Appunti Surface Engeneering

Surface Engineering

Summary

INTRODUCTION............................................................................................................... 5

CHROMIUM PLATING....................................................................................................... 6

Equipment for chromium electro-plating..................................................................7

Reversible rack plating............................................................................................. 9

Main applications of functional chromium:.............................................................14

NICKEL PLATING............................................................................................................ 15

Formulations.............................................................................................................. 15

ADDITIVES.............................................................................................................. 17

ANODES.................................................................................................................. 18

SURFACE IRREGULARITIES and MEASUREMENT............................................................21

ELECTROLESS PLATING................................................................................................. 23

NICKEL-PHOSPHORUS ELECTROLESS DEPOSITION..............................................27

ADHESION PHENOMENA............................................................................................... 30

FRICTION & WEAR......................................................................................................... 31

FRICTION................................................................................................................ 32

WEAR..................................................................................................................... 33

ZINC COATING FOR CORROSION PROTECTION.............................................................34

Hot dip galvanizing................................................................................................. 36

Zinc flakes.............................................................................................................. 36

Mechanical zinc...................................................................................................... 37

Sherardizing........................................................................................................... 37

WEAR OF MATERIALS.................................................................................................... 38

MECHANICAL WEAR................................................................................................... 38

CHROMATE CONVERSION COATINGS FOR ZINC PLATING..............................................41

CLEAR or BLUE-BRIGHT COATING...........................................................................42

YELLOW or IRIDISCENT CONVERSION COATINGS....................................................42

OLIVE DRAB COATING............................................................................................. 43

BLACK CHROMATE COATING...................................................................................43

DYED PARTS............................................................................................................ 43

FORMATION OF CHROMATE FILM............................................................................... 43

SOLUTION FORMULATION....................................................................................... 44

RESISTIVITY OF CHROMATE FILMS..........................................................................45

SALT SPRAY EXPOSURE RESISTANCE......................................................................46

CHROMATE PROCESSING STEPS............................................................................. 46

CHARACTERISTICS OF ZINC DEPOSITS FOR CHROMATING.....................................47

CONTROL PARAMETERS.......................................................................................... 48

Replenishing of chromate solution.........................................................................49

SPOT TESTS............................................................................................................ 50

EQUIPMENT............................................................................................................ 50

SEALING LAYER...................................................................................................... 51

CHROMATE AS PAINT BASE..................................................................................... 52

CONVERSION COATING ON ALUMINIUM AND OTHER METALS.................................52

LUBRICANTS................................................................................................................. 53

Functions & Characteristics of lubricants...............................................................53

STRIBECK CURVE.................................................................................................... 54

TYPES OF LUBRICATIONS........................................................................................... 54

CLASSIFICATION OF LUBRICANTS..............................................................................56

PHOSPHATING CONVERSION COATING on metals.........................................................56

PHOSPHATING PROCESS BASICS............................................................................57

FUNDAMENTAL IRON PHOSPHATE REACTION......................................................57

STAGES OF IRON PHOSPHATING..........................................................................58

FUNDAMENTAL MANGANESE PHOSPHATE REACTION..........................................58

STAGES OF MANGANESE PHOSPHATING..............................................................58

PHOSPHATING STRUCTURE.................................................................................... 59

TYPES OF PHOSPHATE SOLUTIONS.........................................................................60

ELECTRO-PAINTING................................................................................................ 60

LOW ZINC PHOSPHATE advantages........................................................................61

ZINC PHOSPHATE ON ALUMINUM............................................................................61

TROUBLESHOOTING ZINC PHOSPHATING...............................................................61

ANODIZING ALUMINIUM AND ITS ALLOYS......................................................................61

Difference between ANODIZING and ELECTROPLATING.........................................62

Anodizing ALUMINUM vs ALUMINUM-ALLOYS..........................................................63

Chemical composition of the anodic film................................................................63

STRUCTURE of the oxide layer............................................................................... 63

Conventional PARAMETERS for sulfuric electrolyte.................................................64

SOFT & HARD anodizing......................................................................................... 65

Anodizing CONFIGURATION.................................................................................... 65

Anodizing PROCEDURE........................................................................................... 66

SEMINAR: CD & DVD PRODUCTION............................................................................... 68

STRUCTURE of a compact disc............................................................................... 68

How does it work?.................................................................................................. 68

MASTERING PROCESS............................................................................................. 69

RECORDABLE AND REWRITABLE CDs.....................................................................70

DVD (digital versatile disc)........................................................................................ 71

COMPARISON BETWEEN CD, DVD AND BD.............................................................71

CHEMICAL VAPOR DEPOSITION (CVD)...........................................................................72

VARIANTS of CVD processing..................................................................................73

STEPS in CVD processes......................................................................................... 74

REACTION types..................................................................................................... 74

SAFETY ISSUES in CVD........................................................................................... 75

STRUCTURE TYPES of the coatings.........................................................................75

THERMAL SPRAY........................................................................................................... 77

STRUCTURE of thermal spraying coatings..............................................................77

FLAME SPRAYING....................................................................................................... 78

ARC SPRAYING........................................................................................................... 78

PLASMA SPRAYING..................................................................................................... 79

Plasma transferred arc spraying (PTA)...................................................................79

LASER ASSISTED THERMAL SPRAY.............................................................................79

DETONATION GUN SPRAYING..................................................................................... 79

HIGH-VELOCITY OXYGEN FUEL SPRAY (HVOF)............................................................80

COLD SPRAYING......................................................................................................... 80

Final conclusions about spraying............................................................................... 80

PAINTING IN AUTOMOTIVE............................................................................................ 82

Components of the paint........................................................................................ 82

Automotive paint shop........................................................................................... 82

FLAME RETARDANT MATERIALS.................................................................................... 84

How does a fire develop in polymers?.......................................................................84

Flame retardant mechanisms.................................................................................... 85

FR efficiency test....................................................................................................... 86

THERMO-GRAVIMETRIC ANALYSIS (TGA).................................................................86

UL 94 STANDARD.................................................................................................... 86

LIMITING OXYGEN INDEX (LOI)................................................................................86

CONE CALORIMETRY............................................................................................... 86

How are FR applied.................................................................................................... 87

Industrial fabric treatment..................................................................................... 87

New development of flame retardants......................................................................87

Nanocomposites..................................................................................................... 87

Nanocoatings......................................................................................................... 87

INTRODUCTION TO MEMS............................................................................................. 88

Production process.................................................................................................... 90

RELEASE RELATED ADHESION IN MICROMACHINING..............................................90

SOLUTION to RELEASE RELATED ADHESION (STICTION = STATIC FRICTION)......90

MEASUREMENT of ADHESION SURFACES................................................................91

DESIRED PROPERTIES FOR SURFACE MODIFICATION..............................................92

SOLUTION to IN-USE ADHESION (STICTION = STATIC FRICTION).........................92

SELF-ASSEMBLY MONOLAYERS...............................................................................92

CHARACTERISTICS of SAM-coated surfaces.........................................................92

INTRODUCTION

In this course, it will be important speaking about capability of apply coatings or

modify surfaces. Coatings must have constant properties over space and time. Surface

engineering is an independent discipline with important contribution that encompass:

1. Manufacturing processes of surface layers, thus, surface layers and coatings

2. Connected phenomena, what happens to the surface during production or

during use

3. Performance effects obtained by them (corrosion for instance)

Techniques of product superficial layers can be mechanical, thermo-mechanical,

thermal, thermo-chemical, electrochemical/chemical and

physical. Usually they are produced before their service activity.

Techniques of manufacturing surface layers are: a) decremental.

b) non-decremental. c) incremental

In addition, a market classification exists, that divide in new

techniques and old techniques. Interesting is the classification

on thickness of the layer that each technique can reach.

Surface behave differently with respect to the bulk. By Griffith, the definition is “a set

of interruption of macro and microscopic continuity, consisting of crevices, porosity

and irregularities in structure, laminar or mosaic in character or caused by the

inclusion of foreign matter”.

Surface is a discontinuity, a non-ideal structure. Surface are indeed never perfect.

Roughness characterization is fundamental (we define it as the ratio between the true

surface and the nominal area). Typically, we have higher energy states at surfaces, so

we experience an enhanced adsorption. Moreover, we have combination of different

condition with respect to bulk; we are talking about mechanical, thermal, electrical,

physical and chemical effects. These effects have different actions on the surface

during service (cyclic or continuous).

NOMINAL SURFACE is the geometrical one, which we calculate in a usual way.

 TRUE SURFACE is defined as the edge or limit of material bodies. It is different

 from the nominal one since is considering the morphology, so without omitting

roughness, waviness and shape errors; the area of the nominal surface is

usually 5-10% the area of the real one. Asperities are the subject of the

interactions. There are many ways to evaluate an approximation of the true

surface, for instance SEM or other microscopy evaluations.

In these ways I obtain the MEASURED (OBSERVED) SURFACE, which is an

 approximation of the real one but depends on the measurement method.

Surface atoms are more energetic because they are less bounded with respect

to bulk atoms. It is possible to find variations in energetic value of atoms until

layers under the surface. Energetic value depends also on the surface

morphology; I can have edges, corners or ideal plane surfaces.

From the thermodynamic point of view, we can define the surface energy as:

( )

∂G

=

γ ∂A P , V , n

i

We study WETTABILITY with the usual Thomas Young equation, as equilibrium of the

three interfaces with respect to the contact angle:

( )

ϑ =γ −γ

γ cos

For an IDEAL SMOOTH surface: LV SV LS

For a REAL ROUGH surface, we must introduce roughness:

A REAL ( ) ( )

ϑ =R

R= cos cos θ

REAL IDEAL

A IDEAL 13/3/2020

We will discuss the techniques of modification of surfaces. One of the most common

process of deposition onto a metal is electroplating process. My component is

immersed in a solution containing ion of the metal I want to depose. We will discuss

this process for chromium and nickel.

CHROMIUM PLATING

It can be done by an electrodeposition process (for functional chromium) carried out in

wet phase (piece immersed in a solution containing chromium ions). Very important

(more than 60% of the market of surface coatings).

Another way is to deposit by PVD, mainly used for decorative purpose.

Chromium plating falls into three fundamentally different categories (the first two are

the most relevant):

1. FUNCTIONAL CHROMIUM PLATING: commonly referred to as "HARD"

chromium plating is typically performed with the intent of producing a

relatively thick chromium deposit (from 0.5 microns to over 100 microns), which

provides one or more specific functions, such as:

a. Wear resistance: hardness up to 1000 Vickers

b. A layer with decreased friction coefficient

c. Corrosion protection to the substrate

At the present time, all functional chromium plating solutions are based on

chromium trioxide (HEXAVALENT CHROMIUM, toxic and carcinogenic); attempts

to use the trivalent chromium have been made, but results are worse (less

thickness and compactness -> lower hardness).

Typically used on large components.

2. DECORATIVE CHROMIUM PLATING: it refers to relatively thin coatings that

are applied most often over a highly flat and polished surface or over bright

nickel. The purpose of this deposit is to modify the appearance of the surface to

a more pleasing colour ranging from faint blue to black. Decorative chromium

plating thicknesses typically range between 0.1 micron to 1 micron. Decorative

chromium plating solutions are available in both HEXAVALENT and TRIVALENT

formulations (nowadays only trivalent chromium is used for decorative

purpose).

Typically used on small components.

3. "TIN FREE STEEL" Chromium is applied at high current densities over bare

steel as an alternate corrosion protection coating for steel strip to be made into

cans. This plating is performed on a continuous basis using coil coating

technology (a.k.a. strip plating) and is beyond the scope of this lecture.

For both functional and decorative it is possible to make the colour of the chromium

layer black (not made by metallic chromium but by mixed oxide