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MCE Hanshan - Surface Engineering - Week 1 - Coggle Diagram
MCE Hanshan - Surface Engineering - Week 1
Fundamentals
The surface is defined as the first few layers of the substrate
4 main types - ideal flat surfaces, bulk truncated surfaces , high index planes and stepped surfaces.
Ideal flat surface = truncated bulk structure of a perfect crystal (has been directly cut through) - only 1 atomic plane makes up the surface, i.e. there are no steps present in the surface
Bulk truncated surface - still classed as low index, but cuts across several cleavage planes (has been cut along several different directions) (e.g. (111), (110) etc.). Each cleavage plane contains: different atom arrangements, different atom densities and different surface properties. Each cleavage plane will also have its own surface energies. However, each cleavage plane is an ideal surface, i.e. is not stepped.
High index planes - comes from cutting a crystal along a high index plane (i.e one of the Miller indices is greater than 1). Exposes a second or third layer of atoms due to open structure.
Stepped surfaces - consisting of terraces of low index planes. Since these surfaces are stepped, they require a high energy to form, as they are thermodynamically unstable.
Curing the high energy surfaces
Surface relaxation
Changes the position of entire surface layers leading to smaller interlayer spacing - tries to lower energy by moving the surface layers closer together
Surface reconstruction
A change in the 2D structure (only affects the top layer of atoms, interlayer spacing is not affected) - pairs of atoms move closer together to lower the energy.
Surface adsorption
Hertz Knudsen equation describes the sticking of molecules onto a surface by expressing the time rate of change of the concentration of molecules on the surface as a function of the pressure of the gas and other parameters. Look to notes for equation.
Physisorption- Van der Waals forces - weak, low activation energy, negative effect with temperature, multilayer, reversible and fast
Chemisorption - chemical bond, strong, high adsorption heat and activation energy, often irreversible, positive effect with temperature, single layer
Surface contact and friction
Friction
Resistance to motion, energy dissipation (heat, sound, radiation etc.)
Rule 1: F (friction force) = mu*W (mu = coefficient of friction, W = weight or normal contact force)
Rule 2: F is independent of the observed area A (identical blocks on identical surfaces will slide down at the same rate)
Real surface is never flat (asperities)
Real contact area is the sum of the contact areas between the asperities
Normal contact force = yield stress * real contact area
Physical mechanisms of friction
Adhesion (physiochemical) - Adhesion, plastic deformation, junction growth, eventual fracture - area of contact that grows with plastic deformation is known as the adhesion junction of the cold weld
Ploughing (mechanical) - scratching or grooving, accumulation of material in a "bow wave" - mu = 2/ pi* tan 0, where theta is the angle of intrusion.
Estimation of coefficient of friction
F transverse = the shear strength * the real contact area
Normal contact force = yield strength * real contact area
mu = F transverse / F normal
therefore, mu = shear strength / yield strength
Wear
The removal of material from interacting surfaces in motion
Can be measured vie volume loss or weight loss
Mechanisms
Abrasive wear
Adhesive wear
Oxidational wear
Erosion wear
Fatigue wear
Fretting wear
Adhesive wear steps
Asperity contact, plastic deformation.
Adhesion, cold weld
Materials transfer, debris formation
Look to Archard's wear equation
Metallurgical compatability = mutual solubility
Plasticity index
E/H (Young's Modulus and Hardness). Metals are highest, followed by ceramics, and then diamond like carbon is the lowest
Abrasive wear
Micro-cutting, ploughing, brittle fracture
Concerning Archard's Wear equation
If the hardness of the abrasive particle is less than 1.2 times the hardness of the material, then the process is called soft abrasion
If the hardness of the abrasive particles is more than 1.2 times the hardness of the material, then it is known as hard abrasion
Surface engineering
Design of a surface and substrate together to form a functionally graded system (form a chemical gradient)
Surface layer is chemically different to the substrate material
Two main types
Surface coating
A layer of different material is added to the surface - surface layer may have poor bonding to substrate
Physical - physical vapour deposition and thermal spraying
Chemical - Chemical vapour deposition and atomic layer deposition
Electrochemical (electroplating)
Advantages
Can improve both structural and functional properties
No limitations of substrate type
Large number of coatings
Limitations
Weak interface bonding
Thin coating layers (low load bearing capacity)
Duplex surface engineering involves both surface coatings and modifications to get the best of both worlds
Surface modification
Surface composition and / or microstructure is modified. Continuous chemical gradient - no separation between discrete layers
Advantages - Thick case without interface, effective wear/ fatigue protection, cost effective and widely used, high load bearing capacity
Disadvantages - Limited number of processes (needs the substrate to be compatible), not effective for functional materials. (e.g. cannot improve magnetism)
Microstructure modification
Mechanical (laser and shot peening)
Thermal (laser and induction heating
Composition modification
Thermochemical (carburising / nitriding)
Physical (implantation)