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Sliding Wear Properties of HVOF Sprayed WC-10Co4Cr Coatings With Conventional Structure and Bimodal Structure Under Different Loads
Abstract
The WC-10Co4Cr coatings with conventional structure and bimodal structure were sprayed by high velocity oxygen fuel (HVOF) technology. The phase compositions and morphologies of the WC-10Co4Cr powders and coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The microhardness, porosity, bonding strength, elastic modulus and indentation fracture toughness of the conventional coating (Conventional) and the bimodal coating (Bimodal) were also studied. The sliding wear properties of the Conventional and the Bimodal against Si3N4 counterballs under different loads at room temperature (~25 °C) were investigated using a friction and wear tester. Compared with the Conventional, the Bimodal has denser microstructure, lower porosity, more excellent mechanical properties, and the Bimodal has better wear resistance than the Conventional under different loads. The two coatings under 15 N and 30 N only exhibit abrasive and slightly adhesive wear mechanism, while in the load application of 45 N, additional mechanism which is fatigue is detected and causes flaking of the coating.
FeMnCoCr–WC coatings with WC mass fractions of 0 %, 5 % and 10 % were prepared on Ti6Al4V alloy by plasma spraying to improve the tribological properties of FeMnCoCr coating. The effects of WC mass fraction on the microstructure, chemical elements and phases of obtained coatings were analyzed via a scanning electron microscope, super–depth of field microscope, energy dispersive spectrometer and X–ray diffraction, respectively, and the wear behavior of FeMnCoCr–WC coatings at room temperature and 500 ◦C was also evaluated using a
ball–on–disc wear tester. The results show that the WC addition improves the coating hardness through the grain refinement, and the coefficients of friction (COFs) and wear rates of coatings are decreased with the increase in WC mass fraction at roomtemperature and 500 ◦C. The wear rates of FeMnCoCr–WC coatings at 500 ◦C are increased compared with those at room temperature, which are attributed to the softening effect and plastic deformation at high temperature. The wear mechanism of FeMnCoCr–WC coatings at RT is changed from adhesive wear + oxidative wear + abrasive wear to abrasive wear + delamination wear + oxidative wear as the WC mass fraction increases; while those of at 500 ◦C are not changed with the increase in WC mass fraction.
Thermally sprayed ceramic coatings are applied for the protection of surfaces that are
exposed mainly to wear, high temperatures, and corrosion. In recent years, great interest has been garnered by spray processes with submicrometric and nanometric feedstock materials, due to the refinement of the structure and improved coating properties. This paper compares the microstructure and tribological properties of alumina coatings sprayed using conventional atmospheric plasma spraying (APS), and various methods that use finely grained suspension feedstocks, namely, suspension plasma spraying (SPS) and suspension high-velocity oxy-fuel spraying (S-HVOF). Furthermore, the suspension plasma-sprayed Al2O3 coatings have been deposited with radial (SPS) and axial (ASPS) feedstock injection. The results showed that all suspension-based coatings demonstrated much better wear resistance than the powder-sprayed ones. S-HVOF and axial suspension plasma spraying (A-SPS) allowed for the deposition of the most dense and homogeneous coatings. Dense-structured coatings with low porosity (4 vol.%) and good cohesion to the metallic substrate, containing a high content of α–Al2O3 phase (56 vol.%) and a very low wear rate (0.2 ± 0.04 mm3 × 10−6/(N·m)), were produced with the S-HVOF method. The wear mechanism of ceramic coatings included the adhesive wear mode supported by the fatigue-induced material delamination. Moreover, the presence of wear debris and tribofilm was confirmed. Finally, the coefficient of friction for the coatings was in the range between 0.44 and 0.68, with the highest values being recorded for APS sprayed coatings.
The purpose of this study is to investigate the feasibility of replacing cobalt with nickel as a binder in thermal spraying WC-based coatings. Two kinds of coatings WC-13Ni4Cr and WC-10Co4Cr were deposited by the detonation spraying technology in which propane was added into the detonation gases. The relative content of W 2 C and W phases in the coatings was calculated by XRD quantitative analysis method. Wear resistance of the coatings was characterized by ASTM G65 rubber-wheel abrasion test. The results indicate that the decomposition of WC particles in both coatings decreases, while the fracture toughness of the coating increases as the propane flow increases. Wear resistance of WC-based coatings is correlated with the hardness and fracture toughness of the coatings. The wear resistance of both coatings is substantially improved when increasing the propane flow rate. Experimental results show that it is feasible to replace cobalt with nickel in thermal-sprayed WC-based coatings.
Inverse problem of manufacturing is studied under a framework of high performance manufacturing of components with functional surface layer where controllable generation of surface integrity is emphasized due to its pivotal role determining final performance. Surface modification techniques capable of controlling surface integrity are utilized to verify such a framework of manufacturing, by which the surface integrity desired for a high performance can be more effectively achieved as reducing the material and geometry constraints of manufacturing otherwise unobtainable during conventional machining processes. Here, thermal spraying of WC-Ni coatings is employed to coat stainless steel components for water-lubricated wear applications, on which a strategy of direct problem from process to performance can be implemented with surface integrity adjustable through spray angle and inert N2 shielding. Multiple surface integrity parameters are evaluated to identify the major ones responsible for wear performance by elucidating the wear mechanism, involving surface features (coating porosity and WC phase retention) and surface characteristics (microhardness, elastic modulus and toughness). The surface features predominantly determine tribological behaviors of coatings in combination with the surface characteristics that are intrinsically associated with the surface features. Consequently, the spray process could be designed with improved N2 shielding according to the desired surface integrity parameters for higher wear resistance. It is demonstrated a possibility of solving inverse problem of manufacturing if the correlations from processes to performance are fully understood and established, facilitated by effectively reducing the manufacture constraints toward a material and geometry integrated manufacturing.
Conventional and nanostructured WC-12 %wt Co powders were HVOF sprayed on Al 7075-T6. The nanocoatings presented slightly less microporosity, higher microhardness, higher fracture toughness but higher decarburization, than the conventional ones. The nanocoatings exhibited greater resistance to general corrosion and localized corrosion in 3.5% NaCl than their conventional counterparts. Other than an occasional removal of WC nanoparticles, corrosion modes appeared similar for both types of coatings, characterized by selective dissolution of Co, formation of protective surface films and crevice corrosion along interlayer boundaries. Both types of coatings exhibited high dry sliding wear resistance. Nevertheless, the wear performance of the nanocoatings was even superior than that of their conventional counterparts. Main wear modes included plastic deformation of binder, oxidative wear leading to the formation of lubricating oxides and adhesive wear leading to the formation of alumina-rich surface depositions. Coating of Al 7075 with nano WC-12Co was proved effective in terms of microstructural features, corrosion and wear behaviour.
Three coatings have been obtained using nanostructured, bimodal and conventional WC-Co cermet powders by means of High Velocity Oxygen-Fuel (HVOF) technique. Propylene has been used as fuel gas. All the powders have been sprayed under the same spraying parameters. These powders and the coatings obtained have been characterized using XRD and SEM.The friction wear resistance of the coatings has been studied by Ball-on-Disk test (ASTM G99-90), and the abrasive wear resistance has been quantified by Rubber-Wheel test (ASTM G65-91). Finally, the corrosion resistance has been studied by electrochemical techniques and salt fog spray test.The nanostructured coating shows more hardness than the other ones, but the bimodal coating have showed better abrasive and friction wear resistance. Both nanostructured and bimodal coatings have showed good corrosion resistance, better than the conventional coating of WC-Co. The bimodal powder has the advantage of being less expensive than the nanostructured powder, and even providing better properties.
Interest in nanomaterials has increased in recent years. This is due to the potential improvement that size reduction to nanometric scale could provide in properties like hardness, toughness, wear, and corrosion resistance. The current study is focused on WC-Co cermets, materials that are extensively used in applications requiring wear resistance. Two coatings were sprayed by high-velocity oxyfuel using both conventional and nanostructured cermets. Both materials were characterized by means of x-ray diffraction and scanning electron microscopy to study their phases and morphology. Cross-sectional images and microhardness values are given for both coatings. Friction wear resistance of the coatings was studied by ball-on-disk (ASTM G99-90), and abrasive wear resistance was quantified by rubber-wheel (ASTM G65-91). Finally, the corrosion resistance was studied by electrochemical techniques. Comparative results showed that the nanostructured coating has better properties than the conventional coating except in the dry abrasion wear test, where the wear rates are very similar.
The WC–Co cermet bulks were prepared by spark plasma sintering (SPS) using powder mixtures with different-scaled WC particles. The SPS densification process was studied by calculating the current distribution between the powder sample and the die in the SPS system. The microstructures were characterized and compared for different samples by the WC grain size, Co mean free path and contiguity of WC grains. In spite of a weak effect of WC particle size on the SPS densification stages, the WC particle size plays a significant role in the homogeneity of the cermet microstructure. Good mechanical properties of the SPSed cermet were obtained with an optimized WC and Co particle-size combination. The effects of scale combination of WC and Co particles on the microstructure hence the properties of the SPSed cermet were discussed.
Two kinds of WC-10Co4Cr composite coatings with conventional (micron-sized WC particles) and bimodal (a mixture of nano-sized and micron-sized WC particles) structures were successfully prepared on 35CrMo steel (ANSI/ASTM 4135) substrate by high velocity oxygen fuel (HVOF) technology. Scanning electron microscope (SEM) equipped with the energy dispersive spectroscopy (EDS), mechanical testing machine, microhardness tester were used to analyze the characteristics of the coatings. The coatings were immersed in the simulated seawater drilling fluid under the high pressure to simulate the deep-sea environment, and then electrochemical impedance spectroscopy (EIS), potentiodynamic polarization were carried out to investigate the electrochemical properties of the coatings and substrate. The results show the WC-10Co4Cr coating has excellent corrosion resistance and can effectively protect the substrate under the high pressure. Besides, compared with the conventional coating (CC), the bimodal coating (BC) has a denser microstructure, superior mechanical properties, lower porosity, and better corrosion resistance. The corrosion mechanism of the coatings in the simulated seawater drilling fluid under high the pressure is as follows: The micro-galvanic corrosion between the WC phase and the CoCr binder phase is the primary corrosion mechanism. Besides, the electrolyte under the high pressure is more penetrable, so the crevice corrosion also exists.
Rolling contact fatigue (RCF) of sprayed coating is complicated process relating to materials, structures, loading conditions, etc. Herein, the research works over the past decades in RCF of sprayed coatings are summarized here to provide a guidance to further explore the new research interests. Firstly, comprehensive summaries including the common RCF-resisted coating materials, the regular RCF setups, typical failure modes of sprayed coatings, and statistical characterization methods for RCF lifetime are illustrated. Secondly, the influence of surface integrity on RCF behavior of sprayed coating is discussed. The according mechanisms are also discussed by fracture analysis, numerical calculation, and finite element method (FEM). Thirdly, the influence of working condition on RCF behavior of sprayed coating is discussed. The shear stress is recognized as the major contribution of the RCF fracture, and the sliding during rolling contact will deteriorate the lifetime of sprayed coating. Additionally, some new techniques and methodologies have been reported in the investigations of RCF behaviors of sprayed coatings. Nondestructive testing (NDT) and signal processing methods are critical in the detection of micro-fractures within the sprayed coatings. Finally, we have proposed some suggests in future investigation of RCF of sprayed coating based on the previous research achievements.
The aim of this work is to understand the relationship among average grain size, dimple size and tensile properties of 316L stainless steel via directly experimental results. We have successfully prepared samples with the average grain size from a few microns to tens of microns through cold rolling and annealing processes. Uniaxial tensile tests were performed to confirm the Hall-Petch relationship between the grain size and yield strength. In order to uncover the grain size dependence of ductility, the fracture morphologies in details were observed. It revealed that the dimple size is positively related to the value of D1/2 (D is the average diameter of grain size). A larger grain size was believed to result in a larger dimple so as to achieve a higher ductility (uniform elongation).
NiCrBSi alloy coatings were fabricated via internal rotating plasma spraying (IRPS) inside the cylinder liner and in open space using the same processing parameters. The effects of spraying condition on the microstructural, mechanical and tribological properties of the IRPS coatings were investigated. Simultaneously, the effects of the flow rate of primary gas (FRPG) on the coatings at a short spray distance were thoroughly evaluated. The microstructure, Vickers hardness, bond strength and tribological properties of the coatings were experimentally determined and characterized in detail. The results demonstrated that all coatings fabricated by IRPS had a dense structure and high microhardness. The bond strength and tribological properties were effectively promoted inside the cylinder, in contrast to open space condition. It is also found that coefficient of friction and wear volume of the coatings declined with the increasing of FRPG. Especially, the coatings prepared with FRPG at 100 L⸱min⁻¹ exhibited the lowest friction and best wear resistance despite their higher porosity. The main wear mechanisms were abrasive and fatigue wear for the coatings deposited in open space, while the dominant wear mechanisms for the coatings prepared inside the cylinder were adhesive wear and tribolayer delamination.
Slurry erosion is one of the main failure modes of the hydraulic turbine parts, mud pumps and drill pipes. The high velocity oxygen fuel (HVOF) spray process has been widely used for depositing protective coatings especially the cermet coatings that can significantly improve the slurry erosion resistance of substrate materials. In this work, conventional WC-10Co4Cr composite coatings and coatings with bimodal structures were deposited on the 35CrMo steel substrate by HVOF, respectively. Structures of the two kinds of coatings were analysed with SEM and XRD methods, and microhardness, porosity and roughness were also measured. Furthermore, the slurry erosion tests were carried out on the coatings to explore the effect of average particle size, slurry concentration and pH value on the erosion behaviours. The erosion failure mechanisms of the coatings were also explored. The results show that the bimodal coating has a denser microstructure, superior mechanical properties, lower porosity and better slurry erosion resistance than the conventional coating. The erosion-corrosion mechanism of the two kinds of coatings is spalling, caused by crack initiation and propagation under fatigue stress. © 2019, © 2019 Institute of Materials, Minerals and Mining Published by Taylor & Francis on behalf of the Institute.
The present investigation explores the collective outcome of hard particle reinforcement with deep cryogenic treatment (DCT) on wear responses of magnesium metal matrix nanocomposites (MMNC). A multilevel factorial design of experiments with control factors of applied load (20 and 40 N), sliding speed (1.3, 1.7, 2.2, and 3.3 m/s), reinforcement % (0% and 1.5%), and cryogenic treatment (cryogenic-treated and nontreated) was deployed. Around 1.5 wt% WC-reinforced MMNC were fabricated using stir-casting process. DCT was performed at -190 °C with soaking time of 24 h. The dry sliding wear trials were done on pin-on-disk tribometer with MMNC pin and EN8 steel disk for a constant sliding distance of 2 km. The WC reinforcement contributed toward the improvement in wear rate of MMNC appreciably by absorbing the load and frictional heat at all loads and speeds. During DCT of AZ91, the secondary β-phase (Mg17Al12) was precipitated that enriched the wear resistance, only for the higher load of 40 N. Scanning electron microscope analyses of the cryogenic-treated MMNC ensured the existence of both β-phase precipitates and WC in the contact area. As a result, the adhesiveness of this pin was lesser, which attributed to the improved wear resistance (approximately 33%) as compared to base alloy. The coefficient of friction was also less for cryogenic-treated MMNC. A regression analysis was made to correlate the control elements and the responses.
The slurry erosion-corrosion and microbial corrosion electrochemical behavior of high-velocity oxygen-fuel (HVOF) sprayed WC-10Co-4Cr cermet coatings and the reference stainless steel 1Cr18Ni9Ti were both investigated. The slurry erosion-corrosion test was performed in a rotating disk rig facility with circulating system using distilled water and 3.5 wt% NaCl slurries. The microbial influenced corrosion behavior in the presence of SRB was evaluated by electrochemical measurement. The results showed that WC-10Co-4Cr coating exhibited higher slurry erosion-corrosion resistance compared to the stainless steel 1Cr18Ni9Ti in both distilled water and 3.5 wt% NaCl slurries. The evolution of the slurry erosion mechanism of the coatings with the augment of NaCl concentration was cracks and microchipping in distilled water slurry, and a combination of detachment of binder phase, microchipping and fragments in 3.5 wt% NaCl slurry. Potentiodynamic polarization and electrochemical impendence spectroscopy (EIS) results showed that the WC-10Co-4Cr coating had a comparable microbial influenced corrosion resistance in seawater with SRB as compared to the stainless steel 1Cr18Ni9Ti.
In this work, the bimodal WC-Co coatings were sprayed by high-velocity oxygen-fuel (HVOF), and the conventional WC-Co coatings were also fabricated for comparison. The microstructure, mechanical properties and high temperature wear performance were investigated. The bimodal WC-Co coating presented denser structure (porosity lower than 1.0%), higher average hardness (1164 HV0.1) and fracture toughness (11.5 ± 1.4 MPa·m1/2) than that of conventional coating. The Weibull analysis of microhardness data of the bimodal coating presents a mono-modal distribution. The friction coefficient and wear rate of the bimodal coating were 0.61 and 2.96 × 10− 6 mm³·N− 1·m− 1, respectively, which is lower than that of conventional coating at the test temperature of 450 °C. The tribofilm could be formed on the worn surface of bimodal WC-Co coating, which is composed of WO3 and CoWO4. The formation of tribofilm could reduce friction and wear.
This paper described the difference of tribological behaviors and wear mechanism between WC-10 wt%Co and WC-10 wt%Fe3Al. By spark plasma sintering (SPS) method, the two materials were successfully fabricated under lower sintering temperature (1150 °C). Friction and sliding wear tests were carried out under dry conditions. The results showed that Hardness and wear resistance of WC-10 wt%Fe3Al is much higher than WC-10 wt%Co. The spectrums of Raman laser scattering showed WC-Fe3Al posses good oxidation resistance. Energy dispersive spectrums (EDS) verify the existence of Al2O3 in WC-10 wt%Fe3Al and this is the important factor that leads to the better wear and oxidation resistance of WC-10 wt%Fe3Al.
The 30 mm thick WC-Co cemented carbide and AISI 1045 steel were electron beam welded (EBW) with and without Ni-based filler metal. Welding-brazing joints of WC-Co and steel were obtained as the molten pool wet and spread on the unmelted WC-Co base metal. The WC loose areas formed at the WC-Co interface have guaranteed the effective metallurgical bonding. According to simulation results, the thickness of filler metal should be less than 1 mm to avoid unbonding. The direct EBW WC-Co/steel joint was mainly composed of martensite, residual austensite and herringbone M6C carbides. The bulk brittle phases η-Fe3W3C increased the joint brittleness and crack sensitivity. With Ni-based filler metal, the microstructure of the joint was mainly composed of γ-Fe and herringbone carbides FeW3C. No η-Fe3W3C formed at WC-Co interface because of the metallurgical blocking effect. The plastic property of the joint was improved, and WC-Co/filler metal/steel EBW joints with no crack were obtained.
A new type of bimodal-grained WC-Co cemented carbide coating was fabricated, using the raw materials consisting of the in situ synthesized nanoscale WC-Co composite powder as major component and a certain amount of ultra-coarse WC particles as addition. The effects of coarse WC particles on the microstructure, mechanical properties and wear behaviour of the coating were investigated. It was found that with a suitable addition of coarse WC particles, the coating has a decreased decarburization and significantly increased wear resistance, as compared with the coating without the addition. The coarse WC particles allow a certain degree of plastic deformation by dislocation gliding so that under wearing conditions some stress concentration can be released and their fractures are inhibited. A good combination of coarse WC particles and WC-Co nanocomposite facilitates high properties of the coating, with the former mainly bearing the load and resisting against penetration of the wear debris, and the latter providing high hardness and also plastic accommodation in the wearing process.
The sliding wear properties of high-velocity oxygen-fuel (HVOF) sprayed nanostructured WC-CoCr cermet coatings against Al2O3 under dry friction at different temperatures were investigated using a pin-on-disk high-temperature tribometer. The microhardness data of the coatings were also statistically analyzed by using the Weibull distribution. The results showed that nanostructured coatings exhibited a mono-modal distribution under indentation load of 100 g, and a bimodal distribution under indentation load of 300 g. With increasing test temperature, the coatings showed an increase in wear rate. The evolution of the sliding wear mechanism of the coatings with the increase of the temperature was extrusion deformation at room temperature, carbide particle pull-out, oxidation wear and adhesive wear at 200 °C, and a combination of binder extrusion and fatigue delamination coupled with oxidation wear at 500 °C.
In this work, WC–10Co–4Cr coatings were separately deposited on 300M steel by high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) spraying processes. Microstructure, porosity, micro-hardness, bonding strength, wear and corrosion resistance of the coatings after different processing were investigated and compared. The result shows that dense and homogeneous microstructure existed in both two WC–10Co–4Cr coatings. Owing to lower temperature flames and higher particle velocity in the HVAF process, the HVAF-sprayed WC–10Co–4Cr coating exhibited less decarburisation, lower porosity (0.74%), higher micro-hardness (1162 HV0.3) and bonding strength (74.68 MPa) than HVOF-sprayed WC–10Co–4Cr coating. In addition, wear and corrosion resistance of these two coatings were evaluated by sliding wear test and electrochemical corrosion test. The HVAF-sprayed WC–10Co–4Cr coating also exhibited excellent wear resistance and superior corrosion property than HVOF-sprayed WC–10Co–4Cr coating.
Structural parts in gas turbines undergo fretting wear, and as clearances open up, it turns into impact wear. Uncoated cobalt-based substrates that are used in such applications show poor resistance to fretting/impact damage and undergo extensive/unacceptable level of degradation. Commonly used substrate materials such as uncoated cobalt-based materials show poor resistance to fretting/impact wear. This study is focused on assessing the performance of high velocity oxyfuel (HVOF) tungsten carbide (WC) coatings and sintered tungsten carbide inserts as potential solutions for mitigating this issue. Sintered WC12Co with grain size range from 0.2 μm to 4.5 μm and HVOF coating with composition of WC-17%Co was tested. It was found that the HVOF coatings performed better than sintered material and the behavior was attributed to the hard WC particles surrounded by the higher volume fraction of cobalt binder. In the HVOF coating, the normal load was better accommodated by the decarburized WC but a fairly tough binder-surrounding matrix. An additional factor is that the sintered WC had significant volume fraction of undesirable W2C phase, which apparently underwent fracture during the test, thus showing an inferior behavior compared to the HVOF WC coating.
The nanostructured WC–10Co–4Cr coatings were fabricated by high velocity oxy-fuel spraying using the in situ synthesized WC–Co nanocomposite powder with size of 70–200 nm and Cr addition. Through optimization of the processing conditions, the nanostructured WC–Co–Cr coating has only a small amount of decarburized phase, a dense microstructure and an excellent combination of hardness, fracture toughness and wear resistance. A series of sliding wear tests were performed to investigate the wear behavior of the nanostructured cermet coating. The evolution of the friction coefficient, wear characteristics and their mechanisms were studied for the nanostructured WC–Co–Cr coating with the change of the load. The present study proposes a new understanding of the occurrence and the related mechanisms of the wear of the cermet coatings.
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.
The wear behaviour of tungsten carbide-cobalt (WC-Co) high-velocity oxy-fuel coatings within the range of room temperature (RT) to 800 °C in air and RT to 650 °C in argon was studied using a ball-on-disc tribometer. The worn track morphologies, compositions and oxide phases after the tests were analysed by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction analysis and Raman spectroscopy. The correlation between the oxidation and wear of WC-Co coatings was investigated. Results show that oxides, as well as the oxidation of coatings, have an important role during the tribological process of WC-Co coatings. The volume loss of WC-Co coatings in air is low at RT to 600 °C, which indicates that oxidation inhibits wear. Friction promotes the formation of CoWO4. CoWO4 can reduce friction and wear, and enables the coating to maintain good tribological properties even at 600 °C. Above 600 °C, the coating oxidises vigorously and its property deteriorates rapidly, which cause serious wear. In argon, the volume loss of WC-Co coating is significantly higher than that in air at RT to 600 °C. Wear decreases as temperature rises because of the gradual formation of oxides. Consequently, WC-Co coatings should not be used as wear-resistant coatings in O-deficient environments at RT or at elevated temperatures.
In present work, the deposition behaviors of individual bimodal WC-12Co particles impacting on stainless steel and WC-12Co substrates were investigated. The microstructure and properties of the cold-sprayed WC-12Co coating were characterized. The results showed that the WC-12Co particles partially penetrated into the stainless steel substrate. Flattening of the particles occurred with splashes at its periphery when impacting on the WC-12Co substrate. A movement of small WC particles accompanied with a plastic flow of the Co binder occurred during the impacting and flattening process of the powders. The sprayed coating exhibited a characteristic microstructure with a wide range of WC particle sizes varying from less than 100 nm up to a micron scale. The obtained multimodal coating exhibited both a high hardness and high fracture toughness, e.g. the microhardness is similar to that of a cold sprayed nano WC-12Co coating and the fracture toughness is comparable to that of a bulk material of micro WC-12Co. The cold sprayed coating exhibited a greatly enhanced wear resistance in comparison to the conventional high-velocity oxy-fuel (HVOF) sprayed WC-12Co coating.
Nanostructured WC–12Co coating with crystal sizes below 30 nm were deposited on low carbon steel substrate by high velocity oxy-fuel spraying. In this study, corrosion behavior of such coatings was investigated in artificial sea water electrolyte and compared with micron-sized (internal structure) coating of similar composition. Besides that, role of small amount Al alloying on corrosion behavior of such nanostructured coating was also investigated. Nanostructured WC–12Co coatings with or without Al exhibit lower corrosion potential and lower polarization resistance compared to micron-sized (internal structure) coating as well as higher Co dissolution as revealed by electrochemical impedance spectroscopy (EIS) together with other complementary electrochemical techniques.
WC-12Co powders with a bimodal size distributed WC particles were used to produce coating by high velocity oxy-fuel (HVOF) spraying (B coating), and HVOF sprayed WC-12Co coatings from microstructured, submicrostructured and nanostructured powders were also fabricated for comparison. The phase constitution, microstructure, mechanical properties and wear performance of the coatings were investigated. Decarburization occurred during coatings preparation, and the carbide retention of B coating was 0.934, higher than that of nanostructured coating. B coating exhibited typical multimodal microstructure, and had considerably high microhardness and the highest fracture toughness among the four coatings, with the values of 1291 HV0.1 and 10.76 MPa m1/2, respectively. When sliding against GCr15 ring in block-on-ring configuration, B coating exhibited the lowest wear rate and relatively lower friction coefficient compared with other coatings, with the average values of 0.94 × 10−7 mm3 N−1 m−1 and 0.63 at 245 N load, respectively, which could be attributed to the concrete-like structure.
This study compares three types of WC-10Co4Cr coatings deposited with high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) spraying processes. The experimental results indicated that the decarburisation of the WC in the WC-10Co4Cr coating was dramatically influenced by the spraying equipment, and the non-WC phase content in the as-sprayed coatings greatly influenced their performances. The HVAF-sprayed WC-10Co-4Cr coating revealed the lowest degree of decarburisation, achieving the best properties in terms of hardness, fracture toughness, abrasive and sliding wear as well as electrochemical corrosion resistance when compared to the two HVOF-sprayed WC-10Co-4Cr coatings.
In this paper, a novel method, namely, magnetron sputtering and low temperature ion sulfurizing combined technique was used to fabricate the solid lubrication WS2/MoS2 multilayer films. Scanning Electron Microscopy (SEM) was used to observe the surface and worn scar morphologies. X-ray diffraction (XRD) was utilized to analyze the phase structure. The nano-hardness and elastic modulus of WS2/MoS2 multilayer films were surveyed by the nano-indentation tester. The friction and wear test were conducted on a ball-on-disk wear tester under dry sliding condition. The results obtained showed that the WS2/MoS2 multilayer films exhibited a lower friction coefficient and better wear-resistance when compared with single WS2 film and original 1045 steel.
Abrasive wear behaviour of materials can be assessed using a wide variety of testing methods, and the relative performance of materials will tend to depend upon the testing procedure employed. In this work, two cermet type coatings have been examined, namely (i) a conventional tungsten carbide-cobalt thermally sprayed coating with a carbide size of between ∼0.3–5μm and (ii) a tungsten carbide-nickel alloy weld overlay with large spherical carbides of the order of ∼50–140μm in diameter (DuraStell). The wear behaviour of these two materials has been examined by the use of two abrasion tests, namely the micro-scale abrasion test using both silica and alumina abrasives (typically 2-10μm in size), and the dry sand-rubber wheel test (ASTM G65), again with both silica and alumina abrasives (typically 180–300 μm in size). It was found that when the abrasive particles were of the same scale or larger than the mean free path between the hard phase particles, then the matrix phase was well protected by the hard phases. Testing (in both test types) with alumina abrasives resulted in wear of both the hard carbide phases and the matrix phases in both the thermally sprayed coating and the weld overlay, with the thermally sprayed coating exhibiting lower wear rates. The wear behaviour of the materials with the more industrially relevant silica abrasive was more complex; the thermally sprayed coating exhibited a lower wear rate than the weld overlay with the fine abrasive in the micro-scale abrasion test due to effective shielding of the matrix from abrasive action due to the fine reinforcement particle size. In contrast, with the coarser silica abrasive in the dry sand-rubber wheel test, the weld overlay with the large carbides was able to provide matrix protection with low rates of wear, whereas the thermally sprayed coating wore by fracture of the more brittle microstructure. These findings demonstrate the importance of selection of appropriate laboratory test procedures and abrasives to simulate behaviour of materials in service environments.
Development of nano-structured hardmetals is the task of great importance. Nevertheless, in spite of some "euphoria" with respect to nanograined hardmetals, their potential application ranges are yet not clear. In the present work near-nano hardmetals with WC mean grain size of nearly 200 nm and Co contents of 5 to 33 wt. % were produced and examined with respect to their hardness, fracture toughness, transverse rupture strength and wearresistance. These hardmetals are found to have an extremely low wear-resistance compared to that of conventional coarse-grained mining grades in laboratory performance tests on granite-cutting, concrete-cutting and percussion drilling of quartzite. Thus, near-nano and presumably nano WC-Co hardmetals are expected to never substitute conventional mediumand coarse-grained grades. The only application range, where near-nano and nano hardmetals can potentially substitute conventional grades, is a relatively narrow application range of harness of above 18 GPa.
In this work, the spray parameters of the bimodal WC–12Co powder are optimized by Taguchi experiment program by using liquid fuel JP-8000 HVOF spray system with mass flow meter controlling the flux of the medium. The phase composition, microstructure, hardness, porosities, fracture toughness, per-pass deposited thickness and the abrasive wear mechanism of the coatings have been studied in detail. The results indicate that the varying of the spraying parameters shows little effect on the phase composition of the WC–12Co coatings but great influence on their other performance such as hardness, porosity, fracture toughness and the per-pass thickness of coatings. High hardness of the coatings usually means low porosity, low fracture toughness, low per-pass thickness and high wear resistance for the HVOF WC–12Co coatings. The bimodal coating deposited under the optimal spray parameter exhibits excellent performance, which can be attributed to its small mean free path of the cobalt binder resulting from the bimodal distribution of WC particles. As last, three groups of spray parameters, named as economic, moderate and high level spray parameters, are suggested according to the results of Taguchi experiment program and the applied situation of the coated parts as well as their preparation cost.
Conventional and nanostructured WC–12Co coatings were deposited on 1Cr18Ni9Ti substrate using high velocity oxy-fuel spraying. The surface roughness and microhardness of as sprayed coatings were measured, while the phase compositions and microstructures of WC–12Co feedstock powders and corresponding coatings were respectively analysed by means of X-ray diffraction and scanning electron microscopy. Moreover, the microsliding wear behaviour of both coatings sliding against Si3N4 ball from room temperature to 500°C was evaluated under unlubricated condition with an oscillating SRV tribotester. Results reveal that nanostructured WC–12Co coating not only had better wear-resistance than the conventional WC–12Co coating but also led to reduced wear of the Si3N4 counterpart under the same test condition, which could be attributed to the fine microstructure and improved mechanical properties of the nanostructured coating. The detailed wear mechanisms for both coatings have also been discussed.
Conventional and nanostructured WC–12%Co coatings were deposited on 1Cr18Ni9Ti stainless steel substrate using air plasma spraying. The hardness of the coatings was measured, while their friction and wear behavior sliding against Si3N4 at room temperature and elevated temperatures up to 400 °C was comparatively studied. The microstructures and worn surface morphologies of the coatings were comparatively analyzed as well by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDXA). It was found that the as-sprayed WC–12%Co coatings were composed of WC as the major phase and W2C, WC1−x, and W3Co3C as the minor phases. The plasma sprayed nanostructured WC–12%Co coating had much higher hardness and refined microstructures than the conventional WC–12%Co coating. This largely accounted for the better wear resistance of the nanostructured WC–12%Co coating than the conventional coating. Besides, the two types of WC–12%Co coatings showed minor differences in friction coefficients, though the nanostructured WC–12%Co coating roughly had slightly smaller friction coefficient than the conventional coating under the same sliding condition. Moreover, both the conventional and nanostructured WC–12%Co coatings recorded gradually increased wear rate with increasing temperature, and the nanostructured coating was less sensitive to the temperature rise in terms of the wear resistance. The worn surfaces of the conventional WC–12%Co coating at different sliding conditions showed more severe adhesion, microfracture, and peeling as compared to the nanostructured WC–12%Co coating, which well conformed to the corresponding wear resistance of the two types of coatings. The nanostructured WC–12%Co coating with a wear rate as small as 1.01 × 10−7 mm3/Nm at 400 °C could be promising candidate coating for the surface-modification of some sliding components subject to harsh working conditions involving elevated temperature and corrosive medium.
The performance of multimodal and conventional materials in the form of coatings deposited by high velocity oxy-fuel (HVOF) thermal spraying has been studied. WC-12Co coatings were deposited under same conditions using multimodal and conventional WC-12Co powder feedstocks. The phase composition of the feedstock powders and the coatings were analyzed by XRD. Abrasive wear resistances of coatings were carried out on wet sand rubber wheel abrasion tester. The characterizations of spraying feedstock powders, microstructure and surface micrographs of the prophase and anaphase attrition surfaces were performed by SEM. The results indicated the multimodal coating shows slight higher microhardness and better abrasive wear resistance than the conventional counterpart. Also, the thermally sprayed carbide-based coatings have excellent wear resistance with respect to the hard chrome coatings.
Effects of post heat treatment of thermally sprayed nano WC–Co coatings on their mechanical properties were studied. The thermal behavior of WC particles in the coatings was also investigated. WC–Co coatings containing nano carbide particles in the 100–200 nm range were fabricated by detonation gun spraying. Considerable phase decomposition of WC to W2C and amorphous phase was detected, which degrades the mechanical properties of coatings. In order to improve the mechanical properties of the coatings by recovery of dissociated carbide phases, post heat treatment was conducted in an Ar environment in the temperature range of 400–900 °C. Microhardness and fracture toughness were measured by Vickers indentation testing and wear resistance was also evaluated by using a scratch tester. Phase evolution and microstructural changes due to post heat treatment were investigated by optical microscopy, X-ray diffractometry and field-emission scanning electron microscopy. After heat treatment in all temperature ranges, microhardness increased. Fracture toughness and wear resistance of coatings were increased by increasing temperature to 800 °C but decreased after heat treatment at 900 °C. Amorphous phase disappeared and other carbide phases such as W3Co3C and W6Co6C formed during heat treatment above 700 °C. The improved properties were elucidated and discussed in terms of microstructural changes and the relationship between mechanical properties and carbide phase was also discussed.
WC–Co thermally sprayed coatings are now widely used in a range of industries to combat wear. During spraying of WC–Co coatings, there is a decomposition involving dissolution of the hard phase into the molten binder; as such, the coatings are often made up of WC along with other carbides in a matrix consisting of tungsten and carbon dissolved in the cobalt. High velocity oxy-fuel (HVOF) spraying is seen as the pre-eminent process for deposition of such coatings. There have been moves in the industry to spray coatings with liquid-fuelled systems rather than gas-fuelled guns, since the former lead to shorter residence times of the particles in the flame and lower temperatures, both of which lead to less dissolution of WC and decomposition of the coating. This paper presents work concerning the sliding wear behaviour of WC–Co coatings HVOF sprayed with both liquid-fuelled (HVOLF) and gas-fuelled (HVOGF) systems and demonstrates that with a dense powder feedstock, the HVOGF deposited coating is superior to the HVOLF deposited coating. The poorer performance of the HVOLF-sprayed coating is associated with mechanical damage to the WC–Co powder particles as they impact with the substrate resulting in carbide cracking and a reduction in the integrity of the bond between the carbide particles and the matrix phase.
WC–Co coatings have been deposited by high velocity oxy-fuel thermal spraying of conventional and nanocomposite powders which contain WC grains in the size range 2–5 μm and 70–250 nm, respectively. The coatings differed not only in microstructural scale, but also in the nature and proportion of the phases present and in the overall degree of decarburization. A model describing the evolution of microstructure has been developed. As a WC–Co particle is heated in the hot gas jet, the cobalt phase melts and the WC grains begin to dissolve in it. The periphery of the semi-molten particle becomes decarburized by oxidation, promoting further WC dissolution in this region. Particle quenching on impact with the substrate results in precipitation from the melt of W2C and possibly W depending on the local melt composition. The larger surface to volume ratio of the WC in the nanocomposite material promotes more rapid dissolution and thus decarburization. Consequently, W2C is observed in both coatings, whereas W is found only in the nanocomposite deposit.
Fracture by plastic indentation has been studied for a range of brittle materials, and the fractures have been characterized and quantified. An approximate stress analysis has indicated the importance of both the plastic penetration and the interface friction in the development of the observed fractures. The crack extension has been shown to depend primarily on the impression radius and the ratio of the hardness to the fracture toughness. This fracture characterization has been used to examine several important consequences of indentation fracture: notably, fracture toughness determination by indentation, material removal by abrasive processes, and material removal by low velocity solid particle impact.