Highly resistant icephobic coatings on aluminum alloys

Aluminum alloys are widely used for outdoor structures such as ground wires and phase conductors of overhead power lines, as well as aircrafts wings and fuselage. To protect these surfaces against excessive ice accumulation, icephobic coatings must be highly reliable and durable. New coatings with icephobic characteristics and excellent mechanical properties have been developed. The method consisted in depositing an extremely adherent poly(tetrafluoroethylene) (PTFE) coating on an Al₂O₃ underlayer produced by anodizing in a phosphoric acid electrolyte followed by an oxide etching step to enhance surface roughness. PTFE impregnation was carried out at low temperature (320 °C) and coating adhesion was assessed using tape and bend tests. Some of these coatings showed superhydrophobic properties; ice adhesion was around four times lower than bare aluminum. As well, they remained effective after ten ice-shedding events using an aggressive centrifugal technique. Moreover, no sign of PTFE degradation after 14 ice removals was noted and the coatings remained extremely adherent and very hydrophobic. This technique therefore shows very good potential and could be applied to new high-voltage overhead aluminum cables as protection against excessive ice or snow accumulation.     

Preparation and characterization of silica/fluorinated acrylate copolymers hybrid films and the investigation of their icephobicity

Inexpensive hydrophobic and icephobic coatings and films were obtained by a simple method. These coatings were prepared by mixing silica sol and fluorinated acrylate copolymers. There was a phase separation process in the film-forming which can provide the excellent performance. Small amount (about 2 wt.%) of fluorinated (methyl) acrylate was used in all of these coatings. The coatings were eco-friendly by using ethanol as the solvent system. Scanning electron microscopy, atomic force microscope, energy dispersive X-ray fluorescence spectrometer, water contact angle, thermal gravimetric analysis and tests of adhesion and hardness had been performed to characterize the morphological feature, chemical composition, hydrophobicity and icephobicity of the surface, thermal stability and mechanical properties of the coatings. The results showed that the films had good hydrophobicity, high thermal stability and excellent mechanical properties of adhesion strength and pencil hardness. Furthermore, by testing their properties of delaying water droplet from icing, it was found that ice formation was delayed for 90 min compared with the glass surface at − 5.6 °C. The hybrid coatings may be suitable for large-scale and practical application owing to its flexibility and simplicity.     

Facile Fabrication of Robust Ice‐Phobic Polyurethane Sponges

Bio‐inspired superhydrophobicity is a promising anti‐icing (or deicing) strategy, but a superhydrophobic surface may lose its anti‐icing capability once the deposited water freezes. Herein, it is shown that ice can be readily and repeatedly removed from the surface of superhydrophobic polyurethane sponges via a simple mechanical squeezing process. The sponges are fabricated through a mussel‐inspired process and subsequent deposition of Ag nanoparticles. The resulting sponges are able to shed off the ice layers formed on their surfaces up to 90 times, exhibiting robust icephobic properties among the reported superhydrophobic surfaces. The mechanism for the excellent icephobicity is investigated by a highly sensitive microelectrobalance and a fluorescent labeling method. It is revealed that the icephobicity is attributed to low ice adhesion of the superhydrophobic sponges, as well as mechanical durability of their surface textures. The present findings provide a facile strategy to fabricate robust icephobic surfaces for various technological applications.     

Real contact temperatures as the criteria for the reactivity of diamond-like-carbon coatings with oil additives

The operating conditions under which chemical reactions between diamond-like-carbon (DLC) coatings and oil additives occur and the main driving forces, i.e., the “activation criteria” for these chemical reactions, have not yet been defined. In order to clarify the difference between the “test” temperature and “real” contact temperature, and to determine the effect of the real contact temperature for these reactions, we have calculated the contact temperatures using two well-known models and compare them with results of tribological experiments and some state-of-the-art analyses of worn surfaces. The results suggest that the actual surface temperatures are 100−130 °C higher than the test temperatures. A contact temperature of about 250−260 °C was found to be the required key activation criterion for chemical reactions between the dialkyl dithiophosphate extreme-pressure (EP) additive and the DLC coating. Gradual formation of a tribochemical protective film from phosphates and organic sulphur/sulphates suggests a lower chemical reactivity and slower formation of the “optimal” tribochemical protective layer on DLC coatings than on steels. No tribological effect of anti-wear (AW) or EP additives could be found on the DLC coatings when the surface temperatures were below 120−140 °C. The temperature-induced graphitisation of the DLC that occurred in the contacts with the base oils, however, require about 250 °C of contact temperature. Lower surface temperatures or the use of additives suppressed this graphitisation.     

On ice-releasing properties of rough hydrophobic coatings

In this work, ice repellency of rough hydrophobic coatings based on different materials and with different surface topographies is evaluated. The coatings were prepared either from a fluoropolymer incorporated with nanoparticles or by etching aluminum alloy substrate followed by further hydrophobization of the rough surface via an organosilane monolayer adsorbed from solution. This allowed comparing the ice-releasing performance of rough surfaces with high water contact angles (∼ 150-153°) and different dynamic hydrophobicities and mechanical properties. Artificially created glaze ice, similar to naturally occurring glaze, was accreted on the surfaces by spraying supercooled water microdroplets in a wind tunnel at subzero temperature. The ice adhesion strength was evaluated by spinning the samples in a centrifuge at constantly increasing speeds until ice detachment occurred. The results showed that, after several icing-deicing cycles, the more robust surfaces prepared by etching the aluminum substrate maintained their ice-releasing properties better, compared to their counterparts based on nanoparticle-incorporated fluoropolymer. The effect of the dynamic hydrophobicity of the coatings was also examined, clearly demonstrating that the surface with low dynamic hydrophobicity is not ice-repellent, although it demonstrates large values of water contact angle.     

An experimental study on the tensile properties of atmospheric ice

The intrinsic problem of formation and accumulation of atmospheric ice on structures, such as power electric transmission lines and communication equipment, has in recent years resurrected much interest. However, the mechanical properties of the accreted atmospheric ice are not abundantly recognized and, therefore, analytical modeling of circumstantial material is not conceivable. For this purpose, an experimental investigation into the mechanical behavior of atmospheric ice in uniaxial tension has been conducted using conditions generally favorable to brittle fracture and microcracking. The atmospheric ice is grown from supercooled water droplets impinging on an aluminum cylinder rotating at 1 rpm in the test section of the closed-loop wind tunnel. Ice tensile strength was measured as a function of test temperature varying from − 5 to − 15 °C, wind speed during accumulation varying from 10 to 20 m/s, and strain rate ranging from 2.22 × 10^− 6 to 1.67 × 10^− 3 s^− 1. The details of specimen preparation, testing procedure and strength test results are discussed. The fracture mechanism for atmospheric ice is also discussed, and the test results are compared with data reported by previous investigators. A mathematical model was developed using Minitab-15 software to predict the effect of ice accumulation conditions on the tensile strength. Detailed analysis indicates that the interaction coefficients of these variables do not appear to contribute significantly to the tensile strength of atmospheric ice.     

Filament formation during elevated temperature deformation of high purity ultrafine-grained aluminum

Ultrafine-grained aluminum materials were processed by hot isostatic pressing of aluminum nanopowders (99.7 wt.% purity). Quasi-static compression tests were carried out at a strain rate of 2 × 10^− 4 s^− 1 at 200 °C. Scanning electron microscopy investigations of fracture surfaces or cavities that were formed during straining reveal the presence of filaments. The number and dimensions of the filaments depend on the shielding effect of the native amorphous alumina film that forms on the surface of the nanoparticles in the starting powder. After crystallization of the amorphous, extensive filament formation is observed.     

Initiated chemical vapor deposition of pH responsive poly(2-diisopropylamino)ethyl methacrylate thin films

Poly(2-(diisopropylamino)ethyl methacrylate) (PDPAEMA) thin films were deposited on low temperature substrates by initiated chemical vapor deposition (iCVD) method using tertbutyl peroxide as an initiator. Very high deposition rates up to 38 nm/min were observed at low filament temperatures due to the use of the initiator. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy show the formation of PDPAEMA films with high retention of tertiary amine functionality which is responsible for pH induced changes in the wetting behavior of the surfaces. As-deposited PDPAEMA thin films on flat Si surface showed a reversible switching of water contact angle values between 87° and 28°; after successive treatments of high and low pH water solutions, respectively. Conformal and non-damaging nature of iCVD allowed to functionalize fragile and rough electrospun poly(methyl methacrylate) fiber mat surfaces by PDPAEMA, which creates a surface with a switching behavior between superhydrophobic and approaching superhydrophilic with contact angle values of 155 ± 3°and 22 ± 5°, respectively.     

Superhydrophobicity vs. Ice Adhesion: The Quandary of Robust Icephobic Surface Design

The mesoscale, multitier texture of the lotus leaf has served as an inspiration to fabricate surface designs with controllable superhydrophobic properties, targeting a broad range of applications. The choice of material for such designs is directly related to surface performance, in particular under adverse and realistic conditions. Due to its importance in many applications, here aluminium is employed as a material platform and identify key porous hierarchical textures, yielding extraordinary impalement‐resistant behavior: Droplet repellency is demonstrated consistently for water impact velocities up to 12 m s⁻¹ (extreme Weber number, We ≈ 3500). Despite impressive superhydrophobic behavior, if ice forms on such surfaces, ice adhesion is markedly stronger than on less hydrophobic alumina nanotube array structures. In a departure from the findings of the well‐accepted shear stress‐based ice adhesion criterion, a deviation between decreasing ice adhesion strength and increasing hydrophobicity is observed. This is explained with ice adhesion mechanism, depending strongly on the applied stress field orientation and the respective effective ice–substrate contact area. Our results indicate that ice adhesion criteria for the performance of icephobic surfaces should account for the simultaneous presence of shear and tensile stresses, instead of shear stresses alone.     

Electrochemical deposition and superhydrophobic behavior of ZnO nanorod arrays

Vertically aligned ZnO nanorod arrays with different heights are grown on the ZnO seeded indium tin oxide substrate by cathodic electrochemical deposition from zinc nitrate at two temperatures of 60 °C and 80 °C. As-grown ZnO nanorods exhibit wurzite crystal structure and their heights can be well controlled by different deposition times. The fluorination coating tends to induce a superhydrophobicity of ZnO nanorods, i.e., the maximal value of contact angle: 166.9°. The super water repellency can be attributed to the fact that an air layer is confined in the nanorod arrays, and thus leads to water droplets sitting on the ZnO surfaces, referring as Cassie state. Interestingly, their water contact angles are found to vary with the heights of ZnO nanorods, ranged from 99.8 to 746 nm. The superhydrophobicity of ZnO surfaces can be well predicted by a proposed model that is capable of determining the wetted fraction of ZnO pillars. This satisfactory result would shed one light on how the variation of rod height would induce the superhydrophobic behavior of ZnO nanorod arrays.