2 Preparación 2012

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UNIVERSIDAD NACIONAL DE SAN AGUSTN DE AREQUIPA

FACULTAD DE INGENIERA DE PROCESOS

ESCUELA PROFESIONAL DE INGENIERA METALRGICA

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Cap. 2 Tcnicas de Preparacin de Muestras

Cap. 2 Tcnicas de Preparacin de Muestras2.1. Introduccin

2.2. Objetivos y Parmetros de Preparacin de Muestras

2.3. Extraccin de Muestras2.4. Seccionamiento de Muestras

2.5. Montaje de Muestras

2.6. Desbaste y Pulido Mecnico

2.7. Pulido Qumico y Electroltico2.8. Tcnicas de contraste y Ataque

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Etapas de preparacin: muestreo,

seccionamiento, y

preparacin de la superficie

Criterios de preparacin de una muestra

Preparacin Sistemtica: reglas para muestras similares

Reproducibilidad del mtodo de preparacin y el control de

los parmetros (rpp, fuerza, aditivos, tiempo)

Estructura real: caractersticas de la superficie preparada

Resultados aceptables de la preparacin

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Remover la deformacin inducida por el seccionamiento,

desbaste y pulido.

Eliminar las rayas del desbaste inicial.

Evitar la formacin de huecos, erosin, agrietamiento de

partculas duras, manchas y otros defectos de preparacin.

Minimizar el relieve

La superficie debe ser plana, particularmente en los bordes.

Se deber mantener planas las superficies revestidas

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Para revelar la estructura real de la muestra se deber

Eliminar la deformacin inducida por corte, esmerilado y pulido o

debe ser poca profunda para que sea eliminada por el reactivo de

ataque (rayas, extracciones, picaduras, grietas, manchas, relieves, etc.)

limpiar las muestras entre las etapas de preparacin, despus de lapreparacin y despus del ataque.

El atacante seleccionado debe ser o general o selectivo en su accin

para producir fases o lmites de grano limpios, precisos y un contraste

alto.

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La preparacin de muestras materialogrficas requiere de

cinco operaciones:

corte,

montaje (opcional),

esmerilado,

pulido y

Ataque (opcional)

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Superficie del disco o paos Usar el recomendado por el mtodo a aplicarse

Abrasivo debe tener una dureza de 2.5 a 3 veces la dureza del material a ser

preparado

Tamao de grano del abrasivo al inicio ms pequeo y entre etapas, el intervalo ms grande posible.

Lubricante

alta cantidad con poca cantidad de abrasivo para materiales blandos, menos cantidad con alto contenido de abrasivo para materiales duros,

el pao debe estar hmedo pero no mojado

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Velocidad rotacional usar la recomendada de acuerdo al material a prepararse

Fuerza

El rea de la muestra montada deber ser 50% del soporte

Tiempo Debe ser lo mas corto posible, examinar cada minuto

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La muestra debe ser representativa del material a caracterizar Todos los procesos de seccionamiento producen dao;algunos mtodos (como el oxicorte o corte abrasivo seco)

producen cantidades extremas de daos.

Las normas ASTM E 45, E 1122 y 1245 E aconsejan que sebene tomar varias muestras para estudiar las inclusiones enaceros.

Para estudiar el tamao de grano, es comn usar una muestrade un lote.

Para estudiar el efecto de la deformacin en la forma delgrano de los metales forjados, se requiere tomar un mnimode dos secciones - una perpendicular a, y otro paralelo a, ladireccin de deformacin

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Mtodos de seccionamiento muestras metalogrficas

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Mtodos dependiente de la profundidad deformacin de corte del metal

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La seleccin del disco abrasivo es importante evitar la introduccinde niveles innecesarios de dao al cortar las muestras

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Materiales Disco de corte: abrasivo/ligante

Acero, materiales ferrosos, aceros

endurecidosAl2O3 (corundum)/bakelita

Aceros de alta aleacin Nitruro de boro cbico (CBN)/bakelitaMetales no ferrosos, metales duros Carburo de silicio (SiC)/bakelita

Materiales duros y tenaces, cermets,

cermicosDiamante/bakelita

Materiales duros y frgiles, ceramicos,

minerales

Diamante/metal

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Limitado a materiales o compuestos conductres que tienen una cantidad

menor de materiales no conductores.

Se utiliza para seccionar metales duros, metales con cristales simples y

para cortar tambin cermicos.

nodo

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En el caso de materiales con baja conductividad trmica y baja resistencia

a la fractura (por ejemplo, vidrios y cermicos) el proceso est

acompaado por un choque trmico considerable.

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metales duros, fundicin gris

o vidrios y cermicos

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Probablemente, el mtodo ms cuidadoso para el corte demetales no metales con enlace inico con casi ningn dao enla superficie

El corte y fresado se realiza con hilos o cuerdas infiltrados concido o con chorros de cido de 150 m de ancho.

El procedimiento es asistido con electricidad con el uso de unhilo o boquilla como ctodo y la muestra como nodo.

Estos tratamientos toman bastante tiempo y as se limitan a loscristales individuales de corte o de secciones

Estos tratamientos toman bastante tiempo y por lo tanto estnlimitados al corte de monocristales o de secciones delgadas.

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Materiales biolgicos, polmeros

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Montaje de muestras en un dispositivo de sujecin es necesario cuando se

preparan muestras irregulares, pequeas, muy blandas, frgiles, y en los

casos en que se requiere una retencin de borde.

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(a) molde flexible. (b) molde de plstico

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Easy-to-use resins with short curing times

Negligible shrinkage

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Curing times are relatively short and the hardened sample is duroplastic

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Two mounts can be produced at one time using the duplexing spacer.

Cycle times range from 8-12 minutes.

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Requirements of hot and cold mounting materials Specimens thoroughly cleaned either with an ultrasonic cleaner orlightly brushed with water containing a detergent

Inert to the specimen, mold, and etchant

Moderate viscosity during the actual embedding process; nogeneration of air

bubbles after solidification Low linear shrinkage and good adhesion to the specimen

Grinding and polishing behavior similar to those of the specimen, aswell as

similar chemical resistance to all reagents used during the specimen

preparation

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Hot compression mountingmaterials

Cold/room-temperature-curing

Thermosetting plasticsPhenol resins (Bakelite)

Epoxy resinsDiallyl phthalate

Epoxy resins

Polyester resins

Polymerization is irreversible;

material cannot resoften. Heat to

~150 C under pressure. Samples

can be removed hot from themounting press, but cooling under

pressure is recommended.

Polymerization is irreversible; material

will not resoften. Hardening reaction may

increase the temperatures! This is directly

related to the mixing ratio, the ambienttemperatures, the volume of the mixed

components, and the heat transfer by the

molds.

Comparison of hot and cold mounting materials

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Properties of some important mounting materials

Material PropertyCold mounting materialsEpoxy

resins

Good adhesion. High viscosity, fills cracks, gaps, and pores easily

and is therefore well suited for infiltration. Resistant to etchants

and solvents. Nearly transparent. Mold material should be madeof silicon rubber, polyethylene, or Bakelite. Curing time at least 8

h. Work under a fumehood because poisonous fumes are being

generated. Skin irritant

Polyester

resins

Good abrasion resistance, therefore well suited for hard materials.

Shrinkage. Chemical resistance varies with the product.

Acrylics Shrinkage. Short curing times. Poor resistance to alcohol and

chlorohydrocarbon

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When the thickness of the coated sample is known (Fig. 3.19a), thenthe following equation applies: L = S L/S, or

if the angle (Fig. 3.19b) ofthe supporting wedge is known, then L =L sin

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When analyzing thin layers and surface defects, it is importantto have a sample with good edge retention

Rinsing the sample with a wetting agent and using a material

generating low shrinkage during the curing cycle is beneficial

the sample surface is reinforced with an additional layer.

By addition of ceramic particles, such as alumina powder, thehardness of the mounting material can be increased and

adjusted to the hardness of the sample

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the sample must have an electric contact.

This can either be a hole drilled through the mounting materialor a wire attached to the sample before mounting

They are plastic materials, to which metal powders such asiron, silver, copper, or graphite have been added.

Another excellent technique is to embed the sample in low-melting bismuth alloys

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Specimens with voids, pores, and small cracks, and sample material that

are fragile or brittle are prone to breakouts

Only Epoxy resins can be used for vacuum impregnation due to their low

viscosity and their low vapour pressure

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Metal powders with a particle size above 2 m should be coldmounted in epoxy resin with pressurizing equipment.

Powders that tend to agglomerate with a particle size less than2 m (e.g., ceramic powders) can be embedded as follows:

the powder is placed in a test tube and put under vacuum with a three-

way tap using running water;

it is then infiltrated with a mixture of methacrylic acid methyl ester and

1% (wt%) of benzole peroxide.

This mixture cures in 12 h at 50C, with the test tube kept closed. The test tube is then broken and the powder sample is processed as

usual.

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Mechanisms of material removal

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Proper grinding removes damaged or deformed surfacematerial, while introducing only limited amounts of new

deformation..

The lapping process is an alternative to grinding, in which theabrasive particles are not firmly fixed to paper. Instead a pasteand lubricant is applied to the surface of a disc..

Grinding can be divided into two individual processes:

plane grinding and fine Grinding

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Copper specimen ground with 180 grit paper

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Manual grinder/polisher

machine

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conducted with particles of6- or 3-m

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With particles of 1-m

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Copper specimen polished to 6 micron level

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Copper specimen polished to 1 micron level

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Specimen Movement: rotate in a direction counter to the rotation of the polishing wheel.

the specimen is continually moved back and forth between the centerand the edge of the wheel,

After each step, the specimen is rotated 45 to 90 so that the abrasion is

not unidirectional Polishing Pressure The correct amount of applied pressure must be determined by

experience

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Vibratory polisher

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Electrolytic polishing to grind the surface to a 600-grit finish and then electropolish for about

1- o 2-minutes

soft single-phase metals and alloys

Suitable electrolytes: diluted phosphoric, sulfuric or perchloric acid,acetic acid or alcohol

Chemical polishing The agents are usually strongly oxidizing

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Circuitos elctricos y arreglos de los equipos usados para el pulido

electroltico

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Combined techniques, such as alternating mechanical andelectrolytic polishing or etch-polishing, give improved results

in the case of soft or tough metals since deformation and

reaction layers at the surface of the sample are easily removed.

Moreover, refractory metals, multiphase alloys and coatedmaterials can be satisfactorily treated.

Etch-polishing is also applicable to some ceramics, e.g. grainboundary phases in AlN or Al2O3 become visible if NaOH is

added to the abrasive suspension

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Etched copper specimen

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Specimen levelling press

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Heat Tinting: Grain structure of annealed CP titanium revealed by heat tinting on a hot

plate

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Characteristic curve for an electrolytic cell

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Microstructure of thermally etched Al2O3-15 vol.% ZrO2 ceramic. The

white particles are zirconia

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Effect of ion milling on titanium. Mechanically prepared surface on left and

ion milled surface on right

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Instead of an epoxy resin,

Canadian Balsam is often

used, especially if the index of

refraction of the resin has to be

known

In geosciences, the thicknessof the thin section is

normalized to 20-30 m

according to the interference

colors of quartz.

For the characterization ofceramics the thickness may be

chosen according to the

particle size

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Preparation of thin sections

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