Mat.-wiss. u. Werkstofftech. 2009, 40, No. 12
DOI: 10.1002/mawe.200900531
Processing and characterization of a cobalt based alloy for use in diamond cutting tools Fertigung und Charakterisierung von Kobaltbasis-Legierungen fu¨r Diamantschneidwerkzeuge H.C.P. de Oliveira, S.C. Cabral, R.S. Guimara˜es, G.S. Bobrovnitchii, M. Filgueira
Most of the alloys used as bonding matrix in diamond tools show a high content of cobalt - which is undesirable due to a series of reasons. In the last decade, some attempts were made towards the reduction of the Co content in these alloys. NEXT100J alloy (50wt%Cu-25wt%Fe-25wt%Co) by Eurotungstene is an example of it. This study aims to characterize the structure, microstructure and some mechanical properties of the NEXT100 - this is an comercial alloy widely used as bonding matrix for diamonds in cutting tools, and the informations about its properties are very scarce in the literature. The NEXT100 powder was hot pressed in a graphite matrix at 35MPa/800 C/3 minutes. It was performed structural and micro-
structural analyses by x-ray diffraction and scanning electon microscopy, respectivelly. Wear resistance and hardness HRB tests were carried out – these are the most important mechanical tests for a diamond bonding matrix. It was demonstrated that the samples presented good mechanical properties, XRD analysis showed the presence of two phases, Cu (CFC) and the solid solution a-Fe (CCC). Microstructural aspects are high densification, homogeneous distribution of phases, and little presence of pores. Keywords: Fe-Cu-Co alloys, sintering, structure, microstructure, mechanical properties. Schlu¨sselwo¨rter: Fe-Cu-Co-Legierungen, Sintern, Aufbau, Mikrostruktur, mechanische Eigenschaften
1 Introduction
2 Experimental
Nowadays, the majority of the diamond cutting tools employs Co as bonding metal matrix, as stated by Filgueira and Pinatti [1], and Barbosa et al [2]. Co combines good chemical compatibility with diamond at the processing temperatures, excellent diamond retention, associated with a satisfactory wear resistance after some cutting operations. Nevertheless, the trade price of Co is subject of large variations, and it is a strategic metal – just a few countries produces it – so cobalt is no longer the best choice in some diamond tools’ applications [3]. Beside this, Co is toxic in the point of view of its processing. Considering all these negative points, researchers have recently developed or proposed new alloys that have been extensively used as metal matrix in diamond tools, with the reduction of the Co content – exemplified by the Fe-Cu-Co alloys seen in refs.[4 – 7]. The selection of the bonding metal for working as the bonding matrix in the impregnated diamond composite also depends on the abrasiveness and hardness of the material to be cut [8]. In this context, Fe-Cu-Co alloys are used in diamond tools for cutting several types of stones. This paper is dedicated to the study of the structure, microstructure and mechanical properties of the alloy NEXT100 (50wt%Cu-25wt%Fe-25wt%Co) – this is a commercial alloy widely used as bonding matrix in diamond cutting tools, and its properties are poorly referenced in the literature. It is worth to say that Oliveira [9] and Filgueira [10] stated that the mechanical properties such as hardness and wear resistance are the main properties for the selection of a metal matrix for bonding diamonds in impregnated diamond cutting tools.
NEXT100 (50wt%Cu-25wt%Fe-25wt%Co) atomized (prealloyed) powder was acquired from Eurotungstene (France) mean particle size of 3 lm. Seven samples were processed by using hot pressing technique at 35MPa/800 C/3 minutes, where the powder was confined into a graphite mould, and it was processed Ø10 x 10 mm cylinders. Densification was measured by the conventional method of weight/volume, using a digital balance by Scaltec – 0.001 g resolution and a digital micrometer by Mitutoyo – 0.01 mm resolution. Samples were submited to microstructural analysis – using a scanning electron microscope (SEM) – by Shimadzu. Structural aspects were performed through x-ray diffraction (XRD) – Shimadzu, and by chemical analysis via x-ray dispersive energy spectroscopy (EDS) coupled to the SEM. Rockwell B hardness measurements were performed under a load of 63kgf - device by PANTEC RBS. Wear tests were performed using a special physical simulator by Contenco AB800E (details in ref.[11]) – each sample was vertically fixed on a granite disc (abrasion surface), which rotated at 20RPM, and samples were vertically loaded at 2kgf during 2 minutes. These conditions were defined by Oliveira [11] as optimal for the type and dimensions of the samples, considering the used equipment. All samples were weighted before and after wear tests for the weight loss (WL) calculations, according to eq.1 – where Mi and Mf are, respectively, the samples’ weights before and after testing for posterior calculation of the wear resistance (WR) – eq.2.
F 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Mi Mf x100 Mi WR ¼ 1 WL WL ¼
ðeq: 1Þ ðeq: 2Þ 907
3 Results and discussions Figures 1 and 2 show the microstructural aspect of the hot pressed NEXT100 samples. One can observe that the final stage of sintering was reached – homogeneously distributed spheroidal porosity. Other important aspect is the presence of two phases – one darker and other brighter – these were semi-quantitatively measured by EDS in fig.2 (points A and B). Point A presents 52.76 %wtFe and 47.24 %wtCu, that is almost 1:1. Point B presents 41 %wtFe, 33.5 %wtCu and 25.5 %wtCo. These results suggest the formation of Fe-Co solid solution and a Cu matrix, once Cu presents low solubility into Fe and Co. XRD patern in fig.3 makes clear the cited solid solutions – this pattern is very similar to that found by del Villar et al [5]. Figure 4 shows that the NEXT100 alloy lays in the Cu(fcc) + a-Fe(bcc) region. This diagram also informs that the Cu solubility is very limited at this temperature (850 C). According to ref.[4], it is about 2 %wt, so at 800 C (present work) it is even lower. It gives rise to a microstructure comprised of a FeCo solid solution (a-Fe(bcc) – Fe rich) embedded into a Cu matrix – continuous phase, that is, two phases – as seen in figs.1 and 2. very similar micrographs from Fe-Cu-Co systems were found in refs.[5,12 – 14].
Figure 3. XRD pattern of the NEXT alloy.
Figure 4. Fe-Cu-Co ternary phase diagram for the 850 C isothermal section. Extracted from ref.[5].
Figure 1. SEM micrograph of the NEXT alloy. Magnification 4,000X.
Figure 2. SEM micrograph of the NEXT alloy. Magnification 7,000X. Points A and B are regarded to EDS analysis. 908
H.C.P. de Oliveira et al.
Table 1 presents the results of relative density, hardness and wear resistance of the hot pressed samples, compared to results from other works on Fe-Cu-Co alloys for diamond cutting tools. It can be seen that density values from refs.[6] and [15] are quite superior than those from other works, just because these were measured by the water immersion method – Archimedes, thus neglecting the bulk porosity. The other works measured the densities by the conventional method hereby described, which consider the whole porosity. Regarding the hardness values, NEXT100 is inferior to the pressureless sintered Fe-60wt%Cu-wt%20Co alloy. It is due to the lower amount of Cu in the NEXT100 alloy (10wt%). This increment in Cu, when sintered at high temperature – 1150 C, produces an extensive liquid phase sintering, thus enabling an effective pore closure – higher density, leading to higher hardness. This Cu effect is also verified for samples hot pressed at 35MPa/800 C/3 minutes – see NEXT100 versus Cobalite HDR and DIABASE V07. This Cu liquid phase aids someway the Fe-Co solid solution formation, as stated by Barbosa et al [15] – which is responsible for the hardness improvement. Concerning the wear resistance, despite of the lower hardness of the NEXT100 alloy, it presented a higher wear performance than the pressureless sintered Fe-60wt%Cu-wt%20Co alloy. It is explained by the fact that the former presents a lower amount of Co, which is responsible for the wear resistance of the alloy, due to the Fe-Co solid solution. So, as higher the Co content, as better the wear resistance behavior. It is also observed for the alloy with 30wt%Co – it presented the best wear resistant result [15]. Mat.-wiss. u. Werkstofftech. 2009, 40, No. 12
Table 1. Results of densification, hardness and wear resistance of Fe-Cu-Co alloys. Material
Density (%)
Rockwell B hardness (HRB)
Wear resistance, WR (%)
Reference
NEXT100
76.64 0.77
118.9 0.2
2.096 0.055
This work
98
108
-
[6]
98.5
94 – 97
-
[14]
Fe-60Cu-20Co
78.9
125.8 0.7
1.70 0.01
[15]
Fe-15Cu-30Co4
-
-
2.5 0.2
[14]
Cobalite HDR1 DIABASE V07
2
3
1: pre-alloyed Fe-7 %wtCu-27 %wtCo, hot pressed at 35MPa/800 C/3 minutes; 2: pre-alloyed (45 – 65wt%)Fe-40wt%Cu-(20 – 40)wt%Co, hot pressed at 35MPa/800 C/3 minutes; 3 and 4: elemental blending of powders. Pressureless sintering at 1150 C/25 minutes/10-2mbar vacuum.
4 Conclusions The main conclusions of this exploratory work can be drawn as follows: The microstructure of the hot pressed samples has revealed that the final stage of sintering was achieved; DRX pattern along with EDS analises proved that the obtained microstructure refers to a Fe-Co solid solution embedded into a Cu matrix; Hardness and wear resistance are mechanical properties that are strongly affected by the cobalt content in the FeCu-Co alloys, due to the Fe-Co solid solution formation, which strengthens the alloy, as compared to the references’ data.
Acknowledgements The last author express his gratitude to faperj (jovem cientista de Nosso Estado – grant e-26/103.135/2008).
5 References 1. M. Filgueira and D.G. Pinatti, Production of diamond wire by Cu-15 %vol.Nb in situ Process. Proc. of the 15th Plansee Seminar, 2001, p.360 – 374. 2. A.P. Barbosa, L.J. Oliveira, G.S. Bobrovnitchii, R.S. Guimara˜es, A.S. Crespo, and M. Filgueira, Journal of Materials Science Forum, 2008, 591 – 593, 247. 3. M. Filgueira and D.G. Pinatti, Journal of Materials Science Forum, 2003, 416 – 418, 228. 4. B. Kamphuis and A. Serneels, Industrial Diamond Review, 2004, 1, 26.
Mat.-wiss. u. Werkstofftech. 2009, 40, No. 12
5. M. Del Villar, P. Muro, J. M. Sanchez, I. Iturriza and F. Castro, Powder Metallurgy, 2001, 44, 82. 6. I.E. Clark, Industrial Diamond Review 2002, 3, 177. 7. G. Weber and C. Weiss, Industrial Diamond Review, 2005, 2, 28. 8. K. Przyklenk, 1993, 4, 192. 9. L. J. de Oliveira, R. P. da R. Paranhos, R. da S. Guimara˜es, G. S. Bobrovnitchii and M. Filgueira, Powder metallurgy, 2007, 50, 148. 10. M. Filgueira, Production of “in situ” diamond wires. DSc thesis. PPGECM/UENF - Brazil. 2000. 154p. (in Portuguese). 11. L.J. Oliveira, Processing and characterization of the system FeCu-diamond for use in diamond wire beads. MSc dissertation. PPGECM/UENF - Brazil. 2005. 137p. (in Portuguese). 12. M. Palumbo, S. Curiotto and L. Battezzati, A Termodynamic analysis of the stable and metastable Co-Cu and Co-Cu-Fe phase diagrams. Calphad 30 (2006) 171 – 178. 13. S. Curiotto, R. Greco, N.H. Pryds, E. Johnson, L. Battezzati, The liquid metastable miscibility gap in Cu-based systems. Fluid phase Equilibria 256 (2007) 132 – 136. 14. A.P. Barbosa, Processing by powder metallurgy and characterization of Fe-Cu-Co alloys for use in diamond tools. MSc dissertation. PPGECM/UENF - Brazil. 2008. 113p. (in Portuguese). 15. A.P. Barbosa and M. Filgueira, Structure, micro-structure and mechanical properties of PM Fe-Cu-Co alloys. Proc. of the 17th Plansee Seminar, 2009, p.HM67/1 – 10. Corresponding author: Prof. Marcello Filgueira, Universidade Estadual do Norte Fluminense – UENF, Centro de Cieˆncias e Tecnologia – CCT, programa de Po´s Graduac¸a˜o em Engenharia e Cieˆncia dos Materiais – PPGECM, Grupo de Compo´sitos e Ferramentas de Materiais de Alta Dureza - GFer, Av. Alberto Lamego, 2000, 28013 – 620, Campos dos Goytacazes/RJ, Brazil, E-Mail:
[email protected]
Received in final form: September 8, 2009
Processing and characterization of a cobalt based alloy
[T 531]
909
