(Invited) Fundamental Origin for Crack Formation Cr Films Electrodeposited from Cr3+ Solutions
Authors: Ahmadi Kamyar , Chaudhari Nikhil , Hall Timothy D , Taylor E. J. , Brankovic Stanko 
Published: February 2020

The electrodeposited thin film coatings are materials commonly used today in almost all fields of technological enterprise. Generally, mechanical, chemical and physical properties of electrodeposited films are largely function of electrodeposition methods used in their synthesis (DC, pulse current, pulse potential deposition etc..) [[i]], solution design (additive content, anions, complexing agents, ligands, supporting electrolyte) [[ii],[iii],[iv]] and temperature of the process. One of the main challenges that electrodeposition is facing today in producing the high quality films is their mechanical integrity and conformal coating of the substrates. Often, this goal is difficult to achieve due to appearance of the cracks in the films which compromise their stability and the intended application.
The presented work investigates the fundamental origin of crack formation in Cr thin films electrodeposited from Cr3+ solutions. For this purpose, we have developed a methodology for insitu stress measurements and insitu impedance measurements of Cr films, which allowed us to indirectly monitor the crack formation in Cr films during deposition, aging and annealing process. This was of crucial importance to distinguish different processes in the Cr films that contribute to their stress state, mechanical integrity, cracking and change in their electrical properties. The observed growth stress are negative, i.e. compressive with steady state values in the range below -100 MPa. Yet, we have observed a blistering in Cr films and accumulation of different defects on the Cr film surface during deposition which indicated significant amount of dissolved hydrogen either as interstitial impurity or as a part of chemically bonded H as Cr-hydride. Our studies clearly indicate that the tensile relaxation in Cr films after electrodeposition is directly related to the quality of the environment. During relaxation in the solution, the maximum change of stress observed was very modest, just few tens of MPa. The stress data were also complemented with our impedance measurements of Cr films during relaxation in solution which show very modest change in resistance, i.e. disappearance of the hydride phase seems not to be the main driver of the change in electrical properties. However, Cr films aging in air/oxygen shows much greater tensile relaxation and increase in resistance, The insitu stress measurements did not show any direct indication of the crack formation during this process yet, the stress levels observed were becoming close to the values of fracture toughness for pure Cr films[1]. Therefore, presence of oxygen is found as fundamentally important for qualitatively different stress and resistance change of the Cr films during aging process.
Our insitu stress, impedance and optical results undoubtedly suggest that annealing process (250 °C) is solely responsible for visible crack formation in Cr films. This is a consequence of the weakened structure of the Cr films by oxidation process which has a true origin in Cr-hydride formation and subsequent oxidation of its degradation by-products (H and Cr). Although the insitu stress data do not show catastrophic failure of Cr films, or brittle fracture propagation that would be obvious signature of the cracking, a closer inspection of the stress data show that their complex behavior in deed represents an interplay of internal structural changes related to chemical processes, as well as formation of micro and macro cracks. The observed levels of tensile stress in some samples are close or exceed the fracture toughness or ultimate tensile strength of the Cr films (>250 MPa)2 at the temperatures of 250 °C. The stress data are in agreement with impedance studies during annealing which also show the largest resistance change. The main conclusion of this work is that the electrodeposition process that produces less Cr-hydride in the deposit should be more desirable when the crack free Cr films are the goal.
This work is supported by Gift Grant from NASF-AESF Research Foundation.

[1] U. Holzwarth, H. Stamm / Journal of Nuclear Materials 300 (2002) 161–177

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