Barrier for a metal substrate

A barrier is formed on a metal substrate by coating the substrate with a metal oxide, calcining the substrate, impregnating the coated substrate with an acid, and calcining the impregnated coating at a temperature high enough to cause the metal oxide to form the barrier. The resulting barrier acts as an excellent electrical insulator, and also provides improved resistance to abrasion, and improved adhesion to the substrate. The particles forming the barrier also have improved cohesion. The metal substrate having the barrier of the present invention can be used in electrically heated catalytic converters, where it is necessary to provide closely spaced layers of metal foil that must be electrically insulated from each other. The invention can also be used in other metal structures intended to be placed in the exhaust stream of a chemical or manufacturing process or an engine.

BACKGROUND OF THE INVENTION 
This invention provides a barrier for a metal substrate, the barrier being 
electrically insulating and having improved abrasion resistance, and 
improved adhesion to the substrate. As used in this specification, the 
term "barrier" means a layer that is applied to the substrate and which 
remains with the substrate after heat treatment. The invention is useful 
in electrically heated catalytic converters (EHCs), wherein it is 
necessary to provide metal strips which are electrically insulated from 
each other. However, the invention is not limited to use with EHCs, but 
can be used in any application requiring a tightly-adhered, durable 
barrier on a metal substrate. 
U.S. Pat. No. 5,288,470 describes an electrically insulating barrier that 
can be formed on a metal strip, such that the strip can become part of an 
electrically operated heater, such as a heater mounted in the exhaust 
stream of a chemical or manufacturing process, or in the exhaust stream of 
a mobile or stationary engine. The disclosure of the above-cited patent is 
hereby incorporated by reference into this specification. 
The present invention provides an improved barrier for a metal strip, such 
as a metal foil. The barrier of the present invention is not only an 
excellent electrical insulator, but also is very abrasion-resistant and 
durable as compared with the products available in the prior art. 
SUMMARY OF THE INVENTION 
The metal substrate having the barrier of the present invention can be made 
according to the following process. First, a metal substrate is coated 
with a metal oxide, such as alumina, titania, hafnia, or zirconia, and the 
coating is calcined at a temperature of at least about 400.degree. C. 
Next, the oxide coating is impregnated with an acid. The acid can be 
selected from the strong acids, i.e. acids having a pKa of &lt;0.1, including 
but not limited to hydrochloric acid or nitric acid, or any combination 
thereof, or it can be selected from the weaker acids, or combinations of 
weaker acids, i.e. acids having a pKa&gt;0.1 and less than 2.5, such as 
phosphoric acid. Then, the impregnated coating is calcined at a 
temperature high enough to cause the metal oxide to form the desired 
barrier. The latter temperature may be about 400.degree. C., but can vary 
depending on the particular coating used. 
The coating of metal oxide must have a thickness sufficient to provide the 
desired properties, such as electrical resistance and abrasion resistance, 
in the barrier. Preferably, the thickness of the coating should fall 
within the range of about 5-40 microns, and most preferably 10-30 microns. 
The latter thickness is measured after the substrate and oxide coating 
have been calcined. In general, it may be desirable to apply the metal 
oxide layer in more than one coating, depending on the amount of metal 
oxide supplied in each coating. 
The metal oxides used in the present invention include, but are not limited 
to, the oxides of metals such as aluminum, titanium, zirconium, or 
hafnium, or a mixture of oxides. The barrier formed by the present 
invention is more resistant to attrition than the barrier described in 
U.S. Pat. No. 5,288,470. 
The present invention therefore has the primary object of providing a metal 
substrate having a barrier formed thereon. 
The invention has the further object of providing a metal substrate having 
a barrier, wherein the barrier is an excellent electrical insulator. 
The invention has the further object of providing a metal substrate having 
a barrier, wherein the barrier is abrasion-resistant. 
The invention has the further object of providing a metal substrate having 
a barrier, wherein the barrier exhibits excellent adhesion to the metal. 
The invention has the further object of providing a barrier on a metal 
surface, wherein the particles comprising the barrier exhibit improved 
cohesion. 
The invention has the further object of enhancing the reliability and 
service life of an electrically heated catalytic converter (EHC), by 
providing an electrically-insulating barrier to coat the metal strips 
forming the EHC. 
The invention has the further object of providing a barrier for a metal 
substrate, wherein the metal substrate forms part of a structure placed in 
the exhaust stream of an engine or of a chemical or manufacturing process. 
The invention has the further object of providing a method of making the 
metal substrate with the barrier described above. 
The reader skilled in the art will recognize other objects and advantages 
of the invention, from a reading of the following brief description of the 
drawing, the detailed description of the invention, and the appended 
claims.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is a substrate which has a barrier formed thereon. 
The invention also includes a method of forming the barrier on the 
substrate. The barrier is an excellent electrical insulator, and adheres 
very tightly to the substrate. The barrier is also abrasion-resistant. The 
particles forming the barrier also exhibit a high degree of cohesion. 
The substrate and barrier can be made according to the following method. 
First, one coats a metal substrate with a slurry of metal oxide. The metal 
oxide may be an oxide of aluminum, titanium, hafnium, or zirconium. Before 
applying the slurry, it is preferable to pre-heat the metal substrate to 
form a thin layer of oxide which provides a better bond for the oxide 
coating. Then, one calcines the substrate and the slurry, at a temperature 
of at least about 400.degree. C. The slurry is applied in an amount such 
that, when the substrate has been calcined, the thickness of the oxide 
coating is in the range of about 5-40 microns, and preferably 10-30 
microns. The slurry can be applied using any of various methods known to 
those skilled in the art, such as painting, dipping, spraying, etc. 
Next, one impregnates the coated substrate with an acid. The acid can be a 
weak acid, such as phosphoric acid, or a strong acid, such as an acid 
selected from the group consisting of nitric acid, hydrochloric acid, and 
sulfuric acid. One could also use any combination of weak acids or a 
combination of strong acids. 
Finally, one calcines the impregnated structure at a temperature 
sufficiently high to cause the metal oxide to form the desired barrier. 
The entire process can be repeated after completion of the final calcining 
step. 
When the barrier is formed, the acid partially dissolves the metal oxide 
coating, forming salts which decompose to produce metal oxide upon 
calcining. In the case of phosphoric acid, the the acid does not dissolve 
the metal oxide coating, but instead forms a metal phosphate after 
calcining. 
When using a weak acid such as phosphoric acid, the acid should be applied 
in an amount such that the weight gain of the strip due to the acid is at 
least 0.25 times the weight gain due to the addition of the metal oxide. 
Various devices known to those skilled in the art can be used to test the 
barrier formed on the metal substrate according to the present invention. 
For example, the FIGURE shows an apparatus to measure the resistance to 
attrition. The apparatus includes a lower rail 10 of insulating plastic. A 
strip of metal foil 11 is coated with the barrier to be tested, and the 
strip is stretched along the lower rail 10. Strip 11 is held in place by 
clamps 12. The barrier is cleaned off the ends of strip 11 so that clamps 
12 make electrical contact with strip 11. 
A corrugated strip of metal foil 13 is stretched along upper rail 14 and is 
held in place by clamps 15. Upper rail 14 reciprocates over lower rail 10 
with a one-way travel of one-half inch, as indicted by arrows 16. The 
total travel is 60 inches per minute. Strips 11 and 13 are in contact over 
a length of 6 inches. The weight of upper rail 14 is about 400 gm. The 
width of the upper corrugated strip is one inch, which is wider than the 
strip on the lower rail. The latter relationship prevents the edge of the 
corrugated strip from scoring the coating on the lower flat strip. 
The test procedure may be to apply a voltage between strip 11, which is the 
strip being tested, and corrugated strip 13, and to record the time when 
the barrier on strip 11 fails, i.e. when current flows from one strip to 
the other. This procedure gives a reproducible measure of the attrition 
resistance of the barrier. 
Alternatively, the test procedure can involve abrading strips 11, 13 for a 
given period of time, and thereafter measuring the weight loss from strip 
11. 
The following examples clarify the details of the invention, and provide 
information showing the degree of electrical insulation, 
abrasion-resistance, and adhesion exhibited by the barrier formed on the 
metal substrate. 
EXAMPLE 1 
To establish a basis for comparison of the present invention with the prior 
art, the attrition apparatus described above, and shown in the FIGURE, was 
used to test a barrier made by the method of U.S. Pat. No. 5,288,470. The 
test strip was of Haynes Alloy 214 with the following composition: 
16% chromium 
2.5 Iron 
4.5 Aluminum 
Balance nickel 
The lateral dimensions of the strip were 0.7.times.8.25 inches, and the 
strip was 0.002 inches thick. 
In this example, and in many other examples in this specification, the 
process steps are concisely described by a table which indicates, on the 
left-hand side, what was done with the strip, and, on the right-hand side, 
the weight of the strip (in grams) after a particular step. Thus, the 
left-hand column of each table describes the process applied to the strip, 
and shows the order of the process steps, the first step simply being 
providing a bare metal strip. Details on the nature of the oxide coating 
are given in other examples, below. 
The process steps and applicable weights for this example are as follows: 
______________________________________ 
Bare strip 1.4773 
Strip with four coats of alumina 
1.5878 
washcoat, calcined at 400.degree. C. 
Above strip calcined at 110.degree. C. 
1.5917 
Above strip with ends cleaned off 
1.5896 
for electric contact 
______________________________________ 
The strip was stroked for 65 minutes. The weight loss was 0.0193, and 
electric contact between the strips started sometime during the 65 
minutes. 
The following example shows the performance of a barrier made according to 
the present invention. 
EXAMPLE 2 
This example shows the increased hardness of the barrier of this invention. 
The strip had the same size and composition as in Example 1. 
The process steps and applicable weights were: 
______________________________________ 
Bare strip 1.4784 
Above strip with four coats of 
1.5893 
alumina washcoat, calcined at 470.degree. C. 
Above strip impregnated three times 
1.6769 
with phosphoric acid, and calcined at 
460.degree. C. 
Above strip calcined at 110.degree. C. 
1.6667 
Above strip with its ends cleaned 
1.6575 
______________________________________ 
The phosphoric acid (commercial 85%) was diluted to one part (by weight) 
H.sub.3 PO.sub.4 to two parts (by weight) H.sub.2 O. 
In this barrier, the weight gain from the phosphoric acid divided by the 
weight gain from the alumina was (1.6769-1.5893)/(1.5893-1.4784) or about 
0.79. In this specification, the weight gain is abbreviated as PO.sub.4 
/Al.sub.2 O.sub.3. 
This strip was stroked for one hour in the attrition machine shown in the 
FIGURE. Then it weighed 1.6577. The apparent weight gain is due to 
moisture pickup. 
The stroking was continued for 4 hours while a voltage was applied between 
the strips. During the 4 hours, the voltage was increased in steps from 12 
to 100 volts. At the end of 4 hours, when the voltage was increased to 
120, the barrier failed and current flowed between the strips. Then the 
strip weighed 1.6575 gm. 
EXAMPLE 3 
The strip had the same size and composition as in Example 1. 
The process steps and applicable weights were: 
______________________________________ 
Bare strip 1.4676 
Above strip with four coats of alumina 
1.6095 
washcoats, calcined at 450.degree. C. 
Above strip impregnated three times 
1.6702 
with one part (by weight) H.sub.3 PO.sub.4 to 
one part (by weight) H.sub.2 O, and calcined 
at 450.degree. C. 
Above strip calcined at 110.degree. C. 
1.6723 
Above strip with ends cleaned 
1.6672 
______________________________________ 
The weight gain PO.sub.4 /Al.sub.2 O.sub.3 was 0.43. 
This strip was stroked for 6.8 hours while the voltage was increased in 
steps to 120. Then the strip was turned over and tested on the other side 
for 10 hours while the voltage was increased in steps to 120. Then the 
strip was heated to 165.degree. C. to expel absorbed moisture. Then the 
strip weighed 1.6663 gm, for a loss of about 0.001 gram in 16 hours. The 
barrier remained intact during these 16 hours. 
EXAMPLE 4 
The strip in this example had the same size and composition as in Example 
1. 
The process steps and applicable weights were: 
______________________________________ 
Bare strip 1.4674 
Above strip with two coats of alumina 
1.5195 
washcoat calcined at 400.degree. C. 
Above strip impregnated three times 
1.5431 
with one part H.sub.3 PO.sub.4 to one part H.sub.2 O, 
and calcined at 450.degree. C. 
Above strip calcined at 110.degree. C. 
1.5480 
Above strip with ends cleaned off 
1.5468 
______________________________________ 
The weight gain PO.sub.4 /Al.sub.2 O.sub.3 was 0.45. 
With this light coating of alumina, the barrier was ineffective, and barely 
withstood 12 volts. After about 30 minutes of stroking, the strip weighed 
1.5470 gm, so there was no measurable loss in weight, even though the 
barrier was ineffective. 
EXAMPLE 5 
The strip had the same size and composition as in Example 1. 
The process steps and applicable weights were: 
______________________________________ 
Bare strip 1.4701 
Above strip with four coatings of 
1.6000 
alumina washcoat, calcined at 420.degree. C. 
Above strip impregnated three times 
1.6527 
with one part H.sub.3 PO.sub.4 to one part 
H.sub.2 O, and dried at 168.degree. C. 
Above strip calcined at 1100.degree. C. 
1.6534 
Above strip with ends cleaned 
1.6485 
______________________________________ 
The weight gain PO.sub.4 /Al.sub.2 O.sub.3 was 0.41. 
The strip was stroked for 4 hours while the voltage was increased in steps 
to 120. The barrier remained intact. The strip weighed 1.6488, with no 
loss. The strip was turned over and tested on the other side. The barrier 
failed after about 40 minutes, when the voltage was 80. Then the strip 
weighed 1.6484 gm, still no loss. An ohmmeter probe was run along the 
edges of the strip, and showed that the barrier had failed on the edge, as 
usual. In this example the strip was dried at the low temperature of 
168.degree. C. after each impregnation with phosphoric acid. Apparently 
calcining at high temperature is not necessary until after the final 
impregnation. 
EXAMPLE 6 
The strip had the same size and composition as in Example 1. 
The process steps and applicable weights were: 
______________________________________ 
Bare strip 1.4718 
Above strip with four coatings of 
1.5655 
alumina washcoat, and calcined 
at 450.degree. C. 
Above strip impregnated once with 
1.5841 
one part H.sub.3 PO.sub.4 to one part H.sub.2 O and 
calcined at 500.degree. C. 
Above strip calcined at 1110.degree. C. 
1.5869 
Above strip with ends cleaned 
1.5860 
______________________________________ 
The weight gain PO.sub.4 /Al.sub.2 O.sub.3 was 0.20. 
This strip was stroked for one hour. After stroking, it weighed 1.5827, for 
a loss of 0.0033 gm. This low level of PO.sub.4 /Al.sub.2 O.sub.3 produces 
some hardening, but no electrically insulating barrier. There was 
electrical contact between the strips from the start of the test. 
EXAMPLE 7 
The strip had the same size and composition as in Example 1. 
The process steps and applicable weights were: 
______________________________________ 
Bare strip 1.4590 
Above strip with six coats of alumina 
1.6455 
washcoat, and calcined at 300.degree. C. 
Above strip impregnated three times 
1.7179 
with one part H.sub.3 PO.sub.4 to one part 
H.sub.2 O, and dried at 165.degree. C. 
Above strip calcined at 900.degree. C. 
1.7123 
Above strip with the ends cleaned 
1.7005 
______________________________________ 
The weight gain PO.sub.4 /Al.sub.2 O.sub.3 was 0.39. 
The strip was stroked for 4.7 hours while the voltage was increased in 
steps to 120. The barrier remained intact. Then the strip weighed 1.7017, 
with no measurable loss. The strip was turned over and tested on the other 
side. The barrier failed after about 1.5 hours, at 80 volts. The strip 
weighed 1.7021 gm, again with no measurable loss. This test indicates that 
the final calcining temperature can be lowered to 900.degree. C. 
EXAMPLE 8 
This example describes the preparation of the alumina washcoat used in the 
foregoing examples. A five liter ball mill is charged with 4600 gm of 
Burundum.TM. grinding medium and: 
384 gm Catapal G 
36.4 gm Disperal 
34.4 gm concentrated nitric acid 
567 gm water 
The mill was turned for 4 hours, and the product washcoat was poured out. 
About 1000 gm of washcoat was produced in each batch. Catapal G is a 
calcined gamma alumina supplied by Vista Chemical Co. Disperal is an 
uncalcined dispersible alumina supplied by Condea Chemie of Germany. 
EXAMPLE 9 
Here we describe the preparation of the alumina washcoat used in Example 
10. Catapal B is an uncalcined nondispersible alumina. This material was 
calcined at 600.degree. C. to produce an alumina equivalent to the Catapal 
G used in Example 8. A 1.1 liter ball mill was charged with 1600 gm 
zirconia grinding medium and: 
100.0 gm calcined Catapal B 
10.0 gm Disperal 
10.0 gm concentrated nitric acid 
170 gm water. 
The mill was turned for 4 hours and 255 gm of washcoat was poured out. 
EXAMPLE 10 
The strip had the same size and composition as in Example 1. 
The process steps and applicable weights were: 
______________________________________ 
Bare strip 1.5634 
Above strip with six coats of alumina 
1.7688 
washcoat, and calcined at 300.degree. C. 
Above strip impregnated once with 
1.8235 
three parts H.sub.3 PO.sub.4 to one part H.sub.2 O, and 
calcined at 300.degree. C. 
Above strip calcined at 600.degree. C. 
1.8198 
Above strip with ends cleaned 
1.8088 
______________________________________ 
The weight gain PO.sub.4 /Al.sub.2 O.sub.3 was 0.27. 
The barrier failed in the first two minutes of stroking, and the electrical 
contact was located on the edge of the strip. The one-inch wide upper 
corrugated strip was replaced with a flat strip 1/4-inch wide. The contact 
on the edge of the test strip was bypassed thereby. Stroking was resumed 
and continued for 10.7 hours while the voltage was increased in steps to 
140. The barrier remained intact. Then the strip weighed 1.8064 gm, with 
an apparent loss of 0.0024. This test indicates that the final calcining 
temperature can be reduced to 600.degree. C. 
EXAMPLE 11 
This example describes the preparation of the titania washcoat used in 
Example 12. 
The preparation begins with a solution of titanyl sulfate, TiOSO.sub.4 that 
assays 9.4 wt % TiO.sub.2. Fifty grams of TiOSO.sub.4 solution was diluted 
to about 540 gm, and the pH was increased to 2.8 with ammnonium hydroxide. 
This precipitates most, but not all, of the TiO.sub.2 as a hydrous oxide. 
Then 0.80 gm of phosphoric acid was added. This reduced the pH to 2.5, and 
also precipitated the last of the titania. The precipitate was collected 
on a filter and washed free of sulfate ion. The filter cake weighed 57 gm. 
The cake was dried under vacuum to a weight of 30 gm. The dried cake was 
charged to a ball mill along with 3.6 gm of concentrated nitric acid. The 
mill was turned until the cake was reduced to water thin consistency. Then 
18 gm of Kemira titania 907 was added to the mill, and the mill was turned 
until the washcoat reached a constant thin consistency. 
EXAMPLE 12 
The strip had the same size and composition as in Example 1. 
The process steps and applicable weights were: 
______________________________________ 
Bare strip 1.4780 
Above strip with four coats of 
1.6036 
titania washcoat, and calcined 
at 400.degree. C. 
Above strip impregnated with undiluted 
1.6482 
H.sub.3 PO.sub.4, (85% concentration) and calcined 
at 400.degree. C. 
Above strip calcined at 900.degree. C. 
1.6454 
______________________________________ 
The weight gain PO.sub.4 /TiO.sub.2 was 0.35. 
Before starting a test on the attrition apparatus, an ohmmeter probe was 
run along both edges of the strip. There was electrical contact all along 
both edges. To make a meaningful test, the upper one-inch corrugated strip 
was replaced with a 1/4-inch flat strip, just as was done in Example 10. 
The stroking test lasted for 3.6 hours while the voltage was increased to 
80. Then the voltage was increased to 100 and the barrier failed in less 
than one-half hour. The strip was turned over and tested on the other 
side. The test (on the other side) lasted for 3 hours while the voltage 
was increased in steps to 100. The barrier failed at 3 hours when the 
voltage was 100. 
EXAMPLE 13 
This example describes a barrier of hafnium oxide. The source of the 
hafnium was the oxychloride HfOCl.sub.2.8 H.sub.2 O, formula weight 409, 
supplied by Teledyne Wah Chang. 
One tenth mol, 40.9 gm, of oxychloride was dissolved into 900 gm of 
solution. The pH was raised to 7.0 with ammnonium hydroxide which 
precipitated a hydrous oxide. The precipitate was collected on a filter 
and washed free of chloride ions. The undried filter cake weighed 220 gm. 
The cake was dried under vacuum to a weight of 24.9 gm. The cake was 
charged to a ball mill along with 3.7 gm of concentrated nitric acid and 
21 gm water. The mill was turned for 1.6 hours. Forty gm of milk white 
water thin washcoat was poured out of the mill. 
The test strip was of Allegheny Ludlum's alloy Alfa IV with the following 
composition: 
20% chromium 
5% aluminum 
balance mostly iron 
The size of the strip was 3.5.times.6 inches and 0.002 inch thick. Strips 
of this size were used early in this work, before the test apparatus 
described above, and shown in the FIGURE, had been built. Therefore, the 
effectiveness of the barrier was measured by dragging the two probes of 
the ohmmeter across the surface of the strip. If there was infinite 
resistance between the probes, the barrier was intact. 
The process steps and applicable weights were: 
______________________________________ 
Bare strip 4.7056 
Above strip with first coat of hafnia 
4.7409 
washcoat, on one side, dried at 185.degree. C. 
Above strip impregnated with one part 
4.7508 
H.sub.3 PO.sub.4 to three parts H.sub.2 O, and dried 
at 185.degree. C. 
Above strip calcined at 850.degree. C. 
4.7457 
Above strip with second coating of 
4.7759 
hafnia washcoat, dried at 185.degree. C. 
Above strip impregnated with one part 
4.7874 
H.sub.3 PO.sub.4 to three parts H.sub.2 O, and dried 
at 185.degree. C. 
Above strip calcined at 850.degree. C. 
4.7816 
______________________________________ 
The weight gain PO.sub.4 /HfO.sub.2 was 0.16. 
After the second coating, but not after the first coating, there was 
infinite resistance between the ohmmeter probes. 
EXAMPLE 14 
This example describes a barrier of zirconium oxide. The source of the 
zirconium was the oxynitrate ZrO(NO.sub.3).sub.2 supplied by Pfaltz and 
Bauer as a water solution. 
Experiments had shown that 100 gm of this solution requires 0.65 
equivalents of alkali to give complete precipitation. This amount of 
anmonium hydroxide was diluted into 2 liters of solution and 100 gm of 
oxynitrate solution was added with stirring. The precipitate was collected 
on a filter and washed. The filter cake was dried in an oven at 90.degree. 
C. to a final weight of 23.6 gm. A second 100 gm of oxynitrate solution 
was precipitated in the same way. The washed undried filter cake weighed 
190 gm. This undried cake plus the 23.6 gm of dried cake, plus 4 gm of 
concentrated nitric acid and 4 gm of water, was charged to a ball mill. 
The mill was turned for 4 hours and then 203 gm of washcoat was poured 
out. 
The metal strip had the same size and composition as in Example 13. The 
process steps and applicable weights were: 
______________________________________ 
Bare strip 4.6713 
Above strip with first coat of 
4.6846 
zirconia washcoat, dried at 175.degree. C. 
Above strip impregnated with one part 
4.6902 
H.sub.3 PO.sub.4 to five parts H.sub.2 O, and dried at 
185.degree. C. 
Above strip calcined at 470.degree. C. 
4.6889 
Above strip with second coat of 
4.7057 
zirconia washcoat, dried at 190.degree. C. 
Above strip calcined at 500.degree. C. 
4.7030 
Above strip impregnated with one part 
4.7131 
H.sub.3 PO.sub.4 and five parts H.sub.2 O, and dried 
at 180.degree. C. 
Above strip calcined at 530.degree. C. 
4.7108 
Above strip with third coat of 
4.7344 
zirconia washcoat, dried at 180.degree. C. 
Above strip calcined at 550.degree. C. 
4.7304 
Above strip impregnated with one part 
4.7418 
H.sub.3 PO.sub.4 and five parts H.sub.2 O, and dried 
at 185.degree. C. 
Above strip calcined at 570.degree. C. 
4.7386 
Above strip calcined at 850.degree. C. 
4.7376 
______________________________________ 
The weight gain PO.sub.4 /ZrO.sub.2 was 0.38. 
After the third coating with zirconia, but not after the second coat, there 
was infinite resistance between the ohmmeter probes. 
EXAMPLE 15 
This Example describes a barrier that contains the oxides of both titanium 
and zirconium. A feature of this titania-zirconia washcoat is that it is 
made in a single step, unlike the titania washcoat of Example 11 or the 
zirconia of Example 14. The washcoat of this Example was made by ball 
milling together a solution of zirconyl nitrate, ZrO(N0.sub.3).sub.2, and 
titanium oxide. In a typical preparation, the ball mill was charged with: 
105.2 gm Kemira 907 titanium oxide 
82.5 gm ZrO(NO.sub.3).sub.2 solution 
72 gm water 
The mill was turned for one hour. The ZrO(NO.sub.3).sub.2 solution contains 
20.6% ZrO.sub.2, and Kemira 907 contains 81.7% TiO.sub.2 so that the mol 
ratio (Zro.sub.2 /TiO.sub.2) was 0.13. 
A strip of Alfa IV, having dimensions of 3.5.times.6 inches, was coated 
with the above-described material. The process steps and applicable 
weights were: 
______________________________________ 
Weight of bare strip 
4.6417 
Above strip with four coats 
5.0418 
of washcoat on just one side, 
calcined at 500.degree. C. 
Above strip impregnated with 
5.1194 
one weight H.sub.3 PO.sub.4 to 0.5 weights 
H.sub.2 O, dried, and calcined at 
500.degree. C. 
Above strip again impregnated, 
5.1920 
dried and calcined at 500.degree. C. 
______________________________________ 
The weight gain PO.sub.4 /(ZrO.sub.2 +TiO.sub.2) was 0.38. 
A narrow strip 1/4 inch wide was cut off the 6-inch side of the coated Alfa 
IV. The 1/4-inch strip was folded upon itself with the coated side on the 
outside of the fold, and the fold was pressed flat. Only a little of the 
barrier peeled off along the fold line, indicating good adherence of this 
barrier. 
Further experiments showed that good adherence is obtained over a mol ratio 
of ZrO.sub.2 /TiO.sub.2 from about 0.11 to 0.15. 
EXAMPLE 16 
This Example provides a frame of reference for testing the effects of 
different acid treatments in making the coated substrate of the present 
invention. In this and in all of the following Examples, the metal 
substrate was made of Haynes 214 nickel-based alloy having a thickness of 
50 microns (about 0.002 inches). The metal substrate was pre-treated to 
form a thin oxide film by heating in air to 550.degree. C. for one minute, 
so as to provide a hydrophilic surface for the alumina washcoat. To the 
preoxidized foil there was applied, by electrophoretic deposition, a layer 
of alumina washcoat, of the type described in Example 8, above. The 
washcoat was dried using a heat gun to form a porous alumina coating 
containing some hydrated alumina species. The coated foil was calcined at 
950.degree. C. for 15 minutes in air to convert all hydrated alumina 
species to the oxide and to form chemical bonds between the coating and 
the foil (i.e. to provide adhesion) as well as between the alumina 
particles themselves (i.e. to provide cohesion). The thickness of the 
coating after calcination was 25 microns. 
The adhesion energy was measured using a Hesiometer blade adhesion tester, 
which is commercially available from Adhesion International, Inc., of 
Spokane, Washington. This instrument measures the adhesion of the barrier. 
The results may differ from those obtained with the abrasion instrument 
shown in the FIGURE. The results obtained with the latter instrument more 
closely correlate with cohesion, i.e. the bonding among the particles of 
alumina. 
The adhesion energy was measured using the Hesiometer blade adhesion 
tester, which used a 5-mm wide blade set at an angle of 30.degree. 
relative to the foil and a normal force of 10N to scrape the coating from 
the metal foil. The energy required to remove the coating is equal to the 
practical adhesion energy. For this Example which involved a substrate 
having a metal oxide coating, unmodified by acid, the adhesion energy was 
199 J/m.sup.2. 
EXAMPLE 17 
This Example and the following Examples involve the use of nitric, 
hydrochloric, and phosphoric acids to harden the alumina coating applied 
to metal foils. The concentrations used were based on a 3:1 dilution of 
concentrated acid and water. However, in general, a normality sufficient 
to cause dissolution of alumina is sufficient. This would include 
concentrations greater than 1 Normal up to concentrated acid. The more 
dilute the acid, the more applications of acid will be required to achieve 
the desired level of adhesion. 
In this Example, and in the subsequent Examples, the pre-treatment of the 
foil and application of the base alumina coating were identical to Example 
16. 
Following the calcination at 950.degree. C. for 15 minutes, the coating was 
treated in the following way, to modify the coating and to improve the 
adhesion energy. The coating was impregnated with 8N HCl acid by brushing 
to saturation. The impregnated coating and foil were then air dried using 
an air gun followed by a second calcination at 950.degree. C. for 15 min. 
A second impregnation with 8N HCl acid, followed by drying and calcination 
steps, were performed to achieve the additional bonding necessary for 
improved adhesion. The adhesion energy was measured as above, and a 
significant improvement due to the acid treatment was observed. The 
adhesion energy was 460 J/m.sup.2 (at 10N force, with a blade angle of 
30.degree.). 
EXAMPLE 18 
Following the calcination to 950.degree. C. for 15 minutes, the coating was 
treated in the following way to modify the coating and improve the 
adhesion energy. The coating was impregnated with 10N HNO.sub.3 acid by 
brushing to saturation. The impregnated coating and foil was then air 
dried using an air gun followed by a second calcination at 950.degree. C. 
for 15 minutes. A second impregnation with 10N HNO.sub.3 acid, followed by 
drying and calcination steps, were performed. The measured adhesion energy 
was 390 J/m.sup.2 (at 10N force, with a blade angle of 30.degree.). 
EXAMPLE 19 
Following the calcination to 950.degree. C. for 15 minutes, the coating was 
treated in the following way to modify the coating and improve the 
adhesion energy. The coating was impregnated with 5.5N H.sub.3 PO.sub.4 
acid by brushing to saturation. The impregnated coating and foil were then 
air dried using an air gun followed by a second calcination at 950.degree. 
C. for 15 minutes. A second impregnation with 5.5N H.sub.3 PO.sub.4 acid, 
followed by drying calcination steps, were performed. The measured 
adhesion energy was 418 J/m.sup.2 (at 10N force, with a blade angle of 
30.degree.). 
Examples 17-19 show that the addition of acid to the oxide coating 
substantially increases the adhesion energy of the barrier formed 
according to the present invention. In the case of the strong acids, the 
alumina was partially dissolved and re-deposited upon calcining. The acid 
(phosphoric acid in the Examples) did not dissolve the alumina, left a 
residue of aluminum phosphate. 
The invention can be modified further, such as by increasing the number of 
oxide coatings, increasing the amount of acid used, and/or increasing the 
calcining temperatures. These and other similar modifications should be 
considered within the spirit and scope of the following claims.