Wear 255 (2003) 115–120

The grain size effect on the empirically determinederosion resistance of CVD-ZnS

C.S. Changa, J.L. Heb,∗, Z.P. Linb

a Department of Physics, Feng Chia University, 100 Wen Hua Road, Taichung 407, Taiwan, ROCb Department of Materials Science, Feng Chia University, 100 Wen Hua Road, Taichung 407, Taiwan, ROC

Abstract

Mechanical properties are important for infrared (IR) windows that may suffer from erosion damage during service. Zinc sulfide is atypical IR material that provides high transparency and satisfactory mechanical properties. Chemical vapor deposition, capable of producinglarge, complex shaped deposits with high purity, density, is commonly used to deposit IR materials. The substrate temperature determinesthe grain size of the deposits, the transparency and the mechanical properties. The substrate temperature is therefore vitally important toproducing a quality deposition. Reports on the hardness and fracture toughness of the CVD-ZnS were found without empirical erosiontests, which give rise to a study on the effect of grain size on the actual solid particle erosion rate. This study evaluates the solid particleimpingement of ZnS with different grain sizes, deposited at different substrate temperatures. The relationship between the mechanicalproperties and grain size is discussed.

The experimental results show that high substrate temperature favors the (1 1 1) texture and large-grain growth with the mass densitynearly independent. The dominating factor that influences the hardness and fracture toughness is the grain size. The empirically determinederosion resistance of these deposits goes out of the scope of the Evans theory probably due to the unsatisfactory homogeneity of theCVD-ZnS. This implicates the unnecessary consideration on the deposition temperature compromised between IR transmittance anderosion resistance. High temperature deposition produces large grain deposits with lower hardness and toughness values, but is stillbeneficial to erosion resistance.© 2003 Elsevier Science B.V. All rights reserved.

Keywords: Zinc sulfide; Chemical vapor deposition; Mechanical properties; Solid particle erosion

1. Introduction

Infrared (IR) dome materials are requisite for erosion re-sistance as well as IR transmittance from the thermal andhydrodynamic impact involved in atmospheric, rain, ice andsand erosion. Crack formation and surface fractures mayform irregular surfaces on these fragile IR materials fromthe large tension caused by the Rayleigh waves from impactevents. These cracks may eventually be detrimental to theIR transmittance due to scattering, reflection, birefringenceand absorption of the incident infrared waves. This has beenan important issue over the entire range of IR materials, in-cluding both ionically or covalently bonded materials[1].

To construct an erosion loss model of a brittle material,Evans[2] considered a single impact event as an erodent in-dentation. A plastic zone is created underneath the materialsurrounded by a larger hemispherical elastic zone, as illus-

∗ Corresponding author. Tel.:+886-4-24517250x5315;fax: +886-4-24510014.E-mail address: jlhe@fcu.edu.tw (J.L. He).

trated inFig. 1. The maximum residual tensile stress occursbetween two zones, bringing about lateral and radial cracks.The lateral crack extends toward the surface and causes ma-terial loss. The volumetric erosion loss from a brittle mate-rial can be derived usingEq. (1) [2]:

e ∝ V = λ3(v2m)7/6(KICH1/6)−1

(E

H

)4/5

(1)

whereV is the volumetric loss from the target material,λ3the material independent coefficient,v the aerial speed of theerodent,m the erodent mass,KIC, E andH are the fracturetoughness, Young’s modulus and hardness of the target ma-terial, respectively. An assumption that the target material ishomogeneous and brittle was made in this model.

The erosion rates for large numbers of materials have beendetermined[3–9] and the contributions of the material me-chanical properties were found to agree with Evans predic-tions[10,11]. All IR materials are mechanically tested. Thehardness and fracture toughness, in particular are the primarycriteria for erosion resistance. For instance, Townsend and

0043-1648/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0043-1648(03)00266-7

116 C.S. Chang et al. / Wear 255 (2003) 115–120

Fig. 1. A schematic of a section through an impact site showing theplastic zone as well as the lateral radial crack[2].

Field [12] used a micro-hardness tester to measure the frac-ture toughness of a variety of CVD-ZnS with different grainsizes grown at different deposition temperatures. Townsendand Field concluded that the optimized mechanical proper-ties for a CVD-ZnS were obtained at 8�m grain size, corre-sponding to a deposition temperature of 700◦C. It is vitallyimportant to empirically determine the erosion resistance ofCVD IR materials as a function of grain size, controlled bythe deposition temperature. The grain size controls the IRtransmittance and the erosion resistance.

The goal of this work was to investigate the effectsof deposition conditions on the erosion wear behavior ofCVD-ZnS. In doing so, the hardness and fracture toughnesswere measured in order to test the applicability of Evanstheory for erosion. Theoretical results were compared toexperimental results.

2. Experimental

The chemical reaction shown inEq. (2)was used[13] toobtain CVD-ZnS.

H2S(g) + Zn(g) → ZnS(s) + H2 (2)

A horizontal type three-zone CVD reactor was used.This device is schematically shown inFig. 2. Each zonewas individually resistance-heated to a precise temperature.The zinc melt is located in zone 1. A suitable amount ofargon carrier gas was admitted into to the deposition zone(constructed by zones 2 and 3). H2S gas was also admittedusing argon carrier gas as the reactive gas into the de-

Fig. 2. A schematic of the hot-wall CVD reactor used in this study.

Table 1Deposition condition used in this study to obtain CVD-ZnS

Working pressure (Pa) 5332Zinc melt temperature (zone 1) (◦C) 685Deposition temperature (zones 2 and 3) (◦C) 560± 850H2S flow rate (l/min) 0.8Ar flow rate to carry H2S (l/min) 1.0Ar flow rate to carry zinc vapor (l/min) 1.2Deposition duration (h) 16

position zone and carefully controlled using a mass flowcontroller. The CVD-ZnS was then grown onto the lining ofthe deposition zone, made of AISI 304 stainless steel. After16 h deposition duration using the deposition conditionsshown inTable 1, a several millimeters thick CVD-ZnS filmwas obtained. To obtain ZnS deposits with differing grainsizes, the deposition temperature was adjusted from 560 to850◦C while keeping the other parameters constant. Thesedeposits were then cut into 7 cm×7 cm×30 cm specimens.Both sides were polished for characterization and mechani-cal test purposes. The continuity of the grain size change,crystallographic textural change and mechanical proper-ties, etc. confirms the reproducibility of each depositionrun.

The crystal structure of the ZnS deposits were identifiedusing an X-ray diffractometer (XRD). An optical microscopewas used to obtain metallographs of these deposits. Thegrain size was determined using an ASTM standard methodbased on the deposit metallographs, which were pre-etchedby boiling 15% KOH+ 15% K3Fe(CN)6 reagent. Vicker’smicro-hardness was measured using an indentation load of1 kgf. The fracture toughness of the deposits was deter-mined using Niihara et al.[14] method in which a Vicker’smicro-hardness indentor was exerted with a force of 1 kgfon the specimen. The crack lengths developed along the fourcorners of the indentation were measured yieldingc/a ratioused to determine the fracture toughness value. A solid par-ticle erosion test was carried out using a Chun-Hui Brand(Taichung, Taiwan) P-SB-01 unit that supplied 0.3 mm Østeel particle erodents with a Vicker’s hardness ofHv = 350.A flying wheel serves to accelerate the erodents to an esti-mated linear velocity at 60 m/s. Two test runs, 40 and 70 s,were carried out. All of the specimens were subjected to auniform erodent flux in each run. The specimen weight losswas measured using an electronic balance. The surfaces ofthe eroded specimens were examined using an optical mi-croscope.

3. Results and discussion

3.1. Microstructure of the ZnS deposits

Fig. 3 shows the XRD patterns of the deposits obtainedat different deposition temperatures. It is quite clear that the

C.S. Chang et al. / Wear 255 (2003) 115–120 117

Fig. 3. XRD patterns of the CVD-ZnS deposited at (a) 560◦C; (b) 600◦C;(c) 680◦C; (d) 720◦C; (e) 760◦C; (f) 800◦C.

texture of the deposits moves towards the (1 1 1) directionas the deposition temperature increases, producing a purezinc blend phase in all cases.Fig. 4 illustrates the metal-lography of the growing surface plane of the deposit. Thegrain size increases from 2 to 20�m with increasing de-position temperature from 560 to 850◦C, behaving as re-ported in[12,15]. Fig. 5grains produced are columnar, withthe direction of orientation toward (1 1 1) as mentioned byTownsend and Field[12]. The grain sizes measured in thisstudy were revealed in the (1 1 1) direction and could not beoriented in any other direction.Fig. 6shows that the densityof the deposited ZnS is nearly identical to the theoreticalvalue 4.09 g/cm3 and is independent of the deposition tem-perature. This confirms the dense structure obtained usingthe CVD route.

The above characterization and observation indicatesthat the grain size is controlled by the deposition tem-perature. The deposition temperature also influences thetexture of the deposits, but is independent of the depositdensity.

Fig. 4. SEM morphology of the CVD-ZnS deposited at (a) 560◦C; (b) 680◦C; (c) 760◦C; (d) 850◦C.

Fig. 5. Effect of the deposition temperature on the CVD-ZnS grain size.

Fig. 6. Deposition temperature effect on CVD-ZnS density.

3.2. Mechanical properties of the ZnS deposits

The surface hardness is influenced by the grain size of amaterial and hence its deposition temperature, as revealedabove.Fig. 7(a)shows the hardness of the growing surfaceof the ZnS deposit as a function of the deposition tempera-ture. The hardness decreases as the deposition temperatureincreases. This is presumably caused by the grain size ef-fect. The grain size increases as the deposition temperatureincreases.Fig. 7(b)uses the grain size as thex-axis illustrat-ing an inverse relationship between the hardness and grain

118 C.S. Chang et al. / Wear 255 (2003) 115–120

Fig. 7. (a) Deposition temperature effect on the CVD-ZnS and (b) hardness. Grain size effect on CVD-ZnS hardness.

Fig. 8. The indentation in the CVD-ZnS deposited at (a) 680◦C; (b) 700◦C; (c) 760◦C by applying a load of 1 kgf.

size. This matches the Hall–Petch equation precisely, as de-scribed inEq. (3).

H = H0 + Kd−1/2 (3)

whereH is the hardness value,H0 andK are the constantsandd is the grain size. To confirm the texture effect on thehardness value, an additional hardness measurement alongthe plane parallel to the growing direction was carried out,also shown inFig. 7(b). The two sets of hardness valuesmeasured at the two different surfaces are identical. Thisalso implies that the crystalline texture of the deposit isindependent of the hardness value. This gives us good reasonto believe that mechanical properties such as the fracturetoughness and erosion resistance are also independent of thecrystalline texture and are solely dependent upon the grainsize.

As mentioned inSection 2, the fracture toughness was ob-tained using the micro-indentation method. Specimens de-posited at different deposition temperatures show differentresponses to the indentation, as revealed inFig. 8. Thosespecimens deposited at lower temperatures exhibited sharpcracks extended from the corners of the indentation. Net-work microcracks were produced around the periphery of theindentation in specimens deposited at higher temperatures.Higher deposition temperatures produce difficulties withc/adetermination, used for theKIC fracture toughness calcula-

tion. Based on Townsend and Field[12], and Evans[16]approaches, thec-value can be determined by measuringthe diameter of the microcrack network area. An “effectiveKIC” is obtained using this calculation method. ZnS depositvalues with different grain sizes deposited at different depo-sition temperatures can also provide substantial informationon the erosion behavior, which will be discussed later.

The calculation results shown inFig. 9show that theKICvalue decreases with increasing grain size almost identicalto the hardness value. This is because when the material issubjected to the applied stress, three response modes occur;dislocation generation and movement, grain slide and crack

Fig. 9. Grain size effect on CVD-ZnS fracture toughness.

C.S. Chang et al. / Wear 255 (2003) 115–120 119

Fig. 10. Overall grain size effect on CVD-ZnS mechanical properties: line 1, theoretical erosion weight loss calculated from measuredKIC and Hv; line2, measuredKIC; line 3, measured Vicker’s micro-hardness; line 4, erosion weight loss in 40 s; line 5, erosion weight loss in 70 s.

formation. The impact force is absorbed by these responsemodes. Fine grain deposits tend to exhibit dislocation gen-eration and movement and grain slide, thereby prohibitingcrack formation.

3.3. Solid particle erosion behavior

By treatingv, m andE in Eq. (1)as constants when dealingwith CVD-ZnS deposited at different temperatures,Eq. (4)isobtained, which gives us a relative erosion rate for CVD-ZnSwith different grain sizes.

e ∝ 1

KICH0.9(4)

Introducing the previously measuredKIC and H into Eq.(4), normalized erosion ratee versus (grain size)1/2 isplotted as curve 1 inFig. 10. The data points shown inthis figure are somewhat scattered because the empiricalhardness and fracture toughness values were introduced.The empirical fracture toughness and hardness values werealso plotted as a function of (grain size)1/2 in Fig. 10 ascurves 2 and 3 for comparison. The fracture toughness andhardness values increase as the grain size decreases. In

contrast, the normalizede decreases as the grain size de-creases. This is exactly what the Evans theory predicted. InCVD IR material a compromise between the IR transmit-tance and erosion resistance should be reached by adjustingthe deposition temperature. High deposition temperaturecan, although beneficial to the IR transmittance due to lessgrain boundary scattering, be detrimental to its erosionresistance.

The empirical erosion test revealed inFig. 10, wherecurves 4 and 5 (tested for 40 and 70 s, respectively) demon-strate that higher erosion resistance (namely, lower erosionweight loss) results from deposits with larger grain size. Acontroversy arises due to the unsatisfactory homogeneity ofthe CVD-ZnS in converse to the Evans theory, in which ahomogeneous brittle material should be used. Such a mate-rial possesses fine grains that glide easily to produce plasticdeformation. Townsend and Field, who observed the areaunderneath an indentation in brittle material created by aVicker’s indenter, confirmed this. A larger-grained materialis unlikely to produce glide and crack generation. Lateralcracks will dominate instead to absorb the impact energy.This brings the erosion behavior of larger CVD-ZnS beyondthe Evans theory.

120 C.S. Chang et al. / Wear 255 (2003) 115–120

Fig. 11. Metallographs of the erosion tested CVD-ZnS deposited at (a) 560◦C; (b) 680◦C; (c) 850◦C.

Indications of our explanation can be shown inFig. 11, inwhich the eroded surface of the CVD-ZnS deposited at lowertemperatures (Fig. 11(a) and (b)) present a high population ofsmall craters. A CVD-ZnS deposited at higher temperatures,such as inFig. 11(c), shows a lower population but largercraters. Each impact event on a small grain of depositedCVD-ZnS induces a higher probability of material loss dueto grain glide. However, only localized damage is producedwithin large CVD-ZnS deposit grains, therefore reducing theerosion rate.

The empirically determined erosion rate for CVD depositsgoes out of the scope of the Evans prediction due to the un-satisfactory homogeneity of the CVD-ZnS. An extension ofthis summary implicates the unnecessary consideration ofthe deposition temperature compromised between IR trans-mittance and erosion resistance. High temperature deposi-tion although producing larger grain deposits with lowerhardness and lower toughness values is still beneficial toerosion resistance.

4. Conclusion

The mechanical properties of CVD-ZnS with differentdeposited grain sizes at different deposition temperatureswere measured. The hardness values of these deposits re-sembled their effective fracture toughness as a functionof the grain size or deposition temperature, in agreementwith the Hall–Petch equation. The empirically determinederosion resistance of these deposits goes out of the scopeof the Evans theory due to the unsatisfactory homogene-ity of the CVD-ZnS. This clearly demonstrates the truththat CVD-ZnS deposition, although at a higher tempera-ture producing larger grain deposits with lower hardnessand lower toughness values, yet is still beneficial to the

erosion resistance than those low temperature depositedones.

Acknowledgements

The authors wished to thank Chung-Shan Institute of Sci-ence & Technology for its financial support under contractNSC 86-2623-D-035-002.

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