The tribological and electrochemical behavior of HVOF-sprayed Cr3C2–NiCr ceramic coating on carbon steel

High Velocity Oxygen Fuel (HVOF) is an excellent approach to prepare a good, wear-resistant lamella of Chromium Carbide-Nickel Chrome (Cr₃C₂–NiCr) on carbon steel for high temperature application. This research investigates the effect of a thin, deposited layer of Cr₃C₂-NiCr on carbon steel in terms of wear and corrosion properties. The microstructure of the HVOF-sprayed Cr₃C₂–NiCr coating was characterized at each step by scanning electron microscopy. Wear testing was performed with a pin-on-disk tester. Wear weight loss was examined by applying different loads over a 9048.96 m sliding distance. Experimental results show that the wear resistance of the coated sample reduced the risk of seizure compared to the uncoated sample. An electrochemical test was also performed and the behavior of the substrate in the coated sample was investigated in 3.5% NaCl for 27 days. Electrochemical Impedance Spectroscopy (EIS) showed that the HVOF coating has high corrosion resistance and protects the substrate from NaCl electrolyte penetration. So deposition this layer of ceramic composite is protected oil piping from synergistic attack of seawater during the transport of crude oil to the refinery.     

Microstructure of HVOF-sprayed Ag–BaF2⋅CaF2–Cr3C2–NiCr coating and its tribological behavior in a wide temperature range (25 °C to 800 °C)

Ag–BaF₂?CaF₂–Cr₃C₂–NiCr composite powders were prepared by physically blending commercial BaF₂?CaF₂–Cr₃C₂–NiCr and Ag powders. Ag–BaF₂?CaF₂–Cr₃C₂–NiCr composite coatings were deposited on Inconel 718 alloy substrate by high velocity oxy-fuel (HVOF) spraying. The friction and wear behavior of the coatings under dry sliding against Si₃N₄ balls from 25 °C to 800 °C was evaluated with a ball-on-disk high temperature tribometer. The microstructure and composition of the samples were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectrometer. Results showed that the composite coatings were mainly composed of hard phase of Cr₃C₂, binder phase of NiCr, high-temperature lubrication phase of fluorides and low-temperature lubrication phase of Ag. The fluorides existed in the forms of both crystal particles and amorphous state, while the silver featured as typical thermally sprayed splats. Due to the high flame temperature, some fluorides have been oxidized to chromates and around 30 wt% of Ag was lost during spraying. In addition, it was found that Ag content had an important influence on the composite coating, and an appropriate dosage of metallic silver could effectively improve the tribological performance of the coating. The generation of AgCrO₂ at moderate (500 °C and 650 °C) temperature and BaCrO₄ at high temperature (800 °C) could contribute to the decline in friction coefficients and wear rates of Ag–BaF₂?CaF₂–Cr₃C₂–NiCr coatings.     

Sintering behavior of Cr2O3–Al2O3 ceramics

We investigated the sintering behavior of Cr₂O₃–Al₂O₃ ceramic materials. In our observation of the isothermal shrinkage behavior of Cr₂O₃–Al₂O₃ ceramic, the activation energy of sintering reaction was measured to be 102 kJ/mol, that is, the near value of the activation energy of diffusion of Al ions in Al₂O₃ single crystal. Therefore the diffusion of cations is believed to control the sintering behavior of this material. With the addition of TiO₂, (the compound chosen to accelerate the diffusion of cations) to Cr₂O₃–Al₂O₃, the sintering behavior was accelerated.     

Electrodeposition and corrosion behavior of Zn–Ni and Zn–Ni–Fe2O3 coatings

A thin film of Zn–Ni–Fe₂O₃ on steel substrates was prepared by electrodeposition technique using Zn–Ni alloy plating solution with nano-sized Fe₂O₃ particles. The cathodic polarization and cyclic voltammetry techniques were used to explain deposition process. The corrosion behavior of deposits was evaluated by polarization and impedance studies. Scanning electron microscope (SEM) images were used to study the surface morphology of coating. The grain size and amount of Fe₂O₃ particles present in composite coating were measured by X-ray diffraction pattern (XRD) and energy dispersive X-ray diffraction spectrometer (EDS), respectively.     

Cyclic ablation behavior of C/C–ZrC–SiC composites under oxyacetylene torch

Cyclic ablation behavior of C/C–ZrC–SiC composites prepared by precursor infiltration and pyrolysis process was studied using oxyacetylene torch. After repeated 30 s ablation for four times, the composites exhibited better ablation properties than those under single ablation for 120 s because of the lower surface temperature, and their linear and mass ablation rates were −3×10^–4 mm/s and −2.29×10^–3 g/s, respectively. A continuous ZrO₂–SiO₂ layer formed on the surface of center ablation region and acted as an effective barrier to the transfer of heat and oxidative gases into the inner material. Thermal stress induced by repeated impact of oxyacetylene led to some cracks on the ZrO₂–SiO₂ layer; however its destructive power was weaker than that of higher temperature. Stick like silica as grown silica nanowires were generated in the transition ablation region due to the evaporation of silicon oxide at appropriate temperature.     

Development and protection evaluation of two new,advanced ceramic composite thermal spray coatings,Al2O3–40TiO2 and Cr3C2–20NiCr on carbon steel petroleum oil piping

Carbon steel is the most commonly used material in the petroleum industry owing to its high performance and relatively low cost compared with highly alloyed materials. The corrosion resistance of carbon steel in aqueous solutions is dependent on the surface layer created on carbon steel. This layer often consists of siderite (FeCO₃) and cementite (Fe₃C), but it is neither compact nor dense. To improve the carbon steel surface resistance against corrosion and wear, a compact and dense layer can be deposited onto the surface by thermal spray coating. In this research, Al₂O₃–40TiO₂ and Cr₃C₂–20NiCr were deposited onto mechanical part surfaces by HVOF spray technique. The present study describes and compares the electrochemical behavior of carbon steel, Cr₃C₂–20NiCr and Al₂O₃–40TiO₂ in 3.5% NaCl using open-circuit potential measurement (OCP) and electrochemical impedance microscopy (EIS) for 36 days. The tribological and mechanical properties are also investigated using a tribometer (pin-on-disc). The results indicate that these chemical composition coatings facilitated significant anti-corrosion and anti-wear improvement. However, the samples coated with Al₂O₃–40TiO₂ exhibited the lowest corrosion rate. In terms of wear performance, both coated samples displayed similar behavior under different loads. Scanning electron microscopy (SEM) showed the distinctive microstructure of the HVOF-sprayed samples before and after corrosion and wear testing.     

Microstructure and phase transformation behavior of Al2O3–ZrO2 under microwave sintering

Zirconia is an inorganic, nonmetallic material with excellent properties. However, the brittleness of the zirconia, resulting from the thermal performance during the heating and cooling process, seriously limits the application of zirconia in the metallurgical, military, and aerospace industries. Al₂O₃ doped ZrO₂ was developed to improve the potential material's toughness. This paper studied the evolution of the surface functional groups, phase composition, toughening mechanism, and particle morphology of Al₂O₃ doped ZrO₂ during the heating process. Especially microwave heating was selected as the heating method during the experiments to save energy consumption. The results showed that the phase transition temperature was reduced by the microwave sintering technique, which also promoted the transformation between the m-ZrO₂ and t-ZrO₂, advancing the crystallinity and structural properties of the samples. The specific surface area shows a positive relationship with the microwave heating temperature, while the particle size of the powder decreased with the temperature increase. The optimized sintering effect appears at 1000 °C in the studied roasting temperature range (800 °C–1200 °C) for Al₂O₃–ZrO₂ powders. With the optimized sintering temperature, the void of the granular zirconia material was controlled, and the best micromorphology was obtained. In practical production, the application of microwave sintering and alumina doping is beneficial to saving costs and protecting the environment.     

Erosive wear behavior of spark plasma-sintered SiC–TaC composites

Spark plasma sintering of SiC-10, 20, or 30 wt% TaC composites was performed at 1800°C. Microstructures of sintered composites revealed uniform dispersion of TaC particles in SiC matrix. With the increase in TaC content, hardness decreased from 25.75 to 23.30 GPa and fracture toughness increased from 3.48 to 3.85 MPa m^1/2. Erosion testing was performed to evaluate the potential of sintered composites at room temperature and 400°C by a stream of SiC particles impinging at different angles (30°, 60°, or 90°). The erosion rate varied from 25 to 166 mm³/kg, with change in TaC content, impingement angle, or temperature. The erosion rate increased as the angle of impingement and temperature increased, but reduced when the TaC concentration increased. Worn surfaces revealed that the material was dominantly removed via fracture of SiC grains and TaC particles pull-out. SiC-30 wt% TaC composites exhibited superior erosive wear resistance at low impingement angle and room temperature.     

Tribological behavior and wear mechanism of C/C–SiC brake discs based on third-body layer

C/C–SiC composites are promising candidates for heavy-duty tracked vehicle brake discs. A third-body layer (TBL) can be formed on the surface of C/C–SiC self-mated brake discs, which has an important impact on tribological behavior and wear mechanism of brake discs. Herein, the formation conditions and evolution process of TBL and its effect on friction and wear properties were investigated. An appropriate braking pressure and speed (P and V) are beneficial to the cutting of asperities and refinement of wear debris on the contact surface, which are preconditions for the formation of original TBL. The original TBL can be formed under the P·V of 12, 15, and 16, which effectively improve braking stability and reduce the wear rate. During the continuous braking process, the original TBL undergoes growth, stabilization, destruction, and regeneration. Under the frictional heat and compressive stress, wear debris gradually evolves into a uniform and dense TBL. The average coefficient of friction and wear rate reach to the lowest value of .446 and 38.5 × 10⁻³ cm³/MJ, respectively. A continuous high temperature in the later stages of braking leads to severe oxidative wear. The newly formed TBL covers the original surface to form a multilayered structure, indicating the TBL undergoes destruction and regeneration.     

AlCrN/Cr3C2–NiCr duplex coating towards high load-bearing and dry sliding antiwear applications

In this study, experimental analysis and finite element modeling (FEM) were employed to investigate the microstructure and mechanical properties of duplex coatings composed of a Cr₃C₂–NiCr interlayer and a top AlCrN film in comparison with these of a single AlCrN film. Results showed a significant improvement in the adhesive strength, load-bearing capacity, H/E, and H³/E² ratios, and hardness of the AlCrN/Cr₃C₂–NiCr duplex coatings compared to the single AlCrN film, especially the wear resistance that increased by nearly eight times under heavy loads. Moreover, FEM analysis revealed that the duplex design reduced the stress concentration area on the surface of AlCrN film and kept it far away from the contact interface during load-bearing.     

Influence of Ti3SiC2 content on erosion behavior of Cu–Ti3SiC2 cathode under vacuum arc

In this study, a series of Cu–Ti₃SiC₂ composites with different Ti₃SiC₂ contents were prepared by spark plasma sintering. Their mechanical properties and electrical resistivity were investigated. Through analyzing the morphology and composition of the eroded regions, the effect of Ti₃SiC₂ content on the erosion behavior of Cu–Ti₃SiC₂ cathodes under vacuum arc was studied. Results show that the relative density and bending strength of the Cu–Ti₃SiC₂ composites decrease with the increasing Ti₃SiC₂ content, while the opposite holds for hardness and electrical resistivity. The morphology and phase composition of the erosion zone is dominated by the decomposition process and the amount of Ti₃SiC₂ in the cathode. Cu–Ti₃SiC₂ cathodes containing 10 mass%Ti₃SiC₂ or less displayed relatively flat eroded surface morphology. Cathodes with high Ti₃SiC₂ content suffered more serious erosion with voids, cracks, and severe decomposition of Ti₃SiC₂, all of which contribute to impairing the arc ablation resistance of the composite. Ti₃SiC₂ particles decomposed into TiC and Si vapor; eventually, this TiC also decomposed into Ti vapor and C, leaving a considerable amount of C on the arc affected cathode surface. Excess addition of Ti₃SiC₂ particles not only deteriorates the strength but also the electrical and thermal conductivity of the composite, both of which in turn harms the arc erosion resistance of the material. These results suggest that the optimal Ti₃SiC₂ content is below 10 mass% in the composite.     

Microstructure,interfacial characteristics,and wear performances of Cu–Fe–SiC cermet composites

The Cu–Fe metal-based ceramic grinding wheel material with SiC as abrasive was prepared by the powder metallurgy process of ball milling and hot pressing sintering. Cu–Fe–SiC cermets with Cu:Fe mass ratios of 4:1, 1:1, and 1:4 were designed by changing the composition of metal binder. The phase composition, microstructure, mechanical properties, and grinding properties of Cu–Fe–SiC cermets were systematically studied. The effect of Cu–Fe binder ratio on the microstructure and properties of cermets was analyzed. The results show that with the increase of Fe content, the density and hardness of cermets increase gradually, indicating that the mechanical properties are improved. Because the Fe in the adhesive can react with the abrasive SiC to form the reaction bonding interface, the Cu–80Fe–SiC cermets with higher Fe content have better adherence. The grinding test results of Cu–80Fe–SiC cermet show that the friction coefficient is .341, the surface roughness is 6.64 μm, the residual stresses parallel to the grinding direction are 353.3 MPa, and the residual stresses perpendicular to the grinding direction are 140.9 MPa. With the increase of Fe content, the wear mechanism changes from ploughing and cutting to friction.     

Investigation of wetting behavior of Cr–Mn–Ni steels on hBN-SiC-ZrO2-substrate

The present study investigates the wetting behavior of Cr–Mn–Ni-alloys with TRIP/TWIP-effects on the hBN-SiC-ZrO₂-substrate using the sessile drop method. The wetting behavior was studied in a 90 vol% N₂/10 vol% H₂ atmosphere in the temperature range of 1500 °C–1600 °C. Experiments were conducted in the reactive steel/ceramic system. Results demonstrate the effect of the nickel and the sulfur content on the contact angle between hBN-SiC-ZrO₂-substrates and Cr–Mn–Ni-alloys. The increase in the nickel content from 3 to 9 mass% caused the increase of the contact angle from 129 to 138° at 1600 °C. Whereas, the increase in the sulfur content caused the decrease of the contact angle. The contact angle of the melts alloyed with the sulfur increases as the temperature rises. The increase in the contact angle was related to the evaporation of the manganese. In addition, the study discusses the chemical reaction between the hBN-SiC-ZrO₂-substrate and Cr–Mn–Ni-alloys through (i) SEM-EDX investigations and (ii) thermodynamic calculations. The SEM-EDX analysis of the steel/ceramic interface shows the chemical degradation of the hBN-SiC-ZrO₂-substrate. As a result of the reaction, a transition layer with a thickness of around 0.7 mm was formed in the hBN-SiC-ZrO₂-substrate. To characterize the chemical reaction, thermodynamic calculations were conducted using the Thermo-Calc software. Results show that the chemical reaction was caused by the chemical instability of the SiC with regard to elements in Cr–Mn–Ni-alloys.     

Evaluation of tribological behavior of B4C–hBN ceramic composites under water-lubricated condition

The aim of this study is to evaluate the influence of hexagonal boron nitride (hBN) addition on the tribological behavior of B₄C-based ceramic composites under distilled water lubrication. Water-lubricated sliding tests of hot-pressed B₄C–hBN ceramic composites with different hBN amounts against pure B₄C ceramic were carried out on a pin-disc type wear apparatus. It was found that the addition of hBN into B₄C ceramic matrix resulted in a severe decrease of the friction coefficient from 0.373 for B₄C/B₄C sliding pair to 0.005 for B₄C–20 wt% hBN/B₄C sliding pair. A B₂O₃ tribochemical film formed on the worn surface of the B₄C–hBN specimen protected both B₄C–hBN and B₄C and facilitated the frictional surfaces to smooth. Therefore, the tribological behaviors of the pairs were significantly improved. The formation process of the film and its antifriction mechanism are discussed.     

Study of the degradation of hybrid sol–gel coatings in aqueous medium

The design and development of suitable multilayered functional coatings for delaying corrosion advance in metals and become controlled-release vehicles requires that the properties of the coatings are known. Coatings prepared by the sol–gel method provide a good approach as protective layers on metallic surfaces. This kind of coating can be prepared from pure chemical reagents at room temperature and atmospheric pressure, with compositions in a very wide range of environmentally non-aggressive precursors. Sol–gel coatings based on siloxane bonded units were prepared, starting with an organic–inorganic hybrid system. Synthesis procedures included acid-catalysed hydrolysis, sol preparation, and the subsequent gelation and drying. The alkoxide precursors used were methyl-triethoxysilane (MTMOS) and tetraethyl-orthosilicate (TEOS) in molar ratios of 10:0, 9:1, 8:2 and 7:3. After determination of the optimal synthesis parameters, the materials were characterised by solid ²⁹Si nuclear magnetic resonance (²⁹Si NMR), Fourier transform infrared spectroscopy (FTIR), contact angle measurement and electrochemical impedance spectroscopy (EIS) test. Finally, the materials were assayed by controlling their weight in contact with water, to determine their ability to degrade by hydrolysis. Electrochemical analysis reveals the formation of pores and water uptake during the degradation. The quantity of TEOS is one of the principal parameters that determine the kinetics of degradation. There is a correlation between the degradation process obtained for long periods and the electrochemical parameters obtained by EIS in short times. The study tries to incorporate knowledge that can be used for designing the degradation process of the functional coatings and to control their properties in short times.     

Preparation of multilayered C–Si–Al2O3 coatings on continuous carbon fibers and C–Si–Al2O3-coated carbon-fiber-reinforced hydroxyapatite composites

Multilayered C–Si–Al coatings with various morphologies were deposited on carbon fibers (CFs) using magnetron sputtering. The thickness of the coatings was increased from 0.5 to 1.5 μm by magnetron sputtering between 90 and 120 min. C–Si–Al coatings of suitable thickness were heat-treated at 600 °C and transformed into C–Si–Al₂O₃ coatings by one-step anodic oxidation (AO). The oxidation time for the one/two-step anodic oxidation and the ratio of oxidation time for the two-step anodic oxidation significantly influenced the morphologies of the C–Si–Al₂O₃(AO) coatings. Al₂O₃ coatings with satisfactory morphologies and structures were prepared by two-step anodic oxidation with a total time of 30 min and a ratio of 1:1 between the initial and secondary oxidation times. The multilayered C–Si–Al₂O₃(AO) coatings were modified to C–Si–Al₂O₃ coatings by secondary heat treatment at 1050 °C. Subsequently, hot-press sintering was used to prepare CFs with multilayered C–Si–Al₂O₃ coating-reinforced hydroxyapatite (CF/C–Si–Al₂O₃/HA) composites. The multilayered C–Si–Al₂O₃-coated CFs demonstrated good resistance to oxidation and thermal shock. This could effectively protect CFs from oxidative damage and maintain its strengthening effect during sintering. The multilayered C, Si, and Al₂O₃ coatings effectively reduced the difference between the coefficient of thermal expansion of the CFs and HA matrixes. The interfacial gaps between the multilayered coatings and HA were reduced. This could enhance the mechanical performance of the composites. The CF/C–Si–Al₂O₃/HA composites exhibited improved mechanical properties with a bending strength of 83.94 ± 12.29 MPa, and fracture toughness of 2.45 ± 0.08 MPa m^1/2. This study can broaden the application of CF/C–Si–Al₂O₃/HA biocomposites as bone-repair materials and help obtain CF-reinforced composites with excellent mechanical properties that are fabricated or serviced at high temperatures.     

Improving the tribological properties of plasma sprayed NiAl–Bi2O3–Ag–Cr2O3 composite coatings by hot isostatic pressing

In this study, the hot isostatic pressing (HIP) process was adopted to enhance the tribological response of plasma-sprayed NiAl–Bi₂O₃–Ag–Cr₂O₃ coatings under different temperature conditions. The HIP process was performed at a temperature of 800 °C, under a pressure of 100 MPa using argon gas. When compared with as-sprayed NiAl–Bi₂O₃–Ag–Cr₂O₃ composite coatings, the results revealed that the post-HIP process greatly reduced the porosity to a sufficiently low level of 2.7%, and led to a significant transformation from the splat lamellar to composition homogeneity across the entire coating. As highlighted in the hot isostatically pressed (HIPed) coating, more NiBi intermetallic compounds emerged. The mechanical hardness and adhesive strength increased considerably by 15.9% and 22.7%, respectively. The HIPed coating exhibited improved running stability in friction when exposed to different temperatures. In particular, the wear resistance increased significantly by one level of magnitude at the temperature range of room temperature (25 °C) to 400 °C, compared to the as-sprayed composite coating. This was attributed to the presence of the NiBi intermetallic compound and structural restoration after the HIP process. A protective tribo-layer was always present under alternating temperature conditions, and this allowed for continuous inhibition of wear. The mechanical evolution of the tribo-layer was further determined to clarify its effect on the resulting tribological behavior of the HIPed NiAl–Bi₂O₃–Ag–Cr₂O₃ coatings.     

Effect of impact angle on wear behavior of Mo2NiB2–Ni cermets with different Ni content

Herein, the influence of the impact angle and Ni content on the wear behavior of Mo₂NiB₂–Ni cermets was studied using an erodent-carrying slurry comprising artificial seawater and SiO₂ sands. The results reveal that the material loss may be attributed to the wear damage caused by SiO₂ sands because cermets are expected to exhibit good corrosion resistance in artificial seawater. The relative density of cermets markedly influences their resistance to wear damage, and the material loss experienced by cermets with poor relative density is 2–4 times higher than that of cermets with good relative density; this occurs because a higher relative density can markedly enhance the mechanical properties and reduce the defects in the cermets. Moreover, the results indicate that as the impact angle increases from 0° to 60°, the manifestation of the wear mechanism changes from damaging the Ni binder phase (caused by single cutting wear) to damaging both the Mo₂NiB₂ ceramic and Ni binder phases due to the combination of cutting wear and impact wear. The wear damage is dominated by the cutting wear and impact wear from SiO₂ sand at the low and high impact angles, respectively. Furthermore, the severe deterioration of the single ceramic skeleton at high impact angles indicates that the synergistic influence of the Mo₂NiB₂ ceramic and Ni binder phases on enhancing the wear resistance of the cermets intensifies at high impact angles.     

Oxidation-induced crack-healing behavior of SiC-Al2O3-TiB2 composites at 600°C–800°C

It is necessary to give self-healing function to ceramic materials because of their notch sensitivity. In the past, studies on self-healing ceramics have mainly focused on the high-temperature stage, and less research has been done below 1000°C. In this study, SiC/Al₂O₃/TiB₂ ceramic composites were prepared by spark plasma discharge sintering, and cracks were introduced on the surface of the polished specimens. Crack healing at 600°C–800°C was investigated, and the recovery of macroscopic bending strength and the change of microscopic crack morphology after heat treatment were used to evaluate the crack-healing effect. It was found that the surface cracks of the material were completely filled and healed by oxidation products after heat treatment at 700°C for 60 min, and the highest healing efficiency exceeded 95% for both specimens with different crack lengths, and the main mechanism of crack by Si-Al-B-Na-Ca-O type glass produced by the reaction of TiB2 and a small amount of SiC with oxygen to produce oxides and glass powder. Good healing effect and fast healing speed effectively improve the service life and reliability of ceramic materials, which has very far-reaching significance for the practical application with ceramic materials.     

Mechanical properties of (TixW1−x)C–Co cermet prepared by carbothermal reduction of high energy ball milled TiO2–WO3–C

A series of solid–solution carbides, (Ti_xW_1−x)C (x=0.9, 0.8, 0.7, 0.6), was prepared by the high-energy milling of TiO₂–WO₃–C mixtures via subsequent carbothermal reduction. With high-energy milling, only the size reduction of the constituent powders was apparent without any chemical reaction. The milled mixture powder was transformed to a single–phase (Ti_xW_1−x)C solid solution by heat treatment in a vacuum at 1200 °C. (Ti_xW_1−x)C–Co cermets were consolidated by isothermal sintering at 1300, 1400, and 1500 °C. The powders were fully densified by liquid-phase sintering at 1500 °C because the Co melted at 1430 °C. The mechanical properties of the (Ti_xW_1−x)C–Co cermet (H_v: ~24 GPa) were significantly better than those of the conventional WC–Co (H_v: ~13 GPa) or TiC–Co cermets (H_v: ~16 GPa). The use of a solid–solution carbide instead of conventional WC almost doubled its hardness values without a loss of toughness. It is indicated that the improved hardness of the (Ti_xW_1−x)C–Co cermet originates from the high hardness of (Ti_xW_1−x)C, and the solid–solution carbide would be a valuable substitute for conventional carbide cermets.