Surface Preparation and Surface Chemistry in Adhesion

Metals

John G. Dillard

Chemistry Department

Virginia Tech

Surface Preparation “General Outline”

1. Surface Chemistry and Contamination

2. The Objectives of Surface Preparation

3. The Methods, Chemistry, and Characteristics- Mechanical

- Physical

- Chemical and Electrochemical

1) H. M. Clearfield, D. K. McNamara, and G. D. Davis, “Surface Preparation of Metals”, in Engineered Materials Handbook, Volume 3, Adhesives and Sealants, H. F. Brinson, ed., ASM International, Materials Park, OH, 1990, pp. 259.

2) J. Shields, “Surface Preparation”, in Adhesives Handbook. Adhesives, Butterworth & Co., London, 1984, p 89.

3) R. F. Wegman, Surface Preparation Techniques for Adhesive Bonding, Noyes Publications, New Jersey, 1988

4) G. W. Critchlow and D. M. Brewis, “Review of surface pretreatments for titanium alloys”, Int. J. Adhesion and Adhesives, 1995, 15, 161.

5) J. R. J. Wingfield, “Treatment of composite surfaces for adhesive bonding”, Int. J. Adhesion and Adhesives, 1993,

13, 151.

6) J. G. Dillard, “Surface Preparation of Composites”, in Engineered Materials Handbook, Volume 3, Adhesives and Sealants, H. F. Brinson, ed., ASM International, Materials Park, OH, 1990, pp. 281.

7) J. D. Minford, “Handbook of Aluminum Bonding Technology and Data”, Marcel Dekker, Inc., New York, 1993.

8) J. H. Zaat, “25 Years of Plasma Spraying” Ann Rev. Mater. Sci., 1983, 13, 9.

Selected References for Surface Preparations

Model of a Metal Surface

Surface Cleaning - Surface Modification WHY??

1. Remove contaminants

2. Incorporate selected reagents- corrosion inhibitors- adhesion promoters- primer- hydration inhibitors

3. Alter adherend surface chemistry- promote adhesive bonding- enhance durability- promote specific oxide chemistry- generate specific chemical functionality

Surface Cleaning Methods“Mechanical”

1. Mechanical - remove outer scale and coatings

a) abrasion: ScotchBrite pad, metal brush, metal scouring pad, sandpaper, etc.

b) shot or grit blasting: alumina, silica, glass beads, wood

chips, silicon carbide, etc.

Surface Cleaning Methods“Physical”

2. Physical methods - remove surface coating via dissolution

chemical reactions.

a) solvent cleaning: remove greases, oils, polymer coatings

b) aqueous washes: soap or alkali, grease and dirtremoval, power wash-detergent

c) alkali or acid washes: remove contaminants, remove oxide surface layer

d) electrolytic cleaning: cathodic or anodic, gas bubble formation-agitation, dissolution of surface coatings

e) ultrasonic cleaning: agitation and surface cavitation –chemical processes

Surface Cleaning Methods“Chemical and Electrochemical”

3. Chemical and electrochemical - remove coating, specific chemical species, derivatize surface

a) aqueous salt: phosphate salt, thin layers, inhibit water up-take

b) acid/base etching: hydride-reduce MOx to M peroxide-oxidize M to MOx

c) conversion coating: caustic treatment-oxide gel; sulfuric,chromic acid-oxide gel coating,water wettable

d) coupling agents: silane reaction-functionalize the adherendsurface

e) flame and plasma treatments: various flames, surfaces cleanedor oxidized, functionalized

f) anodization: form porous or barrier oxides: acids - phosphoric,chromic, sulfuric; bases - sodium hydroxide

General Procedure for Surface Preparations“Metals”

1. Remove surface contaminants: grease or oils or lubricants

or other “things”- abrasion or grit blast- organic solvents or organic vapors- aqueous solutions

2. Remove residual oxide - deoxidize the surface

3. Rinse – water

4. Apply surface treatment- oxidation; chemical or electrochemical- functionality incorporation

5. Rinse - water

6. Dry - oven or room temperature

1. Mechanical - remove outer scale and coatings

a) abrasion: ScotchBrite pad, metal brush, metal scouring pad, sandpaper, etc.

b) shot or grit blasting: alumina, silica, glass beads, wood chips, silicon carbide, etc.

Surface Cleaning Methods“Mechanical”

Surface Cleaning Methods“Mechanical “

Illustrative example - abrading Al metal

Abrasive - coarse abrasive paper

Results - surface roughened- contaminants removed- surface wets more readily - increased surface energy- adhesion altered

1. Mechanical - remove outer scale and coatings

a) abrasion: ScotchBrite pad, metal brush, metal scouringpad, sandpaper, etc.

b) shot or grit blasting: alumina, silica, glass beads, woodchips, silicon carbide, etc.

Surface Cleaning Methods“Mechanical”

Mechanical Cleaning “Grit Blasting”

Grit blasting surfaces with different grits

- roughening of the surface ?

- removal of surface coatings ?

- chemically altering the surface ?

Surface Cleaning Methods“Grit Blasted Aluminum and Titanium”

As rec’d Al G-blasted Al As rec’d Ti G-blasted Ti

Scanning Electron Photomicrographs“Grit Blasted Titanium Surface”

Acetone cleaned Ti Ti grit blasted with TiO2

XPS Surface Analysis Results for Grit-Blasted Ti Adherends

As rec’d C O Ti Al M

Ti 51.4 37.0 7.3 2.7

Ti/Cr2O3 35.8 44.8 3.9 3.3 9.1

rinse (MeOH) 37.7 45.0 3.7 3.2 9.2

sonicate (MeOH) 39.2 40.7 3.4 3.6 9.2

Ti/SiO2 25.9 50.8 2.6 7.5 12.4

rinse(MeOH) 34.9 39.1 4.1 8.3 12.9

sonicate (MeOH) 38.7 40.2 3.0 6.8 10.2

Surface Cleaning Methods“Physical”

2. Physical methods - remove surface coating via

dissolution chemical processes

a) solvent cleaning: remove greases, oils, polymer coatings

b) aqueous washes: soap or alkali, grease and dirt removal, power wash-detergent

c) alkali or acid washes: remove contaminants, remove oxide surface layer

d) electrolytic cleaning: cathodic or anodic, gas bubble formation-agitation, dissolution of surface coatings

e) ultrasonic cleaning: agitation and surface cavitation – chemical processes

Surface Cleaning Methods“Physical”

Illustrative example - alteration of the surface of Al

metal - etching in sodium hydroxide

Reagent - 5% (w/w) NaOH in water at room temperature

Results - surface appears cleaner- oxide removed, but new oxide formed- surface wets more readily - adhesion altered

3. Chemical and electrochemical - remove coating,

specific chemical species, derivatize surface

a) aqueous salt: phosphate salt, thin layers, inhibit water up-take

b) acid/base etching: hydride-reduce MOx to M; peroxide-oxidizeM to Mox

c) conversion coating: caustic treatment-oxide gel; sulfuric, chromic acid-oxide gel coating, water wettable

d) coupling agents: silane reaction-functionalize the adherendsurface

e) flame and plasma treatments: various flames, surfacescleaned or oxidized, functionalized

f) anodization: form porous or barrier oxides: acids - phosphoric,chromic, sulfuric; bases – sodium hydroxide

Surface Cleaning Methods“Chemical and Electrochemical”

Optimized Forest Products Lab“ Surface Preparation”

Acidic Chromate [Cr(VI)] Treatment

1. Solvent degrease2. Alkaline deoxidize3. Aqueous rinse4. Etch: sodium dichromate in acidic solution;

30 min., 65°C5. Aqueous rinse6. Dry - oven 110°C

Surface Morphology of a FPL Etched Aluminum Specimen

Comparison of the native oxide and the FPL

produced oxide

Native: grain boundaries, 20 nm oxide thickness

FPL: grain boundaries, 5 nm oxide thickness,

~40 nm whiskers, greater surface area

P2 Etch - Fe(III) - Eliminates Cr(VI) Reagents

The P2 etch is carried out in an acidic (sulfuric acid) Fe(III)

solution. Fe(III) is the oxidizing agent replacing Cr(VI).

Procedure:1. Remove organic contaminants - organic solvent or vapor

2. Etch the adherend in base or acid to remove residual oxide

3. Immerse the adherend in Fe(III) sulfate solution at 65°C for 8 minutes.[Fe(III) solution preparation: 122.5 g Fe2(SO4)3

. 4H2O combined with 0.185 L concentrated sulfuric acid; dilute the solution to one liter with water].

4. Rinse the specimen in running deionized water; 2-3 min.

5. Dry the specimen in an oven at 110°C.

6. Store adherend in a desiccator until bonded.

P2 Etch - Fe(III) - Eliminates Cr(VI) Reagents

The P2 etch is carried out in an acidic (sulfuric acid) Fe(III)

solution. Fe(III) is the oxidizing agent replacing Cr(VI).

Procedure:1. Remove organic contaminants - organic solvent or vapor

2. Etch the adherend in base or acid to remove residual oxide

3. Immerse the adherend in Fe(III) sulfate solution at 65°C for 8 minutes.[Fe(III) solution preparation: 122.5 g Fe2(SO4)3

. 4H2O combined with 0.185 L concentrated sulfuric acid; dilute the solution to one liter with water].

4. Rinse the specimen in running deionized water; 2-3 min.

5. Dry the specimen in an oven at 110°C.

6. Store adherend in a desiccator until bonded.

Chemical Processes for FPL and P2 Etching

1. FPL: Acidic Chromate - Aluminum Oxide

Al + H2SO4 + Na2Cr2O7 -> Al2O3 + Na2SO4 + Cr2(SO4)3 + H2O

also Al2O3 + H2SO4 -> Al2(SO4)3 + H2O (slow)

addition of Cu(II) facilitates roughened oxide formation

2. P2 Etch : Acidic Ferric Ion - Aluminum Oxide

Al + Fe3+ + H2O -> Al3+ (Al2O3) + Fe2+

Al + H+ -> Al3+ (Al2O3) + H2

In these processes Al2O3 is in various states of hydration

Coupling Agents and Surface Modification Silicon and Other Alkoxides

The generic reaction is hydrolysis of substituted silanes (alkoxides) and reaction of the product with the adherend surface.

(RO)3Si-R’ + H2O -> (HO)3Si-R’ + ROH

(HO)3Si-R’ + HO-Surface -> R’-Si-O3 -Surface + H2O

R’ = any organic functional group: amine, sulfide, epoxy, etc.

Other coupling reagents include titanium, aluminum, and zirconium alkoxides.

Illustrations of the Coupling Reactions

Surface Cleaning Methods - Electrochemical Chromic Acid Anodized Titanium

As rec’d Ti CAA-Ti

anodization: form porous or barrier oxides; acids -phosphoric, chromic, sulfuric

bases – sodium hydroxide TiO2

Surface Modification “Anodization - Electrochemical Oxidation”

The adherend is the anode of an electrochemical cell and is coupled with an appropriate cathode so that the surface of the adherend is oxidized.

A porous - columnar oxide is produced on aluminum or titanium.The oxide morphology and pore diameter are dependent on the electrolyte and the thickness of the oxide is dictated by the electrolysis time.

Phosphoric Acid Anodic Oxide on Aluminum

Rendition of the structure of anodic oxides on Al anodization in phosphoric and chromic acid

Clearfield, McNamara and Davis

PAA CAA

Performance Data for Surface Treated Titanium

Clearfield, McNamara, Davis

Schematic Diagram of a Plasma Treatment Unit

R.F.

Argon

Valve

RF power supply

Inductivecoupling

Plasma reactor

Vacuum pump

Vacuum gage

Liquid N2trap

Polymer gas

Surface Preparation“Plasma Polymerization”

The adherend is maintained in a plasma reactor at reduced pressure and reagent monomers are introduced into the reactor and polymerized on the surface.

Reagents: olefins, acetylene, allyl organics, silanes. acetonitrile polymerized on silicon

The coating material (solid) is introduced into a plasma and upon exiting the plasma is deposited onto the adherend. The concept is that the material melts in the plasma before decomposition occurs.

Coating materials: metals, inorganic oxides, polymers,

etc.

Sample Preparation“Plasma Spraying”

Sample Preparation - Plasma Spraying“Polyimide on Titanium”

Grit Blased Ti Plasma Sprayed Ti

Plasma Spraying“Surface Morphology”

Scanning electron photomicrographs of as received polymer and plasma sprayed polyimide polymer on aluminum

Polyimide: evidence of melting, polymer particle aggregation

Plasma Spraying“Surface Morphology”

Scanning electron photomicrographs of as received polymer and plasma sprayed epoxy polymer on aluminum.

Epoxy: evidence of melting, smooth polymer film

Plasma Spraying“Surface Morphology”

Scanning electron photomicrograph of titanium dioxide plasma sprayed on aluminum.

Titanium dioxide: evidence of melting, particle agglomeration, adherend surface covered

Surface Preparation of Metals“Titanium”

General approach1. Degrease, remove organic contaminants2. Deoxidize, remove native or mill oxides3. Prepare suitable surface coating; oxide,

derivatized layer, polymer film, etc.

Specific treatments1. Grit blast, abrasion2. Anodization: chromic, phosphoric acid, sodium

hydroxide, alkaline hydrogen peroxide 3. Acid-base treatments: alkaline or acid etches,

phosphate-fluoride, TURCO and Pasa Jell4. Alkoxide derivatization: silicon, aluminum, titanium5. Plasma spraying: Ti alloy, inorganic oxides,

polyimides

Surface Preparation of Metals“Steel”

General approach - etching and bonding to oxide1. Grit blast: remove native or mill oxides2. Alkaline or acid treat: modified layer3. Etch: phosphoric acid, chromate, silicate4. Rinse and dry

Conversion coatings - introduce inhibitor - thin oxide1. Grit blast, machine, sand: remove mill oxide2. Clean and rinse: alkaline etch - oxide layer3. Grain refine: titanium salt4. Coating: zinc phosphate salt, etc.5. Rinse: anti-corrosion reagent, chromate, etc.6. Dry

Surface Preparation of Metals“Copper”

General approach:1. Degrease2. Abrade: sand, wire brush3. Clean: acid; nitric, phosphoric4. Rinse: water, or water and acetone 5. Etch:

a) acidic ferric saltb) alkaline chlorite solution (phosphate)c) proprietary solution - alkaline(?)

Surface Cleaning of Copper“Illustrative Example”

Acidic Fe(III) Solution Cleaning of Copper

Observations:

• Contaminants removed • Surface brightened• Water contact angle decreased surface energy increased