Page 218 - Fiber Fracture

STRENGTH AND FRACTURE OF METALWC FILAMENTS 203

Table 4. Tensile strength of micro-wires produced by the Taylor process (Nixdorf, 1967)
Metal Max. tensile strength Bulk tensile strength
(MPd (MPd
Fe 2800 180-250
Pt-30% Rh I500
Pt 500 I 40
*g 650 I60
cu 400 220-350
Zn I50 30

Various attempts have therefore been undertaken to develop alternative methods to
produce metallic filaments of pm dimension having the excellent properties of whiskers.
Schladitz (1968, 1976) describes the production and the properties of polycrystalline Fe
and Fe-C whiskers. These whiskers grow, similarly to the monocrystalline whiskers,
from the gas phase but their production rate is several orders of magnitude greater.
With diameters in the range of 0.1 to 30 vm their external appearance is similar to
monocrystalline whiskers. Their internal structure, however, is completely different.
They are made of nanocrystallites (a-Fe with a diameter of 8 nm and carbide particles)
and their dislocation density is estimated to be as large as 1.5 x 10l2 cm-2. The ultrafine
microstructure with the carbide particles gives them excellent mechanical properties. Fe
whiskers with less than 1.2% C have tensile strengths between 7 and 8 GPa.
A quite different method to produce wires was invented by Taylor in 1924. Nixdorf
(1968), in an effort to produce high-strength filaments, considerably contributed to its
perfection. In this method hot glass tubes containing the liquid metal are drawn until the
internal diameter of the tube reduces to the desired diameter. After solidification of the
metal and removal of the glass, one obtains micro-wires with round cross-sections and
a smooth surface. The tensile strength of such wires cannot compete with whiskers but
is nevertheless respectable. Table 4 lists some singular values together with values for
bulk polycrystalline samples. Similar to monocrystalline whiskers the dispersion in the
tensile strength is large. It extends from values characteristic of the corresponding bulk
material to the extreme values given in Table 4.

Polycrystalline Micro- Wires

In this section we will discuss the behavior of micro-wires that were produced by
the conventional drawing technique. Very thin Cu wires are extensively used for the
fabrication of flexible electrical cables. Wires with diameters between 20 and 30 pm
made from high-purity Au and Cu or slightly alloyed AI are used as in microelectronics
for electrical connections on chips (bonding wires). Probably the oldest application
of thin wires was the use of W wires as incandescent filament in light bulbs. These
filaments are operated near 2000°C and are likely to recrystallize to a bamboo structure
in which state they become extremely fragile. A great number of studies have been and
continue to be devoted (Schade, 1998) to this subject. The problem is tackled by adding
various grain growth inhibitors to W prior to drawing. Even though we will briefly