Studies of the high temperature tribological behaviour of some superalloys

Rose, Simon Robert (2000) Studies of the high temperature tribological behaviour of some superalloys. Doctoral thesis, University of Northumbria at Newcastle.

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Abstract

High temperature wear is a serious problem in many applications in which relative motion and contact of components occur at elevated temperature. Under certain wearing conditions compacted debris layers known as 'glaze' layers which are highly resistant to further wear are formed. The principal aim of this project was to understand the mechanisms of such layer/glaze formation particularly during sliding of unlike materials against unlike materials. The influence of temperature, load and material compositions was of particular interest in this work.
This thesis examined the wear behaviour of four combinations of Superalloys worn using a block-on-cylinder type wear rig, developed in this laboratory which allows accelerated wear testing simulating the wear processes experienced in valve and valve seats in natural gas fuelled combined heat and power engines. The alloys studied included MA956 and Nimonic 80A predominately as samples and Stellite 6 and Incoloy 800 as predominantly counterfaces.
'Glazed' layers were found to form on three of the combinations tested: MA956 with Stellite 6 and Incoloy 800 counterfaces from 510°C and 690°C and with Nimonic 80A worn with Incoloy 800 from 630°C. In any system before the onset of 'glaze' formation, increasingly severe wear was encountered due to the loss in strength of the materials with increasing temperature, giving rise to high rates of material loss up to a certain maxima, dependent on the materials tested.
With MA956/Stellite 6 the maxima in the temperature/weight loss curve occurred at 0.13g at 510°C. The maxima was at 0.66g for MA956/Incoloy 800 at 630°C. A maximum of 0.49g was recorded at 630°C for Nimonic 80A worn with Stellite 6 counterfaces. However Nimonic 80A worn with Incoloy 800 counterfaces did not show such a maximum. The low wear of the sample was caused by the transfer of a metallic layer from the counterface providing protection below the minimum temperature of 'glaze' layer formation (630°C). However the counterfaces used suffered severe damage.
Testing over a range of loads (7 to 25N) revealed that the layers formed on MA956 with Stellite 6 and Incoloy 800 counterface demonstrated an increased rate of breakdown of the 'glaze' layers at higher loads (25N). For Nimonic 80A worn with Incoloy 800 no layers were formed above 10N; the wear mechanism became oxidational in nature.
Sliding combinations involving like-on-like and sample/counterfaces reversals demonstrated that dissimilar materials tend to undergo less adhesion and so promote layer formation and low wear.
Information on wear damage, debris characteristics, layer formation and subsurface deformation was obtained by optical microscopy, SEM, SEM/EDX and XRD analyses. These data were complemented by hardness and hot hardness measurements on worn and 'as-received' materials.

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