"Metallographic evaluation and process correlation of thermally sprayed coatings”

Carlos Roberto Camello Lima, Flávio Camargo, Carmo Roberto Pelliciari de Lima. São Paulo / Brazil Artigo apresentado no ITSC 2004- International Thermal Spray Conference – ASM International - TSS

Thermal sprayed coatings are widely used for industrial applications. One of the main characteristics that have to be evaluated in a coating is its microstructure that finally determines the coating performance. Several techniques and processes are available for coatings deposition and new materials have been incorporated in the long list of the available ones. Therefore, since microstructure is a key factor to be evaluated, its preparation has to follow some rules in order to inhibit incorrect statements that can arise from wrong interpretation of an incorrect sample preparation. In this work, a series of distinctive materials are thermally sprayed onto low carbon steel substrates. The metallographic preparation of samples for the different coatings is presented and the effects of correct or wrong sample preparation are discussed and correlated to coating microstructure and process characteristics. The coatings were applied by Air Plasma Spray (APS) and Electric Arc Spray. 1 Introduction Thermal sprayed coatings are used for several appli-cations, including wear and corrosion protection, thermal barrier, thermal conductivity and difusivity [1,2,3]. In this sense, to get such special characteris-tics, a wide range of distinctive materials is utilized like several metals and metallic alloys, polymers, ce-ramics and cermets, applied by distinct processes that include Air Plasma Spray (APS), Detonation Gun, Flame Spray (FSP), Arc Spray and High Velocity Oxy-Fuel (HVOF). THE intensive use of such types of coatings in industries like aerospacial, petrochemical, and automotive, to mention just some, makes the quality demand for coatings extremely high. Microstructural analysis is used to characterize both the substrate and the coating, as well as adhesion and cohesion interactions. The main microstructural characteristic observed through microscopy they are morphology of the coating, thickness of the layers, po-rosity, unmelted particles, cracks, coating/substrate interface and oxides. A good contact between the coating and the substrate is a fundamental factor for good adhesion, integrity and coating performance. The interface should be homogeneous, completing all the irregularities. Pores, strange particles, like dirt, and oxides are harmful to the adhesion and should be kept minimum. Through Optical Microscopy and Scanning Electron Microscopy (SEM) the interface can be studied in a detailed way, provided the sample has been carefully prepared so that artifacts are not present. Therefore, the use of several methods of coatings evaluation is motivated and, among them, metallography is highly important as stated by several authors [4,5,6] and it is fundamental in the coating quality management and consequently in the effec-tiveness of its application [7,8]. A better understanding of coatings microstructure definitively helps a most scientific development of the sprayed coatings. This improves the process product development, quality and methods and processes management. 2 Experimental Procedure For the development of this work, several coatings of different feedstock materials were applied on AISI

1020 carbon steel substrate. The spraying processes for metallic and ceramic coatings were Electric Arc and Air Plasma Spray (APS), respectively. Al2O3 coatings were applied By Air Plasma Spraying (METCO 7MB, Westbury, NY, USA). Copper, Alumi-num, 420 Stainless Steel, Molybdenum o and Babbit were applied by Electric Arc (8830 - Tafa, Inc., Con-cord, NH, USA). The materials were applied in com-mercial conditions of use with optimized parameters, generating coatings in optimized conditions. The cut of samples was accomplished in a precision cutoff machine Isomet 2000 (Buehler, USA). Abrasive disc was used for non-ferrous materials. Al-though there are some recommendations for the use of diamond disks in the cut of ceramic materials, an abrasive disc was chosen due to the ferrous nature of the substrate that would quickly wear away the ex-pensive diamond disk. It should be pointed that the great chemical affinity between carbon and iron pro-vokes excessive waste of diamond tools. To guaran-tee the flatness minimizing edge rounding of the pol-ished surface, hot mounting was accomplished using a resin-containing fiberglass, which is harder than common Bakelite. This is a simple and very useful procedure. Grinding and polishing were carried out in an auto-matic polish machine Motopol 2000 (Buehler, USA), allowing the control of applied load, wheel rotation speed, specimens holder and wheel rotation direction control, among other things. The control of these pa-rameters allows better results compared to manual grinding and polishing. The parameters used in the metallographic preparation for the applied materials are presented in Table 1. Grinding papers of SiC and diamond pastes were used for polishing. Some samples were chemically etched after polish-ing. These attacks will be specified later on. The coat-ings were analyzed by optical microscopy in an Opti-cal Microscope Carl Zeiss, model Neophot 32. The porosity, amount of oxides and images were accom-plished through an Image Analysis System Leica Q500.

Table 1. Grinding and polishing used parameters.

Abrasive

Lubricant

Load (N)

Wheel rotation

speed (rpm)

Time (min)

Paper 220 Water 20 300 2 Paper 320 Water 20 300 2 Paper 420 Water 20 300 2 Paper 600 Water 20 300 3 Paper 800 Water 20 300 3,5

Grinding

Paper 1200 Water 20 300 4 Paste 6 µm Alcohol 20 150 5 Paste 1 µm Alcohol 20 150 6

Polishing

Paste .25 µm Alcohol 20 150 7 3 Results and discussion Figure 1 presents a representative image of a Copper coating. This sample was manually prepared and only polished, without chemical attack.

Fig. 1. Microstructure of a Copper coating showing some porosity from metallographic preparation

Fig. 2. Microstructure of a Copper coating showing

normal process porosity Some porosity can be observed (6,87 % according to image analysis measurement), which is typical of de-tachment of material from incorrect procedures in

metallographic preparation. Figure 2 exhibits the same coating prepared in an automatic machine. Now, only the normal coating porosity (1,41 %) result-ing from the spray process can be observed. Figure 3 exhibits a representative image of a coating of 1080 carbon steel that was chemically etched after polishing with Nital 2%. Chemical etching in such ma-terial facilitates the identification of existent phases in the carbon steel structure, as well as porosity.

Fig. 3 Microstructure of 1080 carbon steel coating with Nital 2% etching Figure 4 shows a micrography of the same sample prepared without etching.

Fig. 4. Microstructure of coating of 1080 carbon steel only polished, without chemical attack Comparing the images of Fig. 3 and 4, it can be no-ticed that in Fig. 3 present phases are much clearer facilitating its distinction and even quantification. In Fig. 4 from the sample without attack is more evident and easy to verify the amount of present oxides. The oxidation is relative to the use of compressed air in the Electric Arc Spray process as carrier gas. The oxidation degree can be minimized using an inert gas instead of compressed air, even making the process a little bit more expensive.

Figures 5 and 6 show representative images of a 420 stainless steel coating. In Fig. 5, it can be noticed the debonding of the coating from the substrate. This debonding could lead to the erroneous conclusion of process application problems and consequent low adhesion of the coating to the substrate. In fact, it is a debonding that happened due to problems in the met-allographic preparation, specifically in the specimen cutting process.

Fig. 5. Microstructure of a 420 stainless steel coating debonded from the substrate

Fig. 6 Microstructure of a 420 stainless steel coating with good adhesion to the substrate Even the high porosity level in the coating observed in Fig. 5 can be credited to inadequate polishing, since metallographic preparation of this sample was done with less steps of grinding, generating an aggressive specimen preparation. The sample showed in Fig.6 was correctly prepared and its cutting was carried out in precision cutting machine, evidencing the good ad-hesion of the coating. Figures 7 and 8 show, respectively, Molybdenum coatings only polished and polished and chemically etched. Etching used for Molybdenum was done with a mixture of 100 ml of distilled water, 3 g K3Fe (CN) 6 and 10g of KOH. This sample was automatically pre-pared. Automatic or semiautomatic preparation facili-

tates reproducibility and repeatability in the metal-lographic preparation process, minimizing undesired artifacts appearing.

Fig. 7. Polished Molybdenum coating

Fig. 8. Polished and etched Molybdenum coating The sample prepared without etching, as seen in Fig. 7, facilitates porosity observation, mainly macroporos-ity that can be in the range of 1 to 10 µm [9] or be considered varying from 10 to 30 µm [10]. Chemical etching helps in grains structure and layers boundary observation. Figure 9 shows an Aluminum coating with central coating particle detachment caused by excessive pressure in the grinding process. Aluminum is very sensitive to grinding and polishing work and it is im-portant to keep this in mind while preparing speci-mens for observation at the microscope. Figure 10 exhibits a coating of Zinc. Metallographic preparation of this kind of material type is quite diffi-cult, since soft materials like Zinc, Aluminum and Babbit scratch out easily. To minimize the problem of risks carefully cleaning of the samples and polish cloths is fundamental. Another problem that can occur is smearing. The use of worn-out SiC papers favors smearing, therefore new SiC papers have to be used for such materials.

Fig. 9 Microstructure of an aluminum coating with par-ticles detachment

Fig. 10. Coating of Zinc only polished Figure 11 shows an Al2O3 coating applied with correct standoff distance (100 mm). In that illustration it can be seen that there is good contact between coating and substrate.

Fig. 11 Al2O3 coating applied with correct standoff distance exhibiting good adhesion.

On the other hand, Figure 12 shows a coating of the same material, deposited with the same power but with a longer standoff distance (200 mm). In this case, the contact between coating and substrate is not so good, presenting some flaws in the interface. This happens since a longer standoff distance means that particles tend to be colder when reaching the sub-strate presenting, therefore, lower plasticity and more difficulty to fill the irregularities. In these two cases, the problem is a real process problem since metal-lographic was correct, not influencing in final result. In this metallographic preparation the sample was only polished, without any chemical attack.

Fig.12 Al2O3 coating applied with 200 mm standoff presenting adhesive flaws. Conclusions The obtained results and the topics discussed in this work allow the following conclusions: - Microstructure of coatings defines, in ultimately,

coatings performance when in service for a wide va-riety of applications.

- Metallographic preparation is a fundamental aspect

for correct interpretation of coatings microstructure evaluation for the several spraying processes used.

- Despite of long time discussion of the subject, new

materials, new processes and their consequent pe-culiarities turn even more important the discussion on cares that have to be considered in the evalua-tion of tests and all quality rehearsals.

- This work has presented some practical aspects on

the metallographic preparation and interpretation showing different materials and problems considera-tions related both the spraying process and the met-allographic procedure.

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