Cr 3 C 2 -NiCr HVOF-Sprayed Coatings: Microstructure and Properties Versus Powder Characteristics and Process Parameters
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Maria Prudenziati, Gian Carlo Gazzadi, Marcello Medici, Gregorio Dalbagni, and Marco Caliari (Submitted February 18, 2009; in revised form September 24, 2009) Two 75%Cr3C2-25%NiCr feedstock powders with the same size distribution but different production process were characterized and found quite different in terms of morphology and phase composition. The powders were sprayed in a HVOF Diamond Jet (Sulzer Metco DJ-2600) torch with five different values of the oxygen-to-hydrogen ratio in order to assess the influence of this parameter on the microstructure and properties of the coatings. The results show that the closed and dense microstructure of one powder (Woka 7302) results in coatings with lower amount of decarburization, less oxide formation and higher toughness compared to coatings from the other powder (Praxair 1375). It was found that the O2/ H2 ratio impacts mainly on the YoungÕs modulus, which almost doubled by changing the ratio from 0.40 to 0.50, and on toughness, but does not notably affect the Vickers hardness.
Keywords
carbon loss, hardness, microstructure of powders and coatings, oxidation, phase composition of powders and coatings, toughness, YoungÕs modulus
1. Introduction Thermal sprayed carbide coatings, both WC-Co and Cr3C2-NiCr cermets, are extensively used for wear- and corrosion-resistant applications (Ref 1-10). Coatings of the WC-Co system generally have a high hardness and wear resistance (Ref 8, 11). However, the decarburization of WC, primarily into W2C, leads to degradation of properties and limits the application of the coatings to temperatures in the range of 500 C (Ref 5). On the other hand, Cr3C2-NiCr can be used in corrosive environment at service temperatures up to 800 C; its hardness decreases by increasing the temperature above 600 C but exhibits good erosion resistance up to 800 C (Ref 12). Hence, Cr3C2NiCr coatings are ideally suited as protective layers in corrosive environment at elevated temperatures (Ref 7); coatings on steam turbine blades and boiler tubes against erosive wear and corrosion attack are just only a few
Maria Prudenziati, Department of Physics, University di Modena and Reggio Emilia, Via G. Campi 213/A, I-41100 Modena, Italy; Gian Carlo Gazzadi, CNR-INFM S3 National Research Center on nanoStructures and bioSystems at Surfaces, Via Campi 213/A, I-41100 Modena, Italy; and Marcello Medici, Gregorio Dalbagni, and Marco Caliari, Zocca Officine Meccaniche S.r.l., Via Agucchi 41-45, Funo di Argelato, Bologna, Italy. Contact e-mail: maria. [email protected].
Journal of Thermal Spray Technology
examples of applications relying on these properties (Ref 11-13). Extensive research on these carbide cermets has contributed to processing and microstructure characterization as well as to the evaluation of corrosion resistance, mechanical and tribological properties, including effects of oxidation, decarburization and phase transitions. Correlations were reported between coating properties and fuel type (Ref 2), thermal spray method as for example detonation-gu
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