Influence of the coating preparation method and of nanosilica addition on the bonding character of Si3N4 layer on a silica crucible

An improved preparation method of coating, by directly brushing a Si3N4 slurry with nanosilica on a green body, was successfully practiced to increase the binding strength and to prevent shedding of the coating from the crucible sidewall during the directional solidification of polycrystalline silicon. Two types of coatings were prepared: a pure Si3N4 coating achieved by brushing the slurry on a sintered silica substrate, which is a standard process in industrial production, and Si3N4 coatings with different nanosilica contents applied to the green silica substrate. The properties of two types of coatings such as slurry permeation, the binding strength with substrate, and the related binding mechanisms, were discussed. The results showed that the permeation ability of the slurry to the silica substrate was an important factor for the binding strength, and the addition of an appropriate amount of nanosilica to the slurry with green silica as the substrate was an effective way to promote the permeation ability. After heat treatment at 1200 °C in air and 1500 °C in an argon atmosphere, the Si3N4 coating with 2.8 wt% nanosilica brushed on a green silica substrate exhibited the highest binding strengths of 0.33 MPa and 0.75 MPa, respectively. Furthermore, the introduction of nanosilica increased the coating density which is conducive to reducing the wetting kinetics of liquid silicon.