Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
20~)03Z0
C330/T
Title: Surface Treatment of Metals and Alloy~ !
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De~cription
' Thi8 invention concerns the surface treatment of metals
and alloys, particularly titanium and titanium alloys.
Background to the invention ~
It i8 well known that titanium and its alloys have some ~ -
very useful properties: they are strong, corrosion
resistant, light in weight and easily machinable, weldable ~ -
and foregeable. However the wear properties of such
materials, either between themselves or with other ,~
materials, are very poor. In consequence such materials~ --
are generally unsuitable for application in circumstances
in which rubbing takes place, although they are used in;--~
some special situations where no other alternative i5 -~
practicable, with the material being coated and/or treated
in various ways. , ~-~
None of the coatings and treatments available so far has ! -:
proved really satisfactory. Coatings generally lacX
adhesion or hardness. Metallurgical treatments generally
damage the fatigue resistance, surface finish, or hardness
and/or cause distortion of the bulk material due to the
high temperatures at which the treatments are carried
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out.
In particular plasma nltridlng, a process u~ed widely for
surface hardening of~ some steel alloys, has proved to be
disappointing in treatment of titanium. In this process
as applied to steel a finished component is introduced
into a vacuum ~ystem and bombarded with nitrogen ions, by
holding the component at a negative DC voltage of some 0.1
to 1 kilovolts. The energy dissipated at the surface of
the material by the ion bombardment raises the temperature
gradually to 800-900-C (although the heating can be
accelerated by additional heating). In this temperature
range the nitrogen reacts at the surface with iron, ~-~
producing iron nitrides. The nitrogen diffuses slowly
into the surface during treatment. After a treatment at
temperatures up to lOOO-C and;lasting up to 60 hours a --
hard nitride skin some tens of microns thick is
established on the component. The nitrogen reacts m~re
rapidly with other alloying elements such as titanium
producing titanium nitrides (TiN and Ti2N), which are very
hard and ~table compounds.
However there are various drawbacks to this process. The
effect is inevitably accompanied by changes in dimension,
i.e. distortion, generally necessitating re-worXing or re-
finishing of the component by machining, grinding or
polishing. In addition, and re importantly, there is
a reduction in fatigue resistance of the material probably
due to microcracks being opened up in the surface by
preferential attack by nitrogen ions at grain boundaries.
Moreover the surface finish of the component deteriorates
and takes on an "orange peel" appearance.
In another process, a component is coated with titanium
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nitrlde by phy~lcal vapour depo~ltlon ~PVD). Tltanlum i~
vaporlsed from a eruclblo in a vaeuum sy~tem by electron
beam hombardment. A~ a firet step, the fini~hed machined
eomponent, which has~already been ultra~onleally eleaned,
iB ion bombarded by argon ions for about 30 minute~ in the
vaeuum system, to remove any residual eonta-ination and to
raise the temperature of the eomponent to about 300-400-C.
Then with the component again at negative voltage titanium
evaporation io eommeneed by bombarding the titanium ~ouree
with eleetrons. At the same time nitrogen and argon gas
are admitted to the vaeuum system 80 as to ereate a gas
pressure of some 5-15 ~ 10 3 millibars ~3.3-10 mierons):
a plasma is created of electrons and ionised nitrogen, ~
argon and titanium. Operating eonditions sueh as plasma -- -
pressure and voltage are generally regulated to maintain ~ ~ ;
the temperature of the eompone`nt in the range 350-450-C. ¦ -
- The eomponent attraets the ions, whieh give up their
eharge on arrival at the surface, and the titanium and
nitrogen combine to form a stoichio-etrie eompound of TiN.
After an hour or 80 a titanium nitride eoa-ting 2 or 3 -
mieron~ thiek is built up on the surfaee of the eomponent.
Under the correet conditions the adhesion is satisfaetory.
m On steel surfaces this treatment greatly improves the
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; eutting and wear performance of many manufacturing tools.
On titanium alloys such treatment provides a wear ~ -
resistant surface which is adequate for many specialized
! i ~ applieations: indeed this proeess transforms the
performance and makes the use of titanium alloys
practicable in vehicles (at present mainly racing and
rally cars).
Nevertheless, the coating is relatively thin and brittle. -~
Under high load condition6 sub surface deformation of the
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component can cause breakdown of the coating. In
contrast, the comparatlvely deep diffu~ed layers produced
by pla~ma nitrldlng do not suffer from hlgh load ~ub
surfaee deformatlon. The metallurgleal propertie~ of the
material are hardly affected by PVD ~the treatment
temperature is not high enough), there is no distortion,
the fatigue resistance i8 not wor~ened and can even be
improved, and the surface is left with a highly reflecting
polish as good as before coating.
If the wear propertie~ of titanium and its alloy~ could be
further improved in a cost-effective way this would
signifieantly open up the range of potential uses for such
materials in many industries, particularly the aerospace,
automobile and chemical industries for which combination~ -
of the other properties of su¢h materials are of
outstanding value.
The Invention
According to one aspect of the present invention there is -
provided a method of treating a component made of metal or
alloy in whieh the component is bombarded with nitrogen -~
:;:
ions in a vacuum system at a temperature below that
eonventionally used for plasma nitriding.
The nitrogen ion bombardment, or nitriding, i8 generally ~-~
;~ carried out at a temperature below 850-C, typically in the
range 600-700-C.
:
Nitriding treatment time i8 not critical, but will
typically be in the range 1-2 hours, which is a small
fraction of the time of conventional plasma nitriding :~
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treatment.
The nitriding may be carried out using nitrogen alone, or
nitrogen in combination with other gases such as argon.
After nitriding, a surface coating, particulary of a
nitride, may be formed on the component. Such a coating
may be formed in conventional manner, e.g. using PVD as
described above. Preferred coatings include titanium
nitride, although other materials such as aluminium
nitride and titanium aluminium nitride may also be used.
By way of example, when coating with titanium nitride by
PVD, titanium metal in the vacuum system is vaporised by
electron beam bombardment in parallel with continued
nitrogen ion bombardment. Treatment is generally carried
out for between 20 and 40 minutes, say 1/2 hour, and
following conventional practice at a temperature in the
range 350-450-C, although rather higher temperatures, say
up to 600-700C, can also be used.
If desired, a further coating, eg of a carbide, oxide or
carbonitride, may be formed on top of the first coating.
Prior to the nitriding treatment the component is ' -
conveniently cleaned by ion bombardment in conventional
manner, e.g. by bombardment with argon ions in a vacuum
system as described above. Such treatment is typically
carried out for between 1/4 and 3/4 hour, say 1/2 hour, at
a temperature in the range 300-400C, although rather
higher temperatures, up to about 500C, can also be used.
Nitrogen or titanium ions could also be used in similar
manner for cleaning purposes. If
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nitrogen ion~ are used some plasma nitriding will take
place during the cleaning proces~.
The invention ie particularly applicable to the treatment
of components of titanium and titanium alloys, but may
also be used for other materials such as steel and other
ferrous alloys.
In a further aspect the present invention provides a
method of treating a component made of metal or alloy,
comprising cleaning the surface of the component by ion
bombardment; bombarding the cleaned component with
nitrogen ions in a vacuum system at a temperature below
that conventionally used for plasma nitriding and then
forming a surface coating on the component.
- I
The invention also includes within its scope a component
. which has been treated by a method. in accordance with the
invention.
1~ The nitriding treatment of the invention allows surface
: treatment of a component to be carried out relatively
: quickly, for example 1/2 hour cleaning, 1-2 hours :
:~ ~ nitriding, then 1/2 hour coating by PVD. ~.
Further, use of the nitriding treatment of the invention
prior to surface coating enables production of a component
w!ith improved properties compared with those obtained
hitherto, having good adhesion, wear and surface finish
~` properties without any distortion of the component
occurring.
~: .
In a typical embodiment of the invention a component of
titanium or titanium alloy i~ cleaned by argon ion ~.
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bombardment ln a vacuum system, ~uch a~ the vacuum eystem~
produced by Tecvac Limlted and marketed a~ their IP35L
model. The component is maintalned at a negative
voltage, and argon gas pumped through the system.
Cleaning i8 carried out for about 30 minutes, with the
temperature of the component rising rapidly due to
dissipation of energy at the surface of the material by
the ion bombardment, and being maintained in the range
300-400-C by suitable regulation of the plasma pressure
and the negative voltage of the component. -~-
,
After cleaning the component is nitrided by introducing
nitrogen gas to the vacuum system and bombarding the
component inten~ely with nitrogen ions for 1-2 hours, with
the temperature of the component maintained in the range
600-700-C by regulation of the plasma pressure and the
voltage of the component.
- - , . .
Immediately following nitriding a titanium nitride coating ~-~
is laid down on the component by PVD, by vaporising ~ '
titanium metal introduced to the vacuum system by electron
beam bombardment in parallel with continued nitrogen ion
bombardment. PVD is carried out for about 30 minutes, -
with the temperature of the component maintained in the
range 350-600-C by regulation of the plasma pressure and ~-
the voltage of the component. -~
The resulting surface appears to have all of the - ~-
advantages associated with both of the known treatments ;~
(plasma nitriding and coating by PVD), without many of the - ;
disadvantages. Compared with results obtained
previously, adhesion of the titanium nitride coating is
improved; the fatigue properties and bulk metallurgical
structure of the component are little affected; there is `~;-
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no distortion (as the temperatures employed have not been
sufficiently high to produce di~tortion)~ and the surface
finish is as good a~ that of the component prior to
treatDent .
Features of preferred treatment methods incorporating the
invention are:
1. To build up a graded structure, with a tough core, an
interlayer of increasing hardness and a top ~urface of
greatest pos 8 ible hardne~ 8 .
2. To produce the interlayer and top surface by
consecutive treatment stages in the same equipment, with
one stage immediately following the other without breaX ~io
as to avoid contamination whic~ can affect adhesion.
:~ .
3. To produce diffused layers on titanium with benefits of
both titanium nitride coating and plasma nitriding at low
temperatures and in very fast cycle times, typically one
tenth of plasma nitriding times.
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4. To produce diffused layers on hardened steel by
nitriding at a lower temperature and for a much shorter
time than customarily used for plasma nitriding (typically
one tenth of that time) followed immediately in the same
equipment without a break (ie with one stage immediately
following the next, as in 2 above) by coating with
titanium nitride or titanium aluminium nitride by a PVD ~.
process.
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5. To produce diffused layers on metals other than
hardened steel by nitriding at lower temperatures and for
times shorter than customarily used for plasma nitriding
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followed immediately ~as in 2 above) by coating with
titanium nitride or titanium alumlnium nltride by a PVD
proce~o.
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