Abstract

Heated surfaces are required in many applications ranging from home scientific equipment to home appliances. Conventional heating solutions mainly include utilization of coiled wire elements that are typically installed in the vicinity of the surfaces that require heating. However, based on the physical location and geometry of these conventional heating elements, they have limited performance and cannot achieve their maximum efficiency. Utilizing thermal spraying techniques is a possible approach that can circumvent some of the shortcomings of previous heating systems. Electrical heating elements can be directly deposited on surfaces as a possible way to overcome geometric limitations and minimize air gaps and thermal contact resistance between the heating system and the surface to be heated.

This thesis investigated the microstructure of various heating systems fabricated by using several thermal spraying techniques, namely, suspension plasma spraying (SPS), flame spraying (FS), high velocity oxy-fuel (HVOF), and air plasma spraying (APS). SPS and FS techniques were utilized to deposit alumina as an intermediary dielectric layer in metal-alumina composite heating element coatings. HVOF and APS processes were used for the fabrication of a nickel-20chromium (Ni-20Cr) metal heating element layer. The resultant microstructure and phase composition of the alumina layers were evaluated by using scanning electron microscope images (SEM), Raman spectroscopy, and X-ray diffraction (XRD) analysis. Dielectric breakdown voltage tests were conducted on selected heating systems and the performance of the alumina layer individually and within the heating system was examined.

The advantages and disadvantages of using each of the thermal spraying processes and the impact of various spraying parameters on the microstructure of the developed systems were studied. In the FS-sprayed alumina, large pores were detected that were randomly distributed in the coatings which led to the penetration of the top heating element layer. Partial melting of the alumina powder particles was observed in the FS alumina samples that led to formation of two different alumina phases in each particle. Dense, cauliflower-like, and porous structures were seen in SPS-sprayed alumina coatings that consequently, resulted in various proportions of the alumina phases in each coating.

It was found that microstructural and phase content characteristics are both influential components in determining the dielectric properties of the sprayed alumina coatings. Dielectric strength of single layered samples (only alumina) was much higher compared to multi-layered samples. The results suggest that the types of alumina phases present in the coatings and microstructural features such as air gaps and interconnected cracks can negatively affect the final electrical properties of the alumina layer in the heating system.