CA1253717A - Tib.sub.2 composite materials and process of producing the same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
There is disclosed a process for synthetizing TiB2 composite materials containing a metallic phase. The preparation of these composites comprises providing mixtures of titanium alloys which in addition to at least 30 wt%
titanium also contain Fe, Ni, Al, Mo, Cr, Co, Cu or mixtures thereof, and boron or ferroboron, reacting these mixtures by local igniting and exothermic reaction or by heating or melting, resulting in the synthesis of composite material containing fine TiB2 crystals dispersed in a fine metallic phase which is derived from the metallic element previously alloyed or combined with the titanium of the titanium alloys or the metallic element or elements contained in the ferroboron.
By leaching out the metallic phase, fine TiB2 powders may be obtained. Parts and coatings can also be obtained. The parts are normally obtained by treating the TiB2 material by powder metallurgic techniques. Coatings may be obtained by thermo or plasma sprayed depositing on a substrate. Hard facing techniques may lead to TiB2 composite overlays.

Description

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 3'7~'~
, .
., , 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:

: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 ., , 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 , ;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 l~S37~

..

TiB2 composite materials possess an abrasion resistance ln the range of that of WC-Co materials.

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 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.

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 ~537~l~7 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.

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. -

Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process of preparing composite materials consist-ing of fine TiB2 crystals dispersed in a metallic phase, said metallic phase containing at least 32.6 wt % TiB2, which comprises providing a mixture consisting essentially of titanium bearing alloys and boron, said titanium bearing alloys containing at least 25 wt % titanium and a member selected from the group consisting of Fe, Ni, Co, Al, Mo, Cr, Cu and mixtures thereof, initiating an exothermic reaction in said mixture at a temperature below the melting points of said titanium bearing alloys and said boron, to give fine TiB2 crystals dispersed in a metallic phase, said metallic phase being derived from metallic elements pre-viously alloyed with titanium.

2. A process of preparing composite materials consist-ing of fine TiB2 crystals dispersed in a metallic phase, said metallic phase containing at least 13.9 wt % TiB2, which comprises providing a mixture consisting essentially of titanium bearing alloys and ferroboron, said titanium bearing alloys containing at least 25 wt % titanium and a member selected from the group consisting of Fe, Ni, Co, Al, Mo, Cr, Cu and mixtures thereof and melting said mixture to give fine TiB2 crystals dispersed in a metallic phase, said metallic phase being derived from metallic elements pre-viously alloyed with titanium and contained in the ferro-boron.

3. A process according to claim 1, which comprises densifying or consolidating the TiB2 composite materials containing a metallic phase to give TiB2 - composite parts.