EXTREME PRESSURE LUBRICANTS FILM-FORMING ACTION OF' LEADNAPHTHENATEANDFREESULFUR G. L. SIMARD, H. W. RUSSELL, AND € R. I.NELSON Battelle Memorial Institute, Columbus, Ohia
HE ability of an oil to eral the fundamentalmechsnism Lead naphthenate plus free sulfur oils on withstand high loada and of their action remained relametal surfaoes tend to form lead sulfate prevent galling or seizure tively obscure. under mild treatments (low temperatures, of bearing surfaces depends upon One purpose of the present short heating periods, or low loads) and investigation is to contribute to propertiea which are not well lead sulfide under more severe conditions. understood. It is generally the meager knowledge of E. P. lubrication hy a fundamental conceded that the essential reSurfaces of hypoid gears operated 20,000 study of a powerful E. P. lubriquirement is to produce and miles exclusively in this lubricant eonsieted maintain a film which will precant. A s e c o n d p u r p o s e i s t o mainly of iron oxide and a p h believed to vent intimate contact and condetermine t o what extent deebe a lead-rich form of lead sulfide. sequent welding of the metal tron diffraction methods are Lead naphthenate oils produced an surfaces. Although experiment applicable to the study of luhrihaa shown that oiliness and d naphthenate cation. The I amorphous 61m on metals of either adfilm strength can be enhanced plue free sulfur hypoid lubrisorbed lead naphthenate or of an oxidid by the addition of chemical subcant was the, one selected for product of lead naphthenate. Iron surfaoes s t a n c e to oils, the diversity study. treated with free sulfur oils formed a superDirsculty in deducing the of such additions m a k possible ficial 6lm of iron oxides. action of this lubricant on gear but few generalizations concerning the physical and chemisurfacas waa anticipated beThe reaulta are discussed in the light of c a w of the complexity of the caI nature of the films they recent theories of boundary lubrication. form. Chemically active adchemical system and the variability of uhwical conditions ditions are believed to react among themselves and the bearing surfaces to form adherent during Bervioe operation. In addiiion, &-extreme smoothness of the bearing surface aftm prolonged service d o & films. Lees chemically active but highly polar additions are but little chance of obtaining well-reaolved diffraction patbelieved to form adsorbed molecular layera a t the oil-metal boundary, whose properties are determined both by the terns. Accordingly, the film-forming p r o p t i e s of the lubripolarity and internal structure of the molecules. cant were studied under controlled laboratory conditions as well as under service conditions. Further,the oil additions Surface analsgis by electron diffraction is a promising method for studying such films. This method has found conwere studied singly and in combinations in mveral oil m e siderable use in other lields of surface chemistry, as in studies d i w . Aside from iron and steel with which E. P. lubrica01 oxidation and corrosion of surface, but has not been extion is concerned, brief studies were also made on copper, bronze, tin, and Babbitt metal for the chemical data which tensively applied to lubrication problems. Several inveatimight be obtained. gations have been made of molecular orientation of orgmic matmiale on metal surface, but it appears likely that many of the organic 6lma studied were too thick to permit penetraPreparation of Surfacea tion of the electron heam to the oil-metal interface. In such cases the resulta give little information about molecular The method of pbring surfaces for electron,diEr~ti011 ponsiated of heating g m e t a l specunenn in the lubricant or subject adsorption. This conclusion is supported by the work of ing them to frict:on under varioue loads, after which oil waa mGarmer (3) who showed that the molecular orientatiofi of moved by o c solvents. surfacehwsrs stearic acid molecules due to adsorption doea not extend then e x n m i n S v dectmn A 2 S n ? Z " & e meultitu d i E m beyond the f t s t molecular layer, and that when several laytion patterns d y d m tbe,wal manner. Heat treatments were camed out at controlled t e m p t u m ers are praeent on polisbed surfaces, a diffracted beam of elec*6' C.) i~? a small tubnlar electric furnace. The ycnnena, trona w i l l indicate the molecular orientation of o d y the top Litid pohshed on m o u e grades of emery paper, own to a most two or three l a y m . Bee&, Givens, and Smith (I) re light , L i o n on NO. 4 0, were entirely immersedm ofi contained oently made use of electron diffraction in a study of the fricinasmallcrucibleandLtedforapRdetermitime., menta c+d out in this manner are referred.to 811 Statac tion behavior of long-chain polar compounds. menta. f i c t i o n treatmen* were -4 out m a wear m 3 E In the present work chemically formed f i l m s - U formed for low loads and on an 8. A. E. lubricant tester for h i g h lo&. by the chemically active extreme-pressure (E.P.) additionsThe wear maohine (Figure 1) consieted of a rotat+ steel dish, 10 mchea (26.4 cm.)in diametex, on which +e ep?ur?en# in&have been studied. ted we^ held under fixed load by a c a n t h e r &t system. Some yeam ago when an immediate need was created for rotasteel dkk WBB lubricat+ by the wi& ,oiler a t the E. P. lubricants, an empirical search for e5ective additions bottom o f t e glass tube. The specunea wly held m the 4 to oils was the most direct and fruitful method of eatisrying nut in back of the wick oiler and wm loaded through the hea the demand. S u c d u l lubricants were found, hut in genvertical bar by m e m of a lever and weight system (not s h o w x
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1352
November, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
1353
Its sulfur content is 0.03-0.04 per cent. The sulfur used was Merck's U. S. P. grade. As hypoid lubrication is chiefly concerned with steel surfaces, iron and steel were the metals chosen for study. A number of experiments were conducted, however, on copper, bronze, tin, and Babbitt metal for what chemical data they might offer. The metals were: Iron
Ingot iron of high purity 5. A. E. lubricant test rings (Timken rings)
Copper Bronze Tin
Bus-bar copper
Steels
Babbitt
1940 Chevrolet hypoid gear steel 10% Sn; 10% Pb; 80% Cu
c. P. quality
3.06% Cu, 88.43% Sn, 0.24% Pb, 7.08% Sb
Diffraction Apparatus The electron diffraction camera (Figure 2 is a modified Thomson and Fraser (12)apparatus. A hot cat)hode was substituted for the discharge tube, and a plate magazine was provided. The plate magazine was entirely evacuated, interchan e ofplate position being accomplished by external controls. ipecimens were generally used in sizes of 1.3 X 1.3 X 0.7cm. except in cases where S. A. E. test rings or gear teeth were employed. A springactuated multiple-specimen holder allowed four of the smaller specimens to be examined at one time without the necessity of o ening the camera after each exposure. Accelerating voltages o f 30-40 kv. were provided by a transformer, rectifier, and filter USEDFOR FRICTION TREATMENTS system. Electron wave lengths were obtained by a wave-length FIGURE1. WBARMACHINE calibration of the primary voltage, using gold and aluminum .4T LOW LOADS diffraction photographs. The razing incidence or reflection method of diffraction was empkyed with a specimen-to-plate The temperature of the specimen was measured by a thermocouple whose leads appear a t the left. The S. A. E. machine was the standard type used for routine testing of hypoid gear lubricants. Specimens for the study of actual service operation were taken from two separate sets of hypoid gears run for 20,000 miles exclusively in a commercial lead naphthenate plus free sulfur lubricant. After being subjected to the desired treatment, all specimens were washed successively in c. P. benzene and c. P. petroleum ether (boiling at 30' t o 60" C.) t o remove every trace of residual oil. By virtue of this procedure, the investigation was limited to surface films insoluble in benzene and petroleum ether.
TABLEI. PROPERTIES OF BASEOILS OILS OF Type A n-Cetane Specific gravity a t 20' C. Refractive index a t 20" C. Mol. weight (cryoscopic) Boiling point at0760 mm., C. Freezing point, C. Viscosity a t 100' F., centistokes Formula Sulfur, %
Materials Studies were carried out using as addition agents lead naphthenate and sulfur, singly and in combination. The free sulfur and free sulfur plus lead naphthenate blends in various base oils were made up on a weight basis. The required amount of sulfur was added slowly to the base oil while heating to a temperature not exceeding 90" C. with continuous stirring until complete dissolution had been effected. I n cases where lead naphthenate was to be present, it was dissolved in the base oil after the sulfur had been added and the temperature greatly reduced. This method of preparation yields both free and combined sulfur. Sulfur concentrations given in this paper are total sulfur. Two types of base oil (Table I) were used: (A) oils of definite composition and physical properties, and (B) oils of less definite composition but of known physicd. properties. The lead naphthenate used was Habhaw's No. 40A. It is a commercial product containing 40 per cent lead which is believed t o be combined with naphthenic acids (average molecular weight 210-220) to form compounds of the following type :
grams
Naphthenes Paraffins Viscosity a t looo F centistokes Over point a t IO m&., 0 C. Dry point a t 10 mm O C. Formula: 25 C ato&mol.. one naphthene ring/mol.; type formula, CnHzn -O.M
OILS OF TYPBB Polarex Continental oil (solvent-refined) A. P. I. gravity Viscosity a t 210' F.. Saybolt Universal sec. Pour point (max.). ' C. Color A. 8. T.M. Flash point, open cup ( m i d ' C. Fire point, open cup (av.), O'C. Hypoid stock oil (blend of unrefined fractions from naphthenic crudes) A. P. I. gravity V+osity a t looo F., Saybolt Universal sec. Viscosity a t 210° F., Saybolt Universal sec. Pour point (mas.). ' C. FlaJ point (min.) ' C. F i e point (av.), 0%. Commercial hypoid oil (commercial oil consisting of lead naphthenate free sulfur in hypoid stock oil)
+
0.7739 1.4355 226 285.3 -8.5 3.09
CioHu
