Chiang Mai Journal of Science

The oxide scale of an Fe-Ni-Co alloy formed by pre-oxidation with a reducing LPG/ O2 or oxyacetylene flame before direct fusion to a borosilicate glass was studied. It was found that the amount of the oxide scale increased as the pre-oxidation time increased. The oxide scale is iron oxides consisting of hematite and magnetite. An investigation of the Fe-Ni-Co alloy to the borosilicate glass joints revealed that the contact angle between the glass and the alloy was affected by the amount of the oxide scale on the alloy surface prior to joining. The thicker the oxide scale resulted in the better wetting of glass to the alloy. It was found from scanning electron microscopy that the oxide scale dissolved into the glass. In the case that there is an excessive amount of the oxide scale prior to joining, fayalite was also observed to crystallise within the glass. This led to a negative effect on the strength of the joints. Transmission electron microscopy revealed a dispersion of fine fayalite and iron silicon carbonyl particles within the iron-rich zone where the oxide scale dissolved into the glass. The optimum condition for joining the borosilicate glass to the Fe-Ni-Co alloy is to preoxidise the alloy with a reducing flame for 2 minutes getting the weight gain of 0.59-0.83 mg/cm² prior to direct fusion. The bonding strength obtained was about 4 N/mm² (MPa).