Ermüdung von Hartmetallen in flüssigen Medien
Final Report Abstract
A study has been conducted on the tribological properties and corrosion behaviour of WC-Co hardmetals by carrying out a series of wear tests from the nanoscale up to the macroscopic scale and by carrying out electrochemical tests using different electrolytes from the alkali to the acid regime. The composition of the hardmetals was varied and the binder content ranged from 6 to 15 wt%. The binder in all the samples was cobalt and one of the samples had small additions of Cr3C2 and VC. The WC grain size in the samples ranged from 250 nm (nano-size) up to 2.65 µm (coarse-grained). A binderless WC sample and a pure cobalt sample were also included in this work and the mechanical properties of all the samples were measured using nanoindentation. The wear tests on the nanoscale consisted of scratch testing using a Nanoindenter XP. Macroscopic wear was investigated by conducting three body abrasive and sliding wear tests. The binderless WC sample was found to have the best performance at loads below 1 N. In this load range a smaller grain size led to an increase in the scratch resistance for samples with 6 wt% binder. On the other hand for the samples with 15 wt% binder, a smaller grain size resulted in a decrease in the scratch resistance. In the 1 to 10 N load range the binderless WC underwent brittle wear which led to very high scratch depths. Whereas for the samples containing 15 wt% Co a smaller grain size resulted in an increase in the scratch resistance. The finer grained hardmetals exhibited lower abrasive wear rates than their coarse grained counterparts. The main wear mechanisms were found to be plastic deformation via glide activity, microcracking, binder extrusion and grain fall out. The most important findings are related to the nano-scale scratch tests: For scratch tests in the load range 5 to 500 mN a smaller grain size was found to lead to an increase in the scratch resistance for samples with a low binder content (6 wt%). On the other hand for the samples with a high binder content (15 wt%), a smaller grain size resulted in a decrease in the scratch resistance with higher scratch depths being measured for UFG15. In the load range 1 to 10 N no grain size dependence was observed for the samples containing 6 wt% cobalt binder. However, in the samples containing 15 wt% binder a smaller grain size resulted in an increase in the scratch resistance. A lower binder content resulted in an increase in the scratch resistance. The main wear mechanisms, are plastic deformation via glide activity, microcracking, binder extrusion and grain fall out. The corrosion behaviour of the WC-Co based hardmetals was studied with electrochemical techniques and surface analysis. The corrosion behaviour of hardmetals is characterized by the behaviour of its components WC and Co, but complex interactions between the phases are observed. These interactions depend on the grain size. The influence of different grain sizes on the corrosion behaviour of hardmetals is most prominently seen in alkaline solutions, where Co is passivated. A clear tendency was observed that the smaller the grain size, the higher the corrosion resistance. On contrary, in neutral and acidic solutions the corrosion process results mainly from a selective binder dissolution, in which the hard phase is cathodically protected. Hence, the corrosion behaviour shows no clear dependence on the grain size in these electrolytes. Laterally resolved Auger Electron Spectroscopy and magnetic measurements revealed that significant amounts of W and C diffuse into the Co binder matrix during the sintering process, and that the W and C concentration in the Co phase increases with decreasing grain size. The distribution of W and C in the binder phase is not homogeneous, instead a seam with a higher concentration is observed near the interface to WC. The dissolved W and C in Co stabilize the fcc crystal structure of the Co binder, which is otherwise not thermodynamically stable at the room temperature. X-ray diffraction experiments demonstrate that the small-grained hardmetals showed a higher fraction of fcc Co in the structure. Thus, the binder phase consists of two regions, in which hcp Co is existent in the center. A two phase region of hcp with a higher fraction of fcc Co is present at the outer border of the binder phase. The higher corrosion resistance of the small-grained hardmetals may therefore be ascribed to the higher amount of fcc Co, which has a better corrosion behaviour than hcp Co due to the higher thermodynamic stability. As VC and Cr3C2 additions were used for grain-refining, the effects of these graingrowth inhibitors on the corrosion behaviour was studied. VC addition is deleterious for the corrosion reistance, whereas Cr3C2 shows a positive effect. TOF-SIMS and STEM analyses verify that VC is precipitated mostly along the WC/binder interface after the liquid-phase sintering process. VC is chemically instable in alkaline solutions and therefore it is dissolving completely out of the binder; hence interfaces between the phases are initiation sites for corrosion. In Cr-alloyed hardmetals, most of the chromium is dissolved homogeneously in the binder leading to passivation of the binder by a stable and highly protective Cr2O3 passive film. This work was able to provide some insight into the wear mechanisms that take place in WC-Co hardmetals at different length scales: on the nanoscale, microscale and on the macroscopic scale. The nanoscratch tests allowed a closer look at the wear mechanisms in individual grains and more especially in the binder phase. The wear mechanisms are identical at the nanoscale and at the macro-scale under three-body abrasion conditions. The project was also able to identify critical internal factors (composition and microstructure) as well as external factors (composition and temperature of the electrolyte, potential) influencing the corrosion resistance of WC-Co based hardmetals. The role of the different factors in mechanisms of corrosion was elucidated. To understand the chemical interactions between the components of the composite material, investigations with techniques probing the materials behaviour with high lateral resolution are required.
Publications
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Investigation of the wear properties of WC-Co hardmetals using nanoscratch testing. Wear 263 (2007), 1602 – 1609
S. Ndlovu, K. Durst, M. Göken
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Deformation of WC-Co hardmetals during scratch testing. International Symposium on Friction, Wear and Wear Protection 2008, A. Fischer, K. Bobzin (eds.), Aachen, Germany, pp.171-177
S. N. Ndlovu, K. Durst, M. Göken
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Influence of grain size and binder content on the sliding wear properties of WC-Co hardmetals. 9th Int Conf. on Tribology 2008, Pretoria, South Africa
S. Ndlovu, K. Durst, H.W. Höppel, M. Göken
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Study on the local damage mechanisms in WC-Co hardmetals during scratch testing. Materials Research Society Symposium Proceedings 1049, 2008, pp. 75-81
S. Ndlovu, K. Durst, H.W. Höppel, M. Göken
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Effect of WC grain size on the corrosion behaviour of WC-Co based hardmetals in alkaline solutions. International Journal of Refractory Metals and Hard Materials, 27 (2009) 806-812
F. J.J. Kellner, H. Hildebrand and S. Virtanen
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The WC grain size dependency on the corrosion behaviour of WC-Co hardmetals in strong alkaline solutions. EuroCorr 09, Nizza, 2009. CD-Proceedings
F.J.J. Kellner, H. Hildebrand and S. Virtanen
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The wear properties of tungsten carbide-cobalt hardmetals from the nanoscale up to the macroscopic scale. Dissertation, Universität Erlangen-Nürnberg, 2009
S. Ndlovu
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Influence of Ca ions and temperature on the corrosion behaviour of WC-Co hardmetals in alkaline solutions. EuroCorr 2010, Moscow, 2010. CD-Proceedings
F.J.J. Kellner, S. Virtanen
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Influence of Ca ions and temperature on the corrosion behaviour of WC-Co hardmetals in alkaline solutions. International Journal of Refractory Metals and Hard Materials, 28, (2010) 370-376
F. J.J. Kellner, R. Lynch and S. Virtanen
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Influence of chromium and vanadium graingrowth inhibitors on the corrosion behaviour of WC-Co hardmetals. EuroCorr 2010, Moscow, 2010. CD-Proceedings
F.J.J. Kellner, E. Spiecker, S. Virtanen
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Korrosionsverhalten und –mechanismen von Hartmetallen mit unterschiedlicher mikrostruktureller Längenskala. Dissertation, Universität Erlangen-Nürnberg, 2010
F.J.J. Kellner
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Localized corrosion of ultrafine-grained Al-Mg model alloys. Electrochimica Acta 55 (2010) 1966-1970
J.G. Brunner, J. May, H.W. Höppel, M. Göken, S. Virtanen
