Effect of Hydrothermal Aging on Damping Properties in Sisal Mat-Reinforced Polyester Composites

Hydrothermal aging is a matter of considerable concern for natural fiber-reinforced polymers; it can alter dimensional stability and induce microcracks and macro strain on the composite structure. This study applied a sorption kinetic model and examined the effects of water on the damping factor of sisal mat-reinforced polyester composites. The experimental data were fitted well using a Boltzmann sigmoid function, suggesting a promising first step toward kinetic water sorption modeling. Additionally, a damping test was carried out using the impulse excitation technique, highlighting the composite material's dynamic response under varying water absorption conditions. The result showed that damping exhibited sensitivity to water absorption, increasing significantly during the first 24 h of immersion in water, then remained steady over time, inferring a critical time interval. An empirical model proved satisfactory with the correlation coefficient for sorption rates and damping of sisal mat polymeric composites.

Effect of artificial aging on optical properties and crystalline structure of high-translucency zirconia

PURPOSE

To investigate the effect of different in vitro aging protocols on the optical properties and crystalline structure of high-translucency (HT) zirconia.

MATERIALS AND METHODS

Thirty-six specimens of HT and extra-high translucency (XT) zirconia were divided into three groups: control (CO)-no treatment; hydrothermal aging (HA)-autoclave aging for 12.5 h at 134°C, 2 bar; clinically related aging (CRA)-aging in the chewing simulator for 1.2 million cycles, followed by 50,000 thermocycles (5-55°C) and immersion in HCl (pH 1.2) for 15 h. Optical properties, crystalline structure, and surface roughness were analyzed and compared using analysis of variance (5% significance level).

RESULTS

There was no statistically significant effect of aging on translucency (p = 0.10), but CRA promoted the development of a high contrast ratio (p = 0.03). Aging did not cause significant color changes for HT (p = 0.65) or XT (p = 0.36). The proportion of monoclinic crystals increased to 40% for HT-zirconia after HA and 5% after CRA. No monoclinic crystals were detected for XT groups. There was no effect of aging on surface roughness (p = 0.77).

CONCLUSIONS

Although hydrothermal aging has been widely used to verify zirconia crystalline stability, it did not generate an effect similar to clinically related aging on the optical properties and crystalline structure of zirconia. HA affected the crystalline structure of HT-zirconia, and CRA compromised the optical properties of XT zirconia.

Effect of Hydrothermal Aging on the Tribological Performance of Nitrile Butadiene Rubber Seals

High temperature and humidity affect the tribological performance of nitrile butadiene rubber (NBR) seals, which affects the precise positioning of cylinder systems. Therefore, it is crucial to study the effect of hydrothermal aging on the tribological performance of the NBR seals. In this study, the changes in the tribological performance of the NBR seals under hydrothermal aging conditions were investigated. The results show that the volatilization of additives and the increase in crosslink density of the NBR seals occurs in the hydrothermal aging environment, leading to the deterioration of their surface quality, elastic deformability, and tribological performance. The formation of surface micropores due to additive volatilization is the main factor in the degradation of tribological performance.

Enhanced bioactivity and hydrothermal aging resistance of Y-TZP ceramics for dental implant

Although yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) ceramics have been widely used as restorative materials due to their high mechanical strength, unique esthetic effect, and good biocompatibility, their general application to implant materials is still limited by their biological inertness and hydrothermal aging phenomenon. Existing studies have attempted to investigate how to enhance the bioactivity or hydrothermal aging resistance of Y-TZP. Still, more studies need to be done on the modification that combines these two aspects. In this study, Y-TZP was prepared by 77S bioactive glass (BG) sol and akermanite (AKT) sol infiltration and microwave sintering, which provided Y-TZP with high bioactivity while maintaining resistance to hydrothermal aging. Results of phase composition evaluation, microstructural characteristics, and mechanical property tests showed that modified Y-TZP specimens exhibited little or no tetragonal-to-monoclinic (t → m) transformation and maintained relatively high mechanical properties after accelerated hydrothermal aging treatment. The in vitro biological behaviors showed that the introduction of 77S BG and AKT significantly promoted cell adhesion, spreading, viability, and proliferation on the surface of modified Y-TZP ceramics. Therefore, this modification could effectively enhance the bioactivity and hydrothermal aging resistance of Y-TZP ceramics for its application in dental implant materials.

Effect of Modification and Hydrothermal Ageing on Properties of 3D-Printed Wood Flour-Poly(butylene succinate)-Poly(lactic acid) Biocomposites

Wood flour-poly(butylene succinate)-poly(lactic acid) biocomposite samples were prepared by fused-deposition-molding 3D-printing technology, and modifications with glycerol and a silane coupling agent (KH550) were carried out. The samples were then hydrothermally aged. Modification with glycerol and KH550 enhanced the hydrophilicity of the samples and increased their tensile strength. Hydrothermal aging clearly whitened the surfaces of all the samples and made them more hydrophobic. Meanwhile, their tensile properties and thermal stability became poor; a higher hydrothermal aging temperature affected the mechanical properties more negatively. The modified samples turned out to be more resistant to the hydrothermal aging, and modification with KH550 could improve the anti-hydrothermal aging properties of the samples better than that with glycerol, where the tensile properties and the cross-sectional morphologies of the fractured specimens were concerned. Generally, the effects of hydrothermal aging temperature on the physico-mechanical properties of the printed specimens were greater than those by hydrothermal aging time.

Hydrothermal Aging Alleviates the Phosphorus Poisoning of Cu-SSZ-39 Catalysts for NH3-SCR Reaction

As a new type of catalyst with the potential for commercial application in NO_x removal from diesel engine exhausts, Cu-SSZ-39 catalysts must have excellent resistance to complex and harsh conditions. In this paper, the effects of phosphorus on Cu-SSZ-39 catalysts before and after hydrothermal aging treatment were investigated. Compared with fresh Cu-SSZ-39 catalysts, phosphorus poisoning significantly decreased the low-temperature NH₃-SCR catalytic activity. However, such activity loss was alleviated by further hydrothermal aging treatment. To reveal the reason for this interesting result, a variety of characterization techniques including NMR, H₂-TPR, X-ray photoelectron spectroscopy, NH₃-TPD, and in situ DRIFTS measurements were employed. It was found that Cu-P species produced by phosphorus poisoning decreased the redox ability of active copper species, resulting in the observed low-temperature deactivation. After hydrothermal aging treatment, however, Cu-P species partly decomposed with the formation of active CuO_x species and a release of active copper species. As a result, the low-temperature NH₃-SCR catalytic activity of Cu-SSZ-39 catalysts was recovered.

Insights into SO2 Poisoning Mechanisms of Fresh and Hydrothermally Aged Cu-KFI Catalysts for NH3-SCR Reaction

The complex poisoning of Cu-KFI catalysts by SO₂ and hydrothermal aging (HTA) was investigated. The low-temperature activity of Cu-KFI catalysts was restrained by the formation of H₂SO₄ and then CuSO₄ after sulfur poisoning. Hydrothermally aged Cu-KFI exhibited better SO₂ resistance than fresh Cu-KFI since HTA significantly reduced the number of Brønsted acid sites, which were considered to be the H₂SO₄ storage sites. The high-temperature activity of SO₂-poisoned Cu-KFI was basically unchanged compared to the fresh catalyst. However, SO₂ poisoning promoted the high-temperature activity of hydrothermally aged Cu-KFI since it triggered CuOx into CuSO₄ species, which was considered as an important role in the NH₃-SCR reaction at high temperatures. In addition, hydrothermally aged Cu-KFI catalysts were more easily regenerated after SO₂ poisoning than fresh Cu-KFI on account of the instability of CuSO₄.

Effect of Thermal and Hydrothermal Accelerated Aging on 3D Printed Polylactic Acid

In the new transformation of 'Industry 4.0', additive manufacturing technologies have become one of the fastest developed industries, with polylactic acid (PLA) playing a significant role. However, there is an increasing amount of garbage generated during the printing process and after prototypes or end-of-life parts. Re-3D printing is one way to recycle PLA waste from fused filament fabrication. To do this process successfully, the properties of the waste mixture should be known. Previous studies have found that PLA degrades hydrolytically, but the time at which this process occurs for 3D printed products is not specified. This work aims to establish the baseline of the degradation kinetics of 3D printed PLA products to predict the service time until which these properties are retained. To achieve this, 3D printed specimens were thermally and hydrothermally aged during several time intervals. Thermal and mechanical properties were also determined. This study reveals that tensile strength decreases after 1344 h of hydrothermal ageing, simulating 1.5-2.5 years of real service time. PLA therefore has the same thermo-mechanical properties before reaching 1.5-years of age, so it could be recycled.

Hydrothermal Aging Treatment Activates V2O5/TiO2 Catalysts for NOx Abatement

Thermal stability is crucial for the practical application of deNO_x catalysts. Vanadia-based catalysts are widely applied for the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). Generally, hydrothermal aging at high temperatures induces the deactivation of deNO_x catalysts. However, in this work, a remarkable increase in low- and medium-temperature NH₃-SCR activity was observed for a V₂O₅/TiO₂ catalyst after hydrothermal aging treatment, especially at 750 °C for 16 h. After the vanadia-based catalyst was hydrothermally treated at 750 °C, the specific surface area decreased and the surface VO_x density and surface V ratio increased significantly. Therefore, the aged catalyst presented more abundant polymeric vanadyl species than the fresh one. Furthermore, the redox capability was improved markedly after hydrothermal treatment due to the strong interaction of vanadia and titania, contributing to the NH₃-SCR reaction. 750 °C is the optimal temperature to activate the V₂O₅/TiO₂ catalyst, improving the SCR performance significantly. This study provides an in-depth understanding of vanadia-based catalysts for practical applications.

Degradation Performance Investigation of Hydrothermally Stressed Epoxy Micro and Nanocomposites for High Voltage Insulation

Epoxy resins have demonstrated remarkable properties with potential for usage as high voltage insulators. However, a loss of these properties has been observed in high temperature and humid environments. In order to enhance the hydrothermal stability of epoxy resins, micro (15% SiO₂) and nano (5% SiO₂) silica-based composites of epoxy were fabricated and subjected to standard long term and short term accelerated hydrothermal conditions. To analyze the effect of these stresses, the samples were analyzed periodically through Fourier transform infrared spectroscopy (FTIR) for structure analysis; scanning electron microscopy (SEM) for surface analysis of long-term aged samples; and optical microscopy for the surface topography of short-term aged samples. The Swedish Transmission Research Institute (STRI) classification and contact angle measurement techniques were used for hydrophobicity analysis of long-term and short-term aged samples, respectively. After aging in both conditions, the nanocomposite showed better results as compared to the other samples. After 1000 h of aging, it showed HC-5 class of hydrophobicity, whereas EMC and NE degraded to the HC-6. In case of short-term aging, the contact angle decreased to the 64.15° and 75.05° from 104.15° and 114.9° for ENC and EMC, respectively. Also, in terms of structural degradation, ENC showed the highest structural stability after 1000 h of aging with the highest stable peak of aromatic ether at 1300–1500 cm⁻¹. Microscopic observation through scanning electron and optical techniques also revealed superior performance of the nanocomposites.

Revealing the Synergistic Deactivation Mechanism of Hydrothermal Aging and SO2 Poisoning on Cu/SSZ-13 under SCR Condition

In real-world application, Cu/SSZ-13 simultaneously suffers severe deactivation from hydrothermal aging and SO₂ poisoning during the periodic regeneration of diesel particulate filter (DPF). Herein, we first investigated the synergistic deactivation mechanism of hydrothermal aging and SO₂ poisoning on Cu/SSZ-13 under SCR condition. Hydrothermal aging alone induces more severe degradation of selective catalytic reduction (SCR) performance than SO₂ poisoning alone, while the presence of SO₂ during hydrothermal aging causes further worse SCR performance compared with hydrothermal aging alone. Hydrothermal aging not only damages Si-OH-Al sites, particularly in four-membered ring (4MR) of the CHA cage, but also brings the conversion of ZCuOH, leading to the formation of inactive CuO/CuAlO_x species. By contrast, SO₂ poisoning alone is more prone to promote the transformation of ZCuOH to Z₂Cu. Synergistic deactivation of hydrothermal aging and SO₂ poisoning would exacerbate the damage of Si-OH-Al sites and then the formation of CuO/CuAlO_x species. These results are expected to assist the knowledge-based catalyst design for diesel aftertreatment applications.

Hydrothermal Aging of ATZ Composites Based on Zirconia Made of Powders with Different Yttria Content

The presented work concerns the development and investigation of three different grades of ZrO₂ materials containing Al₂O₃ particles (ATZ-Alumina Toughened Zirconia ceramics with 2.3-20 vol.% of alumina). The zirconia powders containing 3 mol.% of yttria were synthesized by a precipitation/calcination method and fabricated from two different zirconia powders with different yttria content. Then, the selected ATZ composites (ATZ-B, ATZ-10 and ATZ-20) were prepared by means of conventional mixing, compacting and sintering at 1450 °C for 1.5 h. The phase composition, microstructure, relative density and basic mechanical properties were determined. Uniform microstructures with relative densities over 99% of theoretical density, hardness values between 12.0-13.8 GPa, flexural strength up to 1 GPa and outstanding fracture toughness of 12.7 MPa⋅m^1/2 were obtained. The aging susceptibility of alumina toughened zirconia materials, as a consequence of hydrothermal treatment, was investigated. The aim of this study was to determine the influence of LTD (low temperature degradation) on the tetragonal to monoclinic phase transitions and on the flexural strength of hydrothermally aged specimens. The results were compared to those obtained for commercially available tetragonal zirconia-based materials containing 3 mol.% of yttria. This research shows that ATZ composites that have excellent mechanical properties and sufficient hydrothermal aging resistance can be attained and later used in technical and biomedical applications.

Effect of Accelerated Aging on Some Mechanical Properties and Wear of Different Commercial Dental Resin Composites

The aim of current in vitro research was to determine the effect of hydrothermal accelerated aging on the mechanical properties and wear of different commercial dental resin composites (RCs). In addition, the effect of expiration date of the composite prior its use was also evaluated. Five commercially available RCs were studied: Conventional RCs (Filtek Supreme XTE, G-aenial Posterior, Denfil, and >3y expired Supreme XTE), bulk-fill RC (Filtek Bulk Fill), and short fiber-reinforced RC (everX Posterior). Three-point flexural test was used for determination of ultimate flexural strength (n = 8). A vickers indenter was used for testing surface microhardness. A wear test was conducted with 15,000 chewing cycles using a dual-axis chewing simulator. Wear pattern was analyzed by a three-dimensional (3D) noncontact optical profilometer. Degree of C=C bond conversion of monomers was determined by FTIR-spectrometry. The specimens were either dry stored for 48 h (37 °C) or boiled (100 °C) for 16 h before testing. Scanning electron microscopy (SEM) was used to evaluate the microstructure of each material. Data were analyzed using ANOVA (p = 0.05). Hydrothermal aging had no significant effects on the surface wear and microhardness of tested RCs (p > 0.05). While flexural strength significantly decreased after aging (p < 0.05), except for G-aenial Posterior, which showed no differences. The lowest average wear depth was found for Filtek Bulk Fill (29 µm) (p < 0.05), while everX Posterior and Denfil showed the highest wear depth values (40, 39 µm) in both conditions. Passing the expiration date for 40 months did not affect the flexural strength and wear of tested RC. SEM demonstrated a significant number of small pits on Denfil’s surface after aging. It was concluded that the effect of accelerated aging may have caused certain weakening of the RC of some brands, whereas no effect was found with one brand of RC. Thus, the accelerated aging appeared to be more dependent on material and tested material property.

Effect of air-abrasion at pre- and/or post-sintered stage and hydrothermal aging on surface roughness, phase transformation, and flexural strength of multilayered monolithic zirconia

This study aimed to evaluate the effect of air-abrasion/sintering order and autoclave aging on the surface roughness (Ra), phase transformation, and biaxial flexural strength (BFS) of monolithic zirconia. A total of 104 monolithic zirconia specimens (Katana ML) were divided into eight groups according to airborne-particle abrasion protocols and hydrothermal aging: control (non-aged: C-, aged: C+), air-abrasion before sintering (BS-, BS+), air-abrasion after sintering (AS-, AS+), and air-abrasion before and after sintering (BAS-, BAS+). A steam autoclave was used for accelerated aging, and Ra values were measured with a surface profilometer. All specimens were analyzed by X-ray diffraction to determine any phase transformation on the zirconia surface. BFS was measured by using the piston-on-three-balls method. Scanning electron microscopy and atomic force microscopy were performed on one specimen per group. BS and BAS groups showed higher Ra values compared with groups C and AS. The aging process significantly increased the monoclinic phase content of all specimens. Lower monoclinic levels were found in AS+ and BAS+ compared with other aged groups. The AS groups exhibited higher flexural strength values relative to control groups, whereas BS groups exhibited significantly lower flexural strength values (p < .05). There was no reduction in flexural strength by using the BAS protocol. Air-abrasion of zirconia at the pre-sintered stage only is not recommended in clinical use because of the remarkable decrease in flexural strength.

Catalytic Performances of Cu/MCM-22 Zeolites with Different Cu Loadings in NH3-SCR

The NH₃-SCR activities and hydrothermal stabilities of five xCu/MCM-22 zeolites with different Cu loadings (x = 2–10 wt%) prepared by incipient wetness impregnation method were systematically investigated. The physicochemical properties of xCu/MCM-22 zeolites were analyzed by XRD, nitrogen physisorption, ICP-AES, SEM, NH₃-TPD, UV-vis, H₂-TPR and XPS experiments. The Cu species existing in xCu/MCM-22 are mainly isolated Cu²⁺, CuO_x and unreducible copper species. The concentrations of both isolated Cu²⁺ and CuO_x species in xCu/MCM-22 increase with Cu contents, but the increment of CuO_x species is more distinct, especially in high Cu loadings (>4 wt%). NH₃-SCR experimental results demonstrated that the activity of xCu/MCM-22 is sensitive to Cu content at low Cu loadings (≤4 wt%). When the Cu loading exceeds 4 wt%, the NH₃-SCR activity of xCu/MCM-22 is irrelevant to Cu content due to the severe pore blockage effects caused by aggregated CuO_x species. Among the five xCu/MCM-22 zeolites, 4Cu/MCM-22 with moderate Cu content has the best NH₃-SCR performance, which displays higher than 80% NO_x conversions in a wide temperature window (160–430 °C). Furthermore, the hydrothermal aging experiments (xCu/MCM-22 was treated at 750 °C for 10 h under 10% water vapor atmosphere) illustrated that all the xCu/MCM-22 zeolites exhibit high hydrothermal stability in NH₃-SCR reactions.

Effects of Hydrothermal Aging of Carbon Fiber Reinforced Polycarbonate Composites on Mechanical Performance and Sand Erosion Resistance

Carbon fiber reinforced polycarbonate (CF/PC) composites have attracted attention for their excellent performances. However, their performances are greatly affected by environmental factors. In this work, the composites were exposed to hydrothermal aging to investigate the effects of a hot and humid environment. The mechanical properties of CF/PC composites with different aging times (0, 7, 14, 21, 28, 35, and 42 days) were analyzed. It was demonstrated that the storage modulus of CF/PC composites with hot water aged for seven days has the highest value in this sampling period and frequency. Through the solid particle erosion experiment, it was found that the hydrothermal aging causes the deviation of the maximum erosion angle of composites, indicating the composites underwent ductile–brittle transformation. Furthermore, the crack and cavity resulting from the absorption of water was observed via the scanning electron microscope (SEM). This suggested that the hydrothermal aging leads to the plasticization and degradation of CF/PC composites, resulting in a reduction of corrosion resistance.

Effect of Intra-Ply Hybrid Patches and Hydrothermal Aging on Local Bending Response of Repaired GFRP Composite Laminates

This study investigates the influence of intra-ply hybrid patches and hydrothermal aging on the indentation properties of patch repaired GFRP composites. Fabrics with various proportions of glass and Kevlar fibers were employed as the patches to achieve enhanced mechanical properties by hybridizing Kevlar and glass fibers together. Hydrothermal aging behavior of repaired composite laminates modified with water-resistant glass fibers in the outer layers was further investigated. Specimens were immersed in an environmental chamber containing seawater at temperatures of 30 (ambient), 50, and 70 °C until up to saturation. Damage mechanisms of repaired laminates were monitored using real-time acoustic emission (AE) technique. The experimental results showed that specimens repaired with 50G:50K patches offered superior performance than the virgin specimens. The hydrothermal aging effect on indentation behavior of the modified repaired specimens showed a considerable reduction in indentation properties, with higher strength retention exhibited by the repaired specimens modified with chopped glass fibers compared to the particulate fibers.

Water Absorption and Hygrothermal Aging Behavior of Wood-Polypropylene Composites

Environmentally sound composites reinforced with natural fibers or particles interest many researchers and engineers due to their great potential to substitute the traditional composites reinforced with glass fibers. However, the sensitivity of natural fiber-reinforced composites to water has limited their applications. In this paper, wood powder-reinforced polypropylene composites (WPCs) with various wood content were prepared and subjected to water absorption tests to study the water absorption procedure and the effect of water absorbed in the specimens on the mechanical properties. Water soaking tests were carried out by immersion of composite specimens in a container of distilled water maintained at three different temperatures, 23, 60 and 80 °C. The results showed that the moisture absorption content was related to wood powder percentage and they had a positive relationship. The transfer process of water molecules in the sample was found to follow the Fickian model and the diffusion constant increased with elevated water temperature. In addition, tensile and bending tests of both dry and wet composite samples were conducted and the results indicated that water absorbed in composite specimens degraded their mechanical properties. The tensile strength and modulus of the composites reinforced with 15, 30, 45 wt % wood powder decreased by 5.79%, 17.2%, 32.06% and 25.31%, 33.6%, 47.3% respectively, compared with their corresponding dry specimens. The flexural strength and modulus of the composite samples exhibited a similar result. Furthermore, dynamic mechanical analysis (DMA) also confirmed that the detrimental effect of water molecules on the composite specimens.

Preparation, Microstructure, Mechanical Properties and Biocompatibility of Ta-Coated 3Y-TZP Ceramic Deposited by a Plasma Surface Alloying Technique

A Ta coating has been successfully fabricated on the surface of zirconia polycrystals ceramic (3 mol% yttria, 3Y-TZP) by a plasma surface alloying technique. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) results showed that a α-Ta coating with a continuous and compact surface morphology which consisted of a deposited layer with a thickness of 390 nm and a diffusion layer with a thickness of 200 nm covered the 3Y-TZP. Due to the effect of inhabitation the t→m transformation by the deposited Ta coating, the biaxial flexural strength caused by the phase transformation during hydrothermal aging is reduced e.g. p < 0.05 after 20 h and/or 100 h. In addition, the Ta coating shows non-cytotoxicity and improved proliferation ability of osteoblasts.

Effect of Hydrothermal Aging Treatment on Decomposition of NO by Cu-ZSM-5 and Modified Mechanism of Doping Ce against This Influence

Cu-ZSM-5 and Ce-doped Cu-Ce-ZSM-5 samples were prepared by liquid-phase ion exchange method. The two catalysts were subjected to hydrothermal aging treatment in the simulated flue gas of a coal-fired power station at an ageing temperature of 650-850 °C. The denitration experiment found that the activity of the aged Cu-ZSM-5 was 19.6% to 41% lower than that of the fresh Cu-ZSM-5 at the optimal decomposition temperature of NO at 550 °C, while the aged Cu-Ce-ZSM-5 had only a 14.8% to 31.5% reduction in activity than the fresh Cu-Ce-ZSM-5. The samples were characterized by XRD, BET, H₂-TPR, XPS, NO-TPD, etc. The results showed that hydrothermal aging treatment leads to the dealumination of the ZSM-5 framework and reduces the specific surface area and pore volume of the micropore in the sample. It also exacerbates the isolated Cu²⁺, and the active center {Cu²⁺-O^2--Cu²⁺}²⁺ dimers migrate towards the sample surface and form inactive CuO. Doping with Ce can promote the dispersion of Cu(OH)⁺, which was the precursor of {Cu²⁺-O^2--Cu²⁺}²⁺. Ce³⁺ can preferentially occupy the less active bridged hydroxyl exchange sites, so that copper ions occupy the more active aluminum hydroxyl sites, thereby inhibiting the migration of active centers.

The Influence of Hydrothermal Aging on the Dynamic Friction Model of Rubber Seals

Cylinder has become an indispensable and important pneumatic actuator in the development of green production technology. The sealing performance of the cylinder directly affects its safety and reliability. Under the service environment of the cylinder, hydrothermal aging of the rubber sealing ring directly affects the dynamic friction performance of the cylinder. So, the dynamic friction model of the cylinder has been developed based on the LuGre friction model, which considers the influence of hydrothermal aging. Here, the influences of the static friction coefficient and Coulomb friction coefficient on the friction model are analyzed. Then, the aging characteristic equation of rubber is embedded in the model for revealing the influence of aging on the friction coefficient of the model. Results show that the aging temperature, aging time, and compressive stress affects the friction coefficient; the variation of the static friction coefficient is larger than that of the Coulomb friction coefficient. The improved cylinder friction model can describe the influence of the aging process on the cylinder friction characteristics, which is of great significance in the design of the cylinder’s dynamic performance.

Ion-Exchange Loading Promoted Stability of Platinum Catalysts Supported on Layered Protonated Titanate-Derived Titania Nanoarrays

Supported metal catalysts are one of the major classes of heterogeneous catalysts, which demand good stability in both the supports and catalysts. Herein, layered protonated titanate-derived TiO₂ (LPT-TiO₂) nanowire arrays were synthesized to support platinum catalysts using different loading processes. The Pt ion-exchange loading on pristine LPTs followed by thermal annealing resulted in superior Pt catalysts supported on the LPT-TiO₂ nanoarrays with excellent hydrothermal stability and catalytic performance toward CO and NO oxidations as compared to the Pt catalysts through wet-impregnation on the anatase TiO₂ (ANT-TiO₂) nanoarrays resulted from thermal annealing of LPT nanoarrays. Both loading processes resulted in highly dispersed Pt nanoparticles (NPs) with average sizes smaller than 1 nm at their pristine states. However, after hydrothermal aging at 800 °C for 50 h, highly dispersed Pt NPs were only retained on the ion-exchanged LPT-TiO₂ nanoarrays with the support structure consisting of a mixture of 74% anatase and 26% rutile TiO₂. For the wet-impregnation loading directly on anatase TiO₂ nanoarrays derived from LPT, the Pt catalysts experienced severe agglomeration after hydrothermal aging, with the nanoarray supports consisting of 86% anatase and 14% rutile TiO₂. Spectroscopy analysis suggested that Pt²⁺ cations intercalated into the interlayers of the titanate frameworks through ion-exchange impregnation procedure, which altered the chemical and electronic structures of the catalysts, resulting in the shifts of the electronic binding energy, Raman bands, and optical energy bandgap. The ion-exchangeable nature of LPT nanoarrays clearly provides a structural modification in Pt-doped LPT that has resulted in a strong interaction between the Pt catalysts and LPT-TiO₂ nanoarray supports, leading to the enhanced hydrothermal stability of the catalysts. Considering the wide applications of the LPT and TiO₂ nanomaterials as supports for catalysts, this finding provides a new pathway to design highly stable supported metal catalysts for different reactions.

In Vitro versus In Vivo Phase Instability of Zirconia-Toughened Alumina Femoral Heads: A Critical Comparative Assessment

A clear discrepancy between predicted in vitro and actual in vivo surface phase stability of BIOLOX^®delta zirconia-toughened alumina (ZTA) femoral heads has been demonstrated by several independent research groups. Data from retrievals challenge the validity of the standard method currently utilized in evaluating surface stability and raise a series of important questions: (1) Why do in vitro hydrothermal aging treatments conspicuously fail to model actual results from the in vivo environment? (2) What is the preponderant microscopic phenomenon triggering the accelerated transformation in vivo? (3) Ultimately, what revisions of the current in vitro standard are needed in order to obtain consistent predictions of ZTA transformation kinetics in vivo? Reported in this paper is a new in toto method for visualizing the surface stability of femoral heads. It is based on CAD-assisted Raman spectroscopy to quantitatively assess the phase transformation observed in ZTA retrievals. Using a series of independent analytical probes, an evaluation of the microscopic mechanisms responsible for the polymorphic transformation is also provided. An outline is given of the possible ways in which the current hydrothermal simulation standard for artificial joints can be improved in an attempt to reduce the gap between in vitro simulation and reality.

Effects of Silica Coating by Physical Vapor Deposition and Repeated Firing on the Low-Temperature Degradation and Flexural Strength of a Zirconia Ceramic

PURPOSE

To examine the application of physical vapor deposition (PVD) silica coating as an approach to retard low temperature degradation (LTD) for dental applications. Accelerated aging characteristics of heat- and surface-treated zirconia material were also investigated by exposing specimens to hydrothermal treatment.

MATERIALS AND METHODS

The specimens (90 disc-shaped specimens [15 mm ×1.2 mm]) were divided into 9 groups (n = 10) according to the test protocol: Ctrl, control (no surface treatment); Ag, autoclave aging; GrAg, grinding + aging; SiAg, silica coating + aging; GrSiAg, grinding + silica coating + aging; 3FAg, 3-time firing + aging; Gr3FAg, grinding + 3-time firing + aging; 5FAg, 5-time firing + aging; Gr5FAg, grinding + 5-time firing + aging. Accelerated aging was performed in a steam autoclave (134°C, 2 bars) for 12 hours. Following each treatment protocol, X-ray diffraction (XRD) analysis was used to estimate the relative amount of monoclinic phase and corresponding transformed zone depth (TZD). Additionally, a biaxial flexure test was used to calculate the flexural strength. Statistical analysis was conducted with one-way ANOVA and Fisher's LSD test (p < 0.05).

RESULTS

The tetragonal-to-monoclinic transformation was retarded by PVD silica coating only on ground surfaces. Ground and heat-treated specimens exhibited the lowest monoclinic content after aging. The biaxial flexural strength value of the GrAg group was significantly higher than the values in all of the other groups except the SiAg group. The flexural strength value of the GrSiAg group was significantly higher than that of the 3FAg group. There was no statistically significant difference between the other groups (p > 0.05).

CONCLUSIONS

Grinding decreased the susceptibility of zirconia to LTD and increased the flexural strength. PVD silica coating and repeated firing decreased the monoclinic content only in ground specimens during aging.

Surface and Mechanical Characterization of Dental Yttria-Stabilized Tetragonal Zirconia Polycrystals (3Y-TZP) After Different Aging Processes

Yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) is a ceramic material used in indirect dental restorations. However, phase transformation at body temperature may compromise the material's mechanical properties, affecting the clinical performance of the restoration. The effect of mastication on 3Y-TZP aging has not been investigated. 3Y-TZP specimens (IPS E-max ZirCAD and Z5) were aged in three different modes (n=13): no aging (control), hydrothermal aging (HA), or chewing simulation (CS). Mechanical properties and surface topography were analyzed. Analysis of variance showed that neither aging protocol (p=0.692) nor material (p=0.283) or the interaction between them (p=0.216) had a significant effect on flexural strength, values ranged from 928.8 MPa (IPSHA) to 1,080.6 MPa (Z5HA). Nanoindentation analysis showed that material, aging protocol, and the interaction between them had a significant effect (p<0.001) on surface hardness and reduced Young's modulus. The compositional analysis revealed similar yttrium content for all the experimental conditions (aging: p=0.997; material: p=0.248; interaction material×aging: p=0.720). Atomic force microscopy showed an effect of aging protocols on phase transformation, with samples submitted to CS exhibiting features compatible with maximized phase transformation, such as increased volume of the material microstructure at the surface leading to an increase in surface roughness.