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Lapping and Polishing ➤ Flat Lapping Is Often the Process of Choice When Finishing a Ceramic Surface to Precise Dimensions

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Lapping and Polishing ➤ Flat Lapping Is Often the Process of Choice When Finishing a Ceramic Surface to Precise Dimensions Diamond Lapping and Polishing ➤ Flat lapping is often the process of choice when finishing a ceramic surface to precise dimensions. by Mark Irvin, Hyprez® Product Manager, Engis Corp. s advanced ceramics evolve and ever harder and • Greater planar, spherical or conical surface requirements more stable materials are introduced into the mar- • Improved sealing ket, the challenge to finish parts to precise dimen- • Improved cosmetic surfaces sions grows. Flat lapping is often the process of • Planarizing joined dissimilar materials (e.g., laminates, com- Achoice when it is necessary to achieve dimensional flatness and posites) finish requirements. • Surface deburring, removal of “gummy” materials Unless the proper tools are applied, however, flat lapping can • Thinning/finishing parts with poor aspect ratios be a time-consuming process. Lapping with diamond can pay off Common applications for the flat lapping of precision in lower cycle times and lower slurry consumption. Lapping with ceramics include mechanical seals, seal rings, pump parts, vac- diamond also results in lower slurry cost per hour, lower sludge uum chucks, fixture components, wafers for microelectrome- generation, lower reject rates and fewer process steps. chanical devices (MEMs), and flat glass or mirror substrates. Almost any application of engineered ceramics with high flat- What is Lapping? ness and/or surface finish requirements may benefit from the Free abrasive lapping is a four-body process that involves an abra- diamond lapping process. sive, a carrier (paste or liquid) to be applied between the workpiece Compared to lapping with conventional abrasives (e.g., silicon surface, and a lapping plate. While some of the diamond particles carbide), diamond lapping offers the following benefits: in the carrier can become embedded in the lapping plate to per- form a fine grinding action, abrasive particles in lapping may also be continuously loose and rolling (see Figure 1). The lapping process, which is well-suited to brittle materials like hard ceramics, works by pushing the points of the diamond grains into the work surface to abrade microchips of workpiece material. As a result, lapped surfaces do not have directional marks. This kneading or abrading action on the work surface is repeated mil- lions of times in order to effectively remove material while simul- taneously providing a polishing action (especially when using very fine micron and sub-micron sized abrasive particles). Applications and Advantages Diamond lapping is most appropriate for ceramic machining or finishing projects that require any of the following: • Working with non-oxide advanced ceramics or dense, high- purity oxide ceramics; these materials are “super-hard,” but they are not harder than diamond, which can offer more eco- Figure 1. Lapping uses a combination of embedded and free diamond nomical processing. abrasive particles. Reprinted with permission from the May 2011 issue of Ceramic Industry magazine. Table 1. Diamond lapping vs. conventional silicon carbide for an alumina wafer. Abrasive Powder and Slurry Silicon Carbide Diamond Achieving super-fine finishes requires Number of process steps 2 1 rigid control over diamond particle Removal rate/cycle time .0013 in./hr, 45 min cycle .004 in./hr, 15 min cycle size. For this reason, high-quality slurry Slurry consumption .5 gal/hr 1 pint/8 hr always has at its core a carefully graded Sludge generation 4 gal/8 hr .125 gal/8 hr abrasive with a consistent, predictable Part finish Matte finish Reflective mean particle size and tight standard Part cleanliness Dark, dirty; Relatively clean; deviation. High-quality slurry is also free two cleaning steps one-step light cleaning of oversize particles at the high end of Slurry cost/8-hr shift $166 $47 the distribution. Diamond can be paired Diamond with a specially engineered slurry vehicle Table 2. Diamond lapping examples. that promotes diamond dispersion, wets Seal Ring Pump Slider Hot Water Seal Chuck the plate, keeps debris formation to a Ceramic material 99.7% alumina Alumina Alumina Alumina nitride minimum and lubricates to enhance the Surface area 49 in2 0.35 in2 1.5 in2 104 in2 cutting action. Removal rate .0018 in./hr .0024 in./hr .0038 in./min .0005 in./hr “Ceramics are typically processed with Diamond size 6 micron 3 micron 15 micron 3 micron water-based slurries containing mono- Slurry type Water-based Water-based Water-based Water-based crystalline diamond for greatest econ- Plate type HY iron composite HY copper HY copper HY ceramic omy,” says Rich Pacyna, manager of Engis’ Surface finish 5.7 µin 1.7 µin 6.5 µin 6 µin Process Development Lab. “These are easy to use and clean up well. A diamond size • Aggressive material removal for equal for lapping alumina. However, every case should be selected based on the target fin- or better productivity is a bit different since removal rate is a ish and material removal rate. If every- • Uniform edge-to-edge flatness; sub- function of pressure, speed, and a lapping thing else is equal, denser ceramics will light band (11 millionths of an inch) constant that accounts for material and yield smoother finishes.” 1 results are routine, and up to /20 wave- abrasive efficiency (as explained by Pres- length is achievable under specific con- ton’s Equation*). Machines ditions In some cases, part geometry or equip- To gain the full benefit of the inherent • Better than sub-micron surface finish ment constraints limit the pressure that strength of diamond on ceramic, the ideal (< 0.5 Ra) is routine; sub-nanometer can be applied, so removal rates vary lapping machine design features high surface finishes are achievable accordingly. Table 2 shows the results down-pressure/down-force (5 psi and • High potential to develop a one-step achieved with diamond in four different up), a robust drive system to tolerate the lapping and polishing solution for ceramic lapping applications. In applica- high down-pressure, and a variable speed reduced cycle times tions where surface finish requirements drive with high top-end RPM capability. • Reduced waste supports green initiatives are particularly demanding, parts typically undergo a polishing step after lapping. For Plates and Accessories A Case Study example, with the alumina nitride chuck, Proper plate selection is vital. The As shown in Table 1, diamond provided a the part was brought to a 6 µin2 reflective plate must be of optimal hardness more effective lapping solution than con- finish using 1 micron diamond slurry on a and malleability to maintain flatness, ventional abrasives for an alumina wafer. fine-nap pad. accept a diamond charge, and accel- In this instance, diamond eliminated the erate the rolling abrasive action. Lap- hand-polishing step, reduced the cycle A “Systems Approach” ping ceramics with diamond typically time by 30 min, saved $14.87 per hour When it comes to the fine grinding and requires a lapping plate made from in slurry costs, reduced waste and associ- lapping of ceramics, the best results composite metal iron (X-08) or cast ated disposal costs, and reduced cleaning occur through a “systems approach” iron for roughing operations; compos- time and the use of cleaning solutions. In that examines abrasive powders, slur- ite metal copper (HY Cu) is needed for addition, diamond minimized the messy ries, plates/pads, machine tools and general purpose and finishing opera- appearance that is often associated with accessory options, and matches them tions. The use of a composite ceramic conventional abrasive lapping. Diamond’s against specifications for consistent plate (HY Ce) can eliminate concerns lower slurry volume requirements also part quality, processing time, environ- regarding potential metal contamina- reduced shipping and inventory costs. mental requirements and overall costs. tion, when needed. The removal rates and slurry con- Following are guidelines for each of After a certain period of use, the lap- sumption in this study are fairly typical these system components: ping plate will become worn and require *R=KPv, where R is removal rate, K is Preston’s Coefficient (process dependent), P is pressure (load/area) and v is relative lapping velocity. reconditioning to re-establish plate can be “charged.” Charging is the pro- The lapping force applied is actu- flatness and meet end-part flatness cess of embedding the diamond abra- ally load per unit contact area, so tolerance and surface finish require- sive into the plate. controlling the contact area (or bear- ments. When plates become “glazed,” A facing device can be considered as ing ratio) is vital to consistent per- they no longer remove material at a an alternative to traditional conditioning formance. A consistent bearing ratio consistent rate, which, in turn, intro- rings. Facing devices provide superior results in repeatable removal rates duces undesired production vari- control over the groove pattern (macro and excellent batch-to-batch con- ables. Reconditioning a diamond lap- texture) and land shape (micro texture) sistency. As a corollary, the reduced ping plate requires cutting away the of the plate surface, which allows for contact area provided by a grooved top surface to restore flatness (facing) greater consistency in removal rates and plate increases the load per unit area, and retexturing the plate so that it surface finishes. therefore increasing removal rates. An added benefit is that the groove also aids in clearing away swarf. Note that the development of fac- ing devices was driven by the need to increase first-pass quality and reduce scrap costs. If a manufacturer determines that diamond would add benefit to its operations, a facing device eliminates concerns about finding an operator with the skills to maintain a plate using tradi- tional ring-conditioning techniques, thus simplifying the integration of a diamond lapping system. Consistency is Key The advanced ceramics industry is fac- ing an ever-increasing demand to process harder and more stable materials.
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