High temperature indentation of WC/Co hardmetals

Abstract

WC/Co is the material of choice for most high wear applications such as hot forming operations, drilling, cutting, machining and wire drawing due to its superior wear properties. In the evaluation of its material properties to withstand wear, particularly that of its hardness, most of the available information is based on room temperature investigations. However, when WC/Co is used in the actual applications, it also experiences very high contact temperatures and this localized heating has a detrimental effect on the WC/Co material causing microstructural changes on the surface and sub-micron surface that affect its wear resistance properties. Creep also plays an important role in the damage mechanisms at elevated temperatures especially in prolonged contact to heat such as those used in hot metal forming operations. However, the current information on these high temperature properties is not complete and has contradictory results. Moreover, creep studies of WC/Co have been done at temperatures >800°C and information below this temperature is also lacking.

Therefore, the primary goal of this thesis is to obtain a first time in-depth understanding of its high temperature property–composition–microstructure interaction that affects its wear properties from room temperature up to 800°C using comprehensive scratch tests, high temperature indentation hardness tests and indentation creep tests. These tests are supplemented with advanced 3D imaging, microstructural and composition analysis using SEM, EBSD, FIB, EDX, SIMS and laser confocal microscopy. Moreover, due to the lack of a commercially available high temperature microindentation test system, this thesis also aims to design, build and commission a high temperature and high vacuum microindentation test system to carry out these investigations.

The results of the scratch tests showed new information on the damage mechanisms, particularly on the formation of a tribo-layer on the scratched surfaces and the evolution of surface damages incurred on the indenter tip which is found to affect the coefficient of friction. These provide baseline information on the damage mechanisms occurring at room temperature and were instrumental in the development of the methodology used in the high temperature indentation tests.

The results of the high temperature indentation tests showed that the behaviour of the WC/Co hardmetals were categorised according to two temperature regimes. In the low temperature regime (20°C?500°C), the controlling factor is dominated by the WC grain size and that the damage mechanisms are characterised by plastic deformation of the WC grains via slip, intragranular fracture of the WC grains within the surface of the indent and intergranular fracture along the edges of the indent. In the high temperature regime (>500°C), the controlling factor was dominated by the Co binder and the damage mechanism showed first time observation of severe plastic flow accompanied by shape change on the WC grains. A tribo-layer was also found to exist on the room temperature indents but not on the high temperature indents. In addition, pitting of the indenter tip occurred at test temperatures >700°C and its contribution to the measured hardness were determined.

These results provide significant information on the best combination of WC grain size and Co content that are suitable for applications requiring high hardness retention and creep resistance. In addition, the thorough investigation of the damage mechanisms ensuing at these temperatures is beneficial in the design of WC/Co with better wear properties at elevated temperatures.

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Identifiers

ORCID for Robert J.K. Wood: orcid.org/0000-0003-0681-9239

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