Effects of Applied Pressure on Densification During

Sintering in the Presence of a Liquid Phase by W. D. KINGERY, J. M. WOULBROUN, and F. R. CHARVAT

Ceramics Division, Department of Metallurgy, Massachusetts Institute of Technology, Cambridge, Massachusetts and Technology Department, Union Carbide Metals Company, Niagara Falls, New York

Experimental measurements of the effects of an applied pressure on sintering of powdered ma- terials containing a liquid phase indicate that the applied pressure can b e effective b y : ( a ) in- creasing the extent a n d rate of particle rearrange- ment, ( b ) increasing the rate of solution at particle contacts, a n d (c) causing plastic flow within the solid particles. Which of these processes pre- dominates depends o n the characteristics of each particular system a n d on the level of applied

pressure.

1. Introduction URING the sintering of crystalline materials, densifica-

tion occurs by diffusional processes under the influence D of surface energy as a driving force. The rate at which this process can take place is limited by the diffusion coefficients. To achieve high final densities, extremely small particle size starting material and high sintering temperatures are required. Two general approaches have been taken to form dense products more effectively. One is to adjust the composition so that a small amount of liquid phase is present at the sintering temperature; a second approach is the application of pressure at the sintering temperature

The effectiveness of a liquid phase in increasing the sinter- ing rate depends on the introduction of new densification processes. When a liquid is added which (a ) is present in sufficient amounts at the sintering temperature, ( b ) com- pletely wets and penetrates between the solid particles, and

Presented at the Sixty-Fourth Annual Meeting, The American Ceramic Socicty, New York, N. Y., April 30, 1982 (Symposium on Kinetics of Ceramic Reactions, No. 6-2s-62). Received April 9, 1962; revised copy received March 28, 1963.

A portion oE this work was donc at the Massachusetts Institute of Technology wi th the support of the United States Atomic En- ergy Commission under Contract No. AT(30-1)-2574.

Part of this paper was taken from a thesis submitted by J. M. Woulbroun in partial fulfillment of the requirements for the Master of Sciciicc degree in Ceramics, Massachusetts Institute of Tcchnologp.

The writers are, respectively, professor of ceraniics and graduate studcnt, Ceramics Division, Department of Metallurgy, Massachusctts Institute of Technology, and section manager of ceramics, Technology Department, Union Carbide Metals Com- Danv. Tlivision of Union Carbide Cormration. . ~,

( a ) G. C. Kuczynski, Self-Diffusion in Sintering of Metallic Particles, J . Metals, 1 [a]; Trans. A I M E , 185 [2] 169-78 (1949).

( b ) W. D. Kingery and M. Berg, Study of the Initial Stages of Sintering Solids by Viscous Flow, Evaporation-Condensation, and Self-Diffusion, J . Appl. Ihys., 26 [lo] 1205-12 (1955); Ceram. Abstv., 1956, February, p. 45c.

(c) J . E. Burke, Role of Grain Boundaries in Sintcring, J. Am. Ceram. Soc., 40 [3] 80-85 (1957).

( d ) R. L. Coble, lnitial Sintering of Alumina and Hematite, J. Am. Cevam. Soc., 41 [2] 55-62 (1958).

( e ) R. L. Coble, Sintering Crystalline Solids: I, Intermediate and Final State Diffusion Models, J. Appl. Phys., 32 [5] 787-92 (1961); Ceram. Abstr., 1961, November, p. 274s.

(c) is a liquid in which the solid is partly soluble, rapid densification is obtained first by rearrangement of the solid particles under the forces of capillary action by sliding over one another with little friction between them, and second by solution at the contact points and precipitation elsewhere. Theoretical and experimental studies have permitted evalua- tion of the effects of particle size, surface tension, and solu- bility, which are the major factors influencihg the process rate.2*3 Recently, i t has been shown that the requirement of the presence of a liquid phase was overly restrictive; a solid surface (adsorbed) layer was found effective in signifi- cantly increasing the sintering rate i n the tungsten-nickel system.

Since the driving force resulting from liquid surface ten- sion tends t o compress the solid particles, increased solu- bility results at the contact area and the rate of densification during the rearrangement process is

AV/VO t+u (1) Av/vo = fractional volume shrinkage. t = time. 1 + y = exponent near unity.

For spheroidal grains diffusion in the liquid phase still limits the densification rate during the solution-precipitation process and the rate of densification is given by

Al/lo = fractional linear shrinkage. k , and k2 = geometrical constants. 6 = thickness of liquid film between solid grains. D L = diffusion coefficient in liquid. CO = solubility of solid in liquid. Vo = molar volume of dissolving material. yLv = liquid-vapor surface energy. r = initial particle radius. R = gas constant. I = absolute temperature.

These relations have been experimentally verified in a number of systems.

The effect of applied pressure on densification is additive with the pressure derived from the capillary forces. An

W. D. Kingery, Densification During Sintering in the Pres- ence of a Liquid Phase: I, Theory, J . A p p l . Phys., 30 [3] 301- 306 (1959); Ceram. Abstr., 1960, July, p. 171c.

( a ) W. D. Kingery and M. D. Narasimhan, Densification During Sintering in the Presence of a Liquid Phase: 11, Experi- mental, J . A p p l . Phys., 30 [3] 307-10 (1959); Ceram. Abstr., 1960, July, p. 171d.

(b) W. D. Kingery, E. Niki, and M. I). Narasimhan, Sintering of Oxide and Carbide-Metal Compositions in Presence of a Liquid Phase, J. Am. Ceram. Soc., 44 [1] 29-35 (1961).

J. Wulff, J. H. Brophy, and L. A. Shepard, The Nickel Acti- vated Sintering of Tungsten; p. 113 in Powder Metallurgy- Proceedings of an International Conference, New York, 1960. Edited by Werner Leszynski. Interscience Publishers, New York, 1961. 847 pp.; Ceram. Abstr., 1962, August, p. 203b.

391

392 Journal of The American Ceramic Society-Kingay et al. Vol. 48, No. 8 applied pressure, P, during the solution-precipitation process, can be written

The effects of hot-pressing crystalline powders have been mostly discussed on the basis o plastic flow. Murray, Livcy, and Williams5 observed that densification occurred much more rapidly with applied pressure than in normal sin- tcriiig and interpreted this in terms of a different mechanism than volume diffusion. Based on flow characteristics of a I3ingham solid,6 they arrive a t a rate equation for hot- pressing

(4)

p = relative density. 7 = viscosity

In its integrated form

log (1 - p ) = - k P t + log (1 - po) (4a) where po = relative density at t = 0. This equation has been found to agree with experiments on aluminum oxide, [used silica,8 and powdered metal^.^ In the development ol equation (4) it is assunied that the applied pressure is much largcr than the surface energy compressive force and that the yield point for creep deformation is low. These requirements will be met for the materials and conditions in- vestigated here, but they may not always be true for fine particle size materials and low temperatures.

Feltenlo has reported that the initial part of densification during hot-pressing cannot be adequately represented by equation (4) A complex rearrangement process apparently occurred which required particles sliding over one another and involved concentrated stresses at particle contact points, particle Iragmentation, and other similar mechanisms These processes accounted for a large fraction of the observed total density change, as has also been found For the initial stages of densification in the presence of a liquid. For a particular powder thc fractional shrinkage could be descrihed by a relation of the form

whcrc IZ was a fractional expnncnt varying between 0.1 7 and 0 j X During later stages OF the process, after rearrangc- iiient was assumed completed, the density change followed equation (4).

I n thc present study the writers have investigated thc cffect of applied pressure on the densification rate when a liquid phase is present

atomized copper particle size 40 to 60yo

August 1963

0.50 7

0.20

- 0.10 Q

u

0.05

---_.

O 2 L 0 01 0

DensiJication During Sintering 393

Time (minute)

Fig. 1. Correlation between log ( 1 - p) und time for copper-bismuth compacts hot-pressed a t 60OoC.

Fig. 3. Correlation between log ( 1 - p ) and time for room-temperature sodium chloride-water compacts pressed a t 10,000 psi.

0.20 1 0.0% HZ0 I

Fig. 2. Copper-bismuth compact containing 7.5% liquid at 600C pressed for 4 hours at 1000 psi. (KzCr207 etch, X185.)

stantial amount oC dcformation of the solid has actually occurred.

(2) Sodium Chloride- Water Compacts The experimental studies of the sodium chloride-water

compacts wcrc carried out a t much higher pressures than those used for the copper-bismuth system. At the lowest pressure (1 0,000 psi) the dry sodium chloride samples reached a relative dcnsity of 82% after about 5 hours with almost the entire densification occurring in the first 2 minutes. As shown in Fig, 3, there was increased initial consolidation with increasing amounts of liquid, but the rate of densifica- tion became uniform and independent of liquid content after 5 to 10 minutes. That is, the primary effect of the liquid phase was to increase the initial particle rearrangement and consolidation. For the sodium chloride-water system, an additional feature was that continued densification took place in the presence of a liquid. Liquid films on the sur- face of the sodium chloride particles apparently allowed plastic deformation to occur that was absent in the dry powder. This presumably results from the elimination of an atmosphere-contaminated surface layer described in de- formation experiments on alkali halides by Gorum et 01.~

0.002 1 0 10 20 30 40 50 60 70

Time (minute)

Fig. 4. Correlation between log ( 1 - p ) and time for room-temperature sodium chloride-water compacts pressed at 20,000 psi.

When the pressure was increased to 20,000 psi, similar results were found (Fig. 4) except that complete densification was achieved in a very short time when 10 wt% water was employed. As the pressure was further increased to 40,000 psi, the liquid had much less effect (Fig. 5) and the samples became completely dense and transparent after 2 to 4 hours. Observations of microstructure indicated that a t this pressure there was considerable grain fracture as well as plastic deformation.

In previous studies3 of liquid phase sintering, kinetics corresponding to equation (3) have been observed. For

13 A. E. Gorum, E. R. Parker, and J. A. Pask, Effect of Surfacc Conditions on Room-Temperature Ductility of Ionic Crystals, J . Am. Ceram. SOC., 41 151 161-64 (1958).

394 Journal of The American Ceramic Society-Kingery et al. Vol. 46, No. 8

0 0.0% H20

A 1.0% H20

5.0% HzC

40 (minute)

Fig. 5. Correlation between log ( 1 - p) and time for room-temperature sodium chloride-water compacts pressed a t 40,000 psi.

Time (minute)

Log fractional shrinkage of sodium chloride compacts vs. log time pressed a t 10,000 psi.

Fig. 6.

samples with 5% water pressed at 10,000 psi this relation seems to hold during the first half minute as indicated in Fig. 6. For higher pressures and larger water contents, the effect, if it occurs a t all, is so rapid as not to be observed with the writers experimental technique. Although the data shown in Fig. 6 suggest that solution-precipitation processes may be a contributing factor in the rapid consolidation process, this cannot be regarded as demonstrated.

Time (hour) Fig. 8. Log fractional shrjpkage of ice-methanol compacts plotted vs.

log time.

Empirically it was observed that straight lines were ob- tained when log density was plotted as a function of log time (Fig. 7). The various factors contributing to the early stages of the process make any proposed mechanisms speculative.

(3) Ice-Methanol Compacts Measurements in the ice-alcohol system were carried out

at much lower pressures than those employed for the other systems studied. Under these conditions, the total fractional densification was smaller and the rate was slower. Typical data for samples containing 10 and 20 vol% liquid are illustrated in Fig. 8. These data are similar t o results found for straight liquid phase sintering3 and are characterized by an initial rapid rearrangement followed by a solution-pre- cipitation process, and finally by a much slower solid-state process such as plastic deformation. For the 10% liquid, a plot of the fractional shrinkage vs. log time gives a slope of 1/3 corresponding to equation (3).

In equation (3) taking kl = 1/2, kp = 1, 6 = lo-? cm, D = cm2 sec-l, CO = 0.1, = 26 erg ernw2, P = 3.5 X lo6 dyne emw2, and r = 20p, a rate constant K = 0.0048 can be calculated. Plotting AZ/& vs. t I3 for shrinkage values from 0.8 to 8 hours gives a straight line relation with a slope equal to 0.0016. This is the experimental rate con- stant I

August l!X% A dsorfition of Admixtures on Portland Cement 395 gether. This force is greatest with very fine particles and for a small amount of liquid phase. It is not large for the 10 to 20p particle size used here. More important, it does not lead to shear stresses between particles causing extensive rearrangement. The shear stresses resulting from an ex- ternally applied pressure are, in contrast, most effective in rearranging particles relative to one another.

The level of applied pressure (and resultant shear stress) is critical in determining the extent of rearrangement. It also determines the subsequent mode of deformation. At low values, as in the ice-methanol system, i t leads to solution- precipitation processes similar to those found without externally applied pressure. At higher values, as in the copper-bismuth and NaC1-water systems, it causes plastic deformation not otherwise found.

IV. Conclusion Experimental investigation of the hot-pressing of copper-

bismuth compacts, sodium chloride-water compacts, and ice-methanol compacts in the presence of a liquid phase indicates that applied pressure is most effective in giving rapid densification by the rearrangement of particles relative to one another. Once this process has been completed, densification continues by plastic deformation of individual grains fixed in position relative to their neighbors. In the ice-alcohol system evidence for a solution-precipitation process was also found.

Acknowledgment S. Sarian did the experimental measurements on sodium chlo-

ride pressing at the Metals Research Laboratories, Union Carbide Metals Company.

Adsorption of Admixtures on Portland Cement by B. BLANK, D. R. ROSSINGTON, and 1. A. WEINLAND

State University o f New York College of Ceramics at Alfred University, Alfred, New York

In a n attempt to verify theories on the effect of admixtures on portland cement, the adsorption isotherms for calcium lignosulfonate and sali- cylic acid on cement have been determined. The adsorption isotherms for these compounds on 3Ca0.SiOz,2Ca0.SiOz, 3CaO.AI2O3,and 4Ca0'- Al2O3.Fe2Oa and on a mixture of these compo- nents in the ratio 60/25/5/10 have also been determined. Adsorption isotherms were deter- mined at 24C by a spectrophotometric method, using a Beckman DU spectrophotometer. As a check on the method, previous work of Erns- berger and France on the adsorption of ligno- sulfonates on portland cement was repeated and their results were confirmed. Results indicate that in aqueous solution the constituents of port- land cement mainly responsible for the adsorp- tion of the two compounds studied are the hydra- tion products of 3Ca0.AI2O3 and 4Ca0.AI203.- Fe203. Salicylic acid adsorption on cement com- pounds does not fit the Langmuir form. How- ever, adsorption of salicylic acid on type I portland cement, for equilibrium concentrations above 0.005%, fits the form. Calcium lignosulfonate adsorption on type I portland cement, 3Ca0.Si02, 2Ca0 .SiOz, and partly on 4Ca0.A1203.FePO~ fits the Langmuir form, Adsorption isotherms of salicylic acid on the major cement compounds were determined from ethyl alcohol solution to eliminate the hydration of the compounds oc- curring in aqueous solution. The adsorption of salicylic acid on the materials studied was found to be in the order :

:3C:i0.SiOz > 2CaO.SiO2 > type Iportland cement > 4Ca0.Al2O3.Fee03 = 3CaO.Al208 = 0

The effect of water on the adsorption of salicylic acid from ethyl alcohol solution was studied. It was found that the smaller the amount of water present, above a minimum value, the greater was the adsorption on 3Ca0 .A1203, 2CaO.SiO2, and

3Ca0 .%On. I n the case of 4Ca0 .Al2O3.Pe2Oa the adsorption of salicylic acid was proportional to the amount of water present at all concentrations studied. It is observed in certain cases that it i s probably not surface adsorption but a com-

poun-d formation with salicylic acid.

1. Introduction ORTIAND cement is a product of nonequilibriuni cooling

of a sintered mass in the system CaO-Al203-Si02 with P iron and magnesium impurities. The four major con- stituents are tricalcium silicate (3Ca0. SiOZ), dicalcium sili- cate (2Ca0. SOz), tricalcium aluminate (3Ca0. A1~03), and tetracalcium aluminoferrite (4Ca0 .Alz03. FenO3) ; it is con- ventional to abbreviate these formulas CaS, C2S, C3A, and C4AF, respectively. These compounds are found in ASTM type I portland cement in the following percentages:

C3S c2s C3A C4AF

45-60 20-30 5-10 8-15

Organic materials have long been used to modify the physical and chemical properties of cement pastes. The reduction of

Presented at the Sixty-Fourth Annual Meeting, The American Ceramic Society, New York, N. Y., May 1, 1962 (Basic Science Division, No 7-B-62). Received November 7, 1062; revised copy received March 30,1963.

Based in part on a thesis submitted by B. Blank in partial ful- fillment of the requirements for the degree of Master of Science in ceramic technology, State University of New York College of Ceramics at Alfred University.

At the time this work was done, the writers were, respectively, graduate student, assistant professor of physical chemistry, and associate professor of chemistry, Department of Physical Science, State University of New York College of Ceramics at Alfred University. B. Blank is now a graduate student, Department of Mineral Technology, University of California and research as- sociate for the Lawrence Radiation Laboratory, University of California, Berkeley, California.