Electrical Properties of Thermally Sprayed Ni- and Ni20Cr-Based Resistors
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Maria Prudenziati and Magdalena Lassinantti Gualtieri (Submitted September 15, 2007; in revised form November 12, 2007) Five laboratories were asked to deposit Ni and Ni20Cr powders to obtain resistors; we studied their electrical properties in the temperature range 20-500 °C and interpreted the results in the light of their microstructure. Resistors sprayed from Ni powders consist of NiOx ‘‘islands’’ embedded in a Ni matrix. The temperature dependence of resistance (TCR) is in perfect agreement with that of pure Ni, indicating that the matrix determines the electrical transport. Annealing at temperatures from 200 to 400 °C results in an irreversible decrease of resistance. A multiphase microstructure is observed in resistors prepared from 80Ni20Cr powders. The major phase in these resistors is a NiCr alloy but with a Ni:Cr ratio larger than 80:20. Minor amounts of metal oxides are also detected. The TCR in these samples spans from 180 ppm/°C to 2830 ppm/°C, and is attributed to different degree of oxidation and segregation of the metals in the alloy.
Keywords
annealing, electrical conduction, nickel, NiCr, oxidation, thermal spray
1. Introduction Thermal spray processes, such as Atmospheric Plasma Spray (APS) and High Velocity Oxy-Fuel (HVOF), allow the deposition of a wide variety of materials. A powder feedstock is delivered in a high temperature flame. The melted particles are propelled, in a plastic state as well as in a vapor state due to sublimation or evaporation of the original/reacted feedstock powder, by gas streams at very high speeds toward the substrate (Ref 1, 2). Impinging on the substrate, the particles condense and rapidly solidify to form so-called splats. Coatings are formed by the build-up of successive layers of splats giving rise to a lenticular structure. As a result, sprayed materials are characterized by a complex, anisotropic microstructure comprised of elongated brick-like lamellae separated by interlayer pores. In addition, secondary phases (e.g., interlayer or segregated oxides in the case of metallic coatings) and other (intralayer or interlayer) heterogeneities and defects are distributed in a complex way through the coating (Ref 3). The main application of thermal spray technologies has been for the deposition of protective coatings against wear, heat, corrosion, and oxidation. In particular, Ni and its alloys (e.g., Ni20Cr, NiCrAlY, NiCoCrAlY) are widely used, e.g., for reclamation (re-work and repair of damaged
Maria Prudenziati and Magdalena Lassinantti Gualtieri, Dipartimento di Fisica, Universita` di Modena e Reggio Emilia, Via G. Campi 213/A, 41100 Modena, Italy. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
parts) and bond coats. Therefore, a myriad of investigations on the mechanical, tribological, and thermal properties of these coatings can be found in the literature. Recently, the interest is emerged in new applications of thermal spray metals for heaters (Ref 4-8) and sensors (Ref 3, 9, 10). In this perspective, the electrical prop
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