ASTM E1920-2003(2008) Standard Guide for Metallographic Preparation of Thermal Sprayed Coatings《热喷涂层的金相制备用标准指南》.pdf

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1、Designation: E 1920 03 (Reapproved 2008)Standard Guide forMetallographic Preparation of Thermal Sprayed Coatings1This standard is issued under the fixed designation E 1920; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year o

2、f last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide covers recommendations for sectioning,cleaning, mounting, grinding, and polishing to reveal themicrostruc

3、tural features of thermal sprayed coatings (TSCs)and the substrates to which they are applied when examinedmicroscopically. Because of the diversity of available equip-ment, the wide variety of coating and substrate combinations,and the sensitivity of these specimens to preparation technique,the exi

4、stence of a series of recommended methods for metal-lographic preparation of thermal sprayed coating specimens ishelpful. Adherence to this guide will provide practitioners withconsistent and reproducible results. Additional informationconcerning standard practices for metallographic preparationcan

5、be found in Practice E3.1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prio

6、r to use.2. Referenced Documents2.1 ASTM Standards:E3 Guide for Preparation of Metallographic SpecimensE7 Terminology Relating to Metallography3. Terminology3.1 DefinitionsFor definitions of terms used in this guide,see Terminology E7.3.2 Definitions of Terms Specific to This Standard:3.2.1 linear d

7、etachment, na region within a TSC in whichtwo successively deposited splats of coating material have notmetallurgically bonded.3.2.2 splat, nan individual globule of thermal sprayedmaterial that has been deposited on a substrate.3.2.3 taper mount, na metallographic specimen createdby mounting a feat

8、ure, typically an interface or thin coating, ata small angle to the polishing plane, such that the visible widthexhibited by the feature is expanded.3.2.4 TSC, nthermal sprayed coating, including, but notlimited to, those formed by plasma, flame, and high velocityoxyfuel.4. Significance and Use4.1 T

9、SCs are used in a number of critical industrial compo-nents. TSCs can be expected to contain measurable levels ofporosity and linear detachment. Accurate and consistent evalu-ation of specimens is essential to ensure the integrity of thecoating and proper adherence to the substrate.4.1.1 Example 1:

10、By use of inappropriate metallographicmethods, the apparent amount of porosity and linear detach-ment displayed by a given specimen can be increased, byexcessive edge rounding, or decreased by smearing of materialinto voids. Therefore inaccurate levels of porosity and lineardetachment will be report

11、ed even when the accuracy of themeasurement technique is acceptable.4.1.2 Example 2: Inconsistent metallographic preparationmethods can cause the apparent amount of voids to varyexcessively indicating a poorly controlled thermal spray pro-cess, while the use of consistent practice will regularly dis

12、playthe true microstructure and verify the consistency of thethermal spray process.4.2 During the development of TSC procedures, metallo-graphic information is necessary to validate the efficacy of aspecific application.4.3 Cross sections are usually taken perpendicular to thelong axis of the specim

13、en and prepared to reveal informationconcerning the following:4.3.1 Variations in structure from surface to substrate,4.3.2 The distribution of unmelted particles throughout thecoating,4.3.3 The distribution of linear detachment throughout thecoating,4.3.4 The distribution of porosity throughout the

14、 coating,4.3.5 The presence of contamination within the coating,4.3.6 The thickness of the coating (top coat and bond coat,where applicable),4.3.7 The presence of interfacial contamination,4.3.8 The integrity of the interface between the coating andsubstrate, and,4.3.9 The integrity of the coating m

15、icrostructure with re-spect to chemistry.1This guide is under the jurisdiction ofASTM Committee E04 on Metallographyand is the direct responsibility of Subcommittee E04.01 on Specimen Preparation.Current edition approved Oct. 1, 2008. Published January 2009. Originallyapproved in 1997. Last previous

16、 edition approved in 2003 as E 192003.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.5. Selection of Metallographic Specimens5.1 Selection of specimens for metallographic examinationis critical if their interpretation is to be of va

17、lue. Specimensmust be representative of the coating. Generally, the plane ofpolish should be normal to the coating surface so as to displaythe entire coating thickness, the substrate, and all interfaces.6. Sectioning6.1 Specimens to be mounted for metallographic prepara-tion are generally not larger

18、 than 12 mm by 25 mm (0.5 by 1.0in.). The height of the mounted specimen should be no greaterthan necessary for convenient handling during polishing.6.2 In sectioning TSC specimens, care must be exercised toavoid affecting the soundness of the coating and the interfacebetween the coating and the sub

19、strate. Sectioning damage ofthe coating and interface that cannot be removed by subsequentgrinding and polishing must be avoided.6.2.1 Friable, porous, or brittle coatings to be sectioned maybe vacuum impregnated with epoxy mounting compoundbefore sectioning to protect the specimen.6.2.2 Specimens s

20、hould always be sectioned such that thecoating is compressed into the substrate. Sectioning techniqueswhich place the coating and interface in tension are strictly tobe avoided. Sectioning in tension may cause the coating to bepulled away from the substrate or result in delamination of thecoating. D

21、uring examination of the polished specimens, it islikely that this type of damage will be mistakenly interpreted.When sectioning some specimens, it may not be possible toavoid placing some areas of the TSC in tension. These areasshould be noted and not included in the evaluation of thespecimen.6.2.3

22、 Sectioning with a hack saw will produce significantdamage to the coating and interface and is not consideredacceptable.6.2.4 Using an abrasive cutoff blade with a large particlesize abrasive produces a smoother surface than a hack saw, butstill produces coating damage that may require considerableg

23、rinding in subsequent preparation to remove. The choice ofcutoff wheel, coolant, cutting conditions, and the type andhardness of the coating and substrate will influence the qualityof the cut surface. A poor choice of cutting conditions caneasily overheat some TSC specimens rendering the specimensun

24、usable for proper evaluation.6.2.5 Sectioning can be completed with minimal damage tothe cut surface by selection of one of the two followingabrasive cutoff blades:6.2.5.1 Use a diamond wafering blade with a maximumthickness of 0.63 mm (0.025 in.).6.2.5.2 Use an ultrathin aluminum oxide abrasive bla

25、deapproximately 0.76 mm (0.030 in.) thick, which will breakdown during cutting to help reduce sectioning damage.6.2.6 Faster blade speeds, 1675 m/min. (5500 surface ft/min.) or greater, produce less coating damage. Slower bladespeeds will result in more damage to the cut surface and are notrecommend

26、ed.6.2.7 Generally, an abrasive cutoff blade selected to cut thesubstrate effectively will be the best blade for the combinationof TSC and substrate.7. Cleaning7.1 Cleaning of specimens prior to mounting is essential.All sectioning coolant shall be removed from the surface andfrom any porosity conne

27、cted to the surface. Use of an organicsolvent to aid in fluid removal and thorough drying is neces-sary. Drying in an oven at low temperature (60 to 80C or 140to 176F) can accelerate this process. Any liquid residual mayimpede impregnation of porosity, as well as retard the curing ofmounting compoun

28、ds causing difficulty during grinding andpolishing.7.2 Ultrasonic cleaning of TSC specimens is generally notrecommended, especially for fragile or brittle coatings, becausecoating particles may be lost during this energetic cleaningprocess. If ultrasonic cleaning is found to be necessary,cleaning ti

29、me should be kept to a minimum.8. Mounting8.1 General Information:8.1.1 It is always necessary to mount TSC specimens tomaintain the original structure of the specimen during grindingand polishing. Both compression mounting and castablemounting compounds are commonly used when mounting TSCspecimens.

30、 However, only castable epoxy mounting com-pounds should be used in the initial determination of the truecharacteristics of a coating before considering the use of anyother mounting compound. For some TSC specimens castableepoxy may provide the only acceptable mount. Refer to Table1 and Table 2 and

31、Practice E3for characteristics of variousmounting compounds.8.1.2 By placing pairs of specimens in the same mount, timeand expense can be saved. When using this mounting method,the two coated surfaces should face each other. It may bepossible to place more than one pair of specimens in a singlemount

32、.8.1.3 Mounting expenses may be reduced by employing thesandwich mount technique. A sandwich mount is made byusing a small amount of a better, more expensive, mountingcompound as the critical layer in contact with the specimen andthen topping off the mount with a less expensive compound.Care must be

33、 taken to use only mounting materials that arecompatible.TABLE 1 Characteristics of Compression Mounting CompoundsType of CompoundACharacteristicsAcrylic Cure time 10 to 15 min., optically clear, thermoplastic,good impregnation, low hardness, degraded by hotetchantsDiallyl phthalate Cure time 5 to 1

34、0 min., opaque, thermosetting, minimalshrinkage, good resistance to etchants, high hardnessEpoxy Cure time 5 to 10 min., opaque, thermosetting, minimalshrinkage, good resistance to etchants, high hardness,good impregnationPhenolic Cure time 5 to 10 min., opaque, thermosetting,shrinkage during cure c

35、an leave crevice at specimeninterface, degraded by hot etchants, moderatehardnessAThese compounds often contain filler materials, such as glass fibers or mineralparticulate.E 1920 03 (2008)28.1.4 Taper mounting may be useful for examination of theinterfaces between bond coating and top coating, as w

36、ell asbetween coating and substrate.8.1.5 All components of mounting compounds includingresins and catalysts, as well as any dyes or colorants, should behandled in accordance with the manufacturers instructions.Material Safety Data Sheets are available from the manufac-turer.8.2 Compression Mounting

37、 Compounds:8.2.1 Curing of compression mounting compounds is ac-complished by the use of a heated press designed for anddedicated to metallographic mounting. Compression mountingcompounds require the use of heat (140 to 150C or 284 to302F) and pressure (up to 29 MPa or 4200 psi) to properlycure the

38、mount. Therefore, mounting compounds of this typeshould only be used to mount dense, nonfriable coatings withsubstrates a minimum of 1.5 mm (0.060 in.) thick.8.2.2 Compression mounts should be cooled to below 40C(104F) while under pressure, preferably by use of a watercooled mounting press, to preve

39、nt formation of a crevicebetween the mounting compound and the specimen as theycontract unevenly during cooling.8.3 Castable Mounting Compounds:8.3.1 Castable mounting compounds may be used for allTSC specimens, but are particularly useful for porous speci-mens. These mounting compounds are usually

40、prepared bymixing two components just prior to use. The specimen isplaced in a mold, usually a cup or ringform, into which thecompound is poured.8.3.2 Vacuum impregnation of porous specimens with anepoxy resin is required. The specimen and mold are put into avacuum chamber which will allow the casta

41、ble epoxy to bepoured into the mold after the chamber has been evacuated.Vacuum pressure should be in the range of 630 to 760 mm (25to 30 in.) of mercury. The vacuum should be maintained forabout two to ten min before allowing air into the chamber. Lowviscosity epoxy resins are recommended for best

42、results duringvacuum infiltration and subsequent preparation of TSC speci-mens.8.3.3 Addition of fluorescent dyes (1) or other agents, suchas Rhodamine B (2), that produce color response upon fluo-rescent excitation or with crossed polarized illumination,respectively, to the castable compound during

43、 mixing will aidin the identification of impregnated porosity during micro-scopical examination.8.3.4 Care should be used when handling castable mountingcompounds. Suppliers instructions for the use and handling ofthese materials should be followed.9. Grinding and Polishing9.1 Due to the many differ

44、ent types of TSCs and substratematerials, grinding and polishing sequences will vary. Properchoice of polishing surface, lubricant, abrasive type and size,time, force, polishing wheel speed and relative rotation willproduce accurate and consistent results. Some TSCs mayrequire specialized preparatio

45、n techniques. However, manycoatings can be prepared on automatic and semiautomaticpolishing equipment by use of one of the three procedureslisted in Table 3, Table 4, and Table 5.9.1.1 Use of automatic and semiautomatic polishing equip-ment has been found to reduce inconsistency in polishedspecimens

46、 and is therefore required in the metallographicpreparation for TSC specimens. These machines provide re-producibility in wheel speed, specimen position on the wheel,applied pressure, relative rotation, and polishing time. Manualgrinding and polishing is not recommended because of theinherent incons

47、istency and the sensitivity of these specimens topreparation technique. (3,4,5,6)9.1.2 Most of the equipment for automatic and semiauto-matic grinding and polishing move the specimen around arotating wheel covered with abrasive so that the specimenfollows an epicycloidal path. The scratch pattern co

48、nsists ofrandom arcs, and upon inspection, should be uniform acrossthe entire specimen surface before proceeding to the next stepin the preparation sequence.9.1.3 Each of the methods outlined in Table 3, Table 4, andTable 5 requires the following four basic preparation steps:9.1.3.1 Planar grinding

49、is used to remove large amounts ofmaterial rapidly and to bring the group of specimens in theholder into coplanarity. This step is necessary to avoid asituation where one or more specimens in the holder do notcontact the abrasive while the specimens in contact receiveexcessive pressure. Planar grinding of specimens also shouldremove the layer of deformed material resulting from theprevious sectioning operation.9.1.3.2 Fine grinding removes disturbed material from theplanar grinding step while introducing a layer of disturbedmaterial acceptable for the next step in the