High temperature extrusion behavior of fine-grain ZrO 2
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All extrusion experiments are performed with high purity ZrO2 powder partially stabilized by 3 mole% Y2O3. The Y-TZP powder is tetragonal with an extremely fine crystallite size 0.25 /tm after hot pressing at 1500 °C. The high temperature properties of this alloy are described by the equation: = Cane,
where Cad
6 X p -R?
(1)
address: Institut de Micro et Opto-electroniques, Departement de Physique, EPFL, CH-1015 Lausanne, Switzerland.
where ke is the equivalent strain rate, ae is the equivalent stress (see appendix), d is grain size, and RX has its usual meaning. The grain size exponent/? of this alloy is approximately 2 and the apparent activation energy Q is approximately 600 KJ/mole.6 The stress exponent n varies with grain size, decreasing from a value of about 3 for d =£ 0.4 /xm, to n = 2 for d ~ 0.8 jam and n = 1.6 for d = 1.5 /i-m. Such apparent grain size effects on n are not fully understood but could be due to differences in composition of the grain boundary glassy phase and its redistribution during grain growth. The stress exponent also appears to change with stress. The creep properties of these alloys are indeed complex and poorly understood at the present time. Creep properties (listed in Table I) are therefore measured in compression in a strain rate regime equivalent to that experienced during the extrusion experiments (ee < 0.05 s~\ as calculated by Eq. (A8) of the appendix). Creep properties are measured in vacuum at 1500 °C and under low axial stress ( = 35 MPa, D = 2, = 26.6°). Time (min)
C (MPa""/s)
35 35 100
1.3 x 10"7 1.3 x 10"7 1.5 x 10,-7
2.8 2.8 2.5
y (mm)
(mm/min)
15 25 25
0.2 0.02a 0.08
a) Current
J. Mater. Res., Vol. 5, No. 10, Oct 1990
http://journals.cambridge.org
Downloaded: 13 Mar 2015
aNo
graphite paper lubrication.
© 1990 Materials Research Society
2165
IP address: 131.170.6.51
B. J. Kellett: High temperature extrusion behavior of fine-grain ZrO 2
Fig. 1). Billets are cleaned to remove remnants of the graphite paper and billets are reinserted with fresh graphite foil on both the cylindrical and conical surface of the die. Displacements of the loading ram (piston displacements) are measured from outside the furnace at constant load and temperature. Details of the procedure have been given elsewhere.4'5
loading piston in graphite (diameter dp)
graphite die
III. EXTRUSION EXPERIMENTS ZrO2 billet
FIG. 1. Schematic of graphite die and ceramic billet.
Pictured in Fig. 2 are two billets extruded through a die with a diameter ratio D = 3 (i.e., piston diameter dp is 30 mm and final diameter df, 10 mm). Graphite foil is an effective lubricant and when folded over the surface of the billet prevents surface tearing at the die exit. The black color of the zirconia billets is an artifact of the reducing conditions of the furnace. Graphite paper also significantly increases piston velocity (vp), as shown in Fig. 3 for extrusion through a die of diameter ratio 2. Figure 3 also shows that piston velocity decreases with time and increases with the applied piston stres
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