Structural analysis of the MIT Micro Rocket Combustion Chamber

The micro rocket is one of several power microelectromechanical systems (MEMS) under development at MIT. The micro rocket is experiencing structural failures at operating parameters far below the designed performance level. The deterministic strength of brittle materials, such as silicon, is critically dependent on the local strength and flaw population. Experiments and correlating modeling were used to pursue the root cause of the micro rocket structural failure. This thesis presents the results of these experiments and analysis to characterize the strength of deep reactive ion etched single crystal silicon structures and to clarify the influence of geometry on a structure's characteristic strength. The test specimens used for this work were pressurized cavities with the same geometry as the micro rocket combustion chamber and radiused hub flexure specimens. These geometries were correlated with numerical models and finite element models for determination of failure stress levels. Applying Weibull statistics, the strength of the material and the effect of the specimen geometry were quantified. Scanning electron microscope inspection of the etched surfaces provided visual evidence of surface roughness conditions, supporting the experimental results. Pressure tests were used to identify failure modes of the micro rocket geometry without the complex subsystem geometries required for the operational device. A slight variation in geometric configuration of the chamber pressure ports yielded significant differences in device strength. Radiused hub flexure specimens were used to compare the strength of the micro rocket etch recipe, ADAM06, to a baseline etch, MIT69. Additionally, the radiused hub flexure specimens were used to characterize the role of a secondary isotropic smoothing etch in improving the effective material strength of deep etched surfaces. It was determined that the micro rocket etch was not optimized sufficiently to achieve baseline surface roughness. The role of the secondary isotropic etch was determined to be key in achieving high strengths in etched single crystal silicon. The experimental data was used to establish a scaling correlation for strength values from the two different specimen geometries. Using the effective areas of the two structures, characteristic strength for chambers was predicted based on the characteristic strength of radiused hub flexure specimens. The predicted scaling did not correlate particularly well with the data. However, the limited number of samples and a modeling inaccuracy are suspected to have significant influence on the quality of the prediction. Improvement of these conditions could yet yield a useful predictive tool. The results of this thesis are demonstration of the influence of specimen geometry and surface roughness on the characteristic strength of deep etched single crystal silicon structures. Additionally, the predictive scaling between different specimen geometries was attempted with marginal results that might be improved with further testing. Recommendations for future work include further investigation of the effects of slight variations in specimen geometry in fabrication and material strength, characterization of the smoothing effects of isotropic etching with time, reevaluation of the optimization of the micro rocket etch recipe, and further study of the nature of flaws introduced by primary deep etches and their behavior as surface or volume flaws.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Aeronautics and Astronautics.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2002

Author dc:creator

• Noonan, Erin E. (Erin Elizabeth), 1978-

Advisor dc:contributor.advisor

• S. Mark Spearing.

• Aeronautics and Astronautics.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/8127

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/8127