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Investigation of selected plasma-sprayed coatings for bonding glass to metal in hermetic seal applications
129
Investigation of selected plasma-sprayed metal in hermetic seal applications*
coatings for bonding glass to
S. Dallaire and B. Arsenault A. DeSantis (Received August 27, 1991; accepted
in final form March
6. 1992)
Abstract A study was carried out to investigate the feasibility of joining glass to 304 slamless steel by using aclected coatings applied by plasma spraying in the fabrication of hermetically sealed components. After adapting the plasma spraytng technique to coat the small bore of a ring, it was found possible to deposit two-layer coatings on prototypes. Among the coating systems tested. the thin Al,O,--TiO,/Ni-Cr-AI-Y two-layer coating was selected. Following the bonding with glass which was performed at 1000 C in air atmosphere. the sealed components were found to be leak proof.
1. Introduction
The joining of ceramics is usually carried out by chemical bonding, brazing or diffusion bonding. Upon joining, an intimate contact between the metal and the ceramic parts is a requirement [ 11. It results in an actual interface. Van der Waals attractive forces or electronic forces are expected to play an important role in the formation of this interface, resulting in the formation of a chemical bond [ 11. Epitaxial compatibility w)ould also favour the chemical bonding between the metal and the ceramic parts. This study has been carried out to investigate an alternative method for joining metal, ceramic and glass materials by using selected coatings. It was undertaken more particularly to find a way to join a part made of a high thermal expansion metal (stainless steel 304) to a glass material in the fabrication of hermetically sealed components. Commercial use of this metal has been restricted to compression seals [2]. Because of its flexibility, the plasma spraying technique was chosen. It offers the possibility of depositing different ceramic or metallic materials. Two-layer coatings comprising a metal coating lying under a ceramic coating are expected to match the substrate thermal expansion while being wetted by the glass during the sealing treatment. Because it involved the rapid solidification of *Paper presented at NTSC ‘91. National Pittsburgh. PA, May 4-10. 1991.
025778972’92;SS.OO
Thermal
Spray Conference,
deposited materials, this deposition process avoids the dilution of base materials on which coatings are deposited, thus offering more flexibility in the choice of coating materials. Such a deposition method overcomes the problem of matching surfaces while metallizing [3, 41 or brazing [S].
2. Requirements for forming a hermetic seal For joining a glass to a stainless steel substrate to form a hermetic seal adequate for the fabrication of feedthrough (Fig. I), there are basic requirements that must be satisfied. In a comprehensive review, Pask has described all the fundamental requirements for forming strong and impermeable glass/metal and ceramic/metal assemblies [ 11. Firstly, glass being wetted by most oxides, it is necessary to put in close contact with this glass an oxidebased coating. Secondly, this ceramic coating must be deposited on a metallic compliant layer that will accommodate the thermal stresses induced during the sealing operation. The main reasons for this design, schematically shown in Fig. 2, can be summarized as follows. 2. I. Mechanical compatibility The ceramic coating should be able to withstand the sealing process carried out in a furnace heated at a temperature exceeding 1000 -C. Because there is too great a difference between the thermal expansion coeffi-
(
1992 - Elsevter Sequoia.
All rights reserved
or at Icat impcrmeablc to II pa5 prcssu1-c dilltircncc. In addition. the ceramic coating should stop the penetration of glass throughout its thickness.
coating should be selected for wetting purpox. Ho\+x\cr. acessive dissolution should bc avoided. The metallic coating ensuring the management of t hernial stresses should Lvithstand the oxidiAng atmosphcrc in which is pcrformcd the scaling opcrntion. and should also act ;15 ;i ditfusion barrier preventin, ” too nlLlcl1 o\qgcn from coming in contact with the stainless steel.
III
i I
II ‘~~~//lll ~l~l(IIl~ llIl/1l1l 11I’/l~ll llll~llil~lill~lllljilll~llll~illl~llll~llll~lill~llll~llli~ 50 60 70 80 90 100 110 120 130
140
1~ .As shown in Fig. 3. the fabrication of ;I fccdthrough bawd on the bonding of ;i glass with a stainless steel for making ;I hermetic seal requires the spraying of thin coatings (in the order of 25 11m) within the horc of ;I small ring. This ring is rather small and the bore is onl! 5.0 mm in diameter :lnd S.0 mm in depth. Hccause the thermal spraying is an in-sight process, molten particles have to bc dircctcd towards the internal cylindrical surface on which they will stick. This mc;~ns that the few of particles must be prccisel\ aligned at 45 with respect to the internal cylindrical surface. It is important to point out that any deviation resulting from Librations must be avoided during the coating operation. All the displacements. the rotation of the ring and the trans\;erse movemcnt of the spray gun, must & carefully controlled. It is worth mentioning that spraying inside so small a bore has not yet been done. Engineering practice recommends not sprxying inside ;I bore smaller than 6.5 mm in diameter.
3. Selection of coating materials cients of the metal and the glass ccraniic. an intermediate layer must bc deposited between the glass ceramic and the metal parts. This compliant layer with ;L thermal expansion coellicicnt bctwccn those of stainless steel and the osidc ceramic coating helps prevent the development of cvcessivc stresses that would result in catastrophic failure. The resultant assembly schematically represented in Fig. 3 should have ;t iow helium permeability. This is a very severe requirement if it is considered that plasmasprayed coatings are quite porous. There is an interest in developing LI coating with the porosity being mostly located within the lamellae. thus avoiding interlamellar porosity. as much as possible. The latter type of porosity is mostly composed of open channels through which fluids can pcnetrntc. Therefore the ceramic and the metallic compliant layer should be as dense as possible
TM o oxide po\vders, Ai,O, Ti02 A1,0, MgO (70” cl: 30”0). and four titanium, niobium Ni C‘r Al Y and C‘, wxxc selected for this study. All commercially available. The AlzO,
(60” o : JO’C,jI and metallic powders. Ni Cr Fc H Si these powders arc TiO, powder is ;I
blend of fused Al,O, and sintered TiOz powders while the Al,O,- MgO powder is fused. The chemical composition and particle size distribution of these powders are shown in Table 1. They were plasma sprayed on carbon steel (SAE IO 10) and stainless steel (304) substrates using a Metco 11 MB plasma gun especially chosen for this research work. A1,03-Ti02 was selected to be in contact with glass. It is a stable oxide and is relatively easy to spray in a dense and uniform coating. Its thermal expansion coefficient is 9.5 x 10 ~’ mm mm ’ “C ’ at 923 “C. Al,O,-MgO was also considered as a good candidate. It is a stable spinel. Its thermal expansion coefficient of 10.6 x 10m6 mm-’ ‘C-l is, however, higher than that of Al,O,-Ti02. It is thought to have a good resistance to glass dissolution. Titanium was selected as a compliant layer because of 11.3xlo~6 its thermal expansion coefficient mm mm ’ “C ’ at 923 “C is close to that of stainless steel. It can share oxygen atoms with the ceramic coating, thus forming a strong chemical bond. Niobium was also chosen as a compliant layer because its thermal expansion coefficient is close to that of ceramic materials previously selected (8.8 x 10 - 6 mm mm ~‘“C~’ at 923°C). NiLCr-Al-Y, an alloy coating currently used in jet engines, was selected. Upon oxidation it develops a ceramic network within itself. This type of anchoring favours the adhesion of the oxide coating. A nickel alloy containing silicon and boron was also
TABLE
I Chemical
analyses
and particle
sizes of spray
Chemical
Powder
AI,O,-TiO,
Al@, Fe,O, SiO, TiO,
Al,O,-MgO
Al,O, FeD, M@ SiO, Ti Nb Al C Cr Ni Y B C Cr Fe Ni Si
59.99 0.06 0.08 39.87 70.52 0.10 29.12 0.26 99.9 99.9 10.2 0.024 21.8 67. I 0.91 3.5 1.0 17.0 4.0 70.5 4.0
B-S-C
details
4.1. Modijication and addition to the spraying,facilitJl As mentioned before, the highest coating density and the smallest coating thickness were of prime importance. To achieve this goal, the finest commercially available spray powders were chosen except for the titanium powder. The latter was screened and the fraction -32+ 10 urn was retained. Because the coating surface area is very small and consequently the spraying time is very short, it should be pointed out that the powder feed rate must be kept low to ensure thin and uniform coatings. However, owing to their fineness, all these powders do not flow freely and uniformly in a standard commercial powder feeder. A Metco fluidized bed powder feeder was therefore tested to carry the fine powders to the plasma gun. It was observed that this powder feeder could carry adequately fine - 20 + 5 urn powders at high flow rates (above 20 g mini ‘) with a high carrier gas flow rate. At a low powder feed rate, such as 4 g mini ‘, the powder could not be continuously fed. Therefore a Plasmadyne powder feeder was modified to upgrade its feeding capability for fine powders. A vibrator device was installed at the exit of the powder
analysis Content
Ni-Cr.-Fe-
4. Experimental
powders
Component
Ti Nb Ni&Cr-Al&Y
considered because this alloy has a low melting point of 1024°C. It was expected to act as a brazing agent between the ceramic coating and the metal substrate.
Particle size (pm)
Commercial name
-20+5
Amperit
745.8
-20+5
Amperit
760.000
-32+10 - 22.5 + 5.6 -22.5f5.6
Cerac Ti Amperit 160.090 Amperit 413
-44+5
Metco 2001
(wt.%)
feeder and the feed line was shortened to maintain constant the particle velocity without excessively incrcasing the gas flow rate. This modification resulted in a continuous feeding of fine powders such as the AI,O,j TiO, ceramic angular powder. Consequently, all the powders described in Table I could be uniformly transported to the spray gun lvith the exception of the AI,O,~~MgO powder. This powder. however. is not very much different in particle siLe distribution or morphology from the A120, TiO, powder. In order to climinatc moisture that could have been responsible for this bchaviour. this powder was oven dried. Even then. it was not possible to transport properly this powder to the gun. Thcrcfore the A120, MgO ceramic powder was discarded. Moreover. in order to avoid excessi\,c heat and to narrow the spray cone a Metco 11 MB spray gun was chosen. This gun has ;I short plasma plume and seemed appropriate for coating prototypes.
In order to study the influence of spray angle on the coating microstructure and density, and to determine the adequate spraying parameters. coatings were carried out first at 00 and 45 on fat substrates. For this purpose, the spray gun was fixed to an .\- ~7transvcrsc system. For the fabrication of prototypes. a special set-up was designed. As shown in Fig. 4. it consists basically of ;I rotating prototype holder and a computer-controlled transverse system on which the spray gun is lixed. 4.3. L)etr~rnirltrtior1
of’sprrr~~ing ptirfrtwtc~r.s A preliminary study was undertaken to determine the spraying parameters that would result in the best coating quality after a proper surface preparation. Table 2 sum-
mariLes the paramctcrs that wcrc sclccted for the spraying of six different powders. With these parameters. metal coatings intended to bc LISA as compliant layers between the stainless steel and the ceramic were at (irst obtained by spraying the selected metal powders. Thcrcaftcr the tint .41,0, Ti02 powder was sprayed on plasma-sprayed metal coatings. These coatings were carried out at 45 and 00 with rcspcct to the substrate surface.
Because the coating smoothness and thickness wcrc important as mcntioncd before. it was found necessary to study the influence of substrate surface preparation on these propcrtics. Moreover, the gcomctry of prototypes required (as in the case of the spray gun) the LISC of ;I special apparatus for the small cylindrical surface preparation. An Abrasive 6500 system 2 (Pennwalt) microsandblastcr was used for this purpose with X5 pm AI,O,\ as the grit-blasting medium and ;I 276 kPa air pressure. The influence of spray angle on the coating bond strength \vas also investigated. The adhesion bond strength was determined according to the ASTM (C633-79) standard method.
Wetting tests were first carried out at IO00 C in air with soda lime and borosilicate glasses on (lat coated picas. Coated stainless steel prototypes were bound to the soda lime glass with the same conditions as for fat surfaces.
5. Results and discussion Spraying at 45 does not seem to modify significantI> the microstructure, surface regularity or densit! of coatings. As noticed before. spray angle variation between 90 and 45 has no noticeable effect on the deposited coating [6, 71. At each angle, the ceramic coating contains low porosity and the metal coatings seem to be equivalent. Diffcrcnt grit blast preparations on steel substrates showed that a surface roughness higher than R,=6 pm produces irregularities in the coating thickness. However, a surface roughness between I.6 and 4 kern gives good coating thickness regularity. Despite a low surface roughness of I.6 Llrn obtained by microsandblasting. a very good bond strength was obtained with all these fine powders as shown in Table 3. Coatings produced with fine powders arc gcncrallq thought to be of lower bond strength than those obtained with coarse powders. The bond strength of the ceramic
TABLE
2. Plasma
spraying
parameters Value
Powder
Arc power (kW) Arc current (A) Arc voltage (V) Arc gas flow rate (argon) (I sm‘) Secondary gas flow rate (1smI) Helium Hydrogen Powder gas carrier flow rate (I s- ‘) Argon Helium Spray rate (g s r) Spray distance (cm) Gun transverse speed (cm s ‘) Ring rotation (rad s- ‘) Deposition efficiency (%)
AI,O,-TiO,
Ti
Nb
Ni-Cr-AI-Y, NipCr-Fe-B-Si-C
21 600 35 0.39 0.20
13 400 32 0.23
23 500 45 0.80
14 400 35 0.23
R, Wn)
I.6 1.6 4 4 7 7
3. Bond strength Deposition angle (deg)
45 90 45 90 45 90
of plasma-sprayed Bond strength
0.023
0.017
0.050 0.025 5 76 1.33 96
0.083 0.075 5 76 I .33 63
0.078 0.058 5 76 1.33 64
0.22 0.037 5 76 1.33 60
coating (Al#-Ti02) is, however, adversely affected by an increase in surface roughness. Because it was necessary to spray at 45” inside the bore of prototypes, the influence of spray angle was also investigated. As shown in Table 3, in which is reported the bond strength of metallic and double-layer coatings, the bond strength varies with the deposition angle. Except for Al,O,-TiO, coatings, which show an increase in bond strength when the spray angle is reduced, all the other coatings have a slightly lower bond strength when sprayed at 45”. Hasui et al. [7] observed that molybdenum and Al,O, coatings deposited at various angles have fairly good adhesion. At 45” the adhesion is sufficiently high and the ceramic coating is dense enough to be used in the sealing operation. Wetting studies carried out with glasses showed that the A1,03-TiO, coating deposited directly on the steel substrate spalled during the slow cooling of the specimen. However, no spalling occurred with the Al,O,-TiO, coating sprayed on Ni-Cr-Al-Y or titanium or niobium compliant layers. When titanium or niobium are used as materials for the compliant layer, the heat treatment should be carried out in an inert or slightly reducing TABLE
0.017
coatings
as a function
atmosphere because oxygen induces extensive metal oxidation at 1000 “C. Two-layer coatings performed very well in these wetting tests carried out with soda lime and borosilicate glasses. This indicates that the two-layer coating concept is well adapted to the bonding of a metal with a glass, especially when it is considered that the borosilicate glass has a thermal expansion coefficient of 5.5x10~6mmmm~‘“C~‘. Following the good results obtained with the twolayer Al,O,-TiO,/Ni-CrAl-Y coating, it was decided to coat prototypes with the two-layer Al,O,-TiO,/NiCr-Al-Y coating. This type of coating does not require an inert atmosphere and is more adapted to the current sealing procedure practised in industry. The first attempt to deposit coating materials by plasma spraying on the internal cylindrical surface schematically represented in Fig. 3 did not succeed. The quality of coating was different from that obtained on flat surfaces. After deposition the coating was very porous and the layers were irregular in thickness. Because it had been observed in the course of spraying that particles bounced out of the bore, it was decided
of substrate
surface
roughness
R, and deposition
angle
(MPa)
NiGZrAl-Y
AI@,
Ni-Cr-Al-Y:A120,
Titanium
Ti:A120,
Niobium
Nb/Al,O,
59 63 47 59 54 65
49 45 45 34 39 31
48 56 52 54 45 50
55 54
45 50
56 57
51 52
to make measurements of the mean trajectory and divergence of molten particles as illustrated in Fig. 5 and as previously explained 1x1, After spraying the A120, TiO, and the Ni Cr Al Y powders again. it \vas observed that these powders had somewhat different mean trajectories and divergences. This feature is not very important when spraying on large flat surfaces but becomes of crucial importance when two-layer coatings are produced on a small surface area. Thus these two powders require a different gull alignment. It was also observed that particles bouncing out of the bore wcrc those that deviated from the mean trajectory to a certain extent.
The first transverse system was discarded and ;I new one was designed. Computer-controlled step motors ensured precise and reproducible plasma spray gun displacements. This transverse system is shown in Fig. 4. Furthermore. the gun displacement was adjusted in such ;I way that the flow of molten particles was “screened”. Only the particles that followed it quasi-axial trajectory. i.6~.those crossing the most energetic zones of the plasma 181. were allowed to come in contact with the internal cylindrical surface. With thcsc modifications, adherent coatings with good regularity in thickness were obtained. Figure 6(a) shows an example of these two-layer coatings. The coated stainless steel rings were bound to the soda lime glass during the sealing operation carried out at Quality Hermetics Company. Figures 6(a) and 7(a) show the Al,O,~~TiO, Ni~~Cr Al Y and Al,03~Ti0,, Ni Cr Fe- B--&C double-layer coatings before the sealing operation while Figs. 6(b) and 7(b) show the same coatings after they had been infiltrated with glass. As observed in Figs. 7(a) and 7(b) the nickel alloy containing boron and silicon melted during the scaling operation. A very good interface was formed with the stainless steel substrate. However, cracks and voids were present at the interface between the nickel alloy and the Al,O,PTiO, layers. This type of prototype did not pass the helium leak test. In contrast, the sealing operation performed on protoPowder injection
types coated \vith the two-layer Al,O, TIC>, Ni VI .Ai Y coating rcsultcd in the formation of a good bond bctwccn the ceramic and the glass. As shokvn in Fig. 6(b) the glass infiltrated well the small defect\ within the oxide layer. Two of the three prototypes coated with the above-mentioned materials presented this characteristic. Thus no leak was detcctcd during the helium test.
6.
Conclusion
With good knowledge and ing. it is possible to adapt specific problems. By taking present in the spray cone, streams of molten particles.
practice of plasma spraythis technology to solve cart of what ih actually which contains divergent it was found possible to
make relatively thin two-layer coatings inside the bore of rings having a diameter and a depth of 5 mm each. The concept of a two-layer coating comprising a ceramic layer (ensuring the formation of a chemical and mechanical bond with the glass) and a metallic compliant layer (ensuring the management of thermal stresses during the sealing operation) seems to be appropriate for joining glass to stainless steel. Prototypes plasma sprayed with a two-layer Al,O,TiO,/NiX%AlLY coating were successfully bound to glass and they were found to be leak proof. This demonstrates the feasibility of the technique. Provided they have a short flame plume, other spraying techniques such as high velocity oxygen fuel can be used to deposit the ceramic layers.
Acknowledgments The authors would like to thank the Industrial Research Assistance Program of National Research Council Canada for financial assistance. The authors also wish to thank S. BClanger for his technical help and S. F. Turcotte for reviewing the manuscript.
References
*
-..
.C.d
.._
_c,
,...,
‘
3
i-.
.--.
(b) Fig. 7. Scanning clcctron micrographs of an A120,-Ti02:Ni-Cr-FeB-Si-C coating: (a) after plasma spraying; (b) after being sealed with soda lime glass.
J. A. Pask. Crrctm. Bull.. 66 (I I) (1987) 1587. J. A. Wilder, Glass-ceramics for sealing to high thermal expansion alloys, Rep, S.4RiL)XO-2/Y-7. Sandia National Laboratories, Albuquerque, NM. March 1981. H. Morris, H. W. Rhodes and M. C. Marlow. L’S Parent Appliwion 6,XX7,166. July 1986. P. Rowcroft, J. Ph~,s. E, 3 (1970) 21 I. B. D. Gallagher, L’S Parent Applictrtion 6.76_5.Y7R, August 19X5. D. A. Gerdemen and N. L. Hecht, .4r(, Plosmtr Technology in Motrriuls Science, Springer. New York, 1972. A. Hasui. S. Kitahara and T. Fukushima, Trtms. NbI. Rrs. Inst. !MH.. I-‘(l) (1970) 9. B. Champagne and S. Dallaire, Particle injection in plasma spraying, Thermal
Sprrry:
Adatrnccs
Cor$.
Orkmdo,
mctl Spray
Park. OH,
1988. p. 15.
in Cooring
Technology
FL, American
Proc.
Nat/.
Society for Metals,
Thrr-
Metals
Investigation of selected plasma-sprayed metal in hermetic seal applications*
coatings for bonding glass to
S. Dallaire and B. Arsenault A. DeSantis (Received August 27, 1991; accepted
in final form March
6. 1992)
Abstract A study was carried out to investigate the feasibility of joining glass to 304 slamless steel by using aclected coatings applied by plasma spraying in the fabrication of hermetically sealed components. After adapting the plasma spraytng technique to coat the small bore of a ring, it was found possible to deposit two-layer coatings on prototypes. Among the coating systems tested. the thin Al,O,--TiO,/Ni-Cr-AI-Y two-layer coating was selected. Following the bonding with glass which was performed at 1000 C in air atmosphere. the sealed components were found to be leak proof.
1. Introduction
The joining of ceramics is usually carried out by chemical bonding, brazing or diffusion bonding. Upon joining, an intimate contact between the metal and the ceramic parts is a requirement [ 11. It results in an actual interface. Van der Waals attractive forces or electronic forces are expected to play an important role in the formation of this interface, resulting in the formation of a chemical bond [ 11. Epitaxial compatibility w)ould also favour the chemical bonding between the metal and the ceramic parts. This study has been carried out to investigate an alternative method for joining metal, ceramic and glass materials by using selected coatings. It was undertaken more particularly to find a way to join a part made of a high thermal expansion metal (stainless steel 304) to a glass material in the fabrication of hermetically sealed components. Commercial use of this metal has been restricted to compression seals [2]. Because of its flexibility, the plasma spraying technique was chosen. It offers the possibility of depositing different ceramic or metallic materials. Two-layer coatings comprising a metal coating lying under a ceramic coating are expected to match the substrate thermal expansion while being wetted by the glass during the sealing treatment. Because it involved the rapid solidification of *Paper presented at NTSC ‘91. National Pittsburgh. PA, May 4-10. 1991.
025778972’92;SS.OO
Thermal
Spray Conference,
deposited materials, this deposition process avoids the dilution of base materials on which coatings are deposited, thus offering more flexibility in the choice of coating materials. Such a deposition method overcomes the problem of matching surfaces while metallizing [3, 41 or brazing [S].
2. Requirements for forming a hermetic seal For joining a glass to a stainless steel substrate to form a hermetic seal adequate for the fabrication of feedthrough (Fig. I), there are basic requirements that must be satisfied. In a comprehensive review, Pask has described all the fundamental requirements for forming strong and impermeable glass/metal and ceramic/metal assemblies [ 11. Firstly, glass being wetted by most oxides, it is necessary to put in close contact with this glass an oxidebased coating. Secondly, this ceramic coating must be deposited on a metallic compliant layer that will accommodate the thermal stresses induced during the sealing operation. The main reasons for this design, schematically shown in Fig. 2, can be summarized as follows. 2. I. Mechanical compatibility The ceramic coating should be able to withstand the sealing process carried out in a furnace heated at a temperature exceeding 1000 -C. Because there is too great a difference between the thermal expansion coeffi-
(
1992 - Elsevter Sequoia.
All rights reserved
or at Icat impcrmeablc to II pa5 prcssu1-c dilltircncc. In addition. the ceramic coating should stop the penetration of glass throughout its thickness.
coating should be selected for wetting purpox. Ho\+x\cr. acessive dissolution should bc avoided. The metallic coating ensuring the management of t hernial stresses should Lvithstand the oxidiAng atmosphcrc in which is pcrformcd the scaling opcrntion. and should also act ;15 ;i ditfusion barrier preventin, ” too nlLlcl1 o\qgcn from coming in contact with the stainless steel.
III
i I
II ‘~~~//lll ~l~l(IIl~ llIl/1l1l 11I’/l~ll llll~llil~lill~lllljilll~llll~illl~llll~llll~lill~llll~llli~ 50 60 70 80 90 100 110 120 130
140
1~ .As shown in Fig. 3. the fabrication of ;I fccdthrough bawd on the bonding of ;i glass with a stainless steel for making ;I hermetic seal requires the spraying of thin coatings (in the order of 25 11m) within the horc of ;I small ring. This ring is rather small and the bore is onl! 5.0 mm in diameter :lnd S.0 mm in depth. Hccause the thermal spraying is an in-sight process, molten particles have to bc dircctcd towards the internal cylindrical surface on which they will stick. This mc;~ns that the few of particles must be prccisel\ aligned at 45 with respect to the internal cylindrical surface. It is important to point out that any deviation resulting from Librations must be avoided during the coating operation. All the displacements. the rotation of the ring and the trans\;erse movemcnt of the spray gun, must & carefully controlled. It is worth mentioning that spraying inside so small a bore has not yet been done. Engineering practice recommends not sprxying inside ;I bore smaller than 6.5 mm in diameter.
3. Selection of coating materials cients of the metal and the glass ccraniic. an intermediate layer must bc deposited between the glass ceramic and the metal parts. This compliant layer with ;L thermal expansion coellicicnt bctwccn those of stainless steel and the osidc ceramic coating helps prevent the development of cvcessivc stresses that would result in catastrophic failure. The resultant assembly schematically represented in Fig. 3 should have ;t iow helium permeability. This is a very severe requirement if it is considered that plasmasprayed coatings are quite porous. There is an interest in developing LI coating with the porosity being mostly located within the lamellae. thus avoiding interlamellar porosity. as much as possible. The latter type of porosity is mostly composed of open channels through which fluids can pcnetrntc. Therefore the ceramic and the metallic compliant layer should be as dense as possible
TM o oxide po\vders, Ai,O, Ti02 A1,0, MgO (70” cl: 30”0). and four titanium, niobium Ni C‘r Al Y and C‘, wxxc selected for this study. All commercially available. The AlzO,
(60” o : JO’C,jI and metallic powders. Ni Cr Fc H Si these powders arc TiO, powder is ;I
blend of fused Al,O, and sintered TiOz powders while the Al,O,- MgO powder is fused. The chemical composition and particle size distribution of these powders are shown in Table 1. They were plasma sprayed on carbon steel (SAE IO 10) and stainless steel (304) substrates using a Metco 11 MB plasma gun especially chosen for this research work. A1,03-Ti02 was selected to be in contact with glass. It is a stable oxide and is relatively easy to spray in a dense and uniform coating. Its thermal expansion coefficient is 9.5 x 10 ~’ mm mm ’ “C ’ at 923 “C. Al,O,-MgO was also considered as a good candidate. It is a stable spinel. Its thermal expansion coefficient of 10.6 x 10m6 mm-’ ‘C-l is, however, higher than that of Al,O,-Ti02. It is thought to have a good resistance to glass dissolution. Titanium was selected as a compliant layer because of 11.3xlo~6 its thermal expansion coefficient mm mm ’ “C ’ at 923 “C is close to that of stainless steel. It can share oxygen atoms with the ceramic coating, thus forming a strong chemical bond. Niobium was also chosen as a compliant layer because its thermal expansion coefficient is close to that of ceramic materials previously selected (8.8 x 10 - 6 mm mm ~‘“C~’ at 923°C). NiLCr-Al-Y, an alloy coating currently used in jet engines, was selected. Upon oxidation it develops a ceramic network within itself. This type of anchoring favours the adhesion of the oxide coating. A nickel alloy containing silicon and boron was also
TABLE
I Chemical
analyses
and particle
sizes of spray
Chemical
Powder
AI,O,-TiO,
Al@, Fe,O, SiO, TiO,
Al,O,-MgO
Al,O, FeD, M@ SiO, Ti Nb Al C Cr Ni Y B C Cr Fe Ni Si
59.99 0.06 0.08 39.87 70.52 0.10 29.12 0.26 99.9 99.9 10.2 0.024 21.8 67. I 0.91 3.5 1.0 17.0 4.0 70.5 4.0
B-S-C
details
4.1. Modijication and addition to the spraying,facilitJl As mentioned before, the highest coating density and the smallest coating thickness were of prime importance. To achieve this goal, the finest commercially available spray powders were chosen except for the titanium powder. The latter was screened and the fraction -32+ 10 urn was retained. Because the coating surface area is very small and consequently the spraying time is very short, it should be pointed out that the powder feed rate must be kept low to ensure thin and uniform coatings. However, owing to their fineness, all these powders do not flow freely and uniformly in a standard commercial powder feeder. A Metco fluidized bed powder feeder was therefore tested to carry the fine powders to the plasma gun. It was observed that this powder feeder could carry adequately fine - 20 + 5 urn powders at high flow rates (above 20 g mini ‘) with a high carrier gas flow rate. At a low powder feed rate, such as 4 g mini ‘, the powder could not be continuously fed. Therefore a Plasmadyne powder feeder was modified to upgrade its feeding capability for fine powders. A vibrator device was installed at the exit of the powder
analysis Content
Ni-Cr.-Fe-
4. Experimental
powders
Component
Ti Nb Ni&Cr-Al&Y
considered because this alloy has a low melting point of 1024°C. It was expected to act as a brazing agent between the ceramic coating and the metal substrate.
Particle size (pm)
Commercial name
-20+5
Amperit
745.8
-20+5
Amperit
760.000
-32+10 - 22.5 + 5.6 -22.5f5.6
Cerac Ti Amperit 160.090 Amperit 413
-44+5
Metco 2001
(wt.%)
feeder and the feed line was shortened to maintain constant the particle velocity without excessively incrcasing the gas flow rate. This modification resulted in a continuous feeding of fine powders such as the AI,O,j TiO, ceramic angular powder. Consequently, all the powders described in Table I could be uniformly transported to the spray gun lvith the exception of the AI,O,~~MgO powder. This powder. however. is not very much different in particle siLe distribution or morphology from the A120, TiO, powder. In order to climinatc moisture that could have been responsible for this bchaviour. this powder was oven dried. Even then. it was not possible to transport properly this powder to the gun. Thcrcfore the A120, MgO ceramic powder was discarded. Moreover. in order to avoid excessi\,c heat and to narrow the spray cone a Metco 11 MB spray gun was chosen. This gun has ;I short plasma plume and seemed appropriate for coating prototypes.
In order to study the influence of spray angle on the coating microstructure and density, and to determine the adequate spraying parameters. coatings were carried out first at 00 and 45 on fat substrates. For this purpose, the spray gun was fixed to an .\- ~7transvcrsc system. For the fabrication of prototypes. a special set-up was designed. As shown in Fig. 4. it consists basically of ;I rotating prototype holder and a computer-controlled transverse system on which the spray gun is lixed. 4.3. L)etr~rnirltrtior1
of’sprrr~~ing ptirfrtwtc~r.s A preliminary study was undertaken to determine the spraying parameters that would result in the best coating quality after a proper surface preparation. Table 2 sum-
mariLes the paramctcrs that wcrc sclccted for the spraying of six different powders. With these parameters. metal coatings intended to bc LISA as compliant layers between the stainless steel and the ceramic were at (irst obtained by spraying the selected metal powders. Thcrcaftcr the tint .41,0, Ti02 powder was sprayed on plasma-sprayed metal coatings. These coatings were carried out at 45 and 00 with rcspcct to the substrate surface.
Because the coating smoothness and thickness wcrc important as mcntioncd before. it was found necessary to study the influence of substrate surface preparation on these propcrtics. Moreover, the gcomctry of prototypes required (as in the case of the spray gun) the LISC of ;I special apparatus for the small cylindrical surface preparation. An Abrasive 6500 system 2 (Pennwalt) microsandblastcr was used for this purpose with X5 pm AI,O,\ as the grit-blasting medium and ;I 276 kPa air pressure. The influence of spray angle on the coating bond strength \vas also investigated. The adhesion bond strength was determined according to the ASTM (C633-79) standard method.
Wetting tests were first carried out at IO00 C in air with soda lime and borosilicate glasses on (lat coated picas. Coated stainless steel prototypes were bound to the soda lime glass with the same conditions as for fat surfaces.
5. Results and discussion Spraying at 45 does not seem to modify significantI> the microstructure, surface regularity or densit! of coatings. As noticed before. spray angle variation between 90 and 45 has no noticeable effect on the deposited coating [6, 71. At each angle, the ceramic coating contains low porosity and the metal coatings seem to be equivalent. Diffcrcnt grit blast preparations on steel substrates showed that a surface roughness higher than R,=6 pm produces irregularities in the coating thickness. However, a surface roughness between I.6 and 4 kern gives good coating thickness regularity. Despite a low surface roughness of I.6 Llrn obtained by microsandblasting. a very good bond strength was obtained with all these fine powders as shown in Table 3. Coatings produced with fine powders arc gcncrallq thought to be of lower bond strength than those obtained with coarse powders. The bond strength of the ceramic
TABLE
2. Plasma
spraying
parameters Value
Powder
Arc power (kW) Arc current (A) Arc voltage (V) Arc gas flow rate (argon) (I sm‘) Secondary gas flow rate (1smI) Helium Hydrogen Powder gas carrier flow rate (I s- ‘) Argon Helium Spray rate (g s r) Spray distance (cm) Gun transverse speed (cm s ‘) Ring rotation (rad s- ‘) Deposition efficiency (%)
AI,O,-TiO,
Ti
Nb
Ni-Cr-AI-Y, NipCr-Fe-B-Si-C
21 600 35 0.39 0.20
13 400 32 0.23
23 500 45 0.80
14 400 35 0.23
R, Wn)
I.6 1.6 4 4 7 7
3. Bond strength Deposition angle (deg)
45 90 45 90 45 90
of plasma-sprayed Bond strength
0.023
0.017
0.050 0.025 5 76 1.33 96
0.083 0.075 5 76 I .33 63
0.078 0.058 5 76 1.33 64
0.22 0.037 5 76 1.33 60
coating (Al#-Ti02) is, however, adversely affected by an increase in surface roughness. Because it was necessary to spray at 45” inside the bore of prototypes, the influence of spray angle was also investigated. As shown in Table 3, in which is reported the bond strength of metallic and double-layer coatings, the bond strength varies with the deposition angle. Except for Al,O,-TiO, coatings, which show an increase in bond strength when the spray angle is reduced, all the other coatings have a slightly lower bond strength when sprayed at 45”. Hasui et al. [7] observed that molybdenum and Al,O, coatings deposited at various angles have fairly good adhesion. At 45” the adhesion is sufficiently high and the ceramic coating is dense enough to be used in the sealing operation. Wetting studies carried out with glasses showed that the A1,03-TiO, coating deposited directly on the steel substrate spalled during the slow cooling of the specimen. However, no spalling occurred with the Al,O,-TiO, coating sprayed on Ni-Cr-Al-Y or titanium or niobium compliant layers. When titanium or niobium are used as materials for the compliant layer, the heat treatment should be carried out in an inert or slightly reducing TABLE
0.017
coatings
as a function
atmosphere because oxygen induces extensive metal oxidation at 1000 “C. Two-layer coatings performed very well in these wetting tests carried out with soda lime and borosilicate glasses. This indicates that the two-layer coating concept is well adapted to the bonding of a metal with a glass, especially when it is considered that the borosilicate glass has a thermal expansion coefficient of 5.5x10~6mmmm~‘“C~‘. Following the good results obtained with the twolayer Al,O,-TiO,/Ni-CrAl-Y coating, it was decided to coat prototypes with the two-layer Al,O,-TiO,/NiCr-Al-Y coating. This type of coating does not require an inert atmosphere and is more adapted to the current sealing procedure practised in industry. The first attempt to deposit coating materials by plasma spraying on the internal cylindrical surface schematically represented in Fig. 3 did not succeed. The quality of coating was different from that obtained on flat surfaces. After deposition the coating was very porous and the layers were irregular in thickness. Because it had been observed in the course of spraying that particles bounced out of the bore, it was decided
of substrate
surface
roughness
R, and deposition
angle
(MPa)
NiGZrAl-Y
AI@,
Ni-Cr-Al-Y:A120,
Titanium
Ti:A120,
Niobium
Nb/Al,O,
59 63 47 59 54 65
49 45 45 34 39 31
48 56 52 54 45 50
55 54
45 50
56 57
51 52
to make measurements of the mean trajectory and divergence of molten particles as illustrated in Fig. 5 and as previously explained 1x1, After spraying the A120, TiO, and the Ni Cr Al Y powders again. it \vas observed that these powders had somewhat different mean trajectories and divergences. This feature is not very important when spraying on large flat surfaces but becomes of crucial importance when two-layer coatings are produced on a small surface area. Thus these two powders require a different gull alignment. It was also observed that particles bouncing out of the bore wcrc those that deviated from the mean trajectory to a certain extent.
The first transverse system was discarded and ;I new one was designed. Computer-controlled step motors ensured precise and reproducible plasma spray gun displacements. This transverse system is shown in Fig. 4. Furthermore. the gun displacement was adjusted in such ;I way that the flow of molten particles was “screened”. Only the particles that followed it quasi-axial trajectory. i.6~.those crossing the most energetic zones of the plasma 181. were allowed to come in contact with the internal cylindrical surface. With thcsc modifications, adherent coatings with good regularity in thickness were obtained. Figure 6(a) shows an example of these two-layer coatings. The coated stainless steel rings were bound to the soda lime glass during the sealing operation carried out at Quality Hermetics Company. Figures 6(a) and 7(a) show the Al,O,~~TiO, Ni~~Cr Al Y and Al,03~Ti0,, Ni Cr Fe- B--&C double-layer coatings before the sealing operation while Figs. 6(b) and 7(b) show the same coatings after they had been infiltrated with glass. As observed in Figs. 7(a) and 7(b) the nickel alloy containing boron and silicon melted during the scaling operation. A very good interface was formed with the stainless steel substrate. However, cracks and voids were present at the interface between the nickel alloy and the Al,O,PTiO, layers. This type of prototype did not pass the helium leak test. In contrast, the sealing operation performed on protoPowder injection
types coated \vith the two-layer Al,O, TIC>, Ni VI .Ai Y coating rcsultcd in the formation of a good bond bctwccn the ceramic and the glass. As shokvn in Fig. 6(b) the glass infiltrated well the small defect\ within the oxide layer. Two of the three prototypes coated with the above-mentioned materials presented this characteristic. Thus no leak was detcctcd during the helium test.
6.
Conclusion
With good knowledge and ing. it is possible to adapt specific problems. By taking present in the spray cone, streams of molten particles.
practice of plasma spraythis technology to solve cart of what ih actually which contains divergent it was found possible to
make relatively thin two-layer coatings inside the bore of rings having a diameter and a depth of 5 mm each. The concept of a two-layer coating comprising a ceramic layer (ensuring the formation of a chemical and mechanical bond with the glass) and a metallic compliant layer (ensuring the management of thermal stresses during the sealing operation) seems to be appropriate for joining glass to stainless steel. Prototypes plasma sprayed with a two-layer Al,O,TiO,/NiX%AlLY coating were successfully bound to glass and they were found to be leak proof. This demonstrates the feasibility of the technique. Provided they have a short flame plume, other spraying techniques such as high velocity oxygen fuel can be used to deposit the ceramic layers.
Acknowledgments The authors would like to thank the Industrial Research Assistance Program of National Research Council Canada for financial assistance. The authors also wish to thank S. BClanger for his technical help and S. F. Turcotte for reviewing the manuscript.
References
*
-..
.C.d
.._
_c,
,...,
‘
3
i-.
.--.
(b) Fig. 7. Scanning clcctron micrographs of an A120,-Ti02:Ni-Cr-FeB-Si-C coating: (a) after plasma spraying; (b) after being sealed with soda lime glass.
J. A. Pask. Crrctm. Bull.. 66 (I I) (1987) 1587. J. A. Wilder, Glass-ceramics for sealing to high thermal expansion alloys, Rep, S.4RiL)XO-2/Y-7. Sandia National Laboratories, Albuquerque, NM. March 1981. H. Morris, H. W. Rhodes and M. C. Marlow. L’S Parent Appliwion 6,XX7,166. July 1986. P. Rowcroft, J. Ph~,s. E, 3 (1970) 21 I. B. D. Gallagher, L’S Parent Applictrtion 6.76_5.Y7R, August 19X5. D. A. Gerdemen and N. L. Hecht, .4r(, Plosmtr Technology in Motrriuls Science, Springer. New York, 1972. A. Hasui. S. Kitahara and T. Fukushima, Trtms. NbI. Rrs. Inst. !MH.. I-‘(l) (1970) 9. B. Champagne and S. Dallaire, Particle injection in plasma spraying, Thermal
Sprrry:
Adatrnccs
Cor$.
Orkmdo,
mctl Spray
Park. OH,
1988. p. 15.
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