Nano WC-Co Research Articles

Corrosion and wear protection of AISI 4140 carbon steel using a laser-modified high-velocity oxygen fuel thermal sprayed coatings

Inconel and micro and nano WC-12Co powders were deposited on AISI 4140 carbon steel by high-velocity oxy fuel (HVOF) coating and followed by laser surface modification. Laser power and scan speed were varied at different levels. Microstructure and microhardness were investigated. Nanocoatings performed better than microcoatings. Nanostructured WC powder coatings exhibited greater hardness compared to microstructured powder coating. When the laser power is increased to 170 W, a small cellular dendrite microstructure through multiphase solidification is formed due to the difference in thermal properties of Inconel 625 and WC particles. Adequate laser power and low scan speed were preferred to produce a high-quality coating. From the electrochemical corrosion test results, it was observed that the corrosion rate of laser-modified HVOF sprayed coating is lower than the carbon steel sample. This shows that the Inconel sprayed by laser-modified HVOF coating enhanced the corrosion resistance of the substrate steel material. The porosity percentage was higher for all the samples when laser scan speed was increased.

Core-shell structural iron based metal matrix composite powder for laser cladding

WC is considered as a good choice for the reinforced phase of many composite alloys because of its excellent hardness and good wear resistance. In the past, micro WC particles physically mixed with micro metal particles were also widely used in laser cladding, however, micron WC particles often sink down to the bottom and may cause uneven element distribution, holes, cracks, and other defects in the cladding layer due to its high density and high laser absorption rate. Here, we prepare a core-shell structural 420-nano WC-Co composite powder by high energy ball milling. The composite powder has high circularity and high fluidity, which is suitable for laser cladding. The cladding layer with no holes or cracks can be attained using the core-shell structural composite powders, moreover, the nano WC-Co particles are evenly distributed in the cladding layer and the microhardness of the cladding layer has enhanced significantly.

Simultaneous increase of friction coefficient and wear resistance through HVOF sprayed WC-(nano WC-Co)

Generally, well wear resistance corresponds to low friction coefficient. While, well wear resistance and high friction coefficient are needed in brakes for short braking distance and long serving life. Ceramics possess high hardness which correspond well wear resistance. Cobalt is often used as bond phase of tungsten carbide ceramic, which will increase the toughness of tungsten carbide. While, the stiffness of WC-Co will decrease after cobalt is introduced into tungsten carbide, which will lead to the decrease of friction coefficient. To increase the stiffness and the friction coefficient of WC-Co, nano WC particles were introduced into cobalt binder phase which formed WC-(nano WC-Co).In this work, WC-(nano WC-Co) coatings were prepared through high velocity oxygen fuel (HVOF) spray on the vermicular iron substrate in order to increase friction coefficient and wear resistance simultaneously. The microstructure, mechanical property and wear performance of the coating were investigated.It was found that the microhardness of WC-(nano WC-Co) coatings had the highest hardness of 1393 ± 13 HV with 50% nano WC-10Co-4Cr. Meanwhile, its friction coefficient was obviously increased to 0.7, which was three times higher than that of the vermicular cast iron and higher than both of the micro-sized and nano sized WC-Co. Its wear mass loss of WC-(nano WC-Co) coating was much lower than that of the vermicular cast iron and lower than both of the micro-sized and nano sized WC-Co. The friction coefficient and wear resistance of WC-(nano WC-Co) were increased simultaneously.

Microstructure and Erosion Properties of HVOF Sprayed Cermet Coatings via Different Feedstock Powders

In the present study, Cr3C2-NiCr and WC-Co coatings have been deposited using high velocity oxygen-fuel (HVOF) spray technology from five available powders with various bonding phase content or manufacturing process. The microstructure of coatings was observed by scanning electron microscopy (SEM). Microhardness and erosion performance of the coatings were studied. The influence of powder characteristics on the microstructure and erosion performance of coatings was also investigated. The results indicated that an independent bonding phase distributed in the feedstock powder can effectively improve the erosion resistance of Cr3C2-NiCr coatings although the microhardness of the coatings may be lower. Deformation of the free NiCr binder layer in the coating is probably to prevent nucleation and propagation of cracks, which may result in improving the erosion resistance of the coating. Nano WC-Co coatings reached lower erosion resistance than micro WC-Co coatings due to the higher porosity and lower microhardness of nano WC-Co coatings.

Weibull analysis of hardness distribution in detonation sprayed nano-structured WC-12Co coatings

WC based cermet coatings were deposited by detonation spray coating (DSC) technique using nano WC-12Co feedstock at various oxygen to acetylene (oxy-fuel: OF) ratios on mild steel (MS) substrate. Microstructure and phase composition of feedstock and coatings were analyzed using scanning electron microscope (SEM) and X-ray diffraction (XRD) techniques. Microhardness of coatings was evaluated and analyzed statistically as a function of applied load (100g and 300g), while a total of 300 indentations performed at each load. Three body rubber wheel abrasive wear tests using SiC as an abrasive medium were carried out and the coating wear rates were calculated. The extent of decarburization in nano WC-12Co coatings increased with increase in OF ratio. The characteristic hardness (Ho) and Weibull modulus (m) were found to be influenced by the indentation load and coating microstructure. An increase in ‘m’ value and decrease in ‘Ho’ value were noticed with increasing indentation load. Coatings with similar characteristic hardness values have exhibited different Weibull modulus values. Based on the results obtained, the minimum number of indents required for arriving at the characteristic hardness value is calculated, the structure-property correlations were examined and the strategies based on Weibull parameters to achieve best possible properties and performance was proposed.

A comparative study on the microstructure and surface property evaluation of coatings produced from nanostructured and conventional WC–Co powders HVOF-sprayed on Al7075

Conventional and nanostructured WC-12wt.% Co powders were HVOF sprayed on Al7075-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 (from the Al2O3 counter balls). Coating of Al7075 with nano WC–12Co was proved effective in terms of microstructural features, corrosion and wear behaviour.

Characterization of cold-sprayed multimodal WC-12Co coating

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 100nm 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.

A Comparison of Mechanical and Tribological Behavior of Nanostructured and Conventional WC-12Co Detonation-Sprayed Coatings

In the present study, WC-12Co coatings were deposited by detonation-spraying technique using conventional and nanostructured WC-12Co feedstock at four different oxy/fuel ratios (OF ratio). The coatings exhibited the presence of phases like W2C and W due to the decarburization of the WC phase, and the proportions of these phases were higher in the nano WC-12Co coatings compared with conventional WC-12Co coatings. Coating hardness and fracture toughness were measured. The tribological performance of coatings was examined under dry sand rubber wheel abrasion wear, and solid particle erosion wear conditions. The mechanical and wear properties of coatings were influenced by degree of decarburization and more so in the case of nanostructured WC-Co coatings. The results indicate that the extent of decarburization has a substantial influence on the elastic modulus of the coating which in turn is related to the extent of intersplat cracking of the coating.

Simultaneous strengthening and toughening effects in WC–(nanoWC–Co)

Simultaneous hardening and toughening effects were found for WC–(nanoWC–Co) with microWC strengthening particles in nanoWC–Co cermet matrix. The high microhardness of 1683 ± 176 Hv comparable to that of nanoWC–12Co was attributed to the hardening effect from both microWC particles and nanoWC dispersion in the Co binder phase. The high fracture toughness of 18.9 ± 4.0 MPa m1/2, comparable to that of microstructured WC–12Co, resulted from the larger area of the bimodal-sized fracture surface morphology and the crack bridging due to the microsized strengthening WC particles.

Influence of nano-WC–12Co powder addition in WC–10Co–4Cr AC-HVAF sprayed coatings on wear and erosion behaviour

WC–10Co–4Cr coatings deposited by activated combustion-high velocity air fuel (AC-HVAF) spray process are the most commonly used system for different components prone to wear/erosion. The aim of this work is to investigate the influence of nano-WC–12Co powder addition in WC–10Co–4Cr AC-HVAF sprayed coatings on wear and erosion behaviour. Therefore, in this work, nano-WC–12Co powder was added to the sub-micron WC–10Co–4Cr powder in order to improve the hardness of WC based AC-HVAF coatings. The result shows that 15% addition of nano-WC–12Co will improve the hardness of AC-HVAF coating from 1677 to 1873 HV0.3 due to the embedding of nano-WC–12Co powder. Sliding wear test and slurry erosion test were chosen to evaluate the effect of adding nano-WC–12Co powder. The wear and erosion resistance of AC-HVAF coated samples go up with the addition of nano-WC–12Co powder, in addition, the coating hardness has also increased. The coatings have been characterised by scanning electron microscope (SEM), microhardness tester and X-ray diffractometer (XRD). The results indicated that the sample with 15% addition of nano-WC–12Co to WC–10Co–4Cr based AC-HVAF coating has the best wear and erosion resistance. In addition, this also demonstrated that it is possible to fabricate the nanostructured WC–10Co–4Cr coating with low porosity and high hardness by AC-HVAF spray deposition with reasonable thermal spraying parameters.

Friction Behavior of High Velocity Oxygen Fuel (HVOF) Thermal Spray Coating Layer of Nano WC-Co Powder

High Velocity Oxygen Fuel (HVOF) thermal spray coating of nano size WC-Co powder (nWC-Co) has been studied as one of the most promising candidate for the possible replacement of the traditional hard plating in some area which causes environmental and health problems. nWC-Co powder was coated on Inconel 718 substrates by HVOF technique. The optimal coating process obtained from the best surface properties such as hardness and porosity is the process of oxygen flow rate (FR) 38 FMR, hydrogen FR 57 FMR and feed rate 35 g/min at spray distance 6 inch for both surface temperature <TEX>$25^{\circ}C\;and\;500^{\circ}C$</TEX>. In coating process a small portion of hard WC decomposes to less hard <TEX>$W_2C$</TEX>, W and C at the temperature higher than its decomposition temperature <TEX>$1,250^{\circ}C$</TEX> resulting in hardness decrease and porosity increase. Friction coefficient increases with increasing coating surface temperature from 0.55-0.64 at <TEX>$25^{\circ}C$</TEX> to 0.65-0.76 at <TEX>$500^{\circ}C$</TEX> due to the increase of adhesion between coating and counter sliding surface. Hardness of nWC-Co is higher or comparable to those of other hard coatings, such as <TEX>$Al_2O_3,\;Cr,\;Cr_2O_3$</TEX> and HVOF Tribaloy 400 (T400). This shows that nWC-Co is recommendable for durability improvement coating on machine components such as high speed spindle.

Improvement in Wear Resistance of Nano WC-Co Coatings Fabricated by Detonation Gun Spraying

Nano structured WC-Co coatings with carbide size of 100−200nm were fabricated by detonation gun spraying. The fabricated nano coatings showed improved hardness and wear resistance compared to micron WC-Co coatings. Considerable phase decomposition of WC to W2C and amorphous phase in nano coatings was detected, which is known to degrade wear resistance of coatings. In order to improve the wear resistance of the coatings by recovering of dissociated carbide phases, post heat treatment was conducted in Ar environment at temperature range of 400−900°C. Harness, fracture toughness and wear resistance of nano coatings were significantly improved by post heat treatment. The improved properties were elucidated and discussed in terms of microstructure and phase compositions.

A Feasibility Study on Advanced Methodology to Produce High-Performance Nano WC-Co Granule Feedstock for Thermal Spraying

A feasibility study was conducted, in order to fabricate WC-Co powder granule as high-performance feedstock for thermal spraying process, by spray drying weakly flocculated nano-particulate slurries under various conditions. The defects were evaluated with respect to the amount of additives to prepare spherical WC-Co feedstock powders with few morphological defects and homogeneous microstructures. The characteristics of feedstock powders heat-treated at 850~ 1200ı were analyzed using SEM, EPMA, and Hg-porosimetry. The microstructure investigation revealed that the relative portion of larger pores around 10 µm increased with increasing heattreatment temperature. On the other hand, the relative portion of smaller pores of 1 µm decreased due to the coalescence of WC particles, leading to dense feedstock powders with less defects such as hollow or craters. The hardness of thermal-sprayed coat developed with the feedstock in this study, which had exceptionally high feedstock strength, was superior to that of any other sprayed coat obtained with commercial feedstock powders.