Sintered Silicon Nitride (Si3N4 )

In the early stages of its development, silicon nitride, Si₃N₄, powder compacts could not be sintered without the application of high mechanical pressure.  If Si₃N₄ is compacted into a green shape and heated in air, it does not sinter and form a dense, strong material. Rather, it decomposes to silicon and nitrogen and fails to give the improved properties. 

A major step in developing high density silicon nitride was the accidental discovery in the mid 1970’s that Si₃N₄ can be sintered if the additives developed for hot-pressed silicon nitride are included in the compact and precautions are taken to limit the loss of silicon nitride.  The most commonly used additives are magnesium oxide (MgO) and yttrium oxide (Y₂O₃), which can be used separately, in combination or with aluminium oxide (Al₂O₃).  The additives promote liquid phase sintering.

Precautions must also be taken to limit the decomposition of silicon nitride or loss of the additives.  The compact is sintered in a bed of silicon nitride powder and/or under a high pressure nitrogen atmosphere (typically 1 to 8 MPa). The protective atmosphere suppresses the evaporation of silicon, nitrogen and additives, and enables the sintering reaction to take place.  Higher temperatures (1825 - 2080°C) and longer times (up to 5 hours) are needed than used to produce hot-pressed silicon nitride, and strengths approaching those of hot-pressed silicon nitride can be achieved.

Key Properties

Applications exploit the following properties of sintered silicon nitride:

• Low density
• High temperature strength
• Superior thermal shock resistance
• Excellent wear resistance
• Good fracture toughness
• Mechanical fatigue and creep resistance
• Oxidation resistance

Table 1. Typical physical and mechanical properties of sintered silicon nitride.

Applications

The sintering process gives similar properties to hot pressed silicon nitride, but is considerably more economical.

The material is used currently in niche market applications for example in reciprocating engine components and turbochargers, bearings, metal cutting and shaping tools and hot metal handling.

The sintering process can also be used to post-sinter reaction bonded silicon nitride components.  Shrinkage of the component is less (5% compared with 15%) giving better control of dimensions and less need for expensive machining to achieve final dimensions.