Ames Laboratory ISC Technical Reports Ames Laboratory

9-1-1953 Stability of zirconium oxide in graphite surroundings D. R. Wilder Iowa State College

E. S. Fitzsimmons Iowa State College

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Recommended Citation Wilder, D. R. and Fitzsimmons, E. S., "Stability of zirconium oxide in graphite surroundings" (1953). Ames Laboratory ISC Technical Reports. 69. http://lib.dr.iastate.edu/ameslab_iscreports/69

This Report is brought to you for free and open access by the Ames Laboratory at Iowa State University Digital Repository. It has been accepted for inclusion in Ames Laboratory ISC Technical Reports by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Stability of zirconium oxide in graphite surroundings

Abstract Zirconium oxide specimens which have been fired at high temperatures in graphite enclosures are known to fail mechanically upon oxidation. A review of the literature failed to completely explain such failure. Possible causes of failure, arising both from internal and external influences are discussed. Experimental investigation established the presence of zirconium carbide in all specimens where failure later occurred. Compressive strengths were measured both before and after oxidation and are discussed. Limitations and recommendations for graphite firing of zirconium oxide are given.

Keywords Ames Laboratory

Disciplines Ceramic Materials | Engineering | Materials Science and Engineering | Metallurgy

This report is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/ameslab_iscreports/69 row0- 5C Ptl­ rsc- J/-03 UNITED J TES ATOMIC ENERGY COMMISSION

ISC-403

STABILITY OF ZIRCONIUM OXIDE IN GRAPHITE SURROUNDINGS

By D. R. Wilder E. S. Fitzsimmons

September 1, 1953 Subject Category, METALLURGY AND CERAMICS.

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STABILITY OF ZIRCONIUM OXIDE IN. GRAPHITE SURROUNDINGS

By

' D. R. Wilder and E. s. Fitzsimmons

ABSTRACT

Zirconium oxide specimens which have been fired at high tempera­ tures in graphite enclosures are known to fail mechanioally upon oxidation. A review of the literature failed to completely explain such failure. Possible causes of failure, arising both from internal and external influences~ are discussed. Experimental investigation established the presence of zirconium carbide in all specimens where failure later occurred. Compressive strengths were measured both before and after oxidation and are discussed. Limitations and recommendations for graphite firing of zirconium oxide are given.

INTRODUCTION

Zirconium oxide, due to its high melting point and other refractory properties, has become a popul ar high temperat ur e f urnace material wit hin the past few years. The cryst a llographic i nver sion which takes place is the only major defect in i t s r efr ac t ory proper­ ties and its effect has been minimized by the use of s t abilizing addi t i ons of other refractory oxides.

While using zirconium oxide 9 it was noted t hat r eaction with graphite destroyed some of its more desirabl e phys i ca l properti e s . After zirconia had been fired in a graphite susceptor i nduct i on furnace and oxidized to remove carbon, complete failure oft en occurred. Considering the lack of information in the lit erat ur e and the otherwise excellent properties of stabilized zirconia » a short study of the cause of such failure was made.

PREVIOUS WORK

A review of the literature failed to yield suf f icient i nforma­ tion to explain the problem. Prescott (1 ) found an equil ibrium pressure of 1 a;,mosphere of CO gas at 1930°K (1657° C) in the reaction 6 ISC-403 between carbon and zirconium oxide. The reaction wafl reported as very slow. George (2) reported reaction at as low a temperature as 1500°C. In direct contradiction to this» Wainer (3,4) was issued two patents pn a method of producing zirconium oxide refractory articles employing a mixture of the oxide and up to 5% of another oxide and silica. The mixture was fired in reducing atmosphere to 2200°C. Jennings (5) also patented a zirconium oxide-silica mixture which was fused in a carbonaceo~s atmosphere at 2200°C. The zirconia was reported to be completely stabilized by this treatment. Kroll and Schlechten (6) found zirconium oxide reacted with carbon at 1300°c under high vacuum and at 1400°C at atmospheric pressure. Zirconium carbide was formed and found to be stable. Schofiel d and co-workers (7) reported loss of strength or complete failure of specimens fired to 3500°F and 3900°F (1930°C and 2150°C) respectively when oxidized at 1800°F (982°C). A carbon resistance furnace with argon atmosphere was employed for the high temperature firing. Whittemore and Marshall (8) described the use of stabilized zirconia insulating grain around graphite tubes. They claim absence of carbide at 2300oc. At a temperature of 2600°C, no loss of lime was noted, indicating no decrease in the stabilizing agent at high temperatures. In a study of hot pressing of refractory oxides» Murray, Rodgers, and Will iams (9) found zirconium oxide could be hot pressed in graphite molds at temperatures of '1200°C to 2000°C. A thin skin was observed on all specimens but presence of carbide was not reported. Extremely rapid heating was employed. Thus the literature search does not predict or explain failure of graphite-fired zirconia. The information is somewhat contradic­ tory and does not indicate any clear solution.

POSSIBLE CAUSES OF FAILURE

There are several possible causes of failure of a refractory material~ some due to external influence and others due to the nature of the refractory itself. It is wise to consider the latter carefully before examining external causes of failure.

Zirconia~ in its pure form» has a monoclinic structure which changes to tetragonal upon heating to approximately 1000°C. Other higher temperature inversions have been reported by Cohn (10) but have not been fully confirmed and accepted. The transformation at 1000°C is accompanied by a volume change which is sufficient to eliminate the possibility of a pure zirconia refractory. ISC-403 7

Fortunately, it has been found that small additions of other oxides, usually lime or magnesia, cause the formation of a predom­ inately cubic structure at about 1700°C. This structure is stable and excellent refractories may be made in this manner. Zirconia with additions of this type is termed stabilized. It has been noted by several workers that a small amount of monoclinic material is desirable as it gives a structure with a lower coefficient of expansion than that obtained by an entirely cubic form. Consider­ able work has appeared in the literature concerning stabilized zirconia, notably a paper on additions by Geller and Yavorsky (11) and a bibliography by Kreidl (12). From the above discussion, it may be concluded that if the stabilizing influence is removed from the cubic zirconia structure, failure would be expected to occur on heating. This would be failure from the action of the material itself, although influenced by external factors. Thermal shock may cause failure of the type observed, i.e. severe spalling. AS stabilized zirconia is reasonably resistant to thermal shock, oxidation had been carried out by placing the carbon fired specimens in a hot furnace. This was a possible cause of failure which was easily eliminated by the use of a low firing rate during oxidation. The formation of another compound whose structure or volatiliza­ tion would be detrimental to the stability of zirconia was next considered. The most obvious of these was zirconium carbide wpich, having a cubic structure, would be expected to be reasonably compatible with cubic zirconia. However, on oxidation, t his carbide might be expected to cause a disturbance in the zirconia structure.

EXPERIMENTAL PROCEDURE

To determine which of the above factors, or all three 9 caused failure, a small graphite induction furnace was constructed. A four inch tube of vitreous silica, forming the outer shell of the furnace, was placed inside a four inch water-cooled induction coiL This coil wa s activated by a 6 Kw Ajax mercury arc high frequency converter. A graphite slug, 2~ inches in diameter a nd six inches in length, was cut and fitted to form a tight enclosure for three specimen bars, each t x t x 3 inches. This slag was packed in the silica cylinder with graphite powder. A small alumina tube was placed on the top of the slug and positioned so it was possible to see into a ~ inch opening drilled into the center of the susceptor. Graphite powder was packed around this tube, insuring reducing conditions all around the zirconia. The furnace temperature was measured with an optical pyrometer sighted on the center opening in the susceptor. This furnace was found to be very satisfactory for operation up to temperatures of 2400°C. 8 ISC-403

The zirconia used for this study was a commercial grade of stabilized material, supplied by the Norton Company, for which production methods have been described elsewhere (8,13). Spectro­ chemical analysis of this material showed the presence of ironp titanium, silicon, and slight amounts of alkalies as well as calcium, the stabilizing agent. This oxide was ball milled in a steel mill revolving at 60 rpm for 28 hoursp then washed with hydrochloric acid to remove the iron. Analysis showed no con­ tamination from the ball mill after washing. The particle size distribution of the ground material, as determined by the hydrometer method , (l4), is given in Figure 1. The curve is the result of two individual analyses.

Specimens, ~ x ! x 3 inches, were made by both slip casting and dry pressing. The slip cast pieces were cast at a pH of 4.0 and at a specific gravity of approximately 3.5. In all cases the slip cast bars were extremely fragile and difficult to handle. It was found that dry pressing at 3,000 to 4,000 psi with 5 to 10% moisture gave excellent specimens with good handling qualities. These were fired to 1200°C in air before firing in the graphite induction furnace. After this prefiring, all mold stains and surface defects were carefully removed with sandpaper so surface effects could be more accurately observed. Compressive strength measurements were made using the fired specimens described. They were cut to approximately one inch lengths and the ends ground parallel. Loading was with a Baldwin hydraulic testing machine at no more than 40 lb./sec.

RESULTS AND DISCUSSION

The physical results observed after firing are given in Table I. The induction furnace was heated at approximately 1900°C/hr. The oxidation firing rate was approximately 100°C/hr. with a one hour soaking period at 700°C. In addition to the tabulated observations it was noted that where the bars were in direct contact with loose graphite, as in the bottom of the holes where they were setp reaction occurred locally at temperatures as low as 1600°C. This was uniform in all specimens, with the amount increasing only slightly with increase in temperature. The results of the compressive strength measurements are given in Figure 2. All specimens failed with a splintering fracture which is characteristic of brittle materials of this typeo The upper curve in Figure 2 shows the ultimate strength of unoxidized speci0 mens. There is an appreciable increase in strength at about 1850 C which reaches a maximum at about 1~5°C. This increase and maximum occurs in the region of decreasing strength of oxidized material. This would indicate a similar cause for both conditions. Such a cause is changed by oxidation at 700°C. ISC-403 9

~ ~ N 0w ..J ..J ::1 ..J ..J C( CD C/) z z 0 0 t= a:: ;:::) (,) CD ~ ....a:: C/) Oct: 0 -w .... w w N ~ C/) C( 0 w ..J (,) .... a:: C( Q. w ;:::) C!) ii:

0 0 0 0 0 ~ 8 CD ~ 01 MiNI.:I% 10 ISC-403

Table I

Description of Specimens after Firing and Oxidizing

Temp. 0 C Appearance Induction After graphite After oxidation firing firing at 700°c

Slip Cast Specimens 1750 white to grey red-brown core, yellow surface 1800 slightly darker same, smaller core 1900 medium grey same , much smaller core 1950 some black, grey no core }Jresent 2000 black, charred appear­ badly spalled ance. Homogeneous. 2100 same as 2000°C completely shattered

Dry Pressed Specimens

1600 light grey yellow, cored as above 1700 light grey yellow., cored as above 1800 medium-light grey yellow (repeated oxidation had no effec t) 1900 medium grey yellow; smaller core 1950 dark grey yellow-brown 2000 very dark no core., yellow-brown

2050 very dark no core ,I) some spalling ·-·~ I IJ_ / 4 (/) 4 UNOXIDIZED H 4 4 Cfl 0 0 ~~~ I 0 +=' 0 0 \.A) ~ I OXIDIZED 2 0 --- 01 0 ~ ~ 2 ~ ~

0 1600 1700 1800 HEAT TREATMENT (OC)

FIGURE 2 ULTIMATE COMPRESSIVE STRENGTHS OF Zr0 2

I-' I-' 12 ISC-403

The above curves are not to be considered fundamental but were made on a qualitative basis and are to be used for comparison only. Much higher strength values have been reported in the litera­ ture and are not to be confused with the information presented above. To determine whether or not zirconium carbide was formed during the graphite firing, x-ray analyses were made of the raw ma t erial, bars fired to 20oooc, bars fired to 2100°C and later oxidized at 700°C, and zirconium carbide. These were exposed to nickel fil ter ed CuK radiation in a Debye-Scherrer camera. As may be observed in Fi gure 3, numerous carbide l1nes are mtably prominent in the film of the s peci- men which had been fired to 2000°C and had not been oxidi zed. This gives positive evidence of the existence of carbide in the f i red pieces which later failed in the oxidizing cycle. The oxidized bars ex2ibit the same structure as the original material, i ndicat- ing no loss in sta~ilization due to the high firing. This informa­ tion was checked by actual measurement of the line spacings. Spectrochemical analysis of the specimens failed to show any change in the amount of calcium, i.e., the stabilizing agent, between various specimens. It did, however, give excellent reasons for the coring effect noted ~n Table I. The iron content decreased from 0.150% Fe at the center of one of these cores to 0.028% at the surface. This coring effect was unchan$ed during the oxidizing cycleJ) as illustrated by repeated oxidations.

CONCLUSIONS

From the data and observations presented on the zirconia­ graphite reaction, it is possible to make the following conclusions:

L Zirconium carbide is formed when zirconia is hea~ed in graphite surroundings at temperatures above 1600 c . 2. The ultimate compressive strength increases with heat treatment but decreases when the carbide is removed by oxidation. Specimens have been found to disintegrate completel y when oxidized after firing to temperatures above 2000°C. This is likewise attributed to the formation and oxidation of zirconium carbide. 4. A core, the dimensions of which decrease with increasi ng temperatureJ) may be observed in the specimens. Thia is thought to be due to the presence and volatilization of iron. Sufficient volatilization occurs to groduce a uniform color throughout the specimen at about 2000 c, i.e., the core disappears. 13

Stabilized Zirconia (Norton Company)

Stabilized Zirconia (2000°C)

Zirconium Carbide

Stabilized Zirconia (2100°C + 700°C)

Figure 3 14 ISC-403

5. Graphite firing does not decrease the amount or stabilizer present in the zirconium oxide nor change q the crystallographic structure. 6. For best results, zirconium oxide should not be fired in · graphite enclosures at temperatures above 1900°C nor in direct contact with graphite at temperatures above 1600°C. Carbide formation at these temperatures is sufficient to cause failure on oxidation.

ACKNOWLEDGMENT

The authors are indebted to Professor Richard T. Othmer who performed the compressive strength determinatioas~

LITERATURE CITED

1. Prescott, C. H., J. Am. Chem. Soc. 48, 2534-50, 1926.

2. George, H., J. Four. Elec. ~7, 127-8, 1938. 3. Wainer, E., Zirconia Refractories. U. S. Patent 2,335,326. Nov. 30, 1943. 4. Wainer, E., Method of Making Zirconium Oxide Refractories. U. s. Patent 2,335,325. Nov. 30, 1943. 5. Jennings, H. W. K., Zirconium Oxide Refractories. Brit. Patent. 478,409. July 10, 1946. 6. Krollt W. J. and Schlechten, A. w., J. Electrochern. Soc. 93, 247-51:5, 1948. 7. Schofield, H. z., Slyh, J. A., Duckworth, W. H. and Sesler, E. C., Jr., BMI-T-28. June, 1950. 8. Whittemore, 0. J. and Marshall, D. w., J. Am. Ceram. Soc. 35, 85-9, 1952. 9. Murray, P., Rodgers, E. P., Williams, A. E., AERE M/R 893, 1952. 10. Cohn, W., Trans. Electrochem. Soc. 68, 65, l935. 11. Geller, R. F. and Yavorsky, P. J ., J. Res. Natl. Bur. Stds. 35, 87-llOJ) 1945. 12. Kreidl, N. J ., J. Am. Cerarn. Soc. 25, 129-39, 1942. ISC-403 15

13. Ballard, A. H. and Marshall, D. w., Stabilized Zirconia . U. S. Patent 2,535,526. Dec. 26, 1950. 14 . Casagrande, A.J Hydrometer Method for Determination of Fineness Distribution in soiis. Julius Springer, Berlin" 19~.