Zirconia Milling Research Articles

Impact of Digital Manufacturing Methods on the Accuracy of Ceramic Crowns.

The purpose of this study was to assess the accuracy of full coverage crowns produced by two manufacturing methods: additive 3D printing and subtractive milling utilizing three different predefined cement spaces. Six groups were allocated based on the manufacturing method and the predefined cement space: printed wax with a 20 µm space (PW1); printed wax with a 50 µm space (PW2); printed wax with a 100 µm space (PW3); milled wax with a 20 µm cement space (MW1); milled wax with a 50 µm cement space (MW2); milled wax with a 100 µm cement space (MW3); milled zirconia coping with a 20 µm cement space (MZ1); milled zirconia coping with a 50 µm cement space (MZ2); milled zirconia with a 100 µm cement space (MZ3). All fabricated specimens were scanned using an Identica Blue 3D scanner and saved as standard tessellation language (STL) files. A triple scan method was performed using 3-matic software to assess accuracy. The discrepancy values were recorded in micrometers, and the analysis was conducted using one-way ANOVA. The wax printing method, with a cement gap design of 100 μm, demonstrated a significant improvement in the accuracy compared to the other methods (P<.01). In contrast, the zirconia milling method exhibited significantly lower accuracy relative to the other techniques (P<0.01). Moreover, different cement spaces resulted in various accuracy levels but the only statistically significant difference was observed for the 100 µm cement space in the printed wax group. The additive 3D printing method exhibited greater accuracy than the subtractive milling approach. Furthermore, altering the cement gap was found to impact the accuracy of both techniques, albeit without statistical significance. The accuracy of each CAM technique was investigated in this study, highlighting the promising accuracy of 3D-printed patterns for the lost-wax technique over direct crown milling. However, all methods produced reliable and accurate crown restorations.

Impact of CAD/CAM burs lifetime on surface properties of dental zirconia: Implications for biocompatibility of custom abutments.

To evaluate the effect of the deterioration of computer aided design/computer aided manufacturing (CAD/CAM) burs during zirconia milling, on surface roughness, contact angle, and fibroblast viability. Ceramic blocks were milled and 75 ceramic disks (8 × 1.5 mm) made and allocated into three groups (n = 25): G1-brand new 2L and 1L burs, G2-2L bur at the end of lifetime and brand new 1L bur and G3-both burs at the end of their lifetimes. Roughness (Ra, Rq, and Rz) was evaluated using a 3D optical profilometer, the contact angle by the sessile drop method and the cell viability of the mouse NIH/3T3 fibroblast, using the Alamar Blue assay at intervals of 24, 48, and 72 h (ISO 10993-5). Data were analyzed by one-way ANOVA and Kruskal-Wallis tests (p ≤ 0.05). Roughness increased as the burs deteriorated and G3 (0.27 ± 0.04) presented a higher value for Ra (p < 0.001). The highest contact angle was observed in G3 (86.2 ± 2.66) when compared with G1 (63.7 ± 12.49) and G2 (75.3 ± 6.36) (p < 0.001). Alamar Blue indicated an increase in cell proliferation, with no significant differences among the groups at 24 and 72 h (p > 0.05). The deterioration of the burs increased the surface roughness and decreased the wettability, but did not interfere in cell viability and proliferation. The use of custom zirconia abutments represents an effective strategy for single crowns restorations. Our findings suggest that these abutments can be efficiently milled using CAD/CAM burs within their recommended lifetime.

Parametric study of surface characteristics of laser micro-channel milling of zirconia (ZrO2) at defocused condition

Laser beam micro-channel milling (LBMM) is a recently emerged novel micromachining approach, which can be a useful technique for the machining of hard-to-machine advanced engineering ceramics. LBMM provides better competence compared with remaining non-traditional machining processing regarding the material removal rate, better surface characteristics, and ability to cut different nature of materials disregarding of electrical conductivity and hardness. Moreover, micro-channel with high accuracy and precision can be fabricated by this method. This process on zirconia (ZrO 2 ) ceramic materials was investigated in this paper using different parametric combinations. The surface roughness (Ra and Rz) was observed using a precision profilometer. Besides, laser milling operation at defocused condition was performed to reduce the surface irregularities of the machined surface. It was seen that the defocusing condition machining of laser beam results in a low value of surface finish (Rz and Ra) compared to results obtained in focused condition machining of the laser. The optical microscopic view of the machined surface also confirmed the better-machined surface at defocused conditions.

Laser-Assisted Milling of Zirconia with Systematically Determined Machining Conditions

Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) is a promising material for dental restoratives. Although grinding or polishing with diamond tools is widely used to machine Y-TZP, the processing efficiency and cost of the process are problematic. In this study, we applied laser-assisted machining (LAM) to Y-TZP, in which non-diamond tools were used. Unlike LAM applied to other materials, decrease of the fracture toughness at elevated temperatures which is a unique feature of the Y-TZP was adopted as a key mechanism for machinability enhancement. In addition, a systematic method to determine the LAM conditions was proposed. In this study, we explain the LAM condition-determining method, which is based on numerical simulations of the temperature distribution. Secondly, the determining method was evaluated through a series of LAM experiments to obtain the appropriate LAM conditions. Using the determined conditions, LAM of Y-TZP was demonstrated to be effective; the thrust force was reduced by 51.3% and the tool wear was significantly reduced, while no cracks formed on the Y-TZP.

A comparative study of additive and subtractive manufacturing for dental restorations

Statement of problemDigital systems have recently found widespread application in the fabrication of dental restorations. For the clinical assessment of dental restorations fabricated digitally, it is necessary to evaluate their accuracy. However, studies of the accuracy of inlay restorations fabricated with additive manufacturing are lacking. PurposeThe purpose of this in vitro study was to evaluate and compare the accuracy of inlay restorations fabricated by using recently introduced additive manufacturing with the accuracy of subtractive methods. Material and methodsThe inlay (distal occlusal cavity) shape was fabricated using 3-dimensional image (reference data) software. Specimens were fabricated using 4 different methods (each n=10, total N=40), including 2 additive manufacturing methods, stereolithography apparatus and selective laser sintering; and 2 subtractive methods, wax and zirconia milling. Fabricated specimens were scanned using a dental scanner and then compared by overlapping reference data. The results were statistically analyzed using a 1-way analysis of variance (α=.05). Additionally, the surface morphology of 1 randomly (the first of each specimen) selected specimen from each group was evaluated using a digital microscope. ResultsThe results of the overlap analysis of the dental restorations indicated that the root mean square (RMS) deviation observed in the restorations fabricated using the additive manufacturing methods were significantly different from those fabricated using the subtractive methods (P<.05). However, no significant differences were found between restorations fabricated using stereolithography apparatus and selective laser sintering, the additive manufacturing methods (P=.466). Similarly, no significant differences were found between wax and zirconia, the subtractive methods (P=.986). The observed RMS values were 106 μm for stereolithography apparatus, 113 μm for selective laser sintering, 116 μm for wax, and 119 μm for zirconia. Microscopic evaluation of the surface revealed a fine linear gap between the layers of restorations fabricated using stereolithography apparatus and a grooved hole with inconsistent weak scratches when fabricated using selective laser sintering. In the wax and zirconia restorations, possible traces of milling bur passes were observed. ConclusionsThe results indicate that the accuracy of dental restorations fabricated using the additive manufacturing methods is higher than that of subtractive methods. Therefore, additive manufacturing methods are a viable alternative to subtractive methods.

In vitro evaluation of marginal discrepancy of monolithic zirconia restorations fabricated with different CAD-CAM systems

Statement of problemDental laboratories use different computer-aided design and computer-aided manufacturing (CAD-CAM) systems to fabricate fixed prostheses; however, limited evidence is available concerning which system provides the best marginal discrepancy. PurposeThe purpose of this in vitro study was to evaluate the marginal fit of 5 different monolithic zirconia restorations milled with different CAD-CAM systems. Material and methodsThirty monolithic zirconia crowns were fabricated on a custom-designed stainless steel die and were divided into 5 groups according to the type of monolithic zirconia crown and the CAD-CAM system used: group TZ, milled with an MCXL milling machine; group CZ, translucent zirconia milled with a motion milling machine; group ZZ, zirconia milled with a dental milling unit; group PZ, translucent zirconia milled with a zirconia milling unit; and group BZ, solid zirconia milled using an S1 VHF milling machine. The marginal fit was measured with a binocular microscope at an original magnification of ×100. The results were tabulated and statistically analyzed with 1-way ANOVA and post hoc surface range test, and pairwise multiple comparisons were made using Bonferroni correction (α=.05). ResultsThe type of CAD-CAM used affected the marginal fit of the monolithic restoration. The mean (±SD) highest marginal discrepancy was recorded in group TZI at 39.3 ±2.3 μm, while the least mean marginal discrepancy was recorded in group IZ (22.8 ±8.9 μm). The Bonferroni post hoc test showed that group TZI was significantly different from all other groups tested (P<.05). ConclusionsWithin the limitation of this in vitro study, all tested CAD-CAM systems produced monolithic zirconia restorations with clinically acceptable marginal discrepancies; however, the CAD-CAM system with the 5-axis milling unit produced the best marginal fit.

Reactive sintering of (K0.5Bi0.5)TiO3-BiFeO3 lead-free piezoelectric ceramics

Ceramics based on BiFeO3 are potential lead-free replacements for Pb(Zr,Ti)O3 in a variety of applications such as sensors, transducers and actuators. Recently, ceramics in the (K0.5Bi0.5)TiO3-BiFeO3 system were developed which have excellent piezoelectric properties. However, these ceramics are difficult to sinter to high density. The present work studies the use of reactive sintering to prepare 0.4(K0.5Bi0.5)TiO3-0.6BiFeO3 ceramics. Undoped and MnO-doped powders were prepared by ball milling K2CO3, (BiO)2CO3, TiO2, α-FeO(OH) and MnCO3 in ethanol with zirconia milling media. The decomposition and calcination reactions of the starting materials were studied using differential scanning calorimetry/thermogravimetric analysis, X-ray diffraction and Fourier transform infra-red analysis. Samples were sintered in the temperature range from 1000 to 1075°C and their structures and microstructures examined using X-ray diffraction, micro-Raman scattering and scanning electron microscopy. MnO doping reduced the rhombohedral distortion of the unit cell. The dielectric, ferroelectric and piezoelectric properties of selected undoped and MnO-doped samples were measured. Both undoped and MnO-doped samples displayed relaxor-type behavior. MnO doping reduced the conductivity of the samples, which exhibit a well-defined activation energy of 1.21 eV. Undoped samples have strain vs. electric field properties comparable to those reported in the literature.

Simulation of Fully Sintering Dental Zirconia Milling Process Based on Discrete Element Method

Discrete element model of fully sintering dental zirconia was constructed and calibrated. Based on the model, the dynamic process of low-speed milling of zirconia was simulated, and the effects of different cutting speeds, cutting widths and federates on the formation of surface cracks were also analyzed. Results show that residue cracks number and maximum depth increases significantly with increase of the cutting width, while the influence of cutting speed and federates is not distinct. That shows the possibility of high-speed machining on fully sintering dental zirconia with development of coating technology of cutting tool.

Micro-EDM Milling of Electrically Nonconducting Zirconia Ceramics

Micro-EDM is a well-known process for precision micro machining of difficult to cut materials. It is especially beneficial when machining complex micro structures such as deep bores, grooves or undercuts in highly wear resistant materials. One constraint is its limitation to electrically conductive materials.Advanced materials such as high-temperature resistant engineered ceramics are gaining more and more importance, nonetheless due to their manifold applications in transport, energy and also biomedical areas.An adaption of micro-EDM to nonconducting ceramics could bring together the design freedom implemented in the EDM process and the beneficial material properties of engineered ceramics.In the last years, research has successfully applied the assisting electrode method to zirconia material, allowing for the micro-EDM to proceed notwithstanding the electrically nonconducting properties. Using suitable dielectric and process parameters, the machining continues even after the electrically conducting starting layer has been removed.Within this study the micro-EDM milling of zirconia is investigated in order to further understand the mechanisms of ceramics machining. Roughness and surface characteristics are analyzed and compared to the equivalent metal parts.The voltage and current signals are recorded using high speed deep memory A/D converters to allow for an insight into the process. With the findings the process can be improved in terms of speed, accuracy and achievable structures.

Micromachining of ceramic surfaces: Hydroxyapatite and zirconia

Computerised Numerical Control (CNC) precision machining can be employed as a fast and reproducible method for surface micropatterning. For biomedical applications an efficient and reproducible micropatterning of zirconia and calcium phosphate based materials is highly sought in order to guide implant interactions with surrounding biological tissues for a better osseointegration. Therefore, CNC precision machining of zirconia and hydroxyapatite substrates is investigated in this study and optimised process parameters are reported. By microgrinding and micromilling microgrooves with a minimum width of 100 μm were obtained and process parameters such as cutting tool diameter and feed velocity discussed. As all samples were sintered prior to the micropatterning process, the influence of sintering temperature on the pattern quality, size and hardness of the obtained samples are studied. Vickers hardness of the different sintered ceramic surfaces was measured to correlate the possible wear impact on the tip of the cutting tools. The stiffness and the hardness of the used cutting tools were measured and their effect on the cutting results was discussed. The pattern quality and the average roughness in the machined microgrooves were analysed by 3D-profilometry and imaged by SEM. Comparison of the two machining techniques yielded more defined and less fractured micropatterns for microgrinding. The process efficiency for both methods was limited by the economic life time of the tool tips. For CNC grinding the life time was downsized due to more pronounced abrasive wear. For both materials the hardness was the crucial process parameter, which was adjusted by the sintering temperature. For milling of zirconia the sintering should not exceed a temperature of 1100 °C to minimize tool wear. A temperature of lower 1200 °C is suggested for the milling of HA. For sintering temperatures higher than 1200 °C the machining of both ceramic surfaces was hardly possible. The feed velocity was found not having a significant influence on the obtained micropattern width. The preset line pitch of 100 μm was excellently reached for both applied machining processes. It was found that lower feed velocities and smaller tool diameters caused deeper micropatterns.

Strength influencing variables on CAD/CAM zirconia frameworks

Purpose Many studies in the dental literature look at the effect of different surface treatment methods on the flexure strength of zirconia where polished zirconia has been used as control. However, zirconia is subjected to different types of surface damage as a result of the CAD/CAM milling procedure and also to damage produced by other laboratory procedures in use daily. Aim of the work The aim of this work was to evaluate the effect of different surface treatment methods and in particular the effect of the CAD/CAM milling procedure on the flexure strength of zirconia frameworks. Materials and methods At least 20 zirconia bars (17 mm × 2 mm × 1 mm) for each group were prepared by either cutting and polishing zirconia milling blocks or by using a CAD/CAM device (Cercon) which left behind characteristic surface features related to the milling process. The fully sintered bars received either of the following surface treatments: air-borne particle abrasion (with 50 and 120 μm aluminum oxide particles, or both). Some bars received a heat treatment commonly used in baking veneer ceramics before or after particle abrasion. The surface roughness was measured for all bars, which were finally loaded in a three-point device. The fractured bars were examined using scanning electron microscopy. Data were analyzed using one-way analysis of variance and survivability was estimated using Weibull analysis ( α < 0.05). Results There were significant differences in the flexure strength (in MPa) between the tested groups subjected to different surface treatments which can be categorized into four strength levels: (1074–1166 MPa) for polished zirconia and the CAD/CAM bars that were particle abraded (50 μm Al 2O 3) whether with or without heat treatment (936 MPa) for the ground bars that were particle abraded (50 μm Al 2O 3), (708–794 MPa) for CAD/CAM bars and for the polished bars that were particle abraded (120 μm Al 2O 3), and (546 MPa) for the ground bars that were particle abraded (120 μm Al 2O 3) being the weakest. There was a strong correlation between flexure strength and the severity of surface damage as indicated by surface roughness ( R 2 = 0.912). Scanning electron microscopy revealed different types of surface and subsurface damage produced by the different surface treatments. Conclusions The surface damage produced by the CAD/CAM milling procedure significantly reduced the strength of zirconia which could be further weakened by different surface treatment methods resulting in unexpected failures at stresses much lower than the ideal strength of the material. It is advised to consider the effect of the CAD/CAM procedure on the characteristic strength when designing zirconia-based fixed partial dentures.

A study in the mechanical milling of alumina powder

The mechanical milling of alumina in order to reduce grain sizes to ≤100 nm has been proposed as a means of reducing sintering temperatures and improving pressureless sintered density, particularly as a means of allowing co-firing with metallic components for biomedical implants. There is a persistent problem with contamination from the milling media, usually hardened steel which can be only partially alleviated by acid leaching. We have explored the use of alternative milling media with a view to reducing the levels of contamination. Alumina powders were milled with hardened steel, tungsten carbide, alumina and zirconia milling media under identical conditions of ball mass:powder mass ratio 10:1 and target milling times of 32 h. All of the milling media were found to cause unacceptable levels of contamination. Zirconia media gave the lowest contamination (3–4%) and in some circumstances, the addition of a small amount of zirconia may lead to increased toughness without loss of bio-compatibility.

Pressureless sintering of silicon nitride/boron nitride laminate composites

Silicon nitride (Si3N4) and boron nitride (BN) are materials commonly used in the production of layered ceramics [1–3] and their derivatives [4]. In composites made from these two materials, the Si3N4 provides strength and the BN deflects cracks that intersect its basal planes at right angles [5]. The process of crack deflection greatly increases the length of the crack path through the sample, resulting in greater energy absorption as compared to the relatively straight crack path often observed in a monolithic ceramic. Composites made from these materials are densified by hot-pressing because of the reluctance of BN to sinter [2, 4]; For example, h-BN similar to that used in this study was hot-pressed for 1.5 hrs at 1740 ◦C under 25 MPa with no apparent sintering observed between BN particles [6]. It is the strong covalent bonds on the basal planes that make for an unreactive, non-wetting compound which severely hampers the densification of BN. Previous research on Si3N4/BN composites has shown that during the hot-pressing step, the liquid phase formed in the Si3N4 cells is forced into the BN cell boundaries [5]. This glassy phase, which has a composition that is a function of the sintering aids used and the silica on the surface of the silicon nitride, acts as a “glue” to hold the BN platelets together. It is proposed that if the sintering aids are added in the appropriate proportion directly to the BN powder [7], as well as to the Si3N4 [8–15], laminate composites may be densified by pressureless sintering. α-silicon nitride (UBE Industries E-10, ∼0.5 μm diameter) was ball milled with 4 wt% Al2O3 (Alcoa A16SG, ∼0.4 μm in diameter) and 4 wt% Y2O3 (Alfa Aesar REacton, ∼10 μm in diameter) to create the silicon nitride layers (presently identified as Si3N4-4A4Y). From prior work [16], the Si3N4-4A4Y composition investigated for the Si3N4 was determined to sinter to 92.3% and be composed of entirely β-Si3N4. The BN layer was made with h-BN (Advanced Ceramics HCP) to which 34.3 wt% each of alumina and yttria powders, 17.1 wt% α-Si3N4 and 2 wt% SiO2 (Alfa Aesar REacton, <45 μm) were also added. Prior work showed this composition to sinter to 66% of its theoretical density [16]. This composition of BN powders will be referred to as 88BN in the rest of the text. The powders that were used to form the Si3N4-4A4Y and 88BN layers were ball-milled in methanol for 1 hr using zirconia milling media prior to assembling the layered ceramic. The compact was fabricated by adding alternating layers of 2 g of Si3N4-4A4Y powder, followed by a 0.9 g layer of 88BN powder, to a steel die with inner dimensions of 76 mm × 38 mm × 6 mm. Note that a total of 15 layers were stacked, with the top and bottom layers composed of Si3N4-4A4Y. In addition, the 88BN powder intentionally did not completely cover the 76 mm × 38 mm area; rather, the perimeter was kept free of the 88BN powder so that adjacent layers of Si3N4-4A4Y would touch. The thickness of the perimeter was ∼3 mm. It is thought that sintering of the Si3N4-4A4Y powder would provide an additional compaction force to the 88BN layers. After all the layers were added, the composite was pressed in a laboratory press at ∼7 MPa. The compact was removed from the die and packed in a powder bed consisting of 60 wt% α-Si3N4 and 40 wt% BN, and sintered in a BN crucible at 1800 ◦C for 3 hrs in flowing nitrogen gas in a Centorr Testorr furnace. In addition to the layered ceramic, individual compacts of Si3N4-4A4Y powder and the 88BN powder were pressed and sintered similarly to the layered ceramic. Table I presents the properties of these monolithic ceramics [17]; these properties were used to predict the expected mechanical properties of the layered ceramic. The average density of three 15-layered test specimens was ∼2.1 g/cm3, approximately 66% of its theoretical density of ∼3.4 g/cm3. The theoretical density (TD) was calculated based on the composite having ∼65 vol% Si3N4-4A4Y (TD ∼3.26 g/cm3) and ∼35 vol% 88BN (TD ∼3.66 g/cm3), as determined from point-count analysis. X-ray diffraction of the faces parallel to the pressing direction, using Cu Kα radiation, revealed only β-Si3N4 and h-BN peaks. Standard 4 pt. bend mechanical test specimens [18], 3 mm × 4 mm × 50 mm, were machined from the Si3N4-4A4Y/88BN layered composite. The as-sintered surface was not polished prior to testing due to minor surface curvature. A fully articulated SiC 4 pt-bend flexural fixture with inner and outer spans of 20 and 40 mm, respectively, was used to fracture the speci-

Bismuth–Ceramic Nanocomposites with Unusual Thermal Stability via High‐Energy Ball Milling

AbstractElectrically conducting nanocomposites of bismuth metal and insulating ceramic phases of SiO2 and MgO were generated via high‐energy ball milling for 24 h using zirconia milling media. The resulting nanocomposites contain Bi nanoparticles with sizes down to 5 nm in diameter. The morphology is a strong function of the oxide phase: specifically, the Bi appears to wet MgO while it forms spherical nanoparticles on the SiO2. X‐ray diffraction measurements indicate a nominal bismuth grain size of 50 nm, and peak fitting to a simple bidisperse model yields a mixture of approximately 57 % bulk bismuth and 43 % 27 nm diameter crystallites. Nanoparticles as small as 5 nm are observed in transmission electron microscopy (TEM), but may not constitute a significant volume fraction of the sample. Differential scanning calorimetry reveals dramatic broadening in the temperatures over which melting and freezing occur and a surprising persistence of nanostructure after thermal cycling above the melting point of the Bi phase.

Effect of Milling Media on the Reaction Kinetics of the Mechanochemical Synthesis of Pentatitanium Trisilicide

The mechanochemical synthesis of pentatitanium trisilicide (Ti5Si3) by the ball‐milling of elemental mixed powders using zirconia, stainless steel, and tungsten carbide milling media was performed. These reactions were mechanically induced self‐propagating reactions. The induction period during the reaction using zirconia milling media was the shortest among these reactions. Because the thermal conductivity of zirconia is the lowest, it is assumed that the heat produced during milling using the zirconia milling media was effectively stored, and the ignition temperature was attained within the shortest time. The ignition temperature was estimated to be ∼500°C. The particle size of the Ti5Si3 obtained using the tungsten carbide milling media was the finest because of the induction time and specific gravity.

Formation of supersaturated solid solution in ZrO2CeO2 system induced by mechanical alloying

To date, studies of mechanical alloying appear to be mainly focused on metallic systems. In fact, mechanical milling of materials has long been a major component of ceramic processing. The structure variation of ceramics during milling process had been observed long before the wide applications of mechanical alloying. Despite this, many recent MA researchers, unfortunately, take it for granted that ceramics can only be refined while their structure cannot be changed during milling. The Recent work proved that mechanical alloying of ceramics is also possible. This result is important not only for the broader applications of ceramics but also for the synthesis of new structure ceramic materials. In this paper the authors report the formation of a supersaturated solid solution of zirconia in fluorite-structure ceria starting from the milling of zirconia and ceria crystalline powder mixtures.