CA1332373C

CA1332373C - Laser surface treatment

Info

Publication number: CA1332373C
Application number: CA000586377A
Authority: CA
Inventors: Walter Winston Duley; Grant Kinsman
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-12-17
Filing date: 1988-12-19
Publication date: 1994-10-11
Anticipated expiration: 2011-10-11
Also published as: WO1989005707A1

Abstract

ABSTRACT OF THE DISCLOSURE

A method of modifying the reflectivity and emissivity of a surface of a material comprises the steps of irradiating the surface with a beam of coherent pulsed radiation at a power sufficient to generate a surface plasma and scanning said beam across said surface. Successive pulses of radiation are caused to overlap and chemical change at the surface is promoted by provision of a localized atmosphere. The surface produced has features on a scale of less than 50 microns and 18 restricted to a depth of less than 10-3 cm. The body of the material is not affected.

Description

The present lnvention relates to a method of modifylng the reflectivlty and/or of a surface of a metal.

Most surfaces reflect radiation over a wide range of wavelengths and do so with a varying reflectivity at different wavelengths. This characteristic can be used in a number of ways and has dlffering effects. For example, in an armaments envlronment, a laser is used to interrogate a vehicle and the signature of the vehicle is determined by the reflectivity of the surface. This is used to identify an enemy and the type of vehicle. The emissivity of a surface, which is related physically to the surface reflectivity is also an important parameter in determining the rate with which a surface loses or gains heat by radiation. The emissivity of a surface is for example critical in the cooling of spacecraft where maximum emissivity is desirable to reduce the temperature of the spacecraft and avoid damage to the vehicle. Such surfaces would have a low reflectivity at -~
wavelengths in the infrared.
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The emissivity of a surface can be modified in ~
a number of ways. To enhance cooling of a surface in a ~-spacecraft it is known to use special paints but these ~ 30 are prone to erosion due to the radiation, atoms and `~ lons present in space. Similarly, specific materlals or paints have been used on armaments to change their reflective characteristics and reduce their reflectivity to radar and laser range-finding devices. However, these treatments tend to be expensive and not permanent.

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:~: ' ' ' ''' 133~373 It is, therefore, an object of the present invetion to provide a method of modifying the reflectivity and hence the emissivity of a surface that obviates or mitigates at least some of the above disadvantages~

According to one aspect of the present invention there is provided a method of modifying the reflectivity of a surface of a material comprising the steps of:
irradiating the surface with a beam of coherent pulsed radiation, said pu1sed radiation being at a pawer level sufflcient to melt the surface and thereby to generate a surface plasma so that the shock wave associated with the surface plasma produces a roughening of said surface; and repeatedly scanning said beam across said surface to form successive closely spaced lines on said surface as a result of said roughening of said surface.
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In another aspect, the present invention provides a method of ~educing the average re~ectivity of a surface of a material comprising the steps o~:
irradiating said surface with a beam of coherent pulsed radiation, said pulsed radiation being at a power level sufflcient to melt the surface and thereby to generate a surface plasma so that the shock wave associated with the surface plasma produces a roughening of said surface; and repeatedly scanning said beam across selected areas of said surface to form successive closely spaced lines on said selected areas of said surface thereby to reduce the reflectivity of said selected areas, the average reflectivity thereby being reduced as a function of the ~eflestivity and the surface area of the selected areas.

In still yet another aspect of the present invention there is provided a method of changing the signature of a surface when interrogated by a laser radiation comprising the steps of treating selected areas of said surface to reduce the reflectivity of said selected areas wherein the re~ectivity of said selected areas is reduced by irradiating the surface with a beam of coherent pulsed radiation, said radiation being at a power level sufflcient to melt the surface and thereby to generate a surface plasma so that the shock ~ ~ wave associated with the surface plasma produces a roughening of said surface and `~:

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repeatedly scanning said beam across said selected areas to form successive closely spaced lines on said surface as a result of said roughening of said surface.

It has also been found that the surface produced leads to a significant increase in the convection losses thereby improving the heat transfer characteristics of the material.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a perspective representation of a laser surface treatment technique;
Figure 2 is a graph illustrating the reflective characteristics of the surface of one type of material.
Figure 3 is a graph illustrating the wavelength dependence of the reflectivity of the surface represented in Figure 2 after the laser treatment technique of Figure l;
Figure 4 is a graph illustrating the relfective characteristics of a surface of a second material (aluminum);
Figure 5 is a graph illustrating the reflective characteristics of the surface represented in Figure 4 following the laser treatment technique represented in Figure l;
Figure 6 is a graph illustrating the effect on the re~ective characteristics of an aluminum surface caused by varying the periods of treatment by the technique represented in Figure l;
Figure 7 is a curve similar to Figure 6 showing the relationship between emissivity efflciency and the number of pulses of laser radiation where the pulses partially overlap.
Figure 8 is a curve similar to Figure 7 showing relationship between emissiivity efflcienc~ and number of pulses for a stationary sample of material.

~ , ~ -3 -~, , , . ,. ,, -Flgure 9 ls a curve showing the effect of-different energies of the lncldent beam on the absorptlvity of an alumlnum surface.

Figure 10 ls a curve showing the effect of , varying amounts of the treatment of Figure 1 on the convective loss of the material used in Figures 6 to 9.

Figure 11 is a graph similar to Figure 6 showing the effect of the treatment on copper;

Figures 12 to 22 are reproductions and electron microscope scans of samples described in chart 1 below.

Figure 23 is a schematic representation of an ~: alternative apparatus to that shown in Flgure l;

Figure 24 is a representation of a material :~ having selected areas of its surface treated by the , ~ technique of Flgure 1.
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Referring therefore to the drawings, in Figure 1 there is shown a laser 10 irradiating a surface 12 of ' a material. The laser is finely focused to a point 14 ': typically having a surface area in the order of 1 mm2.
The material 12 can be moved relative to the laser 10 ~ 30 about two mutually perpendicular axes so that the laser `:~ can be moved to any point on the surface 12.
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The laser 10 is operated to produce pulsed radlation and focused to the point 14 so that the intenslty of the beam ls sufflcient to generate a laser , E,~ ; . , -supported detonatlon wave. Typlcally, the threshold for plasma lgnltlon leading to a detonation wave ls about 5 x 10 8 watts/cm2, i.e. a fluence in the order of 20J/cm2 In use, the laser 20 produces pulsed radlation and ~e ma~ w~n ~* ~X 14is~T~a~d ~ ~ -sufficient lntenslty so as to form a surface plasma.
Focu881ng of this radlatlon to produce an lntenslty ~ -~
exceedlng the threshold for plasma productlon results in an evaporation and melting of surface material.
Material removed forms a vapor over the surface that ls heated by the laser beam to form a plasma. ~he shock wave assoclated wlth thls plasma lmpacts onto the heated surf w e and produces a surface roughening as llguld materlal ls eJected away from the focal spot. It i8 this surface roughenlng together with oxldatlon of the ~-hot materlal that produces a change ln the reflectivlty and emlsslvlty of the material. As the laser 10 18 operated, the spot 14 ls scanned across the surface 12 by movement about one of the axes. After completion of -~
a row along one axis, the materlal is moved along the other axis and the scan repeated. This continues until the whole surface has been treated and a roughening of the entire surf w e has been effected.
Hence an oxtended bulk area is built up by successive scans.

The preferred apparatus is an Excimer laser, -operating at a wavelength of 308 nm and providing an energy per pulse of between lO and lO00 mJ and a pulse wldth of less than lO0 ns, typically in the order of 30 ns. This is operated at a repetition rate of 50 Hz with ~; soan sp-ed across the surface of 7 in/min. Typically ~:, .:
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each area on the treated sur$ace will be lrradlsted by a plurality of pulses, preferably more than 5 an~ moré
preferably more than 7. In the example noted above, the number of pulses ls 17. The effect of irradiating the surface as noted above is to prov~de a number of overlapping pits indicated at 16 arranged in lines lndicated at 18 across the surface. As will be descrlbed in further detail below, the surfaces produced exhibit a characteristic roughness with gross features produced by the overlapping pulses and, within the gross features, a small scale structure with a scale of less than 50 microns.

1~ The chan~e of reflectivity as a result of thls treatment particularized above on the surface of stainless steel may be seen with reference to Figures 2 and 3. Figure 2 shows a plot of reflectivity versus wave number (1/ (cm)) for untreated material. It will be seen that the reflectivity varies between 67% and 80%, between 4000 and 400 wavenumbers with characteristic peaks exhibited at particular wavenumbers.

As seen from Figure 3, after being treated in a manner described above, the reflectivity has been reduced to a range of 18 - 69~, between the same wavenumber limits with the peaks significantly diminished.

Figures 4 and 5 show the effect of the treatment on aluminum. In figure 4 it can be seen that the reflectivity for untreated material varies between 47 and 57% between 4000 and 400 wavenumbers. After r~

` 1332373 treatment as shown in Figure 5, the reflectivity has been reduced over the entlre range between 4000 and 400 wavenumbers. Of particu~ar lmportance in the curve shown in Flgure 5 ls the dramatlc reductlon ln reflectivity at 3600 wavenumbers and at 800 wavenumbers.
Thedocnu~eat3600 wa~numbe~ maybeattnbuted ~ ~c formation of surface hydroxlde during treatment.
Slmllarly, the formation of a surface oxide causes a correspondlng reduction ln the reflectlvlty of the treated sample at 800 wavenumbers.

It wlll be apparent, therefore, that the surface treatment can be enhanced by provlding a lS locallzed atmo~phere, such as an oxygen-rlch atmosphere, at the focal polnt of the laser beam durlng lrradlatlon.
Thls would promote the formatlon of oxides and reduce the reflectlvlty further. Similarly, it may be preferred to utillze nltrogen-rich atmospheres to generate nltrldes at the surface if they show a slgnificant reduction in surface reflectivity at useful wavelengths.

~he effect of the number of overlapplng pulses is best illustrated in Flgure 6. This curve show~ the refloctivity at a fixed wavelength, typically that used by a laser radar, l.e. 10.6 micron versus the number of overlapping la~er pulses for alumlnum treated wlth a pulsed exclmer laser operating under the condltions no~dax~e~nthanoNogyofl60 m~/p~ Itw~be ~xa that as the number of pulses lncreases, l.e. the scanning speed is reduced or the pulse repetition rate increased, the reflectivity of the surface to radiatlon at a wavelength of 10.6 micron is progressively decreased, i.e. the emissivity at 10.6 micron is ; - 7 -.
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progresslvely lncreased. Once the materlal has been subJected to a certain number of pulses, ln thls case 60 or so, there is tendency for the curve to become non-linear, lndicating that minimum reflectivity has been attained. The number of pulses reguired to attain minlmum reflectivity will of course vary from material to material and with the wavelength used to interrogate the surface. In each case however a signlficant effect ls obtained after several pulses, more particularly 5 pulses and more particularly after 70 to 100 pulses.

This effect is attrlbutable to the surface roughening and to the chemical change that occurs at the surface. As noted on Figure 6, as the number of pulses increase, the oxide creatin~ process is enhanced whlch ; contributes to the decrease in reflectivity until such time as æaturatlon of the surface w$th oxide occurs at between 60 and 70 pulses. As can be seen from Flgure 7 a corresponding increase in the emissivity of the surface is obtained. The curve of Figure 7 plots the emissivlty efficiency expressed as a percentage of black body radiation against the number of pulses to which the surface is sub~ected. It will be observed that a rapid increase in emissivity occurs after several pulses with a progre~sive levelling off after 20 to 30 pulses.

Flgure 8 shows similar results where successive pulses are coincident, i.e. the laser and ~i30 material are relatively fixed and shows that whilst a slmilar ef$ect is obtained its magnitude ls approxlmately 50% of that produced with partially overlapping pulses. Accordlngly lt is believed that ~ relatlve movement between the laser and the materlal `~ 35 surface ls beneflcial for producing the optimum surface ~, effect as well as facllitating processing.

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133237~

The effect of intens~ty of the incldent beam is illustrated in Figure 9. It will be noted that with higher energy level~, the initial change in reflectivity is lncreased but rapldly reaches a llmlt. The lower energy level also shows a dramatlc increase but contlnues to increase the absorptivity (i.e. reduce the reflectlvity over a greater number of pulses). This effo~n~ybea~bu~d ~ ~cn~o~ ~m~M~
occurs at higher energy levels that inhibits the ~rma~onor~d~on ~ ~ude~ sF~u~
absorbivlty to radiation emltted from a C02 laser iQ
plotted.

A further beneflcial effect 18 lllustrated ln Flgure 10 where the effect of the resultant ~urface on the convectlve 1099 19 shown. It will be seen that after relatlvely few pulses a slgnlflcant lncrease ln convective 1088 occurs, in the order of 300% to 400%.
Thereafter the conve¢tlve 1098 i8 relatlvely constant.

It may be surmlsed therefore that the initial exposure of the surfaee produces a surface roughening that lncreases surfaee area and reduces reflectlvlty.
Progresslvely lncreaslng numbers of pulses malntaln the lncreased surface area but also promote formation of chemlcal compounds, partlcularly oxide~ that contrlbute to the further reduction in reflectivity.

As may be seen in Flgure 11, a slmilar effect is obtained with copper as with aluminum with a .~

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slgnlficant change in reflectivity occurring after 5 pulses, more slgnificantly after 10 or more pulses.-The parameters effecting surface treatment are illustrated in Table 1 below where tests produced under a variety of conditions are summarized.

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TABL~ I
_ Fxclmer (303 n.M~) Materlal S.~.M. 8ur~ace La~er Parameters Fl~. Character1zatl~n Absorptivity . _ ....... .. ,, Fluence Over- .
lapping pul--~ Al ll urface roughness only .08 (8%) 1 5-20 mlcrons 44J/cm2 50 ~ype llO0 12 underlying surface .25 (25%) features covered wlth oxldlzed mat.
500 i500 mlcrons 13 new structure ln oxldl- ;.gs thlckness zed materlal lO ~lcrons.
14 flne Ytructure, .5 - 20 .3 (30%) mlcrons wlthln coarser structures. Shows begln-nlngs of oxlde growth.
95 J/cm2 14 contlnued oxlde growth, .38 (38~) the fllllng up of the llne structure.
. _ .. _ . ._ Cu 15 Top vlew showlng edge of processed trsck ~
surroundlng reglon.
35 J/cm2 36 126 mlcron 17 slde vl-w of processed .4 (40%) thlcknesQ track, 2-5 micron structure.
18 top vlew showlng structure withln processed reglon.
26 J/cm2 150 18 cross-sectlon show~ that statlonary 150 pulses has removed Cu pulses to a depth of lO0 mlcrons, wlth no modlflcatlon to the ;~ underlylng bulk materlal.
26 J/cm2 750 20 shows redeposlted oxldlze statlonary droplets outslde of pul~es irradiated region.
Partlcle lzc 1-6 microns. Foreground shows untreated ~mooth Cu urface xpo--d ln th ; cross-sectlon.
. 65 J/cm2 930 ^ Al 21 shows recry tallz-d tatlonary structuro withln puls-s lrradlated reglon.A~perit structure 18 microns.
Central depressed shows deformation due shock wave _ _ a~ ociated with ~uch pulse.

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133237~

The sur~ces shawing reduced re~ectivity and increased emissivi~ each h~e a roughness with small scale structures of las ~an 50 micron3 and it will be observed that as the numbe~ of pulses incrase~, o~ida a~ formed fi~her contributing ~ the reduction in reflectivi~.

At the high incident intenQitles used in the present inventlon, the prlmary effect appears to be the production of a surface plasma. This plasma is created when vapourized target material 18 heated by incident radiatlon vla lnverse brems~trahlung. Thls appears to be followed by the formatlon of a laser supported ab~orption wave that further couple~ laser energy lnto ~e t~et, there~ easing ~e ma~rial rema~ a~ Under thae conditions, liquid i5 e~cpelled ~m ~# ~cat point lr~r the shock wave associated with this plasma. The hot liguld droplets are oxldized during the proce~s, likely byn~on wn~ a~n~co~ygen ~ ~eb~o~downp~sm~.
The roughened surface contains ~mall oxide particles that have been entrained in a rough metal matrix.
Increased coupling of 10.6 m~cron radiation to this surface i9 believed to be due to this roughening and to the presence of entrained oxide particle3.

Under heavier processing conditions, i.e.
exposure to a greater number of pulses, particle deposition results in the formation of a thick (i.e.
~ less than or equal to 10 microns) oxide layer that -~ yield~ a high coupllng coefficient (i.e. low ~ 35 reflectivlty) at 10.6 m. In this case, coupling appears , .., ~/~
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to arise primarily by absorption of incident radlation ln this oxide layer. -~ ~

The above-described surface treatment technique is very rapid and relatively low cost.
Moreover, because it is a permanent change in the surface, it is not prone to abrasion and does not change the electrical properties of the surface.

When used on an armament, the treatment can be applied selectively to alter the silhouette of the vehicle or to change the signature obtained from the vehicle under laser radar interrogation. Such treatment is effectively a form of electronic camouflage.

Similarly, when the surface treatment i8 :
~20 applied to a radiating surface such as aluminum, a large number of overlapping pulses produce an almost black oxide coating ae noted in Figure 6, providing a very good emitter and therefore enhancing radiative cooling -efficiency. This iæ particularly beneficial on ~25 spacecraft where radiative cooling is a dominant effect.
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With the treatment described above, the overlapping of the pits 18 produces fine grooves on the surface and with repeated scans a bulk surface area of the desired characteristics may be produced. As best . seen in Figure 22, a particularly beneficial effect may be obtained if the radiation does not impinge normally `:~
to the surface. The axis of the last lOa is inclined to the surface of the material 12a to produce lines 18a as noted above. It has been found that when the laser r radiation does not $mpinge normally on the surface, then the grooves created are undercut or angled with respect to the surface. Such angled grooves produce a change due to both components of reflectivity, that i8 ~pectral and diffuse reflectivity, being altered. This again can produce a pronounced effect leading to a reduction ln the reflectivity of the surface.

By processing only selected areas of a surface so as to decrease the reflectivity of these areas, the average reflectivity over a large area can be effectively reduced. This eliminates the need for processing an entire surface so as to reduce its average value to some desired level. For example, in Figure 9 selected areas of the surface 14b have been treated as indicated at X. If the reflectivity of the untreated surface is RA while that of a treated strip ls Rx, then the average reflectivity, R, of a sample of total area A
containlng area X of treated surface is R = R~X ~ RA (A-X). For example, if A = lm2, X = 0.5 m2, and RA = 0 9 while RX = 0.1; then the average reflectivity of this surface is reduced to R = O. 5. This significant reduction in reflectivity has been accomplished by processing only 50~ of the surface.
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The treated areas may be arranged in random ~, patterns across the surface or may follow a predetermined pattern to produce a desired effect.
Different areas may be treated in different ways to . provide a maximum effect at different wavelengths.
Similarly, the treated areas may be applied to selected areas to change the silhouette of a vehicle when interrogated by laser radar.

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;,.';~ ~ ' It wlll be apparent that the surface treatment may be used in a number of ways to provide a change in the reflectiviq of a surface. The e~act nature of the treatment 18 varlable to produce partlcularly deslrable effects such as a mlnlmum reflectlvity to a particular wavelength or a black oxide coating to maximize emls.~lvlty for black body radlation.

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Claims

1. A method of modifying the reflectivity of a surface of a material comprising the steps of:
irradiating the surface with a beam of coherent pulsed radiation, said pulsed radiation being at a power level sufficient to melt the surface and thereby to generate a surface plasma so that the shock wave associated with the surface plasma produces a roughening of said surface; and repeatedly scanning said beam across said surface to form successive closely spaced lines on said surface as a result of said roughening of said surface.

2. A method according to claim 1 including the step of promoting chemicalchange at said surface.

3. A method according to claim 2 wherein the chemical change is promoted by providing a localised atmosphere at said surface.

4. A method according to claim 3 wherein said localised atmosphere is selected from the group comprising oxygen and nitrogen.

5. A method according to claim 1 wherein said beam scans said surface at a rate to cause successive pulses of radiation to overlap on said surface.

6. A method according to claim 1 wherein said beam is inclined to the surface of the material.

7. A method according to claim 6 wherein said beam scans said surface at a rate to cause successive pulses of radiation to overlap.

8. A method of reducing the average reflectivity of a surface of a material comprising the steps of:

irradiating said surface with a beam of coherent pulsed radiation, said pulsed radiation being at a power level sufficient to melt the surface and thereby to generate a surface plasma so that the shock wave associated with the surface plasma produces a roughening of said surface; and repeatedly scanning said beam across selected areas of said surface to form successive closely spaced lines on said selected areas of said surface thereby to reduce the reflectivity of said selected areas, the average reflectivity thereby being reduced as a function of the reflectivity and the surface area of the selected areas.

9. A method according to claim 8 wherein said selected areas are arranged in a random pattern on said surface.

10. A method according to claim 8 including the step of promoting chemicalchange at said surface.

11. A method according to claim 10 wherein the chemical change is promotedby providing a localised atmosphere at said surface.

12. A method according to claim 11 wherein said localised atmosphere is selected from the group comprising oxygen and nitrogen.

13. A method according to claim 8 wherein said beam scans said selected areas of said surface at a rate to cause successive pulses of radiation to overlap.

14. A method according to claim 8 wherein said beam is inclined to the surface of the material.

15. A method according to claim 14 wherein said beam scans said selected areas of said surface at a rate to cause successive pulses of radiation to overlap.

16. A method of changing the signature of a surface when interrogated by alaser radiation comprising the steps of treating selected areas of said surface to reduce the reflectivity of said selected areas wherein the reflectivity of said selected areas is reduced by irradiating the surface with a beam of coherent pulsed radiation, said radiation being at a power level sufficient to melt the surface and thereby to generate a surface plasma so that the shock wave associated with the surface plasma produces a roughening of said surface and repeatedly scanning said beam across said selected areas to form successive closely spaced lines on said surface as a result of said roughening of said surface.

17. A method according to claim 16 wherein said selected areas are arranged to alter the silhouette of said surface.