Hot-pressing of active magnesia

CERAMURGI,A INTERNATIONAL,

V(~I. 1, n. 2, 1975

81

HOT-PRESSING OF ACTIVE MAGNESIA R. PAMPUCH, H. TOMASZEWSKI and K. HABERKO Unstitute of Materials Science, Academy of Mining and Metallurgy, Cracow, Poland

Isothermal hot-pressing of powders of active MgO formed by thermal decomposition of Mg{OH}2 at temperatures of 500-700°C and pressures of 2000-2800 kgs/cm 2 yields transparent polycrystalline MgO having densities close to theoretical. A detailed characterization of the MgO powders before, during, and after hot-pressing and by an analysis of creep kinetics allowed to indicate the probable dominant mechanisms leading to densification. In systems consisting of crystallites having a strained lattice, densification during hot-pressing at 500°C appears to be due to a point-defect diffusion mechanism; this is possible at such low temperatures because vacancies become mobile in ionic crystals at 0.25 Tin, i.e. at 770°K {500°C} in MgO, and their concentration is considerably increased in strained {stressed} crystallites in comparison with unstrained crystallites. Such a mechanism does not appear to occur in systems consisting of unstrained crystallites, either on hot-pressing at 500°C or at 700°C where crystall'ite boundary sliding by dislocation glide/climb is more probable. A rearrangement ofcrystallites due to crystallite boundary sliding, by dislocation glide/climb, as well as the lubricating action of traces of water may also contribute to the de nsification of the systems studied. Fragmentation does not contribute to the enhanced densification at the low temperatures of hot-pressing utilized in the present work.

1 - INTRODUCTION

As is well known, the hot-pressing technique allows one to obtain polycrystalline materials with densities close to the theoretical and controlled microstructures. In the usual technique well-annealed ceramic powders are pressed at temperatures ranging from 1500 to 2000°C ~,2 where extremely severe requirements are placed on the die and plunger materials. By contrast, hot-pressing of materials which undergo a phase transformation or thermal decomposition while being pressed {reactive hot-pressing} allows the use of much lower temperatures 3~. The theoretical explanation of the enhanced densification at such lower temperatures is far from being complete and consistent. It has been suggested in previous research 7-14 that

the enhanced densification during reactive hot-pressing at temperatures as low as 400-700°C may be attributed to a combination of several superimposed factors such as: vapour-phase lubrication, grain fragmentation during decomposition, enhanced deformability of materials during polymorphic phase transformations, and enhanced neck growth between freshly-formed fine particles. In the few studies aimed at a quantitative analysis of reactive hot-pressing 7-9 it has been suggested that the rate determining step of both reactive hot-pressing of thermally decomposing hydroxides and the thermal decomposition of these compounds without application of pressure are the same. However, it was suggested that grain fragmentation occurring during thermal decomposition may significantly alter the defor-

82

R. PAM.PUCH, H. TOMASZEWSKI, K. HABERKO

mability of particles during application of an external pressure and thus enhance densification. It has been found by the present investigators that the practical application of reactive hot-pressing of hydroxides presents great difficulties due to a build-up of water vapour pressure inside the die. By contrast, transparent magnesia polycrystals of theoretical density can be obtained at temperatures as low as 500-700oC when highly active MgO powders, freshlyformed by the decomposition of MgCOH)2, are pressed, instead of magnesium hydroxide (see. Fig. 1). i

I

i

i

I

I

i

I

9O 8O 70

m

50

co 4o z ec I-.-

30

20 10 i

400

I

500

I

I

600

I

I

I

700

WAVE-LENGTH

I

800

I

I

900

I

l

1000

I-Nm]

FIGURE 1 - Light transmission spectrum of hot-pressed magnesia. Tp ---- T, = 500°C; PA ~- 2000 kgs/cm2; thickness of the specimen1 mm.

The behaviour of the active magnesia powders during hot-pressing can be analysed more easily than hotpressing phenomena accompanied by the evolution of gaseous products of thermal decomposition. In addition, the main structural and textural features believed to cause enhanced densification during reactive hotpressing, such as lattice strains, grain fragmentation, etc., are still comparable to those occurring during the first stages of decomposition during reactive hotpressing. This comparability is due to the fact that the escape of the bulk of the gaseous products under pressureless conditions and at low temperatures is a faster process than the annealing of structural changes in the solid state. The behaviour of active magnesia powders during hotpressing is the subject of the present work. The study of the densification process was also accompanied by a characterization of the magnesia powders.

slightly broadened in its upper part to prevent stress generation in the cooled sample after hot-pressing. Plungers were made from the same material as the die. The diametrical clearance between the die and the plungers was 0.005 mm. The plungers were lubricated with colloidal graphite in order to avoid seizure during pressing and sticking of the sample to the plungers. Temperatures were measured by a Pt-Pt 10%Rh thermocouple inserted into the die within 4 mm of the inner wall. The usual procedure of hot-pressing consisted first of a cold-pressing at 600 kgs/cm 2 of 1 g of the Reachim Mg (OH)2. The original height of the pellet was measured while in position in the cold die. The pellets were then heated under atmospheric pressure in air with a 10°C/rain rate of temperature rise up to the selected temperature level (500-700°C), further referred to as the , soaking temperature Ts-. After reaching the particular temperature level of Ts the selected pressure (600-2800 kgs/cm 2) was immediately applied and hot-pressing conducted at the same temperature as the soaking temperature. In some experiments, after reaching the given soaking temperature, the system was quickly cooled down to a constant temperature of 500°C or 600°C and pressure immediately applied. In order .to distinguish the two series of experiments, the temperature at which the hot-pressing was performed will be referred to as the ,, hot-pressing temperature, Tp ,. In many cases, T, and T, were equal. The duration of isothermal hot-pressing varied in different runs from 1 to 30 minutes. During each run, changes in the height of the pellet were continuously monitored, with an accuracy of ___ 0.0025 mm, by measuring the movement of the hydraulic ram with a dial gauge. A correction was made for the thermal expansion of the die assembly and bolsters. The therreal expansion of the pellet was neglected because of its small thickness (0.5 to 0.7 ram). After the completion of hot-pressing at a given temperature and period of time, the compact was moved into the upper part of the die and cooled down with the furnace. Several pellets were prepared for each hot-pressing cycle. Each pellet was weighted on an analytical balance and its dimensions measured in order to determine its apparent density. After the hot-pressing cycle the pellets were characterized by X-ray phase analysis, as well as by X-ray determination of lattice strains, crystallite size, and texture coefficient {defined below}. In order to assess the state of the system before pressing (as well as for density determination), the pellets, in some experiments, were simply heated up

J

r

7

2 - EXPERIMENTAL PROCEDURE

The starting material was a pure-grade magnesium hydroxide, Reachim {USSR) where the primary impurities, as shown by spectrographic analysis, were Fe < 0.05% and Si < 0.02% {based on MgO). Microscopically determined grain sizes of the Mg(OH)2 ranged from 1 to 10 pm. The average crystallite size, estimated from X-ray line broadening, was 194±20 ~, and 424__+40 • in the [0001] and [1120] directions, respectively. The hot-pressing apparatus is shown in Fig. 2. The main part of the apparatus consists of a cylindrical die made of sintered carbide (65% WC, 10% TiC, 25% Co, Huta Baildon, Katowice, Poland}. The outer diameter of the die was equal to 40 mm, while the inner one was - 18 mm. The inner diameter was

~3

3 8

I FIGURE 2 - Press and die assembly (diagrammatic) - 1. Die, 2. Plungers, 3. Nimonic bolsters, 4. Pt-PtRh therrno-couple, 5. Dial gauge, 6. Split electric furnace with kanthal winding, 7. Screw

piston, 8. Hydraulic jack.

HOT-PRESSING

TABLE

I

-

83

OF ACTIVE MAGNESIA

Material constants used to solve steady-state creep rate equations for dislocation glide and diffusional flow me-

chanisms. Constant

Value

1. Atomic volume, ~, [cm 3] *

Notes and references

2.1952 • 10--23

~ = 8 r ~, r=l.40A, radius of O2-- ion.

,--)

a

2.4 10-~

2. Burgers' vector, b [cm]

-- $ [I[0],

•

[28]

2

3. Shear modu,lus, G [dynes/cm 2] 4. CrystalHte size, d [cm] ** 5. Dorn parameter, A

1.5 • 10'2 2.5. 10-6[Tp=T,=500°C) 3.3. 10-6(Tp=T,=600°C) 4.8. 10~(Tp=L= 70O°C) 1.995

[28] Measured from X-ray profile broadening (see Table [I] Calculated from Stocker and Ashby [29] equation for n=3

* The values for the slower diffusiqng ion were utilized. ** Average values of the erystallite size duri.ng the period of steady-state creep, whi,ch are 5ifferent wi~h systems obtained by heating up to different temperatures T,.

to the selected temperature and then removed from the die for X-ray analysis. In order to determine lattice strains *, crystallite size, and texture coefficient, X-ray profile broadening was analysed using the procedure described by Pampuch and Librant is, with corrections proposed by Wagner 16, ShoeningtT, and Halder and Wagner Zt CUK, radiation was used to determine the (111), (200) and [400) diffraction profiles of MgO. The X-ray diffraction traces were taken from the faces of the pellets oriented perpendicularly to the direction of applied pressure. The crystallite size and lattice strains in the < 100 > directions were determined from the (200) and [400) profiles. The texture coefficient was determined from the ratio of the intensities of the (111) and [200) reflections. The theoretical value of this ratio for randomly oriented crystallites is equal to 1:10 (0.1); deviations from this value can be taken as a measure of deviation from random orientation of the crystallites. The densification curves obtained during hot-pressing were analysed by the procedure described by McDonough and Spriggs~9, according to which the isothermal steady-state creep rate ~ can be written in the general form: ~ = BP, ~

tions for different creep mechanisms, it is possible to delineate several fields in a system of coordinates: effective stress ~e, - temperature, T. A given creep mechanism dominates in each field due to the fact that steady-state creep rate associated with the particular mechanism is higher than for any other one. The Ashby procedure was used by us to estimate the boundary between fields of dislocation glide and diffusional creep [i.e., Nabarro-Herring creep) only over the temperature range of 500 to 700°C. The lack of adequate data makes a further subdivision of the diffusional creep fields not possible. The data used to solve the appropriate rate equations are compiled in Table I.

0.002

0.001

Ts =

500 C

Tp =

500 'C

200

PA = 1000 kgs/cm ~ I

0.003

I

I

I00

~

'

300

O ooL

< 0.002

Tp =

%, o

200

500;C

< 0.001

108

I 0.003

I

J

J

-+--+

.....

0.002 l ~

0.001

I 5

I 10

--~ _ 300

Ts =

500"C

Tp =

500'C

I 200

PA = 2800 kgs/cm

Ad//[ j

I 15 T I M E [min]

L 20

~

J~

D[,oo] _ _ _ ~ . . . . . . . . . .

L * Lattice strains are defined as Adhkt/dhki, where Adhkl is the deviation of the average i,nte.r,plana,rsl~acing from the equil.ibrium value of d~kl.

DD°°iG

I

[1]

where P, is the applied pressure, B and n are constants. This equation may be used for a given material when no changes of the grain (crystallite) size, d, occur and a single mechanism is operative during isothermal steady-state creep. From log-log plots of ~ vs. P, the value of n = (a In ~/a In ~)d;~ was determined. As well known from numerous previous studies each creep mechanism exhibits a specific value for the constant n. For point-defect diffusion mechanisms, corresponding to Newtonian flow, n = 1 while n > 1 implies non-Newtonian flow, e.g., one of the mechanisms in which dislocations are involved. Under the experimental conditions of the present work, a steadystate creep rate was established after three to ten minutes from the start of the hot-pressing run. Since the crystallite size was fairly constant after this time with all MgO systems studied (see Figs. 3-5) the values of n derived on the basis of eqn. [1] are expected to be realistic. According to Ashby m, by equating pairs of rate equa-

300

0.003

100

30

3 - Lattice strains and crystallite size vs. time of hotpressing under indicated conditions. FIGURE

84

R. P A M P U C H ,

D[ioo] ~ ~---~

H.

TOMASZEWSKI,

HABERKO

K.

- 400

600

.D[lOO] ...

30O

T. . . . . .

--i . . . . . . .

0 002

0.001

Ts =

600 C

Tp =

600°C

400 200 Ts = 100

PA = 1000 k g s / c m ~

&did=0

7 0 0 °C

Tp = 700°C PA =

i .

05 0.002 Z

.

.

.

.

.

.

.

.

.

lk_.°,,°°i

0.001 /d

.

.

i

200 ~

I

I

,< 300

r~

Z4.

Ts =

600°C

200 ~

Tp =

600°C

-a 100 ~<

i

kgs/cm

400

.

PA = 2000 k g s / c m ~

I

I

1000

I

__ D,lOO]

.....

+

. . . . . .

600

-~

a 400

&d/d=O

Ts - -

700%

Tp =

700°C

PA - - 2 0 0 0 4OO

I

I

200

uJ" N

<

kgs/cm;

I

300 0.002

0.001

Ts =

600 °C

Tp ~

000 'C

200

PA = 2800 k g s / c m ~

5

110

AlOOL__

600

.......

. . . . . .

100

400

L 15

0

3O

Ad/d=0

Ts =

7 0 0 °C

me

7 0 0 °C

200

T I M E [min.] PA - - 2 8 0 0

FIGURE 4 - Lattice strains end crysteilite size vs. time of hot. pressing under indicated conditions.

I

I

I

I

5

10

2O

30

T IM E

3 - RESULTS AND DISCUSSION X-ray phase analysis of MgO powders heated without application of external pressure up to 500, 600, and 700°C with a rate of temperature rise of 10°C/rain proved that this processing results in a complete decomposition of Mg (OH), with all MgO systems studied. However, TGA curves did show a continuous loss of weight up to 1000°C due to the delayed evaporation of water, which was formed on decomposition and retained in the sample i,n amounts of about 0.5 weight percent. Examination of IR-spectra confirmed the absence of Mg {OH]= and of isolated OH groups in the MgO systems studied. The pellets subjected to hot-pressing were thus composed of MgO containing minor amonuts of retained water, which varied slightly from sample to sample. In Table II are collected the crystallite sizes, lattice microstrains, and texture coefficients measured in the cold-pressed Mg [OH)~ pellets and after their pressureless heating to 500-700°C. In addition to determined

kgs/cm 2

[mini

FIGURE 5 - Lattice strains and crystalllte size vs. time of hot. pressing under indicated conditions.

d<,oo> crystallite dimensions, the d and d<,0> dimensions have been calculated, assuming that the MgO crystallites obtained by decomposition of Mg (OH)2 are cubic, as is usually observed. These calculations were made to facilitate comparison, since Mg (OH)2 decomposes topotaxially on heating to MgO in such a way that the following directions remain parallel: [0001],g(o.), II [111],~o and [11~0]M,~o.)= II < 110 >M~. it can be seen from Table II that in comparison with the original Mg(OH)2 crystallites, the dimensions of the MgO crystallites are two times larger, on the average, in the direction normal to the close-packed anionic layers [0001] in Mg(OH], and < 1 1 1 > in MgO, re, spectively} and decrease by about one-fourth in directions lying in these planes [11~0] and < 1 1 0 > respectively). Hence, the volume of crystallites remained practically constant after decomposition under pre-

TABLE II - Characteristics of initial Mg(OH)2 and of MgO derived from the hydroxide by heating up to different temperatures (T,}, before hot-pressing. Material

T, [°C]

Crystallite size [A]

Lattice microstrains A d/d

Texture coefficient

Mg [OH]=

--

194+--20 [0001] 424+-40 [11~0]

0 0

MgO

500

190+-20 <100> 330+-3O <111> 270+30 <110>

0.0015+-0.0007 <100>

0.09

600

203 + 20 < 100> 351+-30 <111> 287+-30 <110>

0.0015-1-0.0007 < 100>

0.09

700

242___24 <100> 419___40 <111> 342+-30 <110>

0.0008+-0.0004 <100>

0.10

HOT-PRESSING OF ACTIVE MAGNESIA

sureless conditions. Furthermore, as shown in Figs. 3-5, when pressures of 1000-2800 kgs/cm 2 are applied on hot-pressing for different periods of time up to 30 minutes, the MgO crystallite dimensions do not diminish but actually increase slightly, especially during the first three to five minutes of hot-pressing, and then remain fairly constant. According to Pampuch, Librant, and Piekarczyk 2~, the grains and crystallites of MgO obtained by thermal decomposition of Mg[OH)2 in air are pseudomorphs of the initial hydroxide grains and crystallites and no fragmentation of the grains and crystallites is observed after decomposition. The above evidence thus suggests strongly that grain (and crystallite) fragmentation, believed to be a possible source of enhanced densification during reactive hot-pressing, does not occur with the MgO systems studied here. The same conclusion also seems to apply to the reactive hot-pressing of Mg[OHh, since by applying a pressure of 60 kgs/cm 2 during the decomposition of the hydroxide, the < 1 1 1 > dimension of the MgO crystallites increased about two times in comparison with the original Mg[OH)~ crystallites in the [0001] direction. Hence, in further discussion, attention shall be paid to another factor which we found to be important for the enhancement of densification at low temperatures, namely lattice microstrains, it has been suggested that these strains develop in active MgO dispersed systems studied due to the formation and growth of MgO from coherent nuclei in the MgCOH)~ matrix during decomposition ~5, Retention of gaseous products of decomposition in the lattice is regarded by the authors as a less probable cause of lattice microstrains, since MgO obtained by the decomposition of the carbonate and oxalate does not show any lattice strains 2~, although it retains similar amounts of water as the system studied here up to high temperatures. it can be seen from Figs. 3-5 that the lattice microstrains are considerable in the hydroxide-derived MgO obtained by heating up to 500"C, and do not completely disappear even after 30 minutes of hot-pressing at pressures up to 2800 kgs/cm 2. By contrast, with the MgO obtained by heating up to 700°C, no lattice microstrains are detectable after only 0.25 minutes of application of pressure. The microstrains in MgO obtained by heating up to 600°C have intermediate values and disappear completely after 15 minutes of hotpressing at a pressure of 1000 kgs/cm 2 of after 3 minutes of hot-pressing at a pressure of 2800 kgs/cm 2. Hence, in the case of MgO systems obtained by heating up to 500 and 700°C, we are dealing with the hot-pressing of systems of crystallites having a strained and an unstrained lattice, respectively. The crystallite size initially differs in both cases by only about 20 per cent [see Table II). In order to eliminate the possible influence of hot-pressing temperature on the results, the behaviour of the former and the latter system has been compared at the same hot-pressing temperature of 500°C [Fig. 6). It is seen in Fig. 6 that, under identical hot-pressing conditions, the system having unstrained crystallites IT, = 700°C) attains much lower densities than the system having strained crystallites [T, = 500°C). Since the crystallite size is comparable in both systems, both initially [see Table II} and after hot-pressing, namely, about 300 and 320 ~1, after 30 minutes of hot-pressing at 2800 kgs/cm 2 in the former and latter system, respectively, this difference in density may be taken as a measure of the intensification of mass transport due to lattice microstrains. The elastic strain energy due to lattice strains can enhance both dislocation glide and. diffusional processes. More probable, however, is an enhancement of diffusional processes, since pure dislocation glide is

85

I

I

1

I

E 3.0 o >Z LU

tm 2.0 LM £C

< <

1.0

~ ]

i

Tp ~ 500 C ::

I.

.

; Tp

.

.

1000 2000 APPLIED PRESSURE [ k g s / c m ]

.

500 C i I

~_ 3000

FIGURE 6 - Apparent density vs. applied pressure after 30 m i n . of isothermal hot-pressing at 5OO°C of two starting MgO materials obtained at soaking temperatures (T,}, 500 and 700°C, respectively.

very improbable in polycrystalline MgO, especially at the very small crystallite size of the systems studied. (see also later discussion). This conclusion is substantiated by the analysis of the steady-state creep rates. In Table Ill are collected values of the coefficient n, calculated by means of eqn. [1], with indication of the + confidence intervals at a 95% confidence level. By applying Student's t test, it has been ascertained that the average value of n is not significantly different from n - - 1 even at a 99% confidence level for MgO systems having strained crystallites ITs = Tp = 500°C}. in the absence of a liquid phase n = 1 corresponds to several point-defect diffusion mechanisms, i.e., diffusion through the bulk of the lattice and around grain [crystallitel boundary ledges as well as along boundaries 23-26 The temperature at which the systems probably undergo diffusional creep during hot-pressing corresponds to the temperature of 0.25 Tm [Tin - - absolute melting temperature) at which temperature vacancies become mobile in ionic crystals. Hence, diffusional creep is possible here at a non-negligible concentration of vacancies. Such does not appear to be the case with MgO systems having unstrained crystallites, where the equilibrium concentration of vacancies at 500°C and 700°C, respectively, should be of order of 10 -13 - - 10 -12 at an energy of formation of Schottky defects of 4 eV per vacancy pair. In strained and, consequently, stressed MgO crystallites, the equilibrium concentration of vacancies should be considerably greater, according to [V[~}] = IV[o-)) exp [ ~ / K T )

[2]

where [V[~}] and [V[~)] denote the equilibrium vacancy concentration in stressed and unstressed crystallites, respectively; ~ - the stress; £ - the atomic volume; k - t h e Boltzmann constant; T-absolute temperature. If it is assumed that it is only the near-boundary layers [10 A thick), of the MgO crystallites that undergo an expansion in order to attain interatomic distances which occur in the parent Mg[OH)2 from which the MgO domains grow on decomposition, one obtains, by inserting appropriate data compatible with the average microstrains of 0.0015, into eqn [2], that [V(cr)] in the near-boundary layers should be of the order of 10 ``9, i.e. of the order of magnitude of the equilibrium vacancy concentration in unstrained [unstressed) crystallites at 1000°C. Hence, point-defect diffusion, such as around boundary ledges, would not appear to be impossible in strained MgO crystallites at 500°C.

86

R. PAMPUCH, H. TOMASZEWSKI, K. HABERKO

TABLE III - Values of steady-state strain rates ~ and of the constant n = (~)ln~/aln¢) d,~ for hot-pressing at different temperatures {Tp) of MgO dispersed systems obtained by heating to different temperatures (T,) before hot-pressing. Applied pressure

Tp = 500°C T, = 500°C

PA[kgs/cm']

~ [sec -~]

600

0.021 ±0.030

1000

0.035--+0.011

2000

0.070+0.007

2800

0.0132+0.015

Tp = 500°C T, = 700°C n

~ [sec -~]

n

-0.063 + 0.020

1.139 +0.486

I

0,028 + 0.003 --

I

LL

I

700 °C

o

o

600°C

~

O 2.0

~

500 °c

0

F<10 i

600

i

i

$

1000 2000 APPLIED PRESSURE[kgs/cm2]

FIGURE 7 - Texture coefficient vs. applied hot-pressing temperatures, Tp ----- T,.

pressure

2.12

0.034±0.004 0.276 + 0.058 1.091 +0.947

o

uJ3.C

~ [sec -t]

Tp = 600°C T, = 600°C n

0.029 + 0 . 0 1 0

In MgO systems w h e r e microstrains are absent (T, = 700°C), no i m p o r t a n t point-defect diffusion appears to occur during hot-pressing at 500 and 600oC, as indicated by the values of n = 2.12 and n = 2.39 {see Table III), These values differ significantly from 1 but do not differ f r o m 2 at a 95% confidence level. Therefore an i m p o r t a n t contribution due to dislocation g l i d e / c l i m b in the lattice also is not expected. The l o w e r boundary of the Ashby-field for the dislocation c l i m b / g l i d e mechanism in the t e m p e r a t u r e range b e t w e e n 500 and 700°C corresponds to a ¢o, = 2.4 • 10 '° d y n e s / c m 2, w h i l e the highest effective stresses ¢off utilized in the present w o r k {hot-pressing at 500-700°C at 2800 k g s / c m 2) w e r e o n l y 3.3. 109 d y n e s / c m 2. In the absence of both noticeable point-defect diffusion and dislocation g l i d e / c l i m b in the lattice, crystallite boundary sliding by dislocation g l i d e / c l i m b in a zone near the boundary and along the boundary remains as a possible d o m i n a n t mechanism in the case w h e r e microstrains are present. Langdon ~ has calculated a value of n = 2 for such a mechanism. Our experimental values of n = 2.12 and 2.39 compare v e r y w e l l w i t h such a value. C r y s t a l l i t e boundary sliding by dislocation g l i d e / c l i m b in a zone near the boundary and along the boundary should be more effective than diffusional creep in bringing about an increased o r i e n t a t i o n of the MgO I

Tp = 600°C T, = 700°C

2800 for different

c r y s t a l l i t e s w i t h t h e i r < 1 1 1 > axis parallel to the pressing direction during hot-pressing, such as is indicated by the increase of the t e x t u r e coefficient Ti {Fig. 7). A t the same applied pressure, e.g. 2800 k g s / c m 2, Ti increases w i t h increase in hot.pressing t e m p e r a t u r e from a value of 0.1, w h i c h corresponds to randomly o r i e n t e d crystallites, to a value of only 0.15 in MgO systems showing diffusional creep {Tp = T, = 500°C), and to 0.30 in systems for w h i c h a boundary sliding by g l i d e / c l i m b has been suggested (Tp = T, = 700°C}. It is p o s s i b l e that in both systems the w a t e r traces remaining in the pellets may e x e r t a lubricating action during hot-pressing, as suggested by Carruthers and Wheat s for the case of alumina gels, and thus con-

E [sec-']

n

0.015±0.022

2.39 -+-1.43

0.072+0.017 0.253+0.092

3.000--51.29

2.394±0.544

tribute to the increasing orientation. The data for MgO systems obtained by heating up to T, = 600oC have not been discussed since in this case a change of condition in the system from a strained to an unstrained state occurs during hot-pressing. Because of this change, the value of n found exper i m e n t a l l y does not have a precise physical meaning. REFERENCES 1. R.M. SPRIGGS, in High Temperature Oxides, Academic Press, New York, 1970, p. 183. 2. R.M. SPRIGGS and T. VASILOS, in Progress in Ceramic Science, J.E. Burke Ed., Pergamon Press, Oxford, London, New York, Paris, 1968, p. 96. 3. A.C.D. CHAKLADER, Nature 24 (1965) 392. 4. P.E.D. MORGAN and E. SCALA, in Sintering and Related Phenomena G.C. Kuczynski ,Ed., (1965), p. 861. 5. T.G. CARRUTHERS an,d T.A. WHEAT, Proc. Brit. Ceram. Soc. 3 (4966) 259. 6. T.A. WHEAT and T.G. CARRUTHERS, i,n Sci,enoe of Ceramic, G.H. Stewart Ed., Academiic Press, London, New York, 1968, p. 33. 7. A.C.D. CHAKLADER and L.G. MCKENZIE, J. Am. Ceram. Soc. 49 (1966) 477. 8. A.C.D. CHAKLADER and R.C. COOK, Ceram. Bull. 47 (1968) 712. 9. P.W. SUNDERLAND and A.C.D. CHAKLADER, Mat. Res. Bull. 2 (1967) 1111. 10. A.C.D. CHAKLADER, Proc. Bri~. Ceram. Soc. 15 (1970) 225. 11. R.G.S. JACQUES and A.C.D. CHAKLADER, Trans. and J. Brit. Ceram. Soc. 70 (,1971) 269. 12. P.W. SUNDER,LAND and A.C.D. CHAKL,ADER, J. Am. Ceram. Soc. 52 (1969) 410. 13. A.C.D. CHAKLADER and G. BEYNON, ibid. 53 (1970) 577. 14. A.C.D. CHAKLADER and V.T. BAKER, Ceram. Bul,l. 44 (1965} 258. 15. R. PAMPUCH a~ndZ. LIBRANT, Zeszyty Naukowe AGH, Krak6w (Scientific Papers, Academy of Mi,ni,ng aed Metallurgy, Cracow) 4 (1968) 39. 16. G.N.J. WAGNER, in Loeal Atomic Arrangemer~t Studh~l by X,ray Diffraction, J.B. Cohan, J.E. Hi~llriard Eds., Sci,ence Publ., New York, 1966. 17. F.R.L. SHOENING, Acta Cryst. 18 (1965) 975. 18. N.C. HALDER and G.N.J. WAGNER, Acta Cryst. 20 (1966) 312. 19. W.J. MCDONOUGH and R.M. SPRIGGS, i~n Si,ntering and Related Phenomena, G,C. Kuczynski Ed., Plenum Press, N e w York, Lond(~n, 1973, 417. 20. M.F. ASHBY, Acta Metallurgica 20 (.1972) 687. 21. R. PAMPUCH, Z. LIBRANT and J. PIEKARCZYK, Ceramic Papers, Polish Academy of Science, Cracow Fili,al 21 (1974) 354, 22. R.S. GORDON and W.D. KING,ERY, J. Am. Oeram. Soc. 50 (1967) 8. 23. F.R.N. NABARRO, in Rpt. of a Conf. ,on Strength of Solids, The Physical Society, London, 1948. 24. C. HERRING, J. Appl. Phys. 21 (1950) 437. 25. R.L. COBLE, ,ibid. 34 (1,963) 1679. 26. M.F. ASHBY, R. RAY and R.C. GIFKINS, Scrip~a Mket. 4 (1970) 737. 27. T.G. LANGDON, P,hH. Mag. 22 (1970) 689. 28. J. KELLY, G.W. GROVES, in Crystalograph and Crystal Defects, Longman, London, 1970, 163. 29. R.L. STOCKER, M.F. ASHBY, Scripta Met. 7 (1973) 115. P~eoeived January 30, 1975; revised copy received /~pri| 28, 1975.