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Mechanical Testing of Thin-Film Coatings

#1127


Researches on Development of a Method of Surface Micromechanical Tests of Metal Oxide Thin-Film Heat-Resistant Coatings on Metal-Ceramic Substrates for High-Temperature Fuel Cells

Tech Area / Field

  • NNE-BCM/Batteries and Components/Non-Nuclear Energy
  • MAT-SYN/Materials Synthesis and Processing/Materials

Status
8 Project completed

Registration date
08.10.1997

Completion date
21.02.2003

Senior Project Manager
Mitina L M

Leading Institute
VNIITF, Russia, Chelyabinsk reg., Snezhinsk

Collaborators

  • University of New Mexico, USA, NM, Albuquerque\nSandia National Laboratories, USA, NM, Albuquerque

Project summary

The purpose of the project is development of an effective method for quantitative evaluation of resistance to cracking for metal oxide thin-film heat resisting coatings on metal-ceramic substrates for SOFC.

As one of the most perspective and promising approaches to development of electrochemical generator on the basis of high-temperature SOFC is acknowledged a thin-film technology of metal oxide coating - electrolyte YSZ (ZrO2) deposition on metal-ceramic porous substrates - electrodes.

However rigorous researches and development of promising technologies for deposition of SOFC electrolyte coatings are virtually impossible without evaluation and monitoring their mechanical performance.

To most important of them it is necessary to refer fracture toughness and, accordingly, tightness of oxide coatings of all electrolyte cells crucial for serviceability and functioning efficiency of electrochemical generator as a whole.

In this association urgent necessity has arisen regarding researches, development and application of a method for mechanical tests of coatings to the greatest degree adequate to their distinctive features, namely: fragility, optical transparency, small thickness.

The developed method will also allow assessing more objectively influence of high temperatures and adhesion to substrate on strength properties of oxide coatings and their fracture toughness. The established interrelations and influence of structural-technological factors on properties of coatings will be thus one of the optimizing elements of manufacturing technology for tight and heat resistant thin-film SOFC electrolytes.

The method of control tension-sensitive coatings is known, permitting to study distribution of strain-stress fields on specimens surface, as well as on full-scale units and details at application of exterior loads to them.

However critical factor of stresses intensity K1C is considered to be more demonstrative characteristic for evaluation and prediction of mechanical properties of slightly pliable and fragile materials: glasses, ceramics, solid alloys.

Known methods of special specimens tests and of K1C calculation are unacceptable for specimens of small sizes and shapes close to full-scale, and, especially, for an evaluation of fragile coatings properties on such samples.

Interest to development of micromechanical tests methods for slightly pliable materials with determination of fragility and kinetics of cracks formation at local loading has recently increased. The cracks are formed at action of harder indent on the surface, thus generating tensile stresses in a contact zone.

The expediency and necessity of MMT method development for SOFC with thin-film electrolytes is caused by the several reasons:


- The main of these reasons consists in need of an operative quantitative evaluation of coating mechanical properties, including, at action of high temperatures and working stresses. Evaluation of level of resistance to fragile destruction is necessary first of all in view of the functional requirements to SOFC coatings - electrolytes: tightness, thermal strength, adhesion. It will allow to influence actively on technological process of electrolyte cell manufacturing in SOFC composition; to determine optimum design -technology variant of thin-film electrolyte on a carrying substrate-electrode manufacturing;
- Separate evaluation of technological and operational factors contribution to fragile destruction resistance as of electrolyte coatings, and so of substrate - electrode itself also is of practical interest.

The synthesis of the above-stated methods concerning thin-film elements of a system "coating - substrate" will allow the authors of the project:


- to test for resistance to cracking samples of small sizes including with thin-film coatings (approximately 10 microns);
- to take into account influence of carrying elements - substrates on properties of coatings;
- to assess quantitatively influence the main structural-technological parameters, including high temperatures, on K1C and thermal strength of coatings;
- to optimize the elements of manufacturing technology for thin-film oxide ZrO2-based electrolyte for to high-temperature SOFC.

The economic efficiency of MMT method development consists in an optimum choice of designing-technological variant of thin-film coating manufacturing in a pair with a substrate and, hence, timely rejection of ineffective variants.

The development of MMT method is effective not only for high-temperature SOFC, but also represents practical interest for evaluation of fragile coatings properties in any other area of applicability.

Accumulated by the authors of the project technological and research experience, and also methodical developments in the field of coatings deposition, the researches of their surface characteristics also are a basis for development of a surface MMT method for thin-film heat resisting coatings in composition with a substrate.

At a realization of the project it is supposed to apply the following principles, engineering approaches and methods:


- "from analysis of various methods and models to synthesis of an optimum method for evaluation of fragile thin-film coatings properties";
- computer simulation of calculated - theoretical and of experimental dependencies;
- application of factorial and multifactorial mathematical planning of experiment on model samples;
- realization of experimental works as on specialized, and so on developed experimental equipment and tools;

The project provides realization of the following stages of works:


- Analysis of known models for quantitative correlation of resistance to cracking and fragility with loading characteristics. Development a plan-matrix of a factorial experiment with realization of micromechanical tests.
- Computational - experimental simulation of loading process at micromechanical tests ensuring critical tensile stresses at established depths of indent introduction.
- Choice, development and manufacturing of a laboratory loading device, indents for it and model specimens of the carrying elements - substrates.
- Development of technologies of thin-film heat resisting oxide ZrO2-based coatings deposition and contrasting technological coatings of defined composition and thickness.
- Realization of experimental works, including:

- deposition of coatings;
- micromechanical tests;
- thermal cycling specimens with coatings (20-1000 °C);
- study of an adhesion and thermal strength of coatings (X-ray microspectral - analysis, MMT method).

- The computational - experimental modeling of resistance to cracking correlation with loading parameters, exterior actions, characteristics of thin-film ZrO2-based coatings in composition with metal-ceramic substrates.
- Registration of the reporting scientific and technical documentation.

The scientific and engineering problems, covered in the present project, are promising for scientific and industrial organizations having relation to the given sphere of activity in USA, Japan, Europe, CIS. Information exchange and discussion of results obtained at realization of the project is supposed with probable collaborators.


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