Ceramic heat insulating layer and process for forming same

Abstract

A ceramic heat insulating layer formed on an iron-based base material with or without a bonding layer interposed therebetween, comprising: aggregate particles of a nepheline mineral; and a binder composed of silica particles and of a metalloxane polymer, the binder filling spaces between the aggregate particles and chemically bonding the aggregate particles to each other and to the base material or to the bonding layer. Alternatively, the binder leaves voids between the aggregate particles, and a sealing layer seals the voids in a surface region of the ceramic heat insulating layer. A process of forming the ceramic heat insulating layer comprises mixing aggregate particles of a nepheline mineral, a binder of an alcoxide and an organosilicasol, and a dispersing medium to form a slurry; applying the slurry either on the surface of an iron-based base material, or on any bonding layer formed on the surface; and firing the iron-based base material having the applied slurry; wherein the mixing is either carried out in a sufficiently acidic or sufficiently alkaline solution such that the surface potential of particles dispersed in the slurry does not pass an isoelectric point due to an increase in a pH value of the slurry because of alkaline metal ions dissolved from the aggregate particles of the nepheline mineral, or the mixing is carried out after coating the aggregate particles of the nepheline mineral with a coating layer which prevents dissolution of alkaline metal ions from the aggregate particles of the nepheline mineral.