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Thermal residual stresses, hardness and microstructural characterisation of functionally graded WC-Co composites
Linköping University, Department of Mechanical Engineering, Engineering Materials. Linköping University, The Institute of Technology.
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: urn:nbn:se:liu:diva-30048Local ID: 15508ISBN: 91-7373-026-2 (print)OAI: oai:DiVA.org:liu-30048DiVA: diva2:250869
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-11-28
List of papers
1. Aspects of residual stress generation in functionally graded WC-Co composites: a literature survey
Open this publication in new window or tab >>Aspects of residual stress generation in functionally graded WC-Co composites: a literature survey
(English)Manuscript (preprint) (Other academic)
Abstract [en]

This literature survey provides an introduction to functionally graded materials, WC-Co composites and estimations of residual stresses in such material systems.

Functionally graded materials feature gradual transitions in microstructure and/or composition. Gradual transitions are preferred over abrupt transitions which introduce local stress concentrations. The various manufacturing methods for functionally graded materials are divided into two classes. Constructive processes (e.g., powder processes) use an appropriate distribution of constituents, often as a precursor to the component, to create the gradient. Transport based processes (e.g., infiltration) rely on natural transport phenomena such as fluid flow, diffusion of atomic species or conduction of heat.

Liquid phase sintering of mixed and pressed powder is used to produce WC-Co functionally graded composites. The mechanical properties of WC-Cocomposites and their constituent phases are reviewed. A wide range of overall composite behavior can be achieved when the Co content is varied. For example, the hardness is reported to be 1650 HV for WC with 6 wt.% Co and 780 HV for WC with 25 wt.% Co. The corresponding fracture toughness values are 8.5 and 14.5 MPa m1/2, respectively. These values suggest, that WC contributes to the hardness of the composite and Co contributes to the toughness. Therefore, the possibility of tailoring material performance by varying composition in the component arises.

During cooling from the sintering temperature, thermal residual stresses develop because of the differences between the WC and Co thermal expansion coefficients. It is possible to use X-ray diffraction methods to measure residual stresses in composites. Results from measurements, and finite element (FE) analysis of the residual stresses in WC-Co composites are also reviewed.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-102050 (URN)
Available from: 2013-11-28 Created: 2013-11-28 Last updated: 2013-11-28
2. Thermal residues stresses and microstructural characterisation of functionally graded WC-Co composites
Open this publication in new window or tab >>Thermal residues stresses and microstructural characterisation of functionally graded WC-Co composites
(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:nbn:se:liu:diva-102051 (URN)
Available from: 2013-11-28 Created: 2013-11-28 Last updated: 2013-11-28
3. Hardness profile measurements in functionally graded WC–Co composites
Open this publication in new window or tab >>Hardness profile measurements in functionally graded WC–Co composites
2004 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 382, no 1-2, 141-149 p.Article in journal (Refereed) Published
Abstract [en]

Micro- and nanoindentation were used to determine hardness as a function of depth in two functionally graded WC–Co composites. The gradients were continuous (extended over ∼70 and ∼40 μm, respectively) and consisted of varying WC and Co phase volume fractions. Five comparable homogeneous samples with different Co contents and different average WC grain sizes were also used for direct comparison. A relationship between hardness and Co content was established for both the graded and the homogeneous samples wherein the hardness decreased with increasing Co content. The magnitude of the hardness was the same (for a given Co content) for the functionally graded and the homogeneous materials. The hardness measurements were also correlated with X-ray diffraction studies of thermal residual stresses and the absence of any major influence explained. It is generally concluded that the hardness values are dominated by the local Co content. Additionally, the examination of surface cracks around indents suggests that compositional gradients in WC–Co composites offer increased toughness.

Keyword
hardness, functionally graded composites, WC-Co, nanoindentation, microindentation, residual stress
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-46174 (URN)10.1016/j.msea.2004.04.065 (DOI)
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2017-12-13

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