Abrasive silicon nitrides

As a future alternative to glassed steel there is ceramics-coated steel which is resistant to abrasion, corrosion and high temperatures. The base metal is coated with silicon nitride formed in situ. Silicon nitride has resistance to both acid and alkali and it is durable at temperatures up to 1 000°C, suggesting a promising future coating in aggressive operating environments. 

Silicon nitride (Si N ), resistant to oxidation, is an excellent coating for metals, as well as an adhesive and abrasive, and it is used in high-temperature crucibles. It has proven useful as heat-resistant substance for the nozzles on rocket engines. 

Among the useful compounds of silicon are silicon carbide (carborundum) and silicon nitride, which are hard, tough materials used for making cutting tools, abrasives, and engineering... 

Dogan, C.P. and Hawk, J.A. Influence of whisker reinforcement on the abrasive wear behavior of silicon nitride- and alumina-based composites , Wear, 203-204 (1997) 267-277. 

Among nonoxides, silicon carbide and silicon nitride are two very important ceramics. Both are very hard and abrasive materials, and show excellent resistance to erosion and chemical attack in redudng environments. In oxidizing environments, any free silicon present in a silicon carbide or silicon nitride compact will be oxidized readily. Silicon carbide itself can also be oxidized at very high temperatures, the exact temperatures being a function of purity and... 

This reaction aids the mechanical abrasion by the silica nanoparticles. The silicon nitride stop layer is also slowly abraded according to the following reaction (for KOH stabilized slurry) ... 

Silicon carbide, widely employed as an abrasive (carborundum), is finding increasing use as a refractory. It has a better thermal conductivity at high temperatures than any other ceramic and is very resistant to abrasion and corrosion especially when bonded with silicon nitride. Hot-pressed, self-bonded SiC may be suitable as a container for the fuel elements in high-temperature gas-cooled reactors and also for the structural parts of the reactors. Boron carbide, which is even harder than silicon carbide, is now readily available commercially because of its value as a radiation shield, and is being increasingly used as an abrasive. 

Silicon nitride (see Nitrides) is a key material for structural ceramic applications in environments of high mechanical and thermal stress such as in vehicular propulsion engines. Properties which make this material uniquely suitable are high mechanical strength at room and elevated temperatures, good oxidation and creep resistance at high temperatures, high thermal shock resistance, excellent abrasion and corrosion resistance, low density, and, consequendy, alow moment of inertia. Additionally, silicon nitride is made from abundant raw materials. 

At elevated temperature, graphite can be abrasive rather than lubricating, due to desorption of moisture, unless additives (such as dibasic ammonium phosphate) are incorporated. This abrasiveness appears to be more severe in the case of a silicon nitride counterface. Surface profilometry of the silicon nitride disks in the region of contact showed that the ceramics had been polished and, in some cases, grooves had been worn in the ceramic surfaces. 

The key ingredients to a successful STI process are the achievement of well-dispersed abrasive ceramic particles having high oxide-to-nitride selectivity and producing few microscratches on the wafer. Silica slurry had been conventionally used in the STI CMP process, however, ceria (Ce02) slurry with high oxide-to-nitride selectivity has been introduced as the thickness of silicon nitride film is decreased by design rule restrictions. 

The wear tests confirmed substantially higher resistance of coated Si3N4 inserts (Fig. 14a) and CBN tools (Fig. 15b) to abrasive wear in comparison to uncoated tools. The practical findings of the wear tests are that in order to guarantee tool life of several minutes, the cutting speed should be kept at 240 m/min and 320 m/min for coated silicon nitride and CBN tools, respectively. 

The last of the mentioned nonmetallic nitrides is silicon nitride (SijNJ. Since about 50 years, the importance of SijN as a technical ceramic increased permanently. SijN combines high hardness and abrasion resistance with a relatively low density of about 3.24 gcm and good oxidation resistance up to... 

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