Crack Formation During Electrodeposition and Post-deposition Aging of Thin Film Coatings - 3rd Quarterly Report
Prof. Stanko R. Brankovic*
University of Houston
Houston, Texas, USA
Editor’s Note: This NASF-AESF Foundation research project report covers the third quarter of project work (July-September 2016) on this AESF Foundation Research project at the University of Houston. A printable PDF version is available by clicking HERE. It may be helpful to the reader to consult previous reports on this project. The links to these reports are provided at the end of this report.
- Stanko R. Brankovic, PI, Electrical and Computer Engineering and Chemical and Biomolecular Engineering, University of Houston,
- Kamyar Ahmadi, PhD Student, Material Science Program, University of Houston,
- Nikhil Chaudhuri, PhD Student, Material Science Program, University of Houston.
The objective of the proposed work is to study fundamental and practical aspects of crack formation in electrodeposited thin films. The aim is to identify and quantify key parameters of the electrodeposition process affecting the crack formation in thin films. This study should enable development of an effective strategy generally applicable in practice whenever electrodeposition process for crack free films is demanded.
The activities in this period were focused on studies of electrodeposition of chromium thin films of >10 micron thickness on copper and NiCr polycrystalline substrates from Cr+3-containing electrolytes. The main experimental work involved EXDBA 1411 Bath with pH=5 (see http://short.pfonline.com/NASF16Nov2 for description).
Resistance/Impedance change of chromium films exposed to air during room temperature aging
In this period we have performed in situ impedance measurements during chromium film aging in air at room temperature and during annealing at 250°C. The experimental apparatus for in situ room temperature impedance measurements and sample configuration was already described in detail in our previous report (Report #2; http://short.pfonline.com/NASF16Dec2). The procedure for this set of experiments involved an additional step,“removal of solution,” which lasted about 3 seconds. Therefore, the samples were left to age in air while simultaneous impedance (AC resistance) transients of the chromium thin films were recorded.
The observed resistance changes for chromium films deposited with different current densities are shown in Fig. 1(a-d). The optical images of the representative morphology of the chromium surface for each sample are also shown on the right side for each transient. Impedance transients were recorded over the time intervals of 3 hr, while the approximate chromium film thickness was around 15 microns. The maximum change in the impedance value is indicated in the graph.
The common feature of all results in Fig. 1 is the quite significant magnitude of the impedance change. It ranges from 300% to 2200%. There is no obvious trend and correlation of the impedance change with respect to the conditions of the electrodeposition process and deposition current density. Initially, (Fig. 1(a-c), Fig. 2) we see the increasing impedance change with increasing current density, (j = 250-350 mA/cm2) and then, a significant decrease in relative change of impedance with further increase of current density, Fig. 1(d), and Fig. 2.
The optical inspection of the chromium samples as well as the smooth appearance of the impedance transients do indicate that no crack formation in the chromium deposited films occurs at room temperature as a result of the aging process. This is a bit surprising considering the large values of impedance change that are observed. Indirectly the data suggests that the source of such a huge change of the impedance of chromium films must be related to the increased volume concentration of the defects in the chromium films.
The important thing to notice here is a different magnitude of the impedance change for two different aging processes at room temperature. In the case of the aging of chromium films in Cr-formate solution, the change of the impedance was of the order of 1% (Fig. 2). The aging process in air, at the same temperature results in almost three orders of magnitude larger impedance change. The observed trends with respect to the current density used for sample preparation also diverge. In reality, the aging in air is the more relevant process to consider since it is to happen no matter what the application of the chromium coating might be. Yet, the huge difference in results seems to be only related to the environment (solution vs. air).
We have already discussed that the possible mechanism for creating the defects during room temperature aging of chromium films is the decomposition of CrxHy (Cr-hydride) in the bulk phase. This process can liberate the hydrogen molecules (CrxHy = 0.5YH2 + xCr) which are trapped within the bulk of the sample at the grain boundaries, or in the lattice making a gas molecules to agglomerate or create a nano-voids. They represent structural imperfections, and thus can act as electron scattering points.