Sialon

α-SiAlON parts

SiAlONceramicsare a specialist class of high-temperaturerefractorymaterials, with high strength at ambient and high temperatures, goodthermal shockresistance and exceptional resistance to wetting or corrosion by moltennon-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 first reported around 1971.

Forms

SiAlON forms

Form	formula	n	symmetry	space group	No	Pearson symbol	Z
α	Si12–m–nAlm+nOnN16–n		trigonal	P31c	159	hP28	4
β	Si6–nAlnOnN8–n	0–4.2	hexagonal	P63	173	hP14	2
O'	Si2–nAlnO1+nN2–n	0–0.2	orthorhombic	Cmc21	36	oS20	4

• m and n are the numbers of Al–N and Al–O bonds substituting for Si–N bonds

SiAlONs are ceramics based on the elementssilicon(Si),aluminium(Al),oxygen(O) andnitrogen(N). They are solid solutions ofsilicon nitride(Si₃N₄), where Si–N bonds are partly replaced with Al–N and Al–O bonds. The substitution degrees can be estimated from the lattice parameters.The charge discrepancy caused by the substitution can be compensated by adding metalcationssuch as Li⁺,Mg²⁺,Ca²⁺,Y³⁺and Ln³⁺,where Ln stands forlanthanide.SiAlONs exist in three basic forms, which are iso-structural with one of the two common forms of silicon nitride, alpha and beta, and with orthorhombicsilicon oxynitride;they are hence named as α, β and O'-SiAlONs.

Production

SiAlON gear (right) prepared from a billet (left) by forging at 1200 °C within 2 seconds.

SiAlONs are produced by first combining a mixture of raw materials includingsilicon nitride,alumina,aluminium nitride,silicaand theoxideof arare-earth elementsuch asyttrium.The powder mix is fabricated into a "green" compact byisostatic powder compactionorslipcasting,for example. Then the shaped form is densified, typically bypressureless sinteringorhot isostatic pressing.Abnormal grain growthhas been extensively reported for SiAlON ceramics, and results in a bimodal grain size distribution of the sintered material. The sintered part may then need to be machined by diamondgrinding (abrasive cutting).Alternatively, they can be forged into various shapes at a temperature of ca. 1200 °C.

Applications

A variety of SiAlON phosphor powders under UV light

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 acutting toolfor machining chill cast iron and as brazing and welding fixtures and pins, particularly forresistance welding.

Other applications include in the chemical and process industries and theoil and gasindustries, due to sialons excellent chemical stability andcorrosion resistanceandwear resistanceproperties.

Somerare-earthactivatedSiAlONs arephotoluminescentand can serve asphosphors.Europium(II)-dopedβ-SiAlON absorbs inultravioletandvisible lightspectrum 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 whiteLEDs;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.