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Oxidation pitting of pyrolytic graphite and vitreous carbon
196
LE2ITERS
TO ‘llXl-3 EDITUR
significantly higher rate, which apparently contimres indefinitely. The initial and tinal rates both vary with temperature, passing through a maximum near 1900°K; near this temperature the initial rate per&s for such a short time that it is difbcnlt to measure. It mav also be noted (bottom left, Fig. 1) that the ZTA graph&e is not appreciably at&ted by the high velocity flow of argon used during the period, common to all runs, when the %emperature level and umstaney are be&g estab&&ed, The rdatim&ip cllf the i&t&d iind finets rates to the timeindependent rates of PG oxidation@) are shown in Fig. 3.
reaction between coke and CC&. After a pariod of time, the binder reaction has proceeded sufficientlyfar inward to begin to undermine the coke particles, which are then sheared away by the high velocity gas stream. Beyond this point, the ablation rate depends chiefly on the rate of binder consumption, but the measured rate includes rate of particle loss as weir as rate of gasifkzati~nby chemicai reaction. C%n&tent with this picture, the hmiting rate of abgation of ZTA graphite near IQOPK increased v&h approximatefy the 0.4 power of the veiocity of the &r&ring gai when this velocity was varied between 500 and 10.000 cm se@. whereas WALLS and STRICKLAND-~ON~T~"~~(~~ found ihe oxidation of pyrolytic graphite by 0s under similar hydrodynamic conditions to be velocity-independent at gas velocities greater than 2500 cm se&. even though the ablation iates in the two sets of experiments Were nearly the same. The authors are indebted to Dr. assistance in the design of the camera-
microscope.
Frrc, 3, ~~pera~~ dependence of the rate of ablation of ZTA graphite in Bowing COs. CO, stagnation pressure 400 tom. The ablation af ZTA graphite under attack by COs may be qualitatively explained by assuming that the binder phase reacts more rapidly than the coke particles in the graphite and that each sepsrately would exhibit a complex temperature dependence like that in Fig. 3. Since the method of measurement d~~~~sh~ the h@ess* i.e. least reaoted, poiuts on the surface* the IrMai rates observed correspond to the rate of &emicaI
Cmbon 1967, Vol. 5, pp. 196-197.
Pergamon Press Ltd.
Trix OsJECT of this note is to draw attention ta the similarities between oxidation pits observed with two carbons of markedly different micro- and macro-structure, namely pyrolytic graphite and vitreous carbon. The need for further research is indicated. The oxidation behaviour of pyrolytm graphite is far from simple but, as with single erystais of graphite, the general evidence@+ is that tha rate of oxidation is faster on the edge planes paraffeI to the plane of deposition (preferred a-ax& direction] than on the basal $anes
1. WrLsoIi H. w. JR. and LADDL R., Rev. Sci. fvtstu, 37, 1359 (1966). 2. Lmn‘f. R: and WMSH P, N., Ca&ea 4, 539 (19661, 3, Because of this, the term ablation is used here to describe the fess of material when ZTA sranhite is oxidized. It ahonld be noted, however*tharit was not eatabhshed that particle loss occurred in the vicinity of the stagnation point, where the rates were measured. The narticIes collected could have come from the aides of-the sample, where shear forces are higher. 4, This instrument wifi be described in detail elsewhere. 5. WALLS J. R. and STRI~~&~-CO~TABLB R. F., Car&~ 1, 333 (1964).
Printed in Great Britain (preferred o-axis direction), But in addition to general oxidation, irregular pitting oxidation can occur on the basal p~s.~~,~~ This means that though fignres for weight loss (expressed, for example, as mgfcm%ninf in the a-axis direction give a realistic indication of the severity of wastage to be expected under given application conditions, this is not the case in the c-axis direction. Here while oxidation pits may make a relatively low contribution to the measured total wastage, they can cause serious weakening, or even puncturing, of pyrolytic layers used to protect the surfaces of devices from thermal breakdown. Figures 1 and 3@ illustrate the grof&s of pits
* See facing page.
FIG. 1. Photomicrograph
(X
200) of oxidation
pits on surface of pyrolytic
graphite.
FIG. 2. Photomicrograph
(X 200) of oxidation pits on surface of pyrolytic graphite.
..
197
CARBON resulting from exposing a pyrolytic graphite tube (2150°C deposit from methane) to fast flowing air (300 cm/set) at 1600°C for five min. The two examples represent the observed extremes of regular and irregular pit profiles. Similar oxidation pitting occurs in vitreous carbon; Figs. 3 and 4 show the mode and extent of attack observed with a tubular specimen oxidised at 2000°C by a flowing stream of CO*. In a recent paper@) LEWIS mentioned that there is auto-radiographic evidence that the position of similar pits observed with vitreous carbon oxidised in air coincided with metallic impurities initially present on the surface, but no details were given in the paper. He also reports that pitting was not eliminated by using specially purified resins (~5 ppm ash) to prepare the vitreous carbon-but he noted that adventitious contamination occurred during the course of oxidation. It may well be that the original sites of the oxidation pits coincide with the location of catalytic surface impurity centres and it is noteworthy that with the pyrolytic material the frequency of occurrence of oxidation pits decreases (a) with increasing temperature of deposition and (b) for a given temperature of deposition, with subsequent annealing at around 3000”C.(4) But the lateral extension of the pits cannot readily be explained by catalytic oxidation alone (cf. the narrow oxidation channels observed by PRESLAND and HEDLEY@)). Nor does it seem possible, as has been suggested by LEW,@~ that the driving force behind the lateral extension of the pits in pyrolytic graphite is the release of available strain energy.(‘) This explanation would require a cooperative energy release mechanism that acts at highly localised sites and extends over a considerable distance (up to, say, 1 mm) in both the preferred a- and c-axis directions, without regard for the major structural discontinuities at the boundaries of the cones that dominate the macro-structure. These points are well illustrated in Figs. 1 and 2. With vitreous carbon there is no obvious source of available strain energy associated with structural anisotropy.
Our understanding of pitting oxidation in pyrolytic graphite and in vitreous carbon is not sticiently complete, therefore, to suggest possible means of control. Further experiments along the lines of adding surface catalytic impurities to an already well characterised material might give useful information. An alternative approach would be to study the effect of heat-treatment, or, of ion or electron bombardment, from the point of view of the elimination of surface impurities and the reduction in the severity of pitting.
Acknowledgments-Mrs. D. E. Granger obtained the microeranhs and Mr. T. C. Lewis of the Allen Clark Research- Centre prodded the sample of oxidised vitreous carbon. REFERENCES 1. HORTONW. S., Proceedings of the Fifth Conference on Carbon, Vol. 2., p. 233, Pergamon Press, Oxford (1963). 2. LEVYM. and WONG P., J. Electrochem. Sot. 111,1088
(1964). 3. LEVY M., Ind, Eng. Chem. Prod. Res. Develop. 1, 19 (1962). 4. TOMINAGA Y. and NAGAOKI T., Symposium on Carbon, VIII.4.1.
Tokyo (1964).
5. L~wrs J. C. A., Proceedings of the Second Conference on Industrial Carbon and Gtabhite. _ _ London. _ _D. 12.58 (1965). 6. PRESLAN~A. E. B. and HEDLEY J. A., J. Nucl. Mater. 10, 99 (1963). 7. BLACKMANL. C. F., SAUNDERSG. A. and UBBELOHDE A. R., Proc. Roy. Sot. A264, 19 (1961). British Coal Utilisation Research AssocMtion Randalls Road Leatherhead suwey
L. C. F. BLACKMAN
LE2ITERS
TO ‘llXl-3 EDITUR
significantly higher rate, which apparently contimres indefinitely. The initial and tinal rates both vary with temperature, passing through a maximum near 1900°K; near this temperature the initial rate per&s for such a short time that it is difbcnlt to measure. It mav also be noted (bottom left, Fig. 1) that the ZTA graph&e is not appreciably at&ted by the high velocity flow of argon used during the period, common to all runs, when the %emperature level and umstaney are be&g estab&&ed, The rdatim&ip cllf the i&t&d iind finets rates to the timeindependent rates of PG oxidation@) are shown in Fig. 3.
reaction between coke and CC&. After a pariod of time, the binder reaction has proceeded sufficientlyfar inward to begin to undermine the coke particles, which are then sheared away by the high velocity gas stream. Beyond this point, the ablation rate depends chiefly on the rate of binder consumption, but the measured rate includes rate of particle loss as weir as rate of gasifkzati~nby chemicai reaction. C%n&tent with this picture, the hmiting rate of abgation of ZTA graphite near IQOPK increased v&h approximatefy the 0.4 power of the veiocity of the &r&ring gai when this velocity was varied between 500 and 10.000 cm se@. whereas WALLS and STRICKLAND-~ON~T~"~~(~~ found ihe oxidation of pyrolytic graphite by 0s under similar hydrodynamic conditions to be velocity-independent at gas velocities greater than 2500 cm se&. even though the ablation iates in the two sets of experiments Were nearly the same. The authors are indebted to Dr. assistance in the design of the camera-
microscope.
Frrc, 3, ~~pera~~ dependence of the rate of ablation of ZTA graphite in Bowing COs. CO, stagnation pressure 400 tom. The ablation af ZTA graphite under attack by COs may be qualitatively explained by assuming that the binder phase reacts more rapidly than the coke particles in the graphite and that each sepsrately would exhibit a complex temperature dependence like that in Fig. 3. Since the method of measurement d~~~~sh~ the h@ess* i.e. least reaoted, poiuts on the surface* the IrMai rates observed correspond to the rate of &emicaI
Cmbon 1967, Vol. 5, pp. 196-197.
Pergamon Press Ltd.
Trix OsJECT of this note is to draw attention ta the similarities between oxidation pits observed with two carbons of markedly different micro- and macro-structure, namely pyrolytic graphite and vitreous carbon. The need for further research is indicated. The oxidation behaviour of pyrolytm graphite is far from simple but, as with single erystais of graphite, the general evidence@+ is that tha rate of oxidation is faster on the edge planes paraffeI to the plane of deposition (preferred a-ax& direction] than on the basal $anes
1. WrLsoIi H. w. JR. and LADDL R., Rev. Sci. fvtstu, 37, 1359 (1966). 2. Lmn‘f. R: and WMSH P, N., Ca&ea 4, 539 (19661, 3, Because of this, the term ablation is used here to describe the fess of material when ZTA sranhite is oxidized. It ahonld be noted, however*tharit was not eatabhshed that particle loss occurred in the vicinity of the stagnation point, where the rates were measured. The narticIes collected could have come from the aides of-the sample, where shear forces are higher. 4, This instrument wifi be described in detail elsewhere. 5. WALLS J. R. and STRI~~&~-CO~TABLB R. F., Car&~ 1, 333 (1964).
Printed in Great Britain (preferred o-axis direction), But in addition to general oxidation, irregular pitting oxidation can occur on the basal p~s.~~,~~ This means that though fignres for weight loss (expressed, for example, as mgfcm%ninf in the a-axis direction give a realistic indication of the severity of wastage to be expected under given application conditions, this is not the case in the c-axis direction. Here while oxidation pits may make a relatively low contribution to the measured total wastage, they can cause serious weakening, or even puncturing, of pyrolytic layers used to protect the surfaces of devices from thermal breakdown. Figures 1 and 3@ illustrate the grof&s of pits
* See facing page.
FIG. 1. Photomicrograph
(X
200) of oxidation
pits on surface of pyrolytic
graphite.
FIG. 2. Photomicrograph
(X 200) of oxidation pits on surface of pyrolytic graphite.
..
197
CARBON resulting from exposing a pyrolytic graphite tube (2150°C deposit from methane) to fast flowing air (300 cm/set) at 1600°C for five min. The two examples represent the observed extremes of regular and irregular pit profiles. Similar oxidation pitting occurs in vitreous carbon; Figs. 3 and 4 show the mode and extent of attack observed with a tubular specimen oxidised at 2000°C by a flowing stream of CO*. In a recent paper@) LEWIS mentioned that there is auto-radiographic evidence that the position of similar pits observed with vitreous carbon oxidised in air coincided with metallic impurities initially present on the surface, but no details were given in the paper. He also reports that pitting was not eliminated by using specially purified resins (~5 ppm ash) to prepare the vitreous carbon-but he noted that adventitious contamination occurred during the course of oxidation. It may well be that the original sites of the oxidation pits coincide with the location of catalytic surface impurity centres and it is noteworthy that with the pyrolytic material the frequency of occurrence of oxidation pits decreases (a) with increasing temperature of deposition and (b) for a given temperature of deposition, with subsequent annealing at around 3000”C.(4) But the lateral extension of the pits cannot readily be explained by catalytic oxidation alone (cf. the narrow oxidation channels observed by PRESLAND and HEDLEY@)). Nor does it seem possible, as has been suggested by LEW,@~ that the driving force behind the lateral extension of the pits in pyrolytic graphite is the release of available strain energy.(‘) This explanation would require a cooperative energy release mechanism that acts at highly localised sites and extends over a considerable distance (up to, say, 1 mm) in both the preferred a- and c-axis directions, without regard for the major structural discontinuities at the boundaries of the cones that dominate the macro-structure. These points are well illustrated in Figs. 1 and 2. With vitreous carbon there is no obvious source of available strain energy associated with structural anisotropy.
Our understanding of pitting oxidation in pyrolytic graphite and in vitreous carbon is not sticiently complete, therefore, to suggest possible means of control. Further experiments along the lines of adding surface catalytic impurities to an already well characterised material might give useful information. An alternative approach would be to study the effect of heat-treatment, or, of ion or electron bombardment, from the point of view of the elimination of surface impurities and the reduction in the severity of pitting.
Acknowledgments-Mrs. D. E. Granger obtained the microeranhs and Mr. T. C. Lewis of the Allen Clark Research- Centre prodded the sample of oxidised vitreous carbon. REFERENCES 1. HORTONW. S., Proceedings of the Fifth Conference on Carbon, Vol. 2., p. 233, Pergamon Press, Oxford (1963). 2. LEVYM. and WONG P., J. Electrochem. Sot. 111,1088
(1964). 3. LEVY M., Ind, Eng. Chem. Prod. Res. Develop. 1, 19 (1962). 4. TOMINAGA Y. and NAGAOKI T., Symposium on Carbon, VIII.4.1.
Tokyo (1964).
5. L~wrs J. C. A., Proceedings of the Second Conference on Industrial Carbon and Gtabhite. _ _ London. _ _D. 12.58 (1965). 6. PRESLAN~A. E. B. and HEDLEY J. A., J. Nucl. Mater. 10, 99 (1963). 7. BLACKMANL. C. F., SAUNDERSG. A. and UBBELOHDE A. R., Proc. Roy. Sot. A264, 19 (1961). British Coal Utilisation Research AssocMtion Randalls Road Leatherhead suwey
L. C. F. BLACKMAN