MODIFICATION OF THE STRUCTURE AND PROPERTIES OF THERMALLY SPRAYED COATINGS BY VARIOUS POST-PROCESSING METHODS

MODIFICATION OF THE STRUCTURE AND PROPERTIES OF THERMALLY SPRAYED COATINGS BY VARIOUS POST-PROCESSING METHODS Petri Vuoristo Tampere University of Technology, Institute of Materials Science, Tampere, Finland Abstract Post-processing methods available to modify and improve the structure and properties of thermally sprayed metallic and ceramic coatings are described. Sealing of the porosity by inorganic sealants and modification of the coating structure by laser remelting are effective techniques to improve the properties of the coatings. 1. INTRODUCTION Thermal spraying is a well established technique in manufacturing protective and functional coatings for a wide range of industrial applications. Thermally sprayed coatings are used f.ex. in gast turbines, boiler plants, car engines, paper machines, process valves and pumps, seals and sleeves, textile machines, various machine parts and in steel structures of bridges. The large variety of coating materials and spray methods available make thermal spraying attractive for large scale industrial use. Thermal spray processes available are flame spraying (FS) with powder or wire, wire arc spraying (AS), plasma spraying in ambient atmosphere (APS) or in vacuum (VPS), detonation gun spraying (DGS) and high-velocity oxyfuel spraying (HVOF). The materials are either in wire or powder form, and can be pure metals and alloys, hardmetals, ceramics or polymers. It is well known that the structure and properties of thermally sprayed coatings usually differ from those of corresponding wrought or cast materials. This is caused by the specific coating formation mechanism, which causes the presence of various levels of residual porosity, cracks, interlamellar voids, oriented microstucture and chemical inhomogeneity in the sprayed coatings. In most cases, these voids are undesirable and are tried to be avoided by means of selection of appropriate spray technique, optimisation of the deposition process and the properties of the material to be sprayed. Although thermally sprayed coatings are frequently used in their as-sprayed condition, or after finishing by machining or grinding, there remains often needs to carry our various post-treatment procedures before the use of the coatings. These post-treatments are typically different heattreatments, mechanical treatments or filling the residual porosity with sealants. Main objects are to improve mechanical properties or to provide corrosion protection of the substrate and the coating material. Various post-treatments have also been studied extensively during the last years at Tampere University of Technology, and this presentation is a short review of this work. Some topics of post-treatments, including sealing of porosity of plasma sprayed ceramic coatings, and treating the sprayed coatings by means high power laser methods are described in more detail. 2. MICROSTRUCTURE OF THERMALLY SPRAYED COATINGS Thermally sprayed coatings are formed from flattened solidified melt droplets. In the spray process, the molten droplets from the feedstock material are formed by heat and energy from a combustion gas, thermal plasma or electric arc. Then the molten particles are projected towards the surface to be coated by means of a fast flowing gas, typically by the combustion gas, plasma gas stream, or by a separate propelling gas. Upon impacting on the surface, the molten particles spread and solidify rapidly forming a coating with specific features and voids schematically presented in Fig. 1. The structure and properties of thermally sprayed coatings are highly dependent on the particular spray process used and the coating material. In many cases, the microstructure, porosity, hardness and bond strength of the coatings are mainly controlled by the flame temperature and velocity of melted particle which together determine the thermal and kinetic energy of the particle,

respectively. The porosity in thermally sprayed coatings can be classified in three different cathegories, i.e. macropores, micropores and cracks. Macropores can be caused by inadequate placement and spreading of the sprayed droplets, or they are pores caused by interaction of material particles with the gaseous medium and pores caused by the splashing of particles after impacting substrate or previously deposited layer. Typical size of macropores is 0.5-10 µm. Micropores, size less than 0.5 µm, can result from dendritic crystallisation of sprayed particles or, quite often, they are cause by non-bonded areas between boundaries of individual lamels. Cracks can also be classified to macrocracks and microcracks. Cracks are mainly formed due to residual stresses in ceramic and other hard coatings. The connected porosity from the coating surface to the substrate is mainly caused by micropores between solidified particles (non-bonded areas) in metallic coatings and by vertical cracks going through the flattened particles in the case of ceramic coatings. Coating through-porosity is the main concern of substrate corrosion under a plasma sprayed ceramic coating. When easily corroding materials, e.g. mild steels, are used as substrates for plasma sprayed ceramic coatings, corrosion protection is usually provided by deposition of a metallic bond coating with adequate corrosion resistance, and by preventing the corrosive media to penetrate into the pores by various sealants. Figure 2 presents a typical microstructure of plasma sprayed ceramic coating; Fig 2a is a micrograph taken by optical microscope and Fig 2b a scanning electron micrograph showing the lamellar coating structure. Fig. 1. Schematic presentation of the structure of a thermally sprayed coating showing typical features and voids /1/. Fig. 2. a) b) Microstructure of plasma sprayed Al 2 O 3 coating: a) optical micrograph, b) SEM image.

In metallic coatings, the through-porosity and chemical inhomogeneity both can cause coating destruction in conditions where corresponding bulk material may work without problems. This is usually a consequence of the use of spray process with low particle velocity, e.g. flame, arc and plasma spraying, and/or partial oxidation of the sprayed particles in-flight. Figure 3a shows the structure of a plasma sprayed stainless steel coating, in which typical structural features, e.g. porosity, oxide lamels, non-bonded interfaces and partially oxidised particles are present in the coating. Such coatings possess usually limited corrosion resistance due to porosity and inhomogeneous structure, e.g. oxidised metal and low-chromium content areas near the oxidised areas. Selective dissolution of the phases poor in chromium present in such coatings is usually the main cause of the coating corrosion. Coatings sprayed using processes in which the flame temperatures are lower and the particle velocity is high, e.g. in HVOF process, the amount of oxidation can be kept at significantly lower level. HVOF sprayed coatings are also denser than plasma sprayed coatings. Figure 3b shows an example of HVOF sprayed metallic coating, in this case the coating material is NiCrMo alloy Inconel 625. The level of oxidation is very low, but oxide strips are still present in the coating, as can be seen from the SEM micrograph. a) b) Fig. 3. Microstructures of thermally sprayed metallic coatings; a) plasma sprayed stainless steel (Fe-20Cr-18Ni-6.2Mo, SMO 254) /2/, b) HVOF sprayed nickel based alloy coating (Ni- 22Cr-9Mo, Inconel 625) /3/. 3. METHODS TO IMPROVE COATING STRUCTURES AND PROPERTIES It is evident that from such coatings as shown above having various levels of porosity and inhomogeneity can not possess equal properties with corresponding wrought or cast materials. These drawbacks found in thermally sprayed coatings have led to several studies and research in which coating structures and properties have been tried to be modified by means of various posttreatment. Often the aim has been to improve the corrosion resistance of the coating. This has been done either by closing or impregnating the pores of thermally sprayed coatings by various sealants, or by remelting the coating f.ex. by laser beams. In the next, these techniques and achievements will be described. 3.1 Remelting metallic coatings by high power laser beams Remelting of HVOF sprayed NiCrMo coatings by high power Nd:YAG lasers has been subject to studies recently at Tampere University of Technology /3/. Laser surface post-treatment can offer a controlled way to improve the properties of thermally sprayed coatings without changing the substrate properties. The laser devices traditionally employed for surface treatments have been CO 2 laser, because of their high power range (up to 25-45 kw). In recent years, the development in neodymium-doped yttrium aluminium garnet (Nd-YAG) laser technology has resulted in continuous wave multi-kilowatt (up to 5 kw) high power devices, which have certain advantages

compared to CO 2 lasers. Due to shorter wavelength of radiation (1.06 µm vs. 10.6 µm) laser beam can be transmitted through optical fiber, which offers great opportunities to utilize industrial robots and treat components remote from the laser source. Moreover, most of the metals absorb shorter wavelength radiation more efficiently enabling together with high power the use of wide beam optics. Metallurgical properties of the laser remelted coating using wide beam width (10 mm) and HVOF sprayed coating were examined. In addition, corrosion resistance in 3.5 wt.% NaCl of the remelted and HVOF as-sprayed coatings was studied and compared to the wrought Inconel 625. Inconel 625 coatings were produced by using Diamond Jet Hybrid 2700 high-velocity oxy-fuel (HVOF) spray process. The spray powder was commercial Inconel 625 (Ni-Cr-Mo) superalloy (-45 µm). The homogeneity of the HVOF sprayed coating quality and the uniform thickness of the coating are features, which favour the subsequent laser remelting. The substrate material was unalloyed low carbon steel Fe 37. The thickness of the obtained coatings was approximately 300 µm. The remelting experiments of the sprayed coatings were conducted with a 4 kw continuous wave optical fiber coupled HAAS HL 4006 D Nd-YAG laser. The processing end of the fiber was equipped with hardening optics, which consists of a collimator and an integrating mirror having an effective focal length of 100 mm. The rectangular shape of the delivering beam was defocused to a spot size of 10 mm x 8 mm enabling the melt widths of 10 mm. Coating samples were mounted on a moving numerically controlled x-y table below the fixed laser processing head. To remelt the whole surface area of the HVOF sprayed coating, a series of overlapping passes were made using different levels of laser power (W) and scanning speed (mm/min). In the corrosion tests the samples were exposed to 3.5 wt.% NaCl solution at room temperature for 7 days. At the same time open circuit potential of the coating/substrate system was measured with respect to the Ag/AgCl reference electrode using high resistance voltmeter. Cyclic potentiodynamic polarization measurements were performed in 3.5 wt.% NaCl electrolyte at room temperature. The melt depth of the samples was noted to increase from 260 µm to 840 µm when the specific energy absorbed by the surface was increased by slowing down the traverse speed or raising the power, Figure 4. When the thickness of the sprayed coating was measured to be approximately 300 µm before laser treatment, it can be said that samples treated with parameters 2250 W, 600 mm/min and 2000 W, 500 mm/min showed the melt depths close to the original thickness of the coating. Laser parameters with higher specific energy produced melt depths markedly higher than the thickness of 300 µm indicating high dilution and mixing between coating and substrate. The microstructures of the assprayed and laser remelted coatings are shown in Figures 5 a and 5b. The layered microstructure in this HVOF sprayed coating is clearly seen. Layers of oxides at intersplat boundaries as seen in Figure 5a are typical for metallic coatings. The coatings obtained using laser remelting are metallurgically dense, free of cracks and porosity as seen in Figure 5b. It can be seen that the metallurgical bond between coating and substrate was formed indicating better adherence than in the case of as-sprayed coating. Inconel 625 Depth (um) 900 800 700 600 500 400 300 200 100 0 22.5 24 27 30 33.8 Specific energy (J/mm 2 ) Fig. 4. Melt depth of the laser remelted HVOF Inconel 625coatings as a function of specific energy. Original coating thickness was 300 µm.

a) b) Fig. 5. Microstructure of a) as-sprayed HVOF Inconel 625 coatings and b) after remelting with 4 kw Nd:YAG laser. Coating thickness is 300 µm. In the case of HVOF sprayed coating the open circuit potential value vs. Ag/AgCl reference electrode started to change to more negative direction immediately after exposure to the 3.5 wt.% NaCl solution, indicating active corrosion behaviour, as is presented in Fig. 6. At the same time it was noted that after a couple of hours from the beginning of the test, first corrosion products appeared more or less evenly on the whole surface of the exposed coating. Perhaps due to interconnected pores, splat boundaries and microcracks characteristic for sprayed coatings the electrolyte reached fast the substrate and corroded it. It seems that the deterioration of coating has initiated along splat boundaries and corroded the material nearby. E (mv) Ag/AgCl 0-100 -200-300 -400-500 -600-700 -800 Laser(2250/600) Laser(2250/500) Wrought PTA Laser(2250/400) Laser(2500/500) Laser(2000/500) HVOF Fe37(gritblasted) 0 50 100 150 200 Time (h) Fig. 6. Open circuit potential curves of the wrought, PTA (plasma transferred arc welded), laser remelted and HVOF sprayed Inconel 625 alloy. Laser remelted coatings showed another type of behaviour. In the case of 2250/600 (W / mm/min) and 2250/500 coatings, the open circuit potential started to change to more positive direction after exposure to electrolyte indicating passive corrosion behaviour. After one week of immersion the surface of the exposed area was still free of any corrosion products. Open circuit potential was noted to be 700 mv more positive than that of uncoated Fe 37 substrate. Coatings remelted with parameters 2000/500, 2500/500 and 2250/400 allowed the electrolyte to pass the coating through a single interconnected pore causing crevice corrosion-like behaviour. Corrosion products appeared from the substrate through this single pore to the surface after a couple of hours from the beginning of the test. At the same time it was noted that open circuit potential values decreased like in the case of sprayed coating being however 200 mv more positive than values of coating/substrate system.

As a reference, immersion test and OCP measurements were also done with wrought and PTA samples. Wrought Inconel 625 exhibited passivation behaviour identical with the best laser remelted samples (2250/600 and 2250/500) and PTA showed active corrosion behaviour allowing the electrolyte to pass the coating and corrode the substrate in spite of markedly higher coating thickness (2.3 mm) in the PTA sample. Figure 7a shows the cyclic polarization curves for the laser remelted with parameters 2250/600 and HVOF sprayed coatings measured in 3.5 wt.% NaCl solution. The sprayed coating exhibits poorer corrosion resistance in the solution having lower corrosion potential ( 288 mv vs. Ag/AgCl) and higher corrosion current density (1.2 x 10-6 A/cm 2 ) compared to remelted (-197 mv vs. Ag/AgCl, 3.6 x 10-7 A/cm 2 ) coating. Corrosion current densities were estimated from cyclic polarization curves. In addition, HVOF sprayed sample shows a rapid increase in current after narrow passivation area (from 200 mv to 60 mv vs. Ag/AgCl) already at about 60 mv vs. Ag/AgCl, which is breakdown potential. Remelted sample exhibits rapid increase in current at +430 mv vs. Ag/AgCl indicating higher breakdown potential. Remelted sample also shows substantially wider passivation region compared to the as-sprayed sample. Rapid increase in current usually means the breakdown of passivity in the passivation layer on the coating indicating localized corrosion, commonly known as pitting or crevice corrosion. In this case of the remelted and sprayed samples, the rapid increase in current is probably not due to pitting but uniform corrosion of coating material itself in the transpassive region. In addition, both the curves showed negative hysteresis, which does also not support the pitting corrosion. Moreover, the surfaces of the samples were examined visually after the cyclic polarization and both the exposed surfaces proved to be free of corrosion pits. The corrosion performance of the remelted sample is very close to the corresponding wrought material, as is presented in Fig. 7b. Corrosion potential and corrosion current density in the passive region are even slightly better in the case of the remelted sample. The breakdown potential of the remelted sample is obvious at about +430 mv vs. Ag/AgCl and lower than that of the wrought sample for which it is difficult to point out the exact breakdown potential. Perhaps, it is more correct to say that the passivity of wrought sample is maintained until about +600 mv vs. Ag/AgCl. 3.5% NaCl, RT 3.5% NaCl, RT 1.000 1.000 0.500 0.500 LASER(2250/600) LASER(2250/600) E [V] 0.000 HVOF E [V] 0.000 WROUGHT -0.500-0.500-1.000 1.00E-10 1.00E-09 1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02 log I [A/cm2]) log I [A/cm2]) a) b) Fig. 7. Electrochemical polarisation curves of a) HVOF as-sprayed and laser remelted, and b) laser remelted and wrought Inconel 625. -1.000 1.00E-10 1.00E-09 1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02 The corrosion resistance of the coating/substrate system is more or less dominated by the substrate material if interconnected porosity, micropores or microcracks exist in coating material. From the immersion test, open circuit potential and cyclic polarization measurements, it can be said that the corrosion properties of the remelted (2250/600) sample were very close to corresponding wrought material. In immersion test and open circuit potential measurements remelted and wrought samples behaved identically indicating the fact that the influence of substrate was neglected in the case of remelted sample due to uniform, pore- and crack-free coating layer.

It can be concluded from the present study that laser remelting of the sprayed coating improves corrosion performance markedly if appropriate processing parameters are used. Laser parameters (power and traverse speed) generating too high specific energy should be avoided, because of dilution (Fe) from substrate to coating decreasing particularly the pitting corrosion resistance of the coating. However, it seems that it is possible to achieve the excellent performance equivalent to wrought Inconel 625 superalloy with laser remelting of sprayed coating by selection of appropriate laser remelting parameters. 3.2 Sealing of thermally sprayed coatings As was stated earlier, ceramic coatings prepared by plasma spraying consist of a lamellar structure and residual porosity which usually is open and continuos from the top of the surface of the coating down to the coating/substrate interface. This through-porosity can cause substrate corrosion and loss of adhesion, if such coated structures are exposed to wet corrosive environments. A typical method to prevent corrosive liquids from penetration into the coating is the use of various sealants. In this case, the pores are typically filled with a polymeric resin. An extensive study has been reported recently in /4/, but due to space limits will not be reviewed here. In contrast, another topic that has also been subject to intense studies is sealing of plasma sprayed ceramic coatings by using inorganic sealants /5-8/. The sealant materials consisted of aluminum phosphate or orthophosphoric acid. The ceramic coatings sealed were Al 2 O 3, Cr 2 O 3 and ZrO 2. Aluminum phosphate was prepared from aluminum hydroxide Al(OH) 3, orthophophoric acid H 3 PO 4, and purified water. The ratio of thr hydroxide and acid was 1:4.2. The sealant solution was impregnated into the pores of the oxide coating. Next the samples were allowed to dry for 12 hours at room temperature. Then the samples were heated to 200 o C in order to remove the water from pores, which was followed by a curing for 2 hours at 400 o C to form aluminum phosphate AlPO 4 to the coating. Numerous studies including abrasion, erosion, slurry abrasion and slurry erosion, as well as corrosion studies have been done to these sealed coatings. Next only a few results will be presented. Figure 8c is presentation of abrasion wear test results for plasma sprayed, detonation gun sprayed and for alumimum phosphate sealed coatings. These results show the marked improvement which can be received by sealing plasma sprayed alumina coating with aluminum phosphate. c) Fig. 8. Wear test results for APS and DGS alumina coatings as-sprayed and sealed with aluminum phosphate; a) and b) erosion wear test, c) abrasion wear test results /5/.

The aluminum phosphate sealing gives improvement also in the solid particle erosion wear test, although the improvement is not that remarkable as in the abrasion wear resistance. Figure 9 shows the electrochemical polarisation curves of plasma sprayed alumina coating in as-sprayed condition and after sealing with aluminum phosphate inorganic sealant. It is evident that the 2-3 orders of magnitude lower corrosion current arises from significanly lower dissolution rate of the substrate material due to closing of a major amount of the pores. Fig. 9. Electrochemical polarisation curves for plasma sprayed alumina coating in as-sprayed condition (B) and after sealing with aluminum phosphate (A) /5/. LITERATURE 1. A.R. Nicoll, Lecture material, Sulzer Metco A.G., Switzerland. 2. P. Siitonen, T. Kinos, P.O. Kettunen, Corrosion Properties of Stainless Steel Coatings Made by Different Methods of Thermal Spraying. Proc. 7 th Nat. Thermal Spray Conf., 20-24 June, 1994, Boston, MA, p. 105-110. 3. J. Tuominen, P. Vuoristo, T. Mäntylä, M. Kylmälahti, J. Vihinen, P. H. Andersson, Improving Corrosion Properties of HVOF Sprayed Inconel 625 by Using a High Power Continuous Wave Nd-YAG Laser. Journal of Thermal Spray Technology, ASM International, in print. 4. J. Knuuttila, P. Sorsa, T. Mäntylä, Sealing of Thermal Spray Coaitngs by Impregnation. Journal of Thermal Spray Technology, vol. 8(2) June 1999, p. 249-257. 5. K. Niemi, P. Sorsa, P. Vuoristo, T. Mäntylä, Thermally Sprayed Alumina Coatings with Strongly Improved Wear and Corrosion Resistance. Proc. 7 th Nat. Thermal Spray Conf. 20-24 June 1994, Boston, MA, USA, p. 533-536. 6. E. Kumpulainen, M. Vippola, K. Niemi, P. Sorsa, P. Vuoristo, T. Mäntylä, Characteristics of Aluminum Phorsphate Sealed Chromium Oxide Coatings- Proc. 8 th Nat. Thermal Spray Conf. 11-15 Sept. 1995, Houston, Texas, USA, p. 579-582. 7. E. Kumpulainen, M. Vippola, P. Vuoristo, P. Sorsa, T, Mäntylä, Characteristics of Phosphoric Acid Sealed Ceramic Oxide Coatings. Thermal Spray: Practical Solutions for Engineering Problems, ASM International, Materials Park, Ohio, USA, 1996, p. 489-491. 8. J. Knuuttila, S. Ahmaniemi, E. Leivo, P. Sorsa, P. Vuoristo, T. Mäntylä, Weat Abrasion and Slurry Erosion Resistance of Sealed Oxide Coatings. Proc. 15 th Int. Thermal Spray Conf., 25-29 May, Nice, France, p. 145-150.