US3821111A

US3821111A - Solid lubricant

Info

Publication number: US3821111A
Application number: US00278936A
Authority: US
Inventors: A Sugafuji; W Abe; Y Terada
Original assignee: Oiles Industry Co Ltd
Current assignee: Oiles Industry Co Ltd
Priority date: 1971-08-19
Filing date: 1972-08-09
Publication date: 1974-06-28
Anticipated expiration: 1991-06-28
Also published as: JPS5128786B2; JPS4828867A; DE2240829C3; FR2149556B1; SE380287B; GB1409921A; GB1403621A; DE2240829B2; DE2240829A1; FR2149556A1

Abstract

This invention is based on a finding in which an addition of 10 - 30 wt percent of alloy powders to the solid lubricant of graphite-sodium fluoride or graphite-sodium fluoride-tungusten disulfide suitable for use at high temperature contributes to improvements of the mechanical strength as well as the friction coefficient, the preferable alloy being that of the composition of 77Cu-23Pb or 90Cu-5Sn-5Pb. While the addition of 10 - 30 wt percent of a metal salt of tungustate contributes to the formation of strong film or coatings at a high temperature and to prolongation of bearing life.

Description

United States Patent 1191 June 28, 1974 Abe et al.

[ SOLID LUBRICANT 3,607,248 9/1971 Wallace 252/12 [75] Inventors: wataru Abe Chigasaki; Yasuyuki 3,705,450 12/1972 Mor1sak1 252/12 Terada, Kamakura; Akira Sugafuji, Fujisawa, all of Japan Oiles Kogyo Kabushiki Kaisha, Tokyo, Japan Filed: Aug. 9, 1972 Appl. No.: 278,936

Assignee:

US. Cl 252/12, 75/156, 75/163 Int. Cl..... ClOm 7/06, ClOm 7/04, ClOm 7/02 Field of Search 252/12, 12.2, 12.4, 12.6; 75/156, 163

Houston et al. 252/12 I Primary Examiner-Daniel E. Wyman Assistant Examiner-l. Vaughn Attorney, Agent, or FirmOldham & Oldham Co.

57 ABSTRACT This invention is based on a finding in which an addition of 10 30 wt percent of alloy powders to the solid lubricant of graphite-sodium fluoride or graphitesodium fluoride-tungusten disulfide suitable for use at high temperature contributes to improvements of the mechanical strength as well as the friction coefficient, the preferable alloy being that of the composition of 77Cu-23Pb or 90Cu-5Sn-5Pb.

While the addition of 10 30 wt percent of a metal salt of tungustate contributes to the formation of strong film or coatings at a high temperature and to prolongation of bearing life.

7 Claims, 3 Drawing Figures Pmmrmwaza 1114 U" E 220 g 200- E 13o- 55 I60- [I] LL |4o- O I20" I00 2 4 8O 0: 60 E 3 4o FIG.

COMPOUNDING AMOUNT OF NOF (WI lb 2'0 3'0 4'0 53 COMPOUNDING AMOUNT OF NOF (wI FIG. 2

-IOO- OPP(|)SITE MATING SURFACE N WEIGHT CHANGE OF THE BCA sour) LUBRICANT It has been used heretofore a solid lubricant comprising graphite and molybdenum disulfide as a representative solid lubricant.

Graphite tends to bring an increase of a friction coefficient at an atmoshpere of about 400C and also-to be inferior to a film forming ability, while molybdenum disulfide is superior to a film forming ability and a migration property to the mating bearing surface, but molybdenum disulfide comes into question as a lubricating material at the high temperature since an oxidative wear of bearing becomes excessive in an atmospheric temperature of more than about 350C.

Many attempts, have been effected to improve the lubricating property, one of which is a method for coating metallic surface with a paste consisting of molybdenum disulfide and grease resulting in making unbearable to a repeated sliding motion on the bearing surface, another of which is a method for burying and setting into cavities provided on the bearing surface, the solid lubricant together with a binder of a synthetic resin resulting in limitting an use temperature of the bearing owing to a heat resistance property of the binder.

In order to overcome said disadvantages, various methods, namely methods for adding metal oxide to graphite or adding metal sulfide to molybdenum disulfide have been attempted.

However, it has not been found the solid lubricant capable of hearing at high temperature of about 400 500C.

This invention relates to the solid lubricant suitable for the use of high temperature atmosphere having the following compositions, namely i a. 55 97 wt percent of graphite and 3 45 wt percent of sodium fluoride,

b. less than 30 wt percent of tungsten disulfide, 3 45 wt percent of sodium fluoride and a residual amount of graphite,

c. 55 97 wt percent of graphite, 3 45 wt percent of sodium fluoride and 5 20 wt percent of sodium meta-phosphate as the binder on the basis of the total weight of graphite and sodium fluoride,

d. less than 30 wt percent of tungsten disulfide, 3

45 wt percent of sodium fluoride and the residual amount of graphite and 5 20 wt percent of sodium meta-phosphate as the binder on the basis of the total weight of tungsten disulfide, sodium fluoride and graphite,

e. 3 45 wt percent of sodium fluoride and 55 97 wt percent of graphite and i0 30 wt percent of 77Cu-23Pb alloy powder or 90Cu-5Sn-5Pb alloy powder as the binder on the basis of the total weight of sodium fluoride and graphite,

f. less than'30 wt percent of tungsten disulfide, 3- 45 wt percent of sodium fluoride and the residual amount of graphite and 30 wt percent of 77Cu-23Pb alloy powder or 90Cu-5Sn-5Pb alloy powder as the binder on the basis of the total weight of tungsten disulfide, sodium fluoride and graphite,

g. 3 45wt percent sodium fluoride and 55 97 wt percent of graphite and 10 30 wt percent of 77Cu-23Pb alloy powder or 90Cu-5Sn-5Pb alloy powder on the basis of the total weight of graphite and sodium fluoride as the binder and also 5 wt percent of metal salt of tungstate on the basis of the total weight of graphite, sodium fluoride and said metal alloy,

h. less than 30 wt percent of tungsten disulfide, 3 45 wt percent of sodium fluoride and the residual amount of graphite and I0 30 wt percent of 77Cu-23Pb alloy powder or Cu-5Sn-5Pb alloy powder on the basis of the total weight of graphite, tungsten disulfide and sodium fluoride as the binder and also 5 20 wt percent of metal salt of tungstate on the basis of the total weight of graphite, tungsten disulfide, sodium fluoride and said metal alloy.

The solid lubricant of this invention possesses the properties of bearing to the use of high temperature atmosphere (room temperature to about 500C) and variousother properties of protecting the oxidative wear, enhancing the film forming ability and prolonging a film life.

FIG. 1 is a graph of showing an interrelationship between the compounding ratio of sodium fluoride to graphite and a wear rate of the bearing.

FIG. 2 is a graph of showing the interrelationship between the migrating amount of the solid lubricant to the mating surface under the same conditions as those of the FIG. 1. I

FiG. 3 is a graph of showing the interrelationship between the migrating amount of the solid lubricant to the mating surface and a friction time under the same conditions as those of the FIG. 1. The invention is based on the provision of the solid lubricant suitable for the use of high temperature having the compositions a h as mentioned above.

These and other objects, features and advantages of the present invention will become apparent from the following examples.

Various types of the solid lubricants have been manufactured according to the methods as mentioned below.

1. The solid lubricant of buried type manufactured by uniformly mixing a graphite flake having a particle size of less than mesh of Tyler standard slieve with fine powders of sodium fluoride having a particle size of less than 200 mesh and then pressure molding the mixture into a metallic mold under a pressure of about l,000 Kglcm 2. The solid lubricant of buried type manufactured by uniformly mixing the graphite flake having the particle size of less than 150 mesh with fine powders of tungsten disulfide having a mean particle size of 2 microns and sodium fluoride having the particle size of less than 200 mesh and then molding the mixture obtained into the metallic mold under the pressure of 1,000 Kg/cm.

. The solid lubricant of the paste type manufactured by uniformly mixing the graphite flake having the particle size of less than 150 mesh with fine powders of sodium fluoride having particle size of less than 200 mesh and then further mixing an aqueous solution of sodium meta-phosphate (NaPO as the binder into the resulting mixture to form the solid lubricant suitable for covering the sliding surface of the bearing.

. The solid lubricant of the paste type manufactured by uniformly mixing the graphite flake having the particle size of less than 150 mesh with fine powders of tungsten disulfide having the mean particle size of 2 microns and sodium'fluoride having the particle size of less than 200 mesh and then further mixing the aqueous solution of sodium metaphosphate (NaPO as the binder into the resulting mixture to form the solid lubricant suitable for cov- 5 ering the sliding surface of the bearing.

5. The solid lubricant having high strength manufactured by heating the paste obtained from the methods of said items 3 and 4 to remove most of water contained therein and then molding said paste into the metallic mold under the pressure of 1,000 Kg/cm and further heating said paste into an oven at about 200C.

6. A cylinderical solid lubricant of buried type manufactured by uniformly mixing the graphite flake having the particle size of less than 150 mesh with sodium fluoride having the particle size of less than 200 mesh and adding to said mixture, alloy powders of less than 150 mesh consisting 90 wt percent Cu, 5 wt percent Sn and 5 wt percent Pb and after uniformly mixing, premolding under the pressure of 1,000 Kg/cm and then sintering the mixture obtained at the temperature of about 800C in N gas atmosphere. 2

7. The cylinderical solid lubricant of buried type manufactured by uniformly mixing the graphite flake having the particle size of less than l50 mesh with fine powders of tungsten disulfide having the mean particle size of 2 microns and sodium fluo- 3O ride having the particle size of less than 200 mesh and adding to said mixture, alloy powders of less than 150 mesh consisting 90 wt percent Cu, 5 wt percent Sn and 5 wt percent Pb and after uniformly mixing, premolding under the pressure of 1,000 5 Kg/cm and then sintering the mixture obtained at the temperature of about 800C in N gas atmosphere.

8. The cylinderical solid lubricant of buried type manufactured by uniformly mixing the graphite 40 flake having the particle size of less than 150 mesh with sodium fluoride having the particle size of less than 200 mesh alloy powders having the particle size of less than 150 mesh and consisting 90 wt percent Cu, 5 wt percent Sn and 5 wt percent Pb and sodium tungstenate (Na WO having the particle size of less than 200 mesh and premolding the mixture obtained, under the pressure of 1,000 Kg/cm and then sintering said mixture at the temperature of about 800C in N gas atmosphere.

9. The cylinderical solid lubricant of buried type manufactured by uniformly mixing the graphite flake having the particle size of less than 150 mesh with fine powders of tungsten disulfide having the mean particle size of 2 microns, sodium fluoride having the particle size of less than 200 mesh, alloy powders having the particle size of less than 150 mesh and consisting 90 wt percent Cu, 5 wt percent Sn and 5 wt percent Pb and sodium tungstenate (Na WO having the particle size of less than 200 mesh and premolding the mixture obtained, under the pressure of 1,000 Kg/cm and then sintering said mixture at the temperature of about 800C in N gas atmosphere.

The components to be added to the solid lubricant are shown as follows: a. Sodium fluoride powder.

Said powders of Mohs hardness of more than 5 are used as the additives.

Sodium fluoride does not give any influence to the lubricating property of graphite but protects a-change of the friction coefficient even if the temperature change of the solid lubricant occur in the compounding amount of about 1 wt percent of sodium fluoride and also improves the film forming ability in the dry friction when about 3 wt percent of sodium fluoride is mixed into graphite.

When about 10 wt percent of sodium fluoride is mixed into graphite, the effect of addition will remarkably increase. The more the compounding amount of sodium fluoride increases for example to more than 50 wt percent, the more the migrating amount of solid lubricant to the mating surface will increase thereby indicating a reverse effect.

Therefore, it has been found that a preferable amount of mixing sodium fluoride is less than 45 per- 0 cent by weight. The additive effect of sodium fluoride to the solid lubricant of graphite-W8 compositions is similar to that of the graphite composition.

b. Tungsten disulfide powder.

Said powders show a superior lubricating effect but,

5 possess an inferior heat resistance property.

The additive effect of tungsten disulfide will bring an increase of the lubricating property and an improvement of the film forming ability in a relatively low temperature region.

The heat resistance property of tungsten disulfide itself is improved to such extent that the solid lubricant may be used in the high temperature such as 400 500C or more than one by the addition of sodium fluoride.

The additional range or extent of tungsten disulfide is preferable to be less than 30 wt percent.

Tungsten disulfide may be added in an upper limited amount of said range in the use of relatively low temperature. The compounding amount of tungsten disulfide may gradually decrease according to the increase of the use temperature. By the reason of that in a high temperature of about 400 500C, the durability of film becomes more important than the decrease of friction, the addition of tungsten disulfide attributes to the depression of the friction coefficient but the durability of film will decrease.

In order to overcome said disadvantages, the addition of tungsten disulfide will have to be controlled in lesser extent. In view of said matter, the solid lubricant not I containing any tungsten disulfide is applicable in rather high temperature. The table 1 shows the solid lubricant of the present invention having various compositions.

c. Sodium meta-phosphate.

Sodium meta-phosphate may be used for forming the film of the solid lubricant on the mating surface of the hearing as the binder.

Sodium meta-phosphate may be used for forming a block type solid lubricant by heating the paste to remove the most of water contained in the paste, and then molding it into the metallic mold and then sintering and further may be used for forming the film of the solid lubricant on the mating surface of bearing by coating said surface with the paste of the solid lubricant and heating it to about 200C to remove the most of water contained therein and then backing the molded article into the oven at the temperature of 200C or molding into the hot press under a high temperature and a high pressure. As sodium meta-phosphate itself does not give any lubricating property, it is preferable to add 5 wt percent of sodium meta-phosphate in the film forming use, while to add less than wt percent of sodium meta-phosphate in the block type solid lubricant use.

The bearing properties of the solid lubricants containing graphite and sodium fluoride or graphite, tungsten disulfide and sodium fluoride are shown in the FIGS. 1 3.

In FIG. 1, an abrasion amount of the bearing was measured about the solid lubricants containing various amounts of sodium fluoride by burying it into the mating surface of a cylindrical bronze casting (JIS BS 6) using an opposite mating bearing of 545C under the test conditions of the temperature of 500C, the sliding rate of 3.5 m/min and the surface pressure of Kg/cm at every 20 hours.

The composition of said bronze casting corresponds to 81.0 87.0 wt percent Cu, 4.0 6.0 wt percent Sn, 4.0 7.0 wt percent Zn and 3.0 6.0 wt percent Pb.

The area of the solid lubricant to the total area of the mating surface corresponds to 30 wt percent.

From the test results of FIG. 1, it has been found that the effect of adding sodium fluoride appears when about 3 wt percent of sodium fluoride are added to the solid lubricant, on the contrary, the wear amount of bearing increases when more than 50 wt percent of sodium fluoride are added.

In FIG. 2, it has been found that the migrating amount of solid lubricant remarkably increases when the compounding amount of sodium fluoride reaches more than 50 wt percent and that the friction coefficient does not change remarkably showing a value of about 0.2 when said compounding amount reaches less than wt percent and also that the friction coefficient reaches to the value of 0.4 when the compounding amount of sodium fluoride reaches to wt percent of sodium fluoride.

As mentioned above, if the migrating amount is too much, it is celar that any satisfactory result will not be obtained in the effects of the friction and the wear.

In FIG. 3, curve (A) shows the test result of the solid lubricant having the composition of test No. 5 of table 1, curve (B) shows that of the solid lubricant having the composition of test No. l8 of table I and curve (C) shows that of a film of the solid lubricant having the composition of test No. l8 mixed with 8 wt percent of NaPO as the binder.

Curves (D) and (E) show comparative test results of graphite or graphite containing 10 wt percent of tungsten disulfide respectively.

It has been found that a good film of the solid lubricant is formed by migrating the solid lubricant to the mating surface of the bearing thereby resulting in the increase of the weight.

The film formed on the mating surface of the bearing has clearly observed by a. nacked eyes to be a smooth and glossy black film.

The friction coefficient after 40 hours has been equal in the solid lubricants having the compositions as shown in curves (A) and (B).

The friction coefficient having the compositions as shown in the curves (A) and (B) was 0.15 0.20, until a lapse of 20 hours, 0.20 0.25 after 40 hours, while the friction coefficient having the composition as of thickness of the migrated coatings amounts to 4 6 microns when the weight increase of the mating surface reaches 50 mg, while said value amounts to less than I micron when said weight increase reaches to ID mg. A.

The bearing properties of the solid lubricant suitable for a pellet type containing graphite, tungsten disulfide, sodium fluoride and the metallic powders as the binder are explained as follows.

In case that the metal powders, especially the mixture of Cu and Pb powders or the mixture of Cu, Sn and Pb powders are used as the binder, the mechanical strength, hardness, antifriction property and antiwearing property of the molded solid lubricant are remarkably improved without any depression of an inherent lubricating property of graphite, tungsten disulfide and sodium fluoride.

It is a reason for selecting Cu, Sn and Pb powders as the alloy powderssuitable for the binder that said powders are low melting point metal powders having lower temperature than the oxidative temperature or the melting point of the solid lubricant namely graphite, WS and NaF and capable of forming a solid solution thereof and also having a similar component to that of a base metal of bronze castings (JIS BC6). In the present invention, it is preferable to use the alloy of 77Cu- 23Pb or 90Cu-5Sn-5Pb.

The composition and the bearing property are shown in the table 2.

In the table 2, the solid libricant having the composition of graphite l5 tungsten disulfide 10 sodium fluoride was used compounding the alloy powders as the binder.

The test conditions are shown as follows.

The pellets of the solid lubricant having the composition of table 2 were buried on the sliding surface of the cylindrical bronze castings (JIS BC6).

The area of the solid lubricant buried corresponds to 30 wt percent of the total area of the sliding surface.

While the opposite mating material is l3 Cr steel, and the temperature of a heat treatment for forming the pellet is 700 850C.

The test was effected under the conditions of the surface pressure of 50 Kg/cm and the sliding rate of 3.5 m/min at the temperature of 500C for 20 hours.

As shown in the Table 2, the compounding ratio of the metal powders to the solid lubricant is preferable to be 10 30 wt percent. If the compounding ratio of the metal powders is lessthan said range, the mechanical strength of the solid lubricant formed, decreases whereby the solid lubricant buried tends to break during sliding and to increase the friction coefficient. While if e said ratio is more than said range, the mechanical strength of the solid lubricant increases but the friction coefficient increases whereby the friction will increase. The friction coefficient and the abrasion amount of the solid lubricant of the pellet type depend on the treating temperature even if the composition of the solid lubricant is same.

Therefore, it is important to select an appropriate temperature for heat-treating of the solid lubricant, the temperature of about 700 850C is preferable. It has been found in the table 2 that the allowable limitation in the abrasion amount of the solid lubricant amounts to 0.10 mm and that if the abrasion amount of the solid lubricant is beyond the said limitation, a scuffing abrasion will suddenly increase whereby the friction coefficient reaches to more than 0.30.

The similar results have been obtained in the solid lubricant comprising 70 90 wt percent of the base lubricant having the compositions of 55 97 graphite and 3 45 sodium fluoride, and 30 wt percent of said alloy powders as well as the solid lubricant comprising 70 90 wt percent of the base lubricant having 3 45 wt percent of sodium fluoride, less than 30 wt percent of tungsten disulfide and residual amount of graphite, and 10 30 wt percent of said alloy powders.

The satisfactory results were obtained showing the abrasion amount of 0.050 0.095 mm and the friction coefficient of 0.20 0.30.

The bearing properties of the solid lubricant having the composition of graphite, tungsten disulfide, sodium fluoride and the said alloy powders as the binder and also metal salt of tungstate are explained as follows.

Said alloy powder comprises Cu and Pb powders or Cu, Sn and Pb powders, while said tungstate comprises alkali salts of tungstate such as sodium tungstate (Na potassium tungstate K WO,) alkali earthmetal of tungstate such as barium tungstate (BaWO and metal salt of tungstate such as lead tungstate (PbWO zinc tungstate (ZnWO copper tungstate (CuWO and cadmium tungstate (CdWO It has been found that a strong film or coatings is formed on the friction surface of the bearing in especially a high temperature thereby protecting a base metal and assisting the lubricating property of graphite, tungsten disulfide and sodium fluoride, and also prolonging the bearing life.

In the table 3, the solid lubricants having the composition of 90 (75 graphite tungsten disulfide 10 sodium fluoride) 10 (90Cu-5Sn-5Pb) and various sorts of metal salts of tungstate were tested.

It has been found in the table 3 that the solid lubricant having low friction coefficient is manufactured by adding 5 4 20 wt percent of lead tungstate to the said lubricant.

In table 3, the test was carried out using 5 percent by weight of another metal salts of tungstate whereby similar results were obtained.

The friction coefiicient and the abrasion amount of The similar good results have been obtained in the solid lubricant having the composition of 70 90 wt percent of 97 graphite 3 45 NaF) l0 30 wt percent of (77Cu-23Pb or 90Cu-5Sn-5Pb) and also the solid lubricant comprising 5 20 wt percent of the metal salt of tungstate and 80 95 wt percent of the base lubricant having the composition of 90 wt percent of less than 30 wt percent of W8 3 45 wt percent of NaF residual amount of graphite l0 30 wt percent of said metal powders respectively.

The test results showing the wear amount of 0.025 0.050 mm and the friction coefficient of 0.15 0.30 have been obtained.

Table 1 Graphite (wt%) Table 2 -G raphitcl SW52 77Cu-23Pb Cu-5Sn Wear amount of Friction Table 3 90(75-Graphite-l5WS2 -10NaF)-l0(9OCu- PbWOl CuWO; CdWO; K WO NaZWO CaWol Wear amount of Friction No 5Sn-5 Pb) (wt%) (wt%) 'vt%) (wt%) (wt%) (Wt%) (wt%) bearing (nun) coefijcient What is claimed is:

1. A solid lubricant comprising 55 97 percent by weight of graphite, 3 45 percent by weight of sodium fluoride and 10 percent by weight of alloy powders of 77Cu-23Pb or 90Cu-5Sn-5Pb on the total basis of graphite and sodium fluoride.

2. A solid lubricant comprising 3 45 percent by weight of sodium fluoride, less than 30 percent by weight of tungsten disulfide, 25 97 percent by weight of graphite and 10 30 percent by weight of the alloy powders of 77Cu-23Pb or 90Cu-5Sn-5Pb on the total basis of graphite and sodium fluoride.

3. A solid lubricant as claimed in claim 1 in which 10 30 percent by weight of a metal salt of tungstate are added to the solid lubricant of the claim 1.

4. A solid lubricant as claimed in the claim 2 in which 5 20 percent by weight of the metal salt of tungstate are added to the solid lubricant of the claim 2.

5. A solid lubricant as claimed in claim 3 in which less than 30 percent by weight of tungsten disulfide is added to the solid lubricant.

6. A solid lubricant as claimed in claim 4 in which the metal salt of tungstate added is from the group consisting of sodium tungstate, potassium tungstate, barium tungstate, lead tungstate, zinc tungstate, copper tungstate, and cadmium tungstate.

7. A solid lubricant as claimed in claim 3 in which the metal salt of tungstate added is from the group consisting of sodium tungstate, potassium tungstate, barium I tungstate, lead tungstate, zinc tungstate, copper tungstate, and cadmium tungstate.

UNITED S ATES PATENT )FFICE CERTIFICATE OF CORRECTION Patent No. 111 D t d June 28, 1974 Inventor(s) Wataru Abe et at It is certifie dthat error appears in the above-identified patent and that said Letters Patent are hereby correct zed as shown below:

Columh 5, line 42 "celar" sl'nuld be clear Column 6, line 63, 'e" should be the Column 7, Line 33 "Wo -)"'shou1d be W0 Signed and sealed this 1st day of October 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. I C. MARSHALL DANN Attesting Officer Commissioner of Patents FORM PO-1050 (10-69) USCOMM DC 60376d=69 tv us. sovsmmsu'r PRINTING OFFICE: I969 o-ass-334,

Claims

2. A solid lubricant comprising 3 - 45 percent by weight of sodium fluoride, less than 30 percent by weight of tungsten disulfide, 25 - 97 percent by weight of graphite and 10 - 30 percent by weight of the alloy powders of 77Cu-23Pb or 90Cu-5Sn-5Pb on the total basis of graphite and sodium fluoride.
3. A solid lubricant as claimed in claim 1 in which 10 - 30 percent by weight of a metal salt of tungstate are added to the solid lubricant of the claim 1.
4. A solid lubricant as claimed in the claim 2 in which 5 - 20 percent by weight of the metal salt of tungstate are added to the solid lubricant of the claim 2.
5. A solid lubricant as claimed in claim 3 in which less than 30 percent by weight of tungsten disulfide is added to the solid lubricant.
6. A solid lubricant as claimed in claim 4 in which the metal salt of tungstate added is from the group consisting of sodium tungstate, potassium tungstate, barium tungstate, lead tungstate, zinc tungstate, copper tungstate, and cadmium tungstate.
7. A solid lubricant as claimed in claim 3 in which the metal salt of tungstate added is from the group consisting of sodium tungstate, potassium tungstate, barium tungstate, lead tungstate, zinc tungstate, copper tungstate, and cadmium tungstate.