Note: Descriptions are shown in the official language in which they were submitted.
53~7~7
., ,
This invention relates to the production
of -titanium boride. More specifically, the present
inventio~ is directed to the production of TiB2 composite
material for wear-resistant coatings, and parts. The
invention also relates to the production of titanium
boride powder.
~nong the different ceramic compounds, TiB2
is one of the most interesting because of its exceptional
characteris-tics. The transition metal diboride TiB2
combines such important properties as high hardness,
high melting point, good electrical or thermal conduct-
ivity and good corrosion resistance. These properties
are responsible for the fact that TiB2 is attractive
in various fields of engineering where parts must have
wear-resistance as well as good oxidation resistance
or high thermal resistance in different media.
Up to now, TiB2 has been produced by directly
reacting titanium and boron or by the so-called boro-
carbide method, i.e. by heating mixtures of Ti, B4C
and B2O3 or TiO2 at temperatures of 1800C to 2000C.
These methods have important drawbacks since expensive
starting elements such Ti, B or B4C are used. Further-
more, these methods cannot produce TiB2 composite materials
containing a fine dispersion of interesting elements
such as Fe, Ni or Co, A1, Mo, Cr and Cu.
Tt is also known that the applications ~of
TiB2 are limited by the brittleness of pure TiB2 or
because difficulties of fabrication are involved in
order to obtain dense coatings from pure TiB2 powders.
l~S37~l~7
.
It i5 well recognized that the mechan:ical proper-ties
of TiB2 need to be improved in order to enable this
material to be used under industrial conditions. The
best way to enhance the mechanical properties of TiB2
is to associate this material with metallic binders.
This was generally performed by using mixtures of
TiB2 and metallic powders, mainly iron and nickel pow-
ders.
These mixtures can then be used to produce
parts based on TiB2 by slntering or hot pressing.
They can also be used to produce thermal sprayed coatings
buth this requires a careful control of mixing and the
use of fine powders in order to achieve a good distri-
bution of each constituent. Furthermore, a limiting
feature of the thermal sprayed coating is the need
of melting powder during its travel through the flame.
The high melting point of borides, particularly titanium
boride, limits the use of thermal sprayed coatings
of these compounds. Indeed, the temperature required
to melt TiB2 i5 50 high and the times of residence
of particles within the flame are not long enough to
produce an adhexent layer. Because of their high melting
points, coatings based on TiB2 have not been satisfactory
achieved.
U.S. Patent No. 4,014,688 issued to Horst
Schreiner et al on March 29, 1977 discloses the fab-
rication of contact material for high-power vacuum
circuit breaker. This material consists of an alloy
having a base metal and alloying metals which form
a eutectic with the base metal used. Iron and titanium
are mentioned as possible base metal whereas boron
is mentioned as alloying element. The contact materials
described by Schreiner et al consist of a base metal
-- 2 --
3'7~'~
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., ,
wi-th dispersed second phases obtained by ~ormation
oE a eutectic. These con-tact materials are hypo- or
hyper-eutec-tic alloys and are not ceramic ma-terials.
The proportion of iron, titanium and boron used by
Schreiner et al is up to about 90% iron, up to about
90% titanium and about 1% boron.
Canadian Patent No. 686,187 which issued
on ~ay 12, 196~ is directed to a method oE preparing
a titanium powder containing titanium monoboride.
1o ~hich is different from titanium diboride. This ~owder
can be consolidated to proauce parts having a titanium
matrix havlng titanium monoboride dispersed in it.
However, the maximum content in titanium monoboride
particles is limited to about 30 vol. ~. Furthermore,
~ there is no mention of other metallic matrix like Fe,
Ni, etc.
Canadian Patent No. 1,003,246 which issued
on January 11, 1977 relates to a wear-resistant composite
materials. They consist of a dispersion of coarse
(0.3 to 1 mm) particles within a matrix of copper
or nickel. These composites are heterogenous materials
containing about 50 vol. ~ of rich titanium and boron
bearing particles. The hardfacing is the only method
suitable to produce overlays based on those materials.-
They are not constituted of fine dispersion of TiB2
in a metallic matrix.
Canadian Patent No. 1,110,881 relates to
another wear-resistant product, which is made of iron-
molybdenum boride. It is not based on TiB .
It is an object of the present invention
to provide a process for synthesizing the transition
metal diboride TiB2.
L~3'71~7
It is another object oE the present invention
to provide a method to produce TiB2 composite materials.
It is a primary object oE the present invention
to provide thick plasma or thermal sprayed coatings
or parts made by powder metallurgy techniques having
high wear-resistance and good mechanical properties.
It is another object of the present invention
to provide a process for producing TiB2 in a metallic
phase.
It is another o~ject of the present invention
to provide TiB2 composite materials which can be used
in different areas of engineering requiring high wear-
resistance combined with high oxidation, corrosion
or degredation by the attack of molten metal such as
aluminium.
It is another object of the present invention
to produce TiB2 composite coatings synthesized during
the coating operation.
In accordance with a broad aspect of the
invention, there is provided a process which permits
the synthesis of TiB2.
According to the present invention, titanium
alloys or compounds are mixed with boron or ferroboron
to synthesize TiB2 composite material. These mixtures-
are then heated and the synthesis of Tis2 occurs.
The synthesized materials are constituted of fine TiB~
crystals and a fine metallic phase. The mean grain
size of TiB2 and of the metallic phase is not more
than 5 jum.
According to one aspect of the invention,
titanium alloys are mixed with boron. This mixture
is then heated at a temperature sufficient to initiate
an exothermic reaction which leads to the synthesis
~S;~7~7
of TiB2 composlte materials. The reaction occurring during
the synthesis can be expressed by the Eollowing expression:
a (MexTiy) ~ b (~ > c(qliB2) -~ d(Me)
where a, b, c and d express mole fraction while x and
y are atomic ratio and x + y = l, preferably x varies
between 0.05 and about 0.7 and y varies between about
0.3 and about 0.99 and Me designates a metal preferably
Fe, Ni, Co, Al, Mo, Cr and Cu. The boron To-Titanium
atomic ratio preferably varies between l to 2.5, i.e.
ay< b < 2.5 ay.
According to another aspect of the invention,
the synthesis is made by the auxiliary metal bath
process. In this case, titanium alloy are mixed with
ferroboron. The mixture is then heated above the melting
point of the auxiliary metal to promote the synthesis
of TiB2. The endothermic reaction can be expressed by
the following expression:
a(MexTiy) + b(MexBy) ~~- ~ c(TiB2) + d(Me)
where a, b, c and d express mole fraction and x and y
express atomic ratio and x + y = l, preferably x varies
between 0.05 and about 0.7 and y varies between about
0.3 and about 0.99 and Me designates mainly Fe with the
presence of Fe, Ni, Co, Al, Mo, Cr and Cu. The metal
of the titanium alloy and that of the boron compound may
be different. The boron To-Titanium atomic ratio preferably
varies between l to 2.5, i.e. ay < by < 2.5 ay.
A practical embodiment of the invention in-
volves a process for producing TiB2 composite materials,
which include the ~ollowing steps:
-- 5 --
:12S3~7;~7
. ,
Step 1 - Mixing selected amounts of fine
powder of titani.um alloys and o:E boron (either amorphous
or crystalline) or Eerroboron.
Step 2 - Simultaneously ml.xing and milling
mixtures obtained in step 1 using various devices for
this purpose. This operation is preferably performed
- 5a -
lZS3~7~7
. .
in an iner-t liquid or gas media
Step 3 - Making agglomerated particles by
agglomeration techniques including spray dryiny, me-
chanical agglomerating, crushing or granulating, pellet~
izing.
Step 4 - Thermal or plasma spray depositing
agglomerated particles obtained through step 3 onto
a substrate. This operation is performed at a ternper-
ature sufficient to synthesize TiB2. These agglomerated
particles may also be deposited by various hardfacing
technique to obtain Ti~2 composite coatings.
Step 5 - Reacting mixtures obtained through
step 2 or agglomerated particles obtained through step
3 at a sufficient temperature to synthesize TiB2.
The reaction is preferably carried out in an inert
atmosphere. The reaction products may be densified
or consolidated into parts by various powder metallurgy
techniques such as hot isostatic pressing, sintering,
infiltrating,for~ing.
~0 Step 6 - Densifying or consolidating reaction
products obtained through step 5 by various powder
metallurgy techniques such as pressing, sintering,
infiltrating, hot isostatic pressing, forging, rolling,
extruding.
Step 7 - Optionally leaching out the reaction
products obtained through step 5, of their metallic
phase to give substantially pure TiB2.
Step 8 - Optionally reacting and/or rapidly
solidifying agglomerated particles obtained through
step 3 and then leaching out their metallic phase to
give very fine and pure TiB2 crystals.
;1~537~7
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EXAMPLE 1
The startingferrotitanium is a slight hyper-
eutectic Ti~Fe alloy containing 62 wt% titanium. A
typical microstructure of ferrotitanium shows that
this alloy is constituted mostly of FeTi crystals and
eutectic of FeTi and Ti. This is also confirmed by
X-ray diffraction analysis which reveals that ferro-
titanium contained mainly FeTi and metastable ~Ti which
is retained at low temperature. There is also a very
small amount of complex iron-titanium oxide.
The starting materials consisted of ferro-
titanium and boron powders having the following com-
position:
Ferro-titanium Boron
(wt%) (wt%)
Ti = 62.9 B = 94.96
Al = 0.96 Mg - 1 max.
Mn = 0.86 2 = balance
Cr = 0.64
C - 0.09
Fe = halance
The ferro-titanium powder was mixed with
an amorphous boron powder in stoechiometric proportions
according to the following equation:
FeTi+ Ti -~ 4 B ~ 2 TiB2 + Fe (1)
The reactions were carried out in arqon at-
mosphere and a strong exothermic reaction was observed
upon heating. Thermal differential analysis was used
to determine the temperature necessary for initiating
the reaction. The ferro-titanium powder exhibits an
endothermic effect at 1120C which corresponds to the
,
-- 7 --
;L2~3 ~;
melting point of -this alloy. No thermal af~ects are
observed on the -thermogram of boron. ~lowever, the
thermogram of ferro-titanium and boron mixtures revealed
the presence of an exothermic reaction which is initiated
at 675C. An X-ray diffraction analysis shows that
the reaction product is mostly TiB2 and iron according
to equation (1) . It can also be observed that
conversion of ferro-titanium and boron to TiB2-iron
materials is completed at 850C and that heating above
this temperature is not necessary.
The reaction products were ground to a -200
mesh powder and cold pressed into rectangular shapes.
The compacts were then encapsulated into an evacuated
low carbon stell capsule and were not isostatically
pressed. The operating temperatures were between 900
and 1300C and the compacting pressures were between
48 and 150 MPa.
It is possible to densify completely ( 4.82
g/cm3) by hot isostatic pressing synthesized TiB2composite
powders at a temperature of 1300C compared with the
usual 2000C for pure TiB2. The microstructure of
~Iipped parts shows that this material is constituted
of small TiB2 crystals surrounded by a continuous iron
phase The microhardness of this material is 1600
kg/mm . Flexural strength (measured by thç three points
bending test) of TiB2com~osite ~aterials is greatly affected by
hot isostatic pressing conditions which control the
residual porosity. Non-porous specimen exhibits flexural
strength of 1100 Mpa. This value is particularly in-
teresting considering that cemented carbides (WC-Co)
possess flexural strength in the 2000 Mpa range. Abrasion
resistance measurements indicated that fully dense
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l~S37~
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TiB2 composite materials possess an abrasion resistance
ln the range of that of WC-Co materials.
EXAMPLE 2
Commercial ferrotitanium and amorphous boron
powders were used as starting materials for the prepar-
ation of cermets. The chemical analysis of these powders
and the nickel amount used as an addition element appears
in Table I. The as-received Eerro-titanium powders
were first milled using steel balls in methanol in
order to prevent oxidation. Fine ferro-titanium powders
were then dried and batch compositions were done adding
the boron and nickel powders to the ball mill.
Table I Chemical Analysis of Powders
~aterials Element (wt%)
Ferrotitanium Ti C Mn Si Cr Ni Mo Cu
65.79 0.18 3.73 0.45 0-12 0.06 0.20 0.14
Ca V Fe ~ Al
0.07 0.10 28.61 0.04 0.47
Amorphous B ~ O~
Boron 94-961 max Bal
Nickel CS Fe Cu Co -2
0.0610.040 O.llS 0.001 0.09 0.5 Ni -
Agglomerated powders based on these mixtures
are made usinq spray-drying techiques or mechanical
aqqlomeration techniques.
~ Plasma spray powders were prepared through
agglomeration of fine powders of the starting materials.
Reagents stick together when the solvent evaporated
during mechanical agglomeration or spray drying.
g
1;2S;~7~7
The resultant powder was sievecl to eliminate fines
and also to classiEy them in two size fractions (~63
-~ 32 ~m, -125 -~- 63 ~m). These powders were then sprayed
with conventional plasma spray equipment.
The proportion of each constituent (Table
II) was settled in order that the atomic B/Ti ratio
will be slightly less than 2 to prevent the formation
of Fe2B or FeB. Nickel was also incorporated into
the micropellets during the agglomeration for increasing
the density and improve the mechanical properties of
coatings. The nickel amount added to the mixture is
about 20 vol. %.
Table II Composition of agglomerated powders
Ferrotitanium Boron Nickel Atomic ratio
(wt%) (wt%) (wt~) (B/Ti)
54.5 14.6 30.8 1.8
-
The plasma spray experiments were carried out under
ambient atmosphere with conventional plasma sprayin~ equip-
ment. As ~xpected, the X-ray diffraction analysis of coat-
ings confirmed that TiB2 is the main constituent synthetized
during the reaction of agglomerated powders through the plasma.
The ~-ray diffraction analysis also showed that
nickel is retained in its elemental form. Nickel borides
were not identified on XRD patterns of coatings. The
reaction seems to be complete since FeTi and Ti phases
were not observed after the X-ray diffractlon analysis
of coatings.
Very thick TiB2composite coatings can be deposited
~by the plasma spray synthesis (PSS) process. The coat-
ings contain very little porosities, no defect or crack
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1~537~7
even if a thickness ~Ip to 3 mm is deposited in a single
pass. It appears -that appropriate preparation oE agg]o-
merated reactive powders may lead to very dense coatings
with microhardness values higher -than 1550 kg/m2 (100
g). These values are similar to those of fully dense
parts obtained by hot isostatic pressing. The micro-
structure of coatings is considerably finer than that
of the hot isostatically presses specimen. High mechani-
cal properties are thus expected since refinement oE
crystals size is a way to increase mechanical strength.
It should be pointed out that a flexural strength of
1100 MPa and an abrasive wear resistance comparable
with WC-Co cermet have been measured on fully dense
hot isostatically pressed parts. TiB2composite coatings
obtained by the plasma spray synthesis process are
thus very promising for protection against abrasive
wear.
EXAMPLE 3
The starting materials consist of ferrotitanium
and ferroboron powders having the following chemical
analysis in non-iron elements:
TABL~ III
Chemical analysis of ferrotitanium and ferroboron
-
Element (wt %)
Material _ _
Ti C Si Al Mn B
Ferrotitanium 73.69 0.28 0.23 0.94 6.67 --
Ferroboron -- 0.33 1.94 3.32 1.04 14.88
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AlthoucJh different agglomeration techniques
including spray drying can be used to make -the spray
powders, mechanical agglomeration was used to produce
agglomerated powders. The powders were sieved to yield
agglomerates in the following size range:
-90 - 63Jum
-63 - 38 ~m.
These agglomerates were then consolidated by sintering
at 1000C during 3600 s in an argon protective atmos-
phere~ The apparent density and Hall flow measurements
of the resulting micropellets are given in Table cy.
~ - 12 -
3~
TABLE IV
Properties of agglomerated powders
Powder Particle Apparent Hall Flow
Size (~m) Density (g/cm ) (s/50g)
Coarse -90 + 63 1.69 44.5
Fine -63 + 38 1.66 41.8
1. per ASTM B212-82
2. per ASTM B213-77
Conventional plasma spray equipment was used
to spray these micropellets in the subsonic mode. The
process parameters used are given in Table V.
TABLE V
Process parameters for plasma deposition
Working gas Argon - hydrogen (15 vol.~)
Gas flow rare (l/s) 1.06
Arc Current (A) 300-800
Arc Voltage (V) 49-56
Powder feed
:
Spray rate (g/s) 0.03-0.2
Carrier gas Argon
Glas flow rate (l/s) 0.094
1;2~3~7~l'7
., ,
Deposi-tion was carried out on a 3.81x2.54x
1.27cm low carbon steel substrate. Some subs-trates
were also coateA with WC-Co for comparison. These
tungsten carbide coatings were produced by spraying
WC-ll wt% Co powders along recommended Mach II para-
meters.
Abrasive wear measurements were made according
to the Dry Sand/Rubber Wheel Abrasion Test ~ASTM de-
signation : G65-81), and they are reported as volume
10losses in Table VI.
TABLE VI
Dry sand/rubber wheel abrasion test data
Material Volume lossAbrasion res~stance
(mm3 ) factor2
SAE 1018 Steel 294 3.4
Sintered tungsten 4.2 238
carbide
Coatings
Coarse powder 19.3 51.8
Fine powder 23.5 ~2.6
High Energy
WC-Co 13 76.9
1. per ASTM G65-81, Procedure A
2. 1000/volume loss
To prevent the micropellets from being div-
ided into very fine fragments, which will be called
satellites, fine and coarse powders were plasma sprayed
at 20 kW and 30 kW respectively.
The coating is made of major phases consisting
of TiB2 and Fe, with minor amounts of Fe2B and FeTi.
The structure is very fine. The crystal size of TiB2
~ - 14 -
3t~
is less -than 1 ~Im.
It is known that a material may inclent another
only iE its hardness is 20% higher, i.e. Ha/Hm=1.2
(D. Tabor, "I`he Hardness of Solids", Review of Physics
in Technology", 1, (1970), 145-179). Because the hardest
natural abrasive is quarts which has a Vickers of
1100, a hardness of at least 1500 HV is considered
sufficient for most applications. Thus an exceptional
surface hardness is not necessary to obtain extended
wear life.
Hardness measurements of TiB2-Fe coatings
produced according to this EXAMPLE were in the 1400
HV to 1580HV range. These coatings thus exhibit a
good abrasive wear resistance. Table IV summarizes
the abrasion test data of TiB2-Fe coatings as compared
to high energy WC-Co coatings and to other dense ma-
terials. These results indicate that the performance
of TiB2-Fe coatings is nearly equivalent to that of
high energy WC-Co coatings. It was also observed that
~iB2-Fe coatings produced from coarse powder has a
slightly higher abrasion resistance than those produced
from fine powder. It must be pointed out that the
maximum TiB2 content in the synthesized coatings obtained
in this EXAMPLE is about 45 vol.%. This TiB2 content
could be increased by using higher B/Ti atomic ratios
which will enhance their abrasion resistance.
E~.AMPLE 4
A mixture of 79.2 wt% in a titanium nickel
alloy (Table VII) and of 20.8 wt% in amorphous boron
12~;3'7:17
is prepared. This mixture is heated up to 1100C for
900 s in an ar~on atmosphere to synthesize TiB2.
TABLE VII
Chemical Analysis of Titanium - Nickel Alloy
Titanium = 70 wt%
Nickel = 30 wt%
-
The reactions products are constituted of
TiB2 and a metallic phase based on nickel. This powder
is prepared in a manner similar to EXAMPLE 1 in order
to produce parts having a high wear resistance.
EXAMPLE 5
A mixture of 73.~ wt% in a titanium-cobalt
alloy (Table VIII) and of 26.6 wt% in amorphous bGron
; is prepared. This mixture is heated to about 1100C
for 900 s in an argon atmosphere to synthesize TiB2
TABLE VIII
Chemical Analysis of Titanium-Cobalt Alloy
_ _ _ .
Titanium = 80 wt%
Nickel = 20 wt%
The reactions products are constituted of
TiB2 and a metallic phase based on cobalt. This powder
is prepared in a manner similar to EXAMPLE 1 in order
to produce parts having a high wear resistance. -
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