Oxygen-containing organic compounds as boundary lubricants for silicon nitride ceramics

Oxygen-containing compounds, particularly compounds wherein the oxygen is present in OH-groups, such as alcohols, sulfonic and carboxylic acids, or metal salts thereof, serve as lubricants for ceramic materials, particularly silicon nitride materials under high stress and high load conditions.

BACKGROUND OF THE INVENTION
 This invention relates to a method of lubricating silicon nitride ceramics
 and the like.
 Advanced ceramics offer great potential for future engineering
 applications. Their unique blend of strength, wear and corrosion
 resistance and light weight enables technologies that were not possible
 otherwise. Technologies such as low heat rejection engines, advanced gas
 turbines, environmentally compatible fuel efficient diesel engines, and
 space structures all depend on the availability of such advanced
 materials. The brittleness of ceramics, however, causes concern in terms
 of reliability and durability, especially since effective reliable
 lubrication technology of ceramics does not currently exist. This limits
 the load carrying capacity of the ceramics and durability of the ceramic
 component.
 Silicon nitride, e.g., Si.sub.3 N.sub.4, is the most promising ceramic for
 future applications in bearings, tools and engine components. The ability
 to lubricate these materials under high stress (boundary) conditions will
 become critical as the technology continues to mature.
 In general, conventional lubricants rely on special chemical compounds to
 reduce friction and wear under high stress (boundary) conditions. One of
 the key concepts in boundary lubrication is that chemical reactions occur
 with the surface to produce a protective film. This is described in an
 article by S. M. Hsu, "Boundary Lubrication of Materials," MRS Bulletin,
 October 1991, pp. 54-58.
 Many of the concepts of boundary lubrication on metal surfaces rely on
 chemical reaction with the metal surface and/or catalysis by the metal.
 This is easily understood because most metal system are chemically
 reactive. Current lubricants for iron-based systems are based on the
 reactions between P, S and Cl with iron. The resultant protective boundary
 lubricating films are rich in iron-organo-metallic compounds containing P,
 S and Cl.
 Ceramic materials, however, contain substantially no iron (less than 0.2%).
 Further, ceramic materials are generally not considered to be reactive,
 relative to iron-based systems. In fact, ceramics are used in many
 applications because they are considered to be chemically inert and are
 thus, useful in high temperature and corrosive environments. Consequently,
 iron chemistry with conventional antiwear compounds would not be expected
 to be applicable to boundary layer lubrication of ceramic materials, such
 as silicon nitride.
 Furthermore, one of the key roles of boundary lubricants in metal systems
 is to serve as a barrier film separating the two surfaces to prevent
 adhesion--a dominant wear mechanism in metal systems. Ceramic systems, on
 the other hand, are dominated by brittle fracture as a dominant wear
 mechanism. The mechanical strength, surface morphology, and film thickness
 for ceramics therefore are different from metal systems. To minimize
 fracture in ceramics, the dominant role of the lubricating films is to
 redistribute the asperity stresses in a contact. Therefore, the film
 should be thicker, stronger in shear strength, and faster reacting than
 films for metal systems. The distinction in the nature of wear mechanisms
 between metal systems and ceramic systems further supports the expectation
 that conventional concepts of boundary lubrication for metal systems would
 not be applicable to ceramic systems.
 Prior art addressing the issue of Si.sub.3 N.sub.4 lubrication is sparse
 and inconclusive. Willermet used several material pairs (cast iron/cast
 iron, steel/steel, Si.sub.3 N.sub.4) and a pair of formulated lubricants
 to show that an oil soluble molybdenum containing compound reduced
 friction in an LFW-1 bench test for all material pairs, P. Willermet, "An
 Evaluation of Several Metals and Ceramics in Lubricated Sliding," ASLE
 Transactions, 30, 1, pp. 128-130, 1987. The oils were both SAE 30 with Zn
 dithiophosphate antiwear additive and an overbased detergent package
 containing Mg and boron. No wear data were provided. Klaus used a
 Ball-on-Three-Flat (BTF) wear tester to examine the performance of a SAE
 5W-30 commercial oil on Si.sub.3 N.sub.4 specimens. He found that at 40 kg
 applied load, the formulated oil had lower wear than a straight mineral
 oil. It was not determined which components in the commercial oil were
 responsible for the lower wear, E. E. Klaus et al., "Lubricated Wear of
 Silicon Nitride," Lubrication Engineering, 47, pp. 679-687, 1991.
 As to oxygen-containing compounds, Jahanmir, "Friction and Wear of Silicon
 Nitride Lubricated by Humid Air, Water, Hexadecane and Hexadecane+0.5
 Percent Stearic Acid", STLE Transaction, 31, pp. 32-43 (1988), indicated
 that 0.5% stearic acid in hexadecane under low load reduced the wear of
 Si.sub.3 N.sub.4, however, no experimental evidence was offered to link
 direct chemical reaction with Si.sub.3 N.sub.4. Indeed, Habeeb, I. Mech. E
 C132/87, p. 555-564 (1987), found that 0.4% lauric acid in a 150 neutral
 base oil increased the wear of Si.sub.3 N.sub.4. Tsunai "Tribochemical
 wear of silicon Nitride in Water, n-Alcohols and Their Mixtures", Wear of
 Materials, p. 369-374 (1989) suggested that alcohols might react with
 Si.sub.3 N.sub.4 and SiC for lubrication, however, experiments were
 conducted only at unrealistically low sliding speeds of 0.002 m/s, and the
 chemicals were limited to linear alcohols of carbon number 10 or less. The
 proposed mechanism was based on a reaction study of C.sub.1 to C.sub.3
 alcohols with Si.sub.3 N.sub.4 performed by Hattori, "Reactions of Silicon
 Nitride with Alcohols", the 56th Annual Meeting of Chem. Soc. Japan,
 Abstract, Vol. I, Tokyo, p. 790, April 1988, who did not present clear
 experimental evidence to support his proposal. Tsunai used Hattori's
 speculation on reaction mechanism even though Tsunai was unable to find
 the same reaction products in his wear tests. See also Hibi "Friction and
 Wear of Silicon Nitride in Water, n-Alcohols, Water-Methanol and
 Water-Glycol, "Bulletin of Mechanical Engineering Laboratory", No. 53
 (1990).
 Attention is also directed to Gates et al., "Effect of Selected Chemical
 Compounds on the Lubrication of Silicon Nitride," Tribology Transactions,
 34, 3, pp. 417-425 (1991), which discloses that certain specific compounds
 show a lubrication effect upon silicon nitride.
 SUMMARY OF THE INVENTION
 An object of this invention, therefore, is to provide novel lubricants
 which are effective under boundary lubrication conditions, and a method of
 utilizing such lubricants for the lubrication of silicon nitrides and the
 like.
 Upon further study of the specification and appended claims, further
 objects and advantages of this invention will become apparent to those
 skilled in the art.
 To attain the objects of this invention, there are provided alcohol
 compounds that can serve as effective lubricants for silicon nitrides
 under high stress and high load conditions, commonly referred to as the
 boundary lubrication regime.
 Tests conducted on a ball-on-three-flat wear tester under boundary
 lubrication conditions demonstrate that additions of as little as, for
 example, 1.0% of these materials to a base oil reduced wear by over 90%.
 The lubricants react with the silicon nitride materials to form boundary
 films which provide a lubricating effect. Such lubrication offers
 significant benefits in developing ceramic technologies in which low
 ashing, alternate fuels-compatible lubricants are desired.
 According to one aspect of the invention, there is provided a method of
 lubricating silicon nitrides in the boundary lubrication regime with
 alcohol compounds.
 Preferred are aromatic alcohols of the formula (I):
 ##STR1##
 wherein the R.sub.3 groups independently are hydroxy, alkyl of 1-15 carbons
 or a polyethoxy chain of 1 to 9 ethoxy units, and n is 1 or 2, provided
 that when R.sub.3 is hydroxy n is 1. Particularly preferred are
 octylphenol having a polyethoxy chain of 1,4 or 9 ethoxy units, catechol
 (1,3-dihydroxybenzene) and 3-n-pentadecylphenol.
 Also, polyaromatic alcohols are useful in the invention. For example,
 compounds of the formula (IA):
 ##STR2##
 wherein R.sub.1 is a long chain alkyl group of, for example, 8 to 18 carbon
 atoms, where the aromatic groups are substituted internally and/or at the
 ends of the alkyl group, preferably both aromatic groups being toward the
 same end. Particularly preferred for R.sub.1 is a C.sub.15 -pentadecane
 group, such as 1,8-bis(hydroxyphenyl)pentadecane.
 Also, preferred are alkanols or alkenols of 6 to 18 carbon atoms,
 particularly, linear primary alcohols, such as, for example, 1-hexanol,
 1-octanol, 1-decanol, 1-dodecanol, 1-octadecanol and oleil alcohol.
 A further class of alcohols useful as lubricants according to the invention
 are alkylaryl-polyether alcohols, for example, alcohols, having the
 formula (IIA):
 ##STR3##
 wherein n is 1-18, particularly 1, 5 or 9, and Triton N alcohols of the
 formula (IIB):
 ##STR4##
 where n is 4-15.
 Mannich reaction products, such as, for example, of the formula (VI):
 ##STR5##
 wherein r is from 1-4 and R is an alkyl chain consisting of 3 to 8 carbon
 atoms, are also effective lubricants for silicon nitrides.
 These compounds are effective in admixture with a base oil, for example,
 purified paraffin oil (PPO) or the like. The organic compound is provided
 in the base oil in a lubricating-enhancing amount, the optimum
 concentration with respect to lubrication and cost being determined in
 each case by routine experimentation and calculations, in general, it is
 preferred for the concentration to be in the range of about 0.1 to 20 wt.
 % and preferably in the range of 1-3 wt. %.
 In addition to being useful in minor amounts in base oils, the alcohol
 compounds described above can be used "neat", i.e. in substantially pure
 form, directly as the lubricant for silicon nitride ceramics, or in any
 amount between the minimum suitable to provide a lubricating effect, for
 example, 1% and 100%.
 These alcohol compounds are effective at bulk fluid temperatures in the
 range of 0.degree. C. to their boiling points, preferably at room
 temperature, and in the boundary lubrication regime.

EXPERIMENTAL APATUS AND PROCEDURES
 Wear tests were conducted using a ball-on-three-flat (BTF) modification of
 a four-ball wear tester. This configuration replaces the lower three ball
 specimens with 6.35 mm (0.25 in) diameter, 1.59 mm (0.0625 or 1/16 in)
 thick disks. The advantage of this configuration is that tests can be
 conducted using only one of the relatively difficult to fabricate 12.7 mm
 (0.5 in) diameter balls. Disks are easily made from 6.35 mm diameter rod
 stock of the selected ceramic material using a sequence of diamond
 cutting, grinding, and polishing steps. Specimens were cleaned just prior
 to testing using a sequence of solvents of hexane, acetone, detergent
 ("Micro") in 18 M.OMEGA. deionized (DI) water, and pure DI water. Wear
 tests were conducted at conditions of 60 kg applied load (240 N normal
 load), 0.23 m/s (600 rpm), 30 minute duration, and 21.degree. C.
 Wear was measured at the end of the test using an optical microscope with a
 calibrated graduated reticle. Each of the wear scars were measured
 parallel to and perpendicular to the direction of sliding. The resulting
 six measurements were averaged to give the wear scar diameter for the
 test.
 Friction was continuously monitored throughout each test by a force
 transducer with output to a strip chart recorder and personal computer.
 The value reported for the coefficient of friction is the steady-state
 value obtained at the end of the test.
 Specimens were cleaned just prior to testing by soaking them in solvents
 and ultrasonically agitating them for 10 seconds. The solvent sequence
 used was hexane, acetone, 5% laboratory detergent in deionized water and
 pure 18 M.OMEGA. deionized water. After each ultrasonic soaking, the
 specimens were rinsed twice with the same solvent. After the final water
 rinse, the specimens were dried with nitrogen gas.
 A detailed description of the friction type (I-III) terminology used in
 this paper is described in the Gates et al. article cited above. Type I
 represents poor lubrication. Friction immediately rises to relatively high
 levels. This behavior is associated with high wear. Type II friction
 represents good lubrication. Friction is low and steady throughout the
 test. This behavior is associated with low wear. Type III friction starts
 out low and becomes high during the test. This behavior is associated with
 high wear and is considered to indicate initial good lubrication with a
 transition to poor lubrication some time during the test. This type of
 friction indicates a lubricant that is close to its limit of
 effectiveness. Under slightly lower severity the lubricant may be more
 effective.
 Materials
 The monolithic Si.sub.3 N.sub.4 's used for this study were commercially
 available, fully dense materials known as NC132 and NBD 100. These two
 materials start from the same powder processing step but differ in their
 heat treatment to form the final dense Si.sub.3 N.sub.4. NC132 is hot
 pressed, while NBD100 is HIP'd (Hot Isostatically Pressed). For the
 purpose of these tests, there is no significant difference between the
 wear properties of these two materials.
 Chemical compounds used in these tests were obtained commercially in as
 pure a form as was available. For example, the octanol used for many of
 the wear tests had a purity better than 99.5%.
 The base oil used in this study was a .apprxeq.27 Cst (measured at
 37.7.degree. C. or 100.degree. F.) paraffinic oil purified by percolating
 it through 200 mesh activated alumina. This resulted in a purified
 paraffin oil (PPO) free of polar impurities that might affect the wear
 results.
 Wear Test Results and Discussion
 Results of wear tests conducted according to the above-described procedures
 using as lubricants 1% of selected alcohol compounds added to a purified
 paraffin oil (PPO) are summarized in Table 1.
 PPO itself exhibits some, limited, boundary-lubricating ability with
 Si.sub.3 N.sub.4, but only at relatively low loads of 20 to 30 kg in the
 BTF test. Under the higher severity of 60 kg, PPO exhibits high friction
 and wear as shown in the optical micrograph of FIG. 1 and friction trace
 of FIG. 2. An immediate increase to higher friction is observed (type I
 friction) and wear is high. The optical micrograph reveals that the wear
 scar is smooth and symmetric. The small striated discolorations within the
 wear scar suggest a micro-abrasion process by opposing asperities. These
 discolorations are brownish and are reminiscent of oxidized organic
 compounds often seen inside the wear scars of paraffin oil lubricated wear
 tests on iron-based metals. A substantial amount of deposit is observed
 surrounding the wear scar in a pattern that indicates the flow field for
 the lubricant (and wear products) during the wear test. The high molecular
 weight, organic, nature of the deposit was confirmed using gel permeation
 chromatography and FTIR.
 Esters had no beneficial effect at 1% in PPO. These included ethyl
 stearate, a polyol ester, and two dibasic acid esters. The polyol ester
 and the alkyl dibasic acid ester (bis(2-ethylhexyl)sebacate) actually gave
 much higher wear than neat PPO.
 Significant antiwear effect was exhibited by several alcohol compounds,
 including aromatic alcohols (mannich product and 3-n-pentadecyl phenyl).
 In these cases, wear was low, friction was low and of type II behavior,
 and films were observed in the wear scar. When the hydroxyl group is
 sterically hindered as in the aromatic alcohol
 2,6-ditertiarybutyl-p-cresol, no lubricating effect is observed.
 Several compounds were tested neat, i.e., in substantially pure form. The
 results of the these tests, shown in Table 2, indicate that several neat
 alcohols can provide lower friction and wear than the neat PPO.
 The friction coefficients of .apprxeq.0.05 observed for some of the alcohol
 compounds is very low for such a severe wear test in which the initial
 mean Hertzian pressures exceed 2.1 GPa (300,000 psi), and suggest an
 elastohydrodynamic component of lubrication may play a partial role in the
 lubrication process. The ether and ester compounds were not effective
 neat.
 One striking feature of many of the anti-wear alcohols was the wear scar
 morphology, as exemplified by oleil alcohol in FIG. 3. The direction of
 sliding in this optical micrograph is from left to right. There is a
 wedge-shaped region on the entrance side, and a smooth, horseshoe-shaped
 region on the exit side. Surface profilometry confirmed that the wedge is
 projecting out of the wear scar surface. The horseshoe-shaped region is
 therefore a deeper, trough in the wear scar. Its smoothness suggests a
 chemical polishing has taken place in this high stress, high temperature
 region of the wear scar.
 Table 3 shows the results of wear tests using neat solutions of a series of
 three long-chain alcohols known as Triton surfactants. These compounds
 have the advantage of being both higher molecular weight, and liquids at
 room temperatures, because of their mixed alkyl/aryl chemical structure.
 All of the these alcohols were successful, with longer chains being more
 effective. Also the final coefficient of friction decreases with
 increasing chain length.
 Table 4 summarizes the data on the described wear tests on neat solutions
 of primary alcohols. The wedge-shaped regions seen in the wear scars of
 the other neat alcohol-lubricated tests were also visible here. See FIGS.
 4 and 5, showing an optical micrograph of the wear scar using decanol as
 lubricant and the coefficient of friction trace therefore. In this case,
 the smaller chain length alcohols had larger wear scars, but smaller
 wedge-shaped regions. The fact that the wedge-shaped region got smaller as
 chain length decreased suggests that there is some relationship between
 chain length, wedge-shaped region and lubrication effectiveness. Attempts
 to remove this wedge-shaped region using solvents were not successful.
 Even acids (including HF) could not remove this feature from the wear
 scar. Finally, Auger analysis with depth profiling was used to show that
 this feature and the surrounding surface were identical in composition
 with depth, leading to the conclusion that the wedge-shaped feature is
 merely unworn silicon nitride. The shiny horseshoe-shaped region at the
 trailing part of the wear car is in fact a region of higher wear. This
 reinforces the hypothesis that some form of reaction and chemical
 polishing has occurred.
 To study the effects of differing functional groups, a series of
 eight-carbon compounds with different functional groups were tested to
 directly compare effectiveness. The results are shown in Table 5.
 1-Octanol was the most effective. A secondary alcohol (2-octanol) was only
 effective for a few minutes, as was octanoic acid. The octyl ketone and
 aldehyde were not effective.
 The advantageous level of friction observed with the alcohol lubricants of
 the invention is not often seen in boundary lubrication. These
 observations and the low viscosity of many of the compounds used suggest
 that a component of elastohydrodynamic lubrication is a contributing
 factor. Although not intending to be bound by this theory, it is
 hypothesized that the elastohydrodynamic contribution proceeds to
 lubrication through a combination of viscous reaction product film forming
 and surface smoothing to reduce surface roughness. Indeed smooth, worn
 areas in the higher temperature exit region of the wear scars have been
 observed for alcohol-lubricated Si.sub.3 N.sub.4. This suggests that
 chemical polishing is occurring through direct chemical reaction between
 the Si.sub.3 N.sub.4 surface and the alcohol compounds. For example, the
 low load boundary-lubricating ability of Si.sub.3 N.sub.4 by paraffin oil
 can be explained in the light of these results. The high temperature
 generated by friction results in oxidation of the paraffin oil to provide
 carboxylic acids and alcohols. These alcohols can then provide (limited)
 boundary lubrication protection.
 TABLE 1
 Friction and Wear Data on Selected Oxygen-Containing Model Compounds at 1%
 in Paraffin Oil
 WSD Dia increase Frict. Wear Scar
 Film in Lubrication
 Chemical Compound.sup.1 (mm) above Hz. mm COF.sup.2 Type.sup.3
 Appearance Scar ? Classification
 NONE (Average of 16 tests.sup.4) 0.645 0.265 0.117 I smooth
 no Poor
 3-n-Pentadecyl Phenol 0.402 0.022 0.071 II roughened Yes
 - plastic Good
 Mannich Product 0.402 0.022 0.088 II smooth Yes
 - spotty Good
 2,6-ditertiarybutyl-p-cresol 0.661 0.281 0.123 I smooth
 no Poor
 Bis (2-ethylhexyl) Phthalate 0.678 0.298 0.134 III (2) smooth
 no Temporary
 Ethyl Stearate 0.696 0.316 0.120 III (3) smooth no
 Temporary
 Bis (2-ethylhexyl) Sebacate 0.729 0.349 0.129 III (2) smooth
 no Temporary
 Polyol Ester 0.749 0.369 0.134 I smooth no
 Poor
 .sup.1 1 wt % in purified paraffin oil
 .sup.2 Measured at the end of the test
 .sup.3 Number in parenthesis indicates time to friction transition in
 minutes
 .sup.4 Repeatability of wear scar measurement: 0.645 .+-. 0.018;
 Repeatability of friction coefficient measurement: 0.117 .+-. 0.010
 WSD: Wear Scar Diameter
 Hz: Hertzian contact diameter
 All Tests conducted on at 600 rpm, 60 kg, 30 minutes, 1% additive in PPO,
 21.degree. C.
 TABLE 2
 Summary of neat Oxygen-Containing Compound BTF Wear Test Data
 WSD
 WSD Incr.sup.1 Frict
 COMPOUND (mm) (mm) Final COF Type
 PPO.sup.2 0.645 0.265 0.117 I
 Oleil Alcohol 0.429 0.049 0.049 II
 C.sub.18 OH (75.degree. C.) 0.495 0.115 0.046 II
 Polyphenyl Ether 0.733 0.353 0.095 I
 Bis (2-ethylhexyl) Sebacate 0.919 0.539 0.148 I
 .sup.1 Wear Scar Diameter Increase above the Hertzian Contact diameter
 (0.380 mm)
 .sup.2 Average of 16 tests
 TABLE 3
 Summary of Neat Alkylarylpolyether Alcohol BTF Wear
 Test Data
 WSD WSD Incr.sup.1
 Compound (mm) (mm) Final COF
 PPO.sup.2 0.645 0.265 0.117
 Alkylarylpolyether (n = 9) 0.420 0.040 0.067
 Alkylarylpolyether (n = 5) 0.465 0.085 0.073
 Alkylarylpolyether (n = 1) 0.528 0.148 0.081
 .sup.1 Wear Scar Diameter Increase above the Hertzian Contact diameter
 (0.380 mm)
 .sup.2 Average of 16 tests
 n Refers to the number of ethoxy "mer" units comprising the ether side
 chain.
 TABLE 4
 BTF Wear Test Results on
 Primary Linear Alcohols
 WSD WSD Incr.sup.1
 Alcohol (mm) (mm) Final COF
 Decanol 0.503 0.123 0.045
 Octanol 0.557 0.177 0.049
 Hexanol 0.633 0.253 0.051
 .sup.1 Wear Scar Diameter Increase above the Hertzian diameter (0.38 mm)
 TABLE 5
 Summary of Neat C.sub.8 Series Oxygen-Containing
 Compound BTF Wear Test Data
 WSD
 WSD Incr.sup.1 Final Frict.
 Compound (mm) (mm) COF Type.sup.2
 PPO* 0.645 0.265 0.117 I
 1-Octanol 0.557 0.177 0.049 II
 3-Octanone 0.970 0.590 0.175 I
 Octanoic Acid 1.068 0.688 0.187 III (1)
 2-Octanol 1.076 0.696 0.126 III (6)
 Octyl Aldehyde 1.250 0.870 0.141 I
 .sup.1 Wear Scar Diameter Increase above the Hertzian Contact diameter
 (0.380 mm)
 .sup.2 Number in parenthesis indicates time to friction transition in
 minutes.
 *Average of 16 tests
 From the foregoing description, one skilled in the art can easily ascertain
 the essential characteristics of this invention, and without departing
 from the spirit and scope thereof, can make various changes and
 modifications of the invention to adapt it to various usages and
 conditions.