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Application Notes Metallographic Preparation of Thermal Spray Coatings

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Application Notes Metallographic Preparation of Thermal Spray Coatings 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.
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