ASTM F1438-93 (2020) pdf free.Standard Test Method for Determination of Surface Roughness by ScanningTunneling Microscopy for Gas Distribution SystemComponents.
4.1 In this test method a sharp,conductive tip is scannedover very closely but not in contact with a conductive surface;that is, they are separated by a gap of several angstroms.A biasvoltage present between them causes a flow of electronsthrough, rather than over, the energy barrier represented by thistip-surface gap. This flow is referred to as the tunneling current. The manner in which the current fluctuates during thescanning process is used to indicate the surface’s topography.Though the tip or sample can be scanned, the method describedhere considers only the tip to be in motion. A more extensivediscussion of the operating principles can be found in otherliterature.In this test method,stainless steel tubing is used as anexample of a component surface.An area of the surface is firstscanned at a width of 500 nA,then 2000 nA. Even thoughlarger areas can be scanned by most instruments, these mag-nifications are chosen to show surface texture in a size rangebeyond that measured by contact stylus type surface profilinginstruments， but not at an atomic scale.The surface scans arethen compared for damage, artifacts,etc. Numerical analysiscan then proceed using these data for roughness or surface areaor both, following the model of other standards such as ANSIB46.1.The use of STM images and data is for purposes oftextural quality assessment and calculation of figures of merit,and for high purity gas system clean room components.This test method defines a standard data presentationformat and suggests figures of merit that utilize STM’s abilityto analyze threedimensional surface features.
Some (stainless steel) component surfaces have an oxidelayer that prevents tunneling from occurring under any condi-tions without affecting tip or surface morphology.This resultsin ambiguous surface data.Such surfaces require the use ofother techniques for topographic measurement.This test method assumes that the images obtained areunperturbed by very thin,non-solid layers (for example,hydrocarbons,moisture) on the surface.ASTM F1438 pdf download.