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A technique for fabrication of coated TiCN-based cermets with functionally graded structure
International Journal of Refractory Metals & Hard Materials 19 (2001) 523±526 www.elsevier.com/locate/ijrmhm
A technique for fabrication of coated TiCN-based cermets with functionally graded structure I. Konyashin
*
Max-Planck-Institut fur Metallforschung, Heisenbergstr. 5, D-70569 Stuttgart, Germany Received 7 March 2001; accepted 21 September 2001
Abstract A technique for fabrication of coated TiCN±Ni±Mo cermets with functionally graded microstructure and composition has been developed. A multilayer coating and the substrate near-surface zone with graded microstructure form as a result of interaction between the cermets and chromium vapour in vacuum. The coating consists of an upper layer of chromium carbide of about 10 lm in thickness and a thin interlayer of less than 1 lm composed of a Ni±Cr alloy between the carbide layer and the substrate. The wide near-surface zone of over 100 lm in thickness with graded microstructure and composition forming under the coatings has an increased Cr content in the Ni-based binder. This zone is characterised by enhanced corrosion- and oxidation-resistance. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Cermet; Coating; Functionally graded structure; Chromium carbide; Performance
1. Introduction There have been many attempts to improving the performance of TiC- or TiCN-based cermets by depositing wear-resistant coatings. Although various techniques of chemical vapour deposition (CVD) and physical vapour deposition (PVD) have been tried, the problem of development of a highly eective technology for coatings cermet cutting inserts is not yet completely solved. It has been found that the employment of conventional CVD techniques leads to a considerable decrease in the fracture toughness and transverse rupture strength (TRS) of cermets [1,2]. Although the use of PVD techniques does not result in decreasing TRS, they are by far less eective for cermets than for conventional WC±Co hardmetals [3]. Recently we developed and implemented a new method for deposition of wear-resistant coatings for cermet cutting inserts, which is based on the interaction between cermets and Cr vapour in vacuum [4]. Some features of such coatings were described in [5], and the *
Present address: Technical Development Centre, Boart GmbH, St adeweg 18-24, D-36151 Burghaun, Germany. Tel.: +49-6652-82412; fax: +49-6652-82490. E-mail address: [email protected] (I. Konyashin).
physicochemical fundamentals of the deposition technique were outlined in [6]. This paper presents some new results on the microstructure and properties of the TiCN-based cermets comprising a bi-layer coating and a functionally graded near-surface layer obtained by the technique developed.
2. Experimental details Commercially available TiCN±Ni±Mo cermets containing 19.5 wt% Ni were used as substrates. Special coarse-grained model TiCN±Ni±Mo samples containing 50 wt% Ni were made to examine the chromium distribution in the near-surface zone of the coated cermets. Deposition of chromium carbide-based layers accomplished by the formation of a substrate near-surface zone with functionally graded structure occurred due to interaction of chromium vapours with the cermets in vacuum. The deposition procedure is described in detail in [4,5]. Microstructure and elemental composition of the cermets graded near-surface zone were examined by optical and scanning electron microscopy, and electronprobe microanalysis (EPMA). The transverse ruptures strength (TRS) of the coated cermets were examined
0263-4368/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 4 3 6 8 ( 0 1 ) 0 0 0 5 8 - 0
524
I. Konyashin / International Journal of Refractory Metals & Hard Materials 19 (2001) 523±526
according to the ISO procedure and their fracture toughness was measured by the Palmquist method. Oxidation-resistance of coated vs. uncoated cermets was examined in air at temperatures of 800±1100 °C; their corrosion-resistance was evaluated by dissolution in diluted H2 SO4 . Tool lifetime of the coated cermets was examined in ®nish and semi-®nish turning of low-alloyed steels.
3. Results and discussion Fig. 1 shows a schematic diagram of the coating process. Chromium vapour interacts with the cermet substrate leading to formation of the hard chromium carbide-based coating of up to 10 lm in thickness and a ductile interlayer composed of a Cr±Ni alloy. In spite of the fact that the carbide layer forms only due to carbon diusion from the substrate, there is no local decarburised zone in the cermet substrate under the coating. The absence of the substrate local decarburisation under the coating is obtained because the coating growth rate is found to be higher than the carbon diusion rate in the cermet substrate [6]. Chromium diuses into the substrate forming the near-surface zone with graded structure and composition. This zone allows obtaining enhanced adhesion of the coatings as well as very high corrosion- and oxidation-resistance of the coated cermets, which will be demonstrated below. The mechanism of formation of the multilayer coating and the substrate near-surface zone with graded microstructure and composition has been established elsewhere [6]. The coating is thought to grow as a result of formation of an insigni®cant amount of a liquid
Fig. 1. Schematic diagram illustrating the formation of coated cermets with functionally graded near-surface zone.
phase on the substrate surface due to melting of a lowmelting-point eutectics in the Cr±Ni±C system. Carbon contained in the Ni±Mo binder of the substrate diuses through the ®lm of the liquid phase and interacts with Cr from the vapour phase resulting in the continuous growth of the chromium carbide coating. Cr atoms diuse from the Ni±Cr liquid phase through the Ni±Mo binder into the substrate resulting in formation of the graded microstructure of the substrate near-surface zone. The liquid phase of the eutectics composition consisting of mainly Ni and Cr, which is present between the carbide layer and the substrate, is thought to form a thin ductile metallic interlayer after its solidi®cation. Fig. 2 shows the microstructure of the coating and the substrate near-surface zone. One can see that the binder of the substrate adjacent to the coating is not etched with Murakhami's reagent. The concentration of Cr diusing into the substrate during deposition is quite high, which can be seen in Fig. 3. Special model samples with a high binder content and coarse-grained microstructure were prepared to examined the Cr distribution in the cermet substrate; the microstructure of such samples are shown in Fig. 4.
Fig. 2. Microstructure of the coated cermet with the graded nearsurface zone.
I. Konyashin / International Journal of Refractory Metals & Hard Materials 19 (2001) 523±526
525
Table 1 Composition of the substrate near-surface zone for the sample containing 50 wt% Ni under the coating
Fig. 3. Chromium distribution in the substrate near-surface zone under the coating at dierent distances from the substrate-coating interface.
Fig. 4. Microstructure of the model TiCN±Ni±Mo sample containing 50 wt% Ni.
Fig. 5 indicates that Cr diuses into a large volume of the model samples and its concentration reaches over 10 wt% in the substrate near-surface zone at the expense of Ti and Ni. Table 1 provides evidence that chromium is
Fig. 5. Chemical elements distribution in the near-surface zone of the coated model sample containing 50 wt% Ni at dierent distances from the substrate-coating interface.
Consistent part of the microstructure
Concentration of chemical elements, at.% Ti
Cr
Ni
Mo
TiCN (core) TiCN (periphery) Binder
99.0 78.3 5.2
0.1 0.3 24.4
0.6 1.1 61.7
0.3 20.3 8.7
dissolved practically only in the Ni-based binder forming a very corrosion- and heat-resistant Ni±Cr alloy. The performance of the cermets, is found, can be signi®cantly improved after coating. The results of the laboratory cutting tests of the coated cermet cutting inserts indicating more than a threefold increase in their tool lifetime are presented in [7]. Fig. 6 shows the results of ®eld tests of the coated cermet cutting inserts in machining of low-alloyed steels which are evidence for the signi®cant improvement in their performance compared to that of the uncoated inserts. The improvement is found to be achieved in turning and milling even under unfavourable cutting conditions. The coated cermet inserts having sharp cutting edges can be used in metal-cutting due to retaining the fracture toughness of the cermets coated by use of the new technology. The bilayer structure of the coating comprising the tough and ductile metallic interlayer is thought to prevent propagation of cracks formed in the carbide coating into the substrate during metal-cutting. The TRS and fracture toughness of the TiCN-based cermet are found almost not to be aected by the coating due to its special bi-layer structure and the presence of the vast Cr-alloyed substrate near-surface zone (Fig. 7).
Fig. 6. Results of ®eld tests on turning of low-alloyed steels by use of indexable cutting inserts made of the coated TiCN±Ni±Mo cermet with graded structure.
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I. Konyashin / International Journal of Refractory Metals & Hard Materials 19 (2001) 523±526
hanced almost without any in¯uence on the TRS and fracture toughness. Cr diusing into the substrate is present in the binder volume partially substituting Ni. The cermets obtained in this way are found to have a high corrosion- and oxidation-resistance.
Acknowledgements The author gratefully acknowledges ®nancial support for the present work from the European Commission, project ICA2-2001-10008 Carbon. Fig. 7. Relative values of properties of the coated TiCN±Ni±Mo cermet with graded structure compared to those of the substrate.
Fig. 7 shows that the coated cermets have a very high corrosion and high-temperature oxidation-resistance due to the presence of the chromium carbide coating and the large substrate near-surface zone with graded structure containing a large amount of chromium dissolved in the binder. Even, when the carbide coating is disturbed, the graded structure of the near-surface zone reliably protects the cermet in the unfavourable environment, like acids, and during high-temperature oxidation in air. 4. Conclusions The technology for coating TiCN-based cermets accomplished by the formation of the Cr-alloyed graded near-surface zone of the substrate allows the cermets performance in metal-cutting to be signi®cantly en-
References [1] Schintlmeiseter W, Pacher O, Pa®nger K, Reine T. Structure and strength eects in CVD titanium carbide and titanium nitride coatings. J Electrochem Soc 1976;123:924. [2] Schintlmeiseter W, Pacher O, Krall T, Wallgram W, Raine T. Manufacture of CVD-coated hardmetals. Powder Metall Int 1981;13:71. [3] Frantsevich I, Gaevskaya L, Beloborodov I, Sereda N, Sedlyar G. Eect of titanium nitride coating on the wear-resistance of the tungstenless hardmetal KTS-2M. Sov Powder Metall Met Ceram 1983;22:373. [4] Konyashin I. A vacuum technology for coating TiCN-based cermets. J Vac Sci Technol A 1995;13(3):1208. [5] Konyashin I. The formation of wear-resistant layers, including a stress-relaxing interlayer, during a chromium surface treatment of TiC or TiCN based cermets. Int J Ref Met Hard Mater 1997; 15:187. [6] Konyashin I. Interaction between the TiC(TiCN)±Ni±Mo hardmetals and chromium vapours. J Mater Sci 1995;30:5723. [7] Konyashin I. Wear-resistant coatings for cermet cutting tools. Surf Coat Technol 1995;71:291.
A technique for fabrication of coated TiCN-based cermets with functionally graded structure I. Konyashin
*
Max-Planck-Institut fur Metallforschung, Heisenbergstr. 5, D-70569 Stuttgart, Germany Received 7 March 2001; accepted 21 September 2001
Abstract A technique for fabrication of coated TiCN±Ni±Mo cermets with functionally graded microstructure and composition has been developed. A multilayer coating and the substrate near-surface zone with graded microstructure form as a result of interaction between the cermets and chromium vapour in vacuum. The coating consists of an upper layer of chromium carbide of about 10 lm in thickness and a thin interlayer of less than 1 lm composed of a Ni±Cr alloy between the carbide layer and the substrate. The wide near-surface zone of over 100 lm in thickness with graded microstructure and composition forming under the coatings has an increased Cr content in the Ni-based binder. This zone is characterised by enhanced corrosion- and oxidation-resistance. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Cermet; Coating; Functionally graded structure; Chromium carbide; Performance
1. Introduction There have been many attempts to improving the performance of TiC- or TiCN-based cermets by depositing wear-resistant coatings. Although various techniques of chemical vapour deposition (CVD) and physical vapour deposition (PVD) have been tried, the problem of development of a highly eective technology for coatings cermet cutting inserts is not yet completely solved. It has been found that the employment of conventional CVD techniques leads to a considerable decrease in the fracture toughness and transverse rupture strength (TRS) of cermets [1,2]. Although the use of PVD techniques does not result in decreasing TRS, they are by far less eective for cermets than for conventional WC±Co hardmetals [3]. Recently we developed and implemented a new method for deposition of wear-resistant coatings for cermet cutting inserts, which is based on the interaction between cermets and Cr vapour in vacuum [4]. Some features of such coatings were described in [5], and the *
Present address: Technical Development Centre, Boart GmbH, St adeweg 18-24, D-36151 Burghaun, Germany. Tel.: +49-6652-82412; fax: +49-6652-82490. E-mail address: [email protected] (I. Konyashin).
physicochemical fundamentals of the deposition technique were outlined in [6]. This paper presents some new results on the microstructure and properties of the TiCN-based cermets comprising a bi-layer coating and a functionally graded near-surface layer obtained by the technique developed.
2. Experimental details Commercially available TiCN±Ni±Mo cermets containing 19.5 wt% Ni were used as substrates. Special coarse-grained model TiCN±Ni±Mo samples containing 50 wt% Ni were made to examine the chromium distribution in the near-surface zone of the coated cermets. Deposition of chromium carbide-based layers accomplished by the formation of a substrate near-surface zone with functionally graded structure occurred due to interaction of chromium vapours with the cermets in vacuum. The deposition procedure is described in detail in [4,5]. Microstructure and elemental composition of the cermets graded near-surface zone were examined by optical and scanning electron microscopy, and electronprobe microanalysis (EPMA). The transverse ruptures strength (TRS) of the coated cermets were examined
0263-4368/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 4 3 6 8 ( 0 1 ) 0 0 0 5 8 - 0
524
I. Konyashin / International Journal of Refractory Metals & Hard Materials 19 (2001) 523±526
according to the ISO procedure and their fracture toughness was measured by the Palmquist method. Oxidation-resistance of coated vs. uncoated cermets was examined in air at temperatures of 800±1100 °C; their corrosion-resistance was evaluated by dissolution in diluted H2 SO4 . Tool lifetime of the coated cermets was examined in ®nish and semi-®nish turning of low-alloyed steels.
3. Results and discussion Fig. 1 shows a schematic diagram of the coating process. Chromium vapour interacts with the cermet substrate leading to formation of the hard chromium carbide-based coating of up to 10 lm in thickness and a ductile interlayer composed of a Cr±Ni alloy. In spite of the fact that the carbide layer forms only due to carbon diusion from the substrate, there is no local decarburised zone in the cermet substrate under the coating. The absence of the substrate local decarburisation under the coating is obtained because the coating growth rate is found to be higher than the carbon diusion rate in the cermet substrate [6]. Chromium diuses into the substrate forming the near-surface zone with graded structure and composition. This zone allows obtaining enhanced adhesion of the coatings as well as very high corrosion- and oxidation-resistance of the coated cermets, which will be demonstrated below. The mechanism of formation of the multilayer coating and the substrate near-surface zone with graded microstructure and composition has been established elsewhere [6]. The coating is thought to grow as a result of formation of an insigni®cant amount of a liquid
Fig. 1. Schematic diagram illustrating the formation of coated cermets with functionally graded near-surface zone.
phase on the substrate surface due to melting of a lowmelting-point eutectics in the Cr±Ni±C system. Carbon contained in the Ni±Mo binder of the substrate diuses through the ®lm of the liquid phase and interacts with Cr from the vapour phase resulting in the continuous growth of the chromium carbide coating. Cr atoms diuse from the Ni±Cr liquid phase through the Ni±Mo binder into the substrate resulting in formation of the graded microstructure of the substrate near-surface zone. The liquid phase of the eutectics composition consisting of mainly Ni and Cr, which is present between the carbide layer and the substrate, is thought to form a thin ductile metallic interlayer after its solidi®cation. Fig. 2 shows the microstructure of the coating and the substrate near-surface zone. One can see that the binder of the substrate adjacent to the coating is not etched with Murakhami's reagent. The concentration of Cr diusing into the substrate during deposition is quite high, which can be seen in Fig. 3. Special model samples with a high binder content and coarse-grained microstructure were prepared to examined the Cr distribution in the cermet substrate; the microstructure of such samples are shown in Fig. 4.
Fig. 2. Microstructure of the coated cermet with the graded nearsurface zone.
I. Konyashin / International Journal of Refractory Metals & Hard Materials 19 (2001) 523±526
525
Table 1 Composition of the substrate near-surface zone for the sample containing 50 wt% Ni under the coating
Fig. 3. Chromium distribution in the substrate near-surface zone under the coating at dierent distances from the substrate-coating interface.
Fig. 4. Microstructure of the model TiCN±Ni±Mo sample containing 50 wt% Ni.
Fig. 5 indicates that Cr diuses into a large volume of the model samples and its concentration reaches over 10 wt% in the substrate near-surface zone at the expense of Ti and Ni. Table 1 provides evidence that chromium is
Fig. 5. Chemical elements distribution in the near-surface zone of the coated model sample containing 50 wt% Ni at dierent distances from the substrate-coating interface.
Consistent part of the microstructure
Concentration of chemical elements, at.% Ti
Cr
Ni
Mo
TiCN (core) TiCN (periphery) Binder
99.0 78.3 5.2
0.1 0.3 24.4
0.6 1.1 61.7
0.3 20.3 8.7
dissolved practically only in the Ni-based binder forming a very corrosion- and heat-resistant Ni±Cr alloy. The performance of the cermets, is found, can be signi®cantly improved after coating. The results of the laboratory cutting tests of the coated cermet cutting inserts indicating more than a threefold increase in their tool lifetime are presented in [7]. Fig. 6 shows the results of ®eld tests of the coated cermet cutting inserts in machining of low-alloyed steels which are evidence for the signi®cant improvement in their performance compared to that of the uncoated inserts. The improvement is found to be achieved in turning and milling even under unfavourable cutting conditions. The coated cermet inserts having sharp cutting edges can be used in metal-cutting due to retaining the fracture toughness of the cermets coated by use of the new technology. The bilayer structure of the coating comprising the tough and ductile metallic interlayer is thought to prevent propagation of cracks formed in the carbide coating into the substrate during metal-cutting. The TRS and fracture toughness of the TiCN-based cermet are found almost not to be aected by the coating due to its special bi-layer structure and the presence of the vast Cr-alloyed substrate near-surface zone (Fig. 7).
Fig. 6. Results of ®eld tests on turning of low-alloyed steels by use of indexable cutting inserts made of the coated TiCN±Ni±Mo cermet with graded structure.
526
I. Konyashin / International Journal of Refractory Metals & Hard Materials 19 (2001) 523±526
hanced almost without any in¯uence on the TRS and fracture toughness. Cr diusing into the substrate is present in the binder volume partially substituting Ni. The cermets obtained in this way are found to have a high corrosion- and oxidation-resistance.
Acknowledgements The author gratefully acknowledges ®nancial support for the present work from the European Commission, project ICA2-2001-10008 Carbon. Fig. 7. Relative values of properties of the coated TiCN±Ni±Mo cermet with graded structure compared to those of the substrate.
Fig. 7 shows that the coated cermets have a very high corrosion and high-temperature oxidation-resistance due to the presence of the chromium carbide coating and the large substrate near-surface zone with graded structure containing a large amount of chromium dissolved in the binder. Even, when the carbide coating is disturbed, the graded structure of the near-surface zone reliably protects the cermet in the unfavourable environment, like acids, and during high-temperature oxidation in air. 4. Conclusions The technology for coating TiCN-based cermets accomplished by the formation of the Cr-alloyed graded near-surface zone of the substrate allows the cermets performance in metal-cutting to be signi®cantly en-
References [1] Schintlmeiseter W, Pacher O, Pa®nger K, Reine T. Structure and strength eects in CVD titanium carbide and titanium nitride coatings. J Electrochem Soc 1976;123:924. [2] Schintlmeiseter W, Pacher O, Krall T, Wallgram W, Raine T. Manufacture of CVD-coated hardmetals. Powder Metall Int 1981;13:71. [3] Frantsevich I, Gaevskaya L, Beloborodov I, Sereda N, Sedlyar G. Eect of titanium nitride coating on the wear-resistance of the tungstenless hardmetal KTS-2M. Sov Powder Metall Met Ceram 1983;22:373. [4] Konyashin I. A vacuum technology for coating TiCN-based cermets. J Vac Sci Technol A 1995;13(3):1208. [5] Konyashin I. The formation of wear-resistant layers, including a stress-relaxing interlayer, during a chromium surface treatment of TiC or TiCN based cermets. Int J Ref Met Hard Mater 1997; 15:187. [6] Konyashin I. Interaction between the TiC(TiCN)±Ni±Mo hardmetals and chromium vapours. J Mater Sci 1995;30:5723. [7] Konyashin I. Wear-resistant coatings for cermet cutting tools. Surf Coat Technol 1995;71:291.