SINTERING 3

Mechanical Processing & Properties of Nanostructure Materials

Subject : Mechanical Processing & Properties of Nanostructured Materials

An Assignment

On

SINTERING MECHANISM

SEM photograph, Sintering of Cu (Left) & Ni (Right)Heating rate 300K/h, sintering temperature 1313K, isothermal sintering

time 600min

Name : V Arunkumar

Roll No :2010413002

Course : M.Tech (Nanoscience & Technology -2 nd Semester)

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CONTENTS

1. Introduction

2. Important Variables in the Process

3. Sintering Mechanism

a. Two Key Mechanisms

b. Factors Affecting Sintering Mechanismi. Sintering Routes

ii. Surface Propertiesiii. Dihedral Angle & Its effect

4. Sintering Stages

5. Properties of Powders & Resultant Sintered Properties

6. Role of Nanoparticles in Sintering

7. Reference

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SINTERING MECHANISM

1. INTRODUCTION

Sintering is the process whereby green compacts are heated in a controlled atmosphere furnace to a temperature below the melting point, but sufficiently high to allow bonding or fusion of the individual particles. Prior to sintering, the compact is brittle and its strength, known as green strength, is low. Consequently, sintering is a heat treatment operation performed on the compact to bond its metallic particles to increase its strength and hardness.

Sintering Process is broadly classified as under

2. IMPORTANT VARIABLES IN THE PROCESS

The driving force for sintering is REDUCTION OF SURFACE ENERGY.

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Sintering

Solid State Sintering

Multi-Phase

Without Reaction

With Chemical Reaction

Single-Phase

Liquid Phase Sintering

Liquid Content <15% by Vol

Liquid Content > 15% by Vol

Pressure Sintering

Hot Pressing Hot Iso-static Pressing


The green compact consists of many distinct particles, each with its own individual surface and hence the total surface area contained in the compact is very high. Under the influence of heat, the surface area is reduced through the formation and growth of bond between the particles with associated reduction in surface energy. The finer the initial powder size , higher the total surface area and hence greater the driving force behind the process.

The nature and the strength of the bond between the particles and hence that of the sintered compact depend on the following :

Mechanisms of diffusion Plastic flow Evaporation of volatile materials in the compact Re-crystallization Grain growth and Pore shrinkage.

The THREE main variables in sintering are:

Temperature - Sintering temperatures are generally within 70% to 90% of the melting point of the metal or alloy.

Time - Sintering times range from a minimum of about 10 minutes for iron and copper alloys to as much as 8 hours for tungsten and tantalum.

The furnace atmosphere.

Continuous sintering furnaces which are used for most production have three chambers.

1. Pre-heat Chamber - A burn-off chamber for volatizing the lubricants in the green compact, in order to improve bond strength and prevent cracking;

2. Sintering Chamber - A high-temperature chamber for sintering; and3. A cooling chamber

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Sintering Temperature and Time for few metals are as under

Materials Temperature (®C) Time (Min)Copper, brass, and bronze 760 - 900 10 – 45Iron and iron- graphite 1000 – 1150 8 – 45Nickel 1000 – 1150 30 – 45Stainless steels 1100 – 1290 30 – 60Alnico alloys (for permanent magnets) 1200 – 1300 120 – 150Ferrites 1200 – 1500 10 – 600Tungsten carbide 1430 – 1500 20 – 30Molybdenum 2050 120Tungsten 2350 480Tantalum 2400 480To obtain optimum properties and for successful sintering, proper control of the furnace atmosphere is important.

An oxygen-free atmosphere is essential, to control the carburization and de-carburization of iron and iron-based compacts and to prevent oxidation of powders.

Vacuum is generally used for sintering refractory-metal alloys and stainless steels.

The gases most commonly used for sintering a variety of other metals are : hydrogen, dissociated or burned ammonia, partially combusted hydrocarbon gases and nitrogen.

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3 SINTERING MECHANISM

Sintering mechanisms are complex, they depend on :

The composition of the Metal Particles Processing Parameters.

Sintering involves mass transport to create necks and to transform them into grain boundaries.

As temperature increases, two adjacent particles begin to form a bond by a diffusion mechanism (solid-state bonding). As a result, the strength, the density, the ductility, and the thermal and electrical conductivities of the compact increase. At the same time, however, the compact shrinks; hence, allowance should be made for shrinkage. This is also called Solid State Sintering or Solid Phase Sintering because the metal remain unmelted at the treatment temperatures (of about 0.7 to 0.9 of metals melting temperatures)

A second sintering mechanism is vapor-phase transport. Because the material is heated to very close to its melting temperature, metal atoms will release to the vapor phase from the particles. At the interface of two particles, the melting temperature is locally higher and the vapor phase pre-solidifies. Thus, the interface grows and strengthens while each particles shrinks as a whole.

If two adjacent particles are of different metals, alloying can take place at the interface of the two particles. One of the particles may have a lower melting point than the other, in that case, one particle may melt and because of surface tension, surround the particle that has not melted. This process is called liquid phase sintering. An example is cobalt in tungsten carbide tools and dies. Stronger and denser parts can be obtained in this way.

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Stage 1 – Neck Formation (Surface Diffusion)

Stage 2 – Shrinkage (Grain Boundary Diffusion)


Shape Accommodation Ostwald Ripening

In liquid phase sintering the concentration of heavier components may be higher at the bottom than at the top of the part because of the effects of gravity. In order to obtain a more uniform distribution, experiments are being conducted in space shuttles under conditions of micro gravity, just as is being tried in metal casting.

Another method, which is still at an experimental stage is spark sintering. In this process, loose metal powders are placed in a graphite mould, heated by an electric current, subjected to a high energy discharge, and compacted, all in one step. The rapid discharge strips contaminants (or any oxide coating such as aluminium) from the surface of the particles and thus encourages good bonding during compaction at elevated temperatures.

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3.a TWO KEY MECHANISMS : SURFACE & BULK TRANSPORT

Surface transport: neck growth without shrinkage or densification

- surface diffusion: lowest activation energy, predominant at low temperature

- vapor diffusion: evaporation from convex surface to concave surface

Bulk transport: net particle movement leading to shrinkage and densification, accompanying shrinkage in dimension

- lattice diffusion: dominant for crystalline materials

- cross grain boundary diffusion: dominant for crystalline materials

- viscous flow: dominant for amorphous materials

Reactive sintering: solid-to-solid (solid phase) reaction - Formation of interface product layer

3.b FAFTORS AFFECTING SINTERING MECHANISM

3.b.i SINTERING ROUTES - Typical Sintering routes involve :

Reduce excess surface free energy A chemical potential difference exist between surfaces of different curvature. Mass transport from convex to concave surfaces. Grain growth

1. Evaporation-condensation (higher vapour pressure over a convex surfacecompared to a concave)

2. Diffusion (differences in vacancy concentration) Surface diffusion(a), grainboundary diffusion(b), volume diffusion(c)

3. Flow (pressure induced)

4. Dissolution-precipitation (liquid phase wetting the surface)

3.b.ii SURFACE PROPERTIES:

Surface tension properties of crystals and liquids are important for sintering kinetics.Governing Equation,

dD/dt = k/DWhere, D - grain diameter,

k -rate constant, & t - time

Solving above equation yields,

D2 = 2kt + C where , C is integration constant

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implies, time, t = (D2 – C)/2kit is apparent that “time, t” increases as square of Diameter of the particle.

CONCLUSION - “Smaller Grains” means “Faster Sintering” (because of high surface area)

3.b.iii DIHEDRAL ANGLE & ITS EFFECTS - ON AMOUNT OF GRAIN TO GRAIN CONACT (Fig – ‘a’ below)

Low dihedral angle: (fig – ‘b’ above)

Small amount of grain-grain contact

Large amount of liquid penetration between grains

Rapid grain growth, large crystals

Large dihedral angle: (fig – ‘c’ above)

Strong solid-solid contacts

Good hot strength

SUMMARY OF TRANSPORT

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4. SINTERING STAGES

Adhesion : Contact Formation, Very small loss in surface area, No densification & No coarsening

Initial stage: Transport from high energy convex particle surfaces to concave surfaces, necks. Fusing, increased contact area. Pore volume and density remains almost constant (4-5% shrinkage, relative density 0.5-0.6)

Intermediate stage: Interparticle neck growth, grain boundary area increase, $interparticle grain boundary flattens, pore diameters decrease. (5-20% shrinkage, relative density up to 0.95)

Final stage: isolated pores may remain at triple points or inside grain matrix. These pores may be gradually eliminated. (relative density > 0.95)

STAGE PROCESS SURFACE AREA LOSS

DENSIFICATION COARSENING

Adhesion Contact Formation

Minimal, unless compacted at high pressures

None None

Initial Neck Growth Significant, Upto 50% loss

Small at first Minimal

Intermediate Pore Rounding & Elongation

Near total loss of open Porosity

Significant Increase in grain size & pore size

Final Pore-closure & Final Densification

Negligible further loss

Slowly & relatively minimal

Extensive Grain & Pore Growth

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5. PROPERTIES OF POWDERS & RESULTANT PROPERTIES

Particle size: Materials transport over smaller distances, higher surface energies. Larger grains grow at the expense of smaller ones. Coarsening of the grains.

Particle packing: Improves the number of contact points between particles. Relative density increased. Faster densification, less volume shrinkage. Particles that pack poorly sinter poorly.

Particle shape: Irregular shaped particles with higher surface area/volume ratio, have a higher driving force for sintering.

PROPERTIES

Depending on temperature, time and processing history, different structures and porosities can be obtained in a sintered compact, to affect its properties. Porosity cannot be completely eliminated because voids remain after compaction and because gases evolve during sintering. Porosities may consist either of a network of interconnected pores or of closed holes. Generally, if the density of the material is less than 80% of its theoretical density, the pores are interconnected. Porosity is an important characteristic for making powder metallurgy filters and bearings. Typical properties of some sintered materials are given here below.

1. Hot isostatically pressed (HIP) powders have properties that are similar to those of cast or forged ones. The key advantage being, the forged components are likely to require additional machining process (unless they have been precision forged to net shape) that a powder metallurgy component may not require. Consequently, powder metallurgy is becoming a competitive alternative to most small forgings.

Mechanical properties of selected Powder Metallurgy materials

Designation Condition Ultimate tensile strength (MPa)

Yield strength (MPa)

Hardness Elongation in 25 mm (%)

Ferrous – FC- 0208

As Sintered 225 205 45HRB <0.5

Aluminium 601 AB, pressed bar

As Sintered 110 48 60HRH 6

Brass CZP- 0220 165 76 55HRH 13Titanium Ti- 6AI-4V

HIP 917 827 13

Superalloys Stellite 19

1035 49HRC <1

Mechanical property comparison for Ti-6AI-4V

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Process (*) Density Yield strength (MPa)

Ultimate strength (MPa)

Elongation (%)

Reduction of area (%)

Cast 100 840 930 7 15Cast and forged 100 875 965 14 40Blended elemental (P+S)

98 786 875 8 14

Blended elemental (HIP)

>99 805 875 9 17

Prealloyed (HIP) 100 880 975 14 26(*) P+S =pressed and sintered, HIP = Hot isostatically pressed

(a) Density of copper- and iron-powder compacts as a function of compacting pressure. Density greatlyinfluences the mechanical and physical properties of P/M parts. Source: F. V. Lenel, Powder Metallurgy:Principles and Applications. Princeton, NJ; Metal Powder Industries Federation, 1980. (b) Effects of densityon tensile strength, elongation, and electrical conductivity of copper powder. IACS means InternationalAnnealed Copper Standard for electrical conductivity.

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6. ROLE OF NANOPARTICLES IN SINTERING

Finer powders densify at lower temperatures and in less time!

Advantages of nanoparticle sintering

- High rate of sintering

- Lowering of sintering temperature

- Addition of nanoparticles accelerates the sintering of larger particles

e.g. W+ 0.1-0.5% of Ni nanoparticles; sintering temperature to 1200~1300oC.

Disadvantages of nanoparticle sintering

- Aggregation results in formation of pores, change in required dimension or

deformation of the sintered body.

High-pressure sintering/ Fast-firing/ Plasma-assisted sintering

Chemical modification of the nanoparticle surface

Driving force for g

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7. REFERENCE

Sr.No Reference

1. Fundamentals of Modern Manufacturing – Materials Process & Systems – 3ed

By Mikell P Groover

2. Manufacturing Engineering & Technology – 4ed by Kalpakjian & Schmid

3.Nanocrystalline Ceramics Produced by SinteringBy Georg Rixecker Max Planck Instiut for Metals Research andInstitute of Nonmetallic Inorganic Materials, University of Stuttgart

4.Metal Powder Processing by Prof J S Colton – Georgia Institute of Technology

5.Sintering – Basics (Densification & Coarsening) - By Wakai – Technische Universtiat Darmstad (JACS 2006)

6.A Ceramist’s Perspective on Processing of Oxide-based Nuclear FuelsBy Dr. Juan Claudio Nino – University of Florida

7.Sintering Mechanism in Mixed Powder Compacts of the Fe-Cu-C Ternary System by Kazuhiko Majima & Hiroyasu Mitani (JACS 1977)

8.Sintering Mechanism of Two Spheres Forming Homogeneous Solid Solubility Neck by Atsuko Shimoaka, et al., KONA No 21 2003

9. Other websites including MIT OCW & UC Berkeley

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SINTERING 3

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