US3158495A - Solid lubricant coatings and coating compositions - Google Patents

Description

United States Patent 3,158,45 S9113) LUBRICANT CGA'HNGS AND COATING CQMPOSITIONS Sylvester F. Murray, Schenectady, and Marshall B. Peterson, Ballston Lake, N.Y., assiguors to General Electric Company, a corporation of New York No Drawing. Filed Sept. 30, 1960, Ser. No. 59,492 1 Claim. (Cl. 11746) This invention pertains to solid lubricant coatings which are efiective to reduce the coefiicient of friction between sliding surfaces at temperatures up to about 1200 F. More particularly, this invention pertains to vitreous lead oxide-silica lubricant coatings, the coating composition from which the solid vitreous coatings are obtained, the method of preparation and application of these compositions and the use of the solid vitreous coatings at the elevated temperatures indicated.

It is known to mix lead oxide and silica in solid form and to subsequently fuse a coating of the mixture to form a solid lubricant coating which has been found'useful to reduce the coefiicient of friction between sliding surfaces at temperatures up to about 1200 F. The known coat ings have been bonded generally to low carbon and stainless steel parts and have been found generally effective to reduce friction and surface damage of the coated parts to the elevated temperatures. The method for bonding of the known coatings to a metallic substrate comprises first pre-oxidation of the metal substrate in air with heat until a thin yellow and blue oxide film has been formed, thereafter applying the dry powdered coating mixture, and finally firing the coated substrate at temperatures of approximately 1650 F. to form the vitreous coating. Although the coatings described satisfactorily reduce the coetficient of friction between sliding metal parts at elevated temperatures, many problems are encountered with the application of these solid compositions to the substrate.

Utilizing the known process above described, it has been found that the pre-oxidation step for preparation of the metal substrates is one of extreme criticality. More specifically, where the pre-oxidation of the metal substrate was insufficient it was found that subsequently applied coatings from a solid mixture of the oxides would not adhere to the oxidized surface. On the other hand, if the metal substrate had been overly oxidized, the final vitreous lubricant coating would thereafter spall at the elevated temperature for use of these coatings. Another important problem associated with the application of a dry powdered mixture to a surface is the difi'iculty encountered With the deposition of the mixture to other than horizontal surfaces. For example, it is often desirable to coat certain circular bearing surfaces with the vitreous lubricant coatings. Great difiiculty is experienced with the initial deposition of the powdered mixture of oxides by reason of the poor adhesive characteristics thereof. The addition of adhesive agents indis criminately to the powdered mixture generally results in only contaminating the subsequently produced vitreous coating and also increases the cost of the coating com-' position.

It has been discovered by the applicants that certain liquid coating compositions can be prepared which eliminate or greatly minimize all of the above outlined problems. These liquid coating compositions comprise a mixture of a lead compound and a sufiicient quantity of a liquid polysiloxane adhesive to yield the vitreous coating at the elevated temperatures employed to fire the coating. At these firing temperatures the lead compound and polysiloxane components of the coating composition are decomposed to form lead oxide and silica respectively and these products thereafter produce the vitreous lubriice cant composition. It has been found that the polysiloxane component in the composition not only provides an adherent coating so that the coated specimen can be han died and even stored without difficulty before firing, but also surprisingly inhibits oxidation of the metal substrate during the firing. The prevention of excessive oxidation to the metal substrate in turn improves the adherence of the vitreous lubricant coating on the final product. The invention may be practiced in its preferred embodiment as illustrated in the following examples and subsequent discussion thereon, but it is not limited thereto. Where parts and percentages appear hereinafter in the specification and claims, they are parts and percentages by weight unless otherwise specified.

Example 1 A stainless steel journal bearing was first cleaned by immersion for approximately 10 minutes in a hot concentrated liquid solution of an alkaline phosphate and thereafter rinsed with Water and dried. The bearing was next pre-oxidized to a light yellow color by heating in air to a temperature of approximately 1615 F. for approximately one minute. The bearing surface was next painted manuallywith an approximately two mill thick film of a mixture comprising grams of lead oxide with 15 grams of the commercial mixture of a 50% solid solution of methylphenylpolysiloxane in xylene, which polysiloxane contained an average of 1.5-1.7 combined methyl and phenyl groups per silicon atom. The coating was thereafter baked for two hours at approximately 500 F. to cure the polysiloxane and remove the solvent. Next, the coated journal hearing was then placed in a furnace heated to approximately 1650 F. for from two to three minutes, whereupon the cured coating had cornpletely decomposed'to form a vitreous lead oxide-silica coating. The fired bearing was removed from the furmace and cooled rapidly to room temperature. a

In order to measure the effectiveness of the vitreous lead oxide-silica coating as a solid lubricant for the bearing, friction tests were conducted on the coated bearing using apparatus especially designed for the purpose. The means by which friction was measured with this apparatus consisted essentially of measuring the amount of friction between the end of a rod being rubbed against the coating surface in a planar manner. More specifically, friction was measured by reciprocating the rod against the coated surface in a direction perpendicular to the circular periphery of the bearing. The rod specimen was mounted in a lever arm, so mounted that the free end of the arm was able to move either in a vertical or horizontal plane. Various loadings were obtained by fastening dead weights to the free end of the arm. Sliding was obtained between the rod and the samples by mov ing the loaded end of the arm back and forth manually and the friction generated therefrom was measured in the conventional manner. Heat was supplied to the apparatus in order to measure the effectiveness of the vitreous lead oxide-silica solid lubricantat elevated temperatures by means of resistance heaters which surrounded the'coated hearing. A refractory cover also surrounded the coated bearing in order to minimize heat loss and to insure uniform heating conditions. The temperature was measured by a thermocouplewhich was spring loaded against the coated hearing. The coefiicient of friction for theabove coating measured at a temperature of 1100 F. and a load of 17,000 psi. on the arm was approximately 0.15. In contrast thereto, the coefficient of friction for a lead oxide-silica'coating prepared from a solid mixture of the oxides in the same proportion as the example ranged from 0.125 to 0.20.

As'a further test of the effectiveness of the present coatings at elevated temperatures, the endurance life of the coating was determined using the above apparatus. The endurance test was conducted by moving the loaded arm back and forth with an air cylinder for a specific number of cycles. The coefiicient of friction for the coating of the example after 1,000 cycles at a velocity of travel of the rod of approximately 3.7 feet per minute was found unchanged.

Example 2 A low carbon steel bearing coated according to the method of Example 1 except that for the liquid coating composition employed therein, there was substituted a mixture comprising 95 parts lead oxide with 30 parts of the organic polysiloxane solution used in that example. The coeflicient of friction measurements made upon the solid vitreous coating obtained by curing and firing the polysiloxane coating in the manner hereinbefore described gave results substantially comparable to the results obtained with the coating of Example 1.

Example 3 To still further illustrate the use of compositions within the contemplation of the invention, a stainless steel bearing was coated according to the method of Example 1 except that for the liquid coating composition employed therein, there was substituted a mixture comprising 95 parts lead oxide with six parts of the organic polysiloxane solution used in that example. The coefficient of friction for the vitreous lead oxide-silica coating obtained by curing and firing the polysiloxane coating in the manner hereiubefore described was substantially comparable to the results obtained with the coating of Example 1.

The liquid coating compositions of the present invention are mixtures of a lead compound and a liquid polysiloxane material which reacts at elevated temperatures in air to form a vitreous lead oxide-silica product. These compositions may be characterized as being storage stable at ordinary temperatures and non-reactive with a low carbon steel or stainless steel material. It is not deemed critical to the successful practice of the invention that the lead compound be dissolved in the liquid coating compo sition, and successful results have been achieved with lead oxide suspensions in the liquid polysiloxane adhesive. Further, it is not believed that the solids content of the liquid coating compositions of the invention is critical, since there are many known ways to achieve a final solid coating of desired thickness from a coating composition having a givensolids concentration. For example, if the coating composition has comparatively low solids concentration, it is still possible to obtain comparatively thick films simply by multiple applications of the coating composition. On the other hand, where it is desired to obtain comparatively thin coatings from a high solids coating composition, excess liquid coating canbe removed after application .to the substrate by means of a roller, doctor knife, or other known means.

The ratio of lead compound to liquid silicone adhesive in the coating composition is important in order to obtain the final vitreous coating which constitutes the solid lubricant film of the invention. More specifically, it is necessary that the ratio of lead compound to polysiloxane in the liquid coating be such that, upon firing of the ap plied coating at the temperatures indicated in air, a vitreous coating of lead oxide-silica be formed. In other Words, the polysiloxane should furnish sufficient silica to thereafter combine with the lead oxide formed in such proportions to form a solid vitreous coating. It is further necessary that the proportions of polysiloxane and lead compound be such to form the vitreous coating upon firing in air, which vitreous coating will be a solid at temperatures up to about 1200 F. The need for a solid vitreous coating at elevated temperatures is to maintain the lubricant on the bearing surface during its use and thereby extend the life of the lubricated bearing. It is obvious that a lubricant which would be a liquid at especially at elevated temperatures.

from the bearing surface and thereby increase wear of the bearing. The ratio of lead compound to polysiloxane which has been found'useful to provide a solid vitreous coating can best be characterized as that which will yield from two to 22 parts silica and 98 to 78 parts lead oxide. The preferred ratio of silica to lead oxide in the vitreous coating is two to 10 parts silica and 98 to 90 parts lead oxide for the reason that optimum low friction and long life characteristics of the coated substrates are obtained in this range. 7

The solid vitreous coatings of the invention can best be characterized by uniformly low friction characteristics and low wear of the coated product atelevated temperatures up to about 1200" F. In addition, the coatings impart greater oxidation resistance to the coated surface Although the thickness of the coating has not been found critical to obtain the improved properties described, it is obvious that for use on bearing surfaces under sufficiently heavy load that a maximum thickness will be dictated above which some of the coating can be removed during use. Coatings up to three mils thickness are preferred to obtain improved life of the coated bearing without suffering loss of the coating due to the effect described. On the other hand, improved life of the coated hearing has been obtained with coatings having a thickness of but one mil. It is, therefore, obvious that the optimum thickness of the vitreous coating will be a function of the particular application for the coated surface and will be affected by such considerations as load, temperature, and even velocity of the moving coated surfaces.

The lead compounds which are useful in the practice of the invention can be selected from the class of organic and inorganic compounds of lead which can be. oxidized at elevated temperatures and the presence of a silicone adhesive to form lead monoxide (PbO), the lead monoxide thereaftcr not converting to oxide (Pb O at the elevated temperatures of use. The reason for distinguishing between the various lead oxides is that the latter mentioned oxide has been found to destroy the lubricating properties of the vitreous lubricant coating. Useful lead compounds may be further characterized as those which in the form applied inthe liquid composition will not react deleteriously with the substrate. For example, those lead compounds which are the salts of strong acids such as lead chloride and lead sulphate are believed unsatisfactory by reason of reacting at elevated temperatures to liberate acidic products from the coating compositions which will attack the substrate. Useful lead compounds include lead carbonate, lead silicate, lead formate, and lead naphthenate. Lead oxide is the preferred lead compound by reason of not requiring conversion at elevated temperatures which would result in the formation of by-products, such by-products although perhaps not acting deleteriously with the substrate could still act as a diluent in the final lead oxide-silica composition.

Useful polysiloxane adhesives comprise polysiloxane compounds which are either adhesive liquids at ordinary temperatures or can be dissolved in organic liquid solvents to form liquid adhesive coating compositions. The useful polysiloxane compounds can further be characterized by an absence of volatilization before conversion to silica at the elevated firing temperatures. Suitable polysiloxane adhesives can be selected from the well-known methylsilicone resins, high molecular weight silicone oils, and polysiloxane elastomers. Polysiloxanes having an average 1.2 to 1.7 combined aliphatic and aromatic groups per silicon atom are preferred by reason of being heat curable to form a solid non-tacky film that permits handling of the coated object without need for immediate firingof the coating to 1650 F.

The class of utilizable polysiloxane compounds described above is understandably a broad one and includes alkyl Silicone polymers, aryl silicone polymers, alkyldensation products composed essentially of silicon, oxygen, and at least one methyl group attached directly to silicon. The methyl group or groups may be attached to any or all (preferably to all) of the silicon atoms contained in the molecule. Ordinarily, the polymer will have in its molecule an average of one to approximately two methyl groups per silicon atoms. Typical aikyl silicone polymers also include compositions composed essentially of oxygen atoms and ethyl radicals each bonded to silicon atoms and wherein the average ratio of ethyl radicals per silicon atom is from 0.5 to 1.5. Useful aryl silicone polymers include dipheny-l silicone and silicones which are the reaction product of such alkyl benzene starting materials as toluene, xylenes, mono-, di-, and triethyl benzenes. cone polymers which are useful include halogenated products such as dimethyldichlorosilane, methyltrichlorosilane, methylchlorophenyl silicone, fiuorophenyl silicone, and aroxyalkyl or aroxyaryl silicones such as methyl phenoxy phenyl silicone and di-(phenoxy phenyl) silicone.

It is not intended to limit the coating compositions of the invention to a mixture of a lead compound and a polysiloxane adhesive merely. For example, as has been shown in the above examples, it is preferred that the coating composition contains a sufiicient quantity of an organic liquid solvent to maintain a comparatively low viscosity for the coating composition. One advantage obtained with a low viscosity coating composition is an ability to coat more irregular shapes at ordinary temperatures than is possible with a higher viscosity coating composition. It is also possible to incorporate other inert agents in the coating composition, such as suspending agents, extenders, etc., if desirable, Without altering the nature or" the final vitreous coating. For example, the addition of many organic compounds can be expected to result in the absence of any residue therefrom by reason of the elevated temperatures employed in the firing operation.

Typical substituted products of the sili- 6 It is also notintended to limit the method of obtaining the final vitreous lead oxide-silica coating to the means disclosed in the above examples. For example, it will be obvious that more elevated firing temperatures may be employed to .form the vitreous coating if needed, since it has been shown that the polysiloxane component or" the coating composition is effective to improve the oxidation resistance of the substrate. On the other hand, firing temperatures as low as approximately 1300 F. have been found adequate to produce the vitreous lead oxide-silica coating. Also, it is obviously not intended to limit the means for application of the liquid coating composition to the method of manual painting illustrated in the above examples. Other'known means, including spray, roller, knife, and dip coating, are suitable for the application of the liquid coating composition to the substrate.

It is, therefore, intended to limit the invention only by the scope of the following claim.

What We claim as new and desire to secure by Letters Patent of the United States is:

A method for providing a bearing lubricant to a substrate which comprises coating the substrate with amixture containing polysiloxane adhesive and an oxidizable non-corrosive solid lead compound in ratios yielding 2 parts si ica and 98 parts lead oxide to 22 parts silica and 78 parts lead oxide and heating the coating in air at temperature of at least 1300 F. for a sufficient time to form a vitreous product of lead oxide and silica.

Referenees titted in the file of this patent UNITED STATES PATENTS 2,198,253 Koehring Apr. 23,. 1940 2,593,817 Waggoner Apr. 22, 1952 2,748,030 Silversher May 29, 1956 FOREIGN PATENTS 749,641 Great Britain May 30, 1956 OTHER REFERENCES Johnson et al.: Lubrication Engineering, vol. 15, No. 12 (December 1959) (pp. 487-91 and 496).