Werkstoffe der technischen Keramik

3.4.2 Oxide Ceramics

3.4.2.1 Aluminium oxide

Aluminium oxide (Al₂O₃) is the most important technical oxide ceramic material and has the widest range of applications. Densely sintered aluminium oxide is characterised by

• high strength and hardness,
  

• temperature stability,
  

• high wear resistance and corrosion resistance even at high temperatures.
  

These synthetically manufactured materials with aluminium oxide contents ranging from 80 % to more than 99 %, have been proven in practice. The choice of this material is determined by technical and economic criteria. A material with higher aluminium oxide content does not necessarily fulfil the needs of an application best.
It is also possible for the mechanical properties of aluminium oxides from different manufacturers to vary, even when the aluminium oxide content is the same, due to different powder formulations.

Figure 6: Surface of an aluminium oxide "as fired"

Figure 7: Microstructure of an aluminium oxide ceramic (99.7 %)

Aluminium oxide materials satisfy all the requirements of insulation materials for applications in electrical engineering. Outstanding values for bending strength and for resistance to wear and to high temperatures suit the material to mechanical applications.

The microstructure, and therefore the other properties, of aluminium oxide ceramics can differ markedly (see Figures 8 and 9).

Figure 8: Microstructure of a micro-crystalline aluminium oxide

Figure 9: Microstructure of a coarse-crystal aluminium oxid

Surfaces with the highest possible bearing area are not always either necessary or desirable. Thread guides used in the textile industry, for instance, should have a small contact area with the thread and should not have any sharp edges, whereas sealing plates require a material proportion of approx. 80 %.

Figure 10: "As fired" surface of a extruded aluminium oxide

Due to their good price/performance ratio and their generally useful properties, aluminium oxide ceramics are used

• in the sanitary industry as a sealing element,
  
• in electrical engineering as insulation,
  
• in electronics as a substrate,
  
• in machine and plant construction as wear protection (wear-resistant material),
  
• in the chemical industry as corrosion protection (corrosion-resistant material, highly resistant to vapours, melting and slag up to high temperatures) and as a filter,
  
• in instrumentation as a protective tube for thermocouples used for high temperature measurements
  
• in human medicine as an implant, and
  
• in high temperature applications as a burner nozzle or as a support tube for heat conductors.
  

	C700
	Oxide