- Email: [email protected]
Silica coatings on strongly passivated substrates
Surface and Coatings Technology,
SILICA COATINGS
ON STRONGLY
W. HANNI, H. E. HINTERMANN, Swiss Centre for Electronics (Switzerland)
463
36 (1988) 463 - 470
PASSIVATED
SUBSTRATES*
D. MOREL and A. SIMMEN
and Microtechnology
Inc., Maladi&-e 71, 2007 Neuchdtel
(Received March 14,1988)
Silicon dioxide can be deposited by pyrolysis of silicon alkoxide onto highly passivating substrates such as stainless steels (e.g. AISI 316L), superalloys (H34CT), and titanium and its alloys. Before silicon dioxide deposition the passivating layer has to be removed, and a strongly adherent, dense oxide layer has to be re-formed (pre-oxidation). The substrate composition, in particular the carbon content, is the determining factor for the type of surface preconditioning used; this pretreatment is necessary for the complete removal of the chromium-rich passivating layer. Mechanical grinding, honing and polishing, chemical wet etching, electrochemical polishing and gas phase etching are typical treatments considered for surface preparation; these treatments replace partially or completely the reduction of the passivating oxide layer by means of the carbon in the substrate, which occurs above 780 “C. For the final mechanical operation only diamond should be considered (tool, abrasive); the use of other materials tends to contaminate the surface and leads to the formation of mixed oxides during the pre-oxidation. The pre-oxidation itself is performed under pure water vapour, moist nitrogen or isopropanol; in some cases (titanium and its alloys) preconditioning under pure nitrogen must be carried out. Internal stresses must be relieved before the pre-oxidation and silicon oxide deposition treatments. It was also found that, during the pyrolysis of silicon alkoxide, the reactor atmosphere must have some water vapour, which can be produced by an adequate evaporator or by dehydration of isopropanol.
1. Introduction The possibilities of depositing SiOz on metallic substrates at temperatures below 1000 “C are described in the literature [l -41. This study was *Paper presented at the 15th International Conference San Diego, CA, U.S.A., April 11 - 15,1988. 0257-8972/88/$3.50
on Metaihmgical Coatings,
@ Elsevier Sequoia/Printed in The Netherlands
464
concerned with SiOz coatings obtained by the thermal decomposition of silicon alkoxide tetraethylorthosilicate (TEOS) at 780 “C. The thermal decomposition of TEOS is strongly influenced by the water vapour concentration in the gas atmosphere. When SiO? is produced in a very dry gas atmosphere ([H,O] < 5 ppm) a cracked coating is obtained. Before the oxide deposition takes place, the surface is oxidized by pure water vapour; the temperature at which this pre-oxidation takes place depends on the substrate. For alloyed and corrosion-resistant steels, and for superalloys, the process temperature can be as high as 900 “C. SiO, coatings are mainly used as an electrical insulator and for corrosion protection. The substrate surface preparation is therefore limited to sandblasting and consequent ultrasonic cleaning in organic solvents. This cleaning process is sufficient for Si02 coatings on alloyed heat- and corrosion-resistant steel, AISI 310, as well as on Incoloy 800. The substrate preparation described above is, however, insufficient for titanium, its alloys, corrosion-resistant steels (e.g. AISI 316L) and superalloys (e.g. H34CT). Spalling of the coating was observed immediately after the silicon oxide deposition treatment. This poor adhesion is observed at the substrate-pre-oxidation interface or within the pre-oxidation layer itself. The pre-oxidation-SiO, interface adhesion appears to be good. The study presented in this paper was mainly concerned with substrate preparation and pre-oxidation. Observation indicated that not only the natural passivation layer, the internal stresses [5] and the mechanical surface preparation but also the pre-oxidation are determining factors in the adhesion of the SiOz layer. These parameters are discussed in more detail.
2. Experimental procedures Water vapour oxidation of corrosion-resistant steels has shown that the properties of the oxide layer are very dependent on the chemical composition of the substrate, especially its carbon concentration. Typical compositions (in weight per cent) of corrosion-resistant steels are 10% - 35% Ni and 18% - 25% with or without substitution elements such as cobalt instead of nickel and additional alloying elements such as molybdenum, titanium or aluminium and with carbon concentrations ranging from 0.02% to 0.05%. When the carbon concentration is below 0.03% (e.g. AISI 316L) the oxide film produced by water vapour consists of two phases and has a thickness of several micrometres; when the substrate contains more than 0.1% C a compact single-phase layer thicker than 1 pm is obtained. The carbon in the substrate (e.g. in austenite) can act as a reducing agent toward the oxidebased passivating layer. The higher the carbon content in the substrates, and the higher the treatment temperature, the more important is this reduction effect. To illustrate this it can be mentioned that on AISI 440C steel (17% Cr, 1% C) the passivating fihn is completely removed after 3 mm under argon at 1000 “C. It is believed that there is a direct relation between the
465
carbon content of the substrate and the passivating layer which automatically forms in air. The adhesion and the morphology of the oxide layer produced by water vapour oxidation are also dependent on the mechanical surface preparation of the substrate. In addition to the internal stresses produced by the machining operation, the substrate surface can be contaminated with “pressed-in” abrasive grains which originate from sandblasting or polishing operations; this is even more pronounced when the considered substrates are relatively soft. Most of these pressed-in abrasive grains (oxides, carbides, nitrides etc.) are not completely removed by ultrasonic cleaning. During the water vapour oxidation at 800 - 900 “C, these foreign particles react with the passivating layer and form mixed oxides; these mixed oxides must be avoided since they decrease the adhesion of the pre-oxidation film to the substrate and the adhesion of the SiOz to the interlayer. When the workpieces to be coated have a complicated geometry, it is often impossible to submit all the surfaces to the same mechanical machining operation; therefore, workpieces may have surfaces with different passivating layer thicknesses. These surface irregularities, leading to poor adhesion, can be improved by carefully selecting and applying adequate chemical and/ or electrochemical treatments. For practical engineering purposes (dimensions, geometry and tolerances of the workpieces) it is important that the selected processes remove a minimum amount of material from the substrate. Mechanical operations induce internal stresses not only near the surface but also deep into the substrate; polishing may affect metallic materials to a depth of 10 - 50 pm, grinding and honing to 50 - 500 pm, and milling and drilling to 200 - 1000 pm. These internal stresses have to be relieved through thermal treatments. These treatments have to be performed before the coating process, especially when the dimensional tolerances of the workpieces are important. Even mild surface finishings, such as turning, honing or grinding, introduce stresses in the substrate by cold working. The oxide layers are too brittle to follow the dimensional changes associated with the stress relieving when it is carried out during the coating process, and they crack or spall off. Depending on the substrate type, pure water vapour oxidation starts at 120 “C and can continue up to temperatures ranging from 350 to 950 “C. Low carbon content substrates (e.g. AISI 316L steel) can be pre-oxidized under “milder” conditions, such as humid Nz at normal pressure or under mild vacuum (35 mbar). Strongly passivating alloys are pre-oxidized by using isopropanol, the carrier gas being N, ; the oxide deposition process continues under the above-mentioned conditions between 750 and 950 “C at normal pressure or under vacuum (35 mbar). Hydrocarbon molecules produced by the thermal decomposition of isopropanol act to reduce residues of the passivating layer. The water vapour oxidation of titanium and its alloys at temperatures above 780 “C is not applicable for SiO? deposition onto these metals. It has been shown that gas phase nitriding at 800 - 850 “C improved the adhesion of the SiO, coating. Apparently the nitride layer acts as a
466
diffusion barrier for the oxidation of the substrate; a gas phase etching is, however, necessary before gas phase nitriding. The actual SiO? depositions take place, as mentioned earlier, through thermal decomposition of TEOS. The metal-organic chemical vapour deposition equipment used in this work operates either manually or semi-automatically. The concept of the unit is such that the gas phase etching, the preoxidation and the oxide deposition are performed in a single run. The SiO, deposition takes place at 750 - 800 “C through thermal decomposition of TEOS transported by N, in a vacuum of 50 mbar. The SiOz layer obtained by this method presents a spider-web-like crack pattern. If, however, water vapour is added to the gas phase, a crack-free coating can be obtained. Isopropanol can also be used as a substitute for water vapour, generating humidity by its’dehydration.
3. Results The AISI 316L stainless steel sample shown in Fig. 1 was lapped prior to oxide deposition. A two-phase oxide interlayer with an SiO, coating on top can be seen. The well-adherent “speckled” interlayer inner portion is irregular and partially diffused into the substrate; a microprobe analysis shows an unusually high chromium concentration, and the microstructure corresponds to (Fe,.,, Cro.&Os. The outside portion of the interlayer, in contrast, consists of the desired Fez03. At the contact area of the two interlayer portions are large pores which decrease the adhesion; the spalling of the coating was mostly observed at this level. The 316L steel has a very stable passivating layer which is not fully reduced at high temperature, which is due to its low carbon content (0.02%). Figure 2 presents a section of another AISI 316L stainless steel sample coated under identical conditions. This sample, however, after lapping, underwent an additional polishing with diamond paste followed by gas phase etching. The neutral diamond polishing helps to remove the last residues of the lapping process. It can be seen that the results can be influenced by the grinding and/or polishing grains pressed into the surface; the same has been observed in the past with other substrates and non-oxide coatings such as TiC and TiN. It is known that stable mixed oxides are formed at the surface during heat treatment in water vapour. Figure 3 shows an AISI 316L steel sample coated with SiO? after lapping, wet chemical etching and water vapour pre-oxidation. The oxide interlayers which are visible on this micrograph are very different from those seen in Fig. 1. The interlayer thickness is mostly below 1 pm, and the SiOz layer appears to be well adherent. The chemical etching mixture consisted of HF:HNOJ:HzO (3:10:87 by volume) at 50 “C; this treatment is carried out in an ultrasonic bath for 3 - 5 min, e.g. until the evolution of H,. However, the reaction time was not sufficient; in some places Cr-Fe oxides were still present. This has a negative influence on the interlayer formation as can be
467
Fig. 1. Metallographic section through an AISI 316L stainless steel sample after lapping, water vapour pre-oxidation and SiOz deposition (unetched). Fig. 2. Metallographic section through an AISI 316L stainless steel sample after lapping, diamond polishing, gas phase etching, water vapour pre-oxidation and SiOz deposition (unetched).
Fig. 3. Metallographic section through an AISI 316L stainless steel sample after lapping, wet etching, wet chemical etching (HF-HNOs), water vapour pre-oxidation and SiOz deposition (unetched). Fig. 4. Metallographic section through an SiOz-coated H34CT superalloy sample. The oxide layer spalled off completely and the near-surface region is strongly deformed (etched).
468
seen from Fig. 1. Hence gas phase etching did contribute to remove these oxides completely. If the etching time is too long, surface roughening takes place. Figure 4 shows an H34CT superalloy sample after an Si02 deposition treatment and an etching and pre-oxidation pretreatment. The SiOz layer spalled-off, together with the interlayer. Etching showed that the substrate surface was stressed and cold worked during the mechanical operations. The results of stress-relieving thermal treatments are shown in Fig. 5; the internal stresses were determined by the sin’$ X-ray diffraction method [ 6,7]. After 1 h at 1000 “C the stress level decreased significantly. If the thermal stress relieving continues for more than 1 h the influence of a phase or structural change can be observed (renewed increase in internal stresses). A treatment of 8 h is necessary to obtain a stress-free surface. To produce a well-adherent Si02 coating on such substrates or workpieces it is therefore necessary to stress relieve them for 8 h before etching, pre-oxidation and actual oxide deposition. It was observed, especially with AISI 316L, that the gas phase etching greatly reduces the spontaneously formed passivating layers, especially in connection with the wet chemical and mechanical metalworking operations. Figure 6 shows an Si02-coated nitrided TiA16V4 sample. The oxide layer obtained at temperatures above 780 “C is no longer compact and adherent, even when the material is pre-oxidized by water vapour in moist NZ. The sample which was nitrided under dry N, at 800 “C, in contrast, produced a dense Si02 deposit. The nitrided layer constitutes an efficient diffusion barrier with respect to the oxidizing atmosphere during Si02 deposition. Between room temperature and 900 “C the oxidation of the titanium alloy occurs in four different modes.
annealing
0
2
4
6
1000°C
0
h
Fig. 5. Surface region stress relieving of a machined H34CT superalloy sample by thermal treatment at 1000 “C for different periods of time.
Fig. 6. Metallographic section through a TiA16V4 sample after wet chemical etching, gas nitriding and SiOz deposition in a water-vapour-containing gas phase (unetched). Fig. 7. Scanning electron microscopy view of the cracked surface of a TiAl6V4 sample after wet etching, nitriding and Si02 deposition in a water-vapour-free gas phase (etched).
When SiOZ layers are obtained under water-vapour-free conditions ([H,O] < 5 ppm) the surface layer exhibits a spiderweb-like crack pattern over its entire surface (Fig. 7). This crack formation can be avoided by introducing sufficient water vapour in the gas phase. To avoid contamination of the gas system with water vapour, and corrosion or oxidation products in the gas lines and valves, it is advisable to replace the water vapour by chemicals which decompose and produce water vapour in situ at elevated temperatures. Secondary and tertiary alcohols are suitable as water vapour donors; for the tests described here, isopropanol was used. It is assumed that the SiOZ deposition through thermal decomposition of TEOS in a water-vapourfree gas phase is incomplete and leads to substoichiometric Si02. As the SiOZ layers are amorphous, X-ray diffraction methods cannot be used and, furthermore, microprobe analysis is not sufficiently precise to determine the exact stoichiometry.
4. Conclusions If the substrate properties are carefully taken into consideration and if the passivating layers can be removed, it is possible to deposit SiOZ on highly passivating substrates such as stainless steels (e.g. AISI 316L), superalloys (H34CT), and titanium and its alloys. An internal reduction of the passivating layer can be obtained with steels having a high carbon content (0.1% or more) and operating at temperatures above 780 ‘2. This is not the case for steels containing little carbon or which contain carbon-stabilizing elements
470
such as chromium and molybdenum. The remaining chromium-rich passivating layer can be removed completely through wet chemical and/or electrochemical and/or gas phase etching. Abrasive particles pressed into the substrate surface during grinding and polishing must be removed using diamond paste; diamond is inert in the reaction systems used here. If the surface is contaminated with abrasive particles, the water vapour oxidation will lead to mixed oxide layers of uncontrolled composition and of poor adhesion, independently of whether the pre-oxidation occurs under pure water vapour, moist nitrogen or isopropanol. The internal stresses induced by the mechanical operations must be relieved before the gas phase etching. The thermal expansion coefficients of stainless steel and amorphous SiOz are very different; therefore, internal stresses and phase and structural changes, especially during cooling, easily provoke spalling of the coating. Water vapour is necessary in the gas phase during SiOz deposition to produce crack-free SiOz coatings.
References 1 A. J. Foster, M. L. Sims and D. Young, Protective metal oxide films on metal or alloy substrate surfaces susceptible to coking, corrosion or catalytic activity, U.S. Patent 4,297,150, October 27, 1981, to British Petroleum. 2 D. E. Brown, J. T. K. Clark, A. I. Foster, J. J. McCarroll and M. L. Sims, The inhibition of coke formation in ethylene steam cracking, Natl. Meet. American Chemical Society, New York, 1981. 3 D. E. Brown, J. T. K. Clark, A. I. Foster, J. J. McCarroll, M. S. Richards, M. L. Sims and M. A. M. Swidzinski, A silica coating process for passivation of steels in high temperature environments, Proc. 8th Znt. Conf. on Chemical Vapour Deposition, Paris, 1981, Electrochemical Society, Pennington, NJ, p. 699. 4 J. T. K. Clark, A. I. Foster, M. L. Sims, J. M. Swidzinnski and D. Young, The development of CVD oxide coatings for the protection of metal surfaces, Proc. 4th Eur. Chemical Vapour Deposition Conf., Eindhoven, 1983, Philips Centre for Manufacturing Technology, Eindhoven, p. 385. 5 Stress relieving of austenitic stainless steels, ASM Metals Handbook, Vol. 4, Heat Treating, American Society for Metals, Metals Park, OH, 9th edn., p. 647. 6 L. Chollet, H. Boving and H. E. Hintermann, Residual stress measurement of refractory coatings as a nondestructive evaluation, Proc. 6th Znt. Conf on Nondestructive Evaluation in Nuclear Industry, Zurich, 1983, in Trans. ASM, (1983) 817. 7 L. Chollet and A. J. Perry, The state of stress both in films and their substrates. In E. Macherauch and V. Hank (eds.), Residual Stresses in Science and Technology, Vol. 1, DGM Verlag, Oberursel, 1987, p. 485.
SILICA COATINGS
ON STRONGLY
W. HANNI, H. E. HINTERMANN, Swiss Centre for Electronics (Switzerland)
463
36 (1988) 463 - 470
PASSIVATED
SUBSTRATES*
D. MOREL and A. SIMMEN
and Microtechnology
Inc., Maladi&-e 71, 2007 Neuchdtel
(Received March 14,1988)
Silicon dioxide can be deposited by pyrolysis of silicon alkoxide onto highly passivating substrates such as stainless steels (e.g. AISI 316L), superalloys (H34CT), and titanium and its alloys. Before silicon dioxide deposition the passivating layer has to be removed, and a strongly adherent, dense oxide layer has to be re-formed (pre-oxidation). The substrate composition, in particular the carbon content, is the determining factor for the type of surface preconditioning used; this pretreatment is necessary for the complete removal of the chromium-rich passivating layer. Mechanical grinding, honing and polishing, chemical wet etching, electrochemical polishing and gas phase etching are typical treatments considered for surface preparation; these treatments replace partially or completely the reduction of the passivating oxide layer by means of the carbon in the substrate, which occurs above 780 “C. For the final mechanical operation only diamond should be considered (tool, abrasive); the use of other materials tends to contaminate the surface and leads to the formation of mixed oxides during the pre-oxidation. The pre-oxidation itself is performed under pure water vapour, moist nitrogen or isopropanol; in some cases (titanium and its alloys) preconditioning under pure nitrogen must be carried out. Internal stresses must be relieved before the pre-oxidation and silicon oxide deposition treatments. It was also found that, during the pyrolysis of silicon alkoxide, the reactor atmosphere must have some water vapour, which can be produced by an adequate evaporator or by dehydration of isopropanol.
1. Introduction The possibilities of depositing SiOz on metallic substrates at temperatures below 1000 “C are described in the literature [l -41. This study was *Paper presented at the 15th International Conference San Diego, CA, U.S.A., April 11 - 15,1988. 0257-8972/88/$3.50
on Metaihmgical Coatings,
@ Elsevier Sequoia/Printed in The Netherlands
464
concerned with SiOz coatings obtained by the thermal decomposition of silicon alkoxide tetraethylorthosilicate (TEOS) at 780 “C. The thermal decomposition of TEOS is strongly influenced by the water vapour concentration in the gas atmosphere. When SiO? is produced in a very dry gas atmosphere ([H,O] < 5 ppm) a cracked coating is obtained. Before the oxide deposition takes place, the surface is oxidized by pure water vapour; the temperature at which this pre-oxidation takes place depends on the substrate. For alloyed and corrosion-resistant steels, and for superalloys, the process temperature can be as high as 900 “C. SiO, coatings are mainly used as an electrical insulator and for corrosion protection. The substrate surface preparation is therefore limited to sandblasting and consequent ultrasonic cleaning in organic solvents. This cleaning process is sufficient for Si02 coatings on alloyed heat- and corrosion-resistant steel, AISI 310, as well as on Incoloy 800. The substrate preparation described above is, however, insufficient for titanium, its alloys, corrosion-resistant steels (e.g. AISI 316L) and superalloys (e.g. H34CT). Spalling of the coating was observed immediately after the silicon oxide deposition treatment. This poor adhesion is observed at the substrate-pre-oxidation interface or within the pre-oxidation layer itself. The pre-oxidation-SiO, interface adhesion appears to be good. The study presented in this paper was mainly concerned with substrate preparation and pre-oxidation. Observation indicated that not only the natural passivation layer, the internal stresses [5] and the mechanical surface preparation but also the pre-oxidation are determining factors in the adhesion of the SiOz layer. These parameters are discussed in more detail.
2. Experimental procedures Water vapour oxidation of corrosion-resistant steels has shown that the properties of the oxide layer are very dependent on the chemical composition of the substrate, especially its carbon concentration. Typical compositions (in weight per cent) of corrosion-resistant steels are 10% - 35% Ni and 18% - 25% with or without substitution elements such as cobalt instead of nickel and additional alloying elements such as molybdenum, titanium or aluminium and with carbon concentrations ranging from 0.02% to 0.05%. When the carbon concentration is below 0.03% (e.g. AISI 316L) the oxide film produced by water vapour consists of two phases and has a thickness of several micrometres; when the substrate contains more than 0.1% C a compact single-phase layer thicker than 1 pm is obtained. The carbon in the substrate (e.g. in austenite) can act as a reducing agent toward the oxidebased passivating layer. The higher the carbon content in the substrates, and the higher the treatment temperature, the more important is this reduction effect. To illustrate this it can be mentioned that on AISI 440C steel (17% Cr, 1% C) the passivating fihn is completely removed after 3 mm under argon at 1000 “C. It is believed that there is a direct relation between the
465
carbon content of the substrate and the passivating layer which automatically forms in air. The adhesion and the morphology of the oxide layer produced by water vapour oxidation are also dependent on the mechanical surface preparation of the substrate. In addition to the internal stresses produced by the machining operation, the substrate surface can be contaminated with “pressed-in” abrasive grains which originate from sandblasting or polishing operations; this is even more pronounced when the considered substrates are relatively soft. Most of these pressed-in abrasive grains (oxides, carbides, nitrides etc.) are not completely removed by ultrasonic cleaning. During the water vapour oxidation at 800 - 900 “C, these foreign particles react with the passivating layer and form mixed oxides; these mixed oxides must be avoided since they decrease the adhesion of the pre-oxidation film to the substrate and the adhesion of the SiOz to the interlayer. When the workpieces to be coated have a complicated geometry, it is often impossible to submit all the surfaces to the same mechanical machining operation; therefore, workpieces may have surfaces with different passivating layer thicknesses. These surface irregularities, leading to poor adhesion, can be improved by carefully selecting and applying adequate chemical and/ or electrochemical treatments. For practical engineering purposes (dimensions, geometry and tolerances of the workpieces) it is important that the selected processes remove a minimum amount of material from the substrate. Mechanical operations induce internal stresses not only near the surface but also deep into the substrate; polishing may affect metallic materials to a depth of 10 - 50 pm, grinding and honing to 50 - 500 pm, and milling and drilling to 200 - 1000 pm. These internal stresses have to be relieved through thermal treatments. These treatments have to be performed before the coating process, especially when the dimensional tolerances of the workpieces are important. Even mild surface finishings, such as turning, honing or grinding, introduce stresses in the substrate by cold working. The oxide layers are too brittle to follow the dimensional changes associated with the stress relieving when it is carried out during the coating process, and they crack or spall off. Depending on the substrate type, pure water vapour oxidation starts at 120 “C and can continue up to temperatures ranging from 350 to 950 “C. Low carbon content substrates (e.g. AISI 316L steel) can be pre-oxidized under “milder” conditions, such as humid Nz at normal pressure or under mild vacuum (35 mbar). Strongly passivating alloys are pre-oxidized by using isopropanol, the carrier gas being N, ; the oxide deposition process continues under the above-mentioned conditions between 750 and 950 “C at normal pressure or under vacuum (35 mbar). Hydrocarbon molecules produced by the thermal decomposition of isopropanol act to reduce residues of the passivating layer. The water vapour oxidation of titanium and its alloys at temperatures above 780 “C is not applicable for SiO? deposition onto these metals. It has been shown that gas phase nitriding at 800 - 850 “C improved the adhesion of the SiO, coating. Apparently the nitride layer acts as a
466
diffusion barrier for the oxidation of the substrate; a gas phase etching is, however, necessary before gas phase nitriding. The actual SiO? depositions take place, as mentioned earlier, through thermal decomposition of TEOS. The metal-organic chemical vapour deposition equipment used in this work operates either manually or semi-automatically. The concept of the unit is such that the gas phase etching, the preoxidation and the oxide deposition are performed in a single run. The SiO, deposition takes place at 750 - 800 “C through thermal decomposition of TEOS transported by N, in a vacuum of 50 mbar. The SiOz layer obtained by this method presents a spider-web-like crack pattern. If, however, water vapour is added to the gas phase, a crack-free coating can be obtained. Isopropanol can also be used as a substitute for water vapour, generating humidity by its’dehydration.
3. Results The AISI 316L stainless steel sample shown in Fig. 1 was lapped prior to oxide deposition. A two-phase oxide interlayer with an SiO, coating on top can be seen. The well-adherent “speckled” interlayer inner portion is irregular and partially diffused into the substrate; a microprobe analysis shows an unusually high chromium concentration, and the microstructure corresponds to (Fe,.,, Cro.&Os. The outside portion of the interlayer, in contrast, consists of the desired Fez03. At the contact area of the two interlayer portions are large pores which decrease the adhesion; the spalling of the coating was mostly observed at this level. The 316L steel has a very stable passivating layer which is not fully reduced at high temperature, which is due to its low carbon content (0.02%). Figure 2 presents a section of another AISI 316L stainless steel sample coated under identical conditions. This sample, however, after lapping, underwent an additional polishing with diamond paste followed by gas phase etching. The neutral diamond polishing helps to remove the last residues of the lapping process. It can be seen that the results can be influenced by the grinding and/or polishing grains pressed into the surface; the same has been observed in the past with other substrates and non-oxide coatings such as TiC and TiN. It is known that stable mixed oxides are formed at the surface during heat treatment in water vapour. Figure 3 shows an AISI 316L steel sample coated with SiO? after lapping, wet chemical etching and water vapour pre-oxidation. The oxide interlayers which are visible on this micrograph are very different from those seen in Fig. 1. The interlayer thickness is mostly below 1 pm, and the SiOz layer appears to be well adherent. The chemical etching mixture consisted of HF:HNOJ:HzO (3:10:87 by volume) at 50 “C; this treatment is carried out in an ultrasonic bath for 3 - 5 min, e.g. until the evolution of H,. However, the reaction time was not sufficient; in some places Cr-Fe oxides were still present. This has a negative influence on the interlayer formation as can be
467
Fig. 1. Metallographic section through an AISI 316L stainless steel sample after lapping, water vapour pre-oxidation and SiOz deposition (unetched). Fig. 2. Metallographic section through an AISI 316L stainless steel sample after lapping, diamond polishing, gas phase etching, water vapour pre-oxidation and SiOz deposition (unetched).
Fig. 3. Metallographic section through an AISI 316L stainless steel sample after lapping, wet etching, wet chemical etching (HF-HNOs), water vapour pre-oxidation and SiOz deposition (unetched). Fig. 4. Metallographic section through an SiOz-coated H34CT superalloy sample. The oxide layer spalled off completely and the near-surface region is strongly deformed (etched).
468
seen from Fig. 1. Hence gas phase etching did contribute to remove these oxides completely. If the etching time is too long, surface roughening takes place. Figure 4 shows an H34CT superalloy sample after an Si02 deposition treatment and an etching and pre-oxidation pretreatment. The SiOz layer spalled-off, together with the interlayer. Etching showed that the substrate surface was stressed and cold worked during the mechanical operations. The results of stress-relieving thermal treatments are shown in Fig. 5; the internal stresses were determined by the sin’$ X-ray diffraction method [ 6,7]. After 1 h at 1000 “C the stress level decreased significantly. If the thermal stress relieving continues for more than 1 h the influence of a phase or structural change can be observed (renewed increase in internal stresses). A treatment of 8 h is necessary to obtain a stress-free surface. To produce a well-adherent Si02 coating on such substrates or workpieces it is therefore necessary to stress relieve them for 8 h before etching, pre-oxidation and actual oxide deposition. It was observed, especially with AISI 316L, that the gas phase etching greatly reduces the spontaneously formed passivating layers, especially in connection with the wet chemical and mechanical metalworking operations. Figure 6 shows an Si02-coated nitrided TiA16V4 sample. The oxide layer obtained at temperatures above 780 “C is no longer compact and adherent, even when the material is pre-oxidized by water vapour in moist NZ. The sample which was nitrided under dry N, at 800 “C, in contrast, produced a dense Si02 deposit. The nitrided layer constitutes an efficient diffusion barrier with respect to the oxidizing atmosphere during Si02 deposition. Between room temperature and 900 “C the oxidation of the titanium alloy occurs in four different modes.
annealing
0
2
4
6
1000°C
0
h
Fig. 5. Surface region stress relieving of a machined H34CT superalloy sample by thermal treatment at 1000 “C for different periods of time.
Fig. 6. Metallographic section through a TiA16V4 sample after wet chemical etching, gas nitriding and SiOz deposition in a water-vapour-containing gas phase (unetched). Fig. 7. Scanning electron microscopy view of the cracked surface of a TiAl6V4 sample after wet etching, nitriding and Si02 deposition in a water-vapour-free gas phase (etched).
When SiOZ layers are obtained under water-vapour-free conditions ([H,O] < 5 ppm) the surface layer exhibits a spiderweb-like crack pattern over its entire surface (Fig. 7). This crack formation can be avoided by introducing sufficient water vapour in the gas phase. To avoid contamination of the gas system with water vapour, and corrosion or oxidation products in the gas lines and valves, it is advisable to replace the water vapour by chemicals which decompose and produce water vapour in situ at elevated temperatures. Secondary and tertiary alcohols are suitable as water vapour donors; for the tests described here, isopropanol was used. It is assumed that the SiOZ deposition through thermal decomposition of TEOS in a water-vapourfree gas phase is incomplete and leads to substoichiometric Si02. As the SiOZ layers are amorphous, X-ray diffraction methods cannot be used and, furthermore, microprobe analysis is not sufficiently precise to determine the exact stoichiometry.
4. Conclusions If the substrate properties are carefully taken into consideration and if the passivating layers can be removed, it is possible to deposit SiOZ on highly passivating substrates such as stainless steels (e.g. AISI 316L), superalloys (H34CT), and titanium and its alloys. An internal reduction of the passivating layer can be obtained with steels having a high carbon content (0.1% or more) and operating at temperatures above 780 ‘2. This is not the case for steels containing little carbon or which contain carbon-stabilizing elements
470
such as chromium and molybdenum. The remaining chromium-rich passivating layer can be removed completely through wet chemical and/or electrochemical and/or gas phase etching. Abrasive particles pressed into the substrate surface during grinding and polishing must be removed using diamond paste; diamond is inert in the reaction systems used here. If the surface is contaminated with abrasive particles, the water vapour oxidation will lead to mixed oxide layers of uncontrolled composition and of poor adhesion, independently of whether the pre-oxidation occurs under pure water vapour, moist nitrogen or isopropanol. The internal stresses induced by the mechanical operations must be relieved before the gas phase etching. The thermal expansion coefficients of stainless steel and amorphous SiOz are very different; therefore, internal stresses and phase and structural changes, especially during cooling, easily provoke spalling of the coating. Water vapour is necessary in the gas phase during SiOz deposition to produce crack-free SiOz coatings.
References 1 A. J. Foster, M. L. Sims and D. Young, Protective metal oxide films on metal or alloy substrate surfaces susceptible to coking, corrosion or catalytic activity, U.S. Patent 4,297,150, October 27, 1981, to British Petroleum. 2 D. E. Brown, J. T. K. Clark, A. I. Foster, J. J. McCarroll and M. L. Sims, The inhibition of coke formation in ethylene steam cracking, Natl. Meet. American Chemical Society, New York, 1981. 3 D. E. Brown, J. T. K. Clark, A. I. Foster, J. J. McCarroll, M. S. Richards, M. L. Sims and M. A. M. Swidzinski, A silica coating process for passivation of steels in high temperature environments, Proc. 8th Znt. Conf. on Chemical Vapour Deposition, Paris, 1981, Electrochemical Society, Pennington, NJ, p. 699. 4 J. T. K. Clark, A. I. Foster, M. L. Sims, J. M. Swidzinnski and D. Young, The development of CVD oxide coatings for the protection of metal surfaces, Proc. 4th Eur. Chemical Vapour Deposition Conf., Eindhoven, 1983, Philips Centre for Manufacturing Technology, Eindhoven, p. 385. 5 Stress relieving of austenitic stainless steels, ASM Metals Handbook, Vol. 4, Heat Treating, American Society for Metals, Metals Park, OH, 9th edn., p. 647. 6 L. Chollet, H. Boving and H. E. Hintermann, Residual stress measurement of refractory coatings as a nondestructive evaluation, Proc. 6th Znt. Conf on Nondestructive Evaluation in Nuclear Industry, Zurich, 1983, in Trans. ASM, (1983) 817. 7 L. Chollet and A. J. Perry, The state of stress both in films and their substrates. In E. Macherauch and V. Hank (eds.), Residual Stresses in Science and Technology, Vol. 1, DGM Verlag, Oberursel, 1987, p. 485.