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- Persson, Daniel H. E., 1974-
(author)
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On the Mechanisms behind the Tribological Performance of Stellites
- 2005
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Doctoral thesis (other academic/artistic)abstract
- This thesis reveals the tribological mechanisms behind the intrinsic low friction potential of the Co-based family of alloys called Stellites. Although being an established and important group of materials, a satisfactory explanation to why they exhibit low-friction properties under severe sliding conditions has not previously been found in the literature. The main part of this thesis is dedicated to the clarification of the tribological performance of Stellites in highly loaded sliding contact. The results should assist the development of Co-free alternatives, suitable for replacing Stellites in nuclear applications. Owing to their beneficial properties they are today the most commonly used material in the sealing surfaces on gate valves in the primary circuits of boiling water reactors (BWR). The underlying reason for the replacement in the nuclear applications is an undesired contribution to the background radiation level, originating from the Co in the Stellite surfaces. The Stellites mainly consist of Cr-rich carbides in a solid solution dominated by Co. The commonly used Stellite 6 and Stellite 21 were chosen as primary test materials and applied by laser cladding, providing a metallically bonded clad layer with a fine dendritic microstructure. By combining information from a series of dedicated tribological tests and modern high-resolution analysis instruments (e.g. SEM, XRD and TEM) available at the Ångström Laboratory at Uppsala University, the following conclusions can be made regarding the tribological performance of Stellites under high load sliding. Mechanisms. The (tested) Stellites form a thick deformation hardened layer, topped with a superficial easily sheared layer of hcp basal planes aligned parallel to the worn surface. The easy-shear layer is continually regenerated, replacing worn off material. Technical benefits. The Stellites offer low-friction properties thanks to their easily sheared surface layers. The risk of severe galling is also avoided by restricting shear and adhesive transfer to very thin superficial layers. In closed sliding contacts, self-generated protective layers formed by re-deposition of wear fragments are also offered.
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- Persson, Daniel H. E., et al.
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Surface analysis of laser cladded Stellite exposed to self-mated high load dry sliding
- 2006
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In: Wear. - : Elsevier BV. - 0043-1648 .- 1873-2577. ; 261:1, s. 96-100
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Journal article (peer-reviewed)abstract
- Materials from the Stellite family of Co-based alloys are commonly used as low friction, galling resistant materials in high load dry sliding contact applications.In the present investigation, the surface region of a Co-based material (Stellite 21) exposed to self-mated high load dry sliding at room temperature has been analysed in detail.During sliding, an approximately 30 nm thick Co-enriched tribofilm is created. It exhibits low friction properties and a high galling resistance. The transformation from an face-centred-cubic structure to easily sheared hexagonal-closed-packed basal planes in the tribofilm combined with the high load carrying capacity of the underlying deformation hardened zone is suggested to explain the excellent low friction properties and galling resistance of this material.
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