Processing of Ceramics-Surface Finishing Studies

The SRI expanded ring test was used to determine the tensile strength of high purity, dense alumina. The test materials were prepared in five different nominal grain sizes ranging from 10 to 50 micrometers. The blanks were diamond ground to final dimensions by a technique developed earlier in the program. It was found that in grinding this material extensive damage occurred which had not been observed in any other alumina body ground under identical conditions. The only difference in strength was found between groups of specimens having grain sizes equal to or smaller than 30 micrometers, and those whose grain size was equal to or larger than 40 micrometers. The vacuum strength of this material was the same as the strength in air regardless of grain size. In all other aluminas investigated previously in this program, the vacuum strengths were considerably higher than the strengths measured in air. Griffiths theory was applied to test the data, assuming that the grain size determined the flaw length, and a good correlation between observed and calculated strength values was obtained for all aluminas tested with the exception of the high purity alumina. In this material the mechanical damage occurring during grinding extended beyond the first grain boundary in the smaller grain size samples, and thus the mechanical damage controls the strength. When the grain size is equal to or larger than the mechanically damaged region, the microstructure controls the strength.