Superhard boride layer deposition on a carbide-cobalt hard alloy

Journal of Alloys and Compounds, 201 (1993) 1-3 JALCOM 740

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Superhard boride layer deposition on a carbide-cobalt hard alloy Z. T. Z a h a r i e v a n d M. I. M a r i n o v Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia (Bulgaria) (Received November 5, 1992)

Abstract The interaction of a carbide--cobalt hard alloy (K 10, International Standards Organization) with powdery B4C and the product Borozar-HM and the effect of this interaction on the structure and hardness of the diffusion layer obtained are investigated. The phase composition of the diffusion layers is shown to depend on the type of the powders and on the temperature and duration of the boronizing process. As a result of thermal treatment with Borozar-HM, the phase CoWB is formed in the diffusion layer whereas in the case of B4C the boron-richer CoW2B2 and CoB phases prevail. The C o - W - B phase diagram, the concentration of dissolved tungsten in the binder and the difference in mass transfer of boron from the two boride powders are used to determine the phase composition of the layer. The homogeneous diffusion layer with a small number of defects, formed during the interaction of the alloy with Borozar-HM, was studied by electron microscope analysis. It is shown that the difference in phase composition of the diffusion layers obtained with the two boron compounds affects the layer hardness. The thermochemicai treatment of the alloy with Borozar-HM leads to the appearance of superhard surface layers exceeding in hardness those obtained with B4C. The maximum Vickers hardness measured HV0~ = 23.4 GPa is attributed to the singlephase diffusion layer of CoWB obtained.

1. Introduction Thermochemical treatment of hard carbide-cobalt alloys with boron-containing substances has been carried out up to now with the boronizing substances usually applied to steels such as activator- and additive-containing amorphous boron or boron carbide [1-3]. These investigations have shown intensification of the boron transfer process (i.e. the boron diffusion in the alloy) in the presence of activators, but the diffusion layer obtained has a high porosity [1]. In addition, the thermochemical treatment leads to the formation of cobalt borides (Co2B, Co3B and CoB) in the surface layer, which possess a low microhardness in comparison with other coatings (Table 1). The above disadvantages can be avoided using the chemical vapour deposition method [8] which leads to the formation of the ternary compound CoWB with a high hardness and considerable wear resistance. The purpose of the present paper was to study the interaction of the WC-Co alloy with the product Borozar-HM and B4C and to elucidate the effect of this interaction on the structure and microhardness of the diffusion layer obtained. 0925-8388/93/$6.00

TABLE 1. Microhardness of various compounds Compound

Microhardnessa (GPa)

Reference

Co3B Co2B CoB WC TIN0.97 A1203 TiCo.96 CoWB

11.30 11.30 11.30 16.85 19.60 20.00 31.10 45,00

4 4 4 4 5 6 4 7

ap = 0.5 N.

2. Experimental details Samples of a standard WC--Co alloy K10 (92 wt.% WC, 6 wt.% Co and 2 wt.% Ta-Nb-C) were packed separately in powdery Borozar-HM (325 mesh) and BaG (F 220 technical grade, ESK Kempten) and heated at 1273-1673 K for 30-120 min in an inert medium (argon). Heating up to 1473 K was carried out in a largescale Bor 6-CM-3 installation for boride coating deposition on WC-Co alloys. Above this temperature, a Degussa vacuum furnace was used. © 1993- Elsevier Sequoia. All rights reserved

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Z . T . Zahariev, M. L Marinov / Superhard boride layer deposition

TABLE 2. Phase compositions of the diffusion layer Temperature (I,:)

1273 1373 1473 1673

Borozar HM-K 10

B4C-K 10

30 min a

120 min

30 min

120 min

WC + CoWB WC + CoWB CoWB + W2Co21B6 W2CoB2 + CoWB + WC

WC + CoWB + W2CoB2 TiC + CoWB CoWB + W2CoB2 CoWB + W2CoB2

WC + W2CoBz WC + W2CoB2 WC + CoWB W2CoB2 + WC + CoWB

WC + W2CoB2 + CoWB WC + W2CoB2+ CoB CoWB + WC + WzCoB2 W2CoB2+ CoWB

aTreatment duration.

! \'

tll\ / \ 21 2O

19 18 17

tE

Co

10

20

30

tO

B, a t . %

~_'3~__N_N\ \ \

\\

/273

1473

I

I

/373

I

\ \\\ I

1573

\1\" I

1673 .~ TK

Fig. 1. Liquidus surface of the Co-W-B phase diagram according to Stadelmaier-Lowder [9] and the probable region of interaction of WC-Co with Borozar-HM or B4C.

Fig. 3. Dependence of the boride layer hardness on the temperature of alloy treatment with Borozar-HM and BaG.

Fig. 2. A single-phase boride layer (CoWB) on a hard K 10 alloy obtained by treatment of the alloy with Borozar-HM (1473 K, 30 min).

Fig. 4. A two-phase boride layer obtained by boronizing the alloy with B4C at 1373 K for 30 min.

X - r a y p h a s e analysis was p e r f o r m e d with a D R O N 3 a p p a r a t u s using C o Kot r a d i a t i o n . X - r a y m i c r o a n a l y s i s o f s o m e o f the s a m p l e s was m a d e with a J E O L - 7 3 3 . V i c k e r s h a r d n e s s was m e a s u r e d with a L e i t z - D u r i m e t a p p a r a t u s (load, 1-5 N) a n d a C a r l Z e i s s a p p a r a t u s (load, 0.5-1 N). B e f o r e t h e h a r d n e s s m e a s u r e m e n t s , the s a m p l e s u r f a c e s w e r e slightly polished. T h e h a r d n e s s

v a l u e o f e a c h s a m p l e r e p r e s e n t e d the m e d i a n o f 15 measurements. T h e p h a s e c o m p o s i t i o n o f t h e diffusion layers o b t a i n e d d e p e n d s on t h e b o r o n i z i n g c o m p o u n d a n d t h e t e m p e r a t u r e a n d d u r a t i o n o f the i n t e r a c t i o n b e t w e e n t h e alloy a n d t h e p o w d e r s . T a b l e 2 illustrates t h e d e p e n d e n c e o f the layer c o m p o s i t i o n s on the t e m p e r a t u r e

Z. T. Zahariev, M. L Marinov / Superhard boride layer deposition

EE o

superhord boride layer

-r

3000 diffusion layer

2500 ~WC-Co

harclalto},

16o

I~o ~

2000

1500

go

3

on the alloy surface, of superhard layers whose hardness exceeds that of the layers obtained with B4C. The maximum hardness value (23.4 GPa) was found for layers with Borozar-HM at 1473 K, which is ascribed to the formation at this temperature of a single-phase CoWB layer (see Table 2). The dependence of the coating hardness on the temperature of alloy treatment with boron carbide also passes through a maximum at a lower temperature (1373 K) at which the diffusion layer is two-phase CoW2B2 and WC (Fig. 4). The porosity of the diffusion layer increases with increasing temperature for saturation by boron carbide. The saturation by Borozar-HM is a reason for obtaining substantially thicker (0.1 mm) and non-porous diffusion layers on WC-Co alloys. The microhardness of a superhard boride CoWB layer as shown in Fig. 5 is equivalent to the microhardness of polycrystal boron carbide material [12].

Fig. 5. Dependence of the microhardness of a boride layer obtained at 1473 K with Borozar-HM on the layer thickness.

3. Conclusions

of thermochemical treatment for a duration of 30 or 120 min. It is worth noting that the use of BorozarHM leads to the formation of CoWB only in the diffusion layer, whereas in the case of B a G the ternary boride CoW2B2, which is richer in boron, prevails. This can be explained on the basis of the Co-W-B phase diagram [9] concerning the boron content (i.e. the boron transfer) from boron carbide (Fig. 1). It is known that in the binding cobalt phase of carbide--cobalt alloys there is 8-10 wt.% of dissolved W [10] whose solubility significantly increases with increasing temperature. In addition, active interaction of WC with boron proceeds above 1373 K [11]. Hence it could be assumed that the interaction should take place in the Co-W-B system and, depending on the liquidus surface of the latter [9], CoW2B2 would be formed at high boron concentrations (above 20 at.%) whereas at lower concentrations CoWB should be found. The much higher concentration and partial pressure of boron in B4C facilitate boron transfer and interaction with the binding phase (Co-W). Electron microscope studies and X-ray microanalysis confirmed the different mass transfer mechanisms for the two powders. The appearance of a eutectic phase during the interaction of the alloy with Borozar-HM facilitates the formation of a homogeneous diffusion layer with a small number of structural defects (Fig.

2). The difference in phase composition of the diffusion layers obtained using the two powders affects the layer hardness (Fig. 3). The thermochemical treatment of the alloy with Borozar-HM results in the formation,

(1) The structure and hardness of diffusion boride layers obtained by the interaction of the hard WC-Co-MeC alloy (K 10) with Borozar-HM or B4C depend on the type of medium, the temperature and duration of the thermochemical process. (2) The superhard boride layers deposited with Borozar-HM can be used for enhancing the wear resistance of the hard WC-CO-MeC alloys. References 1 Yu. P. Kolosvetov, B. S. Navrotskii and G. L. Zhunkovskii, Poroshk. Metall., 11(119) (1972) 33. 2 I. Katsumy, K. Masami and H. Horosi, J. Jpn. Soc. Powder, Powder Metall., 5 (1969) 230. 3 W. Eberhard, DDR Patent 68432, 1967. 4 G. V. Samsonov and I. M, Vinitskii, Tugoplavkie soedineniya, Spravotchnik, Metallurgiya, Moscow, 1976, p. 33. 5 G . V . Samsonov and T. S. Verkhoglyadova, Zh. Strukt. Khim., 2 (5) (1961) 617. 6 G'V'Sams°n°v'Sprav°tchnikFizik°-khirnicheskiesv°istva°kislov, Metallurgiya, Moscow, 1978, p. 203. 7 Z. Zakhariev, R. Zlateva and K. Petrov, J. Less-Common Met., 117 (1986) 129. 8 U. Koenig, B. van den Hendrikus and R. Norbert, Pat. BRD, D E 3332260 AI, 28 March 1985. 9 H. Stadelmaier and J. Lowder, Metall (Berlin), 21 (10) (1967) 1023. 10 H. Suzuki and H. Kubota, Planseeber. Pulvermetall., 14 (2) (1966) 96. 11 H. Hofman and G. Petzow, J. Less-Common Met., 117 (1986) 121. 12 A. Lipp and K. Schwetz, Haerte und Haertebestimmung von nichtmetallischen Hartstoffen, Ber. Dtsch. Keram. Ges., 52 (11) (1975) 337.