Promoting Phase Transformation using Hold Durations

The two pathways of plastic deformation detailed above show that, if phase transformation is the initial mechanism of plastic deformation, subsequent loading will continue to favour this transformation over the nucleation and propagation of crystalline defects. The number of indents that exhibited either form of plastic deformation were also shown to be dependent on maximum load and hold duration. Figure5.19(a) plots the probability of either deformation mechanism occurring as a function of maximum load at a fixed hold duration of 15 minutes. The ratio between phase transformation and crystalline defects (PT/CD) is also included. Both mechanisms appear to increase with maximum load, most likely due to the increased number of plastic deformations (in contrast to indents that respond elastically). Unfortunately, increasing loads appears to favour defect nucleation over PT as the PT/CD ratio decreases. Figure5.19(b) plots the same values as a function of hold duration (at a fixed maximum load of 300 mN). An increase in hold duration also leads to an increase in both forms of plastic deformation. More interestingly, increasing hold duration leads to an increase in the PT/CD ratio. This suggests that a lower maximum load held for a longer duration will maximise phase transformation over defect nucleation in the indents that deform plastically.

This decrease in maximum load to promote phase transformation must also be balanced against the increase in indents responding elastically. For example, at 200 mN with a hold duration of 15 min the only plastically deformed indents are phase transformed indents. This would suggest that, given the tip used in this chapter, 200 mN is the ideal maximum load to promote phase transformation. However, indents that exhibit plastic deformation only consists of 4% of the total indents made (shown in Table. 5.2). Further, a significant increase in hold duration to 60 minutes did not show a significant increase in indents responding plastically as none of

the five indents performed under these conditions exhibited plastic deformation.

To better visualise the relative impact of maximum load and hold duration, the mean contact pressure as a function of load can be considered. The mean contact pressure (Pc) can be

calculated using the equation [91]:

Pc=

Fm

π.a2

c

where Fm is the maximum load, and acis the contact radius. The contact radius is calculated

from the contact depth observed in the load/unload curve as detailed in section3.1. Fig. 5.20

presents the Pressure-Time-Transformation plot of the presented phase transformed indent results. It should be noted that while the mean contact pressure for all indentations presented in this work are below the transformation threshold of 11 GPa, the mean contact pressure is averaged over a wide area that includes many regions where contact is made but no transfor- mation occurs, thus such a low mean pressure is expected. This plot indicates that, within the regimes explored in this work, increasing load has a higher impact on the occurrence of phase transformation than hold duration. However, this greater increase occurs alongside an increase in the nucleation and formation of crystalline defect which may negatively affect the quality of the phase transformed region.

5.5

Summary

In this chapter, results showing that a hold duration promotes plastic deformation have been presented. By using a hold duration, a maximum load regime which had previously only responded elastically to indentation has exhibited plastic deformation. Within this regime, phase transformation and the nucleation of crystalline defects are shown to exist as competing deformation mechanisms. Both processes are nucleation limited and stochastic in nature. For the tip used in this work, it is shown that between 200 mN - 500 mN there exists a pressure regime where, once nucleated, each deformation mechanism is the sole observed deformation. At higher loads (750 mN and above), sufficient pressure is present for both mechanisms to occur. The results suggest that the initial deformation mechanism that occurred remains dominant within the residual indent even at this higher load.

Controlling the initial form of plastic deformation is of great importance, leading to several avenues of further study. Firstly, the pressure distribution within the regime where plastic deformation first occurs leads to buried layers of phase transformed material due to the in- terplay between hydrostatic and shear stress. There is potential to further understand how the two stresses interact to initiate phase transformation. Secondly, during unloading from

Fig. 5.20: A Pressure-Time-Transformation plot showing the percentage of indents that plas- tically deformed via phase transformation across a range of mean contact pressure and hold durations. Each data point is labelled with the percentage of indents that phase transformed under those conditions. Each shaded region approximates a range of transformation percent- ages, with an arrow indicating the direction of increasing phase transformation.

theβ-Sn phase, the pressure distribution also appears to affect the transformation to either a-Si or the bc8/r8 mixed structure. Further study into these affects would allow for greater control over the end phases formed. Finally, it has been shown that both increased load and increasing hold duration lead to increased indents exhibiting phase transformation. However, further study is required to discover an ideal condition under which phase transformation is maximised relative to both elastic indents and the nucleation of crystalline indents.

Chapter 6

Annealing of the bc8/r8 Mixed Structure

This chapter investigates the annealing pathway of the bc8/r8 mixed structure that is formed via nanoindentation. The bc8/r8 structure undergoes furnace, heater stage, and laser anneal- ing ranging from 30 to 750◦C. The resulting transformed material is then characterised via Raman microspectroscopy and SADP analysis to investigate the transformation temperatures and pathways from the bc8/r8 mixed structure to the equilibrium dc-Si phase.

6.1

Introduction

The effects of annealing on the bc8/r8 mixed structure is of interest due to several reasons. Firstly, the formation of a new structure (Si-XIII) which has only been reported from annealing of this mixed phase, is of fundamental interest. Secondly, there is also a technological drive to understand the stability of both the bc8/r8 mixed structure formed using indentation and hd-Si that is formed via annealing of the mixed structure. Interest in the former is due to the potential optical properties of r8-Si [143, 144] and the recent study of bc8-Si that indicated it is a narrow band-gap semiconductor [137]. The ability to write industrially relevant regions of hd-Si onto existing Si surfaces using indentation is also technologically interesting [141]. Realisation of these goals is hindered by the lack of understanding regarding the transformation pathway of the bc8/r8 mixed structure under annealing.