SURFACE FINISH COMPOSITIONS

TECHNICAL FIELD

This invention relates to surface finishes which impart nonabradable and nonetchable, durable dry lubricity, corrosion resistance and improved wet film entrapment characteristics to a substrate and to methods for applying such surface finishes to a substrate.

Although this invention is primarily directed to the surface finishing of metallic substrates, it should be noted that it is likewise applicable to surface finishes for application to other suitable substrate materials such as ceramic compositions. Furthermore, it should be noted that the metallic substrates employed herein may range from very hard metals having a hardness factor measured on the Rockwell C scale of greater than 40 to soft metals having hardness values measured on the Rockwell B scale. Accordingly, a diversity of substrate materials may be utilized in this invention provided that the material has sufficient structural integrity to withstand the high pressure impact application techniques employed herein.

A wide variety of corrosion-resistant coatings as well as methods for the application of such coatings to substrates have been disclosed heretofore. Examples thereof may be found in U.S. Patent Nos. 3,574,658; 3,754,976; 4,228,670; 4,312,900; 4,333,840; 4,415,419; 4,552,784; 4,553,417 and 4,753,094.

In this regard, several of the above-noted patents disclose processes for applying coatings to the surface of work pieces by a peening or blasting procedure in which the coating material is applied to the surface by pellets or other shot material and is impacted at high pressure against the surface of the work piece in order to apply the coating on the pellets or shot to the surface of the work piece. For example, in U.S. Patent No. 3,574,658, a method is disclosed for applying a dry lubricant in the nature of a molybdenum or tungsten disulfide coating to the shot material and then applying this dry lubricant material to the surface of the work piece as a coating. U.S. Patent No. 3,754,976 discloses a coating process wherein shot and powdered metal are peened against the surface of a work piece which has previously been cleaned with a gentle stream of peening particles in the absence of the coating material. U.S. Patent No. 4,228,670 discloses a process wherein steel or glass shot is co-mingled with lubricant and blasted against a work piece in order to apply the lubricant to the work piece surface. U.S. Patent No. 4,312,900 discloses a process wherein the work piece surface is initially pitted by shot blasting using abrasive materials such as glass or sand followed by buffing dry molybdenum disulfide into the pits created in the surface of the work piece by the shot blasting. U.S. Patent No. 4,552,784 discloses a further process for applying a metal powder to the surface of a work piece by a peening technique. Again, in U.S. Patent No. 4,753,094, a process is taught wherein a thin film coating of molybdenum disulfide is applied to a substrate surface by a peening action in order to adhere the molybdenum disulfide to the surface of the substrate as a coating thereon.

However, none of the prior disclosures have provided products demonstrating the combination of characteristics and properties which are achieved by the products of the present invention nor do they provide processes for producing such products. Indeed, the need to prolong the wear-life of substrate surfaces such as metal surfaces and to reduce the frictional properties thereof in order to reduce repair and replacement costs has been and continues to be the focus of intensive research and development efforts. Nonetheless, these efforts have achieved only relatively limited success resulting from the use of previously known coatings, paints and lubricants (both wet and dry) . Each of the known techniques for treating substrates such as metal surfaces has presented significant problems and drawbacks in regard to the cost, difficulties in application, product properties achieved and the like.

Particularly, it is presently believed that the processes of the present invention achieve surface modification whereby plural polymers are bonded with the surface of the treated work piece or substrate. For purposes hereof, the term bond or bonded will apply to either physical or chemical bonds which result in products demonstrating the desired characteristics. Based on this belief, it is presently hypothesized that the surface finishes of this invention are not coatings but are permanently bonded with the substrate and can only be removed by grinding away the substrate surface itself. Accordingly, the surface-finishing processes of the present invention result in products with permanent finishes having a degree of long lasting, durable dry lubricity and corrosion resistance which has not been achieved heretofore.

With regard to prior processes for imparting desirable physical properties of polymers to substrate surfaces such as metal surfaces, it has been common to employ fluorocarbon polymers such as tetrafluoroethylene (TFE) sold, for example, under the tradename "Teflon" by E.I. Du Pont de Nemours & Co. (Inc.), as a coating material. Teflon coated surfaces are known to reduce friction and adhesion but must be applied to the substrate by use of primers such as epoxy. The coated surface, accordingly, abrades under modest pressure, does not coat evenly or thinly and requires high temperatures for application.

SUMMARY OF THE INVENTION

The present invention overcomes many of the known shortcomings of the prior art. The invention comprises preparing a particulate mixture of a sulfur containing metallic compound such as molybdenum disulfide or tungsten disulfide and a fluorocarbon polymer such as tetrafluoroethylene, preferably in a ratio of about 1:1 to about 10:1 parts fluorocarbon polymer to sulfur containing metallic compound (on a weight percentage basis) .

A pressurized stream of the particulate mixture is impacted onto the surface of a substrate at a sufficient pressure and for a sufficient period of time to cause surface modification whereby the particulate mixture interacts with the substrate. As a result of the application of such surface finishes to the surface of the substrate, it has been found that the resulting product demonstrates outstanding corrosion resistance as well as long lasting, durable dry lubricity characteristics. Furthermore, the surface finishes have been found to provide a relatively thin, impermeable, surface hardened exterior on the surface of the substrate or work piece.

These surface finishes have been found to be sufficiently thin so that the finishes do not interfere with critical tolerances of any processed parts or components.

Accordingly, it is a general object of the present invention to provide new and improved surface finishes for application to substrates and to provide methods of applying such surface finishes to substrates.

Another object is to provide corrosion-resistant surface finishes demonstrating long lasting, durable dry lubricity characteristics as well as providing an impermeable, surface hardened outer surface on a substrate.

A further object is to provide methods for producing corrosion-resistant, long lasting, durable dry lubricant surface finishes on substrates. A further object is to provide a surface finished product having a high degree of permanent dry lubricity. Another object is to provide a metal surface exhibiting long lasting, durable dry lubricity and high resistance to temperature extremes. A still further object is to provide methods for producing thin surface finishes which exhibit long lasting, durable dry lubricity; corrosion and heat resistance as well as improved thin film entrapment (or retention) properties. Yet another object is to provide methods for relatively easy and inexpensive application of the surface finishes of this invention to substrate surfaces.

Other objects of this invention, in addition to those set forth above, will become apparent to one of ordinary skill in the art from the following description.

BRIEF DESCRIPTION OF THE DRAWING

Fig. 1 is a schematic flow diagram illustrating the method of the present invention employed to apply a dry lubricating, corrosion-resistant finish to the surface of a substrate.

DETAILED DESCRIPTION

The drawing is a schematic flow diagram showing an embodiment of the methods of the present invention for applying surface finishes to a substrate.

In the embodiment of this invention depicted in Fig. 1, a multistep process is illustrated wherein a substrate surface is first subjected to an optional solvent precleaning step in order to remove any loose surface contamination such as hydrocarbons and other physical and chemical debris from the substrate prior to further processing. This precleaning step is employed in order to reduce contamination which may be encountered and which may thereby interfere with the blast application of the surface finish onto the substrate.

The appropriate solvent to be used for this precleaning is somewhat substrate specific. For example, very dirty, greasy substrates will require a Stoddard solvent to be employed to clean the substrate surface. For substrate surfaces which are non-degassing, such as chrome/molybdenum or stainless steel, 1,1,1 trichloroethane or equivalent solvent may be employed in an ultrasonic cleaning procedure. For degassing substrates, a Branson IS solvent is employed in an ultrasonic cleaning procedure.

In a specific solvent precleaning process employed in the laboratory, a metallic substrate was brush scrubbed in Stoddard solvent with Hurri-Safe Special Formula Degreaser

at a 1:4 dilution in a Hurri-Kleen cold part washing machine and the substrate was then air dried. Thereafter, the material was cleaned in either Branson IS Formulated Cleaning Solution, 1:10 dilution or 1,1,1 trichloroethane precleaner (sold by Brownells) utilizing an indirect method in a Branson 8200 Ultrasonic Cleaner filled with Branson IS Formulated Cleaning Solution, 1:10 dilution. Cleaning time was about 15 minutes at 40° C.

As illustrated in the drawing, after completion of the solvent precleaning step, the substrate is then subjected to an abrasive cleaning/surface disruption step to create a sufficient and appropriate amount of disrupted surface area on the surface of the substrate in order to interact with the surface finish material to be applied thereafter. In this abrasive cleaning step, any oxidation or contamination from the substrate material which was not removed in the precleaning. step is removed.

This abrasive cleaning/surface disruption step may be performed in a blast cabinet environment in accordance with the procedures disclosed for precleaning in U.S. Patent No. 4,753,094 (the disclosure of which is incorporated herein by reference) . The specific parameters of treatment within this step of the process are subject to choice, depending on the substrate material and its intended end use. For example, the delivery pressure/velocity, temperature, angle of delivery, duration of blasting and like parameters of the process are subject to choice and will vary depending on whether final treatment of the substrate is intended to increase dry lubricity, wear resistance, quick release (i.e., non-sticking effect) , operative temperature range and/or corrosion resistance.

In regard to the blast materials to be used for this abrasive cleaning/surface disruption step, it has been found that for softer, nonferrous metals and alloys, (e.g., aluminum, copper, lead, magnesium, zinc, beryllium, gold, tin, bronze, brass, etc.); glass beads, nylon or plastic particles or aluminum shot may be employed for blast cleaning the surface of the substrate. For harder, nonferrous metals (e.g., nickel) and for ferrous metals and alloys, (e.g., iron, molybdenum, chromium, tungsten, vanadium, steels and stainless steel) aluminum oxide particles, silicon carbide particles, glass beads, sand particles, steel shot and the like may be used to provide the peening action in cleansing the surface of the substrate. In this regard, it has been found that less aggressive media (e.g., glass beads) may be used for applications where a characteristic such as quick release or non-sticking is desired, while more aggressive media such as aluminum oxide or silicon carbide are preferred for use in applications where end product characteristics such as increased wear resistance or dry lubricity are desired.

In regard to the delivery pressures to be employed for performing this abrasive cleaning step, it is believed that pressures up to 250 psi may be employed for hard and very hard substrates such as chrome/molybdenum steels and tungsten carbides, whereas lower delivery pressures of as low as about 20 psi may be used in other applications. As employed herein, the term "delivery pressure" is defined as the blast pressure applied to a substrate at a distance of two inches from the nozzle of the delivery device. The temperature range to be employed in performing this abrasive cleaning step appears to be a matter of selection and not to be determinative of the quality of

the surface treatment achieved. However, it has been found that temperatures ranging between ambient temperatures and about 50° C are suitable for this cleaning step. In specific abrasive cleaning/surface disruption processes employed in the laboratory, substrates which were to be cleaned/disrupted with aluminum oxide (extra fine grade-Brownells) utilized a Techni Blast Model 36 Cleaning Machine, sold under the trademark "SURFGARD" at 58 cubic feet per minute at 100 pounds pressure. This cleaning machine was equipped with a 3/16 inch blast gun with a ceramic nozzle. Alternatively, substrates which were to be cleaned/disrupted with glass beads (#270 U.S. Sieve Size-Brownells) were blasted utilizing a Trinco Direct Pressure Cabinet Model 36X30/PC equipped with a 1/4 inch nozzle I.D. and the substrate was blasted at 60-120 psi (preferably about 80-100 psi) at a distance of between about 2 inches and 12 inches (preferably about 6-8 inches) at an angle of about 20° - 90° (preferably about 30° - 60°) until a uniformly disrupted surface was obtained and all surface contamination was removed.

In step three illustrated in the drawing, the substrate is air cleaned with dry, compressed air to remove any residual cleaning/disrupting media thereby avoiding any possible cross-contamination with different media.

Once the preliminary cleaning steps one, two and three are completed, the substrate is then in condition to be processed in accordance with the present invention. In accordance with the present invention, a pressurized stream of a particulate mixture of a sulfur containing metallic compound and a fluorocarbon polymer is directed in a pressurized stream to impact against the

surface of a substrate at a sufficient pressure and for a sufficient period of time to cause the particulate mixture to interact with the substrate and to provide a surprisingly thin, impermeable, surface hardened, corrosion-resistant, durable, dry lubricant finish on the surface of the substrate.

In practice, the substrate surface to be treated is preferably a metallic surface. However, as previously noted herein, the substrate may be any suitable ferrous or nonferrous metal or alloy of a metal or a ceramic composition.

In order to expedite the impacting or peening of the particulate matter against the surface of the substrate, it has been found that suitable peening media having suitable shot sizes should be employed for purposes of conveying the mixture to the previously disrupted surface of the substrate. Another purpose of the peening media in addition to providing a carrier for the surface finish particulate material is to surface harden the substrate through the peening process. A suitable peening medium for purposes of use in the present process is chosen as a function of its compatibility with the substrate and its affinity for the particulate surface finish material which it is carrying. In addition, the size and hardness of the peening media have been found to influence the effective transfer of the surface finish material to the cleansed, disrupted substrate surface. In this regard, we have found that shot sizes ranging from SAE Size No. S70 to about S780 (preferably about Size No. S70 and S230; most preferably about Size S170) may suitably be employed in the processes of this invention. In particular, we have found that with softer metal substrates (such as those on the Rockwell B

scale or on the Rockwell C scale ratings of 40 and below) , use of larger size shot (such as about SAE Size No. 170 and above) is preferred in order to achieve maximum surface finish coverage. Also, we have found that with harder metals (Rockwell C scale ratings of 40 and above) , such large shot (i.e., SAE No. S170 and above) is likewise preferred for purposes of achieving continuous surface coverage. However, it is to be noted that smaller size shot may also be employed in certain applications to avoid surface asperities.

Examples of suitable peening media which may be used herein are steel shot, stainless steel shot, aluminum shot, plastic shot and the like having sufficient structural integrity to withstand impact on the substrate surface.

In general, the surface finish composition of this invention is a particulate mixture of solid lubricants formulated to provide dry lubrication and/or corrosion resistance and/or non-stick properties desired for purposes of the end use of the product. Suitable solid lubricants for use in the particulate mixtures of the present invention include fluorocarbon polymers and carrier or binder polymers.

Exemplary of suitable fluorocarbon polymers are homogenates or mixtures of finely-divided fluorocarbon resins having fully fluorinated carbon backbones such as tetrafluoroethylene homopolymer (TFE) , hexafluoropropylene (HFP) , perfluoroalkoxyvinyl ether (PPVE) , copolymers of TFE and HFP, copolymers of TFE and PPVE. Other suitable fluorocarbon polymers are fluoropolymer resins which are not fully fluorinated such as ethylenetetrafluoroethylene (ETFE) , polyvinylidene fluoride (PVDF) , ethylene- chlorotrifluoroethylene (ECTFE) , copolymers of ethylene

and TFE such as products sold under the trademark "Tefzel" by E.I. Du Pont de Nemours & Co. (Inc.). The molecular weight of the fluorocarbon polymers to be used herein may vary over a relatively wide range although molecular weights of from about 800 to about 2000 are preferred and, particularly about 1000-1800. Furthermore, it should be noted that mixtures of fluorocarbon polymers of varying molecular weights may be advantageously employed herein as, for example, mixtures of tetrafluoroethylenes having molecular weights of 1100 and 1300.

In summary, the fluorocarbon polymers are chosen for their ability to impart their individual characteristics to the substrate and for their affinity to the substrate, the peening media employed, and/or the other solid lubricant material chosen. Furthermore, suitable fluorocarbon polymers for use herein are impermeable and chemically unreactive to water and other solids, UV radiation and gases. The polymers are highly thermally stable and will withstand high upper surface temperatures (i.e., about 204° C - 260° C) as a result of their high C-F and C-C bond strengths and the resulting non-polar nature of the linear polymer. These resins have a low coefficient of friction and a low dielectric constant and dissipation factor. They exhibit a high degree of linear flexibility and are flame resistant.

The other solid lubricant component of the particulate mixture employed herein is a sulfur containing metallic compound which acts as a carrier or binder molecule herein. Suitable metal sulfides for purposes of the present invention possess anti-friction/dry lubrication capabilities, can withstand increased operating temperatures and/or demonstrate high affinity towards metals such as those employed as the substrates herein or

the peening media utilized herein as well as demonstrating high affinity toward the fluorocarbon polymers selected as part of the surface treatment mixture.

Representative of suitable sulfur containing metallic compounds for use herein are sulfides of molybdenum, tungsten, lead, tin, copper, calcium, titanium, zinc, chromium, iron, antimony, bismuth, silver, cadmium and alloys and mixtures thereof.

In a preferred form, molybdenum disulfide is employed as the sulfur containing metal compound in the particulate mixtures employed. Molybdenum disulfide has a high affinity to steel and other base metals and has the ability to increase surface hardness, corrosion resistance, elevated temperature strength and dry lubricity. It also has a high affinity to fluorocarbon micropowders which may be employed advantageously herein. Thus, it has been found that use of molybdenum disulfide herein provides the dual function of a dry lubricant additive as well as a carrier/binder molecule for the fluorocarbon polymer to promote coating of the peening media.

In general, the amount of fluorocarbon polymer to be incorporated in the particulate mixture to provide the requisite surface finish is determined by the amount of such polymer required to saturate the carrier or binder molecule such as molybdenum disulfide. The total amount of the particulate mixture to be employed for applying the surface finish to the substrate via a peening action in a blast cabinet is determined by the amount of material required to keep the peening medium completely coated during the blasting operation in the cabinet.

In a laboratory example of the practice of the present invention, a Techni Blast Model 36 SURFGARD Peen Plating

Machine, 70 cubic feet per minute at 100 pounds pressure, 3/16 inch Suction Blast Gun with Ceramic Nozzle was employed for directing the particulate mixture against the surface of a substrate in a blast cabinet. The cabinet was loaded with 500 ml. (by volume) molybdenum disulfide (Super Fine Grade, Lot #510DS, Climax Molybdenum Co.); 500 ml. (by volume) tetrafluoroethylene having a molecular weight of about 1100 (Teflon Fluoroadditive Type MP1100, Lot #BMAB40D002, Du Pont); 500 ml. (by volume) tetrafluoroethylene having a molecular weight of about 1300 (Teflon Fluoroadditive Type MP1300, Lot #68-86, Du Pont) and 200 pounds of S70 steel shot (Techni Blast) . The blast cabinet temperature was maintained at about 50° C and the delivery pressure at the nozzle of the peen plating machine was 80 psi. The particulate mixture with the peening medium was blasted at a 45° angle at a distance of about 6-8 inches until a uniform, void-free surface treatment had been achieved.

After completion of step four in the drawing wherein the finish surface is applied to the substrate in accordance with the method of the present invention, it has been found that the resulting product may advantageously be subjected to a post-treatment cleaning and preservation step (step five in the drawing) . In this step of the process, the substrates having the inventive surface finish applied therein are cleaned with dry, compressed air to remove any residual surface treatment particles. Thereafter, the substrate is washed with a cleaning solution and preserved with an oil that is compatible with the end use of the material, if so desired. In a preferred embodiment of the present invention, a surface finish is produced in the surface of a two inch by two inch square, 1/4 inch thick chrome/molybdenum steel

sample. The hardness of the chrome/molybdenum steel sample was 53 as measured on the Rockwell C scale. In the process, after subjecting the steel sample to appropriate solvent precleaning, the sample was subjected to an abrasive cleaning/surface disruption step in a cabinet wherein aluminum oxide shot was impacted onto the steel surface at 60 psi at an angle of about 45° under ambient temperature conditions.

Thereafter, the sample was introduced into a blast cabinet and a Techni Blast Model 36 SURFGARD Peen Plating Machine having a 3/16 inch Suction Blast Gun with Ceramic Nozzle was employed to direct a particulate mixture against the surface of this sample. The particulate mixture was prepared by mixing 22 ounces (by weight) tetrafluoroethylene having a molecular weight of about

1500 (Teflon Fluoroadditive Type MP1500J, Lot #999999) in a container with 100 lbs. of SAE No. S170 steel shot. Then, an additional 14.5 ounces (by weight) of tetrafluoroethylene (MP1500J) was admixed with the steel shot in the same container. In a separate container, 30 ounces (by weight) of molybdenum disulfide (Super Fine Grade, Lot #510DS, Climax Molybdenum Co.) was mixed with 100 lbs. of SAE No. S170 steel shot.

The contents of the two containers were then mixed together and an additional 24.3 ounces (by weight) of tetrafluoroethylene (MP1500J) was added to the mixture. The combined mixture contained 60.8 ounces (by weight) tetrafluoroethylene, approximately 30 ounces (by weight) molybdenum disulfide and about 200 lbs. SAE No. S170 steel shot. The resulting combined mixture contained a ratio by weight of tetrafluoroethylene to molybdenum disulfide of about 2:1.

The blast cabinet temperature was maintained at about 50° C and the delivery pressure at the nozzle of the peen plating machine was 80 psi. The steel shot peening medium having the particulate mixture intimately coated on the surface of the shot was blasted onto the precleaned, disrupted surface of the sample at an angle of about 45° at a distance of about 4 inches for a period of about 15 seconds to form a uniform, void-free surface on the surface of the chrome/molybdenum steel sample. Subsequent to the blast treatment, the sample was subjected to a post-treatment cleaning step by subjecting the sample to Stoddard solvent in a Hurri-Kleen Station. This cleaning step was followed by subsequent cleaning of the resulting product in 1,1,1 - trichloroethylene in a 1000 ml. beaker and the resulting cleaned surface finish product was subjected to air drying before evaluation.

The resulting product was found to have a nonabradable, nonetchable surface which was durable, corrosion resistant and demonstrated dry lubricity and exceptional wet film entrapment characteristics.

Thus, a method has been described herein for producing a surface finish on a substrate in a manner such that the resulting product exhibits a wide range of benefits otherwise unavailable. The surface finished product demonstrates permanent dry lubricity and is highly resistant to temperature extremes. Furthermore, the surface finished product provides a natural barrier to normal oxidation and corrosion since it is chemically inert. In addition, the finish in the treated substrate surface exhibits exceptional durability and is extremely thin, being measured as low as about 0.5 micron thickness as opposed to prior art coatings wherein the coat is measured in mils such as the industry standard electroless

nickel coatings which have a thickness of 3/8 mil when submerged in nickel plating solution for 45 minutes at 90.5° C. Still further, the surface finishes of the present invention are applied relatively easily even at relatively low temperatures and inexpensively in order to provide the desired surface modification herein.

The products produced in accordance with this invention have a multiplicity of uses in a variety of industries and in products containing metal on metal friction points or which are subject to metal surface corrosion. Exemplary of the scope of the utilization of the present invention are applications within the automotive industry, fuel handling systems, power tools and equipment, fasteners, ball bearings, rollers and other anti-friction components, consumer products including cookware, houseware and razor blades, turbines, gears and other intermeshing machinery as well as a variety of other potential uses.

Although the invention has been described in its preferred form with a certain degree of particularity, it is to be understood that the present disclosure has been made by way of example only. Numerous changes in the details and operational steps of the methods and in the materials utilized therein will be apparent without departing from the spirit and scope of the invention, as defined in the appended claims.