2. Surface treatment and oxide film formation on metals

When joining glass and metal, pre-treatment of the metal components is also important to achieve a highly airtight and reliable bond. Yamamura Photonics also carries out several treatments, which are introduced here.

Surface treatment of metals

Metal components produced by rolling, pressing, cutting, drawing, etc. usually have contaminated surfaces and require appropriate surface treatment before joining. Depending on the type and condition of contamination, chemical etching or physical honing treatments are used to provide adequate cleaning. Insufficient cleaning can lead to problems affecting the tightness of the joint after joining and the strength of the joint, such as uneven pre-oxide film formation and bubbles in the joint, which will be discussed later in this section.

Heat treatment prior to oxide film formation

The formation of an oxide film on the surface of the metal is effective in ensuring the stability and reliability of the bond. The oxide film improves the wettability of the glass to the metal and the diffusion of the metal oxide into the glass results in an airtight bond. To form an oxide film, the following heat treatment must first be carried out as a pre-treatment. Not all of these treatments are carried out at this time, but those that are selected according to the condition of the metal and the joining method.

(1) Decarburisation.

This method removes trace carbon and organic contaminants from metal surfaces using wet hydrogen. In this method, a slight oxidation reaction also occurs on the surface, which facilitates the subsequent formation of an oxide film.

ａＨ_２＋Ｈ_２Ｏ＋Ｃ→ａＨ_２＋Ｈ_２＋ＣＯ

(2) Nitrogen heat treatment.

This method thermally decomposes organic contaminants on metal surfaces. At the same time, it is also an annealing treatment for metals, but care must be taken as excessive heating can lead to nitriding.

(3) Vacuum treatment.

This method removes trace amounts of carbon from metal surfaces in a vacuum.

Oxide film formation

The metals of choice for glass-to-metal hermetic sealing are mainly Kovar and FeNi alloys. When these metals are heated to a certain temperature, magnetite (Fe3O4) is formed on the surface as an oxide film, which acts as a bonding promoter between glass and metal. It is necessary to form an oxide film of the optimum thickness, as too thick can cause leaks and too thin can lead to a reduction in strength. The formation of this oxide film is a particularly important know-how in the hermetic bonding of glass to metal.