C90500 is a member of the gear bronze family. These tin bronzes are all
known for wear resistance and good thermal conductivity.
High hardness does not always mean a metal is wear resistant. Hardness may
be raised by cold work or alloying with a metal that raises hardness, but not wear
resistance. True wear resistance requires definite properties.
The metal matrix should have a second phase, finely dispersed, that has a
low coefficient of friction and wear resistance. These particles act as small slide
bearings that contact the valve stem, but do not abrade it. Alloy C90500 has a strong
bronze matrix (alpha phase) with a fine dispersion of the hard, slippery delta phase.
As this alloy is poured into the continuous casting die, the liquid is
initially 88% copper - 10% tin - 2% zinc. As the material solidifies, the growing crystals
absorb tin at a decreasing rate as the casting freezes. As a result, the last metal to
freeze between the crystals has a tin content that freezes as a different crystal type.
This last metal to freeze is the delta phase. It is this phase that makes the alloy wear
The hardness of the alpha phase is typically Rockwell 75B, whereas the
hardness of the delta phase is Rockwell 30C (converted from Vickers 294). The delta phase
is superior to other possible second phases (silicides, aluminides, etc.). This delta
phase may not be the "hardest", but it has better wear resistance.
The alpha phase in manganese silicon bronze is prone to galling, whereas
the alpha phase in C90500 is a copper tin alloy that has been used to make strong sleeve
bearings for over 200 years.
We are also of the opinion that continuous cast C90500 is a better choice
for valve guides than any extruded alloy. in an extrusion, the second phase is present as
stringers of particles in the extrusion direction. The phosphor bronzes can not be
extruded over 5% tin content and the stringers do not cover the valve stem as completely
as the random and larger quantities of delta phase found in C90500. whether the stringers
are delta (C54400 phosphor bronze) or manganese silicide stringers - they are all
In sliding or rotary contact, stringers are undesirable. Tribologists have
documented that bronze bearings are more wear resistant if the second phase is randomly
and finely dispersed.
The properties of continuous cast C90500 are strictly derived from
chemistry. There is no applied heat treatment and the metal is not cold worked. Both the
heat treated and cold worked metals will lose their strength when subjected to high
One other feature of the tin bronzes is that the tin content allows the
formation of polar molecules on the bearing surface. This is the most desirable effect,
especially where valve stem lubrication is meager and temperatures are high.