Porcelain enameled metal substrates

The present invention contemplates a novel enamel composition, a novel electrodeposition bath, a novel metal substrate for enameling, and the novel PEMS produced from these materials and procedures. In general, the novel glass composition of the present invention are within the following composition ranges (which are set forth in percent by weight). ______________________________________ SiO.sub.2 9-16% B.sub.2 O.sub.3 20-25% MoO.sub.3 2-10% MgO 30-40% BaO 16-22% ZnO 5-14% SnO.sub.2 0-4% ______________________________________ In the novel application method of the present invention, an improved electrophoretic bath has been found in which the glass particles are suspended in a mixture of isopropyl alcohol and either dichloromethane or trichlorotrifluoroethane together with ethylene glycol and an acrylic additive. Stainless steel electroplated with a thin film of molybdenum and heat-treated at an elevated temperature is employed as the substrate.

The present invention relates to porcelain enameled metal substrates 
("PEMS") and more particularly to novel porcelain enamel compositions for 
preparing PEMS, as well as novel methods of preparing metal substrates for 
application of an enamel coating and novel baths and techniques for 
applying such enamel coatings. 
BACKGROUND OF THE INVENTION 
The use of PEMS as a base upon which to fabricate electronic circuitry is 
well known in the art. The early PEMS comprised a conventional porcelain 
enamel glass applied to a metal substrate, typically a low carbon steel 
substrate, which was fired causing the glass to fuse and flow and form a 
glass film bonded to the metal substrate by various chemical and 
mechanical mechanisms. On cooling to room temperature, the resulting 
porcelain enameled metal substrate was found to be useful for fabricating 
a variety of electronic elements, so long as the subsequent steps in the 
fabrication of the electronic component did not require reheating of the 
PEMS to a temperature higher than the softening temperature of the glass, 
usually between 550.degree. and 650.degree. C. This temperature limitation 
prohibited the use of PEMS in the fabrication of high reliability hybrid 
circuit boards where the printed circuits were desired to be cured at 
temperatures as high as 850.degree. C. 
In preparing such PEMS from porcelain enamel glass compositions, the glass 
was typically prepared from a mixture of precursor oxides which were 
smelted at a temperature of about 1200.degree. to about 1500.degree. C. 
for about 30 to about 60 minutes, then roll quenched to provide a frit 
which was ground in water to an average particle size of about 10 to about 
25 microns. The porcelain enamel overcoat was then applied to the metal 
substrate by a number of different techniques, but usually by 
electrophoresis from a water based bath, after which the enamel was dried 
and fired typically at a temperature of about 760.degree. to about 
880.degree. C. 
As noted earlier, in order to achieve stability and reliability of printed 
hybrid circuit boards, refiring temperatures as high as 850.degree. C. or 
higher are preferred. Since conventional porcelain enamel compositions 
could not function under such parameters, the use of crystallizing glass 
compositions were developed. Crystallizing glass compositions, or 
devitrified glass compositions, behave quite differently from conventional 
porcelain enamel glass compositions when fired. 
The conventional glass enamel, when applied to the metal substrates and 
fired, are fused with increasing temperature to flow and form a glass film 
upon the metal substrate. The crystallizing porcelain enamel coatings on 
the other hand have a normal firing temperature of about 800.degree. to 
900.degree. C. to form a good enamel film, but subsequently crystallize to 
drastically increase the viscosity of the coating. The crystallized 
coating will then behave similar to the crystalline materials and retain 
their rigidity even if refired to the same high firing temperatures. Such 
materials, for example, are taught in U.S. Pat. No. 4,256,796 to Hang et 
al., U.S. Pat. No. 4,358,541 to Andrus et al., U.S. Pat. No. 4,385,127 to 
Chyung, and also in Japanese Patent Publication No. 85/172 102, dated 
Sept. 5, 1985. These glass ceramic systems generally rely on the 
recrystallization of BaO.2MO.2SiO.sub.2, 2MO.B.sub.2 O.sub.3, 
2MO.SiO.sub.2, and MO.SiO.sub.2 crystals from the mother glass based on 
the various oxide compositions of BaO, SiO.sub.2, B.sub.2 O.sub.3, and MO, 
where MO stands for MgO, CaO, or ZnO. 
The use of recrystallizing glass ceramic coatings has overcome the 
softening problems encountered with conventional glass enamel 
compositions, however, problems have been encountered with lack of 
adhesion of the glass ceramic coating to the metal substrate. In 
particular with low carbon steel substrates, it has been noted that the 
enamel cracks on the edges of the substrate, more particularly on the 
corners, especially after being refired repeatedly at high temperatures. 
This failure is due in part to the poor adhesion of glass ceramic coating 
to the metal. However, the main cause of the enamel cracks is due to too 
high a thermal expansion of the core metal compared to that of the coating 
material. The volume change accompanied by the ferrite-toaustenite phase 
transformation in low carbon steel taking place around 870.degree. C., is 
still another factor causing the enamel cracks if fired above this 
temperature. 
The low carbon steel substrates were also found to have a tendency to warp 
when fired repeatedly at temperatures in excess of 850.degree. C. Extended 
exposure to elevated temperatures can cause the grain growth of low carbon 
steel structures and the coarse grain crystal structures adversely affect 
the physical strength of substrates. The volume change associated with the 
phase transformation can further distort the low carbon steel substrate. 
Nevertheless, at present, the most popular substrate for enamel coating is 
still low carbon steel. Typically where low carbon steels are employed as 
the metal substrate, they are subjected to acid pickling and deposition of 
a thin nickel coating prior to the electrophoretic application of the 
porcelain enamel coating. Such techniques, however, have limited efficacy 
in the case of alloy metal substrates such as stainless steel which are 
chemically and electrochemically rather inert, so that little or no 
reaction takes place in the acids and/or the nickel sulfate solution. 
Until now, where stainless steel was to be employed as the metal substrate 
for fabrication of a PEMS the stainless steel was either etched and/or 
sandblasted. While sandblasting is generally considered the state of the 
art method, it is expensive and often deforms or warps the light gauge 
sheet steels commonly used as substrates for enameling. In addition, sand 
particles can become imbedded in the steel surface and cause enamel 
defects. 
U.S. Pat. No. 3,962,490 to Ward teaches a method of applying an enamel 
coating to a stainless steel substrate in which the stainless steel 
workpiece is first dipped into an aqueous solution of a molybdenum salt, 
then heated to thermally decompose the molybdenum compound prior to 
enameling. This worked fine for some applications, but it did not provide 
an even coating of molybdenum, and was not suitable for PEMS being 
prepared for use in fabricating sophisticated electronic components. 
The application of porcelain enamel coating on to the metal substrates can 
be done in various ways, however, electrophoretic deposition technique is 
most preferred for the manufacture of PEMS. Early PEMS were prepared by 
electrophoretically depositing the enamel from a water slurry. The 
chemistry of crystallizing glass ceramic coatings, however, generally 
requires a non-aqueous suspension such as alcohols. 
The preparation of a suspension with the optimum properties may require 
experimentation with the composition, concentration, and dispersing 
procedure. Polar compounds such as alcohols, nitroparaffins, and mixtures 
of these can be employed. Slightly polar organic compounds, such as 
diethylene glycol, dimethyl ether, and pyridine may also be employed. A 
great number of suspension formulations can be made from these various 
organic suspension vehicles, but no single generalized formulation can be 
defined. 
It is therefore one object of the present invention to provide a novel 
crystallizing porcelain enamel composition for use in coating metal 
substrates. 
It is another object of the present invention to provide a novel deposition 
bath for electrophoretically applying crystallizing porcelain enamel 
coatings to a metal substrate. 
It is yet a further object of the present invention to provide improved 
porcelain enameled metal substrates for use in fabrication of electronic 
circuitry. 
It is a still further object of the invention to provide a novel method of 
treating steel substrates to enhance the adhesion of the fired porcelain 
enamel coating to the metal substrates, specifically electrochemically 
rather inert alloy steels such as stainless steels. 
SUMMARY OF THE INVENTION 
The novel enamel compositions of the present invention are prepared by 
adding from about 2 to about 10 weight percent molybdenum oxide to 
recrystallizing glass compositions of the type discussed hereinbefore. The 
addition of molybdenum oxide appears to provide a threefold advantage, 
since it appears to reduce the surface tension of the glass and thus 
improve the flowing property of the enamel in the initial firing stage; it 
improves the opacity of the coating, and, perhaps most importantly, it 
appears to substantially improve the bonding of the enamel coating to the 
substrate. A small amount of tin oxide may also be added if a shiny 
surface appearance is desired. 
In general, the novel glass composition of the present invention are within 
the following composition ranges (which are set forth in percent by 
weight). 
TABLE I 
______________________________________ 
SiO.sub.2 
9-16% 
B.sub.2 O.sub.3 
20-25% 
MoO.sub.3 
2-10% 
MgO 30-40% 
BaO 16-22% 
ZnO 5-14% 
SnO.sub.2 
0-4% 
______________________________________ 
There are a number of possible advantages to using a stainless steel, 
specifically a 400-series stainless steel, as the substrate, including a 
better potential match between the thermal coefficient of expansion of the 
substrate and that of the enamel coating. In the novel method of the 
present invention, a thin film of molybdenum is electroplated onto the 
stainless steel substrate and fired in air at an elevated temperature of, 
for example, 880.degree. C. for 10 minutes, and this provides substantial 
improvement in the adhesion between the stainless steel substrate and 
subsequently applied enamel coating. 
As noted before, early PEMS were prepared by electrophoretically depositing 
the enamel from a water slurry. The recrystallizing enamel compositions 
are typically subject to hydration if they come into contact with water. 
For this reason, while they are still electrophoretically deposited, it is 
from a nonaqueous solvent, typically an isopropyl alcohol suspension. In 
the novel application method of the present invention, an improved 
electrophoresis bath has been found, in which the glass particles are 
suspended in either trichlorotrifluoroethane or dichloromethane containing 
about 10 to 40 volume percent of isopropyl alcohol. These baths also 
contain acrylic additive together with ethylene glycol. 
The first step in preparing porcelain enamels for the manufacture of PEMS, 
including those of the present invention, is to prepare a glass frit 
having the desired composition. This is accomplished using techniques well 
known to those skilled in the art by admixing the specific raw materials 
and then heating the mixture to the melting temperature being employed in 
the glass making process. Particular attention should be paid to the 
exclusion of alkali metal impurities such as sodium oxide which can have a 
marked adverse effect on the electric properties of the porcelain. 
The raw materials are weighed out in any suitable batch size from 
laboratory size to commercial scale and are blended together. The mixture 
is then heated to a temperature of about 1350.degree. to about 
1500.degree. C., and the resultant molten mass is maintained at this 
temperature for about 30 minutes to about 1 hour. A platinum crucible or 
the like should be used because of the highly corrosive nature of the 
present glass compositions. 
The molten glass is then converted into a glass frit. Again, this step is 
not per se critical and any of the various techniques well known to those 
skilled in the art can be employed. In the practice of the present 
invention, the molten stream of glass has been poured over a set of 
revolving water cooled rollers to produce a thin ribbon of solidified 
glass. 
The solidified glass ribbon is then crushed and resulting flakes are placed 
in a ball mill and milled to a slurry using a non-aqueous organic solvent 
as the medium, such as, for example, isopropyl alcohol. 
About 500 ml of the organic solvent is added to the 1 kg of glass flakes 
which is milled for a period sufficient to reduce the particle size of the 
glass to a range of from about 1 to about 10 microns. This generally takes 
anywhere from about 12 to about 20 hours, depending on the specific 
equipment employed and the size of the batch. After grinding, the slurry 
is removed from the ball mill and stored as wet slurry or dried to powder 
for the future preparation of the electrophoretic deposition bath in which 
the glass is applied to the metal substrate. 
The foregoing glass is particularly suitable for the fabrication of 
electronic circuit boards and for coating electrical components, 
especially for those applications which require a high degree of 
performance and reliability under adverse conditions, such as where they 
are subjected to particularly high temperatures, i.e. 700.degree. to 
900.degree. C. during subsequent fabrication and manufacturing steps. 
In general, the manufacture of PEMS using the porcelain enamel of the 
present invention is also similar to known procedures and techniques. A 
metal core or substrate is initially prepared for porcelain enameling, the 
core generally being chosen from a variety of metals and metal alloys 
including stainless steel and low carbon steels. The metal substrate is 
stamped or laser cut into the desired configuration and any required 
apertures, mounting holes or the like are formed in the metal core by 
conventional metal working techniques. Prior to application of the 
porcelain enamel, it is desirable to remove all burrs, sharp edges, 
imperfections or the like from the metal to facilitate the subsequent 
application of a uniform coating of porcelain enamel. 
Typically, where low carbon steel metal cores are employed, they are 
degreased and rinsed, then etched, typically with a sulfuric acid. The 
substrate was then usually given a flash of metal such as nickel or cobalt 
to assist in the adhesion of the porcelain to the metal. 
The low carbon steel metal cores have been successfully used for the 
manufacture of conventional PEMS. However, as described earlier, a number 
of problems such as poor adhesion of coating to the metal, enamel cracks 
on edges and corners, and warpings have been encountered frequently with 
the high temperature glass ceramic coatings applied onto the low carbon 
steel substrates. The low carbon steel is seemingly unable to provide the 
property requirements needed by the glass ceramic coatings where the 
extended exposures at the elevated temperatures in excess of 850.degree. 
C. are required. 
The 400 series stainless steels have been found to possess the adequate 
thermal expansion and heat resisting properties required for the present 
application. In particular, 409 stainless steel was preferred having added 
advantages of cost and commercial availability. Lack of enamel to metal 
bonding has been a major difficulty with stainless steel type substrates 
heretofore used for porcelain enamel coating application. A novel process 
has been used in the present invention which provides improved enamel to 
metal bonding and overcomes prior problems. 
The novel treatment process of the present invention comprises 
electroplating of a molybdenum film onto the stainless steel substrate 
using a 5 weight percent ammonium heptamolybdate tetrahydrate solution, 
with subsequent firing of the dried substrate in air at temperatures 
ranging from 800.degree. C. to 980.degree. C. for 5 to 30 minutes or 
longer. The prepared metal core is then coated with a porcelain enamel, 
preferably by electrophoretic deposition from a non-aqueous bath, i.e., 
preferably, one of the novel bath compositions provided in Table V 
hereinafter. 
Excellent bonding of glass ceramic coating to the metal core was observed 
when the molybdenum plated and heat treated stainless steel substrates 
were electrophoretically coated with the glass ceramic compositions of the 
present invention and subsequently fired. The bonding characteristics were 
tested by the standard drop weight impact method well known in the 
porcelain enamel industries. 
Typically, in electrophoretic deposition, the organic suspension is 
prepared according to the desired bath composition that includes solid 
concentration, type and amount of vehicles, and other additives if needed. 
The suspension medium most widely and commonly referred to and taught in 
the prior art such as U.S. Pat. No. 4,256,796, is isopropyl alcohol. 
Various experiments with the 100% isopropyl alcohol bath showed that 
further improvement was necessary for the satisfactory deposition of the 
porcelain enamel compositions of the present invention. In some early 
tests, problems such as sagging, dripping, beading, heavy edge coating, 
and drain marks were encountered. 
The novel bath composition of the present invention is a mixture of 10 to 
40 volume percent of isopropyl alcohol and 60 to 90 volume percent of 
either trichlorotrifluoroethane or dichloromethane together with 0 to 10 
volume percent of ethylene glycol and 0 to 5 volume percent of an acrylic 
polymer solution. About 5 to 30 weight percent of dried enamel powder is 
added to the bath and stirred continuously to keep the particles in 
suspension. This deposition bath composition minimizes the problems 
encountered with the straight isopropyl alcohol bath, and a satisfactory 
enamel deposit with a uniform coating thickness can reproducibly be 
obtained. 
Stainless steel sheets are typically employed as the two anodes being 
separated by about 2 inches. The metal part to be coated is placed between 
the anodes in the cathode position. A DC potential of 200 to 1000 volts is 
applied across the electrodes through the bath in a conventional manner 
and the glass particles in suspension deposit on the surface of the metal 
part. The coating thickness is a function of the deposition voltage and 
time being employed. When the desired thickness of glass particles has 
been deposited on the metal core, the part is removed from the bath and 
excess solvents are allowed to drain. 
The coated substrates require no special drying, but preferably should be 
placed in a vented hood to avoid problems with regard to solvent vapors in 
the general working area. The coated substrate can be fired in about 15 
minutes as the solvents evaporate rapidly. 
The article can be fired in any conventional manner, preferably using 
either a box furnace or a continuous furnace, techniques commonly used in 
the porcelain enamel industries. The optimum firing schedule for the glass 
compositions of the present invention is 5 to 10 minutes at a temperature 
around 870.degree. C. The enamel coating undergoes a rapid change as it is 
fired. In the case of a box firing, the glass particles begin to fuse with 
time, and a shiny glassy surface is observed in about 1 to 2 minutes. The 
coating immediately turns to a dull matte like appearance, resulting from 
the rapid devitrification of the enamel coating. Several additional 
minutes are needed for the completion of desired crystallization process. 
The glass compositions of the present invention can still form a glassy 
film even if fired at a temperature as low as 720.degree. C. However, the 
coatings are very fragile and have nearly no adhesion. At least 10 minutes 
firing at 820.degree. C. is generally required to obtain a devitrification 
visually similar in appearance to a sample fired at 870.degree. C. for 5 
minutes. 
In the case of a continuous chain furnace firing, the enamel coating 
appears to fuse gradually as it passes through the preheating zone, 
forming a good glass film in the hot zone, and then subsequently 
crystallizing. The very slow crystallization rate at lower temperatures 
and the rapid crystallization rate at higher temperatures as described 
above for the present glass compositions, therefore, suggests use of the 
box furnace for the small quantity work, and the use of continuous chain 
furnace for the large scale production, with nearly identical results from 
both types of firings. 
By following the foregoing firing procedures, there is sufficient initial 
flow for leveling but essentially no extensive flow of the material on the 
surface. As a result of the rapid increase in viscosity, there is no 
substantial change in the uniformity of the thickness of the coating on 
the metal core. The finished fired porcelain will have the same relative 
uniform thickness layer on the surface, and, using the novel porcelain 
enamel of the present invention, also around the holes and on the edges. 
The fired porcelain enamel has been estimated to contain more than 90 
volume percent of crystalline material with the remainder of the 
composition being comprised of vitreous glass. The proportion of the 
crystal material and vitreous glass is dependent upon both the composition 
of the frit employed and to some extent of the firing conditions utilized. 
In the compositions of the present invention, four types of crystalline 
phases have been confirmed to be present by x-ray diffraction analyses, 
namely 2MgO.B.sub.2 O.sub.3, BaO.2MgO 2SiO.sub.2, BaO.2ZnO.2SiO.sub.2, and 
BaO.MoO.sub.3. The amount of BaO.MoO.sub.3 crystals present in the fired 
enamel increases nearly in proportion to the MoO.sub.3 content according 
to the x-ray analyses.

PREFERRED EMBODIMENT 
The preferred enamel compositions of the present invention fall within the 
oxide ranges set forth in Table II. 
TABLE II 
______________________________________ 
SiO.sub.2 
11.0-15.0% 
B.sub.2 O.sub.3 
21.0-24.0% 
MoO.sub.3 
2.0-8.0% 
MgO 30.0-36.0% 
BaO 16.0-22.0% 
ZnO 6.0-12.0% 
SnO.sub.2 
0-4.0% 
______________________________________ 
EXAMPLES 1-8 
A series of specific glass compositions were prepared as described 
hereinafter having the oxide compositions set forth in Table III. 
TABLE III 
______________________________________ 
EXAMPLES OF GLASS COMPOSITIONS 
______________________________________ 
#1 #2 #3 #4 
______________________________________ 
BaO 17.89 17.55 16.90 18.13 
MgO 32.62 32.02 30.83 33.65 
ZnO 10.59 10.38 10.00 10.38 
SiO.sub.2 
14.02 13.75 13.24 11.54 
B.sub.2 O.sub.3 
22.87 22.45 21.62 22.45 
MoO.sub.3 
2.01 3.85 7.41 3.85 
Total 100 100 100 100 
______________________________________ 
#5 #6 #7 
______________________________________ 
BaO 21.00 18.01 21.00 
MgO 33.00 33.14 33.00 
ZnO 7.00 10.48 7.00 
SiO.sub.2 
12.00 12.78 12.00 
B.sub.2 O.sub.3 
23.00 22.66 23.00 
MoO.sub.3 
4.00 2.93 3.60 
SnO.sub.2 
-- -- 0.40 
Total 100 100 100 
______________________________________ 
Batch compositions of raw materials were then calculated based on the oxide 
compositions. An example is given in Table IV for the glass composition 
No. 5 in Table III. 
TABLE IV 
______________________________________ 
BATCH COMPOSITIONS 
______________________________________ 
Barium Carbonate, BaCO.sub.3 
654.4 grams 
Calcined Magnesia, MgO 
829.2 grams 
Zinc Oxide, ZnO 168.0 grams 
Quartz Powder, SiO.sub.2 
288.0 grams 
Anhydrous Boric Acid, B.sub.2 O.sub.3 
560.4 grams 
Molybdenum Trioxide, MoO.sub.3 
96.0 grams 
TOTAL 2596.0 grams 
______________________________________ 
As noted, the corrosive nature of glass compositions of the present 
invention does not permit ordinary refractory crucibles to be used for 
smelting. The glass should be smelted in a platinum crucible preferably at 
1450.degree. C. for 30 to 60 minutes. The glass melt has good flowing 
properties at this temperature and can be roll-quenched readily. 
The glass frits were milled in an isopropyl alcohol medium for 16 hours to 
an average fineness of 5 microns. The milled slip was then dried and 
stored for the preparation of electrodeposition bath. 
Dilatometric analyses show no softening of enamel even at temperatures 
above 900.degree. C. The thermal expansion coefficient and glass softening 
temperature obtained from the dilatometric measurements are shown in Table 
V. In the dilatometric measurements, test samples received a heat 
treatment similar to the initial enamel firing so that the test results 
would represent properties as close to the initially fired enamel coatings 
as possible. 
TABLE V 
______________________________________ 
Thermal Expansion Coefficients (in/in/.degree.C.) and 
Dilatometric Softening Temperatures (.degree.C.) 
of Example Glasses 
Ther. Exp. Softening Ther. Exp. 
Softening 
(25-500.degree. C.) 
Pt.(.degree.C.) (25-500.degree. C.) 
Pt.(.degree.C.) 
______________________________________ 
#1 109.2 .times. 10.sup.-7 
&gt;930 #5 115.8 .times. 10.sup.-7 
930 
#2 109.4 &gt;930 #6 117.8 &gt;930 
#3 106.4 &gt;930 #7 115.8 930 
#4 116.2 &gt;930 
______________________________________ 
Because deposition equipment and conditions vary widely, the preparation of 
a suspension with the optimum properties may require experimentation with 
the composition, concentration, and dispersing procedure. 
The novel bath compositions of the present invention embrace a number of 
specific combinations of solvents. The vehicle systems given in Table VI 
yielded particularly satisfactory deposition results in both laboratory 
beaker and moderately scaled-up deposition baths. 
TABLE VI 
______________________________________ 
BATH COMPOSITION #1 
BATH COMPOSITION #2 
______________________________________ 
Glass Solids 
100 gm Glass Solids 100 gm 
(Ave. 5 Microns).sup.(1) 
(Ave. 5 Microns).sup.(1) 
Isopropyl Alcohol 
200 ml Isopropyl Alcohol 
100 ml 
Dichloromethane 
400 ml Trichlorotri- 500 ml 
fluoroethane 
Ethylene Glycol 
4 ml Ethylene Glycol 
8 ml 
Acrylic Binder.sup.(2) 
2 ml Acrylic Binder.sup.(2) 
1 ml 
Deposition Voltage 
800 VDC Deposition Voltage 
800 VDC 
Deposition Time 
30 sec Deposition Time 
30 sec 
Deposit on Cathode Deposit on Cathode 
______________________________________ 
.sup.(1) Milled in isopropyl alcohol for 16 hours. 
.sup.(2) Acrylic binder was made from 137.33 gm of Acryloid 87 in MEK, 
10.3 gm of Butylbenzyl phthalate and 215 gm of Trichloroethane. 
The above bath compositions are considered optimum, and therefore generally 
to be preferred, but a widely varied composition range (at least .+-.50% 
change) still yielded very satisfactory results. Deposition voltage and 
time can also vary from 200 to 1000 VDC or higher and 10 to 180 seconds or 
longer depending on the desired coating thickness. These bath formulations 
were, however, found to be particularly satisfactory for depositing 
glass/ceramic particles on various metallic substrates such as low carbon 
steel, alloy steels, stainless steels, aluminum, and even chromeplated 
steel substrates. The baths yielded a uniform, dense and consistent 
deposit over a two months test period. 
Ethylene glycol appears to aid in obtaining a uniform, smooth coating 
deposit and the binder seems to help improve the deposit strength and 
minimize flow marks during the sample withdrawal from the bath. The coated 
substrate required no special drying except placing in a vented hood and 
was ready to be fired in about 15 minutes because the solvents evaporated 
rapidly. No adverse effect due to the binders was observed on the fired 
appearance of samples. 
For the low carbon steel substrates which had been pickled and nickel 
flashed, a simple copper activation dipping was necessary, similar to that 
for the aqueous deposition systems. As noted hereinbefore, low carbon 
steel has long been regarded as the most economical and suitable core 
metal for the porcelain enamel coated metal substrates. The low carbon 
steel provides good enamel-metal bonding for the conventional PEMS and has 
a thermal conductivity better than that of any alloy steel. One of the 
major difficulties experienced with the low carbon steel has been a 
tendency to develop enamel cracks on edges or more specifically on 
corners. 
One of the main causes of the edge/corner chipping problem is thought to be 
too great an expansion of the core metal compared to the coating 
materials. Since the stresses on the edges and corners of the panel are 
opposite to those on a flat panel, the edge/corner chippings are thought 
to be closely related to the stress level on the edges and corners 
produced by the expansion difference. The edge/corner chipping problem is 
particularly troublesome where the substrates are to be repeatedly refired 
at the high temperatures. 
Since the thermal expansion of the glasses of the present invention 
(106-117.times.10.sup.-7 in/in/.degree. C.) have been already deemed to 
have been optimized, other metals with an expansion lower than that of low 
carbons steel (146.5.times.10.sup.-7 in/in/.degree. C.) have been 
examined. The 400-series stainless steels have been found to have an 
adequate range of thermal expansion of around 125.times.10.sup.-7 
in/in/.degree. C. and a good heat resisting property. The 409 stainless 
steel (SS-409) has been found to be a particularly good material, having 
an expansion of 127 .times.10.sup.-7 in/in/.degree. C. 
Lack of enamel/metal bonding has been a major difficulty with stainless 
steel type substrates heretofore used for porcelain enamel coating 
application. A new preparation process has been developed which achieves a 
good enamel/metal bonding and overcomes such prior problems. As already 
noted, the conventional metal preparation techniques, such as acid 
pickling and nickel flash, are not applicable to the stainless steel 
substrates because these metals are chemically and electrochemically very 
inert so that little or no reaction will take place in acid or nickel 
solutions. Other prior art techniques such as sandblasting have also been 
found unsatisfactory for use in the present application. Although the 
sandblasting is often considered the best present method for preparing 
stainless steel, it is costly and deforms (warps) the light gauge sheet 
steel commonly used for the enamel substrates. 
The novel treatment process of the present invention utilizes a metallic 
film of molybdenum electrochemically plated onto the stainless steel 
substrate typically under the following electroplating conditions. 
______________________________________ 
Bath 5 wt % ammonium heptamolybdate 
tetrahydrate solution, 
(NH.sub.4).sub.6 Mo.sub.7 O.sub.24.4H.sub.2 O 
Cathode Workpiece (2" .times. 2") 
Anode Inert material 
(stainless steel, 2" .times. 3") 
Plating Voltage 
2.0-5.0 volts DC 
Plating Current 
0.2-3.0 amp per 2" .times. 2" 
(7 to 110 amp/sq. ft.) 
Plating Time 5-60 seconds 
______________________________________ 
A uniform film of molybdenum was plated on to a SS-409 cathode. The piece 
was rinsed, dried, and fired in air at 880.degree. C. for 10 to 20 
minutes. This firing step is important since an adhesion promoting layer 
is formed in this stage, presumably by the oxidation of thin molybdenum 
film and the diffusion of molybdenum into the steel. Particularly good 
enamel bonding was obtained with the enamel of Example 7. Higher firing 
temperatures (up to 980.degree. C.) or longer firing times (up to 30 
minutes) for this heat treatment procedure did not appear to significantly 
affect the enamel bonding characteristics. In general, the heat treatment 
should comprise heating to a temperature of at least 800.degree. C. and 
for at least 5 minutes. 
The preferred PEMS of the present invention comprises a stainless steel 
substrate having a coating of molybdenum electroplated and heat treated on 
at least one surface thereof, and a porcelain enamel coating on said 
molybdenum coated surface, said enamel having a composition within the 
oxide ranges set forth in Table I. 
Novel PEMS were fabricated by using the enamel of Example 7 and SS-409 
metal core. Samples of such PEMS were then examined and provided the data 
described below. 
1. Refire (enamel softening) temperature :&gt;900.degree. C. 
2. Smooth surface, off white color 
3. Good enamel to metal bonding 
4. Good edge coverage, no edge/corner chipping even after ten 900.degree. 
C. refires. 
5. Thermal Expansion (.times.10.sup.-7 in/in/.degree. C. 
______________________________________ 
Core Metal Enamel Low Carbon 
Temp. Range 
(SS-409) No. 7 Steel 
______________________________________ 
20-300.degree. C. 
127.3 108.5 140.0 
20-500 127.2 115.8 146.5 
20-800 130.8 123.3 148.0 
20-900 133.5 124.0 -- 
______________________________________ 
*Close expansion matches between the core metal and enamel should mean a 
low level of stresses (tension/compression) or a good edge/corner chippin 
resistance. 
6. Electrical Resistivity 
______________________________________ 
Below 200.degree. C. 
Higher than 10.sup.15 ohm-cm 
At 250 2.0 .times. 10.sup.14 
At 300 1.6 .times. 10.sup.13 
Dielectric Constant (25.degree. C.) 
8.5 (1 KHz) 
8.2 (1 MHz) 
Dissipation Factor (25.degree. C.) 
0.005 (1 KHz) 
0.005 (1 MHz) 
______________________________________ 
7. Dielectric Strength (5 mils coating) :&gt;3.0 Kv 
______________________________________ 
Sample No. Thickness Breakdown Voltage 
______________________________________ 
#1 5.0 mils 4.5 Kv 
#2 5.0 No breakdown at 6.0 Kv 
#3 5.0 3.0 
#4 5.0 5.5 
#5 6.0 No breakdown at 6.0 Kv 
#6 6.0 No breakdown at 6.0 Kv 
#7 5.5 No breakdown at 6.0 Kv 
#8 5.5 No breakdown at 6.0 Kv 
______________________________________ 
*These properties, 1 to 7, are considered superior to those of known 
enamels and substrates. 
It will, of course, be clearly understood that the novel enamel 
compositions of the present invention have utility in the fabrication of 
PEMS using conventional electrophoresis application techniques and 
conventional low carbon steel or other metal substrates. Likewise, the 
novel bath of the present invention has equal utility with other 
devitrified glass compositions outside the scope of Tables I and II set 
forth hereinbefore and likewise, can be employed in coating conventional 
as well as stainless steel metal substrates. Finally, the specially 
treated stainless steel substrates of the present invention can be used as 
substrates for conventional devitrified glass compositions applied from 
conventional electrophoresis plating baths or by any other application 
techniques. 
It will therefore be understood that the foregoing specific examples are 
presented by way of illustration and not by way of limitation, and that a 
wide variety of changes, alterations, and substitutions can be made in the 
formulations, baths, materials and processes described hereinbefore 
without departing from the scope of the present invention.