Buehler’s Guide to Materials Preparation - …

1 Buehler's Guide to Materials Preparation The Science Behind Materials Preparation and Analysis TM. Introduction Sir Henry Clifton Sorby Anything approaching to a burnished (smeared) surface of polished scratches is fatal to good results.. Scientist - Sheffield, England Father of Petrography and Metallography Understood Effect of Abrasion on Microstructure Introduction Preparation Requirements: To See the True Microstructure Remove cutting, grinding and polishing related deformation Avoid thermal damage Avoid edge rounding Minimize relief and smearing Produce scratch-free surfaces Introduction Preparation Sequence Each Step Must be Performed Properly Sampling Sectioning Mounting (if needed). Grinding Polishing Etching (if needed). Sampling Designating Sampling Planes 1.

2 Transverse section 2. Longitudinal planar section parallel to the rolled surface 3. Longitudinal section perpendicular to the rolled surface n io ct ri e d g llin Ro 1 2 3. Sectioning Sectioning Sample a large component or part by removing a suitably-sized specimen from the larger mass at the desired location and orientation Sectioning plane should be as near to the desired location as possible Aggressive cutting methods will produce excessive damage that must be removed Sectioning 100 m Examples of damage (arrows) from sectioning. Left: cut surface in CP Ti (mod. Weck's reagent) on a plane perpendicular to the cut; Right: residual sectioning damage in the plane-of-polish of a CP Ti specimen (Kroll's reagent). Sectioning Heat-affected zone (left) and melting at the surface (right, arrows) due to abrasive sectioning A2 tool steel without a coolant (nital etch).

3 The cut surface was Ni plated after cutting perpendicular to the first cut (using coolant). Sectioning Sectioning Machines Sectioning Sectioning Parameters Equipment (abrasive cut-off, precision saw). Blade, Wheel (SiC, Al2O3, CBN, diamond). Operating Variables: Load Speed Feed Rate Contact Area Coolant Note: Delicate Materials may require encapsulation Sectioning Precision Saws Precision positioning Small kerf loss Diamond blades, thin abrasive wheels Applications Delicate components Ceramics, Carbides, Nitrides Biomaterials Mounting of Specimens Why Mount Specimens? Protect edges during Preparation process Protect delicate samples Increase life of polishing surfaces Uniformity of shape and size for automation Simplify specimen identification Mounting of Specimens To Maximize Edge Preservation Select the best mounting compound EpoMet resin Use a press that cools under pressure Plate edge with a protective metal EdgeMet Kit Add a filler material to cast resins Flat Edge Filler Use nappless polishing surfaces Introduction Hot Mounting Presses Mounting of Specimens Selecting a Hot Mounting Compound Phenolic (PhenoCure ) Acrylics (TransOptic ).

4 Lowest Price Transparent High Shrinkage Long Curing Cycle Poor Edge Retention High Shrinkage Poor Resistance to Hot Etchants Defect Prone Low Chemical Resistance Epoxy (EpoMet ) Poor Heat Resistance Superb Edge Retention Low Shrinkage Resistant to Heat and Chemicals Abrasion Rate Matches Metals Edge Retention Salt-bath nitrided 1215 carbon steel mounted in a) Epomet resin, b) phenolic resin; and c) methyl methacrylate resin and all prepared in the same holder revealing variations in edge retention (nital etch). The arrows point to the iron nitride surface layer. The needle-like particles are nitrides. Mounting of Specimens Castable Mounting Cold Mounting . Acrylic Resins VariDur SamplKwick . Epoxy Resins EpoKwick . EpoxiCure . EpoThin . EpoColor . EpoHeat . Mounting of Specimens Selecting a Castable Resin Acrylic Resins Epoxy Resins Low Cost Low Shrinkage Rapid Cure Transparent Poor Edge Retention Adheres to Specimen Strong Exothermic Reaction Solvent Resistant High Shrinkage Moderate to slow cure Strong Odor Will Flow into Cracks and Voids (under vacuum).

5 Mounting of Specimens Castable Resin Processing Factors Specimens must be cleaned and dried Do not use products beyond their shelf life Mix resin and hardener by specified weights Resin and hardener must be mixed carefully Large epoxy volumes generate high heat To reduce exotherm, use conductive mold Large specimen size increases cure time Introduction Grinding / Polishing Grinding Initial Grinding Step Goals Remove the damage resulting from sectioning Establish a planar surface Reach a specific plane close to a desired area/feature Extent of sectioning damage determines the selection of the initial abrasive size Grinding Subsequent Steps Remove damage from previous step(s). Decreasing abrasive size Depth of damage decreases Removal rate decreases Depth of damage is greater for soft Materials than hard Materials Polishing Final Polishing Remove any remaining damage or smear Produce a lustrous, scratch-free surface Maintain edge retention and flatness Yield the true structure with sharpness and good contrast Polishing Problems Examples of poor (a) relief control and (b) good relief control around voids in a braze (glyceregia etch) and comet tails at nitrides in H13 tool steel (Nomarski DIC, as polished), shown above.

6 Polishing Problems Embedding of SiC abrasive is a common problem with SiC embedded in Pb (after 1- m diam). low-melting metals. Polishing with 5 min polish with m Al2O3. diamond abrasive does not remove the embedded particles, but alumina does. After 3 min. polish with m Al2O3. Vibratory polish, SiO2, Pollack's etch Polishing Problems This shrinkage gap caused bleed out of water after drying which obscures detail and creates confusion. Polishing Problems Improper drying has left spots of water on the surface. DIC. Grinding / Polishing Preparation Parameters Abrasive type, size and amount Working surface (pad, cloth, etc.). Wheel and head speeds and directions Head position Force applied to specimens Individual force or central force Lubrication Time Grinding / Polishing Abrasives Alumina (powders, suspensions).

7 Diamond (paste, suspensions, aerosols). (natural or synthetic; monocrystalline or polycrystalline). Colloidal silica (pH ). Magnesium oxide (limited use). Cerium oxide (glass). Grinding / Polishing MasterPrep Alumina and MasterMet Colloidal Silica Both are excellent for most metals and non-metals MasterMet Colloidal Silica Preferred for refractory metals, polymers, sintered carbides and aluminum alloys Unsuitable for precious metals; will etch Mg alloys and stains pearlitic cast irons; causes etching problems with stainless steels and Ni-base superalloys when using etchants with Cl- ions MasterPrep Alumina Suspension Free from etching, cleaning and staining problems Sol-gel processing yields agglomeration-free suspension far better than calcined aluminas Grinding / Polishing Wheel and Head.

8 Direction Contra is slightly more aggressive Complementary tends to throw abrasive off the wheel Most methods use a combination of directions Contra Comp Grinding / Polishing Central or Individual Force Central Force Cannot remove any specimens until Preparation is complete Yields best flatness and edge retention Individual Force One or more specimens can be prepared Can examine specimens easily during Preparation Easy to remove etch or repeat last part of cycle Grinding / Polishing Time Each step must remove the deformation from the previous step Increase time; increase material removal Smaller jumps in abrasive size, shorter times required Increases in specimen surface area may require longer times Grinding / Polishing Traditional Method Load Lb. (N)/ Base Speed Time Surface Abrasive/Size Specimen (rpm)/Direction (min:sec).

9 CarbiMet 120 (P120) SiC* 6 (27) 240 300/Comp. Until plane CarbiMet 240 (P280) SiC* 6 (27) 240 300/Comp. 1:00. CarbiMet 320 (P400) SiC* 6 (27) 240 300/Comp. 1:00. CarbiMet 400 (P600) SiC* 6 (27) 240 300/Comp. 1:00. CarbiMet 600 (P1200) SiC* 6 (27) 240 300/Comp. 1:00. Canvas 6- m diamond paste 6 (27) 120 - 150/Comp. 2:00. Billiard 1- m diamond paste 6 (27) 120 - 150/Comp. 2:00. m alumina MicroCloth 6 (27) 120 - 150/Comp. 2:00. slurry Based on 1 mount size *Water cooled Grinding / Polishing Contemporary Method Load Lb. (N)/ Base Speed Time Surface Abrasive/Size Specimen (rpm)/Direction (min:sec). 180, 240 or 320. CarbiMet 240 300. (P180, P240, or P400) 6 (27) Until plane paper Comp. SiC, water cooled 9- m MetaDi 120 150. UltraPol Supreme diamond 6 (27) 5:00. suspension Comp.

10 3- m MetaDi Supreme 120 150. TriDent 6 (27) 4:00. diamond suspension Comp. m MasterPrep 120 150. ChemoMet 6 (27) 2:00. alumina suspension Contra*. Based on 1 mount size *Use contra only with low-speed heads (<100 rpm). Preparation Procedures For Preparation , Metals Are Grouped According to Like Characteristics Preparation Procedures Procedure Development Materials are grouped by common characteristics; periodic table used as a Guide Primary equipment used for development 8 (200 mm) platen six (30 mm) diameter specimens central force holder Copper, Nickel and Cobalt Copper Procedure Load Lb. Base Speed Time Surface Abrasive/Size (N)/. (rpm)/Direction (min:sec). Specimen 320- (P400) grit SiC 240 Until CarbiMet 6 (27). Water cooled Comp. plane 6- m MetaDi Supreme 150.