ACCELERATED WHISKER GROWTH ON IRON-CHROMIUM-ALUMINIUM ALLOY FOIL

Abstract: An accelerated method for growing oxide whiskers on aluminium-containing stainless steel foil comprises heating the foil in air at an initial temperature suitably less than 960 o C, preferably between 900 o C and 950 o C, to initiate whisker growth and thereafter increasing the temperature to above 960 o C, preferably between 960 o C and 990 o C, to continue whisker growth at an accelerated rate and thereby reduce the time and processing expense required to grow mature whiskers of a desired size.

Description: Background of the Invention This invention relates to oxidation of aluminium-containing ferritic stainless steel foil under conditions that form oxide whiskers that substantially cover the foil surface and improve adhesion of an applied coating. More particularly, this invention relates to a method for growing the whiskers as specified in the preamble of claim 1, for example as disclosed in US-A-4,331,631. A metal monolith automotive catalytic converter typically comprises an aluminium-containing ferritic stainless steel foil substrate that carries an alumina coating impregnated with noble metal catalyst. Suitable iron-base alloys for the substrate comprise, by weight, about 15 to 25 percent chromium and about 3 to 6 percent aluminium. The alloy also may contain up to 1 percent yttrium or up to 0.1 percent rare earth metal, most notably cerium, to enhance high temperature corrosion resistance. Prior to coating, the foil is oxidized to form a protective oxide layer. United States Patent Nos.4,331,631 and 4,318,828 describe oxidation treatment of the foil to form an oxide layer characterized by densely spaced whiskers that improve adhesion of the applied coating. The foil may be formed by cold rolling or by a metal-peeling process. For cold-rolled foil, the foil is pre-treated by heating in a low-oxygen atmosphere to form a thin, precursor oxide film suitable for nucleating the whiskers. Although whiskers may be grown on peeled foil without a low-oxygen pre-treatment, such pre-treatment is nonetheless preferred to assure consistent growth. To grow the whiskers, the pre-treated foil is heated in a readily oxidizing atmosphere such as air for an extended time. In general, whiskers of preferred size and shape for bonding an applied coating are formed by heating at temperatures between 900°C and 950°C. At higher temperatures, whisker growth tends to be inhibited, most likely by the formation of non-whisker alpha alumina. The particular uppermost whisker-growing temperature is sensitive to alloy composition. For example, a typical commercial cerium-containing alloy is found to be covered by whiskers after treatment at 960°C, but to form few, scattered whiskers when heated at 980°C. On the other hand, it is desired to reduce the treatment time to reduce the cost. Even within the preferred 900°C to 950°C growing range, treatment times of eight hours or more have heretofore been required to grow whiskers of a suitable size for tightly bonding the applied coating. A method for growing oxide whiskers on a suitably prepared aluminium-containing ferrite stainless steel surface according to the present invention is characterised by the features specified in the characterising portion of claim 1. It is an object of this invention to provide an improved treatment for growing oxide whiskers on aluminium-containing ferritic stainless steel foil, which method employs a multi-temperature whisker-forming oxidation treatment to reduce the time required to grow mature whiskers. Summary of the Invention In accordance with a preferred embodiment of this invention, an improved method for growing alumina whiskers on suitably prepared aluminium-containing ferritic stainless steel foil comprises heating in air at a first temperature for a short period sufficient to initiate formation of the whiskers, and thereafter at a second, relatively higher temperature to continue growth into mature whiskers at an accelerated rate. The bare foil, which may be peeled or cold-rolled, is preferably pre-treated by heating in a low-oxygen atmosphere at between about 875°C and 925°C. Suitable pre-treatment atmosphere contains an oxygen partial pressure not greater than 200 Pa (1.5 torr), preferably less than 100 Pa (0.75 torr). Only a brief pre-treatment is needed to form a blush oxide film suitable for nucleating the whiskers. Thereafter, the pre-treated foil is heated in air to commence formation of the whiskers. This initial treatment step is carried out at a temperature less than 960°C and preferably between 900°C and 950°C. This corresponds approximately to the range heretofore preferred for growing the whiskers. However, in accordance with this invention, this initial growth stage is carried out for a shortened period insufficient to form mature whiskers, for example, less than two hours and preferably less than 15 minutes. Thereafter, the temperature is increased to between 960°C and 990°C to continue growth at an accelerated rate. It is found that whiskers initiated at the lower temperature continue growth at the elevated temperature even though that temperature is above the range of temperature suitable for forming whiskers ab initio on pre-treated foil. Furthermore,it is found that the higher temperature accelerates the rate at which the whiskers form. Thus, this invention can reduce the time, by up to one-half or more, that is required to grow mature whiskers of a desired size and shape. Description of the Figures Figures 1 to 10 of the accompanying drawings are electron photo-micrographs showing oxide layers formed on aluminium-containing ferritic stainless steel foil surfaces oxidized under various conditions set forth in the examples that follow. Detailed Description of the Invention This invention is better understood by reference to the following examples. Example 1 A peeled foil was formed of an alloy comprising, by weight, about 19.7 percent chromium, 5.31 percent aluminium, 0.30 percent silicon, 0.28 percent manganese, 0.03 percent carbon, 0.007 percent nitrogen, not greater than 0.002 percent sulphur, less than 0.002 percent magnesium and the balance iron. The foil was made by a metal-peeling process as described in United States Patent No.4,331,631, issued to Chapman et al in 1982. The bare foil was pre-treated by heating in a vacuum furnace at about 900°C for about 15 minutes. Prior to heating, the furnace was evacuated to about 1 x 10⁻² Pa (7.5 x 10⁻⁵ torr). The oxygen partial pressure was estimated to be about 2 x 10⁻³ Pa (1.5 x 10⁻⁵ torr). Following the pre-treatment, the foil was placed in a furnace having an air atmosphere and heated for one hour at 925° C and then for three hours at 960°C. The resulting oxide is shown in Figure 1 and consists of well-developed oxide whiskers randomly oriented and substantially covering the surface. The whiskers are characterized by a high aspect ratio and are effective for bonding an applied ceramic coating. Comparative Example 1a A sample of the peeled foil in Example 1 was pre-treated as in Example 1 and thereafter oxidized by heating in air for four hours at 925°C. The resulting oxide is shown in Figure 2 and comprises whiskers which, in comparison to the whiskers formed in Example 1, are significantly smaller, even though the respective whisker growth treatments were carried out for comparable times. Comparative Example 1b For purposes of further comparison, a second sample of the foil described in Example 1 was pre-treated also as in Example 1 and thereafter oxidized by heating in air for four hours at 960°C. The resulting oxide is shown in Figure 3 and comprises a rough oxide layer having few, scattered whiskers, in marked contrast to the oxide whiskers shown in Figure 1 that substantially cover the surface. Thus, the method of this invention as set forth in Example 1 was effective to grow whiskers at temperatures that would not normally produce the desired whiskers ab initio. Example 2 A second sample of the foil described in Example 1 was treated in accordance with this invention to grow whiskers thereon. The sample was pre-treated also as in Example 1 and thereafter oxidized in air for two hours at 925°C and then for two hours at 990°C. The resulting oxide is shown in Figure 4. The oxide consists of high aspect ratio whiskers that substantially cover the surface to improve bonding of an applied coating. Comparative Example 2a For purpose of comparison, a sample of the foil described in Example 1 was pre-treated also as in Example 1 and thereafter heated for four hours at 990°C. The resulting oxide is shown in Figure 5. The oxide is formed of nodules, in marked contrast to the dense whisker growth formed in Example 2. Thus, this invention, as shown in Example 2, enabled the growth of whiskers at temperatures above the range heretofore considered suitable for such whisker growth. Example 3 A sample of the foil described in Example 1 was pre-treated as in Example 1 and thereafter heated in air for one hour at 925°C, then for one hour at 960°C, and finally for two hours at 990°C. The resulting oxide is shown in Figure 6 and comprises mature, high-aspect ratio oxide whiskers that substantially cover the foil surface and are well-suited for penetrating an applied coating to improve bonding thereof. Example 4 A sample of cold-rolled foil was treated in accordance with this invention to accelerate whisker growth. The foil was composed of a alloy comprising, by weight, 20.2 percent chromium, 5.06 percent aluminium, 0.37 percent silicon, 0.12 percent manganese, 0.003 percent sulphur, 0.039 percent cerium, 0.079 percent total rare earth (including cerium) and the balance iron and impurities. The foil was pre-treated by heating at 900°C for 10 seconds in an ambient pressure, nitrogen atmosphere containing 100 ppm oxygen. Whiskers were grown by heating the pre-treated foil in air for 15 minutes at 915°C and thereafter for 3.75 hours at 965°C. The resulting oxide is shown in Figure 7 and comprises mature, high-aspect ratio oxide whiskers that substantially cover the foil surface. For purposes of comparison, comparable whisker growth was obtained by heating similarly pre-treated foil at 940°C for eight hours. Thus, the dual temperature treatment of this invention was estimated to accelerate whisker growth by a factor of about two. Example 5 A sample of the cold-rolled foil described in Example 4 was treated in accordance with this invention to accelerate whisker growth thereon. The foil was pre-treated as in Example 4 and thereafter oxidized in air at 940°C for 15 minutes and thereafter at 985°C for 3.75 hours. The resulting oxide is shown in Figure 8 and comprises mature, high-aspect ratio whiskers substantially covering the surface. For purposes of comparison, a sample of similarly pre-treated foil was heated in air at 985°C for eight hours, but produced few, scattered whiskers. Example 6 A sample of cold-rolled foil of calcium-enriched ferritic stainless steel alloy was treated in accordance with this invention to accelerate whisker growth thereon. The alloy contained, by weight, about 18.7 percent chromium, 5.0 percent aluminium, 0.007 percent sulphur, 0.018 calcium and the balance mainly iron. The sample was pre-treated by heating at about 900°C for 10 seconds in an ambient pressure, nitrogen atmosphere containing 100 ppm oxygen. The pre-treated foil was heated for 15 minutes at 915°C and thereafter heated in air for 3.75 hours at 965°C. The resulting oxide is shown in Figure 9 and consisted of an abundance of high-aspect ratio oxide whiskers substantially covering the foil surface. For purposes of comparison, a single temperature oxidation treatment carried out on a similarly pre-treated sample at 965°C produced comparable whiskers, but only after about eight hours. Thus, the method of this invention successfully reduced whisker growing time by about one-half. Example 7 A sample of the foil described in Example 6 was similarly pre-treated and subjected to an accelerated whisker growing process in accordance with this invention. The sample was pre-treated as in Example 6 and heated in air at 940°C for about 15 minutes and thereafter at 985°C for 3.75 hours. The resulting whiskers are shown in Figure 10. For purposes of comparison, a comparably treated sample was heated at 985°C for eight hours, but produced few, scattered whiskers, in marked contrast to the abundance of whiskers substantially covering the foil surface when treated in accordance with this invention. Therefore, the method of this invention accelerates whisker growth on aluminium-containing ferritic stainless steel foil and thereby reduces the time and expense required to treat the foil, for example, to receive an applied coating. The optimum temperature schedule for accelerated whisker growth depends upon several factors including alloy composition. Whilst not limited to any particular theory, it is believed that the whiskers are formed of a transitional alumina composition composed mainly of delta and theta alumina. It is believed that the formation of this transitional alumina on aluminium-containing ferritic stainless steel is favoured between 900°C and 950°C. The formation of alpha alumina, which is not suitable for forming whiskers, is favoured at higher temperatures and predominates at temperatures above 1000°C. Thus, as oxidation temperature increases, the tendency to form alpha alumina also increases, reducing whisker formation. For this reason, it has been heretofore common practice to grow whiskers at temperatures less than 960°C, although the maximum whisker-growing temperature varies depending upon alloy composition and, more particularly, is sensitive to additives that influence oxide formation, such as oxide-adherence agents. It has now been found in accordance with this invention that whisker growth initiated within an optimum transitional alumina-forming temperature range may be continued and indeed accelerated at higher temperatures, including temperatures above the range normally suitable for initiating whisker growth. Thus, once whisker growth is initiated within the preferred transitional alumina-forming temperature, it may be suitably continued at higher temperatures up to the temperature whereat transitional alumina tends to transform to alpha alumina, typically about 1000°C. Thus, whisker growth in accordance with this invention is initiated and carried out at temperatures within distinctive ranges. In a first step, the steel is heated at a temperature between 900°C and 950°C to initiate whisker growth. This initial step may be readily accomplished by heating for about two hours, although longer times are not detrimental to whisker growth. Preferably, whisker growth may be initiated by heating for 15 minutes or less. Thereafter, the temperature is progressively increased to accelerate maturation of the nascent whiskers into whiskers of the desired size. This maturation step is carried out at temperatures between 960°C and 1000°C. Preferably, the oxidation temperature is initially set within the whisker initiation temperature range and progressively increased to a temperature within the suitable whisker maturation temperature range. The time required to complete whisker growth is dependent in part upon the size of the desired whiskers. Whilst in the accompanying Figures large whiskers having high aspect ratios are depicted, those skilled in the art will appreciate that smaller whiskers may be suitable for particular applications. For example, smaller whiskers may be adequate for bonding a ceramic washcoat of the type employed in the manufacture of an automotive catalytic converter and may be formed in a total of about one hour by the method of this invention by a regimen comprising oxidizing for 15 minutes within a suitable whisker initiation temperature range and thereafter oxidizing for the remaining time within a suitable whisker maturation temperature range. In the Examples, whisker growth was carried out in air, which is in marked contrast to the oxygen-starved atmospheres required for the preferred pre-treatment. This invention is intended to be carried out on aluminium-containing ferritic stainless steel alloys comprising chromium and aluminium and optionally containing oxide-adherence agents such as yttrium and cerium. Preferred alloys comprise, by weight, about 15 to 25 percent chromium and about 3 to 6 percent aluminium. This method is particularly suited for growing whiskers on foil, which foil may be formed either by cold-rolling or by a metal-peeling process. For cold-rolled foil, a low oxygen pre-treatment is necessary to form a precursor oxide suitable for nucleating whisker formation. Suitable pre-treatments are described in U.S. Patent No.4,318,828. In the above examples, pre-treatment was carried out either in a vacuum comprising a suitably low residual oxygen partial pressure or in a nitrogen atmosphere containing a controlled oxygen addition. Another example of a suitable pre-treatment comprises heating in a carbon dioxide atmosphere, which dissociates to produce reactive oxygen in amounts suitable for pre-treatment. Although pre-treatment is not necessary for foil formed by a metal-peeling process, it is nevertheless preferred to assure reliable and consistent whisker growth.

Claims: A method for growing oxide whiskers on a suitably prepared aluminium-containing ferritic stainless steel surface which comprises heating the surface in a readily-oxidizing atmosphere, characterised in that the surface is heated in air at an initial temperature in the range of 900°C to 960°C for a time less than two hours to initiate whisker growth, and thereafter is heated in air at a relatively higher temperature in the range of 960°C to 1000°C to continue whisker growth for a time sufficient to grow mature whiskers of a desired size on said surface. A method according to claim 1, characterised in that the aluminium-containing ferritic stainless steel is formed of an iron-base alloy containing, by weight, between 15 and 25 percent chromium, between 3 and 6 percent aluminium, and optionally an oxide-adhesion agent selected from the group consisting of yttrium and rare earth metals. A method according to claim 1 or 2, characterised in that the surface is prepared prior to initiating whisker growth by a pre-treatment process which comprises heating the surface whilst it is exposed to an atmosphere containing oxygen in an amount not greater than 100 Pa (0.75 torr) partial pressure to form a whisker precursor oxide film on said surface. A method according to claim 1, for growing oxide whiskers on an aluminium-containing ferritic stainless steel foil composed of an iron-base alloy containing, by weight, between 15 and 25 percent chromium and 3 and 6 percent aluminium, characterised in that said method comprises pre-treating the foil by heating the foil whilst it is exposed to an atmosphere containing oxygen in an amount not greater than 100 Pa (0.75 torr) partial pressure to form a whisker precursor oxide film on the foil, and heating the pre-treated foil whilst the foil is exposed to air to grow high-aspect ratio oxide whiskers that substantially cover the foil surface, said heating being carried out at an initial temperature in the range of 900°C to 950°C for a time equal to or less than 15 minutes and thereafter at a relatively higher temperature in the range of 960°C to 990°C to grow the desired mature whiskers. A method according to claim 4, characterised in that the alloy further includes an oxide-adhesion agent selected from the group consisting of yttrium and rare earth metals. A method according to claim 4, characterised in that the foil is cold-rolled foil. A method according to claim 4, characterised in that the foil is formed by a metal-peeling process. A method according to claim 4, characterised in that the total time that the foil is heated to grow the whiskers thereon does not exceed four hours. A method according to claim 4, characterised in that the temperature at which the foil is heated to grow the whiskers is progressively increased from a temperature within said initial temperature range to a temperature within said relatively higher temperature range.

Inventors: SIGLER, DAVID R., VANEMAN, GERALD L.
Applicants: GENERAL MOTORS CORPORATION
Application year: 1989
Main classification: C30B29
IPC codes: C22C38, C30B25, C23C8, C30B29