WO2000056537A1

WO2000056537A1 - Ceramic substrate for nonstick coating

Info

Publication number: WO2000056537A1
Application number: PCT/US2000/006500
Authority: WO
Inventors: Louis J. Gazo; Srinivasan Sridharan
Original assignee: Ferro Corporation
Priority date: 1999-03-24
Filing date: 2000-03-13
Publication date: 2000-09-28
Also published as: AU4009200A

Abstract

The present invention provides new and useful nonstick coatings for use on pure aluminum, alloys of aluminum, or aluminized steel surfaces, methods of forming such nonstick coatings, and articles of cookware having such nonstick coatings applied thereto. Nonstick coatings according to the present invention include a ceramic substrate disposed on an aluminium surface and a fluorocarbon polymer top coat disposed on the ceramic substrate. The ceramic substrate is formed by applying a slip containing abrasion resistant particles and one or more glass frits to an aluminum surface and then firing the slip to form a continuous layer of vitreous enamel having surface projections defined by said abrasion resistant particles. A conventional fluorocarbon polymer top coat is applied to the ceramic substrate. The surface projections of the ceramic substrate protect the fluorocarbon polymer top coat from abrasive wear and the continuous layer of vitreous enamel protects the underlying aluminum surface from mechanical wear and chemical attack.

Description

Title: Ceramic Substrate for Nonstick Coating

Cross-Reference to Related Application

This application is a continuation-in-part of parent application Ser. No. 09/275,307, filed March 24, 1999.

Field of Invention The present invention relates to nonstick coatings for use on pure aluminum, alloys of aluminum, or aluminized steel, methods of forming such nonstick coatings, and articles of cookware having such coatings applied thereto. More particularly, the present invention relates to nonstick coatings which are formed by applying a ceramic substrate containing abrasion resistant particles to an aluminum surface and applying one or more fluorocarbon polymer top coats to the ceramic substrate.

Background

Fluorocarbon polymers, such as polytetrafluoroethylene (PTFE), polymers of chlorotrifluoroethylene (CTFE), fluorinated ethylene-propylene polymers (FEP), polyvinylidene fluoride (PVF), combinations thereof and the like, are known to have superior nonstick properties. For this reason, they have been used in a wide variety of applications, including forming nonstick coatings on articles of cookware. However, due to the inherent nonstick nature of these fluorocarbon polymers, it has been difficult to form nonstick coatings that adhere well to substrates such as pure aluminum, alloys of aluminum, and aluminized steel. Moreover, due to the inherent softness of fluorocarbon polymers, it has been difficult to form nonstick coatings that resist abrasion.

In an effort to overcome these difficulties, it has been the conventional practice to apply one or more base coats containing adhesive resins in order to better adhere fluorocarbon polymer top coats to substrates (throughout this specification and in the claims, the terms "bases coat" and "primer coat" are used interchangeably). In general, such base coats comprise a combination of high temperature binder resins, such as polyamideimide resins (PAI), polyethersulfone resins (PES) or polyphenylene sulfide resins (PPS), and fluorocarbon polymer resins. The performance of these conventional nonstick coating systems is based upon a stratification of the applied coatings. This stratification results in a coating that is rich in high temperature binder on the bottom and rich in fluorocarbon polymer at the top. The binder-rich bottom provides adhesion to the substrate while the fluorocarbon polymer-rich top provides a layer to which subsequent fluorocarbon polymer top coats can be fused by heating at high temperature.

The performance of such nonstick coating systems is at best a compromise. The bottom layer of the base coats is not a purely binder resin. Considerable levels of fluorocarbon polymer resins must be included in the base coats in order to provide a layer that is sufficiently rich in fluorocarbon polymer to promote good bonding of subsequent fluorocarbon polymer top coats to the base coat. The presence of fluorocarbon polymer resins in the base coat are disadvantageous because they detract from the adhesion of the base coat to the substrate. Therefore, it has been necessary to roughen substrates by mechanical (e.g. grit blasting) or chemical (e.g. etching) means to assist holding the base coat to the substrate. Moreover, because both the adhesive resins and fluorocarbon polymers are relatively soft, there have been difficulties in making these nonstick coatings resistant to abrasive wear. Efforts to overcome these deficiencies have included the addition of mica particles, ceramic fillers, or metal flakes to the intermediate and top coat in order to increase the hardness. The presence of these fillers can be disadvantageous. For example, incorporation of metal flakes in the applied coatings can actually promote chemical corrosion of the underlying metal substrate due to dissimilarity between the metals. Moreover, these particulate fillers cannot be incorporated into the nonstick coating at high levels because at high levels they diminish the nonstick properties of the coating and the bonding to the substrate. Due to the limitations thus described, articles of cookware coated with conventional fluorocarbon polymer nonstick coating systems are prone to damage and abrasive wear during normal use. Cooking utensils, for example, often cause cuts, slices, or gouges in the nonstick coating which permit acids or alkaline foodstuffs and cleaning agents to corrode the exposed aluminum substrate. Corrosion of the underlying aluminum by these materials can further weaken the adhesion of the nonstick coating adjacent to the cut or slice. Moreover, abrasive forces routinely encountered in cooking and cleaning cause the gradual removal of the soft fluorocarbon polymer top coat resulting in diminished nonstick properties. Conventional nonstick coatings simply do not adequately protect the aluminum substrate from corrosion or the fluorocarbon polymer top coat from routine abrasive wear.

Summary of the Invention

The present invention provides new and useful nonstick coatings for use on pure aluminum, aluminum alloys, or aluminized steel surfaces, methods of forming such nonstick coatings, and articles of cookware having such nonstick coatings applied thereto. Nonstick coatings according to the present invention comprise a ceramic substrate disposed on an aluminum surface and at least one fluorocarbon polymer top coat disposed on said ceramic substrate. Prior to firing, the ceramic substrate comprises a blend of abrasion resistant particles and one or more glass frits. In a preferred embodiment, the ceramic substrate further comprises a base layer that promotes adhesion of the ceramic substrate to the aluminum surface.

Subsequent to firing, the ceramic substrate forms a continuous layer of vitreous enamel having surface projections defined by said abrasion resistant particles. The surface projections hold the fluorocarbon polymer top coat together and protect it from abrasive wear, and the continuous layer of vitreous enamel protects the underlying aluminum surface from mechanical and chemical attack. Nonstick coatings according to the present invention can be applied to aluminum surfaces that have been cleaned only. It is not necessary to grit blast or acid etch the surface in order to attain satisfactory adhesion of the coating.

Nonstick coatings according to the present invention are substantially more durable than conventional nonstick coatings.

Nonstick coatings according to the present invention are formed by applying a ceramic substrate to an aluminum surface. In a preferred embodiment, the ceramic substrate is applied by spraying a slip containing abrasion resistant particles, one or more glass frits, and other optional vehicles, mill additives, and fillers, to the aluminum surface. In the presently most preferred embodiment, an adhesion promoting enamel ground coat layer is disposed between the aluminum surface and the ceramic substrate. The ceramic substrate is fired to form a continuous vitreous enamel that has a very fine sandpaper-like finish. One or more fluorocarbon polymer top coats is then applied to the ceramic substrate and sintered. Nonstick coatings formed according to the present invention exhibit substantially improved corrosion and abrasion resistance performance as compared to prior art nonstick coatings.

The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.

Brief Description of the Drawings

Fig. 1 shows a schematic cross-sectional view of one preferred embodiment of the nonstick coating according to the present invention.

Fig. 2 shows a schematic cross-sectional view of another preferred embodiment of the nonstick coating according to the present invention.

Detailed Description

In accordance with the present invention, nonstick coatings for use on pure aluminum, alloys of aluminum, or aluminized steel are provided that are especially suitable for use in the food industry on cookware and on electrical appliances used in the preparation of food. However, the present invention is also suitable for use in other applications where durable nonstick surfaces are needed, such as on steam irons and in industrial applications. Novel nonstick coatings according to the present invention are formed by applying a ceramic substrate to an aluminum surface and applying at least one fluorocarbon polymer top coat to said ceramic substrate. In one preferred embodiment, the ceramic substrate is formed by ball milling one or more glass frits together with optional vehicles, mill additives, and fillers to form a slip. The composition of the glass frit or frits used in the preparation of the slip is not per se critical, and any one or more of a number of conventional glass frits for use on aluminum or aluminized steel is suitable for use in the invention.

The glass frit or frits may be prepared utilizing conventional glass melting techniques. A conventional ceramic refractory, fused silica, or platinum crucible may be used to prepare the glass frit. Typically, selected oxides are smelted at temperatures of from about 1200°C to about 1400°C for 30 minutes. The molten glass formed in the crucible is then converted to glass frit using water-cooled steel rollers or water quenching. It will be appreciated that the step of producing the glass frit is not per se critical and any of the various techniques well-known to those skilled in the art can be employed.

As noted above, the composition of the glass frits is not critical, and a variety of glass frits suitable for use on aluminum and aluminized steel can be used in the application. For example, glass frits having the following compositional range (by weight percent) can be used in this application:

Constituent Range

SiO, 30-45

TiO, 12-30

Alkali Metal Oxides 5-35

Bi₂O₃ 0-20

B₂O₃ 0-15

Alkaline-Earth Metal Oxides 0-10

V₂O₅ 0-10

Sb₂O₅ 0-5

SnO, 0-5

The glass frit or frits are milled with optional mill additions and fillers. Common mill additions and fillers include, but are not limited to, boric acid, potassium hydroxide, sodium hydroxide, sodium silicate, potassium nitrate, potassium carbonate, potassium silicate, quartz, colloidal silica, ceramic fillers, and pigments. As is well known in the art, there is a wide range of other acceptable mill agents or components that may also be utilized in the present invention to produce the desired resultant product. Preferably, optional mill additions and fillers will comprise from about 0% to about 50% by weight of the solids portion of the slip.

Typically, the slip is milled to a fineness of about 0.3 to about 0.5 grams residue being retained on a 325 mesh sieve from a 50 cubic centimeter sample. Milling can be accomplished by wet or dry techniques. It will be appreciated that milling fineness is not critical, and can be altered without significant impact on the final coating.

After milling, abrasion resistant particles are added to the slip and thoroughly mixed in a high-speed mixer or blender. Preferably, the abrasion resistant particles display a Mohs hardness of at least about 6.0. Abrasion resistant particles suitable for use in the invention include diamond, carbides, borides, nitrides, oxides, silicates, and intermetallic materials. Preferred abrasion resistant particles include one or more selected from the group consisting of diamond, boron nitride, boron carbide, titanium boride, aluminum boride, silicon carbide, titanium carbide, alumina, silicon nitride, zirconium boride, NiAI, TiAI, zirconia, feldspar, and silica.

It will be appreciated that harder abrasion resistant particles will provide a more durable nonstick coating than softer abrasion resistant particles. Accordingly, diamond would be the ideal abrasion resistant particle if cost was not a consideration. However, cost is almost always a consideration in developing a coating system. Therefore, a lower cost alternative to diamond must usually be employed in the invention. Carbides are presently the most preferred abrasion resistant particles. Carbides are less costly than diamond, are generally less reactive with the other materials in the slip than borides, and generally have a higher degree of hardness as compared to oxides and silicates. Intermetallics such as NiAI and TiAI can also be used in the invention, but they are not as preferred as carbides because they can sometimes cause undesired interactions with the glass during firing thereby producing bubbles. These problems can usually be avoided if the intermetallic material is refractory coated.

In view of the foregoing factors, silicon carbide is presently the most preferred abrasion resistant material for use in the invention. It will be appreciated that silica, alumina, zircon, mullite, zirconia, and feldspar are lower cost alternatives to silicon carbide, but such materials are not as hard, and alumina and zirconia are somewhat susceptible to chemical attack. Those skilled in the art will appreciate that the foregoing factors must be considered when selecting a abrasion resistant material for use in producing a nonstick coating according to the present invention. The average diameter of the abrasion resistant particles used in the invention is from about 3 μm to about 150 μm, with an average diameter of from about 35 μm to about 75 μm being preferred. The abrasion resistant particles comprise from about 1% by weight to about 20% by weight of the solids portion of the slip.

After the abrasion resistant particles are added, the slip is applied to an aluminum surface using conventional wet application processes, such as spraying, dipping, and flow coating, which are well-known. The aluminum surface can be pure aluminum, alloys of aluminum, or aluminized steel. The aluminum surface need not be roughened prior to application of the slip such as by grit blasting or etching, although such roughening could be done and would marginally enhance adhesion of the ceramic substrate to the aluminum surface. The aluminum surface need only be cleaned, such as with alkalis, prior to application of the slip. In the presently most preferred embodiment of the invention, an adhesion promoting enamel ground coat layer is applied to the aluminum surface and then the ceramic substrate is applied to the enamel ground coat layer. Thus, the adhesion promoting enamel ground coat layer is disposed between the aluminum surface and the ceramic substrate. The enamel ground coat layer preferably includes one or more glass frits containing a smelted-in metal oxide selected from the group consisting of cobalt, nickel, copper and iron (including a combination or mixture of such metal oxides), which improves the adhesion of the applied coatings to the aluminum surface. A preferred metal oxide is cobalt oxide. A glass frit for use in forming an enamel ground coat layer according to the presently most preferred embodiment of the invention preferably has the following compositional range (in weight percent):

Constituent Range Preferred Range

SiO₂ 20-50 20-50

Na₂O 5-30 10-30

TiO₂ 5-30 5-30 K₂O 0-15 1-15

V₂O₅ 0-15 0-15

M_xO_y ^* 0-5 1-5

Li₂O 0-5 1-5

P₂O₅ 0-5 0-5 B₂O₃ 0-5 0-5

^*wherein M comprises cobalt, nickel, copper or iron The enamel ground coat layer can be dried and/or fired before the ceramic substrate is applied, but such drying and/or firing is not necessary. In a preferred

embodiment, the enamel ground coat layer is merely allowed to air dry until no surface moisture is present before the ceramic substrate is applied.

Whether an enamel ground coat layer is applied or not, the slip containing the abrasion resistant particles is typically applied at a rate of about 200 to about 520 grams per square meter. The slip containing the abrasion resistant particles can be dried prior to firing, although drying is not a necessary step. Firing is typically conducted in an air convection furnace at a temperature from about

1 ,000°F. to about 1 ,100°F. for a period of about 5 minutes to about 18 minutes. Of course, the exact period at temperature will be a function of the gauge of the metal, heavier gauges requiring longer fire times. However, it will be appreciated that the maximum allowable firing temperature will be a function of the melting temperature of the aluminum surface. Care must be taken to avoid the melting temperature of the aluminum surface during firing. Also, it will be appreciated that generally speaking, a longer or shorter firing period could be used depending on the application rate of the slip and the thickness of the article being coated.

After firing, the ceramic substrate will preferably have a thickness of from about 1.0 mil to about 4.0 mils, and more preferably of about 1.5 mils. It will be appreciated that the application rate of the coating composition can be varied to produce thinner or thicker ceramic substrates, and that application rate and thickness is not critical and can be altered without significant impact on the nonstick system.

After firing, the ceramic substrate exhibits a very fine sandpaper-like surface. When viewed under magnification, it is apparent that the abrasion resistant particles in the ceramic substrate define surface projections or peaks that rise above a continuous layer of vitreous enamel. It will be appreciated that by varying the average diameter and/or weight percent of the abrasion resistant particles in the slip, ceramic substrates with varying degrees of surface roughness can be produced. Conventionally, the roughness of a surface is expressed in terms of average surface roughness (R_a), which is the arithmetic average of the absolute deviations of the roughness profile from the roughness center line. The average roughness (R_a) of a ceramic substrate formed according to the present invention is preferably within the range of from about 100 μin to about 250 μin, with about 150 μin being most preferred. By comparison, the average roughness (RJ of a conventional enamel for use on aluminum is typically less than about 15 μin. All of the surface roughness measurements disclosed in this specification were made using an M4Pi- Rk® surface analyzing instrument available from Mahr GmbH, and profileView® surface analyzing software available from Metrex, a division of Extrude Hone of

Irwin, Pennsylvania.

Surface roughness can also be expressed in terms of maximum peak to valley roughness (R_max), which is the vertical distance between the highest peak in the roughness profile and the deepest valley in the roughness profile, and/or mean roughness depth (R_z), which is the mean value of the single roughness depth of consecutive lengths in a roughness profile. A ceramic substrate formed according to the present invention preferably has a maximum peak to valley roughness (R_max) of from about 350 μin to about 1000 μin, with about 600 μin being most preferred, and a mean roughness depth (RJ preferably of from about 750 μin to about 1400 μin, with about 1 125 μin being most preferred. By comparison, the maximum peak to valley roughness (R_maJ and mean roughness depth (RJ of conventional enamels for use on aluminum is about 55 μin, and 46 μin, respectively.

Ceramic substrates made in accordance with the present invention are color stable, resistant to chemical attack (acid and alkali resistance), and display good mechanical (abrasion) resistance properties. Additionally, such ceramic substrates exhibit excellent adherence to aluminum surfaces which have been cleaned only, especially when an enamel ground coat layer is used. The inherent hardness of the abrasion resistant particles makes the ceramic substrate very durable and resistant to abrasive wear. A conventional fluorocarbon polymer top coat is applied to the ceramic substrate and sintered to form a nonstick coating. Throughout the specification and in the claims, the term fluorocarbon polymer top coat refers to conventional fluorocarbon polymers such as polytetrafluoroethylene (PTFE), polymers of chlorothfluoroethylene (CTFE), fluorinated ethylene-propylene polymers (FEP), polyvinylidene fluoride (PVF), combinations thereof and the like. The fluorocarbon polymer top coat may also contain adhesion promoting high temperature binder resins, such as polyamideimide resins (PAI), polyethersulfone resins (PES) and polyphenylene sulfide resins (PPS). The composition of the fluorocarbon polymer top coat is not critical, and a variety of fluorocarbon polymer compositions conventionally used in the formation of nonstick coatings can be employed in the invention. After the ceramic substrate is formed, a fluorocarbon polymer top coat is applied by conventional wet or dry techniques. It will be appreciated that the fluorocarbon polymer top coat can be applied in several layers or in a single layer. After sintering, the fluorocarbon polymer top coat preferably has a thickness of from about 0.25 mils to about 2 mils, and more preferably of about 0.5 mils. Sintering temperatures and times vary depending upon the composition and the thickness of the fluorocarbon polymer top coat. For example, PTFE applied to a thickness of about 25 to about 50 μm can be properly sintered in a convection oven heated to a temperature of at about 810°F. for about 10 minutes. It has been discovered that improved adhesion between the fluorocarbon polymer top coat and the ceramic substrate can be achieved when the surface of the ceramic substrate is enriched with SiO₂ bonding sites and the base or primer coat of the fluorocarbon polymer top coat contains polyamideimide resins. In order to enrich the surface of the ceramic substrate with SiO₂ bonding sites, an overspray comprising about 30 to about 95 parts by weight of the slip and about 5 to about 70 parts by weight of fine quartz can be applied by a wet application technique to a thickness of from about 0.25 mils to about 1 mil, and preferably to about 0.5 mils. The overspray can be applied before or after the first coat of slip is dried. When an overspray is employed, the ceramic substrate will have a surface that is rich in SiO₂ bonding sites that can bond with polyamideimide resins in the fluorocarbon polymer top coat. Fig. 1 shows a schematic representation of a cross-sectional view of one preferred embodiment of the nonstick coating 1 according to the present invention. With reference to the Fig 1 , a ceramic substrate 2 is disposed on the aluminum surface 3 of a base 4. A fluorocarbon polymer top coat 5 is disposed on the ceramic substrate 2. The ceramic substrate 2 is a continuous layer of vitreous enamel 6 having surface projections 7 defined by abrasion resistant particles 8. The portion of the ceramic substrate 2 defined by brackets 9 is enriched with SiO₂ bonding sites and was formed by applying an overspray containing about 50 parts by weight of the slip and about 50 parts by weight of very fine quartz. The region of the fluorocarbon polymer top coat 5 defined by the brackets 10 in contact with the ceramic substrate 2 was formed by applying a primer coat containing a blend of fluorocarbon polymer and polyamideimide resin to the ceramic substrate to promote bonding and adhesion of the fluorocarbon polymer top coat 5 to the ceramic substrate 2. The balance of the fluorocarbon polymer top coat 5 not defined by the brackets 10 contains little, if any, polyamideimide resin. The fluorocarbon polymer top coat 5 is sintered to provide a nonstick surface 11.

Fig. 2 shows a schematic representation of a cross-sectional view of the presently most preferred embodiment of the nonstick coating 1 according to the present invention. The nonstick coating 1 shown in Fig. 2 differs from the nonstick coating 1 shown in Fig. 1 in that an enamel ground coat layer 12 is disposed between the aluminum surface 3 and the ceramic substrate 2. As noted above, use of an enamel ground coat layer 12 improves the adhesion of the applied coatings to the aluminum surface 3 of the base 4.

Thus, according to the method of the present invention, a durable nonstick coating is formed on an aluminum surface by the steps comprising providing a substrate having an aluminum surface, cleaning the aluminum surface, applying a slip comprising a blend of abrasion resistant particles and one or more glass frits to the aluminum surface by a wet application method, firing the applied slip to form a ceramic substrate comprising a continuous layer of vitreous enamel having surface projections defined by said abrasion resistant particles, applying one or more fluorocarbon polymer top coat layers to said ceramic substrate, and sintering said fluorocarbon polymer to form a nonstick coating. Optionally, the method can further comprise one or more additional steps including: roughening the aluminum surface, such as by grit blasting or acid etching, after cleaning; applying an enamel ground coat layer to the aluminum surface and then applying the slip containing the abrasion resistant particles thereto; applying an SiO₂ rich overspray to said applied slip prior to firing; and/or partially or fully drying the applied slip prior to firing.

The following examples are intended only to illustrate the invention and should not be construed as imposing limitations upon the claims: Example 1

A glass frit having the following oxide composition (the frit also contained about 3.95% by weight F₂) was prepared using conventional glass melting techniques:

Constituent Weiαht %

SiO₂ 40.3

TiO₂ 22.2

K₂O 16.1

Na₂O 10.8

Li₂O 4.8

B₂O₃ 3.2

CaO 2.6

A slip was prepared by ball milling the frit together with the mill additions shown below:

Component Grams

Frit 800 '

325 mesh quartz 150 boric acid 10 sodium hydroxide 10 sodium silicate 75

Kasil (potassium silicate) 68

F 6340 black oxide pigment 100

Colloidal silica 10

Water 450

The slip was milled to a fineness of 0.3 to 0.5 grams being retained on a 325 mesh sieve from a 50 cubic centimeter sample. After milling, 50 grams of silicon carbide abrasion resistant particles having an average diameter of about 50 μm were added to the slip and blended using a high speed mixer. The slip was applied to the inner surface of a cookware blank (9" diameter skillet) formed from a 1/8th inch thick sheet of 3003 aluminum alloy that had been cleaned only to a wet thickness of about 1.7 mils. An overspray comprising 50 parts by weight of the slip and 50 parts by weight of 325 mesh quartz was applied to the wet coating to a thickness of about 0.5 mils. The coated 3003 aluminum alloy cookware blank was dried for 15 minutes at 225°F. and then fired in a convection oven at about 1025°F. for about 11 minutes. The fired thickness of the ceramic substrate was about 1.5 mils. The enamel had a very fine sandpaper-like appearance.

A conventional polyamideimide/polytetrafluoroethylene blend fiuorocarbon polymer primer coat was applied to the ceramic substrate by a conventional wet spraying coating method to a thickness of about 10 μm. An additional layer of conventional polytetrafluoroethylene was applied thereto by the same coating technique to a thickness of about 25 μm. The cookware blank was then heated in a conventional oven for about 10 minutes at a temperature of about 800°F. to sinter and cure the fluorocarbon polymer top coat. The inner surface of the cookware blank had excellent nonstick properties.

Example 2

The nonstick coated cookware blank produced in Example 1 and two present state-of-the-art production nonstick cookware pieces (also 9" diameter skillets) were tested for comparative abrasion resistance. The present state-of-the-art production nonstick cookware pieces were formed by first coating a steel cookware blank with a metal-reinforced enamel coating layer, then spraying the metal-reinforced enamel coating iayer with white-hot particles of stainless steel which adhere thereto to form a rough surface, and then applying a conventional three layer fluorocarbon top coat to the roughened surface. The manufacturer of the present state-of-the-art production nonstick cookware pieces used in this Example claims that abrasion testing demonstrates that its nonstick coating is 20 times more durable than conventional fluorocarbon polymer nonstick coated cookware.

The nonstick coated surface of each cookware piece was abraded using a Taber Model 5130 Abraser equipped with C-17-F abrasive wheels bearing a 1000 gram load. The number of revolutions or cycles required for the abrasive wheels to penetrate the fluorocarbon polymer coating to the metal substrate was recorded. The present state-of-the-art cookware pieces were abraded to the bare metal substrate in 500 and 1 ,600 cycles, respectively. Testing was stopped on the coated cookware blank produced in Example 1 after 3,000 cycles. No aluminum metal was exposed, and the surface of the coated cookware blank retained its original nonstick performance capability notwithstanding the abrasive action of 3,000 cycles with the abrasive wheels.

A scanning electron micrograph at 100 magnification showing the nonstick surface of a present state-of-the-art production nonstick cookware piece as detailed in Example 2 after 500 cycles on the Taber Abraser showed abrasive wear. A scanning electron micrograph at 1 ,000 magnification showing the nonstick surface of a present state-of-the-art production nonstick cookware piece as detailed in Example 2 after 500 cycles on the Taber Abraser revealed tearing of the nonstick coating and the exposure of the bare metal substrate. A scanning electron micrograph at 100 magnification showing the nonstick surface of a cookware blank coated with the nonstick coating according to Example 1 after 3,000 cycles on the Taber Abraser revealed no meaningful wear. A scanning electron micrograph at 1 ,000 magnification showing the nonstick surface of a cookware blank coated with the nonstick coating according to Example 1 after 3,000 cycles on the Taber Abraser revealed that the nonstick coating is not torn, and the aluminum surface is not exposed.

Applicants submit that Example 2 demonstrates that cookware pieces coated with nonstick coatings according to the present invention are more than six times more wear resistant than present state-of-the-art production nonstick cookware pieces.

Example 3

A glass frit having the following oxide composition (the frit also contained about 4.00% by weight NO₃) was prepared using conventional glass melting techniques:

Constituent Weiαht %

SiO₂ 33.87

Na₂O 20.44

TiO₂ 20.38 v₂o₅ 9.33

K₂O 7.58

Co₂O₃ 3.13

P₂O₅ 2.82

Li₂O 2.11

B₂O₃ 0.24

A composition for use in forming an enamel ground coat layer was prepared milling the frit together with the mill additions shown below:

Comoonent Grams

Frit 800 boric acid 32 potassium hydroxide 20 sodium silicate 20

KNO₃ 0.8

F 6340 black oxide pigment 100

Colloidal silica 10

Water 280 The composition for use in forming an enamel ground coat layer was milled to a fineness of 0.1 to 0.3 grams being retained on a 325 mesh sieve from a 50 cubic centimeter sample. The composition was applied to the inner surface of a cookware blank (9" diameter skillet) formed from a 1/8th inch thick sheet of 3003 aluminum alloy that had been cleaned only to a wet thickness of about 1.2 mils. The enamel ground coat layer was allowed to partially air dry until no surface moisture was present.

A second glass frit having the following oxide composition was prepared using conventional glass melting techniques:

Constituent Weiαht %

SiO₂ 39.7

TiO₂ 23.0

K₂O 16.1

Na₂O 10.8

Li₂O 4.8

B₂O₃ 3.2

CaO 2.6

A slip was prepared by ball milling the second glass frit together with the mill additions shown below: Component Grams

Second glass frit 800 colloidal silica 16 potassium hydroxide 40 boric acid 40

Kasil (potassium silicate) 110 sodium silicate 120

F 6340 black oxide pigment 160

KNO₃ 1.5

325 mesh silica 800

Water 960

The slip was milled to a fineness of 0.1 to 0.3 grams being retained on a 325 mesh sieve from a 50 cubic centimeter sample. After milling, 80 grams of silicon carbide abrasion resistant particles having an average diameter of about 50 μm were added to the slip and blended using a high speed mixer. The slip was applied to the partially air dried enamel ground coat layer to a wet thickness of about 1.7 mils. An overspray comprising 50 parts by weight of the slip and 50 parts by weight of 325 mesh quartz was applied to the wet coating to a thickness of about 0.5 mils. The coated 3003 aluminum alloy cookware blank was dried for 15 minutes at 225°F. and then fired in a convection oven at about 1025 . for about 11 minutes. The fired thickness of the ceramic substrate was about 2.0 mils. The enamel had a very fine sandpaper-like appearance.

A conventional polyamideimide/polytetrafluoroethylene blend fluorocarbon polymer primer coat was applied to the ceramic substrate by a conventional wet spraying coating method to a thickness of about 10 μm. An additional layer of conventional polytetrafluoroethylene was applied thereto by the same coating technique to a thickness of about 25 μm. The cookware blank was then heated in a conventional oven for about 10 minutes at a temperature of about 800°F. to sinter and cure the fluorocarbon polymer top coat. The inner surface of the cookware blank had excellent nonstick properties.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and illustrative examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.

Claims

What is claimed is:

1. A nonstick coating for use on a pure aluminum, aluminum alloy, or aluminized steel surface comprising a ceramic substrate disposed on said surface and a fluorocarbon polymer top coat disposed on said ceramic substrate, said ceramic substrate prior to firing comprising a slip comprising a blend of abrasion resistant particles and one or more glass frits, said ceramic substrate subsequent to firing comprising a continuous layer of vitreous enamel having surface projections defined by said abrasion resistant particles.

2. The nonstick coating as in claim 1 wherein said ceramic substrate subsequent to firing has an average roughness (RJ of from about 100 μin to about

250 μin and said abrasive particles have a Mohs hardness of at least 6.0.

3. The nonstick coating as in claim 1 wherein said abrasion resistant particles are one or more selected from the group consisting of diamond, boron nitride, boron carbide, titanium boride, aluminum boride, silicon carbide, titanium carbide, alumina, silicon nitride, zirconium boride, NiAI, TiAI, zirconia, zircon, mullite, feldspar, and silica.

4. The nonstick coating as in claim 1 wherein said abrasion resistant particles have an average diameter of from about 3 μm to about 150μm.

5. The nonstick coating as in claim 1 wherein the portion of said ceramic substrate in contact with said fluorocarbon polymer top coat is enriched with SiO₂ bonding sites and the portion of said fluorocarbon polymer top coat in contact with said ceramic substrate contains polyamideimide resin.

6. The nonstick coating as in claim 1 wherein said ceramic substrate prior to firing has an average thickness of from about 1 mil to about 4 mils.

7. The nonstick coating as in claim 1 wherein said aluminum surface is cleaned only prior to application of said ceramic substrate.

8. The nonstick coating as in claim 1 wherein said glass frits comprise, by weight, from about 30% to about 45% SiO₂, from about 12% to about 30% TiO₂, from about 5% to about 35% alkali metal oxides, from about 0% to about 20% Bi₂O₃, from about 0% to about 15% B₂O₃, from about 0% to about 10% alkaline- earth metal oxides, from about 0% to about 10% V₂O₅, from about 0% to about 5% Sb₂O₅, and from about 0% to about 5% SnO₂.

9. The nonstick coating as in claim 1 wherein said fluorocarbon polymer top coat comprises polytetrafluoroethylene.

10. The nonstick coating as in claim 1 wherein said fluorocarbon polymer top coat has an average thickness of from about 0.5 to about 2 mils.

11. The nonstick coating as in claim 1 further comprising an enamel ground coat layer disposed between said surface and said ceramic substrate.

12. The nonstick coating as in claim 11 wherein said enamel ground coat layer prior to firing comprises one or more glass frits comprising, by weight, from about 20% to about 50% SiO₂, from about 5% to about 30% Na₂O, from about 5% to about 30% TiO₂, from about 0% to about 15% K₂O, from about 0% to about 15% V₂O₅, from about 1 % to about 5% of a metal oxide selected from the group consisting of cobalt oxide, copper oxide, iron oxide, nickel oxide or mixtures of such oxides, from about 0% to about 5% Li₂O, from about 0% to about 5% P₂O₅, and 5 from about 0% to about 5% B₂O₃.

13. An article of cookware comprising a base having a pure aluminum, aluminum alloy, or aluminized steel surface, a ceramic substrate disposed on said surface of said base, and a fluorocarbon polymer top coat disposed on said ceramic substrate to define a nonstick cooking surface, said ceramic substrate prior to firing o comprising a slip comprising a blend of abrasion resistant particles and one or more glass frits, said ceramic substrate subsequent to firing comprising a continuous layer of vitreous enamel having surface projections defined by said abrasion resistant particles.

14. The article of cookware as in claim 13 wherein said ceramic substrate 5 subsequent to firing has an average roughness (RJ of from about 100 μin to about

250 μin.

15. The article of cookware as in claim 13 wherein said abrasion resistant particles are one or more selected from the group consisting of diamond, boron nitride, boron carbide, titanium boride, aluminum boride, silicon carbide, titanium 0 carbide, alumina, silicon nitride, zirconium boride, NiAI, TiAI, zirconia, feldspar, zircon, mullite and silica.

16. The article of cookware as in claim 13 further comprising an enamel ground coat layer disposed between said surface and said ceramic substrate.

17. The article of cookware as in claim 16 wherein said enamel ground coat layer prior to firing comprises one or more glass frits comprising, by weight, from about 20% to about 50% SiO₂, from about 5% to about 30% Na₂O, from about

5% to about 30% TiO₂, from about 0% to about 15% K₂O, from about 0% to about 15% V₂O₅, from about 0% to about 5% of a metal oxide selected from the group consisting of cobalt oxide, copper oxide, iron oxide, nickel oxide and mixtures thereof, from about 0% to about 5% Li₂O, from about 0% to about 5% P₂O₅, and from about 0% to about 5% B₂O₃.

18. A method of forming a nonstick coating comprising the steps of providing a pure aluminum, aluminum alloy, or aluminized steel surface, applying a slip comprising a blend of abrasion resistant particles and one or more glass frits to said surface, firing said slip to form a ceramic substrate comprising a continuous layer of vitreous enamel having surface projections defined by said abrasion resistant particles, applying a fluorocarbon polymer top coat to said ceramic substrate, and sintering said fiuorocarbon polymer top coat.

19. The method of claim 18 wherein said ceramic substrate has an average roughness (RJ of from about 100 μin to about 250 μin and said abrasion resistant particles have a Mohs hardness of at least 6.0.

20. The method of claim 18 wherein said pure aluminum, aluminum alloy, or aluminized steel surface is cleaned only prior to application of said slip.

21. The method of claim 18 wherein said abrasion resistant particles are one or more selected from the group consisting of diamond, boron nitride, boron carbide, titanium boride, aluminum boride, silicon carbide, titanium carbide, alumina, silicon nitride, zirconium boride, NiAI, TiAI, zirconia, feldspar, zircon, muliite and silica.

22. The method of claim 18 wherein said glass frits comprise, by weight, from about 30% to about 45% SiO₂, from about 12% to about 30% TiO₂, from about 5% to about 35% alkali metal oxides, from about 0% to about 20% Bi₂O₃, from about 0% to about 15% B₂O₃, from about 0% to about 10% alkaline-earth metal oxides, from about 0% to about 10% V₂O₅, from about 0% to about 5% Sb₂O₅, and from about 0% to about 5% SnO₂.

23. The method of claim 18 wherein said fluorocarbon polymer top coat comprises polytetrafluoroethylene.

24. The method of claim 18 wherein said firing is conducted at a temperature of from about 1 ,000°F. to about 1 ,100°F. for a period of from about 5 minutes to about 18 minutes.

25. A method of forming a nonstick coating comprising the steps of providing a pure aluminum, aluminum alloy, or aluminized steel surface, applying an enamel ground coat layer to said surface, applying a slip comprising a blend of abrasion resistant particles and one or more glass frits to said enamel ground coat layer, firing said slip and said enamel ground coat layer to form a ceramic substrate comprising a continuous layer of vitreous enamel having surface projections defined by said abrasion resistant particles, applying a fluorocarbon polymer top coat to said ceramic substrate, and sintering said fluorocarbon polymer top coat.

26. The method of claim 25 wherein said enamel ground coat layer prior to firing comprises one or more glass frits comprising, by weight, from about 20% to about 50% SiO₂, from about 5% to about 30% Na₂O, from about 5% to about 30% TiO₂, from about 1% to about 15% K₂O, from about 0% to about 15% V₂O₅, from about 0% to about 5% Co₂O₃, from about 0% to about 5% Li₂O, from about 0% to about 5% P₂O₅, and from about 0% to about 5% B₂O₃.