Hardmetals

Hardmetals E A

Almond,

National Physical Laboratory, Teddington, Middlesex, T W l l OLW.

Abstract

Uncertainties of supply, diminishing resources and prospects of improved properties have stimulated research on finding substitutes for the constituents in tungsten-carbide/cobalt hardmetals and for finding replacements. The direction of the research varies in different countries andfor different applications. The materials involved are hardmetals containing alloy binder phases or complex carbo-nitrides, or are cermets or toughened ceramics. Introduction The fact that for many industries, the terms 'hardmetals', 'cemented carbides' 'hard alloys' and even 'carbides' are used synonymously for WC-Co hardmetals is a tribute to the versatility of this particular hard-particle/binder-phase combination which is used extensively in materials shaping, mining, and wearresistant components (Fig 1). There is no evidence of new materials that could achieve a wholesale replacement of the WC based compositions and there is not the pressure of seven years ago for substitution. Nevertheless the stimulus from substitution studies has led to the development of materials with potentially better properties than WC-Co hardmetals for specific applications. Before a discussion of these it is pertinent to refer to some of the major technological stumbling blocks to replacement. Firstly no material has been developed with equivalent impact toughness to the WC hardmetals used for percussion mining and hot forging, which represent the major proportion of the hardmetal market, and very few materials have equivalent abrasion resistance. Secondly, many potential substitutes lack ancillary properties. Amongst these are an amenability to ease of joining by brazing and diffusion bonding and to net-shape forming with the minimum amount of finishing; a sufficiently high electrical conductivity to be machined by electro-discharge methods; a composition that is recyclable and reclaimable; physical properties that suit non-destructive quality control measurements such as predictable magnetic properties; a capacity for some degree of surface hardening and shape correction by heat treatment; and of major importance, the raw materials should be available in a form that can be processed successfully into a uniform product on an industrial scale, preferably by the conventional milling and sintering route.

324

Fig. 1. Examples of hardmetal tools and components: background- carbide tipped circular saw, and wear resistant tiles (below): lower foreground drills and burrs used for wood, composite boards and printed circuit boards: centre- rotary seals: upper l e f t - masonry drill: upper rightindexible insert in holder, and wire drawing die.

MATERIALS & DESIGN Vol. 7 No. 6 NOVEMBER/DECEMBER 1986

Candidate Substitutes The question of materials substitution cannot be divorced from that of materials improvement since there is little point in satisfying a short term need for a substitute if this itself will be replaced by superior materials in the near future. Basically what we are looking for as a direct substance in WC-Co hardmetals is a high volume fraction of very hard constituents bound together by a tough strong metal. The potential hard particles are borides, carbides or nitrides of the transition groups IVB, VB, and VIB metals of the periodic table, which are W, Ti, Ta, Hf, Nb, Zr, Mo, Cr, V; the basic constituents of the binder are the transition group VIII metals Co, Ni, Fe, alloyed to various degrees with the hard particle's soluble constituents and other deliberate additions. Of the candidate compounds in order of hardness the borides are usually hardest, followed by the carbides, then the nitrides but this distinction applies only to metal-C, -N, -B compounds which form the tightly packed Hagg-type interstitial structure with directional atomic bonding (Table I). In this respect the compounds of the group VIII metals are excluded, and transition group oxides are relatively soft because of their nondirectional ionic bonding. This applies equally if selection is extended to the compounds of silicon where the ionicity and softness increases as we go from carbide to nitride to oxide. These rules for selection reflect trends in behaviour rather than scientific guidelines since hardness is not an intrinsic property. Of equal concern are high temperature properties, toughness, chemical inertness and thermal shock resistance. Unfortunately these latter requirements have been expressed even less satisfactorily than hardness. In considering the properties that are needed by a binder-phase to be able to compete with Co, we remark that in WC-Co hardmetals the binder-phase is a Co-W-C alloy with a very high workhardening capability and good corrosion and creep resistance. Also one of the essential characteristics that a substitute should match is cobalt's high wettability for the WC grains, which means that the particle-matrix interfaces are not an easy fracture path. Addressing next the danger of an essential need arising for finding a replacement for a short-term substitute for WC-Co hardmetals we note that the EEC is currently not self sufficient in any of the metals mentioned above. If changes in the economic and political climate created a sufficiently high demand, W and Ti could be extracted

TABLE I Mierohardnesses of Borides, Carbides and Nitrides (Quoted in Cermets, Naukova Dumka, Kiev, USSR 1985) Boride

Carbide

Nitride

2100 2900 2350 2600 2500 3400 2800 2660 2250

1800 2830 (1800) 2600 2380 3170 2480 2100 2950

(1640) 1520 (1220) 2050 1310 1670

Cr Hf Mo Nb Ta Ti V W Zr

These values are presented solely to illustrate trends, since there are no absolute values for microhardness. from native ores in adequate quantities and Fe could be salvaged from scrap. Similarly the introduction of Si- and AIbased compounds could become viable. Complementary to these considerations are the widely varying estimates for the exhaustion dates of the known economically extractable reserves of particular minerals in the world. For present consumption levels, some calculations give 30-50 years for W, Ni and Mo. Before then, increasing costs of raw materials will bring about an increasing degree of substitution and reclamation unless large ore deposits are discovered. The economics of raw materials costs have changed little in the last ten years and most costs have decreased in real terms. For example, WC powder cost £12/kg in 1976 and it is £14/kg today, whilst Co has remained reasonably steady lately despite major fluctuations in the late 1970's and at £28/ kg is still more than three times the cost ofNi, Mo and Cr. Since powder prices are usually quoted in cost per unit mass, it is easy to overlook the fact that the properties of the sintered product will be determined by the volume contents of the constituent phases. The advantage of using light elements becomes evident when comparing the cost per kg of£ 14, £22, £30 and £40 for WC, TiC, TiB2 and TiN respectively with the cost per cm 3 of £0.22, £0.11, £0.13 and £0.22. However as was stressed earlier, when dealing with completely new compositions, an important factor in the economics of manufacture will be whether powders can be produced in a suitable form for milling and sintering into a uniform product. Regarding current research and development in hardmetals for improvement, substitution and replacement, several trends can be identified and categorised but first it is of interest to

MATERIALS & DESIGN Vol. 7 No. 6 NOVEMBER/DECEMBER 1986

consider the WC-Co system itself. Here, a very active area is the development of materials with very fine grain sizes of an average of about 0.3pro to give good edge retention in drilling and cutting wood, composite-boards, soft metals and printed circuit boards. Also there are continual general improvements to obtain more consistent products, and investigations into new techniques for processing the powders. The main categories of development which form the sustenance of this paper are: i) WC hardmetals with the Co binder phase replaced by either ferrous alloys or superalloy compositions, which have good resistance to creep and corrosion, and will probably find a general field of application; ii) Ti(C,N)-based hardmetals which are resistant to creep and diffusion wear and are aimed mainly at cutting applications; iii) various cermets containing either low binder phase contents for metal cutting, or high binder contents for metal forming, wear resistant applications and mining; iv) ceramic composites which have been developed with increased toughness to extend their application beyond the present range of continuous cutting operations. Finally, but not considered here, there is a constant encroachment from hard coatings for metal cutting and wear resistance, from sintered tool steels for metal forming and from compacts of diamond and cubic boron nitride with high volume fractions of metal binder phases for mining and metal forming. To illustrate how some of these developments are influencing production trends in indexable cutting inserts,

325

in Japan the proportion of Ti(C,N)based hardmetais was 27% in 1983 and is increasing by 50% annually, whilst the usage of ceramics is still only 4%.

WC Hardmetals with Alloy BinderPhases An example of a major incentive for substitution is the one-third decrease in Co supplies in the late 1970's which together with other factors caused the price to double within the space of a month. The main candididate substitute was Ni because of it similar chemistry but it lacks equivalent strength and it was necessary to use a 70% C o - 30% Ni mixture or to research suitable strengthening techniques. This latter choice proved to be a blessing in disguise since it has resulted in new hardmetal compositions which may have the potential not only to match but in some aspects to surpass the Co binder phase hardmetals. Of primary interest are the superalloy compositions which fall into the two categories of NiCo-Cr-Mo and Ni-Cr-Mo-A1 based. The former rely for strengthening on solid solution hardening while in the latter it occurs by the precipitation of the Ni 3 A1, gamma prime phase (figs 2a and b). As expected the creep and probably the corrosion resistance are better than those of equivalent WC-Co grades whilst preliminary restults indicate that their toughness may be superior also. The stages in development of these materials are ~hown in Fig 3. Another area where substitution should have been able to exploit a high state of metallurgical understanding is in the ferrous based materials. Developments have followed the obvious routes of using heat treatable binder phase compositions that can be transformed to hard martensitic matrices, whilst high chromium contents can provide corrosion resistant austenitic structures. Unfortunately the risk of quench cracking restricts the use of heat treatable hardmetals to high binder phase compositions, and careful C control is essential to prevent the formation of the numerous carbides that can form in ferrous alloys and cause irreproducibility in properties. So far the very complexity of the Fe-C system and some corrosion problems appear to have defeated any major substitute incursion from this source. An imaginative substitution which employs the concept applied by Hadfield for hardening the surfaces of railway lines uses a 14% Mn steel binder phase which transforms to martensite as it is subjected to work hardening when used

326

a) Fig. 2

b) The typical angular microstructure of a 9% Co/WC hardmetal (a), is unaffected by substituting Ni alloys for the Co binder phase but the incorporation of Ni3A! precipitates (b) is often needed to provide strength at low and high temperatures.

WC-BASED HARDMETALS

WC-Co

WC-Ni

WC-Co-Ni

WC-Ni-Cr-Mo

I

WC-Ni-Cr-Mo-AI

WC-Ni-AI

WC-Co-Ni-Cr-Mo WC-Co-Ni-Cr-Mo-AI

Fig. 3. Stages in development of WC hardmetals with alloy binder-phase for example as a wear surface or cutting edge.

TI(C,N) Hardmetals Research on Ti based hardmetals in the last fifty years has taken place on a scale second only to that for the WCCo grades. The binder phase has usually been Mo and Ni and the course of developments has encompassed many applications including wear resistance where the steel bonded TiC grades has found a small but constant market (Fig 4a). In machining applications a primary incentive for using TiC has been the good resistance to edge blunting from plastic deformation at high temperatures and the resistance of TiC to diffusion wear by iron which

is a weakness that precludes the use of WC-Co hardmetals in an unalloyed or uncoated condition for machining steels (Fig 4b). Though most of the early development occurred in Germany and the USA, the emphasis of research has recently switched to Japan. This has not changed the objectives which are generally to achieve an increase toughness both by modifying the structure of the hard Ti(C,N) phase and increasing its cohesion to the matrix. In addition, as with WC-Co hardmetals the concept of using a superalloy binder phase has been adopted to enable improvements to be obtained in creep and corrosion resistance. As can be seen in Fig 5, these materials can obtain up to six different metals.

MATERIALS & DESIGN Vol. 7 No. 6 NOVEMBER/DECEMBER 1986

a)

b)

..... !

Although the latter development is similar in the two classes of hardmetal the detailed chemistry of the reactions of the binder phase constituent with the Ti(C,N) is far more complicated than with WC. The hard constituent is usually a (Ti, Mo)(C,N) phase with a higher concentration of Mo and N in the outer rim of the particles (Fig 4c), and although it appears to be tougher than TiC, the hardmetals still lack the impact toughness of the WC grades. In attempts to improve toughness further, a range of (Ti, W)( C ,N) hardmetals are being developed but there appear to be major problems with controlling the ratios and distribution of C and N to obtain reproducible properties.

Other Cermets Conventional routes

R

....

e) Fig. 4

d) a) The roundness of TiC grains is claimed to contribute to the low friction and good "lubrlcosity" of TiC-Fe hardmetais. b) The tough WC angular structure is provided with high temperature rigidity and diffusion-wear resistance by TiC and TaC in this 58% WC-13% Tac-16% Tie-13Co hardmetal. c and d) Despite major differences in microstructural appearance, the basic Ti(C,N) hardmetals have similar properties when made by conventional (e), or by a spinodal decomposition (d). However the toughness of the former has been considerably improved by alloying to alter the relative dimensions and properties of the hard core and soft rim of the Ti(CN) particles.

Ti(C,N)BASEDHARDMETALS

TiC-Mo-Ni T

i

C

-

M

~

Tic-TiN-Mo-Ni (Co) N TiC-Mo-Ni-AI

TiC-TJ~N-!o-Ni-A1 Tic-TiN-MIC-M2C-Mo-Ni (Co)

TiC-TiN-MIC-M2C-Mo-Ni-AI Fig 5.

Stages in development of Ti(C,N) hardmetals; M, and M2 represent added carbides of the metals Cr, Hf, Mo, Nb, Ta, V, W and Zr

MATERIALS & DESIGN Vol. 7 No. 6 NOVEMBER/DECEMBER 1986

Although there have been studies of the carbides of Mo and Cr and other metals that have led to the development of new hardmetals, research on the borides and nitrides has not yet enjoyed the same successful outcome. Of particular interest currently is a boride with the most attractive properties, TiB2. This has high elastic modulus, a melting point of 2980°C, and a good electrical conductivity. Interest expressed in this compound in the 1950's has returned lately in the USA, the F R G and Japan where studies are being made of the atomic bonding and of the sintering reactions with Ni to establish the conditions for the formation of intergranular Ni3B and the Tau phase (NixTiy)23B6; these phases relax anisotropic crystallographic shrinkage stresses but they may also be embrittling. Other developments in Japan involve combining TiB2 with Ti(C,N) in a complex boride matrix. In general, both the USSR and Japan appear to be closer to introducing the borides into machining and wear applications than the USA or EEC countries. It is impossible to give a comprehensive coverage of all the potential cermet combinations in a short survey or, indeed, in a major research programme. However, in this respect the USSWs approach is instructive since they have combined their resources at one institute where they are able to consider the cermet area as a whole, embracing not only tools for material shaping and mining but also grinding wheels, wear resistant components, armour, furnace components and heating elements. Additionally they are using high pressures of up to 50 kbar at 1200°C to obtain strong particlematrix interfaces in systems such as WTiC, WA1203 and many others in order

327

to understand the fundamental requirements for producing tough cermets in selected systems. Some idea of the scope of their investigations and consequently of the possibilities for cermets generally can be obtained from the following examples: In oxide-metal systems, existing combinations include 60%W20%TiN-20%AI203 and 70%W30%A1203; whilst new possibifities are 80%A1203-20%Mo; 30% A1203-70%Mo; 30%A1203-50% TiN-20%Mo; 20%A1203-20% TiN-60%W; 40%Zr02-60%Mo. In nitride-metal cermets new possibilities include the ZrN-Mo, ZrN-W and TiN-Mo systems. The boride-metal cermets include the (Ti, Cr)B:Ni, (Ti, C r ) B : C u and(Ti, Cr) BE-Cu-Nisystems. Also there is a potential for development of the W2Bs-(78%Ni22%Fe), W2B:(80%Ni-20%Cr), MoBs-(78%Ni-22%Fe ) and MoB:(80%Ni-20%Cr) cermets which use the metal binder phase compositions of permanent magnets and electric-fire elements. In the B4C range there are the following possibilities: B4C-(90% Cu-9%Al); B4C-AI; B4C-(94% Cu-6%Zr); B4C-Cu-Si andB4CSi.

than is obtained with hard-facing alloys and some WC grades. By making various substitutions with Cr, V, Mn, Nb, Ta, Co and Fe into f~-Mn structure and binder phase, it is likely that a new range ofhardmetals could be developed for a variety of applications. Toughened Ceramics The use of ceramics such as AI203 and AI203-TiC combinations for machining has recently been extended to include the Si3N4, Si-A1-0-N and Zr02 systems. However in their un-reinforced form, the simple ceramics are either adequately hard but too brittle, or reasonably tough but too soft to compete with the hardmetals in the majority of applications. This weakness is also hindering the introduction of ceramics into high temperature structural applications. An example of how the consequence of these limitations can be seen in the requirements for resistance to severe abrasive wear, where the wear rate V

has the form V = h F H -t + 12FaKbH~ where H, K and F are respectively the hardness, fracture toughness and load; a,b and c are positive constants, and and fl are constants of the materials and the system. It is evident that unless the right balance of toughness and hardness is achieved, an inferior performance will be obtained. The mechanisms for property improvements are based on particle or fibre reinforcement. The introduction of Zr02 into AI203 to produce toughening by transformation and microcracking has been studied extensively but other systems are now attracting attention such as TiC particles in A120a, and in SiaN4 (Figs 6a-c). Fibre research has concentrated on SiC because of its hardness and stiffness which it retains at higher temperatures than is possible with other ceramics. It

Novel Approaches During the history of hardmetal development there have been several attempts to bypass the traditional routes for producing a hard constituent which will react favourably with the binder phase. The classic example is the work of Rudy who starts with a nitrided TiCMo master alloy to produce a single solid solution (Ti,Mo)(C,N) phase which upon cooling enters a spinodal two phase region. This separates as a Ti and N-rich phase surrounded by a Mo and C-rich phase (Fig 4d). The microstructure consists of relatively coarse carbo-nitride particles in a matrix hardened by a fine dispersoid. The hardmetal is made commercially and it is interesting to note that the same composition made by conventional routes has a completely different microstructure but similar properties. In a more recent development, Jack starts with a Ni-Mo mixture which is nitrided to give Ni2Mo3N (f$-Mn structure), then milled with Ni before sintering to give a dense compact with very high resistance to corrosion and oxidation. Because of dissolution of Ni during sintering, the binder phase content increases to greater than 10% but this still produces better wear resistance

328

a)

b)

c) Fig. 6

The toughness obtained by adding ZrO 2 particles to AI203 (a) and other ceramics can be explained by energy dissipative mechanisms involving microcracking and phase transformations in the process zones at the tips of cracks. In contrast, there is incomplete understanding of the beneficial effects of TiC particles on the toughness of AI203 (b), and on the high temperature strength ofSisN 4

(c).

MATERIALS & DESIGN Vol. 7 No. 6 NOVEMBER/DECEMBER 1986

a) Fig. 7

b) The toughness and erosion wear resistance of AI~O3 are enhanced by additions of 25 vol% SiC whiskers (a), whilst similar contents of SiC whiskers in Y203 stabilised tetragonal ZrOe (b) improve the toughness and high temperature strength.

is being tried mainly in SisN4, A1203, Zr02and Ti(C,N), and it has been used in at least one commercial cutting tool of hardness 2722 HVO. 2 with an A1203 matrix (Figs 7a and b).

Conclusions The proportion of WC hardmetals with alloy binder phases will increase in all applications when powders in a suitable form become available and when users become aware of their better all-round properties. Hard WC hardmetal grades will be partly replaced by titanium based hardmetals, boride containing cermets and reinforced ceramics for service at low and high temperatures in applications where high impact toughness is not important. The existing tough, but relatively soft, WC hardmetals will encounter increased competition in metal-forming

and wear applications from new cermets and hardmetals made by non-conventional processes. Cermets shoud be researched in a wider perspective embracing broad range of applications in addition to mining, metal shaping and wear resistance. Borides and reinforced ceramics deserve more attention in research for long term replacements for hardmetals.

Acknowledgements The author acknowledges helpful discussions with Professor K H Jack, Professor P S Kisley, Dr Hidekazu Do~ Dr B Roebuck and various members and associate members of the British Hardmetal Association's research groups. Fig 4c and 5 were supplied by Dr H Doi and Figs 6b and 6c by Dr V Sarin. Micrographs 7a and b are examples supplied by Professors J. Routbolt and N Claussen respectively.

MATERIALS & DESIGN Vol. 7 No. 6 NOVEMBER/DECEMBER 1986

Main Reference Sources Cemented Carbides. P Schwarzkopf and R Kieffer, The Macmillan Company, New York, 1970. Cermets: Progress in Science and Technology Series, P S Kisley, N I Bodnaruk, M S Borovikova et al, Ukranian A c a d e m y of Sciences, Naukova Dumka, Kiev. USSR, 1985. Refractory Hard Metals. P Schwaixkopf and R Kieffer. The Macmillan Company, New York., 1953. Science of Hard Materials. Proceedings of International Conference, Jackson, Wyoming, USA, 1981. Ed. R K Viswanadham, D J Rowcliffe and J Gudand Plenum Press, London, 1983. Science of Hard Materials~ Proceedings of 2nd International Conference, Rhodes, Greece, 1984. Ed. E A Almond, C A Brookes and R Warren, Institute of Physics CS75, Adam Hilger Press, Bristol, UK, 1986.

329