The thermal expansion of niobium pentoxide and its effect on the spalling of niobium oxidation films

JOURNAL OF THE LESS-COMMON METALS

THE THERMAL

EXPANSION

ON THE SPALLING

OF NIOBIUM PENTOXIDE OF NIOBIUM

OXIDATION

I.51

AND ITS EFFECT FILMS

D. L. DOUGLASS Vallecitos Atomic Laboratory, General Electric Company, Pleasanton, (Received

October

Calif. (U.S.A.)

17th, 1962)

SUMMARY The thermal expansion coefficients of stoichiometric and non-stoichiometric niobium pentoxide have been determined from zo”-~ooooC and zoo-5oo”C, respectively, and were found to be significantly smaller than that of niobium metal. The thermal stresses were calculated which are induced during cooling of a coherent oxide-coated niobium plate. In the absence of volume mismatch of oxide and metal it was concluded that thermal stresses due to differences in expansion coefficients should not cause spalling when the oxidized sample is cooled from 600°C. A more realistic appraisal, which is consistent with experimentally observed spalling, was made by consideration of the very large strains arising from coherency of the voluminous oxide on the more dense metal. The surface layers of the substrate may deform plastically during oxidation at high temperatures, even though appreciable contamination and strengthening has occurred. Upon cooling, however, the added thermal stresses and decreasing plasticity of the contaminated substrate may lead to spalling, the crack initiating in the contaminated metal and propagating into the brittle oxide.

INTRODUCTION

The protectiveness of an oxide film on a metal is determined by several factors, two of which are the volume ratio of oxide to metal (Pilling-Bedworth ratio) and the thermal expansion coefficients of both metal and oxide. These two factors are perhaps the most important with respect to spalling characteristics of the films. The oxidation of niobium metal usually results in the formation of the pentoxidel-6 which is very voluminous compared to the metal (Pilling-Bedworth ratio of 2.68). Large biaxial compressive stresses will therefore exist in the oxide film during that portion of the oxidation process in which metal and oxide are coherent, whereas tensile stresses exist in the metal substrate. Although the stresses in the film eventually cause spalling or failure of the film under isothermal conditions, thermal cycling tends to magnify the stresses, depending upon the relative coefficients of thermal expansion between metal and oxide. Invariably the film will rupture during cooling or reheating. It was the objective of this study to obtain accurate values for the coefficient of thermal expansion of the oxide and therefore to permit calculations to be made of the interfacial, coherency strains induced by cooling or heating. J. Less-Common Metals, 5 (1963) 151-157

D. L. DOUGLASS

152

PROCEDURES

Fully densified bodies of the oxide were prepared by hot-pressing high purity, 1.3 ,u diameter powder (99.96% Nbz05) in b oron nitride-coated graphite dies. The coating was used to prevent formation of niobium carbide and was applied by painting a slurry of BN in acetone followed by baking at IOO’C. Pressing was performed in a vacuum of I ,u at 13oo’C and 3,000 p.s.i. The pressure was maintained at temperature for 30 min. No reaction occurred between the die and billet or between the BN coating and billet. The billet was 2 in. in diameter and 3 in. high. Rectangular bars of the oxide 40 mm long and 2 mm square were cut from the billet for dilatometer samples. Other samples were used for density measurements (weighing in air and in CCL) and for metallography. The billet had a density of 4.55 g/cm3 and exhibited no porosity as shown in Fig. I. A fine grain, equiaxed structure was obtained. The composition

Fig.

I.

Microstructureof hot-pressedniobium pentoxide. Bright field. (x

1000)

of the billet as determined by oxygen absorption in a Seivert’s apparatus was Nb204.97s and was black in color from the anion-deficient structure. Subsequent oxidation of the non-stoichiometric oxide to the stoichiometric composition was performed in order to determine if any difference in thermal expansion could be detected between the anion-deficient and stoichiometric varieties. The black, anion-deficient sample could not be run at temperatures over 500°C without oxidation. A Leitz UBD dilatometer was used over the temperature range of 2o”-~ooooC. Several runs were made on each sample with heating rates varying between 2.7’ and 50°C per minute. The elastic modulus was measured in compression on a rectangular bar 12 mm J. Less-CommonMetals,

5 (1963)

x51-157

THERMAL EXPANSION

OF

NbsOb

153

square and 35 mm long. Two Tuckerman gauges were used, one on each side of the bar. Copper sheets were used to seat the sample, and the sample was repeatedly repositioned until both strain gauges gave the same deformation readings. The value of the modulus was calculated to be 23.4 * 10s p.s.i. DISCUSSION

The coefficients of thermal expansion of Nb205 are listed for intervals of IOO’C in Table I and increase with increasing temperature as one might expect. However, it may also be noted that a maximum is reached, after which the values decrease to those observed in the lowest temperature interval. The reason for this behaviour is not known. TABLE THERMAL

EXPENSION

COEFFICIENTS

I

OF NIOBIUM

PENTOXIDE

Stoichiometirc NbzOs Temp. interval (“C) *o--200

Heating (2.7”Clmin)

Nba04.878

Cooling (2.7’C/min)

Equilibrium*

Heating (zo’C/min)

Cooling (zo’C/min)

I.07

I.07

I.07

I.30

I.27

1.83 2.10

I.71 2.16 1.96 I .g8

I.91

I.88

2.11

2.12

2.22 **

2.24 **

800+00

goo-1000

I.25

20-600

1.76

300-400 400-500 500-600 600-700 700-800

IO-e)

Nonstoichiometric

1.61 I .a5 2.11 I.90 1.69 I.53 1.41

*oo--300

(IIIIll/mIIl/“c

2.15

2.I9

* Held at constant temperature until movement ceased. ** Values obtained above 5oo’C in air were meaningless due to expansion from oxidation. tion was nil from zo-5oo°C in the time interval used.

Oxida-

The difference in values between the stoichiometric and non-stoichiometric oxides are small with slightly higher values being observed for the latter. The coefficients for niobium pentoxide are very small compared to most materials and considerably smaller than that of metallic niobium. The effect of the difference in thermal expansion of oxide and metal on the stability of coherent oxide films formed during oxidation may be seen by a simple calculation of the stresses which will arise during cooling. For the sake of simplicity let us consider an idealized case initially in which there is no volume mismatch between oxide and metal at the temperature of formation. The stresses for mismatch are thus equal to zero, and the only stresses which will exist will be due to restraint of the metal by the oxide during cooling to room temperature. There are no strains in either oxide or metal at the oxidation temperature (for the idealized case of no volume change during oxidation). Both oxide and metal will contract during cooling from TI to Tz, the amount of contraction being proportional to ATa,, and ATOLNIJ, respectively, where AT = T1 - T2. The greater coefficient of thermal expansion of metal than of oxide indicates that the metal will contract more than the oxide ; however, the interface is coherent and a restraint is exerted as shown in Fig. 2. J. Less-Common

Metals, 5 (1963) 151-157

D. L. DOUGLASS

I.54

The stresses existing in each phase at the interface will be equal but opposite in sign at all temperatures below Ti. The strains will not be the same due to a difference in elastic moduli and due to the fact that the equilibrium lengths of each phase will be different at all temperatures other than Tr. The stress is given by 0~x7 for an infinite plate geometry with an oxide film on both sides of the plate by ax

=

-&,dT(&, 1+*0x-

E

OiNb)

tax

ENS,

i

hb

1

the stress in the oxide elastic modulus of the oxide elastic modulus of niobium thickness of oxide = thickness of niobium. tNb The stress in the oxide coating will be highest for very thin films and will decrease as the film thickens as listed for some representative values of the thickness ratio in Table II. where

ooX E,, ENS tOX

= = = =

TABLE VARIATION

OF STRESS

IN OXIDE

II

COATING

0.01

-78,000 -76,200

0.1

-61,000

0.001

WITH

FILM

THICKNESS

These stresses are the maximum stresses which exist at the interface and are calculated for values of the elastic modulus of 23.4 . 10s and 16.2 * 106 p.s.i.8, respectively, for niobium oxide and niobium. Let us now consider what happens to each phase of the oxide-metal couple when subjected to thermal stresses. Initially the highest stress possible will exist in the film when it is very thin. The tensile stress of 78,000 p.s.i. in the metal far exceeds the tensile strength of the niobium at room temperatureg; however, the initial step in niobium oxidation is oxygen dissolution in the metals, and the contaminated layer near the oxide-metal interface is significantly strengthened. The solid solubility of oxygen in niobium is 0.25 wt. o/o at 5oo”C, the yield strength of Nb-0.2 oxygen is 70,000 p.s.i. and the tensile strength is go,ooo p.s.i.ir. The compressive strength of Nbz05 is 117,000 psi. at room temperature 12, therefore neither the metal nor the oxide would fracture. Some plastic deformation should occur in the metal which will partly relieve the thermally induced stresses. In other words, there should be no spalling in pure niobium during cooling to room temperature from an oxidation temperature of 600% in the absence of mismatch. The heretofore neglected, mismatch strains will be considered. The Pilling-Bedworth ratio of 2.68 (assumed independent of temperature) gives the molar volume ratio of oxide to metal. The relative difference between oxide and metal for one dimensional strain is then equal to the cube root of 2.68, or about 1.39. The strain* * A rough estimate which neglects epitaxial relationships. of the oxide there has been no determination of epitaxy.

Due to the complex

J. Less-Comwmt

crystal structure

i’kft?tds, 5 (1963) 151-157

THERMAL EXPANSION

OF

Nb205

r.55

is given by I.39 - I.OO/I.OO = 0.39. The elastic stress in the metal corresponding to this extremely large strain would be 0.39 * 14 . 10s = 5.4 * 106p.s.i., an unbelievably large value obtainable only in some single crystal whiskers of extreme perfection. Plastic deformation of either metal or oxide would necessarily occur to relieve interfacial stresses. In view of the lower strength of the metal compared to that of the oxide it is not unreasonable to expect the metal to deform plastically during formation of the oxide layer. Plastic deformation can occur only to the point at which the stress is equal to the elastic limit of the metal. Subsequent cooling of the couple then adds the thermal stress to the mismatch stress, and the nearly complete lack of ductility of the oxygencontaminated metal at lower temperatures prevents plastic deformation during cooling. Initiation of fracture would then be expected in the metal. It has been observed in Nb-V alloys oxidized in high temperature steam13 that cracks do initiate in the contaminated surface layers of the metal and propagate into the relatively brittle oxide film. The inability of thick films to resist spalling compared to the comparative stability of thin films arises from an increase in the total strain energy in the film. The flaking of the film occurs in an effort of the oxide to minimize strain energy. The work required to detach a unit area of a strained film is independent of film thickness, whereas, the total strain energy per unit area increases with increasing film thickness. If the interfacial strain remains nearly constant with increasing film thickness, a critical thickness will then be required in order to exceed the energy required for spalling. The only data available on the variation of strain with film thickness are given by

(a)

At oxidation Thermal

temperature, contraction

(b)

Tq of

At room

oxide-metal

r t

+ IOxide

/hletal

temperature,T2

couple

when

aox
Interf..xe

z!KF

ZO ‘,

Distance

Distance (c) At

Id)

TI Tangential

stress

At T2

distribution.

Fig. z. Thermal strains and stresses in oxide-metal J. Less-Common

couple. Metals, 5 (Ig63) 151-157

D. L. DOUGLASS

156

BORIEet ~1.14for thin films of Cu20 on (110) single crystals of copper. They determined the spacing of (110) oxide planes as a function of distance across the oxide for films of five thicknesses. The data, shown in Fig. za, indicate a nearly identical spacing at the oxide-gas interface, and a greatly expanded spacing at the oxide-metal interface, the largest expansion occurring in the thinnest film (140 a). The normally cubic oxide existed as an orthorhombic crystal with the a2 parameter perpendicular to the metal and decreasing from the metal-oxide interface to the oxide-gas interface. The bo parameter was constant, and COincreased from the metal-oxide interface to the oxide-gas interface. The unit cell volume remained constant. The severe distortions resulted from the epitaxial mismatch of IS%, ((IIo)c~~~ // (110)~“; [~io]~~,o // [1Io]~a)l5.

It is important to note that the COlattice parameter at the oxide-gas interface was approximately that of the bulk oxide (cubic oxide was indexed according to an orthorhombic structure) for all thicknesses of the film studies (Fig. zb), and that the strain gradient in the film decreased as the films became thicker. These results are consistent with those calculated in Table II and support the assumption that the outer oxide layers tend to achieve their equilibrium dimensions. One last item should be mentioned in connection with both the present work and that of BORIEand co-workers. In both cases very large strains arise from large volume

3.02 -1 Oxide-metal

lb0 interface (a) Strain

200

300

Distance gradients

400

4

500

in film, S

for various

thicknesses

of films

4.25 OQ 4.20 6. 4.15 4.10

0

100 200 300 Film thickness,

(b) Lattice parameter film thickness

400 8

as a function of

Fig. 3. X-ray analyses of CUZO films on

(110)

Cu after BORIE et al.

J. Less-Common

Metals, 5 (1963) 151-157

THERMAL EXPANSION OF

NbsOs

I.57

mismatches between oxide and metal, and in both cases the oxides are coherent in the thin film region, although severely strained. It is not possible to account for 39% and 18% elastic strain, respectively. BORIE has postulated a dislocation structure of edge dislocations having a [OOI] Burger’s vector and shown that the dislocation density decreases with film thickness, this approach being consistent with the decrease in the strain gradient noted with increasing film thickness. In order to understand completely the spalling of niobium oxide films it would be necessary to obtain similar data, and in particular to know the epitaxial relation of oxide to metal. These data would then provide the basis to calculate all of the stresses existing in the oxide films. The very low values of the thermal expansion coefficient of NbsOb compared to the metal and the large difference in molar volume between oxide and metal tend to preclude the absence of spalling of the metal during either oxidation or thermal cycling. The development of niobium alloys for use in oxidizing atmospheres will have to be based upon the formation of a less voluminous oxide which has a coefficient of thermal expansion closer to that of niobium.

ACKNOWLEDGEMENTS The

sample

preparation

of the elastic modulus ledged.

Helpful

was performed

by F. A. WATSON,

and the determination

of NbsOs by R. E. BLOOD. Their efforts are gratefully

discussions

acknow-

on the nature of the strains and state of stress were held

with L. N. GROSSMAN and M. B. REYNOLDS for which the author expresses his thanks.

REFERENCES H. INOUYE, Scaling of columbium (1956).

W. D. KLOPP, C. T. SIMS AND niobium, Second Intern. Conf.

in air, Reactive

Metals,

AIME Special Report Series, No. 5

I. JAFFEE, Effect of alloying on the kinetics of oxidation of 0% Peaceful Uses of Atomic Energy, Geneva, 1958, Paper No.

15/p/712. W. D. KLOPP, C. T. SIMS AND R. I. JAFFEE, High temperature oxidation and contamination of niobium, Trans. Am. Sot. Metals, 51 (1959) 282. T. HURLEN, Oxidation of niobium, J. Inst. Metals, 8g (1961) 273. H. J. G~LDSCHMIDT, A high temperature X-ray investigation of niobium pentoxide and some problems concerning the oxidation of niobium, J. Inst. Metals, 37 (1958) 235. P. KOFSTAD AND H. KJOLLESDAL, Oxidation of niobium in the temperature range 500-1200°C. Trans. AIME, 221 (1961) 285. G. D. OXX, Which coating at high temperature?, Prod. Eng., 61 Jan. 20 (x958). D. P. LAVERTY AND E. B. EVANS, Young’s modulus of columbium at elevated temperatures, in D. L. DOUGLASS AND F. W. KUNZ, Columbium Metallurgy, Interscience, New York, 1961, P. 299. E. T. WESSEL, L. L. FRANCE AND R. T. BEGLEY, The flow and fracture characteristics of electron-beammeltedcolumbium, in D. L. DOUGLASS AND F. W. KUNZ, Columbium Metallurgy, Interscience, New York, 1961, p. 459. R. P. ELLIOTT, Columbium-oxygen system, Trans. Am. Sot. Metals, 52 (1960) ggo. C. R. TOTTLE, The physical and mechanical properties of niobium, J. Inst. Metals, 85 (1957)

377. D. L. DOUGLASS, unpublished

research. D. L. DOUGLASS, The kinetics and mechanism of columbium alloy corrosion in superheated steam, to beipublished, Corrosion. B. BORIE, C. J. SPARKS JR. AND J. V. CATHCART, Epitaxially induced strains in Cur0 films on copper single crystals I. X-Ray diffraction effects, Acta Met., IO (1962) 691. K. R. LAWLESS AND A. T. GWATHMEY, The structure of oxide films on different faces of a single crystal of copper, Acta Met., 4 (1956) 153.

J. Less-Common

Metals,

5 (1963) 151-157