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Glossary of MinSE


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S

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A term first coined by Ichii and others to describe the shallow (≈3 µm) surface zone produced in the absence of CrN after nitriding austenitic stainless steels at or below 500°C.   The precise nature of S-phase has yet to be conclusively elucidated, but it is characterised by exceptional hardness (≈1300 kg/mm2), high residual stress and outstanding corrosion resistance.   It displays only three or four broadened diffraction peaks when exposed to monochromatic X-rays;  the peak positions being consistent with a tetragonal unit cell.     The phase is meta-stable; it is not observed when  nitriding at ≥550°C.   Instead, the chromium is partitioned out of solution as CrN.   In plasma nitriding, relatively high power densities are required to induce the formation of S-phase.    Also there have been some practical experience with lack of uniformity of treatment, especially when treating large numbers of components.   The diagram shows potentiodynamic sweeps (at 0.5 mV per second) for AISI 316 austenitic stainless steel plasma nitrided in cracked NH­3 for various temperatures and times, e.g., 800/10 indicates 800°C for 10 hours.  All tests were carried out in 3% NaCl solution at ambient temperature relative to a Standard Calomel Electrode.  The data shows that all samples containing S-phase (500/10, 500/36 and 450/36) have superior corrosion resistance to non-nitrided AISI 316.   Samples nitrided at 600°C and above (where CrN predominantly forms) exhibit inferior corrosion resistance.      Data from P. A. Dearnley,  A. Namvar, G. G. A. Hibberd and T. Bell, Proc.Plasma Surface Engineering,  Vol. 1, 219-226, 1989, Oberursel,  DGM.

Hungary flag S-fázis

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A standard microscopy method with exceptional depth of field.   Modern instruments now deploy field emission electron sources which provide excellent illumination.   Especially useful for the examination of worn, corroded or fractured surfaces

Hungary flag Pásztázó elektronmikroszkópia

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A relatively new technique enabling the resolution of lattice points on a sample surface. The method may prove useful in the elucidation of wear and corrosion mechanisms.

Hungary flag Pásztázó alagútmikroszkópia

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A test where a diamond stylus is dragged over a surface.  The load on the stylus is progressively increased until the coating fails.   Some experimental set-ups enable the recording of acoustic emissions which aid interpretation of the recorded forces;  a large 'jump' in acoustic emission often accompanies coating failure.   Hence, the force required to cause coating failure is detected.   Also see adhesive (or adhesion) strength tests.

Hungary flag Karcolásos adhézió teszt

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When two formerly sliding surfaces bond or weld together.  In the case of metal on metal seizure, a metallurgical bond is made across the contacting interface.   Seizure can take place as a result of very high contact loads and/or when lubrication breaks down.    Some metals, like titanium and austenitic stainless steel show a greater tendancy to seize (for a given loading situation) than other materials.   Seizure may take place intermitently and culminates in localised fracture or tearing.  This latter phenonemon is sometimes termed galling or, less appropriately, scuffing.   When on a small scale,  the resulting wear is sometimes called adhesive wear.    In metal cutting a special situation arises where the metal chip or swarf is seized to the tool rake face over most of the contact area.   The chip does not slide over the tool surface but is intensely sheared within a narrow shear zone.   This results in the creation of sufficient heat to commonly raise the surface temperature of cutting tools to well over 900°C.

Hungary flag Beragadás /beszorulás

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Thermochemical diffusion treatment involving the enrichment of a metallic surface with zinc; carried out at temperatures ranging from 500 to 800°C for 2 to 4 hours.  The main use of sherardizing is to increase the corrosion resistance of components constructed from ferrous alloys, especially those exposed to rain and sea water.  The diffusion zone comprises various Fe-Zn intermetallic compounds, depending upon the exact thermal cycle deployed,  although, g-Fe5Zn21 is frequently the dominant phase produced by pack sherardizing and gaseous sherardizing.   No significant distinction can be made between what might be called liquid phase sheradizing and what is actually termed hot dip galvanising.    Processing temperatures for the latter method are somewhat lower than for pack sherardizing  and gaseous sherardizing.   Consequently, the diffusion layer differs in constitution, comprising several Fe-Zn intermetallic compounds (See hot dip galvanising ), not usually found in gaseous or pack sheradized surfaces.

Hungary flag Száraz horganyzás

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A laser surface hardening method that uses very high power densities ≈108 to 109 W/cm2 , with very short interaction times  ≈10-7 seconds, to vapourize a sacrificial overlay coating, and thereby impart shock-waves of such intensity to cause work hardening of the material beneath.   Rarely practised.

Hungary flag Sokk keményítés, edzés

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A mechanical surface engineering treatment in which the surface of a metal object is exposed to the action of a stream of hard metallic shot under controlled conditions.  This treatment results in increasing the hardness of the surface layer by cold working and inducing residual compressive stresses.  The main purpose of shot peening is to increase fatigue strength.   Great skill is required in the use of this technique.   If the peening intensity is too great, surface cracks result;  these can  have detrimental consequences.

Hungary flag sörétszórás

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Secondary ion mass spectrometry.   An incident ion beam of neutral or reactive character is directed at a solid surface causing the removal of surface ions (secondary ions) through sputtering. Although early SIMS instruments used low ion energies, <10KeV, it is now more common to use higher energies, ≈15 keV.     Argon is  a principal neutral ion beam, while oxygen and caesium are frequent reactive ion beams; secondary ions of either negative or positive polarity can be produced and SIMS instruments can be configured to detect either.  The secondary ions are collected by a mass spectrometer and quantified.   Under oxygen ion bombardment the positive secondary ion yield of electropositive elements, like Mg, V, Ti and Cr are at least three orders of magnitude greater than the positive secondary ion yields of electronegative elements like C, S and O.   Conversely when caesium ions are used and negative ions are detected, the reverse situation applies, i.e., the secondary ion yields of the electronegative elements are much higher than for the electropositive elements.   Regardless of polarity, the secondary-ion intensity IA* of a given element A, is related to its atomic concentration [A], viIA* = K.PA*.[A]

where K is a constant related to the ion beam density and PA* is the practical secondary ion yield of element A.  In practice K is difficult to determine.  However, if only the ratio of two elemental concentrations is sought (sputtered under similar conditions), then a knowledge of K is no longer required, since

Hungary flag SIMS

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The wear resistance of two component surfaces sliding over each other under lubricated, wet or dry conditions. 

Hungary flag Csúszás okozta kopással szembeni ellenállás


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