Page 382 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Braking systems C HAPTER 12.1

lead to hot judder problems). Although the compressive
Table 12.1-3 Physical properties of three candidate disc materials
strength is good, the tensile strength is relatively low and
the material is brittle and prone to microcracking in ten- r c P r,c P k
sion. As the proportion of flake graphite in GI is increased, Disc kg Jkg L1 kJ m L3 Wm L1 a 310 L6
the tensile strength reduces but thermal conductivity in- material m L3 K L1 K L1 K L1 K L1
creases as shown in Table 12.1-2. Note that spheroidal
graphite iron (SG) has a higher tensile strength than GI High carbon 7150 438 3132 50 10
but a much reduced conductivity which explains why it is cast iron
rarely used for brake rotors. Generic 20% 2800 800 2240 180 17.5
Currently GI grades used for disc brakes fall into two SiC-
categories reflecting two different design philosophies reinforced Al
(MacNaughton and Krosnar, 1998): MMC
Carbon–carbon 1750 1000 1750 40–150 0.7
1. Medium carbon GI (e.g. Grade 220)
composite
These irons are used for small diameter discs such as on
small- and medium-sized passenger cars. Such discs will
run hot under extreme conditions, and good strength and has been directed at investigating light-weight alterna-
thermal crack resistance at high temperatures are tives to cast iron. Two such alternatives which have re-
therefore required.
ceived serious attention are aluminium metal matrix
2. High carbon GI (e.g. Grade 150) composites (MMCs) and carbon–carbon composites,
These grades tend to be used for larger vehicles where typical properties for each of which are displayed in Table
space constraints are not as content limited. Discs are 12.1-3. together with corresponding properties for a high
larger and, with the improved conductivity due to the carbon cast iron (Grieve, et al. 1995).
high carbon, will run cooler. Strength retention at high Aluminium MMCs normally incorporate 10–30% by
temperature is therefore not as critical and manufactur- volume silicon carbide particle reinforcement within
ability improves with the higher carbon content. a silicon-containing alloy matrix. The resulting composite
Alloying elements can be applied to all grades of cast iron has much lower density than cast iron and much im-
with the general effect of improving strength but at ex- proved conductivity. Thus the thermal diffusivity
pense of thermal properties and manufacturability. The ðk=r,c P Þ is much higher which opens the possibility of
most commonly used elements and their effects are as lighter discs running cooler by being able to rapidly
follows: conduct heat away from the friction interface. However,

aluminium MMCs have a low MOT (c. 500 C) and
chromium increases strength by stabilizing pearlitic there are serious consequences if this MOT is exceeded
matrix at high temperatures (preventing martens- since complete surface disruption may then occur leading
itic transformations) but tends to promote formation to extremely rapid pad wear. Ideally higher re-
of bainitic structures which cause casting/machining inforcement contents or alternative reinforcing materials
difficulties and can reduce pad life;
(e.g. alumina) should be used to increase the MOT but
molybdenum similar to chromium; the former causes severe casting difficulties whilst alu-
copper increases strength without causing manufac- mina reinforcement results in poorer thermal properties.
turing difficulties; It can be seen from Table 12.1-3 that carbon–carbon
nickel as for copper but more expensive; composites have an even lower density than aluminium
titanium reported to influence friction performance MMCs and can have a conductivity almost as high. Their
but rarely used at significant levels. MOT is also very high, raising the possibility of using thin
rotors which run much hotter and lose heat by radiation
12.1.5.3 Alternative rotor materials as well as by conduction/convection. Also the very low
coefficient of thermal expansion of carbon minimizes
Although GI is a cheap material with good thermal thermal distortions. Thus there is the potential for very
properties and strength retention at high temperature, its significant weight savings with carbon–carbon composite
density is high and, because section thickness must be discs. However, the material has a poor low temperature
maintained for both manufacturability and performance, friction performance and moreover is currently much
cast iron rotors are heavy. Currently there are significant more expensive than metallic alternatives. Hence, it is
incentives to reduce rotor weights in order to: (a) reduce likely to remain confined to high performance race car
emissions by improving the overall fuel consumption of applications for the foreseeable future.
the vehicle, and (b) aid refinement and limit damage to When considering alternative materials or designs for
roads by reducing the unsprung mass. Thus much effort disc brakes, reference can be made to the so-called

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