Friction control through solid lubricant films Jens Hardell, Braham Prakash Luleå University of Technology, Sweden jens.hardell@ltu.se
Outline Introduction to solid lubricants Molybdenum disulphide Burnished MoS 2 films Sputtered MoS 2 -Ti films
Introduction to solid lubricants What is a solid lubricant? • An easily sheared solid material that reduces mechanical interaction and friction between two contacting surfaces under normal load and relative sliding • Examples of common solid lubricants: Layered lattice or lamellar Polymers Soft metals Oxides MoS 2 PTFE Au MoO 3 WS 2 Nylon Ag PbO Graphite Sn
Introduction to solid lubricants Some desirable properties of a solid lubricant: Adhesion to substrate Anisotropy Cohesion Solid lubricant Electrical Hardness conductivity Thermo-chemical Volatility stability
Introduction to solid lubricants Why use solid lubricants? • Extreme operating conditions Low temperature High temperature Temperature, °C Temperature, °C Room temperature 0 150 200 250 300 500 -100 -273 Liquid lubricants Liquid lubricants and greases Greases Solid lubricants Solid lubricants High vacuum Radiation Vacuum pressure, Pa Radiation dose, rads Atmospheric pressure 10 2 10 -2 10 -4 10 -6 10 -8 1 10 5 10 6 10 7 10 8 10 9 10 10 Liquid lubricants and greases Liquid lubricants and greases Solid lubricants Solid lubricants
Introduction to solid lubricants Examples of applications Self lubricating bearings Space applications Machining tools
Strong covalent bonds Molybdenum disulphide between S and Mo S • Naturally occurring mineral Mo • Lattice layer structure S • Low friction is achieved through: – Good adhesion to substrate – Weak VdW bonds – Large separation distance between Weak VdW bonds adjacent lamellae between lattice layers Pros Cons Vacuum Oxidation High loads Humidity High temperature www.machinerylubrication.com
Burnished MoS 2 films Materials and film preparation Test setup High purity powder (OKS 110) -Average particle size: 0.6-0.8 μ m F N - Max. particle size: 15 μ m - Abrasive impurities: Insignificant Flat plate F R F R MoS 2 powder AISI 304 Stainless steel - Sand blasted Figure eight pattern Disc specimen A. K. Kohli, B. Prakash, Tribology Transactions 44 (2001) 147-151
Burnished MoS 2 films Effect of temperature on friction Influence of temperature and momentary water vapour input on the frictional behaviour A. K. Kohli, B. Prakash, Tribology Transactions 44 (2001) 147-151
Sputtered MoS 2 -Ti films Closed Field Unbalanced Magnetron Sputter Ion-plating Tribological tests Bearing steel ball F N MoS 2 Bearing steel ball F N MoS 2 Ti F R F R Tool steel with ω MoS 2 self-lubricating coating MoS 2 -Ti Ti MoS 2 is co-sputtered with Ti to improve wear resistance, Tool steel with resistance against oxidation and humid environments and Tool steel self-lubricating coating hardness of the coating J. Hardell, I. Efeo ğ lu, B. Prakash, Tribological degradation of MoS2-Ti sputtered coating when exposed to elevated temperatures , To be communicated
Sputtered MoS 2 -Ti films Reciprocating sliding tests at 25% RH, 0.1 m/s and 50 N 1,0 100 Friction 0,9 90 Load Coeffcient of friction 0,8 80 0,7 70 Load [N] 0,6 60 0,5 50 0,4 40 0,3 30 0,2 20 0,1 10 0,0 0 0 200 400 600 Time [s] MoS 2 -Ti coatings shows load dependent coefficient of friction J. Hardell, I. Efeo ğ lu, B. Prakash, Tribological degradation of MoS2-Ti sputtered coating when exposed to elevated temperatures , To be communicated
Sputtered MoS 2 -Ti films Reciprocating sliding tests at 25% RH , 0.1 m/s and 50 N 1,0 40C 0,9 Preheated 30min 400C Coefficient of friction 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0 1 10 100 1000 10000 Time [s] Increased friction and reduced life of coating after exposure to elevated temperature J. Hardell, I. Efeo ğ lu, B. Prakash, Tribological degradation of MoS2-Ti sputtered coating when exposed to elevated temperatures , To be communicated
Sputtered MoS 2 -Ti films Unidirectional sliding tests at 40% RH , 0.2 m/s and 50 N Friction is increased and “spikes” appear sooner when exposed to elevated temperature J. Hardell, I. Efeo ğ lu, B. Prakash, Tribological degradation of MoS2-Ti sputtered coating when exposed to elevated temperatures , To be communicated
Sputtered MoS 2 -Ti films Surface damage Disc Ball J. Hardell, I. Efeo ğ lu, B. Prakash, Tribological degradation of MoS2-Ti sputtered coating when exposed to elevated temperatures , To be communicated
Concluding remarks Burnished MoS 2 films: – Coefficient of friction is load and temperature dependant – Exposure to moist air increases friction momentarily Sputtered MoS 2 -Ti coatings: – Coefficient of friction is load and temperature dependant – Increased humidity leads to increased friction – After exposure to elevated temperature, the MoS 2 -Ti coating experiences reduced life and induces higher wear on the counter body Application of solid lubricants – Can be an effective way to control friction and reduce wear in certain applications – For successful implementation it is important to keep in mind factors like environment, load, speed, lifetime of component etc.
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