Tungsten carbide -based hard metal coatings are extensively used in demanding industrial applications like for wear protection purposes. Continuously increasing demands set new limits and need for materials with enhanced features. One solution is to improve hard metal properties by nanostructures. Presented study is part of a research where novel and safe route to manufacture nanostructural WC-Co powders starting from water soluble raw materials was developed. In this study powders’ workability in thermal sprayings is studied. WC-12Co powder was manufactured using water soluble raw materials: ammonium metatungstate as a tungsten source, glycine as a carbon source and cobalt acetate as a cobalt source. The powder was manufactured via optimized spray drying and heat treatment method producing a correct phase structure and chemical composition. Experimental powder was sprayed by HVAF-spraying to study its workability and functionality. Morphology, microstructure and properties were analyzed from the experimental nanostructural powder and the HVAF-coatings.

Superior wear performance combined with excellent friction properties against metals makes chromium oxide (Cr 2 O 3 ) an interesting coating material for many industrial applications. However, Cr 2 O 3 is a challenging material for HVOF spraying due to its high melting temperature. Fracture toughness and lamella cohesion of a coating is limited and may be improved by using ceramic-ceramic –nanocomposite powders, which forms phases with improved properties. In this study Cr 2 O 3 -TiO 2 systems were selected aiming to improve the toughness and lamella cohesion of coating without reducing the excellent wear properties.

Residual stresses in plasma-sprayed Al 2 O 3 and Cr 2 O 3 coatings, deposited using commercial powders, and in HVOF-sprayed ceramic coatings, deposited using conventional Al 2 O 3 and Cr 2 O 3 feedstock and nanostructured- Al 2 O 3 feedstock, were studied by combining X-ray diffraction, substrate chemical removal technique and analytical modelling. The in-situ curvature technique was also employed for HVOF-sprayed Al 2 O 3 coatings, for further verification. Both HVOF-sprayed Al 2 O 3 -based coatings display similar, tensile residual stresses (≈120 MPa) near the top surface and possess moderate through-thickness stress gradients (≈10 - 20 MPa). Plasma-sprayed Al 2 O 3 possesses a smaller through-thickness stress gradient and a larger near-surface stress (≈220 MPa): this latter result seems to be due to higher quenching stresses in APS Al 2 O 3 , as determined by analytical computation The analytical model is validated by its fairly good agreement to the experimental results obtained both by substrate chemical removal and by in-situ curvature. Cr 2 O 3 -based coatings possess a lower near-surface residual stress (≈20 MPa); the HVOF one also exhibits a very large stress gradient (≈80 MPa). Machining and sliding processes (like polishing and dry sliding tribological testing) change their surface residual stresses to compressive ones.

The potential of the high velocity oxy-fuel (HVOF) thermal spray process to produce coatings with reduced porosity is well known. The ability to produce high density ceramic coatings offers potential in high performance applications in the fields of wear, corrosion resistance and dielectric coatings. It has been, however, demonstrated that benefits from HVOF ceramic coatings can be obtained only if particles are melted enough and good lamella adhesion is produced. Therefore, due to the operational limits of the HVOF process, the process-structure- relationship must be well optimized. One strategy to improve melting of ceramic particles in the relatively low flame temperatures of the HVOF process is to modify particle crystal structure and composition. In this paper, the effects of the powder structure and the composition on coating microstructure and deposition efficiency of the HVOF spray process are studied. The effect of fuel gas, hydrogen vs. propane, was also demonstrated. The studied materials were agglomerated alumina- and titania-based pure and composite powders. Coating properties such as microstructure, hardness, and abrasive wear resistance, were compared to the coating manufactured by using conventional fused and crushed powders.

Plasma sprayed oxides are effective coatings against wear and corrosion. Low particle velocity in the plasma jet causes a limited interlamellar cohesion. HVOF-sprayed ceramic coatings emerged as an improved alternative. In this paper, microstructural characteristics and tribological performances of HVOF sprayed Al 2 O 3 , nanostructured Al 2 O 3 and Cr 2 O 3 coatings are compared to reference plasma-sprayed Al 2 O 3 and Cr 2 O 3 . The microstructure is analysed by SEM, EDS and XRD. Hardness and fracture toughness are investigated by instrumented indentation and elastic modulus by 3-point bending. Steel wheel and rubber wheel tests have been used to assess dry particle abrasion resistance. Sliding wear resistance is tested by pin-on-disk at room temperature and at 400°C, against SiC and 100Cr6 steel balls. HVOF-sprayed coatings are denser and have better interlamellar cohesion thanks to increased particle velocity. They are harder, tougher, possess a higher elastic modulus and lower porosity. Dry particles abrasion resistance is definitely superior to plasma-sprayed ceramics due to higher toughness; sliding wear resistance is higher, particularly at 400°C.

HVOF thermal spraying has been developed to deposit dense Al 2 O 3 -coatings with improved protective properties for various applications. Nanocrystalline coatings have been found to offer better thermal shock resistance, lower thermal conductivity and better wear resistance than their conventional counterparts. In this paper we describe the development of nanocrystalline Al 2 O 3 and Al 2 O 3 -Ni -coatings, where the grain size of Al 2 O 3 has been decreased and a few percents of nickel has been added in order to toughen the coating. Coatings were manufactured by HV- 2000 HVOF using spray parameters determined based on the on-line spray diagnostics. Parameters were selected aiming at different melting stages of the powder. The resulting microstructure of the coatings and its influence on the coating properties is discussed

This paper presents development of adaptive Abrasive Water Jet (AWJ) removal method for HVOF hard coatings. High pressure water jet using Al 2 O 3 powder was used to remove WC-CoCr and Cr 3 C 2 -NiCr HVOF coatings in a controlled manner. As a result, coating thickness was reduced down to 10-20 µm without damaging the base material. With this novel method use of environmentally problematic chemicals can be avoided or reduced in coating stripping process. To control the coating removal process and to prevent unwanted base material erosion through jetting, a system capable of measuring the coating thickness non-destructively with adequate accuracy was integrated to the AWJ removal cell. Depending on the substrate and coating materials used, the applied measurement technique is based on the eddy current or X-ray fluorescence methods.

In this paper the point of view for the industrial coating manufacturing is presented. The perspective for industrial manufacturing of thermal spray coatings and facts that effect on the processibility of coatings in real life is often requested but seldom published. The perspective is from Pikoteknik Oy, which has a wide and long-term experience in the improvement and repair of machine parts in-situ, containing measurements, machining, coating and balancing of rolls and cylinders. Pikoteknik Oy is able to perform maintenance of paper cylinders on-site including pretreatments, thermal spraying and balancing quickly and skillfully. Due to their versatile state-of-the-art equipment, they are able to work flexibly on-site. To be able to operate on-site there are special demands for equipment, personnel and scheduling. The quality of the final product is in the hands of operator. Destructive testing is not possible to carry out and large amount of variables, such as moisture and positioning of equipment's exist. Grinding and grit blasting must carry out without dust formation due to the surrounding bearings and sensors. Large amount of experience and special knowledge is needed to be able to produce high quality coatings over paper cylinders.

Use of high-chlorine and alkali containing fuels such as biomass and refuse is increasing in production of heat and electricity. Fluidized bed boilers experience severe and harsh conditions, in which high temperatures combined with inhomogenous fuel cause severe material wastage to metallic parts in boilers. Thermally sprayed coatings have been reported to provide protection to the boiler tubes. However, thermally sprayed coatings have encountered serious problems, when corrosion has proceeded to the substrate material through voids and oxides in lamella boundaries. Sealing of the coatings can solve these problems. Sealing of coatings with commercial sealants and laser fusion treatment was investigated. Thermally sprayed HVOF metallic coatings were sealed with different commercial sealing agents and diode laser. The coatings were tested in molten salt, simulating condition of fluidized bed boiler superheaters. Optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive X-ray analysator (EDX) were used for examination of the tested and non-tested specimens. Some of the tested sealants protected the coatings adequately in the short time alkali chloride - alkali sulphate exposure test. Best sealant contained aluminium oxide and aluminium phosphate. Laser treated coatings had good corrosion resistance in a short-term test.

Functionally graded (FG) coatings were manufactured by High Velocity Oxy-Fuel (HVOF) thermal spraying and tested aiming for the high temperature applications. Single layers were manufactured and their elastic modulus measured by using Impulse Excitation Technique (IET). Obtained data was used for modeling of optimal gradient structure. Dual feeding hose for HVOF gun was developed. Calibration procedure for the concurrent use of two powder feeders was performed. NiCr-Al 2 O 3 coatings with coating thickness of 600 µm and 1000 µm were manufactured and tested. Promising results were obtained from high temperature corrosion tests.

Several polymeric coatings, including flame sprayed polyethylene (PE), were evaluated for use in parts of natural gas pipelines. The components of interest were for instance large valves, T-joints, weld joints of pipes and pipe bends. More than 30 different coatings were selected to laboratory scale testing and evaluation. After first preliminary tests, the most potential coatings were selected further for more detailed and long term laboratory scale studies. After these tests were finished, one coating concept, i.e. fusion bonded epoxy (FBE) + flame sprayed PE, was prepared on a small natural gas valve body for demonstration purposes. Besides this coating concept, also some other coatings, e.g. liquid epoxy + flame sprayed PE and some polyurethane coatings were found to be potential coatings for the application. The test methods and results are presented in this paper.