T A C a b c a A R R A A K U A D S N 1 h r c a e t p s i b ( ( h 0 Journal of Materials Processing Technology 231 (2016) 336–356 Contents lists available at ScienceDirect Journal of Materials Processing Technology jo ur nal home p ag e: www.elsev ier .com/ locate / jmatprotec he ultra-precision Ud-lap grinding of flat advanced ceramics rthur Alves Fiocchia,∗, Luiz Eduardo de Angelo Sanchezb, Paulo Noronha Lisboa-Filhoc, arlos Alberto Fortulana University of São Paulo—USP, Department of Mechanical Engineering, Av. Trabalhador Saocarlense 400, 13566-590 São Carlos, SP, Brazil São Paulo State University—Unesp, Department of Mechanical Engineering, Av. Luiz Edmundo Carrijo Coube, 14-01, 17033-360, Bauru, SP, Brazil São Paulo State University—Unesp, Department of Physics, Av. Luiz Edmundo Carrijo Coube 14-01, 17033-360, Bauru, SP, Brazil r t i c l e i n f o rticle history: eceived 10 April 2015 eceived in revised form 1 October 2015 ccepted 4 October 2015 vailable online 23 October 2015 eywords: d-lap grinding dvanced ceramics uctile grinding urface characterization anotechnology a b s t r a c t The present study focuses on the Ud-lap grinding process and its machine-tool design aiming at ultra- precision (UP) manufacturing of advanced ceramics. Impacts of three different overlapping factors on a dressing (Ud) and three abrasive grit sizes of conventional SiC grinding wheels were analyzed on flat nano- metric surface finishing of dense discs of 3Y-TZP in a ductile regime of material removal. Microhardness, contact and optical profilometry, SEM-FEG, Raman spectroscopy, XRD, and confocal epi-fluorescence were applied to characterize the ceramic material. Mechanical and electric-electronic designs of the machine-tool were developed toward the UP ceramic grinding. The design methodology was successful for supporting the achievement of all design requirements of the CNC Fiocchi Lap Grinder. The machine- tool and the Ud-lap grinding process were capable of manufacturing flat 3Y-TZP surfaces with nanometric finishing without introducing critical defects. The best finishing, Ra = 60.63 nm, came from the #300 grind- ing wheel dressed with Ud = 5. A flatness deviation of 0.308 �m was obtained through the #800 grinding wheel and Ud = 3. Differences between theoretical and real macro and micro effects over the grinding wheels after single-point diamond dressing, epoxy bond strength, abrasive protrusion, abrasive grit size and abrasive friability play a key role in the Ud-lap grinding. There is no report of an abrasive process capable of achieving similar nanometric finishing without introducing critical defects with the same micrometric grit size and type of abrasive. The Ud-lap grinding can replace the engagement of processes such as grinding, lapping, and polishing of advanced ceramics. © 2016 Published by Elsevier B.V. . Introduction Compared with metals, advanced ceramics are materials of igher hardness and special properties that present performance, eliability, and safety at elevated temperatures; these are out- omes of their exceptional chemical stability due to strong ionic nd/or covalent atomic bonds (Boch and Nièpce, 2007). How- ver, advanced ceramics suffer from their relatively low fracture oughness, which also significantly affects their cost as a result of ost-sintering processing normally done by expensive superabra- ive tools and manufacturing processes (Janssen et al., 2008). Except for glass, plastic molding and casting of advanced ceram- cs are exceptionally high cost, mainly because of the chemical onds that raise the melting point. Brittleness is also a significant ∗ Corresponding author. E-mail addresses: arthuraf@eesc.usp.br A.A. Fiocchi), sanchez@feb.unesp.br (L.E. de Angelo Sanchez), plisboa@fc.unesp.br P.N. Lisboa-Filho), cfortula@sc.usp.br (C.A. Fortulan). ttp://dx.doi.org/10.1016/j.jmatprotec.2015.10.003 924-0136/© 2016 Published by Elsevier B.V. characteristic of these ceramics below 1000 ◦C. In addition to lim- iting their use, the brittleness imposes restrictions on production techniques. All these limitations of ceramic manufacturing explain why the basic technique of densification is the sintering of com- pacted powder (Boch and Nièpce, 2007). This opposition is quite evident between metals (ductile) and ceramics (fragile). As ceramics are more brittle, the main mechanical failure usu- ally occurs by crack propagation (Argawal and Rao, 2008). Initial critical cracks, precursors of the failure, are defects usually origi- nating in irregularities acquired in the early stages of obtaining the piece (from powder to sintering) (Bukvic et al., 2012) and/or during its post-sintering processing (hard ceramic machining) (Marinescu et al., 2014), hence the design of a product of advanced ceramic needs special care in each phase of its manufacturing. In general lines, in the production of a ceramic artifact, the raw materials (powders) are milled or synthesized, are mixed, and receive additives such as binders and plasticizers to assist the con- formation, compaction, and sintering (Scheller, 1994). The ceramic powder is formed into compacted pieces, also called green parts due to their not heat treated state, which are subsequently sintered to dx.doi.org/10.1016/j.jmatprotec.2015.10.003 http://www.sciencedirect.com/science/journal/09240136 http://www.elsevier.com/locate/jmatprotec http://crossmark.crossref.org/dialog/?doi=10.1016/j.jmatprotec.2015.10.003&domain=pdf mailto:arthuraf@eesc.usp.br mailto:sanchez@feb.unesp.br mailto:plisboa@fc.unesp.br mailto:cfortula@sc.usp.br dx.doi.org/10.1016/j.jmatprotec.2015.10.003 A.A. Fiocchi et al. / Journal of Materials Processing Technology 231 (2016) 336–356 337 Nomenclature ad Depth of dressing (mm) AE Acoustic emission AISI American Iron and Steel Institute ASTM American Society for Testing and Materials bd Dressing width CNC Computer numerical control Cps Counts per second E Elastic modulus (GPa) EESC São Carlos School of Engineering ELID Electrolytic-in process dressing FAPESP São Paulo Research Foundation FEB Bauru College of Engineering HFW Horizontal field width (mm) HV Hardness vickers HV High voltage (kV) IFSC São Carlos Institute of Physics k Parameter for monoclinic phase quantification by Raman spectroscopy KIC Fracture toughness (MPa.m0.5) LATUS Laboratory of Machining Technology ln Total measured length (mm) LTC Laboratory of Tribology and Composite lr Single measured length (mm) m Monoclinic phase MEMS Micro-electromechanical systems mf Final mass (g) mi Initial mass (g) MQF Minimum quantity fluid MRR Material removal rate (g/minute) NEMS Nano-electromechanical systems ng Grinding wheel rotational speed (rpm) nw Workpiece rotational speed (rpm) P Ud-lap grinding pressure (kPa) PABA 4-Aminobenzoic acid PVB Polyvinyl-butyral PVD Physical vapor deposition Ra Arithmetic mean surface roughness (�m) RMS Root mean square rp Dresser nose radius geometry (mm) Rt Total height of the roughness profile (�m) r Stylus tip radius (�m) Sa Arithmetic mean height over the complete 3D sur- face (�m) Sd Dressing feed SE Secondary electron detection SEM-FEG Scanning electron microscope equipped with field emission gun SiC Silicon carbide Sku Kurtosis of the 3D surface texture Sp Max peak height over the complete 3D surface (�m) Sq Root mean square height evaluated over the com- plete 3D surface (�m) Ssk Skewness of the 3D surface texture Sv Maximum valley depth over the complete 3D sur- face (�m) Sz Maximum height over the complete 3D surface (�m) T Tetragonal phase UP Ultra-precision USP University of São Paulo Vcrit Critical velocity of the jar Vm Volume of monoclinic phase (%) Vm max Maximum volume of monoclinic phase (%) Vm min Minimum volume of monoclinic phase (%) XRD X-ray diffraction WD Working distance (mm) W Real waviness (�m) Y-TZP Tetragonal zirconia polycrystals stabilized with yttria ZrO2 Zirconium oxide or zirconia 3Y-TZP Tetragonal zirconia polycrystals stabilized with 3 mol% of yttria � Single abrasive relief angle (◦) � Single abrasive wedge angle (◦) � Single abrasive rake angle (◦) � Parameter for monoclinic phase quantification by Raman spectroscopy � Diffraction angle (◦) �c Cutoff (mm) Density (g/cm3) Bending strength (MPa) Ø Diameter (mm) # Mesh number of the grinding wheel (abrasive grit TZ-3Y-E Tosoh zirconia grade t → m Martensitic transformation Ud Overlap factor or overlap ratio on dressing UNESP Universidade Estadual Paulista size) achieve the desired hardness and density (Richerson, 2006). Each step has the potential to produce undesirable microstructural flaws in the piece, which in itself may limit their properties, reliability, and machinability. Scientific investigations are not exclusively focused on the material itself, but also in its processing into powder, granules, and in the subsequent stages of obtaining a part as well as its machining. For less demanding applications regarding form and finish, the sintering is the last stage of the ceramic processing. Never- theless, several areas of science and technological applications require better finishing, tighter dimensional and geometric toler- ances, and surfaces completely free of damage (Brinksmeier et al., 2010). Such parts must undergo, consequently, further process- ing steps after sintering, especially to confer on them complex geometries and finer finishing. The demands for better surface finishing and high accuracy of optical, electrical, and mechanical components are growing along with the miniaturization of high performance products, such as lenses, electronic components of computers and smartphones, and sensors, as well as micro and nano-electromechanical systems (MEMS and NEMS). Among the available methods of material removal, the abrasive machining is the most widespread in industry. Grinding reconciles stock removal, form correction, and surface quality for large- scale production in an economically feasible way (Malkin, 1989). Although widespread, hard ceramic machining is an extremely difficult task due to the exceptional ceramic properties in the sin- tered state, especially: high hardness, high elastic modulus, high melting point, and fragility (Callister and Rethwisch, 2013). These require superabrasive cutting tools, stiff machine-tools and clamp- ing devices, micro and nanometer depths of cut, and a highly skilled workforce (Brinksmeier et al., 2010). In traditional abrasive technology, the brittle mode is the pri- mary removal mechanism (Marinescu et al., 2004) caused by the propagation of lateral cracks that promote chipping (Xu and Jahanmir, 1995). Higher removal rates can be achieved in the brittle mode in opposition to ductile abrasive cutting (Xu et al., 1995). On 3 Proce t c a c 2 i d ( b t s ( s o i h t d t a 2 o t t s a t a d t S t a i ( a t m e B f m F m S o ( C n w t c 2 p n d m m 38 A.A. Fiocchi et al. / Journal of Materials he other hand, grinding may also generate longitudinal cracks that an remain in the product after grinding (Marinescu et al., 2007) nd act, for example, as stress concentrators that under specific onditions stimulate subcritical crack propagation (Deville et al., 006). The main technological challenge of hard ceramic machining s, thus, to promote material removal without introducing critical efects that will compromise the performance of the component Marinescu et al., 2014). Therefore, the brittle removal mode should e avoided in ultra-precision (UP) grinding or minimized in order o avoid mainly the formation and propagation of surface and sub- urface cracks, microstructural changes, and residual tensile stress Brinksmeier et al., 2010). To minimize the emergence and evolution of cracks, machining hould be conducted with discretion, so as to avoid the introduction f residual tensile stress on surfaces of materials subject to stress- nduced phase transformation (Deville et al., 2006). The search for igh-tech ceramic parts with functional surfaces requires, hence, he most sophisticated manufacturing processes. Over the last three decades, researchers have responded to emands of industry for machine-tools and fabrication processes hat aim to attain high-precision parts in a nanometer (10−9 m) and ngstrom (10−10 m) for both dimensional and finish range (Ohmori, 011). Nevertheless, these quality characteristics are very difficult, r economically unviable, to achieve by concatenating the tradi- ional abrasive processes such as grinding, lapping, and polishing. For finishing flat surfaces, researchers have been putting ogether the main advantages of the traditional abrasive processes uch as: face grinding with constant pressure, fixed abrasives for two-body removal mechanism, total contact of the part with he tool, and lapping kinematics as well as some specific oper- tions to keep grinding wheel sharpness and form. In this way, ifferent dressing and conditioning techniques have been applied o the tool before and/or during grinding processes. According to anchez et al. (2011), these different dressing procedures yield dis- inguished results in grinding performance. Nanogrinding (Gatzen nd Maetzig, 1997), face grinding with lapping kinematics or grind- ng on a lapping machine (Tomita and Eda, 1996), ELID-lap grinding Ohmori et al., 2011), flat honing (Beyer and Ravenzwaaij, 2005), nd Ud-lap grinding (Fiocchi et al., 2015) are some examples of hese high-end grinding processes for finishing flat surfaces. The Brazilian process, named Ud-lap grinding, was created to eet the demands for a gold-standard abrasive processes as an conomical and accessible technique for industry, especially the razilian one, which until now employed only foreign technologies or UP finishing of flat advanced materials (Fiocchi et al., 2015). Ud-lap grinding investigations started with an adapted lapping achine (Sanchez et al., 2011). Fiocchi Lap Grinder I (Fiocchi, 2010 iocchi et al., 2015) came next to establish the first fully CNC achine-tool to grind flat metallic surfaces using epoxy bonded iC grinding wheels dressed according to the overlap factor the- ry (Ud) proposed by König and Messer (1980). The second version Fiocchi, 2014), presented in this paper, represents a cutting-edge NC machine-tool design, improved for grinding materials that are ot only ductile, but also hard and difficult to machine. Products made of tetragonal zirconia polycrystals stabilized ith yttria (Y-TZP) have a noticeable interest in functional surfaces hat require top-quality mechanical (Zhuang et al., 2015), electri- al (Flegler et al., 2014), and tribological properties (Chevalier et al., 007). The 3Y-TZP used under some specific conditions presents a com- lex phenomenon of subcritical crack growth defined, among other ames, as stress corrosion (Sikalidis, 2011) or low temperature egradation (LTD) (Chevalier et al., 2007). This mechanism pro- otes the change of the crystalline structure from tetragonal to onoclinic (t → m) when the ceramic is subjected to stress and/or ssing Technology 231 (2016) 336–356 temperature (Kelly and Rose, 2002), especially in the presence of water (Yoshimura et al., 1987). This structural change of a marten- sitic nature generates volumetric expansion of the monoclinic phase (Kelly and Rose, 2002) that generates cracks and stimulates their propagation (Schmauder and Schubert, 1986), therefore influ- encing the mechanism of material removal. There is concern that 3Y-TZP products suffer unwanted phase destabilization under undersized conditions, even without apply- ing stress (Li et al., 2001). Under circumstances of external loading during either the manufacture or use of the ceramic product, this destabilization can happen. In this context, the transformation can be triggered by temperatures and stresses from the machining pro- cess (Deville et al., 2006). The presence of aqueous-based cutting fluid can accentuate this phenomenon of phase destabilization. The microstructural control capability of this event makes the 3Y-TZP a key material in the evaluation of UP machining pro- cesses, allowing its scientific study by qualifying and quantifying the martensitic transformation induced by chip removal. The present work has as its hypothesis the possibility that the achievement of flat surfaces with nanometric finishing (Ra < 100 nm) in advanced ceramics without critical defects is possible by combining appropriate machine-tool kinematics and grinding pressure, proper cutting tool sharpening, and specific lubrication and cooling. To prove this hypothesis, the Fiocchi Lap Grinder’s design was evolved, aiming at UP face grinding with lapping kinematics of advanced ceramics, meeting the conditions of removal and finish- ing in a single automated process and equipment. The 3Y-TZP was chosen as the workpieces’ material due to its phase transformation under severe removal conditions, for instance, excessive specific cutting pressure and high temperatures in a humid atmosphere. Grinding wheels made of silicon carbide (SiC) grits and epoxy bond were applied under different Ud dressing parameters and water as a dressing fluid. 2. Experimental set-up 2.1. Machine-tool A second version of the Fiocchi Lap Grinder was used in this work, named Fiocchi CNC Lap Grinder II. The machine-tool was conceptualized and designed in a partnership between the Labo- ratory of Tribology and Composite (LTC) at Escola de Engenharia de São Carlos (EESC) of Universidade de São Paulo (USP), São Car- los Campus, and the Laboratory of Machining Technology (LATUS) at Faculdade de Engenharia de Bauru (FEB) of Universidade Estad- ual Paulista (UNESP), Bauru campus. The machine-tool was put together and validated at LATUS. As well as being applied to build the first Fiocchi’s machine-tool during his Master’s course (Fiocchi, 2010), design methodology (Pahl et al., 2007) was used to evolve Lap Grinder I, aiming the ultra-precision (UP) manufacturing of advanced ceramics by improving critical aspects pointed out in early studies of Sanchez et al. (2011), Fiocchi (2010), and Fiocchi et al. (2015). Fig. 1 compares some of the modifications implemented in Fiocchi Lap Grinders. The main improvements from Lap Grinder I (Fig. 1a) (Fiocchi, 2010; Fiocchi et al., 2011) to Lap Grinder II (Fig. 1b) (Fiocchi, 2014) were: (1) substitution of the flexible shaft by a servomotor to control a workpiece’s angular velocity; (2) a new spindle having two tapered roller bearings mounted in a back-to- back configuration, 50 N pre-load, and a hollow shaft for supporting a vacuum chuck, which has a universal joint with two degrees- of-freedom to keep workpiece’s surface parallel to the grinding wheel; (3) new stepping motors and drives to prevent linear axis resonating; (4) new physical configuration of the novel electrical A.A. Fiocchi et al. / Journal of Materials Processing Technology 231 (2016) 336–356 339 der I, c w 2 3 s s c f m c b 5 0 t i T ( f a Fig. 1. (a) Fiocchi Lap Grin omponents inside the main electrical panel; and (5) updated soft- are and grinding strategy to improve linear axis dynamics. .2. Workpiece material Zirconia powder with 3 mol% of yttria (Y2O3) from Tosoh (TZ- Y-E) was chosen as the raw ceramic material due to its attractive urface and mechanical (strength and toughness) properties after intering in the present study. Moreover, tetragonal zirconia poli- ristal stabilized with 3 mol% of yttria (3Y-TZP) is a key material or medical and dental implants as well as the electronic and echanical industries. According to Tosoh, after sintering, 3Y-TZP an achieve hardness of 1250 HV, elastic modulus (E) of 210 GPa, ending strength ( ) of 1200 MPa, fracture toughness (KIC) of .0 MPa.m0.5, and microstructure endowed by fine grains under .3 �m average diameter. The material also presents interesting surface finishing apti- ude as well as the possibility that phase transformation can be nvestigated in outside layers induced by grinding conditions. he occurrence and magnitude of the martensitic transformation tetragonal → monoclinic) can, therefore, be used as a scientific tool or evaluating the quality of the material removal process. In UP manufacturing of advanced materials, all production steps re important and have influence not only on the final mechanical (b) Fiocchi Lap Grinder II. and electrical properties for example, but also on surface finish- ing. Considering that fired advanced ceramics are hard materials that are difficult to machine, and their machining is expensive, an optimized manufacturing route should consider a near-net- shape production, in which material removal processes are used for improving mainly surface finishing. For the best machining results, the material properties and geometry must be well-defined and controlled. In the present research, hence, it was pre-defined that workpieces’ manufactur- ing route should start with TZ-3Y-E powder and aim to accomplish sintered flat ceramic discs with: (1) quasi-zero defects, (2) homoge- nous grain size up to 0.4 �m average diameter, and (3) dense bodies, so that Ud-lap grinding could focus on (4) achieving surface roughness (Ra) under 100 nm, removing little material using sili- con carbide (SiC) grinding wheels. Fig. 2 illustrates the main steps and their parameters for manufacturing and characterization of the ceramic material from powder to Ud-lap ground discs. 2.2.1. Mixing The powder was prepared in a ball mill with a nylon jar of 100 ml internal volume (Carvalho and Fortulan, 2006 Carvalho, 2007) rotating at 104 rpm, corresponding to 65% of the critical velocity (Vcrit = 1338.2/D0.5) for a 70-mm inner diameter jar (D). 30 vol% of the useful volume of the jar were filled up with TZ-3Y- 340 A.A. Fiocchi et al. / Journal of Materials Processing Technology 231 (2016) 336–356 TZ-3Y E d a ( r f b t 2 2 a c t 2 a a t g 2 w p a s p t b m r 2 ( a a Fig. 2. Main steps and characteristics of (� = 6.05 g/cm3), with 0.5 kg of milling media (Tosoh YTZ 10 mm iameter), and 70 vol% of the useful volume filled up with isopropyl lcohol (� = 0.786 g/cm3) as a slurry solvent. 4-Aminobenzoic acid PABA; � = 1.37 g/cm3) was added in as a deflocculant in the atio of 0.5% of the TZ-3Y-E mass. Then, the ball mill operated or 12 h. After deflocculating completed, Polyvinyl-butyral (PVB- utvar B98; � = 1.1 g/cm3) was added to the slurry as a binder in he ratio of 2.0% of the TZ-3Y-E mass, and the mixing continued for more hours for homogenizing the colloidal suspension. .2.2. Drying After the deagglomeration and mixing steps, drying was manu- lly done by agitating the slurry in a hot air current inside a clean hamber. Gloves without talc were used and extreme care was aken in order to avoid any contamination. .2.3. Granulation and selection Manual granulation and selection applied an 80-mesh (180-�m perture) stainless steel sieve with the purpose of standardizing the gglomerate size in preparation for pressing. Granulation and selec- ion were also performed inside the clean chamber while wearing loves. .2.4. Forming Forming took place in two steps; 13.6 g of the zirconia granules ere manually inserted into the cylindrical mold with the help of a lastic container, leveled with a stainless steel spatula, and gently gitated by a vibrating table Labortechnik GmbH (THYR2 model), eeking homogenization of the filling. Next, single-acting uniaxial ress happened at 83.7 MPa during 10 s in a 37-mm diameter mold o shape discs with 4.72 ± 0.03-mm thickness. Subsequent wet- ag isostatic pressing occurred in an AIP wet-bag presser (CP360 odel) at 200 MPa for 30 s to homogenize the green ceramic bodies, educing density gradients and increasing densification. .2.5. Sintering Sintering occurred in an ambient atmosphere Lindberg furnace Blue M model) with 3.89 ◦C/min heating and cooling rates and 2 h t 1400 ◦C, resulting in ceramic discs of 3.77 ± 0.02-mm thickness nd 29.8 ± 0.15-mm diameter. -E manufacturing and characterization. 2.3. Ceramic characterization According to Fig. 3, several characterization techniques were applied in this research. Apparent density and porosity were determined by ASTM C73. TZ-3Y-E powder, granules, and green and sintered workpieces’ microstructure were investigated by SEM-FEG. Green, sintered, and Ud-lap ground workpieces were investigated by confocal microscopy. Optical fluorescence microscopy was applied to the powder, green body, and Ud-lap ground workpieces. Hardness Vickers were measured in sintered pieces. XRD was measured in powder, sintered and Ud-lap ground workpieces. Raman spectroscopy and profilometry were used with sintered and Ud-lap ground parts as well as optical microscopy in grinding wheels’ active surface after dressing and Ud-lap grinding. SEM applied an FEI electronic microscopy (Inspect F50 model) equipped with a field emission gun (FEG), 10 kV electron accel- eration voltage, working distance between 6.7 and 10.3 mm, and secondary electron detection (SE). Above each SEM image there is a legend with the main information and conditions applied, from the left to right side: date and time, dwell time, high voltage (HV), horizontal field width (HFW), working distance (WD), detection mode, and scale. The specimens received a thin layer of gold by physical vapor deposition (PVD). A Nikon microscope (model Eclipse TS100) and NIS Elements software were used to analyze optical fluorescence. Powder visu- alization applied 10× lenses and sintered pieces with 4× objective and 10× ocular lenses. XRD was studied through a PANalytical Diffractometer (X’Pert Pro MRD model) of CoK� radiation (1.78 Å, 40 kV, and 40 mA), con- tinued scanning, 0.02 angular resolution, 6.667 × 10−3 degrees, and 2� ranging from 20◦ to 65◦. Quantification of the volume percentage of the monoclinic phase was accomplished according to the models described by Toraya et al. (1984a,b) and Sato et al. (2008). Vickers hardness was evaluated by a Mitutoyo Microhardness tester (HM-200 model) with pyramidal diamond indentator with 1 kg load for 10 s, and equal loading and unloading time of 4 s in sintered workpieces. Raman spectrum measurements were done in a Witec Raman Confocal Microscope (Alpha 300 A/R model), Witec Control soft- ware, He-Ne laser (632 nm), and UHTS spectrum analyzer. Different points of each workpiece surface were focused on with a 1-�m diameter laser beam during 3 s of integrating time and the average result of 10 integrations was taken. Quantification of the volume A.A. Fiocchi et al. / Journal of Materials Processing Technology 231 (2016) 336–356 341 e fixtu p t K ( w t T s w b s s a e p A m 2 d w g g # d fl o fl l F S i t o T U i r T l a c M n Fig. 3. Workpiec ercentage of monoclinic phase was accomplished according to he models described by Katagiri et al. (1988), Lim et al. (1992), im et al. (1997), Casellas et al. (2001), and Tabares and Anglada 2010). Roughness (Ra, Rt) and flatness deviation of sintered workpieces ere measured by a Form Talysurf Intra i60 contact profileme- er with a 2-�m stylus tip radius (rtip), Taylor Hobson �ltra and alymap Gold software. Due to green bodies’ fragility and low trength, a non-contact confocal Leica microscope (DCM 3D model) as used to measure surface and roughness parameters, controlled y Leica Scan software and the data analyzed by Talymap Gold oftware. Input parameters were a 0.25-mm cutoff (�c), 0.25-mm ingle measured length (lr), 1.25-mm total measured length (ln), nd Gaussian filter to distinguish roughness, waviness, and shape rror according to ISO 25,178 and ISO 4287. Indirect material removal rate (MRR) quantification of the work- ieces was used by means of an analytical Shimadzu scale (model UW-D) after sintering (initial mass, mi) and Ud-lap grinding (final ass, mf). MRR was calculated as (mi − mf)/time. .4. Ud-lap grinding For the dressing operation, brand-new single-point diamond ressers, with a 60◦ taper and 0.5-mm nose radius geometry (rp), ere used for each test. The dresser was kept perpendicular to the rinding wheel. For Ud-lap grinding experiments, three different conventional rinding wheels made of different silicon carbide grit sizes (#300, 600, and #800) and hot pressed with an epoxy bond of 350-mm iameter and 25-mm thickness were used. Filtered water at an 8.33 × 10−5 m3/s flow rate was applied by ood during dressing. A 1:40 emulsion of semi-synthetic soluble il (Rocol Ultracut 370) was sprayed using the minimum quantity uid (MQF) method at 2.78 × 10−7 m3/s during Ud-lap grinding. The ocation of the cutting fluid nozzles was the same as that used by iocchi (2010, 2014), Fiocchi et al. (2011, 2015), and Fiocchi and anchez (2011). A higher cutting fluid flow rate was necessary dur- ng dressing to remove the slurry formed on the grinding wheel, hus cleaning it up before Ud-lap grinding. Ud-lap grinding tests were conducted to analyze the influences f three Uds and three SiC abrasive sizes on surface finishing of 3Y- ZP specimens (3 × 32 experiments). The main parameters were: d = 1, 3, and 5; #300, #600, and #800 SiC; 0.1-mm depth of dress- ng (ad); 100-kPa lap grinding pressure (P); and 100-rpm workpiece otational speed (nw) and grinding wheel rotational speed (ng). hese final parameters, summarized in Table 1, were defined after a ong series of pretests aiming to produce the best surface finishing. The Ud-lap grinding was interrupted at 5, 10, 15, 20, 25, 30, 40, nd 60 min and the workpieces were removed from the holder, leaned with acetone in an ultra-sound bath, dried, weighed for RR quantification, and their surface roughness (Ra, Rt) and flat- ess deviation were measured. ring technique. Both dressing and Ud-lap grinding steps were monitored by measuring the main motor electric power (current sensor), apply- ing a Ciber Power Analyzer (CVM NGR 96 model), described in Fiocchi (2010), which is integrated into the machine-tool. A Nikon and a Carl Zeiss stereo microscope (SMZ 800 and Citoval 2 models, respectively), both linked to a Samtek digital camera (STK 3520 model), as well as PixelView software, were used for grinding wheel optical microscopy. The clamping technique was brought from the electronic ceramic industry by melting of Thermoplastic Quartz Cement (model 70C from Lakeside) on a glass substrate etched by hydroflu- oric acid to enhance adhesion. One workpiece at time was first glued onto the center of the disc using a template to centralize the workpiece on the glass. A QUIMIS hot plate at 100 ◦C (Q.261.2 model) was used to melt the glue. During cooling of the glue, a sec- ond disc and a 2-kg mass were placed over the workpiece in order to homogeneously distribute the molten glue and keep the work- piece’s surface parallel to the glass. Next, the smooth face of the disc was put in contact with the vacuum chuck, the vacuum pump turned on, and the vacuum valve closed to hold the disc in position on the chuck. Fig. 3 shows the workpiece fixture sequence. A 2400-W vacuum cleaner was used to clean up the grinding wheel’s surface during dressing and Ud-lap grinding operations in order to avoid any loose particles on the grinding wheel that may have damaged the finishing and/or the workpiece’s surface integrity. Therefore, only two-body abrasion is expected. Suction nozzles are strategically located before and after grinding and dressing areas according to Fiocchi (2010, 2014) and Fiocchi et al. (2015), as can be seen in Fig. 4. 3. Results and discussion 3.1. Lap Grinder II Fig. 4 shows the grinding area of Lap Grinder II and its key ele- ments, such as control panel, servo motor, grinding wheel, vacuum chuck and valve, as well as cutting fluids and suction nozzles. According to Pahl et al. (2007), the evolution of the machine-tool regarding design methodology is considered an adaptive design, since it kept to known and established solution principles and adapted the embodiment to changed requirements. On the other hand, the original machine-tool based on a modified design that confers on it new features and capabilities is considered a develop- mental design by Kerala (2002). The vacuum chuck and glued workpiece showed themselves efficient and practical, with good resistance to cutting fluids and grinding forces in addition to easy cleaning. The developmental design of the spindle was necessary to clamp the workpiece by a vacuum and to add two degrees-of-freedom to the joint connecting the vacuum chuck and the spindle’s shaft. This articulation dynamically compensates for any misalignment, keeping the workpiece’s surface always parallel to the grinding 342 A.A. Fiocchi et al. / Journal of Materials Processing Technology 231 (2016) 336–356 Table 1 Input and output parameters studied through Ud-lap grinding. Input parameters Output parameters Dressing Ud-lap grinding Workpiece Grinding wheel ad = 0.1 mm P = 100 kPa 3Y-TZP Tosoh Silicon Carbide (SiC) Workpiece roughness (Ra, Rt) 60◦ taper and 0.5 mm dresser nose radius (rp) ng = 100 rpm 1400 ◦C sintering temperature Hot pressed epoxy bond Workpiece flatness deviation (�m) nw = 100 rpm nw = 100 rpm 29.8 ± 0.15 mm diameter Ø350 × 25 mm Dressing and Ud-lap grinding power (W) 8.33 × 10−5 m3/s filtered water 2.78 × 10−7 m3/s 1:40 Rocol Ultracut 370 3.77 ± 0.02 mm thickness #300 Grinding wheel surface microscopy Flood MQF 0.45 �m Ra 7.47 �m Rt #600 Workpiece 3D profilometry Ud = 1, 3, and 5 60 min 29.3 ± 1.7 �m flatness #800 MRR (g/minute) cchi L w i t v p w 3 3 t 6 r d ( f # a a i F a c 3 ( Fig. 4. CNC Fio heel. These features resulted in a stiffer and more stable mechan- cal system. The servomotor offered constant angular velocity to he vacuum chuck, and the new stepping motors and drives pre- ented the linear axis from resonating. Thus, all the irregularities ointed out were overcome and a robust UP machine-tool design as achieved and validated. .2. Ud-lap grinding .2.1. Grinding wheel topography Figs. 5 and 6 show the optical microscopies of 4 × 4 mm areas of he grinding wheels’ active/functional surfaces after dressing and 0 min of Ud-lap grinding, respectively. Ud values are arranged in ows and abrasive grit size in columns. Darker regions represent eeper areas of the grinding wheel. The images showed in Fig. 5 are similar to those found in Fiocchi 2010) and Fiocchi et al. (2011, 2015). The macroeffect was evident or all Ud dressings applied on both #800 and #600 and on the 300 grinding wheel dressed with Ud = 1. After Ud-lap grinding, ll grinding wheels (Fig. 6) kept their structures opened without dhesion of material from the workpiece, as opposed to that seen n the machining of AISI 420 stainless steel in Fiocchi (2010) and iocchi et al. (2011, 2015). The lower ductility, higher hardness, nd lower friction coefficient of the 3Y-TZP as well as the grinding onditions may sustain these findings. .2.2. RMS electric power For those grinding process, i.e., peripheral longitudinal grinding Fig. 6a), in which the active surface is the cylindrical periphery of ap Grinder II. the grinding wheel, at a constant distance from its rotational center, the grinding power (Pc) can be calculated as: Pc = F t × V c (1) where Ft is the tangential grinding force and Vc the tangential veloc- ity of the tool (Tönshoff et al., 2002). In such process, for instance, Ft can be accurately obtained by commercial piezo-dynamometers and, hence, Pc can be calculated. On the other hand, the abrasive process studied does not have a constant relative tangential velocity due to the relative kinematics of the workpiece and grinding wheel (Fig. 6b). Thus, every single point (i) of the tool/workpiece interface has its own Vc,i. In this particular face grinding with constant pressure and lapping kine- matics process, the use of a hall effect sensor is recommended to determine Pc, considering that a dynamometer would not be able to determine local tangential grinding force (Ft,i). Commercial piezo-dynamometers provide the most accurate measurement of cutting forces. These transducers have high stiff- ness, but since they use piezoelectric ceramics, the measurement of static forces over a long period of time may result in significant drift (Byrne et al., 1995), and because of their difficulties for installation in the force loop, the application of piezo-dynameters is restricted (Jemielniak, 1999). Force transducers are also subjected to dynamic influences from the machine/tool/workpiece system (Oliveira and Valente, 2004). According to Oliveira and Valente (2004), current sensors for power measurement are the simplest and lowest-cost alternative for process monitoring; in addition, their installation is simple, with A.A. Fiocchi et al. / Journal of Materials Processing Technology 231 (2016) 336–356 343 Fig. 5. Optical microscopy of the grinding wheel’s active surfaces after dressing. (ad = 0.1 mm, 0.5-mm nose radius, ng = 100 rpm, and filtered water applied by flood). min of n b i ( t h t n Fig. 6. Optical microscopy of the worn grinding wheel’s active surfaces after 60 o influence on the system stiffness. The power is usually obtained y measurement of the voltage and electric current at the grind- ng wheel spindle motor (Malkin and Koren, 1980). Byrne et al. 1995), Jemielniak (1999), and Oliveira and Valente (2004) con- end that this method (hall effect/current sensor) is precise and it as low intrusiveness, but it is damped due to the system iner- ia, i.e., grinding wheel, and the signal processing characteristics eeded. This leads to a delayed response that is not important for Ud-lap grinding. (P = 100 kPa, ng = nw = 100 rpm, and emulsion applied by MQF). this research, since the work is not focused on short events. Thus only the relative magnitude of Ud-lap grinding power among the grinding conditions is important in order to reveal the influence of Ud and grinding wheel grit sizes along 60 min of Ud-lap grinding. 3.2.3. Dressing power The updated machine-tool permits deactivation of the workpiece-holder rotation during dressing. In the older ver- 344 A.A. Fiocchi et al. / Journal of Materials Processing Technology 231 (2016) 336–356 l grin s c t s s d i r A w i i i t l t t 3 t 1 t l e b a d e ( s t d s t e d 3 s r Fig. 7. (a) Discretization of force components in peripheral longitudina ion this was not possible due to the permanent mechanical onnection between the workpiece-holder and the main shaft hrough the flexible shaft (Fiocchi, 2010; Fiocchi et al., 2011). In uch case, Lap Grinder II demands less energy by stopping the ervomotor and, consequently, the vacuum chuck spindle, during ressing. As for the reduction in the amount of energy required for dress- ng, some correlation was expected between the present and earlier esults (Fiocchi et al., 2015) regarding dressing power (Fig. 8). lthough the current research has used precisely the same grinding heels, the same dresser’s geometry, and exactly the same dress- ng parameters applied by Fiocchi et al. (2015), there was a change n the RMS dressing’s electric power. The #600 and #800 grind- ng wheels integrities may have been damaged by the age of the ools purchased in 2004. A softening of these grinding wheels, by oosening abrasives particles more easily, was noticed; this caused heir macro geometries to deteriorate more rapidly, especially for he #800 grinding wheel. .2.4. Ud-lap grinding power Fig. 9 indicates that, on average, the higher the Ud value, he lower the RMS Ud-lap grinding power: 176.71 W for Ud = 1, 73.91 W for Ud = 3, and 172.34 W for Ud = 5. This graphic suggests, herefore, that, on average, higher Uds have a tendency to demand ess power in zirconia Ud-lap grinding using SiC grinding wheels. The difference between theoretical and real macro and micro ffects over the grinding wheel and the weakening of the epoxy ond can explain those variances. Fig. 10 illustrates some important spects changed in grinding wheel topography by a single-point iamond dressing, such as the number of active abrasives (cutting dges interacting with the workpiece), as well as the wedge angle �), rake angle (�), relief angle (�), and macro porosity. Such analysis is by no means a trivial task. It depends on abra- ive size and friability as well as the “health” of the bond holding hese abrasive particles and agglomerates in place, which in a real ressing situation involve diverse macro and micro effects, besides tructural damage in grinding wheel, which can be extremely hard o predict, quantify, and qualify. The predominance of one or more ffects can thus justify the dispersal of Ud-lap grinding powers epicted in Fig. 9. .2.5. Material removal rate A very important consequence of applying different Uds is hown in Fig. 11, which presents different conditions of material emoval as a function of all Ud-lap grinding situations. The range ding (Tönshoff et al., 2002). (b) Geometrical model for Ud-lap grinding. of the results using the same grinding wheel reinforces the capa- bility of the Ud dressing to modify grinding wheels’ aggressiveness. The lowest MRR was 1 × 10−6 g/minute for Ud = 1 and #800. The highest, 19 times bigger, happened with Ud = 5 and #300. 3.3. 3Y-TZP characterization Unfortunately, it was not possible to study/focus on exactly the same regions due to lack of a common reference system among the confocal microscopy, contact profilometer, SEM-FEG, XRD, and Raman spectrometer. All analysis happened in the same vicinity, which also represents the characteristics of the entire surface. 3.3.1. 3Y-TZP preparation Two different magnifications of SEM-FEG of TZ-3Y-E powder are shown in Fig. 12a and b. The raw material presented the pre- dominance of a rounded agglomerate of 0.06 ± 0.03-mm (Fig. 12a) and particle size of 0.08 ± 0.02-�m average diameter (Fig. 12b). Some agglomerate shapes were obtained after drying (Fig. 12c) and granulation/selection (Fig. 12d). Fig. 12e presents the compacted ceramic at 200 MPa, representing the new bigger agglomerates and the residual interagglomerate porosity as described by German (1987). Fig. 12f demonstrates zirconia’s 0.35 ± 0.07-�m average grain size achieved by 2 h of sintering at 1400 ◦C. Table 2 displays the final zirconia’s properties. 3.3.2. Profilometry Uniaxial pressing inducted density gradients in the green bodies, compacted as well explained in literature. Those gradients gen- erated plastic deformations and, therefore, form error in the thin ceramic discs. After the isostatic pressing step those density gradi- ents were considerably reduced, but the form error remained and produced concave and convex opposite surfaces in the same work- piece. The form error increased or decreased depending on the side of the workpiece exposed to the higher temperature inside the fur- nace during sintering. An amplification of form error was noticed when the concave surfaces were exposed to higher temperatures. On the other hand, better form correction happened when convex surfaces were subjected to higher temperatures. Also observed was an important influence of gravity acceleration during sintering as consequence of deformation of thin ceramic discs that had rested on irregular surfaces. In the universe of producing near net shapes in UP ceramic man- ufacturing, the following should be considered: the type of pressing and its associations; the adoption of green ceramic machining to A.A. Fiocchi et al. / Journal of Materials Processing Technology 231 (2016) 336–356 345 Fig. 8. RMS dressing electric power as function of overlap factor and SiC grit size. (ad = 0.1 mm, 0.5-mm nose radius, ng = 100 rpm, and filtered water applied by flood). Fig. 9. RMS Ud-lap grinding electric power as function of overlap factor and SiC grit size. (P = 100 kPa, ng = nw = 100 rpm, and emulsion applied by MQF). Fig. 10. Schematic representation of the real and theoretical macro geometry (dashed lines) of the grinding wheels depending on the abrasive size and overlap factor (Ud). Table 2 Final zirconia discs properties after sintering. TZ-3Y-E Micro hardness (HV) Opened porosity (%) Density (g/cm3) Average grain size (�m) 1414.10 ± 339.28 0.002 ± 0.001 6.0 ± 0.06 0.35 ± 0.07 346 A.A. Fiocchi et al. / Journal of Materials Processing Technology 231 (2016) 336–356 it size s o t t j m fi 3 v o b i U t w 5 f h t a fl m 6 r t w n w e w a e w d p a e Fig. 11. MRR (g/minute) as function of overlap factor and SiC gr hape the part and also remove the highest-density gradients in utside layers; the flatness of the base, and the temperature dis- ribution inside the furnace. All these consideration will minimize he effects of heterogeneous pressing and sintering shrinkage. The udicious choice of these parameters, therefore, shall result in less achining time for correction of form, which will focus mainly on nishing. .3.3. Surface roughness For all tested conditions, there was a sharp diminishing in the alues of surface roughness in the first 5 min and a slight variation f surface quality until the end of 60 min of Ud-lap grinding. As can e seen in Fig. 13, all three graphics had the same scaling. For the #800 dressed with Ud = 1 and 3, the roughness decreas- ng rate is practically the same and several times higher than for d = 5 in the first 5 min. The worst performance can be explained by he premature wear of the #800 grinding wheel dressed with Ud = 5, hich lost its macro effect sooner, as can be noticed comparing Figs. -7 and 6-7. The #600 grinding wheel did not show a visible macro effect or Ud = 3 and 5 (Fig. 5-5 and 5-8). However, the micro effect may ave positively influenced surface roughness by offering new cut- ing edges with different angles and lower depth of cut as well s sufficient grinding wheel porosity to carry debris and cutting uid, therefore showing that the Ud through a single-point dia- ond dressing changes grinding wheel aggressiveness. Fig. 6-5 and -8 show similar topographies and may explain the similarity in oughness produced by the #600 dressed with Ud = 1 and 5 during he last 20 min. The influence of Ud was more evident over the #300 grinding heel. During the first 5 min all Uds produced quite similar rough- ess decreasing rates. The best roughness came from Ud = 5 and the orst one from Ud = 3. Figs. 14 and 15 present Ra (nm) and Rt (�m) roughness param- ters values at the end of 60 min of Ud-lap grinding, respectively. The idea behind Fig. 10 can justify the roughness depreciation ith the substantial increase of deeper risks produced by the #600 nd #800 grinding wheels; higher Rt values are shown in Fig. 15. TZ-3Y-E is harder than the AISI 420 stain steel studied by Fiocchi t al. (2015) and thus no material is impregnated into the grinding heel’s surface, resulting in higher abrasive protrusion that may epreciate finishing, but keeps the tool cleaner and with opened orosity. On the other hand, the constant abrasive exposure associ- ted with the higher hardness of zirconia wears the grinding wheel asily. . (P = 100 kPa, ng = nw = 100 rpm, and emulsion applied by MQF). 3.3.4. Topography Fig. 16 and 17 illustrate confocal microscopy of a green and a fired piece, respectively. Surface finishing improved 36% in arith- metic mean height (Sa) from green (0.71 �m) to the sintered (0.52 �m) stage, as well as producing flatter and smoother areas. Fig. 18 represents workpieces’ surface obtained by confocal microscopy after 60 min of Ud-lap grinding. In this figure can be seen a predominance of ductile cutting, absence of cracks, and pat- tern of aleatoric scratches. At least two levels of scratching can be visualized: one severe with larger width and depth, and another gentler with shallow marks. In both cases the microcutting seems to be the major ductile removal mechanism in all conditions studied. In some workpieces there are still remnants of areas from the sintered genitor surface, such as the presence of opened pores, depressions with spherical geometry, and non-machined areas. In the context of this work, those zones are classified as irregularities and might be removed by increasing the Ud-lap grinding time, since the studied cutting conditions provided low MRR. The condition of Ud = 1 and the #300 grinding wheel raised a suspicion that besides ductile cutting, there was also microplowing and plastic displacement of material, as demonstrated in Fig. 19. The white arrows point to microcutting marks. The white dashed circles delimit areas of plastic deformation at the intersec- tion of the scratches, which reveals the chronological sequence of events. The newest scratches have overshadowed the predecessor ones. The black arrows point out possible microcracks. However, SEM proved that those regions came from plastic dislocation of material toward the center of the scratch. Another fact that supports this thesis is that the white left arrow points to a deeper scratch without cracking. How could a less aggressive condition, pointed out by the black arrows, generate such failure? The presence of plastic deformation put forward the idea that strain hardening may be taking place. In such case, by the time its limit is reached, the material suffers brittle fracture. This discussion will be resumed by SEM analysis. 3.3.5. Flatness deviation Fig. 20 displays a wide variety of flatness deviation and rein- forces the ability that the single-point diamond dressing has to modify grinding wheels’ characteristics in Ud-lap grinding. 3.3.6. SEM-FEG The quality of the images acquired by the SEM-FEG is much bet- ter than those obtained by confocal microscopy; however, the SEM employed was not capable of measuring roughness. Figs. 18 and 21 A.A. Fiocchi et al. / Journal of Materials Processing Technology 231 (2016) 336–356 347 F ated/s 2 r c o r m p b c h m t ig. 12. SEM-FEG of Tosoh TZ-3Y-E: (a and b) raw material; (c) dried and (d) granul h at 1400 ◦C and a heating/cooling rate of 3.89 ◦C/minute. epresent the same surfaces shown by different techniques. Such omparison is important to display the differences in interpretation f the surfaces depending on the microscopy technique used. The confocal and electron microscopy images demonstrate emoval mechanisms such as microcutting, microplowing, and icrogrooving. During both microscopy procedures the work- ieces had all of their surfaces scanned, and no crack introduced y Ud-lap grinding was detected. The microcutting was more active at the beginning of the pro- ess, when the abrasives were still sharp. The other mechanisms appened practically during the entire machining time, becoming ore evident with the wear of the cutting edges that increased the ip’s radius, exactly as described by Marsh (1964). elected by #80 (180 �m) sieve; (e) isostatically pressed at 200 MPa; (f) sintered by Fig. 22a shows the microplowing, which plastically dislocates material to the border of the furrow. The microgrooving can be seen in Fig. 22b; zirconia’s grains are smashed, minimizing the grain boundaries by means of longitudinal plastic deformation of the ceramic material. In some circumstances the abrasive penetration into the work- piece provided a cutting cross section that promoted ductile cutting with low residual plastic deformation. This specific cutting condi- tions did not raise material at the border of the scratch and left parallel abrasive marks in the cutting direction at the bottom of the scratch. Fig. 22c illustrates this microcutting mechanism. According to Schinker and Döll (1987), microplowing and microgrooving mechanisms promote not only lateral deforma- tion, but also the compression of the material in the direction of 348 A.A. Fiocchi et al. / Journal of Materials Processing Technology 231 (2016) 336–356 Fig. 13. Roughness (Ra) versus time for different SiC sizes and Uds. (P = 100 kPa, ng = nw = 100 rpm, and emulsion applied by MQF). Fig. 14. Final roughness (Ra) after 10 min of Ud-lap grinding for different SiC grit sizes and Uds. (P = 100 kPa, ng = nw = 100 rpm, and emulsion applied by MQF). grit s m p t S t t Fig. 15. Final roughness (Rt) after 10 min of Ud-lap grinding for different SiC achining. By increasing depth of cut the microcutting becomes redominant again. This was one of the explanations found to jus- ify the best roughness produced by the largest abrasive grit (#300 iC), which also has higher friability. The SEM images show that Ud-lap grinding has achieved plas- icity conditions of 3Y-TZP, which lead us away to believe that he nanometric surface finishing was produced mainly by ductile izes and Uds. (P = 100 kPa, ng = nw = 100 rpm, and emulsion applied by MQF). microcutting and pulverization removal mechanisms. In the first mechanism, the volume of removed material is equal to the scratch volume left. It is believed that the second mechanism happens due to successive plastic deformation accumulated by 3Y-TZP in dis- tinct directions (defects) that has achieved a critical strain state, in which microcracks were not only nucleated, but also formed a recrystallized layer of nanometric thickness according to Tabares A.A. Fiocchi et al. / Journal of Materials Processing Technology 231 (2016) 336–356 349 Fig. 16. Confocal microscopy (Leica DCM 3D) of a green piece. 20× objective, dark field, and monochromatic light. ntered e m i w m s a 3 t s c o o 1 d s o 3 ( Fig. 17. Confocal microscopy (Leica DCM 3D) of a si t al. (2011). This specific removal condition can be associated with etal hardening as a consequence of cold working (strain harden- ng), which produces very small fragments of material (debris), as ell illustrated in Fig. 22d. It is also believed that the last removal echanism is a consequence of low-cycle fatigue, producing a mooth and practically not scratched surface, as shown in some reas of Fig. 22c. .3.7. Raman spectroscopy Fig. 23 presents Raman spectra of the workpieces submitted to he nine Ud-lap grinding conditions. This figure also contains the pectra of a workpiece as sintered as well as a compacted mono- linic phase zirconia. Vertical black and red lines are superimposed n the graphic in order to help differentiate tetragonal (t) and mon- clinic (m) Raman intensities. The volume percentage of the monoclinic phase was lower than %, according to Table 3, which presents different approaches for etermining the martensitic transformation. The results were quite imilar in each applied theory. Considering those values, the mon- clinic phase may be negligible. .3.8. XRD Fig. 24 presents x-ray diffraction patterns for (1) raw material Tosoh TZ-3Y-E) as supplied, (2) sintered 3Y-TZP, (3) sintered 3Y- piece. 100× objective, bright field, and white light. TZP after 60 min of Ud-lap grinding using #300 SiC dressed with Ud = 5, P = 100 kPa, ng = nw = 100 rpm, and 1:40 emulsion applied by MQF, and (4) compacted monoclinic powder without binder pressed at 80 MPa. The raw material has traces of the monoclinic phase, as indicated by the purple arrows in the vicinity of 2� = 33◦ and 36.5◦ (Fig. 24-1). Sintering resulted in a fully tetragonal material (Fig. 24-2). Fig. 24 shows the XRD patterns for all nine Ud-lap grinding con- ditions studied. The #600 and #800 grinding wheels practically did not alter the spectra in relation to the sintered workpiece, only tenuous, asymmetric, left broadening around 35◦. The singularity was more pronounced using the #600 grinding wheel. Two characteristic phenomena arising from Ud-lap grinding using the largest abrasive grit were observed. These were the asym- metric broadening of tetragonal peaks near 35◦ and 59◦ (black arrows), and reversal of the intensity of the tetragonal peaks at 40◦ and 41◦ (Figs. 24-3 and 25). Virkar and Matsumoto (1986), Mehta et al. (1990), and Tabares et al. (2011) affirm that reversal occurs due to ferroelastic domain switching, which can be explained as a reori- entation of the crystallographic planes at the level of crystallite. The asymmetric broadening may also indicate a superficial grain refine- ment. This finding is in agreement with Tabares et al. (2011) that 350 A.A. Fiocchi et al. / Journal of Materials Processing Technology 231 (2016) 336–356 Fig. 18. Confocal microscopy (Leica DCM 3D) after 60 min of Ud-lap grinding with 9 different conditions. 100× objective, bright field, and white light. Fig. 19. Confocal microscopy (Leica DCM 3D) of a surface produced by a #300 SiC grinding wheel dressed with Ud = 1. 100x objective, bright field, and white light. Fig. 20. Flatness deviation after 60 min of Ud-lap grinding, as a function of the overlap factor for different SiC grit sizes (P = 100 kPa, ng = nw = 100 rpm, and emulsion applied by MQF). A.A. Fiocchi et al. / Journal of Materials Processing Technology 231 (2016) 336–356 351 Fig. 21. SEM-FEG after 60 min of Ud-lap grinding with 9 different conditions. Fig. 22. SEM-FEG of (a) microplowing, (b) microgrooving, (c) microcutting/scratching, and (d) chips produced by Ud-lap grinding. 352 A.A. Fiocchi et al. / Journal of Materials Processing Technology 231 (2016) 336–356 Fig. 23. Raman intensity versus Raman shift (cm−1) of the nine Ud-lap grinding conditions applied in 3Y-TZP, 3Y-TZP as sintered, and a compacted monoclinic phase zirconia. Vertical black and red lines added to the graphic to indicate Raman shift of tetragonal and monoclinic phases, respectively, according to Pezzotti and Porporati (2004). Table 3 Volume percentage of monoclinic phase (Vm) measured by Raman spectroscopy in Ud-lap ground workpieces. Author/equation Parameter Grinding wheel mesh Overlap factor (Ud) Vm (%) Vm max. [%] Vm min. [%] k � Clarke and Adar (1982) 0.97 1 800 1 0.50 600 1 0.47 300 1 0.50 800 3 0.50 600 3 0.50 – – 300 3 0.50 800 5 0.50 600 5 0.50 300 5 0.50 Katagiri et al. (1988) 0 2.2 ± 0.2 800 1 0.48 0.50 0.46 600 1 0.49 0.51 0.47 300 1 0.48 0.50 0.46 800 3 0.48 0.50 0.46 600 3 0.48 0.50 0.46 300 3 0.48 0.50 0.46 800 5 0.48 0.50 0.46 600 5 0.48 0.50 0.46 300 5 0.48 0.50 0.46 (1992) 1 0.33 ± 0.33 800 1 0.75 1.0 0.60 600 1 0.73 1.0 0.57 300 1 0.75 1.0 0.60 800 3 0.75 1.0 0.60 600 3 0.75 1.0 0.60 300 3 0.75 1.0 0.60 800 5 0.75 1.0 0.59 600 5 0.75 1.0 0.60 300 5 0.75 1.0 0.59 (1997) – – 800 1 0.20 600 1 0.20 300 1 0.20 800 3 0.20 600 3 0.20 – – 300 3 0.20 800 5 0.20 600 5 0.20 300 5 0.20 Casellas et al. (2001) – – 800 1 0.53 600 1 0.52 300 1 0.53 800 3 0.53 600 3 0.53 – – 300 3 0.53 800 5 0.53 600 5 0.53 300 5 0.53 A.A. Fiocchi et al. / Journal of Materials Processing Technology 231 (2016) 336–356 353 Fig. 24. X-ray diffraction patterns for (1) Tosoh TZ-3Y-E powder as supplied, (2) sintered 3Y-TZP (2 h at 1400 ◦C), (3) sintered 3Y-TZP after 60 min of Ud-lap grinding using #300 SiC dressed with Ud = 5, P = 100 kPa, ng = nw = 100 rpm, and 1:40 emulsion applied by MQF, and (4) compacted monoclinic powder without binder pressed at 80 MPa. F TZ-3Y U QF. f 3 i o ig. 25. X-ray diffraction patterns for (1) compacted monoclinic powder, (2) Tosoh d-lap grinding using P = 100 kPa, ng = nw = 100 rpm, and 1:40 emulsion applied by M ound a recrystallized zone of grains of ∼20 nm diameter in ground Y-TZP. Tabares et al. (2011) also showed that the asymmetry observed n the spectrum of ground zirconia is related to the overlapping f the tetragonal and rhombohedral phases. The rhombohedral -E powder as supplied, (3) sintered 3Y-TZP, (4-12) sintered 3Y-TZP after 60 min of phase has been associated with distortion of the cubic phase in the [1 1 1] direction according to many authors, such as Hasegawa (1983), Kitano et al. (1988), Mitra et al. (1993), and Tabares et al. (2011). Therefore, the probability of finding a rhombohedral grain is equal to the probability of finding a cubic grain. On the other hand, 354 A.A. Fiocchi et al. / Journal of Materials Processing Technology 231 (2016) 336–356 F ) Tosoh TZ-3Y-E powder, (c) compacted body of Tosoh zirconia monoclinic powder pressed a ng with the #300 SiC grinding wheel dressed with Ud = 5, P = 100 kPa, nw = ng = 100 rpm. H ( h t t t l s t s c i t t a l c i e t p d w a T a n a t Table 4 Volume percentage of monoclinic phase (Vm) measured by XRD in Ud-lap ground workpieces. Author/equations Grinding wheel mesh size Overlap factor (Ud) Vm (%) Toraya et al. (1984a,b) and Sato et al. (2008) 800 1 0.033 600 1 0.042 300 1 0.101 800 3 0.038 600 3 0.034 300 3 0.057 800 5 0.037 ig. 26. Optical fluorescence microscopy of (a) Tosoh zirconia monoclinic powder, (b t 80 MPa, and (d) sintered TZ-3Y-E workpiece subjected to 60 min of Ud-lap grindi asegawa (1983), Burke and Rainforth (1997), and Tabares et al. 2011) argued that the rhombohedral phase is stable only under igh compressive stress at ambient temperature. According to Tabares et al. (2011), from XRD spectra it is clear hat peak broadening is a consequence of an overlap of the (1 1 1) etragonal peak with another symmetrical peak, corresponding to he reflection of the (1 1 1) plane of the cubic phase moved to the eft of its position in as sintered. If the cubic phase undergoes the ame texture observed on the tetragonal and monoclinic phase, heir {1 1 0} plane will be oriented approximately parallel to the urface. This configuration would explain the shift of the (1 1 1) ubic peak to the left side, when a biaxial compressive stress is mposed on the surface plane. The Raman spectroscopy, that analyzed the surface by a confocal echnique, demonstrated practically no t → m phase transforma- ion. XRD is a volumetric technique (cps/volume or mass) and has lso indicated that there was no monoclinic phase inside the ana- yzed volume. Therefore, the association of the two techniques onfirmed that the nine Ud-lap grinding conditions studied did not ncite martensitic transformation. The comparison of these results with those found by Tabares t al. (2011) indicates the superiority of the Ud-lap grinding to he peripheral longitudinal grinding. This last grinding process roduced three severe microstructural changes, which are clearly ivided into three regions from the surface: (1) recrystallized zone ith a grain of ∼20-nm diameter; (2) plastically deformed zone, nd (3) t-m phase transformation zone. Table 4 represents the quantitative analysis proposed by the oraya et al. (1984a,b) and Sato et al. (2008) equations. A maximum volume of 0.1% of monoclinic phase was found in ll Ud-lap ground workpieces. Although it is clear that the two tech- iques cannot be compared without the necessary correction of the nalyzed volume and absorbed radiation, both results indicate that he level of transformation in the lap ground material is practically 600 5 0.035 300 5 0.077 negligible. Hence, the Ud-lap grinding did not form the third region described by Tabares et al. (2011), keeping the desired tetragonal phase, which has superior mechanical properties. Even with the detection of a very low monoclinic phase, it can be noticed that there is a tendency that more aggressive cutting conditions (larger abrasive grit and lower Ud) provoke more phase transformation than gentle ones. The low percentage of martensitic transformation presented in the outside volume can be consequence of tender tribological con- ditions achieved by Ud-lap grinding—for instance, shallow depth of cut, low relative velocity (<2 m/s), cutting temperature close to ambient, predominance of ductile removal, and tetragonal phase stabilization reinforced by compressive residual stress. The #300 grinding wheel offered the best surface roughness but also the highest crystallographic damage on 3Y-TZP, probably due to higher cutting specific pressure that plastically deformed the surface. The lowest Ra roughness should also be a consequence of the higher friability of the #300 SiC abrasive grit. Proce m a d g p p 3 t t p r F o w n t a c a c v a n o t 4 m c i w i e • • • • • • A.A. Fiocchi et al. / Journal of Materials The correct choices of the Ud-lap grinding cutting conditions ust, therefore, take into account not only roughness output, but lso crystallographic changes that may influence the life of the part epending on its application. If it is demonstrated that the Ud-lap rinding causes compressive residual stress, the studied abrasive rocess should occupy a prominent position among the UP grinding rocesses of ceramic parts susceptible to fatigue and/or LTD. .3.9. Optical fluorescence Fig. 26 shows four optical fluorescence images and reveals that he monoclinic zirconia powder fluoresces under violet light exci- ation (Fig. 26a). The zirconia doped with yttria (TZ-3Y-E) did not resent fluorescence (Fig. 26b). Fig. 26c illustrates the strong fluo- escence of a compacted body pressed at 80 MPa without binder. ig. 26d demonstrates homogeneously distributed fluorescence n a sintered workpiece subjected to 60 min of Ud-lap grinding ith the #300 SiC grinding wheel dressed with Ud = 5, P = 100 kPa, w = ng = 100 rpm. The other eight images are not shown because hey are qualitatively similar. Although the literature reports the use of photoluminescence s a tool to study electronic band transitions, structure, defects, hemical composition (Lai et al., 2005), and residual stress (Sheng nd Todd, 2011), there has not been seen either an image or a proto- ol aimed at visual differentiation of the zirconia phases inside the isible electromagnetic spectrum. Considering the XRD and Raman nalysis, the fluorescent spots upon Ud-lap ground workpieces can- ot be considered a consequence of martensitic transformation, but ther defects such as compressive stress that may have stabilized he rhombohedral phase. . Conclusions This paper investigated the Ud-lap grinding process and its achine-tool design aiming at UP manufacturing of advanced eramics. Influences of three different overlapping factors on dress- ng (Ud) and three abrasive grit sizes of conventional SiC grinding heels were analyzed on flat surface finishing of dense 3Y-TZP discs n a ductile regime of material removal. The key findings of this xperimental study were: The design methodology was successful for supporting to achieve all design requirements in the second version of the CNC Fioc- chi Lap Grinder. The efficient machine-tool and Ud-lap grinding process were capable of manufacturing flat 3Y-TZP surfaces with nanometric finishing without introducing critical defects. Differences between theoretical and real macro and micro effects over the grinding wheels after single-point diamond dressing, epoxy bond strength, abrasive protrusion, abrasive grit size and abrasive friability play a key role in Ud-lap grinding. The consequences of applying different Uds are the distinct out- puts regarding roughness, MRR, and flatness deviation; the range of these results, using the same grinding wheel, reinforces the capability of the Ud dressing to modify grinding wheels’ aggres- siveness and surface finishing. Nanometric surface finishing was produced mainly by microcut- ting and pulverization removal mechanisms. The best finishing, Ra = 60.63 nm, came from the #300 grinding wheel dressed with Ud = 5. Flatness deviation of 0.308 �m was obtained through the #800 grinding wheel and Ud = 3. Raman spectroscopy and XRD indicated that the level of marten- sitic transformation in the Ud-lap ground material was negligible. The occurrence and magnitude of phase change induced by removal conditions can be used as a scientific tool for evaluating the quality of UP material removal processes. ssing Technology 231 (2016) 336–356 355 • Monoclinic zirconia powder fluoresced under violet light excita- tion. • In UP manufacturing of advanced materials, all production steps must be controlled. Considering that fired advanced ceramics are materials that are hard, difficult and expensive to machine, an optimized manufacturing route should consider a near-net- shape production, in which material removal processes should be applied mainly for improving surface finishing. • Ud-lap grinding-generated a pattern of isotropic nanometric fin- ishing due to cycloidal trajectories regardless of the surface orientation and form error of the workpiece, was strongly influ- enced by grinding wheel’s flatness. • There is no report of an abrasive process capable of achieving sim- ilar nanometric finishing with the same micrometric grit size and type of abrasive. The Ud-lap grinding can replace the engagement of processes such as grinding, lapping, and polishing of advanced ceramics. Acknowledgments The authors would like to thank Fundaç ão de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Brazil, for financial support (Grants 2002/06452-6, 2008/50233-3, and 2011/22486- 7), Coordenaç ão de Aperfeiç oamento de Pessoal de Nível Superior (CAPES), Faculdade de Engenharia de Bauru (FEB/UNESP), Instituto de Física de São Carlos (IFSC/USP), Departamento de Física da Fac- uldade de Ciências (DF/UNESP), and Escola de Engenharia de São Carlos (EESC/USP). References Argawal, S., Rao, P.V., 2008. Experimental investigation of surface/subsurface damage formation and material removal mechanisms in SiC grinding. Int. J. Mach. Tools Manuf. 48 (6), 698–710. Beyer, P., Ravenzwaaij, M., 2005. Innovative flat honing with vitrified-bond grinding and conditioning tools. Ind. Diamond Rev. IDR 4, 47–50. Boch, P., Nièpce, J.C., 2007. Ceramic Materials: Processes Properties, and Applications, first ed. Wiley. 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