How to Improve Defects in Metal Powder Metallized Ceramics? (1)

The sintered metal powder method is the earliest, most mature and widely used process among ceramic-to-metal sealing processes. So far, the most of R&D institutes and manufacturing facilities of vacuum electronic devices use this process at home and abroad. The vast majority of these companies utilize the Mo-Mn formulation, so it is usually also called the Mo-Mn method, the Ceramic parts after metallizing process with high-temperature cure are called as Metallized Ceramics. In order to make the solder wettability on the metallized layer as well as form a barrier layer, an additional coating of nickel is electroplated on the metallized surface.

Metallized ceramics play a very critical roles in vacuum electronics technology, the typical usage as followings:

Laser applications: RF feedthroughs, Electrical feedthroughs, Pinch off tubes.

Medical applications: Scanners, imaging system, electrode and sensors.

Aerospace applications: Satellite propulsion and engine management as high-temp structural parts.

Energy storage applications: Optoelectronics and power tubes, Gas lasers, Deep Sea repeaters, Rechargeable batteries;

X-ray applications:

Electrosurgical instrumentation, Implantable packages and feedthroughs, Mass spectrometry, Gas detectors, Microscopes (TEM, SEM), High voltage feedthrough;

The Mo-Mn method of Ceramic metallization is related to many procedures; thus, the manufacturing process is much complicated. There are also many factors that affect the quality of Metallized Ceramic Parts. In this article, some common defects in metallized products are summarized and analyzed, and corresponding improvement measures are put forward.

1. Defects of the metallized layer

1.1 Blistering of the metallized layer, this is attributed to the following factors:

a. Imperfect quality of hydrogen resulting in incomplete decomposition and residual OH- ions;

b. Too fast heating rate preventing gas exclusion, or excessively high sintering temperature leading to excessive liquid phase;

c. Excessive solvent and binder content in the metallization paste causing thick coating and incomplete decomposition;

d. Presence of internal impurities or coarse grinding traces in ceramic body resulting in micro cracks.

In light of the aforementioned situation, improvement measures include enhancing hydrogen quality, implementing raw material particle size control, controlling paste thickness, adhering to standard sintering temperature specifications while ensuring the roughness of ceramic brazing surface to be Ra0.8.

1.2 The metallized layer cracks

The metallized layer of ceramic surface often cracks during the drying process after coating. Its characteristics are turtle-shaped cracks, the main reason for the formation of this crack is due to too much binder in the powder paste, also the nonuniform thickness of the coating layer, which will inevitably cause in the drying process of the binder decomposition, volatilization and shrinkage resulting in uneven sizes and cracks.

Three solutions have been proved in the production practice:

a. Add appropriate amount of binder, but also grinding & stirring to make it uniform;

b. The thickness of the coating at applying process should be uniform; c. Sintering heating rate should be properly slow.

1.3 The oxidized metallized layer:The main reasons are as follows:

a. The surface temperature of oven-fresh metallized ceramics is too high; b. The dew point of hydrogen is too high;

c. The cooling zone temperature is low, condensate water appears in the furnace pipe; d. Air enters when the outlet furnace is opened so that resulting in oxidation;

In these cases, the improvement methods are:

a.Reduce the temperature of Metalized ceramics; b.When cooling, dry hydrogen is used; c.The exit door should be opened and closed quickly.

1.4 Scrapes away issue of metallization layer:

The metallized surface looks dark, the surface layer of plating is easy to remove when scraped with ceramic, and the metallized layer can be completely scraped away when it is serious. The reason for scraping away is mostly that the metallization temperature is too low, or the oxidation capacity of the atmosphere is too low so that resulting in weaken adhesion strength. The improvement measures as followings:

a. Increase the metallization temperature; b. Adjust the atmosphere humidity;

c. Check whether the proportion is wrong when the metallization formula is prepared and whether the material properties of the components are changed.

1.5 Peeling off of metallized layer:

The so-called metallized layer peeling refers to the serious shrinkage deformation of the metallized coating during the heating process, which causes the metallized layer to locally bulge out of the ceramic surface, and even most of the metallized layer to fall off. If the coating itself is sintered well, the root cause of this phenomenon is that the coating contains too much binder or the coating is too thick at the heating rate

When it is relatively fast, the coating shrinks quickly and breaks off the porcelain surface. In addition, the paste is too sticky, applied too fast, the paste does not wet the porcelain surface well, and the coating is easy to leave the porcelain surface when heating up. Therefore, the paste viscosity should be appropriate, if the coating speed is accelerated, the paste viscosity should be reduced accordingly, and the rising and falling temperature should be properly controlled.

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