Sialon

SiAlON ceramics are a specialist class of high-temperature refractory materials, with high strength (including at high temperature), good thermal shock resistance and exceptional resistance to wetting or corrosion by molten non-ferrous metals, compared to other refractory materials such as, for example, alumina. A typical use is with handling of molten aluminium. They also are exceptionally corrosion resistant and hence are also used in the chemical industry. Sialons also have high wear resistance, low thermal expansion and good oxidation resistance up to above ~1000 °C. They were invented by Kenneth Henderson Jack in 1984.

Forms

SiAlONs are ceramics based on the elements silicon (Si), aluminium (Al), oxygen (O) and nitrogen (N). They are solid solution of silicon nitride (Si₃N₄) and exist in three basic forms. Each form is iso-structural with one of the two common forms of silicon nitride, beta and alpha and with silicon oxynitride. The relationship between that of sialon and silicon nitride is similar to that between brass and pure copper. The later case, copper atoms are replaced by zinc to give a stronger alloy than the mother metal. In the case of sialon, there is substitution of silicon by aluminium with corresponding atomic replacement of nitrogen by oxygen, to satisfy valency requirements. The resulting 'solution' (sialon) has superior properties to the original pure solvent (silicon nitride).

Production

SiAlONs are produced by first combining a mixture of raw materials including silicon nitride, alumina, aluminium nitride, silica and the oxide of a rare earth element such as yttrium. The powder mix is fabricated into a 'green' compact by isostatic powder compaction or slipcasting, for example. Then the shaped form is then densified, typically by pressureless sintering or hot isostatic pressing. The sintered part may then need to by machined by diamond grinding (abrasive cutting).

Applications

SiAlON ceramics have found extensive use in non-ferrous molten metal handling, particularly aluminium and its alloys, including metal feed tubes for aluminum die casting, burner and immersion heater tubes, injector and degassing for nonferrous metals, thermocouple protection tubes, crucibles and ladles.

In metal forming, sialon is used as a cutting tool for machining chill cast iron and as brazing and welding fixtures and pins, particularly for resistance welding.

Other applications include in the chemical and process industries and the oil and gas industries, due to sialons excellent chemical stability and corrosion resistance and wear resistance properties.

Some rare-earth activated SiAlONs are photoluminescent and can serve as phosphors. Europium(II)-doped β-SiAlON absorbs in ultraviolet and visible light spectrum and emits intense broadband visible emission. Its luminance and color does not change significantly with temperature, due to the temperature-stable crystal structure. It has a great potential as a green down-conversion phosphor for white LEDs; a yellow variant also exists. For white LEDs, a blue LED is used with a yellow phosphor, or with a green and yellow SiAlON phosphor and a red CaAlSiN₃-based (CASN) phosphor.

Further reading

• Thommy Ekström and Mats Nygren (2005). "Sialon Ceramics". Journal of the American Ceramic Society. 75: 259–276. doi:10.1111/j.1151-2916.1992.tb08175.x.
• Sialon Ceramics Guide
• A new class of sialon advanced ceramics