Since the late 90s the use of zirconia based all-ceramic restorations increases. Many manufacturing steps are necessary, like pre-sintering, soft machining (pre-sintered condition), sintering and hard machining (fully sintered) in combination with a final staining or veneering step. All these techniques, especially hard machining, are associated with the production of flaws in different scales, in conjunction with thermal and residual stresses and phase transformations. These are inter alia capable to induce failure. This work investigates the impact of hard machining on the material properties and attempts to establish a correlation to failure. © 2016 Published by Elsevier B.V

Breidenstein, Bernd

Busemann, Sarah

Denkena, Berend

Lehr, Claudius Moritz

Institutionelles Repositorium der Leibniz Universität Hannover

Available online at www.sciencedirect.com2212-8271 © 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the scientific committee of the 3rd CIRP Conference on BioManufacturing 2017doi: 10.1016/j.procir.2017.04.055  Procedia CIRP  65 ( 2017 )  248 – 252 ScienceDirect3rd CIRP Conference on BioManufacturing Impact of hard machining on zirconia based ceramics for dental applications  Berend Denkenaa, Bernd Breidensteina, Sarah Busemanna,*, Claudius Moritz Lehrb  aInstitute of Production Engineering and Machine Tools (IFW), Leibniz Universität Hannover, An der Universität 2, 30823 Garbsen, Germany bDepartment of Prosthetic and Biomedical Materials Science, Hannover Medical School, Carl-Neuberg-Str. 1, 30625 Hannover, Germany  * Corresponding author. Tel.: +49-511-762-18279; fax: +49-511-762-5115. E-mail address: busemann@ifw.uni-hannover.de Abstract Since the late 90s the use of zirconia based all-ceramic restorations increases. Many manufacturing steps are necessary, like pre-sintering, soft machining (pre-sintered condition), sintering and hard machining (fully sintered) in combination with a final staining or veneering step. All these techniques, especially hard machining, are associated with the production of flaws in different scales, in conjunction with thermal and residual stresses and phase transformations. These are inter alia capable to induce failure.  This work investigates the impact of hard machining on the material properties and attempts to establish a correlation to failure. © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 3rd CIRP Conference on BioManufacturing 2017.  Keywords: ceramic; zirconia, hard machining; surface and subsurface properties  1. Introduction The use of zirconia based ceramics for dental applications increased since the late 90s because of their superior mechanical as well as aesthetic properties [1]. The techniques for manufacturing zirconia based all-ceramic dental restorations are in a continuous state of development, modern dentistry is not imaginable without computer-aided-design/computer-aided-manufacturing (CAD/CAM) any more [2]. Nevertheless, manual process steps are still necessary in the technical process chains. Most systems machine partially sintered blanks, this includes designing an enlarged geometry and milling it from a partially sintered zirconia blank. The geometry has a linear shrinkage of 20-25% during sintering until it reaches its final dimensions [3]. Due to the shrinkage, there are sintering distortions, the object has to be reprocessed and adapted, the fully sintered zirconia has to be finished by grinding. This so called hard machining results in complex influences on the near-surface material layers, it can have an effect in two different directions: On the one hand, surface near compressive residual stresses can be induced as a consequence of the plastic deformation and due to tetragonal to monoclinic phase transformation. These stresses can increase the flexural strength. The individual mechanisms are not sufficiently known yet, but many investigations show that mechanical as well as thermal loads form residual stresses [4, 5]. On the other hand, grinding can introduce surface flaws [6, 7]. These can act as stress concentrators and decrease strength, if they are larger than the thickness of the grinding induced compressive residual stress layer [8, 9]. For the machining of fully sintered zirconia considerable time is required. Economic aspects of the machining process as well as the resulting product quality play a decisive role for the application of zirconia. Therefore, an efficient grinding process which results in required material properties like roughness, plastic deformation, damage and residual stresses is necessary. The mechanical properties of zirconia are depending not at least on the ability of phase transformation. For example machining induced cracks up to a critical size can be stopped by transformation from the tetragonal to the monoclinic phase. This effect is based upon the increase of the unit cell size during phase transformation, which induces compressive residual stresses and thus hinders the cracks from propagation [1, 6, 10, 11]. Phase transformation is influenced by the material removal mechanism, which can be more ductile or brittle [12, 13]. A scale for this is the critical uncut chip thickness hcu,crit. The aim of this work is to determine  2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the scientifi c committee of the 3rd CIRP Conference on BioManufacturing 2017249 Berend Denkena et al. /  Procedia CIRP  65 ( 2017 )  248 – 252 the influence of various grinding process parameters on the material properties roughness, residual stresses and phase transformation in grinding zirconia. 2. Experimental conditions 2.1. Experimental setup For these investigations the grinding process was carried out as a standard face grinding process. The experiments are conducted on a 5-axis CNC machine tool (Röders RFM 600 DS). Cutting speed vc, depth of cut ae and abrasive grain size dg are varied, the feed rate f is constant. The parameters are summarized in Table 1. A wet grinding process is applied, as coolant a 5% oil-in-water emulsion is used. The specimens are fixed with dental sticky wax onto a workpiece carrier (see Figure 1). Before the main experiments can be performed, as preparatory work the specimens have to be face ground onto the same level. This is necessary to generate comparable results without an effect of the manual manufacturing of specimens.  Table 1. Parameters – face grinding Process parameter  Cutting speed vc = 10, 30 m/s Depth of cut ae = 0.005, 0.1 mm Feed rate vf = 800 mm/min Specific material removal rate Q’w = 0.07; 1.33 mm³/s·mm  The grinding tool topography is measured by means of an SEM. After grinding surface roughness, monoclinic phase content and resulting residual stresses are measured. Surface roughness of the ground specimens is measured in a tactile way by a stylus instrument (Perthometer Concept) with the parameters given in Table 2.  Table 2. Parameters for roughness measurement. Parameter D15, D251 Cut off length  Oc = 0.8; 2.5 mm Traversing length ltl = 4; 7.5 mm  Residual stresses are measured applying the sin²\method using a GE XRD 3003 ETA diffractometer with a 2 mm collimator and Co KDradiation with 30 kV and 40 mA with a maximum penetration depth of W= 3.1 µm. Residual stresses are measured parallel and transverse to the grinding direction. The quantity of the monoclinic phase is measured in the surface near region to a maximum depth of 1.5 µm.  2.2. Grinding tools For this investigation grinding wheels with 1A1-geometry and natural diamond as abrasive grain with an average grain size of dg = 15 µm and dg = 251 µm (Figure 1) and electroplated bonding are used. The tool properties are summarized in Table 3. Usually resin bond grinding tools are used for grinding zirconia components due to their higher flexibility and porosity. But for small components like dental crowns and bridges, it is technically unconvertible, so state of the art in dental manufacturing are electroplated grinding tools [2].   Figure 1. Tool topography and experimental setup. Table 3. Tool properties. Diameter 40 mm Geometry 1A1, cylindrical Abrasive natural diamond Grain size D15, D251 Bonding electroplated 2.3. Material The material used in this investigation is a commercially available zirconia based dental ceramic. The tetragonal phase is doped with 4.5 - 6 wt.% yttrium oxide (Y2O3). The mechanical properties at room temperature for the dense material are shown in Table 4. Table 4. ZrO2 properties  Mechanical properties   Density g/cm³ > 6.04 Flexural strength MPa 1.500 Compressive strength MPa 3.000 Young’s modulus E MPa 205.000 Fracture toughness KIC MPa m0.5 8 Hardness H HV 0.5 1.300  The material comes as a pre-sintered blank. The needed specimens are separated in an enlarged geometry by a precision table saw and then sintered densely. The specimens are heated up to a desired temperature of 1450 °C with a rate of 8 °C/min. The desired temperature is hold for 2:00 h and then cooled down to room temperature with a cooling rate of 8 °C/min. The parameters are summarized in Table 5. The ready-for-use specimens have a size of 30 mm x 15 mm x 1.5 mm. The microstructure of the dense material has an average grain size of 0.4 µm. 250   Berend Denkena et al. /  Procedia CIRP  65 ( 2017 )  248 – 252 Table 5. Sintering conditions.  step initial temper-ature [°C] desired temper-ature [°C] heating/ cooling rate [°C/min] holding time [h:min] 1 20 1450 8 3:00 2 1450 1450 0 2:00 3 1450 20 8 3:00 3. Experimental results and discussion The uncut chip thickness is varied by changing grain size, grinding speed and depth of cut in order to study the material removal mechanisms when grinding zirconia and its influence on roughness, residual stresses and phase transformation. The critical uncut chip thickness hcu,crit depending on the material properties, has a calculated value of 0.895 µm, according to (1) [12, 14] with < >@and the material properties given in Table 1: 2ICcrit,cu HKHEh ¹¸·©¨§¹¸·©¨§ \   (1) The uncut chip thickness hcu  is calculated according to [15]:  31331¸¸¹·¨¨©§  geGVcfcu laNvvh O             (2) The parameters O(form factor of grain) and NGV (amount of grains per cm³) are measured based on the surface topography of the grinding wheel before the first tests (Figure 1). The form factor of grain is defined as shown in (3), the value for NG (cutting edge aperture angle) is assumed with 150 °.  ¹¸·©¨§ 2tan GNO   (3) For the amount of grains per cm³ (NGV), the grains in the top view of a SEM picture are counted for a an area of a square centimetre and then extrapolated over the grain size to the volume. For the tool with a grain size of 15 µm, NGV has an amount of 10133333/ cm³, for the tool with a grain size of 251 µm, NGV has an amount of 47809/ cm³. The wear caused by the grinding process is not taken into account although the grinding tool conditions change during the process. So the calculated uncut chip thickness shows the initial values and a trend for the absolute ones. In Figure 2 and Figure 3 SEM pictures of ground surfaces are shown. The calculated chip thickness for the surface in Figure 3 is 0.42 µm, thus it is smaller than the calculated critical uncut chip thickness of 0.895 µm. The result is an even surface with evenly distributed grinding marks, the ceramic is removed in a more ductile mode. In Figure 2 the calculated uncut chip thickness is 2.52 µm, so it is higher than the calculated critical    Figure 2. Surface topography of ground surface with hcu = 2.52 µm. uncut chip thickness. The surface shows brittle intercrystalline breakouts, high roughness and bulging at the scratch edges. The roughness increases with uncut chip thickness. Nevertheless, also this surface shows areas, where the material is removed in a more ductile way. The grinding direction can be seen in both figures. The surfaces consist mostly of a series of parallel grinding marks, the width of these marks increases with increa- sing abrasive grain size. In both figures, the specific material removal rate Q’w has a value of 0.07 mm³/s·mm. This shows that at a same productivity level the surface topography can differ in a wide range. -  Figure 3. Surface topography of ground surface with hcu = 0.42 µm. In Figure 4 and Figure 5 the average surface roughness Rz, the maximum surface roughness Rmax and the peak height Rp are depicted. The roughness is measured transverse to the grinding direction. Figure 4 shows the values for dg = 15 µm and the maximum value is about 3 µm for an uncut chip thickness of 1 µm. When the uncut chip thickness is higher than 0.7 µm roughness increases.  251 Berend Denkena et al. /  Procedia CIRP  65 ( 2017 )  248 – 252  Figure 4. Roughness depending on uncut chip thickness, dg = 15 µm Figure 5 shows the values for dg = 251 µm and the maximum value is about 34.5 µm for an uncut chip thickness of 2.4 µm. The overall surface roughness is 10 times higher than the surfaces ground with dg = 15 µm. If the graphs of Rz and Rmax are compared a trend can be suspected: When the graphs for the maximum and average roughness have a similar development (Figure 4) the generated surfaces are even as well (Figure 3). When the graph development is uneven (Figure 5) it indicates a surface with breakouts and an uneven distribution of the grinding marks (Figure 2). In both figures, Rp stays almost constant. The ratio Rp / Rz is an indicator for the surface profile shape. A value smaller than 0.5 describes a round surface profile shape and a value higher than 0.5 a sharp one [16]. For Figure 4 and Figure 5 the Rp / Rz ratio has a range from 0.46 to 0.52, so there is no significant difference.  In Figure 6 the measured values of the residual stresses corresponding to the calculated uncut chip thickness are shown. The residual stresses for the as-fired specimens in both directions (transverse and parallel) have a value of -3 to -9 MPa, so they are, in consideration of the measurement error, “stress-free”. All near-surface measured stresses are compressive residual stresses. Up to a chip thickness of 1 µm the residual stresses show a slight decreasing trend. With further increasing uncut chip thickness the residual stresses are increasing as well, this is in agreement with [4], showing   Figure 5. Roughness depending on uncut chip thickness, dg = 251 µm  Figure 6. Residual stresses versus calculated uncut chip thickness increasing compressive residual stresses with increasing uncut chip thickness. Compressive residual stresses in subsurface area are increasing with increasing surface roughness, this is coupled with increasing amounts of plastic deformation. The stresses transverse to grinding direction are up to 200 MPa stronger than the stresses parallel to grinding direction. As a reason for this strong plastic material deformation due to lateral displacement by the cutting abrasive grain size is assumed. It should be considered that the surface roughness for the specimens with high compressive residual stresses is higher than the penetration depth of the diffraction radiation, so only subsurface stresses are measured, the stresses at the grinding marks’ ground are not taken into account. In Figure 7 the results for the residual stresses and the monoclinic phase depending on the calculated uncut chip thickness for dg = 15 µm are shown. The amount of monoclinic phase was calculated by the levels of intensity according to [17]:      111111111111111tmmmmmIIIIIx¹¸·©¨§¹¸·©¨§   (4) I is for the integral intensity at 2Tt is for the tetragonal peak and m for the main peak of the monoclinic phase. The amount of monoclinic phase shows a small increase up to an uncut chip thickness of 0.7 µm with decreasing residual stress. For an uncut chip thickness of 1 µm the amount decreases, but the values are in a range from 3.1 to 5.2 %, so considering the measured values referring to the measurement error, there is almost no difference. 252   Berend Denkena et al. /  Procedia CIRP  65 ( 2017 )  248 – 252  Figure 7. Residual stresses and monoclinic phase content for dg = 15 µm. 4. Conclusion and outlook This paper shows, that varying the uncut chip thickness plays an essential role in hard machining zirconia based ceramics. The surface properties like e. g.  residual stresses and roughness are depending on the process- and tool-properties, as well as the given material properties themselves. The abrasive grain size has a leading impact on the surface layer properties. For the analyzed material, compressive residual stresses are obtained with calculated uncut chip thicknesses up to 6 µm. Due to the high surface roughness it was not possible to define the amount of the monoclinic phase for the rough surfaces (hcu > 1 µm) reliably, this has to be revised with other parameters. With uncut chip thickness up to 1 µm, however, there are amounts of monoclinic phase up to 5 %. This is absolutely within ISO 13356, which  demands an amount below 20 % [18].  For future work it is necessary to find a possibility to measure the monoclinic phase content for very rough surfaces. Besides, it would be instructive to test the flexural strength as an indicator for inner damage and whether the defects are introduced by hard machining (e. g. cracks) or sintering (e. g. powder agglomerates, pores).  Acknowledgements The authors would like to thank the German Research Foundation (DFG) for its support of the project “Investigation of bonding mechanisms between scaffolding and veneering materials”.  References [1] Chevalier J, Gremillard L, Virkar AV, Clarke DR. 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Impact of Hard Machining on Zirconia Based Ceramics for Dental Applications

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