1.What is Tungsten disulfide (WS2 )? Tungsten Disulfide is one of the most lubricious materials known to science. With Coefficient of Friction (dynamic at 0.07, static at 0.03), it offers excellent dry lubricity unmatched to any other substance. It can also be used in high temperature and high pressure applications. It offers temperature resistance from -450o F (-270o C) to 1200o F (650o C) in normal atmosphere and from -305o F (-188o C) to 2400o F (1316o C) in Vacuum. Load bearing property of coated film is extremely high at 2000Mpa (300,000 psi).When being applied to solid lubricity of spaceflight, only WS2 can endure such high temperature -270℃~1300℃ in cosmic space. Thus, it is extensively used by NASA spaceflight, aviation, military etc. as lubricant material with high performance.

2. Lubricating Mechanism of Tungsten Disulfide Why WS2 has good lubrication performance? Because, the microstructure of WS2 crystal is layered hollow sphere with hexagonal crystal clusters, whose surfaces are formed by hexagonal networks with S―W―S molecules combined up, whose layers are connected by Van de Walls Force, this weak binding force is easy to be sheared between layers, so the low friction factor is put up. Meanwhile, WS2 crystal is very stable at high temperature, even above 550℃, WS2 can build up compact WO3 covering layer by slow oxidation to inhibit further oxidation, and WO3 has low friction factor and can prevent the glue happened between metal surfaces as well.

3. Characteristics comparison among Tungsten Disulfide(WS2 ), Molybdenum Disulfide (MoS2 ) and Graphite Tungsten Disulfide offers excellent lubrication under extreme conditions of Load, Vacuum and Temperature. The properties below show that Tungsten Disulfide offers excellent thermal stability and oxidation resistance at higher temperatures. WS2 has thermal stability advantage of 93oC (200oF) over MoS2. Coefficient of Friction of WS2 actually reduces at higher loads.

Table 1: WS2 / MoS2 physical and Technical Properties

properties

Tungsten disulfide (WS2)

CAS No 12138-09-9

Molybdenum Disulfide (MoS2)

CAS No 1317-33-5

Colour

Silver Gray

Blue-Silver Gray

Appearance

Crystalline Solid

Crystalline Solid

Melting Point (oC)

1250oC, 1260oC (decomposes)

1185oC (decomposes)

Boiling Point

 

450o C

Density (Kg.m-3)

7500

5060

Molecular Weight

248

160.08

Coefficient of Friction

0.03 Dynamic; 0.07 Static

 

Thermal Stability in Air

COF <0.1 till 1100oF (594oC)

COF <0.1 @600oF (316oC) increases to 0.5 @1100oF (594oC)

Load Bearing Ability

300,000 psi for coated film
COF:0.044 @ 20,000 psi
COF reduces to 0.024 between 200,000 to 400,000 psi

250,000 psi

Lubrication Temperature Range

Ambient: from -273oC to 650oC Vacuum(10 -14 Torr): from -188oC to 1316oC

Ambient: from -185oC to 350oC Vacuum: from -185oC to 1100oC

Chemical Durability

Inert Substance, Non-Toxic

Inert Substance, Non-Toxic

Magnetism

Non-Magnetic

Non-Magnetic

Electrical Properties

Has Semiconductor properties

 

Rockwell Hardness

30HRc

 

Coating Film Thickness

0.5 micron

 

Corrosion Stability

Can slow down the corrosion rate, but can not fully prevent substrate corrosion

 

Coatable Substrates

Iron, Steel, Aluminum, Copper, other Metals, Plastics and Manmade Solids

Iron, Steel, Aluminum, Copper, other Metals, Plastics and Manmade Solids

 

Table 2: comparison of the friction coefficient (μ) and the diameter of scuff scar among three materials

Item

 

 

Load(kg)

graphite

MoS2

WS2

friction factor

friction vestige

(mm)

friction factor

friction vestige

(mm)

friction factor

friction vestige

(mm)

21

0.0194

0.241

0.0194

0.268

 

0.197

25

0.0490

0.255

0.0204

0.270

 

0.250

31

0.0526

0.312

0.0180

0.283

0.0066

0.266

34

0.0601

0.316

0.0240

0.288

0.0240

0.270

38

 

 

0.0269

0.301

0.0296

0.282

40

 

 

0.0459

0.312

0.0375

0.309

44

 

 

0.0510

0.324

0.0348

0.318

48

 

 

0.0511

0.331

0.0375

0.325

52

 

 

0.0572

0.349

0.0333

0.343

56

 

 

0.0565

0.370

0.0419

0.353

61

 

 

 

 

0.0452

0.354

66

 

 

 

 

0.0448

0.361

From the data above we can conclude that basing on four-ball extreme pressure test, the item value of the average scuff scar's diameter and the friction factor is ranked as: graphite > MoS2 > WS2; and PB is WS2 > MoS2 > graphite, thus we may draw a conclusion that the lubrication performance is WS2 > MoS2 > graphite.

4. The application of WS2Tungsten Disulfide (WS2) can be used instead of Molybdenum Disulfide (MoS2)and Graphite in almost all applications, and even more. Molybdenum and Tungsten are from same chemical family. Tungsten is heavier and more stable. Molybdenum Disulfide (Also known as Molybdenum Disulfide) till now has been extremely popular due to cheaper price, easier availability and strong and innovative marketing. Tungsten Disulfide is not new chemical and has been around as long as Molybdenum, and is used extensively by NASA, military, aerospace and automotive industry.Till few years ago, price was Tungsten Disulfide was almost 10 times that of Molybdenum Disulfide. But since then price of Molybdenum Disulfide has doubled every six months. Now the prices of both chemicals are within comparable range. Now, it makes more economic sense to use superior dry lubricant (Tungsten Disulfide) and improve the quality and competitiveness of terminal product.
The following are established ways the WS2 powder can be used

1). WS2 powder worked as additives for lubricants (such as oil, grease & other synthetic lubricants) The powder can be mixed 1% to 15% v/v (as required) with grease or oil. This will enhance lubricity of the mixture and also improves High Temperature and Extreme Pressure properties of mixture. During the use, WS2 in the mixture will get coated on mating/moving parts, which in turn reduces friction and improves lubricity and load bearing ability for much longer cycles.

2). Coating the WS2 powder on a substrate requiring (dry) lubricity The powder can be coated by spraying at 120 psi (0.8 Mpa) the substrate with dry (& cool) pneumatic air. It does not require any binders and spraying can be done at normal room temperature. Coated film will be 0.5 micron thick. In an alternative application method, the powder can also be mixed with Isopropyl alcohol and this paste could be buffed to the substrate. The coating applications are already established in many areas such as Automotive parts, Racing Car Engine and other parts, Aerospace parts, Bearings (Linear, Ball, Roller etc), Shafts, Marine parts, Cutting Tools, Blades, Slitters, Knives, Mold release, Precision Gears, Valve components, Pistons, Chains, Machinery components and many other areas.Besides, it can also be applied to antifriction material in carbon industry; super rigidity material and weld thread;Tungsten Disulfide (WS2) can be used instead of Molybdenum Disulfide (MoS2) in almost all applications, and even more. Since the powder offers one of the lowest Coefficient of Friction (Dynamic at 0.03 & Static at 0.07), the applications are unlimited and could be tried with every conceivable idea. .

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