the growth and structure of thin oxide

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Oxidation of Metals, Vol. 40, Nos. 5/6, 1993 The Growth and Structure of Thin Oxide Films on Ceria-Sol-Coated Nickel F. Czerwinski*t and W. W. Smeltzer* Received February 11, 1993;revisedJune 22, 1993 The influence of 14-nm thick ceria ceramic coatings deposited by the sol-gel technique on the early-stage oxidation of polycrystalline nickel at 973 K was studied by analytical electron microscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry and X-ray diffraction. The size of the ceria particles in the coating was modified prior to oxidation by vacuum annealing. It was found that ceria particle size is a crucial factor affecting the oxidation kinetics, oxide microstructure, and distribution of cerium within the oxide film. Coarse ceria particles applied to the nickel surface were ineffective in the inhibition of oxidation and were spread throughout the whole oxide. Coatings with small ceria particles markedly improved the oxidation resis- tance. After oxidation such particles were present in the surface region of nickel oxide, acting as the sources of cerium ions segregated at the nickel- oxide grain boundaries. The stereological analysis of oxide microstructure as well as microchemical examination supported the predominant role of grain- boundary segregation of cerium ions decreasing the oxidation rate. The results are discussed in terms of reactive-element effect on the development of microstructure of nickel oxide film during initial stages of oxidation. KEY WORDS: reactive-element effect; nickel oxide; ceria/cerium; sol-gel coating. INTRODUCTION Sol-gel processing using colloidal dispersions of hydrous oxide was devel- oped initially to prepare spherical particles of oxide nuclear fuel and *Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada, L8S 4M1. tPermanent address: Institute of Metallurgy, Academy of Mining and Metallurgy, 30-059, Cracow, Poland. 503 0030-770X/93/1200-0503507.00/0 1993 Plenum Publishing Corporation

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Oxidation of Metals, Vol. 40, Nos. 5/6, 1993

The Growth and Structure of Thin Oxide Films on Ceria-Sol-Coated Nickel

F. Czerwinski*t and W. W. Smeltzer*

Received February 11, 1993; revised June 22, 1993

The influence of 14-nm thick ceria ceramic coatings deposited by the sol-gel technique on the early-stage oxidation of polycrystalline nickel at 973 K was studied by analytical electron microscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry and X-ray diffraction. The size of the ceria particles in the coating was modified prior to oxidation by vacuum annealing. It was found that ceria particle size is a crucial factor affecting the oxidation kinetics, oxide microstructure, and distribution of cerium within the oxide film. Coarse ceria particles applied to the nickel surface were ineffective in the inhibition of oxidation and were spread throughout the whole oxide. Coatings with small ceria particles markedly improved the oxidation resis- tance. After oxidation such particles were present in the surface region of nickel oxide, acting as the sources of cerium ions segregated at the nickel- oxide grain boundaries. The stereological analysis of oxide microstructure as well as microchemical examination supported the predominant role of grain- boundary segregation of cerium ions decreasing the oxidation rate. The results are discussed in terms of reactive-element effect on the development of microstructure of nickel oxide film during initial stages of oxidation.

KEY WORDS: reactive-element effect; nickel oxide; ceria/cerium; sol-gel coating.

I N T R O D U C T I O N

Sol-gel processing using colloidal dispersions o f hydrous oxide was devel- oped initially to prepare spherical particles of oxide nuclear fuel and

*Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada, L8S 4M1.

tPermanent address: Institute of Metallurgy, Academy of Mining and Metallurgy, 30-059, Cracow, Poland.

503 0030-770X/93/1200-0503507.00/0 �9 1993 Plenum Publishing Corporation

504 Czerwinski and Smeltzer

eliminate the grinding and sieving steps. ~ Currently, the direct calcination of gels is also being utilized to produce ceramics with high purity and homogeneity. Although sols are a relatively expensive source of oxides for fabrication of bulk ceramic materials, their use to produce the thin surface coatings is highly cost effective. Compared to the other precursors for the reactive-elements oxides in coating technology for high-temperature-corro- sion protection, i.e. the nitrate salts in solutions or molten2 or the oxide slurry, 3 the sol coatings are characterized by having the :smallest and best-controlled oxide particle size.

Although the early patent by Pfeil 4 dealt with surface-applied, reac- tive-element oxides, most of the first three decades of research analyzed mainly the reactive element as an alloy addition. The studies in the last 25 years have focused also on other types of additions including ion implanta- tion and coatings. 5 There is a considerable literature dealing with the influence of reactwe-elements-oxide coatings on the high-temperature-oxi- dation behavior of NiO-formers, 6 Cr2 03 -formers, 7 and A1203-formers. 8 The beneficial effect of this mode of applying of reactive elements is reported for NiO and Cr203-formers. The latest study of thin coatings of nitrate-converted, reactive-element oxides showed no beneficial influence at all, however, on scale adhesion of A12 03 -formers. 8

In our previous papers 9,1~ we discussed the effect of ceria-sol coatings, their thickness, substrate pretreatment, preoxidation before coating and reaction temperature on the oxidation kinetics and growth morphology of nickel oxide. In this study we consider the role of ceria particle size in sol-gel coatings and analyze the development of the microstructure of thin oxide films formed at 973 K on sol-coated polycrystalline nickel.

E X P E R I M E N T A L P R O C E D U R E

Substrate, Coatings, Oxidation Test

Specimens with thicknesses of 1 mm and diameters of 9.5 mm were prepared from 99.99% purity of polycrystalline nickel rod supplied by A. D. MacKay, Inc. After grinding through 600-grit SiC paper, the samples were mechanically polished to a 1-#m diamond finish, cleaned ultrasoni- cally and annealed at 1173 K for 1 hr in a vacuum of 10 - 7 torr. This procedure resulted in an average nickel-grain diameter of 100 #m. As a final step a chemical polish was performed for 40 s at 363 K in a solution composed of 30 ml nitric acid, 10 ml sulphuric acid, 10 ml orthophosphoric acid, and 50 ml glacial acetic acid. it To improve surface wetness before coating, the substrates were preoxidized at 673 K for 2 hr in 1 atm oxygen. Measured oxygen uptake was 7.7 #g/cm 2 which corresponds to a NiO thickness of 54 nm.

Thin Oxide Films 505

Ceria sol having a concentration of 5 g CeOE/dm 3 in 0.1 M nitric acid and containing a nonionic surfactant was prepared using the 20 wt.% colloi- dal dispersion supplied by Johnson Matthey, Ltd. and the surfactant Triton X-100 supplied by Aldrich Chemical Co. Inc. Coating deposition by cold dipping was followed by drying at 363 K for 20 hr in a desiccator and cal- cination at 573 K for 1 hr in 1 atm oxygen. The weight of coatings after drying and calcination was 35 and 10 # g/cm 2, respectively. Annealing was performed for 5 hr in the temperature range of 673-1173 K in a vacuum of 10 -7 torr. For all samples annealing followed calcination. The thermal evolution of the ceria grain size was determined by X-ray diffraction (XRD) using nickel- filtered CuK~ radiation on powder specimens prepared from about 15-/~m thick ceria films deposited on and stripped after drying from a glass substrate.

Oxidation kinetics at temperature of 973 K in oxygen at a pressure of 5 x 10 -3 torr and for times up to 4 hr were measured in an ultrahigh-vac- uum manometric apparatus. The samples were heated to the oxidation temperature in oxygen. Details regarding the oxidation-kinetics measure- ments were reported previously. 1~

Oxide Composition and Microstructure

The combination of Auger electron spectroscopy (AES) and Rutherford backscattering spectrometry (RBS) was utilized to characterize the compo- sition of oxide films. The measurements by a scanning Auger microprobe (Perkin Elmer PHI 600) were performed while simultaneously sputtering the oxide film with Ar + ions. The oxide was sputtered from an area of 2 mm a, and a small part ( < 1%) of this region was analysed. The RBS depth profiles of oxide films were measured using 1 MeV 4He+ ions at a scattering angle of 160 ~ and an energy calibration of 1.016 keV/channel. The sample-tilt angle was 0 ~ A maximum current of 2 nA was used to ensure that the detector dead time was less than 5% and that beam-heating effects were negligible.

The morphology of coatings and growth surface of oxide was observed by scanning electron microscopy (SEM). The oxide microstructure was analyzed in details by transmission electron microscope (TEM-Philips CM12). Thin foils were prepared parallel to the oxide-metal interface by two different techniques. For routine analysis the oxide was stripped from the substrate using a saturated solution of iodine in methanol, la For high-magnification analysis of the films near the gas-oxide interface, the samples were prepared by ion thinning using the technique described by Pint. 13 Ion thinning of oxide from the metal side only was conducted in a Gatan 600 duomill with an Ar beam at 4-4.5 keV. The TEM images and selected area diffraction (SAD) patterns, if not otherwise indicated, were taken at an angle of 0 ~ off the sample normal. The individual ceria particles

506 Czerwinski and Smeltzer

and the segregation of reactive element to NiO grain boundaries were analyzed by a field-emission gun scanning transmission electron microscope (STEM, Vacuum Generator Microscopes Ltd. model HB5) with a 3.5 nm electron probe and windowless Link detector (EDX).

RESULTS

The Modification of Ceria Grain Size in Coatings by Vacuum Annealing

To examine the effect of particle size in a surface-applied, reactive-ele- ment oxide on the inhibition of high-temperature corrosion, the ceria-grain diameter in 14-nm thick coatings on nickel substrates was modified by vacuum annealing before oxidation. Because of the difficulties in determin- ing the dimensions of extremely small particles (about 5 nm in size) in thin coatings, the analysis of thermal growth of ceria grains was carried out separately on 15-#m thick ceramic films produced from the same precursor by sol-gel technology. The crystallite size calculated according to Sherrer's equation, 14'15 plotted as a function of annealing temperature is shown in Fig. 1 together with values derived from SEM and TEM observations. The error bars for XRD results were calculated assuming a 5% experimental error in measuring the diffraction breadth.

A high-magnification TEM image of a coating stripped from non- preoxidized nickel substrate after calcination is given in Fig. 2a. It is seen from microscopical observations that for 14-nm thick coating the ceria particles uniformly cover the substrate. The SAD pattern indicates the presence of small amounts of NiO formed during calcination. Traces of Ni were also detected, due to incomplete dissolution of the substrate during thin-foil preparation. The surface morphology of a coating after annealing at 973 K is shown in Fig. 2b. The mean linear size of ceria particles of 40 nm found after this treatment is about two times larger than the value obtained from XRD. The XRD method was quite insensitive to variation in particle size for annealing temperature above 1000 K due to the coarsen- ing of ceria particles.

Oxidation Kinetics

The oxidation kinetics of pure and ceria-coated nickel are illustrated in Fig. 3. The oxidation of all samples followed a parabolic relation after an early transient stage lasting 20-40 rain. The average values of the parabolic rate constants after 4 hr of exposure at 973 K are listed in Table I. Since the oxygen uptake during preoxidation at 673 K for 2 hr was from 10 to 100% of the total oxygen uptake during subsequent oxidation at 973 K, the rate constants were calculated assuming that the average oxygen uptake is

Thin Oxide Films 507

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275 475

Temperature (K) Fig. 1. Ceria crystallite size as a function of annealing temperature determined by X R D and microscopy. TEM-coating stripped from nickel substrate. SEM: surface coating morphology (973 K), fracture cross-sec-

tion of 15-#m thick films (1173 K).

Table I. The Parabolic Oxidation Rate Con- stants Calculated from Oxygen Uptake After 4 hr Oxidation at 973 K in 5 x 10 -3 Torr Oxy-

gen (Annealing Time 5 hr)

Specimen treatment kp (g2 c m - 4 s - 1)

Uncoated 1.80 • 10 -I3 Coated unannealed 3.43 • 10 -14 Annealed at 723 K 2.85 • 10-13 Annealed at 823 K 2.66 x 10 -13 Annealed at 873 K 2.30 • 10 -13 Annealed at 973 K 2.00 x 10 -13

508 Czerwinski and Smeltzer

Fig. 2. Microscopical observations of ceria coatings: (a) the TEM image and SAD pattern of a coating stripped from a nonpreoxidized nickel substrate after calcination at 573 K for 1 hr and (b) SEM surface morphology of the coating after annealing at 973 K for 5 hr, grain boundary in nickel substrate (arrow).

Thin Oxide Films 509

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0 50 100 150 200 250 Time (rain)

Fig. 3. Oxidation kinetics at 973 K of C e O 2 sol-coated nickel without and after vacuum annealing for 5 hr prior to the exposure to oxygen (723-923 K).

the mean of the values at the start and finish of oxidation at 973 K. Details regarding calculations are described elsewhere. 9,1~ The unannealed coating, which consisted of ceria particles about 5 nm in size, markedly decreased the oxidation rate of nickel. The difference in parabolic rate constants is about one order of magnitude. Vacuum annealing resulted in an increase in ceria particle size and a higher oxidation rate compared to uncoated nickel. All values of kp for annealed samples are higher than that for uncoated nickel and the kp values decrease gradually with increase of annealing temperature in the range of 723-973 K (Table I).

The effect of ceria coatings on reduction of oxidation rate was characterized by the ratio R defined as:

R = kpi blank/kpi coated (1)

where kpi blank and kpi coated are the instantaneous parabolic rate constants for samples uncoated and ceria-coated, respectively. The values of kpi were calculated as 2w dw/dt, where w is oxygen uptake per unit area and t is the time of oxidation. Plots of R as a function of oxidation time are given in

510 Czerwinski and Smeltzer

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Fig, 4. The ratios of the instantaneous values of parabolic rate constants kpi for uncoated nickel to that for ceria-coated nickel as a function of oxidation time at 973 K. Vacuum annealing: 973 K, 5 hr.

Fig. 4. In this Figure, an additional plot of R is presented for a ceria coating deposited on non-preoxidized nickel. The base for R calculation in this case was kpi value for non-preoxidized nickel substrate. Oxidation kinetics for this method of substrate pretreatment are given elsewhere. 9'~~ An essential difference is seen between plots of R for unannealed and annealed samples. Values of R for annealed samples are less than 1 at all oxidation times, which indicates that ceria coatings consisting of coarse particles, (larger than 10 nm in size) are ineffective in inhibiting oxidation. Values of R for unannealed coatings are in the range between 10 and 15; however, a higher reduction of oxidation rate is observed for the coating deposited directly on the pure-nickel substrate rather than on a preformed 54-nm-thick, nickel-oxide film.

O x i d e - D e p t h Compos i t ion

The depth location of cerium in nickel-oxide films was examined by RBS and AES. The surface area analyzed for RBS was about ! mm 2. The

Thin Oxide Films 511

RBS spectra for coated nickel before and after oxidation are shown in Fig. 5a. The concentration profiles derived from these spectra are presented in Fig. 5b. The spectrum for a sample oxidized without annealing exhibits a distinct cerium peak. This peak, however in comparison to the unoxidized case, is lower in intensity and has shifted to lower energies. The spectrum for oxide with the coating annealed before oxidation at 973 K does not show the distinct cerium peak. In this case, cerium is not localized in the vicinity of the outer surface of oxide, as in the unannealed samples, but is spread throughout in the oxide (Fig. 5b).

The above RBS spectra were confirmed with AES studies. Whereas the RBS spectrum was derived from oxide film grown on over 100 grains of nickel substrate, the AES signal was generated from oxide grown on one nickel grain. The results obtained by these two methods were fully consis- tent. For oxide formed on nickel with unannealed coatings (ceria particles

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Fig. 5. Results of RBS measurements: (a) energy spectrum of l MeV 4He ions backscattered from ceria-coated before and after oxidation for 4 hr without and with annealing at 973 K for 5 hr, and (b) cerium concentration vs. depth profiles, as derived from RBS spectra shown in (a).

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Fig. 5. Continued.

of 5 nm) the cerium was also concentrated in the surface region of nickel oxide (Fig. 6). This was not the case for oxide formed on nickel with annealed coatings, composed of coarse ceria particles.

Morphology of Oxide Surface

The surface of oxide formed on uncoated chemically-polished nickel is non-uniform. Differently-oriented nickel grains oxidize selectively. Prefer- ential oxide formation at nickel grain boundaries was also observed. On coated nickel, the oxide surface is more uniform; however, preferential oxide growth is still observed at nickel grain boundaries. The development of surface morphology of oxide formed on ceria-coated non-preoxidized nickel during the very initial stages of exposure to oxygen is illustrated in Figs. 7a-c . It is seen that after 3 rain of oxidation, small grains of nickel oxide are present on the coating surface (Fig. 7b). Longer exposure to oxy- gen resulted in an increase of number and size of individual oxide grains growing on the top of the coating. After approximately 40 rain of oxida-

Thin Oxide Films 513

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SPU'ITER TIME (MIN)

Fig. 6. The AES sputtcr-depth profile of coated nickcl after oxidation at 973 K for 4 hr (without annealing).

tion, these changes are relatively small, suggesting that the subsequent growth process takes place mainly under the coating.

In Fig. 7d is shown the morphology of oxide formed on coated nickel, annealed before oxidation at 973 K for 5 hr. The coating, composed of ceria particles with diameters about 20-40 nm, did not protect the sub- strate against oxidation. After 4 hr of exposure, the oxide film is 380 nm thick, which is greater than that formed on pure nickel after the same oxidation time. Also the oxide gram size is much smaller. Once again, the thickness of oxide grown over the nickel grain boundaries is much higher than that observed for unannealed coatings (Fig. 7d).

TEM Microstructure of Oxide

The typical microstructure for 360-nm thick oxide formed on uncoated nickel after 4 hr of oxidation at 973 K presented in Fig. 8 indicates that the thickness and microstructure of oxide depends on the oxide growth direc- tion. In addition to well-defined subgrains and dislocation substructures, areas are present showing Moir~ fringes produced by overlap which occurs between two NiO crystals. The arrowed Moir6 fringes in Fig. 9, with a measured spacing of 17 A, are produced due to angular rotation. Assuming the NiO {220} interplanar spacing as 1.476 A (from ASTM index), a value of 5 ~ is found for the rotation angle in this region (16).

A bright-field image of oxide formed on ceria-coated nickel annealed at 973 K for 5 hr before oxidation is shown in Fig. 10. The SAD pattern

514 Czerwinski and Smeltzer

Fig. 7. Surface morphology of nickel oxide: (a), (b), (c) unannealed coatings: (a) after calcination at 573 K for 1 hr, (b) oxidation for 3 min, (c) oxidation for 30 min, (d) coating annealed at 973 K for 5 hr--oxidation at 973 K for 4 hr.

Thin Oxide Films 515

Fig. 8. TEM micrograph and SAD patterns of 360-nm thick nickel oxide formed on uncoated nickel at 973 K. SAD from region 1 was taken at an angle 28 ~ of the sample normal.

516 Czerwinski and Smeltzer

Fig. 9. High-magnification TEM image and SAD pattern of oxide formed on uncoated nickel.

indicates the presence of randomly-oriented NiO and C e O a phases. Some of the coarse ceria particles with diameters of about 50 nm are marked.

The TEM microstructure of 165 nm thick oxide grown at 973 K on coated nickel without additional annealing before oxidation is shown in Fig. 11. In comparison to oxide formed on pure nickel, coatings with 5-nm

Thin Oxide Films 517

Fig. 10. TEM micrograph and SAD of oxide formed on ceria-coated nickel, annealed at 973 K for 5 hr before oxidation. Some of the coarse ceria particles are marked.

ceria particles markedly inhibited preferential oxide formation at nickel grain boundaries. However, the nickel oxide formed at these sites still seems to be thicker than that observed at grain faces. Also the thickness of oxide is still dependent on the grain orientation of the nickel substrate. The SAD patterns of NiO formed on the two nickel grains 1 and 2, are composed of rings which are superimposed on the intense spots. The comparison of these patterns with those obtained for oxide formed on pure nickel suggests that these maxima are produced by a portion of nickel oxide having an epitaxial relationship with the nickel substrate. The rings expected for CeO 2 are also present (Fig. 11).

The high magnification TEM bright-field image of the near-surface region of this oxide film is presented in Fig. 12a. The structure is composed of nickel-oxide grains and small ceria particles distributed randomly, at grain boundaries and within grains. As is seen from SAD pattern (Fig. 12b), both phases in this portion of oxide are oriented randomly. The nickel-oxide grain boundaries in this region of oxide film were also examined by STEM/EDX to detect the segregation of reactive element. Typical results in Fig. 13 demonstrate the segregation of cerium to NiO grain boundaries.

Stereological Quantitative Analysis of Microstructure

The quantitative analysis of the surface portion of nickel oxide pro- duced on coated nickel after 4 hr oxidation at 973 K, was performed using

518 Czerwinski and Smeltzer

Fig. 11. TEM micrograph and SAD patterns from regions 1 and 2 indicated, of 165-nm thick oxide formed on ceria-coated nickel at 973 K. Oxidation time 4 hr.

Thin Oxide Films 519

Fig. 12. High-magnification TEM image (a), and SAD pattern (b) of the surface region of NiO formed during 4 hr at 973 K on ceria-coated nickel.

images obtained by TEM. Images for examination by the Leco 20001 image analyser were prepared manually from TEM micrographs. Figure 14a gives a schematic representation of the oxide microstructure, manually created on the basis of the bright-field image. The ceria particles seen as a black phase, are superimposed on the network of nickel-oxide grain

520 Czerwinski and Smeltzer

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analysis of cerium segregation in a NiO grain boundary (b).

Thin Oxide Films 521

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Fig. 14. Results of stereological quantitative analysis of Ce-rich portion of nickel oxide after 4 hr oxidation at 973 K: (a) microstructure with elements analyzed (nickel-oxide-grain-boundary network and ceria par- ticles as black phase), (b) distribution of NiO grain diameters, (c) distribution of ceria particle diameters.

522 Czerwinski and Smeltzer

Table II. Results of Analysis of Near-Surface Region of Oxide Formed on Ceria-Coated Nickel After 4 hr Oxidation at 973 K

Phase Parameter Value Remarks

Nickel oxide Mean linear grain size of oxide 29.1 nm 6 = 12.9 nm Surface area of grain boundaries 13.7 • 105 cm i Assuming a one phase

per unit volume of specimen structure Ceria Mean linear particle size 5.5 nm ~ = 3.3 nm

Volume fraction 17.3% Mean linear spacing 12.4 nm 6 = 6.1 nm

between particles

boundaries. A simple, visual assessment of this microstructure supports the random distribution of ceria particles. The stereological parameters evalu- ated for NiO and CeO2 are listed in Table II. The distribution of the size of NiO grains and CeO: particles are illustrated in Figs. 14b and c, respectively. NiO grains in this region are characterized by an average size of 29.1 nm with higher fractions at 24 and 36 nm. The average diameter of ceria particles was measured to be 5.5 nm, however, a high percentage of the particles had a diameter of about 4 nm. The volume fraction of ceria approximating to 17.3% is relatively high in comparison to the volume of 5 - 1 0 % found by Moon 6 after long-time oxidation of nickel at 1173 K. The spacing between particles of 12.4 nm is much smaller than the nickel-oxide grain size of 29.1 nm.

DISCUSSION

The Role of Ceria Particle Size in Sol-Gel Coating

It was stated by George e t a l . 17 that several different reactive elements produce a similar effect. From the literature it is seen that the effectiveness of reactive elements can also depend on the element itself and on the method of application. Some of the important factors affecting the efficiency for a given element seem to be the size and dispersion of reactive element oxide particles.

The particle-size effect was analysed by Rhys-Jones e t al . 18 using an internally oxidized Fe-20Cr-0 .02Ce alloy. Increased spacings of CeO2 particles at the alloy surface gave less chromia with more time being required to form a continuous oxide layer. In this work, the ceria-particle size in 14-nm thick coatings was modified in the range of 5-40 nm by vacuum annealing before oxidation. The experiment clearly demonstrates

Thin Oxide Films 523

that the ceria-particle diameter in the sol coatings is the critical factor affecting the oxidation process. It is seen in Fig. 3 that a coating containing ceria particles with diameters larger than 10 nm is ineffective in the inhibition of oxidation. The oxide-growth rate and oxygen uptake after 4 hr of exposure are even higher than that observed for uncoated nickel. This higher oxidation rate is believed to be due to the refinement of the nickel-oxide grain size and an increase of grain-boundary density as easy-diffusion paths. This factor is probably the main reason reported in the literature for higher oxidation rates observed for some alloying additions of reactive elements. A weight gain in excess of 40-50% in comparison to pure nickel after 142 hr exposure in 1 atm oxygen at 1173 K was noted by Moon 6 for nickel with alloying addition of 0.6 wt.% Ce. Coatings containing large reactive-ele- ment-oxide particles derived from nitrate solutions or slurry are often reported to exhibit low efficiency or no effect of the reactive elements. 2

The same factor seems to be responsible for low-protection properties of thick or non-uniform coatings. Thicker coatings are not so easily incorporated into growing oxide and remain for a longer time on the surface. The size of a reactive-element-oxide particle also increases and reaches a size, which is ineffective in inhibiting oxidation. This effect is more clearly seen for lower oxidation temperatures, as was observed for ceria-sol-coated nickel oxidized at 873 K. 1~ Also, as demonstrated in Fig. 4, the higher efficiency of coatings deposited on nonpreoxidized substrates is due to the rapid incorporation of ceria particles in the growing oxide film. A coating deposited on preformed oxide remains on the surface for a longer time as is seen from kinetics measurements. 9,1~

It is clear from the depth location of cerium in an oxide film that a general correlation exists between the location of cerium in the oxide and the reduction in oxidation rates. 19'2~ Some authors detected the maximum concentration of reactive element at the gas-oxide interface, 2~ whereas the others find it at the oxide-metal interface. 2~,22 There is also literature showing reactive elements disseminated regularly in oxide scales. 6,23 Hussey et al. 24 noted that the location of cerium in an oxidized sputter-coated i ron-chromium alloy depends on coating thickness. For ineffective, thin- ceria coatings, the cerium maximum was present at the al loy-oxide inter- face. With increasing coating thickness, the maximum was shifted toward the outer surface of the oxide film. In the present case when the coarse ceria particles are spread throughout the oxide they do not reduce the oxidation rate compared to the uncoated nickel (Figs. 3 and 5b). From a comparison with the microstructure shown in Fig. 10 and the corresponding oxidation kinetics (Fig. 3) coarse CeO2 particles seem to be ineffective as a source of Ce ion segregants. The size of a CeO2 particle determines its effectiveness as a source of Ce ions for segregation at the oxide boundaries.

524 Czerwinski and Smeltzer

The plot of the ratio R of instantaneous parabolic rate constants indicates that the effect of a reactive element on the inhibition of nickel oxidation increases with time (Fig. 4). Two phenomena taking place in the surface portion of oxide affect these characteristics. During the early stages of oxidation, very small ceria particles deposited on the surface are incorporated into the growing nickel oxide. During further oxidation, nickel-oxide grain boundaries in this region become saturated with Ce ions, inhibiting the flux of Ni 2§ along these boundaries. The principal nickel oxidation mechanism at 973 K is by outward diffusion of Ni 2§ cations via grain boundaries. It is believed that the mechanism of the reactive-element effect is the blocking of these paths by large Ce 4+ cations. The distribution of the ceria-particle sizes after 4 hr oxidation in Fig. 14, depicts that in comparison to relatively uniform size of particles before oxidation (Fig. 2a), after 4 hr exposure the ceria particle size is rather non-uniform. It is characteristic that the highest number of particles have a diameter of approximately 3-4 nm suggesting the preferential dissolution of smaller particles. Some coarse particles, with a diameter of about 15 nm are produced in thicker regions of a coating and lead to its non-uniformity.

Development of Oxide Microstructure During the Early Stages of Exposure to Oxygen

It was possible to deduce the cross-sectional image of an oxide film formed on ceria-coated nickel after 4 hr of oxidation at 973 K on the basis of SEM and TEM planar observations (Fig. 15). The total thickness of oxide, including preoxidation, is 165 nm as estimated from oxygen uptake. The ceria-rich portion is located in the vicinity of the outer surface. As estimated from oxygen uptake during the very initial, transient stage of oxidation, this film should have a thickness of about 30 nm. Some surface overgrowth of nickel oxide is present at the location of the grai n boundaries of the nickel substrate. The major part of the oxide film is present beneath the ceria-modified portion of NiO. After long-time oxida- tion these overgrowths form a continuous outer film of nickel oxide19; however, even after 120hr oxidation at 973K the ceria-rich region is located close to the oxide surface. In this three-layer structure, the thickness ratio of the outer to inner part, as evaluated from cross-sectional TEM observations, is about 0.1.

If we assume that ceria particles act as inert markers, their location indicates the transport mechanism leading to the oxide-structure development as illustrated schematically in Fig. 16. At the beginning of oxidation, a ceria- modified nickel oxide film is formed by outward Ni 2+ diffusion. The nuclea- tion of small NiO grains on the top of ceria coatings after a few minutes of

Thin Oxide Films 525

Oxidation temperature 973 K OxidatiOn time 4 h

02 - gas

NiOovergrowth

- ~ NiO+CeO2 E= randomly oriented

NiO oriented epitaxially with Ni substrate

grain boundary Ni substrate

Fig. 15. Schematic diagram of cross-sectional microstructure de- velopment of NiO grown on ceria-coated nickel at 973 K for 4 hr.

Oxidation temperature 973 K

02 - gas

i i i i l l l l l I I i i i i I ~ I I / I

Nickel

C e O 2

NiO from preoxidation

02 - gas

/

2e /Ni2+ 2e-ll O 2"

i ,, I

Nickel

NiO overgrowth NiO+CeO 2

NiO

Fig. 16. Schematic diagram of mechanism of NiO growth on ceria-sol-coated nickel during the initial stages of oxidation.

526 Czerwinski and Smeltzer

oxidation (Fig. 9) supports this mechanism. This oxide layer contains ceria particles and cerium ions segregated at boundaries. Subsequent growth of nickel oxide occurs above and beneath this ceria-modified layer probably by outward Ni 2+ cations and inward 0 2- anions diffusion. The present measurements and observations do not distinguish whether oxygen diffusion occurs only in the ceria-modified layer or completely through the oxide film.

CONCLUSIONS

1. In surface-applied, sol-gel coatings on polycrystalline nickel, the size of ceria particles is a decisive factor affecting the oxidation kinetics, oxide microstructure and distribution of cerium within the oxide film. Coarse ceria particles with diameters higher than 10 nm in a coating are ineffective in decreasing the nickel oxidation rate and are spread throughout the oxide. Small particles with a mean size of 5 nm markedly inhibit the oxidation rate and after 4 hr oxidation at 973 K are located in the surface region of the oxide film and act as sources of cerium-ion segregation in NiO boundaries.

2. The analyses of oxide microstructure and microchemistry supported the predominant role of grain-boundary segregation of cerium ions in decreasing the nickel oxidation rate at 973 K.

3. Examination of oxide-growth surface, microstructure and cerium location indicates that the following mechanism can be proposed for oxide growth on ceria-sol-coated nickel during the initial 4 hr oxidation at 973 K: (i) during very early stages of exposure to oxygen, a ceria-modified nickel oxide film is formed by outward Ni 2+ diffusion; this portion of oxide with ceria particles and cerium-ion segregant in grain boundaries is believed to be transport controlling for steady-stage oxide growth, and (ii) oxide growth, following this transient period takes place predominantly under the ceria-modified portion of nickel oxide probably by inward oxygen diffusion.

ACKNOWLEDGMENTS

The authors express their appreciation to the Natural Sciences and Engineering Research Council of Canada for financial support of this research. We also wish to thank Dr. R. G. Macaulay-Newcombe for skillful assistance with the RBS analysis and Dr. A. Perovic for the EDX examination.

Thin Oxide Films 527

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