1、Designation: E1920 03 (Reapproved 2014)Standard Guide forMetallographic Preparation of Thermal Sprayed Coatings1This standard is issued under the fixed designation E1920; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of
2、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 themicrostructu
3、ral features of thermal sprayed coatings (TSCs)and the substrates to which they are applied when examinedmicroscopically. Because of the diversity of availableequipment, the wide variety of coating and substratecombinations, and the sensitivity of these specimens to prepa-ration technique, the exist
4、ence of a series of recommendedmethods for metallographic preparation of thermal sprayedcoating specimens is helpful. Adherence to this guide willprovide practitioners with consistent and reproducible results.Additional information concerning standard practices for met-allographic preparation can be
5、 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 prior
6、to use.2. Referenced Documents2.1 ASTM Standards:2E3 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 de
7、tachment, 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 created bymounting a featu
8、re, typically an interface or thin coating, at asmall 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 TS
9、Cs 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: B
10、y 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 reporte
11、d 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 sprayprocess, while the use of consistent practice will regularlydisplay
12、 the true microstructure and verify the consistency ofthe thermal 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 specimen
13、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 co
14、ating,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,1This guide is under the jurisdiction ofASTM Committee E04 on Metallographyand is the direct responsibility of Sub
15、committee E04.01 on Specimen Preparation.Current edition approved May 1, 2014. Published September 2014. Originallyapproved in 1997. Last previous edition approved in 2008 as E192003(2008). DOI:10.1520/E1920-03R14.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM C
16、ustomer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States14.3.8 The integrity of the interf
17、ace between the coating andsubstrate, and,4.3.9 The integrity of the coating microstructure with re-spect to chemistry.5. Selection of Metallographic Specimens5.1 Selection of specimens for metallographic examinationis critical if their interpretation is to be of value. Specimensmust be representati
18、ve 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 than 12 mm by 25 mm (0.5 by 1.0in
19、.). 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 substrate. Sectioning damage ofthe co
20、ating 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 should always be sectioned such tha
21、t 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. During examination of the polished
22、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 Sectioning with a hack saw will p
23、roduce 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 considerablegrinding in subsequent preparation
24、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 specimensunusable for proper evaluation.6.2.5
25、 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 bladeapproximately 0.76 mm (0.030 in.
26、) 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 notrecommended.6.2.7 Generally, an abrasive cu
27、toff 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 connected to the surface. Use of an org
28、anicsolvent 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 compounds causing difficulty during grind
29、ing 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 time should be kept to a minimum.8.
30、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. However, only castable epoxy moun
31、ting 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 Practice E3 for characteristics of
32、 variousmounting compounds.8.1.2 By placing pairs of specimens in the same mount, timeand expense can be saved. When using this mounting method,TABLE 1 Characteristics of Compression Mounting CompoundsType of CompoundACharacteristicsAcrylic Cure time 10 to 15 min., optically clear, thermoplastic,goo
33、d impregnation, low hardness, degraded by hotetchantsDiallyl phthalate Cure time 5 to 10 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 imp
34、regnationPhenolic Cure time 5 to 10 min., opaque, thermosetting,shrinkage during cure can leave crevice at specimeninterface, degraded by hot etchants, moderatehardnessAThese compounds often contain filler materials, such as glass fibers or mineralparticulate.E1920 03 (2014)2the two coated surfaces
35、should face each other. It may bepossible to place more than one pair of specimens in a singlemount.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
36、contact with the specimen andthen topping off the mount with a less expensive compound.Care must be taken to use only mounting materials that arecompatible.8.1.4 Taper mounting may be useful for examination of theinterfaces between bond coating and top coating, as well asbetween coating and substrat
37、e.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 Compounds:8.2.1 Curing of compres
38、sion 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 mount. Therefore, mounting compoun
39、ds 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 prevent formation of a crevicebetween t
40、he 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 prepared bymixing two components j
41、ust 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 castable epoxy to bepoured into the mol
42、d 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 results duringvacuum infiltration
43、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 mixing will aidin the identificat
44、ion 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 different types of TSCs and substratemat
45、erials, 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 preparation techniques. However, manycoating
46、s 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 and is therefore required in the
47、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 inconsistency and the sensitivity of the
48、se 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 consists ofrandom arcs, and upon ins
49、pection, 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 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
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