NiCr Coatings Research Articles

CrO2(OH)2 volatilization rate and oxidation behavior prediction of the NiCr coating in air-H2O environment at 650 ℃

The volatilization rate and lifetime prediction of the NiCr coatings produced by high-velocity oxygen-fuel were investigated at 650 °C in air containing 20 % water vapor. Water vapor could significantly accelerate the consumption of Cr which reduced the service time of the coatings. Oxidation kinetic rate of the NiCr coating and volatilization rate of the CrO2(OH)2 were calculated using classical gas transport theory in the viscous-flow regime. According to the practical oxidation kinetic rate, we built a relationship to predict the lifetime of the NiCr coating.

Microstructure and Properties of Cold Sprayed NiCrAl Coating on AZ91D Magnesium Alloy

Herein, a NiCrAl coating was prepared on the AZ91D magnesium alloy by cold spraying technology. The microstructure, wear resistance, and corrosion resistance of the cold sprayed NiCrAl coating were studied and compared with two NiCrAl coatings prepared by plasma spraying. The results showed that the porosity of the two-plasma sprayed NiCrAl coatings was 3.21% and 2.66%, respectively, while that of the cold sprayed NiCrAl coating was only 0.68%. The hardness of the cold sprayed NiCrAl coating (650 HV0.1) was higher than those of the two-plasma sprayed NiCrAl coatings (300 HV0.1, 400 HV0.1). In the abrasion resistance test, the cold sprayed NiCrAl coating showed a lower friction coefficient (0.346), less wear volume (3.026 mm3), and superior wear resistance accordingly compared with the two-plasma sprayed NiCrAl coatings. Moreover, the scanning electron microscopy (SEM) morphology at the bottom of the wear trace of the cold sprayed NiCrAl coating showed a compact mechanically mixed layers (MML) structure, and its wear mechanism was mainly abrasive wear, with some fatigue wear. In the electrochemical test, the corrosion current density of the cold sprayed NiCrAl coating (4.404 × 10−2 A·cm−2) was much lower than those of two plasma sprayed coatings (25.96 A·cm−2, 26.98 A·cm−2), indicating that the cold sprayed NiCrAl coating had superior corrosion resistance. Therefore, preparing a cold sprayed NiCrAl coating is a feasible method to comprehensively improve the wear resistance and corrosion resistance of the AZ91D magnesium alloy.

Effect of Laser Surface Melting on Microstructure of Cold Sprayed Ni20Cr Coatings

Effect of Laser Surface Melting on Microstructure of Cold Sprayed Ni20Cr Coatings

Erosion-Corrosion Protection Due to Cr3C2-NiCr Cermet Coating on Stainless Steel

Cr3C2–NiCr coatings have been used extensively to combat the erosion corrosion of hydro power turbine blades made of stainless steel. Cr3C2–NiCr coatings are also used in aqueous corrosive environments due to the high corrosion resistance rendered by the NiCr binder. In this investigation, both erosion and corrosion environments are introduced to cermet coating to study corrosion behavior using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The cermet coatings are useful for reducing the risk of deterioration of mechanical properties of hydro power turbines due to the continuous exposure to the erosive and corrosive action of the corrosive environment containing silt. It was observed that Cr3C2–NiCr coating offered a reasonable improvement in corrosion resistance when compared to bare substrate. The corrosion behavior of the coating was studied in a 150 mL solution of 0.1 M NaCl with 2 gms of quartz particles (0.2–0.8 mm) at various rotation speeds (3000, 4500, 6000 rpm) of the solution over a 1 h immersion using potentiodynamic polarization and EIS studies in a specifically designed experimental set-up for erosion corrosion. When compared to the bare stainless steel samples at 3000 rpm and 6000 rpm, the coating showed the highest improvement at 6.57 times and the least improvement at 3.79 times, respectively.

Effect of Ni60-WC on Properties of NiCrAl Coating

Ni60-WC reinforced NiCrAl coatings were prepared on the TC4 alloy by laser cladding technique with different mixed powders: 95%NiCrAl-5%Ni60-WC, 85%NiCrAl-15%Ni60-WC, 75%NiCrAl-25%Ni60-WC (wt.%). The microstructure, phases compositions, microhardness, wear properties were studied. The results showed that the mainly consists of dendrites and bulk like compounds crystal and some of flower-like microstructure formed. The XRD results indicated TiC, VC metalloceramics, Cr0.2Ti0.8C, NiTi2, Ni2W4C and AlFe3C intermetallic compounds generated. The microhardness of the coatings are 881HV0.3, 778HV0.3 and 788HV0.3, respectively. Compared with the substrate hardness about 200HV0.3, the hardness of coatings have been improved significant. The friction coefficient of coatings are 0.35, 0.42, and 0.42, respectively, and the wear rates of the coatings are 3.86×10-7mm3N-1m-1, 4.51×10-7mm3N-1m-1 and 4.34×10-7mm3N-1m-1, respectively.

Influence of the detonation-spraying mode on the phase composition and properties of Ni-Cr coatings

Influence of the detonation-spraying mode on the phase composition and properties of Ni-Cr coatings

Microstructure and Property Modification of Cold Sprayed Coatings Using Different Grain Sizes of Cr3C2-25(Ni20Cr) Composite Powder

This article describes the microstructure and mechanical properties of Cr3C2-25(Ni20Cr) cermet coatings cold sprayed on Al 7075 alloy substrates. The aim of the study was to determine the effect of three different powder grain sizes and three different standoff distances on the microstructure (phase composition, surface topography, volume fraction and distribution of the Cr3C2 phase in the Ni20Cr matrix and porosity) and the mechanical properties (hardness and abrasive wear resistance) of the Cr3C2-25(Ni20Cr) coatings. The examinations revealed that the thickness of the coatings obtained with same duration and feed rate decreased with increasing powder grain size and standoff distance. The coatings produced from powders with smaller particles had higher hardness and more compact structures than the coatings obtained from powders where the particles were larger (more than 40 μm). The coatings produced from finer powders were also more resistant to abrasive wear.

Erosion corrosion behaviour and mechanical properties of wire arc sprayed Ni-Cr and Ni-Al coating on boiler steels in a real boiler environment

ABSTRACT The aim of the current investigation is to evaluate the erosion-corrosion (E-C) performance of wire arc sprayed Ni-20Cr and Ni-5Al coatings on T22 and SA516 boiler steels in a real boiler environment. The microstructure, mechanical properties and high temperature E-C performance of the deposited coatings were investigated. The E-C behaviour of the test samples was determined in the superheater zone of the coal fired boiler at 750°C for fifteen cycles, where each cycle involved 100 hours of heating followed by 1hour of cooling. Thickness loss data was used to determine the E-C behaviour of the coating. XRD, SEM/EDS were used to analyse the test samples. Ni-20Cr coated SA516 steel decreased maximum E-C rates by 98.05%, while Ni-5Al coated T22 steel reduced E-C by 68.63% than the base steel. This higher E-C resistance is due to the formation of protective phases (Cr2O3, NiO and Al2O3) and better mechanical properties of the coatings.

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.

Microstructure and cavitation erosion behavior of HVOF sprayed ceramic-metal composite coatings for application in hydro-turbines

Cavitation erosion commonly occurs in hydro-turbine blades and has been a crucial issue for operation of hydro-turbines. In this paper, high-velocity oxygen-fuel (HVOF) spraying was applied to produce Cr3C2–NiCr and WC-CoCr ceramic-metal coatings. The microstructure and the relationship between the flow velocity and cavitation erosion behavior of the coatings were evaluated, and their cavitation erosion mechanisms at different flow velocities were discussed. Experimental results showed that the WC-CoCr coating exhibited slightly lower porosity and significantly higher microhardness as compared to the Cr3C2–NiCr coating. Both ceramic-metal coatings were combined well with the steel substrate and showed an increase in the volume loss rates with increasing flow velocity. The volume loss rates for the WC-CoCr coatings were lower than that of the Cr3C2–NiCr coatings at each flow velocity. The critical flow velocity of the Cr3C2–NiCr coating was in the region of 33.5–41.9 m s−1. The cavitation erosion process of the Cr3C2–NiCr coatings mainly included removal of chromium carbide particles, coalescence of the craters and brittle detachment of the coating, while the failure mechanism of the WC-CoCr coatings was erosion of the binder phase at 23.4 m s−1, cavitation pits interlinking at 33.5 m s−1, and detachment of the WC particles at 41.9 m s−1.

Tribological assessment of NiCr, Al2O3/TiO2, and Cr3C2/NiCr coatings applied on a cylinder liner of a heavy-duty diesel engine

Internal combustion engines consume about 90% of fuel refined from crude oil which supplies 30% of the annual global flow of energy. Heavy-duty diesel engines are the primary source of power used in highways, marine, railroads, and power stations. The right coating can improve the tribological properties of cylinder liners and increase the mechanical efficiency of an engine. Also, it can help to extend the maintenance periods, and enhance the reliability of the vehicles. In this research, tribological and economic evaluations were performed for coated and uncoated substrates from a cylinder liner of a heavy-duty diesel engine, aiming to lower friction, wear rate, and maintenance cost. A reciprocating friction test was conducted under dry condition using Wolfram carbide (tungsten carbide) ball applied a 10 N normal load on a ball on disk geometry. The cylinder liner was made of gray cast iron, and the substrates obtained were coated with three different coating materials (Cr3C2/NiCr, NiCr, and Al2O3/TiO2) through the thermal spray and high-velocity oxy-fuel coating process. Tribological evaluations showed that the substrates coded with Al2O3/TiO2 and Cr3C2/NiCr had the lowest friction coefficient and wear rate. The most economical coating was Al2O3/TiO2, being able to supply about 61% lower coefficient of friction and 94% less wear rate relative to the uncoated sample, for the price of one-third of the Cr3C2/NiCr coating and one half of a new gray cast iron cylinder liner.

Microstructure and Selected Properties of Cr3C2-NiCr Coatings Obtained by HVOF on Magnesium Alloy Substrates.

In present work the Cr3C2–NiCr coating was deposited on magnesium alloy substrate with high velocity oxygen fuel (HVOF) spraying. The microstructure of the samples has been characterized by means of electron microscopy, SEM and phase composition analysis carried out. The porosity of coatings has been also estimated. Finally, tests of selected mechanical properties, such as instrumented indentation, abrasive erosion have been performed. The results of the investigations confirmed that dense, homogeneous and well-adhered Cr3C2–NiCr cermet coating is possible to obtain onto the magnesium AZ31 alloy substrate. Moreover, the coatings exhibit high resistance to erosion.

Investigation of PVD thin films as hydrogen barriers in aluminized press hardened steels (PHS)

The reduction of hydrogen intake of aluminized boron press hardened steel Usibor®1500 during austenitisation at 900 °C under N2 humid atmosphere has been explored by applying thin physical vapour deposition (PVD) coatings. A wide range of metallic coatings have been explored including Au, Cu, Ti, Y, Mo, W, Cr, Ni, Ni0.8Cr0.2, Ni0.4Cr0.6, Inconel 690 and stainless steel 316L as well as some nitrides WN, CrN and oxides Al2O3 and SiO2. W, Y and Ti enhance the increase of hydrogen intake, SiO2 and Al2O3 have no effect while Au, Cu, Mo, CrN, WN, Cr, Ni and NiCr alloys provide a decrease of the hydrogen ingress. The barrier effects of these coatings are discussed as a function of their thicknesses and chemical composition of the NiCr coatings. The most efficient barrier to hydrogen intake is obtained in this study for Ni0.8Cr0.2 coating with a reduction higher than 50% for an optimum coating thickness of 200 nm. The structure and the surface of the latter coating are investigated after austenitisation by a combination of techniques.

Fireside corrosion on T24 steel pipes and HVOF NiCr coatings exposed to different salt mixtures

Fireside corrosion of T24 steel and HVOF NiCr-coated counterparts is investigated. The samples were exposed to different deposits of NaCl, KCl, Na2SO4 and K2SO4 at 650 °C for 360 h in dry air. The deposit with the highest chloride content (NaCl-KCl) was the most aggressive followed by the ones combining chlorides and sulfates. In contrast, the sulfate-based salts did not produce severe damage. The NiCr coating maintained the integrity of the substrate but the chlorides induced a great attack on the coating. The formation of K2CrO4 and the penetration of chlorides are identified as the main factors of this attack.

Surface Engineering Analysis of D-Gun Sprayed Ni-20Cr Coating in Aggressive Environment

Surface Engineering Analysis of D-Gun Sprayed Ni-20Cr Coating in Aggressive Environment

Evaluation of the corrosion resistance of Ni(P)Cr coatings for bipolar plates by electrochemical impedance spectroscopy

In the present research, the corrosion resistance of Ni–P and Ni–P–Cr coatings on AA7075-T6 aluminum plates under simulated anodic and cathodic conditions of polymer electrolyte membrane fuel cells (PEMFC) has been studied by electrochemical impedance spectroscopy (EIS). Three Ni–P coatings 20 μm, 30 μm, and 40 μm thick applied by electroless deposition were tested. Besides, a two-layer Ni–P–Cr coating 30 μm thick was also analyzed. It was formed by an inner Ni–P layer, and an outer 10 μm thick chromium one added by electroplating. Corrosion tests were combined with interfacial contact resistance (ICR), roughness, contact angle, and SEM-EDX measurements. The best results were obtained for the 20 μm Ni–P and the two-layer Ni–P–Cr coatings, although the latter showed a high ICR value due to the high electrical resistivity of the chromium oxide surface formed. It was verified that coating degradation occurs when the electrolyte penetrates the micro-cracks and the nodular surface interfaces, reaching the base metal and causing the coating delamination. This behavior is associated with a sharp decrease in the polarization resistance (Rp) of the equivalent circuit model fitted to the EIS results.

TiC–NiCr thermal spray coatings as an alternative to WC-CoCr and Cr3C2–NiCr

TiC-based hardmetal coatings containing 25 or 40 vol% Ni-20 wt%Cr matrix (hereafter TiC–25NiCr and TiC–40NiCr) were obtained by High Velocity Oxygen-Fuel (HVOF) and High Velocity Air-Fuel (HVAF) spraying, starting from high-energy ball milled feedstock powders. These coatings are intended as critical raw materials-free solutions against wear and corrosion.HVOF-sprayed coatings contain some more oxide inclusions than do HVAF ones, but, irrespective of the deposition conditions, TiC–40NiCr coatings are usually somewhat harder (800–900 HV0.3) than TiC–25NiCr ones. They also exhibit lower wear rates in ball-on-disc sliding tests against Al2O3 at room temperature. A hard asperity can indeed penetrate slightly deeper into TiC–25NiCr, as it deforms inelastically through microcracking. Bigger abrasive grooves are thus produced.The wear resistance of TiC–40NiCr coatings compares favourably to that of a Cr3C2-25% (NiCr) reference, and even approaches that of WC-10 wt%Co-4wt.%Cr. TiC–40NiCr coatings are also more corrosion resistant than both reference materials when tested by electrochemical polarization in a 3.5% NaCl solution.At 400 °C, to the contrary, TiC–25NiCr coatings exhibit better sliding wear resistance, whilst more severe abrasive grooving and adhesive tearing affect TiC–40NiCr samples. TiC–NiCr coatings are also unaffected by the transverse macro-cracking that was found to compromise the usefulness of WC-CoCr at 400 °C.

High-Temperature Erosive Wear Behavior of High-Velocity Oxy-Fuel Sprayed Cr3C225 (Ni20Cr) Coating on (AISI 316) Austenitic Steel

Abstract AISI 316 austenitic steel is extensively used in various components of power plants like boilers, boiler tubes and pipes, which suffer severe air jet erosion. Fly ash is a major erodent in that case. Present study is conducted to study the effect of Cr3C225 (Ni20Cr) coating on 316 substrates, when it is exposed to thermal power plant erosive conditions. High-velocity oxy-fuel deposition technique is used for coating. Major constituent of Indian fly ash is alumina, so alumina particles are taken as erodent. Erosion wear behavior is investigated for coated and uncoated conditions using an air jet erosion tester at 100 m/s impact velocity. Three impact angles, i.e., 30 deg, 60 deg, and 90 deg, and four working temperatures, room temperature, 200 °C, 400 °C, and 600 °C, were chosen to identify wear mechanism. Scanning electron microscopy and energy dispersive X-ray spectroscopy (EDX) were utilized to characterize the coated, uncoated, and eroded surface. Erosion behavior is correlated with micro hardness, roughness, and microstructure. Results reveal that the coated surface offers better erosion resistance than the uncoated surface. Substrate exhibits ductile wear behavior as it shows higher wear rate at low angle and decreases on increasing the impact angle, while coating offers good wear characteristics at 30 deg and 90 deg impact angles. An increase in working temperature favors wear rate increment for both coated and uncoated samples. However, coated samples exhibit ductile erosive behavior at high temperatures. Uncoated surfaces have micro-cutting and deformation as major erosive mechanisms. Whereas for coated samples at high temperature, oxide layer formation takes place and erosion takes place due to spalling of the oxides from the coated surface.

Wear and friction characteristics of atmospheric plasma sprayed Cr3C2–NiCr coatings

ABSTRACT The present work focuses on investigating the wear and friction characteristics of the Atmospheric Plasma Sprayed Cr3C2-NiCr coatings deposited onto the surface of die steel material. The as-sprayed specimens were characterized. The coating porosity, bond strength and microhardness values were evaluated. Wear tests were performed on the high-temperature pin-on-disc tribometer at room temperatures, 400°C and 800°C under two loads as 25N and 50N in the laboratory. The wear mechanisms of all the worn-out samples were studied by scanning electron microscopy (SEM) technique. The specific wear rates and the coefficient of friction values were analyzed. The developed coating showed better wear resistance than its uncoated counterpart. The coefficient of friction values for coated specimens decreased at elevated temperatures. At room temperatures, the wear mode was observed to be adhesive and further at elevated temperatures of testing, the wear mode was observed to be the combination of oxidative, adhesive and abrasive.

Creation of modified Cr3C2–NiCr hardmetal coating microstructures through novel processing

High velocity thermal spray Cr3C2–NiCr coatings are widely used to mitigate wear at high temperatures and in corrosive environments due to their superior oxidation and corrosion resistance relative to WC-Co coatings. This work explored a novel two–step process of first depositing a coating from feedstock particles having a high liquid phase content by shrouded plasma spraying (SPS), followed by controlled precipitation of the carbide grains through heat treatment. The appearance of additional melt from peritectic decomposition of Cr3C2, protection from oxidation by the shroud, as well as absence from water vapour of hydrocarbon combustion, created a new quality of microstructure. Heat treatment generated uniquely novel coating microstructures of continuous carbide within which pools of Ni alloy formed, in direct contrast to the conventional coating microstructure of isolated carbide grains encapsulated within a continuous Ni binder. The variation in the as-sprayed chemical and phase compositions between the SPS and HVOF coatings played a direct role in the dramatic microstructural variation seen with heat treatment. The mechanism of compositional and microstructural development with heat treatment in the SPS coating is contrasted to that in conventional HVOF coatings. Microhardness values above 1000 VHN300 demonstrate the potential of this new type of coating.