Green Machining to Net Shape Alumina Ceramics Prepared Using Different Processing Routes

Green Machining to Net Shape Alumina CeramicsPrepared Using Different Processing Routes

Santanu Dhara* and Bo Su

IRC in Materials Processing, Department of Metallurgy and Materials, University of Birmingham,Edgbaston, Birmingham B15 2TT, U.K.

Department of Oral & Dental Science, University of Bristol, Lower Maudlin Street, Bristol,BS1 2LY, U.K.

In the present study, the net shape manufacturing of alumina ceramics was carried out using computer numericallycontrolled (CNC) machining of differently processed alumina green bodies using different tool materials and geometries.In case of a carbide-coated tool, severe wear was observed; on the contrary, a diamond-coated tool was used successfully.The profile of the as-machined surface was found to be smooth using diamond-coated flat tool. Near net shape aluminawas demonstrated using CNC green machining from optimized machining conditions. Optical micrograph and the surfaceroughness of the as-machined samples showed good surface finish for the green bodies with higher binder content.

Introduction

Advancement in science and technology has result-ed in the growing development of bulk ceramic com-ponents for existing as well as emerging applications,1–3

which is dependent on processing techniques capable ofproducing the desired shapes and structures. Unlikemetals, most technical ceramics cannot be shaped viamelting and casting, as they possess high melting pointsand poor thermal conductivity. Manufacturing of com-plex shaped ceramic components by machining of sin-tered ceramics4–8 is also not a preferred option as it

results in chipping and sub-surface cracks. Net shapefabrication is thus the most preferred option.9–12

Net shape fabrication of ceramics is generally basedon powder processing technique.13 It could be done intwo ways, i.e., bottom-up approach such as direct cast-ing,14–17 or top-down approach such as green machin-ing.18,19 Various bottom up net shape forming processeshave been developed, e.g., pressure-based process suchas dry powder pressing,1,2,10–12 and several well-estab-lished flow-based processes such as slip casting,14 extru-sion,10–12 injection molding,10–12,15–17 etc. In most ofthe above-mentioned bottom-up processes, the shapesof the ceramics are defined by the molds. However,these processes are more suitable for large batch or massproduction of complex shape ceramic components. Forsmall batch or custom-specific components, mold-free

Int. J. Appl. Ceram. Technol., 2 [3] 262–270 (2005)

Ceramic Product Development and Commercialization

Work funded by EPSRC (GR/S51400/01).

*[email protected]

solid freeform fabrication (SFF) techniques such as 3Dprinting, stereo lithography (SLA), laminated objectmanufacturing (LOM), selective laser sintering (SLS),and fused deposition modelling (FDM) are morepreferred ones.20–23 However, the application of SFFtechniques in rapid prototyping of ceramics has beenless successful than in metals and polymers so far.One of the major obstacles is the stair-steps surface to-pography produced during the layer-by-layer built upprocess, which is detrimental to the mechanical prop-erties because the brittle ceramics are extremely notchsensitive. Any attempts to improve the surface qualityby reducing the layer thickness will inevitably increasethe production time. Top-down approach via greenmachining is thus proposed. However, few of the aboveceramic forming processes are able to produce greenbodies that are strong enough to be machinable.Green machining is impossible until the developmentof new techniques such as gel casting,24–31 protein co-agulation casting (PCC),32–36 and gel forming, whichare able to produce green bodies strong enough to bemachinable.

Gel casting was first developed in the 1990s byMark Janney et al.24 The process of gel casting involvesuse of highly loaded ceramic slurries containing solublemonomers that are polymerized in situ following castinginto molds. The use of gel casting has enabled fabrica-tion of large ceramic components with uniform greendensity and high green strength.29–31 Almost any moldmaterial can be used provided the surface is imperviousand smooth. The total process of gel casting does notrequire much specialized expensive equipment. The gelcast components have reasonably low organic bindercontent that can be eliminated easily without any specialcare during binder burn out steps. For most oxide andeven some non-oxide ceramics that do not react withwater, aqueous slurries can be used thus simplifying thematerial requirement for fabrication of components bygel casting. The high green strength of gel cast parts alsoenables machining in the green state. The biggest con-cern in gel casting is, however, the monomers usedwhich are regarded as neurotoxic.27

Different attempts were made to use naturally oc-curring biopolymers such as gelatine, carrageenan,chitosan, and saccharides, etc., for gel casting of ceram-ics.37–40 Zhang et al.41 and Xie et al.42 have reportedagar consolidation of ceramics using agarose as bindingmaterial. This natural material is soluble in hot waterand gels upon cooling from high temperature. None of

the processes are very promising for green machiningdue to their low green body strength. Recently, Santa-cruz et al.43 have attempted to use the combination ofdifferent binders to improve green body strength. An-other combination of natural materials that has beenevaluated for their gelling ability is the agar and gal-actomannan (locust bean gum).44

A safe and environmental friendly process, PCC hasbeen developed recently.32–36 The PCC process utilizescoagulation of protein molecules resulting from dena-turation that takes place due to changes in pH, ionicstrength or temperature. Denaturation of protein mol-ecules is caused primarily due to disruption of hydrogenbonds within the tertiary structure,45 which leads touncoiling of protein chains that get entangled togetherto form aggregates. The above-described physical chang-es in protein molecules, are utilized towards consolida-tion of ceramic compacts. The coagulation of water-soluble proteins present in ceramic slurries is responsiblefor transforming fluid slurries to green compacts withshape dictated by the molds. The process of PCC hasbeen used to form both dense and porous ceramic bod-ies owing to excellent binding properties and naturalfoaming tendency of ovalbumin. For fabrication ofdense ceramic bodies, antifoaming agent is added tothe slurries. De-aired slurries are cast followed by tem-perature-induced coagulation to produce strong greenceramics.

Gel forming is another net shape fabricationprocessing which is an analog to gel casting, where in-stead of using monomers, linear polymer binder (such asPVA) with intrinsic crosslinking functionalities is used.The polymeric binders are crosslinked to form a net-work after ceramic forming, such as extrusion. Highlyplastic deformable green ceramic bodies are obtainedthrough high shear twin-roll milling prior to gela-tion.46,47 As soon as the green ceramics are consolidat-ed, they have minimum distortion and extremely highgreen strength.

Only few literatures are available on machining ofgreen ceramic compacts using gel casting and other re-cently developed techniques.30,31 Work has been report-ed on the direct ceramic machining on partially sinteredceramics as a rapid prototyping method for such appli-cations as dental bridges.48,49 Shin et al.50 and Caseet al.51 have also reported fabrication of complexinternal meso-scale channels in oxides ceramics viamachining of partially sintered ceramics followed byjoining.

www.ceramics.org/ACT Green Machining to Net Shape Alumina 263

The development of these new ceramic formingtechniques has rendered the possibility of green ma-chining as a new rapid prototyping technique for netshape manufacturing of ceramics, especially with therecent advancement of computer numerically controlled(CNC) machining technology. The top down approachvia CNC green machining may provide a viable optionfor the net shape manufacturing of ceramics in suchapplications as dental and orthopedic implants whereonly one-off custom-specified products are needed.

Preliminary results of CNC machining using greenalumina compacts prepared from three different form-ing processes such as PCC, gel casting, and gel forming,are presented in this work. Different green bodies arecharacterized for green density, strength, and hardnessmeasurement. The machining parameters such as selec-tion of tool materials and tool geometries are initiallycarried out for producing defect-free samples. Using3-mm diameter rounded and flat tools, the quality ofthe as-machined surface are compared for green PCCsamples. A 3-mm diameter diamond-coated flat toolwas used to machine three different green compacts un-der optimized machining conditions. All of the as-ma-chined surfaces are examined using scanning electronmicroscopy (SEM) and optical microscope. The rough-ness profiles of the as-machined surfaces before and aftersintering are examined and the arithmetic average sur-face roughness is characterized.

Experimental Procedure

Sample Preparation

Different green compacts are prepared using thefollowing three fabrication techniques, i.e., PCC, gelcasting, and gel forming. Alumina powder (Alcoa CT3000 SG; d50 5 0.70 mm; surface area, 7 m2/g) was usedas raw material. Duramax D-3005 (aqueous solution ofpoly (acrylic acid) ammonium salt; MW 5 3500; Rohmand Haas, Philadelphia, PA) was used as a dispersant dur-ing fabrication of green compacts. The green-machinedsamples were sintered at 15501C for 2 h. The details ofeach of the above fabrication techniques are given below.

PCC: The process for fabrication of alumina densecompacts involved preparation of aqueous alumina slur-ries with mixed binder of ovalbumin- sucrose system.Optimized amount of Duramax D-3005 is used as adispersant. In the present study, 55 vol% alumina load-

ing was used with 20 vol% of the ovalbumin content inthe premix along with 3 wt% sucrose dry alumina pow-der weight basis.34 The slurries were prepared by ballmilling the above mix in the presence of spherical zir-conia media in a polypropylene container for 24 h. Allmillings were carried out in the 600-mL polypropylenebottles with 200 mL of slurry and zirconia-grinding me-dia (diameter of 3 mm and 1.4:1 of powder to mediaweight ratio) adding to the bottle. Following 24 h ofmilling, the slurry was filtered through a sieve to sepa-rate the zirconia milling media. The slurry was de-airedby rolling for an hour without the milling media. Thede-aired slurries were cast into petroleum jelly-coatedrectangular molds. The molds were placed in an ovenpreheated to 401C for 12 h. The parts were removedfrom the mold and then dried at 801C. Coagulation anddrying were done in a single step.

Gel Casting: An aqueous premix of water-solublemonomer, methacrylamide (MAM) and cross linker,methylene bis acryl amide (MBAM) in 6:1 ratio, wasused to prepare the slurries. Optimized amount ofDuramax D-3005, a commonly used ceramic disper-sant (1.2 mL per hundred gram of alumina) was addedto the aqueous premix followed by addition of the alu-mina powder and antifoaming agent (n-octanol). Allsamples were prepared with 55 vol% alumina loading inthe slurry. The slurry was mixed/milled for 24 h usingzirconia grinding media (diameter B2–3 mm) and thenkept at rest (idle) for 30 min followed by de-airing. Thefollowing amounts of the initiator and catalyst wereadded to the slurry, respectively—1 mL of ammoniumper sulphate (APS) and 0.5 mL of tetra methyl ethylenediamine (TEMED)/g of slurry. The slurry was thenpoured into plastic molds and allowed to gel at 401C.The dried samples are used for green machining ofceramics.

Gel Forming: Alumina powders were first mixed withPAA dispersant (Duramax D-3005) and freeze-driedsubsequently. The surface-treated powders were thenmixed with a high molecular weight polymer binder(PVA, KH17) and water with a small amount of plas-ticizer (Glycerol). Mixing was carried out on a twin-rollmill. After mixing, the alumina green tapes were de-aired under a press for 4 h. The green tapes were finallylaminated together and dried at 401C for 12 h and crosslinked at 1201C for 8 h to form a strong green body formachining operation.

264 International Journal of Applied Ceramic Technology—Dhara and Su Vol. 2, No. 3, 2005

Green Body Characterization

The green densities of the fully dried samples weremeasured. The dried samples were tested for strengthmeasurement in flexural mode. The size of the samplesused for strength testing was 35 mm� 4 mm� 3 mm.The flexural strength of alumina green body was meas-ured in three-point bend configuration with a span of25 mm, at a crosshead speed of 0.5 mm/min using amechanical testing machine (Instron Corporation,Model 5800, Grove City, PA). Hardness of green sam-ples is measured through a Vickers indentation (MicroHardness Tester, Model No. MVK-H1, Mitutoyo,Kawasaki, Kanagawa, Japan) using 200 g force. Priorto indenting, all samples are lightly coated with markerpen ink to make the indentations clearly visible. All in-dentations are symmetrical and had the perfect diamondpyramid shape.

Machining Operation

A bench top milling machine Roland (Model No.MDX 650, Roland DG Ltd., Shinmiyakoda, Hama-matsu-shi, Shizuoka-ken, Japan) was used for the pur-pose of machining in the present study. It was abelt driven fully automated computer-controlled mill-ing machine, which could be operated at variable rpm of3000–12,000 with maximum x–y, and z speed of14 mm/s. In the present study, the x–y and z directionspeeds were optimized to get the crack free samples atminimum machining time. Primarily machining wascarried out using high-speed steel (HSS), carbide- anddiamond-coated tool to examine which tool would besuitable for CNC machining of the green ceramics.Green ceramic samples were mounted on an aluminumplate using wax or glue. The plate was then mechani-cally clamped on the base of CNC machine.

Characterization of Machined Surface

The as-machined surfaces were examined underthe Optical Microscope (Model NO. AXIOSKOP 2,Carl Zeiss Ltd., Welwyn Garden City, Herts, UK) andscanning electron microscope (JEOL, JSM-6060LVWatchmead, Welwyn Garden City, Herts, UK). Thesurface roughness of the as-machined surface was exam-ined using TALYSURF 10 (Rank Taylor Hobson Lim-ited, Leicester, England).

Results and Discussion

Different Green Body Characteristics

Different green body characteristics of the aluminasamples were measured and are listed in Table I. Binderweight percent present in the gel-formed samples were16.5 wt%, which is about four times higher than thePCC and gel-cast ceramics. Green density of gel-castand PCC samples were 2.6 gm/cm3 which 67% of thetheoretical density of alumina. Green density of the gel-formed body was relatively lower (2.3 gm/cm3) in com-parison to gel-cast and PCC samples due to higherbinder content. Green body strength of the gel-formedsamples was about 120 MPa, which was about ten timeshigher than PCC and gel-cast samples. The binderweight percent present in PCC and gel-cast ceramicswere very similar and the green body strength was alsocomparable. The hardness of the green samples was verydifferent in comparison to green strength values. Vickershardness of the green PCC ceramics was 448 MPa,which was very similar to those of green gel-cast ceram-ics (346 MPa) whereas gel-formed green ceramics hadthe Vickers hardness of 199 MPa, which is much lowerthan PCC and gel-casting samples. Though gel-formed

Table I. Binder Content and Green Body Characteristics of PCC, Gel Casting, and Gel-Forming Samples AreEnlisted Along with Their Surface Roughness Values

Fabricationprocesses

Binder(wt%)

Green density(gm/cm3)

Green bodystrength (MPa)

Vickershardness (MPa)

Surface roughness (lm)

//F >F //R >R

PCC 4.7 2.6 1371.1 448 2.3 2.5 2.9 7.7Gel casting 4.3 2.6 1270.63 346 3.1 3.9 � �Gel forming 16.5 2.3 12072.1 199 0.1 0.1 � �PCC, protein coagulation casting; //, along the direction to surface finish; >, along the perpendicular direction to surface finish;F, using flat tool; R, using rounded tool; � , not measured.


green body samples had the maximum green strength, itexhibited something more like viscous polymer flowbehavior during hardness measurement due to the high-volume fraction of polymer binder content and thusresulted in low hardness. On the other hand, the greenPCC and gel-cast ceramics were brittle in nature due tolow-volume fraction of the organic binder content inthe green body and additionally the cross-link polymericnetwork structure of the binder. Therefore, green PCCand gel-cast samples had higher hardness in comparisonto gel-formed green samples.

Effect of Tool Materials’ Properties

Preliminary studies were carried out with the use ofHSS, carbide-, and diamond-coated tools for machiningof the green alumina ceramics. It was observed that boththe HSS and carbide-coated tools wear out very fastduring machining of ceramics in the green state. A blackmark was also observed along the tool movement path(Fig. 1a), due to the wear of tools during green ma-chining of alumina. Also the machined surface wasbrownish black in color, probably due to partial burn-out of the organic binders (Fig. 1a) as a result of heatingup of the carbide-coated tool owing to the friction be-tween the tool and alumina powder during continuousmachining. There was also a smell of burning of theorganic binders during machining. It is thus concludedthat carbide-coated tools are unsuitable for green ma-chining. Moreover wearing of the carbide-coated tool isharmful to the ceramic body due to incorporation of themetallic impurities in the green ceramics. In addition,fast wearing of the tools results in decrease of the ma-chining efficiency.

In contrast, the diamond-coated tool has shownnegligible wear during green machining, even after ma-chining operations of long durations. There was noblack mark along the tool path and no changes in color(Fig. 1b), indicating good lubrication between the tooland green ceramics. Therefore, diamond-coated toolshave been used in the late studies. Figure 1b shows theexample successfully machined using a diamond-coatedtool with negligible defects.

Effect of Tool Geometry on Surface Roughness

The PCC-based green alumina compacts have beenused as a model sample to compare the machining char-acteristics using different tool geometries. Initial studies

involve optimization of the cutting speed such as speedalong x–y and z direction and amount of materials beingcut in one step. These parameters are very critical to getdefect-free samples and better surface finish. After a fewtrials of the experiments, the optimized conditions (x–yand z direction speed of 10 mm/s and the 12,000 toolrpm) were obtained. The optimized conditions wereused for comparison of the all green alumina duringmachining in this study.

Two different diamond-coated tools with flat androunded end shape have been used to investigate theeffect of tool geometries on the surface finish of green-machined samples. It is evident that the machinedsurface is not smooth in case of the rounded tool in

Fig. 1. PCC-based samples were machined using (a) carbide-coated and (b) diamond coated tools.


comparison with flat tool (Table I). There was a regularmark along the path of the tool movement (Fig. 2a andb). This mark was probably due to special tool features.Also the surface roughness of the as-machined samplesmeasured along and perpendicular to the direction ofmovement of the tools indicate that the surface smooth-ness of samples using flat tool is better comparing tosamples machined using rounded tool (see Table I andFig. 3a and b). Though the arithmetic average surfaceroughness values were very similar in both cases alongthe machining direction (Fig. 3a(i) and b(i)), they arevery different along the directions perpendicular to thetool movement (Fig. 3a(ii) and b(ii)), which are about2.5 and 7.7mm for flat and rounded tool, respectively. Itis thus necessary to use the flat tool to obtain bettersurface finish.

Machining of the Different Green Bodies

Different green alumina bodies have been ma-chined using 3-mm diameter diamond-coated flat toolunder optimized machining conditions. In all the cases,x–y and z direction speed of 10 mm/s and the 12,000tool rpm were used during machining and the millingtool moved along x–y directions instead of contour pathand finishing step was done along the same direction asroughening. The optical micrograph of the as-machinedsurface is compared to see the difference in surface mor-phology. Their optical micrograph is shown in Fig. 4. Ahorizontal mark has been observed along the path of thetool movement due to machining of the green ceramics.Surface finish of the as-machined gel-formed samples(Fig. 4c) is the best among three different processed

Fig. 2. Surface features of the machined surface using diamond-coated (a) rounded tool and (b) flat tool.

Fig. 3. The surface roughness profile of the as-machined green PCC alumina using 3-mm diameter diamond coated (a) rounded tool and (b)flat tool. In each of the samples, surface profile (i) indicates the roughness is measured along the direction of the machining and (ii) indicates theroughness is measured along the perpendicular direction of the machining.


samples (Fig. 4), due to, the presence of larger volumefraction of the polymeric binders. The tool movementmark was prominent in case of gel-cast samples, whereasin case of PCC samples the tool movement path wasrelatively less prominent.

Soon after the machining of the green ceramics, theswarfs was collected and observed under SEM. It is ev-ident that the PCC and gel-cast samples were brittle innature and the machined swarfs were irregular and par-

Fig. 4. Optical microscopy shows the surface features of the green-machined surfaces for (a) PCC, (b) gel-cast, and (c) gel-formedsamples using diamond-coated flat tool.

Fig. 5. SEM micrograph of swarfs collected soon after machiningof (a) PCC (b) gel-cast, and (c) gel-formed samples using diamond-coated flat tool.


ticle-like, as seen in Fig. 5a and b respectively. The ma-chined swarfs of the gel-formed samples are very regular,long strips, as shown in Fig 5c. The completely driedPCC based samples are very similar to gel-cast samples.However, the partially dried PCC samples behaved dif-ferently. Their swarfs are more like thin wood strips(Fig 5a). This is probably due to the presence of sucrosein the PCC samples. Probably due to the moisture ab-sorbing ability of the sucrose molecules, the PCCgreen body showed moisture-induced plasticity duringmachining.

Measurement of Surface Roughness of theMachined Surface

Surface roughness profiles of the as-machined sur-face of the different samples were measured along andperpendicular to the direction of the machining. Thearithmetic average surface roughness values are enclosedin the Table I for all of the different samples. The sur-face profiles of the as-machined surfaces are shown inFigs. 3 and 6. It can be seen that the surface profiles ofthe gel-formed samples are very smooth with averagesurface roughness value of about 0.1 mm in any direc-tion (Table I, Fig. 6), whereas the surface morphologyof the as-machined PCC and gel-cast green bodies aremuch rougher and are different along different direc-tions of machining (Table I, Figs. 3 and 6). The arith-metic average surface roughness of the PCC and gelcastsamples were 2.3 and 3.1 mm, respectively, along thedirection of the surface machining, which were much

higher than the gel-formed sample (Table I). Thoughthe as-machined gel-formed sample produced bettersurface finish compared to the PCC and gel-cast sam-ples, gel forming cannot be used for thick section com-ponents due to the difficulty of drying and de-bindingof a large amount of binder content. However, it can bepotentially used in the net shape fabrication of ceramicmicro components.

Finally, it should be noted that, though the as-machined PCC green sample has average surface rough-ness above 2 mm, it is reduced to about 0.3 mm as a re-sult of sintering. Sintering seems to be able to furthersmooth the surfaces of as-machined green ceramics.This is the most distinct advantage of green machiningover current bottom up SFF technology. The surfaceroughness of the SFF fabricated ceramics is at least oneorder of magnitude higher than the one presented in thecurrent work.

Conclusions

Machining study has been carried out for three dif-ferent alumina green ceramics prepared from PCC, gel-casting, and gel-forming methods. Different tool mate-rials are used for surface machining and it is concludedthat only the diamond-coated tool can be used success-fully for CNC machining of the green ceramics withoutsignificant damage of the tool. The use of the diamond-coated flat-tool resulted in a good surface finish of theas-machined surface in comparison to the rounded tool.The surface roughness of the machined surface reducedsignificantly as a result of sintering. The gel-formedsample shows the smoothest surface when using a dia-mond-coated flat tool compared to the PCC and gel-cast samples. It also has the highest green strength, andit is thus suggested that gel forming and green machin-ing can be used for the microfabrication of net shapeceramics. The PCC and gel-casting processing in com-bination with green machining can be used for rapidprototyping of ceramics, because of their low bindercontents and high green strength.

Acknowledgments

The authors would like to thank Mr. Carl Meggsfor his kind technical assistance throughout the work.

Fig. 6. The surface roughness profile of the (a) gel-cast and (b) gel-formed as-machined green alumina using 3-mm diameterdiamond-coated flat tool along the direction of machining.


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Green Machining to Net Shape Alumina Ceramics Prepared Using Different Processing Routes

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