Machinability of IPS EmpressT 2 Framework Ceramic: Potentialfor Ceramic Dentures

Christian Schmidt,1 Paul Weigl2

1Fraunhofer—Institute for Production Technology, Department Process Technology, Steinbachstr. 17, D-52074 Aachen,Germany

2Clinical Centre of Johann Wolgang Goethe–University, Department of Prosthodontics, Theodor-Stern-Kai 7, house 29,D-60590 Frankfurt am Main, Germany

Received 3 November 1999; revised 10 February 2000; accepted 16 February 2000

Abstract: Using ceramic materials for an automatic production of ceramic dentures byCAD/CAM is a challenge, because many technological, medical, and optical demands must beconsidered. The IPS Empresst 2 framework ceramic meets most of them. This study shows thepossibilities for machining this ceramic with economical parameters. The long life-timerequirement for ceramic dentures requires a ductile machined surface to avoid the well-known subsurface damages of brittle materials caused by machining. Slow and rapid damagepropagation begins at break outs and cracks, and limits life-time significantly. Therefore,ductile machined surfaces are an important demand for machine dental ceramics. Themachining tests were performed with various parameters such as tool grain size and feedspeed. Denture ceramics were machined by jig grinding on a 5-axis CNC milling machine(Maho HGF 500) with a high-speed spindle up to 120,000 rpm. The results of the wear testindicate low tool wear. With one tool, you can machine eight occlusal surfaces includingroughing and finishing. One occlusal surface takes about 60 min machining time. Recom-mended parameters for roughing are middle diamond grain size (D107), cutting speedvc 5 4.7m/s, feed speedvft 5 1000 mm/min, depth of cutae 5 0.06 mm, width of contactap 5 0.8 mm,and for finishing ultra fine diamond grain size (D46), cutting speedvc 5 4.7 m/s, feed speedvft 5 100 mm/min, depth of cutae 5 0.02 mm, width of contactap 5 0.8 mm. The results ofthe machining tests give a reference for using IPS Empresst 2 framework ceramic inCAD/CAM systems.© 2000 John Wiley & Sons, Inc. J Biomed Mater Res (Appl Biomater) 53: 348–352,2000

Keywords: IPS Empresst 2 framework ceramic; dental ceramic; machining; grinding;ductile behavior

INTRODUCTION

A new framework ceramic of the IPS Empresst System(Ivoclar AG, Bendererstrasse 2, FL-9494 Schaan, Liechten-stein, http://www.ivoclar.com) was introduced in November1999, named IPS Empresst 2. It is based on a SiO2-Li2Osystem and shows improved mechanical properties, accord-ing the manufacturer, especially a high mean fracture tough-ness (KIC 53.5 MPa m1/2) and flexural strength (350–450MPa). The aim of the study was to evaluate the machinabilityof this ceramic, though it was designed for heat-press form-ing. The machining tests were focused on parameter variationin relation to the surface quality and the tool wear.

MATERIALS AND METHODS

Samples of IPS Empresst 2 framework ceramic ingots andVita Mark II (VITA Zahnfabrik H. Rauter GmbH & Co.KG, Spitalgasse 3, Postfach 1338, D-79704 Bad Sa¨ckin-gen, Germany, http://www.vita-zahnfabrik.de) ceramic(control group,KIC 5 1.7 Pa m1/2, sB 5 135 MPa) wereused for the machining tests. The Empresst 2 ceramic,based on a SiO2-Li 2O system, shows improved mechanicalproperties, according to the manufacturer, especially ahigh mean fracture toughness (KIC 5 3.5 Pa m1/2) andflexural strength (sB 5 350 – 450 MPa). In Figure 1, anetched surface and a polished surface are shown. Etchingremoves the glass matrix, so a deeper look into the materialis possible. The columnar structure of LiO2 crystals givesthe material strength.

The samples were ground with ball-shaped diamond graintools with ultra fine (D46), fine (D76), and middle grain size(D107) on a 5-axis CNC milling machine (Maho HGF 500,

Correspondence to:Christian Schmidt, Fraunhofer—Institute for Production Tech-nology, Department Process Technology, Steinbachstr. 17, D-52074 Aachen, Germany(e-mail: c.schmidt@ipt.fhg.de)

© 2000 John Wiley & Sons, Inc.

348

Gildemeister AG, Gildemeisterstrasse 60, D-33689 Bielefeld,Germany, http://www.gildemeister.com) with a high-speedspindle up to 120.000 rpm; see Figure 2. The machine wasused as a jig grinding machine.

Machining parameters were varied during the tests (Fig.3). For wear tests, real denture forms were produced. SEMimages were used to evaluate the surfaces. The subsurfacedamage indicates the surface quality and was measured bycross-sections.

RESULTS

Decreasing the diamond grit improves the quality of theEmpresst 2 surface. With up to a feed rate of 1500 mm/min,the ultra fine grain size (D46) realized a ductile cuttingbehavior. A ductile behavior is recommended for a longlife-time for ceramic dentures. In contrast, the Vita Mark IIceramic revealed highly significant differences. Figure 4shows the results of cross-section investigations, which showthe crack depth into the surface. Vita Mark II shows overallcrack damages, indicating brittle material removal. In con-trast, for IPS Empresst 2, no crack damages were detected(tool D76 and tool D46). The subsurface damage with toolD107 is minor and can be removed using a finishing process.Little and/or missing crack damages on the surface are evi-dent for ductile material removal. Furthermore, clearly visibletool tracks are a simple sign of this cutting behavior.

In Figure 5, the surfaces machined by the same parametersare shown. The best quality surfaces are on the left side. Onthe top is Empresst 2 with ductile tool tracks; on thebottom is theVita Mark II with its typical brittle surface.Brittle breakouts and cracks are reasons for total damage instress. The Vita Mark II surfaces show no significant differ-ences for a complete variation feed rate (vft 5 100 . . . 2000mm/min). No ductile cutting behavior is visible. You cannot

Figure 1. EmpressT 2 surfaces: (left) polished, (right) etched (magnification: 100003).

Figure 2. Grinding process.

Figure 3. Test parameters for machining.

349MACHINABILITY OF IPS EMPRESSt 2 FRAMEWORK CERAMIC

Figure 4. Subsurface damage of IPS EmpressT 2 and Vita Mark II.

Figure 5. Comparison with surfaces made of IPS EmpressT 2 and Vita Mark II.

350 SCHMIDT AND WEIGL

see any tracks of the grinding tool. You can observe only amix of cracks, split-offs, and surface breakouts. The testedmachining parameters were too strong for the material. TheVita Mark II is a glass ceramic, and the tested tool grain sizewas too rough. Glass materials are very brittle. A ductilecutting behavior is possible, but not with economic parame-ters. For the Empresst 2 surface you can see narrow scaleswithout any breakouts on the left side. On the top right sideare smeared tool tracks with small split-offs. But the 2000mm/min feed rate is a fast machining parameter, whichstresses the surface more than lower feed rates such as 100mm/min. Therefore, surface roughness increases with in-creased feed rate. A feed rate of 2000 mm/min is for roughmachining, and typical characteristics for a ductile cuttingbehavior are still evident. This behavior is possible, becausethe fracture toughness is much higher than for Vita Mark II.The matrix of SEM-pictures in Figure 6 shows the full rangeof tested parameters.

The material shows different machining behavior. Foreach grain size, the surfaces are finer with decreasing feedrate. For a high feed rate (more than 1000 mm/min and agrain size over D46) the surfaces have more brittle breakouts.The complete parameter variation for ultra fine grain size D46shows tracks of diamond grains. For grain size D46, tooltracks indicate a ductile cutting behavior showing a widerange of shapes from big scales on the left to small sickles onthe right. The scale forms are created by an overlaid transla-tion (feed) and rotation of the grinding tool. If you raise thefeed rate, the scales are stretched. For higher grain sizes, partsof ductile cutting behavior decrease. The number of split-offsincreases, especially for a feed rate of 2000 mm/min. Theparameter field grain size of D76, D107, and feed rate of1500–2500 mm/min are too strong, causing increased surfacedamage. These parameters are not recommended.

The results of the wear tests indicate low tool wear.Various tool specifications were tested. The removed materialvolume was measured until the end of life. A ball-shapedgeometric tool with a grain size of D107 (middle grain size)showed the best performance in this test (Fig. 7). With suchtools you can machine eight occlusal surfaces for completemachining (roughing and finishing); see Figure 8. One piecetakes nearly 30 min for roughing and about 30 min forfinishing. The high number of pieces and the low machiningtime represents low cost. Up to now, the calculation of oneocclusal surface was by nearly 100 USD including equipment(5 year write-off time), personnel, scanning, software, mate-rial, and tools. For a complete tooth, the side must be ma-chined. This takes 2 h more machining time and about 50USD additional.

Figure 6. Surface matrix of tested parameters.

Figure 7. “End-of-life” test with various tool specifications.

351MACHINABILITY OF IPS EMPRESSt 2 FRAMEWORK CERAMIC

DISCUSSION

Many technological demands on ceramic material are re-quired for automatic production of ceramic dentures by CAD/CAM. Technical points are mechanical properties, machiningprocess, surface quality, subsurface damage, and economy(tool wear). For use in CAD/CAM systems, the IPS Em-presst 2 framework ceramic meets most of these require-ments. For finishing parameters, brittle surface defects were

not measurable and the tool wear was less. The IPS Empresst2 framework ceramic shows an excellent machinability com-pared to Vita Mark II ceramic. The ductile behavior duringthe grinding process could by compared with the behavior oftransformable zirconia, but without high tool wear. Recom-mended parameters for roughing are middle diamond grainsize (D107), cutting speedvc 5 4.7 m/s, feed speedvft 51000 mm/min, depth of cutae 5 0.06 mm, width of contactap 5 0.8 mm, and for finishing ultra fine diamond grain size(D46), cutting speedvc 5 4.7 m/s, feed speedvft 5 100mm/min, depth of cutae 5 0.02 mm, width of contactap 50.8 mm.

REFERENCES

1. Ivoclar AG. Scientific documentation: IPS Empresst 2. Schaan,Liechtenstein: Scientific Service; 1999.

2. Jahanmir S, Ramulu M, Koshy P. Machining of ceramic andcomposites. New York: Dekker; 1999.

3. Spur G. Keramikbearbeitung. Mu¨nchen: Hanser–Verlag, 1989.4. Konig W, Klocke F. Fertigungsverfahren Band 2: Schleifen,

Honen, Lappen. Dusseldorf: VDI–Verlag; 19965. Tonshoff HK, Peters J, Inasaki I, Paul T. Modeling and simula-

tion of grinding processes. Annals CIRP 1992;41:677–688.6. Kriegesmann J. Technische keramische Werkstoffe. Ko¨ln: Deut-

scher Wirtschaftsdienst; 1999.7. Beitz W, Kuttner KH. Dubbel—Handbook for engineering. New

York: Springer; 1995.

Figure 8. Eight occlusal surfaces made of IPS EmpressT 2 with onetool.

352 SCHMIDT AND WEIGL