Cr3C2-NiCr Coatings Research Articles

HVOF- and HVAF-Sprayed Cr3C2-NiCr Coatings Deposited from Feedstock Powders of Spherical Morphology: Microstructure Formation and High-Stress Abrasive Wear Resistance Up to 800 °C

Chromium carbide-based coatings are commonly applied to protect surfaces against wear at high temperatures. This work discusses the influence of feedstock powder and spray torch selection on the microstructure and high-stress abrasion resistance of thermally sprayed Cr3C2-NiCr coatings. Four commercial feedstock powders with spherical morphology and different microstructures were deposited by different high-velocity spray processes, namely third-generation gas- and liquid-fueled HVOF torches and by the latest generation HVAF torch. The microstructures of the coatings were studied in the as-sprayed state and after various heat treatments. The high-stress abrasion resistance of as-sprayed and heat-treated coatings was tested at room temperature and at 800 °C. The study reveals that the selection of the spray torch mainly affects the room temperature abrasion resistance of the as-sprayed coatings, which is due to differences in the embrittlement of the binder phase generated by carbide dissolution. At elevated temperatures, precipitation and growth of secondary carbides yields a fast equalization of the various coatings microstructures and wear properties.

A novel non-skid composite coating with higher corrosion resistance

Abstract In this work, low-pressure cold-sprayed Ni-Zn-Al2O3 intermediate layers were deposited between supersonic–plasma-sprayed NiCr-Cr3C2 surface layers and underlying low-carbon steel layers to form a sandwich structure that enhances the corrosion resistance of non-skid NiCr-Cr3C2 coatings. The corrosion performance of these bi-layer non-skid coatings and that of a single-layer coating were investigated through electrochemical measurements and observations of their corrosion morphologies. The novel non-skid coating with a top layer possessing a fine powder grain size exhibited the best corrosion resistance because of the pseudopassivation of the interlayer and physical barriers created by the corrosion process. The intermediate layer substantially improved the corrosion resistance of the non-skid coatings.

Effect of pores on the micromechanics of plasma-sprayed Cr3C2–NiCr coating in the nanoindentation testing

The elastoplastic properties of plasma-sprayed Cr3C2–NiCr coatings were obtained through the dimension analysis and inverse analysis by combining the experimental nanoindentation tests and numerical modeling. The digital image processing technique was used to extract the pore distribution inside the coatings based on the actual microstructure. Finally, the effect of pore distribution on the coating residual stress during nanoindentation process was analyzed through simulations. The anisotropic pore microstructure shows different mechanical responses and stress propagation behaviors during nanoindentation process. The elastic modulus of the coating demonstrates anisotropy along the spraying direction and the transverse directions due to the presence of pores.

Influence of Cr addition on the interface purification of vacuum brazed NiCr-Cr3C2 coatings on single crystal superalloy

In order to improve the high temperature wear- and oxidation resistance of single crystal superalloy, NiCr-Cr3C2 coatings were vacuum brazed on the surface. Different weight ratios of Cr were added into the coatings to control the diffusion of Si and C into the base metal, which would be harmful to the base metal stability. The results show that, when no Cr was added into the coatings, a layer of Ni3Si phase at the interface and MC precipitates in base metal were produced. With the increase of the Cr addition, the coating/base metal interface was gradually purified. The Ni3Si layer and the MC precipitates disappeared when the Cr addition increased to 30wt% and an interface was clearly presented with the γ′ phases on both sides epitaxially growing on some extent. After 200 thermal fatigue cycles, the total crack lengths were measured, which decreased with increasing Cr addition, indicating that the thermal fatigue resistance was improved.

Preliminary Microstructural and Microscratch Results of Ni-Cr-Fe and Cr3C2-NiCr Coatings on Magnesium Substrate

Thermal coatings have a large scale application in aerospace and automotive field, as barriers improving wear mechanical characteristics and corrosion resistance. In present research, there have been used two types of coatings, Ni-Cr-Fe, respectively Cr3C2-NiCr which were deposited on magnesium based alloys (pure magnesium and Mg-30Y master alloy). There have been investigated the microstructural aspects through scanning electronic microscopy and XRD analysis and also a series of mechanical characteristics through microscratch and indentation determinations. The results revealed the formation of some adherent layers resistant to the penetration of the metallic indenter, the coatings did not suffer major damages. Microstructural analysis highlighted the formation of Cr3C2, Cr7C3, Cr3Ni2, Cr7Ni3, FeNi3, Cr-Ni phases. Also, the apparent coefficient of friction for Ni-Cr-Fe coatings presents superior values than Cr3C2-NiCr coatings.

Corrosion behavior of HVOF sprayed hard face coatings in alkaline-sulfide solution

The paper focuses on the corrosion behavior of high velocity oxygen fuel (HVOF) sprayed WC-17Co, WC-10Co-4Cr, Cr3C2-25NiCr coatings in alkaline-sulfide solution (S2−, 0.2ml/L, pH=10). Eighteen days of immersion test is carried out and corrosion rate analysis shows that the Cr3C2-NiCr coating of low porosity exhibits the best corrosion resistance. In alkaline-sulfide solutions, porosity, passive film and microgalvanic between hard phase and binder phase have significant effect on the corrosion behavior of coatings. The corrosion mainly occurs in binder phase from SEM, though WO3, WS2, Cr2S3 are detected in XPS. In WC-17Co coating, the binder phase Co transforms to Co oxides and serious corrosion can be observed in binder phase. WC-10Co-4Cr coatings suffer localized corrosion since galvanic corrosion occurs between locations with different solubilities of W in Co binder. Cr3C2-25NiCr coating shows slight corrosion with the formation of NiS/Ni2O3/Cr2O3from the binder and Cr2S3 from the hard phase. The results are verified by the polarization curves, which show the longest passive region and lowest Icorrosion of Cr3C2-25NiCr coating.

High temperature wear performance of HVOF-sprayed Cr3C2-WC-NiCoCrMo and Cr3C2-NiCr hardmetal coatings

A novel Cr3C2-WC-NiCoCrMo and commercial Cr3C2-NiCr thermal spray-grade powders with particle size of −45+15μm were prepared by an agglomeration and sintering process. Cr3C2-WC-NiCoCrMo and Cr3C2-NiCr coatings were deposited by high velocity oxygen fuel (HVOF) spraying. The fundamental properties of both coatings were evaluated and friction wear test against Al2O3 counterbodies of both coatings at high temperatures (450°C, 550°C, 650°C) were carried out ball-on-disk high temperature tribometer. All specimens were characterized by optical microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and 3D non-contact surface mapping profiler. The results have shown that the Cr3C2-WC-NiCoCrMo coating exhibited lower porosity, higher micro-hardness compared to the Cr3C2-NiCr coating. The Cr3C2-WC-NiCoCrMo coating also exhibited better wear resistance and higher friction coefficient compared to the Cr3C2-NiCr coating when sliding against the Al2O3 counterpart. Wear rates of both coatings increased with raising temperature. Both coatings experienced abrasive wear; hard phase particles (WC and Cr3C2) with different sizes, distributed in the matrix phase, will effectively improve the resistance against wear at high temperatures.

Sliding wear behaviour of HVOF and HVAF sprayed Cr3C2-based coatings

Thermally sprayed tungsten carbide (WC) and chromium carbide (Cr3C2) based hard metal coatings are commonly applied on component surfaces as corrosion and wear resistant layers. Typically, WC-Co/Ni with optional Cr addition and Cr3C2-25NiCr powders are sprayed with high velocity oxy-fuel (HVOF) or high velocity air-fuel (HVAF) processes. Due to the poor oxidation resistance of the WC particles, Cr3C2-25NiCr composition is typically selected for high temperature environments, up to 800–900°C. In this study, two distinct Cr3C2-based compositions of Cr3C2-50NiCrMoNb and Cr3C2-37WC-18NiCoCr were selected as interesting alternatives to conventional Cr3C2-25NiCr. Sliding wear behavior of the coatings sprayed with HVOF and HVAF processes were tested with a ball-on-disk configuration against an Al2O3 ball at room temperature and at 700°C. It was found that both alternative materials had comparable coefficients of friction with the Cr3C2-25NiCr coatings. The Cr3C2-37WC-18NiCoCr coatings provided improved wear resistance at room temperature conditions, but at 700°C the wear rate was increased to the level of the Cr3C2-50NiCrMoNb coatings. Cr3C2-25NiCr coatings experienced the lowest wear rates at elevated temperatures, which was even lower than at room temperature.

Influence of Fracture Toughness and Microhardness on the Erosive Wear of Cermet Coatings Deposited by Thermal Spray

An evaluation of the relationship between the microhardness and fracture toughness with resistance to erosive wear of WC10Co4Cr, WC-12Co, and Cr3C2-25NiCr coatings was conducted. Powder and flexible cored wire feedstock materials were applied by high-velocity oxygen fuel (HVOF) and flame spray (FS), respectively. The erosive wear mechanism prevailing in the coatings was found to be brittle, which also explains the higher erosion rate for the experimental condition using the particle impact angle of 90 deg and impact velocity of 9.33 m/s. The best wear performance was for the coatings applied by HVOF that attains 1.83 mm3/kg for the 90 deg/3.61 m/s test condition. The coating obtained with the WC-10Co4Cr material using the FSFC method showed tungsten carbide decarburization, justifying its poor mechanical properties and poor performance in the erosive wear test. Flame-sprayed flexicords proved to be a promising alternative to HVOF in obtaining coatings with low porosity and acceptable mechanical properties, especially in applications where the use of the HVOF technique is inadequate because of inaccessibility or excessively high cost. Values of K c for the coatings obtained by HVOF (7.35 to 10.83 MPa.m1/2) were between two and three times greater than the values obtained for the coatings resulting from FSFC (2.39 to 3.59 MPa.m1/2), in a similar manner as with the microhardness.

Surface Engineering Analysis of D-Gun Sprayed Cermet Coating in Aggressive Environment

Material degradation due to hot corrosion is a common problem in many engineering system, including coal fired turbines, valves and boiler tubes leading to huge economy losses. Detonation Gun process is a novel coating deposition process among the other thermal spray processes due to its very low porosity, strong adhesion to substrates, high cohesive strength and high hardness. In the present investigation, D-Gun process has been used to deposit Cr3C2–25NiCr coating on 23/8N Nitronic steel. The oxidation behavior of D-Gun sprayed Cr3C2–25NiCr coatings subjected to high temperature (700°C) in actual coal fired boiler for 1000 hours has been carried out in the present work. X-ray diffraction, Field emission–scanning electron microscope technique was used to analyze the scales formed on the surface of the oxidized samples. High volume fraction of carbides contributed for the higher microhardness of the coating (970HV). No microspalling of the scales were showed by the coated specimen. The scale remained intact and adherent to the substrate. The development of protective oxide scale of chromium, nickel and spinel of nickel and chromium have contributed the better oxidation resistance for coated specimen. The Cr3C2-25 NiCr coating imparted better hot corrosion resistance than the uncoated one in the given environment.

Hot Corrosion behaviour of Nanostructured Cermet based Coatings Deposited by Different Thermal Spray Techniques: A Review

Hot corrosion is a serious problem in power plants, boilers, gas turbine and internal combustion engines. There are several alloys and super alloys which are capable to protect the components within a certain limit. So many coatings are widely used to improve service life of components by providing better erosion-corrosion resistance to the substrate. This review is essentially based on the hot corrosion behavior of Nanostructured and conventional Cr3C2-NiCr coatings deposited by several thermal spray techniques. This review is done to summarize the various properties of Chrome carbide nickel chrome coatings deposited by different methods and the hot corrosion behavior of both Nanostructured and conventional coatings in molten salts environment.High-velocity oxy-fuel (HVOF) spraying method is effective method to deposit dense and less porous coatings but a new method has been developed known as Detonation gun (D-Gun) coating,deposited by D-gun has low porosity and high bond strength than HVOF. Plasma spray is also thermal spray technique which possess less porosity, high bond strength, good abrasive and wear resistance and high hardness value but the cost of equipment is very high which is the limitation of this method. The main aim of the review is to summarize the performance of Cr3C2-NiCr coatings in various environment and to study the effect of coating parameters such as coating thickness & porosity on the hot corrosion behavior.

Mechanical and Tribological Properties of HVOF-Sprayed (Cr3C2-NiCr+Ni) Composite Coating on Ductile Cast Iron

The aim of the investigations was to compare the microstructure, mechanical, and wear properties of Cr3C2-NiCr+Ni and Cr3C2-NiCr coatings deposited by HVOF technique (the high-velocity oxygen fuel spray process) on ductile cast iron. The effect of nickel particles added to the chromium carbide coating on mechanical and wear behavior in the system of Cr 3 C 2 -NiCr+Ni/ductile cast iron was analyzed in order to improve the lifetime of coated materials. The structure with particular emphasis of characteristic of the interface in the system of composite coating (Cr 3 C 2 -NiCr+Ni)/ductile cast iron was studied using the optical, scanning, and transmission electron microscopes, as well as the analysis of chemical and phase composition in microareas. Experimental results show that HVOF-sprayed Cr3C2-NiCr+Ni composite coating exhibits low porosity, high hardness, dense structure with large, partially molten Ni particles and very fine Cr3C2 and Cr7C3 particles embedded in NiCr alloy matrix, coming to the size of nanocrystalline. The results were discussed in reference to examination of bending strength considering cracking and delamination in the system of composite coating (Cr 3 C 2 -NiCr+Ni)/ductile cast iron as well as hardness and wear resistance of the coating. The composite structure of the coating provides the relatively good plasticity of the coating, which in turn has a positive effect on the adhesion of coating to the substrate and cohesion of the composite coating (Cr3C2-NiCr+Ni) in wear conditions.

Long-term compositional/microstructural development of Cr3C2-NiCr coatings at 500 °C, 700 °C and 900 °C

Cr3C2-NiCr thermal spray coatings are widely used to mitigate wear at temperatures between 500 and 900°C. During deposition, compositional changes occur in the coating material. With exposure at elevated temperature the metastable as-sprayed structure begins to transform back to an equilibrium composition. The aim of this work was to determine the effect of heat treatment temperature on the mechanisms and rate of coating development at 500–900°C for up to 30days. Compositional development was characterised by X-ray diffraction. Image analysis was used to study the mechanisms of microstructure development and quantify the rate of carbide grain growth. Rapid transformation of the metastable coating phases to an equilibrium composition of Cr3C2 and a Ni alloy occurred within 1–5days at 500°C and within 1day at higher temperatures. The steady state composition matched that of the starting powder. The effect of the compositional/microstructural development on the coating microhardness is discussed.

Thermal Shock Resistance of Plasma Sprayed Multi-Layered Functionally Graded Cr3C2-NiCr Coatings

The interface properties of multi-layered functionally graded Cr3C2-NiCr coatings deposited by plasma spraying technique were experimentally studied in this paper. The microstructure and phase structure of coatings were studied with scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The thermal shock resistance was investigated by cyclic heating and cooling tests using an electro-calefaction furnace. The crack appearances of the coatings were observed carefully. Results show that the plasma sprayed multi-layered functionally graded coatings are compact and the adhesion between the layers and the substrate is good. The coatings have better macro-hardness than the substrate, and the 6-layers coating has the highest macro-hardness and the best wear resistance. Besides, the micro-hardness of coatings increases with increasing content of Cr2C3 in coating materials. Results of cyclic thermal shock show that the main failure styles of the coatings are crack and desquamation and the thermal shock resistance of the coatings is improved obviously by increasing the number of coating layers.

Improving the toughness of thermally sprayed Cr 3C 2-NiCr hardmetal coatings by laser post-treatment

Thermally sprayed hardmetal coatings typically exhibit a pronounced embrittlement of the metallic binder matrix due to carbide dissolution during the deposition process. This characteristic renders the coatings prone to wear by brittle fracture, which lays at the core of many relevant wear phenomena. The present work introduces laser post-treatments as a suitable means of “curing” the microstructure of high-velocity oxygen-fuel sprayed Cr3C2-NiCr coatings from this spray-process induced deterioration. While operating well below the remelting threshold, the essential impact of the laser-generated heat flux is precipitation of secondary chromium-carbides from the supersaturated binder matrix. The concomitant transition from a solid-solution to a precipitation-hardened phase significantly increases the fracture toughness of the binder matrix and renders the coatings more resistant against mechanical wear. In the present work, the microstructural modifications of the coatings upon laser post-treatments were investigated by means of scanning electron microscopy and microhardness probing, and the corresponding impact on the abrasive wear resistance was tested under both high- and low-stress conditions. Major improvements of the high-stress abrasive wear resistance by up to a factor of three were determined and discussed in the context of the microstructure of the wear scars.

Degradation behaviour and microstructural characterisation of HVOF-sprayed Cr<SUB align="right">3C<SUB align="right">2-NiCr cermet coatings in molten salt environment

The present work investigated the cyclic high temperature degradation behaviour of high velocity oxy-fuel (HVOF) sprayed Cr3C2-NiCr coating deposited on 310S stainless steel (310S). The coated as well as uncoated specimens were exposed to molten salt (Na2SO4-60%V2O5) environment at 700°C under cyclic conditions. The kinetics of hot corrosion was established using thermogravimetric technique. The corrosion products were characterised using the techniques such as X-ray diffraction, FE-SEM/EDAX and X-ray mapping for elucidating the corrosion mechanisms. The Cr3C2-25%NiCr coating was found to be successful in maintaining its continuous contact with the alloy substrate during cyclic degradation under simulated laboratory condition. HVOF sprayed Cr3C2-25%NiCr coating on 310S alloy substrate is found to be very effective in reducing the corrosion rate in the aggressive environment at 700°C as observed from the present work. The oxide scales for Cr3C2-25%NiCr coated specimen were found to be intact and spallation-free. The parabolic rate constant of the coated specimen was lower than that of the bare specimen. The better hot corrosion resistance imparted by Cr3C2-NiCr coatings may be attributed to the formation of oxides of chromium, nickel and spinel of nickel and chromium.

Comparison between WC–10Co–4Cr and Cr3C2–25NiCr coatings sprayed on H13 steel by HVOF

WC–10Co–4Cr and Cr3C2–25NiCr coatings were deposited on H13 steel by high velocity oxy fuel (HVOF) spraying. To enhance the thermal stability of the WC–10Co–4Cr coating, NiCr powders were sprayed between the surface coating and substrate. The microstructures of the surface and cross section, thermal shock and wear resistance of these two coatings were investigated in detail. The carbon diffusion in the two coatings was explained from the view of the thermodynamic. And the adhesive strength of Cr3C2–25NiCr coating (64.40 MPa) is almost the same as that of WC–10Co–4Cr coating (61.69 MPa). The friction tests show that the Cr3C2–25NiCr coating has higher friction coefficient than the WC–10Co–4Cr coating at both 500 and 600 °C. The wear resistance of the Cr3C2–25NiCr coating is better than that of the WC–10Co–4Cr coating.

Structure and wear behavior of NiCr–Cr3C2 coatings sprayed by supersonic plasma spraying and high velocity oxy-fuel technologies

The 25%NiCr–Cr3C2 coatings in the experiment were sprayed on 1045 steel substrates by supersonic plasma spraying (SPS) and high velocity oxy-fuel (HVOF) technologies. The microstructures of the coatings were evaluated using environmental scanning electron microscopy (ESEM) and transmission electron microscopy (TEM) equipped with energy dispersive X-ray detector (EDX); the phase composition and chemical composition were analyzed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS); the distribution status and quantity of pores were assessed by an software named as ImageJ2x; the bonding strength, micro-hardness, elastic modulus were respectively experimented and analyzed by different equipments and methods. Besides, wear properties of coatings were determined by UMT-3MT. The results showed that both coatings fabricated by SPS and HVOF had a high micro-hardness, elastic modulus and low porosity; The phase composition of both coatings were similar to that of the NiCr–Cr3C2 feedstock, while the coating deposited by HVOF (HVOF coating) produced a new compound Cr23C6 and was somehow oxidized for the emerging of oxygen analyzed by XPS. Both coefficient of friction (COF) and the mass loss of coating deposited by SPS (SPS coating) were lower than those of HVOF coating under same test conditions. The HVOF coating had an obvious spalling and a deeper scratch. And there was a great deal of micro-cracks around the scratch, especially on both ends of scratch. The main wear mechanisms of both coatings were abrasive wear and lamellar spalling.

High-temperature oxidation of HVOF thermally sprayed NiCr–Cr3C2 coatings: microstructure and kinetics

In this study, 25 wt% Ni(Cr)–75 wt% Cr3C2 powder was thermally sprayed onto 304 stainless steel substrate to produce coatings of ~400-µm thick and the oxidation behaviour of the coatings was investigated. The kinetic behaviour of oxidation of freestanding coatings was investigated over short times at 650, 700 and 800 °C using the thermogravimetric analysis technique. The oxidation rate for all samples was parabolic in nature and the parabolic rate constant (k) increased with increasing temperature. The activation energy (Q) for oxidation was found to be around 226 kJ mol−1. Furnace oxidation tests were also carried out on the as-sprayed samples at 700 and 800 °C in laboratory air for times ranging from 1 h up to 16 days (384 h). The oxide scale growth, morphology and phase formation were investigated using scanning electron microscopy, energy dispersive X-ray analysis and X-ray diffraction. An oxide scale covered the surface of the cermet coating following oxidation at both 700 and 800 °C; the scale was significantly thicker at the higher temperature. It was found to be Cr2O3 and it covered both the metal matrix and the Cr3C2 particles present in the coating. The oxide grains had an angular shape after 16 days of exposure at both temperatures but the grain size was significantly larger after exposure at 800 °C.

High Temperature Behavior of Cr3C2-NiCr Coatings in the Actual Coal-Fired Boiler Environment

Erosion-corrosion is a serious problem observed in steam-powered electricity generation plants, and industrial waste incinerators. In the present study, four compositions of Cr3C2-(Ni-20Cr) alloy coating powder were deposited by high-velocity oxy-fuel spray technique on T-91 boiler tube steel. The cyclic studies were performed in a coal-fired boiler at 1123 K ± 10 K (850 °C ± 10 °C). X-ray diffraction, scanning electron microscopy/energy dispersive X-ray analysis and elemental mapping analysis techniques were used to analyze the corrosion products. All the coatings deposited on T-91 boiler tube steel imparted hot corrosion resistance. The 65 pctCr3C2 -35 pct (Ni-20Cr)-coated T-91 steel sample performed better than all other coated samples in the given environment.