Paper 5: Preparation and Properties of Molecularly Flat Surfaces

1967 ◽  
Vol 182 (11) ◽  
pp. 65-69
Author(s):  
R. H. S. Winterton
Keyword(s):  
Shear Strength ◽  
Surface Damage ◽  
Surface Forces ◽  
Calcium Stearate ◽  
Smooth Surfaces ◽  
Flat Surfaces ◽  
Surface Energies ◽  
Multiple Beam ◽  
Strong Adhesion

Very smooth surfaces may be prepared by cleaving suitable crystals of mica and other lamellar solids. Large areas of these surfaces are free from steps, even 1 molecule high, as may be shown by multiple beam interference techniques. In this paper the nature of contact between mica surfaces has been investigated and strong adhesion has been found. The application of tangential loads to cause sliding results in severe surface damage—Amonton's law is not obeyed. If a monolayer of calcium stearate is spread on the surfaces, damage is eliminated and the shear strength of the interface falls to one-fortieth of its previous value. Surface energies of mica, and mica coated with monolayers, have been measured and, in addition, these very well-defined surfaces are suitable for measuring the range of action of surface forces.

The area of real contact and the shear strength of monomolecular layers of a boundary lubricant

1955 ◽  
Vol 227 (1171) ◽  
pp. 500-515 ◽  
Keyword(s):  
Shear Strength ◽  
Molecular Layer ◽  
Tangential Force ◽  
Surface Damage ◽  
Calcium Stearate ◽  
A Value ◽  
Boundary Lubricant ◽  
Tangential Forces ◽  
Monomolecular Layers ◽  
Single Molecular Layer

A method has been developed by which molecularly smooth surfaces may be placed together and the area of contact formed between them measured. Selected sheets of mica were cleaved to be free from cleavage steps on both sides of the sheets. These were bowed up and mounted as crossed cylinders in an apparatus in which normal and tangential loads could be applied. The area of contact formed between the surfaces has been examined using multiple-beam interference techniques. Examination of the hue and intensity of the central region of the interferograms enables a separation of the mica sheets to be detected even if this is of only a few ångströms. This means that the boundary of the region of contact may be determined with a greater precision than has hitherto been possible. Fringes of equal chromatic order provide a sensitive means for the detection of small particles of accidental contamination and have been used to show when the contact is intimate over the whole region. Normal and tangential forces have been applied to the mica specimens, both when in a clean condition and when covered with a monomolecular layer of calcium stearate. The area of contact was observed simultaneously. The area and the force required to shear it being known, the shear strength of the junctions has been calculated. The damage occurring during the process has been examined using reflexion electron microscopy. At certain values of the tangential force smooth sliding takes place on the lubricated surfaces and no surface damage can be detected. Further increase of the tangential force produces a rapid slip causing fragments to be torn out of the surface and some damage occurs. No smooth sliding was detected on unlubricated surfaces of mica and the damage produced during the slip was extensive. The force required to shear the junction formed between clean smooth mica surfaces is very high. A value of 10 Kg/mm 2 has been obtained. For the monomolecular layers of calcium stearate the value obtained is about forty times lower but is by no means negligible. This means that a considerable force is required to shear a film of boundary lubricant and accounts for the otherwise unexplained observation that when metals are lubricated with a single molecular layer of soap or fatty acid the frictional force is reduced by only a factor of 10 while the wear decreases by a factor of 10000 or more.

Surface Forces Derived from Surface Energies

2019 ◽  
pp. 140-180
Author(s):  
C. Mathew Mate ◽  
Robert W. Carpick
Keyword(s):  
Surface Roughness ◽  
Rough Surfaces ◽  
Surface Forces ◽  
Adhesion Forces ◽  
Flat Surfaces ◽  
Surface Energies ◽  
Contact Points ◽  
Starting Point ◽  
Jkr Theory ◽  
Flat Geometry

As it more practical to measure the forces acting between two contacting surfaces then the energies of surfaces, this chapter covers those surface forces that are derived from surface energies. The starting point is Derjaguin’s approximation, which relates the energy between two flat surfaces to the force in other geometries: sphere/flat, sphere/sphere, and crossed cylinders. Next is a discussion of the surface forces in dry contacts with no liquid menisci around the contact points. This discussion covers the cases where adhesion causes significant deformation (JKR theory), where deformation is insignificant (DMT theory), and the cases in between. How surface roughness impacts adhesion is also discussed. The second half of this chapter deals with how liquid menisci around contacts contribute to adhesion forces, both for the sphere-on-flat geometry and for contacting rough surfaces.

Friction of diamond, graphite, and carbon and the influence of surface films

1951 ◽  
Vol 208 (1095) ◽  
pp. 444-455 ◽  
Keyword(s):  
Shear Strength ◽  
Large Scale ◽  
Physical Adsorption ◽  
Surface Films ◽  
Low Friction ◽  
Real Area Of Contact ◽  
Carbon And Graphite ◽  
Strong Adhesion ◽  
Frictional Behaviour ◽  
Real Area

This paper describes an experimental study of the frictional behaviour of diamond, graphite and of carbon which have been outgassed in vacuo . The removal of surface films which are normally present causes a large increase in the friction. The admission of a small amount of oxygen, water vapour or other contaminant will reduce the friction. Both physical adsorption and chemical adsorption are important. There is evidence that with clean graphite surfaces there is strong adhesion at the interface, so that when sliding takes place slip and shearing occurs beneath the surface. Carbon and graphite have a negative tem perature coefficient of friction. The low friction normally observed with diamond is due to the presence of adsorbed oxygen and other gases. The friction of clean diamond on diamond is high, and the shear strength at the interface is comparable with the shear strength of diamond. Large-scale seizure does not occur because the deformation of the diamond in the region of contact is elastic and the real area of contact necessarily remains small.

Syntheses of hyperbranched liquid-crystalline biopolymers with strong adhesion from phenolic phytomonomers

2012 ◽  
Vol 84 (12) ◽  
pp. 2559-2568 ◽  
Author(s):  
Siqian Wang ◽  
Daisaku Kaneko ◽  
Kai Kan ◽  
Xin Jin ◽  
Tatsuo Kaneko
Keyword(s):  
Molecular Weight ◽  
Shear Strength ◽  
Liquid Crystalline ◽  
Hydroxycinnamic Acid ◽  
Carbon Substrate ◽  
High Shear ◽  
High Glass Transition Temperature ◽  
Degradation Temperature ◽  
Strong Adhesion ◽  
High Shear Strength

A novel thermotropic liquid-crystalline (LC) biocopolymer, poly{trans-3-methoxyl-4-hydroxycinnamic acid (MHCA: ferulic acid)-co-trans-3,4-dihydroxycinnamic acid (DHCA: caffeic acid)}, was synthesized by a thermal acidolysis-polycondensation of MHCA and DHCA, efficiently catalyzed by Na2HPO4. When the MHCA composition of poly(MHCA-co-DHCA) was 60, 75, and 90 mol %, the copolymers showed a nematic LC phase although individual homopolymers such as polyMHCA and polyDHCA did not exhibit LC phase. Poly(MHCA-co-DHCA)s showed high molecular weight (Mw) ranged between Mn 2.6 × 104 to 3.7 × 104 and Mw 8.2 × 104 to 13.1 × 104, respectively, high glass-transition temperature (Tg) with the range of 115 to 140 °C and high degradation temperature T10, from 315 to 356 °C. In the adhesive test of copolymers against the surface of carbon substrate, the copolymers showed high shear strength at fracture.

The shear strength of thin lubricant films

1973 ◽  
Vol 333 (1592) ◽  
pp. 99-114 ◽  
Keyword(s):  
Shear Strength ◽  
Stearic Acid ◽  
Sebacic Acid ◽  
Activation Energies ◽  
High Density ◽  
Thin Layers ◽  
Calcium Stearate ◽  
Effect Of Temperature ◽  
Similar Process ◽  
Shear Process

This paper describes a study of the effects of pressure and temperature on the shear strength of very thin layers of a number of lubricants. The shear strength is deduced from measurements of the tangential (frictional) force required to slide glass spheres over glass plates coated with the lubricant. It is assumed that no glass-glass contact occurs through the lubricant film and that the area of lubricant sheared is the geometrical contact area determined by the elastic constants of the sliding surfaces. Consequently by varying the radius of the sphere and the normal load the contact pressure can be varied from 10 7 to 5 x 10 9 Pa (1 Pa = 1 Nm -2 ). The results show that the shear strength of Langmuir-Blodgett films of calcium stearate is constant at low pressures, but at pressures greater than 5 x 10 7 Pa it increases approximately in proportion to the pressure. At higher temperatures the behaviour is similar but the shear strength is reduced at all pressures. Similar experiments have been carried out on retracted stearic acid films, on vapour deposited sebacic acid films and on smeared films of copper stearate and anthracene. Thin films of high density polythene and PTFE have also been studied. For these materials both the absolute value of the shear strength and its variation with pressure resemble the behaviour of calcium stearate. Experiments on the effect of temperature on the shear strength show that calcium stearate, stearic acid and high density polythene exhibit similar ‘activation energies’ for the shear process. This suggests that these materials all shear by a similar process involving the sliding of linearly orientated methylene groups over one another. PTFE behaves in a similar way although its ‘activation energy’ is a little higher. By contrast anthracene and sebacic acid have much lower activation energies suggesting that a different shear process is involved.

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