Appendix F
Example of a Coating Designation System
Any standard designation system for coatings must capture the essential features of the materials and processes being used and be sufficiently flexible so that future materials and processes can readily be added without requiring a new framework. A potential designation system would also have to be agreed on by all interested parties before it could be adopted. The purpose of this appendix is to suggest some possible approaches to such a system.
An example of a standard coating designation system is a code with four elements. The code would be "XX-####X###-T##(M or E)." The code has the flexibility to be used by the first part alone, the first two parts, the first three parts, or the entire code. The coating is progressively better defined as more parts of the code are used.
The first part of the code, "XX," refers to the class of coating, such as AL for a simple aluminide, AS for a silicon modified aluminide, AP for a platinum aluminide, CR for a chromide, NI for a nickel-base overlay, CO for a cobalt-base overlay, and NC for nickel/cobalt-base overlay.
The second part of the code, "###," is four numbers that give the nominal chemical composition. For a simple aluminide coating, this would be the aluminum level in weight percent, such as "2300" for 23-weight-percent aluminum. For a platinum aluminide this would be the aluminum level followed by the platinum level, such as "2310" for 23-weight-percent aluminum and 10-weight-percent platinum. For an overlay coating, the numbers would be the chromium and aluminum levels in weight percent, such as "2308" for 23-weight-percent chromium and 8-weight-percent aluminum.
The third part of the code, "X###," refers to processing, chemistry, or base-metal requirements. They are defined by reference to a table of terms, such as for the aluminum standards. Processing codes would start with a "P" and would refer to a slurry process, an inward diffusion coating, an air plasma spray, a low-pressure plasma spray, etc. Chemistry codes would start with a "C" and refer to special alloying elements, such as hafnium or silicon. Base-metal codes would start with a "B" and would refer to the specific superalloy types for which the coating is intended. Multiple designators would be used as needed. This portion of the code has the greatest potential for creating a very long code.
The last part of the code, "T##(M or E)," is the nominal thickness of the coating, expressed in either millimeters or mils. The "M" is used for millimeters and the "E" is used for mils to identify the units of thickness.
Duplex coatings and thermal barrier coatings (TBCs) require that the individual parts of the coating be separately specified. Thus a duplex coating would be called "DPXX/YY" and a TBC would be "TB-XX/YY," where XX would be the top coating and YY the bottom coating. As an example, an aluminide on top of a NiCrAlY would be called "DP-AL/NI." A TBC over the same bondcoat would be "TB-ZR/NI." The codes for the component coatings would still be required after this designation.
The advantage of this coding system is that it captures the essential features of the coating. The disadvantage is its potential for a long and ungainly designator.
Alternatives to this system are to use either a number for each coating, similar to what is currently used by vendors, or a two-digit letter to identify the class of coating (e.g., AL, AP, CO) followed by a number for the specific type of coating. Both of these alternatives have the advantage of a simpler designator but the disadvantage of providing little information about the coating from the designator.
