Moisture degradation mechanism of silver-based low-emissivity coatings

Abstract

Moisture deterioration of low-emissivity (Low-E) coatings with layer structures of oxide/silver/oxide/glass, oxide/silver/glass, silver/oxide/glass, and silver/glass were investigated. All the films were formed by r.f. magnetron sputtering. Zinc oxide (ZnO) and tin oxide (SnO2) films were used as oxide layers. The coatings were analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The storage for 144h at 50°C, 95% relative humidity was adopted as a moisture test. Only the silver film formed on the ZnO underlayer showed a crystalline state of the Ag(111) peak before the moisture test. The coating with the crystalline silver layer was more stable against moisture, however the moisture test generated silver migration partly, so that shallow dimples were formed. On the other hand, the coating with an amorphous silver layer was less stable, and the moisture test changed its structure into the crystalline form and brought about strong silver agglomeration. Judging from these results, it was concluded that the silver migration induced by moisture triggered a reduction of the ZnO–Ag interfacial adhesive force and caused the moisture-induced deterioration (the peeling of the top oxide layer).