Page 365 - Introduction to Marine Engineering

336 Engineering materials

bronze or propeller brass. Additions of aluminium and zinc result in
aluminium bronze and gunmetal respectively.

Cupro-nickel
An alloy of copper and nickel with 20 or 30% of nickel. Good strength
properties combined with a resistance to corrosion by sea or ri¥er waters
make this a popular alloy. Monel metal is a particular cupro-nickel alloy
with small additions of iron, manganese, silicon and carbon.

White metal
Usually a tin based alloy with amounts of lead, copper and antimony. It
may also be a lead based alloy with antimony. White metal has a low
coefficient of friction and is used as a lining material for bearings.

Non-metallic materials

Many non-metallic materials are in general use. Their improved
properties have resulted in their replacing conventional metals for many
applications. The majority are organic, being produced either synthetic-
ally or from naturally occurring material.
Ceramics are being increasingly considered for marine use particular-
ly where galvanic corrosion is a problem. Sintered alpha silicon carbide
and other silicon-based ceramics have good strength properties and are
inert in sea water.
The general term 'plastic' is used to describe many of these
non-metallic materials. Plastics are organic materials which can be
moulded to shape under the action of heat or heat and pressure. There
are two main classes, thermoplastic and thermosetting, although some
more modern plastics are strictly neither. Thermoplastic materials are
softened by heat and can be formed to shape and then set by cooling,
e.g. perspex, polyvinylchloride (PVC) and nylon. Thermosetting
materials are usually moulded in a heated state, undergo a chemical
change on further heating and then set hard, for example Bakelite,
epoxy resins and polyesters.
Some general properties of plastic materials are good corrosion
resistance, good electrical resistance and good thermal resistance; but
they are unsuitable for high temperatures. To improve or alter
properties, various additives or fillers are used, such as glass fibre for
strength. Asbestos fibre can improve heat resistance and mica is
sometimes added to reduce electrical conductivity.
Foamed plastics are formed by the liberation of gas from the actual
material, which then expands to form a honeycomb-like structure. Such