G.S.G.K.S Bharadwaj       Reg. No. : 14ETMM02 

    

 Lab Report  

Scanning electron microcopy   

Objective:To make microstructural observations of sample observed under scanning electron microscope   Equipment handled: Hitachi S3400N   Theory : Scanning electron microscopy is a microscopic technique used to enhance linear                         magnification by using electron beam as source radiation. Apart from mere observation                       SEM is also used to find chemical signature of sample using EDX (Energy dispersive                           X­ray spectroscopy ) technique..  

Though resolution of SEM is still limited by Abbe’s                   equation , it is much narrowed down as compared to optical microscopy. Thus minute                           features on surface can be observed using SEM. Important feature that sets SEM stand                           alone in microscopy techniques is it’s high depth of field ( with high resolution). This feature                               is important to analyse fracture surfaces of specimen. Important features viz. mode of                         fracture, epicenter of fracture and a lot of information can be deciphered from fractographs.                           Apart from above listed information it is also possible to predict qualitatively the depth of                             so collected 3D surfaces.  Principle : Scanning electron microscope produces images of samples due to                     interactions between electron beam and sample surface. Different kinds of information can                       be extracted from SEM based on interaction signal generated due to sample surface­                         electron gun interactions.  

The main signals which are generated by the interaction of                     the electron beam and the specimen´s bulk are secondary electrons (SE) and                       backscattered electrons (BSE) and furthermore X rays. They come from an interaction                       volume in the specimen which differs in diameter according to different energies of the                           electron beam (typically between 200 eV and 30 keV). The SE come from a small layer on                                 the surface and yield the best resolution, which can be realized with a scanning electron                             microscope. The well known topographical contrast delivers micrographs which                 resemble on conventional light optical images.           

  The BSE come from deeper regions of the investigated material thus giving a lower                             resolution. The typical compositional contrast gives material specific information since                   the signal is brighter for regions of a higher middle atomic number of the investigated area.                               As a byproduct of the image giving signals X­rays are produced. They result from                           ionization processes of inner shells of the atom leading to electromagnetic radiation. The                         characteristic X­rays give information about the chemical composition of the material. The                       

method energy dispersive X­ray spectroscopy (EDS) as exemplified earlier enable                   the detection of chemical elements from in a qualitative and even quantitative manner. They                           differ in the energy resolution (the energy values are correlated to the line energy of a                               special chemical element) in the processing and in time of measurement.  Pros and cons : Added advantage with lies in fact that no sample preparation is required                               to pursue observations on SEM ( for metallographic study ). Samples ( say metal samples)                             can be observed as they are without any modifications as compared to conventional                         

optical microscopy i.e. sample need not be polished ,etched to reveal the features.                         However a point that deserves mention is that SEM is not compatible with non­conducting                           samples. As it is the electron beam interaction that we are concerned with , a non                               conducting sample could accumulate charge thereby blurring the vision. Above problem                     can be circumvented by applying a thin coat of conducting sample over the non conducting                             specimen. A carbon tape can also used for this purpose.  Observations : SEM micrographs are obtained from samples that are prepared for                       metallographic observation in optical microscope. 

 

  Following information is extracted from SEM micrograph  

1) Acceleration voltage is 15.0 Kv 2) Magnification at which image is captured is 2000x 3) SEM is set for secondary electron imaging. 

 Above details extracted from micrograph image are of little use and of no interest.                           Microstructural information is what that is required. Following microstructural information is                     inferred from above SEM image.  

1) There are evidences of etch pits on sample surface, that are caused due to over                             etching of the sample. These are artefacts and not defects. 

2) There are also small pores observed in the bulk of grain.  

  

1) Magnification is 1000x. 2) Acceleration voltage is 15.0 kv 3) Twin boundaries are seen in the micrograph. These are mechanical twins that are                         

characteristic of austenitic steels.   

  

1) Imaging mode is different in both micrographs. Micrograph (a) is imaged in BSE                         mode where as micrograph (b) is imaged using transmitted electrons. 

2) Morphology of object under observation can be deciphered from micrograph (a) (                       one on the left side) 

3) Center portion of micrograph (b) is dense relative to the edges as there is no                             available signal from that portion. 

                  

1) This is high resolution ( scale bar reads 1 nm) image obtained perhaps through                           TEM. 

2) This image refers to diamond film grown epitaxially on Si wafer. 3) Slight lattice misfit is observed as there is mis orientation in atomic positions. 

                          

1) Epitaxially deposited layer on other is observed. 2) (1 1 1) is more amenable to plastic deformation than (2 0 0) planes. 3) Based on given data lattice parameter is evaluated to be 3.60 angstorm units. 4) (1 1 1) planes are closepacked as compared to (2 0 0) planes. 

  

   

1) A cylindrical film is grown on a substrate. 2) In micrograph (b) operated in STEM mode it can be observed that the film is                             

permeable to electron flow as opposed to metal substrate which is opaque. 3) Acceleration voltage is 30 kv in both cases. 4) Field of view is same in both micrographs. 5) Micrograph (a) is operated in secondary electron imaging mode. 6) working distance in both cases is same. 

 Conclusions : Metallography is studied with help of scanning electron microscope 

2) SEM is operated hands­on and images are obtained. 3) Aspects related to microstructural analysis are realized.