Co-based Coatings Research Articles

CAVITATION EROSION BEHAVIOR OF WC–Co COATINGS ON TURBINE STEEL DEPOSITED BY COLD SPRAY

A renowned hard coating material (WC–Co) has the potential to overcome cavitation erosion (CE) in hydromachinery components. Hydro turbines that employ silt-filled water for electricity generation create a lot of metal waste due to erosion and cavitation caused by the particles. There is an urgent need for repair of hydropower components that have undergone damage due to cavitation. In this investigation, tungsten carbide composite coatings were produced on 13Cr4Ni (CA6NM) stainless steel substrates using the cold spray method. In order to create high-quality coatings, nitrogen was used as the propellant gas. The coating microstructures were characterized by EDS, SEM XRD, and microhardness tests. Cold-sprayed WC–Co-based coatings were exposed to enhance the cavitation erosion resistance of CA6NM steel. Cold spray coating prepared with WC-12/17Co powder has shown excellent wear-resistant properties due to higher hardness and lower porosity.

Artificial Neural Networks in Spray Coatings for Protection of Hydroturbines in Silt Conditions

Background: Artificial intelligence techniques are able to predict the erosion in various coatings. In recent times, the erosion of the coating was successfully predicted using artificial neural networks (ANN). Aim: This patent aims to present the critical findings in the area of spray coatings used for the protection of hydroturbines. This critical review explores the surface erosion of numerous hydroturbine materials and coatings. Moreover, it covers the evolution of coating techniques used in different industries such as hydropower plants and materials handling industry. Objective: The objective of this patent is to present the critical finding in the field of erosion wear in coatings utilizing an artificial neural networks (ANN) technique. Methods: The effect of influencing factors on silt erosion was assessed. However, the evolution of various coating processes was explored comprehensively. Furthermore, the properties of various coatings, namely WC, Cr2C3, and Ni/Co coatings, were reported. A number of comparative analyses were carried out that can help in the selection of coatings on the basis of their mechanical as well as chemical properties. The applications of ANN in the spray industry were explained extensively. Results: The results show that the HVOF coating is the most widely used deposition method. HVOF is an appropriate technique for both ceramics and pure metals. Ceramics, as well as cermet, are deposited to enhance the wear and tear resistance of hydroturbine materials. As a result, the HVOF, laser cladding, D-Gun, and plasma spray technique are optimal in these circumstances. Coatings based on WC have been employed by several researchers to increase stainless steels' resistance to erosion and wear. Conclusion: With the application of appropriate models and ML methods, it is expected that coatings with specific desired properties can be manufactured with fewer experiments. There is no surprise that WC-10Co-4Cr finds widespread usage in a variety of industrial contexts. Ni- and Cobased coatings show promise for use in tribological settings. Stellite is suggested by several researchers for high-erosion duty conditions in hydroturbines. Co-based coatings are more erosion resistant as compared to No-based coatings. Coatings formulated with chromium (Cr) are resistant to erosion and wear in harsh mining environments.

Effects of B content on wear and corrosion resistance of laser-cladded Co-based alloy coatings

Abstract To improve the surface properties of Ti alloy, (Co34Fe8Cr29Ni8Si7)100–x B x alloy, coatings were prepared by laser cladding. The coatings—mainly composed of TiCr, Fe0.1Ti0.18V0.72, CoTi, Ti2Ni, and TiB—and amorphous phases were investigated in terms of microstructure, wear resistance, and corrosion resistance. The results showed that the microhardness of the Co-based coatings first increased and then decreased with the increase of B content. When the B content was 6%, the microhardness of the coating increased up to 1210 HV0.2 which was 3.4 times that of TC4 alloy substrate. The coatings exhibited diverse wear mechanisms that gradually transitioned from severe fatigue spalling and oxidative wear to slightly abrasive wear. The corrosion current density of Co-based coatings in 3.5 wt% NaCl solution first increased and then decreased as B contents increased. Coatings with 4% B content, however, exhibited the best corrosion resistance, which was most suitable for improving the corrosion resistance of Ti alloy.

Tribological Properties Laser-Cladded Spherical WB-Reinforced Co-Based Coatings under Low-Temperature Friction.

Three groups of spherical WB-reinforced Co-based coatings (Co coating, Co + 15%WB coating, Co + 45%WB coating) were fabricated by laser-cladded technology. The microstructure and constituent phase of spherical WB-reinforced Co-based coatings were examined through scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). The low-temperature tribological properties were analyzed by coefficient of friction, 2D and 3D profiles across the wear track, and wear surface morphology, respectively. The results showed that the phases in the WB-reinforced Co-based coatings are mainly γ-Co, carbides Cr23C6 and Cr7C3, WB, and WO3. Under dry sliding friction at -20 °C, the more spherical WB, the lower the friction coefficient. The wear rate of Co + 45%WB coating was as low as 3.567 × 10-4 mm3/N·m-1, indicating the outstanding wear resistance. Abrasive wear was observed on the rough surface of the WB-added coatings. Compared with dry sliding, due to the plastic deformation of micro-convexes and lubrication function in the 3.5 wt.% NaCl solution, the wear tracks on the surfaces of the three tested coatings were shallower, exhibiting distinct elongated plough grooves.

Corrosion Resistance Mechanism of Stellite-6 Coating on 42CrMo Substrate by Overlap Remelting

To delay the corrosion rate of 42CrMo steel in oil and gas drilling industry, Co-based coatings were prepared on 42CrMo steel through laser cladding at different overlap rate (20%, 50% and 60%). The relationships between microstructure and corrosion behaviour of Stellite-6 coating in Cl ion corrosion medium were analysed by means of SEM, polarization curve and EIS. The results showed that the microstructures of Stellite-6 coating were mainly equiaxial crystal, and the corrosion form of cladding Stellite-6 alloy was pitting corrosion. The best corrosion resistance of Stellite-6 coating was the coating prepared at the overlap rate 50%. The formation of passivation films (Cr2O3 and Cr(OH)3) on the coating could obviously improve the corrosion resistance in Cl ion corrosion medium.

Effect of CeO2 on microstructure and properties of Ni–Co-based coatings

To improve the property and service life of continuous casting copper mould, Ni–Co–CeO2 composite coatings were prepared on the surface of the copper plate by electroplating technology. The effect of CeO2 addition on the phase composition, microstructure, hardness, wear resistance at 600 °C and corrosion resistance of the coatings was studied using X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) combined with an energy dispersive X-ray spectroscopy (EDS), microhardness tester, electrochemical workstation, high-temperature friction and wear tester. The results indicated that the appropriate amount of CeO2 can increase the Co content of Ni–Co–CeO2 composite coatings with a kind of dense cellular surface structure. CeO2 particles accelerated the deposition rate of the coatings and the coatings thickened with an increase of CeO2 particles added in the bath. The maximum Vickers hardness of Ni–Co–CeO2 composite coating was obtained after 5 g/L CeO2 was added in the electroplating bath, which was 51% higher than that of Ni–Co alloy coating. The corrosion rate of Ni–Co–CeO2 (10 g/L) composite coating was 83% lower than that of Ni–Co alloy coating, and the impedance of the coating was much higher than that of the others. Ni–Co–CeO2 composite coatings with uniform structure exhibit excellent high-temperature wear resistance at 600 °C. The minimum wear volume appeared in the Ni–Co–CeO2 (10 g/L) composite coating, which was 40% lower than that of Ni–Co alloy coating. The high-temperature wear resistance at 600 °C and corrosion resistance of the Ni–Co–CeO2 coatings have been improved because of the CeO2 particle addition.

Effect of Electrically Assisted Preheating on Microstructure and Properties of Laser-Cladded Co-Based Coating on CP-Ti Alloy Substrate

Co-based coatings were prepared on commercially pure titanium (CP-Ti) with electrically assisted pre-heating (EAPH), followed by laser cladding with the assistance of pulsed current. Conventional pre-heating (CPH) laser cladding was carried out as a control to investigate the effects of a pulsed current on the phase composition, microstructure, microhardness, and wear resistance of the coatings. The results showed that periodically varied pulsed currents generate an induced magnetic field. This field fragments bottom dendrites and transforms columnar dendrites into equiaxed crystals through the influence of the Lorentz force. The phase composition of the coatings remained unchanged under the pulsed current, as well as unassisted and CPH condition, consisting of γ-Co, α-Ti, CoTi2 solid solution, and TiC, Cr7C3 hard phases. The microhardness of the coating increased at 720 A due to grain refinement, compared to unassisted and preheated coatings at the same temperature. Moreover, a suitable preheating temperature can reduce crack generation and improve the wear properties of the coating.

Исследование содержания кислорода и азота в плазменных покрытиях

A study was carried out on the content of oxygen and nitrogen in plasma coatings deposited from Co powder and its alloys (wt. %) Co – 32 Ni – 21 Cr – 8 Al – 0.5 Y, Co – 27.9 Cr – 7.04 Al – 3.25 Si – 2 Y. To do this, we used a conical nozzle attached to the plasma torch, which performs the extraction of the plasma flow after heating and accelerating the sprayed particles. As the power of the plasma jet increases, the content of oxygen and nitrogen in the Co coating increases. When alloying cobalt alloys with Cr, Al, Si, and Y, the oxygen content in the coatings increases less rapidly with increasing plasma jet power, but the nitrogen content in these coatings is higher than in the Co coating due to the interaction of alloying elements with nitrogen. The results for Co-based coatings were analyzed along with their previous data for coatings sprayed from powders based on Fe and Ni. Coatings based on Fe, Co, Ni were divided into four groups according to the mechanism of their interaction with oxygen. The first group includes pure metals, the second includes alloys alloyed with Cr, the third is additionally alloyed with Al, the fourth is alloys with complex alloying of Cr, Mn, C, B, Si with elements that actively interact with oxygen, including the formation of gaseous oxides. Doping with Cr reduces the oxygen content in coatings by 35 – 46 %, additional alloying with Al reduces the oxygen content in the coating by 57 – 78 %, doping with Cr, C, B, Si reduces the oxygen content in the coating by 92 – 95 %. We performed a detailed analysis of the nozzles used for the plasma torch.

Fabrications and microstructure analysis of cobalt-based coatings by an easy-coating and sintering process

Abstract In order to enhance the service life of special complex shaped workpieces under severe working conditions, a new coating preparation process, namely, easy-coating and sintering technology, has been proposed. In this study, the feasibility of preparing Co-based coatings on 38CrMoAl substrates using the easy-coating and sintering process was investigated, and the microstructure and properties of the Co-based coatings were studied. By optimizing the parameters of the sintering process, the prepared coating is dense, and the coating forms a good metallurgical bond with the substrate with few pores, but no other defects. As the process has good applicability to the surface of complex-shaped parts, the inner wall of small serpentine tubes and large thicknesses, and is low-cost and easy to operate, the easy-coating-sintering process has great application prospects.

Laser cladding Co-based coating coupled with electromagnetic/ultrasonic compound energy field

Co-based coatings were fabricated by laser cladding coupled with electromagnetic/ultrasonic compound energy field. The microstructural evolution and wear resistance were investigated. The experiment results indicate that the number of coarse dendrites is continuously reduced and that of fine grains is gradually increased as the energy field changes from single to compound energy field. The compound energy field makes the cladding layer microstructure more uniform and refined, leading to the cladding layer showing the best wear resistance, in terms of highest microhardness (531.95 Hv0.2), lowest friction coefficient (0.259), minimum wear rate (8.96 × 10−2mm3 (Nm)−1). The solidification structure of cladding layer can be effectively controlled by applying an external physical field in the process of laser cladding with obvious improvements in mechanical properties, especially for the compound energy field.

Hot-Corrosion and Particle Erosion Resistance of Co-Based Brazed Alloy Coatings

Tape brazing constitutes a cost-effective alternative surface protection technology for complex-shaped surfaces. The study explores the characteristics of high-temperature brazed coatings using a cobalt-based powder deposited on a stainless-steel substrate in order to protect parts subjected to hot temperatures in a wear-exposed environment. Microstructural imaging corroborated with x-ray diffraction analysis showed a complex phased structure consisting of intermetallic Cr-Ni, C-Co-W Laves type, and chromium carbide phases. The surface properties of the coatings, targeting hot corrosion behavior, erosion, wear resistance, and microhardness, were evaluated. The high-temperature corrosion test was performed for 100 h at 750 °C in a salt mixture consisting of 25 wt.% NaCl + 75 wt.% Na2SO4. The degree of corrosion attack was closely connected with the exposure temperature, and the degradation of the material corresponding to the mechanisms of low-temperature hot corrosion. The erosion tests were carried out using alumina particles at a 90° impingement angle. The results, correlated with the microhardness measurements, have shown that Co-based coatings exhibited approximately 40% lower material loss compared to that of the steel substrate.

Effect of graphite/CeO2 on microstructure and tribological property of plasma cladded Co-based coatings

In this study, graphite/CeO2 reinforced Co-based coatings were prepared on the surface of HT250 gray cast iron by the plasma cladding technology. The influence of graphite and CeO2 on microstructure and properties of Co-based coatings were studied in detail. The phase composition and microstructure of coatings were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The micro-hardness, wear resistance and corrosion resistance of coatings were evaluated using micro-hardness tester, friction wear testing machine and electrochemical workstation. The results showed that the addition of graphite was beneficial to enhance micro-hardness and wear resistance of coatings, but there was still shortcoming in improving corrosion resistance. On this basis, CeO2 was further introduced together with graphite. The microstructure of coatings was found to be more uniform under the synergy of graphite and CeO2. The micro-hardness increased from 684.5 HV0.3 (coatings adding graphite alone) to 767.4 HV0.3 (coatings adding graphite and CeO2), which was 29.7% and 115.1% higher than that of as-cladded coatings and substrate respectively. More importantly, the corrosion current density of coatings was an order of magnitude lower than that of coatings adding graphite alone. Therefore, adding CeO2 together with graphite was beneficial to the synergistic improvement of micro-hardness, wear resistance and corrosion resistance of Co-based coatings.

Tribological study of Ti3SiC2/Cu5Si/TiC reinforced Co-based coatings on SUS304 steel by laser cladding

For the needs of the key moving parts of SUS304 steel in extreme environments, the composite coatings (Co-Ti 3 SiC 2 /Cu) were successfully fabricated by laser cladding. The tribological behaviors of three coatings at room temperature and 600 °C were systematically studied. Results indicated that the microhardness of the three coatings was 1.2–1.8 times higher than the SUS304 substrate. With the addition of the Ti 3 SiC 2 and Cu, the coating had good tribological capabilities, especially the anti-wear at room temperature was increased by 96%, and the friction reduction at 600 °C was increased by 73%, even it was found that Cu could promote the lubrication of Ti 3 SiC 2 . The wear of the three coatings changed from severe plastic deformation to slight oxidative adhesive wear . • The wear resistance of N3 coating was increased by 96% at 27 ℃, and the anti-friction was increased by 73% at 600 ℃. • The temperature cloud map could predict whether the material and the coatings formed an excellent metallurgical bonding. • At 600 °C, the Co-Ti 3 SiC 2 /Cu coatings formed the multi-layer of membrane-like sliding lubrication and oxide film wear. • The oxide film affected the overall lubrication effect and the Cu could enhance the lubrication of Ti 3 SiC 2 .

Tribological Properties of Laser Cladding Co-Based Alloy Coating Under Different Loads

Tribological Properties of Laser Cladding Co-Based Alloy Coating Under Different Loads

Effect of Freezing–Corrosion Treatment on Wear Resistance of Laser-Cladded Cobalt-Based Coatings on EH32 Steel

Laser cladding on polar icebreaking ship steels that can work under low-temperature conditions has rarely been investigated. Herein, three types of the cobalt (Co)-based coatings with different chemical compositions and structures were fabricated on EH32 steel substrate by laser cladding. The influence of corrosion treatment at room temperature and freezing–corrosion treatment at a low temperature of −80 °C on the wear resistance of the EH32 substrate and the Co-based coatings was investigated. Results showed that the as-cladded Co-based coatings were composed of Co-based solid solutions and hard carbides such as Cr7C3, Cr23C6, Mo2C, WC, and/or W2C. Columnar dendrites with a 3D honeycomb-like structure and equiaxed dendrites occurred in the coatings containing WC and/or W2C. The Co-based coatings could significantly improve the hardness and wear resistance of the EH32 substrate, and the coating with the highest hardness possessed the best wear resistance. Both the corrosion and freezing–corrosion treatments could enhance the wear resistance of the Co-based coatings but weakened the wear resistance of the EH32 substrate. The Co-based coatings after corrosion treatment had superior wear resistance compared to the untreated coatings and the coatings after freezing–corrosion treatment.

The effect of laser energy density on the geometric characteristics, microstructure and corrosion resistance of Co-based coatings by laser cladding

In view of the urgent problem of oil pipeline corrosion in the petroleum industry, Co-based coating was prepared on 42CrMo pipeline steel through laser cladding technology to realize the surface strengthening. The objective of the paper is to investigate the effects of process parameters (Led = 64–100 J/mm2) on the geometric characteristics, microstructure and corrosion resistance of the coating. It can be seen from the results that the increase of Led led to the increase of the dilution ratio and the Marangoni convection effect. At the Led of 90 J/mm2, the grain size of the coating was refined, and the dendrite growth direction was clearly visible. In the electrochemical test, the corrosion resistance of the coating first increased and then decreased with the increase of Led. Using the appropriate process parameters, the corrosion current density of the coating was only 1.740 × 10−7 (A/cm2), and a dense passive film was formed on the Co-based coatings. The Led was closely related to the properties of the coating. While at the Led of 64 J/mm2, the passive film resistance reached 6.474e+003 Ω cm2, and many microcracks appeared on the surface of the coating after corrosion. However, at the Led of 90 J/mm2, the corrosion products on the coating surface were less with excellent corrosion resistance. Therefore, the process parameters of laser cladding showed great effect on the forming quality and corrosion resistance of the coating. The coating with excellent corrosion resistance could be obtained by the adjustment of the process parameters.

Effects of CeO2on Microstructural Evolution, Corrosion and Tribology Behavior of Laser Cladded TiC Reinforced Co-based Coatings

Effects of CeO2on Microstructural Evolution, Corrosion and Tribology Behavior of Laser Cladded TiC Reinforced Co-based Coatings

Microstructure, High-Temperature Wear Resistance, and Corrosion Resistance of Laser Cladded Co-Based Coating

Co-based coating has been fabricated on Ti-6Al-4V (TC4) titanium alloy by laser cladding. The phase composition and microstructure of the coating were analyzed using metallurgical microscope (MM), scanning electron microscope (SEM), x-ray diffraction (XRD), energy-dispersive spectroscopy (EDS) and transmission electron microscopy (TEM), respectively. The microhardness, high temperature wear resistance and corrosion resistance of the coating were investigated by performance tests. The results show that the coating was mainly composed of CoTi, CoTi2 and Co0.5Cr1.5Ti crystalline phases and an amorphous phase. The microhardness of the coating was twice that of the titanium alloy substrate. The value of friction and wear coefficient of the coating was lower and more stable, and the wear mass loss decreased by 52% as compared with the substrate in the high-temperature friction and wear test. The oxidative wear mechanism in the condition of high temperature friction and wear was studied. Moreover, the coating exhibited better corrosion resistance than the substrate in both NaCl and H2SO4 electrolyte solutions.

The Gradient Microstructure and High-Temperature Wear Behavior of the CoCrMoSi Coating by Laser Cladding

Recently, laser cladding has been widely investigated for the preparation of Co-based coatings for high-temperature applications. However, there is a highly cracking tendency in Co-based alloys due to the reinforcing hard phases. In this work, a single layer of CoCrMoSi coating was prepared with laser cladding on 304 stainless steel. By proper preheating, cracking was avoided during deposition. The high-temperature wear behaviors of the coating were evaluated by ball-on-disk test. In terms of morphology and chemical composition, the coating exhibited a gradient microstructure across the section. From the interface with the substrate to the top surface, the structure was characterized with three types of morphology in sequence, i.e., coarse cellular crystals in the interdiffusion zone, nanosized lamellar eutectic and Laves-phase dendrites embedded within nanosized lamellar eutectic. The surface hardness of the coating is 835HV0.3, which was mainly attributed to the Laves phase. Compared with the 304 stainless steel, the volume loss of the coating decreased by 76.5%at 400°C. The wear mechanisms of the coating were oxidation wear and abrasive wear. The excellent coating wear resistance at high-temperature can be traced to the microstructure of Laves phase embedded within the nanosized eutectic.

Influence of processing conditions on the microstructure and sliding wear of a promising Fe-based coating deposited by HVAF

Thermal spray is a versatile and cost-effective process to deposit wear and corrosion resistant coatings. In this work, a relatively new ‘Fe-based’ chemistry comprising boride and carbides, is explored as a ‘greener’ alternative to the relatively expensive and carcinogenic Co-based coatings to mitigate wear. The emergent thermal spray process of high-velocity air-fuel (HVAF) spraying was chosen to deposit the Fe-based coatings, with the high-velocity oxy-fuel (HVOF) also being employed solely for the purpose of preliminary comparison. Detailed characterization of the HVOF and HVAF sprayed Fe-based coatings was carried out. Microstructure, porosity, hardness and phase analysis results demonstrate the influence of processing conditions, where specific spray conditions yielded minimal undeformed particulates content, high hardness, low porosity and feedstock phase retention. Differences in microstructural features of the as-deposited coatings in relation to their processing conditions are discussed in detail. The coatings were subjected to ball-on-disc tribometry tests at different load conditions and their friction and wear performance were evaluated. The coefficient of friction results of investigated coatings concurred with their respective microstructural features. Post-mortem of the worn coating surface, the mating alumina ball surface and wear debris was performed using SEM/EDS analysis to understand the associated wear mechanisms and material transfer. This work provides new insights on identifying appropriate HVAF processing conditions to achieve acceptable microstructural features and phases in Fe-based coatings for improved wear performance.