CoCr Coatings Research Articles

Effect of B4C addition on microstructure and tribological performance of laser cladded WC−10Co4Cr coatings

Effect of B4C addition on microstructure and tribological performance of laser cladded WC−10Co4Cr coatings

Influence of WC Particle Size on the Mechanical Properties and Residual Stress of HVOF Thermally Sprayed WC-10Co-4Cr Coatings.

Cermet coatings deposited using high-velocity oxy-fuel (HVOF) are widely used due to their excellent wear and corrosion resistance. The new agglomeration–rapid sintering method is an excellent candidate for the preparation of WC–Co–Cr feedstock powders. In this study, four different WC–10Co–4Cr feedstock powders containing WC particles of different sizes were prepared by the new agglomeration–rapid sintering method and deposited on steel substrates using the HVOF technique. The microstructures and mechanical properties of the coatings were investigated using scanning electron microscopy, X-ray diffraction, nanoindentation, and Vickers indentation. The through-thickness residual stress profiles of the coatings and substrate materials were determined using neutron diffraction. We found that the microstructures and mechanical properties of the coatings were strongly dependent on the WC particle size. Decarburization and anisotropic mechanical behaviors were exhibited in the coatings, especially in the nanostructured coating. The coatings containing nano- and medium-sized WC particles were dense and uniform, with a high Young’s modulus and hardness and the highest fracture toughness among the four coatings. As the WC particle size increased, the compressive stress in the coating increased considerably. Knowledge of these relationships enables the optimization of feedstock powder design to achieve superior mechanical performance of coatings in the future.

Corrosion and Wear Behavior of WC-10Co4Cr Coating under Saturated Salt Drilling Fluid.

Coating on the surface is one of the main ways to improve the corrosion resistance and wear resistance of materials. In this work, the corrosion, erosion, and wear resistance of WC–10Co4Cr coating and 27CrMoV substrate were compared by simulating the actual working conditions of the drill pipe. The simulation results show that the most serious corrosion occurred at the pipe body and the dominating erosion arose at the pipe joint closing to the inlet of the flow field. WC–10Co4Cr coating has excellent protection to 27CrMoV substrate, resulting in a 400 mV increase in corrosion potential, a two-orders-of-magnitude decrease in the corrosion current, and four times the improvement of the impedance value. The erosion resistance of the WC–10Co4Cr coating increased to more than 30% higher than that of the 27CrMoV substrate. The friction coefficient of the WC–10Co4Cr coating was much lower than that of the 27CrMoV substrate, and the wear resistance of the coating was higher than that of the substrate.

Wear and corrosion of Co-Cr coatings electrodeposited from a trivalent chromium solution: Effect of heat treatment temperature

Electrodeposition from a trivalent chromium bath was employed to obtain the cobalt‑chromium coatings with a thickness of ~25 μm. The effect of heat treatment temperature on characteristics, mechanical properties , and electrochemical behavior of the coatings, were examined. Heat treatment was performed in a wide range of temperatures (200–800 °C). Heat-treating below 400 °C increased the crystallinity of alloy coatings and had a negligible effect on nodular and cracked morphology. Raising the heat-treating temperature changed the X-ray diffraction patterns by thickening the surface oxide films , which consisted of different oxides (e.g., cobalt chromite spinel). The surface nodules were faded out above 700 °C. The heat-treating below 300 °C decreased the hardness and wear resistance of Co-Cr electrodeposits. The 400 °C heat-treated sample had the highest hardness of ~1000 HV, and the best tribological behavior. The weight loss and coefficient of friction of this sample were 24 and ~2 times smaller than the as-deposited coating. The hardness and wear resistance dropped again with further raising the heat-treating temperature. While detachment of coating during sliding occurred at low-temperature heat-treated samples, the abrasion was the main mechanism of material loss at those heat-treated at higher temperatures. According to the corrosion results, the optimum heat-treating temperature that the highest corrosion resistance could be achieved was 300–500 °C. The corrosion current density of 400 °C heat-treated coating was ~3.7 times smaller than the as-deposited Co-Cr film. • The relatively thick Co-Cr coatings were produced from a trivalent chromium bath. • Effect of heat treatment on Co-Cr films' characteristics and properties was studied. • Different types of oxides and morphologies were observed at various temperatures. • The 400 °C heat-treated sample showed the highest hardness and wear resistance. • Optimum heat treatment temperatures for the best corrosion resistance was 300–500 °C.

Influence of Test Conditions on Sliding Wear Performance of High Velocity Air Fuel-Sprayed WC-CoCr Coatings.

Sliding wear performance of thermal spray WC-based coatings has been widely studied. However, there is no systematic investigation on the influence of test conditions on wear behaviour of these coatings. In order to have a good understanding of the effect of test parameters on sliding wear test performance of HVAF-sprayed WC–CoCr coatings, ball-on-disc tests were conducted under varying test conditions, including different angular velocities, loads and sliding distances. Under normal load of 20 N and sliding distance of 5 km (used as ‘reference’ conditions), it was shown that, despite changes in angular velocity (from 1333 rpm up to 2400 rpm), specific wear rate values experienced no major variation. No major change was observed in specific wear rate values even upon increasing the load from 20 N to 40 N and sliding distance from 5 km to 10 km, and no significant change was noted in the prevailing wear mechanism, either. Results suggest that no dramatic changes in applicable wear regime occur over the window of test parameters investigated. Consequently, the findings of this study inspire confidence in utilizing test conditions within the above range to rank different WC-based coatings.

Microstructure, dry sliding friction performances and wear mechanism of laser cladded WC–10Co4Cr coating with different Al2O3 mass fractions

To enhance the hardness and wear resistance of cold work mold, Al2O3 reinforced WC–10Co4Cr coatings were fabricated on Cr12MoV steel using a laser cladding (LC). The morphologies, chemical elements and phase compositions of obtained coatings were analyzed using a scanning electron microscope (SEM), energy disperse spectroscopy (EDS), and X–ray diffraction (XRD), respectively. The results show that the Al2O3 reinforced WC–10Co4Cr coatings are composed of Co4W2C, WC and C–depleted W2C phases, and the chemical elements of Al2O3 reinforced WC–10Co4Cr coatings are diffused to the substrate, which form the metallurgical bonding at the coating interfaces. The hardness of WC–10Co4Cr coatings with the 2, 4 and 6% Al2O3 mass fractions increases from 538% to 640% than that of substrate. The average coefficients of friction (COFs) of WC–10Co4Cr–2%Al2O3, –4%Al2O3 and –6%Al2O3 coatings decrease from 71% to 56% than the substrate; and the corresponding wear rates also decrease from 12.4% to 4.2% than the substrate. The average COF and wear rate of WC–10Co4Cr–6%Al2O3 coating are the lowest among the three kinds of coatings, showing that the addition of Al2O3 is the main factor to reduce the COFs and increase the wear resistance of WC–10Co4Cr coatings.

Wear properties of WC–Co and WC–CoCr coatings applied by HVOF technique on different steel substrates

Abstract Low alloy and stainless steel are the most used types of iron-based materials world wide. Their use against in machine element work, reclamation, corrosion and wear resistance are still challenging. To overcome this problem, many steel alloys are coated with cermet coatings to protect the parts from wear and corrosion. In the present study, WC-Co and WC-CoCr coatings were applied by means of a high velocity oxy-fuel (HVOF) technique on AISI 304, AISI 1040, and AISI 4340 steel alloys used as substrates. The aim was to investigate surface properties and wear resistance of the coatings and to determine their relationship with the type of coating and substrate. In accordance with this purpose, hardness and thickness of the coatings were measured, sliding wear tests were performed, scanning electron microscope (SEM) images and X-ray diffractions (XRD) were taken, surface roughness and friction coefficients were determined. The results showed that the WC-CoCr coatings had higher hardness and lower thickness than the WC-Co coatings. Maximum hardness was obtained in the WC-CoCr coating applied to AISI 4340 steel, which was also the hardest alloy among those studied. After wear resistance tests, it was revealed that the wear resistance of the WC-CoCr coatings was better than that of the WC-Co coatings for each steel substrate. During the coating, the new phases resulting from the decomposition of the WC phase in the WC-CoCr coatings contributed more to wear resistance than those of the WC-Co coatings. A lower friction coefficient and lower surface roughness of the WC-CoCr coatings during wear were obtained, resulting in higher wear resistance. A WC-CoCr coating on AISI 4340 alloy which has the highest hardness, lowest surface roughness and lowest friction coefficient resulted in the highest wear resistance among all types studied.

Wear properties of WC–Co and WC–CoCr coatings applied by HVOF technique on different steel substrates

Abstract Low alloy and stainless steel are the most used types of iron-based materials world wide. Their use against in machine element work, reclamation, corrosion and wear resistance are still challenging. To overcome this problem, many steel alloys are coated with cermet coatings to protect the parts from wear and corrosion. In the present study, WC-Co and WC-CoCr coatings were applied by means of a high velocity oxy-fuel (HVOF) technique on AISI 304, AISI 1040, and AISI 4340 steel alloys used as substrates. The aim was to investigate surface properties and wear resistance of the coatings and to determine their relationship with the type of coating and substrate. In accordance with this purpose, hardness and thickness of the coatings were measured, sliding wear tests were performed, scanning electron microscope (SEM) images and X-ray diffractions (XRD) were taken, surface roughness and friction coefficients were determined. The results showed that the WC-CoCr coatings had higher hardness and lower thickness than the WC-Co coatings. Maximum hardness was obtained in the WC-CoCr coating applied to AISI 4340 steel, which was also the hardest alloy among those studied. After wear resistance tests, it was revealed that the wear resistance of the WC-CoCr coatings was better than that of the WC-Co coatings for each steel substrate. During the coating, the new phases resulting from the decomposition of the WC phase in the WC-CoCr coatings contributed more to wear resistance than those of the WC-Co coatings. A lower friction coefficient and lower surface roughness of the WC-CoCr coatings during wear were obtained, resulting in higher wear resistance. A WC-CoCr coating on AISI 4340 alloy which has the highest hardness, lowest surface roughness and lowest friction coefficient resulted in the highest wear resistance among all types studied.

Effect of Co-Cr coating by EB-PVD on the bonding strength between titanium and porcelain.

To investigate the effect of the deposited Co-Cr coatings on the bonding strength of titanium-porcelain restoration, Co-Cr coatings with different thickness were deposited onto the pure titanium substrate by electron beam physical vapor deposition (EB-PVD). Sixty pure titanium specimens (25.0 mm×3.0 mm×0.5 mm) were randomly divided into 5 groups. Four Co-Cr coatings with different thickness, including 20, 40, 60 and 80 nm, were deposited onto the specimens (group A, B, C and D, respectively). Sandblasting treatment was selected as a control group (group E). After porcelain fusion, bonding strengths of titanium-porcelain restoration were measured by three-point bending test according to ISO 9 693. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were employed to evaluate the morphology and elemental composition of both Co-Cr coatings and interfaces of the titanium-porcelain restoration. The bonding strengths in the group E and the test groups (groups A, B, C, and D) were (30.23±1.94), (36.11±1.17), (43.33±1.17), (39.95±2.22), (38.85±1.10) MPa, respectively. In contrast to the group E, the Co-Cr coatings significantly improved the bonding strength between titanium and porcelain (P<0.05). Moreover, some cracks and pores were found at the titanium-porcelain interface in the group E, while the titanium-porcelain interface in the group B was tight and compact (P<0.05). Meanwhile, comparing to the group E, more residual porcelains were found on the substrate of group B (P<0.05). Co-Cr coating with appreciate thickness using EB-PVD could significantly improve bonding strength between titanium and porcelain.

Experimental investigation and study of HVOF sprayed WC-12Co, WC-10Co-4Cr and Cr3C2-25NiCr coating on its sliding wear behaviour

An experimental investigation followed by a comparative study of HVOF sprayed WC-12Co, WC- 10Co4Cr and Cr3C2-25NiCr coatings were conducted at three different loads of 20, 40 and 60 N. Sliding wear of coated specimens against hardened EN-32 disc was performed as per the G 99–5 standard at room temperature. The feedstock powders and corresponding coatings were characterized for microstructural studies along with porosity, microhardness, and adhesive bond strength. The experimental results suggest that the stability of the transfer layer plays a significant role in stable wear. The WC-12Co coating shows the best sliding wear resistance, maximum microhardness which was found to be 56.6% and 9.6% higher than WC-10Co-4Cr and Cr3C2-25NiCr coatings. The maximum adhesive bond strength was found to be 2.03% and 10.5% higher in WC-12Co than WC-10Co-4Cr and Cr3C2-25NiCr. Tribo oxide layer counterbalances to stabilize the wear during high heat generation at higher loads. Various aspects and mechanisms of these improvements are discussed in this paper.

Microstructure, mechanical and tribological properties of cold sprayed Ti6Al4V–CoCr composite coatings

Metal-matrix composite (MMC) coatings are advanced materials, composed of two or more phases that can be tailored based on the industrial needs such as lightweight and wear-resistance. Cold spray (CS) technology is highly suitable to develop these MMC coatings as it is a low temperature deposition method, which does not involve particle melting, hence retaining the original properties of the materials. Ti–6Al–4V (Ti64) is a common alloy used in aerospace applications owing to its lightweight and high yield strength. Recently, cold sprayed Ti64 coatings have shown promise to repair damaged or enhance surfaces of Ti components. However, there are still several issues with cold sprayed Ti64 coatings, such as relatively high porosity level and low wear-resistance. To solve these, a novel approach is to incorporate CoCr (ASTM F75) as reinforcing metal alloy particles into the Ti64 matrix to form a Ti64–CoCr composite coating. Pure Ti64 and pure CoCr coatings were also deposited as a comparison. The Ti64–CoCr composite coating achieved low porosity level (0.9%), high hardness (434 HV) and high wear resistance (1.7 × 10−4 mm3/Nm) compared to the Ti64 coatings. These properties were obtained while maintaining the original phases of the feedstock Ti64 and CoCr powders and achieving adhesion strength above 60 MPa. A 20 nm thick Ti–Co interdiffusion layer was also observed at the interface of CoCr and Ti64 particles under TEM. These results showed that Ti64–CoCr composite coating is a promising lightweight and wear resistant metallic coating for aerospace application.

Numerical Modeling of Transient Temperature and Stress in WC–10Co4Cr Coating During High-Speed Grinding

Based on the classical theory of heat conduction and thermoplastic mechanics, the three-dimensional heat transfer and thermal stress models of high-velocity oxygen fuel WC–10Co4Cr coating under the action of a moving heat source during grinding were developed. Considering temperature-dependent material properties, the transient temperature field and the thermal stress field generated in workpiece were simulated by a finite element method (FEM). As both the thermal conductivity and the specific heat capacity of WC–10Co4Cr are larger than 300 M, temperature rise mainly occurred in the coating. The rise in grinding temperature was discontinuous along the depth of the workpiece, and the temperature gradient at the coating–substrate interface was found to be the largest. Due to the large difference in thermal expansion coefficient between coating and substrate material, considerable thermal stress was generated at the bonding surface. Grinding temperature and thermal stresses at the coating–substrate interface started to increase with the decrease in coating thickness. Grinding surface temperatures were measured experimentally for different coating thicknesses, and simulation results were compared with the obtained experimental values. It was found that FEM results were well consistent with experimental observations.

Bone ingrowth comparison of irregular titanium and cobalt-chromium coatings in a translational cancellous bone model.

The use of cobalt-chromium (CoCr)-bearing surfaces in total joint replacement (TJR) remains the predominate bearing surface. The conundrum with using this biomaterial has been selecting an ideal porous coating to assure reproducible skeletal attachment. There has been evidence that smooth CoCr beads may be inferior for skeletal attachment compared to identically shaped titanium (Ti) beads. Recent in vitro studies have demonstrated that an increased surface area and roughness favors osteoblast adhesion to metallic biomaterials. Therefore, we hypothesized that an irregular shape CoCr bead with an increased surface texture would help correct the negative bone responses that have been reported with smooth beaded CoCr coatings and thus allowing for bone ingrowth equivalently as an irregular commercially pure Ti porous coating with similar porosity. This investigation employed a weight-bearing translational sheep cancellous bone model to accurately simulate a cancellous bone response as it would be clinically in a human TJR. The data analyses obtained from this investigation revealed similar bone responses between the porous coatings. By 12 weeks the irregular shape CoCr coating was able to achieve similar bone ingrowth with skeletal interlock when compared to a clinically proven Ti porous coating.

Effect of grinding on surface characteristics of HVOF-sprayed WC–10Co–4Cr coatings

ABSTRACT The aim of this research was to investigate the effect of the grinding depth of cut on surface quality and corrosion behaviour of WC–10Co–4Cr cermet coatings. Accordingly, a WC–10Co–4Cr coating with 400 μm thickness was deposited on the carbon steel substrate using a High-Velocity Oxygen Fuel (HVOF) process. Consequently, the effect of different depths of cut on the coating properties was evaluated. Porosity, surface roughness and microhardness of as-sprayed and ground coatings were measured in order to investigate the effect of grinding on the coating characteristics. The corrosion behaviour of the coatings was evaluated using open circuit potential, electrochemical impedance spectroscopy, and potentiodynamic polarisation tests. The results indicated that after grinding, the porosity and microhardness of the coatings were increased and the surface roughness was decreased. Furthermore, the increase in the depth of cut increased the coating porosity and microcracks. Therefore, the corrosion resistance of the coating was decreased.

Effect of Tribo-Oxidation on Friction and Wear Behaviour of HVOF Sprayed WC–10Co–4Cr Coating

The effect of tribo-chemical reactions under varying sliding speed and load conditions on the friction and abrasive wear response of high-velocity oxy-fuel-sprayed WC–10Co–4Cr coating was studied. The abrasive wear rate and friction coefficient decreased with the increase in sliding speed while friction coefficient displayed increasing trend with increase in load. The decrease in friction coefficient and wear rate was attributed to formation of tribo-oxides and surface films with good lubricating properties. Severity of abrasive wear increased with increasing load which was associated with transition in wear mechanisms from plastic deformation and fatigue to delamination cracking, intergranular fracture and splat fracture. Increase in friction coefficient with load irrespective of sliding speed was due to increasing contribution of fracture-assisted mechanical wear as compared to oxidative wear. The nature, composition and properties of tribo-films imparted crucial role to influencing friction and abrasive wear of WC–10Co–4Cr coating.

Structure and cavitation erosion behavior of HVOF sprayed multi-dimensional WC–10Co4Cr coating

Abstract A new kind of multi-dimensional WC–10Co4Cr coating which is composed of nano, submicron, micron WC grains and CoCr alloy, was developed by high velocity oxy-fuel (HVOF) spraying. Porosity, microhardness, fracture toughness and cavitation erosion resistance of the multi-dimensional coating were investigated in comparison with the bimodal and nanostructured WC–10Co4Cr coatings. Moreover, the cavitation erosion behavior and mechanism of the multi-dimensional coating were explored. Results show that HVOF sprayed multi-dimensional WC–10Co4Cr coating possesses low porosity (≤0.32%) and high fracture toughness without obvious nano WC decarburization during spraying. Furthermore, it is discovered that the multi-dimensional WC–10Co4Cr coating exhibits the best cavitation erosion resistance which is enhanced by approximately 28% and 34%, respectively, compared with the nanostructured and bimodal coatings in fresh water. The superior cavitation resistance of multi-dimensional WC–10Co4Cr coating may originate from the unique micro–nano structure and excellent properties, which can effectively obstruct the formation and propagation of cavitation erosion cracks.

Microstructure and Properties Characterization of WC-Co-Cr Thermal Spray Coatings

WC-Co-Cr coatings are widely employed due to their improved wear resistance and mechanical properties, however, the properties and performance of these coatings are compromised by the processing parameters of each spraying technique. Therefore, this study is aimed to evaluate and determine the effect of the deposition parameters on the properties and microstructural characteristics of WC-Co-Cr coatings using a more economical thermal spray technique. In particular, the influence of flame spray parameters on the microstructure, crystal structure, hardness, and sliding wear resistance of WC- Co-Cr coatings was examined. Two parameters were considered: Type of flame (reducing, neutral and oxidizing), and the spray torch nozzle exit area. Results indicated that WC particles undergo considerable degree of decarburization and dissolution during spraying, showing substantial amounts of W2C, W, and Co3W3C, for all the considered conditions. However, the extent of phase transformation depended largely on the flame chemistry. The microstructure of the coatings was mainly affected by the spray nozzle. Regarding the sliding wear behavior, the coatings with uniform distribution of hard particles provided the best wear resistance. The decomposition of WC into W2C phase seems to have meaningless significance in the mass loss, nevertheless, the WC phase transformation to metallic tungsten and η-phase (Co3W3C) produce higher wear rates due to deficiency of carbide particles and embrittlement of the binder phase which induces cracking and delamination of the splats.

Comparative Evaluation of Hot Corrosion Resistance of 83WC–17CO and 86WC–10CO–4Cr Coatings on Some Boiler Steels in Actual Boiler in Thermal Power Plant

This research work was focused on the performance of HVOF-sprayed 83WC–17CO and 86WC–10CO–4Cr coatings on boiler steel alloys ASME SA213 T22 and ASME SA213 T91 in a coal-fired boiler environment. Both coated and bare steel alloys were subjected to cyclic exposures, in the superheater zone of a coal-fired boiler for 10 cycles at 900 °C to compare the effect of the coatings in actual boiler environment. During the study, each cycle consisted of 100-h heating followed by 1-h cooling at ambient conditions and thereafter thermogravimetric method was used to establish the kinetics of corrosion. Corrosion products were examined by x-ray diffraction, scanning electron microscopy/energy-dispersive spectroscopy techniques. The hot corrosion resistance of both the coatings 83WC–17CO and 86WC–10CO–4Cr coating was found better on ASME SA213 T22, whereas 86WC–10CO–4Cr coating showed the higher corrosion resistance on ASME SA213 T22 as compared to 83WC–17CO coating.

An investigation of corrosion resistance of HVOF coated ASME SA213 T91 boiler steel in an actual boiler environment

PurposeThis paper aims to use the high-velocity oxy fuel (HVOF) spraying process for depositing 93(WC–Cr3C2)–7Ni, 75Cr3C2–25NiCr, 83WC–17CO and 86WC–10CO–4Cr coatings on ASME SA213 T91 to study the corrosion resistance of these coatings in an actual boiler environment.Design/methodology/approachThe HVOF spraying process was used for depositing 93(WC–Cr3C2)–7Ni, 75Cr3C2–25NiCr, 83WC–17CO and 86WC–10CO–4Cr coatings on ASME SA213 T91. All the coatings obtained are found to be uniform, dense and having thickness between 200 and 250 μm. All the coatings were exposed in an actual boiler environment at 900°C temperature for 10 cycles. Each cycle consisted of 100 h heating followed by 1 h cooling at ambient conditions. X-ray diffraction, scanning electron microscopy and energy-dispersive spectroscopy techniques were used to analyse corrosion products.FindingsAll the coated samples were found to be having higher corrosion resistance than the uncoated samples. Among coated specimens, 93(WC–Cr3C2)–7Ni coating has shown maximum and 75Cr3C2–25NiCr coating has shown minimum resistance to corrosion.Originality/valueThis paper is original research.

Comparative analysis of tribological behavior of plasma- and high-velocity oxygen fuel-sprayed WC-10Co-4Cr coatings

PurposeThe purpose of this study is to prepare WC-10Co-4Cr coatings using two processes of plasma spraying and high-velocity oxygen fuel (HVOF) spraying. The decarburization behaviors of the different processes are analyzed individually. The microstructural characteristics of the as-sprayed coatings are presented and the wear mechanisms of the different WC–10Co–4Cr coatings are discussed in detail.Design/methodology/approachThe WC–10Co–4Cr coatings were formed on the surface of Q235 steel by plasma and HVOF spraying.FindingsPlasma spraying causes more decarburizing decomposition of the WC phase than HVOF spraying. In the plasma spraying process, η(Cr25Co25W8C2) phase appears and the C content decreases from the top surface of the coating to the substrate.Practical implicationsIn this study, two WC–10Co–4Cr coatings on Q235 steel prepared by plasma and HVOF spraying were compared with respect to the sliding wear behavior.Originality/valueThe wear mechanisms of the plasma- and HVOF-sprayed coatings were abrasive and oxidation, respectively.