Surface oxidation, crystallization, wetting, and magnetic properties of FeNi-based metal amorphous nanocomposite

 FeNi-based metal amorphous nanocomposites (MANC) used in high-speed motor (HSM) applications exhibit significantly reduced eddy current losses while maintaining good mechanical properties and glass-forming abilities. A protective native surface oxide layer on FeNi-MANCs provides sufficient electrical insulation to reduce interlaminate eddy current losses and lower overall losses in magnetic components. The primary focus of this work is on 1) detailed characterization of the surface oxide and crystallization layer at various annealing conditions, 2) surface and bulk magnetic anisotropies affected by the surface crystallization and oxidation, 3) epoxy wetting of these oxides and strain dependencies upon tension annealing and 4) optimized annealing conditions to improve soft magnetic properties such as grain refinement to reduce coercivity and lower losses. To investigate these topics, traditional materials characterization tools, including x-ray diffraction, differential scanning calorimetry, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, vibrating sample magnetometry, and magneto-optical Kerr effect. In addition, a custom-made contact angle goniometer and flash annealing apparatus for rapid heat treatment were constructed in the laboratory to measure contact angles. 

Major accomplishments presented in this thesis which have been published in journal articles or presented at technical conferences include the following: 

i. Detailed characterization of the surface oxide layer in FeNi amorphous magnetic ribbon (AMR)  and MANC, showing Fe-rich crystalline and amorphous oxide layers of varying thickness and composition upon heat treatment.   

ii. Identified transverse uniaxial magnetic anisotropy in the bulk of FeNi-based MANC, which evolved into isotropic properties upon stress relief conventional annealing near primary crystallization temperatures. Strain annealing induced relatively large uniaxial anisotropy compared to all the FeNi-based MANCs samples tested. 

iii. Strain-dependent surface energies of pre-strained FeNi-based MANC were determined by contact angle measurements which had not been previously reported in literature. 

iv. Improved soft magnetic properties, including lower coercivity, of a newly developed high induction FeNi-based MANC by nanostructure refinement via optimized rapid annealing heat treatment techniques. 

v. Identification of preferential nucleation of fcc phase in FeNi-based MANC during crystallization by rapid annealing contrary to the previously observed predominant bcc phase during conventional annealing at similar annealing temperatures. vi. Identification of passivating Si-oxide content inhibiting the growth of the Fe-rich surface oxide layer on FeNi-MANC surfaces