by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
2005 by George Langford
INTRODUCTION - Lesson 3 - Surface Coatings and Heat Resisting Alloys
Surface coatings are often sufficiently thick that you can resolve their metallurgical microstructures.  What is different is that the transition zone between the coating and the substrate is often quite thick and is affected by imperfect wetting, heat flow, diffusion, or other invasion of the substrate by the coating metal.
The photomicrographs made by me of these miscellaneous specimens in the M.I.T. collection all illustrate 
heat resisting alloys.  Their microstructures range from quite simple to rather complex, but they often show transition type behavior due to interactions with their environments, similarly to the coated metals.
Allow plenty of time to study and to take good notes about each specimen.  About two hours per lesson would be appropriate.  You will be expected to interpret some of these specimens during the final examination.  Feel free to use the Internet to find additional information about the alloys and applications mentioned here.
Whenever the narrator asks a question, be sure to commit yourself to an answer before going to the next page.  You can backtrack with your browser's BACK button at any time, of course.  Just be sure to click the FORWARD button to return to the proper page before proceeding so that you don't get lost.