Studies on the Formation of Intermetallic Compound Layers …
671
As aforementioned, the tungsten content of the binder material in the cemented
carbides strongly depends on its structure, the carbon balance, sintering process, etc.
Therefore, a further purpose of this study was to investigate the influence of the
W content in Co on the Co–Zn phase formation and draw conclusions concerning
the recyclability of certain cemented carbide grades. Extensive SEM/EDS analyses
revealed that the transition zone consists of different and overlapping Co–Zn phases
resulting in a layered structure in which tungsten is practically nonexistent. However,
tungsten accumulates outside of the Co–Zn layer in the area of liquid zinc. The ability
of Zn to form alloys with Co and not with W allowed us to conclude that if Zn atoms
reach the interface, they will react with Co atoms to a Co–Zn compound and the
W atoms stay unaffected. As a result of the volume expansion and the progressive
Co–Zn formation due to the mutual diffusion processes of Co and Zn, the W atoms
are pushed together and transported outwards of the adherent layer. This process
leads to the development of a W seam on the interface between the Co–Zn layer and
the surrounding Zn as well as to tungsten accumulations in outer areas.
These locally higher W concentrations can constitute certain challenges regarding
the recyclability of cemented carbides throughout the zinc process. If cemented carbides with a higher W content in the binder are treated by this method, these remaining
W accumulations will stay in the material. As a consequence, these enrichments can
lead to a blocking of pores between the WC grains, which impedes the distillation of
Zn in the second stage of the process, especially for cemented carbides with low Co
contents. Moreover, the remaining W enrichments may result in a higher stability of
the generated product and cause problems in crushing and milling. Since the W and
C content in the binder of cemented carbides rises with decreasing grain size and
are enriched in a narrow seam in the binder in the vicinity of the WC/Co interface
[13], the occurrence of W accumulations and challenges in recyclability are more
likely to arise for cemented carbides with lower grain size. Future studies should
provide further insight concerning the composition of W enrichments since tungsten
can form various compounds with cobalt like Co 3 W or Co 7 W 6 . If Co(W) came into
contact with liquid zinc, it is assumed that Co partly reacts with Zn resulting in an
enrichment of W until the exceeding of the solubility of tungsten in cobalt Co. Subsequently, the formation of Co–W phases takes place according to Co 3 W → Co 7 W 6
→ W with decreasing Co supply. Follow up investigations aim to clarify on which
side of the reaction 3Zn + Co 3 W ↔ 3CoZn + W the system is located, whereas
Co 3 W and CoZn represent arbitrary Co–Zn and Co–W phases, respectively. This
study will involve thermodynamical approaches as well as XRD measurements for
phase identification. Furthermore, the disintegration behavior of cemented carbides
with higher W content in the binder throughout the zinc process needs to be focused
since the current study indicates certain challenges regarding the recyclability due
to the occurrence of W enrichments.
Acknowledgements The financial support by the Austrian Federal Ministry for Digital and Economic Affairs and the National Foundation for Research, Technology and Development is gratefully
acknowledged.
671
As aforementioned, the tungsten content of the binder material in the cemented
carbides strongly depends on its structure, the carbon balance, sintering process, etc.
Therefore, a further purpose of this study was to investigate the influence of the
W content in Co on the Co–Zn phase formation and draw conclusions concerning
the recyclability of certain cemented carbide grades. Extensive SEM/EDS analyses
revealed that the transition zone consists of different and overlapping Co–Zn phases
resulting in a layered structure in which tungsten is practically nonexistent. However,
tungsten accumulates outside of the Co–Zn layer in the area of liquid zinc. The ability
of Zn to form alloys with Co and not with W allowed us to conclude that if Zn atoms
reach the interface, they will react with Co atoms to a Co–Zn compound and the
W atoms stay unaffected. As a result of the volume expansion and the progressive
Co–Zn formation due to the mutual diffusion processes of Co and Zn, the W atoms
are pushed together and transported outwards of the adherent layer. This process
leads to the development of a W seam on the interface between the Co–Zn layer and
the surrounding Zn as well as to tungsten accumulations in outer areas.
These locally higher W concentrations can constitute certain challenges regarding
the recyclability of cemented carbides throughout the zinc process. If cemented carbides with a higher W content in the binder are treated by this method, these remaining
W accumulations will stay in the material. As a consequence, these enrichments can
lead to a blocking of pores between the WC grains, which impedes the distillation of
Zn in the second stage of the process, especially for cemented carbides with low Co
contents. Moreover, the remaining W enrichments may result in a higher stability of
the generated product and cause problems in crushing and milling. Since the W and
C content in the binder of cemented carbides rises with decreasing grain size and
are enriched in a narrow seam in the binder in the vicinity of the WC/Co interface
[13], the occurrence of W accumulations and challenges in recyclability are more
likely to arise for cemented carbides with lower grain size. Future studies should
provide further insight concerning the composition of W enrichments since tungsten
can form various compounds with cobalt like Co 3 W or Co 7 W 6 . If Co(W) came into
contact with liquid zinc, it is assumed that Co partly reacts with Zn resulting in an
enrichment of W until the exceeding of the solubility of tungsten in cobalt Co. Subsequently, the formation of Co–W phases takes place according to Co 3 W → Co 7 W 6
→ W with decreasing Co supply. Follow up investigations aim to clarify on which
side of the reaction 3Zn + Co 3 W ↔ 3CoZn + W the system is located, whereas
Co 3 W and CoZn represent arbitrary Co–Zn and Co–W phases, respectively. This
study will involve thermodynamical approaches as well as XRD measurements for
phase identification. Furthermore, the disintegration behavior of cemented carbides
with higher W content in the binder throughout the zinc process needs to be focused
since the current study indicates certain challenges regarding the recyclability due
to the occurrence of W enrichments.
Acknowledgements The financial support by the Austrian Federal Ministry for Digital and Economic Affairs and the National Foundation for Research, Technology and Development is gratefully
acknowledged.