Comparison of the Crystallization Behavior of Silica Between Mg- and Al-Phosphate Used in Tension Coatings of Grain-Oriented Electrical Steel

The effect of the phosphate component on the thermal stability of tension coatings was investigated with a focus on the crystallization behavior of amorphous silica in the tension coating. After stress-relief annealing, core loss of samples coated with a Mg-phosphate was improved, while that coated with an Al-phosphate was deteriorated. The domain wall spacing of the samples coated with Mg- and Al-phosphates was respectively increased and decreased after stress relief annealing. This means that the stress relief annealing did not much diminish the tensile stress for Mg-phosphate coating but much diminished the tensile stress for Al-phosphate coating. Based on FTIR and XRD results, we found that the crystallization temperature of silica with Al-phosphate was lower than that of silica with Mg-phosphate. The crystallization of silica was accompanied by an abrupt volume change, which formed cracks in the tension coating and deteriorated the tensile stress.