Plasma spray synthesis of TiB2–Fe coatingsB. Champagne and S. Dallaire Citation: Journal of Vacuum Science & Technology A 3, 2373 (1985); doi: 10.1116/1.572884 View online: http://dx.doi.org/10.1116/1.572884 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/3/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Plasma sprayed Nd–Fe–B permanent magnets J. Appl. Phys. 93, 7987 (2003); 10.1063/1.1558590 Laser remelting of plasma sprayed coatings J. Laser Appl. 12, 175 (2000); 10.2351/1.521930 Formation of TiB surface alloys by excimer laser mixing AIP Conf. Proc. 231, 652 (1991); 10.1063/1.40801 Magnetron sputtered boron films and Ti/B multilayer structures J. Vac. Sci. Technol. A 8, 3910 (1990); 10.1116/1.576419 Transmission electron microscopy characterization of plasma sprayed TiC coatings J. Vac. Sci. Technol. A 3, 2475 (1985); 10.1116/1.572861

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.174.255.116 On: Sun, 30 Nov 2014 05:33:10

Plasma spray synthesis of TiB2-Fe coatings B. Champagne and S. Dallaire National Research Council Canada, Industrial Materials Research Institute, 75 De Mortagne Boulevard, Boucherville, Quebec J4B 6Y4

(Received 30 April 1985; accepted 18 May 1985)

Titanium boride possesses so high a hardness that it could be considered a good candidate for the production of wear-resistant coatings. However, conventional coating processes failed to achieve thick TiB2 coatings with appropriate mechanical strength for industrial application. To overcome their brittleness and take advantage of their hardness, titanium boride should be adequately dispersed in an appropriate metal matrix. To meet these requirements, TiB2-Fe coatings were produced via a synthesis process. In this work, TiB2-Fe coatings have been synthesized through the endothermic reaction offerrotitanium with ferroboron. The fabrication technique consists in plasma spraying micropellets comprising the reagents and in depositing the as-reacted products onto a substrate. Spraying parameters were studied and optimized to ensure the synthesis ofTiB2. By this one-step process, thick abrasion resistant TiB2-Fe coatings have been obtained.

I. INTRODUCTION

Due to their high hardness borides are considered good can­didates in wear resistance applications. Different fabrication processes such as diffusion, chemical vapor deposition and electric spark alloying are currently used to develop boride coatings onto materials. These processes have some draw­backs which seriously limit their practical interest. Indeed, the substrate"temperature is usually high (diffusion, chemi­cal vapor deposition) and reactant materials are often toxic and expensive (gaseous boriding). Moreover, parts of com­plex shape or large dimension are difficult or impossible to coat uniformly, the coating thickness is usually thin (25-250 ,urn) and some of these processes require complex equip­ment.

A different coating process is proposed here to develop a wear-resistant coating. It consists in producing TiB2-Fe coatings by the plasma spray synthesis process. These coat­ings are obtained through the endothermic reaction between ferrotitanium and ferroboron alloys. The principle of the method is explained and the way by which the synthesis of TiB2 occurs during the spraying operation is described. The influence of process parameters are also examined. Finally, the microstructure of the resulting TiB2-Fe coatings is de­scribed and their wear resistance is evaluated.

II. COATING PRINCIPLE

Conventional spraying techniques failed to achieve thick cermet coatings based on TiB2 because they have not per­formed very well with materials that are difficult to melt. A different approach was previously proposed for producing cermet coatings based on TiB2. It consists in synthesizing TiB2 through the reaction of a ferro alloy with elemental bo­ron and in depositing the reacted products onto a substrate by plasma spraying. 1 The exothermic reaction which occurs between reactants can be represented by the following equa­tion:

a[FeTi] + b [Ti] + c[B]-d [TiB2] + e[Fe] , (1 )

where prefixes a, b, c, d, and e express mole fractions. Thick TiB2-Fe cermet coatings have been deposited by using the plasma spray synthesis.

Another means which could be less expensive was devel­oped. This different route uses the principle of the auxiliary metal bath process to synthesize TiB2. The auxiliary metal bath process, known also as the Menstruum process, consists of promoting the reaction of elements by dissolving them in a liquid metal. 2-5 TiB2 crystals can be synthesized in an iron auxiliary bath by melting ferrotitanium and ferroboron mix­ture.5 The reaction which occurs between constituents can be represented by the following equation:

e[Fe] + f[FeTi]:[B]I[Ti] <2

a[FeTi] + b [Ti] + c[FeB]-d [TiB2] + g[Fe]:[B]I[Ti] = 2 (2)

h [Fe] + i[Fe2B]:[B]I[Ti] >2

where a, b, c, d, e,/, g, h, and i express mole fractions. The synthesized products depend on the reaction tem­

perature as well as on the [B]I[Ti] atomic ratio. When the [B]/[Ti] atomic ratio is less than 2, the reaction products contain Fe and FeTi compounds. Similarly, Fe and Fe2B compounds are found in the products when this ratio is greater than 2. Figure 1 shows the microstructure of a solidi­fied bath formed by arc melting a mixture of ferrotitanium and ferroboron having a [B]/[Ti] atomic ratio of 2. In this

~ase the temperature was sufficiently high to complete the reaction and the microstructure is constituted of an angular TiB2 phase surrounded by an iron based matrix. The tem­perature necessary to complete the reaction was found to be above 1700 0c. 5 This temperature can be easily achieved during plasma spraying. Thus it is possible to produce TiB2-Fe coatings by plasma spraying micropellets comprising fer­rotitanium and ferroboron. Indeed, these micropellets may act as "microbaths" in which the synthesis of TiB2 takes

2373 J. Vac. Sci. Technol. A 3 (6), NovlDec 1985 0734-2101/85/062373-05$01.00 2373 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.174.255.116 On: Sun, 30 Nov 2014 05:33:10

2374 B. Champagne and S. Dallaire: Plasma spray synthesis

FIG. I. Scanning electron micrograph of an arc· melted bath wherein the [B]I[Ti] atomic ratio is 2.

place during the plasma spraying resulting in the deposition of TiB2 based coatings (Fig. 2).

III. MATERIALS AND EXPERIMENTAL DETAILS

Commercial ferrotitanium and ferroboron powders were used for the preparation of micro pellets. The chemical anal­ysis of the as-received powders appears in Table I. The ferro­titanium alloy contains FeTi and Ti while the ferroboron is mostly constituted of FeB. The starting powders were sepa­rately ball milled in methanol. The resulting powders were dried and mixed on a weight basis to give a [B]/[Ti] atomic ratio of 1, i.e., a mixture containing 46 wt. % ferrotitanium and 54 wt. % ferroboron. This ratio was selected to ensure the complete transformation of FeB.

Micropellets were prepared by agglomeration of ferroti­tanium and ferroboron and sieved to yield two different size fractions: - 90 + 63 11m, - 63 + 3811m. The two size frac­tions were sintered at 1000 °C under an argon protective at­mosphere in order to increase their mechanical strength. The apparent density and flowability of micropellets were measured in accordance with the ASTM B212-82 and B213-77 methods. X-ray diffraction patterns were obtained by us­ing filtered MoKa radiation.

Plasma spraying was done using process parameters shown in Table II. Three different working gases (Ar, Ar + H2, Ar + He) and various power levels were tried for the plasma spraying of micropellets.

The abrasion wear resistance of coatings was measured in accordance with the low stress ASTM G-65 method. The recommended procedure A for evaluating the abrasion resis-

Agglomerated particles

fJ/;J~ ~ Injection

Gas .bCJ"'-· .~ .• ~ .. "" •...• .,. .. ,.., .. ~D Melted particles

~ 0\0 -. -. -c:::::::::: <m> <::> <>=:::- <> <>

Gas 'rl p~sma \

Substrate

Plasma gun Coating

FIG. 2. Schematic view ofthe plasma spray synthesis process.

J. Vac. Sci. Technol. A, Vol. 3, No.6, Nov/Dec 1985

2374

TABLE I. Chemical analysis of powders.

Element (wt.%) Material Ti C Si AI Mn B Fe

Ferrotitanium 73.69 0.28 0.23 0.94 6.67 Bal. Ferroboron 0.33 1.94 3.32 1.04 14.88 Bal.

tance of coatings was used. The measurement method con­sists in abrading a specimen with a grit of controlled size and composition. Figure 3 shows a schematic representation of the test apparatus. A force of 130 N maintained the specimen against the rotating wheel. The 50170 mesh quartz sand was introduced between the specimen and the rotating wheel at a flow rate ranging between 250 and 350 g/min. The test lasted the time required for 6000 wheel revolutions. Wear is report­ed as volume loss.

IV. RESULTS AND DISCUSSIONS

The typical appearance of the sintered micropellets is shown in Fig. 4. The micropellets possess a good flowability: 44.5 s/50 g for the - 90 + 63 11m fraction; 41.8 s/509 for the - 63 + 38 11m size fraction. However, their low appar­ent density make them difficult to inject into the plasma: 1.66 g/cm3 for the - 90 + 63 11m fraction; 1.6 g/cm3 for the - 63 + 38 11m fraction. As mentioned earlier, the micropel­

lets must be totally melted to insure the synthesis of TiB2. This requires a careful control of spraying conditions. The effect of spraying parameters on the melting of micropellets was thus carefully studied. Micropellets were sprayed into water and the proportion of melted micropellets was deter­mined by metallographic counting. As shown in Table III, the proportion of melted micropellets varied with the work­ing gas and the power level. Argon working gas containing hydrogen seemed to be more efficient. Higher power merely increases the proportion of melted particles. It can also be pointed out that the finest micropellets melt more easily.

Figure 5 shows the morphology of completely melted mi­cropellets collected into water. The microstructure of these "microbaths" consists of fine TiB2 crystals dispersed in an iron-based matrix (Fig. 6).

The melting of most of the particles is generally consid­ered as an important criterion to set the spraying parameters for the production of a good coating. However, the amount of melted micropellets is not the only factor determining the

TABLE II. Plasma spraying parameters.

Working gas Ar, Ar + He, Ar + H2 Gas flow rate (I/s) 1.06 Arc current (A) 300-800 Arc voltage (V) 49-56 Spraying rate (g/s) 0.03-0.2 Powder carrier gas Argon Powder gas carrier flow rate (I/s) 0.094

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.174.255.116 On: Sun, 30 Nov 2014 05:33:10

2375 B. Champagne and S. Dallaire: Plasma spray synthesis

Hopper

Ottawa sand

Weights

Specimen Rubber lined wheel (1" x 3" x 1/2 ")

FIG. 3. Schematic view of the abrasive test apparatus (ASTM G-65).

adequate conditions to obtain good TiB2-Fe coatings. In­deed, it was observed that the plasma spraying at high power leads to fragmentation of micropellets. Figure 7 illustrates the particle size distribution of micropellets sprayed into wa­ter at power levels of 17 and 40 kW. The proportion of fine particles increases significantly when the power is raised. The morphology of the micropellets plasma sprayed into wa­ter (Fig. 8) clearly indicates that a satellization phenomenon occurred when using a power level of 40 kW.

Whatever the mechanism of this phenomenon, it was con­sidered objectionable. Indeed, satellization during spraying may change the synthesis process resulting in variation of

FIG. 4. Scanning electron micrograph of sintered micropellets: (a) - 63 + 38 flm; (b) - 90 + 63 flm.

J. Vac. Sci. Technol. A, Vol. 3, No.6, Nov/Dec 1985

2375

TABLE III. Influence of spraying parameters on the melting of micropellets.

Size of Gas Power Proportion of melted micropellets (kW) micropellets (%)

(.urn)

- 90+ 63 Ar/5 vol % H2 17 67 - 90+ 63 Ar/5 vol % H2 25 84 - 90+ 63 Ar/5 vol % H2 40 92 - 63 + 38 Ar/5 vol % H2 17 95 - 63 + 38 Ar/5 vol % H2 25 97 - 63 + 38 Ar/5 vol % H2 40 96 -90+ 63 Ar/50 vol % He 21 29 - 90+ 63 Ar/50 vol % He 40 32 - 63 + 38 Ar/50 vol % He 21 46 - 63 + 38 Ar/50 vol % He 40 61 - 90+ 63 Ar 40 63

FIG. 5. Scanning electron micrograph of micro pellets sprayed into water.

FIG. 6. Microstructure of micropellets sprayed into water.

the microstructure of coatings. In order to minimize this phenomenon the - 90 + 63 J-lm and the - 63 + 38 J-lm mi­cropellets size fractions were sprayed at 30 and 20 kW, re­spectively.

When the micropellets in which the synthesis of TiB2 oc­curs are adequately sprayed, they flatten on a substrate forming a coating. Figure 9 shows the microstructure of a TiB2-Fe coating. X-ray diffraction analysis confirmed that the deposit is mostly constituted of TiB2' iron and a small amount of FeTi (Fig. 10). The sprayed coating contains also Fe and FeTi since the [B]I[Ti] atomic ratio in micropellets was 1.

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.174.255.116 On: Sun, 30 Nov 2014 05:33:10

2376

UJ

" z ~ ~

5

4

E 3 ~ J.

Ui ' ~ ~ 2

~ v

UJ ~

5

4

5

4

B. Champagne and S. Dallaire: Plasma spray synthesis

o 25 50 7S

a 2S 50 7S

M I CROPELLETS

DIAMETER , pm )

DIAMETER < ,,",m )

150 175 200 225

150 175 200 225

PL .... SM .... SPRAYED I

........ /l':5vol. XH2

100 125 150 175 200 225 DIAMETER

< pm )

a

b

c

FIG. 7. Particle size distribution of micro pellets ( - 90 + 63 ,urn): (a) Before plasma spraying; (b) plasma sprayed at 17 kW; (c) plasma sprayed at 40 kW.

Sprayed coatings generally have the structure of rapidly solidified materials. Impinging microbaths also undergo a rapid solidification resulting in a considerable refinement of the microstructure. Due probably to this fine dispersion of hard TiB2 compounds in the matrix, the TiB2-Fe coatings exhibit a high abrasion resistance. Table IV gives the volume losses measured by the dry sand/rubber wheel abrasion test. For comparison, the test was also done on a WC-Co coating sprayed along Mach II parameters. These results indicate that the performance of TiB2-Fe coatings approach that of high energy WC-Co coatings. The TiB2-Fe coatings synthe­sized via the exothermic reation offerrotitanium with boron [reaction (1)] are slightly more abrasion resistant than those obtained by the auxiliary metal bath process [reaction (2)].

J. Vac. Sci. Techno!. A, Vol. 3, No.6, Nov/Dec 1985

2376

FIG. 8. Typical morphology of micropellets ( - 90 + 63 ,urn) sprayed into water at 40 kW.

FIG. 9. Optical micrograph of a TiB2-Fe coating.

I I 35

+TiB2 TFe o FeTi

I I 30

I , I I 25 20

28 (DEGREES)

FIG. 10. X-ray diffraction pattern of a TiB2-Fe coating.

TABLE IV. Wear resistance of TiB2-Fe coatings.

Material Micropellets size (,urn)

TiB2-Fe (exothermic) - 125 + 63 TiB2-Fe (exothermic) 63 + 32

TiB2-Fe (endothermic) 90+ 63 TiB2-Fe (endothermic) 63 + 38

WC-\1 wt. % Co (Mach II parameters)

+

15 10

Volume loss (mm3

)

15.8 18.9

19.3 23.5

\3

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.174.255.116 On: Sun, 30 Nov 2014 05:33:10

2377 B. Champagne and S. Dallaire: Plasma spray synthesis

Their higher TiB2 content (60 vol %) is probably the reason for their better performance. TiB2-based coatings synthe­sized through microbath have only 45 vol % TiB2.

V. CONCLUSION

Hard coatings which comprise TiB2 have been deposited by the plasma spray synthesis process. The main feature of this technique is to combine in a one-step operation, both the synthesis and the deposition process. Coatings constituted of very fine TiB2 crystals dispersed in an iron-based matrix have been obtained through the reation offerrotitanium and

J. Vac. Sci. Technol. A, Vol. 3, No.6, Nov/Dec 1985

2377

ferroboron. MicropeUets comprising reactants act as micro­baths in which the TiB2 synthesis takes place. The resultant coatings are thick (several millimeters) and possess a good abrasion resistance which could make them useful in low­stress applications. Their performance could also be en­hanced by a further increase of the TiB2 content.

IS. Dallaire and B. Champagne, Thin Solid Films 118, 477 (1984). 2R. Kieffer and H. Rassaerts, Int. J. Powder Metall. 2,15 (1966). 3G. Jangg, R. Kieffer, and L. Usner, J. Less-Common Met. 14,269 (1968). 'G. Jangg and R. Kieffer, Monastsh. Chern. 104,266 (1973). 5B. Champagne, S. Dallaire, and A. Adnot, J. Less-Common Met. 98, L21

(1984).

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.174.255.116 On: Sun, 30 Nov 2014 05:33:10