Metallographic preparation of thermal spray coatings Application Notes Thermal spraying was invented in the early 1900s using zinc for „metallizing” substrates for corrosion protection. The development of the plasma spray gun in the late 50s and 60s made it commercially viable to use high temperature materials such as ceramics and refractory metals for coating materials. In addition to fl ame and plasma spraying, today thermal spray methods include high velocity and detona- tion spraying using a multitude of different spray materials for the most diverse and demanding applications.

Thermal spray coatings are applied to a substrate to give a specifi c surface quali- ty, which it originally does not have. Thus Metallography of thermal spray coatings the bulk strength of a part is given by the can have several purposes: substrate, and the coating adds superior - To defi ne, monitor and control spraying surface qualities such as corrosion, wear conditions for quality control or heat resistance. - For failure analysis Therefore thermal spray coatings are wide- - For developing new products. ly used in the aerospace and power gene- ration industry for new and refurbished The procedure normally involves coating sections and parts for jet engines and a test coupon to defi ne and optimize the gas turbines, compressors and pumps. process for the part to be sprayed. Sec- The properties of some coatings can only tions of this test coupon are then metal- be fabricated by thermal spraying, using lographically prepared and examined to mainly metals, ceramics, carbides and assess coating thickness, size and distribu- composites as well as mixtures of various tion of porosity, oxides and cracks, adhe- materials. sion to base material, interface contamina- Electric arc metal spray coating, showing grey oxides and round, unmelted particles tion and presence of unmelted particles.

Diffi culties during metallographic preparation Solution: Cutting: Cracks in the coating due to Grinding and polishing: Because of smear- - Precision cutting clamping the sample and using coarse ing of soft materials and pull-outs in brittle - Vacuum impregnation with epoxy resin cut-off wheels; materials, it is diffi cult to establish and - Standardized, reproducible preparation Delamination from substrate evaluate true porosity methods for thermal spray coatings Mounting: Insuffi cient penetration of mounting resin

Crack between a plasma spray coating 500x Fig.1: Ceramic spray coating, 200x Fig. 2: Same coating as Fig.1, 200x and the substrate. The crack originates insuffi ciently polished polished correctly from cutting Spray methods and applications of thermal spray coatings

Fig. 3: Flame sprayed coating; Ni5Al

and therefore the oxide content in these coatings is relatively high (Fig. 3); adhesion and density are moderate (subsequent fus- ing to increase the density is possible). Flame sprayed coatings are used for corro- sion protection and/or wear protection of structures and components, surface build- up and repair of worn shafts, for coating small parts and spots. Brass synchronising rings fl ame-sprayed with Unmelted molybdenum for wear resistance particles Electric Arc spraying uses the heat Voids of an electric arc between two con- Oxides tinuous consumable wire electrodes Substrate made of coating material to melt the wires. In the spraying process the coating mate- The wires intersect in front of a jet of rial, wire or powder, melts in a high tem- compressed air. As the heat from the arc perature heat source in a spray gun and is melts the wires, the compressed air blows accelerated by the fl ame or plasma jet and the molten droplets of the coating material projected towards the substrate. A stream onto the substrate. The high arc tempera- of molten and semi-molten particles im- ture and particle velocity gives this coating pinges onto the substrate and forms a a bond strength and density superior to coating. When the particles hit the work- fl ame sprayed coatings. However, because Fig. 4: Electric arc wire-sprayed metal coating FeCrSiNi and Mn piece they mechanically lock onto the sur- of the use of compressed air the arc face, deform and cool rapidly. The bonding sprayed coatings have a higher percentage of single particles is through mechanical of oxides (Fig. 4). the workpiece with extremely high kinetic interlocking, or in some cases metallurgical The advantage of arc wire spraying is its energy. These coatings have an excel- bonding or diffusion. High velocity of the high deposition rate which makes it suit- lent density, integrity and adhesion to the particles leads to better bonding and higher able for large areas or high volume produc- substrate. Due to the process conditions density of the coating. For good adhesion tion applications: spraying of large struc- this method is limited to the application of to the substrate it is essential that the tures like bridges and off-shore structures carbide coatings, mainly in the aerospace surface is roughened by sandblasting and with corrosion resistant zinc or aluminium and aviation industry for wear-resistant thoroughly degreased and cleaned before coatings, reclamation of engineering com- coatings. spraying. ponents and spraying of electronic compo- The various spraying techniques display nent housing with conductive coatings of In High Velocity Oxy-Fuel Combustion different temperatures at the heat source copper or aluminium. spraying (HVOF) fuel gas and oxygen are and different particle velocities, which, fed into a chamber in which combustion together with the economical aspect, need For Detonation spraying small amounts of produces a supersonic fl ame, which is to be taken into consideration for specifi c carbide powder, fuel gas and oxygen are forced down a nozzle increasing its veloc- applications. In the following the main introduced in a closed tube and exploded. ity. Powder of coating material is fed into spraying techniques are briefl y described The detonation ejects the powder with this stream and the extreme velocity of the and some of the most well-known applica- multiple sonic speed and shoots it onto particles when hitting the substrate creates tions of the resulting coatings mentioned: Principle of layer formation Flame spraying is the oldest method of ap- plying thermal spray coatings. The coating material is either wire or powder, which is fed into an oxygen-fuel gas fl ame. The mol- ten and atomized particles are ejected in a directed stream through the spraying gun nozzle. Due to the relatively low particle velocity the oxygen exposure is increased Flying drops of molten Impact on substrate Heat dissipation to Solidifi cation and shrinking coating material substrate of coating material Diffi culties in the preparation of thermal spray coatings

Fig. 5: HVOF coating of WC/12Co

a very dense, strong coating (Fig. 5). The Cutting: Clamping of spray coated work- very high kinetic energy of the particles pieces for sectioning can introduce cracks when striking the substrate ensures an in brittle coatings or compress very soft adequate mechanical bond even without coatings. the particles being fully molten. This makes this spraying method particularly well-sui- ted for spraying of coatings with carbides. Typical applications are tungsten carbide coatings on air engine turbine components and valves, and nickel-chromium coatings Combustion chamber with APS thermal barrier coating, bond coat NiCrAlY, topcoat ZrO² + Y² O³ for oxidation resistance.

Plasma spraying is the most common method for thermal spray coatings, and is

applied as Air Plasma Spraying (APS) or Cracks introduced through sectioning spraying under controlled atmosphere. An electric arc is formed between a cathode Mounting: Cold mounting resins with high and the concentric nozzle of the spray gun. shrinkage can cause damage to coatings A mixture of gases with a high fl ow rate with weak adhesion to the substrate; due along the electrode is ionised by the arc, to the shrinkage gap the coating is not and forms plasma. This plasma stream is supported by resin, which can lead to dela- pushed out of the nozzle, where the pow- Fig. 6: APS coating with NiCr bond coat and titanium mination of the coating during grinding and der of the coating material is injected into oxide top coating polishing. the plasma jet. The heat and velocity of the plasma jet rapidly melts and accelerates high quality coatings and used for a wide Grinding and polishing: Edge-rounding can the particles so that they are propelled onto range of applications, including coatings on lead to uneven polishing and subsequent the substrate and form a coating. Plasma traction surfaces, thermal barrier coatings misinterpretation of the coating density sprayed coatings have a denser structure on turbine combustion chambers, vanes (Fig. 7). Relief between coating and sub- than fl ame sprayed coatings (compare and blades, biocompatible hydroxylapatite strate creates a shadow that can be misin- Figs. 3 and 6). coatings for implants and ceramic coatings terpreted (Fig. 8). Plasma spraying has the advantage that on print rolls. it can spray materials with high melting How to estimate the true porosity in a me- points such as ceramics or refractory met- tallographically prepared spray coating is als. It is a versatile spraying method for still a reason for debate, as metallographic grinding and polishing, if not carried out

Water cooled electrode, Powder injector (external) cathode Insulator Plasmajet

Coating material

Fig. 7: Incorrect polish suggests less porosity in the middle of the coating Cooling water Water cooled nozzle, anode

Plasma Plasma gases, gases primary gases: Ar1, N2 secondary gases: H , He – + 2

Current 250 - 1000 A

Fig. 8: WC/Co spray coating with relief polish Schematic drawing shows dark line at resin/coating interface. of plasma spray gun Can lead to misinterpretation. Nickel fl ame spray coating with 15% graphite

a) Metal spray coating after fi ne grinding

correctly, can introduce artefacts which are When cutting pieces other than test cou- not part of the coating structure. For exam- pons, for instance samples for failure ple, in metal or metal/ceramic coatings, the analysis, it is important to ensure that the softer metal is smeared into pores during workpiece is clamped into the cut-off ma- grinding and if not polished properly can chine in such a way that the cut-off wheel cover up the true porosity (see Figs. a-c). is cutting into the coating towards the In comparison, ceramic coatings are brit- substrate, and not from the substrate into tle and particles break out of the surface the coating. As the bond of the coating is during grinding. If not polished thoroughly, mainly mechanical, it can delaminate from b) Same coating as in a) polished with 3 µm these break-outs leave an incorrect impres- the substrate due to the drag of the cut-off sion of a high porosity (see Figs. d-f). wheel. Particularly fragile or thin coatings can fi rst be vacuum impregnated with cold Recommendations for mounting epoxy resin, and then the micro the preparation of thermal sections are cut and remounted for grind- spray coatings ing and polishing. This ensures maximum As there are many different spraying support to the coating during sectioning. materials with sometimes unusual com- binations, it is important to know the The appearance of cracks in a coating correct spraying and substrate material. It after fi nal polishing may or may not be c) Same coating as in b) after fi nal polish facilitates to estimate how the materials will the result of cutting. It is recommended to behave under mechanical abrasion. As dif- regrind and polish the sample. If the crack ferent spraying processes result in different is from cutting it will usually disappear, if coating densities and structures it also it is inherent in the coating it will reappear, helps to know the spraying method used or cracks will surface in other areas of the on a particular sample in order to estimate coating. the expected porosity and oxide content. Mounting: Cold mounting with epoxy resin Cutting: Selection of the cut-off wheel is (EpoFix, CaldoFix) is recommended as based on the substrate material, which spray coatings are very easily damaged is usually metallic. A wheel with a looser during hot compression mounting d) Ceramic spray coating after fi ne grinding bond (soft) is preferable to a denser bond (Figs. 9 and 10). (hard) as brittle particles of the coating are In general, vacuum impregnation is rec- dragged out by a hard cut-off wheel. This is ommended for all coatings. The depth of particularly important impregnation varies when cutting parts with the degree of with ceramic coatings. open porosity and Even if the coating is interconnections ceramic, it constitutes between the pores. only a small percent- Very porous coatings age of the total cross can be easier impreg- sectional area and does not need to be cut nated than denser ones, and coatings with e) Same coating as in d) polished with 3 µm with a diamond cut-off wheel. Usually sec- less than 10% porosity can not be impreg- tioning is possible with a soft aluminium nated successfully. As it can be diffi cult to oxide wheel. If the ceramic coating is very distinguish voids fi lled with transparent thick and dense a resin-bonded diamond or translucent mounting resins from the cut-off wheel can be used as an alternative. structural elements of the coating, it helps to mix a fl uorescent dye (Epodye) into the A thin piece of styrofoam between clamps cold mounting resin. Viewed with a long and sample can help to protect brittle and pass blue fi lter and a short pass orange very soft coatings from being damaged. fi lter in the microscope, the fl uorescent dye

f) Same coating as in e) after fi nal polish Standard preparation method for thermal spray coatings

Grinding

Step PG FG

Surface SiC-paper 220# MD-Largo

Fig.9: Damage to ceramic spray coating 200x DiaPro Suspension due to hot compression mounting Allegro/Largo*

examined as a whole, plane grinding with a stone may be preferred as it is faster. Lubricant Water Whichever method is use, one must always be aware that the fi rst preparation step rpm 300 150 should aim to remove any cracks that arise Force [N] 180 180 from cutting without introducing new damage from coarse Time Until plane 5 min.

Fig.10: Same coating as in Fig. 9, cold mounted 200x grinding.

To retain fl atness and assure a good mate- Polishing rial removal rate, fi ne grinding is preferably done with diamond on a composite fi ne Step DP 1 DP 2** grinding disc. For ceramic coatings the fi ne grinding disc MD-Allegro is recommended, Surface MD-Dac MD-Nap and for metal coatings MD-Largo. A tho- rough polishing on a silk cloth (MD-Dur or Suspension DiaPro Dac* DiaPro Nap B* MD-Dac) will retain the fl atness of the sam- ple and guarantee the removal of smeared metal. rpm 150 150

Fig.11: WC/Co plasma spray coating in bright fi eld Metal coatings can be fi ne polished either Force [N] 180 120 with 1 µm diamond or a colloidal silica (OP-U) on a soft cloth. It is not recom- mended to use the colloidal silica suspen- Time 5 min. 1 min. sion OP-S for polishing metal spray coat- ings as it creates too much relief. However, Valid for 6 mounted samples, 30 mm diam. clamped in a holder. OP-S is suitable for the fi nal polishing of Remarks: ceramic coatings as it gives a good con- *Alternatively DiaPro diamond suspension can be replaced trast to the structure. by DP-Suspension, P, 9 µm, 3 µm and 1 µm respectively, applied with blue lubricant. In the trial stage for establishing prepara- **Alternatively, this diamond polishing step can be Fig.12: Same coating as in Fig.11 in fl uorescent light replaced by a polishing step with colloidal silica (OP-U for tion methods both silicon carbide and metal, OP-S for ceramic coatings) for 30-60 sec. diamond grinding can be tried to fi nd out will colour those voids yellow which have which is the more suitable plane grinding been fi lled with resin by the impregnation method. The same applies to the fi nal pol- (Fig.11 and 12). The preparation method in the table ishing step, for which 1µm diamond might Unfortunately this method is not always above has successfully been used for the in some cases be preferable to colloidal applicable for ceramic coatings, because most common coatings. The data are for silica. ceramics are translucent and the whole 6 mounted samples, 30 mm diameter, coating appears fl uorescent. In general it is recommended that, if pos- clamped into a holder. DiaPro diamond sible, a standard procedure is always used suspension can be replaced by DP-Sus- Grinding and polishing: As a general rule for all coatings. With automatic preparation pension 9 µm, 3 µm and 1 µm respectively, plane grinding should start with the fi nest equipment it is possible to control prepara- applied with blue lubricant. possible silicon carbide paper to avoid cre- tion parameters, which guarantees consist- ating artifi cial porosity by fracturing brittle ent results and excellent reproducibility. By Etching: In general, etchants that are rec- particles. Exceptions can be very dense or keeping the preparation conditions con- ommended for a specifi c material can also thick ceramic coatings, which are plane stant, it can then be assumed that sudden be used for spray coatings of this material. ground more effi ciently with diamond differences in the microstructure in most It can be expected that the more similar (with e.g. MD-Piano 220). For high sample cases refl ect differences in the spraying the substrate and coating materials are, the volumes or large parts, which need to be process and not in the preparation process. more even the etching attack will be. Struers A/S Pederstrupvej 84 DK-2750 Ballerup, Denmark Phone +45 44 600 800 Fax +45 44 600 801 [email protected]

Thermally sprayed acetabular cup shell

Coatings sprayed in a controlled atmos- USA and CANADA DEUTSCHLAND Struers Inc. Struers GmbH phere have few or no oxides and it is dif- 24766 Detroit Road Karl-Arnold-Strasse 13 B Westlake, OH 44145-1598 D- 47877 Willich fi cult to recognize the coating structure. Phone +1 440 871 0071 Telefon +49(02154) 486-0 Fax +1 440 871 8188 Telefax +49(02154) 486-222 Therefore these types of coatings need to [email protected] [email protected] be contrasted with chemical etching. SWEDEN ÖSTERREICH Struers A/S Struers GmbH Vacuum sprayed coatings on nickel and Smältvägen 1 Zweigniederlassung Österreich P.O. Box 11085 Ginzkeyplatz 10 cobalt based superalloys can be etched SE-161 11 Bromma A-5020 Salzburg Telefon +46 (0)8 447 53 90 Telefon +43 662 625 711 with the same solutions used for the sub- Telefax +46 (0)8 447 53 99 Telefax +43 662 625 711 78 strate, or electrolytically with 10% aqueous SEM photomicrograph of thermally sprayed surface [email protected] [email protected] of acetabular cup shell oxalic acid. FRANCE SCHWEIZ The structure of coatings containing mo- Struers S.A.S. Struers GmbH 370, rue du Marché Rollay Zweigniederlassung Schweiz lybdenum can be revealed by using the and that the diamond polish needs to be F- 94507 Champigny Weissenbrunnenstrasse 41 sur Marne Cedex CH-8903 Birmensdorf following etchant: long enough to reveal the true porosity. Téléphone +33 1 5509 1430 Telefon +41 44 777 63 07 Télécopie +33 1 5509 1449 Telefax +41 44 777 63 09 50 ml water [email protected] [email protected] 50 ml hydrogen peroxide (3%) The recommended preparation procedure is based on experience and gives excel- BELGIQUE THE NETHERLANDS 50 ml ammonia Struers S.A.S. Struers GmbH Nederland lent results for the majority of common 370, rue du Marché Rollay Electraweg 5 F- 94507 Champigny NL-3144 CB Maassluis Caution: Always follow the recommended thermal spray coatings. However, it should sur Marne Cedex Tel. +31 (0) 10 599 72 09 Téléphone +33 1 5509 1430 Fax +31 (0) 10 599 72 01 safety precautions when working with be noted that for some specifi c proprietary Télécopie +33 1 5509 1449 [email protected] chemical reagents. coatings the polishing times may need to [email protected] CZECH REPUBLIC be adjusted. UNITED KINGDOM Struers GmbH Summary Struers Ltd. Ocelářská 799 Erskine Ferry Road, CZ-190 00 Praha 9 Thermal spray coatings are widely used to Old Kilpatrick Tel. +420 2 84 818 227 Application Notes Glasgow, G60 5EU Fax +420 2 660 32 278 give or improve a specifi c surface quality Metallographic preparation of thermal spray coatings Phone +44 1389 877 222 [email protected] or function to a workpiece. Different spray- Fax +44 1389 877 600 Elisabeth Weidmann, Anne Guesnier, Struers A/S, [email protected] Copenhagen, Denmark POLAND ing methods result in different character- Struers Sp. z o.o. Brigitte Duclos, Struers S.A.S., Champigny, France istics of the coatings, and they are mainly JAPAN Oddział w Polsce Acknowledgement Marumoto Struers K.K. ul. Lirowa 27 applied for corrosion, heat and wear resist- Takara 3rd Building PL-02-387 Warszawa We wish to thank Sulzer Metco AG, Wohlen, Switzerland, 18-6, Higashi Ueno 1-chome Tel. +48 22 824 52 80 ance. Metallographic examination of spray for its cooperation and supplying information material. Taito-ku, Tokyo 110-0015, Fax +48 22 882 06 43 coatings includes estimation of porosity, Special thanks go to J. Hochstrasser and P. Ambühl for Phone +81 3 5688 2914 [email protected] sharing their extensive knowledge with us and supplying Fax +81 3 5688 2927 [email protected] oxides and unmelted particles as well as the following images for reproduction: photo of spraying HUNGARY adhesion to the substrate. Because incor- process and large micrograph on page 1; drawing: Struers GmbH Principle of particle movement, photo synchronising CHINA Magyarországi fi óktelep rect grinding and polishing procedures can Struers (Shanghai) Ltd. Puskás Tivadar u. 4 rings and micrographs on page 2; drawing, photo Room 2705, Nanzheng Bldg. H-2040 Budaörs infl uence the evaluation of the true porosity combustion chamber and all micrographs on page 3 and 580 Nanjing Road (W) Phone +36 (23) 428-742 it is very important that metallographic micrograph of nickel fl ame sprayed coating on page 4. CN - Shanghai 200041 Fax +36 (23) 428-741 We thank Richard Compton, Zimmer, Inc. USA, for Phone +86 (21) 5228 8811 [email protected] preparation is carried out systematically Fax +86 (21) 5228 8821 the photo of the acetabular cup shell and the SEM [email protected] and that the results are reproducible. Preci- photomicrograph on page 6. sion cutting with the correct cut-off wheel Bibliography SINGAPORE Metallographic preparation of thermally sprayed Struers A/S is recommended to avoid cracks in the 10 Eunos Road 8, orthopaedic devices, Richard C. Compton, Zimmer, Inc., #12-06 North Lobby coating. Mounting should follow with slow USA, Structure 28, 1995 Singapore Post Centre curing epoxy. Coarse grinding introduces Summary Report of the Plasma Spray Coatings Singapore 408600 Symposium at Struers, Copenhagen, May 25th to 27th, Phone +65 6299 2268 the most damage to the coating and should 1988 Fax +65 6299 2661 [email protected] therefore be carried out with the fi nest grit Universal metallographic procedure for thermal spray coatings, S. D. Glancy, Structure 29, 1996 possible. To avoid relief fi ne grinding with Materialographic characterization of modern multilayer diamond on a rigid disc is recommended, coating systems used for hot-gas components in large followed by a thorough diamond polish on gas turbines for static power generation, A. Neidel, S. Riesenbeck, T. Ulrich, J. Völker, Chunming Yao, Siemens a silk cloth. Power Generation, Berlin, Structure 2/2004 It is particularly important to be aware that metal coatings behave differently to cera- mic coatings under mechanical abrasion www.struers.com

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