Coarse Polishing Research Articles

The impact of different polishing systems on yttria-stabilized tetragonal zirconia polycrystalline crowns in relation to heat generation.

To investigate the heat generation on yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) crowns during polishing with coarse and fine polishing systems at various speeds. Two polishers (coarse and fine) at three polishing speeds were investigated. Two simulation models of the first mandibular molars were prepared for full coverage Y-TZP restorations with different reduction dimensions. Preheated water was pumped into the abutment chamber, to simulate the intrapulpal temperature and blood flow. Twelve Y-TZP crowns (3M™ Lava™ Esthetic) were milled for each prepared tooth abutment and each cusp (n = 10) was individually ground for 30 s and polished for 2 min. Thermocouple wire was secured to the intaglio surface of the crown and linked to a data logger for recording temperature changes. Selected scanning electron microscopy (SEM) images of the treated surfaces and polishers were analyzed. The data was statistically analyzed using Prism 9. The highest temperature rise was observed in the 20,000 RPM polishing speed groups for both coarse and fine polishing, and higher than the threshold value of 5.5°C for pulp damage. The Kruskal-Wallis test, revealed statistically significant differences (p < .0001) in heat generation between low (10,000 RPM) and high (20,000 RPM) polishing speeds. High-speed polishing at 20,000 RPM generated the most heat over the threshold of 5.5°C, which would threaten the dental pulp. The results suggest that a cautionary approach should be taken to high-speed intraoral polishing. Dental clinicians should be aware of the choice of polishing systems and speeds to avoid pulp damage from intraoral polishing of Y-TZP restorations.

Surface Roughness and Hardness of CAD/CAM Ceramic Materials after Polishing with a Multipurpose Polishing Kit: An In Vitro Study.

This article evaluates the effect of multipurpose polishing kit on surface roughness and hardness of three computer-aided design/computer-aided manufacturing (CAD/CAM) ceramic materials at different polishing durations. Weight changes of the polishing bur were also determined. Three CAD/CAM ceramic materials were lithium disilicate glass ceramic (IPS e.max CAD), translucent zirconia (VITA YZ), and zirconia-reinforced lithium disilicate ceramic (Celtra Duo). Ceramics were ground with a diamond bur, and polished with the multipurpose polishing kit (Eve Diacera HP), which comprises coarse and fine polishing burs. Surface roughness value (Ra) was measured using a noncontact optical profilometer (n = 10 per group) after grinding and every 15 seconds of coarse and fine polishing until 60 seconds. The complete polishing Ra was compared with the lab as-received specimens and human enamel. Surface morphology was examined using a scanning electron microscope after 60-second coarse and fine polishing and compared with the lab as-received specimens. Hardness was measured using a Vickers hardness tester on the lab as-received specimens and after the final polishing process (n = 4 per group). Changes in surface roughness and polishing bur weight of each material were analyzed using one-way repeated-measures analysis of variance (ANOVA) and dependent t-test. One-way ANOVA was used to detect differences in surface roughness, Vickers hardness, and bur weight among materials within the same polishing duration (α = 0.05). From grinding to complete polishing, the greatest Ra reduction was found in VITA YZ, followed by Celtra Duo and IPS e.max CAD. Final Ra values of all ceramics after 60-second fine polishing were not significantly different, and were similar to that of enamel and lab as-received specimens. Vickers hardness of ceramic materials did not change after grinding and polishing. Coarse polishing bur demonstrated the highest weight loss after polishing VITA YZ, followed by Celtra Duo and IPS e.max CAD. The multipurpose polishing kit reduced surface roughness of CAD/CAM ceramic materials to the similar level of the lab as-received specimen and enamel regardless of material's hardness. The reductions of surface roughness and a coarse polishing bur weight were highest in VITA YZ, followed by Celtra Duo and IPS e.max CAD.

Influence of Surface Modifications on Bacterial Adherence to Implant Abutment Materials.

The purpose of this study was to investigate the effects of various titanium and zirconia polishing protocols on the colonization of oral bacteria. Titanium and zirconia discs were divided into five groups: unpolished (control, UNP) and polished with Brownie only (BRO), Brownie plus Greenie (BPG), Brownie plus Greenie plus Supergreenie (BGS), and CeraMaster Coarse plus CeraMaster polishing tips (CER). The samples were sterilized and immersed in unstimulated saliva, then incubated in a liquid suspension of Streptococcus gordonii (S gordonii). The number of attached bacteria were counted 48 hours after the diluted suspensions were inoculated. Data were analyzed with ANOVA and Tukey test (P < .05). For titanium discs, the average number of bacteria from each group (CFU/mm2) was 1.51 x 103 for UNP; 3.71 x 103 for BRO; 5.65 x 103 for BPG; 8.99 x 102 for BGS; and 8.49 x 102 for CER. For zirconia, the averages were 2.87 x 102 for UNP; 3.16 x 102 for BRO; 3.50 x 102 for BPG; 1.83 x 102 for BGS; and 8.73 x 101 for CER. Inadequate polishing roughens surfaces and promotes microbial adhesion to titanium and zirconia. Sequential polishing to the finest-finish polishing tips minimizes bacterial adherence to abutment surfaces. Zirconia exhibited less bacterial adhesion than titanium.

The Effect of Different Abrasives on the Shear Bond Strength of Adhesive Resin Cements to Dentin

Aim: The aim of this study was to evaluate the influence of dentin surface preparation with coarse diamond, extra-fine diamond, and polishing discs on the shear bond strength (SBS) of two resin cements (RCs) to dentin. Materials and Methods: Sixty-six recently extracted human mandibular third molar teeth that were free of cracks, caries, or restorations were collected. Sixty teeth were used for the SBS test. The occlusal part of the crowns was cut to expose mid-coronal dentin. The teeth were divided into three dentin preparation groups: coarse diamond (Meisinger, Germany), extra-fine diamond, and polishing discs (SofLex, 3MESPE, USA), and further conventional RC combined with a self-etch adhesive (Panavia F2.0/ED Primer II, Kuraray, Japan) or self-adhesive RC (Maxcem Elite, Kerr, USA) subgroups ( n = 10). Sixty composite resin blocks were prepared (diameter: 3 mm, height: 4 mm) and bonded to the teeth according to the groups. The SBS test was performed with a universal testing device. Six teeth were used for the observation of prepared dentin surfaces and the resin–cement dentin interfaces using field emission scanning electron microscopy (Mira 3 XMU, Tescan). Statistical analyses were performed using the Mann–Whitney U and Kruskal–Wallis tests ( P &lt; .05). Results: Panavia F2.0/ED Primer II exhibited significantly higher SBS values for coarse and extra-fine diamond than for the polishing discs ( P &lt;.05). Regarding Maxcem Elite, no significant difference was observed for the preparation methods ( P &gt; .05). Panavia F2.0/ED Primer II had higher bond strength than Maxcem Elite for the dentin surface prepared with the coarse diamond ( P &lt;.05). The differences were not significant for the extra-fine diamond and the polishing disc groups ( P &gt; .05). Conclusion: The use of polishing discs did not increase the SBS of the RCs to the dentin.

Two-Step Chemical Mechanical Polishing of Stainless Steel

Stainless steel with high surface quality is required in many industries and chemical mechanical polishing can achieve both local and global planarization of the substrate surface. However, it is difficult to realize both high material removal rate and high surface quality by a single step polishing. In this regard, a two-step polishing process, coarse polishing with α-Al2O3 abrasives first and then fine polishing with silica abrasives, was proposed to solve the trade-off between material removal and surface quality. The effects of pH (1 ∼ 12) and H2O2 (0 ∼ 0.5 wt%) on the polishing of 304 stainless steel disk (area ∼ 6.7 cm2) were systematically studied and CMP mechanism of stainless steel was discussed. The results indicated that, at pH 4, with the addition of 0.01 wt% H2O2, the surface roughness of stainless steel was successfully reduced from 0.702 μm to 44.6 nm (the first step using α-Al2O3 abrasives) and 1.61 nm (the second step using silica abrasives). Finally, an ultra-smooth surface was obtained with decent material removal rate.

Influence of surface roughness on concrete nanoindentation

While the application of nanoindentation technology has made great progress in cemented materials, the nanoindentation data show great discreteness. This study proposes an efficient procedure for concrete with coarse aggregate grinding and polishing to provide low surface roughness through a lot of polishing experiments. An integrated investigation, using SEM-EDS and nanoindentation, is performed to study the causes of the discrete nanoindentation data. Thousands of nanoindentation experiments show that surface roughness should be tested before indentation, and the indentation matrix should be within the tested area due to the significant variability of surface roughness. Both surface roughness and micropores in concrete cause indentation data discretisation. The influence of surface roughness which is carried out through quantitative study is the main cause. The mean of the maximum indentation depth of aggregates and cement paste both decrease with the surface roughness and tend to be stable. The influence of surface roughness can be negligible if the root-mean-squared (RMS) roughness of indentation area is less than 120 nm. The corresponding irregular load-depth curves in indentation results should be abandoned. HIGHLIGHTS The discreteness of concrete nanoindentation data is quantitatively investigated through a multi-technique approach. Both surface roughness and micropores could affect the accuracy of nanoindentation results, and amongst which surface roughness is the main factor. Whilst the root-mean-squared of surface roughness of tested area is less than 120 nm for concrete specimens, the influence of surface roughness could be ignored.

MAIN PROPERTIES OF ABRASIVE DENTISTRY MATERIALS UTILIZED IN POLISHING OF REMOVABLE DENTURE BASES

The requirements to the quality of dental prostheses are currently very high. All dental prostheses need thorough final finishing through filing and polishing in order to obtain smooth, polished, shiny and esthetic surface. In addition to comfort and aesthetics this guarantees high hygienic quality of dental prostheses and extends their operational characteristics. Smooth and even surface of plastic or thermoplastic prostheses can more efficiently resist aging and decay caused by the change of temperature and bacterial flora activity which typically occur in the mouth cavity.&#x0D; Modern orthopedic dentistry utilizes various abrasive materials for filing and polishing of removable dentures made from different materials. Filing is the method of removing roughness and unevenness from the surface of dentures. Polishing is the final process of finishing aimed at obtaining smooth, shiny surface of dentures. It has been proved that rough and badly polished surface of dentures can cause significant patient’s discomfort and complicate hygiene of the mouth cavity. Together with the aggressive environment of the mouth cavity, such surfaces can lead to dental plague and tartar deposit, which, correspondingly, leads to bad smell, stomatitis and other dental and general illnesses. It has been proved that high quality finishing of the surface of removable dentures contributes to the improvement of their sustainability, durability and aesthetic characteristics. Well-polished surface of removable dentures more efficiently resists the processes of destruction, such as aging and decay, caused by the change of temperature and bacterial flora activity.&#x0D; The analysis of bibliography has shown that the process of polishing and the quality of the surface under work depend on a number of factors. The most significant role belongs to the size of abrasive particles, the pressure of abrasive on material and its thermal conductivity, the speed at which abrasive particles move along the surface. Grains of the size 0,15-0,75 mm are used most often. It has been specified that coarse polishing can utilize grains of bigger size reaching 1,5-2 mm. It has been proved that the pressure of abrasive should be light in order to prevent the damage of dentures or instruments. Moreover, excess pressure can cause overheating of an instrument or the surface of dentures being polished. It has also been demonstrated that even short-term high temperature exposure can result in polymer deformation which reduces sustainability and durability of removable dentures. It is known that overheating can be avoided on condition of following the basic rules of polishing.&#x0D; Orthopedic dentistry utilizes pumice, silicate or aluminum oxide, silicone dioxide as abrasive filler. The particular interest presents the study of the efficiency of zirconium silicate as an abrasive component of polishing paste for finishing of thermoplastic polymer bases and providing full gloss of the surface. Stearin, paraffin, wax, petrolatum are used as the components of surfactants, as they demonstrate low surface tension and facilitate spreading the paste on the polished surface.&#x0D; There is not a wide range of locally produced polishing pastes utilized in dentistry. They leave traces, microscratchings; do not allow obtaining even, smooth shiny surface of dentures with thermoplastic polymer basis. Finishing of removable dentures requires a lot of time and expenses from professionals.

Effects of Polishing Bur Application Force and Reuse on Sintered Zirconia Surface Topography.

Limited information is available on how to polish and finish zirconia surfaces following computer-aided design/computer-aided manufacturing (CAD/CAM), specifically, how differing application forces and reuse of zirconia polishing systems affect zirconia topography. To determine the effect of differing, clinically relevant, polishing application forces and multiple usages of polishing burs on the surface topography of CAD/CAM zirconia. One hundred twenty 220-grit carbide finished zirconia disks were sintered according to manufacturer's directions and divided into two groups for the study of two coarse polishing bur types. Each group was divided into subgroups for polishing (15,000 rpm) at 15 seconds for 1.0 N, 4.5 N, or 11 N of force using a purpose-built fixture. Subgroups were further divided to study the effects of polishing for the first, fifth, 15th, and 30th bur use, simulating clinical procedures. Unpolished surfaces served as a control group. Surfaces were imaged with noncontact optical profilometry (OP) and atomic force microscopy (AFM) to measure average roughness values (Ra). Polishing burs were optically examined for wear. Scanning electron microscopy (SEM) was performed on burs and zirconia surfaces. One-way ANOVA with post hoc Tukey HSD (honest significant difference) tests (α=0.05) were used for statistical analyses. AFM and OP Ra values of all polished surfaces were significantly lower than those of the unpolished control. Different polishing forces and bur reuse showed no significant differences in AFM Ra. However, significant differences in OP Ra were found due to differing application forces and bur reuse between the first and subsequent uses. SEM and optical micrographs revealed notable bur wear, increasing with increasing reuse. SEM and AFM micrographs clearly showed polished, periodic zirconia surfaces. Nanoscale topography, as analyzed with kurtosis and average groove depth, was found dependent on the specific polishing bur type. These in vitro results suggest changes in OP Ra due to bur reuse and polishing application force. Within the parameters of this study, the resultant topography of zirconia polishing is force-dependent and the reuse of coarse polishing burs is possible without statistically significant differences in Ra values after initial use. Nanoscale and microscale topography were shown to depend on specific polishing bur type.

Study on chemical mechanical polishing of silicon wafer with megasonic vibration assisted

Chemical mechanical polishing (CMP) is the primary method to realize the global planarization of silicon wafer. In order to improve this process, a novel method which combined megasonic vibration to assist chemical mechanical polishing (MA-CMP) is developed in this paper. A matching layer structure of polishing head was calculated and designed. Silicon wafers are polished by megasonic assisted chemical mechanical polishing and traditional chemical mechanical polishing respectively, both coarse polishing and precision polishing experiments were carried out. With the use of megasonic vibration, the surface roughness values Ra reduced from 22.260nm to 17.835nm in coarse polishing, and the material removal rate increased by approximately 15–25% for megasonic assisted chemical mechanical polishing relative to traditional chemical mechanical polishing. Average Surface roughness values Ra reduced from 0.509nm to 0.387nm in precision polishing. The results show that megasonic assisted chemical mechanical polishing is a feasible method to improve polishing efficiency and surface quality. The material removal and finishing mechanisms of megasonic vibration assisted polishing are investigated too.

Effect of varying concentrations of home bleaching agents on hardness of a resin composite: An in vitro study

Aim: To evaluate the effect of 10%, 20% and 35% Carbamide Peroxide home bleaching agent on hardness of a microfilled resin composite. Materials and Methods: Forty specimens of micro filled resin composite(Z250, 3M ESPE) were prepared using moulds with dimensions of 4mm diameter x 2mm thickness and cured with a LED polymerization unit (LEDition - Ivoclar Vivadent). The specimens were polished with coarse, medium, fine and super fine polishing discs (Sof-lex, 3M ESPE) and were stored in distilled water at 37?C in an incubator for 24 hours. Specimens were then divided into four groups of 10 specimens each. One group was selected as the control group (not treated with bleaching agents) and was stored in artificial saliva for 7 days at 37?C. The other three groups were treated with 10%, 20% and 35% carbamide peroxide gels respectively (Opalesence PF 10%, Opalesence PF 20% and Opalesence PF 35%, Ultradent) as per the manufacturer’s instructions over a period of 7 days. The specimens were subjected to Shore Durometer for 15 seconds and checked for hardness. The results were analyzed using ANOVA and Post hoc Bonferroni tests. Results: There was no significant difference in the hardness of the specimens belonging to Group A, Group B or Group C when compared with the Control Group. Conclusion: It can be confirmed based on the results of this study that the hardness of the Micro filled Composite resin was not affected by10%, 20 % or 35% carbamide peroxide bleaching gels when used as per the manufacturer’s instructions. Restorations done using microfilled composite resins can be retained after bleaching procedures.

A study on the influence of surface topography on the low-speed tribological performance of port plates in axial piston pumps

Axial piston pumps are widely used under severe conditions because they are able to operate highly efficiently at high pressure and various ranges of speed. However, pump efficiency is relatively low at low speed because of insufficient lubrication. The contact between the port plate and cylinder barrel is critically important because insufficient lubrication from this pair causes rapid wear and high friction loss which significantly reduces pump efficiency. Ring-on-disc testing was conducted to investigate the friction and wear behaviours of the port plate–cylinder barrel contact. Four types of surface finishing processes were used: coarse grinding, fine grinding, polishing, and laser texturing. A confocal laser scanning microscope and energy dispersive spectroscopy were used to examine the surface. Results indicate that the polished surfaces have the best tribological performance. Finely ground surfaces have lower wear but higher friction than coarsely ground surfaces. The number and size of the brass particles transferred to the steel surface are related to the surface topography. Laser textured surfaces have the highest friction and wear. Brass particles aggregate inside micro-dimples. Increasing contact pressure does not change the wear mechanism but increases abrasion.

The Effect of Surface Finish on Low-Temperature Acetylene-Based Carburization of 316L Austenitic Stainless Steel

We observed a strong influence of surface finish on the efficacy of low-temperature acetylene-based carburization of AISI 316L austenitic stainless steel. Steel coupons were prepared with different surface finishes prior to carburization, from P400 SiC grit paper to 1-µm-diameter-diamond-paste. The samples with the finer surface finish developed a thicker “case” (a carbon-rich hardened surface layer) and a larger surface carbon concentration. Transmission electron microscopy revealed that the differences arose mainly from the nature of the deformation-induced disturbed layer on the steel surface. A thick (>400 nm) disturbed layer consisting of nano-crystalline grains (≈10 nm diameter) inhibits acetylene-based carburization. The experimental observations can be explained by assuming that during machining or coarse polishing, the surface oxide layer is broken up and becomes incorporated into the deformation-induced disturbed layer. The incorporated oxide-rich films retard or completely prevent the ingress of carbon into the stainless steel.

Swing arm profilometer: improving the testing accuracy of large mirrors with shorter arms

Swing arm profilometer is a useful metrology tool for large optics. For larger mirrors, the testing accuracy decreases as the arm becomes longer, while the testing accuracy requirement remains the same. We introduce a simple solution to make testing of large mirrors with shorter arms possible, which improves the testing accuracy, especially reduces the uncertainty of low-order shapes like astigmatism and trefoil. Simulation and experiment results show that testing uncertainty of low-order shape and high-frequency errors reaches 0.1 m RMS, which faithfully meets the requirement of profile testing to guide the grinding and coarse polishing process.

Fracture toughness and hardness of a Y-TZP dental ceramic after mechanical surface treatments

The aim of the investigation was to evaluate the effects of surface mechanical treatments of all-Y-TZP restorations. In order to do that, surface roughness (Rs), fractal dimension (Df), fracture toughness (Ft), Vickers hardness (Hv), crack length (Cl), and opening (Co) were determined. Y-TZP bar-shaped specimens (n = 30) were subdivided into three groups: as received by milling center (M), after coarse polishing (CP), and after fine polishing (FP). The specimens were examined under both confocal laser and electron scanning microscopes. One-way ANOVA and post hoc tests were used to analyze the data (α = 0.05). The FP group showed a significantly lesser amount of Rs (P < 0.05). The Df analysis confirmed the significantly lesser amount of surface complexity within the FP group (P < 0.05). The Hv values were not significantly different among the groups. The Ft was significantly lesser within the FP group (P < 0.05). At the same time, both Cl and Co were significantly higher within the FP group (P < 0.05). Likewise, the FP treatment showed a significant decrease in roughness and surface complexity associated to a decrease of fracture toughness with a significant microcrack increase. The present study suggests that the fine polishing procedure in all-YTZP restorations produces surface embrittlement. Because of the surface embrittlement, mechanical surface polishing of all-zirconia restorations should be avoided.

Effects of different surface treatments on stainability of ceramics

Final adjustments may result in a loss of ceramic glaze, a situation which must be corrected by reglazing or polishing to obtain clinically successful restorations; such restorations may be susceptible to staining. The purpose of this study was to determine the stainability of ceramics exposed to coffee after different surface treatments and to correlate the surface roughness with the color differences. Sixty-six ceramic (IPS e.maxCeram) disks (15 × 2 mm) were fabricated and glazed according to the manufacturer's instructions, then assigned to 6 groups. Group Glaze (Group G), the control, was not subjected to any procedure. All the others were abraded with a diamond rotary cutting instrument. Group Reglaze (Group R) was reglazed; others were polished with different polishing materials; Group Shofu (Group S) was polished with abrasive stone (Dura-Green Stones), coarse silicon polisher (Ceramaster Coarse), silicon polisher (CeraMaster), and polishing paste (Ultra II) with polishing disks (Super-Snap Buff Disks); Group Ultradent (Group U) was polished with 1.0 and 0.5-μm polishing pastes (Ultradent Diamond) with a goat hair brush (Jiffy) and Group Bredent (Group B) was polished with an abrasive stone (Diagen turbo grinder), a round polishing brush (Abraso-fix), and polishing paste (Diamond) with felt wheels. Group Diamond rotary cutting instrument (Group D) was not treated after abrasion with a diamond rotary cutting instrument. Surface roughness was evaluated by profilometer (n=10), and 1 specimen from each group was evaluated by scanning electron microscopy (SEM). Color difference was measured by a spectrophotometer before and after 12 days of immersion in a coffee solution. Data were analyzed by 1-way ANOVA, the Tukey HSD test, and the Pearson rank correlation tests (α=.05). The surface roughness (Ra) values were ordered from the highest to the lowest value, which were Group D, B, U, S, R, and G, respectively. Significant differences among groups (P<.01) were found, except for G and R. There were no significant differences between the color difference values of Groups G and R, both of which were significantly lower than the other groups (P<.01). Group D showed the highest ΔE values (>2), which is considered clinically unacceptable (P<.01). No significant differences were found among the S, U, and B groups. There was an 83% positively significant relationship between Ra and ΔE values (P<.01). Rough surfaces stained more after coffee immersion than did smooth surfaces. Surface treatments affected surface roughness and color stability. Smooth surfaces showed better color stability after discoloration. Ceramic staining may be related to surface texture changes after different surface treatments.

Interlaminar shear strain measurement on angle-ply laminate free edge using digital image correlation

Because only coarse polishing is required in order for Digital Image Correlation (DIC) to be used, such a technique has been used to study interlaminar shear strain on the free edges of carbon/epoxy laminates. Pure UD and quasi UD have been considered. The measurement uncertainties are small when compared to measured fields. The correlation parameters of DIC have been optimized to highlight the strain gradients. DIC results for pure-UD samples show high interlaminar strain gradients with a non-linear behaviour and local damage close to inter-ply interfaces. Measurements of quasi-UD samples highlight the microstructure influence on the spatial repartition of free edge strain gradients.

Improving processing of large prisms

The process schemes for processing large prisms from blocking in gypsum and treatment with free abrasives with conversion to individual strengthening and use of a combined diamond instrument in coarse and fine polishing operations were examined. The results of improving processing with conversion to fine polishing operations from a spot to a lamellar instrument and from glass wool to PTS cloth in polishing are reported.

Chemical-Assisted Mechanical Polishing of Diamond Film on Wafer

Diamond has been well recognized a strategic engineering material. It possesses excellent physical and chemical properties including the highest hardness and thermal conductivity, and good resistance to chemical erosion. Although CVD diamond film has good potential outstanding properties, its industrial applications have been limited by the non-uniform thickness and rough surface. In the current study, the CVD diamond film is polished by the chemical-assisted mechanical method with different slurries. These slurries contain strong oxidation chemical and diamond powder. During the process, the diamond film was held against the rotational ceramic plate with transverse oscillation at 90 °Cor lower. The profilometer, atomic force microscope and scanning electron microscope were used to evaluate the surface integrity of the diamond films before and after polishing. Based on the experimental results, the slurry containing potassium persulfate (K2S2O8) produces the highest material removal rate while potassium permanganate (KMnO4) develops the best local surface roughness. The strategy of using potassium persulfate for coarse polishing followed by potassium permanganate for fine polishing yields the diamond films of the best global surface roughness. The average surface roughness of the diamond film produced by the proposed technique is below 10 nm after 5 hours.

An evaluation of the effects of handpiece speed, abrasive characteristics, and polishing load on the flexural strength of polished ceramics

Many studies on the strengthening effects of grinding and polishing, as well as heat treatment on ceramics, are not well standardized or use commercially available industrial polishing systems. The reported effectiveness of these strengthening mechanisms on ceramics may not be applicable to clinical dentistry. The purpose of this study was to evaluate the effects of controlled polishing on the flexural strength of dental ceramics by using a custom-made machine that applied standardized loads and speeds that coincided with the mean loads and speeds used by experienced prosthodontists. A total of 140 aluminous dental ceramic bar-shaped specimens (Vitadur Alpha Enamel) measuring 1.5 x 2.0 x 25 mm were fabricated and divided into 12 groups (for most groups, n=10). Specimens were untreated, polished with different polishing systems, polished at different speeds, ground and autoglazed, polished and autoglazed, autoglazed and polished, polished with loose (paste) and bonded abrasives, or overglazed. Simulated clinical polishing was performed on the ceramic specimens by using a customized polishing apparatus that allowed independent control over the relevant polishing parameters (abrasive hardness, applied load, linear speed, rotational velocity, and wheel stiffness). Flexural strength (MPa) was measured with a 4-point bending test, and subjective surface roughness was assessed with scanning electron microscopy. Autoglazing was performed at various stages of the polishing sequence to determine the effects of polishing on surface stresses. Mean values, standard deviations, independent-sample t tests, 1-way and 2-way analyses of variance, Dunnett t tests and Kruskal-Wallis tests were applied to the data (alpha=.05). Under a clinical load of 0.6 N for a coarse polishing wheel, 1.0 N for a medium polishing wheel, and 1.3 N for a fine polishing wheel, a linear speed of 499 mm/min, and a rotational velocity of 10,000 rpm, the use of clinical polishing instruments did not affect the flexural strength of the aluminous ceramics studied (P=.274). At higher rotational velocity (20,000 rpm), specimens polished with the diamond polishing system produced statistically weaker specimens compared with those that had been polished at 10,000 rpm (P=.019). Autoglazing treatment of the diamond-polished specimens did not reverse the strength degradation (P=.125). Conversely, diamond polishing of the autoglazed specimens resulted in significant flexural strength reduction (P=.029). Fine-diamond-bonded abrasive significantly reduced flexural strength (P=.025). Simulated clinical polishing at 10,000 rpm did not appear to substantially strengthen or weaken the ceramic specimens. Polishing at 20,000 rpm reduced flexural strength of the ceramic bars.

Airborne ultrasonic electrostatic transducers with conductive grooved backplates: tailoring their centre frequency, sensitivity and bandwidth

Several electrostatic transducers with metallic backplates have been manufactured. Some backplates have been subjected to coarse polishing, whereas others were first polished and then rectangular grooves were machined. Experimental investigations have been performed in order to study the influence of the groove widths, their layout and the value of DC bias on the transducer behaviour. These results are compared with theoretical predictions using the Helmholtz resonator model. The equation of the vibration of a membrane submitted to a mechanical stress, but neglecting the effects of its elasticity, was also compared with experimental results. This shows that the Helmholtz resonator model is adapted to the behaviour of the polished backplates, whereas the membrane model explains the vibration over the grooves. By choosing the groove widths and layout (i.e. polished- to grooved-surface ratio), the centre frequency, the bandwidth and the sensitivity of the transducer can be optimised for a given application.