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The relationship between flake graphite orientation, smearing effect, and closing tendency under abrasive wear conditions
Department of Mechanical Engineering, Materials and Manufacturing - Casting, School of Engineering, Jönköping University, Sweden.ORCID iD: 0000-0003-2698-5445
Department of Mechanical Engineering, Materials and Manufacturing - Casting, School of Engineering, Jönköping University, Sweden.
2014 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 317, no 1–2, p. 153-162Article in journal (Refereed) Published
Abstract [en]

Abstract Plastic deformation of the matrix during the wear process results in closing the graphite flakes. In this study, the relationship between the deformation of the matrix and the closing tendency of flake graphite was investigated, both qualitatively and quantitatively. Two representative piston rings, which belonged to the same two-stroke marine engine but were operated for different periods of time, were studied. Initial microstructural observations indicated a uniform distribution of graphite flakes on unworn surfaces, whereas worn surfaces demonstrated a tendency towards a preferred orientation. Approximately 40% of the open flakes of the unworn surfaces were closed during sliding, which may result in the deterioration of the self-lubricating capability of cast iron. Moreover, flakes within the orientation range of 0 to 30° relative to the sliding direction showed a maximum closing tendency when subjected to sliding. The closing tendency gradually decreased as the angle increased, approaching a minimum between 30 and 70°. A slight increase in the closing tendency was observed for flakes with orientations between 70 and 90°. A similar trend was observed on both rings. Furthermore, SEM and EDS analysis indicated substantial deformation of the matrix in the area around the flakes. An insignificant corrosion attack was observed on both worn piston ring surfaces.

Place, publisher, year, edition, pages
Elsevier, 2014. Vol. 317, no 1–2, p. 153-162
Keywords [en]
Grey cast iron, Piston ring-cylinder liner, Flake graphite orientation, Sliding wear, Abrasive wear, Graphite closing tendency
National Category
Metallurgy and Metallic Materials Manufacturing, Surface and Joining Technology Other Mechanical Engineering
Identifiers
URN: urn:nbn:se:his:diva-22005DOI: 10.1016/j.wear.2014.05.015ISI: 000341340700015Scopus ID: 2-s2.0-84902973160OAI: oai:DiVA.org:his-22005DiVA, id: diva2:1708134
Projects
European project entitled Helios
Funder
EU, FP7, Seventh Framework Programme, 265861
Note

The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007–2011) under grant agreement no. 265861 (Helios).

Available from: 2014-07-07 Created: 2022-11-02 Last updated: 2025-02-14Bibliographically approved
In thesis
1. Tribological and Mechanical Behaviour of Lamellar and Compacted Graphite Irons in Engine Applications
Open this publication in new window or tab >>Tribological and Mechanical Behaviour of Lamellar and Compacted Graphite Irons in Engine Applications
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

There has been much discussion about the beneficial uses of lamellar graphite iron in piston rings–cylinder liner systems, where a good combinations of both thermal and tribological properties are essential. The excellent tribological performance of lamellar iron under such sliding conditions is principally associated with lubrication behaviour of the graphite particles which are distributed as lamellas throughout the matrix. During sliding, graphite particles are extruded and smeared onto the counterfaces, act as solid lubricating agents and form a thin graphite film between the sliding surfaces. Although this process especially, during the running-in period significantly changes the sliding wear response of the components, the exact mechanism behind of this phenomenon has rarely been discussed in previous studies.

It is tribologically beneficial to keep the graphite open, particularly in applications where the scuffing issues do matter. In this thesis, the main causes involved in closing the graphite lamellas are discussed, with a focus on matrix plastic deformation that occurs during sliding. In first step, the relationship between graphite lamellae orientation and plastic deformation was investigated. To do so, two piston rings, belonging to the same two-stroke marine engine operated for different periods of time, were selected and compared to the unworn sample. The worn piston rings displayed a substantial decrease in both frequency and area fraction of the graphite lamellas. Most of the lamellas were closed as a result of plastic deformation of matrix. This happening was caused mainly by the interaction between abrasive particles and metallic matrix. Additionally, it was found that graphite lamellas parallel or near-parallel to the sliding direction exhibited maximum closing tendency under sliding condition.

In next step, to have a better understanding of the graphite film formation mechanism and matrix deformation role in closing the graphite lamellas, microindentation and microscratch testing were performed on typical lamellar iron. The qualitative results showed a similar mechanism involving in graphite contribution to lubricate the sliding surfaces. Moreover, microindentations made nearby the graphite lamellas demonstrated that the deformation of the matrix causes the formation of cracks in the centre of the graphite lamellas, compressing and then extruding the graphite from its natural position, irrespective of the lamellas′ size. Furthermore, it was found that subsurface graphite orientation had a large influence on the extrusion behaviour, in that, for graphite lamellas oriented towards the indenter, the effect was observed more pronounced.

Furthermore, an improved fully ferritic solution strengthened compacted graphite iron was produced for future wear studies. The effects of different Si levels and section thicknesses on tensile properties and hardness were investigated as well. The influence of Si content and section thickness on mechanical properties was revealed by improving the materials strength and slightly enhancing the hardness through increasing Si content. Besides, Si addition up to 4.5 wt% significantly affected the strength and elongation to failure of cast samples.

Place, publisher, year, edition, pages
Jönköping: School of Engineering, Jönköping University Department of Materials and Manufacturing, 2015. p. 49
Keywords
Sliding wear, abrasive wear, graphite lubricating performance, matrix deformation, lamellar graphite iron, high-Si compacted graphite iron
National Category
Other Mechanical Engineering
Identifiers
urn:nbn:se:his:diva-22011 (URN)978-91-87289-06-4 (ISBN)
Presentation
2015-02-12, E1405, Gjuterigatan 5, Jönköping, 12:24 (English)
Opponent
Supervisors
Projects
HeliosFFI
Funder
EU, European Research Council, 265861
Available from: 2022-11-02 Created: 2022-11-02 Last updated: 2025-02-14Bibliographically approved
2. The influence of microstructure on mechanical and tribological properties of lamellar and compacted irons in engine applications
Open this publication in new window or tab >>The influence of microstructure on mechanical and tribological properties of lamellar and compacted irons in engine applications
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Lamellar graphite iron (LGI) is commonly used in diesel engine applications such as piston rings–cylinder liner where an excellent combination of physical and tribological properties is essential to avoid scuffing and bore polishing issues. The excellent tribological behaviour of LGI alloys is related to the graphite lamellas, which act as solid lubricant agents by feeding onto the tribosurfaces under sliding conditions. However, increasingly tighter emissions and fuel economy legislations and the higher demands on enhanced power and durability have encouraged both engine designers and manufacturers to introduce pearlitic compacted graphite irons (CGI) as an alternative material replacing LGI, although the poor machinability of pearlitic CGI alloys compared to the LGI remains a challenge.

The focus of this study is placed on investigating how the microstructure of LGI and CGI alloys affects their mechanical and tribological properties. This was initially undertaken by investigating representative, worn lamellar cast iron piston rings taken from a two-stroke large-bore heavy-duty diesel engine. As known that it is tribologically essential to keep the graphite open under sliding conditions, in particular under starved lubrication regimes or unlubricated conditions to avoid scuffing issues; however, this study revealed the closure of a majority of graphite lamellas; profoundly for those lamellas that were parallel to sliding direction; due to the severe matrix deformation caused by abrasion. Both microindentation and microscratch testing, which were used to crudely simulate the abrasion under starved lubricated condition in combustion chamber, suggested a novel mechanism of activating the graphite lamellas to serve as lubricating agents in which the matrix deformation adjacent to the graphite initially resulted in fracturing and then extrusion of the graphite lamellas.

Additionally, in order to investigate the relation between matrix constituents, mechanical properties and machinability of cast iron materials, solution-strengthened CGI alloys were produced with different levels of silicon and section thicknesses. The results showed significant improvements in mechanical properties and machinability while deteriorating the ductility. Moreover, multiple regression analysis, based on chemical composition and microstructural characteristics was used to model the local mechanical properties of high Si ferritic CGI alloys, followed by implementing the derived models into a casting process simulation which enables the local mechanical properties of castings with complex geometries. Very good agreement was observed between the measured and predicted microstructure and mechanical properties.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering, 2016. p. 80
Series
Dissertation Series ; 17, 2016
Keywords
Cast iron, Si solution-strengthened CGI, microstructure, mechanical properties, modelling and simulation, tribology, abrasive wear, scratch testing
National Category
Metallurgy and Metallic Materials Other Mechanical Engineering
Identifiers
urn:nbn:se:his:diva-22012 (URN)978-91-87289-18-7 (ISBN)
Public defence
2016-11-25, E1405, Jönköping University, School of Engineering, Jönköping, 10:00 (English)
Opponent
Supervisors
Available from: 2022-11-02 Created: 2022-11-02 Last updated: 2025-02-14Bibliographically approved

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Ghasemi, RohollahElmquist, Lennart

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