7/30/2019 CoatingPro Jan2011 - Blasting Abrasive Blasting Myths

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BY TOM GIBBO NSEditor's m>U': A good coatings jub beginswith fJroper surfaceprep. Bttt it seems thatthere are ns maHJ' myths abl)ut acMev-ing proper profile rts thert are CoM ingsc ~ m t r a c w r s . It all boils dowu to a simplefact: toomuch otto litllepainr in l'tt weenthost: pettks and 1 allqswill lead to pnmwture fa ilure. Catt att equation help liS allaw id these "'NtverAgain" scenarios?Does Depth of Profile Increasethe Surface Area of Profilean d Dead Volume o fCoatingDue to Abrasve Basting?One of the stated reasons for abras i\'cblasting of substratts is to p ro\ldeadditional acthe sur-face for successful adhesion of coatiJl8S and ljnings. Thelarger the surface area. the more areaexists for friction o r active s. tcs for chc:m i-c:.l .-.nd Jncchan ical bondi ng. On thisbasis, some people assume that the deeperthe profi le produ ced by abrasiYc blasting, the larger the active sur fact ae a tedto increase ad hesion to the substrate. Butthis a.ssumption js erroneous in tha t eachmetal has its own unique fracture angle,and once the entire surface is abrasivelyblasted , no furt her s urface a rea can be

70 Coo r ngsP'ro Jonuorv 2011

provided. regardless of the dep th of theprofi le. And that is whe re tr oub le canbegin. ' f his a rti cle a\ dresses the Olv:tilable surface area increase due to abrasheblasting of metals; the \'Oiumc of he peaksin relation to the volume of he voids; andtotal volume of stet! as measured by theunbl.-.sted sort3ce are

,,Ao ..,81'2: h t1 x h tan'N "' ll1.711btt of Pu b L !t Ar ,..(UA}/ A,N, UA/Sh'r..rr' u n /2

\.

= n ~ = - 8 1 1$ $")111 1-@lf ol Pq.)lc S i d ~ h / M

Jv .. )(81'(2h 1.)1\ l; rl 2flt h/ COS!1v . jshl .JI' liln/2) / cos

I . W t h l c e A n : l l o i P e l l l ! s : A "'" 'A.,XII,A 11>" (U t M ~ / l )I )((3h' Un/2 I 00')

A ., VA / eo$ Ull ,.VA/$iflRlltlo of$urlikl!.)rqQ of Peale$ IOUA" (U A/ ,!n 11 1/l) A '" 1/ &InA w/UA . I / $111 l/ s!n 4S" . I /0.1071067811 .4 14213562

F ,4 ~ W ) ~ ' d , 1'1. J. , " M O ' O I I " " ' 4 ' 1 ' . . , . - ~ " . IJSS. lntfO'IVIII onof7fl;PA,2M/ rl 19S(i

with coinciMntal base edges, tri.-.ngles,squares, h

7/30/2019 CoatingPro Jan2011 - Blasting Abrasive Blasting Myths

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Th is is the maximum increast in area inan ideal sit1.1ation. Pn-actic:al increases willbe sonlewhatl ess han the idea. (Ste ChartI: Numerical Calculations ofPenkSSurfnceArea vs. P l a n ~ : AreaofSubstrate) .

The conclusion is tha t regardless ofthe height of the profile, the increase intotalS\ltfuce area cannotexceed the inverseof the sine of he:- fracture angle of the:

beiJlg blasted (l / sin n}. You canincrease the heightof he profile. but hat isconcomitant with n decrease in the numberof p-eaks; or )'0\l can n c r ~ a s e the munberof peaks ami at the same time reduce theheight of the profJie. The fin