Wang, Xiang
(2022)
Atomic-Scale In-situ TEM Investigation on Mechanical Deformation and Friction in Nanocrystals.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
Revealing the deformation and friction mechanisms of nanomaterials at atomic scale, remains challenging due to the technology limitation. In this dissertation, in-situ transmission electron microscopy (TEM) experiments have been carried out to study the mechanical deformation of nanocrystals and friction between nanocrystals.
Twinning in body cubic centered (BCC) metals requires to be activated by extreme high stress due to its high activation energy barrier. The stability of the deformation twins remains largely elusive, which is crucial for employing twinning or twin structures to improve BCC metals’ mechanical properties. Here, it is found that the twin stability in BCC W is strongly dependent on the type of twin boundary. A unique high-energy interface structure called the inclined twin boundary provides the driving force for spontaneous detwinning in BCC nanocrystals. On the other hand, it is well-known that materials follow a “smaller is strong” trend when the sample size is below submicron and nanoscale. But the ultrahigh surface-to-volume ratio in nanoscale materials can dramatically activate surface diffusion to mediate the plastic deformation, resulting in the softening of nanoscale materials. Here, it is found that the diffusion-assisted dislocation nucleation is responsible for the transition from “smaller is stronger” to “smaller is weaker” and diffusion-mediated plastic deformation induces the decline of the flow stress in face-cubic centered (FCC) nanocrystals.
Up to now, visualizing the friction process between nanocontacts to reveal the friction mechanism at the atomic scale is barely achieved. Here, by designing the nanocontact and performing relative motion between nanoscale asperities under high-resolution TEM, the real-time atomistic friction process is realized, revealing that atomic friction displays a discrete stick-slip behavior. Interface atoms exhibit an asynchronous motion, consistent with the non-uniform strain distribution in contact. This work proposes a new methodology to achieve in-situ atomic-friction research and sheds light on the fundamental mechanisms of friction at atomic-scale.
This dissertation gives insights into the mechanical behaviors of BCC and FCC nanocrystals and provides feasible nanotechnology to investigate the atomic friction in nanocrystals, which are of importance for advancing the understanding of nanomaterials degradation and extending the service life of nanodevices.
Share
| Citation/Export: |
|
| Social Networking: |
|
Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
|
| Date: |
10 June 2022 |
| Date Type: |
Publication |
| Defense Date: |
18 January 2022 |
| Approval Date: |
10 June 2022 |
| Submission Date: |
18 February 2022 |
| Access Restriction: |
No restriction; Release the ETD for access worldwide immediately. |
| Number of Pages: |
175 |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Swanson School of Engineering > Mechanical Engineering |
| Degree: |
PhD - Doctor of Philosophy |
| Thesis Type: |
Doctoral Dissertation |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
Deformation Friction In-situ TEM Twinning BCC FCC |
| Date Deposited: |
10 Jun 2022 18:10 |
| Last Modified: |
10 Jun 2022 18:10 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/42245 |
Metrics
Monthly Views for the past 3 years
Plum Analytics
Actions (login required)
 |
View Item |
![[feed]](/images/feed-icon-32x32.png){kind=icon}