The designs of experiments (DOE) methodology useful for thermal spraying and associated processes of post-spray treatment are thoroughlyreviewed. The designs Hadamard (Plackett–Burman), two-level full and fractional factorial and also the response surfaces methodology arebriefly described. The designs enable to obtain a polynomial regression equation which expresses the influence of process parameters on theresponse. The methods of determining of the significant coefficients of the regression equation (factors) are discussed. Examples of the applicationof different designs to determine the response equations with the responses related to microstructure, mechanical, electrical and other propertiesofcoatings deposited using different thermal spray and post-spray processes are presented and discussed.© 2008 Elsevier B.V. All rights reserved. Keywords: Design of experiments; Parameters of thermal spraying processes 1. IntroductionThermal spraying is the family of coating depositionprocesses in which molten, semi-molten or solid particles aredeposited onto a substrate. The microstructure of the coatingsresults from their solidification and sintering [1]. The processesuse hot gas, flame or plasma to accelerate the particles and toheat them up. Obtained coatings have alamellar microstructure,which determines many of coatings properties. Since theoptimization of coatings properties for a given specificationneeds a careful control of the operational spray parameters andthe statistical methods can be useful in preparation of theexperiments. The initial step in the design of experiments is achoice of variables being process parameters. In the simplestcase, the parameters are fixed at low (−1) and high (+1) level.The experimental space is defined inside these parametersvalues. The choice of parameters needs some understanding ofthe processas thereare as many as 50processvariables[2].Thefirst group of variables is related to the feedstock and concerns,supposing powder feedstock, mainly [3]:• size distribution of powder;• powder feed rate;• powdermorphology,suchase.g.shape,internalporosity,etc.Typical variables for atmospheric plasma spraying are asfollows:• composition of working gas, such as e.g. vol.% of hydrogenin argon–hydrogen mixture;• spray distance;• powder feed rate;• electric power input;• carrier gas flow rate.For high velocity oxy-fuel spraying, it is useful to add alsoanother variable which is the ratio of fuel gas to oxygen [4]as well as other parameters for arc spraying, flame sprayingor cold gas spraying. Another problem concerns pre-spray(sand blasting) and post-spray treatment (laser glazing, hotisostatic pressing, infiltration…), which also is frequently

Design of experiments in thermal spraying: A review

In the current study, aluminum coating was deposited on mild steel by arc spraying. A well-adhered coating with low level of porosity was successfully obtained. To evaluate the corrosion behavior of the coating, electrochemical impedance spectroscopy (EIS) and polarization tests in 3.5% NaCl solution were carried out. The as-coated samples were also subjected to a 1500-h salt spray assay. Polarization tests indicated that the corrosion current density of the aluminum coating is more than that of bulk aluminum. This could be due to the penetration of the electrolyte through open pores, resulted in the acceleration of aluminum corrosion. EIS measurements showed that the corrosion performance of the coating is improved during a long time immersion and exposure to saline mist. This could be due to plugging of pores by corrosion products which hinder further penetration of the electrolyte through the coating. The results obtained indicated that twin wire arc sprayed aluminum coatings can reliably protect steel structures against corrosion in chloride-containing aqueous solutions.

Study of Corrosion Behavior of Arc Sprayed Aluminum Coating on Mild Steel

Pore blocking characteristics of corrosion products formed on Aluminum coating produced by arc thermal metal spray process in 3.5 wt.% NaCl solution

This paper gives a bibliographical review of the finite element methods applied to the analysis and simulation of coatings, their mechanical and material properties from the theoretical and application points of view. The surface modification technologies in the context of coatings are also included. The added bibliography at the end of this paper contains 1032 references to papers and conference proceedings on the subject that were published in 1995–2005. The following topics are included: coating property simulations; surface modification process simulations and practical coating applications.

https://iopscience.iop.org/article/10.1088/0965-0393/13/6/011/pdf

Coatings and surface modification technologies: a finite element bibliography (1995–2005)

Arc thermal metal spray coating provides excellent corrosion, erosion and wear resistance to steel substrates. This paper incorporates some results of aluminum coating applied by this method on plain carbon steel. Thereafter, coated panels were exposed to an environment known to form stable corrosion products with aluminum. The coated panels were immersed in Society of Automotive Engineers (SAE) J2334 for different periods of time. This solution consists of an aqueous solution of NaCl, CaCl2 and NaHCO3. Various electrochemical techniques, i.e., corrosion potential-time, electrochemical impedance spectroscopy (EIS) and the potentiodynamic were used to determine the performance of stimulants in improving the properties of the coating. EIS studies revealed the kinetics and mechanism of corrosion and potentiodynamic attributed the formation of a passive film, which stifles the penetration of aggressive ions towards the substrate. The corrosion products that formed on the coating surface, identified using Raman spectroscopy, were Dawsonite (NaAlCO3(OH)2) and Al(OH)3. These compounds of aluminum are very sparingly soluble in aqueous solution and protect the substrate from pitting and uniform corrosion. The morphology and composition of corrosion products determined by scanning electron microscopy and energy dispersive X-ray analyses indicated that the environment plays a decisive role in improving the corrosion resistance of aluminum coating.

/pdf/corrosion-resistance-properties-of-aluminum-coating-applied-36g9tx66pj.pdf

Corrosion Resistance Properties of Aluminum Coating Applied by Arc Thermal Metal Spray in SAE J2334 Solution with Exposure Periods

An experimental design matrix was set up in which carbon steel coatings were deposited with a twin wire arc spray gun (TAFA 9000™), using either compressed air or nitrogen as spraying gas. The coating's mechanical properties were studied. Some correlations were made between these properties, spraying conditions and the microstructure of the deposits. Young's modulus was estimated by the single beam method using finite element modeling. Results show that direct relationships do exist between spray conditions, oxide content in the coating and microhardness. Young's modulus of the coatings depends on the lamella thickness and the oxide content. When increasing the compressed air flow rate, Young's modulus increases at first because smaller particles and finer lamellae were made and it decreases later because of a higher oxide content. The increase of nitrogen flow rate lowers the oxide content and increases Young's modulus.

/pdf/correlations-between-operating-conditions-microstructure-and-33g8tl0n6l.pdf

Correlations between operating conditions, microstructure and mechanical properties of twin wire arc sprayed steel coatings

This paper describes a simple and efficient method to identify Young's modulus of thermally sprayed coatings. The mechanical method is based on the cantilever beam test, without the use of a strain gauge, combined with a finite element numerical modelling and optimization technique. The influence of coating roughness was taken into account in this work. Several types of coatings were tested and their Young's moduli were calculated. A sensibility analysis was also performed.

Determination of the Young's modulus of thermally sprayed coatings using a modellized cantilever test

Cet article presente une description de la fabrication directe de moules par l'utilisation du procede de projection thermique. Cette technique permet la fabrication directe de coquilles metalliques ou ceramiques sur des modeles. En principe, aucune etape intermediaire n'est necessaire entre la fabrication du modele et celle du moule. C'est la raison pour laquelle les delais de fabrication et la qualite de replication sont considerablement ameliores. Cependant, la necessite de ne pas endommager le modele et de garder un bon controle dimensionnel necessitent de travailler a temperature proche de l'ambiante. Ceci conduit generalement a de mauvaises proprietes mecaniques des coquilles principalement a cause d'une forte proportion d'oxydes. De plus, des distorsions geometriques et des fissures apparaissent lorsque les contraintes residuelles, liees aux conditions de projection, ne sont pas maitrisees. Une technique permettant de resoudre ces problemes est presentee ainsi que des exemples de realisations actuelles.

G. Jandin

Papers

Correlations between operating conditions, microstructure and mechanical properties of twin wire arc sprayed steel coatings

Determination of the Young's modulus of thermally sprayed coatings using a modellized cantilever test

Principles of rapid tooling using an improved thermal spray process