Please enable JavaScript.
Coggle requires JavaScript to display documents.
MCE Hanshan - Surface Engineering - Week 2 - Coggle Diagram
MCE Hanshan - Surface Engineering - Week 2
Fundmamentals
Stress is highest in the surface of a component, hence the need for surface modification in certain scenarios
Archard's Equation (again)
Heat treatments
Increased hardness increases wear and fatigue properties
Martensite is a supersaturated form of C in Fe - carbon interstitially hardness, preventing quick diffusion from 1 crystal structure to another. Can be formed via quenching (rapid cooling)
Hardness profile is generally higher but also has a much steeper drop off for quenched samples when compared to slow cooled samples
Carburising vs Nitriding
Nitriding is carried out at lower temperatures
Quenching and tempering is not necessary for nitriding, as carbon steels are martensite hardened, but nitrogen steels are precipitation hardened
Nitriding depths are > 1mm, carburising is 0.25-4mm
Higher achievable hardness using nitriding
Limitations of thermochemical treatments
Thermal distortions and residual stresses
Reduced surface quality - oxidation and surface roughening
Long treatment time and high energy consumption
Difficult for selective hardnening
Ion implantation
Very expensive equipment
Can be used for doping (interstitial/ alloying) - very useful for semiconductors
Drawback is the generation of defects, such as voids and pores
Applications - moulds and tool steels, high performance bearings on satellites and rocket engines, body implants
Other drawbacks include that it makes a very thin case with a very low load bearing capacity (less than 1 micron), and is also very expensive
Principles of carburising
Only way to achieve desired properties - quenching does not give desired compromise of hardness at surface and toughness in core.
High C surface - very hard surface
Low C substrate - very tough core
3 stages
Dissociation - breaking of molecular bonds in diffusing species
Adsorption - employs physisorption followed by chemisorption
Diffusion - affected by temperature and vacancy concentration in material
Is a diffusion controlled process - concentration of carbon gradually decreases as you get further away from the surface, as diffusion will slowly get more difficult.
Different types of carburising
Pack (solid) carburising
Placing parts into a container filled with charcoal, gets heated and undergoes multistep reaction to produce carbon that will then diffuse into the components that have been rammed in.
Very simple, cheap process.
However, surface quality is not very good, as solid lumps of charcoal may not be in contact with the parts at all time
Gas carburisation
Placing the part into a vapour atmosphere, highly concentrated with a hydrocarbon gas, e.g. methane
Applications
Machine parts, small. large and heavy duty gears, pins etc.
Nitriding
Epsilon phase = Fe 2-3 N (HCP)
Gamma prime phase = Fe4N (FCC)
Compound layer - contains iron nitrides
Diffusion layer = layer where nitrogen has diffused into (not necessarily form intermetallics)
As the N content increases, we move from solid solution strengthening to grain boundary strengthening to precipitation strengthening
Alloying elements can be added to help promote precipitation strengthening, as these will be likely to react with nitrogen to form nitrides
However, these alloying elements will essentially trap the nitrogen and prevent it from diffusing. Therefore, with increasing alloying element concentration, surface hardness increases, but diffusion depth decreases.
Processes
Liquid Nitriding
Molten salt bath containing NaCN - environmental and working safety problems - not commonly used
Gas nitriding
Formation of surface brittle compound "white layer" - has to be removed.
Plasma nitriding
Highly efficient, environmentally friendly
High speed argon ions impact nitriding medium, causing nitrogen to be ejected (sputtering).
Nitrogen atom is then ionised by electron beam, where it is then attracted towards the negatively charged cathode. It then impacts the target (which is the component we are trying to nitride) and becomes embedded.
Property enhancement
Surface hardness
Wear resistance
Fatigue strength
Load bearing capacity
Physical Vapour deposition
3 main steps - creation of the material vapours, transport of the vapours, condensation of the vapours and growth of the coating
Heated in a vacuum so that vapour particles do not interact with atmosphere particles
Electron beam evaporation
Coating medium is placed in graphite or tungsten crucible.
Top surface of material is melted so that there is minimised contamination from crucible.
Due to high electron beam power available, high melting point materials can be evaporated
Material vapours are then deposited onto target
Crucible is designed to be very high Tm, to prevent melting and contamination
Sputtering
Momentum transfer from argon ions attracted to and impacting on the coating medium, which is used as the negatively charged cathode.
Thermal Spraying
The spray material is melted or nearly melted and projected against a substrate surface.
Carried out in normal atmosphere, allowing for oxidation of molten particles
Can produce lots of different types of defects, such as voids, oxidised particles and unmelted particles
Disadvantages
The relatively low bond strength (low adhesion, coherence and chemical bond strength, leads to very easy delamination)
The porosity of the coating - detrimental to the mechanical properties e.g. the load bearing capacity
Line of sight nature - if you can't see it, you can't spray it
Environmental concerns -- lots of noise, uses fossil fuels, potentially toxic metallic fumes
Flame spraying
Electrode continuously fed into heating nozzle
Electrode heated via flame of oxygen and acetylene, propylene, propane or hydrogen
Forms a thin tip at the electrode which will be easier to melt
Molten particles then sprayed onto substrate material
Advantages
Low capital investment
High deposition rates
Very flexible
Disadvantages
Low bonding strength
High porosity
High oxide level
High velocity oxyfuel spraying (HVOF)
Fuel gas and oxygen are burned in a sealed chamber
The product expands through the nozzle with high speed (supersonic, mach 4)
High density, high bonding and low oxide levels due to high speed of molten particle travel
Applications - valves for energy industry which are exposed to wear, oxidation and corrosion, and roll surfaces for paper production (erosion, abrasion and corrosion)
Aircraft components
Abradable seals - need to get thickness of surface modified layer spot on - too thick, and it adds weight to the component and increases intertial forces, too thin, and it exposes the substrate material to abrasion
Need to withstand high temperatures and oxidation, high cyclic stresses, vibration and creep and erosion.
Thermal barrier coatings
Yttria (Y2O3) stabilised tetragonal zirconia
Avoids volume change induced cracking due to tetragonal / monoclinic transformation.
Low thermal conductivity and high thermal shock resistance (does not transfer large amounts of heat energy to substrate, can withstand large thermal gradients within itself)
Formed of top coat, bond layer, naturally formed oxide layer and then finally substrate
Comparing electron beam PVD vs plasma sprayed thermal barrier coatings
EB PVD coating has more compliant columnar structure (allows for easier changes in substrate dimensions), has a higher thermal conductivity and has a longer life, but is more expensive.
The plasma sprayed coating has a layered structure that is less compliant, has a lower thermal conductivity (so accumulates larger amounts of thermal energy, leading to larger distortions) and is relatively inexpensive. Less compliant due to shear forces being aligned parallel, as oppose to perpendicular in columnar grains
Powder plasma spraying
Light, inert gases (Ar, He, H2, N2) are accelerated using a tungsten electrode - stripped of outer electrons to form a plasma
Powder is sprayed upwards through nozzle into plasma stream, where it is melted and can be deposited.
Nozzle is cooled using water to preventing overheating or melting in presence of high temperature plasma
Comparisons of spraying techniques
HVOF is highest velocity
Plasma is highest temperature
HVOF has lowest porosity, flame has highest
HVOF has highest bond strength, flame has lowest
Flame and HVOF both have high deposition rates, plasma has a low deposition rate
Flame is cheapest, plasma and HVOF are most expensive