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Friday, 11 March 2011

Surface chemistry: A close look at hydrophobicity_Wetting_non-Wetting_Bulk Steelmaking Macro to Nanotechnology and Biomimicry

This post was motivated by a recent publication (14 Feb. 2011 in Nature Asia Materials) entitled

 Surface chemistry: A close look at hydrophobicity : research highlight : NPG Asia Materials (ref. 1)

Much progress has been made in understanding the phenomena involved in the wetting of solid surfaces by liquids, in the characterisation of wetting phenomena since Charles Macintosh (FRS) chemist and engineer famous for the impermeable named after him (1766 – 1843) for that that matter since my very first study as young,high temperature physical chemistry,research scientific officer involved in "wetting- non-wetting of refractory surfaces by liquid steel (mpt.1500°C) so fundamental to steelmaking and it's manufacturing process improvement.(1969-71)  It appeared to provide a surprising historical insight into the study of wetting, hydrophobia-hydrophilic as well as an occasion to revisit themes treated pragmatically in my very first study project involving the formation of gas bubbles on refractory surfaces in steel, perhaps re-situate it in what has today become a flourishing inspirational approach to many biomimetic material innovations.  Our focus at the time (1970) was the then new vacuum degassing DH and RH processes whereby liquid steel is recycled through a vacuum chamber. Deoxidation is by carbon forming CO/CO2 gas bubbles formed under the prevailing vacuum conditions. Often the liquid steel circulation was hindered in the narrow recirculation legs by unwanted CO/CO2 gas bubbles. We confirmed the role of liquid wetting, active or unwetted pore size, the influence of choice of refractory materials, and the combined influence of the overhead atmospheric pressure and the pressure of the head weight of liquid steel. The total pressure was varied by reducing the atmospheric pressure. Data is shown below:


 





Ref. 2 The growth of carbon monoxide bubbles on refractory surfaces during vacuum degassing of iron melts.  J. Alexander, G.S.F Hazeldean, M.W. Davies Sheffield Conf. 1971  and BISRA -Corp Labs of British Steel Corp. Report CH/28/71.



If I personally did not follow-up this applied research in bulk liquid metal degassing, it did stand me in good stead for rapidly coming to terms with gas bubble phenomena in liquid steel and special alloys. For example Fe-Ni and Fe-Ni-Co alloys, Invars and Covars highly sensitive to CO gas solubility and rimming or degassing during solidification. The larger the ingot the more difficult it is to solidify and subsequently remove remaining traces of gas blow-holes. Nevertheless ingot sizes were increased from 4T to 10T and even to 18T. Similarly improvements were made in VIM-vacuum induction melting and refining and VAR-vacuum arc remelting etc. all stemming from intimate knowledge of C deoxidation reaction its theoretical and practical limitations and of the physics and chemistry of wetting.

If I and worse the reader feels that this is old-hat stuff, I and hopefully the reader like me will be most encouraged by the historical background referenced in the title paper:
Surface chemistry: A close look at hydrophobicity : research highlight : NPG Asia Materials  (14 Feb. 2011) Ref. 3.


Wenzel's referenced work is "Wenzel RN (1936) Resistance of solid surfaces to wetting by water. Ind Eng Chem" and Cassies referenced work is Cassie ABD, Baxter S (1944) Wettability of porous surfaces. Trans Faraday Soc 40:546–551.28:988–994. [ WENZEL STATE _ WENZEL-CASSIE-TRANSITION_free from PNAS.ORG [Pdf format] (Ref. 3)

Of course lower temperature (RT) phenomena and modern computing techniques and computer technological advance readily allow molecular dynamic (MD) simulations to be carried out. If accent in the 1960-1980's focused on macro-phenomena and increasing productivity and economies of size. Recent approaches focus more and more on the infinitely small-nanoscience and technology first driven by micro-electronics (Moore's Law ) and much more recently inspired by biomimicry cf for example The Biomimicry Institute

The types of  applications, inventions, innovations arising from nanotechnology and the biomimetic approach are given in ref. 4 below.

NB. Recent great mind who moved from solid state physics to explore  
"Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves"
 and on to soft materials is the late and much regreted Pierre-Gilles de Gennes, who associated with Francoise Brochard-Wyart and David Quere authored the book in the above title. cf also amazon's offer in Books below.


Refs:
1. Surface chemistry: A close look at hydrophobicity : research highlight : NPG Asia Materials

2. Ref. 2 The growth of carbon monoxide bubbles on refractory surfaces during vacuum degassing of iron melts. J. Alexander, G.S.F Hazeldean, M.W. Davies Sheffield Conf. 1971 and BISRA -Corp Labs of British Steel Corp. Report CH/28/71.

3. WENZEL STATE - WENZEL CASSIE [Pdf]

4. Hydrophobicity - Superhydrophobicity

5. Good overall introduction to physics of Wetting, Adhesion, Biomimicry, Friction:
Nick Fang's Lecture_Wetting_Adhesion_Biomimicry_Friction_Macro to Nano [pdf]

RELATED POSTS:

1. Whisky - Chemical up-date from the RCS-Chemistry World

2. Water repellent properties, Biomimicry, Self Assembling Molecules, Network of micro- nanowires, excellent imagery in "Nanomaterials: Cu Water Strider .

3. Metaklett-steel grips, Biomimicry and Shape Memory Alloy meanders

4. Nanotechnology - to many to list - use blog search tool - top left.  


GOOGLE BOOKS:

Thursday, 10 March 2011

Etching of Stainless Steels and Special Alloys for Optical Microscopic Study

A colleague recently asked what etching agent or agents should be used in order to identify which phases are present in an unspecified commercial grade stainless steel (SS) and presumably study theses phases?

Two main types come to mind:
1 Austenitic typically 18Cr/10Ni room temperature phase austenite a face centered cubic crystal structure. Austenitic SS are non-magnetic.
2 Ferritic typically Fe-12%Cr little or no Ni, room temperature phase is ferrite, a body centered cubic structure. Ferritic SS is magnetic.

1. So to tell which is which, use a magnet.
(NB slight magnetism may occur in austenitic SS)
2. Use etch agents (Table 1 &2) which are not suitable for both categories cf. below for Tables, references and full explanation.

Etched Microstructure of austenitic stainless steel
By Katharine B. Small, David A. Englehart and Todd A. Christman*Carpenter Technology Corp., Wyomissing, PA, USA
*Member of ASM International, Microstructure of a stainless Type 330 sample in annealed condition at 100x, using a tint etch consisting of a solution of 40 ml hydrochloric acid (MCL) + distilled water (H2O) + one gram potassium meta bisulfite (KS2O5) + 4gms ammonium biflouride (NH4F–HF) at room temperature.

LIGHT OPTICAL MICROSCOPIC METHODS

Carpenter Tech gives a useful list of light optical microscopic methods in  Fig 1: Light optical microscopy methods of illumination used in microstructural examination.
Light Optical Microscopy Methods

Bright-Field Illumination

The most commonly encountered method of illumination in which the light reflection is perpendicular to the specimen being viewed. Generally, microstructural features such as grain boundaries are dark and matrix regions are bright.

Dark-Field Illumination

The light is obliquely reflected back through the objective so that what appears bright and dark in bright-field illumination is reversed in dark-field illumination.

Oblique Illumination

The illustration source is decentered at an oblique angle producing shadows on microstructural features. This method is extremely helpful if the operator knows the illumination direction; thereby, knowing which features are raised and which are recessed by the shadow orientation.

Differential Interference-Contrast (DIC)

A beam-splitting prism, polarizer and analyzer are inserted into the light path producing shadowing variations that reveal height differences in the microstructural features.

Polarized Light

The light is passed through a polarizing filter and can be adjusted to enhance the color contrast obtained with stain etchants.

ETCHING

A comprehensive list of etching agents (Table 1) together with those recommended for different grades of stainless steels and special alloys. (Table 2) A careful examination of these data shows that some etching agents are common to both austenitic (fcc) and ferritic steels (bcc). Let me suggest that by using agents applicable to one type but not the other will distinguish one type from the other.

cf. Tables 1 and 2. _Carpenter Tech Corp
As we have now see (above) the notion of commercial grade is rather vague when used for a large markets such as stainless steels (and special corrosion resistant alloys

A full list of stainless steel families based on their phase-crystalline structure are given below together with references for further reading and information.


MORE_ALL STAINLESS STEEL TYPES:

Austenitic stainless steels :
have an austenitic, face centered cubic (fcc) crystal structure. Austenite is formed through the generous use of austenitizing elements such as nickel, manganese, and nitrogen. Austenitic stainless steels are effectively nonmagnetic in the annealed condition and can be hardened only by cold working. Some ferromagnetism may be noticed due to cold working or welding. They typically have reasonable cryogenic and high temperature strength properties. Chromium content typically is in the range of 16 to 26%; nickel content is commonly less than 35%.

Ferritic stainless steels:
Ferritic stainless steels are chromium containing alloys with Ferritic, body centered cubic (bcc) crystal structures. Chromium content is typically less than 30%. The ferritic stainless steels are ferromagnetic. They may have good ductility and formability, but high-temperature mechanical properties are relatively inferior to the austenitic stainless steels. Toughness is limited at low temperatures and in heavy sections.

But also
Martensitic stainless steels
Duplex (ferritic-austenitic) stainless steels
Precipitation-hardening stainless steels.

Refs:
1. Main reference due to Carpenter Technology Corp

2. Microstructures of Stainless steels
Umist.ac.uk Internet Microscope_micrographs-microstructures_micrographs_stainless-steel

3. Stainless-Steel Main Types from Materials Engineer

Other references on etching :

4. Technical Information_Etching from Buehler Book [pdf]

5. Struers_Application Notes Stainless Steel English [pdf]

High Purity Cr sources for Superalloys

Energy for th Future:Phil.Trans.A-Vol. 365, N° 1853 / April 15, 2007, curtesy The Royal Soc. London

Engineered foams and porous materials: Phil Trans A. Vol 364, N° 1838 / 06 curtesy_The R Soc. Lond