Corrosive wear behavior of HVOF-sprayed micro-nano-structured Cr3C2–NiCr cermet coatings under aqueous media

A novel micro-nano-structured Cr₃C₂–NiCr cermet coating was prepared on 316L stainless steel by high-velocity oxygen fuel spraying technology (HVOF). Cermet coatings with different contents of micro-and nano-sized Cr₃C₂ particles as the hard phase and a NiCr alloy matrix as the bonding phase were prepared and characterized in terms of porosity, microhardness, and corrosive wear resistance in a 3.5% NaCl solution and artificial seawater. Compared to nanostructured coatings, micro-nano-structured coatings avoid decarburization and reduce nanoparticle agglomeration during the spray process, and mechanical and electrochemical properties were improved in comparison with those of conventional coatings. The micro-nano-structured Cr₃C₂–NiCr coating rendered low porosity (≤0.34%) and high microhardness (≥1105.0HV_0.3). The coating comprising 50% nano-sized Cr₃C₂ grains exhibited the best corrosive wear resistance owing to its densest microstructure and highest microhardness. Furthermore, compared to static corrosion, the dynamic corrosion of the coatings led to more severe mechanical wear, because corrosion destroyed the coating surface and ions promoted corrosion to invade coatings through the pores during corrosion wear.     

Slurry erosion behavior of thermally sprayed ceramic nanocomposite coatings on turbine steel

The aim of the present study is to investigate the slurry erosion behavior of nano-yttria-stabilized zirconia (YSZ) reinforced Cr₃C₂–25NiCr ceramic nanocomposite coatings deposited on turbine steel. The Cr₃C₂–25NiCr coating powder, 95% (Cr₃C₂–25NiCr) + 5% YSZ, and 90% (Cr₃C₂–25NiCr) + 10% YSZ nanocomposite coating powder deposited on CA6NM steel samples by using high-velocity oxy-fuel coating technique. L₉ orthogonal array Taguchi method was used to design the experiment. Erosion tests were performed on erosion test rig under hydro accelerated conditions at different levels of various parameters. Erosion tests and analysis of variance resulted that for coated samples, velocity is the major influencing factor followed by slurry concentration, impact angle, and particle size. Velocity was the largest contributor to the mass loss, whereas particle size has the least contribution to mass loss of the coated samples. The scanning electron microscopy analysis of eroded samples revealed that craters, micropores, platelets, plowing, spalling, and so on were responsible for the mass loss of uncoated and coated samples. The incorporation of YSZ nanoparticles decreased the porosity; erodent particles cannot penetrate more deeply inside the workpiece and resulted in less erosion. It has been resulted that 95% (Cr₃C₂–25NiCr) + 5% YSZ and 90% (Cr₃C₂–25NiCr) + 10% YSZ ceramic nanocomposite coatings exhibited better erosion resistance as compared to Cr₃C₂–25NiCr coatings due to high microhardness and low porosity.     

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.     

Effect of load on the friction and wear characteristics of Si3N4-hBN ceramic composites sliding against steels

The tribological behaviors of Si₃N₄-hBN ceramic composites sliding against steels (austenitic stainless steel (ASS) and 45 steel) under dry friction conditions at different loads were investigated by using an MMW-1 type vertical universal friction and wear tester. The experimental results showed that the friction coefficients and wear rates first showed a decrease and then an increase with an increase in the load under dry friction conditions. The better tribological performance was exhibited by the SN10/ASS sliding pair under a load of 20 N (the friction coefficient was as low as 0.27 and the wear rates of both pin and disc had a magnitude of 10⁻⁶ mm³ N⁻¹ m⁻¹). This may be attributed to the formation of a black surface film (consisting of B₂O₃, SiO₂, and Fe₂O₃). For the same sliding pair, when the load was 10 N, the dominating wear mechanism was abrasive wear. Hence, the friction coefficient was higher (0.7). When the load increased to 30 and 50 N, the wear mechanism of the SN10/ASS sliding pair was a combination of abrasive and adhesive wears, and higher friction coefficients (0.48 and 0.72 under loads of 30 and 50 N, respectively) were obtained. On the other hand, the contents of hBN also showed a significant impact on the tribological behaviors of the Si₃N₄-hBN/ASS sliding pairs. When the hBN content was less than 10%, the friction coefficients of the Si₃N₄-hBN/ASS sliding pairs decreased with an increase in the hBN content. On the other hand, at hBN contents of 10% or more, the friction coefficients of the sliding pairs increased with an increase in the hBN content. Under the same experimental conditions, the Si₃N₄-hBN/45 steel pairs showed poor tribological properties as compared with the Si₃N₄-hBN/ASS pairs.     

Wet abrasive wear behavior of WC-based cermet coatings prepared by HVOF spraying

In this study, three kinds of WC-based cermet coatings including WC–CoCr coating, WC–Ni coating and WC–Cr₃C₂–Ni coating were prepared by the high-velocity oxygen-fuel (HVOF) spraying process. Scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and Vickers hardness tester were used to analyze the microstructure and mechanical properties of these coatings. The WC–CoCr coating presented the highest average microhardness of 1205 HV_0.3, and then followed by the WC–Cr₃C₂–Ni coating (1188 HV_0.3) and the WC–Ni coating (1105 HV_0.3). The abrasive wear behavior of the WC-based coatings under the conditions of different applied loads and sediment concentrations were studied by a wet sand-rubber wheel tester. The results indicated that the abrasive wear loss rates of all the coatings increased with the increment of applied load or sediment concentration. In addition, the coatings with higher microhardness appeared to have higher abrasive wear resistance. The abrasive wear resistance of the WC-based coatings was 4–90 times higher than that of AISI 304 stainless steel under the same testing condition. The abrasive wear mechanism of the WC-based coatings was deduced to be the extrusion and removal of binder phases, as well as the fragmentation and peel-off of hard phases.     

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.     

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 and oxidation behavior of NiCr-chromium carbides coating prepared by powder-fed laser cladding on titanium aluminide substrate

In the present study, a NiCr–Cr₃C₂ powder mixture was prepared by mechanical alloying and then coated on titanium aluminide substrates by the powder-fed laser cladding process using a set of optimum parameters. The high temperature oxidation behavior of the substrate and coating was studied by isothermal annealing at 900 °C for 5 h. It was found that the microstructure of the coating is composed of γ solid solution with different chromium carbide phases (Cr₃C₂, Cr₇C₃ and Cr₂₃C₆). The presence of different chromium carbides in the microstructure of coating can be attributed to the partial melting of primary Cr₃C₂ and the formation of non-equilibrium carbide phases during rapid cooling of laser cladding. The NiCr-chromium carbide laser cladded coating samples showed superior oxidation resistance compared to the substrate. The oxidation mechanism of both coating and substrate follow the parabolic law, where the parabolic rate constant of the coating was 20% of that of the substrate at 900 °C. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) and Grazing Angle X-Ray Diffraction (GAXRD) analysis revealed that the surface of the oxide layer formed on the NiCr-chromium carbides coating and the substrate is mostly composed of Cr₂O₃ and TiO₂, respectively.     

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.     

Preparation and characterization of Ni-coated Cr3C2 powder by room temperature ultrasonic-assisted electroless plating

Ni-coated Cr₃C₂ powder was prepared by ultrasonic-assisted electroless plating process without conventional sensitization and activation treatments at room temperature. The growth mechanism of Ni layers and the effects of Cr₃C₂ powder concentration on plating rate and coating quality were studied. Surface morphologies and composition of initial Cr₃C₂ powder, pre-treatment Cr₃C₂ powder and Ni-coated Cr₃C₂ powder were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that the Ni layers on the Cr₃C₂ powder had cell structure with dense and uniform distribution. The growth mechanism of Ni layers appears as follows: the pretreated surfaces of Cr₃C₂ powder act as activated sites, nucleation and the growth of nickel grains take place on the powder surfaces, and the process repeats continuously on the lath particles with reticulate structure on the as-coated surfaces of previously deposited Ni-cells, finally Ni cells grow up and merge into a layer. With the increase of Cr₃C₂ powder concentration, the plating time becomes shorter accordingly and the optimal powder concentration is 40 g L^?1.     

Tribological properties of duplex coatings of chromium-vanadium carbide produced by thermo-reactive diffusion (TRD)

Carbide coatings are very important in molding industries due to their good anti-wear properties increasing the life of molds of hot and cold forging, extrusion and powder metallurgy exposing to abrasive forces. Diverse processes are applied to produce carbide coatings. One of them is the thermo-reactive diffusion method (TRD) using a molten salt bath. This process has many advantages including cost-effectiveness over other similar surface coating methods. The aim of this study is the formation of carbide-composite coatings using molten salt baths containing oxides of carbide forming elements individually and in a mixed form on SKD-11 cold work tool steel at 1000 °C. For this purpose, two sets of experiments were considered in this study. In experiment A, a chromium oxide bath and then a mixed chromium oxide and vanadium oxide bath with a molar ratio of Cr to V equal to 0.66 were used. In experiment B, at first a mixed chromium oxide and vanadium oxide bath (Cr/V ratio = 0.66) and next a single bath of vanadium oxide were utilized. To evaluate and compare the produced coatings, FE-SEM, EPMA (point, line and map), XRD analysis, nano-indentation and wear tests were performed. The results showed that the coatings include chromium carbides (Cr₃C₂, Cr₇C₃ and Cr₂₃C₆) and vanadium carbides (V₂C, V₆C₅ and V₈C₇) as well as a carbon-chromium-vanadium triple phase with the composition of Cr₂C₂V. Moreover, the best hardness and abrasion resistance were gained for the coating produced in the molten bath containing chromium oxide and vanadium oxide after experiencing a chromium oxide bath. For the sample, the hardness was between 17.2 and 19.6 GPa and the lowest amount of COF was 0.32.     

Electrochemical behaviour of thermally sprayed Cr3C2–NiCr coatings in 0.5 M H2SO4 media

The electrochemical behaviour of coated Cr₃C₂–NiCr steel in aerated 0.5 M H₂SO₄ solution was studied by means of electrochemical a.c. and d.c. measurements. A complete structural characterization of the coated steel before and after electrochemical tests was also carried out to access the corrosion mechanism of coated steel, electrolyte penetration through the coating, and to confirm the results obtained using electrochemical techniques. Two types of Cr₃C₂–NiCr coatings produced by a high velocity oxy-fuel spraying system (HVOF) were studied. Differences between coated steels are related to the spraying parameters reflecting their behaviour against corrosion phenomena. The electrochemical behaviour of the coated steel was strongly influenced by porosity and the presence of microcracks in the coating. Once the electrolyte reaches the steel substrate, it corrodes in a galvanic manner resulting in coating detachment from the steel.     

Influence of Al content and post-annealing on synthesis and mechanical properties of plasma sprayed Cr–Al–C composite coatings

Due to ultra-high temperature and short reaction time, it was very challenging to produce high purity MAX phase by plasma spraying. In this study, Cr–Al-graphite agglomerated powders with different Al additions (x = 0.2–1.5) was used to prepare Cr–Al–C composite coatings by atmospheric plasma spraying followed with annealing. Results showed that the as-sprayed coatings displayed typical lamellar structure, mainly composed of Cr–C binary carbides (Cr₇C₃ and Cr₂₃C₆) and residual Al. After annealing at 700 °C, the newly formed Cr₂AlC phase increased significantly in the coatings. The higher addition of Al, the more Cr₂AlC phase formed after annealing. The enhanced atomic diffusion, sufficient Al source and existence of (Cr, Al)C_x contributed to the formation of Cr₂AlC under annealing. Annealing treatment improved the hardness of the coating, but with the increase of Cr₂AlC phase content, the hardness decreased slightly. The Al content and post-annealing had a synergistic effect on the formation of Cr₂AlC phase in the sprayed coatings. This provided an effective route to control the Cr₂AlC content in sprayed Cr–Al–C composite coatings.     

Tribological properties of selected ceramic coatings

The paper presents the characteristics of some ceramic coatings obtained by a plasma spray method. The ceramic coatings Al₂O₃, Cr₂O₃ and Cr₂O₃?+?5% TiO₂ were evaluated. Also the influence of the NiCr interlayer on the functional properties of sprayed coatings was studied. Other parameters studied included: thickness; microhardness; adhesion of the coatings; resistance to abrasive wear and thermal cyclic loading. The addition of TiO₂ to the Cr₂O₃ material increased the coating density, but did not substantially reduce the hardness. On the other hand, the lowest loss of material thickness was seen for Cr₂O₃; while the Al₂O₃ and the Cr₂O₃?+?5 wt.% TiO₂ material showed a higher loss. The loss in the case of the latter two was about the same. Relatively, higher values of abrasive wear resistance were observed in the Cr₂O₃ coatings, as compared to the reference material (Al₂O₃ coating), and the highest microhardness values were measured in the Cr₂O₃ coating. Finally, the metal interlayers in all coatings increased their resistance to thermal shock. All the coatings, using the interlayer to reduce differences in coefficients of thermal expansion, were suitable for the purpose of the thermal loading up to 1000?°C.     

Phase formation and microstructure evolution of plasma sprayed Cr2AlC MAX phase coatings under post annealing

In this study, thick Cr₂AlC coatings were first synthesized via plasma spraying of Cr₃C₂–Al–Cr agglomerated powders and post annealing. The microstructure evolution and mechanical properties of the Cr₂AlC coatings annealed at 500–1000 °C were investigated. The as-sprayed coatings exhibited a lamellar structure, primarily consisting of Cr₂AlC, Cr₇C₃, Cr₂₃C₆, and (Cr, Al)C_x solid solutions. The short residence time during spraying led to incomplete reactions in the Cr₃C₂@Al–Cr agglomerates, resulting in the formation of (Cr, Al)C_x. Post annealing provided sufficient energy for the transition of (Cr, Al)C_x → Cr₂AlC. With an increase in the annealing temperature (<900 °C), gradual transition of the (Cr, Al)C_x phase led to a slight increase in the Cr₂AlC content, and thus, the as-annealed coatings maintained high hardness (>1000 HV_0.2) with improved fracture toughness. Higher annealing temperatures (>900 °C) promoted clear enhancement of the Cr₂AlC content, thus reducing the coating hardness. The transition phase (Cr, Al)C_x and high temperature annealing were the primary factors to promoting the formation of the Cr₂AlC phase in sprayed coatings. This study indicates that the Cr₃C₂@Al–Cr agglomerates can be effective alternatives to expensive MAX phase powders as feedstock for plasma spraying of Cr₂AlC coatings.     

Friction and wear properties of Si3N4-hBN ceramic composites using different synthetic lubricants

This paper presents a tribological investigation of Si₃N₄-hBN composite ceramics using synthetic lubricants. The friction and wear properties of Si₃N₄-hBN ceramic composites sliding against TC4 titanium alloy (Ti6Al4V) were investigated via pin-on-disc tests. An axial compressive load of 10?N was applied with a sliding speed of 0.73?m/s. Three different lubrication conditions including simulated body fluid (SBF), physiological saline (PS) and bovine serum (BS) were used. For SBF lubrication, the friction coefficients and wear rates of Si₃N₄-hBN/Ti6Al4V pairs were varying with the increase of hBN contents. When using 20?vol% hBN, the average friction coefficient and wear rate of Si₃N₄ (0.28 and 3.5?× 10^?4 mm³ N^?1 m^?1) were as good as that of the pure Si₃N₄ (0.34 and 3.69?× 10^?4 mm³ N^?1 m^?1). Meanwhile, the processability of the Si₃N₄ material would be improved by adding hBN. It was worth to mention that when using 30?vol% hBN, the tribological performance of bearing combination deteriorated with extensive wear from the ceramic pin. This may due to the reduction of mechanical property caused by adding hBN and the occurring of tribochemical reaction. According to the worn surface examination and characterization, the main wear mechanism was abrasive and adhesion wear. Scratch grooves were observed on the metal disc, and metallic transform layers were seen on the ceramic pin. Moreover, surface lubrication film consisting of TiO₂, SiO₂·nH₂O, Mg(OH)₂, and H₃BO₃ were formed on the metal disc when using SBF lubrication and 20?vol% hBN content. Among the three lubrication conditions, SBF generally led to the best tribological performance. No surface lubrication film was found during BS and PS lubrications. This may be resulted from the absence of essential ions to promote the formation of surface lubrication film (PS lubrication) and the formation of a protein barrier on the surface of the metal disc (BS lubrication).     

Effect of various morphology of in situ generated NbC particles on the wear resistance of Fe-based cladding

To improve the wear resistance of Fe-based cladding, NbC particles with different morphology are generated in situ by adding Nb, Cr₃C₂ and C with different content. The composition of samples and the morphology of NbC particles generated in situ are revealed by XRD, SEM and EDS. The wear resistance is studied by a reciprocating friction and wear testing machine. The wear mode and the wear mechanism of each sample are investigated. The results show that although NbC particles are generated in situ in samples with different Nb, Cr₃C₂ and C content, the morphology of NbC particles is varied. The wear resistance of samples containing cross-shaped NbC particles is more outstanding than that of samples containing only rectangular NbC particles. In addition, changing Nb, Cr₃C₂ and C content does not result in a change in wear mode, but leads to the formation of continuous lattice structure of Cr_0.19Fe_0.7Ni_0.11 and Cr₂₃C₆ compounds at grain boundaries and a change in the wear resistance. When the additions of Nb, Cr₃C₂ and C are 11.2 wt%, 8.6 wt% and 0.2 wt% respectively, the coefficient of friction of the sample is the lowest, and the wear resistance is the most outstanding.     

Improving the properties of remanufactured wear parts of shield tunneling machines by novel Fe-based composite coatings

With the aim of remanufacturing high-value wear parts of shield tunneling machines, novel Fe-based composite coatings were prepared by collaborative modification with nano-TiC and nano-CeO₂ particles. This work aims to improve the wear properties of Fe-based alloy coatings by regulating the morphology and dispersion of TiC through the addition of different contents of nano-TiC and nano-CeO₂. First, the coatings with different contents of nano-TiC (from 5 wt% to 15 wt%) and nano-CeO₂ (from 1 wt% to 2 wt%) were prepared by laser cladding. Subsequently, the microstructure, phase composition, microhardness, and wear properties of the coatings were examined. Furthermore, the wear morphology and the influence mechanism of nano-particles on the wear resistance of the coatings were investigated. It was found that the addition of nano-TiC eliminates the macro-defects of Fe55 alloy coating. Meanwhile, the morphology and dispersion of TiC particles in coatings were affected by the content of nano-TiC and nano-CeO₂. Specifically, the addition of 1 wt% nano-CeO₂ facilitates to the formation of near-spherical tiny TiC particles with low agglomeration in the coating. Therefore, the Fe55 + 10 wt% nano-TiC+1 wt% nano-CeO₂ coating exhibits the best wear property among all the prepared Fe-based coatings. This paper provides theoretical guidance for the preparation of the modified Fe-based coating with excellent wear resistance.     

Synthesis of chromium carbide coatings on carbon steels in molten salts and their properties

Chrome plating from the melt of NaCl-KCl-CrCl₂ on substrates of carbon steels is studied and the chromium carbide formation with the Cr₂₃C₆, Cr₇C₃, and Cr₃C₂ composition is revealed. The controlled potential electrolysis parameters resulting in the chromium carbide formation with various compositions on the steel surface are determined. The currentless transfer method is used for the synthesis of chromium carbide coatings with the Cr₇C₃ composition on grades St3 and U9 steels. The coating with the given composition is demonstrated to reduce by some orders the corrosion rate of steel substrates within the concentrated mineral acids. It is found that the composition of St3-Cr₇C₃ coating is stabler to concentrated acids than the composition of U9-Cr₇C₃ coating. The Cr₇C₃ coating has a high microhardness (21–23 GPa) and increases the wear resistance of samples practically by an order.     

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.