Effects of alumina in nonmetallic brake friction materials on friction performance

The effects of alumina (Al₂O₃) as an abrasive on brake friction performance and friction layers of nonmetallic brake friction materials were evaluated. Five experimental compositions containing from 0 to 14.6 vol% alumina were tested (Al₂O₃—0, 3.4, 5.6, 9.0, and 14.6 vol%). The experimental results indicated that alumina enhances friction coefficient and improves wear performance. The formation and development of friction layers were characterized using X-ray fluorescence spectrometry and scanning electron microscopy with energy dispersive X-ray analysis. Phenomena of baryte film and transferred iron-containing film formed on the friction surfaces were observed. Baryte films were detected on specimens containing from 0 to 5.6 vol% alumina. Iron-containing films were detected on surfaces of all alumina-containing specimens but not on the material without alumina. The role of abrasive in nonmetallic friction materials was studied in relation to formulation, friction performance, and friction surfaces.     

Nanostructural effect of acid-etching and fluoride application on human primary and permanent tooth enamels

This study examined the nanostructural effects of fluoride application and the acid-etching time with respect to the time elapsed after fluoride application on the primary and permanent tooth enamel layers using atomic force microscopy (AFM) and scanning electron microscopy (SEM). 192 non-carious teeth were assigned to sixteen experimental groups (n = 12) including primary (1 to 8) and permanent (9 to 16) teeth, based on the timing of acid-etching with 37% phosphoric acid after an acidulated phosphate fluoride (APF) pre-treatment. The APF pre-treatment led to a decrease in surface roughness in both the primary and permanent teeth. After the APF treatment, the roughness in both primary and permanent teeth increased with the time elapsed. An acid-etching time of 40 s led to increased nanostructural changes in the enamel surfaces compared to the conventional acid-etching time of 20 s. This acid-etching process led to a higher roughness changes in the primary teeth than in the permanent teeth. To obtain proper enamel adhesion of a sealant after APF pre-treatment, it is important to apply acid-etching two weeks after pre-treatment. In addition, the acid-etching time should be prolonged to apply etching more quickly than two weeks, regardless of the primary and permanent teeth.     

齿面粗糙纹理方向对齿轮润滑效应的影响

 不同的机械加工方法会形成相异的表面粗糙纹理.为了探析不同粗糙纹理对齿轮传动润滑效应之影响,基于实际测量所获得的一系列粗糙度数值,进行了45组数值计算.计算结果显示：当粗糙度均方根值σ≤0.15 μm时,粗糙度对齿面油膜厚度与接触应力影响甚微;当σ>0.15 μm后,随着σ的增大,纵、横向纹理粗糙齿面接触应力均呈线性规律增加,但前者的接触应力始终大于后者.尤其是当σ≥0.55 μm时,与光滑齿面相比,纵、横向纹理粗糙齿面接触应力分别增大90%和80%;与此同时,纵向纹理膜厚值减低40%,而横向纹理膜厚值增大80%以上.在此基础上,针对纵、横向两种粗糙纹理齿面,分别建立了轮齿接触应力与齿面粗糙度之间的回归方程.最后,给出了横向纹理粗糙齿面的润滑效果要优于纵向纹理粗糙齿面的研究结论.     

Surface modifications to improve Ti–porcelain bonding

Investigations of surface modifications on cast titanium surfaces and titanium-ceramic adhesion were performed. Cast pure titanium was subjected to surface modification by preoxidation and introduction of an intermediate layer of SnO _x by sol–gel process. Surfaces only sandblasted with alumina were used as controls. Specimen surfaces were characterized by XRD and SEM/EDS. The adhesion between the titanium and porcelain was evaluated by three-point flexure bond test. Failure of the titanium–porcelain with preoxidation treatment predominantly occurred at the titanium-oxide interface. Preoxidation treatment did not affect the fracture mode of the titanium–ceramic system and did not increase the bonding strength of Ti–porcelain. However, a thin and coherent SnO _x film with small spherical pores obtained at 300 °C served as an effective oxygen diffusion barrier and improved titanium–ceramic adhesion. The SnO _x film changed the fracture mode of the titanium–ceramic system and improved the mechanical and chemical bonding between porcelain and titanium, resulting in the increased bonding strength of titanium–porcelain.     

Transparent magnesium aluminate spinel: a prospective biomaterial for esthetic orthodontic brackets

Adult orthodontics is recently gaining popularity due to its importance in esthetics, oral and general health. However, none of the currently available alumina or zirconia based ceramic orthodontic brackets meet the esthetic demands of adult patients. Inherent hexagonal lattice structure and associated birefringence limits the visible light transmission in polycrystalline alumina and make them appear white and non transparent. Hence focus of the present study was to assess the feasibility of using magnesium aluminate (MgAl₂O₄) spinel; a member of the transparent ceramic family for esthetic orthodontic brackets. Transparent spinel specimens were developed from commercially available white spinel powder through colloidal shaping followed by pressureless sintering and hot isostatic pressing at optimum conditions of temperature and pressure. Samples were characterized for chemical composition, phases, density, hardness, flexural strength, fracture toughness and optical transmission. Biocompatibility was evaluated with in-vitro cell line experiments for cytotoxicity, apoptosis and genotoxicity. Results showed that transparent spinel samples had requisite physico-chemical, mechanical, optical and excellent biocompatibility for fabricating orthodontic brackets. Transparent spinel developed through this method demonstrated its possibility as a prospective biomaterial for developing esthetic orthodontic brackets.     

Lapping assisted dissolved wafer process of silicon for MEMS structures

Dissolved wafer process (DWP) is being extensively used to fabricate complex micro-electro-mechanical system (MEMS) structures. Etching non-uniformity, increased surface roughness and duration of DWP is often influence MEMS devices yields. This paper presents a modified DWP involving lapping and polishing followed by chemical etching of silicon to release MEMS based structure. The lapping experiments are performed using silicon-carbide (SiC) and alumina (Al₂O₃) abrasive. The polishing of the silicon samples is also done. The lapped and polished surfaces are compared with etched silicon surfaces in KOH and EDP solutions. The lapping-polishing process is found to be 2.5 (Al₂O₃)–3 (SiC) times faster than a standard etching processes based on KOH and EDP solutions. The average roughness (R_a) of the lapped–polished silicon surfaces are found to be 19.2 and 32.9 nm corresponding to SiC and Al₂O₃ abrasive respectively. The R_a value of EDP and KOH etched silicon surfaces are found to be 16.2 and 238.3 nm respectively. Based on the lapping—polishing results, SiC based lapping followed by polishing of silicon surface can be used as an alternate of etching of silicon during DWP. In this paper, a two-step DWP, involving lapping-polishing followed by EDP chemical etching of silicon, is used to fabricate suspended comb-type microaccelerometer structure.     

Influence of topographical features on the fluoride corrosion of Ni-Ti orthodontic archwires

Different manufacturing processes of Ni–Ti archwires respond differently to corrosion due to the surface conditions involved. In this study, several topographical features and their influence upon fluoride corrosion were studied. Four topographies (smooth, dimple, scratch, and crack) according to the main surface defect were characterized (n = 40). Static corrosion tests were performed in artificial saliva with fluorated prophylactic gel (12500 ppm) for 28 days. The surface was characterized by SEM and laser confocal microscopy. Standard electrochemical corrosion (open circuit potential, corrosion potential and corrosion current density) was performed. Statistical analysis was carried out using the ANOVA test (α ≤ 0.05). An increase was observed in the surface defects and/or roughness of the cracked and scratched surfaces. These defects produced an important increase in corrosion behavior. The best surfaces for the orthodontic archwires were the smooth and dimpled surfaces, respectively. The increase in defects was independent of roughness. Manufacturing processes that produce surface cracks should be avoided in orthodontic applications.     

The effect of surface roughness on tensile strength of the medium density fiberboard (MDF) overlaid with polyvinyl chloride (PVC)

The objective of this study is to determine the effect of the surface roughness on the tensile strength perpendicular to the surface of medium density fiberboard (MDF) overlaid with polyvinyl chloride (PVC). The surfaces of the MDF panels were sanded with four different grit abrasives, 220, 240, 280 and 320-grit, to create different roughness values. The roughness of the panel surfaces were determined in accordance with ISO 4288. The MDF panels were laminated with polyvinyl chloride (PVC) using a polyurethane-based adhesive. A total of 70 samples having the dimensions of 50 × 50 mm from the panels were prepared based on EN 326-1. The universal test machine was used for tensile strength. The test results were statistically analyzed and it was found that the sanding process decreases the surface roughness of the MDF and as the grit size of the abrasives increases, the surface roughness decreases. The surface roughness influences the tensile strength perpendicular to the surface of the overlaid samples. The most suitable surfacing type for lower cost and the highest tensile strength is the sanding with the 240-grit abrasive.     

Porous ceramic lamellae for orthodontic ceramic brackets: Part I Fabrication and characterization

This study was undertaken to test a new and original orthodontic bracket base, consisting of a porous lamella, which was designed to facilitate removal of ceramic brackets from the enamel surface after treatment. In the phase of the study presented here, porous pellets were made by bonding coarse alumina particles (calcined or fused) with mullite, formed by their reaction with fine quartz particles during firing of the mixture at 1700°C. After machining the pellets to the desired shape, nine types of lamellae with different porosities were attached to the brackets using two different adhesive resins, and also bonded to bovine enamel using the same adhesives. The tensile bond strengths for the assemblies were determined so that the lamellae and the bonding adhesive that might be suitable for clinical application could be selected for testing in vitro in the second phase of the study.     

<Emphasis Type="Italic">Streptococcus sanguinis</Emphasis> adhesion on titanium rough surfaces: effect of shot-blasting particles

Dental implant failure is commonly associated to dental plaque formation. This problem starts with bacterial colonization on implant surface upon implantation. Early colonizers (such as Streptococcus sanguinis) play a key role on that process, because they attach directly to the surface and facilitate adhesion of later colonizers. Surface treatments have been focused to improve osseointegration, where shot-blasting is one of the most used. However the effects on bacterial adhesion on that sort of surfaces have not been elucidated at all. A methodological procedure to test bacterial adherence to titanium shot-blasted surfaces (alumina and silicon carbide) by quantifying bacterial detached cells per area unit, was performed. In parallel, the surface properties of samples (i.e., roughness and surface energy), were analyzed in order to assess the relationship between surface treatment and bacterial adhesion. Rather than roughness, surface energy correlated to physicochemical properties of shot-blasted particles appears as critical factors for S. sanguinis adherence to titanium surfaces.     

Preparation of AlN-Cu composite powders by electroless plating of controlled oxidized nitride particles to prevent degradation by hydrolysis

The present work describes the preparation of AlN-Cu composite powders by electroless plating. Initially, the hydrolysis reaction of the ceramic particles in the electroless solution was studied as a reference element for the design of a protective surface barrier that enabled the coating process, with no ceramic phase degradation. The metal source of the electrolytic bath was copper sulfate, with formaldehyde as the reducing agent, under alkaline conditions of pH 12. The microstructural characterization indicated the formation and growth of aluminum hydroxides from AlN particles, inhibiting the coating of Cu by increasing the OH⁻ ions in the solution. As the exposure time increased, the ellipsoidal bayerite grew from AlN and transformed into prismatic particles of the thermodynamically more stable gibbsite phase. To prevent the degradation of AlN, a controlled oxidation stage was implemented to form a protective barrier of non-reactive alumina on the surface through thermal treatment in oxidizing atmospheres. An atmosphere of dry air was found to be more appropriate than pure oxygen for the formation of a continuous and dense layer of crack-free alumina on nitride surfaces, and a temperature of 1000 °C for 1 h enabled the formation of 3.9 by weight of α-Al₂O₃, capable of reducing the hydrolysis reaction of AlN. The process of autocatalytic deposition on the passivated particles, applied in three consecutive steps of metallization, led to AlN-Cu composite powders with 29 wt% Cu. Finally, the coated powders were treated in a hydrogen-reducing atmosphere at 400 °C to remove traces of the Al(OH)₃ phase encountered, as well as to improve the adhesion of the nanostructured deposit of the cauliflower-like structure to the AlN surfaces, obtaining AlN-Cu composite powders suitable for the preparation of metal/ceramic composites.     

Effect of aging on the abrasive wear properties of AlMgSi1 alloy

In this paper, the wear performance of the aged AlMgSi1 alloy was investigated. Great improvements in mechanical properties of Al alloys can be achieved by suitable solution treatment and aging operations. A pin-on-disk wear machine was designed and developed for abrasive wear tests. The wear resistance was evaluated using a pin-on-disk wear testing method with a SiC abrasive paper counterface. The variation of wear volume is presented as a function of applied normal load, abrasive grit size and sliding distance for running speed. Mass losses were measured within a load range of 6.45–11 N, a sliding velocity range of 0.078–0.338 m/s and abrasive grit size of 5–30 μm. The effects of different sliding speeds and loads on wear resistance and surface roughness were also examined. It was measured amounts of mass loss and examined worn surfaces. Metal microscope was used to study the microstructures of the wear scars. Natural aged specimen observed maximum wear resistance.     

Biological response of human bone marrow stromal cells to sandblasted titanium nitride-coated implant surfaces

Titanium nitride (TiN) coating has been proposed as an adjunctive surface treatment aimed to increase the physico-mechanical and aesthetic properties of dental implants. In this study we investigated the biological response of primary human bone marrow stromal cells (BMSC) to TiN-coated sandblasted (TiN-SB) compared to uncoated sandblasted (SB) surfaces. SB and TiN-SB disks were qualitatively and quantitatively analyzed by atomic force microscopy. BMSC were obtained from healthy donors and their adhesion and proliferation on the titanium disks were evaluated by scanning electron microscopy and viability assay. The osteoblastic differentiation, in terms of alkaline phosphatase activity, osteocalcin synthesis, and extracellular mineralization, was assessed by specific immunoenzymatic or spectrophotometric assays. No difference (P > 0.05) between TiN-SB and SB disks was found in terms of any of the investigated parameters. TiN-coating showed to maintain the topographical characteristics of sandblasted titanium surfaces and their biological affinity toward bone precursors.     

Polyetheretherketone and titanium surface treatments to modify roughness and wettability-Improvement of bioactivity and antibacterial properties

Among the materials available for implant production,titanium is the most used while polyetherether-ketone (PEEK) is emerging thanks to its stability and to the mechanical properties similar to the ones of the bone tissue.Material surface properties like roughness and wettability play a paramount role in cell adhesion,cell proliferation,osteointegration and implant stability.Moreover,the bacterial adhesion to the biomaterial and the biofilm formation depend on surface smoothness and hydrophobicity.In this work,two different treatments,sandblasting and air plasma,were used to increase respectively roughness and wettability of two materials:titanium and PEEK.Their effects were analyzed with profilometry and contact angle measurements.The biological properties of the material surfaces were also investigated in terms of cell adhesion and proliferation of NIH-3T3 cells,MG63 cells and human Dental Pulp Stem Cells.Moreover,the ability of Staphylococcus aureus to adhere and form a viable biofilm on the samples was evaluated.The biological properties of both treatments and both materials were compared with samples of Synthegra(R) titanium,which underwent laser ablation to obtain a porous micropatterning,character-ized by a smooth surface to discourage bacterial adhesion.All cell types used were able to adhere and proliferate on samples of the tested materials.Cell adhesion was higher on sandblasted PEEK samples for both MG63 and NIH-3T3 cell lines,on the contrary,the highest proliferation rate was observed on sandblasted titanium and was only slightly dependent on wettability;hDPSCs were able to proliferate similarly on sandblasted samples of both tested materials.The highest osteoblast differentiation was ob-served on laser micropatterned titanium samples,but similar effects,even if limited,were also observed on both sandblasted materials and air plasma treated titanium.The lowest bacterial adhesion and biofilm formation was observed on micropatterned titanium samples whereas,the highest biofilm formation was detected on sandblasted PEEK samples,and in particular on samples not treated with air-plasma,which displayed the highest hydrophobicity.The results of this work showed that all the tested materials were able to sustain osteoblast adhesion and promote cell proliferation;moreover,this work highlights the fea-sible PEEK treatments which allow to obtain surface properties similar to those of titanium.The results here reported,clearly show that cell behavior depends on a complex combination of surface properties like wettability and roughness and material nature,and while a rough surface is optimal for cell adhesion,a smooth and less hydrophilic surface is the best choice to limit bacterial adhesion and biofilm formation.     

Preparation of porous alumina abrasives and their chemical mechanical polishing behavior

Porous alumina abrasives with different pore sizes were prepared using hydrothermal synthesis method by different hydrothermal temperatures. The pore structure, pore size and pore volume of the products were characterized by transmission electron microscopy and nitrogen adsorption desorption isotherm measurement. The chemical mechanical polishing (CMP) performances of porous alumina abrasives in hard disk substrate CMP were investigated. The results show that, the polished surface average roughness (Ra) decreases when the pore diameter of porous alumina abrasive increases. By comparison with solid alumina abrasive, the prepared porous alumina abrasives give lower Ra, and the porous alumina abrasive with 8.61 nm pore diameter has higher material removal rate under the same polishing conditions.     

Porous ceramic lamellae for orthodontic ceramic brackets: Part II In vitro performance testing

This study was undertaken to test a new and original orthodontic bracket base, consisting of a porous lamella, which was designed to facilitate removal of ceramic brackets from the enamel surface after treatment. In the phase of the study presented here, two types of lamella and the adhesive resin used to bond them to brackets and teeth, were evaluated in vitro. Two types of test were carried out on bracketed teeth. The tensile bond strength was measured for specimens that had been either kept in water for 24 h at 37°C or subjected to 18000 cycles in water between 6°C and 55°C. The stress required to remove brackets with debracketing pliers was measured and the mode of failure recorded for specimens that had been kept in water for 24 h at 37°C. The results indicate that bracket/lamella assemblies can be bonded to enamel sufficiently strongly for clinical application and can be safely removed without damage to enamel.     

A study on the abrasive and erosive wear behavior of arc-deposited Cr-N-O coatings on tool steel

In this study, the cathodic arc evaporation technique, by using the chromium target and controlling the flow rate of nitrogen/oxygen reactive gases, was utilized to deposit three different Cr-N-O coatings (CrN, CrN/Cr(N,O), CrN/Cr₂O₃) on AISI M2 tool steel. Two types of wear tests were applied to evaluate the abrasive and erosive wear behavior of the coated and uncoated specimens. One was the ball-on-disk abrasion test to measure the friction coefficient of these specimens. The other was the erosion test using Al₂O₃ particles (~ 177 µm in size and Mohr 7 scale) of about 5 g, and then the surface morphologies of the eroded specimens were observed. To further understand the coating effects on the two wear behaviors of M2 steel, coating structure, morphology, and adhesion were analyzed using XRD, SEM, and TEM, respectively. The results showed that surface roughness and adhesion of the double-layered coatings (CrN/Cr(N,O) and CrN/Cr₂O₃) were inferior to those of monolithic CrN, but their hardness and elastic modulus were superior to those of CrN. In the abrasive behavior, Cr-N-O coatings reduced the friction coefficient of M2 substrate. In particular, the CrN/Cr₂O₃ has the highest hardness/elastic (H/E) modulus ration, therefore the lowest friction coefficient, among all the coated-specimens tested. In the erosive behavior, the coated specimens exhibited better erosion resistance as compared to the uncoated ones, at the impingement angles of either 30^o or 90^o. Moreover, the erosion resistance of CrN/Cr(N,O) coatings was superior to that of CrN/Cr₂O₃ coatings due to its better adhesion.     

Dispersion effect and auto-reconditioning performance of nanometer WS<Subscript>2</Subscript> particles in green lubricant

This paper reported on dispersion effect and dispersing techniques of nanometer WS₂ particles in the green lubricant concocted by us. And it also researched on auto-reconditioning performance of nanometer WS₂ particles to the abrasive surfaces of steel ball from four-ball tribology test and piston ring from engine lubrication test. The treated and untreated nanometer WS₂ particles were analysed by infrared spectrum. And the elementary component and interior elementary distribution of abrasive surface repaired by nanometer WS₂ particles were analysed by multifunction electron spectrometer. The results showed that the combinative method of ultrasonic dispersion, mechanical agitation and surface modification could improve the dispersion uniformity and stability of nanometer WS₂ particles in the green lubricant effectively. And the optimal ratio of the mass between surface modifier and nanometer WS₂ particles was 1: 2.5, the optimal treating time was 5 h. And IR analysis indicated that surface modifier could react with hydroxide radicals on surfaces of WS₂ particles and modify the surfaces, and the long lipophilic groups on surfaces of nanometer WS₂ particles could stretch in oil adequately and form steric hindrance layers between particles which prevented particles from conglomerating and depositing. In addition, tribological tests and surface analysis indicated that there were WS₂ adsorption film and reaction film on abrasive surfaces during the tribological tests, which could fill and level up the furrows on abrasive surfaces. As a result, the abrasive surfaces were repaired effectively by nanometer WS₂ particles.     

Laser polishing of aluminum by remelting with high energy pulses

Laser polishing is a contact‐free, quick and automated method to smooth surfaces. The method has been applied to different forging and casting aluminum alloys. The surfaces of the samples were belt‐grinded with a grain size of mesh 240. The samples are protected from ambient air in a gas shield chamber. The used laser system is an Nd : YAG Laser with maximum pulse energy of 65 J. The initial and the laser polished surfaces have been analyzed by microscopy, roughness spectroscopy, white light interferometry and cross‐section polishes. The surfaces of the laser polished forging alloys are covered by multiple lateral and horizontal cracks. Unlike the forging alloys, the casting alloys could be processed well by laser polishing. The initial surface roughness of Ra₂₄₀ = 1.37 µm was reduced up to Ra_LP ≈ 0.47 µm. This represents a roughness reduction of 66%. The roughness spectroscopy of the laser polished surface shows for structural wavelengths from 2.5 µm to 500 µm a Ra‐value close to 0.1 µm and from 500 µm to 800 µm higher values. The remelted area extends up to100 µm into the material.     

Tribological response of sterilized and un-sterilized orthodontic wires

The aim of this work was first to study frictional forces between brackets and different arch wires, second to determine the influence of sterilization and surface treatment on the orthodontic arch wire friction coefficient. One type of bracket was chosen (stainless steel) and four kinds of wires were tested (NiTi and Ti–Mo alloys). Half of the wires were sterilized by autoclaving. Vickers hardness and roughness measurements were performed on each surface. The friction coefficient was calculated for various bracket/wire pairs at different times during the tribological test. EDX-SEM analysis was performed before and after friction tests on the surfaces and on wear debris.It was shown that TMA samples' friction coefficient and hardness are higher than those of NiTi whereas roughness is lower. The overall results highlighted two types of tribological response: abrasive and adhesive behavior. The sterilization process and surface treatment induced no significant modification of the tribological properties of any of the samples. Thus, it is important to encourage dental practitioners to practice autoclaving onto arch wires before clinical application; however, these results did not show clearly the advantage of using surface treated wires (no details available on the treatment process).