Page 226 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 8.4 Glasses 205

EXAMPLE 8.3 Ceramic Ball and Roller Bearings
Silicon-nitride ceramic ball and roller bearings are They have high wear resistance, high fracture
used when high temperature, high speed, or margin- toughness, perform well with little or no lubrication,
ally lubricated conditions occur. The bearings can be and have low density. The balls have a coefficient of
made entirely from ceramics, or just the ball and thermal expansion one-fourth that of steel, and they
rollers are ceramic and the races are metal, in which can withstand temperatures of up to 1400°C.
case they are referred to as hybrid bearings (Fig. 8.2). Produced from titanium and carbon nitride by
Examples of machines utilizing ceramic and hybrid powder-metallurgy techniques, the full-density
bearings include high-performance machine tool titanium carbonitride (TiCN) or silicon nitride
spindles, metal-can seaming heads, high-speed flow (Si3N4) bearing-grade material can be twice as hard
meters, and the Space Shuttle’s main booster rocl<et’s as chromium steel and 40% lighter. Components up
liquid oxygen and hydrogen pumps. to 300 mm in diameter can be produced.
The ceramic spheres have a diameter tolerance
of 0.13 ,um and a surface roughness of 0.02 pm.

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fi”
ia,
WE If

aa
.ff ii5f~;5§ffi` "'_

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(H) (b)
FIGURE 8.2 A selection of ceramic bearings and races. Source: Courtesy of The Timken Company.

Bioceramics. Because of their strength and inertness, ceramics are used as bioma-
terials (bioceramics) to replace joints in the human body, as prosthetic devices, and
in dental work. Commonly used bioceramics are aluminum oxide, hydroxyapatite,
tricalcium phosphate, silicon nitride, and various compounds of silica. Ceramic im-
plants can be made porous, so that bone can grow into the porous structure (like-
wise with porous titanium implants) and develop a strong bond with structural
integrity.

8.4 Glasses

Glass is an amorphous solid with the structure of a liquid. It has been supercooled
(cooled at a rate too high to allow crystals to form). Technically, glass is defined as
an inorganic product of fusion that has cooled to a rigid condition without crystal-
lizing. Glass has no distinct melting or freezing point; thus, its behavior is similar to