Performance in Resistance to Surface Fatigue for Cr3C2–25%NiCr Coatings by Plasma Spray and CDS Spray

Three Cr₃C₂–25%NiCr coatings, prepared by Air Plasma Spray (APS)-Ar/H₂, APS-Ar/He and Continuous Detonation Spraying CDS, have been tested on a double-point-contact fatigue-test machine to examine their surface fatigue limit. Results show that the CDS coating exhibits the best performance in resistance to contact fatigue, while the other two coatings tested in our study are unacceptable as fatigue-resistant materials. On the basis of SEM observations, different failure mechanisms for the coatings are suggested. The fatigue limit of the Cr₃C₂–25%NiCr coatings decreases with the reduction of coating thickness, and the causes for the phenomenon have been discussed.     

Comparative Study of Thermally Sprayed Coatings Under Different Types of Wear Conditions for Hard Chromium Replacement

The tribological properties of part surfaces, namely their wear resistance and friction properties, are decisive in many cases for their proper function. To improve surface properties, it is possible to create hard, wear-resistant coatings by thermal spray technologies. With these versatile coating preparation technologies, part lifetime, reliability, and safety can be improved. In this study, the tribological properties of the HVOF-sprayed coatings WC–17%Co, WC–10%Co4%Cr, WC–15% NiMoCrFeCo, Cr₃C₂–25%NiCr, (Ti,Mo)(C,N)–37%NiCo, NiCrSiB, and AISI 316L and the plasma-sprayed Cr₂O₃ coating were compared with the properties of electrolytic hard chrome and surface-hardened steel. Four different wear behavior tests were performed; the abrasive wear performance of the coatings was assessed using a dry sand/rubber wheel test according to ASTM G-65 and a wet slurry abrasion test according to ASTM G-75, the sliding wear behavior was evaluated by pin-on-disk testing according to ASTM G-99, and the erosion wear resistance was measured for three impact angles. In all tests, the HVOF-sprayed hardmetal coatings exhibited superior properties and can be recommended as a replacement for traditional surface treatments. Due to its tendency to exhibit brittle cracking, the plasma-sprayed ceramic coating Cr₂O₃ can only be recommended for purely abrasive wear conditions. The tested HVOF-sprayed metallic coatings, NiCrSiB and AISI 316L, did not have sufficient wear resistance compared with that of traditional surface treatment and should not be used under more demanding conditions. Based on the obtained data, the application possibilities and limitations of the reported coatings were determined.     

High temperature wear of cermet particle reinforced NiCrBSi hardfacings

The aim of the current study was to investigate erosive and impact/abrasive wear behaviour of TiC–NiMo and Cr₃C₂–Ni reinforced NiCrBSi hardfacings at temperatures up to 700 °C.Coatings were produced using plasma transferred arc cladding process. It was shown that the high temperature wear behaviour of TiC–NiMo and Cr₃C₂–Ni NiCrBSi hardfacings is influenced by oxidation. The formation of mechanical mixed layers and oxide films was observed for both investigated coatings. TiC–NiMo and Cr₃C₂–Ni reinforced hardfacings show high wear resistance at all testing temperatures for both impact/abrasion and erosion conditions.     

Microstructure and tribological properties of laser clad γ/Cr7C3/TiC composite coatings on γ-TiAl intermetallic alloy

In order to improve the wear resistance of the γ-TiAl intermetallic alloy, microstructure, room- and high-temperature (600 °C) wear behaviors of laser clad γ/Cr₇C₃/TiC composite coatings with different constitution of NiCr–Cr₃C₂ precursor-mixed powders have been investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive spectrometer (EDS), block-on-ring (room-temperature) and pin-on-disk (high-temperature) wear tests. The responding wear mechanisms are discussed in detail. Results show that microstructures of the laser clad composite coatings have non-equilibrium solidified microstructures consisting of primary hard Cr₇C₃ and TiC carbides and the inter-primary γ/Cr₇C₃ eutectic matrix, about three to five times higher average microhardness compared with the TiAl alloy substrate. Higher wear resistance than the original TiAl alloy is achieved in the clad composite coatings under dry sliding wear conditions, which is closely related to the formation of non-equilibrium solidified reinforced Cr₇C₃ and TiC carbides and the positive contribution of the relatively ductile and tough γ/Cr₇C₃ eutectics matrix and their stability under high-temperature exposure.     

Role of the strengthening phases in abrasive wear resistance of laser-clad NiCrBSi coatings

The influence of the strengthening phases on the tribological characteristics (wear intensity, specific work of wear, coefficient of friction) and the wear mechanisms in two-body abrasion tests with abrasives of different hardnesses (corundum Al₂O₃, ~2000 HV and silicon carbide SiC, ~3000 HV) has been investigated for PG-SR2 (Cr₂₃C₆, 1000–1150 HV), PG-10N-01 (Cr₇C₃, 1650–1800 HV; CrB, 1950–2400 HV), and 75% PG-SR2 + 25% TiC (TiC, 2500–2900 HV; (Cr,Ni)₂₃(C,B)₆ and (Ti,Cr)(C,B), ~2000 HV) coatings. The dominant role of the strengthening phases (compared with the role of the metal matrix) in the abrasive wear resistance of laser-clad NiCrBSi coatings has been estimated. Different wear mechanisms have been identified and, accordingly, different levels of coatings wear resistance have been achieved depending on the ratio between the hardness of the strengthening phases (carbides, borides, carboborides) and abrasive particles.     

Friction and wear behaviour of Thordon XL and LgSn80 in sliding against plasma-sprayed Cr2O3 coatings

This paper studies the friction and wear behaviour of two important bearing materials, Thordon XL and LgSn80, in dry and lubricated sliding vs. plasma-sprayed Cr₂O₃ coatings. As a reference, AISI 1043 steel is also studied under the same conditions. SEM, EDS and surface topography were employed to study the wear mechanisms. The results indicate that the Thordon XL/Cr₂O₃ coating pair gives the lowest dry friction coefficient (0.16) under a normal load of 45.3 N (pressure 0.453 MPa) at a velocity of 1 m/s. The dry friction coefficient of Thordon XL/Cr₂O₃ coating increases to 0.38 under a normal load of 88.5 N (pressure 0.885 MPa). The dry friction coefficients of the LgSn80/Cr₂O₃ coating are in the range of 0.31–0.46. Secondly, both dry wear rate under low normal load (45.3 N) and lubricated wear rate under a load of 680 N for Thordon XL are lower than those of LgSn80 in sliding against plasma-sprayed Cr₂O₃ coatings at a speed of 1 m/s. However, under a normal load of 88.5 N the dry wear rate of Thordon XL is much higher than that of LgSn80. Thirdly, a high viscosity lubricant (SAE 140) leads to lower wear for Thordon XL and LgSn80 than a low viscosity lubricant (SAE 30). Finally, the dominating wear mechanism for Thordon XL is shear fracture when against the plasma-sprayed Cr₂O₃ ceramic coating. For LgSn80 against plasma-sprayed Cr₂O₃ ceramic coating, abrasive wear is the governing failure mechanism.     

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.     

Influence of Al2O3 reinforcement on the abrasive wear characteristic of Al2O3/PA1010 composite coatings

In the present study, the abrasive wear characteristics of Al₂O₃/PA1010 composite coatings were tested on the turnplate abrasive wear testing machine. Steel 45 (quenched and low-temperature tempered) was used as a reference material. The experimental results showed that when the Al₂O₃ particles have been treated with a silane coupling agent (γ-aminopropyl-triethoxysilane), the abrasive wear resistance of Al₂O₃/PA1010 composite coatings has a good linear relationship with the volume fraction of Al₂O₃ particles in Al₂O₃/PA1010 composite coatings and the linear correlation coefficient is 0.979. Under the experimental conditions, the size of Al₂O₃ particles (40.5-161.0 μm) has little influence on the abrasive wear resistance of Al₂O₃/PA1010 composite coatings. By treating the surface of Al₂O₃ particles with the silane coupling agent, the distribution of Al₂O₃ particles in PA1010 matrix is more homogeneous and the bonding state between Al₂O₃ particles and PA1010 matrix is better. Therefore, the Al₂O₃ particles make the Al₂O₃/PA1010 composite coatings have better abrasive wear resistance than PA1010 coating. The wear resistance of Al₂O₃/PA1010 composite coatings is about 45% compared with that of steel 45.     

Comparison of self-mated hardmetal coatings under dry sliding conditions up to 600 °C

Hardmetal coatings prepared by high velocity oxy-fuel (HVOF) spraying represent an advanced solution for surface protection against wear. In the current systematic study the high-temperature oxidation and unidirectional sliding wear in dry and lubricated conditions were studied. Results for a series of experiments on self-mated pairs in dry conditions as part of that work are described in this paper. Coatings with nominal compositions WC-10%Co4%Cr, WC-(W,Cr)₂C-7%Ni, Cr₃C₂-25%NiCr, (Ti,Mo)(C,N)-29%Ni and (Ti,Mo)(C,N)-29%Co were prepared with an ethylene-fuelled DJH 2700 HVOF spray gun. Electrolytic hard chromium (EHC) coatings and bulk (Ti,Mo)(C,N)-15%NiMo (TM10) hardmetal specimens were studied for comparison. The wear behaviour was investigated at room temperature, 400 and 600 °C. For the coatings sliding speeds were varied in the range 0.1–1 m/s for a wear distance of 5000 m and a normal force of 10 N. In some cases the WC- and (Ti,Mo)(C,N)-based coatings showed total wear rates (sum of wear rates of the rotating and stationary samples) of less than 10^?6 mm³/Nm, i.e., comparable to values typically measured under mixed/boundary conditions. Coefficients of friction above 0.4 were found for all test conditions. The P × V values as an engineering parameter for coating application are discussed. The microstructures and the sliding wear behaviour of the (Ti,Mo)(C,N)-based coatings and the (Ti,Mo)(C,N)-15%NiMo hardmetal are compared.     

Tribological properties of plasma sprayed and laser remelted 75/25 Cr3C2/NiCr coatings

A CO₂ laser was used to fuse based Ni–Cr Cr₃C₂ coatings for the purpose of homogenizing their microstructures and eliminating their porosity. Tests of layer control and wear resistance were carried out on the samples treated with the laser. The results have shown that laser remelting improves the microstructure of the coatings, increases the coating microhardness, and improves coating–substrate adherence. The dry sliding wear behaviour was characterized by the existence of two periods. During the first period the square of the wear volume is proportional to the sliding distance. During the second, the wear volume is proportional to the sliding distance.     

Wear study of Ni–WC composite coating modified with CeO2

In the present investigation, Ni–WC composite powder was modified with the addition of CeO₂ in order to form a new composition of Ni–WC–CeO₂. The Ni–WC and Ni–WC–CeO₂ compositions were used for coating deposition by high-velocity oxy-fuel (HVOF) spraying process so as to study the effect of CeO₂ addition on microstructure, distribution of various elements, hardness, formation of new phases, and abrasive wear behavior. Further, the effect of load, abrasive size, sliding distance, and temperature on abrasive wear behavior of these HVOF-sprayed coatings was investigated by response surface methodology. To investigate the abrasive wear behavior of HVOF-sprayed coatings four factors such as load, abrasive size (size in micrometers), sliding distance (meters), and temperature (°C) with three levels of each factor were investigated. Analysis of variance was carried out to determine the significant factors and interactions. Investigation showed that the load, abrasive size, and sliding distance were the main significant factors while load and abrasive size, load and sliding distance, abrasive size and sliding distance were the main significant interactions. Thus an abrasive wear model was developed in terms of main factors and their significant interactions. The validity of the model was evaluated by conducting experiments under different wear conditions. A comparison of modeled and experimental results showed 4–9% error. The abrasive wear resistance of coatings increases with the addition of CeO₂. This is due to increase in hardness with the addition of CeO₂ in Ni–WC coatings.     

Two-body dry abrasive wear of cermets

The present paper concerns the two-body dry abrasive wear phenomenon of a series of cermets on the base of titanium and chromium carbides with different composition, using a “block on abrasive grinding wheel” test machine. WC–Co hardmetals were used as reference material. Abrasive wear resistance of WC-base hardmetals is superior to that of TiC- and Cr₃C₂-base cermets. The wear coefficient of the cermets reduces with the increase of carbide content in the composites. The volume wear decreases with the increase in bulk hardness. At the first period volume wear of cermets increases linearly with the sliding distance up to the first 100 m; after that the alumina grits become blunt. Scanning electron microscopy examination of the wear tracks in the worn blocks suggests that abrasive wear mechanisms of different cermets are similar and occur through surface elastic-plastic and plastic deformation (grooving). The fracturing of bigger carbide grains and carbide framework the formation of sub-surface cracks by a fatigue process under repeated abrasion is followed by loss of small volumes of the material.     

Microstructure and Dry-Sliding Wear Behavior of Thermal Sprayed and Fused Ni-Based Coatings with the Addition of La2O3

A Ni-based alloy with 1.5 wt% of La₂O₃ powders was thermal sprayed onto steel substrate. The microstructure and dry sliding wear behavior of the coatings were studied by XRD, field emission gun scanning electron microscope (FEGSEM) and SEM analyses. The microstructure of the coating with 1.5 wt% of La₂O₃ differs widely from the coating without La₂O₃; the typical microstructure with 1.5 wt% of La₂O₃ is composed of net-like dendrite (Cr, Fe)₂₃C₆ and Cr₇C₃, cellular-dendrite Fe₂₃(C, B)₆, γ-Ni + Ni₅Si₂ interdendritic lamellar eutectic. Interestingly, significant amounts of net-like (Cr, Fe)₂₃C₆ and Cr₇C₃ hard phases as a wear-resistant skeleton were formed and uniformly dispersed in the coating. Meanwhile, blocky and rod-like hard-phase CrB scattered in the coating can also contribute to improving the wear resistance. The novel microstructure, therefore, is beneficial for wear resistance. Friction and wear tests without lubricant show that the friction coefficients of the coating are less than 0.57. There is an approximately linear relationship between friction coefficients and sliding speed. The wear rate slightly increases with an increase of load, and the wear rate of the coating slightly decreases with sliding speed.     

Comparative evaluation of ambient temperature friction behaviour of thermal sprayed Cr3C2–25(Ni20Cr) coatings with conventional and nano-crystalline grains

Due to their improved hardness and toughness properties, nano-crystalline materials have become attractive for a wide variety of applications. However, as the processing of nano-crystalline materials is quite expensive, efforts have been made to develop coatings using nano-crystalline grains. The present study investigates the ambient temperature friction behaviour of Cr₃C₂–25(Ni20Cr) coatings with nano-crystalline grains (CNG). In the technical process, both, Cr₃C₂–25(Ni20Cr) coatings with conventional grains (CCG), and nano-crystalline grains were deposited using an HVOF technique. Micro-structural characteristics and mechanical properties of the coatings were determined. Friction behaviour of the coatings as a function of different test conditions was evaluated. The specimens worn were examined by a scanning electron microscope (SEM). The results showed that the coefficient of friction of the CNG against 100Cr6 steel is lower than that of CCG against 100Cr6 steel. The principal types of material removal of CNG on the one hand, and CCG on the other, differ. Maps showing the prevalence of different wear mechanisms for different wear conditions were established for both types of coatings.     

Effect of temperature and load on three-body abrasion of cermets and steel

The effect of temperature and load on three-body abrasion resistance has been examined for stainless steel, Cr₃C₂-Ni cermet, plain WC-Co hardmetal and yttria stabilized zirconia doped WC-based composites. Series of tests at various tribo-conditions were performed on a recently developed device. Coefficient of friction and materials response to abrasive actions have been analyzed and positive effect of zirconia addition on materials wear resistance has been shown. The low wear rates of ZrO₂ containing cermets are due to lower susceptibility of zirconia to transgranular crack propagation, smaller mean free path between ceramic grains and formation of lubricating glazed silica-rich layer.     

Direct laser cladding of SiC dispersed AISI 316L stainless steel

The present study concerns development of SiC dispersed (5 and 20 wt%) AISI 316L stainless steel metal-matrix composites by direct laser cladding with a high power diode laser and evaluation of its mechanical properties (microhardness and wear resistance). A defect free and homogeneous composite layer is formed under optimum processing condition. The microstructure consists of partially dissociated SiC, Cr₃C₂ and Fe₂Si in grain refined stainless steel matrix. The microhardness of the clad layer increases to a maximum of 340 VHN (for 5% SiC dispersed) and 800 VHN (for 20% SiC dispersed) as compared to 150 VHN of commercially available AISI 316L stainless steel. Direct laser clad SiC dispersed AISI 316L stainless steel has shown an improved wear resistance against diamond surface with a maximum improvement in 20% SiC dispersed AISI 316L stainless steel. The mechanism of wear was predominantly abrasive in nature.     

Friction and wear behaviour of plasma-sprayed Cr2O3 coatings against steel in a wide range of sliding velocities and normal loads

This study investigates the influence of sliding speed and normal load on the friction and wear of plasma-sprayed Cr₂O₃ coatings, in dry and lubricated sliding against AISI D2 steel. Friction and wear tests were performed in a wide speed range of 0.125–8 m/s under different normal loads using a block-on-ring tribometer. SEM, EDS and XPS were employed to identify the mechanical and chemical changes on the worn surfaces. A tangential impact wear model was proposed to explain the steep rising of wear from the minimum wear to the maximum wear. The results show that the wear of Cr₂O₃ coatings increases with increasing load. Secondly, there exist a minimum-wear sliding speed (0.5 m/s) and a maximum-wear sliding speed (3 m/s) for a Cr₂O₃ coating in dry sliding. With the increase of speed, the wear of a Cr₂O₃ coating decreases in the range 0.125–0.5 m/s, then rises steeply from 0.5 m/s to 3 m/s, followed by a decrease thereafter. The large variation of wear with respect to speed can be explained by stick-slip at low speeds, the tangential impact effect at median speeds and the softening effect of flash temperature at high speeds. Thirdly, the chemical compositions of the transfer film are a-Fe₂O₃ in the speed range 0.25–2 m/s, and FeO at 7 m/s. In addition, the wear mechanisms of a Cr₂O₃ coating in dry sliding versus AISI D2 steel are adhesion at low speeds, brittle fracture at median speeds and a mixture of abrasion and brittle fracture at high speeds. Finally the lubricated wear of Cr₂O₃ coating increases sharply from 1 to 2.8 m/s.     

Influence of Heat Treatment on Microstructure and Sliding Wear of Thermally Sprayed Fe-Based Metallic Glass Coatings

Fe₆₂Ni₃Cr₄Mo₂W₃Si₆B₁₇C₃ amorphous coatings were thermally sprayed by a high velocity oxygen fuel spraying system (DJ-2700) and heat-treated at the temperatures ranges from 873 to 1,173 K in vacuum for 1 h. Differential scanning calorimetry, X-ray diffraction (XRD), and scanning electron microscopy were used to study the microstructural characteristics of the coatings. Vickers hardness tester was used to measure the hardness of the coatings. At the same time, the sliding wear behavior of the coatings was evaluated in a reciprocating ball-on-disk system. Within the resolution of XRD, amorphous structure without apparent crystalline phases was obtained in the as-sprayed coating. The heat treatments above 873 K led to the crystallization of amorphous phase. With the increase of heat treatment temperature, diffusion and sintering could occur between the layers of the coatings. The highest microhardness was obtained in the coating heat-treated at 973 K. When wear tested at a relative low load of 2 N, a direct correlation between the hardness and wear resistance of the coatings seems to be reasonable. However, at relative high loads, the wear resistance of the coatings is dependent on the resistance to crack initiation and growth between the layers rather than the hardness.     

Tribological and corrosion behaviors of Al2O3/polymer nanocomposite coatings

In this paper, the tribological and electrochemical corrosion properties of Al₂O₃/polymer nanocomposite coatings were studied by using micro-hardness test, single-pass scratch test, abrasive wear test, and finally electrochemical technique such as potentiodynamic polarization measurement. The coatings containing Al₂O₃ nanoparticles showed improvement in scratch and abrasive resistance compared with that of polymer coating. The improvement in scratch and abrasive resistance is attributed to the dispersion hardening of Al₂O₃ nanoparticles in polymer coatings. Corrosion test results showed that the embedded Al₂O₃ nanoparticles in polymer matrix do not sacrifice the corrosion resistance of the polymer itself.     

Cr2N/CrN multilayer coatings for wood machining tools

The present paper describes results from a recent research project aimed at forming a wear resistant coating based on chromium on tools to wood machining. Cr₂N/CrN multilayer coatings deposited on HS6-5-2 steel substrates using cathodic arc evaporation were tested. These coatings were formed from 7 bilayers being ca. 340 nm thick and equally thick Cr₂N and CrN layers. For comparison, Cr₂N and CrN monolayer coatings were also prepared. Hardness measurements, indentation and scratch tests, friction and wear were performed to characterize the mechanical properties. The wear tracks and Rockwell indentations enable to assess wear mechanisms of the coatings. The results of the Cr₂N/CrN coatings investigated show high hardness: ca. about 22 GPa and a critical force being higher than 95 N and a low wear rate.The industrial tests of planer knives with Cr₂N/CrN multilayer coatings were carried out on a down-spindle milling machine to determine the durability of tools with wear resistant coatings for woodworking. These tools show increase of “life time” two times. Another positive feature of the use of such tools is the increase of the quality of wood surface machined when compared with uncoated tools.