MACHINING HIGH PURITY ALUMINA.

The rate of attack by 52 aqueous hydrofluoric acid on commercial grades of high purity alumina 96 and 94 are discussed. The composition and the method of formation used appear to be parameters controlling the rate of attack. All 96 and 94 alumina engineering parts have been rendered friable by hydrofluoric acid treatment, machined and reconstituted. The 96 alumina parts have physical properties which suggest that they have been successfully reconstituted when fired to P.C.E. cone 33 1745C for four hours and were simultaneously transformed into parts of at least 99 alumina or better. Although the 94 alumina has been capable of being rendered friable and machinable it has not been successfully reconstituted. Reconstitution at P.C.E. cone 33 resulted in dense bodies which were warped and distorted. The warping of plates, cylinders and rods was not improved upon by reconstituting under the less severe conditions of P.C.E. cone 31 1680C in addition they had an open pore porosity of about 8 and, hence, would be unsatisfactory when measured against Mil I-10 A. Author