Heli Koivuluoto

High Pressure Cold Sprayed (HPCS) and Low Pressure Cold Sprayed (LPCS) Coatings Prepared from OFHC Cu Feedstock: Overview from Powder Characteristics to Coating Properties

Cold spraying enables high quality Cu coatings to be deposited for applications where high electrical and/or thermal conductivity is needed. Fully dense Cu coatings can provide an effective corrosion barrier in specific environments. The structure of cold-sprayed Cu coatings is characterized by high deformation which imparts excellent properties. Coating properties depend on powder, the cold spray process and post treatments. First of all, powder characteristics have a strong influence on the formation of pure coatings. Secondly, cold spraying provides dense, adherent, and conductive coatings by using HPCS and LPCS. Furthermore, an addition of Al2O3 particles to the Cu powder in LPCS process significantly improves coating properties. Also, heat treatments improve electrical conductivity. This study summarizes optimal characteristics of Cu powder optimized for cold spraying, achieving high coating quality and compares properties of HPCS Cu, LPCS Cu and Cu+Al2O3 coatings prepared from the same batch of OFHC Cu powder.

High-Pressure Cold-Sprayed Ni and Ni-Cu Coatings: Improved Structures and Corrosion Properties

Cold spraying is a promising technique for the production of dense metallic coatings. In cold spraying, coating formation is through high velocity impacts of solid particles with high kinetic energy. During impact, particles deform plastically and adhere to the substrate, gradually building-up the coating. This makes it possible to form pure and dense coating structures. These impermeable coatings are advantageous in many applications such as those where corrosion protection is required. Nickel and nickel-copper alloys have good corrosion resistance and therefore, as dense coatings, have high potential for employment as corrosion barrier layers. In this study, the structural and corrosion properties of high-pressure cold-sprayed (HPCS) Ni and NiCu coatings are characterized. NiCu alloys are known to have good corrosion resistance in sulphuric and hydrochloric acids, whereas Ni is resistant to caustic soda and alkaline salt solutions. This study also shows the effect of heat treatments on coating properties. FESEM studies of cross-sectional samples reveal structural details of the HPCS coatings while corrosion properties are evaluated with polarization measurements. The corrosion behavior of both the bulk and substrate material is determined in order to assess the real corrosion protection potential of the coatings.

Influence of the Spray Gun Type on Microstructure and Properties of HVAF Sprayed Fe-Based Corrosion Resistant Coatings

The aim of this study is to evaluate the microstructural details and corrosion properties of novel Fe-based coatings prepared using two different generations of HVAF spray guns. These two generations of HVAF guns are Activated Combustion HVAF (AC-HVAF, 2nd generation) M2 gun and Supersonic Air Fuel HVAF (SAF, 3rd generation) M3 gun. Structural details were analysed using x-ray diffractometry and field-emission scanning electron microscope. Higher denseness with homogeneous microstructure was achieved for Fe-based coating deposited by the M3 process. Such coatings exhibit higher particle deformation and lower oxide content compared to coatings manufactured with M2 gun. Corrosion properties were studied by open-cell potential measurements and electrochemical impedance spectroscopy. The lower porosity and higher interlamellar cohesion of coating manufactured with M3 gun prevent the electrolyte from penetrating through the coating and arriving to the substrate, enhancing the overall corrosion resistance. This can be explained by the improved microstructures and coating performance.

Analysis of single spray beads prepared using low-pressure cold spraying

Abstract Cold spraying is a coating deposition method for the production of high-quality metallic coatings. Copper, for instance, is widely used as a coating material due to its suitability for cold spray process. Cold-sprayed copper coatings are known to have interesting technical properties such as high electrical and thermal conductivity. Low-pressure cold spraying (LPCS) is a cold spray technique of preparing relatively dense and pure copper coatings. The deposition behaviour of low-pressure cold-sprayed spherical and dendritic copper particles impacting the substrate was evaluated in this study. The effect of powder type was clearly detected in the observations of single spray beads. Higher deposition efficiency and denser coatings were prepared by using the spherical copper powder.

Wear and Corrosion Resistance of High-Velocity Oxygen-Fuel Sprayed Iron-Based Composite Coatings

Thermally sprayed iron-based coatings are being widely studied as alternative solution to conventional hardmetal (cermet) and Ni-based coatings for wear and corrosion applications in order to reduce costs, limit environmental impact and enhance the health safety. The aim of the present work is to study the cavitation erosion behaviour in distilled water and the corrosion properties in acidic solution of four high-velocity oxy-fuel (HVOF) sprayed Fe-based composite coatings. Fe-Cr-Ni-B-C powder was selected for its good sliding wear properties. In addition, a powder composition with an addition of Mo was studied in order to increase the corrosion resistance whereas additions of 20 wt. % and 40 wt. % WC-12Co as blended powder mixtures were investigated in order to increase wear resistance. Improvement of coating properties was significant with the advanced powder compositions. Dense coating structures with low porosity were detected with microstructural characterization. In addition, good cavitation wear resistance was achieved. The cavitation resistance of customized Fe-based coating with Mo addition was reported to be twice as high as that of conventional Ni-based and WC-CoCr coatings. The corrosion properties of HVOF Fe-based coatings were also evaluated by studying electrochemical behaviour in order to analyse their potential to use as corrosion barrier coatings.Copyright

Anti-icing Behavior of Thermally Sprayed Polymer Coatings

Surface engineering shows an increasing potential to provide a sustainable approach to icing problems. Currently, several passive anti-ice properties adoptable to coatings are known, but further research is required to proceed for practical applications. This is due to the fact that icing reduces safety, operational tempo, productivity and reliability of logistics, industry and infrastructure. An icing wind tunnel and a centrifugal ice adhesion test equipment can be used to evaluate and develop anti-icing and icephobic coatings for a potential use in various arctic environments, e.g., in wind power generation, oil drilling, mining and logistic industries. The present study deals with evaluation of icing properties of flame-sprayed polyethylene (PE)-based polymer coatings. In the laboratory-scale icing tests, thermally sprayed polymer coatings showed low ice adhesion compared with metals such as aluminum and stainless steel. The ice adhesion strength of the flame-sprayed PE coating was found to have approximately seven times lower ice adhesion values compared with metallic aluminum, indicating a very promising anti-icing behavior.

Achieving a slippery, liquid-infused porous surface with anti-icing properties by direct deposition of flame synthesized aerosol nanoparticles on a thermally fragile substrate

Slippery, liquid-infused porous surfaces offer a promising route for producing omniphobic and anti-icing surfaces. Typically, these surfaces are made as a coating with expensive and time consuming assembly methods or with fluorinated films and oils. We report on a route for producing liquid-infused surfaces, which utilizes a liquid precursor fed oxygen-hydrogen flame to produce titania nanoparticles deposited directly on a low-density polyethylene film. This porous nanocoating, with thickness of several hundreds of nanometers, is then filled with silicone oil. The produced surfaces are shown to exhibit excellent anti-icing properties, with an ice adhesion strength of ∼12 kPa, which is an order of magnitude improvement when compared to the plain polyethylene film. The surface was also capable of maintaining this property even after cyclic icing testing.

Characterization of High-Velocity Single Particle Impacts on Plasma-Sprayed Ceramic Coatings

High-velocity impact wear can have a significant effect on the lifetime of thermally sprayed coatings in multiple applications, e.g., in the process and paper industries. Plasma-sprayed oxide coatings, such as Cr2O3- and TiO2-based coatings, are often used in these industries in wear and corrosion applications. An experimental impact study was performed on thermally sprayed ceramic coatings using the High-Velocity Particle Impactor (HVPI) at oblique angles to investigate the damage, failure, and deformation of the coated structures. The impact site was characterized by profilometry, optical microscopy, and scanning electron microscopy (SEM). Furthermore, the connection between the microstructural details and impact behavior was studied in order to reveal the damage and failure characteristics at a more comprehensive level. Differences in the fracture behavior were found between the thermally sprayed Cr2O3 and TiO2 coatings, and a concept of critical impact energy is presented here. The superior cohesion of the TiO2 coating inhibited interlamellar cracking while the Cr2O3 coating suffered greater damage at high impact energies. The HVPI experiment has proven to be able to produce valuable information about the deformation behavior of coatings under high strain rates and could be utilized further in the development of wear-resistant coatings.

Corrosion Resistance of Cold-Sprayed Coatings

This chapter presents the corrosion properties of cold-sprayed coatings. Cold spraying has shown its potential producing corrosion-resistant coatings for several operation conditions due to the fact that dense and protective metallic and composite coatings can be manufactured by using cold spray processes, examples are presented in Fig. 13.1. This enables the use of cold-sprayed coatings as corrosion barrier coatings. In addition to corrosion resistance, other advantages of cold-sprayed coatings are dealing with high strength, electrical conductivity and minimal or compressive residual stresses as well as repairing and additive manufacturing (Papyrin et al., Cold spray technology, 1st edn. Elsevier, printed in the Netherlands, 328 p, 2007; Champagne and Helfritch, Int Mater Rev 61(7):437–455, 2016; Champagne, The cold spray materials deposition process: fundamentals and applications. Woodhead Publishing Ltd., Cambridge, 362 p, 2007; Koivuluoto, Microstructural characteristics and corrosion properties of cold-sprayed coatings, Doctoral Thesis, Tampere University of Technology, Tampereen Yliopistopaino Oy, Tampere, 153, 2010; Villafuerte, Modern cold spray – materials, process, and applications. Springer International Publishing, p 429, 2015). Corrosion properties and behavior of the cold-sprayed coatings are increasingly reported in the literature during last years. Recently, review papers concerning corrosion properties of cold-sprayed coatings have been published by Bala et al. (Surf Eng 30(6):414–421, 2014), Koivuluoto and Vuoristo, (Surf Eng 30:404–414, 2014) and Hassani-Gangaraj et al. (Surf Eng 31(11):803–815, 2015). Furthermore, cold spraying can be used for corrosion protection as well as repairing of corrosion defects (Vardelle et al., J Therm Spray Technol 25(8):1376–1440, 2016).

Novel Online Diagnostic Analysis for In-Flight Particle Properties in Cold Spraying

In cold spraying, powder particles are accelerated by preheated supersonic gas stream to high velocities and sprayed on a substrate. The particle velocities depend on the equipment design and process parameters, e.g., on the type of the process gas and its pressure and temperature. These, in turn, affect the coating structure and the properties. The particle velocities in cold spraying are high, and the particle temperatures are low, which can, therefore, be a challenge for the diagnostic methods. A novel optical online diagnostic system, HiWatch HR, will open new possibilities for measuring particle in-flight properties in cold spray processes. The system employs an imaging measurement technique called S-PTV (sizing-particle tracking velocimetry), first introduced in this research. This technique enables an accurate particle size measurement also for small diameter particles with a large powder volume. The aim of this study was to evaluate the velocities of metallic particles sprayed with HPCS and LPCS systems and with varying process parameters. The measured in-flight particle properties were further linked to the resulting coating properties. Furthermore, the camera was able to provide information about variations during the spraying, e.g., fluctuating powder feeding, which is important from the process control and quality control point of view.