Cr3C2-NiCr Coatings Research Articles

Microstructural evolution of carbides and its effect on tribological properties of SAPS or HVOF sprayed NiCr–Cr3C2 coatings

The wear resistance of NiCr–Cr3C2 cermet coatings is highly dependent on the feature of carbides. In the present work, NiCr–Cr3C2 coatings were deposited by two typical high-velocity spraying methods including supersonic atmospheric plasma spraying (SAPS) and high velocity oxygen-fuel (HVOF) spraying. The microstructural evolution of carbides and its effect on the tribological properties of as-sprayed coatings were systematically studied. The results suggested that a large number of carbides dissolved or diffused with molten metal binder and then precipitated as the form of Cr23C6 from supersaturated molten metal binder, leading to the formation of interface transition zone and network carbides. Compared with HVOF-coating, although SAPS-coating had slightly higher porosity and lower hardness, a large number of large-sized network carbides and a high content of interface transition zone resulted in a lower friction coefficient (0.14) and wear rate (0.77 × 10−5 μm/N·s) under heavy load, owning to the high temperature and reducing atmosphere of supersonic plasma jet. The main wear mechanism of SAPS-/HVOF-coating was associated with abrasive and slight adhesive wear, and a small amount of delamination was also observed on the wear tracks of SAPS-coating. The wear debris of both coatings mainly consisted of Cr2O3 and NiCr2O4 accompanied by a small amount of free graphite.

Nanocrystalline Cermet Coatings for Erosion–Corrosion Protection

The processing techniques, microstructural characteristics, and erosion corrosion behaviour of Cr3C2–NiCr and tungsten carbide (WC)-based cermet coatings are reviewed in this work. Conventional and nanocrystalline Cr3C2–NiCr and WC-based cermet coatings are generally synthesized using thermal spray technique. The wear, erosion, and corrosion protection ability of conventional and nanocermet coatings are compared based on available literature. In Cr3C2–NiCr coatings, the corrosion resistance is offered by NiCr metal matrix while the wear resistance is provided by the carbide ceramic phase, making it suitable for erosion–corrosion protection. The nanocrystalline cermet coatings exhibits better erosion–corrosion resistance as compared to the conventional coatings. The nanocrystalline coatings reduces the erosion–corrosion rate significantly compared to conventional coatings. It is attributed to the presence of the protective NiCr metallic binder that allows easier and faster re-passivation when the coating is subjected to wear and the fine-grain structure with homogeneous distribution of the skeleton network of hard carbide phases. In addition, corrosion-accelerated erosion dominates the reaction mechanism of erosion–corrosion and, therefore, higher hardness, strength, and better wear resistance of nanocermet coating along with its faster repassivation kinetics accounts for improved corrosion resistance as compared to conventional coatings.

Influence of plasma spraying parameters on microstructure and corrosion resistance of Cr3C2-25NiCr cermet carbide coating

PurposeIn this work, Cr3C2-25NiCr coatings were deposited on 410 stainless steel substrate by using the atmospheric plasma spray technique, at varying spaying parameters. The porosity and microhardness, adhesion strength and corrosion behaviour of coatings were examined in relation to these spraying parameters.Design/methodology/approachThe microstructure of prepared coatings was examined by using scanning electron microscopy. The coating compositional analysis was performed by using X-ray diffraction (XRD) technique. The corrosion resistance of coated steel was investigated by potentiodynamic polarization. Results indicate that optimal factors for minimalizing the porosity were as follows: 10 g/min feed rate, 600 A plasma current and 100 mm spraying distance. The spraying factors influencing corrosion resistance of coating were also evaluated.FindingsUnder this optimal condition, the porosity of coating reached its minimal value of 3.1 per cent. The microhardness and adhesion of coatings also reached their maximum values of 64.8 Rockwell Hardness scale C and 60 MPa, respectively. XRD results indicated the transformation of Cr3C2originating from Cr3C2-25NiCr source powder into Cr7C3and Cr23C6crystalline phases, due to the high temperature during spraying process. The undetectable Cr3C2peaks indicating that this phase was remained in coating at very low concentrations. The potentiodynamic polarization and salt spray tests confirmed the highest corrosion resistance for the coating prepared by optimal spraying parameters.Originality/valueThe application of Cr3C2-NiCr cermet carbit coating for protection of steel from corrosion-erosion is very promising.

Erosion wear performance of WC-10Co4Cr and Cr3C2-25NiCr coatings sprayed with high-velocity thermal spray processes

Thermally sprayed hardmetal coatings are widely used to protect components and surfaces against wear in various applications. Hard and wear resistant coatings increase the component lifetime and can generate significant savings promoting ecological manufacturing. This study focuses on the performance of tungsten carbide (WC-10Co4Cr) and chromium carbide (Cr3C2-25NiCr) based hardmetal coatings sprayed with gaseous and liquid fuelled high-velocity oxygen-fuel (HVOF) spray processes and a modern high-velocity air-fuel (HVAF) spray process. The coating characterisation revealed reduced carbide dissolution with decreasing process temperature and denser feedstock powder particles. Smaller carbide size in the Cr3C2-25NiCr material significantly reduced the carbide rebounding leading to higher carbide content in the sprayed coating and improved erosion wear resistance. Most significant improvements were observed in cavitation erosion for HVAF sprayed WC-10Co4Cr coatings (0.4 μm/h) compared to the HVOF sprayed coatings (1.5–3.7 μm/h). The cavitation erosion resistance of the HVAF sprayed coatings was almost at the level of the WC-10Co sintered bulk (0.2 μm/h).

Effects of laser power on friction–wear performances of laser thermal sprayed Cr3C2–NiCr composite coatings at elevated temperatures

Cr3C2–NiCr composite coatings were fabricated on H13 hot work mould steel using a laser thermal spraying (LTS) at the laser power of 1200, 1400 and 1600 W. The surface and cross–section morphologies, chemical compositions, bonding strengths, phases and roughness of obtained coatings were analyzed using a scanning electronic microscopy (SEM), energy dispersive spectroscopy (EDS), scratch, X–ray diffractometer (XRD), and atomic force microscopy (AFM), respectively. The friction–wear properties of Cr3C2–NiCr composite coatings at 600 °C were analyzed using a high temperature wear test, and the wear mechanism were also discussed. The results show that the Cr3C2–NiCr composite coatings are primarily composed of Cr7C3 and Cr3C2 hard phases, and NiCr binder phase, the average bonding strengths is 46.25 N, which forms the metallurgical bonding at the coating interface. The average coefficients of friction (COFs) of Cr3C2–NiCr coating fabricated at the power of 1200, 1400 and 1600 W is 0.65, 0.34, and 0.67, respectively; the corresponding wear volumes are 111.3, 101.1, and 143.2 mm3, respectively, the wear mechanism of Cr3C2–NiCr coatings is primary abrasive wear and oxidation wear.

Tribological behavior of SAPS sprayed Al2O3-TiO2 and NiCr-Cr3C2 coatings under severe load conditions

Plasma sprayed Al2O3-TiO2 and NiCr-Cr3C2 coatings have been widely used in various components with severe wear conditions. In this study, the microstructure and friction-wear behavior of both coatings were comparatively studied in order to provide an insight into their tribological properties under heavy load. The results suggested that although both of the coatings showed almost the similar porosity and hardness, the wear rate of NiCr-Cr3C2 coating was only 44% as much as that of Al2O3-TiO2 due to a higher toughness. The delamination, grooves and pits were observed on the worn surface of Al2O3-TiO2 or NiCr-Cr3C2 coating. The wear behavior of both coatings was associated with abrasion, accompanied by a certain extent of delamination and adhesion. For Al2O3-TiO2 coating, the high hardness Al2O3 particles that deeply embedded in the surface of softer friction pair (Ni55) caused serious adhesive tearing and scratch under normal and radial stresses.

Wear-resistant coating spraying technique for wear-proofing cover of coke dry quenching furnace

During the operation of a coke dry quenching system of coking plant, wear-proofing cover of the boiler was seriously eroded, and even the ceiling tube of the boiler was worn and torn. Thus, attentions were paid to the research, development and application of the wear-resistant coating spraying technique for wear-proofing cover of the coke dry quenching furnace. The Cr3C2–NiCr coatings for wear-proofing cover were fabricated via supersonic arc spraying process, and the residual coating of wear-proofing cover would be sampled for being analyzed after two years. The coating presents a dense microstructure with few pores and micro-cracks (the porosity is 4.65%). After thermal spraying of the wear-proofing cover, the hardness and the wear resistance of the surface are improved. The remaining coating is continuous and compact, with an average thickness of about 147 μm. Transverse micro-cracks parallel to the spreading direction of the coating surface are displayed on different areas of the coating, which demonstrates its serious erosion wear effect. The main chemical component of white zone is Fe–18Cr–Ni. The Cr level of light gray phase is about 77.57 wt.% and the O level is 22.43 wt.%. And the main chemical components of dark gray phase are C, O, Al, Si and Ca. X-ray diffraction patterns were adopted to implement phase analysis on the surface of coating sample, which indicated that the coating was composed of a large amount of Cr3O2 and a small amount of metal Cr.

Effect of Experimental Parameters on Wear Response of Thermally Sprayed Carbide Based Coatings

Friction and abrasive wear response of WC-12 Co and Cr3C2-25 NiCr coatings was studied. Abrasive wear experiments were conducted with SiC abrasive particles with varying load and sliding speeds. The results were analyzed using SEM, XRD and XPS observations of worn surfaces, wear debris and worn out abrasive paper. Friction coefficient diminished with rise in sliding speed and increased for increase in load while wear rates decreased for increasing sliding speed for both coatings. WC-12 Co coating had better wear resistance than Cr3C2-25 NiCr coating while Cr3C2-25 NiCr coating displayed lower friction coefficient. The variation in wear rate and friction response of coatings was due to combined effects of adhesion, abrasion and tribo oxidation effects. The composition of tribo films was strongly influenced by load and sliding velocity and altered friction and wear response of the coatings.

Properties of HVOF-sprayed TiC-FeCrAl coatings

As an alternative to WC-CoCr and Cr3C2-NiCr coatings for wear and corrosion protection, a TiC – 25 vol% (Fe-20 wt%Cr-5 wt%Al) powder, free from hazardous and/or supply-critical elements (Ni, Co, W), was produced by high-energy ball-milling and processed by High Velocity Oxygen-Fuel (HVOF) spraying, obtaining dense (<1 vol% porosity), hard (HIT > 12 GPa) layers with reasonably good deposition efficiency of ≈ 54%.Tribological testing revealed that the TiC-FeCrAl coatings are particularly promising for sliding contacts, as their ball-on-disc wear rates against an Al2O3 counterpart were lower than those of an HVOF-sprayed Cr3C2-NiCr reference, both at room temperature and at 400 °C, although they could not match the performance of WC-CoCr. At room temperature, brittle fracture along oxidized lamellar boundaries caused localized spallation, releasing debris in the contact region, but, in the incubation period before spallation cracks could propagate, remarkably low friction (≈0.27) was recorded. At 400 °C, spallation was largely suppressed by thermal softening, whilst coarser abrasive grooving became the dominant wear mechanism.TiC-FeCrAl coatings appeared less suited to high-stress abrasion, since extensive brittle fracture resulted in higher wear rates than HVOF-sprayed Cr3C2-NiCr, and to (acidic) corrosive environments. Electrochemical polarisation tests in 0.1 M HCl indeed revealed limited corrosion resistance of the FeCrAl matrix.

Inter-diffusion between thermally sprayed Cr3C2-NiCr coatings and an Alloy 625 substrate during long-term exposure at 500 °C, 700 °C and 900 °C

Long-term operation of Cr3C2–NiCr thermal spray coatings at elevated temperatures, including service in oxidative and corrosive environments, leads to significant changes in the coating composition and microstructure. A critical, but often overlooked, aspect of high temperature service conditions is interdiffusion between the coating and substrate. This leads to changes in the coating/substrate compositions and microstructures with significant consequences in terms of the functionality of the coating. The aim of this work was to characterise the magnitude and mechanism of interdiffusion between a Cr3C2–NiCr coating and a Ni-based Alloy 625 substrate at 500–900 °C for up to 30 days. Interdiffusion was not observed at 500 °C, but occurred to a limited extent at 700 °C, and very intensively at 900 °C. The precipitation of M23C6 and M6C grains within the alloy grain boundaries was proposed to occur due to carbon diffusion from the coating into the substrate. It is proposed, that this carbon was liberated from the Cr3C2 grains in the coating due to formation of (Cr,Ni)7C3. Chromium diffusion from the substrate into the coating resulted in the formation of a continuous oxide layer at the coating–substrate interface and the formation of a continuous (Cr,Ni)7C3 layer in the coating adjacent to the interfacial oxide. Loss of Cr from the substrate led to the formation of a barren, precipitate free zone adjacent to the interface.

Microstructure and Erosion Properties of HVOF Sprayed Cermet Coatings via Different Feedstock Powders

In the present study, Cr3C2-NiCr and WC-Co coatings have been deposited using high velocity oxygen-fuel (HVOF) spray technology from five available powders with various bonding phase content or manufacturing process. The microstructure of coatings was observed by scanning electron microscopy (SEM). Microhardness and erosion performance of the coatings were studied. The influence of powder characteristics on the microstructure and erosion performance of coatings was also investigated. The results indicated that an independent bonding phase distributed in the feedstock powder can effectively improve the erosion resistance of Cr3C2-NiCr coatings although the microhardness of the coatings may be lower. Deformation of the free NiCr binder layer in the coating is probably to prevent nucleation and propagation of cracks, which may result in improving the erosion resistance of the coating. Nano WC-Co coatings reached lower erosion resistance than micro WC-Co coatings due to the higher porosity and lower microhardness of nano WC-Co coatings.

Elevated temperature solid particle erosion behaviour of carbide reinforced CoCrAlY composite coatings

CoCrAlY+WC-Co and CoCrAlY+Cr3C2-NiCr coatings are deposited on nickel based alloy using atmospheric plasma spray technique. Mechanical properties such as microhardness, adhesion strength and fracture toughness of coatings are evaluated. Elevated temperature solid particle erosion behaviour of these coatings are investigated at 600 °C using alumina erodent at 30 and 90° impact angle. Coatings are characterized utilizing Scanning electron microscope (SEM), x-ray diffraction (XRD) and Energy dispersive spectroscopy (EDS). CoCrAlY+WC-Co coating shows higher hardness, adhesion strength and fracture toughness than CoCrAlY+Cr3C2-NiCr coating. CoCrAlY+WC-Co coating exhibited approximately 3 times higher erosion resistance than CoCrAlY+Cr3C2-NiCr coating at 90° and 30° impact angles. SEM images of eroded surfaces of coatings reveals the combination of ductile and brittle fracture. CoCrAlY+Cr3C2-NiCr coating shows severe cracks, craters, carbide pull out and chipping than CoCrAlY+WC-Co coating. High temperature erosion is a combination of simultaneous building up of material by oxidation and removal of material by erosion process. Thus reforming the erosion process to oxidation modified erosion process.

Wear of grinding rotors with thermally-sprayed coatings in a high-speed mill

In this paper, the erosion behavior of three types of protective thermally-sprayed coatings and non-coated substrate steel was investigated under semi-industrial test conditions using a laboratory high-speed pin mill DESI-11. The grinding in the mill was performed by two counter rotors, on which protective coatings were deposited either by atmospheric plasma spraying (APS) (Cr3C2-NiCr and NiCrBSi coatings) or by high velocity oxy-fuel (HVOF) process (WC-CoCr coating). The grinding rotors with deposited coatings were used for milling of the Portland cement, and rotors' weight loss was monitored after milling of 1, 3, 5, 10, and 15 kg of this material. The lowest weight loss in the mixed impact erosion conditions was exhibited by WC-CoCr coating, which was followed by Cr3C2-NiCr and NiCrBSi coatings. The greatest material removal on the anterior and the right lateral faces of rotor pins was a result of erosion damage at high impact angles through surface fatigue wear and the following failure of protective coatings down to the substrate. In contrast, the top and the left lateral faces of the pins were subjected mostly to the ploughing and microcutting at oblique impact angles that resulted in significant erosive damage only if hardness of the pin was lower than that of the Portland cement (Cr3C2-NiCr-coated and non-coated steel pins). The study also found a significant disproportion between the volumetric wear losses of various rows of pins of grinding rotors. The central part of the grinding tool consisting of two counter rotors (both rows of 2-row rotor and a middle row of 3-row rotor) suffered more intensive erosion wear than the external part (outer rows of 3-row rotor). The design of the mill and the resulting variability in parameters of milled powder particles at different sites of the grinding tool (such as particle size, particle flux and particle velocity) were considered as main reasons of this phenomena.

Effect of Nozzle Geometry on the Microstructure and Properties of HVAF-Sprayed WC-10Co4Cr and Cr3C2-25NiCr Coatings

Thermally sprayed hard metal coatings are the industrial standard solution for numerous demanding applications to improve wear resistance. In the aim of improving coating quality by utilising finer particle size distributions, several approaches have been studied to control the spray temperature. The most viable solution is to use the modern high velocity air-fuel (HVAF) spray process, which has already proven to produce high-quality coatings with dense structures. In HVAF spray process, the particle heating and acceleration can be efficiently controlled by changing the nozzle geometry. In this study, fine WC-10Co4Cr and Cr3C2-25NiCr powders were sprayed with three nozzle geometries to investigate their effect on the particle temperature, velocity and coating microstructure. The study demonstrates that the particle melting and resulting carbide dissolution can be efficiently controlled by changing the nozzle geometry from cylindrical to convergent–divergent. Moreover, the average particle velocity was increased from 780 to over 900 m/s. The increase in particle velocity significantly improved the coating structure and density. Further evaluation was carried out to resolve the effect of particle in-flight parameters on coating structure and cavitation erosion resistance, which was significantly improved in the case of WC-10Co4Cr coatings with the increasing average particle velocity.

High-Temperature Erosive Behavior of Plasma Sprayed Cr3C2-NiCr/Cenosphere Coating

This research examines the deposition of Cr3C2-NiCr/cenosphere and Cr3C2-NiCr coatings on MDN 321 steel through the process of plasma spray. In this process, the solid particle erosion test is established at 200, 400, 600 °C with 30° and 90° impact angles. Alumina erodent is adopted to investigate the erosive behavior of the coating at higher temperatures. The properties of the Cr3C2-NiCr/cenosphere coating are established based on the microhardness, the adhesive strength, the fracture toughness, and the ductility. To quantify volume loss as a result of erosion, an optical profilometer is used. At higher temperature, decrease in the erosion volume loss of Cr3C2-NiCr/cenosphere and Cr3C2-NiCr coatings is observed. The erosion-resistive property of Cr3C2-NiCr/cenosphere coating is higher than that of MDN 321 steel by 76%. This property is influenced by high-temperature stability of mullite, alumina, and protective oxide layer that is formed at elevated temperatures. The morphology of eroded coating discloses a brittle mode of material removal.

Improving the high temperature abrasion resistance of thermally sprayed Cr3C2-NiCr coatings by WC addition

Two experimental agglomerated and sintered (a&s) feedstock powders were prepared, in order to reveal the role of WC addition on the microstructure, hardness, and the abrasion resistance of HVOF-sprayed Cr3C2-NiCr coatings. These powders contained 10wt.% of sub-micron WC, 20 or 10wt.% of nickel binder, and Cr3C2 as balance. Experimental coatings were deposited by a liquid fueled high velocity oxygen-fuel (HVOF) spray process and subsequently heat treated at 800 °C for 8h to simulate elevated temperature service conditions. The microstructures of the powders and coatings were studied by SEM and X-ray diffraction, and the hardnesses of coatings were probed by means of micro and nanoindentation. In addition, the high stress abrasion resistance was tested in a temperature range from room temperature up to 800 °C. The microstructural characterization of the coatings displayed the presence of WC and tungsten containing Cr3C2 grains. The coating hardness increased after heat treatment, which stemmed from precipitation of secondary carbides and solid solution strengthening of the binder by tungsten. In addition, the study revealed that both experimental coatings have high wear resistance at room and elevated temperatures.

Improving the Wear Resistance of Thermally Sprayed Nanocomposite Cr3C2-25NiCr Coatings by Pulsed Plasma Treatment

Improving the Wear Resistance of Thermally Sprayed Nanocomposite Cr3C2-25NiCr Coatings by Pulsed Plasma Treatment

Effect of heat treatment on friction and abrasive wear behaviour of WC-12Co and Cr3C2-25NiCr coatings

Abstract The effect of heat treatment (300–950 °C) on friction and abrasive wear behaviour of WC-12Co and Cr3C2–25NiCr coatings was investigated in the present work. The abrasive wear rate of heat treated coating decreased with increase in heat treatment temperature till 550 °C at lower loads, whereas an increasing trend was observed at higher loads. The coefficient of friction decreased till 550–750 °C and then showed an increased trend. The phase transformations and variation in mechanical properties predominantly influenced abrasive wear resistance. The friction behaviour of heat treated coatings was rationalized on the basis of relative contribution of mechanical wear and oxidative wear. Severity of contact map was used to identify abrasive wear regime and transitions in wear mechanisms and failure modes.

Electrochemical Behavior of Bilayer Thermal-Spray Coatings in Low-Temperature Corrosion Protection

Cr3C2-NiCr coatings are greatly used to protect critical components in corrosive environments and to extend their lifetime and/or improve functional performance. However, the pores formed during spraying restrict the coating’s applicability area for many corrosion protection applications. To overcome this technical challenge, bilayer coatings have been developed, in which an additional layer (the so-called “intermediate layer”) is deposited on the substrate before spraying the Cr3C2-NiCr coating (the so-called “top layer”). The corrosion behavior of the bilayer coating depends on the composition and microstructure of each layer. In the present work, different single-layer coatings (i.e., Cr3C2-NiCr, Fe- and Ni-based coatings) were initially sprayed by a high-velocity air fuel (HVAF) process. Microstructure analysis, as well as electrochemical tests, for example, open-circuit potential (OCP) and polarization tests, were performed. The potential difference (ΔE) had a great influence on galvanic corrosion between the top and intermediate layers, and thus, the coatings were ranked based on the OCP values (from high to low) as follows: NiCoCrAlY &gt; NiCr &gt; Cr3C2-NiCr &gt; NiAl &gt; Fe-based coatings (alloyed with Cr) &gt; pure Ni. The Ni-based coatings were chosen to be further used as intermediate layers with the Cr3C2-NiCr top layer due to their capabilities to show high OCP. The corrosion resistance (Rp) of the bilayer coatings was ranked (from high to low) as follows: NiCoCrAlY/Cr3C2-NiCr &gt; NiCr/Cr3C2-NiCr &gt; NiAl/Cr3C2-NiCr &gt; Ni/Cr3C2-NiCr. It was shown that splat boundaries and interconnected pores are detrimental for corrosion resistance, however, a sufficient reservoir of protective scale-forming elements (such as Cr or/and Al) in the intermediate layer can significantly improve the corrosion resistance.

Thermally induced metallurgical processes in Cr3C2-NiCr thermal spray coatings as a function of carbide dissolution

Cr3C2-NiCr thermal spray coatings are widely used to mitigate wear and corrosion at high temperatures. The aim of this work was to determine what minimum treatment temperature is required to transform the non-equilibrium as–sprayed coating composition back to an equilibrium composition, with the aim to improve the wear resistance. Cr3C2-NiCr coatings were sprayed using two HVOF techniques and a shrouded plasma spraying technique to produce samples with a broad spectrum of carbide dissolution and peritectic decomposition of Cr3C2. Shrouded plasma spraying was found to be highly effective in decreasing the carbon loss and the oxygen uptake. Differential scanning calorimetry (DSC) was used to characterize the exothermic solid-state phase transformations within the coatings. For the HVOF coatings, one main exothermic peak was observed, which was attributed to the crystallization of the metastable Ni binder material. In the plasma spray coating an additional higher temperature peak was also observed. This was attributed to the transformation of (Cr,Ni)7C3 and a high Cr content Ni phase, into the equilibrium phases Cr3C2 and a low Cr content Ni binder.