MECHANICAL PROPERTIES OF THIN ALUMINA COATINGS (SOL-GEL)

Porous alumina coatings were applied to dense alumina and stainless steel substrates to investigate the effect of the coating on mechanical properties. An alumina sol was used as the coating material and applied to the substrates. Variations in firing temperature, from 700 to 1000 C, were used to change coating structure and alter physical properties.; Several areas of mechanical property measurement were investigated. A novel stress measurement technique incorporating the use of indentation-induced fracture provided a simple means to evaluate coating stress in the sol-gel coatings as well as sputtered coatings (Al{dollar}sb2{dollar}O{dollar}sb3{dollar}, SiO{dollar}sb2{dollar}, Si{dollar}sb3{dollar}N{dollar}sb4{dollar}, and ZrO{dollar}sb2{dollar}) on glass. Measurements made with this technique gave compressive sol-gel coating stresses on the order of 100-200 MPa and compressive sputtered coatings on the order of 50-100 MPa. Independent substrate deflection experiments for both the sputtered and sol-gel coatings indicated the presence of compressive coating stresses.; Porous sol-gel coatings were shown to act as thermally induced stress barriers to underlying alumina substrates. Protection by the sol-gel barrier (coating) allowed for an increase in the critical temperature, T{dollar}sb{lcub}rm c{rcub}{dollar}, of alumina during post thermal shock bend strength tests. The measured increase in T{dollar}sb{lcub}rm c{rcub}{dollar} is in good agreement with a model proposed in this thesis which relates T{dollar}sb{lcub}rm c{rcub}{dollar} to thermally induced stresses.; Porosity decreased from about 80% in the 800 C coating to approximately 60% in the 1000 C coating, with a large portion of the porosity lying in the two to four nanometer radius range for each coating firing temperature. Coating densification can be related to the measured increase in true coating hardness from approximately 400 kg/mm{dollar}sp2{dollar} for an 800 C coating to about 800 kg/mm{dollar}sp2{dollar} for a 1000 C coating. Higher fired coatings also showed slightly less than lower fired coatings in pin-on-flat wear tests.