Posted by Gordon England on April 29, 2004 at 13:58:41:
In Reply to: Affect of Grit Size on Pull off Adhesion Strength posted by sandeep jaikishen on April 29, 2004 at 05:16:31:
: Dear Sir
: We had conducted a test with different Cast grit size 24 (2.5 -3 mm),44 (1.5 to 2 mm) & 65 (0.5 to 1.2 mm) sizes on test panels and metallized the same with Zinc Flame Spray to 40 microns thickness. We were expecting higher the grit size better the adhesion but surprisingly test results are opposite as Grit 65 is showing the best adhesion strength. Now we feel that most suitable size for is 0.8 to 1.2 mm. Please clarify.
: Regards
: Sandeep
Hi Sandeep
Thanks for posting your findings. I am not surprised by your results and have found this trend with many thermal spray coatings. The general assumption that the rougher the grit blast preparation the better the bond in my opinion is false.
I think the quality of the grit blast etch in terms of "sharpness", cleanliness and surface area exposed are important factors.
I think in the case of a very course grit blast etched surface, the small coating particles strike what is effectively a smooth surface(relative to the particle). Also, particle impact angles relative to the surface may be more extreme, increasing chances of shadowing effects producing more voids and poor bonding along substrate/coating interface.
A word of warning, be careful in interpreting your bond strength results, as test done on coatings this thin can be effected by the adhesive penetrating the coating and interacting with the substrate usually giving false higher than expected results.
Regards Gordon
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