Cr 3C Research Articles

Microstructure dependent erosion in Cr 3C 2–20(NiCr) coating deposited by a detonation gun

Cr 3C 2–20(NiCr) coating powder was deposited on a low carbon steel by pulsed detonation spray gun technique. The coated samples were heat-treated at 600 °C for 11/2 h and allowed to cool in air. A systematic microstructural study was carried out using SEM and TEM to understand the microstructural changes. The mechanical properties like hardness, indentation fracture toughness and adhesion strength of the coating in the as-sprayed and heat-treated conditions were also determined. The change in solid particle erosion of the coating was correlated with the microstructural and subsequent mechanical property changes. It was observed that exposure of the as-sprayed coating to elevated temperature improves the wear resistance. It is concluded that crystallisation of the amorphous phase into nanocrystalline composite combined with better bonding between the adjacent splats through sintering contributes to improved hardness, fracture toughness and wear resistance of the heat-treated coating. The deformation characteristics of the binder phase, amorphous vis-à-vis crystalline, also influence the wear behaviour of the coating.

Tribocorrosion behaviour of HVOF cermet coatings

The main purpose of this work is to analyze the degradation mechanisms induced on industrial HVOF cermet coatings by tribocorrosion. Tribocorrosion of cermet coatings is a subject that has not been widely analyzed in research studies: in fact, while many works dealing with wear or corrosion of HVOF cermet coatings are published, studies relevant to the combined processes (wear and corrosion) are relatively few. The tribocorrosion mechanisms of the cermet coatings were studied in a sodium chloride solution under sliding wear, trying to combine and integrate differently produced mechanical and electrochemical damage phenomena. Electrochemical techniques such as potentiodynamic polarization curves as well as potentiostatic ( I vs t) or galvanostatic ( E vs t) methods were used in order to stimulate and to interprete tribocorrosion degradation mechanisms. It was shown that coating post grinding, which is a mechanical operation usually performed after the deposition of conventional cermet coatings in order to obtain a desired roughness, could produce structural damages, which can greatly affect the mechano-chemical behaviour of the cermet coatings. Mainly abrasive–adhesive wear mechanisms were observed on the coating surface and sometimes, depending on coatings mechanical properties (fracture toughness), cracks developed during wear causing the coating continuity breaking. In the latter case, the degradation mechanism is no longer governed only by surface tribocorrosion, but undermining corrosion can occur, greatly affecting sample performances and promoting coating detachment. Cr 3C 2–NiCr coatings, under all the selected experimental conditions, showed good barrier properties and substrate corrosion was never observed. Moreover, when chromium was added to the metal matrix of WC–Co based systems, tribocorrosion behaviour was enhanced and the lower tribocorrosion rates were measured. Finally, it was shown that electrochemical techniques can be used to govern the coating corrosion processes and to interpret the main degradation mechanisms, even though they seem not to provide a precise quantitative analysis of tribocorrosion.

Processing of hot pressed Al 2O 3–Cr 2O 3/Cr-carbide nanocomposite prepared by MOCVD in fluidized bed

Nanosized particles dispersed uniformly on Al 2O 3 particles were prepared from the decomposition of precursor Cr(CO) 6 by metal organic chemical vapor deposition (MOCVD) in a fluidized chamber. These nanosized particles consisted of Cr 2O 3, CrC 1− x , and C. A solid solution of Al 2O 3–Cr 2O 3 and an Al 2O 3–Cr 2O 3/Cr 3C 2 nanocomposite were formed when these fluidized powders were pre-sintered at 1000 and 1150 °C before hot-pressing at 1400 °C, respectively. In addition, an Al 2O 3–Cr 2O 3/Cr-carbide (Cr 3C 2 and Cr 7C 3) nanocomposite was formed when the particles were directly hot pressed at 1400 °C. The interface between Cr 3C 2 and Al 2O 3 is non-coherent, while the interface between Cr 7C 3 and Al 2O 3 is semi-coherent.

Rare earth effect on the microstructure and wear resistance of Ni-based coatings

Eight different Cr 3C 2–NiCr coatings doped with rare earth oxide were deposited with supersonic plasma spraying. Surface contact profiler, X-ray diffractometer, environmental scanning electron microscopy, multi-functional tribometer, microhardness tester were employed to investigate the microstructure and friction and wear performance of sprayed coatings. The results show that the wear volume loss of coatings doped with 2 wt.% La 2O 3 and 2 wt% La 2O 3 + 2wt.% CeO 2 is three orders of magnitude smaller than that of the coating without rare earth, respectively, correspondingly the friction coefficient of two coatings is only two third of that of the coating without rare earth. According to the SEM worn surface observations, the wear mechanism of the coating doped with 2 wt% La 2O 3 is strain crushed wear induced by plastic deformation and fatigue during the sliding process, which is distinct with abrasive wear and delamination fracture wear mechanisms of the coating without rare earth. Furthermore, rare earth can make the microstructure more compact, less porous and denser, leading to the increase of average microhardness and decrease of its fluctuation range of sprayed coatings.

Sliding wear properties of HVOF sprayed WC–20%Cr 3C 2–7%Ni cermet coatings

WC–20 mass%Cr 3C 2–7 mass%Ni powder was sprayed onto low-carbon steel substrates by a commercial high velocity oxygen-fuel (HVOF) spray process as well as by an improved HVOF process equipped with a gas shroud attachment. The latter process utilizes a nitrogen gas flow to shield the region between the spray gun and the substrate in order to suppress the material's degradation caused by reaction with air such as oxidation and decarburization. Some coatings were further heat-treated in air at 773 K for 30 h to form a thin oxide film on the surface. The sliding wear properties of these coatings against an iron pin were evaluated by using a pin-on-disk wear tester. The specific wear rate of the as-sprayed cermet coatings prepared under the conventional spray condition was about three times higher than that of the chrome plating but by using the gas shroud, the wear rate was reduced to the same level with the chrome plating. The specific wear rate could be further decreased by the oxidation heat-treatment. It was found that a proper amount of oxides existing on the surface or within the coatings have a great beneficial effects on the wear properties such as to promote the transition from severe wear to mild wear and thus to reduce the wear rate remarkably. XPS analysis of the transfer particles collected from the wear track revealed a shift in the oxidation state of iron depending on the wear condition.

Mechanical–thermal synthesis of chromium carbides

The present investigation deals with the synthesis of chromium carbides (Cr 3C 2 and Cr 7C 3), starting from metallic chromium (obtained from the reduction of Cr 2O 3 with Al) and carbon (graphite). The synthesis was carried out via high energy milling, followed by heat-treating of pellets made of different milled mixtures at 800 °C, for 2 h, under an atmosphere of argon. A SPEX CertPrep 8000 Mixer/Mill was used for milling under argon atmosphere. A tool steel vat and two 12.7 mm diameter chromium steel balls were used. The raw materials used and the products were characterized by differential thermal analysis, thermo gravimetric analysis, X-ray diffraction, electronic microscopy and X-ray fluorescence chemical analysis. The following variables were investigated: the quantity of carbon in the mixture, the milling time and the milling power. Mechanical activation of the reactant mixture depends upon the milling power ratio used for processing. The energy liberated by the reduction of the chromium oxide with aluminium exhibits a maximum for milling power ratio between 5:1 and 7.5:1. Self-propagating reaction occurred for all heat-treated samples whatever the carbon content of the sample and the milling power ratio used. Bearing carbon samples exhibited hollow shell structures after the reaction. The level of iron contamination of the milled samples was kept below 0.3% Fe. The self-propagated reaction caused high temperatures inside the samples as it may be seen by the occurrence of spherules, dendrites and whiskers. The carbon content determines the type of chromium carbide formed.

Effects of surface pretreatments on the deposition of adherent diamond coatings on cemented tungsten carbide substrates

Well-adhered microcrystalline diamond (MCD) coatings have been deposited on WC–Co substrates by the microwave plasma enhanced chemical vapor deposition (MPECVD) method. A multi-interlayer system Cr/CrN/Cr was deposited on the cemented carbide substrate before diamond deposition to act as a diffusion barrier. The interlayer-coated substrate was shortly peened by friable diamond powders with an average size of 150 μm to roughen the surface. Diamond coatings deposited on short peened substrates show higher nucleation density and stronger adhesion properties. The X-ray diffraction (XRD) pattern showed that an additional carbide compound layer (Cr 3C 2 and Cr 7C 3) was formed during the CVD diamond deposition to work as an intermediate bonding layer for better adhesion. Rockwell indentation tests with a load of 1470 N were conducted to investigate the coating's adhesion. No delamination outside of the indentation zone was observed for the diamond coating deposited on the roughened sample. Electron probe microanalysis (EPMA) results showed that the delamination in the indentation zone occurred mainly at the diamond/Cr interface and very little Co (less than 1 wt.%) was detected on the Cr failure surface. This suggests that during the CVD process Co/C inter-diffusion was successfully prevented by the Cr/CrN/Cr buffer layers.

Evaluation of hot corrosion resistance of HVOF coatings on a Ni-based superalloy in molten salt environment

High-velocity oxy-fuel (HVOF) has the advantage of being a continuous and most convenient process for applying coatings to industrial installations at site. In this study, Cr 3C 2–NiCr, NiCrBSi, Stellite-6 and Ni–20Cr coatings were deposited on a Ni-based superalloy (19.5Cr–3Fe–0.3Ti–0.1C–balance Ni) using an HVOF process. Hot corrosion studies were performed on bare as well as coated superalloy specimens after exposure to a molten salt environment at 900 °C under cyclic conditions. The thermogravimetric technique was used to establish the kinetics of corrosion. XRD, SEM/EDAX and EPMA techniques were used to analyse the corrosion products. The hot corrosion resistance of all the coatings were superior to bare superalloy. Among the coatings studied, the Ni–20Cr coated superalloy imparted maximum hot corrosion resistance, whereas the Stellite-6 coated indicated minimum resistance. The hot corrosion resistance of all coatings may be attributed to the formation of oxides and spinels of nickel, chromium or cobalt.

Characterizations of WC–10Co nanocomposite powders and subsequently sinterhip sintered cemented carbide

Ultrafine WC–Co cemented carbides, combining high hardness and high toughness, are expected to find broad applications. In this study, WC–10Co–0.4VC–0.4Cr 3C 2 (wt.%) nanocomposite powders, whose average grain size was about 30 nm, were fabricated by spray pyrolysis-continuous reduction and carbonization technology. The as-prepared nanocomposite powders were characterized and analyzed by chemical methods, scanning electron microscopy (SEM), transmission electron microscopy (TEM), BET analysis and atomic force microscopy (AFM). Furthermore, “sinterhip” was used in the sintering process, by which ultrafine WC–10Co cemented carbides with an average grain size of 240 nm were prepared. The material exhibited high Rockwell A hardness of HRA 92.8, Vickers hardness HV 1 1918, and transverse rapture strength (TRS) of 3780 MPa. The homogeneously dispersed grain growth inhibitors such as VC, Cr 3C 2 in nanocomposite powder and the special nonmetal–metal nanocomposite structure of WC–10Co nanocomposite powder played very important roles in obtaining ultrafine WC–10Co cemented carbide with the desired properties and microstructure. There was an abundance of triple junctions in the ultrafine WC–10Co cemented carbide; these triple junctions endowed the sintered specimen with high mechanical properties.

Design of powder metallurgy titanium alloys and composites

Low cost and good performance are two major factors virtually important for Ti alloy development. In this paper, we have studied the effects of alloying elements, thermo-mechanical treatment and particle reinforcement on microstructures and mechanical properties of powder metallurgy (PM) Ti alloys and their composites. Our results indicate that low cost PM Ti alloys and their composites with attractive properties can be fabricated through a single compaction-sintering process, although secondary treatments are required for high performance applications. Three new PM Ti alloys and one TiC/Ti composite of high performance are developed, and new design principles are also proposed. For design of PM Ti alloys, addition of alloying elements has the beneficial effect of enhanced sintering and/or improved mechanical properties. For example, Fe element accelerates the sintering process, Mo and Al are good candidates for solution strengthening, and rare earth elements effectively increase the material ductility by scavenging oxygen from the Ti matrix. For the design of Ti-based composites, in situ formation of strengthening particles and solid solution hardening of the matrix both should be considered simultaneously for alloy development. Cr 3C 2 is found to be a very suitable additive for processing particle reinforced Ti composites.

WC grain growth during the early stages of sintering

During sintering of ultrafine-grained WC–Co hardmetals significant WC grain growth takes place already on heating to the isothermal hold. This growth occurs both isotropically as well as in an anisotropic growth mode, where comparatively large trigonal WC plates are formed. Growth inhibitors, such as VC or Cr 3C 2 are added to suppress this early WC growth, but only VC proved to be successful in exploiting the full potential of today’s finest-grained WC powders.

Investigation of chromium carbide/alumina nano-composite prepared via MOCVD in fluidized bed and densification process

Nano-sized Cr-species particles with a size about 20–40 nm, uniformly coated on alumina particles, has been prepared by metal organic chemical vapor deposition (MOCVD) in the fluidized chamber, using the pyrolysis of Cr(CO) 6 precursor. The Carbothermal reactions of Cr-species in an Ar atmosphere in a graphite furnace at 1250 °C for 2 h and the microstructure of high pressure sintering specimen are reported. The crystalline phase of the powder deposited in fluidized bed was characterized by an X-ray diffractometer, and transmission electron microscopy. The analysis indicates that the deposited powders were metastable CrC 1− x and Cr 2O 3. These Cr-species were transferred into stable chromium carbide (Cr 3C 2 and Cr 7C 3) after the thermal treatment as well as high pressure sintering. Microstructure of sintering specimen shows that the cluster of coating nano-particles were formed in the fluidized bed which coarsen during densification and generally located on the grain boundary to inhibit the alumina grain growth. Further, the independent coating nano-particle was also incorporated into the alumina grains.

Tribological performance of chromium/chromium carbide multilayers deposited by r.f. magnetron sputtering

Hard ceramic and ductile metal materials combined in multilayered structures are well-known to produce coatings with improved mechanical and tribological behavior. In this paper, we report the tribological properties of nanometric chromium/chromium carbide (Cr/CrC) multilayer coatings deposited by r.f. magnetron sputtering. Adhesion, sliding and abrasive wear tests were performed on Cr/CrC multilayers as well as on Cr 3C 2 reference coatings. The dry sliding wear behavior of Cr/CrC multilayers was investigated using the ball-on-disk technique with alumina and steel ball counterparts and the abrasive wear was tested against diamond powder wheel. Multilayer coatings have a higher adhesion on steel substrates than the single layer coatings. The multilayered coatings exhibited a dry sliding wear resistance three times better than the reference. The Cr/CrC multilayers showed dry friction coefficients around 0.65 and the Cr 3C 2 single layer around 0.3, for extended sliding tests. The debris resulting in ball-on-disk tests were analyzed by energy dispersive spectroscopy (EDS) and Raman. The main mechanisms of sliding wear are discussed.

Mechanical and microstructural properties of HVOF sprayed WC–Co and Cr 3C 2–NiCr coatings on the boiler tube steels using LPG as the fuel gas

The characteristics and the phase composition of high velocity oxy fuel (HVOF) sprayed WC–Co and Cr 3C 2–NiCr coatings applied onto boiler tube steel substrates using liquid petroleum gas (LPG) as the fuel gas, have been evaluated. The coatings were examined using metallography, SEM/EDAX and XRD techniques. An attempt has been made to describe the transformations that take place during HVOF spraying. The purpose of this study was to compare the microstructure, porosity, surface roughness and microhardness of HVOF sprayed WC–Co, Cr 3C 2–NiCr coatings deposited on boiler steels. The paper will also investigate if there is an application of the deposit in energy generation plants with a view to enhance the life of the component and reduce tube failures in such power plants.

Correlation of microstructure with hardness and wear resistance in Cr 3C 2/stainless steel surface composites fabricated by high-energy electron beam irradiation

Correlation of microstructure with hardness and wear resistance in Cr 3C 2/stainless steel surface composites fabricated by high-energy electron beam irradiation was investigated in the present study. Three kinds of powder mixtures, i.e., 50Cr 3C 2–50CaF 2(flux), 100Cr 3C 2 and 87.5Cr–12.5C (wt.%), were placed on an AISI 316L stainless steel substrate, which was then irradiated with electron beam. In the specimens fabricated without the flux addition, the surface composite layer of 3.5–4.8 mm in thickness was successfully formed without defects by the self-fluxing effect, and contained a large amount (up to 18 vol.%) of Cr 7C 3 carbides in the austenite matrix. The composite layer fabricated with Cr + C powders having lower melting points than Cr 3C 2 powders was thicker than that fabricated with Cr 3C 2 powders because of more effective melting of Cr + C powders during electron beam irradiation. The hardness and wear resistance of the surface composite layer were directly influenced by hard Cr 7C 3 carbides, and thus were about two times greater than those of the stainless steel substrate.

The temperature ranges for maximum effectiveness of grain growth inhibitors in WC–Co alloys

In order to more effectively control the WC grain growth during liquid phase sintering of submicron WC–Co alloys, the temperature ranges where common grain growth inhibitors are most active have been determined. In the study, small additions (0.5% by weight) of TiC, TaC, NbC, VC, and Cr 3C 2 were added to WC–6Co and WC–10Co alloys. In some cases, a small amount of coarse WC powder was added to simulate the effect of discontinuous grain growth. The powder compacts were vacuum sintered at temperatures ranging from 1300 to 1575 °C. The magnetic coercivity and microstructure results indicated that VC is equally effective in limiting continuous grain growth for all temperatures within the temperature range while the other inhibitors become more effective with temperature. VC was also found to be the most effective inhibitor of discontinuous grain growth. The results of the investigation are expressed in terms of the effectiveness of grain growth inhibition as determined by microstructural analysis, magnetic coercivity and hardness.

Simultaneous synthesis and consolidation of chromium carbides (Cr 3C 2, Cr 7C 3 and Cr 23C 6) by pulsed electric-current pressure sintering

Chromium carbides (Cr 3C 2, Cr 7C 3 and Cr 23C 6) have been synthesized and consolidated simultaneously from mixtures of Cr and amorphous carbon powders by pulsed electric-current pressure sintering (PECPS). Dense ceramics thus obtained were composed of chromium carbides with a small amount of Cr 2O 3, which originates from a trace amount of oxygen adsorbed on the as-received starting Cr powder. Synthesis and consolidation processes, which were observed from their shrinkage curves during PECPS, have been examined using X-ray diffraction (XRD) and scanning electron microscopy for the powder compacts. Cr 3C 2 ceramics sintered at 1300 °C for 10 min under 30 MPa have a 98.9% of theoretical density and fine structures with a 3.6 μm grain size. They exhibit excellent mechanical properties: a bending strength σ b of 690 MPa, a Vickers hardness H v of 18.9 GPa and a fracture toughness K IC of 7.1 MPa m 1/2.

Continuous impact wear resistance of duplex surface-modified hot work tool steel H10

An alternative possibility to increase exposure time of hot work tools is the modification of the tool's surface. The most common method of surface modifying for this kind of tools is nitriding. Unfortunately, nitrided cases often suffer from severe impact wear. Therefore, the development of graded surface layers for backing of the nitrided cases by hard metal monocarbides like TiC, Cr 3C 2, WCCo and WCCoCr, alloyed in the tool's surface could be promising. To cut back expensive production trials, impact wear tests of the duplex-coatings were carried out by a special apparatus where a fixed tungsten carbide ball periodically stroke on the specimens’ surface. In time, an indentation dome was developed which could be evaluated by its diameter. Results have shown that the best impact wear resistance is provided by duplex-layer WCCoCr-plasmanitrided. After the laboratory experiments some of the duplex-layers were tested on simple high speed forging tools for screw-production. With duplex-layer WCCoCr-plasma a 50% increase of tool life time was achieved, though the transferability of laboratory experiment and industrial application proved to be rather unsteady.

HIP after sintering of ultrafine WC–Co hardmetals

This paper analyses the changes induced by HIP after sintering on the microstructure and mechanical properties of WC–7wt.%Co ultrafine hardmetal grade. The well known correlation between porosity reduction and fracture strength improvement is confirmed in these compositions, this being more effective at HIP temperatures above the eutectic point of the alloy. In absence of GGI, hardness decreases continuously as the HIP temperature increases. However, for specimens containing VC and Cr 3C 2 additions, hardness increases as the HIP temperature increases from 1200 °C to 1400 °C. This anomalous trend, confirmed by WC grain size observations, could be related to the activation of coalescence mechanisms during solid state HIPing, which are inhibited by the presence of a liquid phase.

A study on kinetics of Cr xC-coated high-chromium steel by thermo-reactive diffusion technique

There are a lot of technologically interesting characteristics of chromium carbide coating deposited by pack method which is the production of hard, wear-resistant, oxidation- and corrosion-resistant coating layer on the steel substrates. In the present study, kinetic study of chromium carbide coating of AISI D2 steel coated by pack method at the temperature of 1223, 1273 and 1323 K for 1–4 h was realized. The presence of phases Cr 7C 3 and Cr 3C 2 formed on the surface of steel substrates were confirmed by X-ray diffraction analysis. Cross-sectional observation of optical and SEM back-scattered electron images (SEM-BEI) showed that chromium carbide layer formed on the steel substrates were smooth and compact. The hardness of chromium carbide layer is 1850±35 HV 0.025. The kinetics of chromium carbide coating by pack method showed that a parabolic relationship between carbide layer thickness and treatment time, and activation energy for the process is 278 kJ/mol. Moreover, an attempt was made to investigate the possibility of predicting the contour diagram of chromium carbide layer variation and to establish some empirical relationships between process parameters and chromium carbide layer thickness.