Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~s~
P-1980
~PROCESS FOR APPLYING HARD COATINGS
AND THE LIRE ~0 METALS AND RESULTING PRODUCT~
This invention relates to the coating of Ketals
(hereinafter referred to as ~substratesU or ~substrate
metals~) with co~tings that serve to provide hard surfaces,
chemically res~stant coatings, etc.
Hard coatings were developed for the purpose of
providing a combination of high performance properties ~uch
as resistance to friction, wear and corrosion to less
expensive metal component~. Early techniques used in the
application of these coatings were based on surface
treatment of metallic ~ubstrates by the diffusion of carbon,
nitrogen, boron, or silicon, thus generating the hard
material~ ~irectly in the ~urface of the ~ubstrate. Most
of the more recent application techniques involve the
deposition of ~n overlay hard layer as Rn external coating.
Examples of techniques include- Chemical v~por deposition
~CVD), physical vapor deposition ~PYD), laser fusion,
sputtering, flame or plasma 6pr~ying, and de~onation gun.
With the possible exception of C~D processes, these
techniques are expen~ive and limited to the line of ~ight
which may lead to ~ariable thickness and unequal cover2ge
particularly at corners, holes and complex shapes.
.
~L2~
-- 2 --
It is an ob~ect of the present invention to provide
an improved method of applying to substrate metals coatings of
MlXn where Ml is the metal whose compound is to b~ applied to
the substrate, X is an element such as nitrogen, carbon, boron
or silicon, and n is a number indicating the atomic proportions
of X to M.
It is a further object of the invention to provide
coated substrate metals in which the coatings, MlXn as
described above, are uniform and adherent to the substrate.
The above and other objects of the invention will
be apparent from the ensuing description and the appended
claims.
The invention provides a method of coating a metal
substrate with a protective coating which comprises: (a)
providing a substrate metal to be coated, (b) providing an
alloy or mixture of at least one metal Ml, and at least one
other metal M2 selected according to the following criteria:
(1) Ml is susceptible to reaction with a reactive gaseous
species of an element X tX being nitrogen, carbon, boron or
silicon) to form a stable compound of Ml and X at a selected
temperature and pressure of such reactive species, (2) M2 does
not form a stable compound with X under such conditions and it
bonds to the substrate on heat treatment of the coated material;
(c) applying such alloy or mixture to a surface of the substrate
to provide a coating and (d) effecting selective reaction of M
with such gaseous species at an elevated temperature under
conditions to produce a compound of Ml and X and to avoid or
minimize formation of a compound of M2 with X.
The invention further provides a coated metal article
comprising~ (a) a metal substrate and Ib) a protective coating
on and adherent to at least one surface of the metal substrate,
such coating comprising an outer layer of a compound MlXn
~. .
- 2a -
wherein X is nitrogen, carbon, boron or silicon and n represents
the atomic proportion of X to M1 and an inner layer of at least
one metal M2 bonded to the substrate, said metals Ml and M2
being selected according to the following criteria: (1) Ml is
susceptible to reaction with a reactive gaseous species of an
element X (X being nitrogen, carbon, boron or silicon) to form
a stable compound of M1 and X at a selected temperature and
pressure of such reactive species, (2) M2 does not form a
stable compound with X under such conditions and it bonds the
coating to the substrate.
In accordance with the present invention, an alloy
or a physical mixture of metals is provided comprising two
metals Ml and M2 which are selected in accordance with the
criteria described below. This alloy or metal mixture is then
melted toprovide a uniform melt which is then applied to a
metal substrate by dipping the substrate into the melt.
Alternatively, the metal mixture or alloy is reduced to a
finely divided state, and the finely divided metal is
incorporated in a volatile solvent to form a slurry which
is applied to the metal substrate by spraying or brushing.
The resulting coating is heated in an inert atmosphere to
accomplish evaporation of the volatile solvent and the
fusing of the alloy or metal mixture onto the surface of
the substrate. (Where physical mixtures of metals are used,
they are converted to an alloy by melting or they are
alloyed or fused together in situ as in the slurry method
of application described above.) In certain instances, as
where the alloy melts at a high temperature such that the
substrate metal might be adversely affected by melting a
coating of alloy, the alloy may be applied by plasma spraying.
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The metals Ml and M2 are 6elected according to the
following criteria: Ml ~orms ~ thermally fitable compound
with X (i.e., a nitride, n carbide, a boride or ~ ~ilicide)
when exposed at a high temperature to an ~tmosphere
csntaining a ~mall concentr~tion of X or of a dissociable
molecule or ~ompound of X. The 6table compound that Ml
forms with X may be repre ented as MlXn where n represents
the atomic ratio of X to Ml.
~ he metal M2, under such conditions, does not
form a stable compound with X and remains entirely or
~ubstantially entirely in metallic form. Further, M2 is
compatible with the substrate metal in the ~ense that it
results in an intermediate layer between the MlXn outer
layer (resulting from reaction with X) and the substrate,
~uch intermediate layer 6erving to bond the MlXn layer to
the substrate. Interdiffusion of M2 and the substrate metal
aids in this bonding effect.
It will be understood that Ml may be a mixture or
alloy of two or more metals meeting the requirements of M
and that M2 may Also be a mixture or alloy of two or more
metals meeting the requirements of M2.
The coating thus formed and applied i~ then
preferably subjected to an annealing step. The ~nnealing
step may be omitted when annealing occurs under conditions
of use.
When ~ coating of suitable thickness has been
applied to the substrate alloy by the dip coating process or
by the slurry process described above ~and in the latter
case after the solvent has been evaporated and the M/M2
metal alloy or mixture is fused onto the surface of tbe
substrate) or by any other suitable process the surface is
then exposed to a selectively reactive atmosphere at an
appropriate elevated temperature.
-4-
~ o form ~ nitride, carbide, boride or ~ilicide
layer on t~e substrate metal, ~n appropriate, thermally
di~soci~ble c~pound or molecule of nitrogen, carbon, boron
or ~ilic~n may be used. Examples ~f ~uitable gaseous media
~re ~et forth in Table ~ below $ncluding ~edia where X
nitrogen, etc.
Table I. Gaseous Media for Forming
Nitrides, Carbides,
- Borides and Silicides
X Gaseous Media
N N2, NH3 or ~ixtures of the two.
C Methane, ~cetylene.
B Borane, diborane, borohalides.
Si Silane, trichloro silane,
tribromosilane, silicon tetrachloride.
Where a very low partial pressure of the reactive
species is needed, that species may be diluted by an inert
gas, e.g. argon . If the RCtive species results from a
gaseous reaction of two precursor species, the concentration
of the active species may be controlled by adjusting the
ratio of the precur~or ~pecies.
~ here results from this process a structure such
as shown in Figure 1 of the drawin~s.
-5-
Referring now to Pigure 1, this ~igure represents
a cross-section through a substrate alloy indicated at 10
coated with a laminar coating indicated at 11. -The laminar
coating 11 consists of an intermediate metallic layer 12 and
an outer MlXn layer 13. The relative thicknesses of the
layers 12 and 13 are exaggerated. The substrate layer 10
is as thick as required for the intended service.
The layer6 12 and 13 together typically will be
about 300 to 400 microns thick, the layer 12 will be about
250 microns thick, and the layer 13 will be about 150
micron~ thick. It will be understood that,the layer 12
will have a thickness adequate to form a firm bond with
the substrate and that the layer 13 will have a thickness
~uiting it to its intended use.
Figure 1 is a simplified representation of the
coating and substrate. A more accurate representation is
6hown in Pigure lA in which the ~ubstrate 10 and outer
layer MlXn are as described in Figure 1. However there
is a diffusion zone D which may be an alloy of one or more
substrate metals and the metal M2 or it may be an inter-
diffusion layer resulting from diffusion of substrate metal
outwardly away from the ~ubstrate and of M2 inwardly into
the 6ubstrate. There i~ al80 an intermediate zone I which
may be a cermet formed as a composite of MlXn and M2.
The metals Ml and M2 will be se~ected according
to the intended use. Table II below lists metals which may
be used as Ml and Table III lists metals that may be used
as M2. Not every metal in Table II may be used with every
metal in Table IIIS it is required that M2 be more noble
than Ml in any Ml/M2 pair. Another factor is the intended
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use, e.g. whether a hard sur~ace, or a ~urface which i5
resistant to ~queous environments lc desired, ~ surface
which acts ns a lubricant, etc. Also the n~ture of the
substrate should be considered. It will be ~een that some
~etals appear in both tables; that i~ a metal Ml appearing
in Table II ~ay be used a~ M2 (the more noble ~etal) with
a less noble metal Ml from Table III.
Table $I (Ml~
Actinium Neodymium
Aluminum Niobium
Larium Praseodymium~
Beryllium Samarium
Calcium Scandium
Cerium Silicon
Chromium Tantalum
Dysprosium Terbium
Erbium Thorium
Europium Thulium
Gadolinium Titanium
~afnium Tungsten
Holmium Van~dium
Lanthanum Ytterbium
Lithium Yttrium
Magnesium Zirconium
Molybdenum
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Table III (M2)
Cobal~ Palladium
Copper Platinum
Gold Rhenium
Iridium Rhodium
Iron Rubidium
Manganese Ruthenium
Molybenum Silver
Nickel ~in
Osmium Zinc
It will be understood that two or more metal
chosen from Table II ~nd two or more metal~ chosen $rom
~able III may be employed to form the coating alloy or
mixture. Examples of suitable Ml/M2 metal pairs including
mixtures of two or more metals Ml and two or more metals M2
are fiet forth in Table IV.
~able IV
Ml M2 ~51 kl2
_ . . . .
Ti Ni Th Ni
~i Fe ~h Pe
Ti Co ~h Co
Ti Cu Th Mg
Ti Pd
Ti + Nb Ni
Ti ~ Zr Co Th . Cu
Ti + Zr Fe Th Al
Ti ~ Zr Cu Sc Al
Zr Fe
Zr Cc~ Sc Cu
Zr Cu Sc Fe
Zr Pd
Zr Pt Sc Pd
Zr Rh Sc Ru
Zr + Y Ni Y Al
Zr + Y Co Y Co
Zr ~ Y Fe
Zr + Y Pd y S::u
y Fe
Zr + Nb ~ Ni
Zr + Hf Ni Y Ni
Hf N i : Y Pd
CU Y ~1
Si Nb
Si Co
Si ~e
Si Mo
5i Ni
Si Pd
Si Pt
Sr Ni
Cr Pd
Si Ru
~S~ll
--9
It will be under~t~d that not every metal pair
will be 6uitable for all purposes. ~or example, where Ml
is silicon the coating tends to be brit~1e; 6~me p~ir& ~re
better ~uited for hardness,others for s~rvice as ther~31
barriers, others for oxidat~on ~nd corro~i~n resi~tance,
etc.
Examples o eutectic ~lloy~ ~re li~ted in Table V.
~t will be understood that not ~11 of the~e alloys.~re
useful on ~ ubs~rate6. In some cases the melting
points are apprcximate. Numbers indicate the approximate
percentage by weight of H2.
Table V
Eutectic Allov Melting Point ~C)
Ti - 28.5 Ni 942
Ti - 32 Fe 1085
Ti - 2B Co 1025
Ti - 50 Cu 955
Ti - 72 Cu 885
~i - 48 Pd 1080
Zr - 17 Ni 960
Zr - 27 Ni 1010
Zr - 16 Fe 934
Zr - 27 C~ 1061
Zr - 54 Cu B85
Zr - 27 Pd 1030
Zr - 37 Pt 1185
Zr 25 Rh 1065
~f - 72 Ni 1130
Hf - 38 Cu 970
Th - 36 Ni 1037 .
~h - 17 Fe 875
10-
Table V ~t:ont 'd. ~
Eutectic AllovMelting Point (C)
Th - 30 Co 975
~h - 22. 5 Cu 880
Th - 75 Al 632
Sc - 45 Al 1150
S~ - 77 Cu 875
Sc - 24 Fe 910
Sc - 22 Pd 1000
;~c ~ 17 Ru 1100
Y - 93 Al 64û
Y - 19 Al 1100
Y - 9. 5 Al 960
Y - 28 Co 725
Y - 88 Cu 890
Y - 66 Cu 840
Y - 50 Cu B30
Y - 27 Cu 760
Y - 25 Fe 900
y _ 47 Ni 9
Y - 25 Ni 802
Y - 34 Pd 903
Y - 28 Pd 907
Y - 17 Ru 1080
Nb - 76. 5 Ni 1270
Nb D 48. 4 Ni 1.175
Si - 88. 3 Al 577
Si - 37. B Co 1259
Si - 8~ Cu 802
Si - 42 Fe 1200
Si 12 Mo 1410
Si -- 62 Ni 964
Si - 74 Pd B70
Si - 77 Pt 979
Table VA li~ts ~rtain tertiary ~lloys that are
useful in the practice o~ the present invention.
Table VA
55.18 ~i - 23.13 Nb - 21.69 Ni
40.38 Ti - 43.52 Zr - 16.10 Ni
~0.07 ~i - 44.35 Zr - 15.58 Co
25.37 Ti - 65~69 Zr - 11.94 Fe
17.36 ~i - 38.01 Zr - 44.63 Cu
69.65 Zr - 16.07 Y ~ 14.26 Ni
55.96 Zr - 23.34 Y - 20.70 Ni
43.08 Zr - ~0.98 Y ~ 15.94 ~o
56.76 Zr 32.43 Y - 10.81 Fe
47.B9 Zr - 34.39 Y 17.72 Pd
S6.68 2r - 22.35 Nb 20.97 Ni
49.33 Zr - 32.43 Hf - 43.94 Ni
24.20 Zr - 48.51 Hf - 27.29 Ni
Yttrium, calcium and magnesium are especially
beneficial in zirconium-noble metal ~M2) alloys because
they stabilize zirconia in the cubic form. Examples of
such ternary alloys are as follows.
Zr Y Ca Mg Ni
76 8 16
77 7 1~
79 5 16
Table VI provides examples of metal substrates
to which the metal pairs may be applied.
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Table YI
Superalloys
Cast nickel base such ~s IN 738
Cast cob~lt base ~uch n~ MAR-M509
Wrought nickel ba~e 6uch ~s RRné 95
Wroughe cobalt b~se such ~s ~aynes ~lloy No. lB8
Wrought iron base ~uch ~s Discaloy
Hastalloy X
RSR 185
Incoloy*901
Coated superalloYs (coated ~or corrosion resistance)
Superalloys coated with Co(or Ni)-Cr-Al-Y alloy,
e.g. 15-25~ Cr, 10-15~ Al, 0.5~ Y, balance is
Co or Ni
Steels
~ool Steels (wrought, cast or powder metallurgy)
~uch as AISIM2; AISIWl
Stainless Steels
Austenitic 304
Ferritic 430
Martensitic 410
Carbon Steels
AISI 1018
AlloY Steels
AISI 4140
Maraging 250
* trade mark
.~
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C~-t irons
Gray, ductile, malleable, alloy
UNSF 10009
Non-ferrous Metals
~itanium and titanium alloys, e.g. ASTM Grade l;
Ti-6Al-4V
Nickel and nickel alloys, e.g. nickel 200, Monel 400
Cobalt
Copper and its ~lloys, e.g. C 10100; C 17200;
C 26000; C 952C0
.
Refractory metals and allovs
Molybdenum alloys, e.g. TZM
Niobium alloys, e.g. FS-85
Tantalum alloys, e.g. T-lll
Tungsten alloys, e.g. W-Mo alloys
Cemented Carbides
Ni and cobalt bonded carbides, e.g. WC-3 to 25 Co
Steel bonded carbides, e.g. 40-55 vol.S TiC, balance
steel; 10-20~ TiC-balance ~teel
The proport$ons of Ml to M2 may vary widely
depending upon ~uch factors as the choice of Ml and M2,
the nature of the ~ubstrate metal, the choice of the
reactive gaseous species, the conversion temperature,
the purpose of the coating (e.g. whether $t is to serve
as a thermal barrier or as a hardened surface), etc.
~2451~L1
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The dip coating method is pre~erred. I~ is easy
to carry out and the molten ~lloy Femoves 6urface oxides
(which tend to cause Epallation)~ In thi~ ~ethod ~ molten
Ml/M2 ~lloy ~ provided and the ~ubstrate alloy i8 dipped
into a body of the coatlng alloy. The temperature of the
~lloy ~nd the time dur~ng which the substrate is held in
the molten alloy will control the thickness ~nd smoothness
of the coating. If an ~erodynamic ~urface or ~ cutting
edge ~8 being prepared ~ ~moother rurface will be desired
than for some other purposes. ~he thickness of the applied
coating can range between a fraction of one micron to a few
millimeter~. Preferably, a coating of about 300 microns to
400 microns is ~pplied if the purpose i8 to provide a
thermal b~rrier. A hardened ~urface need not be ~s thick.
It will be understood that the thickness of the coa~ing
will be provided in accordance with the requirement6 of a
particular end use.
The slurry fusion method has the advantage that
it dilutes the coating alloy or metal mixture and therefore
makes it possible to effect better control over the
thickness of coating ~pplied to the ~ubstrate. Also complex
~hapes cDn be coated and the proGess can be repeated to
build up a coating of desired thickness. Typically, the
slurry coating technique may be applied ~s follows: A
powdered alloy of Ml ~nd M2 is mixed with ~ mineral ~pirit
and an organic cement ~uch as Nicrobraz 500* (Well Colmonoy
Corp.) and MPA-60 (Baker Caster Oil Co.). Typical
proportions u~ed ln the slurry are coating alloy 45 weight
percent, mineral ~pirit 10 weight percent, ~nd organic
cement, ~S weight percent. This mixture is then ground,
for example, in ~ ceramic ball mill using aluminum oxide
* trade mark
: s"
ILZ~
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balls. After separation of the resulting 61urry from the
al~mina ball~, it ~ applied IkeePing ~t ~tirred to insure
uniform dispersion of the particle~ of alloy ln the liquid
medium) to the 6ubstrate surface and the solvent i~
evaporated, for example, ~n a$r at amb$ent temperature or
at a ~omewhat elevated temper~ture. The residue of alloy
and cement i8 then fused onto the surface by heating ~t to
a suit~ble temperature $n an ~nert atmosphere ~uch as argon
that has been passed over bot calcium chips to getter
oxygen. The cement will be decomposed and the products
of decomposition are volatilized.
If the alloy of Ml and M2 h~s ~ melting point
which i~ suf f iciently high that it exceeds or closely
approaches the melting point of the substrate, it may be
~pplied by qputtering, by vapor deposition or some other
technique.
It is advantageous to employ Ml and M2 in the
form of an alloy which is a eutectic or near eutectic
mixture. This has the advantage that a coating of definite,
predictable composition is uniformly applied. Also eutectic
and near eutectic mixtures have lower melting points than
non-eutectic mixtures. Therefore they are less likely than
high melting alloys to harm the substrate metal and they
sinter more readily than high melting alloys.
~ he following specific examples will serve further
to illustrate the practice and advantages of the invention.
3L2~
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Example 1~
The substrate metal was ~ool ~teel in the form of
rod. The coating ~lloy was a eutect$c ~lloy containing
71.5~ nd 28.5~ Ni. Thifi eutectic has a melting point
of 942C. The rod wa~ dipp~d ~nto this alloy at 1000C ~or
10 ~econds ~nd was removed and annealed or 5 hourR ~t
800C. It was then exposed to oxygen free nitrogen for
15 hours at 800C. ~he nitrogen was passed slowly over
the rod at atmospher~c pressure. The resulting coating
was continuous ~nd adherent. The composition of the
titanium nitride, TiNX, depends upon the temperature and
the nitrogen pressure.
Example 2.
Example 1 was repeated using mild steel as the
substrate. A titanium nitride layer was applied.
The coatings of Examples 1 and 2 are useful
because the treated surface is hard. This is especially
helpful with mild steel which ~s inexpensive but soft.
This provides a way of providing an inexpensive metal with
a hard ~urface.
Example 3.
The same procedure was carried out as in Example 1
but at 650~C. The coating, 2 microns thick, was lighter in
color than the coating of Exampie 1.
Darker colors obtained at higher temperatures
indicated a ~oichiometric composition; TiN.
Similar coatings were applied to stainless steel.
~24~
Example 4.
A eutectic ~lloy of B3~ Zr ~nd 17~ Ni (melting
point ~ 961C) is employed. ~he ~ubstrate ~etal (tool
steel) is dip coated at 1000C, ~nnealed 3 hours ~t 1000C
and exposed to nitrogen as $n Exampl~s 1 ~nd 3 at 800C.
A uniform ~dherent coating 2 to 3 ~icron~ thick resulted.
Example 5.
~ ~84 Zr - 52% Cu eutectic alloy, ~elting point
885DC was used. ~ool steel was dipped into the ~lloy for
10 ~econds at 1000C and was withdrawn ~n~ annealed-5 hours
~t lOOO~C. It was then exposed to nitrogen ~t one
atmosphere for 50 hours at 800C. A uniform adherent
coatlng resulted.
An advantage of copper ~s the metal M2 is that
it is a good heat conductor which is helpful in carrying
away heat (into the body of the tool) in cutting.
Example 6.
A 77% Ti - 23~ Cu ~lloy, a eutectic alloy,
melting at 875C was used. Hot dipping was at 1027C for
10 seconds; anne~ling at 900C for S hours; exposure to
N2 at 900C for 100 hours. An adherent continuous coating
resulted. The ~ubstrate metal was high speed steel.
~L2~511~
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.
~E~.
Tool steel was coated with a Ti-Ni ~lloy and
annealed as in Example 3. ~he reactive 9a5 species is
methane which may be used with or without ~n inert g2s
diluent Euch as argon or helium. The co~ted steel rod
is expo~ed to methane at 1000C ~or 20 hours~ A hard,
adherent coating of tit~nium carbide result~.
Example 8.
The procedure of Example 7 may be repeated using
BH3 as the reactive gas species ~t ~ temperature above
700C, e.g. >700C to 1000C, for ten to twenty hours.
A tit~nium boride coating i8 formed which i6 hard ~nd
adherent.
Example 9.
The procedure of Example 7 is repeated using
silane, Si H4, as the reactive gas Epecies, with or
without a diluting inert gas such as argon or helium.
The temperature and time of exposure may be >700C to
1000C ~or ten to twenty hours. A titanium silicide
coating is ~ormed which is hard and adherent.
~Z~5~
--lg-
Among other con6ider~tions ~re ~he ollowing:
The metal M2 ~hould be compatible with the
~ub~trate. For example, ~t ~hould not form brittle inter-
me~allic compound wieh metal~ of ~he ~ub~tr~te. Prefer~bly
it doe~ nvt alter Eeriou~ly the mechanical pr~per~ie~ ~f the
6ubstrate ~nd ha~ a large rDnge of ~olid ~olubility in the
substrate. Al~o $t prefer~bly ~orm~ ~ low nelt~ng eutectic
with Ml. Al~o $t should not form a highly stable carbide,
nitride, boride or æilicide. ~or example, if Ml ls to be
converted to a nitride, M2 ~hould not form a ~table nitride
under the conditions employed to form the ~1 nitride. -
In the hot dipping method of application of anMl/M2 alloy, uneven ~urface application may be ~voided or
diminished by ~pinning and/or wiping.
~ he annealing ~tep after application of the alloy
or mixture of Ml and M2 ~hould be carried out to 6ecure a
good bond between the alloy and the substrate.
Convérsion of the ~lloy coat$ng to the final
product i~ preferably carried out by expo~ure to a ~lowly
flowing stream of the react$ve gas at a temperature and
pressure ~ufficient to react the reactive gaseous molecule
or compound with Ml but not such as to react with M2.
It is also advantageous to employ a temperature slightly
above the melting point of the ~oat$ng ~lloy, e.g. El$ghtly
~bove its eutestic melting pGint. ~he presence of a liquid
phase promote6 migration of Ml to the ~urface ~nd
di~placement of M2 in the outer layer~
20-
If the temperature i~ below the melting point of
the coa~ing alloy and ~f the compound ~ormed by Ml and the
reactive gaseous ~pecies grQws ast, M2 will be entrapped
in the growing compound, thus bonding the particles of
MlXn. In this case ~ cermet will Ibe formed which ~ay t~e
~dvant~gec~us, e.g. ~ W or Nb carb~de cemented by cob~lt
or n ickel .
It will therefore be apparent ~h~ new ~nd
useful method of ~pplying MlXn co~ting to ~ metal sub~trate,
~nd new ~nd useful products are provided.
