Ionic Liquids as Additives in Water-Based Lubricants: From Surface Adsorption to Tribofilm Formation

Tribology Letters - Thickener plays a significant role on the performance of grease, including rheological property, chemical and thermal stability, colloidal and mechanical stability, and so...     

Cr3C2-NiCr and WC-Ni thermal spray coatings as alternatives to hard chromium for erosion-corrosion resistance

Cr₃C₂-NiCr and WC-Ni coatings are widely used for wear applications at high and room temperature, respectively. Due to the high corrosion resistance of NiCr binder, Cr₃C₂-NiCr coatings are also used in corrosive environments. The application of WC-Ni coatings in corrosive media is not recommended due to the poor corrosion resistance of the (pure Ni) metallic matrix. It is well known that the addition of Cr to the metallic binder improves the corrosion properties. Erosion-corrosion performance of thermal spray coatings is widely influenced by ceramic phase composition, the size of ceramic particles and also the composition of the metallic binder. In the present work, two types of HVOF thermal spray coatings (Cr₃C₂-NiCr and WC-Ni) obtained with different spray conditions were studied and compared with conventional micro-cracked hard chromium coatings. Both as-sprayed and polished samples were tested under two erosion-corrosion conditions with different erosivity. Tungsten carbide coatings showed better performance under the most erosive condition, while chromium carbide coatings were superior under less erosive conditions. Some of the tungsten carbide coatings and hard chromium showed similar erosion-corrosion behaviour under more and less erosive conditions. The erosion-corrosion and electrochemical results showed that surface polishing improved the erosion-corrosion properties of the thermally sprayed coatings. The corrosion behaviour of the different coatings has been compared using Electrochemical Impedance Spectroscopy (EIS) and polarization curves. Total material loss due to erosion-corrosion was determined by weight loss measurements. An estimation of the corrosion contribution to the total weight loss was also given.     

Influence of HVOF parameters on the corrosion and wear resistance of WC-Co coatings sprayed on AA7050 T7

Thermal spray WC-based coatings are widely used in the aircraft industry mainly for their resistance to wear, reworking and rebuilding operations and repair of worn components on landing gear, hydraulic cylinders, actuators, propeller hub assemblies, gas turbine engines, and so on. The aircraft industry is also trying to use thermal spray technology to replace electroplating coatings such as hard chromium. In the present work, WC-Co coatings were built up on an AA 7050 aluminum alloy using high velocity oxygen fuel (HVOF) technology and a liquid nitrogen cooling prototype system. The influence of the spray parameters (standard conditions, W19S, increasing the oxygen flux, W19H, and also increasing the carrier gas flux, W19F) on corrosion, friction, and abrasive wear resistance were also studied. The coatings were characterized using optical (OM) and scanning electron (SEM) microscopy, and X-ray diffraction (XRD). The friction and abrasive wear resistance of the coatings were studied using Rubber Wheel and Ball on Disk tests. The electrochemical studies were conducted using open-circuit potential (E_OC) measurements and electrochemical impedance spectroscopy (EIS). Differences among coated samples were mainly related to the variation of the thermal spray parameters used during the spray process. No significant differences were observed in the wear resistance for the coatings studied, and all of them showed a wear rate around 10 times lower than that of the aluminum alloy. The results of mass loss and wear rate were interpreted considering different mechanisms. Comparing the different spray parameters, the oxygen flux (higher flame temperature) produced the sample which showed the highest corrosion resistance in aerated and unstirred 3.5% NaCl solution. Aluminum ions were detected on the surface almost immediately after the immersion of samples W19S and W19F in chloride solution, showing that the electrolyte reached the substrate and galvanic corrosion probably occurred. For sample W19H, aluminum ions were not detected even after 120 min of immersion in NaCl solution.     

Corrosion and Wear Studies of Cr3C2NiCr-HVOF Coatings Sprayed on AA7050 T7 Under Cooling

In this work, cermet coatings were prepared by high-velocity oxygen-fuel (HVOF) technique using a Diamalloy 3007 powder. The influence of the spray parameters on corrosion, friction, and abrasive wear resistance was studied. The samples were obtained using the standard conditions (253 L/min of oxygen and 375 L/min of compressed air), higher oxygen flux (341 L/min), and higher carrier gas flux (500 L/min). The coatings were characterized using scanning electron microscopy (SEM), and x-ray diffraction (XRD). X-ray diffraction and SEM studies showed well-bounded coating/substrate interface, pores, metallic matrix, chromium oxides, carbides, and carbides dissolution into the matrix. In comparison with the standard condition, the sample prepared using higher oxygen flux showed the highest carbide dissolution because of the high temperature achieved in the spray process. When the carrier gas flux was increased, the sample showed denser structure because of the higher particle velocity. The friction and abrasive wear resistance of the coatings were studied using rubber wheel and ball-on-disk tests. All samples showed similar sliding and abrasive behavior, and all of them showed better performance than the aluminum alloy. The electrochemical behavior was evaluated using open-circuit potential (E _OC) measurements, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization curves. The coating prepared with higher carrier gas flux showed the highest corrosion resistance in 3.5% NaCl solution and probably no pitting attack to the substrate occurred even after around 26 h of test. Tests performed for longer immersion times showed that the total impedance values significantly decreased (6 and 4 times) for samples sprayed using standard and higher oxygen flux, and no great change for sample sprayed using higher carrier gas flux was observed. The last sample presents a corrosion resistance around 200 times higher than the others.     

Comparative study of Cr3C2-NiCr coatings obtained by HVOF and hard chromium coatings

In the present work the corrosion resistance of micro-cracked hard chromium and Cr₃C₂-NiCr (HVOF) coatings applied on a steel substrate have been compared using open-circuit potential (E_OC) measurements, electrochemical impedance spectroscopy (EIS) and polarization curves. The coatings surfaces and cross-section were characterized before and after corrosion tests using optical microscopy (OM) and scanning electron microscopy (SEM). After 18 h of immersion, the open-circuit potential values were around −0.50 and −0.25 V/(Ag∣AgCl∣KCl_sat) for hard chromium and Cr₃C₂-NiCr, respectively. The surface analysis done after 12 h of immersion showed iron on the hard chromium surface inside/near surface cracks, while iron was not detected on the Cr₃C₂-NiCr surface even after 18 h. For longer immersion time hard chromium was more degraded than thermal sprayed coating. For hard chromium coating a total resistance values between 50 and 80 kΩ cm² were measured and two well-defined time constants were observed, without significant change with the immersion time. For Cr₃C₂-NiCr coating the total impedance diminished from around 750 to 25 kΩ cm² as the immersion time increased from 17 up to 132 h and two overlapped time constants were also observed. Polarization curves recorded after 18 h of immersion showed a lower current and higher corrosion potential for Cr₃C₂-NiCr coating than other samples studied.     

Tribocorrosion of Pulsed Plasma-Nitrided CoCrMo Implant Alloy

In the present study, a forged CoCrMo (ISO 5832-12) has been subjected to pulsed plasma treatment in a N₂/H₂ atmosphere at low temperatures (below 500 °C). This treatment resulted in the formation of a layer composed by dispersed chromium nitride particles in a N-enriched metal matrix. The materials were tested for corrosion and tribocorrosion performance in 0.9 wt% NaCl at room temperature under controlled electrochemical conditions. After the treatment, the alloy loses its passive nature. The electrode potential was found to critically affect the corrosion and the tribocorrosion rates. In the nitrided alloy, a significant increase of corrosion rate was found at high potentials, while tribocorrosion was determined mainly by mechanical wear and not affected by potential. On the other hand, the untreated CoCrMo alloy exhibited stable corrosion over a wide range of potentials. Its tribocorrosion rate was similar to the nitrided alloy samples at low potentials, but it increased dramatically at high potentials where passivity triggered severe wear-accelerated corrosion and promoted mechanical wear. The present study shows that the electrochemical conditions determine material deterioration and should therefore be considered when selecting materials for tribocorrosion applications such as biomedical implants.     

Effect of Contamination on the Friction and Wear of Carboxylic Acids in Aqueous Lubricants

The friction and wear mechanisms of water-based fluids consisting of various types of carboxylic acids have been systematically investigated using a ball-on-disc tribometer on a super duplex stainless steel under boundary lubricating conditions. Under water-based lubrication, a decrease in friction was found for each of the tested carboxylic acids with a hydrocarbon chain longer than 12 carbon atoms. However, wear reduction was only seen for the carboxylic acids with a hydrocarbon chain longer than 16 carbon atoms or with shorter chain carboxylic acids in concentrations above a critical value. In addition, it was found that when the lubricant is contaminated with bivalent metal ions like Ca²⁺ or Mg²⁺, the frictional performance worsened; however, wear improved. Increasing the concentration of bivalent alkaline metal ions reduced the adsorption ability of carboxylic acids to the steel surface leading to higher friction and the presence of Na⁺ ions facilitated the wear improvement. This is explained by the formation of polyelectrolyte complexes between the metal ions dissolved in the lubricant and the carboxylic acid molecules. To compensate for the detrimental effects in friction, a chelating strategy has been implemented and explained in detail.     

Tribocorrosion behaviour of overlay welded Ni-Cr 625 alloy in sulphuric and nitric acids: Electrochemical and chemical effects

The tribocorrosion behaviour of overlay welded Ni-Cr 625 alloy sliding against alumina in 0.5 M H₂SO₄ and 0.5 M HNO₃ solutions under potentiostatic conditions was investigated. The passivation of the samples in the two electrolytes at different potentials (−1.5, −0.3 and +0.3 V_MSE) was studied by means of potentiodynamic polarization curves and XPS. The tribocorrosion behaviour of the alloy in sulphuric acid was similar to the behaviour of stainless steels found in other studies, with much lower wear at cathodic potential compared to passive conditions. However, the behaviour in nitric acid was highly influenced by the reduction of nitric acid at the electrode-electrolyte interface. This reaction leads to the oxidation of the alloy even at cathodic potential. This feature influenced the tribocorrosion properties of the alloy in nitric acid, the wear rate at cathodic potential being in the same range as at passive potentials.     

Liquid-Solid Self-Lubricated Coatings

Self-lubricated coatings have been a major topic of interest in thermal spray in the last decades. Self-lubricated coatings obtained by thermal spray are exclusively based on solid lubricants (PTFE, h-BN, graphite, MoS₂, etc.) embedded in the matrix. Production of thermal spray coatings containing liquid lubricants has not yet been achieved because of the complexity of keeping a liquid in a solid matrix during the spraying process. In the present article, the first liquid-solid self-lubricating thermal spray coatings are presented. The coatings are produced by inserting lubricant-filled capsules inside a polymeric matrix. The goal of the coating is to release lubricant to the system when needed. The first produced coatings consisted solely of capsules for confirming the feasibility of the process. For obtaining such a coating, the liquid-filled capsules were injected in the thermal spray flame without any other feedstock material. Once the concept and the idea were proven, a polymer was co-sprayed together with the capsules to obtain a coating containing the lubricant-filled capsules distributed in the solid polymeric matrix. The coatings and the self-lubricated properties have been investigated by means of optical microscopy, Scanning Electron Microscopy, and tribological tests.     

Thermally Sprayed SiC Coatings for Offshore Wind Turbine Bearing Applications

Tribological tests were conducted on thermally sprayed silicon carbide (SiC) coatings to investigate its potential on reducing wear in offshore wind turbine bearings. The tests were carried out under dry conditions, 3.5 wt.% NaCl solution, and polyalfaolefin (PAO)-lubricated conditions. In order to obtain good quality SiC coatings, it is compulsory to modify the feedstock to limit SiC decomposition during atmospheric spraying process. The SiC feedstock used in this research has been modified with yttrium aluminum garnet (Y₃Al₅O₁₂) oxide additives that originated from its metal salt precursors. High-frequency pulse detonation (HFPD) technique has been utilized to produce coatings of around 100 μm in thickness. The sliding tests have recorded the lowest coefficient of friction (COF) of 0.15 in PAO condition and the highest COF of 0.50 in dry sliding. The wear tracks morphology show that during dry sliding test, the coatings experience abrasive wear accompanied by tribo-oxidation reaction that initiates crack formation along the splat boundaries. On the other two sliding test conditions (NaCl and PAO), polishing of asperities and some grain plowing from the splats were observed in the wear tracks. Tribochemical wear was found to be the main mechanism producing smooth surfaces. Nevertheless, in all cases, the wear losses were negligible.