1、Microstructure and solid particle erosion of carbon based materials used for the protection ofhighly porous carbon-carbon composite thermal insulationR.I.BAXTER， R。D.RAWLINGSDepartment of Materials, Imperial College of Science, Technology and Medicine,LondonSW7 2BP， UKMultiparticle erosion tests wer

2、e performed on candidate coating（colloidal graphitepaints) and cladding（dense carbonc-arbon composites and graphite foil)employed to protect porous carbon carbon composite thermal insulation in vacuum and inertgas furnaces that utilize inert gas quenching. The dependence of theerosion rate on the an

3、gle of incidence of the erodent was examined and related to the microstructure and the mechanisms of material removal as observed by SEM. Inaddition，the effect of a thin chemical vapour deposited（CVD）carbon layer on topof a colloidal graphite paint coating and a graphite foil clad was investigated.

4、Thecoating and cladding materials displayed a greater erosion resistance at all angles of incidence compared to the porous carbon carbon composite. In general, the greatesterosion rate was found at an angle of incidence of 90 ， where the erodent stream isperpendicular to the erosion surface， and bri

5、ttle fracture was the predominantmechanism of material removal。 The exception was the graphite foil material whichdisplayed maximum erosion at an angle of incidence of 60 。 For this material, twomechanismswereeffectivedisruptionof thegraphiteakeswhicharemainly held together by mechanical locking, an

6、d a ploughing-like mechanism 。 Theaddition of a thin CVD carbon layer to colloidal graphite paint improved performance,whereas the erosion resistance of the graphite foil was slightly degraded as theCVD layer was too thin to prevent the ploughinglike mechanism.IntroductionAclassofhighlyporouscarbon-

7、carbon（C-C)composites ， withdensities in the range 0.1 0.4 Mg m3, are utilized as thermal insulation in vacuum andinert-gas furnaces attemperatures up to2800 C。 A consequence of thevacuummoulding process used in the production of the composite is that thediscontinuousfibresareorientatedintolayerstof

8、ormatwo dimensionalplanar random structure. The vast majority of the volume of the composite consists of interconnectedporesandthefibrenetworkis bondedattheintersectionsoffibresbydiscreteregions of the carbon matrix as opposed to a continuous matrix.Forthis reason these composites are alsoknown as c

9、arbon bonded carbon fibre (CBCF). As a result of the highporosity and the fibre orientation,the thermal conductivity perpendicular tothe fibre layer planes is low, a typical value for a material with a nominal density of0.20 Mg m3 is 0.24 W m1 K1 at 2000 C in vacuum 。 Investigations into the microst

10、ructure 3， 4, mechanical properties 2, 59 and thermal properties 10， 11 of these materials have been reported. ( 1997 Chapman & Hall CBCF is used in furnaces employed in high technology applications such as single-crystal growing (for example， silicon orgalliumarsenide） ormetalheat treatment。Theheat

11、 treatment ofmetals,such astool steels，isincreasinglycarriedoutinfurnacesthatutilizegasquenching(typicallynitrogenisused） 12, 13.Thegasquenchmaybeusedtoreducetheturnaround time of batch processes or as an integral part oftheheattreatmentregime。The advantage of gasquenching during heat treatment， as

12、opposed to an oil quench, is that thecooling rate can be controlled； therefore, it is possible to reduce warping andcracking in the component 。 During gas quenching， parti culate mattermay become entrained in the gas ows, and impingement with the insulationmay result in material removal。 In the chal

13、lenging environment of gasquenching, there is a requirement for erosion protection of the CBCF bythe use of higher densitycarbon-basedcoating and cladding materials。Generally， ductile and brittle materials exhibit different erosioncharacteristics； of particular interest is their relationship between

14、 theerosion rate and the angle of incidence 15。 Ductile materialstend to display maximum erosion at glancing angles of impact,approximately 30 for metals， and material removal is thought tooccur by a micromachining mechanism with a contribution ofdeformation wear at higher angles. On the other hand,

15、 for brittle materials, maximum erosion is found where the erodent stream is perpendicular to the erosion surface ，and material removal typically results from the formation of Hertzian or lateralcracks 。 Although it is a convenient approach to idealize materialserosion behaviour in this manner， it i

16、s an oversimplification， becauseerosion is found to depend on other factors， including the erosionconditions, such as erodent par ticle size and shape， as well asthe details of the microstructure of the target material 。 This paper isconcerned with the examination of the microstructure and the efect

17、iveness inimproving the erosion resistance of several candidate coatings andcladdings. The results presented involve the steady state erosion rate as afunction of impingement angle under defined conditions. The overall aim of thiswork is to relate the microstructure to the erosion data by means ofa

18、mechanistic approach 。materials included the Fiber Materials Inc.C3 composite，which is resin impregnate，and the Toyo Tanso G3470 . In addition， a highdensity carboncarbon composite was producedby employing CVD over a period of 800 h to infiltrate a 5 mmthick section of the CBCF substrate to a densit

19、y of 1 Mg m3. TheCVD process used natural gas as the carbon precursor and nitrogen as the carrier gas. Thedensificationwascarriedoutatapproximately 1100Cunderareduced.Experimental procedure2。1。MaterialsThe CBCF used as the substrate was a standard commercial material (density 0.18 Mgm3) manufactured

20、 byCalcarbLtd.Thecoatingandcladdingmaterials wereappliedtothexyplaneoftheCBCFsubstrate （seeschematicdiagramofCBCFstructurein Fig. 1); the xy plane isperpendiculartothedirectionofminimumthermalconductivityandhenceismost likelytobetheexposedsurfaceoftheinsulationin afurnace.Thecoatingandcladdingmateri

21、alsexam ined in thispaper were all carbon based and they are listed in Table I. The coating materials are defined as those that bond independently to the CBCF substrate,whereas the claddingsare bonded bymeans of acar bonizing cement. Calcoat and Calcoat M arecolloidalgraphite paint coatings that wer

22、e applied to the CBCF substrate by brushing. Thematerial was subsequently heat treated at 900 C in nitrogen to carbonize the resin constituentofthecolloid.Higherdensitycarbon-composites(1.3 Mg m3） usedas of 5 kPa。(Note that theCVD of carbon in the interiorofaporousmediumistermed chemicalvapourinfilt

23、ration,CVI)Anothercladding materialgraphitefoilwhichwasproducedby Toyo Tanso by compressing exfoliated graphite akes in a rolling operation 23. The foil is exible in nature andispredominantlyheldtogetherbymechanical locking, as no binder isused。 Further samples were produced by subjecting the Calcoa

24、tcoating and the graphite foil to a CVD treatment (samplesdesignatedCVD in Table I) for a period of 75 h under the conditions described above. A more extensive descrip- tion of the materials will be forthcomingin the dis- cussion on the microstructures。2。2.Erosion testingMultiparticle erosion tests

25、were performed on a gas blast type rig， asdescribed by Carter et al. 24。 In this apparatus the erodent particles enterthe rig via an aperture in the base of an open hopper. A venturi fitted in the system allows the particles to be entrained in the compressed airow. After passing through a nozzle wi

26、th an 8 mm internal diameter ， the particles strike the target at a stand of distance of 40 mm 。 The target specimenshad nominal dimensions 25 mm; 12.5 mm；5 mm.The erodent used was angular equiaxed silica sand obtained from Hepworth Minerals and Chemicals Ltd， Redhill， UK。 The erodent was sievedto p

27、article sizes between 150 and 300 lm, the mean size (byweight) was 230 lm which was found by a laser difrac- tion method （Mastersizer1005, Malvern Instruments Ltd， Malvern, UK）. The velocity of theparticles was 6 m s1 ， found by the streaking camera technique at the method involved expo sing the fil

28、m for a known lengthof time and measuring the length of the line that the particle produces on thefilm。 Erosion tests were carried out at angles of 30, 45, 60， 75 and 90.Generally，thesampleswereimpactedbyafixed massoferodent， thencleanedandreweighed.Thisprocesswasrepeatedandthe accumulatedmassloss p

29、lotted against the accumulated mass of erodent. The erosionrateexpressedintermsofmass removedperunit mass of erodent，was calculated from the gradient of these plots。 However, in the case of thelowdensityCBCFsubstratematerial,whichwasinvestigatedforcomparisonpurposes,asignificantmassoferodent penetra

30、ted and wasretained within the porous structureofthecomposite.Whencalculatingtheerosion rate, the mass of this penetrated erodent must be taken into account and therefore the erosion rate was found inthefollowingmanner.Eachsamplereceivedonly a single dose of erodent. The total mass change of each sa

31、mple， isequaltothemassofcompositepressure of 5 kPa。（Notethat the CVD of carbon in the interior of a porous medium is sometimes termedchemical vapour infiltration, CVI） Another claddingmaterial was graphite foil which was produced by Toyo Tanso by compressing exoliated graphite snarolling operation 2

32、3。efoil sin nature and is predominantly held together by mechanical locking, as no binder is used。 Further samples were produced by subjecting theCalcoat coating and the graphite foil to a CVD treatment under the conditionsdescribed above。 A more extensive description of the materials will beforthco

33、ming in the discussion on the microstructures.2。3。 Micro structural and surface observationsSamples for optical microscopy were vacuum impregnated with resin and subsequently polished to a 1 lmfinish。 Samples for SEM were mounted onto aluminium tabs and examined at an accelerating voltage of 20 kV.

34、In the majority of cases， coating was not required due to the sufcient electrical conductivity of the carbon samples; however, where charging of retained silica erodent was evident in the eroded samples, they were splutter coated with gold。3。 Results and discussion3。1。MicrostructureThe structure of

35、CBCF insulation material is shown in Fig.1；thecontentofthisfibrenetworkis exceptionallyhighwith87%ofthevolumeofthe compositeconsistingofopenandinterconnectedpores。 Theorientationofthefibresisevidentinthe micrograph inwhich the fibres lie preferentially in xy planes（i.e.perpendiculartothe zdirection）

36、 but are random in direction within these planes. Thethickness of the Calcoat colloidal graphite paint coating is variable，due to the brushing method of application， but generally is in the range 4060 lm (Fig. 2a)。 However， as a result of the high porosity content and theinterconnected nature of the

37、 porosity in the CBCF substrate, some of thepaint penetrates up to a depth of 600 lm （Fig。 2b。 Calcoat M consists of Calcoat colloidal graphite paint ， which containssubmicrometre carbon particles ， with the addition of coarsercarbon particles and short fibres (50 lm)。 The coarser carbon particlesin

38、crease the viscosity of the paint which results in a thicker surfacecoating （80-200 lm) by minimizing the extent of penetration into theinterior of the porous substrate (Fig。 2c)。The CalcoatCVD is produced by depositing carbon from the gaseous phase on to the Calcoat coating intheCVDfurnace.Thisproc

39、essproducesalayerofdensepyrolyticcarbonabout5 lmthickonthe surfaceofpaintcoatingwithlittlepenetration （Fig.2d。TheFMIC3CCcompositeisproducedfrompolyacrylonitrile（PAN） precursorcarbonfibrecloth,whichisabout12 mmthick 21. The cloth isimpregnatedwithphenolicresinbutitisevidentthatthe resindoesnot adequa

40、tely penetratethefibre bundles（Fig. 3a Large platelets of resin-based carbon (500 lm； 500 lm;40 lm）arebetweenthelayersof woven cloth, as can be seen in the plan section micro- graphinFig.3b.Thismayresultfromtheuseof a highviscosityresinor a low impregnation pressure。外文資料譯文碳結構和固體顆粒侵蝕的愛護高度多孔炭碳復合保溫材料的使用材料系，英國皇家理工學院，技術和醫學，倫敦SW7 2BP，英國用來愛護多孔碳材料碳復合保溫在真空和惰性氣體熔爐,利用惰性氣體淬火。依靠性侵蝕率的發生率的角度考察了從微觀結構與機制SEM(CVD）碳層上的油漆涂料和膠體石墨石墨鋁箔復合進行了檢驗.涂層和熔覆材料顯示一個更大的抗侵蝕對全部角度的發病率比多孔碳復合材料。一般來說,最大的侵