New Technology

Silicon Nitride for highly demanding thermal and wear applications

Introduction

Morgan Advanced Ceramics, Stourport, England, is unique within the Morgan group in offering Silicon Nitride. The company has over 25 years of experience in the manufacture of Silicon Nitride, with experienced and highly skilled machinists providing solutions to the most demanding of customer requirements.

What is Silicon Nitride?

Silicon Nitride (Si₃N₄) is a relatively new member of the advanced ceramics family, having been developed in the 1960’s. It is used in many varied applications, but lends itself particularly to applications where high temperatures are involved. This is not limited to continuous temperatures, as with many ceramics, but due to Si₃N₄’s exceptional thermal shock resistance, it is also widely used in cyclic thermal applications, for example induction coil supports and fixtures. Silicon Nitride is also resistant to attack by many molten metals leading to its use in metal handling applications.

What is so special about Silicon Nitride?

• Manufacture by a range of methods to suit volumes from one-off requirements to very high volume applications
• Excellent wear resistance/hardness
• Low thermal expansion
• Good chemical corrosion resistance
• High temperature performance
• Good thermal shock behaviour
• Good flexural strength
• Low weight
• High Stiffness
• Electrically insulating

What are the types of Silicon Nitride?

There are two main types of Silicon Nitride; Reaction Bonded Silicon Nitride (RBSN) and Hot Pressed Silicon Nitride (HPSN). Morgan Advanced Ceramics specialises in the Reaction Bonded Silicon Nitride which, supported by customer endorsements, is considered to be one of the very best on the market.

Reaction Bonded Silicon Nitride

Reaction Bonded Silicon Nitride is quite unique in its manufacture and ability to take advantage of ‘near-net shape’ forming. Components are initially made from very high purity Silicon metal and formed to the desired geometry using a variety of techniques (see table below). Parts are then fired in a Nitrogen controlled environment, which converts the Silicon metal to Si₃N₄ ceramic, retaining about 20% porosity. Once converted, the component can be further machined to achieve an improved surface finish using specialised diamond tooling. There are no sintering aids used in the manufacture of RBSN leading to the final ceramic having very high purity (>99% Si₃N₄). Also there is minimal shrinkage during the entire process (~0.1%) and very accurate dimensions can therefore be achieved cost effectively. 

Features of Reaction Bonded Silicon Nitride

• High purity (>99%) Si₃N₄
• Low weight
• Excellent Thermal Stability
• Sputter resistant
• Not wetted by many metal and alloys
• Excellent thermal shock capability (ie; cycles from RT to 700°C in 7 seconds and repeated every 30 seconds)

What are the forming techniques for Reaction Bonded Silicon Nitride?

A number of factors are considered before choosing the most suitable forming technique. These include complexity of geometry, overall size of the component, quantity required, surface finish and component tolerance. Table 1 below lists some of the typical factors considered with determining the most suitable forming techniques. Note that, although prototype parts cannot be made via the ‘Injection Moulding’ and ‘Die Pressing’ route, prototypes can be made by the ‘Green Machining’ route.

Table 1: - Comparison of forming techniques for Reaction Bonded Silicon Nitride