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Thermal residues stresses and microstructural characterisation of functionally graded WC-Co composites
Linköping University, Department of Mechanical Engineering, Engineering Materials. Linköping University, The Institute of Technology.
Linköping University, Department of Mechanical Engineering, Engineering Materials. Linköping University, The Institute of Technology.ORCID iD: 0000-0002-2286-5588
(English)Manuscript (preprint) (Other academic)
Abstract [en]

X-ray diffraction was used to determine the thermal residual stresses that develop in a functionally graded WC-Co composite. The gradient was continuous within a distance of about 40 Jlm below the surface and consisted of varying WC and Co phase volume fractions. The composite was characterised by scanning electron microscopy, optical microscopy and X-ray diffraction. Thermal residual stresses develop due to the thermal mismatch between the WC and the Co phase during cooling from the liquid-phase sintering temperature (1450°C). Pole figures were obtained in order to determine optimal sample orientations that provided adequate intensity for measurements in the Co phase. Stresses were measured in both WC and Co phases at various depths. For WC, the in-plane compressive residual stresses varied approximately between -300 MPa to -500 MPa with depth below the surface. For the low volume fraction Co phase, the tensile residual stresses were approximately 600 MPa. The changes in the magnitude of the residual stresses can be related to the changes in the Co phase content through the graded zone. Microstresses in the graded zone are attributed to the thermal mismatch between WC and the Co phase. The compressive macrostresses were determined to be a result of the compositional gradient. This conclusion is strongly supported by the result that almost no macrostresses were measured in a similar homogenous sample i.e., without the corresponding compositional gradient. Therefore, varying the composition gradient during fabrication is expected to directly affect the macrostresses.

National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-102051OAI: oai:DiVA.org:liu-102051DiVA: diva2:668023
Available from: 2013-11-28 Created: 2013-11-28 Last updated: 2013-11-28
In thesis
1. Thermal residual stresses, hardness and microstructural characterisation of functionally graded WC-Co composites
Open this publication in new window or tab >>Thermal residual stresses, hardness and microstructural characterisation of functionally graded WC-Co composites
2001 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Functionally graded materials (FGMs) incorporate gradual transitions in their microstructure and/or composition with distance. The gradients provide means to favourably engineer their thermomechanical properties by changing e.g., the local modulus, yield strength, state of residual stress etc. However, prior to optimising such gradients, an understanding of their effect on thermomechanical properties is necessary. WC-Co composites are commonly used as tool bits, where the presence of Co improves the toughness of the wear resistant WC. In this thesis, emphasis has been placed on investigating the thermal residual stresses, hardness, and toughness of functionally graded WC-Co.

The first paper in this thesis reviews existing literature on the subject while introducing FGMs and WC-Co composites. Additional topics addressed include: fabrication of FGMs, mechanical properties of WC-Co and estimates of residual stresses.

In the second paper, X-ray diffraction was used to determine thermal residual stresses that develop in a functionally graded WC-Co composite. The gradient was continuous during a distance of about 40 Jlm below the surface and the stresses were measured in both WC and Co phases at various depths. Due to the difference in coefficient of thermal expansion between the two phases, tensile residual stresses where found in the Co phase and balancing compressive stresses in WC. Pole figures were obtained in order to determine optimal sample orientations that provided adequate intensity for measurements in the Co phase. Microstresses in the graded zone were attributed to the thermal mismatch between WC and the Co phase. The compressive macrostresses were determined to be a result of the compositional gradient.

The third paper reports micro- and nanoindentation experiments to determine hardness as a function of depth in two different WC-Co FGMs. A relationship between hardness and Co phase content was established for the two graded and five homogeneous samples wherein the hardness decreases with increasing Co phase content. For a given Co phase content, no significant differences were observed between the FGMs and homogeneous samples. The measured thermal residual residual stresses did not appear to influence the hardness. Additionally, examination of indents using a Vickers indenter with different loads suggested that larger loads were required to initiate cracks in the FGMs compared to the homogeneous materials. While the investigated WC-Co FGMs did not offer advantages in hardness properties (which seem to be dominated by the local Co content in the graded zone), they appear to offer tribological advantages through increased toughness in terms of crack suppression.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet, 2001. 12 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 884
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-30048 (URN)15508 (Local ID)91-7373-026-2 (ISBN)15508 (Archive number)15508 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-11-28

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Larsson, CeciliaOdén, Magnus

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