The oxides of cerium, yttrium, silicon, magnesium, and calcium were added in various proportions to zirconia of 99-percent purity, and the effects of these additions, combined with preheating at various temperatures ranging from 1,450° to 1,950° C, on thermal length changes of the zirconia during heating and cooling between room temperature and a maximum of 1,700° C were observed. The results are compared with similar observations on zirconia of 96-, of 98-, and of 99-percent purity without oxide additions.The results show that the irregular thermal lcngth changes accompanying phase transformations in zirconia may be prevented by changing the crystal to the stable cubic form. This was accomplished by (a) 11.5-and IS-percent additions of Y20a and heating at 1,700 0 C or higher; (b) 8-and 15-percent additions of MgO and heating at 1,550 0 C or higher, but only for the range from room temperature to 1,200° C; and (c) 5-, 6-, 8-, and IS-percent additions of CaO and heating at 1,550° C or higher, but additions of more than 6 percent caused the specimens to be very porous and proportionately weak. In a ll cases, however, the expansion during heating and contraction during cooling was relatively high. The coefficients of linear thermal expa nsion of the specimens that were all, or nearly all, cubic ranged from 8.8 to 11.8 X 10-6 • Also, zirconia has a low thermal conductivity relative to such materials of high thermal expansion as alumina, magnesia, and beryllia (unpublished data).

Barium oxide (BaO).-Introduced as reagent quality BaCOa. Beryllia (BeO).-(1) Commercial "SP grade," usually used in making the porcelain specimens, and nominally of 99.7-percent purity. Spectrograms showed only traces of copper, iron and magnesium, and very weak lines for silicon.(2) Commercial "CP grade," usually used in specimens for fusionpoint and softening-range determinations, nominally of over 99.9-percent purity.Calcium oxide (CaO).-Introduced as reagent quality CaCOa. Magnesium oxide (MgO).-(l) Fused MgO (periclase) of approximately 97-percent purity, usually used in making porcelain specimens.(2) Calcined MgCOa (calcining temperature given by producer as 2,000° F). Very fine grained, purity about 99 percent, and used in a relatively small number of specimens.(3) Especially selected, water clear, crystals of periclase,2 which were hand ground in agate or mullite mortars and usually used in bodies for fusion-point and softening-range tests. The purity is nominally over 99 percent.Silica (Si02).-Pulverized natural quartz of 99.6-percent purity. Strontia (SrO).-Introduced as reagent quality SrC03• Titania (Ti02).-Trade designation "pigment grade," purity about 99 percent, with Fe20a given by the producer as less than 0.2 percent.Thoria (Th02).-Calcined thoria, nominally of over 99-percent purity. Spectrograms showed only very weak lines for calcium, copper, and silicon, and no other impurities were indicated.Zirconia (Zr02).-A CP grade of calcined zirconia, and recalcined at 1,440° C. The spectrogram showed medium lines for columbium and titanium and only very weak lines or traces for calcium, copper, iron, magnesium, lead, and silicon.In order to obtain the desired fineness of particle size, it was necessary to grind the SP BeO, the commercial grade AI20 a, and the Th02• The BeO described above had been ground wet for 2 hours in porcelain-lined mills with flint pebbles. The A120 a and the Th02 had been ground for 48 hr. in a steel mill ",ith steel rods, and then purified by treatment with diluted HOI until the wash water gave a negative test for iron. 3 III. METHODSSpecimens were prepared during the first stages of the work by hand-mixing the ingredients in slip form, drying, pulverizing in an agate or in a mullite mortar, remL'{ing with a 5-percent solution of soluble starch, and pressing in hardened steel moulds. About 14 g of the starch solution was used for 100 g of dry batch, and the forming pressure was 9,300 Ib/in 2 • Later, the method was improved by machine-blunging the slip with a chemical-Iaboratory-type stirrer, and by vibrating the dampened mixture in the steel moulds before pressing. The pressed specimens were matured in an electric furnace, heated with eight Th02-Y20a resistors, described in Bureau Research Paper RP1662 [12]. The heating and cooling rates were maintained very close to 3°0 per min, and the holding time at the maximum temperature usually was 1 hr. A typical setup with three specimens , Furnisbed by tbo Vitrefrax Corporation and by the Norton Co .• Comparative tests were made ...

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Stabilized zirconias exhibit pronounced photoelastic effects when viewed with transmitted polarized light. Thin sections reveal stress concentrations around pores and inclusions and at grain boundaries. The yield point of these materials, at some high temperature, is exceeded and plastic deformation tends to relieve these stress concentrations. The microhardness measurements of magnesia‐stabilized zirconia indicate both solution hardening and solute segregation and help to explain the physical behavior of this system as the amount of MgO is varied.