Abstract This paper compares the wear properties of HVOF-sprayed WC/12Co hardcoatings produced from different powder feedstock materials, including conventional, nanophase, and mixed powders. The mixed or multimodal feed powder is designed to minimize the amount of material that goes through a high temperature cycle during spraying, thus potentially limiting the extent of decarburization in the resulting coating. As will be shown, decarburization is indeed minimal in a multimodal coating, which translates into exceptional resistance to abrasive and sliding wear. Another favorable factor is the ability to increase the volume fraction of hard WC phase in such a coating, thereby further enhancing its wear resistance.

Abstract A ball-milled mixture of glass and alumina powders has been plasma sprayed to produce alumina-glass composite coatings. The coatings have the unique advantage of a melted ceramic secondary phase parallel to the surface in an aligned platelet composite structure. The alumina raises the hardness from 300HV for pure glass coatings to 900HV for a 60wt% alumina-glass composite coating. The scratch resistance increases by a factor of three and the wear resistance by a factor of five. The glass wears by the formation and intersection of cracks. The alumina wears by fine abrasion and supports most of the sliding load. The wear resistance reached a plateau at 40-50vol% alumina, which corresponds to the changeover from a glass to a ceramic matrix. Keywords: glass composite coatings, wear, thermal spraying

Abstract The HVOF technology is well known to provide a wide variety of coating materials having excellent performance characteristics under different aggressive conditions such as wear, erosion by impact of particle and corrosion. Carbides, as a family, constitute a big segment of materials used by the thermal spray industry. Although their material properties may be well known since they are often used in wear or corrosive-wear industrial applications, aqueous corrosion of such coatings are not well characterized. Moreover, thermal spray process technology being in constant evolution, past literature on these coatings may not be directly applicable as newer produced coatings have higher adhesive and cohesive strength. Recent technology allows a better control on density and oxides content that are important parameters to consider for corrosion applications. The success of a coating is related to judicious material selection for various applications. However, the choice of the starting materials for producing a coating is often difficult since there is a lack of data on the corrosion performance of thermal spray coatings. The present paper addresses the performance of various carbide HVOF coatings in terms of corrosion rate and degradation mode in two corrosive environments — HCl and HNO3. Behavior of the coatings is compared using bulk SS316 and SS316 HVOF coating as a benchmark.

Abstract Coatings have been prepared using the Diamond Jet hybrid and JP5000 high velocity oxyfuel (HVOF) systems with the objectives of improving corrosion resistance and reducing costs through increasing deposition efficiency. Models relating deposition efficiency, coating oxygen content and corrosion resistance to process parameters including fuel flow rate, oxygen flow rate and stand-off distance have been developed. A corrosion test cell has been designed and a procedure determined for studying the corrosion behaviour of large numbers of thermally sprayed coatings in an efficient manner. A significant improvement to the corrosion resistance of HVOF sprayed coatings has been achieved by spraying parameter optimisation and investigation of powder size and distribution. The project has also investigated the influence of spray angle on coating performance with a view to future onsite application. Coating materials tested and compared include nickel alloys Hastelloy C276 and 59, cobalt alloy Ultimet, duplex stainless steel S32750 and an experimental iron-based spray-fuse composition.

Abstract Borides are promising materials with good wear and corrosion resistance properties. Boride coatings are expected to perform better where wear and corrosion resistances are simultaneously required. Zirconium diboride is an important emerging material for such applications, due to its high hardness, high melting point, good wear resistance and corrosion as well as high temperature oxidation resistance. Special properties of laser beam like beam directionality, high intensity and spatial resolution makes laser alloying a fast and efficient technique for producing improved wear resistance coatings. In the present work, mild steel was laser alloyed with ZrB2, using "two-stage" technique of laser alloying. These coatings after characterization by optical microscopy, SEM, EDAX and XRD techniques were tested on a "Pin-on-Disk" machine for determining their wear resistance.

Abstract The isothermal and cyclic oxidation of freestanding Ni-20Cr-10Al-lY thick coatings has been investigated at 1200°C using TGA, SEM, XRD and XPS techniques. Coatings produced by HVOF are dense and remain crack free after thermal treatments. The protective oxide layer formed did not flake off upon cyclic oxidation as confirmed by SEM analysis. In addition, three oxidation regimes were identified after analyzing TGA data: two below 1000 °C and a third one at approximately 1200°C. The regimes below 1000°C correspond to the selective oxidation of elements on the surface and at the subsurface of the coatings whereas the third regime involves element diffusion from the bulk of the coating to the surface. The oxidation regime became asymptotic at 1200 °C as stable oxides formed. The presence of water vapor affects neither the thickness nor the orientation of oxide crystals formed on the surface as confirmed by the X-ray analysis. The XPS and X-ray results show an inter-diffusion between the coating and substrate with a slight increase in chromium concentration at the interface. Element distribution within the oxide layer was found to follow the order: Al-(oxide)Y-(oxide)/Cr-(oxide)/Ni-(oxide)/NiCrAlY from the outermost oxide layer to the bulk of the coating. These results show that HVOF dense Ni-20Cr-10Al-lY sprayed coatings can be used as anti-oxidant barriers in both isothermal and cyclic oxidation at 1200°C.

Abstract Thermal spraying of corrosion resistant alloys onto low alloy steel substrates has received much attention as a method to protect against corrosion in seawater or corrosive solutions, such as mineral or organic acids. The need to ensure high coating quality with minimal porosity and cracking, and with low oxide levels is best achieved in metallic alloys using the high velocity oxyfuel (HVOF) spraying process. This article investigates the electrochemical corrosion behavior of HVOF sprayed coatings, covering coating preparation and characterization, immersion testing, and electrochemical testing. The discussion provides information on immersion test results, polarization plots for coatings, comparison of corrosion performance, influence of microstructure on corrosion performance, and comparison with bulk alloy materials. The results reported in this article have been selected to demonstrate the use of the cyclic potentiodynamic polarization method to rank the corrosion performance of HVOF sprayed Ni-alloy coatings.

Abstract Thermal spray committee of Japan Association of Corrosion Control (JACC) has been conducting a corrosion test of thermal sprayed Zn, Al and Zn-Al coatings at coastal area since 1985. Twelve kinds of sprayed coating were deposited onto steel pipe by arc- and flame-spraying at varied thickness and subjected to various post-spray treatment. The samples were set vertically into the seawater at a port 80 km south from Tokyo. Corrosion performance were inspected annually by recording the appearance and coatings' thickness at sea air-, splash- and tidal-zones. No significant changes were observed for five years exposure. After 7 years, however, Zn coatings with and without sealing started to suffer degradation in the immersed portion. Contrary to this, Al and Zn-Al coatings still exhibit superb corrosion performance. The test will be continued till 2001 to finish a 15 years field test. This paper reports the summary of corrosion performance of sprayed coating during the first 10 year period.

Abstract The work reported herein deals with the interaction among the thermally sprayed coatings and steel substrate in acid aqueous solutions investigated by using the electrochemical process. Thermally sprayed coatings on steel substrate for such as the tribological applications except for the sacrificial anode property have been extended. The environment of these machinery components is often utilized accompanied with the aqueous solutions. The galvanic corrosion occurred in aqueous solutions among the metals or alloys have been well known. In the actual spray process, the formation of penetrated pores or crevice defects are not generally avoided. In the aqueous solution environment, the penetrated defects cause the galvanic corrosion between coating and steel substrate, and tend to be occurred the coating spalling. In this work, the electrochemical process was employed to determine the corrosion phenomena. The preferable combination of sprayed coatings and steel substrate are discussed and the examples of design of the intermediate layers are proposed.

Abstract This paper describes and discusses aspects of the erosion-corrosion behaviour of a WC-Co-Cr, HVOF sprayed coating when subjected to an impinging jet of an aqueous solution of 3.5% NaCl containing solid particles at 18°C. Although pure mechanical erosion has been found to contribute to a large part of material degradation, a considerable amount of material loss can be attributed to the direct and indirect (synergy) effect of corrosion. Aspects of the influence of time, solids loading and impingement angle have been investigated and the mechanisms of erosion-corrosion are discussed.

Abstract This paper discusses the principles of high velocity arc spraying (HVAS) and its use in surface engineering. It also reviews applications in marine environments and power plant boiler pipes, where HVAS coatings provide corrosion protection.

Abstract Structural damage in concrete structures caused by corrosion is widespread and demand comprehensive repair work. The additional installation of an active corrosion protective systems for structures being located in unfavourable conditions is imperative. Thermally sprayed coatings serving as anode have been adapted from the cathodic protection of steel. These systems have gained attention as they offer advantages in efficiency and lower cost. Thermally sprayed zinc coatings are applied to new steel reinforced concrete structures or those which are subject to re-structuring. In this contribution, the capability of various systems is examined by field tests in a marine structure and in different laboratory tests under natural and under accelerated conditions.

Abstract In the sieve section of a paper producing machine, where the formation of paper occurs, a polyethylene sieve is guided over a set of ceramic blades to remove the water in the paper slurry. These sieves are running with velocities of 70 km/h and more. The high velocities lead to defects in the tissue structure of the sieve as well as on the blades. Moreover the presence of high abrasive particles like kaolin and carbonates in the paper slurry as well as its corrosive characteristics enforce the material wear. Present solutions include bulk ceramic blades but they cause defects on the sieves and they are quite expensive. New solutions are desired where these bulk ceramic blades can be replaced by intelligent coating systems that on one hand exhibit an extraordinarily high wear resistance but can minimize the wear of the PE sieve on the other hand. The presentation gives first results obtained by different types of ceramic and cermetic coating systems manufactured by thermal spray technology.