Effect of spraying parameters on the microstructural and corrosion properties of HVAF-sprayed Fe-Cr-Ni-B-C coatings

Thermally sprayed Fe-based coatings have been extensively studied as future solution in order to replace more expensive, harmful and environmentally dangerous Ni- and WC-based coatings for several industrial applications where high corrosion and wear resistance are required. The aim of the present study is to investigate the effect of spraying parameters on the microstructure and the corrosion resistance of Fe-based coatings manufactured with the High Velocity Air Fuel (HVAF) thermal spray process. Six sets of thermal spraying parameters have been chosen and their effect on the overall quality of coatings was investigated. All HVAF coatings showed comparably dense microstructure with near-zero oxidation, proving the high quality of the deposition process. However, higher anti-corrosion and mechanical properties were achieved by increasing the spraying air pressure and decreasing the particle feeding rate without altering the thickness and the overall deposition rate. Powder feeding rate was reported to have a remarkable effect on microstructure and corrosion properties. Coatings with beneficial compressive residual stresses were successfully obtained by increasing air pressure during spraying which resulted in improved microstructural and corrosion properties

Durable Icephobic Slippery Liquid-Infused Porous Surfaces (SLIPS) Using Flame- and Cold-Spraying

Icing and ice accretion cause severe problems in different industrial sectors, e.g., in aircrafts, aviation traffic, ships, solar panels, and wind turbines. This can lead to enormous economic losses and serious safety issues. Surface engineering can tackle these problems by designing surface structures to work as icephobic coatings and, this way, act as passive anti-icing solutions. In this research, slippery liquid-infused porous structures were fabricated using flame- and cold-spraying to produce polymer (LDPE and PEEK) coatings, and impregnated with a silicone lubricant. Microstructural details, surface properties, wetting behavior, and cyclic icing–deicing behavior were evaluated via ice adhesion measurements, which show the potential performance of SLIPS designs. All these SLIPS showed low or medium-low ice adhesion after the first icing-deicing cycle and the best candidate showed stable performance even after several icing-deicing cycles.publishedVersionPeer reviewe

Cold-Sprayed Al6061 coatings. Online spray monitoring and influence of process parameters on coating properties

Process optimization and quality control are important issues in cold spraying and coating development. Because the cold spray processing is based on high kinetic energy by high particle velocities, online spray monitoring of particle inflight properties can be used as an assisting process tool. Particle velocities, their positions in the spray jet, and particle size measurements give valuable information about spraying conditions. This, in turn, improves reproducibility and reliability of coating production. This study focuses on cold spraying of Al6061 material and the connections between particle inflight properties and coating characteristics such as structures and mechanical properties. Furthermore, novel 2D velocity scan maps done with theHWCS2 online spray monitoring system are presented as an advantageous powder and spray condition controlling tool. Cold spray processing conditions were similar using different process parameters, confirmed with the online spray monitoring prior to coating production. Higher particle velocities led to higher particle deformation and thus, higher coating quality, denser structures, and improved adhesions. Also, deposition efficiency increased significantly by using higher particle velocities

Microstructure-based thermo-mechanical modelling of thermal spray coatings

This paper demonstrates how microstructure-based finite element (FE) modelling can be used to interpret and predict the thermo-mechanical behaviour of thermal spray coatings. Validation is obtained by comparison to experimental and/or literature data.Finite element meshes are therefore constructed on SEM micrographs of high velocity oxygen-fuel (HVOF)-sprayed hardmetals (WC-CoCr, WC-FeCrAl) and plasma-sprayed Cr2O3, employed as case studies. Uniaxial tensile tests simulated on high-magnification micrographs return micro-scale elastic modulus values in good agreement with depth-sensing Berkovich micro-indentation measurements. At the macro-scale, simulated and experimental three-point bending tests are also in good agreement, capturing the typical size-dependency of the mechanical properties of these materials. The models also predict the progressive stiffening of porous plasma-sprayed Cr2O3 due to crack closure under compressive loading, in agreement with literature reports.Refined models of hardmetal coatings, accounting for plastic behaviours and failure stresses, predict crack initiation locations as observed by indentation tests, highlighting the relevance of stress concentrations around microstructural defects (e.g. oxide inclusions).Sliding contact simulations between a hardmetal surface and a small spherical asperity reproduce the fundamental processes in tribological pairings. The experimentally observed "wavy" morphologies of actual wear surfaces are therefore explained by a mechanism of micro-scale plastic flow and matrix extrusion

Durability of Lubricated Icephobic Coatings under Various Environmental Stresses

Icephobic coatings interest various industries facing icing problems. However, their durability represents a current limitation in real applications. Therefore, understanding the degradation of coatings under various environmental stresses is necessary for further coating development. Here, lubricated icephobic coatings were fabricated using a flame spray method with hybrid feedstock injection. Low-density polyethylene represented the main coating component. Two additives, namely fully hydrogenated cottonseed oil and paraffinic wax, were added to the coating structure to enhance coating icephobicity. Coating properties were characterised, including topography, surface roughness, thermal properties, wettability, and icephobicity. Moreover, their performance was investigated under various environmental stresses, such as repeated icing/deicing cycles, immersion in corrosive media, and exposure to ultraviolet (UV) irradiation. According to the results, all coatings exhibited medium-low ice adhesion, with slightly more stable icephobic behaviour for cottonseed oil-based coatings over the icing/deicing cycles. Surface roughness slightly increased, and wetting performances decreased after the cyclic tests, but chemical changes were not revealed. Moreover, coatings demonstrated good chemical resistance in selected corrosive media, with better performance for paraffin-based coatings. However, a slight decrease in hydrophobicity was detected due to surface structural changes. Finally, paraffin-based coatings showed better resistance under UV irradiation based on carbonyl index and colour change measurements.publishedVersionPeer reviewe

Slurry and dry particle erosion wear properties of WC-10Co4Cr and Cr3C2-25NiCr hardmetal coatings deposited by HVOF and HVAF spray processes

Thermally sprayed hardmetal coatings were produced to provide improved erosion wear compared to conventional cast GX4CrNi13-4 martensitic steel (CA6NM) used in hydro turbine components. Sprayed coatings and reference materials were tested with high-speed slurry pot tester using either fine or coarse quartz as the erosive media. Additional erosion tests were carried out with centrifugal dry erosion tester. Tungsten carbide based coatings provided the highest wear resistance due to the high hardness and even distribution of the fine carbide particles. The cast 13-4 steel samples experienced up to 180 times higher wear rates in fine quartz slurry and up to 36 times higher wear rates in coarse slurry compared to the sprayed coatings

Effects of multiple wetting incidents, shear and sliding friction on lubricant stability in SLIPS

Surface icing almost invariably derives from the precursory step of liquid water encountering the surface. Thus, slippery liquid infused porous surfaces, SLIPS, must possess steady wetting durability, and lubricant stability to function as a reliable hydro−/icephobic surface design especially in outdoor applications. Additionally, they should maintain their phobic performance under shear forces, and possess low sliding friction to act as a slippery, multirepellent surfaces. These characteristics are needed in variable applications ranging from moving and rotating blades to steady surfaces, operating in altering climate conditions. More profound durability testing is needed to examine the loss of surface functionality when the lubricant is depleted from the structure via various routes. In addition, the durability tests should be designed to serve the application-related purposes and thus, to reveal performance differences between slippery surfaces for further analysis and targeted end-use development. Here, we tested the wetting durability and stability of SLIPS with multicycle Wilhelmy plate by dipping the surfaces multiple times in water bath. Additionally, we examined the effects of centrifugal and friction-based shear stress to investigate the lubricant depletion from the structure. Tests that measure the durability and the stability of SLIPS designs are in great need in further developing functional slippery surfaces for real outdoor application coatings which encounter environmental stresses, e.g., wetting and icing. Acknowledging the material differences under specific stresses will guide designing the slippery surfaces towards more specific and functionable end-use applications.publishedVersionPeer reviewe

Wetting Behavior and Functionality Restoration of Cold-Sprayed Aluminum-Quasicrystalline Composite Coatings

Coating design is an efficient strategy to engineer wettability of surfaces and adjustment of the functionality to the necessities in industrial sectors. The current study reveals the feasibility of functional aluminum/quasicrystalline (Al-QC) composite coatings fabrication by cold spray technology. A commercially available Al-based quasicrystalline powder (Al-Cr-Fe-Cu) was combined with aluminum alloy (Al6061) feedstock materials to make coatings with various compositions. A set of cold spray process parameters was employed to deposit composite coatings with different QC-Al ratios and Al6061 coatings as counterparts. The wettability of the coating surfaces was measured by static water droplet contact angles using a droplet shape analyzer and investigation of the dynamic of water droplet impact by high-speed imaging. Through microstructural studies, the Al-QC composites revealed dense structure, well-integrated and adherent deposits, providing structural reliability and enhanced hydrophobic behavior. In the last step of this work, composite coatings were deposited over eroded cold-sprayed Al6061 and a selected composite to demonstrate the feasibility of repairing the damaged part and function restoring. The results and approach used in this work provide understanding of cold-sprayed Al-QC composite coatings manufacturing and their wetting behavior state for cross-field applications.publishedVersionPeer reviewe