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Home , Detonation spraying, Thermal spraying

Metallographic preparation of thermal spray Application

Thermal spraying was invented in the Notes early 1900s using zinc for „metallizing” substrates for protection. The development of the spray gun in the late 50s and 60s made it commercially viable to use high temperature materials such as and refractory for materials. In addition to flame 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 specific 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 define, monitor and control spraying surface qualities such as corrosion, 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 a sections and parts for jet engines and test coupon to define and optimize the pro- turbines, compressors and pumps. cess for the part to be sprayed. Sections The properties of some coatings can only of this test coupon are then metallographi- be fabricated by , using cally prepared and examined to assess mainly metals, ceramics, and coating thickness, size and distribution of composites as well as mixtures of various porosity, and cracks, to materials. base material, interface contamination and Electric arc spray coating, showing grey oxides presence of unmelted particles. and round, unmelted particles

Difficulties 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 - impregnation with epoxy resin cut-off wheels; materials, it is difficult to establish and - Standardized, reproducible preparation Delamination from substrate evaluate true porosity methods for thermal spray coatings Mounting: Insufficient penetration of mounting resin

Crack between a plasma spray coating 500x Fig.1: spray coating, 200x Fig. 2: Same coating as Fig.1, 200x and the substrate. The crack originates insufficiently polished polished correctly from cutting

Abb. 1: Keramische Spritzschicht, 200x 28.02.2019 / 62142005 unzureichend poliert Riss zwischen Plasmaspritzschicht und 500x Abb. 2: Selbe Beschichtung wie Abb. 1, 200x Trägermaterial. Der Riss wurde beim Trennen korrekt poliert verursacht

Fig. 1 : revêtement par projection de 200x céramique insuffisamment poli Fig. 2 : le même revêtement 200x que fig. 1, poli correctement Fissure entre un revêtement plasma et le 500x substrat. La fissure vient du tronçonnage.

Fig. 2: Mismo recubrimiento que en 200x Fig. 1: Recubrimiento cerámico pulverizado, 200x la Fig. 1, pulido correctamente con pulido insuficiente

Fractura entre un recubrimiento de 500x pulverización por plasma y el substrato. La fractura se origina en el corte 图 2：与图 1 相同的涂层，正确抛光 200x

图 1：陶瓷喷涂层，抛光不足 200x

等离子涂层与基板之间的裂缝。裂缝源于切割 500x . Spray methods and applications of thermal spray coatings

Fig. 3: Flame sprayed coating; Ni5Al

and therefore the 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 flame-sprayed with Unmelted for wear resistance particles Electric Arc spraying uses the heat of Voids an electric arc between two continu- Oxides ous consumable wire electrodes made Substrate of coating material to melt the wires. The In the spraying process the coating mate- wires intersect in front of a jet of com- rial, wire or powder, melts in a high tem- pressed air. As the heat from the arc melts perature heat source in a spray gun and is the wires, the compressed air blows the accelerated by the flame or plasma jet and 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- flame 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 coatings, mainly in the aerospace surface is roughened by sandblasting and with corrosion resistant zinc or 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- Combustion different temperatures at the heat source copper or aluminium. spraying (HVOF) fuel gas and are and different particle velocities, which, fed into a chamber in which combustion together with the economical aspect, need For small amounts of produces a supersonic flame, which is to be taken into consideration for specific 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 briefly 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 flame. The molten 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 Solidification and shrinking coating material substrate of coating material Difficulties 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 carbide coatings on air engine turbine components Combustion chamber with APS , and valves, and - coatings bond coat NiCrAlY, topcoat ZrO + Y O for oxidation resistance. ² ² ³

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 with a high flow 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 de- pushed out of the nozzle, where the pow- Fig. 6: APS coating with NiCr bond coat and lamination of the coating during grinding der of the coating material is injected into oxide top coating and 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 flame 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 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 flame spray coating with 15% graphite

a) Metal spray coating after fine 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 Recommendations for first be vacuum impregnated with cold the preparation of thermal mounting epoxy resin, and then the micro spray coatings sections are cut and remounted for grind- As there are many different spraying ing and polishing. This ensures maximum materials with sometimes unusual com- support to the coating during sectioning. binations, it is important to know the correct spraying and substrate material. It The appearance of cracks in a coating c) Same coating as in b) after final polish facilitates to estimate how the materials will after final polishing may or may not be behave under mechanical abrasion. As dif- the result of cutting. It is recommended to ferent spraying processes result in different regrind and polish the sample. If the crack coating densities and structures it also is from cutting it will usually disappear, if helps to know the spraying method used it is inherent in the coating it will reappear, on a particular sample in order to estimate or cracks will surface in other areas of the the expected porosity and oxide content. coating.

Cutting: Selection of the cut-off wheel is Mounting: Cold mounting with epoxy resin based on the substrate material, which (ProntoFix , EpoFix, CaldoFix-2) is recom- is usually metallic. A wheel with a looser mended as spray coatings are very easily d) Ceramic spray coating after fine grinding bond (soft) is preferable to a denser bond damaged during hot compression mount- (hard) as brittle particles of the coating ing (Figs. 9 and 10). are dragged out by a hard cut-off wheel. In general, vacuum impregnation is rec- This is particularly ommended for all important when coatings. The depth cutting parts with of impregnation var- ceramic coatings. ies with the degree Even if the coat- of open porosity and ing is ceramic, it interconnections be- constitutes only a tween the pores. Very small percentage of the total cross section- porous coatings can be easier impregnated e) Same coating as in d) polished with 3 µm al area and does not need to be cut with a than denser ones, and coatings with less diamond cut-off wheel. Usually sectioning than 10% porosity can not be impreg- is possible with a soft nated successfully. As it can be difficult to wheel. If the ceramic coating is very thick distinguish voids filled with transparent and dense a resin-bonded diamond cut-off or translucent mounting resins from the wheel can be used as an alternative. structural elements of the coating, it helps to mix a fluorescent 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 filter and a short pass orange very soft coatings from being damaged. filter in the microscope, the fluorescent dye f) Same coating as in e) after final polish Standard preparation method for thermal spray coatings

Grinding

Step PG FG

Surface Foil/Paper MD-Largo

Type SiC Diamond Fig.9: Damage to ceramic spray coating 200x Abrasive due to hot compression mounting Size #220 9 μm

examined as a whole, plane grinding with a Suspension/ Water DiaPro stone may be preferred as it is faster. Lubricant Allegro/Largo 9* Abb. 9: Beschädigung einer keramischen 200x Spritzschicht durch Heißeinbetten Whichever method is use, one must always be aware that the first preparation step rpm 300 150 should aim to re- move any cracks that Force [N]/ 30 30 specimen Fig. 9 : endommagement d'un revêtement 200x arise from cutting par projection de céramique dû à un enrobage à chaud avec compression without introducing new damage from Time (min) Until plane 5 coarse grinding. Fig.10: Same coating as in Fig. 9, cold mounted 200x Polishing To retain flatness and assure a good mate-

Fig. 9: Daño en recubrimiento con spray cerámico 200x rial removal rate, fine grinding is preferably Step DP 1 DP 2 ** debido a embutición en caliente done with diamond on a composite fine Abb. 10: Dieselbe Beschichtung 200x grinding disc. For ceramic coatings the fine Surface MD-Dac MD-Nap wie in Abb. 9, in Kalteinbettung grinding disc MD-Allegro is recommended, and for metal coatings MD-Largo. A tho- Type Diamond Diamond Abrasive rough polishing on a silk cloth (MD-Dur or Size 3 μm 1 μm 图 9：由于热压缩镶样对陶瓷喷涂层造成的损坏 200x MD-Dac) will retain the flatness of the sam- ple and guarantee the removal of smeared Suspension/ DiaPro DiaPro Lubricant Dac 3* Nap B 1* metal. Fig. 10 : même revêtement que fig. 9, 200x Fig.11:enrobé àWC/Co froid plasma spray coating in bright field rpm 150 150 Metal coatings can be fine polished either with 1 µm diamond or a colloidal silica Force [N]/ 30 20 (OP-U NonDry) on a soft cloth. It is not specimen recommended to use the colloidal silica suspension OP-S NonDry for polishing Time (min) 5 1 Fig. 10: Mismo recubrimiento que en la Fig. 9; 200x metal spray coatings as it creates too much embutición en frío relief. However, OP-S NonDry is suitable Valid for 6 mounted samples, 30 mm diam. clamped for the final polishing of ceramic coatings in a holder. as it gives a good contrast to the structure. Remarks: *Alternatively DiaPro diamond suspension can be replaced by DP-Suspension, P, 9 µm, 3 µm and 1 µm respectively, In the trial stage for establishing prepara- applied with blue lubricant. Fig.12: Same coating as in Fig.11 in fluorescent light tion methods both silicon carbide and **Alternatively, this diamond polishing step can be diamond grinding can be tried to find out replaced by a polishing step with colloidal silica (OP-U NonDry will colour those voids yellow which have which is the more suitable plane grinding for metal, OP-S NonDry for ceramic coatings) for 30-60 sec. been图 10：与图 filled 9 相同的涂层，冷镶嵌with resin by the impregnation 200x method. The same applies to the final 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 fluorescent. 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 finest equipment it is possible to control prepara- applied with blue lubricant. possible silicon carbide foil/paper to avoid tion parameters, which guarantees consist- creating artificial porosity by fracturing brit- ent results and excellent reproducibility. By Etching: In general, etchants that are rec- tle particles. Exceptions can be very dense keeping the preparation conditions con- ommended for a specific material can also or thick ceramic coatings, which are plane stant, it can then be assumed that sudden be used for spray coatings of this material. ground more efficiently 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 reflect 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. Grinding

Step PG FG Struers ApS Pederstrupvej 84 DK-2750 Ballerup, Denmark Phone +45 44 600 800 Fax +45 44 600 801 Thermally sprayed [email protected] acetabular cup shell www.struers.com

DiaPro Coatings sprayed in a controlled atmos- Allegro/Largo 9* AUSTRALIA & NEW ZEALAND NETHERLANDS phere have few or no oxides and it is dif- Struers Australia Struers GmbH Nederland 27 Mayneview Street Zomerdijk 34 A ficult to recognize the coating structure. Milton QLD 4064 3143 CT Maassluis Australia Telefoon +31 (10) 599 7209 Therefore these types of coatings need to Phone +61 7 3512 9600 Fax +31 (10) 5997201 be contrasted with chemical etching. Fax +61 7 3369 8200 [email protected] [email protected] NORWAY BELGIUM (Wallonie) Struers ApS, Norge Vacuum sprayed coatings on nickel and Struers S.A.S. Sjøskogenveien 44C 370, rue du Marché Rollay 1407 Vinterbro based superalloys can be etched F- 94507 Champigny Telefon +47 970 94 285 sur Marne Cedex [email protected] with the same solutions used for the sub- Téléphone +33 1 5509 1430 Télécopie +33 1 5509 1449 AUSTRIA strate, or electrolytically with 10% aqueous SEM photomicrograph of thermally sprayed surface [email protected] Struers GmbH of acetabular cup shell Zweigniederlassung Österreich Polishing oxalic acid. BELGIUM (Flanders) Betriebsgebiet Puch Nord 8 Struers GmbH Nederland 5412 Puch The structure of coatings containing mo- Zomerdijk 34 A Telefon +43 6245 70567 3143 CT Maassluis Fax +43 6245 70567-78 lybdenum can be revealed by using the and that the diamond polish needs to be Telefoon +31 (10) 599 7209 [email protected] Fax +31 (10) 5997201 following etchant: long enough to reveal the true porosity. [email protected] POLAND Struers Sp. z o.o. 50 ml water CANADA Oddział w Polsce Struers Ltd. ul. Jasnogórska 44 50 ml peroxide (3%) The recommended preparation procedure 7275 West Credit Avenue 31-358 Kraków Mississauga, Ontario L5N 5M9 Phone +48 12 661 20 60 50 ml ammonia is based on experience and gives excel- Phone +1 905-814-8855 Fax +48 12 626 01 46 Fax +1 905-814-1440 [email protected] lent results for the majority of common [email protected] Caution: Always follow the recommended ROMANIA thermal spray coatings. However, it should CHINA Struers GmbH, Sucursala DiaPro safety precautions when working with be noted that for some specific proprietary Struers Ltd. Bucuresti Nap B 1* No. 1696 Zhang Heng Road Str. Preciziei nr. 6R chemical reagents. coatings the polishing times may need to Zhang Jiang Hi-Tech Park 062203 sector 6, Bucuresti Shanghai 201203, P.R. China Phone +40 (31) 101 9548 be adjusted. Phone +86 (21) 6035 3900 Fax +40 (31) 101 9549 Fax +86 (21) 6035 3999 [email protected] Summary [email protected] SWITZERLAND Thermal spray coatings are widely used to CZECH REPUBLIC & SLOVAKIA Struers GmbH Application Notes Struers GmbH Organizační složka Zweigniederlassung Schweiz give or improve a specific surface quality Metallographic preparation of thermal spray coatings vědeckotechnický park Weissenbrunnenstraße 41 Přílepská 1920, CH-8903 Birmensdorf or function to a workpiece. Different spray- Elisabeth Weidmann, Anne Guesnier, Struers A/S, CZ-252 63 Roztoky u Prahy Telefon +41 44 777 63 07 Copenhagen, Denmark Phone +420 233 312 625 Fax +41 44 777 63 09 ing methods result in different character- Fax +420 233 312 640 [email protected] Brigitte Duclos, Struers S.A.S., Champigny, France [email protected] istics of the coatings, and they are mainly [email protected] SINGAPORE Acknowledgement Struers Singapore applied for corrosion, heat and wear resist- We wish to thank Sulzer Metco AG, Wohlen, Switzerland, GERMANY 627A Aljunied Road, for its cooperation and supplying information material. Struers GmbH #07-08 BizTech Centre ance. Metallographic examination of spray Carl-Friedrich-Benz-Straße 5 Singapore 389842 coatings includes estimation of porosity, Special thanks go to J. Hochstrasser and P. Ambühl for D- 47877 Willich Phone +65 6299 2268 sharing their extensive knowledge with us and supplying Telefon +49 (0) 2154 486-0 Fax +65 6299 2661 oxides and unmelted particles as well as Fax +49 (0) 2154 486-222 [email protected] the following images for reproduction: photo of spraying [email protected] adhesion to the substrate. Because incor- process and large micrograph on page 1; drawing: SPAIN Principle of particle movement, photo synchronising FRANCE Struers España rect grinding and polishing procedures can Struers S.A.S. Camino Cerro de los Gamos 1 rings and micrographs on page 2; drawing, photo 370, rue du Marché Rollay Building 1 - Pozuelo de Alarcón influence the evaluation of the true porosity combustion chamber and all micrographs on page 3 and F-94507 Champigny CP 28224 Madrid micrograph of nickel flame sprayed coating on page 4. sur Marne Cedex Teléfono +34 917 901 204 it is very important that metallographic Téléphone +33 1 5509 1430 Fax +34 917 901 112 We thank Richard Compton, Zimmer, Inc. USA, for Télécopie +33 1 5509 1449 [email protected] preparation is carried out systemati- the photo of the acetabular cup shell and the SEM [email protected] FINLAND cally and that the results are reproducible. photomicrograph on page 6. HUNGARY Struers ApS, Suomi Struers GmbH Hietalahdenranta 13 Precision cutting with the correct cut-off Bibliography Magyarországi Fióktelepe 00180 Helsinki Metallographic preparation of thermally sprayed 2040 Budaörs Puhelin +358 (0)207 919 430 wheel is recommended to avoid cracks in orthopaedic devices, Richard C. Compton, Zimmer, Inc., Szabadság utca 117 Faksi +358 (0)207 919 431 Phone +36 2380 6090 [email protected] the coating. Mounting should follow with USA, Structure 28, 1995 Fax +36 2380 6091 epoxy. Coarse grinding introduces the Summary Report of the Plasma Spray Coatings Email: [email protected] SWEDEN Symposium at Struers, Copenhagen, May 25th to 27th, Struers Sverige IRELAND Box 20038 most damage to the coating and should 1988 Struers Ltd. 161 02 Bromma Universal metallographic procedure for thermal spray Unit 11 Evolution@ AMP Telefon +46 (0)8 447 53 90 therefore be carried out with the finest grit Whittle Way, Catcliffe Telefax +46 (0)8 447 53 99 coatings, S. D. Glancy, Structure 29, 1996 Rotherham S60 5BL [email protected] possible. To avoid relief fine grinding with Materialographic characterization of modern multilayer Tel. +44 0845 604 6664 diamond on a rigid disc is recommended, coating systems used for hot-gas components in large Fax +44 0845 604 6651 UNITED KINGDOM [email protected] Struers Ltd. followed by a thorough diamond polish on gas turbines for static power generation, A. Neidel, Unit 11 Evolution @ AMP S. Riesenbeck, T. Ulrich, J. Völker, Chunming Yao, ITALY Whittle Way, Catcliffe a silk cloth. Siemens Power Generation, Berlin, Structure 2/2004 Struers Italia Rotherham S60 5BL Via Monte Grappa 80/4 Tel. +44 0845 604 6664 20020 Arese (MI) Fax +44 0845 604 6651 It is particularly important to be aware that Tel. +39-02/38236281 [email protected] Fax +39-02/38236274 metal coatings behave differently to cera- [email protected] USA Struers Inc. mic coatings under mechanical abrasion JAPAN 24766 Detroit Road Marumoto Struers K.K Westlake, OH 44145-1598 Takanawa Muse Bldg. 1F Phone +1 440 871 0071 3-14-13 Higashi-Gotanda, Fax +1 440 871 8188 Shinagawa [email protected] Tokyo 141-0022 Japan Phone +81 3 5488 6207 Fax +81 3 5488 6237 [email protected] 28.02.2019 / 62142005