Tribological behavior of graphene nanoplatelet reinforced 3YTZP composites

The tribological behavior of graphene nanoplatelet (GNP) reinforced 3 mol% yttria tetragonal zirconia polycrystals (3YTZP) composites with different GNP content (2.5, 5 and 10 vol%) was analyzed and discussed. Their dry sliding behavior was studied using a ball-on-disk geometry with zirconia balls as counterparts, using loads between 2 and 20 N at ambient conditions and compared to the behavior of a monolithic 3YTZP ceramic used as a reference material. The composites showed lower friction coefficients and higher wear resistance than the monolithic 3YTZP. An outstanding performance was achieved at 10 N, where the friction coefficient decreased from 0.6 to 0.3 and the wear rates decreased 3 orders of magnitude in comparison with the monolithic ceramic. A layer adhered to the worn surface was found for all the composites, but it did not acted as a lubricating film. The composites with the lowest GNP content showed an overall improved tribological behavior.     

Wear behaviour of partially stabilized zirconia at high sliding speed

The wear behaviour of three partially stabilized zirconias (PSZs) against steel was investigated on a pin-on-disk wear machine under dry conditions (from 5 m/s to 50 m/s, at 5 N load). In order to maintain plane-on-plane contact, the ceramic pins (dia. = 5 mm) were ground on the wear machine before the wear tests. Particular behaviour of PSZ ceramics was found with extremely heavy wear only in a special range of sliding speed between 10 m/s and 20 m/s and much lower wear was found outside this range. The heavy wear was accompanied by a phase transformation (tetragonal-cubic), which was identified by analysing the wear debris by means of XRD. Delamination mechanism was observed by analysing the worn surfaces of the pin and the wear debris with SEM attached to an EDAX system. The wear resistance of Mg-PSZ was generally better than that of Y-PSZ under the present conditions, especially at high sliding speeds (> 20 m/s).     

Grain-Size-Dependent Transformation Behavior in Polycrystalline Tetragonal Zirconia

Experimental observations of the tetragonal phase transformation behavior in polycrystalline zirconias and the related toughening contribution are presented. An analysis which considers transformation thermodynamics and residual stresses is developed to describe the influence of grain size on tetragonal-to-monoclinic transformation temperature. The model is based on the promotion of the transformation by local internal tensile stress concentrations whose effects scale with grain size. The analysis is supported by observations of the martensite start temperature–grain size behavior in polycrystalline tetragonal zirconia containing 12 mol% ceria (12CeTZP). Next, the analysis considers the grain-size-dependent behavior of the transformation-toughening contribution, Δ K ^T, and the transformation zone size, r _T. The tetragonal-to-monoclinic ( t -to- m ) formation temperature, M _s, increases with increase in tetragonal grain size, d . Experimental results for zirconia-12 mol% ceria (12CeTZP) and 2YTZP ceramics illustrate the predicted forms of the grain size dependencies for Δ K ^T and r _T. The analytical model also describes the temperature dependence of the transformation-toughening contribution Δ K ^T observed in 2 mol% yttriadoped polycrystalline tetragonal zirconias (2YTZP).     

Valence state and ionic conduction in Mn‐doped MgO partially stabilized zirconia

The mechanism of the enhancement in the ionic conductivity resulting from cubic phase stabilization in MgO partially stabilized zirconia (MgPSZ) by Mn doping was studied by examining the local Zr‐O structure. Cubic phase (14 vol%) in MgPSZ was increased with the addition of MnO₂, and 10 mol% Mn‐doped MgPSZ exhibited the highest cubic phase fraction (98.72%), which was analyzed by Rietveld refinement. In addition, only the cubic phase, not the monoclinic and tetragonal phases, was observed in the TEM‐SAED pattern of 10 mol% Mn‐doped MgPSZ. Doped Mn exhibited a high Mn²⁺/Mn⁴⁺ ratio, which was identified by X‐ray photoelectron spectroscopy (XPS). In addition, it indicates that oxygen vacancy formation by substitution of Mn²⁺ in the Zr⁴⁺ site in MgPSZ increased cubic phase fraction. Ionic conductivity of MgPSZ was improved by the cubic phase increase attributed to Mn doping, and 10 mol% Mn‐doped MgPSZ exhibited higher ionic conductivity than MgPSZ. To investigate the mechanism of the ionic conductivity improvement, Zr‐O local structure in Mn‐doped MgPSZ was analyzed by Zr K‐edge EXAFS of MgPSZ, and the number of bonding of the Zr‐O first shell decreased with increased Mn substitution. Therefore, it was considered that the oxygen vacancy generation led to an increase in the cubic phase and the number of ionic conduction sites.     

Cathodoluminescence: A Microstructural Technique for Exploring Phase Distributions and Deformation Structures in Zirconia Ceramics

The scanning electron microscope cathodoluminescence behavior of two partially stabilized zirconias is presented. Monoclinic material, either occurring on grain boundaries or produced by deformation, appears strongly luminescent, whereas in cubic-plus-tetragonal material grain boundaries appear dark, as do deformed regions associated with hardness indentations and wear tracks. These preliminary observations suggest that cathodoluminescence may be a valuable microstructural technique for characterizing the microstructure and deformation behavior of zirconia-based ceramics.     

Stabilized Zirconias Prepared by Mechanical Alloying

Cubic zirconias stabilized by various additive oxides have been obtained by mechanical alloying using high-energy ball-milling. Starting materials are powders of monoclinic zirconia mixed with magnesia, calcia, or yttria. Solid-state reaction is induced by ball-milling and, under given experimental conditions, a single phase consisting of cubic-zirconia nanocrystals is prepared from constituent oxides. Energy dispersive analyses in electron microscopy on ball-milled powders confirm that cubic zirconias have the composition corresponding to that of starting powders.     

Processing of alumina-coated tetragonal zirconia materials and their response to sliding wear

Alumina-coated tetragonal zirconia stabilised with 3 mol% of Y₂O₃ (YTZP) specimens (30 mm × 30 mm × 6 mm) have been obtained by dipping of pre-sintered YTZP compacts in alumina suspensions and subsequent sintering. The coated specimens present hardness values and a wear resistance similar to those of reference dense alumina specimens and significantly higher than those of the YTZP substrates.     

莫来石对Y-TZP陶瓷摩擦磨损性能的影响

用环-块摩擦磨损试验机在室温下研究了莫来石弥散钇稳定四方氧化锆多晶陶瓷(mullite dispersed yttria stabillized tetragonal zirconia polycrystal,MDZ)与高铬铸铁(high chromium cast iron,HCCI)摩擦副在2％(质量分数)SiO2磨粒的5％NaOH溶液中的摩擦磨损性能。结果表明：载荷在100-500N范围内,含15％(质量分数,下同)莫来石的15 MDZ复合陶瓷的耐磨性明显优于20 MDZ(含有20％莫来石)。载荷500N下,虽然3Y-TZP(yttria stabillized tetragonal zirconia polycrystal,含3％摩尔分数氧化钇)中弥散15％莫来石后力学性能下降,但耐磨性提高。柱状莫来石对耐磨性的主要贡献是：具有承载作用;阻碍裂纹扩展;折断的柱状莫来石的“滚针轴承”作用减轻磨擦副之间的摩擦磨损。MDZ复合陶瓷的主要磨损机制为微观犁削和柱状莫来石晶粒脱落。     

A combined structural,microstructural and dilatometric analysis of MgPSZ

MgO-doped partially stabilized zirconia (MgPSZ) is a ceramic where three phases (cubic, tetragonal and monoclinic zirconia) might coexist due to non-equilibrium conditions and proper quantification of phase content is difficult. Here, a few selected compositions (in the 7–10?mol% MgO range) and firing profiles were studied in order to cover several phase compositions and microstructural features. An original attempt is made to correlate data obtained by X-ray diffraction (XRD), dilatometry (DIL) and scanning electron microscopy (SEM). Distinct software packages used in the analysis of XRD patterns of different samples confirmed the shortcomings of the assessment of phase content in sintered bodies while proper handling of DIL data provided complementary quantitative information on their phase content. The suggested procedure can be further used to obtain a subtle insight on phase development with temperature, subject of major relevance with respect to tuning of the mechano-thermal behavior of these ceramics.     

Low-Temperature Treatment of Transformation-Toughened Partially Stabilized Magnesia-Doped Zirconia–A Solid Particle Erosion Study

It is now well established that the mechanical behavior of transformation-toughened partially stabilized zirconias (PSZs) is controlled by R -curve effects arising from the martensitic transformations. Recent studies have induced the martensitic transformation via a liquid-nitrogen-cooling treatment. The result has been the transformation of the tetragonal phase to an orthorhombic phase. The effect of this treatment on solid particle erosion properties is investigated here for a set of commercial partially stabilized magnesiadoped zirconia (Mg-PSZ) samples.     

Wear Perfomance of Ceramics in Ring/Cylinder Applications

In support of efforts to apply ceramics to advanced heat engines, a study was performed of the wear mechanisms of ceramics at the ring/cylinder interface. A laboratory apparatus was constructed to reproduce most of the conditions of an actual engine. Ring and cylinder specimens having a simple geometry were used to facilitate their fabrication. Attempts to operate three zirconias, α-SiC, Si₃N₄, and several plasma-sprayed coatings without lubrication were unsuccessful because of high friction and wear rates. The ceramics responded well to liquid lubricants at a temperature of 260°C. Plasma-sprayed coatings of Cr₂O₃ and hypersonic flame-sprayed coatings of Co-bonded WC performed particularly well as ring coatings. Similar performance was obtained with these coatings operating against SiC, Si₃N₄, SiC-whisker-reinforced Al₂O₃, and Cr₂O₃ coatings. The study demonstrated the critical need for lubrication.     

Ni-P复合镀层摩擦磨损性能的研究

采用化学复合镀在碳钢基体上共沉积（Ni-P）-SiC和（Ni-P）-PTFE两种复合镀层,重点研究了两种复合镀层在相同对磨条件下的摩擦磨损性能及磨损机理表现形式,并与化学镀镍磷层进行对比。结果表明,本实验条件下所制备的（Ni-P）-SiC和（Ni-P）-PTFE两类复合镀层分别具有优异的耐磨和减磨性能,均能对所镀覆基体材料起到良好的保护作用;对磨实验过程中主要出现磨料磨损、粘着磨损和氧化磨损三种磨损方式,而且磨损方式不同,镀层的摩擦磨损性能表现也不尽相同。     

Boron Carbide–Zirconium Boride In Situ Composites by the Reactive Pressureless Sintering of Boron Carbide–Zirconia Mixtures

The heating of B₄C–YTZP (where YTZP denotes yttria-stabilized zirconia polycrystals) mixtures, under an argon atmosphere, generates B₄C–ZrB₂ composites, because of a low-temperature (<1500°C) carbide–oxide reaction. Composites derived from mixtures that include ≥15% YTZP are better sintered than monolithic B₄C that has been fired under the same conditions. Firing to ∼2160°C (1 h dwell) generates specimens with a bulk density of ≥91% of the theoretical density (TD) for cases where the initial mixture includes ≥15% YTZP. Mixtures that include 30% YTZP allow a fired density of ≥97.5% TD to be attained. The behavior of the B₄C–YTZP system is similar to that of the B₄C–TiO₂ system. Dense B₄C–ZrB₂ composites attain a hardness (Vickers) of 30–33 GPa.     

Tribological behavior of carbon fiber reinforced ZrB2 based ultra high temperature ceramics

The tribological behavior of ultra-high temperature ceramic matrix composites (UHTCMCs) was investigated to understand these materials in friction applications. Samples consisting of pitch-based randomly orientated chopped carbon fiber (CF) reinforced ZrB₂-10 vol% SiC were prepared (ZS). The tribological behavior was tested on a self-designed dynamometer, coupling the UHTCMC pads with either carbon fiber reinforced carbon−silicon carbide (C/C-SiC) or steel disks, with two applied contact pressures (1 and 3 MPa) and the surface microstructures were analyzed to unravel the wear mechanisms. Even at high mechanical stresses, tests against the C/C-SiC disk showed stable braking performance and wear. The abraded material from a steel disk formed a stable friction film by fusing together harder pad particles with abraded steel, which reduced wear and stabilized the braking performance. The high values of coefficient of friction obtained (0.5–0.7), their stability during the braking and the acceptable wear rate make these materials appealing for automotive brake applications.     

Friction and wear behaviors of several polymers under oil-lubricated conditions

The friction and wear properties of polytetrafluoroethylene (PTFE), polyimide (PI), and polyamide 66 (PA66) sliding against GCr15 bearing steel under both dry and oil-lubricated conditions were studied by using an MHK-500 ring-block wear tester (Timken wear tester), and then Stribeck's curves of PTFE, PI, and PA66 under lubrication of the oil were given out. The worn surfaces of these polymers and the transfer films formed on the counterfaces were examined by using a scanning electron microscope (SEM) and an optical microscope, respectively. Experimental results show that the friction and wear-reducing properties of PTFE, PI, and PA66 can be greatly improved by lubrication with liquid paraffin, and the friction coefficients can be decreased by 1 order of magnitude compared to those in dry friction condition. Under lubrication of liquid paraffin, the friction coefficients of PTFE, PI, and PA66 decrease with the increase of load, but the wear increases with the increase of load. The variations of friction coefficients with load for PTFE, PI, and PA66 under lubrication of liquid paraffin can be described properly by the Stribeck's curves, as given out in this article. Under higher loads and sliding speeds in liquid paraffin lubrication, the friction and wear reducing properties of PA66 are the best, and those of PTFE are the worst; therefore, PA66 is also very suitable for applications in oil-lubricated conditions. Meanwhile, SEM and optical microscope investigations show that the wear and transfer of PTFE, PI, and PA66 can be greatly reduced by lubrication of liquid paraffin, but they still take place. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 2175–2182, 1998     

Friction and wear performances of Si3N4 ceramic matrix composites: A review from the perspectives of doped phase,layered structure design,and laser surface texturing

Si₃N₄ ceramic matrix composites (SN-CMCs) have been designed and widely used in many engineering fields under externally loading conditions. It is well known that the wear of materials is closely related to components’ mechanical reliability and service life. Understanding of the friction and wear performances is very important to provide insights into how to improve the wear resistance of materials. Coefficient of friction and wear rate, in general, are the most critical parameters of tribological behavior of materials. In this paper, friction and wear performances of SN-CMCs are reviewed from the perspectives of doped phase, layered structure design, and laser surface texturing. The article describes the change of friction and wear performances of SN-CMCs sintered with different additive phases. Tribological behavior of SN-CMCs with engineering designed layered structure is also analyzed from the aspects of surface coating and gradient structure. In addition, friction and wear performances of SN-CMCs under different lubrication conditions are also discussed. As an ideal processing method for hard and brittle ceramic materials, laser surface texturing has been proved to be an effective way to improve the wear resistance of SN-CMCs. Researches have shown that the better wear resistance can be obtained by combining laser surface texture with layered structure. At the end of paper, studies on friction and wear performances of SN-CMCs are summarized and prospected.     

氧化锆陶瓷的摩擦磨损性能

研究了ＺｒＯ２砂同工况条件下的摩擦磨损特性。运用扫描电镜，Ｘ射线衍射技术等来观察和分析磨损前后摩擦副的表面形貌、截面特征、磨悄形状及其相组成，从而分析磨损的机制。实验结果表明：干摩擦磨损和加水润滑的摩擦磨损主要机制是粘着磨损和疲劳磨损机制，并随着工况的     

Braking behaviours of C/C–SiC mated with iron/copper-based PM in dry,wet and salt fog conditions

C/C–SiC composites have enormous potential as a new generation of brake materials. It is worth studying the friction and wear behaviours of these materials in special environments to ensure the safe and effective braking of trains in practical applications. In this study, the braking behaviours and wear mechanisms of C/C–SiC mating with iron/copper-based PM in dry, wet and salt fog conditions are compared in detail. The results show that the coefficient of friction (COF) in the wet condition is reduced by 14.13% compared with that under the dry condition. The COF value of the first braking under salt fog condition is increased by 12.27% and 30.75% compared to the dry and wet conditions, respectively. Additionally, the tail warping phenomenon of the braking curve disappears in wet condition, which is attributed to the weak adhesion of friction interfaces and the lubrication of the water film. The main wear mechanisms of C/C–SiC mating with iron/copper-based PM under dry condition are adhesive, fatigue and oxidation wear. However, the dominant wear in wet condition is abrasive wear. The cooling and lubrication of water reduce the tendency of thermal stress, and weaken adhesive and fatigue wear. Furthermore, salt fog can accelerate the corrosion of alloy friction film, leading to the damage of friction film. Meanwhile, the third body particles formed in salt fog condition participate in the braking process. The wear mechanisms in salt fog condition are dominated by abrasive and delamination wear.     

Tribological behavior of poly(ether ether ketone) composites filled with potassium titanate whiskers sliding in different media

The friction and wear properties of poly (ether ether ketone) (PEEK) composites filled with potassium titanate whiskers (PTWs) under alkali, water, and dry conditions were investigated. The wear mechanisms in different lubrication situations were studied on the basis of examinations of the worn and counterpart surfaces with scanning electron microscopy and optical microscopy. The results showed that PTWs could obviously increase the wear resistance and reduce the friction coefficient of the PEEK composites under dry sliding conditions. Only when the PTW content was greater than 35 wt % did the wear resistance and friction coefficient deteriorate. Sliding in water caused increases in the wear rate and friction coefficient of the PEEK composites, and the PTW‐filled PEEK composites showed the highest friction coefficient and wear rate under this lubrication condition. On the contrary, sliding in an alkaline solution, the PTW‐filled PEEK composites showed the lowest friction coefficient and almost the same level of wear resistance as that found under the dry condition. Furrows and abrasive wear were the main mechanisms for the PTW‐filled PEEK composites sliding in water. The transfer onto the counterpart rings was significantly hindered with sliding under water and alkali conditions. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Development of stabilized zirconia–alkali salts dual membranes for carbon dioxide capture

Molten Na₂CO₃–K₂CO₃ (NKC, 56–44 mol%) eutectic compositions were vacuum-impregnated, at the eutectic temperature, into two porous ZrO₂:8.6 mol% MgO (magnesium-partially stabilized zirconia, MgPSZ) and ZrO₂:8 mol% Y₂O₂ (yttria-fully stabilized zirconia, 8YSZ) ceramics. Thermogravimetric analyses were performed in mixtures of that composition with MgPSZ and 8YSZ ceramic powders. Before impregnation, porosity was achieved in the two compounds by addition and thermal removal of 30 vol.% NKC. To ascertain the carbonates had filled up through the ceramic body, both sides of the parallel and fracture surfaces of the disk-shaped impregnated compositions were observed in a scanning electron microscope and analyzed by energy-dispersive X-ray spectroscopy. The electrical conductivity of the two ceramics, before and after impregnation, was evaluated by electrochemical impedance spectroscopy in the 5 Hz–13 MHz frequency range from approximately 530 to 740°C. The permeation of the carbonate ions through the membranes via the eutectic composition was assessed by the threshold temperatures of the onset of the carbonate ion percolation. The objectives were to prepare dual-phase membranes for the separation of carbon dioxide and for the development of carbon dioxide sensors.