MICROSTRUCTURAL ASPECTS RELATED TO THE QUALITY OF THERMALLY SPRAYED CERAMIC COATINGS

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1 MICROSTRUCTURAL ASPECTS RELATED TO THE QUALITY OF THERMALLY SPRAYED CERAMIC COATINGS C. Zografou, G. Dhupia, W. KrÖnert, M. Protogerakis To cite this version: C. Zografou, G. Dhupia, W. KrÖnert, M. Protogerakis. MICROSTRUCTURAL ASPECTS RELATED TO THE QUALITY OF THERMALLY SPRAYED CERAMIC COATINGS. Journal de Physique Colloques, 1986, 47 (C1), pp.c1-171-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1986 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque Cl, supplément au n 2. Tome 47, février 1986 page ci-171 MICROSTRUCTURAL ASPECTS RELATED TO THE QUALITY OF THERMALLY SPRAYED CERAMIC COATINGS C. ZOGRAFOU, G.S. DHUPIA, W. KRONERT and M. PROTOGERAKIS* instltut fiir Gesteinshiitterikunde der RWTH, Mauerstrasse 5, D-5100 Aachen, F.R.G. 'Bayer AG, D-5090 Leverkusen, F.R.G. Résumé: Les paramètres du procédé ainsi que les matériaux employés sont déterminants pour la qualité des revêtements obtenus par projection thermique appliquée à* la protection contre la corrosion et l'usure des pièces métalliques. Pour obtenir les critères de classification concernant le pouvoir adhérent, on a étudié la structure du plasma et de la flamme. Les matériaux étudiés se composent d'oxydes, de carbones et d'alliages Ni-Cr. Abstract: The quality of thermally sprayed coatings for corrosion and wear protection of metallic components depends on the process parameters and of course on the coating materials employed. The microstructure of plasma and flame sprayed ceramic and alloy coatings is investigated to obtain assessment criteria for coating structure, adhesive bonding, denseness and hardness. The investigated materials consisted of oxides, carbides and Ni-Cr alloys. Introducti on: Within the broad field of surface technology, thermal spraying techniques are becoming increasingly important for the protection of metallic components against wear and corrosion. In the past years a large number of ceramic and alloy based materials have become available. They are generally deposited through plasma and flame spraying techniques for the protection of ceramic as well as metallic substrates of highly stressed large-scale components /1-3/. The scope of this study was to investigate the suitability of a series of complex ceramic and alloy materials coated under industrial conditions on steel work-pieces (St 37). Experimental Conditions: The following materials^ were coated by the flame technique: (1)67 Ni/16 Cr and (2)50 WC/33 Ni/9 Cr with subsequent densification (1050 ) and the following plasma sprayed (weight fractions): (3)83 WC/12 Co, (4)94 Al-0,/2,5 Ti0 9, (5)60 Al? 0./40 Ti0, (6)96 Cr 0 3 /2 Ti0 2, (7)80 IrOy/ZOYio,, (8)90 Ni/5 At/5 Mo The chemical, mineralogfcal and microstructural characterisation of the spraying materials in powder form was done through XRD, SEM and XRF. A mixture of acetylene and oxygen was used for flame spraying and a mixture of argon and hydrogen for plasma spraying. The spraying Metco Inc. N.Y.: (1) 16 C, (2) 34 F, (3) 71 VF, (4) 83/20, (5) 131 VF, (6) 106 NS, (7) 202 NS, (8) 447 Article published online by EDP Sciences and available at

3 cl-172 JOURNAL DE PHYSIQUE distance varied between mm and the spraying rate between kg/h (500 A175 V). The phase compositions of the deposits were determined by XRD. Before spraying the roughness profiles of the substrates were determined after the work-pieces had been roughened under a sand-stream. In some cases an intermediate Ni-based adhesive layer was applied to the substrate before spraying. The following physical properties of the composites were measured: microhardness (vickers, brinell, rockwell), adhesion tensile strength (DIN 50160), modulus of rupture and wear resistance. Chemical corrosion tests are presently being carried out. These physical properties were correlated to extended microstructural studies of the coatings. Results: Due to the limitations imposed by the publishers, only some selected properties and microstructural examples are discussed here. XRD analyses of the deposited materials show a quite different phase composition, except for material 6, as compared to as-recei ved powders. The powders can be divided into two morphological groups: loose splintery, compact grains and spray-dried granules with varying microporosity: Spray-dried, dense granules with a narrow granule size distribution, similar granules however partly h01 low with a wide range of granule size distribution, whereby the average granule size is much smaller. Another example of* spray-dried granules consists of compact, dense granules of uniform size with coarse primary particles. The microstructure of the second group exhibits irregular, almost dense grains of varying sizes with very smooth surfaces. The purpose of microstructural investigations was to ascertain the crystalline composition of the sprayed layer, layer thickness phase composition and denseness, mechanism of bonding with the substrate. The thickness of coatings varied from 400 to 500 microns. The following micrographs show an interesting cross-section of the wide spectrum of microstructural formation. In fig. 1 a,b the matrix of powder 2 consists of irregular WC grains imbedded in Ni-metal and small amounts of CO W C. From the x-ray maps in fig. 1 b it is evident that due to diffusqofi of Fe into the sprayed layer and W into the substrate, an intermediate diffusion layer is created resulting in an excel lent metal1 urgical bond. The adhesion tensile strength of this specimen was so good that it was not measurable according to DIN (adhesive: HTK Ultra bond 100). Fig. la/b: Polished cross section micrograph and corresponding digital EDX maps of composite with 50 WC133 Ni powder

4 The surface of the sprayed layer (fig. 2) (powder 5) contains rounded alumina aggregates with aluminium titanate as the second phase. Some microporosity and microcracks are evident. The cross-section shows a pronounced lamellar structure (fig. 3) with a sandwiched texture consisting of aluminium titanate with varying amounts of titania exsolutions and alumina inclusions. An intermediate Ni -rich adhesive layer is coated on the substrate in order to enhance adhesive properties (fig. 4 a,b). Fig. 2: SEM migrograph of the surface of coating with 60 A1203/ 40 TiO powder Fig. 3: Cross gection micrograph (reflected light) of coating with 60 A1 0 / 40 TiO powder Fig. 4a/b: p01 is6ed cross sgction micrograph and corresponding digital EDX maps of composite with 60 A1203/ 40 Ti02 powder The pronounced lamellar layered structure of the monophase coating with powder 6 is evident in fig. 5 a. The large vertical cracks are due to specimen preparation of the brittle coating and are not inherent to this composite. Here the Ni-rich layer (Fig. 5 b) results in a satisfactory interface between the coating and the substrate leading to very good wear resistance and good adhesive tensile strength. For powder 7, in spite of the intermediate Ni-rich layer, the sprayed coating has the lowest wear resistance and adhesive tensile strength. This is caused by the strong lamellar structure of the dense Y-stabi l i zed zi rconia deposits separated by large elongated pores parallel to the substrate surface (fig. 6 a,b).

5 JOURNAL DE PHYSIQUE Fig. 5aJb: Polished cross section micrograph and corresponding digital EDX maps of composite with 96 Cr203/ 2 TiOp powder Fig. 6a/b: Polished cross section micrograph and corresponding digital EDX maps of composite with 80 Zr02/ 20 Y203 powder The comparison of microhardness tests results in the division of the investigated powders into 3 groups. The flame sprayed powders exhibit the highest microhardness ( HRA), followed by the oxides and carbides except zirconia ( HRA). Zirconia and Ni-alloys (powder 8) had the lowest hardness values of 42.1 and The wear tests were carried out on a ~abler-abraser. The abrasion rate was measured as a function of the number of revolutions (fig. 7). Here too the powders can be divided into 3 groups. The flame sprayed specimens together with chromia show the highest wear resistance. The second group comprises of WCI aluminium titanate coatings and the Ni/AI/Mo composition. The highest wear is shown by the alumina and zirconia coatings. No correlation was found between microhardness and wear resistance of the coatings, The adhesive tensile strengths of all plasma sprayed materials exhibit similar values (fig. 8) of about 40 N/mm2. Zirconia is here an exception with a comparitively very low value. The values for the flame sprayed coatings were higher than the adhesive used so that no valuescould be measured. In comparison to plasma sprayed coatings where the

6 adherence is based on adhesion and clasping forces, the flame sprayed materials undergo an intimate metallurgical bond with the substrate resulting from diffusion and direct bonding. g. 7 Ioo Revolutions + 7O00 Fig Sample-Nr. Fig. 7: Fig. 8: Adksion resistance test of composites Adhesion tensile strengths of composites Conclusions: It is evident that the flame sprayed coatings do not exhibit the characteristic lamellar structure of thermally sprayed coatings due to subsequent thermal densification. These specimens have very low porosity resulting in an increase in density and hardness. All other specimens exhibit the typical lamellar structure due to the absence of subsequent thermal densification. Thi S has a detrimental effect on the physical and chemical~properties of these coatings that would exhibit much higher values as compact materials. This is particularly true for the zirconia composite, where the very high melting point of the particles leads to an unsatisfactory coalescence between the individual grains. Generally, the properties of the plasma sprayed composites are much lower those of the flame sprayed material. However, it must be considered that the investigations of the physical properties were carried out at room temperature. At higher temperatures the inherent properties of ceramic materials should result in improved performance of plasma sprayed composites. References: 1. "Wear Protection through Surface Coatings", German Engineers Assoc. (VDI) Publication no,333,stuttgart 1979, VDI-Publ., Dusseldorf Thermal Spraying Technology 1977, Congress of the German Welding Soc. (DVS), DVS-Pub1., D usseldorf th International Thermal Spraying Conference, May 1983, Essen, DVS-Pub1., D usseldorf 1983