For those working in the field of fingerprint recognition, finding the right material to coat the device has been a significant challenge. The material chosen must be durable, because it will be touched many times over the course of its lifetime. It must be able to be made into a thin layer to keep the size and weight of the device down. Silicon Nitride can be machined into incredibly thin yet durable sheets highly suited to this type of application.

Products are now available that use a Silicon Nitride layer which the user touches in order for their print to be read. These devices can be attached to or built into desktop and laptop computers, PDAs, and other wireless products to prevent unauthorised use. There are also larger versions designed to replace door locks. They do away with the need to remember number based codes or passwords. Authentec, who produce them, have developed a technology that allows the prints to be read even when the fingers are dirty. The products are designed to withstand a million impressions. Advances in technology have allowed the size of these devices to reduce to an extent where they are not much bigger than a fingertip, allowing them to be built into all kinds of devices unobtrusively. This is particularly important in the mobile telephone and PDA markets, where companies compete on the size of product.

.The most widespread application for Si3N4 is currently in the manufacture of bearings. Ceramic bearings, made principally from Si3N4 are the component of choice in many highly demanding circumstances. Although the advantages of Silicon Nitride bearings have been known since 1972, high costs prevented them from being taken up by manufacturers until the early nineties. Those costs have now fallen significantly due to technological advances and these bearings are being used more widely. These advances have largely been driven by the Department of Energy, who have co funded much of the research into Silicon Nitride since the 1960s.

Advantages of Si3N4 bearings over steel bearings:

	Good rolling contact fatigue
	High hardness
	High stiffness
	Low weight
	60% lower density
	50% higher Young's Modulus
	Low thermal expansion
	Low coefficient of friction
	Longer operating life
	Develop lower centrifugal forces
	Resistant to corrosion

One area where Si3N4 bearings make a significant difference to performance is in Formula 1 racing cars. Almost all of the Formula 1 teams use ceramic bearings in their gearboxes and wheel bearings. Formula 1 cars reach speeds of around 200 miles per hour, and so generate massive forces within the car. Ceramic bearings have a clear advantage over metal ones in this harsh environment. Their longer operating life, resistance to corrosion and ability to withstand very high temperatures, make them the perfect choice for these technologically advanced vehicles. The wheel bearings are hybrid, using ceramic balls with high nitrogen steel races. A set of ceramic bearings for a Formula 1 gearbox costs around $5,000 and lasts for about 1240 miles, or 6 complete Grand Prix.

An even more demanding application for ceramic bearings was in Thrust SSC, the car which broke through the sound barrier. On 15th October 1997, the car travelled at 766 mph (Mach 1.020) across the Black Rock Desert, Nevada. The wheels of this car rotated at 8,500 rpm, and at the rim were subjected to forces of 35,000 times that of gravity. Naturally the wheel bearings had to be incredibly tough, and of course Silicon Nitride was the obvious solution.

There are also more everyday applications for the automotive industry, which have been developed from these kinds of project. For example, they have led to the development of Silicon Nitride cam rollers and rolling elements for Diesel engines. These components are used in big trucks and almost eliminate wear compared with traditional metal components.

Silicon Nitride is increasingly being used in a variety of applications that exploit its incredible durability. But surely the most demanding application of all is the one that sends it into space - Silicon Nitride is an important component in the Space Shuttle. It is difficult to imagine a more hostile environment than inside the engine of a Space Shuttle. To get off the ground and into space, the shuttle, which weighs 4.5 million pounds, has to accelerate from 0 to 18,000 mph. It is apparent that the forces inside the engines are massive. In order to achieve lift-off the three main engines provide about 1.2 million pounds of thrust with an additional 6,600,000 pounds from the rocket boosters. Burning liquid hydrogen and oxygen, the main engines can reach 6,000 degrees Fahrenheit. The life span of one of these engines is about 7.5 hours. Silicon Nitride bearings within the fuel pump are lubricated by liquid oxygen, and last about 10 times longer than metal bearings in that unbelievably exacting environment.

In conclusion, it is apparent that the very properties that have led to the use of Si3N4 in the Crystal-Technica Hot Surface Igniter have been recognised and used in many other branches of engineering.