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Fine powders and CBN coatings to the fore at plansee
Fine p o w d e r s and CBN coatings to the fore at Plansee or b o t h refractory metals a n d h a r d materials, the four-yearly Plansee Semin a r is t h e 'can't miss' conference for b o t h manufacturers a n d researchers. Held in t h e Walter Schwarzkopf Hall j u s t outside t h e Metallwerk Plansee p l a n t at Reutte, in t h e Austrian Tyrol, t h e 14th International Plansee Semin a r offered ideal opportunities to discover a n d discuss t h e latest developments in cutting tools a n d wear resistant materials. Though the papers pres e n t e d covered a wide r a n g e of interests, of p a r t i c u l a r i n t e r e s t to MPR readers were t h e sessions devoted to ultrafine tungsten carbide a n d to coatings of s u p e r h a r d cubic b o r o n nitride (CBN). C o n s u l t a n t editor E u r Ing Kenneth J ~ . Brookes provides a sample of w h a t was o n offer.
F
Arnold's keynote A keynote p a p e r on 'the challenges of d i f f i c u l t - t o - m a c h i n e materials', c o n t r i b u t e d by Dave Arnold, vice p r e s i d e n t of Kennametal, characterized m a c h i n i n g difficulty in t h r e e ways -- in general terms, scientifically a n d as seen by t h e m a c h i n e tool operator. Difficult m a t e r i a l s include aerospace alloys, titanium, high silicon a l u m i n i u m alloys a n d certain types of cast irons. M o s t s u c h m a c h i n i n g is nowadays carried o u t by coated WC-base inserts, t h e coatings ranging from p u r e d i a m o n d to multilayer physical v a p o u r dep o s i t i o n ( P V D ) / c h e m i c a l vapour deposition (CVD) c a r b i d e / n i t r i d e . On A1-Si alloys, d i a m o n d coated carbide is already economically competitive w i t h solid polycrystalline diamond, with t h e a d d e d advantage of following c o m p l e x chip breaking geometries. Coated d i a m o n d drills have similar adv a n t a g e s . O t h e r c u t t i n g tool m a t e r i a l s include sialons, CBN a n d whisker-reinforced a l u m i n a
ceramics. In automotive m a n u f a c t u r ing, grinding is being substit u t e d by h a r d machining. Dry m a c h i n i n g is yet a n o t h e r signific a n t trend, particularly in Germany, w h e r e t h e d i s p o s a l of cutting fluids and grinding sludge h a s been s h o w n to cost as m u c h as, or even more t h a n , t h e cutting tool or insert. T h e f o l l o w i n g p a p e r , by Klocke a n d Gerschwiler of RWrH Aachen, Germany, discussed t h e m a c h i n i n g of aerospace materials -- n o t a b l y t i t a n i u m a n d superalloys - in greater detail, coming to similar conclusions. Where t h e r m a l loading was high a n d h e a t was mostly removed t h r o u g h t h e tool r a t h e r t h a n in t h e chips, SiC proved a b e t t e r b i n d e r for polycrystalline diam o n d (PCD) t h a n WC/Co. Cobalt promoted graphitization of d i a m o n d a t t e m p e r a t u r e s above IO00°C.
Fine a n d s u p e r f i n e powders In a d d i t i o n to a n u m b e r of p o s t e r contributions, t h e Semin a r featured a dedicated works h o p session entitled 'Ultrafinegrained and Nanocrystalline C e m e n t e d Carbides'. The keynote p a p e r comprised a n e x a m i n a t i o n of t h e hardnesst o - t o u g h n e s s r e l a t i o n s h i p in fine-grained h a r d m e t a l s by W.D.
Schubert a n d colleagues of Vienn a University. 65 grades were investigated a n d compared on t h e basis of Vickers i n d e n t a t i o n h a r d n e s s a n d Palmqvist indentation toughness (crack length). Variables considered by t h e a u t h o r s included initial carbide grain size, b i n d e r content, sintering temperature and both a m o u n t a n d c o m p o s i t i o n of grain-growth inhibitor. P e r h a p s surprisingly, they did n o t consider h o w a n d w h e n t h e inhibit o r was added, t h o u g h distinct differences are k n o w n to be found when, for example, additions are m a d e as a salt to a precursor solution, as oxide before carburizing, a n d as carbide after carburizing. In general, h i g h e r h a r d n e s s w e n t w i t h lower f r a c t u r e t o u g h ness. Ultrafine g r a d e s were n o t necessarily t o u g h e r t h a n coarser ( c o n v e n t i o n a l s u b m i c r o n ) varieties of c o m p a r a b l e h a r d ness, p a r t i c u l a r l y in t h e r a n g e 1450-2000 HV30. As was k n o w n empirically in t h e 1950s, sampies d o p e d with CraC2 showed excellent hardness/toughness c o m b i n a t i o n s up to 2000 HV30, while a p p r e c i a b l e a m o u n t s of VC give excellent t h o u g h somew h a t erratic h a r d n e s s b u t t e n d t o ~ x d s e m b r i t t l e m e n t . In alm o s t all cases, sintering conditions were found to be critical in optimizing properties.
14 j INTERNATIONAL PLANSEESEMINAR "97
MPR October 1997 11
t2:
>A08
24
A06
High Low None
With decreasing grain size, t h e s a m e h a r d n e s s could be attained with substantially greater b i n d e r c o n t e n t but, surprisingly, f r a c t u r e t o u g h n e s s stayed m u c h t h e s a m e a t similar hardness. H.C. S t a r c k a n d Dow Chemicals contributed interesting, but nevertheless 'promotional' p a p e r s , f e a t u r i n g t h e i r respective u l t r a f i n e powders. S t a r c k c u r r e n t l y m a n u f a c t u r e s a single 0.4 I~m grade of WC in t h i s category, while Dow's new process c a n a c h i e v e c o n s i s t e n t grain sizes a r o u n d 0.2 as well as 0.3-0.4 lam. A c c o r d i n g to S t a r c k ' s research, h a r d m e t a l s m a d e from t h e new 'DS40' p o w d e r w i t h 10 wt% cobalt could have hardness up to 2000 HV30, and hardness of 1870 HV30 could be combined with Palmqvist toughness of 810 N/mm 2 or 10.5 MPa.m 1/2. U n l i k e s o m e other investigations in this critical area, Starck a n d cooperator Fraunhofer IK'TS looked at grain size distribution (using a high resolution scanning electron microscope) as well as average grain size, t h e n at the effect of different d o p a n t s on the distribution. Not unexpectedly, VC had the greatest effect and TaC t h e
Scanning electron micrograph (10 O00X) of Dow's superfine WC powder.
12 MPR October 1997
540 580 6t0
2003 2120 2150
least, with Cr3C2 intermediate between the two. This applied equally to commercial powder of 0.6 lain aw~rage grain size. Optimal properties were achieved with c o m b i n a t i o n s of additives, as shown -- t h o u g h n o t m e n t i o n e d in t h e p a p e r -w i t h t h e early (1940s) submicron (though not ultrafine) Teco grade 'CC', b a s e d on a Krupp Widia developed combin a t i o n of VC a n d TaC. Sintering is a crucial processing o p e r a t i o n for tfltrafine WC/Co h a r d m e t a l s , e x p l a i n e d a n d analysed in great detail by t h e authors. Suggested a p p l i c a t i o n s include microdrills, e n d m i l l s , w o o d c u t t i n g tools, drawing dies a n d wear parts. By contrast, D.F. Carroll's p a p e r for Dow Chemical, pres e n t e d by Cynthia Conner, highlighted the problems of sintering :high quality WC-base h a r d m e t a l s w i t h n o m i n a l 0.2 lam g r a i n size. This c o n t r i b u t i o n showed t h a t e n h a n c e d reactivity could cause u p to 90% of densification to take place below t h e 1280°C sintering t e m p e r a t u r e . In t h e absence of a p p r o p r i a t e grain growth i n h i b i t i n g additives, e x a g g e r a t e d grain growth t o o k place a t t h e same time. Both t h e e n h a n c e d densification and low temperature grain growth were "quite different" from t h a t observed in coarser WC/Co powders. Again, VC a n d CraC2 were found to be t h e m o s t effective :additives, w h i c h t h e author compared with other "typically u s e d " inhibitors such as TaC, TiC, NbC, HfC a n d ZrC, though, with t h e e x c e p t i o n of TaC, t h e s e are more likely to be found in p a t e n t literature t h a n in commereial practice. The n o m e n c l a t u r e for very fine WC powders is controversial, a n d was t h e s u b j e c t of considerable discussion. Dow's in-house classification is t h a t 0.2-0.3 lain WC is superfine, 0.40.5 I~m is ultrafine, a n d 0.8 lam (typical 'submicron') merely 'fine'. By c o m p a r i s o n , Israeli
r e s e a r c h e r s distinguish between 'micron' (1 to 20 lim), 'submicron' (0.5 to 1 p m ) , 'ultrafine' (0.1 to 0.5 lain), a n d ' n a n o ' ( u p to 0.1 ~m or 100 n m ) . Table 1 gives t h e m a j o r p r o p e r t i e s of Dow's powders within its own defined categories. Though employed by some investigators, the expressions 'nanofme' and 'nanocrystalline', w h e n used for n o m i n a l grain sizes above 0.1 ~m, were generally disliked as giving t h e wrong i m p r e s s i o n of u l t i m a t e particle size. Milling t i m e of grade powders h a d a s u b s t a n t i a l effect on sintered properties, as shown in Table 2 for a superfine m i x with 6% cobalt b i n d e r a n d 0.6% VC grain-growth inhibitor. Investigations showed t h a t sintering could be carried o u t over a range of sintering temperatures, w i t h i n w h i c h 1380°C gave n e a r o p t i m a l properties, a n d t h a t a c o m b i n a t i o n of VC a n d Cr3C2 c o m b i n e d g r a i n growth inhibition with a gain in corrosion resistance. Reuben Porat a n d colleagues at the Technion in Haffa, Israel, h a d also compared the mechanical p r o p e r t i e s of h a r d m e t a l s made from WC with a wide range of grain sizes. Reduction in the initial grain size below 0.5 I~n resulted in a p r o n o u n c e d improvement in the h a r d n e s s and fracture toughness, better consolidation in tile solid state with lower activation energy and higher final density. 'Nanocrystalline' powders were said to exhibit ten times t h e grain growth in the solid state a n d twice in the liquid phase when compared with 'micron-size' powders, though this might well be related to wider grain size d i s t r i b u t i o n in t h e spray conversion process used to manufacture the finest starting materials. In fact, of all t h e presenters, only Technion's m e n t i o n e d t h e critical i m p o r t a n c e of grain size distribution. This is very difficult to m e a s u r e a t t h e finest sizes, with t r a n s m i s s i o n electron microscopy offering t h e only route a t p r e s e n t . A l t h o u g h t h i s is possible with s t u d e n t l a b o u r in universities, it is p r o b a b l y n o t feasible as a r o u t i n e p r o c e d u r e in industry. An interesting p a p e r from Widia GmbH described t h e concept of mic~)wave sintering for ultraline h a r d m e t a l compacts. B e c a u s e m u c h of t h e g r a i n growth takes, place in t h e early
stages of sintering, t h e striking s h o r t e n i n g of s i n t e r i n g t i m e s h e l p s to m a i n t a i n t h e initial grain size, even with m i n i m a l a d d i t i o n s of inhibitors. A sugg e s t e d d e v e l o p m e n t was t h e reaction sintering of a m i x t u r e of metallic tungsten, c a r b o n a n d cobalt. In discussion, Hugo O r t n e r asked how, since m e t a l s reflect microwaves, it was possible to apply t h e necessary energy i n p u t in t h e l a t e r stages of microwave sintering. In reply, it was exp l a i n e d t h a t p e n e t r a t i o n was greater at h i g h e r t e m p e r a t u r e s . It followed, however, t h a t it was n o t possible to r e s i n t e r from cold, e.g. for straightening. In r e s p o n s e to o t h e r questions, a u t h o r s s t a t e d t h a t ultrafine o r n a n o s i z e d m a t e r i a l s were n o t p y r o p h o r i c u n d e r norreal conditions, n o r w a s w a x difficult to remove. Typical p r e s s u r e for die p r e s s i n g was 8 g/cm 3 and shrinkage during s i n t e r i n g a b o u t 21%. T. N a k a m u r a a n d his colleagues a t M i t s u b i s h i Materials, J a p a n , i n t r o d u c e d a submicrongrained titanium carbonitride cermet. Additional c o n s t i t u e n t s were tungsten, m o l y b d e n u m a n d t a n t a l u m carbides, as well as 10% cobalt-nickel binder. TiCN-based hardmetals have long been recognized as useful tool materials, especially for cutting steels, with high wear resistance and imparting good surface finish. What they lacked was toughness. According to Nakamura, the submicron variant comb i n e s g r e a t l y i m p r o v e d Kic toughness with enhanced wear resistance, applicable to such cutring tool applications as milling, t u r n i n g , b o r i n g a n d grooving. Apart from hardness and fracture toughness, property d a t a have largely conventional values.
Superfine multUayer coatings An interesting p a p e r from a t e a m r e p r e s e n t i n g Sandvik, Balzers a n d Uppsala University, demonstrated a new t e c h n i q u e for PVD coating. In essence, the s u b s t r a t e holder is rotated above a m u c h smaller electrode in a n evacua t e d d e p o s i t i o n c h a m b e r . As each insert or tool passes above t h e electrode, a very t h i n layer is deposited. Depending on coating deposition rate a n d speed of rotation, individual lamellar coatings can be held to a nanometric or superlattice scale, ty-
p i c a l l y 30 a n d 150 n m for alternating Ti a n d TiN, for 10 a n d 60 seconds deposition time respectively. Hardness was substantially enhanced, a n d good results were o b t a i n e d in milling s t a i n l e s s steels with lamellar TiN/NbNcoated carbide. Turning operations with similar materials h a d not yet been tried.
WE HAVE A COMMON TARGET
Prospects for CBN coating For many years it was t h o u g h t i m p o s s i b l e to synthesize diam o n d except u n d e r extremes of h e a t a n d pressure. Once it h a d been accomplished a t n o r m a l a n d reduced pressures, however, half-a-dozen assorted m e t h o d s were soon developed. All t h a t was necessary was localized energy concentration. Several pap e r s d e a l t w i t h r e c e n t developments in synthesized diam o n d coatings, as well as amorp h o u s diamond-like coatings. The biggest d r a w b a c k for d i a m o n d as a cutting tool material is t h a t it c a n n o t be used to cut steels or o t h e r ferrous materials. Indeed, t h i s is recognized by a special category in t h e ISO a p p l i c a t i o n codes for c u t t i n g tool materials. Cubic b o r o n nitride is a n alternative s u p e r h a r d m a t e r i a l which can cut iron-based workpieces, b u t is currently only available as a solid synthesized polycrystalline m a t e r i a l (PCBN). In an international collaborative project, largely sponsored by the Austrian Science Foundation, Benno Lux and colleagues from TU Wien looked a t d i a m o n d growth with boron additions, at pathways to CBN from melts at atmospheric pressure (where LiaBN2 might be a solvent/catalyst) a n d a t t h e d e p o s i t i o n of B-C-N coatings from trisdimethylaminoboran by hot filament and microwave p l a s m a activation. T h o u g h results so far h o l d promise for t h e long term, curr e n t research is a long way from commercial CBN coatings. Only very t h i n coatings are possible, a n d are largely h e x a g o n a l b o r o n nitride a n d carbon. Once t h e processes are b e t t e r u n d e r s t o o d and controlled, the ultimate p r o d u c t may be a multilayered c o a t i n g of p u r e c u b i c b o r o n n i t r i d e on a s u b s t r a t e of ultrafine, h e a t r e s i s t a n t alloy carbide or carbonitride. K.J.A. Brookes •
Quality coupledwith efficiencyis everyone'starget. Pometonand its Customershave achievedthis by following sophisticatedstatisticalcontrolsand investingin the Imst technology.Thesecombinedefforts guaranteesuccess. Both your successand ours. A wide range of iron, copper (electrolyticand ntomised), bronze (fully alloyed and pre-mixed), brass, tin, ~ lead powdersis producedunderthe above concept.
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M P R O c t o b e r 1997 1 3
F
Arnold's keynote A keynote p a p e r on 'the challenges of d i f f i c u l t - t o - m a c h i n e materials', c o n t r i b u t e d by Dave Arnold, vice p r e s i d e n t of Kennametal, characterized m a c h i n i n g difficulty in t h r e e ways -- in general terms, scientifically a n d as seen by t h e m a c h i n e tool operator. Difficult m a t e r i a l s include aerospace alloys, titanium, high silicon a l u m i n i u m alloys a n d certain types of cast irons. M o s t s u c h m a c h i n i n g is nowadays carried o u t by coated WC-base inserts, t h e coatings ranging from p u r e d i a m o n d to multilayer physical v a p o u r dep o s i t i o n ( P V D ) / c h e m i c a l vapour deposition (CVD) c a r b i d e / n i t r i d e . On A1-Si alloys, d i a m o n d coated carbide is already economically competitive w i t h solid polycrystalline diamond, with t h e a d d e d advantage of following c o m p l e x chip breaking geometries. Coated d i a m o n d drills have similar adv a n t a g e s . O t h e r c u t t i n g tool m a t e r i a l s include sialons, CBN a n d whisker-reinforced a l u m i n a
ceramics. In automotive m a n u f a c t u r ing, grinding is being substit u t e d by h a r d machining. Dry m a c h i n i n g is yet a n o t h e r signific a n t trend, particularly in Germany, w h e r e t h e d i s p o s a l of cutting fluids and grinding sludge h a s been s h o w n to cost as m u c h as, or even more t h a n , t h e cutting tool or insert. T h e f o l l o w i n g p a p e r , by Klocke a n d Gerschwiler of RWrH Aachen, Germany, discussed t h e m a c h i n i n g of aerospace materials -- n o t a b l y t i t a n i u m a n d superalloys - in greater detail, coming to similar conclusions. Where t h e r m a l loading was high a n d h e a t was mostly removed t h r o u g h t h e tool r a t h e r t h a n in t h e chips, SiC proved a b e t t e r b i n d e r for polycrystalline diam o n d (PCD) t h a n WC/Co. Cobalt promoted graphitization of d i a m o n d a t t e m p e r a t u r e s above IO00°C.
Fine a n d s u p e r f i n e powders In a d d i t i o n to a n u m b e r of p o s t e r contributions, t h e Semin a r featured a dedicated works h o p session entitled 'Ultrafinegrained and Nanocrystalline C e m e n t e d Carbides'. The keynote p a p e r comprised a n e x a m i n a t i o n of t h e hardnesst o - t o u g h n e s s r e l a t i o n s h i p in fine-grained h a r d m e t a l s by W.D.
Schubert a n d colleagues of Vienn a University. 65 grades were investigated a n d compared on t h e basis of Vickers i n d e n t a t i o n h a r d n e s s a n d Palmqvist indentation toughness (crack length). Variables considered by t h e a u t h o r s included initial carbide grain size, b i n d e r content, sintering temperature and both a m o u n t a n d c o m p o s i t i o n of grain-growth inhibitor. P e r h a p s surprisingly, they did n o t consider h o w a n d w h e n t h e inhibit o r was added, t h o u g h distinct differences are k n o w n to be found when, for example, additions are m a d e as a salt to a precursor solution, as oxide before carburizing, a n d as carbide after carburizing. In general, h i g h e r h a r d n e s s w e n t w i t h lower f r a c t u r e t o u g h ness. Ultrafine g r a d e s were n o t necessarily t o u g h e r t h a n coarser ( c o n v e n t i o n a l s u b m i c r o n ) varieties of c o m p a r a b l e h a r d ness, p a r t i c u l a r l y in t h e r a n g e 1450-2000 HV30. As was k n o w n empirically in t h e 1950s, sampies d o p e d with CraC2 showed excellent hardness/toughness c o m b i n a t i o n s up to 2000 HV30, while a p p r e c i a b l e a m o u n t s of VC give excellent t h o u g h somew h a t erratic h a r d n e s s b u t t e n d t o ~ x d s e m b r i t t l e m e n t . In alm o s t all cases, sintering conditions were found to be critical in optimizing properties.
14 j INTERNATIONAL PLANSEESEMINAR "97
MPR October 1997 11
t2:
>A08
24
A06
High Low None
With decreasing grain size, t h e s a m e h a r d n e s s could be attained with substantially greater b i n d e r c o n t e n t but, surprisingly, f r a c t u r e t o u g h n e s s stayed m u c h t h e s a m e a t similar hardness. H.C. S t a r c k a n d Dow Chemicals contributed interesting, but nevertheless 'promotional' p a p e r s , f e a t u r i n g t h e i r respective u l t r a f i n e powders. S t a r c k c u r r e n t l y m a n u f a c t u r e s a single 0.4 I~m grade of WC in t h i s category, while Dow's new process c a n a c h i e v e c o n s i s t e n t grain sizes a r o u n d 0.2 as well as 0.3-0.4 lam. A c c o r d i n g to S t a r c k ' s research, h a r d m e t a l s m a d e from t h e new 'DS40' p o w d e r w i t h 10 wt% cobalt could have hardness up to 2000 HV30, and hardness of 1870 HV30 could be combined with Palmqvist toughness of 810 N/mm 2 or 10.5 MPa.m 1/2. U n l i k e s o m e other investigations in this critical area, Starck a n d cooperator Fraunhofer IK'TS looked at grain size distribution (using a high resolution scanning electron microscope) as well as average grain size, t h e n at the effect of different d o p a n t s on the distribution. Not unexpectedly, VC had the greatest effect and TaC t h e
Scanning electron micrograph (10 O00X) of Dow's superfine WC powder.
12 MPR October 1997
540 580 6t0
2003 2120 2150
least, with Cr3C2 intermediate between the two. This applied equally to commercial powder of 0.6 lain aw~rage grain size. Optimal properties were achieved with c o m b i n a t i o n s of additives, as shown -- t h o u g h n o t m e n t i o n e d in t h e p a p e r -w i t h t h e early (1940s) submicron (though not ultrafine) Teco grade 'CC', b a s e d on a Krupp Widia developed combin a t i o n of VC a n d TaC. Sintering is a crucial processing o p e r a t i o n for tfltrafine WC/Co h a r d m e t a l s , e x p l a i n e d a n d analysed in great detail by t h e authors. Suggested a p p l i c a t i o n s include microdrills, e n d m i l l s , w o o d c u t t i n g tools, drawing dies a n d wear parts. By contrast, D.F. Carroll's p a p e r for Dow Chemical, pres e n t e d by Cynthia Conner, highlighted the problems of sintering :high quality WC-base h a r d m e t a l s w i t h n o m i n a l 0.2 lam g r a i n size. This c o n t r i b u t i o n showed t h a t e n h a n c e d reactivity could cause u p to 90% of densification to take place below t h e 1280°C sintering t e m p e r a t u r e . In t h e absence of a p p r o p r i a t e grain growth i n h i b i t i n g additives, e x a g g e r a t e d grain growth t o o k place a t t h e same time. Both t h e e n h a n c e d densification and low temperature grain growth were "quite different" from t h a t observed in coarser WC/Co powders. Again, VC a n d CraC2 were found to be t h e m o s t effective :additives, w h i c h t h e author compared with other "typically u s e d " inhibitors such as TaC, TiC, NbC, HfC a n d ZrC, though, with t h e e x c e p t i o n of TaC, t h e s e are more likely to be found in p a t e n t literature t h a n in commereial practice. The n o m e n c l a t u r e for very fine WC powders is controversial, a n d was t h e s u b j e c t of considerable discussion. Dow's in-house classification is t h a t 0.2-0.3 lain WC is superfine, 0.40.5 I~m is ultrafine, a n d 0.8 lam (typical 'submicron') merely 'fine'. By c o m p a r i s o n , Israeli
r e s e a r c h e r s distinguish between 'micron' (1 to 20 lim), 'submicron' (0.5 to 1 p m ) , 'ultrafine' (0.1 to 0.5 lain), a n d ' n a n o ' ( u p to 0.1 ~m or 100 n m ) . Table 1 gives t h e m a j o r p r o p e r t i e s of Dow's powders within its own defined categories. Though employed by some investigators, the expressions 'nanofme' and 'nanocrystalline', w h e n used for n o m i n a l grain sizes above 0.1 ~m, were generally disliked as giving t h e wrong i m p r e s s i o n of u l t i m a t e particle size. Milling t i m e of grade powders h a d a s u b s t a n t i a l effect on sintered properties, as shown in Table 2 for a superfine m i x with 6% cobalt b i n d e r a n d 0.6% VC grain-growth inhibitor. Investigations showed t h a t sintering could be carried o u t over a range of sintering temperatures, w i t h i n w h i c h 1380°C gave n e a r o p t i m a l properties, a n d t h a t a c o m b i n a t i o n of VC a n d Cr3C2 c o m b i n e d g r a i n growth inhibition with a gain in corrosion resistance. Reuben Porat a n d colleagues at the Technion in Haffa, Israel, h a d also compared the mechanical p r o p e r t i e s of h a r d m e t a l s made from WC with a wide range of grain sizes. Reduction in the initial grain size below 0.5 I~n resulted in a p r o n o u n c e d improvement in the h a r d n e s s and fracture toughness, better consolidation in tile solid state with lower activation energy and higher final density. 'Nanocrystalline' powders were said to exhibit ten times t h e grain growth in the solid state a n d twice in the liquid phase when compared with 'micron-size' powders, though this might well be related to wider grain size d i s t r i b u t i o n in t h e spray conversion process used to manufacture the finest starting materials. In fact, of all t h e presenters, only Technion's m e n t i o n e d t h e critical i m p o r t a n c e of grain size distribution. This is very difficult to m e a s u r e a t t h e finest sizes, with t r a n s m i s s i o n electron microscopy offering t h e only route a t p r e s e n t . A l t h o u g h t h i s is possible with s t u d e n t l a b o u r in universities, it is p r o b a b l y n o t feasible as a r o u t i n e p r o c e d u r e in industry. An interesting p a p e r from Widia GmbH described t h e concept of mic~)wave sintering for ultraline h a r d m e t a l compacts. B e c a u s e m u c h of t h e g r a i n growth takes, place in t h e early
stages of sintering, t h e striking s h o r t e n i n g of s i n t e r i n g t i m e s h e l p s to m a i n t a i n t h e initial grain size, even with m i n i m a l a d d i t i o n s of inhibitors. A sugg e s t e d d e v e l o p m e n t was t h e reaction sintering of a m i x t u r e of metallic tungsten, c a r b o n a n d cobalt. In discussion, Hugo O r t n e r asked how, since m e t a l s reflect microwaves, it was possible to apply t h e necessary energy i n p u t in t h e l a t e r stages of microwave sintering. In reply, it was exp l a i n e d t h a t p e n e t r a t i o n was greater at h i g h e r t e m p e r a t u r e s . It followed, however, t h a t it was n o t possible to r e s i n t e r from cold, e.g. for straightening. In r e s p o n s e to o t h e r questions, a u t h o r s s t a t e d t h a t ultrafine o r n a n o s i z e d m a t e r i a l s were n o t p y r o p h o r i c u n d e r norreal conditions, n o r w a s w a x difficult to remove. Typical p r e s s u r e for die p r e s s i n g was 8 g/cm 3 and shrinkage during s i n t e r i n g a b o u t 21%. T. N a k a m u r a a n d his colleagues a t M i t s u b i s h i Materials, J a p a n , i n t r o d u c e d a submicrongrained titanium carbonitride cermet. Additional c o n s t i t u e n t s were tungsten, m o l y b d e n u m a n d t a n t a l u m carbides, as well as 10% cobalt-nickel binder. TiCN-based hardmetals have long been recognized as useful tool materials, especially for cutting steels, with high wear resistance and imparting good surface finish. What they lacked was toughness. According to Nakamura, the submicron variant comb i n e s g r e a t l y i m p r o v e d Kic toughness with enhanced wear resistance, applicable to such cutring tool applications as milling, t u r n i n g , b o r i n g a n d grooving. Apart from hardness and fracture toughness, property d a t a have largely conventional values.
Superfine multUayer coatings An interesting p a p e r from a t e a m r e p r e s e n t i n g Sandvik, Balzers a n d Uppsala University, demonstrated a new t e c h n i q u e for PVD coating. In essence, the s u b s t r a t e holder is rotated above a m u c h smaller electrode in a n evacua t e d d e p o s i t i o n c h a m b e r . As each insert or tool passes above t h e electrode, a very t h i n layer is deposited. Depending on coating deposition rate a n d speed of rotation, individual lamellar coatings can be held to a nanometric or superlattice scale, ty-
p i c a l l y 30 a n d 150 n m for alternating Ti a n d TiN, for 10 a n d 60 seconds deposition time respectively. Hardness was substantially enhanced, a n d good results were o b t a i n e d in milling s t a i n l e s s steels with lamellar TiN/NbNcoated carbide. Turning operations with similar materials h a d not yet been tried.
WE HAVE A COMMON TARGET
Prospects for CBN coating For many years it was t h o u g h t i m p o s s i b l e to synthesize diam o n d except u n d e r extremes of h e a t a n d pressure. Once it h a d been accomplished a t n o r m a l a n d reduced pressures, however, half-a-dozen assorted m e t h o d s were soon developed. All t h a t was necessary was localized energy concentration. Several pap e r s d e a l t w i t h r e c e n t developments in synthesized diam o n d coatings, as well as amorp h o u s diamond-like coatings. The biggest d r a w b a c k for d i a m o n d as a cutting tool material is t h a t it c a n n o t be used to cut steels or o t h e r ferrous materials. Indeed, t h i s is recognized by a special category in t h e ISO a p p l i c a t i o n codes for c u t t i n g tool materials. Cubic b o r o n nitride is a n alternative s u p e r h a r d m a t e r i a l which can cut iron-based workpieces, b u t is currently only available as a solid synthesized polycrystalline m a t e r i a l (PCBN). In an international collaborative project, largely sponsored by the Austrian Science Foundation, Benno Lux and colleagues from TU Wien looked a t d i a m o n d growth with boron additions, at pathways to CBN from melts at atmospheric pressure (where LiaBN2 might be a solvent/catalyst) a n d a t t h e d e p o s i t i o n of B-C-N coatings from trisdimethylaminoboran by hot filament and microwave p l a s m a activation. T h o u g h results so far h o l d promise for t h e long term, curr e n t research is a long way from commercial CBN coatings. Only very t h i n coatings are possible, a n d are largely h e x a g o n a l b o r o n nitride a n d carbon. Once t h e processes are b e t t e r u n d e r s t o o d and controlled, the ultimate p r o d u c t may be a multilayered c o a t i n g of p u r e c u b i c b o r o n n i t r i d e on a s u b s t r a t e of ultrafine, h e a t r e s i s t a n t alloy carbide or carbonitride. K.J.A. Brookes •
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