Electroless Ni-P based Alloy and Composite Coatings

By

Prof. R.C. Agarwala (Retd.)

Surface Engineering Laboratory,

Metallurgical and Materials Engineering Department,

Indian Institute of Technology, Roorkee, India

Lay Out of the Presentation

Introduction

Experimental Procedure

Reaction Mechanisms

Results and Discussions

Summary

Electroless Ni-P Coating

Credited to Brenner and Riddell (1946)

Electroless Ni-P coatings (EL)It is plating “without the use of electrical energy”- An autocatalyticchemical reduction process which depends upon the catalytic reductionprocess of metal ions in an aqueous solution.

Subsequent deposition of the metal on the substrate, repeated nucleationand grows first laterally followed to vertically by virtue of atom-by-atomdeposition.

EL Composites are preferred for improving the physical and mechanicalproperties.

X=Al2O3, ZrO2 ,TiO2, Al2O3-ZrO2-Al3Zr is embedded into Ni-P matrix toform Ni-P-X nanocomposites for improving the mechanical properties suchas hardness, wear and corrosion resistance of the composite.

Ni-P-Ba/Sr hexaferrite for microwave absorption application.

Background

Uniformity (Even in blind corners)

Simple Processing

High Hardness with heat treatment

Good Wear Resistance

Good Corrosion Resistance

Almost non-porous deposits

Good Microwave Absorption

MERITS OF EL Ni-P COATING

Comparison of deposit uniformity

Electroless Electrolytic

Pipe fittings for piping

connection

Carburetor

Mud pumps

Ball Valve

Applications of EL coatings

Electroless

Coating Technology

Metallic Coating(Cu , Ag etc.)

Alloy Coating(Ni-P , Ni-B etc.)

Nano Composite

Coating

(Ni-P-X)

Conventional

(Co-deposition)(Ni–P–Al2O3, ZrO2 etc.)

Non-Conventional

(In-situ)(Ni-P-ZrO2-Al2O3-Al3Zr etc.)

Classification of Electroless (EL) Coatings

5

Calcined at 400oC for 4 h

Characterizations- DTA, TGA,

XRD, SEM, TEM,Wear track

Mixture

Step I-:Preparation of Zirconia nanoparticles for EL conventional coatings

Synthesis of Zirconia Nanoparticles By Precipitation

Distilled H20

EL Ni-P and Ni-P-ZrO2

conventional

Stir

Dried at 100oC

Wash with Distilled H20

Diluted NH4OHZrOCl2.8H2O

(Commercially pure)

+

Dropwise

15 min

pH=10

Basic Electroless Plating Procedure

Surface Preparation PreTreatment Electroless Bath

Major Steps

1 32

(Dipping time (sec))

Identify Base Metal

(Mild Steel)

Degreasing Agent Acetone (180)

Rinse

Etching ChemicalDil. HCl Or Dil. HNO3 (30)

Rinse with

Deionized water

(Dipping time (sec))

Sensitizing Agent 0.5ml HCl + 0.1% SnCl2 (120)

Activating Agent

0.05ml HCl+ 0.005% PdCl2 (30)

Complexing agent (100)

Stabilizer (50)

Metal salt (30)

Reducing agent (10)

Coprecipitants (ZrO2)

(X)

(Conc. (gm/litre))

Universal EL Ni-P Bath

Step II-: EL Ni-P & Conventional Ni-P-ZrO2 Nanocomposite Coatings

EL Ni-P Coating Reaction Mechanism

I. H2PO2- + H2O H2PO3

- + 2 H+ + 2 e-

(Hypophosphite) (Orthophosphite)

II. Ni++ + 2 e- Ni

III. 2H+ + 2 e- H2

IV. H2PO2- + 2H+ + e- P + 2H2O

Oxidized at Catalytic surface

Heat Treatment (HT) is done at 400oC for 1h at argon atmosphere

Reduced

Reduced

Reduced

Precipitation Reaction Mechanism for

Synthesis of ZrO2 Nanoparticles

I. ZrOCl2.8 H2O+ NH4OH Zr(OH)4 + NH4Cl + H2O

II. Zr(OH)4 ZrO2

400oC,4h

ZrO2 Nanoparticles Synthesized by

Precipitation

DSC/TG traces from room temperature to 1500oC

• In ZrOCl2, Endothermic Peak (100oC, 145oC) (DSC) : Evaporation of

chlorine and hence weight loss of ~20% in 50-150oC (TG curve).

• In dried ZrO2, Exothermic Peak (234oC) (DSC) : Transformation of

Zr(OH)4 to tetragonal ZrO2 & (434oC) (DSC) : Tetragonal to monoclinic

phase transformation of ZrO2.

TG

DSC

DTGDTG

DSC

TG

··

··

··

··

Morphology Studies of ZrO2 Nanoparticles

Micrographs of calcinated Al2O3 powder (a) TEM and (b) FESEM

• Particle size: 5- 10 nm

• Shape: Spherical

TEM FESEM

XRD Analysis of ZrO2 Nanoparticles

XRD patterns of ZrOCl2.8H2O, Dried ZrO2 and Calcined ZrO2 nanopowder at 400oC for 4h

• Phases (Dried ZrO2 ) : Amorphous ZrO2 (Tetragonal)

• Phase (ZrO2 Calcined 400oC,4h) : Crystalline ZrO2 (Monoclinic)

ZrOCl2.8H2O

Dried ZrO2

Cal. ZrO2

Morphology Studies of EL Ni-P and Ni-P-ZrO2

Nanocomposite Coatings

In Ni-P-ZrO2

Region1: ZrO2 nanoparticles on Ni-P globules

Region 2 : ZrO2 nanoparticles are itself coated with Ni-P globules

Globules

Coarser

Globules

Ni-P Ni-P-ZrO2

Ni-P (H.T) Ni-P-ZrO2(H.T.)

XRD Analysis of EL Ni-P and Ni-P-ZrO2

Nanocomposite Coatings

XRD patterns of EL Ni-P and EL Ni-P-ZrO2 in ‘as coated’ and ‘heat treated’ condition

Ni-P

• Major Phase (As coated):

Microcrystalline (mc) Ni

• Major Phase (HT):

Crystalline Ni & Ni3P

As coat

Heat Treated

Ni-P- ZrO2

• Major Phase (As coated):

mc Ni & ZrO2

• Major Phase (HT): Crys. Ni, Ni3P &

ZrO2

Ni-P-ZrO2Ni-P

Heat Treated

As coat

Hardness of EL Ni-P and Ni-P-ZrO2

Nanocomposite Coatings

Microhardness of EL Ni-P & Ni-P-ZrO2 nanocomposite coating at 10gf load &15min dwell time

• Increased Hardness on HT (510 VHN): Precipitation of Ni3P phase &

Crystalline Ni transformation

• Max. Hardness (1011 VHN): ZrO2 reinforcement in Ni-P matrix & HT

0

200

400

600

800

1000

Ni-P-ZrO2(HT)Ni-P-ZrO

2Ni-P(HT)

Mic

roh

ard

ne

ss

(VH

N)

Electroless Deposits

Load= 10 gf

Indentation Time = 15 sec

Ni-P

Friction Coefficient of EL Ni-P and Ni-P-ZrO2

Nanocomposite Coatings

Friction Coefficient of mild steel, EL Ni-P & Ni-P-ZrO2 nanocomposite coating in HT condition

against chromium steel ball under 1N load and 0.1m/s rotational speed

• Friction coefficient of EL Ni-P-ZrO2 : Decreased to about 0.2 (average)

Due to Higher Hardness (1011 VHN)

100 200 300 400 500 600

-0.2

0.0

0.2

0.4

0.6

Co

eff

icie

nt

of

Fri

cti

on

Sliding Distance (m)

(a)

Load= 1N

Velocity=0.1m/s

Ni-P-ZrO2

Ni-P

Mild Steel

Wear Rate of EL Ni-P &Ni-P-ZrO2

Nanocomposite Coatings

Specific Wear Rate of EL Ni-P and Ni-P-ZrO2 nanocomposite coating in HT condition against

chromium steel ball at 0.1m/s rotational speed under 1N,1.5, 2N loads

• Wear Rate of EL Ni-P-ZrO2 : Lower than Ni-P coating

Due to its higher hardness after suitable HT

0

1

2

3

4

5

6

(b)

We

ar

Ra

te x

10

-6(m

m3/N

m)

Load (N)

Ni-P

Ni-P-ZrO2

1.0 1.5 2.0

Velocity = 0.1m/s

Wear Track studies of EL Ni-P &Ni-P-ZrO2

Nanocomposite Coatings

FESEM micrographs of wear tracks of EL coatings in HT condition against chromium steel

ball at 0.1m/s rotational speed under 1N load

• Wear Track : Microcutting and Microploughing effect - Ductile fracture

• Predominant Mechanism: Mild Adhesive wear

ZrO2

SUMMARY

The EN coating layer develops by the lateral and vertical growth of globules

and auto-catalytically cover the substrate surface to form a pore-free deposit.

Heat treatment enhances the strength of the EL coating.

Electroless nickel bath with in-situ coprecipitation reaction within it can be

successfully used for synthesis of Ni-P-X composite coatings.

The highly dispersive second phase fine particles co-deposited in EL Ni-P

matrix by using insitu/codeposition appears to be uniformly distributed and

these particles are in the size range from sub micron to nano level.

Developing upon various physical and mechanical properties of ‘X’, Ni-P-X

composite coating are found to have various applications in the area of wear,

corrosion and microwave absorption.

Acknowledgement

Authors are thankful to WOS-A, DST, UCOST for the research fund

Thank You