Patent Application
20150315090
This invention relates generally to the field of materials technology, and more specifically to a process for glazing a surface of a material.
Glazing has been used since ancient times for creating smooth and decorative textures on ceramic objects. Green ceramic objects are typically covered with a dry or aqueous glaze mixture before inserting them into a kiln for firing. The glaze mixture may contain a glass forming agent such as silica, a fluxing agent such as sodium, calcium or potassium metal oxide to lower the melting temperature of the silica, and a stiffening agent such as alumina to prevent runoff of the glaze from the part.
More recently, laser energy has been used as a heat source for the glazing of ceramic thermal barrier coating materials over superalloy gas turbine engine components to protect the coatings from oxidation, corrosion and infiltration of contaminants. However, the extreme local thermal transients generated during laser melting can cause cracking of the glazed surface. U.S. Pat. No. 5,576,069 describes a two-step process to heal such cracking involving the application of a thin layer of zirconia powder followed by a secondary laser remelting step while the substrate is preheated in order to minimize thermal gradients.
The invention is explained in the following description in view of the drawings that show:
The present inventors have found the prior art solution to the problem of laser glazing cracking to be unsatisfactory because such two-step processes and special heat treatments add time and expense. Accordingly the inventors have developed an innovative glazing process and glazed product that overcome the problem of cracking of a glazed surface. Instead of repairing cracks and limiting thermal transients during the repair, as is currently done in the art, the present inventors avoid the generation of the cracks in the first place. This is accomplished not by limiting thermal stresses with special heat treatment, but by accommodating the thermal stresses that do occur. This is accomplished by introducing small hollow objects into the glaze melt. The hollow objects provide a degree of mechanical compliance to accommodate shrinkage stresses during solidification of the glaze, thereby preventing cracking.
Advantageously, the hollow objects 18 are not melted by the beam 22, such as by being formed of a material having a higher melting temperature than that of the powdered glaze material 16. The molten material is allowed to solidify around the hollow objects 18 behind the beam 22 to create the glazed surface 26. Flexing of the hollow objects 18 during the solidification process accommodates shrinkage stresses, thereby preventing cracking. Incidental melting of some portion of the surface of the hollow objects 18 is included in the condition of “not melted” described herein, as long as the objects 18 retain their geometric form of being hollow in order to provide the degree of mechanical compliance described.
The hollow objects 18 may be nano, micro or milli sized, with smaller objects typically being used for thinner glazed layers 10. In one embodiment, the hollow objects 18 may be hollow silica spheres from 1.5-5 microns in diameter which are commercially available from Microspheres-Nanospheres, a Corpuscular company (http://microspheres-nanospheres.com). Other oxide materials may be used to form the hollow objects 18, such as SiO2, TiO2, Al2O3, or ZrO2, for example. Hollow shapes other than spheres may be used, such as cubic silica particles that have been developed for use in lithium ion battery construction.
While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
