The manufacture of components made of advanced ceramics is divided into several steps
1 Creating the base materials
The base materials for a wide variety of metals and semi-metals must first be chemically created, for example from bauxite ore for aluminum oxide. Using the Bayer method, a solution with aluminum hydroxide (aluminate solution) is dissolved from bauxite with lime and caustic soda and processed into pure aluminum oxide powder (Al2O3) via filtration, precipitation and heating (calcination). The amount of bauxite used corresponds to about seven times the mass of aluminum oxide powder to be produced.
2 Manufacturing the powder
The raw materials are then ground into different grain sizes with different degrees of purity and categorized according to quality classes. The grain size is typically between 0.1 and 10 micrometers. However, powders below and above these limit values can also be produced for special applications. The base material for pressing now has a consistency similar to flour and is therefore difficult to dose and combine. However, to fill the tool molds homogeneously and precisely without excessive work, the powder is mixed into a suspension with around 3% organic binders and then immediately dried again via atomization. The resulting beads with a diameter of around 100 micrometers now have excellent pourability.
3 Pressing the powder
There are four basic methods for pressing the ceramic powder: uniaxial pressing, isostatic pressing, injection molding and extrusion.
Uniaxial pressing is suitable for the manufacture of smaller components in larger quantities. With this type of shaping, the dry powder is put into a mold and pressed with two stamps from above and below under high pressure.
Cold isostatic pressing is suitable for simple shapes and/or larger geometries. The powder is placed in a rubber mold, sealed tightly and then pressure of up to 4,000 bar is applied from all sides in a liquid. Since the high pressure is applied equally from all sides, the best material quality is achieved.
Extrusion is used for simpler, mostly symmetrical components. For this process, it’s not the loose granulate but rather a viscous mass that is required as the base material. In this process, the granulate is mixed and kneaded with around 30% thermoplastics or organic polymers. The mass is then pressed through a mold die under high pressure using a screw press or hydraulic press. Extrusion is particularly suitable for rotationally symmetrical components.
Injection molding (CIM = ceramic injection molding)
Injection molding of ceramics is mainly used for complicated components in large quantities. Since a viscous mass is also required for injection molding, the preparation of the base material is essentially the same as for extrusion. The mass is then pressed into the injection molding tool under high pressure.
4 Green machining
The result of the initial shaping process is referred to as the green body. At this stage, the components can still be further processed using conventional machining methods such as turning, milling and drilling. The parts are still soft and brittle.
Before the components can be sintered, the binder (thermoplastics or organic polymers) must be removed. This is achieved using a chemical and/or thermal process. After this step, the component is very brittle.
Sintering is the term for the actual firing process during which the material becomes hard. In this process step, the time-temperature progression is crucial and, depending on the ceramic, the firing reaches temperatures of over 2,000°C. During the sintering process, the volume of the ceramic component shrinks by up to 50%, depending on the material and the firing process. This makes it very difficult to maintain accurate dimensions.
Hot isostatic pressing (HIP)
During sintering, most of the pores in the component can be closed. However, in order to further increase the density and thus the material quality, an overpressure can be applied using argon gas and the densification process can be continued at an increased temperature. This process is called hot isostatic pressing.
7 Hard machining
For certain applications, the tolerances that can be achieved through the sintering process with regard to shape, position and surface quality are not sufficient. Hard machining of the finished fired ceramic component then becomes necessary. Advanced ceramics can only be machined using diamond-tipped grinding tools. This machining step requires a wealth of expertise and is highly time-consuming. Hard machining is based on experience and specially developed machining processes – the most important are grinding (e.g. cylindrical, flat and profile grinding), lapping, honing and polishing. The achievable tolerances are in the micrometer range and the surface quality has an Ra value of 0.025 (N1).