Cavitation and rolling wear in silicon nitride

Rolling contact and cavitation testing were integrated in a testing methodology to study their combined effect on silicon nitride rolling elements. This testing methodology is well suited to perform controlled cavitation and rolling contact experiments. Acoustic cavitation method and a rotary tribometer were utilized to achieve this objective. Silicon nitride degradation initiated with the formation of blisters in rolling contact. This blisters underwent cracking in the subsequent testing, thereby allowing the lubricant to squeeze in due to rolling contact. Erosion pits formed and their density increased along with their growth by detaching grains and bridging into adjacent pits. As the erosion severity in the rolling contact increased the material eventually failed in a short period of time with respect to the testing conditions. The effect of cavitation and rolling contact parameters on material damage is detailed in this paper thereby allowing an evaluation of material systems for cavitation-rolling contact fatigue (RCF) conditions.     

Cavitation erosion of structural ceramics

Various aluminas, zirconias, silicon carbides, silicon carbide-boron carbide composites, boron carbide and a sialon were eroded for up to 30 h in distilled water at 18°C using an ultrasonic vibratory facility operating at 20 kHz with a peak-to-peak amplitude of 50 μm, and the progress of erosion was followed by using weight loss and microscopical techniques. The measured incubation period (weight loss less than 0·1 mg) varied from 15 min for reaction-bonded silicon carbide to 150 min for 99·9% alumina, and the maximum erosion rate varied from 0·11 mg/h for sialon to 8·25 mg/h for 97·5% alumina. Overall, sialon had the highest resistance to cavitation erosion. Some results depended critically on the surface finish. Microscopical details are presented, and their relationships to the erosion damage are discussed.     

Highly porous hybrid particle-fibre ceramic composite materials for use as diesel particulate filters

This paper describes an investigation into the potential of a novel type of ceramic composite for use as Diesel Particulate Filters (DPFs). Materials for this application need to be highly permeable to gas flow, capable of filtering and retaining fine particulate and be stable under demanding thermo-mechanical conditions. Novel materials have been produced, via a conventional blending, cold pressing and sintering route, containing coarse alumina particles and/or fine alumina fibres. Several thermal and mechanical properties are measured, for a range of fibre contents. Thermal Shock Resistance (TSR) has been investigated both experimentally and via evaluation of a TSR merit index, based on a combination of properties. This type of material shows promise in terms of thermo-mechanical stability, particularly with high fibre content. There are also indications that such composites would be suitable for DPF usage in terms of permeability levels and the filtration of fine particulate.     

Negative permittivity in titanium nitride-alumina composite for functionalized structural ceramics

The study on novel physical properties of structural ceramics or ceramic composites could make them more conducive to be function- and structure-integrated materials. Herein, titanium nitride-alumina (TiN–Al₂O₃) duplex ceramics were prepared and the dielectric spectra of the ceramics were studied from 10 MHz to 1 GHz. Negative permittivity appeared when TiN content exceeded 40 wt% due to the induced plasmonic state of massive delocalized electrons in connected TiN grain networks. Meanwhile, alternating current conduction behaviors of the duplex ceramics were discussed with percolation theory. Furthermore, the analysis of reactance by equivalent circuit models indicated that negative permittivity ceramics exhibited inductive character. This work realized negative dielectric behaviors in TiN–Al₂O₃ duplex ceramics and would promote the study of electromagnetic functionalization in wave shielding or attenuation for structural ceramics.     

Reinforcement effect of textured GnPs on advanced ceramics densified by SPS

Graphene nano-platelets (GnPs) are attractive reinforcement fillers for advanced ceramic materials. GnPs present interesting properties like mechanical reinforcement media, additive for improved electrical or thermal properties. The present study reports on the processing and performances of GnPs reinforced alumina based and ZrB₂ based ceramic matrices. In both systems, the effect of the GnPs has been determined for content up to 7.2 vol.%. The influence on the resulting properties due to the preferential orientation of the GnPs has been assessed. The high degree of orientation was tailored by the extrusion method used to pre-form the green compact.The densification was performed by Spark Plasma Sintering (SPS), allowing achievement of near full density compacts.The evolution of mechanical properties as a function of the GnPs content and their dependence on the reinforcing phase orientation is discussed. The evaluation of the electrical conductivity is reported.     

High temperature compressive strength and creep behavior of SiTiCO fiber-bonded ceramics

Fiber bonded silicon carbide ceramic materials provide cost-advantage over traditional ceramic matrix composites and require fewer processing steps. Despite their interest in extreme environment thermostructural applications no data on long term mechanical reliability other than static fatigue is available for them. We studied the high temperature compressive strength and creep behavior of a fiber bonded SiC material obtained by hot-pressing of SiTiCO fibers. The deformation mechanism and onset of plasticity was evaluated and compared with other commercial SiC materials. Up to 1400 °C, plasticity is very limited and any macroscopic deformation proceeds by crack formation and damage propagation. A transient viscous creep stage is observed due to flow in the silica matrix and once steady state is established, a stress exponent n ∼ 4 and an activation energy Q ∼ 700 kJ mol⁻¹ are found. These results are consistent with previous data on creep of polymer derived SiC fibers and polycrystals.     

氧化物纤维/氧化物陶瓷基复合材料研究概述

氧化物纤维，氧化物陶瓷基复合材料可以在高温氧化环境下长时间工作，是最有发展潜力的高温结构陶瓷材料之一。决定氧化物纤维，氧化物陶瓷基复合材料性能最主要的2个因素是氧化物纤维的性能和界面材料的组成与结构。笔者介绍了氧化物纤维和界面材料的发展，以及界面材料涂覆方法，并探讨了氧化物纤维，氧化物陶瓷基复合材料的发展趋势。     

A review of the research progress on the interface between oxide fiber and oxide ceramic matrix

Oxide ceramics have excellent high temperature performance, superior thermal and chemical stability, which can be used in high temperature oxidizing environments, while oxide ceramics generally have low toughness and are prone to catastrophic damage... Oxide ceramics can be reinforced by high performance ceramic fibers to improve the fracture strength and fracture toughness, thus expanding its application in high-temperature components such as aero-engine combustion chambers and tail nozzles. The interface between fiber and matrix is an important factor that determines the performance of the composites. By tailoring the interface, the energy dissipation mechanisms such as fiber debonding and fiber pull-out can be brought into play to avoid the catastrophic damage of the composites. This review paper summarizes the recent research progress of the oxide fiber/oxide ceramic matrix composites interface. The mechanical properties of the interface and the design principles of the interface engineering are discussed, and types of interfaces and coating preparation methods are reviewed.     

Accelerated aging versus realistic aging in aerospace composite materials. V. The effects of hot/wet aging in a structural epoxy composite

Samples of an aerospace‐grade, carbon‐fiber epoxy composite (Hexcell 8552/IM7) were subject to long‐term (≈ 1 year) hot/wet aging and thermal spiking under different humidity levels and temperature conditions related to the “in‐service” conditions seen in military aircraft. Changes to the chemical and physico‐chemical structure of the composite were analyzed by a range of experimental techniques including gravimetric analysis, FTIR, and DMA, to compare the effects of the various aging conditions. The results indicated that, while the chemical changes (as seen by FTIR) in this well cured composite appeared to be only significant at the surface, they did appear to have a deeper influence on some of the major physical property changes observed, such as microcracking, glass‐transition‐temperature (T_g) variations, and Tan δ curves. These physical changes could not be fully explained by standard water‐absorption effects alone but could also be influenced by chemical changes similar to those seen at the surface. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Friction and wear behavior of alumina-based graphene and CNFs composites

Alumina – carbon nanofibers (CNF) and alumina – graphene oxide (GO) composites were prepared by spark plasma sintering using freeze-dried powders prepared from optimized suspensions of the mixtures. The tribological behavior was studied using the ball-on-disk technique in dry sliding at ambient conditions and compared to a monolithic alumina used as a reference. At low loads there was little difference between friction and wear behavior, whereas at moderate loads the composites showed a noticeable reduction in wear rate over monolithic alumina, five and 2.5 times for the GO and the CNF composite respectively; the friction coefficient slightly decreased for the alumina – GO material. This behavior is related to the presence of a carbon-rich protecting tribofilm. The film present in the alumina – GO showed better tribological performance due to the absence of coalescence of cracks that led to delamination events in the case of the alumina – CNF composite.     

Friction and wear behaviour of silicon carbide/graphene composites under isooctane lubrication

The tribological performance of silicon carbide (SiC)/graphene nanoplatelets (GNPs) composites is analysed under oscillating sliding tests lubricated with isooctane, looking to explore their potential as components for gasoline direct injection (GDI) engines. High graphene filler contents (20?vol.% of GNPs) are required to substantially reduce the friction coefficient of SiC ceramics, attaining decreases on friction up to 30% independently of the applied load. For all materials and testing conditions a mild wear regime is evidenced. SiC/20?vol.% GNPs composite also enhances the wear resistance up to 35% at low load, but the addition of GNPs produces a deleterious effect as the load augments. The tribological behaviour depends on the formation and destabilization of a solid lubricant carbon-based tribofilm and strongly correlates with the mechanical properties of the tested materials.     

Phase and structural evolution of zirconolite ceramics prepared by solid-state reaction sintering

The evolution of phase assemblage and microstructure of stoichiometric zirconolite (CaZrTi₂O₇) ceramics, prepared by a solid-state reaction sintering route, was systematically investigated as a function of sintering temperature. Using powder XRD and quantitative phase analysis data, it was determined that the formation of zirconolite was a one-step reaction, without formation of intermediate phases. The accompanying fractions of secondary CaTiO₃ and ZrO₂ phases were reduced to approximately 2 wt % each after sintering at 1200 °C, with zirconolite formed as the major phase (> 99 wt%) after reaction at 1300 °C. Notable product densification only occurred at T ≥ 1400 °C, at which it was possible to achieve a relative density of 96.97% which is highly desirable for applications as a nuclear wasteform. The zirconolite-2M polytype structure (space group: C2/c) was formed in all products as expected, confirmed by combined high resolution TEM-ED analyses.     

Promising argillaceous materials for fine and structural ceramics

The results of studying the composition, structure, and properties of Kailinskoe clay are considered. The possibility for producing ceramic materials with a light-colored crock based on mineral material from the Siberian region is demonstrated. Translated from Steklo i Keramika, No. 8, pp. 12–15, August, 1999.     

Microstructure and dielectric properties of Ag-BaTiO3 composite ceramics

Di-phase composite ceramics based on BaTiO₃ with 5?vol% of Ag filler have been prepared by sintering the mixture of powders at temperatures above the silver melting point (1000?°C–1300?°C/2?h). As predicted by finite element calculations, the addition of metallic particles should produce a field concentration in some regions of the BaTiO₃ matrix and therefore, an enhanced dielectric response with respect to pure BaTiO₃. The role of oxygen vacancies on the dielectric relaxation mechanisms of Ag-BaTiO₃ composites has been investigated. The sintering temperature of 1200?°C provided optimized ceramics with excellent dielectric properties, i.e. with low losses (tanδ?<?3%) and room temperature permittivity measured at 50?kHz exceeding 6500 (and above 13,000 at the Curie temperature), as result of a good densification (94% relative density) and a synergy effect of the metallic particles inclusions and ceramic grain size in the range of ≈1?μm, where BaTiO₃ has a well-known maximum of its permittivity.     

Theoretical prediction on structure evolution and optimal properties of silicon modified hexagonal boron nitride as interphase in SiCf/SiC composite

To predict the effects of Si doping on hexagonal boron nitride (h-BN) and to achieve a balance between mechanical and oxidation properties for the interphase modification in SiC_f/SiC composites, we herein calculate and analyze the crystal structures and mechanical properties of (BN)₆₄Si_x (x = 4, 8, 16, 32) models by means of density functional theory (DFT) calculations and ab initio molecular dynamics (aiMD) simulations. The possible trends of crack deflection and self-healing ability are discussed. The modeling shows an obvious transition of (BN)₆₄Si_x from the layered crystal structure and anisotropic mechanical property to amorphous structure and isotropic mechanical property as the Si doping content up to 36.1 wt%. Regarding to the application of interphase in SiC_f/SiC composites, (BN)₆₄Si₁₆ model structure possess the highest debonding potential according to Cook and Gordon’s criteria and illustrates the higher self-healing capacity at elevated temperature.     

Tensile creep behavior of three-dimensional four-step braided SiC/SiC composite at elevated temperature

This article presents experimental results for tensile creep deformation and rupture behavior of three-dimensional four-step braided SiC/SiC composites at 1100 °C and 1300 °C in air. The creep behavior at 1300 °C exhibited a long transient creep regime and the creep rate decreased continuously with time. The creep behavior at 1100 °C exhibited an apparent steady-rate regime and the creep deformation was smaller than that at 1300 °C. However, the creep rupture time at both temperatures showed little difference. The mechanisms controlling creep deformation and rupture behavior were analyzed.     

Modelling thermal degradation of composite materials

A one–dimensional, transient thermal degradation heat transfer model for the response of composite materials when exposed to fire is presented. The model can handle layers of different materials. Material properties are functions of temperature. The reaction can be specified using Arrhenius‐type parameters or by inputting a density–temperature relationship determined by any experimental technique such as thermogravimetric analysis. The model is validated against the experimental data presented in Boyer's 1984 dissertation. Overall, the model provides excellent agreement with the experimental data. It is shown that very little difference is found between results arrived at by Arrhenius kinetics and results obtained by specifying the easier to measure density–temperature relationship. From this it is concluded that this technique is a viable alternative to Arrhenius‐type models. Copyright © 2006 John Wiley & Sons, Ltd.     

模板汽蚀问题研究

经过热力学计算，判断出在正常工作状态下模板发生汽蚀的可能性。又经过流体力学计算，找出了切实可行的改造方案并进行了实施，减缓了模板汽蚀速度。     

Effect of hafnium oxide on continuous aluminum oxide-mullite-hafnium oxide composite ceramic fibers

The Al₂O₃-mullite-HfO₂ (AMH) ceramic fiber with a 20 wt% of HfO₂ has demonstrated good tensile strength and good high-temperature stability due to the tiny diameter and small grains even at high temperatures. To investigate the effect of HfO₂ on crystal behavior and high-temperature performance, continuous AMH ceramic fibers with different HfO₂ contents (0 wt%, 10 wt%, and 50 wt%) were prepared by melt-spinning of polymer precursors. The effect of HfO₂ on the crystal form transition process, mechanical properties, and high-temperature resistance of AMH fibers was studied by in-situ XRD and STEM. The AMH fibers with 50 wt% HfO₂ had the highest strength retention rate of 78.33% after heat treatment at 1200 °C for 0.5 h. After 0.5 h of heat treatment at 1500 °C, the grain size of the AMH fibers with 50 wt% HfO₂ was still less than 200 nm.     

Cavitation during tensile deformation of high-density polyethylene

Cavitation process of high-density polyethylene during tensile deformation was studied. It has been shown that the crystallinity and perfection of HDPE crystals govern whether plastic deformation of the polymer is associated with cavitation or deformation occurs without cavitation. The strength of crystals may be controlled by crystallization conditions during preparation of a polymer. If the crystals are thin and the degree of crystallinity is low then the plastic deformation of crystals occurs before reaching the level of stress that initiates cavitation. On the other hand, if the crystals are thick, more perfect, and crystallinity is high, then the cavitation in an amorphous phase is initiated first, and later followed by the deformation of crystals. The cavitation process is usually initiated at the stress level close to the yield point. However, the level of stress necessary for cavitation may be decreased substantially by the orientation of crystalline lamellae, as it was observed in the skin layers of injection-molded material. Voids formed in the skin layer do not influence the yielding process. Typically, cavitation was initiated in volume at the stress of 29-30 MPa, but in the skin of injected samples voids were observed even at the macroscopic stresses of 2 MPa only. The development of voids with deformation was studied both for skin and volume of injection-molded HDPE sample. The shape of voids is strictly connected with deformation of crystalline phase around them.