zirconia-yittria_phase_diagram

zirconia-yittria_phase_diagram - Phase analysis of sintered...

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Phase analysis of sintered yttria-zirconia ceramics by x-ray diffraction A. Paterson National Institute for Materials Research, CSIR, P.O. Box 395, Pretoria, South Africa R. Stevens Department of Ceramics, University of Leeds, Leeds LS2 9JT, England (Received 13 November 1985; accepted 18 December 1985) Determination of the amount of cubic and tetragonal phase in yttria-zirconia using high-angle XRD (x-ray diffraction) has been complicated by problems of resolution and interpretation. The evidence, from electron diffraction studies, for a c-^-t shear transformation also needs to be taken into account. Two compositions, a 3 and a 5.7 mol % Y 2 O 3 -ZrO 2 , were sintered and thermally treated at different temperatures between 1450° and 1700 °C. X-ray diffraction traces revealed the anticipated tetragonal (400) and (004) reflections. The region of the diffraction pattern that was thought to arise from the cubic phase could be best interpreted as a second tetragonal phase t'. The original amount of cubic phase computed from the t' reflections showed good agreement with the phase diagram of Scott. The lattice parameters of the t' phase were determined, and the volume of the tetragonal (t ') unit cell was calculated. I. INTRODUCTION Phase analysis of partially stabilized zirconias by x- ray diffraction (XRD) using low-angle (111),.,, (lll) m , and (lll) m reflections has permitted the quantitative determination of monoclinic contents pres- ent in a system.' The assumptions on which the calcula- tions were made have been shown to be invalid, and a detailed analysis of the problem has been made by Evans et al. 2 For the low-angle determinations, a number of alternative theoretical and experimental formulas are now available. 2 ^* High-angle reflections have been used to determine the relative proportions of the cubic and tetragonal phase. 5 The most complete description of high-angle, cubic-tetragonal analysis has been due to Miller et al. 6 in an investigation of plasma-sprayed, yttria-zirconia coatings. The method used was based on the low-angle analysis of Porter and Heuer 7 to derive an equation of the form M c = 0.88- '(400)c Mt -"(400); + •'(004)1 where M c and M, are the mole fractions of the cubic and tetragonal phase, respectively, and the integrated inten- sities / of the various reflections are indicated with the appropriate Miller indices. In that study the cubic re- flection was considered to be relatively composition in- variant, and the angles of the reflections were standard- ized for the (400) c reflection at 73.29°. The XRD patterns were deconvolved and the d spacings of the (400), and (004), reflections calculated. The angular separation of the tetragonal reflections increased with heat treatment as did the integrated intensity of the cu- bic reflection. The interpretation of this information suggested that the as-sprayed yttria-zirconia has a te- tragonal phase of relatively high yttria content present, which transforms to the expected tetragonal and cubic
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zirconia-yittria_phase_diagram - Phase analysis of sintered...

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