handbook of carbon, graphite, diamond and fullerenes

Structure and Properties of Diamond 275
The by-product of the reaction is carbon dioxide which, being a gas,
provides no surface passivation. A fresh diamond surface is always
exposed and oxidation proceeds parabolically with temperature. Oxidation
in air is less rapid with the onset of the reaction at approximately 500°C.
Under normal conditions, oxygen is adsorbed on the surface of
diamond after exposure to air (or oxygen) for a period of time. However, no
adsorption occurs if the temperature is below -78°C. From 0 -144°C, oxygen
is chemisorbed. CO2 is formed from 244 to 370°C by the interaction of O2
with the diamond surface. [24] Adsorbed oxygen and carbon oxides account
for the hydrophillic characteristic of diamond. As mentioned in Sec. 10.4,
the formation of surface oxides is also an important factor in the control of
frictional properties.
At 600°C and low pressure, the presence of residual oxygen results in
the formation of a dense film of graphite.
11.2 Reaction with Hydrogen
Like oxygen, hydrogen is chemisorbed on the surface of diamond but
not until a temperature of 400°C is reached. This chemisorption is probably
the result of the formation of surface hydrides. Diamond is generally
considered inert to molecular hydrogen (as opposed to graphite). However
attack by atomic hydrogen occurs above 1000°C. Yet diamond is far less
reactive than graphite, a characteristic which is used to good advantage in
the deposition of diamond films and the selective elimination of the codeposited
graphite (see Ch. 13).
11.3 General Chemical Reactions
Diamond is resistant to all liquid organic and inorganic acids at room
temperature. However it can be etched by several compounds including
strong oxidizers such as sodium and potassium nitrates above 500°C, by
fluxes of sodium and potassium chlorates, and by molten hydroxides such
asNaOH. It is resistant to alkalis and solvents. At approximately 1000°C,
it reacts readily with carbide-forming metals such as Fe, Co, Ni, Al, Ta, and
B. This characteristic provides the mechanism of high-pressure synthesis
(see Ch. 12, Sec. 4.5). Generally speaking, diamond can be considered as
one of the most-chemically resistant material.