Mechanical and Thermo-Mechanical Protection by Alumina Sol-Gel Coatings
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MARK F. GRUNINGER, JOHN B. WACHTMAN, JR., and RICHARD A. HABER Rutgers University, Center for Ceramics Research, Box 909, Piscataway, N. J., 08854
ABSTRACT Alumina coatings made by the sol-gel process using aluminum sec butoxide can be made with variable surface area and porosity by firing on dense alumina substrates in the firing range 700 to 1000 C. These coatings are found to have large (over 100 MPa) compressive stresses and to cause a small but significant strengthening of the substrates. Within this firing range the surface area decreases while both the internal stress and strengthening effect increase with increasing firing temperature. Wear rate values are high but decrease with increasing firing temperature.
INTRODUCTION Dense coatings have long been used to enhance the properties of materials. Increased wear and thermal shock resistance, greater hardness and strength, as well as resistance to chemical or other environmental attack have caused increased usage of coatings in applications such as engine components, cutting tools, electronic components, and optical components. Porous plasma-sprayed coatings have been used for thermal protection of metals; i.e. to reduce the surface temperature.[1] High porosity is desirable to give good thermal protection but other desirable properties, including mechanical properties, are usually degraded by increased porosity so that
a
tradeoff is
involved.
Optimum material design
requires better knowledge of the dependence of properties on the amount and character of the porosity. At high temperatures photon conductivity becomes important. For this mechanism of conductivity decrease in pore size should give lower conductivity in contrast to phonon conductivity which should be pore size independent for given total pore volume.[2] The microporous character of oxide coatings made by the sol-gel process makes them interesting candidate materials for thermal protection. The basic aspects of preparation of inorganic gels and their use as coatings has been extensively reviewed.[3,4] Catone and Matijevic [5] prepared aluminum hydrous oxide sols and Yoldas [6,7] developed a process for making transparent alumina coatings and studied their processing, structure and selected properties, especially optical properties. Phase changes, sintering, and microstructure development in similar alumina coatings have been studied by several authors [8,9] and a few studies of processing of composite coatings involving alumina have appeared [10,11]. The thermal and mechanical properties of alumina coatings made by the solgel process have not been extensively studied, however. This paper presents results on some physical properties of alumina coatings made by the Yoldas process.
EXPERIMENTAL PROCEDURE In this work the pre-firing stages of preparation were held constant.
Mat, Res. Soc. Symp. Proc Vol. 54.
1986 Materials Research Society
824
Dense commercial polycrystalline 96% alumina electronic substrates were used as a base for the coatings; their dimensions were 1 by 1 by 0.025 inche
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