Ceramic metal composite

To grasp the complexity of a ceramic-metal composite, in addition to knowledge of the most important material properties, a basic understanding of different materials is crucial. In contrast to metals, for ceramics, the term “material strength” is replaced by the term “probability of fracture.”

The larger the volume of the ceramic component subjected to tensile load, the higher the probability of fracture. The reason for this is that the larger the component, the greater the likelihood of critical defects in the material. In addition to the rule that ceramic components should be subjected to compressive rather than tensile loads, it is important to note that the thermal expansion coefficients of ceramic materials differ significantly from those of metals. This must be taken into account when designing composite systems, both with regard to thermal expansion at the maximum operating temperature and for the joining process. An example will illustrate the crucial differences that can occur with an increase in temperature.

Ceramic metal composite (fit)

A sleeve made of Al₂O₃ ceramic is to be connected to a metal shaft using a fitting. The coefficient of thermal expansion (CTE) of Al2O3 ceramic (99.7%) differs significantly from the CTE of stainless steel 1.4571, at α=7.3*10‐6/K, at α=16.5*10‐6/K. With a steel shaft diameter of D=50mm and a temperature increase of 100K, the expansion is 50mm*(16.5*10‐6/K)*100K = 0.083mm.

For the Al₂O₃ sleeve, the expansion is 50mm*(7.3*10‐6/K)*100K = 0.037mm. If the connection is designed as a tight fit, the larger expansion of the metal shaft would cause the ceramic sleeve to fracture. This design is therefore not suitable for ceramics.

An example of a suitable ceramic-metal composite is the press fit between a ceramic core and a metal sleeve. To slide the sleeve over the oversized ceramic rod, the metal must be heated. Upon cooling, a press fit is formed by shrinking onto the ceramic. Since ceramic materials can absorb compressive forces very well, this design principle is fundamentally suitable for the production of a ceramic-metal composite.

Ceramic metal composite (screw connection)

The most common force-locking connection is the screw connection. This can also be realized with ceramic materials. At room temperature, metal external thread carriers can be easily screwed into a ceramic internal thread.

However, external threads in ceramics essentially represent a long notch or crack in the ceramic. Therefore, the radius at the base of the notch should be as large as possible to reduce the risk of a predetermined breaking point. It is more sensible and usually also cheaper if the design is sufficiently adaptable so that standard metal screws can be used or a metal threaded bushing can be glued or soldered into the ceramic.

Ceramic metal bonding

Organic adhesives can be divided into two classes. Chemically curing adhesives achieve curing through polymerization, polyaddition, or polycondensation reactions. Depending on the reaction type, a distinction is also made between cold-curing and warm-curing, as well as one- or two-component systems. In physically curing adhesives, the solvent evaporates and the base materials harden.

Epoxy resin-based adhesives have proven highly effective for bonding ceramic-metal composite components. They are characterized by the following properties:

Good mechanical properties
Good chemical resistance
Low shrinkage during curing
Suitable for larger bonding gaps thanks to the use of fillers
Easy bonding of different materials

Adhesive bonds should only be bonded under shear, tensile, or compressive stress. Bending loads should be avoided. Thorough cleaning and chemical or mechanical activation of the bonding surfaces is essential. However, it should be noted that bonds made with organic adhesives are subject to aging and should generally not be heated above 150°C for extended periods.

Bonded metal-ceramic composite components are not suitable for use in vacuum technology due to permanent outgassing.

Metal-ceramic and ceramic-ceramic composite (soldering with glass solders)

Glass solders can be used for gas-tight joining of ceramic components, as well as platinum-ceramic composite components, for example. This joining process is characterized by good chemical resistance and operating temperatures of up to approximately 1,000°C, depending on the glass solder. Neither a protective gas atmosphere nor metallization of the ceramic is necessary for joining with glass solder.

Ceramic metal composite (soldering with metallic solders)

The standard joining process at Alumina Systems for producing ceramic-metal composites is brazing ceramics with metals. During brazing, atoms from the solder diffuse into the lattice of the material, and when brazing metals, atoms from the material also diffuse into the structure of the solder. A more or less pronounced diffusion zone forms along the wetting surface. Unlike during welding, the material remains in a solid state.

The range of possible ceramic-metal joining partners is severely limited by the different thermal expansion coefficients of the materials. The greater the difference in the thermal expansion coefficient between ceramic and metal and solder, the higher the bond stresses that arise when the solder joint cools.

When soldering ceramics, two processes are distinguished:

Passive soldering of ceramics
Active soldering of ceramics

1. Passive soldering of ceramic metal composite

To ensure sufficient wetting of the solder on the ceramic, the ceramic surface must first be metallized. For this purpose, MoMn (molybdenum, manganese) or WMn (tungsten, manganese) paste is applied, for example, by screen printing and fired at approximately 1,400°C under forming gas (5% hydrogen, 95% nitrogen).

This layer serves two functions. The Mn-containing glass bonds to the Al₂O₃ ceramic. The metallic Mo particles contained in the glass establish the bond to the metal. After firing, the metallization layer is electroplated with nickel. The Ni also serves several functions. It protects the metallization from corrosion and enables good wettability by the metallic solder.

Passive soldering of ceramics takes place under a reducing atmosphere or in a vacuum, depending on the metal partner. The most common passive solders are based on silver-copper eutectics.

2. Active soldering of ceramic-metal composites

A second option for brazing ceramics is the use of so-called active solders. These contain, for example, titanium, zirconium, or hafnium as reactive components. Active solders bond to the ceramic even without metallization. However, compared to passive solders, active solders hardly flow on the ceramic surface. Thus, no fillet, which is beneficial for bond strength, is formed. Even if brazing without metallization appears more favorable, active brazing requires a high-temperature vacuum or inert gas furnace. To ensure an optimal brazing gap during brazing, the metal and ceramic must only move apart slightly due to thermal expansion during the brazing process. Therefore, iron-nickel alloys such as Ni₄₂ (1.3917) or Kovar (NiCo2918 = 1.3981) are predominantly used for brazing with Al₂O₃. Here, the difference in the thermal expansion coefficient compared to Al₂O₃ is relatively small at room temperature. This results in only low bond stresses. This allows for highly reproducible strengths of the metal-ceramic composite of greater than 200 MPa. These iron-nickel alloys can be easily welded to stainless steels in subsequent processes. This opens up a wide range of applications for ceramic-metal composite components.