Note: Descriptions are shown in the official language in which they were submitted.
1~64~1!37
This invention relates to cemented carbide articles,
in particular cutters, having a coating comprising at least one
layer of a refractory material, covering at least part of the
surface of the article.
The invention also relates to a method of increasing
the wear resistance of the surface of a cemented carbide article.
Articles of cemented carbide ~a term corresponding to
the German term "Hartmetall") consist of a mixture of at least
one binder metal and at least one metal carbide of great hardness.
This carbide may in particular be chosen from the following:
tungsten carbide, titanium carbide, tantalum carbide,niobium carbide and the
mixed carbide of tantalum and niobium. The binder metal may for
example be cobalt, iron, or nickel. The surface of a cemented
carbide article has very great hardness and a very high abrasion
resistance, greater than that of ordinary metals and alloys, in
particular steel. This enables such articles to be used for
various purposes in which the surface of the article must have
great hardness and high abrasion resistance, in particular as
non-resharpenable cutting tools (cutting blades) intended for
machining hard materlals, for example steel used as dies, etc.
It is obviously desirable to increase the wear resis-
tance of cemented carbide articles. In particular, in the case
- of cutting blades, such an increase would enable the life of these
blades to be prolonged for a given cutting speed, or the cutting
speed to be increased for a given life, or both the cutting speed
and life of the blades to be increased.
Processes are already known which enable the wear
resistance of cemented carbide cutting tools to be increased. One
such process consists of covering the surface of the tools with
a coating having a wear resistance greater than the wear
resistance of the original tool surface, and consisting of at
least one carbide chosen from the same constituents as those used
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forming the cemented carbide, in particular titanium carbide TiC.
According to a further process, described in Swiss
Patent Specification No. 540,991, at least part of the surface
of the article of which the wear resistance is to be increased,
is covered with a layer, of at most 50 microns thickness, formed
from at least one refractory compound. In the said Swiss patent,
and likewise in the present description and claims, the term
"refractory compoundl' or "refractory material" is used with its
usual meaning i.e. "solid compound (or material) which is
resistant to high temperatures and chemical agents without sig-
nificant alteration".
Swiss Patent 540,991 mentions, as refractory material,
a simple oxide, nitride or boride of an element belonging to
groups III to VI of the periodic table of elements, a mixed oxide
of the spinel type, an oxide formed from a solid solution of at
least two simple oxides, or mixtures or combinations (these two
latter terms embracing solid solutions and definite compounds) of
at least two oxides.
Finally, according to a process described in Swiss
Patent 540l990, th~ coating material used is at least one of the
following refractory oxides: aluminium oxide, zirconium oxide,
: and zirconium oxide stabilised in the cubic phase.
The object of the present invention is to allow a
cemented carbide article to be prepared provided with a surface
coating with very high wear resistance and excellent resistance
to thermal shock, together with high chemical inertness towards
certain metals at high temperature, the use of such a coating
combining the advantages of a refractory coating such as silicon
nitride Si3N4 (Swiss Patent No. 540,991) and an alumina coating
(Swiss Patent No. 540,990).
To this end, the re~ractory-coated cemented carbide
article according to the invention is characterised in that the
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~L~6~7~7
refractory coating material consists at least partly o~ at least
one solid phase based on the following elements: silicon,
aluminium, oxygen and nitrogen.
Preferably, the refractory material comprises at least
one solid phase consisting of a combination of at least two of
the following compounds: Si3N4, AlN, Al2O3 and SiO2, in which
the four elements silicon, aluminium, oxygen and nitrogen are
simultaneously present.
For example, the material may consist, at least to a
major extent, of at least one solid, crystalline or amorphous
phase, the composition of which lies within the phase diagram
Si3N4 - AlN - Al2O3 - SiO2. This phase diagram has been
described, for example in the following publication: L.J.
GAUCKLER, H.L. LUKAS and G. PETZOW, Journal of the American
Ceramic Society~ pages 346-347; vol. 5~; No. 7-8 (July-August
1975).
The refractory material may in particular comprise a
solid solution of silicon nitride Si3N~l and aluminium oxide Al2O3.
This material may be in the crystalline state (mono-
crystal or an assembly of polycrystals) in the amorphous state,
or in the form of a mixture of crystalline parts and amorphous
` parts.
- Preferably, the refractory material consists, at least
to a major extent, of a solid crystalline phase consisting of a
solid solution having the structure of the compound beta-Si3N4,
and derived from this compound by replacing not less than 5 per
cent and not more than 80 per cent of the silicon atoms by alum-
inium atoms, and replacing part of the nitrogen atoms by oxygen
atoms.
Such a solid phase is referred to in the aforementioned
publication of L.J. GAUCKLER et al., under the name "beta-prime
Si3N4 phase" and its composition is represented there by the
~0647~37
basic general formula Si6 xAlxOxN8 x' in which x lies between O
and 4.2.
Said solid phase may equally consist of a solid
solution having the structure of the compound AlN, and derived
from this latter compound by replacing part of the aluminium
atoms by silicon atoms and replacing part of the nitrogen atoms
by oxygen atoms.
In addition to this solid solution, the refractory
material may comprise aluminium oxide A1203 in mixture with said
solid solution.
Preferably, the proportion of aluminium oxide in said
mixture is at most equal to 70 mole per cent.
Besides the aluminium oxide A1203, the refractory
material may equally comprise a solid phase with the composition
corresponding to the ~ormula Si A1505N3 in mixture with said
solid solution.
Besides the main elements Si~ Al, O and N indicated
heretofore, the refractory material may contain additional
elements such as Li, Be, Mg or Ga.
For example, the material may have a composition lying
within one of the phase diagrams MgO - Si3N4 - A1203; Li20 -
3 4 2 3; 2 3 Si3Ng - A1203; or Be2SiO4 - Si N - Al O
described in the ollowing publication: "Nitrogen Ceramics" by
K.H. JACK, Mellor Memorial Lecture page 376 - 384 (17th Mellor
Memorial Lecture, Institute of Ceramics Ltd. 1973).
Refractory materials essentially consisting of the
elements Si, O and N, in particular materials in which the com-
position may be represented within the phase diagram Si3N4 - AlN -
A1203 - SiO2, have already been described, for example in the
aforementioned publications and also in the following publications:
. OYAMA and 0. KAMIGATITO, Japan J. appl. Phys. 10, page 1963
(1971); Y. OYAMA and 0. KAMIGAITO, Yogyo Kyokai Shi 80, page 327
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(1972); K.H. J~CK and W.I. WILSON, Nature Phys. Sci. 238, page 28
(1972); W. WRUSS; R. XIEFFER; E. GU~EL and B. WILLER "Technol-
ogische Untersuchangen im. System Si3N4 - A12O3" Sprechsaal fur
Keramik, Glas, Baustoffer Cahier No. 13-14 (1975). They are
usually designated by the term "Si-Al-O-N ceramic".
It has been recognised that such refractory materials
have a high abrasion resistance, but their use as a coating to
increase the wear resistance of a cemented carbide part has
never been described or suggested.
In fact, the improvement in wear resistance, due to the
layer of refractory material according to the invention, results
not only from the great hardness of this material but seems
equally to derive from the fact that the presence of this
material opposes the reaction between the cemented carbide
article and any metal article, in particular a steel article,
coming into contact with it at high ternperature, as in the case
of high speed friction between the cemented carbide article and a
metal article.
~ he refractory material used according to the invention
has chemical inertness towards the metals of the iron family (i.e.
iron, cobalt and nickel) and alloys containing at least one of
- these metals, which is greater than the chemical inertness of a
cemented carbide.
Maximum increase in wear resistance is obtained if the
thickness of the layer of refractory material is between 0.5 and
10 microns. In general, if the thickness of the layer is less
than one micron the layer wears too quickly, and if the thickness
is greater than about ten microns, the strength of the coating
decreases.
In addition to said layer of refractory material, the
coating can comprise at least one intermediate layer of at least
one material chosen from the group consisting of carbides,
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106~L7~
nitrides, borides and oxides of the elements of one of groups
III to VI inclusive of the periodic table of elements, between
the substrate and said layer of refractory material.
Preferably the thickness of said intermediate layer is
not less than one micron and not more than ten microns.
The refractory material may be deposited on the
surface of the cutting tool by any appropriate technique which
enables a compact, coherent uniform and adherent coating to be
obtained with a substantially uniform thickness over all the
surface to be covered. For example, particles of the powdered ~-
refractory material or a powdered material or mixture of
- materials capable of generating this material may be sprayed onto
said surface while the particles are at least par~ially melted
by any appropriate known means, such as a plasma torch.
Electrophoresis may also be used. However the method preferably
used is to deposit the layer of solid :refractory material from a
gaseous phase, in particular by evaporation - condensation under
vacuum, cathodic sputtering, or chemical vapour deposition ~C.V.
D.) This latter method is used in particular in one preferred
em~odiment of the invention, and it enables a layer of
refractory material to be deposited which possesses the afore-
- mentioned desired ~ualities to a high degree.
Various chemical reactions may be used to deposit the
refractory layer by the C.V.D. process.
Preferably a mixture of at least one volatile silicon
halide and at least one ~olatile aluminium halide, for example
a mixture of silicon tetrachloride SiC14 and aluminium chloride
AlC13, is reacted with a gaseous mixture capable of simultaneousl~
forming, by reaction in the gaseous phase with said halide
mixture, at least one solid phase the composition of which lies
within the phase diagram Si3N4-AlN-A12O3-SiO2.
Alternatively gaseous mixtures of different compositions
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~6~7B7
could be used, so as to successively deposit at least two
different layers of material on the surface of the cutting tool,
for example firstly a layer of alumina A1203, then a layer of
silicon nitride Si3N4, or a silica layer SiO2 then a layer of
aluminium nitride AlN, or an alumina layer, a silicon nitride
layer and an aluminium nitride layer etc., the assembly of layers
thus obtained being then subjected to heat treatment to induce
the mutual diffusion and/or reaction of the materials in the
layers so as to form the final required refractory material.
One or more of the following reactions can for example
be used to form a layer of refractory material with the overall
composition Si3A1303N5:
a) Reactions between a mixture of silicon chloride and
aluminium chloride, and a mixture of hydrogen, nitrogen and
carbon dioxide CO :
. . 2
3 SiC14 + 3 AlC13 + 2.4N2 + 3 C2 + 10.5H2
Si3A1303N5 + 3 CO + 21 HCl
b) Reactions between a mixture of silicon chloride and
aluminium chloride, and a mixture of ammonia NH3, hydrogen
and carbon dioxide CO :
3 SiC14 + 3 AlC13 + 5 NH3 + 3 C02 + 3H2
. Si3A1303N5 3CO + 21 HCl
- c) Reactions between a mixture of silicon chloride and
aluminium chloride, and a mixture of nitrogen, hydrogen and
water vapour:
3 SiC14 + 3 AlC13 + 2.5N2 + 7'5H2 + 3 H20
Si3A1303N5 + 21 HCl
d) Reactions between a mixture of silicon chloride and
aluminium chloride, and a mixture of ammonia NH and water -
vapour:
3 SiC14 + 3 AlC13 + 5 NH3 + 3 H20
Si3A1303N5 + 21 HCl
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In depositing the refractory material by a chemical
reaction in the yaseous phase, any apparatus may be used
appropriate to the nature of the starting compounds and to the
dimensions and number of articles to be covered. Such apparatus
is known, and have been described in specialist technical litera-
ture~
The accompanying drawing is a diagrammatic represent-
ation, by way of example, of apparatus for deposition of a layer
of refractory material on the surface of a cemented carbide
article by a chemical reaction in the gaseous phase.
In the drawing:
Figure 1 is a general diagrammatic view of the
apparatus; and
Figure 2 is a section, on an enlarged scale, through
the reaction chamber of the apparatus.
The apparatus shown in Figure 1 comprises a reaction
chamber 1 of quartz, provided with a movable support bar 2 of
aluminium, slidably mounted in a gasket 3 which is cooled by a
flow of cold water. A coiled copper pipe 4, which is cooled by a
flow of water and connected to a high frequency electric
current generator, permits the article 5, which is to be covered
with refractory material (in this case a cemented carbide cutting
blade for a cutting tool) and which is placed on the support bar
2, to be heated by induction.
The reaction chamber 1 is supplied through a conduit
6 with part of the gaseous substances to participate in the gas-
phase chemical reaction (for example a mixture of silicon and
aluminium halides diluted in a current of dry hydrogen or inert
gas such as argon, acting as a carrier gas) from a suitable mixing
3~ device 7.
A further conduit 8supplies the chamber 1 with another
part of the gaseous substances to participate in gas-phase
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chemical reaction tfor example a mixture of ammonia vapour NH3
and water vapour, also diluted in a current of inert gas) from a
suitable mixing device 9.
The conduits 6 and 8 are maintained by suitable heating
means at a temperature (for example 200C) sufficient to
maintain in a gaseous state the substances fed through these
conduits into the reaction chamber.
The devices 7 and 9 are provided with means for purging
and rinsing by an inert gas such as argon, supplied from an out-
slde storage container not shown in the drawing.
A conduit 10 connected to a pumping unit 11 permits apressure which can be adjusted, for example, between 1 and 760
torr to be established in the reaction chamber.
Figure 2 shows in greater detail the manner i~ which
the cutting blade 5 is arranged on the support bar 2, if
necessary by way of an intermediate removable support 12 consist-
ing of an aluminium plate. Figure 2 a:Lso shows a device for mix-
ing the gas flows reaching the chamber 1 through the conduits 6
and 8. This device comprises essentially a bell-mouthed tube 13
having a smaller cross-section than the conduit 6. A thermo-
couple 14 enables the temperature of the blade 5 to be measured.
Examples of the coating process will now be given.
Example 1:
A layer, 5 microns thick, of a refractory material
consisting of an amorphous solid phase based on the elements Si,
Al, O and N, the composition of which material corresponds to the
following atomic ratio: Si, 34.8; Al, 11.6; N, 44.7; 0, 8.9, was
deposited on the surface of cemented carbide cutting tools (cut-
ting blades) of the following composition, expressed in percentage
by weight (cemented carbide corresponding to the ISO P30 standard):
Cobalt : 9.5
Titanium carbide TiC : 11.9
~g_
1C~647~7
Niobium carbide NbC : 6
Tungsten carbide WC : 6~.6
This was done by simultaneously reacting a mixture of
silicon chloride SiC14 and aluminium chloride AlC13 with a
mixture of carbon dioxide CO2, nitrogen and hydrogen, in the
reaction chamber of apparatus as heretofore described. The
silicon chloride was mixed with the nitrogen, acting as a carrier
gas, before being mixed with the gaseous aluminium chloride, this
latter being preferably mixed with the hydrogen, also acting as a
carrier gas.
The operating conditions were as follows:
Total pressure in the chamber : 50 Torr
Temperature of the cutting tool surface : 1,000C
Duration of reaction : 240 minutes
Gas flow rates (reduced to 760 Torr
and 20C):
SiC14 : 50 mg/minute
Nitrogen : 200 cm3kninute
AlC13 : 40 mg/minute
2~ Hydrogen : 200 cm3/minute
: C2 : 200 cm3/minute
Example 2:
Proceeding in a manner similar to that described in
: Example 1, a layer, 5 microns thick, of a refractory material
consisting of a solid crystalline phase of the structure of the
compound AlN, and a solid phase based on the elements Si, Al, O
and N was deposited on the surface of cutting tools identical to
those of Example 1.
The operating conditions were as follows:
Total pressure in the chamber : 50 Torr
Temperature of the cutting tool surface : 1,050C
Duration of reaction : 15 minutes
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7~37
Gas flow rates (reduced to 760 Torr
and 20~C)
SiC14 : 50 mg/minute
Argon : 200 cm /minute
The argon serves as a carrier gas for the silicon chloride
AlC13 : 40 mg/minute
Hydrogen : 200 cm3/minute
The hydrogen serves as a carrier gas for the aluminium chloride.
~ C2 200 cm3/minute
NH3 : 5 cm /minute
The carbon dioxide and ammonia are mixed together before their
introduction into the reaction chamber.
Example 3:
The process was carried out in a manner similar to that
described in Example 1, but comprised, in turn reacting gaseous
aluminium chloride AlC13 with a mixture of carbon dioxide CO2 and
hydrogen for 5 minutes, and reacting gaseous silicon chloride
SiC14 with a mixture of hydrogen and alNmonia NH3 for 10 minutes.
The flow rates of the gas streams (reduced to 760 Torr
and 20C during each reaction period were as follows:
a) A12C13 : 20 mg/minute
C~2 : 200 cm3/minute
- ~ydrogen : 200 cm3/minute
b) SiC14 ; 25 mg/minute
Hydrogen : 400 cm3/minute
Ammonia NH3 : 10 cm3/minute
The other operating conditions were as follows:
Total pressure in the chamber : 50 Torr
Temperature of the cutting tool
surface : 1,050C
Total time of deposition : 60 minutes
Total thickness of coating : 5 microns
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a~ter which the gaseous streams were cut off and
replaced by a single stream of argon, and the cutting
tools were maintained at l,200C for 5 hours at a
pressure of 50 Torr in the reaction chamber, to homo-
genise the coating~
In this manner a coating was obtained consisting of a
solid solution of silicon nitride and aluminium oxide of "beta
prime-Si3N4" structure as heretofore defined, of which the
composition corresponds to a content of the order of 50 mole per
cent of Si3N4 and 50 mole per cent of A12O3.
Example 4: ~
The procedure described in Example 3 was followed, but
the aluminium chloride was reacted with the mixture of CO2 and
H2 for 10 minutes instead of 5 minutes, and the SiC14 was reacted
with the mixture of H2 and NH3 for 5 m:inutes, instead of 10
minutes.
A coating was obtained consisting of a mixture of a
solid solution of the "be~a prime-Si3N~" type and alpha alumina
~1203.
~O
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