Bonding thermosprayed silicon to glass-ceramic

Thin Solid Films, 47 (1977) 241-247 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands

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B O N D I N G T H E R M O S P R A Y E D SILICON TO GLASS-CERAMIC* W. G. DORFELD Coming Glass Works, Research and Development Laboratory, Coming, N. Y. 14830 (U.S.A.) (Received April 15, 1977; accepted July 26, 1977)

Many materials, among them silicon, fail to form adherent coatings when thermosprayed onto smooth oxide glasses and glass-ceramics. Mechanical bonding is usually possible if the substrate surface is abraded before spraying, but this process also weakens the crack-sensitive substrate. This paper gives the results of examining alternate surface preparations which create a roughened substrate without the usual weakening of the finished article. The methods rely on the ability of mechanically damaged glass articles to heal, i.e. to decrease the flaw population during crystallization. The surface roughness essential to the mechanical bond is produced when the article is in the green or glassy state, and in a subsequent processing step the glass is converted to glass-ceramic by a crystallization heat treatment. The simplest application is the fabrication of roughened durable ceramic substrates for spraying after the crystallization step. Profilometry and measurements of silicon metal coating adhesion to a lithium aluminosilicate glass-ceramic indicate that for hard glasses the crystallization step does not reduce the surface roughness. When high temperature materials are to be used as coatings, the spraying step can be performed while the substrate is in the glassy state, and then the entire article is subjected to the crystallization heat treatment. For a silicon-coated glass-ceramic, this method produces significantly improved adhesion over simple mechanical bonding.

1. INTRODUCTION Glass-ceramic articles, made by forming and crystallizing a parent glass, are used in many applications which require the high strength and thermal durability of ceramic materials. Often these properties alone are not enough; in addition the material must have metallic qualities, e.g. thermal or electrical conductivity. Composites made by coating a glass-ceramic with metallurgical-grade silicon metal are notable in this respect, because silicon, in addition to its transport properties, offers good oxidation resistance and a coefficient of thermal expansion, 35 x 10 -7 °C-1, which matches commercial glass-ceramic materials more closely than other pure metals do. *Paper presentedat the InternationalConferenceon MetallurgicalCoatings,San Francisco,California, U.S.A., March 28-April 1~1977.

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Smooth glass-ceramic surfaces generally cannot .be plasma or flame spray coated with silicon and many other metals because the coating does not form a chemical bond with the unreactive ceramic. Roughening the substrate surface by mechanical abrasion promotes adhesion 1, but it also seriously decreases the strength of the substrate. As a result, the finished article is often unacceptable because o f poor thermal or mechanical durability. The loss of strength is caused by cracks and flaws which are introduced into the brittle substrate by abrasion. This study explores several methods of minimizing the effects of mechanical damage, utilizing the usual glass-ceramic crystallization heat treatment as a flaw-healing step. 2. EXPERIMENTAL A lithium aluminosilicate glass-ceramic (Corning code 9608) served as the substrate material. The substrates were in the form of either rolled coupons approximately 10 cm square or shallow vessels. The latter have a large flat surface, approximately 25 cm in diameter, which is suitable for coating trials. In addition, the use of a standard shape allows the strength to be judged by test methods developed in this laboratory. The vessels were pressed in two-piece molds and had a bottom thickness of 0.5 cm and a rim height of 5 cm. The glass was crystallized by a heat treatment which consisted of a nucleation hold at 750 °C and crystal growth for 2 h a t I100°C. Flame-sprayed silicon coatings were applied with a Metco powder spray apparatus. A slightly reducing hydrogen-oxygen flame was used with nitrogen as the carrier gas. Gun stand-offs from 18 cm to 20 cm and substrate preheats of 500 °C were used. Plasma-sprayed articles were coated by the WaU-Colomoy Corp. (Santa Fe Springs, California). Substrates were not preheated, and spray parameters for the Avco plasma equipment were similar to those recommended for Nichrome. Silicon powders, metallurgical grade, were obtained from Kawecki-Berylco, Inc., and CeracPure, Inc., and were sized to obtain 80 ~ in the range from + 200 mesh to - 325 mesh. The median particle size was 50-60 ~tm. The coating adhesion was measured by the tensile failure of a plug bonded to the coating. The configuration differed from standard practice 2 in that the coating at the plug periphery was not removed by machining. It was judged to be more important to exclude machining cracks and flaws from the brittle substrate and coating than to avoid stress build-up around the plug. The quoted bond strengths may be slightly in error because of this. Tensile failure always occurred at the silicon-ceramic interface. Surface roughness after abrasion was determined by stylus profilometry as the arithmetic average roughness. The strength of uncoated and coated vessels was evaluated with two thermal down-shock tests. In one test the vessel was heated in a furnace to 500__+10 °C and was then plunged into cold (10°C) water. In the other test, the bottom coated surface was heated on an electrical resistance heater to an inside surface temperature of approximately 420 °C and was then lowered edgewise into cold water at a rate of approximately 5 cm s-1. The first test causes heat flow normal to the surface, while the second allows a thermal gradient'to form parallel to the coating, creating a

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THERMOSPRAYED

Si TO GLASS-CERAMIC

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larger component of shear stress. The object of the tests was to place the vessel under thermal stresses which approach the normal rupture stress of the ceramic. Any significant weakening or residual stresses produce breakage. It should be emphasized that the relationship between the number of successful down-shocks and strength is non-linear. 3. RESULTSAND DISCUSSION The coating adhesions for several different as-abraded surface finishes are shown in Fig. 1. Mechanical interlocking is the bonding mechanism, as evidenced

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Fig. 1. Coating adhesions for as-abraded surfaces. The surface finish of 3 x 10-4 was produced by a •Bianchard grind; all others are blasted. Fig. 2. A scanning electron micrograph of a flame-sprayedsilicon coating. (Magnification, 210 x .) by the loss of adhesion for both smooth surfaces and coarse finishes a. Although these data indicate that peak strength is associated with a typical surface irregularity of about 5 I~m, the median particle size is about ten times this value. This seems to indicate that the finer particles are responsible for the mechanical bond. Within the limit of good spraying practice there is little noticeable dependence of adhesion on spray parameters. The structure of the coatings is shown in Fig. 2. The silicon particles are present as granules rather than as typical lenticular particles, which indicates that only the surface of the silicon melts. In addition, larger particles are conspicuously absent in the micrograph. Larger particles do not adhere well to either the substrate or the coating, which is partly responsible for the low deposition efficiency (of the order of one-quarter). Although silicon coatings will bond to abraded surfaces, the effect of abrasion on strength is often catastrophic. An uncoated glass-ceramic vessel of the type used in this study will ordinarily withstand a minimum of five cold water quench treatments from 500 °C without breakage. Coarse abrasion to about a 4 x 10-3 cm finish generally leads to failure on the first down-shock, while reducing the severity of abrasion to about a 4 x l0 -4 cm finish increases the average durability to only

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two or three 500 °C down-shocks. The decrease in strength is a direct result of flaws, generally small cracks, that are introduced by abrasion. These flaws must be removed to restore the strength of the substrate. One way of making a roughened ceramic substrate is by abrading the glassy article and by then devitrifying it. Crystal growth will heal the abrasive flaws to the extent that a vessel made in this way will withstand five down-shocks from 500 °C. At the same time the surface must retain its original roughness, or the adhesion will suffer. Figure 3 shows the frequency distribution of surface roughness of the parent

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Fig. 3. The distribution of surface roughness for a series of code 9608 surfaces, as abraded in the glassy state and after heat treatment to produce a glass-ceramic. Fig. 4. Adhesion results: , bond strengths for as-abraded surfaces; 0 , surfaces which were blasted, crystallized and coated by flame spraying; II, blasted, crystallized and plasma-spray-coated surface; A, abraded, coated and crystallized articles.

glass, grit blasted with 90 mesh SiC, and of the resulting glass-ceramic surface. No more than a 5 ~ decrease in roughness has occurred during the heat treatment. A glass-ceramic surface prepared in this way is suitable for silicon thermospray coating. Coating adhesion would be expected to be comparable with that observed for an abraded glass or ceramic surface. Measurements of bond strength for coatings of from 0.04 cm to 0.07 cm thickness on surfaces with a (3-5) x 10 -4 cm AA roughness are of the order of 3 MPa for a flame-spraying process and 5 MPa for a plasma-spraying process, confirming that bonding is similar to coatings sprayed onto freshly abraded surfaces. In addition, the coated vessels will withstand five quenches from 500 °C and more than 102 slow immersions from 420 °C with no ceramic breakage. In some cases both flame- and plasma-sprayed coatings were observed to debond in spots, but the substrate remained intact. The coating delamination which sometimes occurs during thermal cycling indicates the need tbr a means o f increasing bond strength. One way is to promote the chemical interaction between coating and substrate. Because the silicon-silicate

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ceramic combination will not react, the interaction must occur through interdiffusion or through an increase in contact area between the coating and substrate. The crystallization heat treatment, which includes 2 h at 1100 °C, can be used to bring this about. An abraded glassy surface can be coated; then when the coated article is crystallized the heat treatment serves both to crystallize and heal abrasion flaws and also to increase the bond strength. A series 0f vessels was coated by abrading the green glass to a (3-5) x 10 -4 cm surface roughness, by flame spray coating it with silicon and by finally crystallizing the coated glass. No breakage occurred during the heat treatment. No measurable loss of silicon occurred through oxidation, although some silicon color changes were noted. A summary of the adhesion results is shown in Fig. 4. The heat treatment causes an increase in bond strength which is two to three times greater than that caused by mechanical interlocking. This is probably due to a combination of glass flow and diffusion near the areas of contact of coating and substrate. The actual interface chemistry is complicated because of the large number of components in the parent glass and impurities in the silicon. However, the variation of silicon concentration across the interface should give an overall indication of the extent of consolidation. Figure 5 is a microprobe profile across a fracture plane which is perpendicular to the macroscopic silicon-ceramic interface. This shows the expected gradual decrease in silicon content from coating to substrate, supporting the idea that adhesion is enhanced by interdiffusion at the crystallization temperature.

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Fig. 5. A microprobe profile of the silicon concentration perpendicular to the silicon-ceramicinterface after the compositehad been heat treated at 1100°C for 2 h. Fig. 6. The number of thermal down-shocksfrom the designated temperature to cold water required to break a silicon-coatedglass-ceramicvesselas a functionof the coatingthickness. A vertical arrow implies that the vessel survived the indicated number of shocks. The strength of articles coated by this process is strongly dependent on coating thickness. The results of thermal down-shock tests are shown in Fig. 6. The number of down-shocks required to break the glass-ceramic is plotted against the silicon

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coating thickness. A vertical arrow means that the indicated number of down-shocks did not lead to fracture. Silicon coatings less than approximately 0.04 cm thick do not lead to appreciable weakening, while articles with thicker coatings are prone to breakage. The expansion mismatch between the silicon coating (whose coefficient of thermal expansion is 35 x 10-7 °C- 1) and the glass-ceramic (12 x 10- 7 °C- 1) is not believed to be the direct cause of the breakage. The point labelled A in Fig. 6 corresponds to a vessel prepared by coating a roughened and crystallized substrate. Although this substrate experiences the same thermal stresses as a vessel made by the high adhesion process, it had not failed after more than 102 down-shocks. This shows that the substrate material is capable of enduring the expansion mismatch stresses caused by down-shock. The most probable reason for the breakage is residual stress caused by the density change which occurs during crystallization. The difference in density between the parent glass and the glass-ceramic leads to a 1.5 ~ linear shrinkage during the ceramicing cycle, while little or no sintering of.the silicon occurs. For thicker coatings (greater than 0.04 cm) large stresses build up in the glass-ceramic as it contracts during ceramicing, and when thermal stresses are imposed in addition during a down-shock the strength of the substrate is exceeded. It is also possible that thinner coatings microcrack more easily to relieve the shrinkage stresses. In either case it should be possible to apply thicker coatings with no loss in strength by choosing a substrate material which does not undergo dimensional changes during ceramicing. Glass compositions are known which have this property 4.

4. CONCLUSIONS Flame- and plasma-sprayed silicon metal coatings can be applied to glassceramic materials with good adhesion and no loss in substrate strength by several processes. The surface of the parent glass article is first abraded to a finish of approximately 5 x 10 -4 cm average surface roughness. If the parent glass is then crystallized, the resulting glass-ceramic will be suitable for thermospray coating. The coating adheres by forming a mechanical bond, and the substrate no longer shows the weakening effects of the abrasion step. Alternatively, the glassy article can be coated, again forming a mechanical bond. If the coated glass article is subjected to the crystallizing heat treatment the coating adhesion increases by a factor of 2-3, indicating that some chemical interaction has occurred. This method can only be applied to silicon coatings less than 0.04 cm thick when using Code 9608 substrates if the production of a highly stressed article is to be avoided.

ACKNOWLEDGMENTS

I wish to express my appreciation to J. E. Nitsche for the flame spraying and to H. F. Dates and F. J. Marusak for many helpful discussions and for the down-shock testing.

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REFERENCES 1 E. Groshart, Met. Finish., 71 (1973) 60. 2 1976 Annual Book of A S T M Standards, American Society for Testing and Materials, Philadelphia, 1976, Part 17, p. 636. 3 R.C. Tucker, Jr., J. Vac. Sci. Technol., 11 (1974) 725. 4 P.W. McMillan, Glass Ceramics, Academic Press, New York, 1964, p. 97.