Structural Integrity Part 2: Wear and Coatings

Surfaces: the contact area of 2 surfaces ~10%

• can quantify the roughness of a surface in various ways
• Machining processes can change the properties of a surface compared to the bulk
• The affinity for Oxygen and diffusivity of oxygen on a surface determines the oxides present

Surface Profilometry: A Sharp stylus is drawn over the surface; Doesn't represent the actual profile of the surface due to the finite tip radius of the stylus. Output:

• Peak to valley height (Rt): not useful, different roughnesses may have the same value
• Centreline average (CLA/Ra): more useful, the average value of the vertical deviation of the profile from the centre line
• Root mean square (Rs): square root of the arithmetic mean of squares of the deviations from the centre line 

Distribution curve --> cumulative distribution:  shows how the contact area changes as the surface wears; increase in wear results in an increase in contact

Wear: loss of materials from a surface; Abrasion is the most important mechanism

Adhesive: surfaces stick at asperity junctions

• shearing occurs when the junction is stronger than both materials
• Rapid wear when P =H/3
  • plastically deformed regions overlap producing a higher wear rate

Abrasive wear

• 2 body: scoring of the soft surface by a hard one
• 3 body: indentation of soft surface 
• derivation of wear volume does not take into account the distribution of asperity heights and shapes
• Abrasion is negligible if Hs/HA > 1.3
• elasticity affects abrasion
• The elastic limit of strain: E' /Y < 70 for a smooth surface; E'/Y < 35 for a rougher surface; at room temperature and when Y: the yield stress

Corrosive wear: corrosion increases if the products are removed from the surface e.g. by rubbing action; products may be abrasive so increases wear

Fatigue wear: rolling contact fatigue beneath the surface; formation of large wear fragments after a critical number of cycles

Sliding contacts: asperities may be deformed repeatedly until they beak away

Fretting wear: A combination of adhesive and fatigue wear; low amplitude vibrations between two metal surfaces which are loaded together 

Erosive wear: Impingement of solid particles

• Dependent on the angle of incidence of the particle and if it is ductile or brittle
• Ductile: low angle particles dig into the material and it abrades away
• Brittle: High angle particles can cause cracking 

Fluid corrosion: high-speed fluid exceeds the yield stress of the material leading to plastic deformation or fracture

Cavitation Erosion: gas bubbles on the surface collapse due to pressure changes in a liquid

Wear rate vs load

• An increase in load leads to an increase in the wear rate but oxide creation decreases the wear rate
• An increase in the speed equates to an increase in the temperature and thus the likelihood of creating an oxide protective layer/wear increases
• Wear rate: volume of removed material divided by the sliding distance 

Thermal effects in sliding contacts: can generate heat at the contact between surfaces; The speed influences the heat produced and the wear behaviour

Wear test methods: different test methods e.g. pin-on-disc, Block on pin: anti-seizure resistance;  twin disc: surface rolling +sliding, Hammer wear: impact testing, Rubber wheel abrasion: abrasion; Dry abrasion: abraided wheel, Wet slurry abrasion: like pin on wheel 

Contact stresses

• Rule: stress by the surface load is less than the yield stress and fracture limit
• treatment: Thermal hardening

Sliding: low temperature, roughness, wear particle generation

• stable sliding: low shear strength and minimum wear

Surface fatigue: high elasticity and thickness allow deflection and avoid high-stress peaks and distributes stress

Fretting: enhance the generation of low friction surface; elastic; surface inertness; hinder crack initiation

Abrasion: high hardness to inhibit groove formation; Thick to prevent abradant penetration

Impact: elastic normal to the impact, to absorb energy

Chemical dissolution/corrosion

• Corrosion: inert, dense, and pore-free to prevent chemical attack: sacrificial properties
• Chemical dissolution: high thermal and chemical stability, high toughness, and high hardness 

Lubricant: coating should function beneficially with lubricator

Modell factor

• H/E --> wear decreases with increasing factor; wear decreases with decreasing E  (for the same H); Wear increases with increasing H

Elastic strain to failure: The amount of strain a coating can take before yielding; increasing strain --> increases H/E and decreases E/H

• Resilience: ability to absorb elastic energy; H²/E² --> indicator of resistance to plastic deformation [coated and uncoated systems]
• Ultimate resistance: ability to resist cavitation; shows that some wear mechanisms --> a degree of plastic deformation can be beneficial
• Toughness: ability to absorb energy in the plastic range

Multilayered structure: Combine low and high modulus phases

• Soft layers shear, hard layers slide over each other
• Bending stress in hard layers is prevented from rising excessively

Nanoparticles

• Nano-composite of hard particles in a ductile matrix
• Advantages: high hardness and yield stress; Hall-petch effect; grain hardening; E unchanged so H/E is high; E decreases as the fraction of intergranular phase becomes substantial
  • Increase in corrosion resistance 
  • Better wear resistance 

Wet Plating: wear and corrosion protection; In an aqueous bath with chemical salts (organic liquid or molten media); Can use an electrical bias

Electroplating: Cathode: Workpiece; Electrolyte: the passage of current, solution of H₂O and salt of the metal;  Anode: metal to be deposited

Bath plating

• Operation temperature does not exceed 100 degrees celsius
• Thickness is proportional to the current density and the deposition time
• Current density is not uniform leading to the coating being thicker at the edges and corners
• size of the bath limits the dimensions of the workpiece 
• can mask areas that do not want to be deposited
• Can be automated
• Properties:  rate of deposition is determined by the density of electric current per unit area of the cathode surface
• current flow to  projections is greater than to recesses 

Brush/Selective plating: restore dimensions of work parts; localized deposits

• Anode enveloped with absorbed pad (soaked in the electrolyte)
• Bring cathode workpiece in contact to get deposition; Manually applied; No masking needed

Coating materials: conductive;  consider electro potentials

• Binary + ternary alloys: plating conditions need to equalize the deposition potentials of the constituents
• Metals with high negative potentials (~1.2) cannot be deposited from aqueous solutions e.g. Al and Ti --> electroplated

Hard chromium: 800-1000 HV

• resistance to abrasion, adhesive wear, corrosion, and oxidation
• High strength
• > 50 microns in thickness

Electroless deposition: from aqueous solution --> chemical reaction; initial layer catalyzes subsequent deposition; nonconductive materials

Electroless Ni: high hardness

• no edge effects, local current density; uniform thickness
• mask areas not depositing
• dimensions of the workpiece are limited by tank size
• can create ternary alloys and composite coatings

NiP: 400-500 HV, corrosion and wear resistance; Raw material expensive

NiB: 500-750HV

• less ductile and high strength than NiP
• Better wear resistance, greater operating cost, lower deposition rates, requires lower temperatures

Hard anodizing: thickens and strengthens naturally formed thin oxide on metals

• Workpiece: anode in a tank with acidic electrolyte 
• Oxygen is generated at the anode oxidises the metal and there is a balance between action and dissolution in the electrolyte 
• High current density, low operating temperature,
• Can densify the coating by applying  AC on top of DC
• Complex equipment, can be automated
• 400-500 HV
• Brittle - not resistant to shock loading
• Microstructure: cell-like structure 

Phosphate conversion coating

• Treatment with dilute phosphate acid
• Surface forms with an insoluble, protective crystalline phosphate layer
• small items --> tumbled on barrels
• Larger --> sprayed 
• Carbides--> immersed

Types: Phosphoric acid +dissoved metal phosphates

• Fe-Phosphate: Tough, corrosion-resistant; used as a base coat for painting
• Zn Phosphate: base coat for painting; lubricant to aid cold forming and tube and wire drawing; resistance to mild wear
• Mn Phospahe: applied by immersion; porous and acts as a base for oil film retention

Chromate Conversion coatings: Chemical attack from chromic acid producing a protective film; high corrosion resistance

• coating dissolves in water BUT excellent protection in marine and high humidity environments
• Protection increases with thickness to a limit
  • As porous, non-adherent film produced 
• provides a non-porous bonding surface

Galvanizing: immersion of Fe and Steel in molten Zn bath

• sublayers with different Zn content; metallurgical bond
• Applications: sacrificial protection against corrosion
• use where steel is exposed in the atmosphere/ water 

Thermal spraying: produce thick, wear-resistant coatings

• material fed into torch/gun, droplets accelerated to substrate
• lamellar structure, overlapping and interlocking of particles as they solidify
• There is a size limitation: can only coat what the torch gun can see (lin-of-sight)
• requires roughened substrate; impact velocity may rupture protective film on the substrate
• Interdiffusion and localized fusion of particles
• may form a direct chemical bond or Van der Waals 
• increase in wear and corrosion resistance; restore work parts
• Processes: Flame, HVOF, Detonation gun

Weld hard facing: hard wear and corrosion-resistant surfaces via welding, thick coatings

• used where large amounts of wear can be tolerated
• coating materials : filler rod/wire/consumable electrode
• Applications: repair worn components, where the lubricant is absent

Materials

• carbides and Boron resist abrasive wear
• compositional changes occur during welding so need to pick the choice of electrode carefully
  • Austenitic Manganese steels
  • Martensitic and high-speed steels
  • Nickel and cobalt-based alloys
  • Tungsten carbide composites
  • Martensitic, austenitic, and high chromium iron

Vapour deposition: PVD: from solid, CVD: from gas, need exhaust scrubber to get rid of contaminants

• Plasma Assisted CVD: operated at a lower temperature, gas is ionised and accelerated to sample which is biased

• can use gases to maintain suitable pressure or provide reactive species
• Substrate temperature < 500 degrees Celcius, determined by source heat

Vacuum Evaporation: line fo sight, Deposition rate in microns

• Sources of vapour: resistively heat sources, current heats component (from Tungsten filament), Radio frequency heated sources, electron beam evaporator (need low thermal conductivity)
• Feed material to the crucible to get long deposition runs
• Applications: turbine blades, ZrO2 barrier coatings etc.

Coating properties: Heat substance during deposition to enhance density and adhesion. The Structure has 3 zones 

• 1) grain structure , low density Ts/Tm  = 0.28
• 2) Columnar structure , Ts/Tm = 0.4
• 3) Equiaxed grains Ts/Tm = 0.8
  • increase wear resistant (dense) need Ts/Tm > 0.5
  • To do so, require high operating temperature --> may damage the substrate
• Solution: use Plasma assisted PVD 

Plasma Assisted PVD

• Argon DC diode --> can see a glow in the chamber 
  • atoms ionised and accelerated to the substrate to clean surface --> and improve adhesion
• Vapour sources: Electron beam;  Ion plating PAPVD: the substrate is subjected to a flux of high energy ion bathe
• Sputtering: bombardment  to create coating atoms, momentum transfer process; Use to clean also; Most materials used; Poor efficiency and deposition rates

Cathodic Arc Evaporation

• The voltage applied to target produces vapour, large amounts of ionisation can also occur
• High rates, all conducting materials can be used
• High costs May form macro-droplets

Electron beam: produces high rates, high efficiency, low cost of the source material, retain the surface finish of the substrate 

Advantages: uniform thickness, adhesion, control structure, etc.

Disadvantages: cost, line-of-sight, substrate strength, requires a chamber which limits its size