American Mineralogist, Volume 58,pages 727-732, 1973

The Toughnessof

Rrcnnno C. Bnlnr. CeramicScience Section, Moterial SciencesDepartment RosEnr E. NrwNHnM, ANDJ. V. Blccens Materials ResearchLaboratory The PennsyluaniaState Uniuersity,Uniuersity Park, Pennsyluania16802

Abstract

The mechanical properties related to the toughness of jade are measured for both and nephrite. surface energies are an order of magnitude greater than most com- mercial ceramics about 120,000 ergs/cm' for jadeite and 225,00O ergslcm' for nephrite. Iadeite is the harder of the two , but nephrite is the tougher and the stronger. Scanning electron microscopy of the fracture surfaces indicates that the exceptional toughness results from the fibrous texture of nephrite and extensive transgranular cleavage of the blocky microstructure of jadeite.

Introduction surface energy, and the fracture toughness(Tetel- Ornamental include two different minerals, man and McEvily, 1967). Fracture surface energy jadeite and nephrite. Nephrite is an in is the amount of energy required to create a unit the -actinoliteseries, Ca2(Mg,Fe) s(Si+Orr )z area of fracture surface. Fracture toughness,desig- (OH)2, derived from alpine-typeperidotite-dunite nated K1" for the openingmode of fracture common intrusives. The rarer jadeite is a of com- to brittle materials.is a measureof the resistanceof position NaAlSizOo,found in Burma as stream-worn the material to unstable crack propagationor frac- boulders.Jadeite is the harder of the two minerals, ture. Fracture toughnessis derived by analyzingthe 7 on Mohs' scalecompared to 6Vz, aTthoughneph- stressdistribution at a crack's tip and is equal to the rite, "the axe stone," is generallyconsidered to be square root of twice the product of the elastic tougher and more resistantto fracture. modulus and the fracture surface energy of the Beauty and durability are the essentialattributes material. Often, as is the casein this study, fracture of a .Durability requires both hardness toughnessis reported as just K when the entire and toughnessin order to withstand abrasionand fracture processmay not be an opening mode. impact. Hardnessand toughnessare physicallyir- Among mineralogiststhere is no widely accepted reversibleproperties, since their tests leave the ma- scale of toughness,although some minerals such as terial in a pennanently altered condition. Although jade are regarded as much tougher than others. scientists and engineers generally describe these Becauseof their exceptionaltoughness, jade bould- propertiesin a rather qualitative fashion, there are, ers are often resistant to breakagewith a hammer in fact, specific quantitativemethods of measure- and may be frequently found in the nearly pure ment.To the mineralogist,hardness means resistance state, sinceweathering has worn away the surround- to scratchingand as such may be empirically defined ing minerals.Early man utilized jade extensivelyfor by the Mohs' scale.Hardness may alsobe measured making tools, finding it almost ideal to shapewith- by indentationusing the Vickers or Brinell tests. out fracture. This sentiment has been shared for The three technqiuesusually agree on the relative centuriesby Chinesecraftsmen who have given the ranking of differentmaterials, although the numerical world many exquisite jade carvings, shaped to ex- scalesappear quite different (Taylor, 1,949). tremely delicateforms. It is the purposeof this study Qualitatively, toughnessmeans the resistanceto to quantitativelyassess the toughnessof severaljades breakage.Quantitatively, it can be describedby two and to compare them with synthetic materials and related parameters,the work of fracture or fracture with other minerals. 727 R. C. BRADT, R. E. NEWNHAM, AND I. V. BIGGERS

Experimental indicatingthe presenceof large crystallites(Fig. 1). The nephrite jades studied in this investigation The individual crystallitesize is much larger than were obtained from several American dealers and the nephrite fibers, generallywithin the 50 to 100p.m are thought to originate from Russia and Alaska, range.The grains exhibit a blocky morphologywhen while the jadeite specimenswere obtained from comparedto the nephrite fibers. Jadeite densities generally streetshops in Hong Kong, and are probably Bur- are about 10 percentgreater than nephrite, patterns mese in origin. The nephrites are a translucent about 3.33 gms/cm3.The X-ray diffraction mottledgreen with a specificgravity of 3.01, indicat- are identical with those reported for NaAlSi2O6 (Yoder, ing appreciableiron content. X-ray diftraction pat- 1950). quantitative terns are in good agreementwith the tremolite To comparetoughness on a basis,it pattern (Stempleand Brindley, 1960). The texture is necessaryto measurecertain mechanicalprop- erties such as elastic modulus and fracture surface (Fig. 1) of the matted fibers is characteristicof '1.f8" nephrite. The fibers vary widely in thickness and energy. Strengths are measured by breaking length,but are generallyless than 10pmin diameter, square bars in three point flexure over a 0.4 inch with length-to-diameteraspect ratios from 10 to 50. span with an Instron testing machine operated at In some regions,the fibers show a strong preferred 0.02 inchesper minute cross-headspeed. Fracture orientation, being grouped into orderly bundles, strength, o1, is calculated from the standard engi- while other regions exhibit a whirlpool-like appear- neering-mechanicsformula : anceof a more randomnature. The jadeites vary in color from a pale green to (r) icy white, with numerous glints of reflected light "':*k' where P is the breaking load, and L, b, and h are the specimen'slength, breadth, and height dimen- sions (Marin, 1962). The elastc moduli are measured by a flexural resonancetechnique on /q" squareby 3" long bars employing a Nametre Model XII B acoustic spec- trometer. This involves vibrating the sample in a flexural mode with an oscillator and observing the natural resonanceof the specimen.The resonant frequenciesdepend on the sample dimensionsand elasticmodulus (Spinnerand Teft, 1961), and are usually in the audio-frequencyrange. Damping ca- pacities of the jades are determinedfrom the reso- nance peaks by the bandwidthmethod (Kennedy, t963). Fracture surface energies are evaluated by the work of fracturetechnique (Nakayama, 1965). The work required to propagate a stable crack over a specific area is measured using rectangular bars similar to thoseused in the strengthmeasurements. The bars are notched at the midsectionwith a 0.010' blade, leaving a triangular cross section for the fracture area. After fracturing the bars in a non-catastrophic,stable fashion on the testingmachine at a crossheadspeed of 0.002 inches per minute, the work, or energy, of fracture is de- Fto. l. Photomicrographsof nephrite (top) and jadeite. termined by integration of the force-displacement Note the matted fibrous texture of the nephrite, and com- curve. Surfacefracture energy is then calculatedby pare it with the blocky grain texture of the jadeite. dividing the work by twice the triangular cross- TOUGHNESS OF ]ADE 729 section area, since two surfacesare formed during Tesr.s l. Mechanical Properties of Jades the fracture process.A comparisonof this technique with other methodshas been given (Coppola and Jade1t e 1972). The fracture surface energy and the Efastic Mqdulus, E t.3 x 1012 2,0 x Lo12 Bradt, (dynes,/cn') strengthresults are reported as the averageswith the -?- -?- Danplng 1 x 10 1.5 x 10 95 percentconfidence limits as describedby the "t" Fracture qtrength, or, z.tz+0.55 x :ro9 1.02+0.21x109 distribution. Thin-sections of jadeite and nephrite (dynes,/cm') were also examined optically on a standard petro- Fracture surlace Energy, 226,000+155,000 121'000+32,000 (er|s/cmz 1 r, ) graphic microscopeand the fracture surfacesphoto- R Fracture Tqughness, K^' 7.7 x 108 7.1 x 10- graphed with a scanning electron microscope after ( dynes,z cm- J/ 2 ) coating the samples with a thin layer of vapor- depositedgold to promote conductivity. Scanningelectron micrographs of the fracture sur- Results and Discussion faceof jadeite(Fig. 2) reveala very high percentage of transgranularcleavage fractures. That is, when a The mechanicalproperties of jadeiteand nephrite crackpropagates through jadeite, it doesnot proceed are listed in Table 1. Although jadeite has the in an intergranular fashion following the boundaries higher elastic modulus, nephrite is superior in between the grains, rather it proceeds directly strength,o1, fracture surfaceenergy, ond fracture 7r, through the grains in a transgranularfracture mode. toughness, IQ, substantiating the general opinion Cleavagestep patternsare especiallyobvious at the that nephrite is tougher than jadeite. The confidence higher magnification,indicating that the fracture is limits for the fracture surface energies and the almost wholly restricted to certain crystallographic toughnessesare comparable to that observed for planes.The elongatednature of some of thesecleav- most brittle materials, with the exception of the age stepsstrongly suggeststhat the fracture is pref- fracture surface energies of the nephrite. A wide erentially occurring parallel to the pyroxene chains, range of scatter was observedon these specimens, probably {110} planes.It is this extensivetrans- with occasionalvalues near 400,000ergs/cm'; how- granular cleavagefracture mode that imparts high ever, no obvious correlationswith place of origin or toughnessto jadeite. structural featureswere apparent.The jade fracture Nephrite has a vastly different appearingfracture surfaceenergies and calculatedfracture toughnesses topography(Fig. 3). Fibers and bundlesof fibers are compared with other polycrystalline and single can be seen protruding from the surface,even at crystalminerals in Table 2. It is apparentthat jade low magnification.The random orientation of in- is deservingof its reputation as a very tough ma- terial. Comparing these results with the fracture surfaceenergies of other materials (Duga, 1969) Tesr-e 2. Comparisonof Toughness indicatesthat jade is about an order of magnitude FractureSurface Fracture tougherthan most ceramicmaterials. Energyr y. Toughnessr (^ Jade mineralsare chain silicates,and the differ- ence in their averagefracture surface energiessug- x 107 geststhat perhapsa network silicate, such as Jade 1te 121,000 7f x Io7 or glass,might be even tougher.This conceptproves Nephrlt e 226,0OO 77 Quart z i te 4,32A 7 xto7 quite erroneous,for the reported fracture surface (l,llederhorn,1969b)

energiesof (see Table 2) and glass(Wie- Alunlna 15,000-50,ooo 35-64 x :'O7 (Gutshal1 derhorn, I969c) are only about 5,000 ergs/cm2,or & Gross, 1969) Maqnesla 20,000 35 x Io7 less. On the same scale of fracture toughnessas (Clarke et a1., 1966) Table 2, both glassand quartz are only about 7 x Mlca ( 001) tr x to7 10'dyne cm-3/2.These comparisons suggest that the (Bryant et al.,1963)

high fracture surface energy and fracture toughness Quartz (loTo) r,030 5xto7 (Brace 1962) of jade are not directly relatedto the atomic bond- and Walsh, corundum (lofl) 600 7xIa7 ing per se, but are apparentlyrelated to texture and (t/1ederhorn,1969a)

the restrictionswhich it imposeson crack propaga- Perlclase (100) L,2oo 9 x 107 (westwood tion. and Go1dhelm, 1963) 730 R. C, BRADT, R. E. NEWNHAM, AND I. V. BIGGERS

tions; however,a correlation of thesetextural param- eterswith toughnessundoubtedly exists, and is prob- ably the main criterion by which jade carvers are able to select particularly "tough" jade specimens. While the exact contribution of a fibrous texture to increasedtoughness is not well understood,it has

*4

FIc. 2. Scanning electron micrographs of the fracture surfaces of jadeite. Note the extensive transgranular cleav- age characteristics of the crack propagation. dividual fibers and the wide distribution of fiber sizesbecomes apparent at higher magnification.It is these variations in fiber and fiber bundle sizes and in orientationsthat are probably responsiblefor the Frc. electron micrographs the fracture variations 3. Scanning of in fracture energy reported in Table 1. surfaces of nephrite. Note the fibrous nature of the frac- No attempt was made to characterizethe nephrites ture surface, the difference in fiber sizes, the occasional on the basis of fiber or fiber bundle size distribu- bundles of fibers, and the regions of random orientation. TOUGHNESS OF JADL 73r recently become a well documentedphenomenon, magnitude is still comparable to commercial ce- having been reported in fiber composites (Lynch ramics and about twice that of common glass. It and Kershaw, 1972), in solidified eutectic alloys appears(Fig. 1) that the jadeite-bearingrock has a (Piekarski and Helmer, 1972), in nitride secondaryphase between the jadeite grains, acting (Lange, 1972) and in silicon carbide (Coppola and as a bond between crystallites; however, no sec- Bradt, 1972\. ondary phaseswere identified in the X-ray patterns There appearto be severalmechanisms by which or thin sections, and the relatively high modulus transgranularcleavage, such as evidencedin the and low damping suggestthat any secondaryphase jadeite, and the fibrous texture, such as that of the is a strong, coherentone. The relatively low damp- nephrite, might increasefracture surfaceenergy and ing suggestsgood structural integrity in both jades, enhance toughness.It almost seems contradictory with relatively little po'rosity. to considercleavage as a tough process,particularly The strengthsof the two jades exceedmost com- with reference to mica. However, mica is a single merically available ceramics;however, they are not crystalwhere the cleavageplanes traverse the entire comparableto the ultra high strength of hot'pressed specimen,whereas the cleavageplanes in poly- oxidesand nitrides usedfor cutting tools and turbine crystalline jadeite traverse only a single grain' A vanes.The Griffith relation, a, : k\/ Ett/c, offers propagatingcrack in polycrystallinejadeite must an explanationof the strengths.This equationrelates change direction to a differently oriented cleavage the fracture strength or, to the elastic modulus E, plane each time it crossesa grain boundary. This the fracture surface energy 71, &[d the flaw sizn c, directionalchange can be up to 47o in jadeite and through a proportionality constant k. (Kingery, very likely causes secondary cracks or fractures 1960).From the data in Table l, it is apparent that which consumeadditional energy. (Ffiedman et a:1, nephrite has the higher fracture surfaceenergy and 1972) These directionalchanges also result in a strength,even though the modulusis lower. However, zig-zagfracture path that yields a roughenedfracture the quantity t/Ett in the Griffith equation accounts surface, so that the real fracture surface area is for only about l0 percentof the observeddifferences greater than the apparent fracture surface area. in the jade fracture strengths.The remainder may (Hoagland et al, l97I) This increaseof surface be attributed to the smallerflaw sizein the nephrite. area, probably only about 50 percent, does not Flaw size is generallyrelated to the size of micro- appearto be as important as secondarycracking or textural features,and in this case the nephrite fiber crack-branchingin jadeites. diameter is considerably smaller than the jadeite Nephrite fractures in accordancewith its fibrous grain size. texture, a structural feature that has alreadybeen Acknowledgment cited to enhancetoughness. Several factors may be The authors would like to acknowledge the contributions responsiblefor this high fracture surfaceenergy, one of G. Gardopee, T. Cline, R. Wolfe, and J' Lebiedzik of the of which is undoubtedly the high ratio of real-to- Pennsylvania State University for assistance in the experi- apparent fracture-surfacearea. This contribution is mental work, and Hok Shing Liu of Chung Hing Mining' University, Tainan, Tai- most certainly greater in nephrite than in jadeite. Hong Kong, and the Cheng-Kung wan, ROC, and N. Lambert for assistancein obtaining the However, probably more important are the frictional jades. effects when individual fibers or bundles of fibers are extractedduring fracture.Furthermore, there is References little doubt that the propagatingcracks in nephrite Bnlce, W. F., lNo J. B. Wlrsn (1962) Some direct mea- quartz are often forced to make drastic directional changes surements of the surface energy of and orthoclase. . 47' llll-1122' permit pullouts, so secondary crack Amer. to the fiber BnveNr. P. J.. L. H. Tevron, lno P' L' Gurss.ru (1963) branchingalmost certainly occurs. Fresently it is not Cleavage studies of lamellar solids in various gaseous possibleto define clearly which processcontributes environments. Trans. 10th Annu. Nat. Vacuum Sympo- most to the jade'shigh fracturesurface energies, but siuln, American Vacuum Society, pp. 2l-26' (1966) it is clear that jade fully deservesits reputationas a Cllme, F, J. P., H. C. T.lrrrnsAlL, ANDG. TeppIN Toughness of ceramics and their work of fracture, Proc. very tough mineral. Brit. Ceram. Soc.6, 163-172' In regard to the other mechanical properties of Conrolr, I. A., eNo R. C. Bneor (1972) Measurementof jade,the lower elasticmodulus of nephriteis almost fracture surface energy of SiC. I. Amer. Ceram. Soc. 55' certainly a result of its lower density;however, its 455-460. 732 R. C. BRADT, R. E. NEWNHAM, AND I. V. BIGGERS

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