February 2009 | Volume 159 | Issue Number 2 www.ceramicindustry.com

2009 R&D Lab Equipment and Instrumentation

Directory ➤ Sample Preparation ➤ Glass Manufacturing ➤ Impact Testing

Special Section | Brick & Clay Record PerfectPrep ➤ New technologies have improved the speed and Sectioning of cermets was only accuracy of specimen preparation for materials such as possible with diamond-tipped wafer- ceramics, cermets, nitrides, borides and sintered carbides. ing blades with a low-speed saw. Although this was a slow process, by George F. Vander Voort, Director, Research & Technology, the cuts were of high quality (i.e., Buehler Ltd., Lake Bluff, Ill. relatively damage-free and smooth). However, as the cermets were very icroscopic examination ing at no more than 300 rpm and low hard, SiC paper could not be used to is an important inspec- applied loads. grind the specimens. At that time, the tion technique widely Grinding with silicon carbide (SiC) only harder abrasive products avail- used in the research, abrasives was possible, but the cut- able for grinding were metal-bonded Mquality control and failure analysis of ting rate was very low. Abrasive cutting discs covered with diamond, available very hard materials. To properly reveal theory has shown that the hardness of in several diamond sizes. The surface the microstructure of these very hard the abrasive must be at least 1.5 times tension between the specimen and materials, it is critical to use the proper that of the material to be abraded, and the disc surface was very high, which equipment (usually automated), along preferably more. SiC has a hardness made them impossible to hand grind. with suitable high-quality consum- of around 2200 HV, close to the mini- A grinder with an automated speci- able products, such as cutting blades, mum value. Polishing with several steps men holder had to be used. The use of mounting compounds, abrasives, lubri- using finer and finer diamond abrasives several diamond grinding discs with cants and working surfaces with proven (such as 9-, 6-, 3- and 1-µm diamond) decreasing particle size, followed by preparation methods. on a napless cloth (usually nylon at that polishing with finer diamond sizes on time) was a slow process but yielded cloths, obtained good results. Historical Perspective good results. Since the 1980s, a great deal of About 35 years ago, evaluating failed A program in the late 1970s evalu- research has been devoted to developing

sintered carbide (WC-Co) cutting ated cermet cutting tools (α-Al2O3 with ceramic materials and, more recently, tools was a slow process. Cutting these 30% TiC) for machining cast alloy steel nitrides. The only suitable abrasive for tools, some of which had hardnesses of rolls for hot rolling round bars. This most of these hard materials is dia- around 1500 HV, was impossible with mill was designed to roll bars to less mond. Cubic boron nitride (CBN) an ordinary abrasive cutoff saw. How- than half the normal dimensional tol- abrasive could be used to prepare some ever, they could be successfully sec- erance of hot-rolled steel bars. Carbide of the lower-hardness ceramics and tioned using diamond-tipped wafering cutting tools wore too much during the nitrides, but little use has been made blades with a low-speed saw. This was a machining of the mating roll passes, of CBN except on wafering blades for slow process at that time, as these were and the lathes did not have tool-wear steels. Fortunately, substantial improve- basically first-generation saws operat- compensation capability. ments have been made in the equipment

14 February 2009 ➤ www.ceramicindustry.com Table 1. DGD approach for preparing very hard materials. Surface Abrasive Load (Lb/N) rpm/Direction Time (min.) Resin-bonded DGD 70 µm (water cooled) 8/36 240-300/Contra Until Planar Resin-bonded DGD 40 µm (water cooled) 8/36 240-300/Contra 3 Silk cloth 9 µm diamond with fluid 8/36 120-150/Contra 3 Chemotextile cloth 3 µm diamond with fluid 8/36 120-150/Contra 3 Polyester cloth 1 µm diamond with fluid 8/36 120-150/Contra 3 Synthetic rayon cloth 0.05 µm slurry 8/36 120-150/Contra 3

Table 2. Rigid grind disc approach to preparing very hard materials. Surface Abrasive Load (Lb/N) rpm/Direction Time (min.) Rigid grind disc 30-µm diamond 8/36 240-300/Contra 5 Figure 1. Microstructure of an as-polished Rigid grind disc 9-µm diamond 8/36 240-300/Contra 5 cermet cutting tool consisting of an α-Al2O3 Silk cloth 3-µm diamond 8/36 120-150/Contra 3 matrix with 30% TiC carbides (white). The magnification bar is 10 µm in length. Silk cloth 1-µm diamond 8/36 120-150/Contra 3 Synthetic rayon cloth 0.05-µm slurry 8/36 120-150/Contra 3

Table 3. Etchants for sintered carbides. Name Composition Approximate Time Effect

Ferric chloride 3g FeCl3 in 100 mL water Swab ~ 10s Attacks Co (or Ni) binder

Murakami’s 10 g NaOH ~ 3 s Colors eta

reagent 10 g K3Fe(CN)6 10-30 s Colors alloy carbides 100 mL water 2 min. Outlines WC

Groesbeck’s 4 g NaOH 30 s Colors alloy carbides

reagent 4 g KMnO4 100 mL water used to prepare hard materials and in the Today, a variety of fixed diamond prod- abrasive products themselves. Specifically, ucts are available for grinding that make the low-speed saw has been enhanced to preparation of these materials rather sim- increase its speed and capabilities. ple. It is possible to grind specimens using rather coarse diamond abrasive on hard Preparation Methods cloths, but this is not a good procedure. The cutting of very hard materials Basically, the coarse grinding procedure still requires diamond abrasive blades. is performed with two types of products: Though they are rather expensive, it rigid grinding discs where diamond slur- is possible to buy large-diameter dia- ries are applied, or fixed abrasive discs Figure 2. Microstructure of barium titanate (BaTiO3): (top) as-polished, (bottom) after mond abrasive blades for large saws. where the diamond is incorporated into etching with Keller’s reagent (magnification For sectioning rather large specimens, the disc permanently. bar is 10 µm in length). these blades are indispensible. If the Of these later discs, the diamond is part to be cut is less than approximately attached by either metal deposition (e.g., Polishing with diamond particle sizes 1 in. (25 mm) in diameter, then less- electroless nickel) or with a resin bond. ≤ 9 µm is generally done using diamond expensive diamond-edged wafering Resin-bonded diamond discs are less paste or diamond slurries applied to hard blades and precision or low-speed aggressive than the metal-bonded variety. woven cloths. Diamond particle sizes saws may be used. These are more Within these diamond discs there are still down to 1 µm and occasionally smaller cost-effective but cannot be used to other options as to how the diamond is (0.5 or 0.1 µm) are used on cloths. In section large specimens. Wafering distributed on the disc surface. Fixed dia- some cases, these fine diamond sizes are blades have not changed dramatically mond discs generally feature diamond the last step in the process. It is also pos- over the past 40 years, although they particle sizes from about 70 to 15 or 9 sible to use oxide polishing compounds have gotten larger in diameter, from 3 µm, although both coarser and finer discs for the final step, even though their abra- to 8 in. (75 to 200 mm). are available. sives are not nearly as hard as diamond.

ceramic industry ➤ February 2009 15 PSUERBHFECTEAD PREP

in Tables 1 and 2 have been used to prepare a variety of very hard materi- als. The following examples of micro- structures are presented to illustrate the wide range of very hard materials that exist and the microstructural results that can be obtained with properly pre- pared specimens. Figure 1 (p. 15) shows a typi- cal cermet cutting tool consisting of 30% titanium carbide (TiC) in an

Figure 3. As-polished structure of a PZT [Pb(Zrx, Ti1-x)O3] – Ag-Pd ceramic multilayer actuator: (top) α-alumina (Al2O3) matrix. Examina- bright field, (bottom) Nomarski DIC. tion of the as-polished microstruc- ture shows a very strong contrast between the dark α-alumina matrix and the bright, highly reflective tita- nium carbides. Figure 2 (p. 15) shows the microstructure of barium titan-

ate (BaTiO3), a ferroelectric ceramic, in the as-polished condition and after etching with Keller’s reagent. Figures 3 and 4 illustrate the micro- structure of a lead zirconium titan- ate (PZT)-Ag-Pd multilayer ceramic actuator. PZT [Pb(Zr , Ti )O ] is a Figure 4. Grain structure of the PZT ceramic shown in Figure 5. As-polished microstructure of boron x 1-x 3 Figure 3 revealed by etching with 100 mL water, 10 nitride (BN). ferroelectric ceramic perovskite mate- mL HCl and three drops HF (magnification bar is 10 rial with a strong piezoelectric effect µm in length). with lapping action that produces a dull (a voltage differential is created across Table 1 shows one approach to prepar- surface; use of a cloth and diamond of its faces when compressed). Figure 3 ing very hard oxides, carbides, cermets and the same size, on the other hand, pro- shows as-polished images of the actua- nitrides. Specimens are cut with the duces a shiny, polished surface. tor in bright-field illumination and in precision saw using diamond-tipped Because of the difference in cutting Nomarski differential interference con- wafering blades and are then mounted action, if the first polishing step after trast (DIC) illumination. DIC reveals in a polymeric resin for ease of han- use of the RGD is with 1-µm diamond, a roughness of the PZT ceramic sur- dling and identification. The method greater time will be required to obtain a face due to its grain structure, which is in Table 1 uses two resin-bonded dia- properly polished surface quality. Final revealed in Figure 4 after etching with a mond grinding discs (DGD) and then polishing with the oxide slurry could solution consisting of 100 mL water, 10 uses three polishing steps with dia- also be performed on a vibratory pol- mL HCl and three drops of HF. mond abrasive paste or slurry applied isher. This is particularly worthwhile Figure 5 shows an as-polished cubic to hard woven cloths. The 9-µm when preparing sintered carbide mate- boron nitride specimen (CBN), a very step, and even the 3-µm step, can be rials, regardless of the complexity of hard material that is an electrical replaced with DGDs of the same par- the carbides in these materials, as the insulator but conducts heat well. Fig- ticle size, if desired. interfaces between the binder and the ure 6 shows as-polished microstruc-

Table 2 (p. 15) shows an alterna- carbides will be revealed much more tures of titanium diboride (TiB2), a tive approach using a rigid grind disc clearly, permitting very high magnifica- very hard ceramic with a hexagonal (RGD). The process could be started tion examination. Etchants for sintered crystal structure. Note that polarized with a coarser diamond abrasive if the carbides have been developed and used light reveals more of the microstruc- initial grinding step must remove more for many years. Table 3 (p. 15) shows ture than bright field. Figure 7 shows material. The 3-µm step could also be three of the most useful etchants for the microstructure of a molybdenum

conducted using a rigid grind disc. If sintered carbides. disilicide (MoSi2), a refractory ceramic this is done, the 1-µm step should be in a SiC matrix. Polarized light, plus a increased to 5 minutes. The use of any Preparation Examples sensitive tint filter, reveals the grain diamond on a RGD is more aggressive The preparation procedures described structure very well.

16 February 2009 ➤ www.ceramicindustry.com Figure 6. As-polished microstructure of titanium diboride (TiB2): (top) bright field, (bottom) polar- ized light (same field and magnification). Best Practices retention while avoiding damage to

Metallographic examination of very the material due to plucking or exces- Figure 7. As-polished composite of MoSi2 in SiC hard, inert ceramics, cermets, nitrides, sive fracturing. The metallographer viewed with polarized light plus sensitive tint borides and carbides can be readily should not overlook the use of polar- (magnification bar is 50 µm in length). performed using modern equipment ized light or Nomarski DIC to exam- For more information regarding sample and consumable products. Due to these ine specimens. While many of these preparation for hard materials, contact materials’ high hardness, diamond is materials may be rather difficult to Buehler Ltd., 41 Waukegan Rd., Lake the chief abrasive used. etch to reveal the grain structure, Bluff, IL 60044; (847) 295-6500; fax (847) Specimens can be prepared with solutions and procedures are avail- 295-7979; e-mail [email protected]; or a high degree of flatness and edge able for most of these materials. c visit www.buehler.com.

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