ENR 116 Engineering Materials Module 5 Surface Engineering

ENR 116 Engineering Materials Module 5 Surface Engineering Dr Tracie Whittle e. Tutor

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Surface Engineering

Intended Learning Outcomes At the end of this section, students will be able to: - • Understand that surface engineering involves modifying the properties of a surface, while maintaining the bulk properties of the material. • Describe key techniques used to modify or characterise surfaces

Surfaces are important • Bulk properties vs surface properties – We interact with surfaces – Interface with the environment • Drive towards smaller devices – Increased surface : volume ratio • New materials

What is it? • Surface engineering involves modifying the properties of a surface, while maintaining the bulk properties of the material. Surface modification Surface coating

Which properties? • • • Hardness Roughness / friction Wettability Optical / reflectivity Chemical functionality

Techniques • • Heat treatment / Diffusion Ion bombardment Sputtering Polymeric coatings

Heat treatment / Diffusion • Knife hardening – Heat to Austenitic temperature 1. Quench to form surface martensite 2. Or hold in charcoal for ~24 hrs to allow carbon to diffuse into the surface Hard outer layer Soft, flexible bulk

Polymeric coatings Chemical grafting Grafting to - polymer adsorption Grafting from Surface specific

Plasma Deposition RF Power heats electrons (>100, 000 K) Electron impacts fragment gas molecules, making them reactive A + e- A+ + 2 e. B-B + e- B∙ + e. Reactive molecules deposit on substrate

Plasma Deposition • Highly cross-linked “polymer-like” layer • Chemical functionality can be retained • Ultra-thin layers deposited (<10 nm)

Ion bombardment • High energy ions impact surface – Alter surface chemistry – Alter roughness Tissue Culture PS

Sputtering • Ions bombard a target (metal) • Metal atoms etched from target • Metal atoms accelerated to substrate • Metallic coatings can be very thin – Few nanometres

Surface Analysis • X-ray Photoelectron Spectroscopy (XPS) • Atomic Force Microscopy (AFM) • Time-of-Flight Secondary Ion Mass Spectrometry (To. F-SIMS) • Scanning Electron Microscopy (SEM)

X-ray Photoelectron Spectroscopy • Surface technique (<10 nm) • Chemical analysis – Elemental analysis – Chemical bonding • Can be used for – Powders (minerals, sand etc) – Surfaces (wafers, polymers) – Fibres

Photoelectric effect Light e- A Increasing frequency increased current Below a threshold frequency, no current was measured

History • Hertz discovered photoelectric effect 1887 • Einstein described “quanta” in 1905 (Nobel Prize 1921) • Rutherford (almost) described XPS equation in 1914 – EK = hn – EB • Resolution increased by 1954 (Seigbahn Nobel prize 1981) • Commercial instruments available 1970 • High energy resolution instruments ~1980

Instrumentation Analyser X-ray source Vacuum chamber Sample Detector UHV Pump

Instrumentation

Photoemission Kinetic energy Fermi level Binding energy hn Core levels hn must be greater than BE for photoemission

Basic method -V 2 -V 1 e- X-ray

Binding Energies Compare Binding Energies BE(1 s) > BE(2 p BE(C 1 s) < BE(N 1 s) < BE(O 1 s)<…

Typical spectra - Polymers C 1 s O 1 s N 1 s

Atomic Force Microscopy • Developed in 1986 (Binning and Quate) • Measures the topography of surfaces – Maximum scan area ~100 x 100 mm – Sub-nanometer resolution

Atomic Force Microscopy • A cantilever with a fine tip is rastered over the surface • Movements in the cantilever are measured via a laser AFM Tip

AFM of graphene

AFM of treated PS • Quantify roughness • Quantify friction

AFM Mechanical Properties Cantilever acts like a spring Force (n. N) Hard surface Measure the depth of the indentation vs force Soft surface

To. F-SIMS • Measures surface chemistry Heavy, high energy ions

To. F-SIMS spectra

Chemical mapping

Scanning Electron Microscopy (SEM) Electron beam • Nanometre resolution imaging Back scattered electrons X-rays Sample

Summary • Surface Engineering involves modifying the properties of a surface, while maintaining the bulk properties of the material. • Techniques used to modify surfaces include heat treatments, polymeric coatings, plasma polymerisation, ion bombardment and sputtering • Surfaces can be characterised using a number of different methods. Common techniques include XPS, AFM, To. F SIMS and SEM

Thank you