2/8/2018. Friction. The Laws of Friction MSE 454 SURFACE TREATMENT OF MATERIALS. Ing. Anthony Andrews (PhD) Friction testing. Why is there friction?

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1 Kwame Nkrumah University of Science & Technology, Kumasi, Ghana Friction MSE 454 SURFACE TREATMENT OF MATERIALS Ing. Anthony Andrews (PhD) Department of Materials Engineering Faculty of Mechanical and Chemical Engineering College of Engineering Website: Friction Resistance to relative motion of contacting bodies in a tangential direction Two classes of relative motion: sliding friction rolling friction Degree of friction is expressed as coefficient of friction (μ). Work, which is needed to overcome friction is usually the "loss" of energy and most of it is discharged into the environment as heat. The Laws of Friction 1. The friction force is proportional to normal load (a,b) First law is expressed as F = μ. W Friction testing 2. The friction force is independent of the apparent area of contact (a,c) 3. The friction force is independent of the sliding velocity Why is there friction? Friction is affected by the following: 1. Presence of wear particles and externally introduced particles at the sliding interface 2. Relative hardness of the materials in contact 3. Externally applied load and/or displacement 4. Environmental conditions such as temperature and lubricants 5. Surface topography and contact area 6. Microstructure or morphology of materials 7. Kinematics of the surfaces in contact (i.e., the direction and the magnitude of the relative motion between the surfaces 1

2 Lubricants CoF of sliding friction for dry metals, ceramics and polymers are rarely below 0.5. Such high values of CoF in engineering applications lead to intolerably high friction forces and frictional energy losses Lubricants are used to reduce the frictional force between surfaces Function of Lubricants Reduce friction Separate surfaces, reduce wear Provide cooling Remove wear particles and debris Reduce vibrations Prevent corrosion Types of Lubricants Lubricants consist of Base oil (additive free liquid base) Additives (enhance, reduce or create certain properties (5 20%) Base oil determines the main physical properties of a lubricant Chemically inert (contains some impurities) Main base oils: Biological (of plant or animal origin) Mineral oils (contain hydrocarbons) Synthetic oils Surface engineering involves the enhancement of certain properties of the surface of the component independently from those of the underlying substrate material Surface treatment are sometimes called postprocessing They affects either thin layer on the surface of the part itself or add a thin layer on top of the surface of the part Important uses of surface engineering: improve corrosion resistance and aesthetics improve surface hardness control friction, reduction of adhesion, and improve lubrication Why Improve Surface Hardness? 1. Increase wear resistance 2. Increase surface strength for load carrying crush resistance 3. Produce tough core for resistance to impact 4. Impart favourable residual compressive stresses 5. Improve fatigue resistance 2

3 Nature of interfaces in surface engineered components Properties of surface engineered component depends on Properties of surface engineered layer Properties of the substrate Properties of the interface Interface can be grouped into 2 sharp and diffuse Nature of interfaces in surface engineered components Interface debonding will depend on the strength of the substrate-coating bond intensity of the tribological conditions and thickness of the coating layer Progressive wear Delamination Taxonomy of Methods Methods to Surface Harden a Component Modification of the component surface with no compositional change Transformation hardening Surface melting Coatings deposited on the component surface Solid and liquid state Vapour phase Modification of the component surface involving compositional change Solid solution, precipitation thermochemical and electrochemical reactions Transformation Hardening Produced hard surface material of a softer carbon steel component (i.e. applies to medium and high carbon steels: %C). No chemistry changes. Transformation takes place by first heating to austenite followed by quenching to form martensite and subsequently tempered. Used to harden gear teeth, camshafts and crankshafts, cutter blades Transformation Hardening In both bulk and surface hardening of steels, heat diffuses into the body from the surface Except electrical resistance heating where heat is relatively uniform throughout the body. Steep temperature gradient can be developed Depth of surface transformation hardening depends on the rate of heat input into the surface layer the time for which it occurs, rate of heat loss High power input for short time favours shallow martensitic layer 3

4 Flame Hardening High intensity oxy-acetylene flame is applied to selective region Temperature is high enough to be in the γ region The heated region is quenched (water jets) to achieve desired hardness Induction Hardening Steel part is placed inside electrical coil which has a.c. through it. This energizes the steel part and heats it up. Rate and depth of heating can be controlled better than flame hardening Advantages?? Large gear Comparison of Flame and Induction Characteristics Flame Induction Equipment Application material Speed of heating Depth of hardening Oxyfuel torch, quench system Ferrous alloys, carbon steels, cast iron Few seconds to few minutes mm Power supply, quench system Same 1-10s mm Processing One part at a time Same Part size No limit Must fit in coil Contrl of process Attention required Very precise Cost Low High Laser Beam Hardening Another variation of flame hardening. Laser used as a heat source (eg. CO 2, Nd-YAG, diode) Selected based on wavelength and beam shape A phosphate coating is applied over the steel to facilitate absorption of laser energy. Selected area is exposed to laser energy heating it up. Parts are then quenched and tempered. Electron Beam Hardening Heat source is a beam of high energy electrons. Quenching Annealed Beam is manipulated using electromagnetic coils. Performed under vacuum conditions since electron beams dissipate easily in air. Surface can be hardened precisely both in depth and location. Very hard and brittle Tempered 4

5 Quenching Media The most commonly used quenching media are: 1. Water (plain or salt water) 2. Oil 3. Air Phase Transformations Microstructure and Properties 5