A process for enhancing the adhesion of coatings applied to the surfaces of glass-ceramic articles. The surface area to be coated is roughened before the article is completely cerammed. When the article is subjected to elevated temperatures for the purpose of converting the green glass to a glass-ceramic, flaws produced by the abrasion step are healed. The coating can be applied before or after the ceramming step.

BACKGROUND OF THE INVENTION 
This invention relates to a method of making thermally durable 
glass-ceramic articles having strongly adherent coatings thereon. 
The term "glass-ceramic" refers to a polycrystalline ceramic prepared by 
the controlled crystalization of a glass in situ. The invention of such 
materials has enabled the formation of intricately-shaped polycrystalline 
articles by forming "green glass" into the desired shape and thereafter 
ceramming the green glass article to convert it to a glass-ceramic. The 
ceramming step conventionally involves heating the green glass article to 
an elevated temperature, e.g. a temperature in the range of about 
650.degree.-800.degree. C., to cause nucleation or the formation of nuclei 
and subsequently heating the article to a higher temperature, e.g. a 
temperature in the range of about 800.degree.-1175.degree. C. to cause 
crystallization and growth of crystals. The resultant glass-ceramic 
material is known for its good mechanical and thermal durability. For 
additional information pertaining to the formation of glass-ceramic 
materials, reference may be made to U.S. Pat. Nos. 2,920,971; 3,148,994 
and 3,157,522. For example, U.S. Pat. No. 3,157,522 indicates that the 
nucleation step can be performed immediately after the article is formed 
while the article is still hot. Thus, the shaped green glass article may 
be cooled to a temperature in the range of 650.degree.-800.degree. C. and 
held at that temperature for a period of time, usually between about 2 
hours and about 10 minutes, depending upon the temperature. This type of 
nucleation step may be performed in the method of the present invention 
when the article is to be abraded while in the nucleated state. 
Furthermore, the abraded article may be subjected to two or more crystal 
growth temperature schedules in accordance with the aforementioned U.S. 
Pat. No. 3,148,994. However, it is to be noted that the method of the 
present invention is not limited to any particular ceramming schedule, and 
any ceramming schedule may be employed since all such schedules are 
capable of alleviating damage caused by a prior, surface abrasion step. 
Glass-ceramic articles have been coated with electrically and/or thermally 
conductive material to form such devices as resistors, heaters, stovetop 
cooking units, cookware and the like. The following references are 
illustrative of these applications: U.S. Pat. Nos. 3,330,940; 3,813,520; 
3,848,111 and 3,883,719 and my U.S. Patent Application Ser. No. 727,893, 
entitled "Low TCR Resistor" filed on Sept. 29, 1976. Any of the articles 
disclosed in the aforementioned references may be subjected to temperature 
changes which can cause the formation of stress therein which can result 
in breakage of the article and/or separation of the conductive coating 
from the glass-ceramic substrate. 
Although glass-ceramic materials have been commonly employed as cookware, a 
primary disadvantage of such ware is the low thermal conductivity thereof. 
For example, direct contact of glass-ceramic cookware with burner elements 
is disadvantageous in that food disposed therein can be burned if it is 
immediately above the burner element, and food which is only a short 
distance away, but not directly over a burner element, may be undercooked. 
The application of coatings to glasses and glass-ceramics is widely 
employed to impart desirable physical properties such as thermal and 
electrical conductivity thereto, and many processes for applying such 
coatings are known. U.S. Pat. Nos. 3,523,013; 3,220,870; 3,296,012; 
3,914,517 and 3,741,780 disclose methods of providing glass-ceramic 
substrates with conductive coatings. 
The principal problems in the art of metal-coating glass-ceramic cookware 
arise out of the substantial differences in thermal expansion behavior 
between the conventional glass-ceramic materials used for the fabrication 
of cookware and more highly conductive materials which might be considered 
for use as coatings in combination with these glass-ceramics. For example, 
glass-ceramic materials typically employed for cookware fabrication 
exhibit rather low coefficients of thermal expansion, e.g., on the order 
of 10-25.times.10.sup.-7 /.degree.C., whereas aluminum, for example, which 
has a desirable thermal conduction capability, has a coefficient of 
thermal expansion of about 230.times.10.sup.-7 /.degree.C. Theoretical 
stresses which may arise as a result of this expansion mismatch over the 
typical temperature range of use of an aluminum metal-glass-ceramic 
composite cooking vessel approach 700 MPa. Increases in the thermal 
expansion of the glass-ceramic material to alleviate this expansion 
mismatch are not possible without sacrificing the excellent thermal shock 
resistance of this material, a major desirable feature of glass-ceramic 
cookware. Even silicon, which has a coefficient of thermal expansion of 
about 35.times.10.sup.-7 /.degree.C., is subjected to stresses which tend 
to cause delamination to occur when silicon-coated cookware is subjected 
to thermal cycles normally encountered in cooking. Moreover, such 
glass-ceramic ware possesses an extremely smooth surface that is not 
normally receptive to many coating materials. One example of a material 
that can be self-bonding to smooth glass-ceramic surfaces is aluminum, 
provided that it is properly applied, an example of such a self-bonding 
material being described in U.S. Patent Application Ser. No. 712,479 
entitled "Process for Making Aluminum-Coated Glass-Ceramic Cooking Vessel" 
filed Aug. 9, 1976. 
Three techniques conventionally employed to improve adhesion of non-bonding 
coatings are surface roughening, substrate preheating, and precoating the 
substrate with a material which is self-bonding and which is compatible 
with the desired coating material. However, each of these techniques 
possesses disadvantages which tend to discourage its utilization. For 
example, subjecting glass-ceramic articles to high preheat temperatures 
was found to effect coating bonds of marginal strength, but all but the 
thinnest coatings spalled as the coated articles cooled. Vacuum deposited 
films of chromium having thicknesses of about 500 A formed good bonding 
layers between glass-ceramic substrates and silicon coatings, but the 
vacuum deposition technique for applying such films is unduly expensive. 
It is knwon that although surface roughening of glass-ceramic substrates 
improves the adhesion of subsequently applied thermally conductive 
coatings, such roughening introduces undesirable stress raisers and 
degrades the thermal durability properties of the glass-ceramic. For 
example, glass-ceramic skillets, the bottom surfaces of which were 
roughened prior to applying thermally conductive coatings thereto, could 
not pass a thermal downshock test because of the damage imparted thereto 
by the roughening procedure. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a method for 
enhancing the adhesion of both self-bonding and non-selfbonding coatings 
on glass-ceramic substrates. 
Another object of the present invention is to provide an improved process 
for manufacturing metal-coated cookware which overcomes many of the 
disadvantages of prior art processes for producing such ware. 
Other objects and advantages of the invention will become apparent from the 
following summary and detailed description thereof. 
The present invention generally pertains to a method for coating a surface 
of a glass-ceramic article. Such a method is often carried out by abrading 
the surface of the finished glass-ceramic article and then applying a 
coating to the abraded surface. In accordance with the present invention 
the surface abrading step is characterized in that it comprises abrading 
the surface prior to completely ceramming the glass-ceramic material. 
Thereafter, the article is subjected to heat treatment to complete the 
conversion of the green glass to a glass-ceramic material and to 
simultaneously heal that damage which was caused by the surface abrading 
step. 
DETAILED DESCRIPTION 
The method of the present invention will be described by reference to 
certain specific embodiments thereof whereby glass-ceramic articles are 
provided with strongly adherent coatings. These articles may consist of 
any glass-ceramic material which can be formed by pressing or otherwise 
shaping the green glass into the final shape and thereafter subjecting the 
green glass to elevated temperatures to cause nucleation and growth of 
crystals in situ. The resultant material consists of a multiplicity of 
inorganic crystals dispersed in a glassy matrix which consists essentially 
of the uncrystallized glass. 
Articles made by the method of the present invention possess good 
mechanical and thermal durability. The tests described hereinbelow may be 
employed to ascertain the durability of an article coated in accordance 
with the method of this invention. 
A standard test for adhesion consists of bonding circularly shaped plugs to 
opposite sides of the article with a suitable adhesive such as an epoxy 
resin cement. The plugs are held in an alignment fixture to ensure the 
axial alignment thereof until the cement cures. A force is then applied to 
the two plugs which tends to pull them apart. The load is steadily 
increased until failure occurs at the substrate-coating interface. 
Thermal durability is determined by two types of quenching experiments. A 
simple thermal downshock test is performed by rapidly immersing a coated 
article which has been heated to about 500.degree. C. into cold water 
which is at a temperature of about 15.degree. C. In this test, thermal 
gradients are predominantly normal to the surface. It is believed that 
conventional borosilicate glass ovenware is unable to withstand a 
500.degree. C. to cold water downshock test since the expansion 
coefficient thereof is too high for the thickness of the glass at the heel 
of such ware. 
"Dunker" testing subjects an article to a different set of thermal 
conditions. During this test, a coated article is heated on an electric 
burner for 20 minutes to an inside surface temperature of 
400.degree.-450.degree. C. and is then lowered edge first into circulating 
cold water. In this case thermal gradients can develop a component along 
the heel and bottom of an article of cookware which is parallel to the 
surface. 
Attempts have been made to coat glass-ceramic substrates with various 
materials to produce printed circuits, thermally conductive cookware, 
resistors, heaters and the like. Such coatings generally adhere poorly to 
the surface of the glass-ceramic material due to the lack of an effective 
anchorage in the surface of the material which is normally smooth and 
substantially non-porous. For example, adherent silicon coatings of 
thicknesses greater than about 0.05 cm applied with preheat temperatures 
less than 600.degree. C. cannot be obtained on a smooth, as-pressed 
surface of a glass-ceramic article of the type described in U.S. Pat. No. 
3,157,522. Although an apparent bond is formed as the silicon is sprayed 
onto the surface, it is not strong enough to withstand the stresses which 
are generated by differential contraction of the coating as the article 
cools to room temperature. Roughening a surface of a glass-ceramic 
substrate improves the adhesion of a coating to that surface, but it also 
produces mechanical damage in the substrate which results in a degradation 
of the physical and thermal properties thereof. 
In accordance with the present invention such mechanical damage is healed 
by abrading the article before it is completely cerammed so that the 
subsequent heat treatment required to complete the ceramming step heals 
the damage previously introduced by the mechanical abrasion. High 
temperature coating materials can generally be applied to the substrate at 
any time after the surface has been abraded. Thus, a green glass article 
can be mechanically roughened, coated while still in the green glass state 
and then cerammed, or the roughened green glass article can be nucleated, 
coated and then cerammed. Alternatively, the article can be roughened 
while in either the green glass or the nucleated state, and it can 
thereafter be cerammed and then coated. Finally, the article can be 
roughened in the nucleated state, coated and then cerammed. Obviously, the 
coating material must be able to withstand the high temperatures of the 
ceramming cycle if it is applied before ceramming occurs. In accordance 
with the teachings of the aforementioned U.S. Pat. No. 3,157,522, the 
minimum ceramming temperature is 800.degree. C., and most ceramming 
processes for manufacturing glass-ceramic cookware require a minimum 
temperature of about 900.degree. C. 
If the coating is to be applied to the article while it is in the green 
glass or nucleated state, the coating material must be able to withstand 
the high temperature to which it will be subjected during the crystal 
growth process. High temperature materials such as silicon carbide, 
alumina, zirconia, silicon and the like are suitable for this purpose. The 
adhesion of non-selfbonding materials such as iron, stainless steel and 
the like, which cannot withstand the high temperatures of the ceramming 
process, is enhanced by applying such coating materials to a glass-ceramic 
article that has been previously roughened and cerammed. The adhesion of 
selfbonding materials is also enhanced by employing the surface roughening 
and damage healing technique of the present invention. Thus, even though 
aluminum, for example, can be applied to the smooth surface of a 
glass-ceramic article, the adhesion thereof will be enhanced by first 
roughening the surface of the article in accordance with the method of the 
present invention. 
In any of the aforementioned embodiments a second coating of the same or 
another material can be subsequently applied. For example, a relatively 
thin layer of coating material can be applied to the roughened surface of 
a green glass article. After the article has been cerammed, an additional 
layer of the same material can be applied to provide a coating of the 
desired thickness. This may be a desirable procedure if the glass-ceramic 
material is one that shrinks during ceramming. Whereas a thick coating 
applied to the abraded surface of a green glass article may result in an 
excessively stressed article after the shrinkage occurs in the ceramming 
step, a thin layer of the same coating material, which is preferably less 
than about 0.025 cm thick, may result in an acceptable article. An 
additional layer of coating material applied after the shrinkage has 
occurred is obviously unaffected by such shrinkage. 
The bond between the substrate and coating is enhanced regardless of the 
particular abrasion technique employed. Course abrasion by sandblasting to 
an estimated 0.008 cm r.m.s. finish yields a weak but uniform bond between 
a glass-ceramic surface and a silicon coating sprayed thereon. An 
additional threefold increase in adhesion can be obtained by subjecting 
the surface to a 90 mesh Blanchard grind or gritblast, both of which can 
give a surface roughness estimated to be about 3.times.10.sup.-3 cm. The 
relative weakness of coatings sprayed on coarsely abraded surfaces results 
from poor bonding over large, smooth fracture areas which occur when chips 
are broken away by large, e.g. 0.5 mm, sand particles. The more controlled 
abrasive action of a 90 mesh gritblast or grind produces a surface covered 
with very small fracture surfaces and many angles and corners. Flame 
sprayed silicon, for example, is able to more securely interlock with the 
substrate in the latter case. In general, an optimal condition of surface 
roughness for thermosprayed coatings can be obtained by grinding or 
blasting with an abrasive powder of a material such as silicon carbide, 
the powder being of such a particle size that a surface roughness between 
about 0.002 cm and 0.005 cm is achieved. The effect of the degree of 
surface roughness on coating bonding strength is well known in the art. 
Additional information on this aspect of the coating process may be 
obtained by referring to the publications: E. Bardal et al., British 
Corros. Journal, vol. 8, p. 15 (1973) and R. L. Apps, Journal of Vacuum 
Science and Technology, vol. 11, p. 741 (1974). 
The roughened surface of a substrate may be provided with a suitable 
coating by any well known method such as thermospraying, dipping, brushing 
or the like, a thermospray technique being preferred. Conventional 
equipment may be used for thermospraying a coating onto the surface of the 
glass-ceramic article. During the operation of such equipment, the coating 
material, which is in wire or powder form, is fed to a plasma or flame gun 
from which it can be sprayed onto the article to be coated. Suitable flame 
spraying equipment is described in U.S. Pat. Nos. 3,055,591 and 3,148,818, 
and plasma spraying equipment is described in U.S. Pat. No. 3,814,620. 
Following is a comparison of the durabilities of glass-ceramic skillets 
coated with silicon by various techniques. Skillets which were sandblasted 
on the bottom surface after ceramming could not pass the thermal downshock 
test because of damage imparted thereto. However, when the green glass was 
sandblasted, provided with a coating of silicon and then cerammed, 
durability was greatly increased. Typical bond strength for as-sprayed 
coatings on abraded substrates of glass ceramic material of the type 
disclosed in U.S. Pat. No. 3,157,522 falls in the range of 1-3 MPa for a 
flame spray process. Plasma sprayed coatings have bond strengths in the 
range between 10 and 20 MPa, as do flame sprayed coatings which are 
applied to a roughened green glass substrate which is then subjected to a 
ceram schedule. It is theorized that the increase in strength of nearly an 
order of magnitude is due to the formation of a diffusion or chemical bond 
between the thermal oxide which coats the silicon particles and the glass 
on glass-ceramic ware. This extra bonding action, which augments the 
mechanical bonding of an as-sprayed coating, would be promoted by the 
higher temperatures reached in the plasma process and in the crystal 
growth portion of the ceram cycle. An electron microprobe study of the 
interface between the silicon and the glass-ceramic revealed evidence 
indicative of the formation of a diffusion bond between the high silica 
content thermal oxide on the silicon particles and the more complex oxide 
mixture of the glass-ceramic. If the 10-20 MPa bonding is generated by the 
same diffusion mechanism in plasma sprayed and in cerammed flame sprayed 
coatings, it appears that very high temperatures are attained at the 
coating-substrate interface during a plasma spray operation. It is well 
known that the plasma itself is highly energenic, but the aforementioned 
bonding strength indicates that energy transfer to the particles and 
subsequently to the interface, takes place with reasonable efficiency. If 
the bonding occurs through a diffusion mechanism, then to obtain the same 
chemical profile at the interface in a plasma spray process as is found 
for heat treatment of 1100.degree. C. for two hours (the crystal growth 
phase of a ceramming process), an extremely high interface temperature 
would be required, assuming that such a temperature was held for a period 
of time on the order of a few seconds. It is known that plasma 
temperatures greater than 6000.degree. C. are obtained in typical spraying 
operations. Thus, formation of a diffusion bond in addition to mechanical 
interlocking seems to be the most likely explanation for the increase in 
bond strength achieved by both plasma spraying and by ceramming after 
flame spraying. The adhesion of the silicon coating after ceramming was 
about 10 MPa as compared with an adhesion of about 1.3 MPa for a bond 
between a roughened glass-ceramic substrate and a flame sprayed silicon 
coating. Some of the stress to which a coating is subjected during the 
process of applying the same results from shrinkage of the glass-ceramic 
during the ceramming process. The glass-ceramic material employed in this 
test was of the type disclosed in the aforementioned U.S. Pat. No. 
3,157,522. This material undergoes a 1.52% linear contraction during the 
conversion from the green glass state to the glass-ceramic state. Of this 
contraction, 1.05% is incurred during the transition between the green 
glass and nucleated states while only 0.47% is incurred during the crystal 
growth phase. It appears that the thicker coatings, i.e., thicker than 
about 0.025 cm are excessively unyielding, and internal stresses generated 
by shrinkage during ceramming cause a degradation of the mechanical and 
thermal properties of the coated articles. To avoid problems associated 
with shrinkage, the article can be abraded while in the green state and 
coated while in the nucleated or the completely cerammed state. An 
alternative solution is to utilize a glass-ceramic material which 
undergoes less change in volume during ceramming and to coat such ware 
prior to the ceramming step. An example of a glass-ceramic composition 
that undergoes substantially no volume change during ceramming is 50.9% 
SiO.sub.2, 23.2% Al.sub.2 O.sub.3, 18.5% ZnO and 7.4% TiO.sub.2, wherein 
the percentages given are calculated from the glass batch on the oxide 
basis. This composition is one of the examples set forth in the 
aforementioned U.S. Pat. No. 2,920,971.

The process of the present invention may be further understood by reference 
to the following detailed examples thereof. The glass-ceramic compositions 
employed in these examples are Corning Code 9608 and Corning Code 9617, 
the the approximate analyses therefor being set out below in weight 
percent. 
______________________________________ 
Code 9608 Code 9617 
______________________________________ 
SiO.sub.2 69.5 66.7 
Al.sub.2 O.sub.3 
17.6 20.5 
Li.sub.2 O 2.7 3.5 
MgO 2.6 1.6 
ZnO 1.0 1.2 
TiO.sub.2 4.7 4.8 
ZrO.sub.2 0.2 0.05 
As.sub.2 O.sub.3 
0.9 0.4 
F 0.03 0.22 
Fe.sub.2 O.sub.3 
0.06 0.035 
B.sub.2 O.sub.3 
0.07 -- 
MnO.sub.2 0.03 -- 
______________________________________ 
EXAMPLE 1 
A circular glass skillet approximately 4.7 cm in depth, 25 cm in diameter, 
and having an average bottom thickness of about 0.48 cm, is selected for 
treatment. This skillet is composed of the lithium alumino-silicate glass 
referred to hereinabove as Corning Code 9608. 
The bottom portion of the skillet which is to be coated is Blanchard ground 
using 90 mesh SiC abrasive to a surface roughness of approximately 0.003 
cm. The roughened surface is then thoroughly cleaned with a 
commercially-available detergent and rinsed with water to remove all 
traces of grease, oil, dirt or other foreign matter which could affect 
bonding. Before the bottom surface of the skillet is coated, it is furnace 
preheated to about 450.degree. C. A commercially-available Metco Type 2P 
Thermo Spray Gun is employed to coat the bottom of the skillet with 
silicon, a steel mask being used to cover the side portions thereof. The 
spray gun, which employs a hydrogen-oxygen flame produced by combustion of 
hydrogen and oxygen at flow rates of 3.7.times.10.sup.6 cc/hr of hydrogen 
and 7.7.times.10.sup.5 cc/hr of oxygen, has a standoff distance of 18 cm. 
Silicon powder is fed into the gun which is mounted in an automatic 
traverse mechanism. The rate of deposition of silicon is about 20 grams 
per minute. The silicon powder is commercially available from 
Kawecki-Berylco Corp. This powder is 98.5% pure and is free flowing, 80% 
thereof being in the +200 to -325 mesh (U.S. Standard Sieve) size range. A 
silicon coating about 0.025 cm in thickness is provided on the skillet 
bottom by the flame-spray application of this powder in the manner 
described. 
The coated glass skillet is then cerammed by initially heating it from room 
temperature to 750.degree. C. at the rate of 400.degree. C. per hour. After 
holding the temperature at 750.degree. C. for 1/2 hour, it is heated to 
1100.degree. C. at the rate of 400.degree. C. per hour. The skillet is 
then held at a temperature of 1100.degree. C. for 2 hours and thereafter 
furnace cooled to room temperature. 
The silicon-coated skillet prepared as described has excellent thermal 
shock resistance, withstanding thermal downshock from about 500.degree. C. 
to cold (8.degree. C.) water without breakage. In fact, the thermal shock 
resistance of this silicon-coated skillet is not degraded by the coating 
process, i.e., its thermal shock resistance is essentially the same as 
that of conventional glass-ceramic skillets having no coating thereon. The 
coating adhesion is also excellent, being about 7 MPa. Moreover, the 
silicon remains electrically conductive through the high temperature 
ceramming cycle. 
EXAMPLE 2 
A silicon-coated skillet is formed in the manner described in Example 1 
except that the silicon powder is manufactured by Cerac-Pure Inc. and the 
thickness of the silicon coating is about 0.06 cm. The characteristics of 
this skillet are similar to those of Example 1 except that this skillet 
exhibits a decreased ability to withstand thermal downshock as measured 
both by rapid quenching from 500.degree. C. to cold water and by "dunker" 
testing. 
EXAMPLE 3 
A silicon-coated skillet is formed in accordance with Example 1 except for 
the surface roughening and silicon deposition steps. Instead of employing 
a Blanchard grind, the surface to be coated is roughened by grit blasting 
with 80 mesh SiC at 70 lbs. air pressure to form a 0.004 cm r.m.s. 
surface. In the silicon deposition step, the spray gun standoff distance 
is 16 cm. The characteristics of this skillet are similar to those of the 
skillet coated in accordance with Example 1. 
EXAMPLE 4 
A circular glass skillet having a configuration and composition such as 
described in Example 1 is selected for treatment. The surface to be coated 
is first grit blasted with 90 mesh SiC at 70 lbs. air pressure until a 
0.004 cm surface roughness is achieved. The green glass is then nucleated 
by subjecting it to a 1/2 hour heat treatment at 750.degree. C. After 
being washed with detergent and water, the skillet is subjected to a 
450.degree. C. furnace preheat after which the bottom surface thereof is 
provided with a 0.071 cm thick silicon coating by process similar to that 
described in Example 1, the gun standoff distance being 16 cm. The 
resultant silicon coating exhibits an adhesion of about 3.5 MPa, but the 
thermal durability thereof is poorer than that of Example 1. 
EXAMPLE 5 
A coated skillet formed in accordance with the method of Example 4 is 
cerammed by heating the skillet at a rate of 400.degree. C. per hour to 
1100.degree. C., maintaining that maximum temperature for 2 hours and then 
cooling. The nucleating step is performed prior to coating in accordance 
with the method of Example 4. The coating adhesion increases to about 9.7 
MPa. 
EXAMPLE 6 
A green glass skillet having the dimensions and composition set forth in 
Example 1 is Blanchard ground as in Example 1 to a surface roughness of 
0.004 cm and is thereafter cerammed as in Example 1. The skillet is then 
furnace-preheated to 550.degree. C. and flame sprayed in accordance with a 
method similar to that of Example 1. The standoff distance of the gun is 18 
cm and the silicon powder employed is of the type described in Example 2. 
The silicon coating, which has a thickness of 0.051 cm, exhibits an 
adhesion of 3.3 MPa. 
EXAMPLE 7 
A flat 25 cm by 25 cm coupon of Corning Code 9617 glass is grit blasted in 
the green state with 80 mesh SiC abrasive at 70 lbs. air pressure to form 
a surface having a 0.004 cm roughness. The coupon is then cerammed on a 
schedule similar to that of Example 1. Silicon powder of the type employed 
in Example 1 is used. Without being preheated, the roughened surface is 
plasma arc sprayed with standard Avco equipment by employing multipass, 
rotary (spiral) and x-y scanning until a thickness of 0.08 cm is achieved. 
An argon sheath is employed during spraying to prevent air aspiration and 
attendant coating oxidation. A strongly adherent coating is achieved. 
EXAMPLE 8 
A green glass skillet of the type employed in Example 1 is roughened by 
grit blasting with 80 mesh SiC at 70 lbs. air pressure and is thereafter 
cerammed in accordance with the schedule employed in Example 1. After the 
roughened skillet is cerammed, a silicon coating having an average 
thickness of 0.1 cm is applied by plasma coating as in Example 7. The 
coating adhesion is 7 MPa. Moreover, the thermal shock resistance of this 
skillet is excellent, it being able to withstand thermal downshock from 
500.degree. C. to cold water and more than 100 cycles of "dunker" testing 
without breaking.