Coatings

Question 1

Name 3 methods to attach a very thin (1 molecule thick) layer to a surface.

Answer 1

• Radiation grafting/photografting
• Surface patterning
• Monolayer structures

Question 2

What ca plasma etching achieve

Answer 2

• Clean
• Sterilise
• Cross link surface polymers
• Activate a previously unactive surface

Question 3

What is the use of plasma etching/plasma deposition?

Answer 3

Form barrier films
Protective coating
Electrically insulating coatings
Reduce absorption from environment • Inhibit release of leachables
Control drug delivery rate

Question 4

How does plasma etching/deposition modify cell and protein reactions?

Answer 4

• Improve biocompatiility
• Promote selective protein absorption
• Enhance cell adhesion and growth
• Produce non-fouling surface
• Reduce friction

Question 5

What are the specific requirements for plasma processing polymers?

Answer 5

• Plasma must be of low energy and low temperature (e.g. 200 W, room temperature, 60 s).
• Typical conditions in the plasma chamber: 25-60 °C, operating pressure 0.025-1.0 Torr

Question 6

What does Plasma etching achieve?

Answer 6

Plasma treatment functionalises surface by removing atoms from surface layer, creating reaction and anchorage sites
-combined with-
Reactive small molecules in the gas phase combine to form higher molecular weight units or particulates that may settle or precipitate onto surface.

Question 7

What is the mechanism of plasma etching?

Answer 7

Removal of atoms and precipitation onto surface

Question 8

What are the three main methods of grafting?

Answer 8

Using ionising radiation; cobalt-60 source
Using UV radiation
Using high energy electron beams

Question 9

What is the mechanism of radiation grafting?

Answer 9

• Radiation breaks chemical bonds to give free radicals
• Reactive species on surface are exposed to a monomer
• Monomer reacts with the free radicals at the surface and propagates as a free radical chain reaction incorporating other monomers into a surface grafted polymer

Question 10

What are the Grafting routes?

Answer 10

• Substrate immersed in monomer solution and then irradiated
• Substrate irradiated under inert atmosphere or low temperatures and then exposed to monomer solution
• Substrate irradiated in oxygen or air
Peroxide groups are formed on surface
Surface is heated then exposed to redox reactant such as Fe2+
Causes decomposition of the peroxide groups to form free radicals that initiate graft polymerisation

Question 11

Immobilisation of molecules using plasmas

Answer 11

• Plasmas - introduce organic functional groups to the surface of a polymer
• These functional groups (amine, hydroxl etc) can then attach biomolecules to the surface
• Surfactants can also be immobilised to the surface

Question 12

Name the two methods used to pattern a surface with functional groups, proteins or peptide sequences.

Answer 12

Micro-contact printing
To fabricate PDMS stamps with relief features, the precursor is mixed with a curing agent, poured onto a template, and cured to crosslink the polymer.
• Patterns with features down to 50 nm have been reproduced using microprinting with PDMS stamps.

Flow channels

Question 13

Why is PDMS a good material for micro-printing?

Answer 13

Low surface energy, due to flexibility of siloxane chain and low intermolecular forces between methyl groups
• Easily separated from the template during fabrication
• Binds reversibly to substance to be transferred during printing
• Facilitates peeling of stamp from substrate after printing (without smearing)
• Surface energy can be lowered further by binding fluorinated silanes to it • Relatively inert
• Does not react with many chemicals
• Does swell in organic solvents, limiting its use in those solvents

Question 14

What are the challenges of micro-printing?

Answer 14

• Protein molecules undergo conformational changes during inking step to adsorb onto the PDMS surface
• For efficient micro patterning to occur the receiving surface needs to have properties that make it more favourable for the protein to transfer than to remain on the stamp.

Question 15

What are Self assembled monolayers?

Answer 15

Surface coating films that form spontaneously as highly ordered structures(2-D crystals) on specific substrates

Examples include:
• n-alkyl silanes on hydroxylated surfaces such as glass, silica, alumina
• alkane thiols such as CH3(CH2)nSH and di-thiols on some metals e.g. Au, Ag, Cu • Amines and alcohols on platinum
• Carboxylic acids on aluminium oxide

Question 16

What are the advantages of Self assembled monolayers?

Answer 16

• Ease of formation
• Chemical stability
• Multiple options for changing the chemistry of the outermost group that interfaces with the outside environment

Question 17

Why is Hydroxyapatite a good coating for bone implants?

Answer 17

• HA is the primary structural component of bone
  o Consists of Ca2+ ions surrounded by PO42- and OH- ions
• One of the most important bio ceramic matierals for its bioactivity and stability
• Unlike other calcium phosphates, HA does not break down under physiological conditions and s stable at physiological pH
• Forms strong chemical bonds with surrounding bone

Question 18

What are the advantages of nano sized HA?

Answer 18

• Nanosized HA has higher surface area and surface roughness resulting in superior surface functional properties compared to its microphase counterpart
• Mimics the bone mineral in composition and structure
• Promotes osteointegration and subsequent bone tissue formation
• Much higher bioactivity than micron sized ceramics
• Capability of decreasing apoptotic cell death and hence improving cell proliferation and cellular activity related to bone growth

Question 19

What is the thermal spray coating process?

Answer 19

Power feedstock -> Powder particle melting -> Molten particle acceleration -> Particle/substrate impact -> Coating

Takes a materials, melts it, atomises it and its then propelled by compressed gas onto the surface. It sticks because of a mechanical bond to the surface being sprayed

Question 20

At what temperature does plasma spraying occur?

Answer 20

Up to 30,000

Question 21

What are the conditions for Arc spraying?

Answer 21

6500°C

18-40V current 50-150V

Question 22

Explain the process of PVD.

Answer 22

A material is converted to its vapour has in a vacuum chamber and condensed onto a substrate surface as a thin film
The vapour is transported across a region of low pressure from its source to the substrate
Vapour undergoes condensation on the substrate to form the thin film

Question 23

Explain thermal evaporation.

Answer 23

• A metal is evaporated by passing a high current through a highly refractory material contaminant structure
• Once the metal is evaporated, its vapour undergoes collisions with the surrounding gas molecules inside the evaporation chamber
• As a result, a fraction is scattered within a given distance during their transfer through the ambient gas
• Therefore, pressure lower than 10-5 is necessary to maintain for a straight-line path for evaporated molecules (i.e. line of sight)

Question 24

Explain e-beam evaporation

Answer 24

• In this mode of operation high intensity electron beam gun is focused on the target material i.e. placed in a water-cooled copper
• The process begins under a vacuum. A tungsten filament is heated so that it will give e- which forms a beam i.e. deflected & focus onto the material to be evaporated by the magnetic field
• When the E-beam strikes the target material, the kinetic energy of the motion is transferred into thermal energy

Question 25

Explain the 4 steps of sputtering

Answer 25

1) Ions are generated & directed at the target material 2) The ions sputter atoms from the target material 3) The sputter atom gets transported to the substrate through a region of reduced pressure 4) The sputter atoms condense on the substrate forming a thin film RF Sputtering

Question 26

What is RF sputtering?

Answer 26

RF sputtering allows the sputtering of targets that are electrical insulators • The target attracts Ar ions during one half of the cycle • The electrons are more mobile & build up a negative charge called a ‘self bias’ which helps in attracting Ar ions which do the sputtering

Question 27

What is DC sputtering?

Answer 27

* Sputtering can also be achieved by applying a large DC voltage (approx 2000v) * A plasma discharge is established & the argon ions will be attracted to an impact sputtering of the target atoms * In DC sputtering the target must be electrically conducted otherwise the target surface will charge with the collection of ion & repel other Ar ions

Question 28

What is magnetron sputtering?

Answer 28

* In this technique, a magnetic field, mainly parallel to the target surface, is superimposed to the applied electric field so that the secondary electrons (emitted by the target during its bombardment) are trapped near the target surface * One single electron can induce several argon ionizations before being lost by recombination on the chamber walls * This results in a large increase of the plasma ionization rate at the target surface and then in a significant increase of the deposition rate * Magnetron sputtering has been widely used in the literature, with a great deal of research coming from Nottingham Uni. The research conducted focusses on multiple areas, such as electrical wire coatings, degradable biomedical glasses, as well as bioactive and antibacterial titanate structures (an example pictured below)

Question 29

What are the advantages and disadvantages of magnetron sputtering?

Answer 29

ADVANTAGES • Materials can be deposited with improved properties compared to the substrate material • Almost any type of inorganic material can be used as well as some kinds of organic materials • Great variety of coatings can be produced • High wear resistance • Low frictional co-efficient • NO toxic reaction product • Excellent adherence • Uniform coating thickness DISADVANTAGES • It is a line of sight technique meaning that it is extremely difficult to coat undercuts and similar surface features • High capital cost • Some processes operate at high vacuums and temperatures requiring skilled operators and specialist equipment • Processes requiring large amounts of heat require appropriate cooling systems • The rate of coating deposition is usually quite slow

Question 30

What are the types of sources used for CVD?

Answer 30

Gasses Volatile liquids Sublimable solids Combination

Question 31

Source materials for CVD should be:

Answer 31

o Stable at room temperature o Sufficiently volatile o High enough partial pressure to get good growth rates o Reaction temperature < melting point of substrate o Produce desired element on substrate with easily removable by-products o Low toxicity

Question 32

What are the four types of CVD?

Answer 32

Atmospheric pressure CVA Low pressure CVD Plasma enhanced CVD Metal-organic cVD

Question 33

What are the advantages of LPCVD?

Answer 33

• Low deposition rate limited by surface reaction, so uniform film thickness (many wafers stacked vertically facing each other; in APCVD, wafers have to be laid horizontally side by side. • Gas pressures around 1-1000mTorr (lower P => higher diffusivity of gas to substrate) • Better film uniformity & step coverage and fewer defects • Process temperature 500°C

Question 34

What are the advantages of PECVD?

Answer 34

• Plasma helps to break up gas molecules: high reactivity, able to process at lower temperature and lower pressure (good for electronics on plastics). • Pressure higher than in sputter deposition: more collision in gas phase, less ion bombardment on substrate • Can run in RF plasma mode: avoid charge buildup for insulators • Film quality is poorer than LPCVD. • Process temperature around 100 - 400°C.

Question 35

What are the advantages and disadvantages of CVD?

Answer 35

Advantages: • High growth rates possible, good reproducibility • Can deposit materials which are hard to evaporate. • Can grow epitaxial films. In this case also termed as “vapor phase epitaxy (VPE)”. For instance, MOCVD (metal-organic CVD) is also called OMVPE (organo-metallic VPE) • Generally better film quality, more conformal step coverage Disadvantages: • High process temperatures • Complex processes, toxic and corrosive gasses • Film may not be pure (hydrogen incorporation...)

Question 36

Explain what Atomic layer deposition is.

Answer 36

A thin film synthesis process based on sequential, self-limiting surface reactions between vapours of chemical precursors and a solid surface to deposit films in an atomic layer-by-layer manner. - ALD involves the use of a pair of reagents. - each reacts with the surface completely - each will not react with itself - This setup eliminates line of site requirements - Application of this AB Scheme - Reforms the surface - Adds precisely 1 monolayer - Pulsed Valves allow atomic layer precision in growth - Viscous flow (~1 torr) allows rapid growth

Question 37

What are the differences between PVD and CVD/ALD?

Answer 37

PVD 1. Atoms as a source material 2. Solid source materials 3. Vacuum/high vacuum 4. Elemental films mostly 5. Room temperature 6. Alloy films easily 7. One process, many materials CVD 1. Molecules as source material 2. Solid, liquid, gas precursors 3. Fluid dynamics important 4. Molecular/compound films mostly, chemical bonds broken and formed 5. Needs elevated temperatures 6. Elements and compounds ok, alloys more difficult 7. Each process materials specific

Question 38

Sketch a coating.

Answer 38

``` Substrate at the bottom Showing surface roughness of substrate and then coating with arrow pointing to adhesion with the substrate within coating arrows to: - Oxides Pore coloured in black Particle cohesion between particles ```