# Presentation: 478758

## Mechanisms of wear revealed by nanoscale analysis of sub-surface

- MECHANICAL MIXING IN METALLIC ALLOYS DURING BALL MILLING AND SLIDING WEARPascal Bellon, University of Illinois, DMR MET Award# 0304942

-  • We are investigating the role of chemical transfer and chemical mixing in severe wear regime.
-  • We perform pin-on-disc testing of high-wear resistant Cu-Ni-Sn bronze, against martensitic stainless steel 440C under severe conditions (10kgf). Wear testing in air leads to high wear rate, while wearing in flowing Ar reduces rate by factor 5.

**We propose that nanostructured mechanically mixed layers promote delamination and thus high wear rate** (manuscript submitted to Wear).
**Competition between scale-dependent shear mixing and thermally activated decomposition leads to self-organization in ball-milling and wear** (Acta Mater. 2006)**.**

**Notes:**
• Main goals of the project:
Elucidate the physical origins of the self-organization of alloys driven by sustained plastic deformation into nanocomposites
Transfer the knowledge gained on the evolution of alloy powders during ball milling to the evolution of transfer layers in dry friction wear
Investigate the possible correlation between dynamical phase transition of the microstructure of such transfer layers with transition of wear regimes, for instance from mild to severe wear.
Apply this knowledge to the design of alloys with improved wear resistance.

Recent important results
For a bronze with high wear resistance, a spinodally-decomposed Cu-15Ni-8Sn alloy used for advanced bearing applications,  we have obtained an extensive structural and chemical characterization of the surface of worn pins. We have shown that the transfer layer is comprised of 3 nanocrystalline mechanically mixed sublayers (MML), the top two ones being shown on Fig. 1 above.  A remarkable result is that these two layers are stabilized by an oxide, Fe2O3, that comes from the counterface material (here a stainless steel disc). We propose that the high wear rate is due to the fact that delamination initiates in the deeper sublayer (MML3), due to the gradient of hardness and fracture toughness introduced by the presence of the 3 sublayers. This analysis predicts that in the absence of oxidation, the wear rate should be reduced, a fact that we already know from our experiments, due to the reduction or even the absence of the MML2 sublayer. We are currently testing this prediction.
We have introduced a new analysis of the mixing induced by plastic deformation and its competition with thermally activated decomposition. We show that the plasticity-induced mixing is length scale-dependent, and as a result the competition with thermal decomposition is also length scale-dependent. This analysis provides a simple and direct explanation for the self-organization of the composition field into patterns, and for the selection of a characteristic length scale in these patterns. These predictions are in good agreement with results obtained by analyzing ball-milled Cu-Ag powders by atom probe tomography (collaboration with Prof. D. N. Seidman, Northwestern University).
Wear testing of the above bronze indicates that there exists a transition from very mild wear to severe wear at a threshold load value. We are currently investigating the possible role of oxidation on this wear regime transition.
Significance of the results
The new analysis of the mixing promoted by plastic dislocation is of broad applicability. Besides elucidating the formation of nanocomposites in materials subjected to severe plastic deformation, it shows that these alloy systems share properties identified in other dynamical systems, but at very different length and time scales, in particular the drift and diffusion of artic sea ice and the mixing of upper and lower mantle of the Earth crust.
The detailed nanoscale characterization of the transfer layer is shedding new light on the concept of transfer layers, on their formation as well as their role on wear rate. Elucidating these points should shed new light on the design of wear-resistant materials. 

## Broader impacts:

- MECHANICAL MIXING IN METALLIC ALLOYS DURING BALL MILLING AND SLIDING WEARPascal Bellon, University of Illinois, DMR MET Award# 0304942

_**2 new undergraduate students **__**	**__**Active-learning teaching method**__**have joined this research program**_

**•** Viralyne Ven and Justin Canniff, junior students from MSE-UIUC, have worked this summer in our group as research assistants (REU program), and are continuing this fall. They have been trained to pin-on-disc wear testing, XRD and SEM characterization .

**•** Implementation and evaluation of an active-learning teaching method in Metals senior courses taught by PI since fall 2003
- Very positive feedback from students through survey
-  PI on the list of “teachers ranked as excellent by their students” every semester since the introduction of active-learning
-  PI served as classroom observer for new faculty in College of Engineering
-  PI selected for classroom visits by new Engineering faculty (FastStart program)

**Notes:**
• Aim of the project:
Offer research opportunities for undergraduate students.
Develop and implement an active-learning teaching method.

Recent important results
In addition to a continuing undergraduate student, Tim Tyler, we have hired two new undergraduate research assistants, Alicia Kuhl and David Narbutas. They joined the project in Jan. 2005, worked as summer interns on the project, and this fall 2005 semester, they  continue to contribute to the research effort. These two new undergraduate students have received training on XRD, SEM, atomistic kinetic Monte Carlo simulations, and on pin-on-disc wear testing.
The PI has received excellent feed back from the students. Surveys are continuously collected and a reflective analysis of this teaching method will be prepared for publication.

Significance of the results
New undergraduate students have been exposed to the challenges and the excitement of scientific research. This is particularly important since there is a growing gap in PhD granted in the US versus abroad, as discussed, for instance, by Richard Smalley in the June 2005 issue of the MRS bulletin.
The PI feels that developing teaching methods that are appealing to the students and effective helps to attract students to traditional areas of Materials Science and Engineering (in this particular case the area of Metals and Metallurgy), especially when so many students are initially drawn to areas such as nanoscience and biomaterials.