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Grinding and Polishing 04_Ceramography_ASM 10/11/02 1:23 PM Page 35 © 2002 ASM International. All Rights Reserved. www.asminternational.org Ceramography: Preparation and Analysis of Ceramic Microstructures (#06958G) CHAPTER 4 Grinding and Polishing GRINDING removes saw marks and levels and cleans the specimen surface. Polishing removes the artifacts of grinding but very little stock. Grinding uses fixed abrasives—the abrasive particles are bonded to the paper or platen—for fast stock removal. Polishing uses free abrasives on a cloth; that is, the abrasive particles are suspended in a lubricant and can roll or slide across the cloth and specimen. A book edited by Marinescu et al. describes in detail the scientific aspects of ceramic abrasion processes. Some companies do not distinguish between grinding and polishing, as in the previous paragraph, but use the term lapping to mean grinding or coarse polishing with an abrasive slurry against a hard metal platen. Lapping is used in ceramography and ceramic manufacturing to produce extremely flat surfaces. Ceramographic specimens can be ground and polished manually, but automatic machines usually yield better-quality, faster, more reproducible results. Manual grinding allows better control of grinding depth than au- tomatic grinding, which could be important when the cross section at a specific depth is of interest. Automatic equipment is much more expensive than manual machines. Diamond abrasives are recommended for grinding most ceramics, but silicon carbide (SiC) paper and cubic boron nitride (CBN) platens can also be used. End each abrasive step when the artifacts (e.g., cracks or scratches) imparted by the previous step are completely removed. Grit sizes of abrasives and micron sizes are correlated in Appendix C. Reference 1 provides a number of material-specific automatic grinding and polishing methods. Automatic Grinding The pressure, time, and starting abrasive size depend on the number of mounts being ground, the abrasion resistance of the ceramic, the amount 04_Ceramography_ASM 10/11/02 1:23 PM Page 36 © 2002 ASM International. All Rights Reserved. www.asminternational.org Ceramography: Preparation and Analysis of Ceramic Microstructures (#06958G) 36 / Ceramography of wear on the abrasive particles, and the smoothness of the as-sawed sur- face. An automatic grinding and polishing machine is shown in Fig. 4.1. Automatic grinding method steps are: 1. Symmetrically load three to six mounted specimens into the specimen holder of an automatic grinding-polishing machine, with the flat sur- face of the ceramic section downward. Most manufacturers provide a leveling tool for loading the mounts into the holder. Attach the holder to the polishing head. 2. Grind the specimens at a contact pressure of 40 to 150 kPa on a bonded diamond platen for approximately 60 s or until the exposed surface of each specimen is flat and clean. Note that the pressure indicated on the grinding machine gage is usually the incoming air pressure, which is not necessarily equal to the pressure of the specimens against the platen. Perforated or grooved platens are available that aid in the re- moval of swarf. Experiment with the abrasive size, contact pressure, relative rotation directions (same or opposite), and frequencies shown subsequently to attain the best results. Typical machine settings: Abrasive 240–400-grit (60–40 µm) metal- or resin-bonded diamond Time 30–60 s (or until specimen is flat and saw marks are removed) Lubricant Water sprayed continuously Head air pressure (e.g., 210 kPa, 2.1 bar, or 30 psi Buehler machines) Head force (e.g., 200 N Struers machines) Platen frequency 200–300 rpm Head frequency 100–150 rpm Head direction Opposite to platen Contact pressure or frequency that is too high could damage the spec- imens or machine and shorten the life of the polishing cloth in the pol- ishing steps. Contact pressure or frequency that is too low slows the rate of stock removal and can prevent any significant abrasion at all. 3. Remove the specimen holder from the machine and clean the speci- mens, as in Subroutine 4.1, but do not remove the specimens from the holder until the last polishing step is complete. Once clean, return the specimen holder to the machine for polishing or more grinding in suc- cessive steps on ever-finer abrasives and follow each step with thor- ough cleaning. In many cases, all the grinding can be accomplished in a single step, such as in the procedure described in Table 4.1. Automatic Polishing After the finest grinding step, polish the specimens on napless polishing cloths loaded with lubricant and progressively smaller diamond abrasives. 04_Ceramography_ASM 10/11/02 1:23 PM Page 37 © 2002 ASM International. All Rights Reserved. www.asminternational.org Ceramography: Preparation and Analysis of Ceramic Microstructures (#06958G) Grinding and Polishing / 37 Subroutine 4.1: Cleaning Ceramographic Mounts After each abrasive step, rinse each specimen in warm tap water. Do not remove specimens from the holder if an auto- matic polishing machine is being used. Use distilled or deion- ized water if the tap water is too hard. Keep a 250 to 400 mL beaker of distilled water containing a laboratory detergent, such as Micro-90 or Alconox, on a hotplate at 60 to 80 °C. Swab each specimen with a cotton ball soaked in the warm, soapy water. If the ultrasonic bath is wide enough, ultrasonicate the entire specimen holder. Support the holder on a ring mold or something similar to prevent the polished faces from touching the basket or tank. Rinse each specimen again in warm tap water or deionized water. After the last abrasive step, remove the mounts from the holder if automatically polished, swab as mentioned previously, and clean one more time in warm distilled water in an ultra- sonic bath for 1 to 2 min. Quickly remove each mount from the ultrasonic bath, rinse with distilled water, spray with ethanol, and dry under a heat gun. Fig. 4.1 Automatic grinding and polishing machine 04_Ceramography_ASM 10/11/02 1:23 PM Page 38 © 2002 ASM International. All Rights Reserved. www.asminternational.org Ceramography: Preparation and Analysis of Ceramic Microstructures (#06958G) 38 / Ceramography Diamond polishing abrasives are typically available in 30, 15, 9, 6, 3, 1, and 0.25 µm sizes, in liquid suspensions, pastes, and aerosols. The sus- pensions can be automatically sprayed by some machines at timed inter- vals. Not every diamond size available is used or necessary in the proce- dure in Table 4.1. The transition from grinding to polishing may require additional time on the coarse polishing step to remove the artifacts of grinding. If paste is used, reapply it to the polishing cloth every few min- utes. All types of diamond abrasives break down quickly and should be re- plenished frequently. Follow each polishing step with a thorough clean- ing, as in Subroutine 4.1. Use napless cloth for diamond pastes or suspensions and napped cloth for the alumina slurry or colloidal silica. Napless cloth is a stiff, nonwo- ven PVC chemotextile sold under such trade names as Texmet, Pellon, DP-Plan, MD-Plan, and Pan-W. Nonwoven, fiber-reinforced-resin perfo- rated pads and woven silk also work well for polishing ceramics with di- amond pastes and suspensions. Flocked twill or napped cloth has a fuzzy texture that conforms to the surface being polished. Spread diamond paste, if used, on the cloth with a clean, gloved finger, along with additional lubricant. Polishing lubricants come under various names, including lapping oil, diamond extender, and blue lubricant. Be careful not to contaminate the paper with larger-size abrasive particles. Replace torn cloths immediately, being careful to smooth out any wrinkles or bubbles in the new cloth. Use xylene to dissolve the adhesive when re- moving worn-out cloth from the platen. Wear rubber gloves when using xylene. A worn-out cloth is easier to remove if the platen is first warmed with a heat gun. Platens tend to heat up during polishing and may require air cooling between intervals in order to prevent the polishing cloth from peeling or rupturing. Step 5(a) in Table 4.1, relief polishing, is optional. Relief polishing is not recommended when the specimen is to be tested for microhardness; Table 4.1 A typical ceramographic grinding and polishing procedure for an automatic polishing machine Platen Head Step Abrasive and lubricant Time, min frequency, rpm frequency, rpm 1. Plane grinding 240-grit bonded diamond disc sprayed 0.5–1 (or until specimen is flat 200–300 120–150 opposite continuously with water and saw marks are removed) to platen 2. Coarse polishing 15 µm diamond suspended in water-soluble 5–10 120–150 120–150 opposite oil, sprayed every 20–30 s on napless paper to platen 3. Polishing 6 µm diamond suspended in water-soluble oil, 5–10 120–150 120–150 opposite sprayed every 20–30 s on napless paper to platen 4. Fine polishing 1 µm diamond suspended in water-soluble oil, 5–10 120–150 120–150 opposite sprayed every 20–30 s on napless paper to platen µ γ 5(a) Relief polishing 0.05 m -Al2O3 slurry sprayed every 1–5 120–150 120–150 (optional) 20–30 s on napped cloth 5(b) Vibratory Colloidal silica slurry, replenished every 60–480 . polishing (optional) 30–60 min on napped cloth Note: For machines without timed spraying, the slurries can be poured from squeeze bottles or aerosols, or diamond pastes can be used instead. Source: Ref 2 04_Ceramography_ASM 10/11/02 1:23 PM Page 39 © 2002 ASM International. All Rights Reserved. www.asminternational.org Ceramography: Preparation and Analysis of Ceramic Microstructures (#06958G) Grinding and Polishing / 39 when edge retention is critical, such as on thin plates; or when the speci- men will be viewed in high magnification, such as fine-grained mi- crostructures.
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