Crack Formation during Electrodeposition and Post-deposition Aging of Thin Film Coatings - 1st Quarterly Report
First Quarterly Report - AESF Research Project #R-118. This new NASF-AESF Foundation research project report covers the First quarter of project work (January-March 2016).
Prof. Stanko R. Brankovic*
University of Houston
Houston, Texas, USA
Editor’s Note: This NASF-AESF Foundation research project report covers the first quarter of project work (January-March 2016) on this new project at the University of Houston. A printable PDF version is available by clicking HERE.
The objective of this 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 performed in this period are focused on initial studies of electrodeposition of chromium thin films of arbitrary thickness on copper, gold and nickel polycrystalline substrates from Cr+3-containing electrolytes. The bath formulation for this work is based on standard Cr+3 baths developed by Faraday Technology (Clayton, Ohio), labeled as EX DBA 1411 and EX DBA 1318 in Table 1. The first experiments were set to explore experimental boundaries of both bath systems, and to test the proposed experimental approach for detecting the onset of crack formation in chromium films.
Table 1 - Operating parameters of trivalent chromium baths under study.
|Designation||j (mA/cm2)||pH||T (°C)|
|EX DBA 1411||250-450||5.2-5.4||35|
|EX DBA 1318||250-450||2.5||27-54|
The current understanding of crack formation in chromium films identifies the high stability of Cr+3 in water as a root cause. The Cr+3 forms a strong complex with water molecules. This causes its solubility at the interface to be strongly dependent on interfacial pH (pHi) having a great tendency to form an insoluble hydroxide.1 The precipitation of the oxide phase on the growing metal surface during electrodeposition results in incorporation of amorphous oxide phase at the grain boundaries.2 This is expected to negatively effect the fracture toughness of chromium films. The overall effect of oxide phase incorporation is manifested through the increased oxygen content in the chromium films and crack formation as a consequence of reduced fracture toughness, and/or oxide particles acting as stress concentration inhomogeneity. To study this phenomenon it is of crucial importance to accurately identify the critical thickness of chromium-films at which the onset of crack formation starts. In our work we proposed several in situ methods to identify the moment of crack formation in chromium films during electrodeposition process. The initial work on their implementation is described in proceeding text.
In situ measurements of stress evolution in chromium films
The most important part of the characterization work represents in situ measurements of stress-thickness and stress-time evolution and identification of the crack formation/propagation moment in chromium films. At the same time, this allows one to identify the critical thickness of the chromium film for crack formation which is important parameters for phenomenological description of the crack formation process.
These measurements will be performed using cantilever bending method.3 For this purpose our in-house system has been modified and re-fitted with a 10 mW laser, and custom cantilever holder system (Fig. 1). The main parts of the system are labeled in Fig. 1(a), indicating the laser path though the beam splitter, electrochemical cell, position sensitive detector (PSD), PSD-signal amplifier and laser beam focusing iris. The custom designed electrochemical cell is positioned on a special stage that can be adjusted in terms of the height and inclination along two axes parallel to the path of the laser beam. This allows one to acquire the best possible tuning of the laser reflection to the center of the PSD, which is mounted on an X-Y stage, allowing the position change with micrometer precision.
The fully mounted cell with all elements, except the solution, is shown in Fig. 1(b), with laser reflection from the back side of the cantilever. The front side of cantilever is deposited with a gold, nickel or copper thin film facing the counter electrode. This means that the laser reflection is from the glass cantilever side that is not affected by the chromium deposition and thus the reflection of the laser beam does not change in intensity as the chromium film is grown on top of the gold, nickel or copper seed on the front side. The volume of the cell is 150 mL which is sufficient for most of the experiments without fear that conditions in the solution will change during chromium thin film growth.