May 21-24,1995 A STUDY OF THE EFFECT OF GRAIN SIZE ON THE BALLISTIC PERFORMANCE OF SILICON CARBIDE. Carl F. Cline

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1 w UCRL-JC PREPRINT A STUDY OF THE EFFECT OF GRAIN SIZE ON THE BALLISTIC PERFORMANCE OF SILICON CARBIDE. Carl F. Cline This paper was prepared forthe 15th International Symposium on Ballistics Jerusalem, Israel May 21-24,1995 March 1995 This is a preprint of a paper intended for publication in a journal or proceedings. Since ps changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author. -

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4 5 A STUDY OF THE EFFECT OF GRAIN SIZE ON THE BALLISTIC PERFORMANCE OF SILICON CARBIDE Carl F. Cline Lawrence Livermore National Laboratory Livermore, CA The depth of penetration method was used to ballistically evaluate the performance of silicon carbide as a function of grain size. The hot pressed silicon carbide was backed by 4340 steel Rc=35 and impacted by tungsten heavy metal projectiles of L/D = 4 at velocities of 1.6 and 1.75 km/s. The hot pressed silicon carbide was also compared with reaction sintered silicon carbide of identical thickness in the current study. Results are compared with data previously reported by others. INTRODUCTION A preliminary study has been completed on silicon carbide ceramic tiles for use as ceramic composite armor. The purpose of the study was to determine what affect, if any, the grain size of the Sic would have on its ballistic performance. Having a fine grain size powder to start with, has several features that are worthy of note, namely: 1. A sub-micron grain size has a lot of surface area which normally brings about a low density for a tile or disc in the unsintered state. 2. The high surface area also promotes a very high initial shrinkage rate which commonly results in a low density tile. 3. The technology required to keep a very fine grain size and still have a high density (Le. 98% or more) means that an additive would have to be used to keep grain growth from occurring. Previous work at the U.S. Army's Materials Technology Laboratory by Woosley[lP2] had shown a difference in the ballistic performance for silicon carbide (Sic) prepared by different processes. The ballistic experiments at MTL used the depth of penetration method to determine performance of the two types of The tests were conducted using a 91% W long rods L/D = 10 and a mass of 65 grams. The impact velocity was 1.5 km/s. The results MAST

5 - indicated that the hot pressed Sic produced by Cercom with an average grain size of 4 microns was superior to the reaction sintered Sic produced by SOHIO Co. An important point with the MTL work is that the Sic tiles were backed with RHA which had a hardness of & = 27. Later work by Wilkins et al.[3] showed that the hardness of the steel backing material contributed significantly to the performance of Sic in a depth of penetration (Dol?)test. Experimental Results The hot pressed 4 x 4 x 1" tiles of silicon carbide purchased from the Cercom was characterized by microstructural examination for average grain size (see Figures 1-3). Knoop hardness was measured on all three grain size materials using a 1000 gram load which produces an indent with a length of 80 microns which is sufficient to interrogate a number of grain boundaries. The hot pressed Sic is compared with reaction sintered Sic from SOHIO. The results are summarized in Table 1. Examination of the results was surprising in that there is very little hardness difference between the samples. The normal range of reproducibility of hard materials is kg/mm2. The three SIC materials from Cercom had been hot pressed with the same proprietary additive to control grain size as this writer was concerned that the additive was "toughening" the grain boundaries. - The tiles were then ballistically tested with L/D = 4 tungsten heavy metal projectiles of the W-2 (97.5% W) composition. A nominal velocity of 1.65 km/s was chosen to allow us to compare these results with Sic and other ceramics[4] which had been produced by Dow Chemical Co. A control material from MTC was also tested to allow us to compare our results with Woolsey et al.[*] using the depth of penetration technique. The samples from MTL, were in the form of 6 x 6 x 1"thick tiles and were from the same SOHIO (see Fig. 4) lot that Woosley[2] have previously reported on and observed a significant ballistic performance difference. Figure 5 illustrates a typical target ensemble. The shot results are tabulated in Table 2. Figure 6 is a plot of the data from Reference 4 with our recent data points. Examination of the results leads this writer to conclude that there is essentially no effect of grain size on the ballistic performance of silicon carbide, a fact hinted at by the Knoop hardness results and secondly the reaction bonded Sic from SOH10 had similar penetration into the steel backing plate. This result is surprising when one examines Woosley et al.'sf2] results but it should be mentioned that we used a stronger steel backing material & = 35 vs I& = 27 and were at a slightly higher velocity, i.e. 1.7 vs. 1.5 km/s. The CERCOM people[s] assert that there is a significant ballistic

6 performance difference between projectile impact speeds of 1.1 and 1.3 km/s so that they feel their conclusions may be velucity dependence. The conclusion that this writer comes to with a very limited number of experiments, namely one test, that a program to produce and fabricate Sic tiles of submicron grain sizes for heavy armor applications is probably not warranted. REFERENCES 1. Woolsey, P., Mariano, S., and Kokidko, D., "Alternative Test Methodology for the Ballistic Performance Ranking of Armor Ceramics," Proc. 5th TACOM Armor Conference, March Woosley, P., "Residual Penetration Ballistic Testing of Armor Ceramics," Second Annual TACOM Combat Vehicle Survivability Symposium, April Wilkins, M. et al., "Ballistic Performance of AIN, Sic, and A Ceramic Tiles Impacted by Tungsten Alloy Long Rod Projectile," TACOM Combat Vehicle Survivability Conference, April Reaugh, J., Holt, A,, Wilkins, M., Cunningham, B., Word, B., and Kusubov, A., "Ballistic Evaluation of Five Ceramic Materials and Pyrex as a Function of Velocity and Thickness," Hyperveloaty Impact Symposium, December (19891, San Antonio, Texas, Page Richard Policka, Cercom Co., private communication. *This work was performed under the auspices of the U.S. Department o f Energy by Lawrence L i vermore National Laboratory under contract No. kl-7405-eng-48.

7 Table 1. Physical properties of Sic targets. Grain Size (m> Knoop Hardness (kg/mm*) (1000 gm load) 1.4 pm" 4.6 pm* 8.0 pm** pm* * Hot pressed +* Reaction sintered Table 2 SIC Ceramic Proj. V e l. ( k i d s ) Esp ; Exp. 2 [ I I 1.67 I, Penetration into Steel ( m m ) Exp. 1 ' Esy a 0.3 i I!

8 Figure 1 Sic PIn grain size 500X Sic prn grain size 500X Figure 2

9 Figure Sic 15.4 pm grain siz? 50UX SOHI0 Sic 8 pm grain size 500X Figure 4

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11 W 2 Penetrators I n t o 4340 Re35 Backed Sic L/D=4 LLNL SIC Data 40 h E E Ec yrn A e ct X 0 4 F: H pm 15.4pm M Dow (12 2 ) P G 0 10 ZU Ceramic Thickness (mm) Figure 6