24 Silicon Nitride

Silicon nitride refers to a family of ceramics whose primary constituent is Si3N4. The ceramics in this family have a favorable combination of properties that includes high strength over a broad temperature range, high hardness, moderate thermal conductivity, low coefficient of thermal expansion, moderately high elastic modulus, and unusually high fracture toughness for a ceramic [5, 6]. This combination of properties leads to excellent thermal shock resistance, ability to withstand high structural loads at high temperatures, and superior wear resistance.

Silicon nitride has been under development continuously in the United States since the late 1960s. Initial efforts were directed toward development of components for gas turbine engines, but this turned out to be a very difficult challenge. Although extensive testing has been conducted, silicon nitride has not yet reached a significant level of production for turbine engines [7]. However, silicon nitride ceramics have reached large-scale production for cutting tools, bearings, tur-bocharger rotors, diesel cam follower rollers, and diesel prechambers and have reached moderate levels of production for other applications such as thermocouple protection tubes, grit-blast nozzle liners, wire-forming rolls and guides, paper-making dewatering foil segments, check valve balls, downhole oil well parts, aluminum die-casting tooling, and a variety of custom wear parts [8]. Figures 2.4 and 2.5 illustrate some silicon nitride parts.

Although the initial driver for silicon nitride development was gas turbine engine components, the first major application was cutting tool inserts [9]. Cutting hard metals such as cast-iron, tool steels, and superalloys results in high temperature at the tool-workpiece interface. Tool failure was usually caused by a combination of wear and high-temperature corrosion. WC-Co, the traditional workhorse for metals machining, wears/corrodes rapidly if the temperature gets too high, so the cutting speed must be limited to around 120 m/min (~400 surface feet per minute) and sometimes even down to around 25 m/min. Silicon nitride is much more resistant that WC-Co to temperature and chemical corrosion. Cutting speeds higher than 1520 m/min have been demonstrated with silicon nitride at a depth of cut of 5 mm and feed rate of 0.4 mm per revolution. Such a rapid rate of metal removal heats the silicon nitride cutting edge to around 1100° C and imposes extreme conditions of thermal shock, impact, contact stress, and erosion/corrosion. This gives an indication of the severe conditions that silicon nitride materials can survive.

The use of silicon nitride cutting tool inserts has had a dramatic effect on manufacturing output [10]. For example, face milling of gray cast-iron gear-case housings with silicon nitride inserts doubled the cutting speed, increased tool life from one part to six parts per edge, and reduced the average cost of inserts by

FIGURE 2.4 Silicon nitride parts including blast nozzle liners, wire-forming rolls and guides, papermaking dewatering foil segments, check-valve balls, downhole oil well parts, custom wear parts, and centrifugal dewatering screen and scraper blade for potash and coal dewatering. (Photo courtesy of Ceradyne, Inc., Costa Mesa, CA).

FIGURE 2.4 Silicon nitride parts including blast nozzle liners, wire-forming rolls and guides, papermaking dewatering foil segments, check-valve balls, downhole oil well parts, custom wear parts, and centrifugal dewatering screen and scraper blade for potash and coal dewatering. (Photo courtesy of Ceradyne, Inc., Costa Mesa, CA).

FIGURE 2.5 Experimental silicon nitride gas turbine engine components. (Photos courtesy of Honeywell Engines, Systems, and Services, Phoenix, AZ).

50%. Outside grinding of diesel truck cylinder liners increased the number of parts machined per tool index from around 130 to 1200 and totally eliminated a prior problem with insert breakage. As a result, tool life was increased to achieve 9600 cylinders per cutter load of inserts compared to 450. The decreased downtime alone increased the output per shift by 25%.

A more recent application for silicon nitride that is having major impact on many industries is bearings [5, 11]. Silicon nitride was first demonstrated as a superior bearing material in 1972 [12] but did not reach production until nearly 1990 because of challenges in reducing the cost. Since 1990 the cost has been reduced substantially as production volume has increased. Although silicon nitride bearings are still 2-5 times more expensive than the best bearing steel, their superior performance and life have resulted in rapid escalation in their use. About 15-20 million silicon nitride bearing balls were being produced in the United States by 1996, and the number has increased dramatically each year since. Table 2.3 lists some applications for silicon nitride bearings.

One of the most important applications of silicon nitride bearings is in machine tool spindles [5]. Because of their light weight (60% lighter than steel), silicon nitride bearings can be operated at much higher speed than metal bearings without generating a critical level of centrifugal stress. Because of their low thermal expansion (one-fifth that of steel) and high elastic modulus, the silicon nitride bearings can operate to much closer tolerances than metal bearings, which enables machines with higher precision and lower vibration. Because of their high hardness and smoother surface, the silicon nitride bearings run smoother and wear at about one-seventh the rate of the best metal bearings. All of these factors together result in 3-10 times the life of metal bearings, up to 80% higher speed capability, about 80% lower friction, higher operating temperature, and 15-20% reduction in energy consumption.

In addition to cutting tool inserts, bearings and check valves, silicon nitride is being vigorously evaluated for diesel and auto engine valves, valve guides, stator vanes and rotors for turbines, a variety of wear parts, forging dies for aluminum, and many other potential products. As additional production applications are achieved and current production levels increase, it is anticipated that the cost of silicon nitride will be significantly reduced, which will remove the primary barrier that has limited broad use of advanced silicon nitride materials.

TABLE 2.3 Some Applications of Silicon Nitride Bearings

Machine tool spindles High-speed hand grinders Food-processing equipment CAT scanners

Spectroscopes Photo copier roll bearings Medical centrifuges Aircraft anti-icing valves

Gearboxes

Helicopter pitch blades

Gas turbine engines High-speed compressors High-speed train motors Air-driven power tools

Gyroscopes

Optical-kinematic mounts Racing cars

Semiconductor processing equipment Actuators

Aircraft wing flap ball screws

Pumps Gas meters Check-valve balls Chemical-processing equipment Galvanizing lines High-speed dental drills In-line skates Textile equipment

Radar

Butterfly valves

Shuttle liquid oxygen pumps

Shuttle main engine

Instruments

The key message from the above examples is that the silicon nitride family is a new generation of ceramics that are much more durable and resistant to brittle fracture than many engineers may realize and may be viable options to consider. The key properties that distinguish silicon nitride from traditional ceramics are the high toughness, thermal shock resistance, and both chemical and structural stability at high temperature.

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