Si Coatings Research Articles

Structural Features of the Cr3C2–NiCr and Ni–Cr–Fe–B–Si Coatings Produced by Multichamber Detonation Spraying

Structural Features of the Cr3C2–NiCr and Ni–Cr–Fe–B–Si Coatings Produced by Multichamber Detonation Spraying

Low-temperature oxidation behavior and mechanism of hot-dip Al and Al[sbnd]Si coatings on Mo substrate at 600 °C in static air

The hot-dip Al and AlSi coatings are synthesized on Mo substrate at different temperatures. The microstructure and low-temperature oxidation behavior both of the coatings are investigated. The results indicate that hot-dip Al coating is mainly made up of an Al4Mo outer layer and an Al8Mo3 interface layer, while the hot-dip AlSi coating is consists of a Mo(Si, Al)2 inner layer and AlSi alloy outer layer containing Mo(Si, Al)2 grains. After exposed at 600 °C for 40 h, a large number of cracks initiate on surface and inside of the hot-dip Al coating, its mass gain per unit area (Δm) reach to 10.32 mg/cm2, and the RSa (average surface roughness) and Sdr (surface area growth rate) values increase from 0.216 μm and 6.602% before oxidation to 1.214 μm and 35.297%, respectively. During the oxidation process, the structure of the hot-dip AlSi coating is preserved intact, and almost no cracks are observed in the coating. After oxidized at 600 °C for 150 h, the Δm of hot-dip AlSi coating is only 1.56 mg/cm2, and the RSa and Sdr values increase from 0.458 μm and 7.083% to 1.509 μm and 43.586%, respectively. Besides, the oxidation rate constant (Kp) of hot-dip Al and AlSi coatings are 7.42 × 10−4 and 4.47 × 10−6 mg2·cm−4·s−1, respectively, and the latter is only 6.17 × 10−3 times than that of the former. The excellent oxidation resistance of hot-dip AlSi coating is attributed to its small grain size, stable and dense coating structure, and complex oxide layer composition.

Characteristics of Si (C,N) Silicon Carbonitride Layers on the Surface of Ni-Cr Alloys Used in Dental Prosthetics.

Chromium- and cobalt-based alloys, as well as chrome-nickel steels, are most used in dental prosthetics. Unfortunately, these alloys, especially nickel-based alloys, can cause allergic reactions. A disadvantage of these alloys is also insufficient corrosion resistance. To improve the properties of these alloys, amorphous Si (C,N) coatings were deposited on the surfaces of metal specimens. This paper characterizes coatings of silicon carbide nitrides, deposited by the magnetron sputtering method on the surface of nickel-chromium alloys used in dental prosthetics. Depending on the deposition parameters, coatings with varying carbon to nitrogen ratios were obtained. The study analyzed their structure and chemical and phase composition. In addition, a study of surface wettability and surface roughness was performed. Based on the results obtained, it was found that amorphous coatings of Si (C,N) type with thicknesses of 2 to 4.5 µm were obtained. All obtained coatings increase the value of surface free energy. The study showed that Si (C,N)-type films can be used in dental prosthetics as protective coatings.

Substructural Changes in the Surface Layers of Composites Gas-Thermal Cu–(Al–Si) Coatings During Friction under Various Conditions

Substructural Changes in the Surface Layers of Composites Gas-Thermal Cu–(Al–Si) Coatings During Friction under Various Conditions

Effect of pre-oxidation and sea salt on the hot corrosion behavior of MCrAlY coatings and Al[sbnd]Si coatings

This paper investigated the hot corrosion behavior of two types of coatings, namely NiCoCrAlYHfSi and AlSi coatings, used in gas turbines operating in a moderate-temperature sea salt environment. Prior to the corrosion experiment, pre-oxidation treatment was employed to generate a continuous α-Al2O3 thin layer on the surface of the coatings, leading to a significant reduction in coating roughness. The results demonstrated distinct failure behaviors of the two coatings in the presence of sea salt, providing insights into the mechanism of hot corrosion. The NiCoCrAlYHfSi coating effectively resisted corrosion through a mixed oxide layer consisting of Al2O3 and Cr2O3, while the AlSi coating utilized the continuous formation of Al2O3 to combat corrosion. In comparison to the NiCoCrAlYHfSi coating, the AlSi coating exhibited superior resistance to sea salt erosion. In addition, pre-oxidation is able to effectively reduce the erosion of the coating by sea salt, thereby extending the lifespan of the NiCoCrAlYHfSi coating and the AlSi coating by 40 % and 30 % respectively.

Novel Cr/Si-Slurry Diffusion Coatings for High Temperatures

Surface enrichment in Al, Si, and Cr can greatly improve high temperature oxidation resistance of many alloys. Al, Si, and Cr coatings are commonly applied via simple slurries or more complex pack cementation processes. Due to the high melting point of Cr, the deposition of Cr-based diffusion coatings by the slurry technique has proved challenging, and to date, Cr has mostly been applied by pack cementation. Here, a novel Cr-Si coating process via the slurry technique is described which has been developed and then demonstrated on two Ni-based superalloys, Rene 80 and Inconel 740H. The addition of Si to the slurry lowers the melting point via a Cr-Si eutectic and enables the formation of a liquid phase during heat treatment. Through this Cr-Si slurry coating process diffusion layers enriched by Cr and Si of about 150 µm were achieved. Oxidation behavior was studied through isothermal exposures at 900 °C for 1000 h in lab air. Uncoated Rene 80 and IN740H both showed formation of a Ti-containing Cr2O3 scale below a thin TiO2 top layer. Underneath the external scale a zone of internally oxidized Al grew over the exposure time and reduced the load-bearing cross-section progressively. In comparison, the Cr/Si-coated samples did not show internal Al oxidation, but a slow-growing Si-rich oxide film underneath the external Cr2O3 scale. This subscale represents an additional oxygen diffusion barrier. Thus, the weight gain during exposure for the coated samples was significantly lower than for the uncoated materials.

Formation of silicon carbide nanowire coatings on C/C composites using an in situ catalytic strategy for improved oxidation resistance

A low-cost in situ catalytic reaction strategy was employed to successfully prepare SiC–Si coatings on the C/C matrix. The addition of catalysts enabled the fabrication of SiC nanowire coatings at lower temperatures. The effect of catalyst amount (Fe(NO3)3·9H2O) and preparation temperature on the interface microstructure and oxidation resistance of SiC–Si coated C/C composites was systematically investigated. Results demonstrated that the nanowires with a addition of Fe(NO3)3–9H2O at 2 wt% showed maximum density and yield. Additionally, it was observed that the oxidation resistance of the coatings exhibited an initial rise followed by a subsequent decline as the sintering temperature increased. The superior oxidation resistance of the coatings obtained at 1300 °C can be attributed to the in-situ formation of a substantial silicon oxide layer on the surface. This layer effectively fills the cracks and pores within the coating. Furthermore, the failure of the coating was mainly attributed to oxygen diffusion through the microcracks and pores.

Influence of calcium on the morphology and corrosion performance of hot-dip Al–Si coatings

Hot-dip aluminizing coatings are gaining attention owing to their corrosion resistance under harsh conditions and economic viability. Although Ca incorporation into metal alloy coatings has been demonstrated to improve the corrosion resistance, Ca addition to hot-dip aluminizing coatings is yet to be reported. Here, we investigated the effect of the Ca content on the microstructures and electrochemical properties of hot-dip aluminizing coatings on commercial quality steel. Defect-free coatings were obtained by optimizing the hot-dipping parameters. Ca promoted the precipitation of CaAl2Si2, altering the morphology of the coating and improving the corrosion resistance. However, owing to the limited stability of the passive layer, the inclusion of Ca did not significantly affect the long-term corrosion characteristics of the Al alloy coating.

Influence of antireflection Si coatings on the damage threshold of fused silica upon irradiation with mid-IR femtosecond laser pulses.

Recent progress in the development of high-power mid-IR laser sources and the exciting laser driven physical phenomena associated with the irradiation of solids via ultrashort laser pulses in that spectral region are aimed to potentially create novel capabilities for material processing. In particular, the investigation of the underlying physical processes and the evaluation of the optical breakdown threshold (OBT) following irradiation of bulk dielectric materials with mid-IR femtosecond (fs) pulses have been recently presented. In this Letter, we will explore the conditions that generate sufficient carrier excitation levels which lead to damage upon irradiation of a dielectric material (SiO2) coated with antireflection (AR) semiconducting films (Si) of variable thickness with fs pulses. Simulation results demonstrate that the reflectivity and transmissivity of the Si/SiO2 are thickness dependent which can be employed to modulate the damage threshold of the substrate. The study is to provide innovative routes for selecting material sizes that can be used for antireflection coatings and applications in the mid-IR region.

In situ characterization of high-temperature mechanical behaviors of freestanding Si, BSAS and Yb2SiO5 environmental barrier coatings by three-point bending tests based on digital image correlation

The high-temperature mechanical properties and failure mechanisms of Si, BSAS and Yb2SiO5 ceramic materials are important for environmental barrier coatings. The Si coating, BSAS coating and Yb2SiO5 coating were prepared by the atmospheric plasma spray (APS) technique. The elastic modulus, fracture strength and fracture strain are determined by three-point bending combined with the digital image correlation (DIC) method. The results show that as the test temperature increases from 25 °C to 1000 °C, the elastic modulus of the Si coating and Yb2SiO5 coating decreases while the elastic modulus of the BSAS coating increases. And the fracture strength and fracture strain of the Si coating, BSAS coating and Yb2SiO5 coating increase. The elastic modulus is related to the interatomic distance and bonding strength of atoms. The fracture strength and fracture strain increase with the densification of the coating.

Influence of Silicon and Chromium on the Na2SO4-Induced Hot Corrosion Behavior of Titanium Alloys

Titanium alloys are widely used as construction materials in the aerospace and automotive industries. They have many advantages but also have limitations related to their susceptibility to high-temperature oxidation and hot corrosion. Many efforts to increase the lifetime of components made of titanium alloys have been reported in the literature; the most promising ones involve the deposition of coatings. The present paper is focused on the development of coatings containing chromium and silicon, and their further evaluation in hot corrosion tests. It was proved that the Cr-Si coatings were more effective than Si coatings alone in protecting the titanium alloys against Na2SO4-induced hot corrosion at 800 °C. The enhanced corrosion resistance was attributed to the preferential formation of a thick and continuous SiO2 layer on the surface and—in the case of titanium aluminide alloy—the growth of an Al2O3-rich inner layer of the scale, promoted by chromium.

Microstructure Characterization of Al–Si Coatings on Hot Stamping Steel under Different Heat Treatment Processes

The microstructures of Al–Si coatings under different heat treatment processes were characterized by scanning electron microscopy and transmission electron microscopy. The research shows that with the increase of heat treatment temperature, the Fe element in the matrix continuously diffuses into the Al–Si coating, the higher the heat treatment temperature, the more sufficient the diffusion, which promotes the evolution of the intermetallic compounds in the Al–Si coating. The EBSD Kikuchi pattern calibration and the transmission electron microscope multi-band axis calibration method can accurately determine the various phases of the Al–Si coating at different heat treatment temperatures, especially the phases with the same or similar crystal structure. The phases of Al–Si coating without heat treatment are Al, Si, Al13Fe4(θ) and Al167.8Fe44.9Si23.9(τ5); when the heat treatment temperature increased to 500°C, the phase of the Al–Si coating remained basically unchanged, and only part of Al167.8Fe44.9Si23.9(τ5) evolved into Al9Fe2Si2(τ6); when the heat treatment temperature increased to 600°C, Al5Fe2(η) was newly formed in the Al–Si coating and Al2Fe3Si3(τ1) was precipitated on its edge; when the heat treatment temperature increased to 700°C, the Al2Fe3Si3(τ1) precipitated evolved into AlFe in the Al–Si coating; when the heat treatment temperature increased to 800°C, the diffusion layer (α-Fe) is formed in the Al–Si coating next to the iron substrate; When the heat treatment temperature increased to 900°C, the phase of the Al–Si coating is basically the same as the former, but its morphology changes significantly, the diffusion layer is significantly thicker, and the band-shaped AlFe phase is obviously discretized.

Characterization and Control of Residual Stress in Plasma-Sprayed Silicon Coatings on SiC/SiC Composites

In order to reveal the relationship between residual stress in Si layers of SiC/SiC composites and the different parameters used in their preparation, the residual stress of the coating surface was tested using X-ray sin2ψ technology and laser Raman spectroscopy. Then, the Raman shift–stress coefficient (P) and the Raman shift with free stress (ω0) were calculated as −201.41 MPa/cm−1 and 520.591 cm−1 via linear fitting with the least squares method. The results showed that all the as-sprayed Si coatings exhibited tensile stress on the surface, ranging from 53.5 to 65.9 MPa. The parameters of the spraying distance and second gas (H2) flow rate were considered to be the most important for controlling the residual stress on the coating surface. Additionally, the surface tensile stress of the Si layers could be eliminated and even changed into compressive stress by annealing above 800 °C. Furthermore, the residual stress distribution in the cross-section of the Si layers was evaluated using laser Raman spectroscopy. Additionally, the particle characteristics, such as in-flight velocity and temperature, were investigated using a diagnostic system. The results of this research contribute to increasing the understanding and control of residual stress in APS Si bond layers.

Wear behavior and atomic competition mechanism of flexible wear-resistant coating with oxygen-free zone in Ti6Al4V alloy

The flexible wear-resistant coating on Ti6Al4V substrate was successfully fabricated by plasma powder cladding with Al–Si powder. The microstructure, element distribution and phase component of the flexible wear-resistant coating were characterized by the field emission scanning electron microscope (FESEM), energy dispersive spectroscopy (EDS) and the X-ray diffractometer (XRD). The micro-hardness of coating was measured by micro-hardness tester. The wear-resistance of Al–Si coating was evaluated by sliding friction experiment. The prepared coatings have no cracks and a good metallurgical bonding (MB) with the substrate. The wear-resistance of the coating surface is about twice as much as that of Ti6Al4V substrate. Furthermore, the in-between MB zone has the more excellent wear-resistance (about 7 times). The oxygen free zone can be formed in the MB zone, providing a good toughness for the flexible wear-resistant coating. The wear mode is adhesive wear for the flexible wear-resistant Al–Si coatings. Based on the self-consistent bond length difference (SCBLD) method, the competition among atoms in MB zone as well as the formation mechanism of flexible oxygen free zone is investigated.

Effects of carbon addition on Ti–Al–Si coatings synthesized by self‐generated gradient hot‐dipping infiltration method

AbstractThe effects of carbon addition on the microstructure and oxidation resistance of Ti–Al–Si coatings synthesized by the self‐generated gradient hot‐dipping infiltration method were studied. The microstructure, phase structure, as well as composition of the coating, were observed by using X‐ray diffraction, scanning electron microscope, and energy dispersive spectrometer. It was found that when carbon was added into the Al‐SiO2 system, the coating was composed of L‐(Al,Si) phase and Ti(Al,Si)3 phase. The thickness of the Ti(Al,Si)3 phase layer increased along with the dipping time. The cycle oxidation tests at 900°C for 160 h showed that the weight gains of the coating samples were only 8.1% of that of the Ti–6Al–4V alloy samples. This result indicates that the coatings remarkably enhanced the oxidation resistance of Ti–6Al–4V alloy, which could be ascribed to two reasons: (1) the mixed oxidation zone of Al2O3, Ti(Al,Si)3, and Ti5Si3 on top of the oxidized coating could act as a barrier to retard the diffusion of oxygen to the substrate; and (2) several new phase layers (Ti3Al, TiAl, and Ti5Si3 + TiAl2) near the substrate during oxidation could reduce the cracks at the bottom of the oxidized coating.

Influence of Si and SiC Coating on the Microstructures and Mechanical Properties of C/C Bolts

High-performance bolts made of carbon/carbon (C/C) composites are necessary for connecting thermally-insulating structural components of aerospace vehicles. To enhance the mechanical properties of the C/C bolt, a new silicon infiltration-modified C/C (C/C-SiC) bolt was developed via vapor silicon infiltration. The effects of silicon infiltration on microstructure and mechanical properties were systematically studied. Findings reveal that dense and uniform SiC-Si coating has been formed after silicon infiltration of the C/C bolt, strongly bonding with the C matrix. Under tensile stress, the C/C-SiC bolt undergoes a tensile failure of studs, while the C/C bolt is subject to the pull-out failure of threads. The breaking strength of the former (55.16 MPa) is 26.83% higher than the failure strength of the latter (43.49 MPa). Under double-sided shear stress, both the crushing of threads and the shear failure of studs occur within two bolts. As a result, the shear strength of the former (54.73 MPa) exceeds that of the latter (43.88 MPa) by 24.73%. According to CT and SEM analysis, matrix fracture, fiber debonding, and fiber bridging are the main failure modes. Therefore, a mixed coating formed by silicon infiltration can effectively transfer loads from coating to carbon matrix and carbon fiber, thereby enhancing the load-bearing capacity of C/C bolts.

EFFECTS OF PRE-OXIDATION ON HOT CORROSION RESISTANCE OF Al–Si COATINGS

In this study, effects of pre-oxidation on the hot corrosion behaviour of Al–Si coatings in a Na2SO4 and 25 w/% NaCl mixture at 900 °C were investigated. The results showed that after 120 h of corrosion, a large amount of β-NiAl residual was found with no obvious sulphides or oxidation in the pre-oxidised Al–Si coating. The pre-formed oxide film on the pre-oxidised coating prevented external O2– and species such as S2– and Cl– from the molten salts from directly contacting the coating and reduced the rate of TiO2 formation in the oxide layer, thus forming an Al2O3-based oxide film and reducing the growth rate of the oxide film on the coating surface. Cracking and peeling of the oxide film, caused by the internal stress owing to the difference in the thermal expansion coefficient between the oxides in the oxide layer, are suppressed; thus, the hot corrosion resistance of the coating is significantly improved.
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Suppressing the phase-transition-induced cracking of SiO2 TGOs by lattice solid solution

The Si coating lifetime at 1350 ℃ is limited by the phase-transition-induced cracking of the SiO2 scale. In this study, Si-stabilizer duplex coatings were proposed to eliminate the unexpected phase transition. The results show that the diffusion of Al from the stabilizer to the SiO2 scale makes the high-temperature β-SiO2 retain metastability to room temperature, thus avoiding phase-transition-induced cracking. Compared with the service lifetime of the pure Si coating, the lifetime of the duplex coating at 1350 ℃ is increased by more than 4 times. This research points to a way to design Si coatings with improved service lifetimes.

Modification of anode material VK8 by silicon for electric spark alloying of steel 35

Metal-ceramic coatings on steel 35 were obtained by the method of electric spark processing with different duration of control pulses, using a new electrode material WC—7.5 % Co—7.5 % Si for applying electric spark coatings on steel 35. It is shown that WC particles were deposited into the coating as a result of solid-phase transfer, which intensified with increasing discharge duration. The addition of silicon to WC—Co electrode materials can significantly reduce the coefficient of friction of the formed coatings. In general, the use of WC—Co—Si coatings can increase the heat resistance of steel up to 5 times, reduce the coefficient of friction of steel 35 to 47 %, and increase the wear resistance of its surface from 3 to 10 times.

Microstructure and Wear Resistance of Laser-Clad Ni-Cu-Mo-W-Si Coatings on a Cu-Cr-Zr Alloy.

To improve the wear resistance of high-strength and high-conductivity Cu-Cr-Zr alloys in high-speed and heavy load friction environments, coatings including Ni-Cu, Ni-Cu-10(W,Si), Ni-Cu-10(Mo,W,Si), and Ni-Cu-15(Mo,W,Si) (with an atomic ratio of Mo,W to Si of 1:2) were prepared using coaxial powder-feeding laser cladding technology. The microstructure and wear performance of coatings were chiefly investigated. The results revealed that (Mo,W)Si2 and MoNiSi phases are found in the Ni-Cu-10(Mo,W,Si) and Ni-Cu-15(Mo,W,Si) coating. WSi2 phases are found in the Ni-Cu-10(W,Si) coating. The degree of grain refinement in Ni-Cu-10(Mo,W,Si) was greater than that of the Ni-Cu-10(W,Si) coating after the effect of Mo. The excellent wear resistance and micro-hardness of the Ni-Cu-15(Mo,W,Si) coating were attributed to the increase in its dispersion phase, which were approximately 34.72 mg/km and 428 HV, 27.1% and 590% higher than the Cu-Cr-Zr substrate, respectively. The existence of silicide plays an important role in grain refinement due to the promotion of nucleation and the inhibition of grain growth. In addition, the wear mechanism transformed from adhesive wear in the Ni-Cu coating with no silicides to abrasive wear in the Ni-Cu-15(Mo,W,Si) coating with high levels of silicides.