Section 13. qxd

CUTTING-TOOL MATERIALS
13-53
Fig. 13.4.6
Hardness of tool materials as a function of temperature.
Fig. 13.4.7
Ranges of properties of various groups of tool materials.
and (3) titanium carbide with nickel and molybdenum as a binder, for
use where cutting temperatures are high because of high cutting speeds
or the high strength of the workpiece material. Carbides are classified
by ISO and ANSI, as shown in Table 13.4.2 which includes recommen-
dations for a variety of workpiece materials and cutting conditions. (See
also Sec. 6.4.)
Coated carbides
consist of conventional carbide inserts that are coated
with a thin layer of titanium nitride, titanium carbide, titanium carboni-
tride, ceramic, polycrystalline diamond, or diamondlike carbon. The
coating provides additional wear resistance while maintaining the
strength and toughness of the carbide tool. Coatings are also applied to
high-speed steel tools, particularly drills and taps. The desirable prop-
erties of individual coatings can be combined and optimized by using
multiphase coatings.
Carbide tools are now available with, e.g., a layer of
titanium carbide over the carbide substrate, followed by aluminum
oxide and then titanium nitride. Various alternating layers of coatings
are also used, each layer being on the order of 80 to 400 
m
in (2 to 10 
m
m)
thick.
Stiffness is of great importance when using carbide tools. Light
feeds, low speeds, and chatter are deleterious. No cutting fluid is needed,
but if one is used for cooling, it should be applied in large quantities and
continuously to prevent heating and quenching.
Ceramic,
or 
oxide,
inserts consist primarily of fine aluminum oxide
grains which have been bonded together. Minor additions of other ele-
ments help to obtain optimum properties.
Other ceramics include silicon nitride, with various additives such as
aluminum oxide, yttrium oxide, and titanium carbide. Silicon-nitride-
based ceramics include 
sialon
(from silicon, aluminum, oxygen, and
nitrogen) which has toughness, hot hardness, and good thermal-shock
resistance. More recent developments include