Question

Coefficient of friction between cast iron and steel

Answer 1

The friction force is the force exerted by a surface when an object moves across it - or makes an effort to move across it.

The frictional force can be expressed as

Ff = μ N                     (1)

where

Ff = frictional force (N, lb)

μ = static (μs) or kinetic (μk) frictional coefficient

N = normal force between the surfaces (N, lb)

There are at least two types of friction forces

kinetic (sliding) friction force- when an object movesstatic friction force - when an object makes an effort to move



For an object pulled or pushed horizontally the normal force - N - is simply the gravity force - or weight:

N = Fg

    = m ag                         (2)

where

Fg = gravity force - or weight (N, lb)

m = mass of object (kg, slugs)

ag = acceleration of gravity (9.81 m/s2, 32 ft/s2)

The friction force due to gravity (1) can with (2) be modified to

Ff = μ m ag                      (3)

Friction Coefficients for some Common Materials and Materials Combinations

Materials and Material CombinationsStatic Frictional Coefficient 
- μs -
Clean and Dry SurfacesLubricated and Greasy SurfacesAluminumAluminum1.05 - 1.350.3Aluminum-bronzeSteel0.45AluminumMild Steel0.61Brake material2)Cast iron0.4Brake material2)Cast iron (wet)0.2BrassSteel0.350.19BrassCast Iron0.31)BrickWood0.6BronzeSteel0.16BronzeCast Iron0.221)Bronze - sinteredSteel0.13CadmiumCadmium0.50.05CadmiumChromium0.410.34CadmiumMild Steel0.461)Cast IronCast Iron1.1, 0.151)0.071)Cast IronOak0.491)0.0751Cast ironMild Steel0.4, 0.231)0.21, 0.1331)Car tireAsphalt0.72Car tireGrass0.35Carbon (hard)Carbon0.160.12 - 0.14CarbonSteel0.140.11 - 0.14ChromiumChromium0.410.34Copper-Lead alloySteel0.22CopperCopper10.08CopperCast Iron1.05, 0.291)CopperMild Steel0.53, 0.361)0.181)DiamondDiamond0.10.05 - 0.1DiamondMetal0.1 - 0.150.1GlassGlass0.9 - 1.0, 0.41)0.1 - 0.6,
0.09-0.121)GlassMetal0.5 - 0.70.2 - 0.3GlassNickel0.780.56GraphiteSteel0.10.1GraphiteGraphite (in vacuum)0.5 - 0.8GraphiteGraphite0.10.1Hemp ropeTimber0.5HorseshoeRubber0.68HorseshoeConcrete0.58IceIce0.02 - 0.09IceWood0.05 IceSteel0.03IronIron1.00.15 - 0.20LeadCast Iron0.431)LeatherOak0.61, 0521LeatherMetal0.40.2LeatherWood0.3 - 0.4LeatherClean Metal0.6Leather fiber Cast iron0.31Leather fiberAluminum0.30MagnesiumMagnesium0.60.08MasonryBrick0.6 - 0.7NickelNickel0.7 - 1.1,
0.531)0.28, 0.121)NickelMild Steel0.641)0.1781)NylonNylon0.15 - 0.25OakOak (parallel grain)0.62, 0.481)OakOak (cross grain)0.54, 0.3210.0721PaperCast Iron0.20Phosphor-bronzeSteel0.35PlatinumPlatinum1.20.25PlexiglasPlexiglas0.80.8PlexiglasSteel0.4-0.50.4 - 0.5PolystyrenePolystyrene0.50.5PolystyreneSteel0.3-0.350.3 - 0.35PolytheneSteel0.20.2RubberRubber1.16RubberCardboard0.5 - 0.8 RubberDry Asphalt0.9  (0.5 - 0.8)1)RubberWet Asphalt0.25 - 0.751)RubberDry Concrete0.6 - 0.851)RubberWet Concrete0.45 - 0.751)SilverSilver1.40.55SapphireSapphire0.20.2SilverSilver1.40.55SkinMetals0.8 - 1.0SteelSteel0.5 - 0.80.16Straw FiberCast Iron0.26Straw Fiber Aluminum0.27Tarred fiberCast Iron0.15Tarred fiberAluminum0.18Polytetrafluoroethylene (PTFE)Polytetrafluoroethylene (PTFE)0.040.04, 0.041)Polytetrafluoroethylene (PTFE)Steel0.05 - 0.2Tungsten CarbideSteel0.4-0.60.1 - 0.2Tungsten CarbideTungsten Carbide0.2 - 0.250.12Tungsten CarbideCopper0.35Tungsten CarbideIron0.8TinCast Iron0.321)Tire, dryRoad, dry1 Tire, wetRoad, wet0.2 WoodClean Wood0.25 - 0.5WoodWet Wood0.2WoodClean Metal0.2 - 0.6WoodWet Metals0.2WoodStone0.2 - 0.4WoodConcrete0.62WoodBrick0.6Wood - waxedWet snow0.14, 0.11)Wood - waxedDry snow0.041)ZincCast Iron0.85, 0.211)ZincZinc0.60.04

1) Kinetic or sliding frictional coefficient - only when there is a relative motion between the surfaces. Without motion the values are somewhat higher.

2) Note! It is commonly thought that the static coefficients of friction are higher than the dynamic or kinetic values. This is a very simplistic statement and quite misleading for brake materials. With many brake materials the dynamic coefficient of friction quoted is an "average" value when the material is subject to a range of sliding speeds, surface pressures and most importantly operating temperatures. If the static situation is considered at the same pressure, but at ambient temperature, then the static coefficient of friction is often significantly LOWER than the average quoted dynamic value. It can be as low as 40 - 50% of the quoted dynamic value.

Kinetic (Sliding) versus Static Frictional Coefficients

Kinetic or sliding frictional coefficients are used with relative motion between objects. Static frictional coefficients are used for objects without relative motion. Note that static coefficients are somewhat higher than the kinetic or sliding coefficients. More force