SINTERING 3

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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 ._________________________________________________________________ ______________

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Transcript of SINTERING 3

Mechanical Processing&Properties ofNanostructure Materials

Subject : Mechanical Processing & Properties of Nanostructured Materials

An Assignment On SINTERING MECHANISM

SEM photograph, Sintering of Cu (Left) &Ni (Right)Heating rate300K/h,sintering temperature1313K, isothermal sintering time 600min

Name : V Arunkumar Roll No :2010413002 Course :M.Tech (Nanoscience& Technology -2nd Semester)

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Mechanical Processing&Properties ofNanostructure Materials

CONTENTS

1. Introduction 2. Important Variables in the Process 3. Sintering Mechanisma. Two Key Mechanisms b. Factors Affecting Sintering Mechanismi. ii. iii. Sintering Routes Surface Properties 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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Mechanical Processing&Properties ofNanostructure Materials

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

Sintering

Solid State Sintering

Liquid Phase Sintering

Pressure Sintering

Multi-Phase

Single-Phase

Liquid Content 15% by Vol

Hot Pressing

Hot Iso-static Pressing

Without Reaction

With Chemical Reaction

2.

IMPORTANT VARIABLES IN THE PROCESS

The driving force for sintering is REDUCTION OF SURFACE ENERGY.

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Mechanical Processing&Properties ofNanostructure Materials 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 :y y y y y

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; and 3. A cooling chamber

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Mechanical Processing&Properties ofNanostructure Materials

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

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.

y

y

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As temperature increases two adjacent particles begin to form a bond by a diffusion mech nism (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)

Stage 1 Neck Formation (Surface Diffusion) Stage 2 Shrinkage ( rain Boundary Diffusion) A second sintering mechanism is vapo -phase t anspo t. 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 cantake 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 sinterin . An e ample is cobalt in tungsten carbide tools and dies. Stronger and denser parts can be obtained in this way.

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Sintering involves mass transport to create nec s and to transform them into grain boundaries

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The composition of the Metal Particles Processing Parameters

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Mechanical Processing&Properties ofNanostructure Materials

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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Mechanical Processing&Properties ofNanostructure Materials 3.aTWO 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.bFAFTORS AFFECTING SINTERING MECHANISM 3.b.iSINTERING ROUTES - Typical Sintering routes involve :y y y

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

1. Evaporation-condensation (higher vapour pressure over a convex surface compared to a concave) 2. Diffusion (differences in vacancy concentration) Surface diffusion(a), grain boundary diffusion(b), volume diffusion(c) 3. Flow (pressure induced)4.

Dissolution-precipitation (liquid phase wetting the surface)

3.b.iiSURFACE PROPERTIES: Surface tension properties of crystals and liquids are important for sintering kinetics. Governing Equation, dD/dt = k/D Where, D - grain diameter, k -rate constant, &t - time Solving above equation yields, D2 = 2kt + C where , C is integration constant

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Mechanical Processing&Properties ofNanostructure Materials implies, time, t = (D2 C)/2k it is apparent that time, t increases as square of Diameter of the particle. CONCLUSION - SmallerGrains means Faster Sintering (because of high surface area)

3.b.iiiDIHEDRAL ANGLE & ITS EFFECTS - ON AMOUNT OF GRAIN TO GRAIN CONACT(Fig

a below)

Low dihedral angle: (figy y y

b above)

Small amount of grain-grain contact Large amount of liquid penetrationbetween grains Rapid grain growth, large crystals

Large dihedral angle: (figy y

c above)

Strong solid-solid contacts Good hot strength

SUMMARY OF TRANSPORT

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Mechanical Processing&Properties ofNanostructure Materials

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 concavesurfaces, necks. Fusing, increased contact area. Pore volume and densityremains 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 Adhesion

PROCESS Contact Formation Neck Growth Pore Rounding & Elongation Pore-closure & Final Densification

Initial Intermediate Final

SURFACE AREA LOSS Minimal, unless compacted at high pressures Significant, Upto 50% loss Near total loss of open Porosity Negligible further loss

DENSIFICATION None

COARSENING None

Small at first Significant Slowly & relatively minimal

Minimal Increase in grain size & pore size Extensive Grain & Pore Growth

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Mechanical Processing&Properties ofNanostructure Materials

5.

PROPERTIES OF POWDERS&RESULTANT PROPERTIES

Particle size: Materials transport over smaller distances, higher surfaceenergies. Larger grains grow at the expense of smaller ones. Coarseningof the grains. Particle packing: Improves the number of contact points between particles. Relative density increased. Faster densification, less volumeshrinkage.Particles that pack poorly sinter poorly. Particle shape: Irregular shaped particles with higher surfacearea/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 evo during sintering. lve 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) 225 110 165 917 1035 Yield strength (MPa) 205 48 76 827 Hardness Elongation in 25 mm (%)