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NASA Technical Paper 21 61

April 1983 Spherical Particle Impingement Studies of Materials

'~ Lg,+lNCopy: RETURN m .. AFWL TECHNICAL Li5Rfiffl P. Veerabhadra Rao, -'. KIRTtAND AFB, N M. Stanley G. Young,

and Donald H. Buckley 1..

25th Anniversary 1958-1983 TECH LIBRARY KAFB, NM ii NASA' Technical Paper 21 61

1983 Solid Spherical Glass Particle Impingement Studies of Plastic Materials

P. Veerabhadra Rao, Stanley G. Young, and Donald H. Buckley Lewis Research Center Cleveland, Ohio

National Aeronautics and Space Administration Scientific and Technical Information Branch fv: Summary (refs. 3, 14, and IS), Tufnol (ref. 7), i?: perspex(refs. 7 and 13), andpolyurethane (ref. 15) have .., Optical and scanning electron microscope studieswere been tested. Onthe other hand, and conducted to characterize the erosionresistances of fluorocarbon (refs. 18 and 20) coatings have been tried to polymethyl methalcrylate (€“MA), polycarbonate, and investigatetheir resistance during solid particle (PTFE). Erosion was caused by a impingement. Solid impingement erosion of bulk plastic jet of spherical glass beads at normal impact. During the materials has received little emphasis (ref. 10). However, initial stages of damage, the surfaces of these materials in view of their widespread use as coatings for aircraft were studied using a profilometer.Material buildup radomes, antenna covers, and external skin protection, abovethe original surface was observed on there are many instances of damage and erosion in real polycarbonate and PMMA. As testing progressed, this situations (refs. 6 and 20). buildup disappeared as the erosion pit became deeper. This workstudies the erosion of threecommonly Little or no buildupwas observed on PTFE. PTFE is the known plastic materials(polymethyl methacrylate most resistant material and PMMA the least. At early (PMMA),polycarbonate, and polytetrafluoroethylene stages of damage and at low-impact pressures, material (PTFE)) using spherical glass bead erodent particles at removal mechanisms appear to be similar to those for normalincidence to gaininsights intothe surface metallic materials. The flake-like debris observed on the transformationsand morphology. Possibleerosion surface, which is indicativeof deformation wear by mechanisms are discussed, based on surface analyses by repeatedimpact and eventual fatigue, caused material optical andscanning electronmicroscopy and surface loss. At higher impactpressures, evidence ofsurface profilometry. melting was noted, and it is believed to be the result of heat generated by impact. Materials, Apparatus, and Procedure Introduction Materials The damage and erosion causedby the impact of solid Specimenmaterials were polymethylmethacrylate particles on the surfaces of aircraft engine components (PMMA),polycarbonate, and polytetrafluoroethylene (refs. to helicopterblades (ref. 6), rocket motor 1 S), (PTFE). The mechanical and other properties of these nozzles (ref. 7), missiles (ref. 5), Earth satellites (ref. 8), three materials are presented in tableI (ref. andspace vehicles (ref. have received increased 9) 21). The specimens, which were 25 millimeters wide, 37 attention in recent years. Inaddition to metallic millimeters long, and 6.4 millimeters thick, were cleaned materials,nonmetallic materials are being usedin with and dried with compressed air. increasing amounts in structures as well as for viewing screens, windows, and metallic surface protection. Some Apparatus and Experimental Procedure deleterious effects of erosion on nometallic surfaces are the loss of visibility, degradation of electromagnetic The investigations in this report were conducted with a properties, and interference with communications as well commercialsandblasting facility. Test specimens of as material damage and loss. Material degradation can plastic materialswere eroded by normal incidence ofhigh occur during operation of tactical aircraft during severe velocity, commercial grade number9 spherical glass bead weather (e.g., dust and sand storms). Solid impingement particles (average diameter, -20 pm; standard deviation, erosion is also of vital interest in defense applications 12 pm). The glass bead particle distribution is shown in such as in optically guided missiles, for laminated plastic figure 1. Figure 2 shows a SEM energy dispersive X-ray transparent , and for canopies (ref. 10). spectrographof the glass beads. Table I presents the Many studies on the erosion of ceramic materials and properties of glass which are similar to the composition due tosingle and multiple solid particle impactare of the glass beads used in this study. reviewedin theliterature (refs. 9 to 11). However, a Aschematic diagram of the steady-jet-impingement limited numberof studies have concentrated on the apparatus is shown in figure 3. The distance between the erosioncharacteristics and resistanceofpolymeric, specimen andthe nozzle (diameter, 1.18 mm) was 13 elastomeric, and plastic bulk materials (refs. 2, 3, 7, and millimeters.Argon was used asthe drivinggas at a 12 to 19) and coatings (refs. 7, 18, and 20). In the various 0.27-megapascal (gage) pressure. Theaverage particle studies, the type of device, erodent particle shape and velocity was 72 meters per second. The glass bead flow at size, impingement velocity, angle of incidence, etc., have this pressure was approximately 0.98 gram per second. been changed. Normal impingement studies of various Beforeexposure all specimens were cleaned with plastic materials are mentioned here. Thus, epoxy resin distilled water and alcohol. The materials were tested in (refs. 2and lS), (refs. 2, 14, IS, and 17), the as-received condition. The original surface roughness TABLE I. - MECHANICAL AND OTHER PROPERTIES OF TEST MATERIALS

~~ Properties Methyl- Poly- Polytetra- Alumino- methacrylate carbonates f luoro- silicate (PMMA) ethylene glass (PTFE) (a) (a) (a) (b 1 Modulus of elasticity, MPa 2380 to 3400 1975 to 2210 224 to 442 77340 Tensile stren th, MPa 48 to 75 54 to65 14 to 31 """""_ Ultimate resi!ience,c MN-m/t$ 0.48 to 0.83 0.74 to 0.9 0.44 to 1.09 """""- Ultimate elongation, percent 2 to 10 20 to100 200 to400 """""- Yield stress, """""""" 54 to 68 11 to 14 """""_ Strain energy,jP;N-m/d """""""" 11 to 65 25 to 90 """""_ Yield strain, percent """""""" """"""" 50 to 70 """""_ Rockwell M80 to M105 M70 to M180 D50 to D65 """""_ Notched Izod impact strength, J 0.41 to 0.81 10.85 to 21.70 3.39 to 8.14 """""_ Specific gravity 1.18 to 1.2 1.2 2.1 to 2.3 2.5 Heat distortion at 1.8 MPa, "C 66 to 99 135 to 145 60 """""- Specific heat, cal/g 0.35 0.3 0.25 """""_ Linear coefficient, C 5~10-~to 9x10m5 6.6~10~~ 10x10-5 0.5~10'~ Maximm continuous service temperature, OC 60 to 93 138 to 143 260 e1210 1.48 to 1.5 1.6 """""" 1.5 to 1.55 Softening point, "C """""""" """"""" """""" 955 aRef. 21. bKimble Glasses Technical Data Owens-Illinois. CDefined as (tensile strength)2/(2 x modulus ofelasticity). dWhen the stress-straiq curves are not available, strain energy can be approximated as elongation x ((yield stress + tenslle strength)/2). eWorking point.

was 0.53 micrometerCLA forthe PMMA and polycarbonateand 2.06 micrometers CLAfor PTFE. Specimens were weighed before and after each exposure to theimpinging jet of glass beads, and weight loss values were converted to volume loss by dividing by . Tracesof the eroded surfaces were recorded with a profilometer, and the eroded surfaces were observed and photographed with opticaland scanning electron microscopes. The specimens that were prepared for SEM examination were gold sputter-coated in order to make them conductive.

Results and Discussion

Erosion Progress and Morphology The effects of glass bead impingement on PMMA and polycarbonate during the initialphases of erosion (from3 to 60 sec) areshown in figures 4 and 5, respectively. Figure 6 presents SEM micrographs of erosion on PTFE as functions of time from 1 to 15 minutes. 1 to L a6 to 41 to 46 to 1 to 3 to Separate specimens were tested for each exposure time 5101520 2530 35 40 45 50 1 Size range, pn n, shown in these figures in order to eliminate the effects of Figure 1. - Particle size distribution of glass beads. interrupted tests. The apparent reduction in the damage area with respect to exposure time, as observed in some II cases in figures 4 and 5, are artifacts due todifficulties in

2 Energy, keV Figure 2 - EDS analysis of glass beads.

valleys from 30 to 100 micrometers deep. Region 3 is a slightly raised regionwhich slopestoward the original surface. Region 4 is a depressed area 5 to 10 micrometers below the original surface level. Evidence formaterial buildup can beseen in the surface traces of PMMA and polycarbonate (figs. 7 and 8). These are believed to be due to heat distorsion or partialmelting and redeposition ofmaterial during impingement. A temperature rise as high as 190" C during impact conditions hasbeen reported (ref.7). Also, increased levels of the glass bead material are observed in this area. Material buildup was negligible for PTFE (fig. 9). However, the surfaces of the PTFE specimens were observed to havechanged color (from white to light brown) after glass bead impact. This color changeis also believed to bedue tothe heat generatedduring impingement. It is reasonable that PTFE would be most affected by heat in view of its lower heatdistortion temperature(table I). Darkening of nylon and Nozzle polypropylene surfaces due tosolid impingement (ref.14) has been attributed to a chemical change to the surface possibly associated with localized heating.Figure 10 --, ---1.18-mrndiam. L orifice shows SEM micrographs of an eroded PMMA specimen &Ytmm/z& exposed to glass bead impingement for 15 seconds (the Figure 3, - Schematic diagram of nozzle holder arrange- sameasthe surface profile in fig. 7(c)). These ment for impingement apparatus at normal incidence. micrographsshow material buildup, a fissure between regions 2 and 3 (fig. 10(a)), and layers or bands in some focussing all areas of the pit with an optical microscope. areas (fig. lo@)). Thesebands are,general,in Surfaceprofiles at selected erosionintervals for these circumferentialarcs surrounding thecenter of impact materials are shown in figures 7 to 9. with decreasingelevations away fromthe center of The damage pattern may be divided into four regions impact. They are believed to be formed by melting and (indicated in figs. 7 and 8). Region 1 is a central irregular resolidification of the plastic material. However, further pit surrounded by region 2, a nonuniformbuildup of studies are necessary to identify the mechanism(s) plastic material andglass. Region 2 consists of peaks and involved with the formation of these stratified layers.

3 t - 3 sec t - 6 sec

t = 15 sec t - 30 sec

t - 45 sec t - 60 sec Figure 4. - Photomicrographs of PMMA surfaces exposed to glass bead impingement at o.~-megapascal pressure as function of time.

4 t - 3 sec t 9 6 sec

t - 15 sec t = 30 sec

t-45sec t - 60 sec Figure 5. - Photomicrographs of polycarbonate surfaces exposed to glass bead impingement at 0.27-megapascal gas pressure as function d time.

5 t .1 min t 2 min

t . 3 min t - 4 min

t-5min t 15 min Figure 6. - Photomicrographs of PTA surfaces exposed to glass bead impingement at 0.27-megapascal gas pressure as function of time.

6 'I

Surface

of time. on polycarbonate as function Figure 8. Surface traces t-

t

t-

8 (a) Details of regions 2 and 3. I 1 (b) Material pileup with possible stratification.

(c) @ tilt photomicrograph of material flw of eroding pit

Figure 10. - SEM photomicrographs (46' tilt) d PMMA specimen exposed to glass beads for 15 seconds at 0.27-megapascal gas pressure.

As erosion progresses for PMMA, the pit in region 1 Erosion-Time Curves deepens and broadens (figs. 4 and 7). Region 2 gradually Cumulativeerosion-time curves forPMMA, deepens and disappears. After a 10-minute exposure all polycarbonate, and PTFE are shownin figures 12 and 13. regionsmerge intothe mainpit. Forpolycarbonate, Table I1 presents the data for the threeplastics at 5-, lo-, regions 3 and 4 are clear at first but gradually disappear 15-, and 20-minuteexposures. PMMAerodes rapidly withvery longexposures (fig. 8). In both cases, at compared with polycarbonateand PTFE (also evident advancedstages of erosion,the pit slopes (region 1) from profiles in figs. 7 to 9). PTFE is observed to be the become very smooth (fig. 11). Figure 11 shows SEM mostresistant of thethree thermoplastic polymeric micrographsoferoded specimens ofPMMA, materials. Theresults in table I1 and figure 12 show good polycarbonate, and PTFE after 10-minute exposures. reproducibility forthe erosionprocess of plastic Deepholes are observed for PMMA and materials under normal impingement. The scatter of data polycarbonate.PTFE appears to retain a relatively increased with increased volume loss. unstructured damage pattern after10 minutes (fig. 1 l(c)). Additional data have been added tothe curve for However, after a 15-minute exposure a layered structure PMMA in figure 12 to show that typical erosion volume is observed along the pit sides (fig. 6, t = 15 min). losscurves can be divided intodifferent stages. The

9 4-

fa) PMMA. 0 5 10 15 25 Time, T. min Figure 12 - Cumulative erosion -time curves for plastic materials at 0.27-megapascal gas pressure.

0 PTFE: V - 0.23T + 0.36

Ib) Polycarbonate.

Time, T, min. Figure 13. - Least squares fit of data derived from glass bead ero- sion of plastic materials at 0.27 -megapascal gas pressure.

induction or incubation period exists when there is little or no weight loss. The accelerationperiod is thetime during which the weight loss rate increases rapidly until it reaches a peak. After this there is often a constant or steady-state period, but in these experiments erosion rates increase for PMMA. Analysis of the data (refs. 14, 15, and 17) indicates that erosion rate-time curves on nylon, polypropylene, and polyurethane exhibit incubation (with and without deposition), acceleration, and steady- state periods. Plots of carbon andglass reinforced nylon, however,show incubation, acceleration, peak erosion (13 PTR. rate, and deceleration periods. The deceleration period is Figure 11. - SEhl photomicrographs (400 tilt) of eroded specimens exposed to glass beads at 0.Zi"megapascal gas pressure for 10 minutes. Corre- the time during which the weight loss rate decreases from sponding surface profiles shown in figures 7 to 9. a peak value.

10 TABLE 11. - SCATTER AND STATISTICALPARAMETERS OF EXPERIMENTAL DATA ~- kmber of Average Standard Variance specimens volume deviation tested losp mm " S PMMA 5 8 8.39 0.20 0.03. 10 6 18.89 1.09 .95 15 4 33.69 1.92 2.77 20 2 53.80 N/A~ N/A Polycarbonate 5 8 1.10 .32 .09 10 6 3.04 .22 .04 15 4 5.19 .10 .008 20 2 7.03 N/A N/A PTFE 5 8 1.37 .10 .009 10 6 2.83 .19 .03 15 4 3.91 .12 .01 20 2 4.85 N/A N/A L L I L aNot applicable, N/A.

In order to compare the volume loss data for these different investigators (refs. 2, 3, 7, and 12 to 15). This materials on a common basis, figure 13 presents least table shows bulk composite materials, nylon, epoxy, and squares fit straight lines through the average cumulative polypropylene to beless resistant to erosion thanthe volume loss data from table 11, and linear equations are metals tested. Table 111 also presents the order of erosion included to show the approximate slopes and intercepts resistance of different and related materials and a for the lines drawn through the four data points for each brief explanation of erosion mechanism and resistance by material. The values of the slopes of the lines in cubic the variousinvestigators. When PMMA was tested by millimeters per minutecan beused for comparative other investigators (refs. 12, 13, 16, and 19) using entirely purposes: PMMA, 3.02; polycarbonate, 0.40; and PTFE, differentshapes and sizesof abrasive particles and 0.23 cubic millimeter per minute. different experimental devices, verylow erosion resistance was indicated when compared to other Erosion Resistance nonmetallic materials (consistent with our results). For elastomers, these investigators found that filled rubber Erosion resistance can also be defined as the ratio of tire tread showed the highest resistance of all nonmetallic the volume ofimpacting particles to the volume of materials tested to erosive wear (refs. 12 and 22). Other material removed; the higher theratio, the higher the investigations (ref. 23) have shown that naturaland erosion resistance. Eachmaterial tested was impacted synthetic rubbers exhibit good erosion resistance because with the same volume of glass beads per unit time (0.98 of their low modulusof elasticity andthat some g/sec). Therefore,approximate erosion resistance correlation existswith ultimate resilience (defined as numbersfor the three materials are PMMA, 20; (tensile strength)2/2(modulusof elasticity)) and with polycarbonate, 150; and PTFE, 260. density of materials (ref. 16). From the datain table I, the The following discussion on ranking the erosion ultimate resilience does not vary sufficiently to arrive at resistance the testedmaterials is based on the of thesame conclusion in the currentstudy. Many more properties listed in table I. The erosion resistance varies materials with carefully measured properties need to be directly with the ultimate elongation, strain energy, and tested to draw any clear conclusions regarding material maximum service temperature; it varies inverselywith property correlations with erosion resistance. tensile strength, yield strength, and modulusof elasticity. No single property is clearly dominant in its effect on Material Removal Processes erosion resistance. It isbelieved, however, that some combinationof high ultimateelongation, impact To morethoroughly study features of the material strength,maximum service temperature,and low removalprocess during exposure to glass bead modulus of elasticity contributes to high erosion impingement, 500X SEM micrographs were taken of the resistance. eroded specimens (fig.14). Platelets or flakes were Table 111 presents a summary of erosion results and observed which looked similar to those observed on conditions on elastomeric and plastic materials by aluminumalloy (ref. 24) under identical impingement

11 e N TABLE 111. - EROSION CHARACTERISTICS OF PLASTIC AND OTHERRELATED MATERIALS TESTED BY VARIOUS INVESTIGATORS USING DIFFERENT DEVICES AND ERODENT PARTICLES - Investigator resting Nonmetallic Abrasiveor Size, Impact Angle of Eroion, Mechanism/erosion (jevice material )articles used Ilm velocity, impact, cm5 /kg resistance m/sec deg Ti lly (ref. 2)a 'I dhirl ing Epoxy resin 3.1 artz 25 128 90 1.27 Resilient plastics ex-

1mi g 115 6.31 particle size until the 140 6.61 onsetof a saturation 170 6.59 plateauwhere erosion is 190 8.71 independentof size. i ~ ~ ~- Goodwin et a1 . LJhirling Fiberglass 3 artz 125 61I 90 1.6 Fiberglassincurs the (ref.3)a arm to 122 6.3 mosterosion, and the 2rosion 150 146 14.9 other plastics tested rig 244 39.1 (polypropylene and lass reinforcednylon 667 are Glasslnylon 52 .2 poorer erosionwise than 76 .4 the metals. 99 .9 137 2.1 183 3.9 Polypropylene 61 .1 76 .2 99 .3 122 .5 137 .8 192 1.4 v 23 7 v 2.5 Neilson and Air Tufnol Iron grid 90 For brittle materials, Gi Gi lchrist stream Perspex Iron grid 90 failure appears to be (ref.7) nozzle caused by cracking fol- method lowedby crazin and subsequent spa1 9 ing of -c material. Russeland Lewis Air jet Rubber tire tread I No. 180 grit fi:.004 Rubber tire tread shows (ref.12) impact Red rubber emery abrasive by far the least abra- test Lucite f51 sion of any of the ma- : Bake1 ite f99 terials tested. Graphite J 250 aValuespresented herein calculated from curves presented in reference cited. bAir pressure, 0.17 MPa (gage)(25 psi). :Erosionrate, 3.77 9/50lb/0.5 min. Erosionrate, 3.22 9/50lb/2 min. eAir pressure, 0.53 MPa. fErosionunits, cm /10kg. TABLE 111. - Continued.

Nonmetallic Abrasive or !, Size, hgle of Ero ion, Mechanism/erosion , pin x device material particles used velocity, impact, I cm 5/kg resistance I m/sec i- Nei 1 son and Air Perspex A1 2O3 210 From the tests on per- Gilchrist stream spex it appears that it (ref. 13) nozzle is a material of the met hod j 40 type where neither cut- ' 45 92.8 ting wear nor deforma- I 50 92.7 tion wear predominates. I 60 j 92.5 For this material no de- 1 70 i 92.5 position effects are ob-

~ EO I 92.4 served and erosion is ' 90 I 92.4 detected at small angles

iI of attack. " Tilly (ref. 14) I Air I Nylon QJartz 6C 103.6 I 20 0.064 Polypropylene incurs blast tc I 30 .099 slightly more deposition I erosion I 1 2E ! 45 e028 than the other plastics 60 'I .003 at 90° (i.e., large in- 90 "026 cubation period). For ' rig I the plastics the surface Carbon 20 .078 topography is less well ' reinforced 30 .064 defined but there seems nylon 60 .018 to be evidence of groov- 90 -.021 ing in the direction of impact. Typical samples G1 ass 20 .066 of the nylon chippings reinforced 30 a079 are fwnd to be flakes nylon 45 .025 $10 vm thick. The 60 .005 fiberglass 1s fwnd to 90 -e013 break into its separate constituents because the Polypropylene 20 .012 glass fibers are broken 30 .014 cleanly with no epoxy 45 .010 attached. The .epoxy 90 -.035 chippings are in the form of flat flakes (do mm thick). Ero- sion processes for plas- tics are not clear. -

~ Ti 1 ly and Sage I Whirling 1 Nylon QJ art z 125 53.3 90 0.22 Reinforcement of plas- (ref. 15)a arm15)a (ref. to 152.4 .68 tics can either improve I erosion I 150 243.8 3.10 or worsen their erosion 289.6 3.80 resistance pro erties, depending on tc e nature 30-Percent 50.3 .20 of reinforcing materi- lass rein- 74.7 .45 als. Composite materi- 9 orced nylon 100.6 .85 als are generally rather 134.1 2.57 poor. Nylon, which-is v 21 3.3 1 4.57 one of the best avail- e P TABLE 111. - Concluded. ______Investigator Testing Nonmetallic Abrasive or Size, Impact Angle of Ero ion, Mechanismlerosion device material particles used !Jm velocity, impact, cm5 /kg resistance mlsec deg ~~ Tilly and Sage Whirling 25-Percent Qartz 125 57.9 90 0.24 able plastics, deteri- (ref. 15)a arm carbon rein- to 94.5 .43 orates by about 4 to 1 erosion forced nylon 150 121.9 1.20 when reinforced by glass rig 243.8 5.89 or carbon fibers. On 295.7 9.10 the other hand, com-a 342.9 12.88 mercial variety of epoxy resin is one of the Epoxy resin 61 .O 1.15 poorest materialsero- 91.4 8.51 sionwise. A very sub- 121.9 12.50 stantial improvement is 160.0 19.95 obtained with steel pow- 196.6 70.85 der reinforcement, al- 304.8 90.11 though it is still ten times worse thanthe 70-Percent 61 .O 1.47 sol id steel. Unfortu- lass rein- 121.9 6.91 nately, at present there ? orced epoxy 144.8 13.80 are insufficient data to 243.8 35.49 draw any generalized conclusions on the be- 80-Percent 59.4 .35 havior of composites. steel rein- 61 .O .28 forced epoxy 121.9 1.68 182.9 3.89 Y 253.0 7.08 Nylon 17 243.8 .35 I 25 .34 37 1.19 50 1.90 70 2.44 98 2.84 13 5 3.12 200 2.91 200 2.58

I Type 66 nylon 125 244 2.3 I' Polypropylene to 1 3.1 ! : Glassreinforced , 150 .' nylon i 7.0 I, Fiberglass i 7.0 Polvurethane V 1 128 53.0 -Presentstudy I PMMASand : Glassbeads 72 0.045 See Material Removal 20 90 I blasting ' Polycarbonate ' Glassbeads 20 72 90 .006 Processes section. I\ devicePTFE i beadsGlass 20 72 90 .004 J- aValues presented herein calculatedfrom curvespresented in reference cited. 4I:

I conditions.However, onthe aluminum alloyflakes, random impact impressions were noticed which were not observed on the plastic material flakes. Tilly (ref. 14) has also reported observations of flakes on nylon,fiberglass, and epoxy resin surfaces dueto angular particle impingement. Flakes were also observed by Evans and Lancaster for slidingof polyphenylene oxide and polyacetal in n-hexane and n-propanol against stainless steel (ref. 25); by Shen and Dumbleton for dry sliding of against stainless steel (ref. 26), and by Shallamach for abrasion ofparticles and pins against ,different types offilled and unfilled rubber (refs. 22,27, and 28). Theflakes observed in the present investigation are believed to form due torepeated direct impact and deformation followed by shear and fatigue resulting in completeremoval during outflow of particles. The extentof heat distortion and melting, (a) PMMA. which may play animportant role in thisprocess, is unknown at present,but it isbelieved to bemore extensive at the bottom of the crater than at the sides. Ithas also been observed fromthe literature that plastic and elastomeric materials such as polypropylene, nylon, PMMA, natural rubber, and Vulkallan B (refs. 2, 13, 14, and 23) behave as either brittle or ductile materials depending onthe angleof impingement (table 111). Maximum erosionrates have been observed at angles between 15" and 35", and cutting wearis indicated by sharp facetedsurfaces. For higherincidence angles (including normal), the damage patterns (including the observation of flakes) appear similar to those for ductile metals. This is indicative of the predominance of wear due to deformation as opposed to cutting. In our studiescutting wear (ref. 29) appears to be absent for all three plastic materials. Thiswas expected as most of the spherical glass beads were not broken even (b) Polycarbonate. after impact (ref. 24), and the material was worn due to deformation by repeatedimpact and eventual fatigue rather than by cutting the surface. For PTFE the flakes appear thinner than those for the othertwo materials (fig. 14). Thinflakes were also reported for the more resistant nylonin reference 14, and large flat flakes were reported in reference 15 for heavily eroded epoxy resin. Hence, thinner flake formation may be related to higher erosion resistance.

Summary of Results Studies of glass bead impingement at normal incidence on polymethylmethacrylate (PMMA), polycarbonate, andpolytetrafluoroethylene (PTFE) havebeen conducted.Argon wasused asthe drivinggas at a (c) PTR. pressure of 0.27 megapascal (gage). Changes in surface Figure 14. - SEM photomicrographs (400 tilt) of eroded specimens showing morphology,material removal with time,erosion material dislodging process. Specimens exposed to glass beads at 0.27- resistance, and surface chemistry were studied. megapascal gas pressure for 10 minutes. 1. Initially, a buildup of material composed of a 7. Neilson, J. H.; and Gdchrist, A.: An Experimental Investigation combination of target materials and erodent particles was into Aspects of Erosion in Rocket Motor Tail Nozzles. Wear, vol. 11, 1%8, pp. 123-143. observed.around the pit for all materials during the early 8. Hoenig, S. A.: Meteoric Dust Erosion Problem and Its Effect on stages of damage. The maximum was observed on the Earth Satellite. Aeronaut. Eng. Rev., vol. 16, no. 7, July 1957. polycarbonate andthe least forPTFE. After further pp. 37-40. exposure material buildup and any other features on the 9. Tilly. G. P.: Erosion by Impactof Solid Particles. Treatise on surface disappeared as the main pit developed. and Technology, vol. 13, D. Scott, ed., Academic Press, 1979, pp. 287-319. 2. Some evidence for melting at the pit centers was 10. Schmitt, G. F., Jr.: Liquid and Solid ParticleImpact Erosion. observed as was subsequent solidification on the sides. Wear Control Handbook, M. B. Peterson and W. 0. Winer, eds., 3. PTFE was found to be the most resistant to erosion American Society of Mechanical Engineers, 1980, pp. 231-282. and PMMA the least resistant. 11. Adler, W. F.: Assessment of the State of Knowledge Pertaining to 4. A dimensionless parameter for erosion resistance, Solid ParticleErosion. Rept. ET1 CR 79-680, U.S. Army ' Research Office, 1979. (AD-A073034.) which is the ratio of the volume of erodant particles to 12. Russel, A. S.; and Lewis, J. E.: Abrasive Characteristics of the material lost, resulted in numbers of 20 for PMMA, Alumina Particles. Ind. Eng. Chem., vol. 46, no. 6, June 1954, pp. 150 for polycarbonate, and 260 for PTFE. 1305-1310. 5. A combination of high ultimate elongation, strain 13. Neilson, J. H.; and Gilchrist, A.: Erosion by a Stream of Solid energy, maximum service temperature, and low modulus Particles. Wear, vol. 11, 1968, pp. 111-122. 14. Tilly, G. P.: Erosion Caused by Airborne Particles. Wear, vol. 14, of elasticity appears to be consistent with high erosion 1969, pp. 63-79. resistance. 15. Tilly, G.P.; and Sage, W.: The Interaction of Particleand Material 6. SEM micrographs show flake-type debris in the Behaviour in Erosion Processes. Wear, vol. 16, 1970, pp. 447-465. eroding pits of the thermoplastic materials. Material loss 16. Kayser, W.: Erosion by Solid Bodies. Proceedings of the Second is believed to be due primarily to the breakup and Meersburg Conference on Rain Erosion and Allied Phenomena, vol. 2, A. A. Fyall and R. B. King, eds., Royal Aircraft removal of these flakes. Smaller, thinner flake formation Establishment, Farnborough, England, 1967, pp. 427-447. appears to correlate with higer erosion resistance. 17. Tilly, G. P.; and Sage, W.: A Study of the Behavior of Particles and Materials in Erosion Processes. ASME Paper No. Lewis Research Center 69-WA/Met-6, Nov. 1969. National Aeronautics and Space Administration 18. Behrendt, A.: SandErosion of Dome and Window Materials. InternationalConference on Rain Erosion and Associated Cleveland, Ohio, January 4, 1983 Phenomena. Royal Aircraft Establishment, Farnborough, England, 1975, pp. 845-861. 19. Soderberg, S.; et al.: Erosion Classification of Materials Using a Centrifugal Erosion Tester. Tribol. Int., vol. 14, no. 6, Dec. 1981, pp. 333-343. 20. Zahavi, J.; and Schmitt, G. F., Jr.: Solid Particle Erosion of References Polymeric Coatings.Wear, vol. 71, no. 1, Sept. 1981, pp. 191-210. 1. Sage, W.; and Tilly, G. P.: The Significance of Particle Sizein 21. Weast, R. C., ed.: CRCHandbook of Chemistry and Physics. Sand Erosion of Small Gas Turbines. J. R. Aeronaut. SOC.,vol. 55th. ed. CRC Press, Inc., 1974, pp. C-735 to C-742. 73, May 1969, pp. 427-428. 22. Schallamach, A.: On the Abrasion of Rubber. Proc. Phys. SOC., 2. Tilly, G. P.: Sand Erosion of Metals and Plastics: A Brief Review. London, Sect. B, vol. 67, part 12, Dec. 1954, pp. 883-891. Wear, vol. 14, 1969, pp. 241-248. 23. Eyre, T. S.: Wear Characteristics of Metals. Tribol. Int., vol. 9, 3. Goodwin, J. E.; Sage, W.; and Tilly, G. P.: Study of Erosion by Oct. 1976, pp. 203-212. Solid Particles. Proc. Inst. Mech. Eng., vol. 184, no. 15, Part 1, 24. Rao, P. Veerabhadra; Young, S. G.; and Buckley, D. H.: 1969-70, pp. 279-292. Morphology of Ductile Metals Eroded by aJet of Spherical 4. Williams, J. H., Ji.; and Lau, E. K.: Solid ParticleErosion of Particles Impinging at Normal Incidence. Wear, vol. 85, no. 2, Graphite-Epoxy Composites. Wear, vol. 29, no. 2, Aug. 1974, pp. April 1983. 219-230. 25. Evans, D. C.; and Lancaster, J. K.: The Wear of . 5. Schmitt, G. F., Jr.:Impact Erosion-A SeriousEnvironmental Treatise on Materials Science and Technology, D. Scott, ed., vol. Threat to Aircraft and Missiles. ASME Paper 75-ENAS-45, July 13, Academic Press, 1979, pp. 85-139. 1975. 26. Shen, C.; and Dumbleton, J. H.: The and Wear Behavior 6. Hibbert, W. A.: Helicopter Trials Over Sand and Sea. J. R. of Polyoxymethylene in Connection with Joint Replacement. Aeronaut. SOC., vol. 69, no. 659, Nov. 1965, pp. 769-776. Wear, vol. 38, 1976, pp. 291-303.

16 ERRATA

NASA Technical Paper2161 SOLID SPHERICAL GLASS PARTICLE IMPINGEMENT STUDIES OF PLASTIC MATERIALS P. Veerabhadra Rao, StanleyG. Young, and Donald H. Buckley Apri 1 1983 Page 16: The following references should be added: 27. Schallamach, A.: Abrasion of Rubber by a Needle. Polym. Sci., vol. 9, no. 5, Nov. 1952, pp. 385-404. 28. Schallamach, A.: Friction and Abrasion of Rubber. Wear, vol. 1, 195711958, pp. 384-417. 29. Hutchings, I. M.: Mechanisms of the Erosion of Metals by Solid Particles. Erosion: Prevention and Useful Applications, W. F. Adler, ed., ASTM STP 664, American Society for Testing and Materials,1979, pp. 59-76.

Date issued: June 1983 -

" 1. Report 1. No. 2. Government Accession No. 3. Recipient's Catalog No. 7 NASA TP-2161 Subtitle 5. Report Date Report Title 5. 4. and Subtitle SOLID SPHERICAL GLASS PARTICLE IMPINGEMENT OF PLASTIC MATERIALS

" 7. Author(s1 Report Organization 8. Performing No. P. VeerabhadraRao, Stanley G. Young, andDonald H. BuckleyE-1122 10. Work UnitNo. 9. Performing Organization Name and Address National Aeronautics and Space Administration Lewis Research Center 11.Contract or Grant No. Cleveland, Ohio 44135 13. Typeof Reportand Period Covered 12. Sponsoring Agency Name and Address National Aeronautics and Space Administration Washington, D. C. 20546 14. Sponsoring Agency Code

I 15.Supplementary Notes P. Veerabhadra Rao, National Research Council - NASA Research Associate; Stanley G. Young and Donald H. Buckley, Lewis Research Center.

16. Abstract Erosion experiments on polymethyl methacrylate (PMMA), polycarbonate, and polytetra- fluoroethylene (PTFE) were conducted with spherical glass beads impacting at normal incidence. Optical and scanning electron microscopic studies and surface profile measure- ments were made on specimens at predetermined test intervals. During the initial stage of damage to PMMA and polycarbonate, material expands or builds up above the original surface. However, this buildup disappears as testing progresses. Little or no buildup was observed on PTFE. PTFE is observed to be the most resistant material to erosion and P"A the least. At low-impact pressures, material removal mechanisms are believed to be similar to those for metallic materials. However, at higher pressures, surface melting is indicated at the center of impact. Deformation and fatigue appear to play major roles in the material removal process with possible melting or softening.

_____^.. . ~- ~ ._-__ Wear;Erosion; Impingement; Morphology; Unclassified - unlimited ;Erosionresistance; STAR Category 27 Glass bead impact

.~~ . ~ ~. . "" -~ ~ ~-~- ~i 19. Security19. Classif. (of this report) 20. Security Classif. (of this page) 21. NO. of Pages 22. Price' Unclassified UnclassifiedA01

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