PROCEDE POUR PRODUIRE DES COUCHES DE BORURE RESISTANTES A L'USURE SUR DES SURFACES DE MATERIAUX METALLIQUES

PROCESS FOR PRODUCING WEAR-RESISTANT BORIDE LAYERS ON METALLIC MATERIAL SURFACES

Abrégé français

L'invention concerne un procédé pour produire des couches de borure résistantes à l'usure sur des surfaces de matériaux métalliques. Ce procédé est caractérisé en ce que de l'halogénure de bore choisi dans le groupe comprenant le trifluorure de bore, le tribromure de bore, le triiodure de bore et leurs mélanges est mélangé avec de l'hydrogène et éventuellement de l'argon et/ou de l'azote en vue de produire un gaz de réaction contenant entre 0,1 et 30 % en volume d'halogénure de bore. Le mélange obtenu est activé par une décharge de plasma de telle manière que le bore peut être transféré du plasma à la surface métallique.

Abrégé anglais

A process is provided for producing wear-resistant
boride layers on metal material surfaces. The process is
characterized in that a boron halide selected from the group
comprising boron trifluoride, boron tribromide, boron
triiodide and their mixtures is mixed with hydrogen and
optionally argon and/or nitrogen, in order to produce a
reaction gas containing between 0.1 and 30 vol% boron halide.
The resultant mixture is activated by a plasma discharge
whereby boron is transferred from the plasma to the metal
surface.

Revendications

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What is claimed is:
1. A process for producing wear-resistant boride layers
on metallic material surfaces, which comprises mixing at
least one boron halide selected from the group consisting
of boron trifluoride, boron tribromide and boron triiodide
is boron source with hydrogen and argon and/or nitrogen to
generate a reaction gas containing from 1 to 35% by volume
of boron halide and activating the resulting mixture by
means of a plasma discharge so as to enable boron to be
transferred from the plasma to the metal surface.
2. The process as claimed in claim 1, wherein the boron
source used is boron trifluoride.
3. The process as claimed in claim 1 or 2, wherein. the
reaction gas contains 20-90% by volume of H2.
4. The process as claimed in one or more of claims 1 to
3, wherein the reaction gas is fed into the treatment space
in an amount of from 0.5 to 2 1 per minute.
5. The process as claimed in one or more of claims 1 to
4 carried out at temperatures of from 400°C to 1200°C.
6. The process as claimed in one or more of claims 1 to
5, wherein the treatment time is from 30 to 240 minutes.

Description

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PROCESS FOR PRODUCING WEAR-RESISTANT BORIDE LAYERS
ON METALLIC MATERIAL SURFACES
BACKGROUND OF THE INVENTION
1) Field of the Invention
The invention relates to a process for producing wear-
resistant boride layers on metallic material surfaces.
Wear-resistant boride layers are usually produced in
practice using solid boriding agents, for example in the
form of powders, pastes or granules.
A disadvantage of these processes is that they are
labor-intensive in terms of packing, unpacking and cleaning
the parts. Cleaning is carried out using a combination of
washing and brushing or abrasive-blasting. Since the
powders, pastes and granules can be used only once,
problems also arise in disposing of the spent boriding
agents.
In addition, the use of liquid boriding agents, for
example in the form of salt melts, is also known. However,
all these processes have not been able to become
established owing to the problems generally associated with
salt baths, viz. those relating to safety of handling,
cleaning of the parts after treatment and disposal of the
baths or their waste products.
In the past, there have been various attempts at
boriding using gaseous boriding agents (CVD processes).
When using organic boron compounds (trimethylboron,
trialkylborons), carburization occurred predominantly
instead of boriding; when diborane is used, safety problems
occur because of the extreme toxicity and the risk of
explosion.
The use of boron trichloride as a boron donor medium
has not been able to become established because of process-
inherent problems in layer formation. The cause of these
problems is the hydrogen chloride formation which always

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occurs in boriding using BC13/HZ mixtures.
In the boriding of ferrous materials using boron
trichloride, the following fundamental reactions occur:
2BC13 + 3H2 + 2Fe ---> 2FeB + 6HC1
2BC13 + 3H2 + 4Fe ---> 2Fe2B + 6HC1
The hydrogen chloride gas formed in boriding using BC13
reacts with the iron of the base material to form volatile
iron chlorides:
2HC1 + Fe ---> FeClz
The iron chlorides have high vapor pressures at the
treatment temperatures in the range 500°C-1200°C which are
employed, resulting in substantial, ongoing evaporation of
iron chloride. This leads to hole formation between boride
layer and substrate, as is always criticized in the case of
the BC13 process . Suppression of the hole formation is only
possible if one succeeds in generating an impermeable
boride layer within a very short time at the beginning of
boriding. This is technically difficult in that to the
present time it cannot be achieved reliably and
reproducibly.
Apart from the purely thermal variant of CVD boriding,
work on plasma-aided boriding (PACVD boriding) is also
known. Hitherto, only diborane and boron trichloride have
been used in this process variant, accompanied by the
disadvantages which are already known from thermal CVD. An
overview of the processes mentioned may be found in the
review "Engineering the Surface with Boron Based
Materials", Surface Engineering 1985, Vol.l, No. 3, pp.
203-217.
It is therefore an object of the invention to provide
a process for producing wear-resistant boride layers on
metallic materials, which process does not suffer from the
abovementioned disadvantages.

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Summary of the Invention
According to the invention, this object is achieved by
a process which comprises mixing at least one boron halide
selected from the group consisting of boron trifluoride,
boron tribromide and boron triiodide as boron source with
hydrogen and possibly argon and/or nitrogen to generate a
reaction gas containing from 1 to 35% by volume of boron
halide and activating the resulting mixture by means of a
plasma discharge so as to enable boron to be transferred
from the plasma to the metal surface.
Description of the Preferred Embodiments
The reaction gas can further comprise boron
trichloride as boron source.
The reaction gas preferably contains from 5 to 20o by
volume of boron halide, particularly preferably from 5 to
15o by volume of boron halide.
The reaction gas preferably contains from 20 to 90 o by
volume of Hz, particularly preferably from 20 to 50o by
volume of H2.
The reaction aas nreferahlv cnmr~ri sac hnrr-,n
trifluoride.
Boron trifluoride is particularly preferably used as
the boron halide.
The reaction gas is fed to the treatment space in an
amount of preferably from 0.5 to 2 1 per minute,
particularly preferably about 1 1/min.
Boriding is preferably carried out in the pressure
range of 1-10 mbar under the action of a plasma discharge
as is known, for example, from plasma coating units. The
plasma discharge can be pulsed or unpulsed.
The required treatment temperatures of preferably from
400°C to 1200°C, particularly preferably from 850 to
950°C,
are generated by the plasma itself or, especially in the
high-temperature range above 900°C, with the aid of
additional heating.

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The treatment time is preferably from 30 to 240 min,
particularly preferably from 30 to 120 min.
The thickness of the boride layers is usually
controlled via the treatment time, with the thicknesses

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4 ~ . . ._
of the layers increasing with increasing treatment
t ime .
As further gases, the reaction gas can
additionally compxise argon and/or nitrogen: They
enable the activity of boron transfer to be controlled
and sufficient heating of the specimens by the plasma
to be achieved. The composition of the reaction gas can
thus vary within wide limits depending on the treatment
conditions and the material to be borided.
The process of the invention is particularly
suitable for boriding ferrous materials.
In the process of the invention,' molecular
hydrogen present in the reaction gas is converted into
atomic hydrogen by means of a plasma discharge. The
atomic hydrogen reduces the boron halide (BY3) and thus
enables boron to be transferred to the workpiece
surface,
F
BY3 + 3 H --__> B + 3 FiY
H + x ME --_--y MexH
However, conversion of BY3 into BYZ by the
plasma can also. occur, in which case the following
reactions can then proceed:
H
3 BYa __-> B + 2 BY3
B -F- x ME ----> MexB
Subsequent to boriding, the borided material
can be subjected to an aftertreatment to convert any
FeB formed into Fe2B. This can be achieved, for
example, by a heat-treatment process subsequent to the
boriding treatment by. stopping the supply of boron
halide and holding the workpiece at the treatment
temperature for"'-~a further time. The duration of this
diffusion treatment depends on the amount of FeB
present and is ~isual~y 20-60 min.
~5_
The process can be carried out, for example, in
a unit which is suitable for plasma coating and is

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known per se, This consists essent~.ally of the
following components:
The vacuum vessel (reactor) for accommodating
the parts to be treated. The reactor should be heatable
and allow operation in the temperature range from 400°C
to 1200°C.
The pumping system for evacuating the reactor
and setting the working pressure.
The gas s~.zpply unit for mixing and metering in
the zeaction mixture,
The pulsed pl8sma power supply for generating
and, maintaining the plasma discharge in the vacuum
vessel, such that the power introduced can be varied
within a wide range by means of the pulse frequency or
pulse width.
The system for neutralizing and disposing of
the gas and the system for controlling and monitoring
the operating parameters,: the latter system controls
and monitors the course of the process.
Example 1
After ir_troducing a 100Cr6 steel specimen into
the reactor, it .s heated in the plasma of a DC glow
discharge having:a constant pulse frequency (4kHz) at a
pressure of 10 mbar. The specimen is additionally
heated by means cf the heating of the reactor, thereby
shortening the heating time. The heating and cooling of
r
the specimen is. carried out in a 1:1 mixture of argon
and hydrogen. After the treatment temperature of 850°C
. is reached, the boron source boron trifluoride is added
at such a rate that a reaction gas mixture composed of
45~ by volume of hydrogen, 40~ by volume of argon and
15~ by volume of boron trifluoride is formed. The gas
mixture is fed to the vessel in an amount of 1. 1/min.
The duration of the plasma treatment is 200 min.
A metallogxaphic section shows a boride layer
having a mean thickness of 42 um. Tha microhardness is
1800 HVo.os- The Payer is free of FeH.

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Example 2
After introducing a 'Hastelloy B specimen into the
reactor, it is heaved in the plasma of a DC glow discharge
having a constant pulse frequency (4kHz). The specimen is
heated tc a temperature of 850°C by means of the plasma
discharge at 10 mb<:~r. The power density is controlled via
the pulse width. The specimen is heated exclusively by
means of the glow discharge. The heating and cooling of
the specimen is carx-ied out in a 1:1 mixture of argon and
hydrogen. After tlne treatment temperature is reached, the
boron source boron t:rifluoride is added in such an amount
that reaction mi:~ture composed of 45% by volume of
hydrogen, 45o by volume of argon and loo by volume of boron
trifluoride is formed. The gas mixture is fed to the
vessel in an amount of 1 1/min. The treatment time is 240
mm .
A metallographic section shows a boride layer having
a mean thickness of 50 ,um.
The micro hardness is 2400 Hvo 05-
* Trade Mark