Development and the implementation of high-temperature reliable heaters in plasma spray technology

Many problems have been encountered during development of reliable high-temperature heaters by means of atmospheric plasma spray and procedures commonly adopted in thermal spray technology, especialy due to poor steel substrate corrosion resistance, notably affected by grit-blasting operations, but also deriving from contamination of insulating layers, dielectric arcs, and failures due to hot spots in the heating elements. While seeking the origin of these problems, a close scrutiny of every single step of the preparation process and analyses of the coatings were carried out using laser confocal scanning microscopy, optical and electronic microscopy, fluorescence analysis, X-ray diffraction, and ancillary techniques. The electrical properties of both alumina layers and metal strips prepared with Ni, NiCr, NiAl commercial powders for the heating elements were studied and cross-related to the failures in the heaters. The article reports the main results of these investigations, delineates the innovations introduced to overcome or circumvent the problems, and underlines the distinct characteristics of new heaters, whose reliability has been proven up to now with temperatures of up to 600 °C in air.