences of structure and composition of the multilayer coatings on the thermal properties are discussed.The thermal conductivity of nanoscale thin film is remarkably lower than that of bulk materials because of its various size effects.     

Cr1−xAlxN coatings deposited by lateral rotating cathode arc for high speed machining applications

In this work, a series of Cr_1−xAl_xN (0 ≤ x ≤ 0.7) coatings were deposited on high speed steel substrates by a vacuum arc reactive deposition process from two lateral rotating elemental chromium and aluminum cathodes in a flowing pure nitrogen atmosphere. The composition, structural, mechanical, and tribological properties of the as-deposited coatings were systematically characterized by energy dispersive analysis of X-rays, X-ray diffraction, nanoindentation, and ball-on-disc tribometer experiments. All of the as-deposited CrAlN coatings exhibited a higher hardness than CrN, showing a maximum hardness of about 40 GPa (at around X = 0.63) which is twice higher than that of the CrN. The wear performance under ambient conditions of the CrAlN coatings was found much better, with both lower friction coefficient and wear rate, than TiAlN coatings deposited by the same technique. The wear rate of the CrAlN coatings against alumina counterpart was about 2-3 orders in magnitude lower than that of the TiAlN coatings. Selected CrAlN coatings with the highest hardness were also deposited on some WC-based end-mills. An evident better performance of the CrAlN-coated end-mills was observed than the TiAlN-coated ones for cutting a hardened tool steel material under high speed machining conditions.     

CrAlN coatings deposited by cathodic arc evaporation at different substrate bias

CrAlN is a good candidate as an alternative to conventional CrN coatings especially for high temperature oxidation-resistance applications. Different CrAlN coatings were deposited on hardened steel substrates by cathodic arc evaporation (CAE) from chromium-aluminum targets in a reactive nitrogen atmosphere at negative substrate bias between − 50 and − 400 V. The negative substrate bias has important effects on the deposition growth rate and crystalline structure. All our coatings presented hardness higher than conventional CrN coatings. The friction coefficient against alumina and tungsten carbide balls was around 0.6. The sliding wear coefficient of the CrAlN coatings was very low while an important wear was observed in the balls before a measurable wear were produced in the coatings. This effect was more pronounced as the negative substrate bias was increased.     

CrN/AlN superlattice coatings synthesized by pulsed closed field unbalanced magnetron sputtering with different CrN layer thicknesses

CrN/AlN superlattice coatings with different CrN layer thicknesses were prepared using a pulsed closed field unbalanced magnetron sputtering system. A decrease in the bilayer period from 12.4 to 3.0 nm and simultaneously an increase in the Al/(Cr + Al) ratio from 19.1 to 68.7 at.% were obtained in the CrN/AlN coatings when the Cr target power was decreased from 1200 to 200 W. The bilayer period and the structure of the coatings were characterized by means of low angle and high angle X-ray diffraction and transmission electron microscopy. The mechanical and tribological properties of the coatings were studied using the nanoindentation and ball-on-disc wear tests. It was found that CrN/AlN superlattice coatings synthesized in the current study exhibited a single phase face-centered cubic structure with well defined interfaces between CrN and AlN nanolayers. Decreases in the residual stress and the lattice parameter were identified with a decrease in the CrN layer thickness. The hardness of the coatings increased with a decrease in the bilayer period and the CrN layer thickness, and reached the highest value of 42 GPa at a bilayer period of 4.1 nm (CrN layer thickness of 1.5 nm, AlN layer thickness of 2.5 nm) and an Al/(Cr + Al) ratio of 59.3 at.% in the coatings. A low coefficient of friction of 0.35 and correspondingly low wear rate of 7 × 10^− 7 mm³N^− 1m^− 1 were also identified in this optimized CrN/AlN coating when sliding against a WC-6%Co ball.     

Deposition of nano-scaled CrTiAlN multilayer coatings with different negative bias voltage on Mg alloy by unbalanced magnetron sputtering

In a magnetron sputtering system, the negative substrate bias voltage has been used as a basic process parameter to modify the deposition structure and properties of coatings. In this paper we report the effect of bias voltage ranging from −40 V to −90 V on nano-scaled CrN/TiN/CrN/AlN (CrTiAlN) multilayer coatings synthesized on a Mg alloy by a closed-field unbalanced magnetron sputtering ion plating system in a gas mixture of Ar + N₂. The technological temperature and atomic concentration in the multilayer coatings were controlled by adjusting the current density of different metal magnetron targets and the plasma optical emission monitor. The composition, crystallographic structure, deposition model and friction coefficient of multilayer coatings were characterized by X-ray photoelectron spectrometry (XPS), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and ball-on-disc testing. The experimental results show that the deposition model and friction coefficient of nano-scaled CrTiAlN multilayer coatings were significantly affected by the negative bias voltage (V_b). The nitride species in multilayer coatings mainly involve CrN, AlN and TiN, and XRD analysis shows that the crystallographic structure was face-centered cubic. Under different bias voltage conditions, the multilayer coating composition shows a fluctuation, and the Al and Cr concentrations respond in the opposite sense to the bias voltage, attaining their greatest values at V_b = −70 V. The surface and cross-sectional morphology shows deposition model change from a columnar model into non-columnar model with the increase in negative bias voltage. The friction coefficient of the nano-scaled multilayer coatings at V_b = −55 V stabilize after 10 000 cycles.     

Oxidation resistance of TiN,CrN, TiAlN and CrAlN coatings deposited by lateral rotating cathode arc

In this paper, four kinds of hard coatings, TiN, CrN, TiAlN and CrAlN (with Al/Ti or Al/Cr atomic ratio around 1:1), were deposited on stainless steel substrates by a lateral rotating cathode arc technique. The as-deposited coatings were annealed in ambient atmosphere at different temperatures (500–1000 °C) for 1 h. The evolution of chemical composition, microstructure, and microhardness of these coatings after annealing at different temperatures was systematically analyzed by energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and nanoindentation experiments. The oxidation behaviour and its influence on overall hardness of these four coatings were compared. It was found that the ternary TiAlN and CrAlN coatings have better oxidation resistance than their binary counterparts, TiN and CrN coatings. The Cr-based coatings (CrN and CrAlN) exhibited evidently better oxidation resistance than the Ti-based coatings (TiN and TiAlN). TiN coating started to oxidize at 500 °C. After annealing at 700 °C no N could be detected by EDX, indicating that the coating was almost fully oxidized. After annealed at 800 °C, the coating completely delaminated from the substrate. TiAlN started to oxidize at 600 °C. It was nearly fully oxidized (with little residual nitrogen detected in the coating by EDX) and partially delaminated at 1000 °C. Both CrN and CrAlN started to oxidize at 700 °C. CrN was almost fully oxidized (with little residual nitrogen detected in the coating by EDX) and partially delaminated at 900 °C. The oxidation rate of the CrAlN coating is quite slow. After annealing at 1000 °C, only about 19 at.% oxygen was detected and the coating showed no delamination. The Ti-based (TiN and TiAlN) coatings were not able to retain their hardness at higher temperatures (≥ 700 °C). On the other hand, the hardness of CrAlN was stable at a high level between 33 and 35 GPa up to an annealing temperature of 800 °C and still kept at a comparative high value of 18.7 GPa even after annealed at 1000 °C, indicating a very promising applicability of this coating for high speed dry machining and other applications under high temperature environments.     

Influence of bi-layer period thickness on the residual stress, mechanical and tribological properties of nanolayered TiAlN/CrN multi-layer coatings

TiAlN/CrN nanoscale multi-layered coatings have been deposited using cathodic arc evaporation system. The coatings were deposited using one Ti₅₀Al₅₀ alloy target and one Cr target with a fixed target power in all the processes, while the bi-layer thickness was varied by various rotation speeds of the substrate holder in order to produce different nanoscale multi-layered period thickness. The texture structure, residual stress, and nanoscale multi-layer period thickness of the coatings were determined by X-ray diffraction using both Bragg-Brentano and glancing angle parallel beam geometries. Hardness and adhesion strength of the coatings were measured by Nano-indentation and Rockwell-C indentation methods, respectively. It has been found that the structural and mechanical properties of the films correlate with nano-scaled bi-layer thickness and crystalline texture. The maximum hardness of nano-scaled TiAlN/CrN multi-layered coatings was approximately 36 GPa with highest residual stress of −6.2 GPa, for a bi-layer thickness ranging from 6 to 12 nm.     

Enhancement of mechanical and tribological properties in AISI D3 steel substrates by using a non-isostructural CrN/AlN multilayer coating

Enhancement of mechanical and tribological properties on AISI D3 steel surfaces coated with CrN/AlN multilayer systems deposited in various bilayer periods (Λ) via magnetron sputtering has been studied in this work exhaustively. The coatings were characterized in terms of structural, chemical, morphological, mechanical and tribological properties by X-ray diffraction (XRD), electron dispersive spectrograph, atomic force microscopy, scanning and transmission electron microscopy, nanoindentation, pin-on-disc and scratch tests. The failure mode mechanisms were observed via optical microscopy. Results from X-ray diffraction analysis revealed that the crystal structure of CrN/AlN multilayer coatings has a NaCl-type lattice structure and hexagonal structure (wurtzite-type) for CrN and AlN, respectively, i.e., made was non-isostructural multilayers. An enhancement of both hardness and elastic modulus up to 28 GPa and 280 GPa, respectively, was observed as the bilayer periods (Λ) in the coatings were decreased. The sample with a bilayer period (Λ) of 60 nm and bilayer number n  =  50 showed the lowest friction coefficient (∼0.18) and the highest critical load (43 N), corresponding to 2.2 and 1.6 times better than those values for the coating deposited with n = 1, respectively. The best behavior was obtained when the bilayer period (Λ) is 60 nm (n = 50), giving the highest hardness 28 GPa and elastic modulus of 280 GPa, the lowest friction coefficient (∼0.18) and the highest critical load of 43 N. These results indicate an enhancement of mechanical, tribological and adhesion properties, comparing to the CrN/AlN multilayer systems with 1 bilayer at 28%, 21%, 40%, and 30%, respectively. This enhancement in hardness and toughness for multilayer coatings could be attributed to the different mechanisms for layer formation with nanometric thickness such as the Hall–Petch effect and the number of interfaces that act as obstacles for the crack deflection and dissipation of crack energy.     

Structure, hardness and thermal stability of nanolayered TiN/CrN multilayer coatings

Nanolayered TiN/CrN multilayer coatings were deposited on silicon substrates using a reactive DC magnetron sputtering process at various modulation wavelengths (Λ), substrate biases (V_B) and substrate temperatures (T_S). X-ray diffraction (XRD), nanoindentation and atomic force microscopy (AFM) were used to characterize the coatings. The XRD confirmed the formation of superlattice structure at low modulation wavelengths. The maximum hardness of the TiN/CrN multilayers was 3800 kg/mm² at Λ=80  Å, V_B=−150 V and T_S=400°C. Thermal stability of TiN, CrN and TiN/CrN multilayer coatings was studied by heating the coatings in air in the temperature range (T_A) of 400-800°C. The XRD data revealed that TiN/CrN multilayers retained superlattice structure even up to 700°C and oxides were detected only after T_A?750°C, whereas for single layer TiN and CrN coatings oxides were detected even at 550°C and 600°C, respectively. Nanoindentation measurements showed that TiN/CrN multilayers retained a hardness of 2800 kg/mm² upon annealing at 700°C, and this decrease in the hardness was attributed to interdiffusion at the interfaces.     

Nanoindentation study of magnetron-sputtered CrN and CrSiN coatings

CrN and CrSiN coatings were deposited on stainless steel substrate by reactive magnetron sputtering. The coatings were characterized for phases, chemical composition, microstructure, and mechanical properties by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM)/energy dispersive spectroscopy (EDS), atomic force microscopy (AFM), and nanoindentation technique, respectively. The cubic phase was the only phase observed in both the coatings as observed in XRD results. A dense morphology was observed in these coatings deposited with high nitrogen and Si contents, 50:50 and 18.65 at.%, respectively. Nanoindentation measurement of CrN coatings, with Ar + N₂ proportions of 60:40, showed maximum hardness (H) and modulus (E) of 21 ± 0.85 GPa and 276 ± 13 GPa, respectively. The CrN coatings deposited in pure N₂ atmosphere showed H and E values of 27 ± 1.62 and 241 ± 10 GPa, respectively. The measured H and E values of CrSiN coatings were found to be 28 ± 1.40 GPa and 246 ± 10 GPa, respectively. The improved hardness in both the coatings was attributed mainly to a reduction in crystallite size, decrease in surface roughness, and dense morphology. The incorporation of Si into the CrN coatings has improved both hardness and Young’s modulus.     

Thermal stability of nitride coatings formed by ion-plasma deposition

Titanium nitride and chromium nitride coatings were formed on a gray cast iron by condensation from a plasma phase in vacuum with the ion bombardment of the sample surface by titanium or chromium plasma flows in a residual nitrogen atmosphere. The element and phase composition of coatings were studied before and after annealing for 1-43 h in air at temperature 700 °C using Auger electron spectroscopy (AES) and X-ray diffraction (XRD). It is established that titanium nitride coatings are single-phase TiN system with a (1 1 1) preferred growth orientation, and chromium nitride coatings—a two-phase system: CrN and Cr₂N phases. The annealing of coatings at the atmospheric pressure and the temperature of 700 °C in the range of 1-43 h results in the deceleration of the oxidation process of the material substrate for chromium nitride with respect to titanium nitride coatings.     

An analysis of macroparticle-related defects on CrCN and CrN coatings in dependence of the substrate bias voltage

Chromium nitride coatings with and without a carbon content being assigned as CrCN and CrN were prepared by cathodic arc evaporation. The effect of negative substrate bias voltages (10-300 V) on the microstructure, phase composition and morphology of the coating surface was studied. X-ray diffraction data show that almost all coatings crystallized in the cubic structure with (111) and (200) diffraction lines appearing only for low negative bias voltage and a (220) diffraction line being present for the coatings deposited at higher negative bias voltages. For CrN coatings obtained at −300 V a hexagonal structure was also observed. In case of CrCN coatings the (220) diffraction line shows much higher intensity than in case of CrN coatings and was significantly broadened. On the surface of the coatings a large number of macroparticles of different size was observed. An increase of bias voltage causes a reduction of the areal density of macroparticles and a decrease of the mean surface roughness Ra.     

Residual stresses and thermal fatigue in CrN hard coatings characterized by high-temperature synchrotron X-ray diffraction

The aim of this work is to analyze thermal fatigue in hard coatings/substrate composites (i) during slow heating and cooling and (ii) after local cyclic thermal laser pulse experiments. As a model system, CrN coatings with a thickness of 3 µm deposited on steel, hard metal and Si(100) substrates using reactive magnetron sputtering at a temperature of 350 °C are used. The coatings are at first characterized by means of in-situ high-temperature X-ray diffraction (XRD) using a commercially available temperature attachment and by applying heating and cooling rates of less than 0.3 °C/s. The treatment results in the expected reduction of intrinsic stresses which are independent of substrate material but strongly influenced by substrate roughness. To simulate local thermal fatigue, selected coating/substrate composites are thermally cycled using a laser beam of 6 mm in diameter in a temperature range of 50-850 °C applying up to 10⁴ cycles and using heating and cooling rates of about 10³ °C/s. Subsequently, laser cycled samples are analyzed using synchrotron XRD, scanning electron microscopy and focused ion beam technique. Laser pulses cause a reduction of compressive stresses in the coatings and a development of tensile stresses in the substrates accompanied by formation of cracks and ripples. The results show that the changes of the local macro- and micro-strains/stresses in the coatings and in the underlying substrates are strongly interlinked. The stress relaxation in the coatings is caused by recovery effects, by micro-cracks formed in the tensely-stressed coating and by plastic deformation of the metallic substrates.     

氮化物硬质涂层中Cr、Ti和Al元素对摩擦磨损特性的影响

利用四靶闭合场非平衡磁控溅射(CFUBMS)技术在石英玻璃和抛光不锈钢片两种基底上制备含有Cr、Ti和Al元素组合的各种氮化物涂层.采用摩擦磨损仪测试涂层摩擦系数,应用金相显微镜对各个涂层磨痕形态进行分析,结果表明TiN、CrN、TiAlN、CrAlN以及CrTiAlN涂层的摩擦系数依次减小,耐磨特性依次提高;结合涂层的X射线光电子能谱分析,可以得到含有Al元素涂层中形成了AlN的结构,提高涂层的硬度,增加耐磨特性;在涂层中含有Cr元素形成了氧化物Cr2O3可以提高涂层自排屑能力,减小摩擦系数,增加耐磨特性,含Ti元素形成的氧化物TiO2则不利于涂层的摩擦磨损特性;由于CrTiAlN本身具有比三元氮化物更高的涂层硬度,且含有Al和Cr元素,因此该涂层具有最好的摩擦磨损性能.     

Nanometric-layered CrN/TiN thin films: mechanical strength and thermal stability

Nanometric-layered CrN/TiN coatings were deposited using unbalanced magnetron sputtering. The layered coating structure was characterised by X-ray diffractometry, and the mechanical properties were measured by nano-indentation and scratch test. High temperature annealing at 400-750 °C was carried out to investigate the thermal stability of the coating structure and mechanical properties. For comparison, samples of TiN and CrN deposited under similar conditions were also annealed and tested. The results showed that nano-layered CrN/TiN has excellent mechanical and thermal properties. Nano-hardness of 40 GPa and scratch adhesion of 80 N were achieved at a wavelength of 7.5 nm and a substrate bias of −80 V. The coating demonstrates application prospects in the stamping/cutting tools industry.     

A study of the nanostructure and hardness of electron beam evaporated TiAlBN Coatings

TiAlBN coatings have been deposited by electron beam (EB) evaporation from a single TiAlBN material source onto AISI 316 stainless steel substrates at a temperature of 450 °C and substrate bias of − 100 V. The stoichiometry and nanostructure have been studied by X-ray photoelectron spectroscopy, X-ray diffraction and transmission electron microscopy. The hardness and elastic modulus were determined by nanoindentation. Five coatings have been deposited, three from hot-pressed TiAlBN material and two from hot isostatically pressed (HIPped) material. The coatings deposited from the hot-pressed material exhibited a nanocomposite nc-(Ti,Al)N/a-BN/a-(Ti,Al)B₂ structure, the relative phase fraction being consistent with that predicted by the equilibrium Ti-B-N phase diagram. Nanoindentation hardness values were in the range of 22 to 32 GPa. Using the HIPped material, coating (Ti,Al)B_0.29N_0.46 was found to have a phase composition of 72-79 mol.% nc-(Ti,Al)(N,B)_1 − x+ 21-28 mol.% amorphous titanium boride and a hardness of 32 GPa. The second coating, (Ti,Al)B_0.66N_0.25, was X-ray amorphous with a nitride+boride multiphase composition and a hardness of 26 GPa. The nanostructure and structure-property relationships of all coatings are discussed in detail. Comparisons are made between the single-EB coatings deposited in this work and previously deposited twin-EB coatings. Twin-EB deposition gives rise to lower adatom mobilities, leading to (111) (Ti,Al)N preferential orientation, smaller grain sizes, less dense coatings and lower hardnesses.     

Influence of silicon content on the microstructure and hardness of CrN coatings deposited by reactive magnetron sputtering

CrN and CrSiN films were deposited on the stainless steel and silicon substrates by DC magnetron sputtering and their microstructural features were investigated by X-ray diffraction (XRD), scanning electron microscope (FE-SEM/EDS), and atomic force microscopy (AFM). The influence of Si content along with process parameters such as power on the microstructural characteristics of Cr–Si–N and CrN films were investigated and compared between each other. The power and increasing Si contents strongly influence the microstructural and hardness of the deposited films. XRD analysis of the coatings indicates a grain refinement with increase in Si content during deposition of coatings, which is tandem with AFM and SEM results. Also, the surface roughness and particle size are decreasing with addition of Si in the films. The hardness of CrN and CrSiN was measured by microhardness tester and found that introduction of Si content in the CrN system increases its hardness from 1839 Hv to 2570 Hv.     

Deposition and characterization of TiAlN/Si3N4 superhard nanocomposite coatings prepared by reactive direct current unbalanced magnetron sputtering

Superhard nanocomposite coatings of TiAlN/Si₃N₄ with varying silicon contents were synthesized using reactive direct current (DC) unbalanced magnetron sputtering. The Si and TiAl targets were sputtered using an asymmetric bipolar-pulsed DC power supply and a DC power supply, respectively, in Ar+N₂ plasma. The structural and mechanical properties of the coatings were characterized using X-ray diffraction (XRD) and nanoindentation techniques, respectively. The elemental composition of the TiAlN/Si₃N₄ nanocomposite coatings was determined using energy-dispersive X-ray analysis and the bonding structure was characterized by X-ray photoelectron spectroscopy. The surface morphology of the coatings was studied using atomic force microscopy. The XRD data showed that the nanocomposite coatings exhibited (1 1 1) and (2 0 0) reflections of cubic TiAlN phase. The broadening of the diffraction peaks with an increase in the silicon content in the nanocomposite coatings, suggested a decrease in the average crystallite size. The TiAlN/Si₃N₄ nanocomposite coatings exhibited a maximum hardness of 43 GPa and an elastic modulus of 350 GPa at a silicon concentration of approximately 11 at%. The hardness and the elastic modulus of the nanocomposite coatings decreased significantly at higher silicon contents. Micro-Raman spectroscopy was used to characterize the structural changes as a result of heating of the nanocomposite coatings in air (400-850 °C) and in vacuum (900 °C). The Raman data of the nanocomposite coatings annealed in air and vacuum showed better thermal stability as compared to that of the TiAlN coatings. Similarly, the nanocomposite coatings deposited on mild steel substrates exhibited improved corrosion resistance.     

Effect of grain size on mechanical properties in CrAlN/SiNx multilayer coatings

Both CrAlN and SiN_x coatings were deposited sequentially by RF magnetron sputtering. During sputtering, thickness of SiN_x layer was set to be 1 nm, while that of CrAlN layer was controlled to be 4, 20, 40, 100, and 200 nm. According to XRD results, it was revealed that grain size of the CrAlN coatings increased from 3.6 nm to 24.2 nm with the increasing thickness. From HRTEM images, the variation on grain size was attributed to the amorphous SiN_x layer, which significantly retarded the continuous growth of CrAlN layer. Hardness of the CrAlN/SiN_x coatings with various bilayer thicknesses was measured by nanoindentation. The relationship between grain size and hardness could be interpreted by the Hall-Petch equation, and an improved hardness around 32 GPa was achieved.     

Corrosion behaviour of sol–gel coatings doped with cerium salts on 2024-T3 aluminum alloy

The corrosion behaviour of the sol–gel coatings doped with cerium chloride or cerium nitrate on 2024-T3 aluminum alloy was investigated by using electrochemical impedance spectroscopy (EIS) and immersion tests. The sol–gel matrix was obtained through hydrolysis, condensation of 3-glycidoxypropyltrimethoxysilane (GPTMS) and tetramethoxysilane (TMOS), using diethylentriamine as curing agent. The results indicated that cerium nitrate with concentration of 1 × 10⁻³ mol L⁻¹ in the silane solution was excellent on self-healing for the sol–gel coating, while cerium chloride had no obvious effect. This result suggested that the introduction of Cl⁻ promoted the under-film pitting of 2024-T3 substrate. It was found that Ce(OH)₃ and Ce(OH)₂²⁺ simultaneously existed in the silane solution by X-ray diffraction (XRD) analysis. Ce(OH)₂²⁺ transformed to CeO₂ due to high-temperature curing of sol–gel matrix demonstrated by X-ray photoelectron spectroscopy (XPS) analysis. Therefore, it can be considered that Ce(OH)₃ and CeO₂ played inhibition roles in the corrosion process of the sol–gel coatings.