Thermal Spray Erosion Testing

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Thermal spray coatings that fall under the abradable coatings family such as nickel-graphite flame sprayed coatings are sometimes required to be tested to obtain an erosion number. This post addresses the concerns regarding the erosion testing of thermal spray abradable coatings from a practical standpoint as would be faced by a typical thermal spray coatings production facility. As one might know, this erosion testing procedure involves testing a standard panel made of Lexan and then testing the actual thermal sprayed coupons coated at the same time as the actual hardware in the same test rig with the same setup. The variables in this testing procedure are way too many to obtain consistent results. While the claim is that normalizing the erosivity number against the lexan sample would eliminate variations in the testing process, the actual depth measurement of either the lexan sample or the actual thermal spray coated coupons themselves may be suspect. The reason I say this is because the erosion pattern in both the lexan sample and the thermal spray sample is a smooth curve and then how do you put a flat anvil micrometer to get the actual depth measurement without deviations. The air pressure that is driving the erosion media may vary albeit in small amounts; sometimes the operator re-uses the media and that may cause variations in the erosion number since there might be foreign particulates in the media. Additionally, one needs to put the thermal spray booth production on hold, until the erosion number is obtained as opposed to coating all parts and then perform the erosion testing after the fact, because what if the erosion number is not in the specified range and you have coated all the parts. I know what the immediate answer is: strip all the parts and re-do the process just like shampooing and rinsing ( repeat the process over and over again ) or blame it on the thermal spray production facility being out of control and so on. The erosion number obtainable from an erosion testing rig as referred to above may be fine for an R and D facility but is cumbersome to say the least to a production thermal spray shop. The erosion testing rig itself is cumbersome to set up quickly and besides the fine powder used in the erosion process flies all over the place even though you may have set it up inside a grit blasting cabinet. I am sure all the readers that have had hands on experience dealing with this headache on a frequent basis agree and those readers that deal with this headache on a sporadic basis take the stand that “if you know what you are doing then it should pose no problem”. To the latter, I say, tell that to the boss who is screaming that the thermal spray booth that is to coat the abradables has been on hold for two hours waiting for the erosion guy to come up with the erosion number!
In my humble opinion, the erosivity testing rigs and erosivity numbers obtained therefrom, need to thrown out of the window and spec out a simple quick method such as using an R15Y number to be met. If an OEM still is interested in the erosion number to be within a specific range for his application, then the burden to obtain the corresponding R15Y number falls on his shoulders and not on the poor lowly thermal spray shop. The R15Y ball is bigger, relatively speaking, and will compensate effectively for the peaks and valleys in the as coated sample and if the hardness testing machine is under calibration, then you wont need no lexan samples. There is a significant correlation between hardness and erosion and while the correlation may not be linear and simple, the simplicity and speed of the test far outweighs the accuracy claim of the erosivity test rig erosion numbers. Then again, many of the abradable coatings used in clearance control applications are not designed for abrasion by blasting media anyways, so why are we playing the silly game of thrusting down a test full of suspect results in place of using a time tested hardness number such as an R15Y value. It is the abrasion of the blades on to the abradable coating that forms the perfect seal groove in these applications with the proper clearance. Tell me where the erosion number, normalized or not, is a better indication of performance as opposed to an R15Y value.