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On the role of tribofilm formation on the alumina drive components of an ultrasonic motor
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Tribologi)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Microsystems Technology. PiezoMotor AB.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Tribologi)
2009 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 267, no 5-8, 1295-1300 p.Article in journal (Refereed) Published
Abstract [en]

Ultrasonic motors typically have a friction drive system to transfer the movement. The miniaturized motor type investigated here has a friction drive system consisting of two drive pads that transfer the high frequency oscillating movement of two piezoelectric elements to a linear drive rail. The pads and rail consist of alumina.

Fiction tests were carried out to investigate how the coefficient of friction between the drive pads and the drive rail depends on the number of strokes of the rail. It was found to initially increase with the number of strokes and then stabilize.

Scanning electron microscopy studies of the friction drive surfaces show how a tribofilm forms and develops with the number of strokes. Interestingly, the smooth tribofilm surface gives a higher coefficient of friction than the original rougher surface. To further investigate the nature of the tribofilm. cross section samples were produced with a focused ion beam instrument. The tribofilms show different characters and appear to form gradually by agglomeration and sintering of wear debris. Transmission electron microscopy showed the tribofilm to be amorphous and partly nano-crystal line. This high resolution investigation also clearly demonstrated that the tribofilm bonds very well to the underlying alumina grains.

The processes of friction increase and tribofilm build-up stabilize early compared to the lifetime of the motor.

Place, publisher, year, edition, pages
Elsevier , 2009. Vol. 267, no 5-8, 1295-1300 p.
Keyword [en]
tribofilm, alumina, friction, ultrasonic motor
National Category
Materials Engineering
Research subject
Materials Science
Identifiers
URN: urn:nbn:se:uu:diva-111717DOI: 10.1016/j.wear.2009.01.020ISI: 000267580700084OAI: oai:DiVA.org:uu-111717DiVA: diva2:282564
Available from: 2009-12-21 Created: 2009-12-21 Last updated: 2016-04-14Bibliographically approved
In thesis
1. Friction and Wear Mechanisms of Ceramic Surfaces: With Applications to Micro Motors and Hip Joint Replacements
Open this publication in new window or tab >>Friction and Wear Mechanisms of Ceramic Surfaces: With Applications to Micro Motors and Hip Joint Replacements
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Surfaces exposed to wear always transform and typically a layer of new structure and composition is formed. This layer, often called tribofilm, changes the friction and wear properties. Tribofilms formed on ceramic surfaces may consist of products from chemical reactions between the materials in contact and the environment or consist of compacted wear debris.

In this thesis, focus has been to understand the friction and wear mechanisms of ceramic surfaces, as well as acquiring knowledge about the properties of the new surfaces created during wear. Ultimately, this understanding can be used to develop ceramic systems offering high or low friction, while the material loss in both cases should be minimised. Such ceramics could improve numerous tribological systems and applications, out of which ultrasonic motors, low-friction ceramic coatings and hip joint replacements have been treated in this thesis. Friction and wear tests, and subsequently various surface analyses have been essential for the knowledge about the friction, wear and tribofilm formation.

 For ultrasonic motors of the studied type, the highest driving force is achieved when the friction is high between the alumina components in the friction drive system. The highest friction was here accomplished with a thick tribofilm on the surfaces. The formation of such tribofilms was favoured by dry conditions, and using an initially rough surface, which increased the initial generation of wear debris.

In a detailed investigation of the importance of microtopography on tribofilm formation and friction behaviour, a low-friction, PVD coating of TaC/a-C was studied. This coating showed a very low, stable friction. High sensitivity to the microtopography was demonstrated, smooth coating exhibited a faster build-up of a dense tribofilm of fine ground material on the counter steel surface and subsequently a faster running in and friction decrease. 

The life span for total hip joint replacements can be prolonged by minimising the wear particles that cause inflammation and subsequent implant loosening. In this work coatings of amorphous/nanocrystalline silicon nitride have shown low wear rate, and hence produce a minimum of wear particles. Furthermore, these particles that are expected to resorb in vivo. This system therefore has potential to reduce problems with inflammation and osteolysis connected to wear particles.    

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2011. 65 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 841
Keyword
Tribology, Friction, Wear, Ceramic, Tribofilm, Alumina, Silicon nitride, Ultrasonic Motors, Hip Joint Replacement, PVD coating
National Category
Materials Engineering
Research subject
Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-156801 (URN)978-91-554-8123-0 (ISBN)
Public defence
2011-09-23, Häggsalen, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2011-09-02 Created: 2011-08-09 Last updated: 2011-11-03Bibliographically approved

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Olofsson, JohannaJohansson, StefanJacobson, Staffan

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