机械密封覆层密封环端面性能分析

机械密封覆层密封环能够综合利用耐磨覆层与韧性基体材料的优良特性,但其应用主要依靠经验,缺乏针对其性能的研究。利用ANSYS软件建立釜用机械密封动环、静环和静环座组成的热-结构耦合模型,综合考虑覆层端面变形、液膜反压和密封环温度之间的相互作用,并试验验证了分析模型的正确性。分析覆层结构和材料组合对密封端面最大端面比压与速度的乘积(PbV)max、最高端面温度Tmax,覆层表面最大拉应力σmax、主界面最大切应力τmax、侧界面最大切应力σcmax和最大法向拉应力τcmax的影响,并确定最佳的覆层结构和材料组合。分析结果表明:覆层厚度、覆层与基体的热膨胀系数比和弹性模量比的变化主要影响覆层表面最大拉应力;覆层端面设计中,覆层厚度取值宜在0.4~0.6 mm,喷涂角度宜取15°~30°,覆层与基体的热膨胀系数比宜在0.5以上,弹性模量比宜在2.5以下。     

密封环支撑边界条件对机械密封端面变形的影响

机械密封的密封环是通过辅助O形圈支撑在轴上或者密封腔内，不同的结构设计会改变密封环支撑边界。针对三种机械密封结构模型，利用ANSYS有限元分析软件，模拟机械密封摩擦副端面的变形，讨论了橡胶O形密封圈不同受力边界条件下机械密封端面变形的规律。研究发现当动环、静环均采用SiC时，在静态（结构分析）时，该三种不同支撑结构的摩擦副端面均形成发散间隙，端面变形受支撑边界接触应力的影响较大；热结构耦合分析发现其动环、静环端面间隙呈收敛间隙运转时，热边界条件影响更大。当动环采用石墨，静环采用SiC时，发现其端面间隙可能为收敛型也可能为发散型，这与支撑边界有关。故密封环支撑边界条件的不同会影响动环端面变形，同时动、静环材料的弹性模量对端面的变形有较大影响，从而会影响密封性能。该研究对机械密封设计有指导意义。     

螺旋槽干气密封环端面摩擦试验及其性能分析

干气密封环端面在启停阶段和由于制造装配误差等造成非正常运行时存在严重的端面接触摩擦，有必要对干气密封动静环进行摩擦学试验，从而分析并探讨其摩擦学特性。利用端面摩擦磨损试验机，选定合适的工况参数与相应的测试技术对螺旋槽干气密封环进行测试，研究不同工况下的摩擦学特性。结果表明：在特定工况下的试验中，螺旋槽干气密封端面存在明显的磨合现象；当工况从226 N、150 r·min^-1增大至1130 N、500 r·min^-1时，石墨环磨损量最大增加193.3%，摩擦系数最大降低22.3%，说明石墨环的自润滑性影响密封端面的摩擦性能；由于端面间螺旋槽的存在，石墨环内圈磨损大于外圈。试验结果可为今后端面摩擦学性能的优化提供依据。     

颗粒介质用机械密封热力耦合变形及摩擦磨损研究

在含有颗粒介质的工作环境中下,硬质材料配对机械密封环的热力耦合变形和摩擦磨损对机械密封的泄漏和使用寿命起着至关重要的作用。考虑动静环和颗粒介质的摩擦,试验测定了摩擦系数,建立了动静环热力耦合的有限元计算模型,研究了WC-Co硬质合金和无压烧结碳化硅（SSiC）陶瓷两种硬质材料密封的温度场和端面变形规律,分析了不同工况下的密封间隙变化规律。试验测试分析了密封环温度、磨损前后的泄漏及表面粗糙度,讨论了端面的磨损机理,验证了计算模型的准确性。结果表明：考虑动环磨粒摩擦热的有限元模型能准确地预测密封的温度和端面变形;耦合作用下动静环端面呈现外径脱离、内径贴合的变形,且变形差异程度随压差和转速的增大而加剧;变形导致端面磨痕分布不均匀,内径磨痕较严重。WC-Co硬质合金配对密封环的端面变形小、泄漏量小,高硬度WC颗粒对Co基体能产生很好的“阴影效应”,具有良好的耐磨粒磨损性能。SSiC陶瓷材料韧性差,易产生片状磨屑,形成过渡型磨粒磨损,材料耐磨性较差,泄漏量增加明显。在磨粒工况下,WC-Co硬质合金机械密封具有泄漏小、耐磨性强的特点。研究结果为颗粒介质中机械密封的材料应用及设计优化提供了参考。     

不同形状方向性型孔液体润滑端面密封性能对比

表面微孔的方向性可以改变密封间隙中流体的流向,在孔区末端汇聚产生明显的流体动压效应,使摩擦副端面打开,形成全膜润滑。以不同开孔形状（圆形、菱形、椭圆形、长方形）型孔端面密封为研究对象,考虑润滑液膜中的空化现象,基于质量守恒JFO空化算法建立数值模拟模型,采用有限差分法求解Reynolds控制方程,获得端面膜压分布。对比分析了在不同操作参数和几何结构参数下不同开孔端面密封性能。结果表明：相比于圆孔,在低速或高压下,方向性型孔都具有较好的动压效应,且长方形孔的动压开启力最好,菱形孔泄漏率较小。当膜厚h₀=1.5～2.5 mm,孔深h_p=2～3 mm,长短轴比γ=3～4,反向开孔比β=0.5,倾斜角α₁=30°～50°、α₂=120°～140°时,不同形状方向性型孔可获得最佳的动压密封性能。     

螺旋槽干气密封环端面摩擦试验及其性能分析

干气密封环端面在启停阶段和由于制造装配误差等造成非正常运行时存在严重的端面接触摩擦,有必要对干气密封动静环进行摩擦学试验,从而分析并探讨其摩擦学特性。利用端面摩擦磨损试验机,选定合适的工况参数与相应的测试技术对螺旋槽干气密封环进行测试,研究不同工况下的摩擦学特性。结果表明:在特定工况下的试验中,螺旋槽干气密封端面存在明显的磨合现象;当工况从226 N、150 r·min~(-1)增大至1130 N、500 r·min~(-1)时,石墨环磨损量最大增加193.3%,摩擦系数最大降低22.3%,说明石墨环的自润滑性影响密封端面的摩擦性能;由于端面间螺旋槽的存在,石墨环内圈磨损大于外圈。试验结果可为今后端面摩擦学性能的优化提供依据。     

椭封内支撑环对疲劳容器力学性能影响数值分析

高周疲劳工况下，以内壁带全焊透支撑环的椭封为模型，采用ANSYS结构优化模块，分析了支撑环5个几何参数：角度A、厚度T、内侧焊角r、外侧焊角R、至设备中心线距离L，对模型疲劳寿命表征参数最大应力值变化幅σ_max和刚度表征参数最大形变量Δy的影响。结果表明：σ_max对L的变化敏感，σ_max在椭封长轴方向存在明显的稳定区、衰减区和发散区；Δy对T、L的变化敏感，T、L增大模型刚度增加。     

热变形对高压干气密封结构件的影响及材料的选用

某合成气压缩机组存在的主要问题是干气密封泄漏较大、产量损失大,影响机组稳定运行,需要对干气密封进行改造。分析了密封机理、影响密封性能的关键因素、结构、材料等,分别对动环、静环和动静环配对进行了温度及热应变计算,结果表明:环体最高温度出现在端面内侧,最低温度出现在环体外环面上;密封环材料的热传导率对密封环的热变形有影响;动静环配对正常工作时,端面热变形会导致从外径向内径形成一个收敛形间隙。根据热应变的影响来进一步优化动环槽型,与热变形位置进行合理匹配,目的是提高密封高压状态下的稳定性,从而保证机组的稳定运行。     

氨压缩机机械密封失效原因分析和处理

1　氨压缩机工况我厂冰机是由意大利辛比隆公司设计制造的多级离心式压缩机 ,型号为 2ＭＣＬ5 2 8/1 ,轴端密封采用德国伯格曼公司生产的Ｈ -Ｄ1 /1 42 -Ｋｂ1型机械密封。整个机械密封由 1套双端面主机械密封和 1套单端面辅助机械密封组成 (见图 1 )。图 1　机械密封结构简图1-灯笼环 ;2 -“Ｏ”形密封环 ;3-动环 ;4-“Ｏ”形密封环 ;5-动环定位套 ;6-机械密封套 ;7-防松螺丝 ;8-锁紧套 ;9-锁紧套 ;10 -密封垫　　双端面主机械密封动环 (3 ) ,由锁紧套 (8)压紧在机械密封套 (6 )上 ,动环下面装有“Ｏ”形密封环 (4)。动环和轴套间无驱动销 …     

基体条件对Sm2Zr2O7/YSZ双陶瓷层热障涂层界面残余热应力的影响

采用有限元分析软件ANSYS对等离子喷涂Sm2Zr2O7/YSZ双陶瓷层热障涂层界面残余热应力分布进行了数值仿真。结果表明:基体厚度不同时,涂层界面Sm2Zr2O7/YSZ及界面YSZ/NiCoCrAlY对应应力及应力梯度基本不变,表明应力及应力梯度与基体厚度无关;但基体材质热膨胀系数对涂层系统界面的径向、轴向及剪切应力梯度有决定性的影响,且各应力梯度随金属基体的热膨胀系数差异增加而增大,表明基体材质是影响涂层界面径向残余热应力及应力梯度的根本原因。采用多层陶瓷结构并合理选择各层材质的热膨胀系数将更加有利于降低涂层应力梯度,进而改善涂层性能,延长涂层寿命。     

Perovskite-Type Strontium Zirconate as a New Material for Thermal Barrier Coatings

Perovskite-type SrZrO₃ was investigated as an alternative to yttria-stabilized zirconia (YSZ) material for thermal barrier coating (TBC) applications. Three phase transformations (orthorhombic↔pseudo-tetragonal↔tetragonal↔cubic) were found only by heat capacity measurement, whereas the phase transformation from orthorhombic to pseudo-tetragonal was found in thermal expansion measurements. The thermal expansion coefficients (TECs) of SrZrO₃ coatings were at least 4.5% larger than YSZ coatings up to 1200°C. Mechanical properties (Young's modulus, hardness, and fracture toughness) of dense SrZrO₃ showed lower Young's modulus, hardness, and comparable fracture toughness with respect to YSZ. The "steady-state" sintering rate of a SrZrO₃ coating at 1200°C was 1.04 × 10⁻⁹ s⁻¹, which was less than half that of YSZ coating at 1200°C. Plasma-sprayed coatings were produced and characterized. Thermal cycling with a gas burner showed that at operating temperatures ∼1250°C the cycling lifetime of SrZrO₃/YSZ double-layer coating (DLC) was more than twice as long as SrZrO₃ coating and comparable to YSZ coating. However, at operating temperatures >1300°C, the cycling lifetime of SrZrO₃/YSZ DLC was comparable to the optimized YSZ coating, indicating SrZrO₃ might be a promising material for TBC applications at higher temperatures compared with YSZ.     

Intermediate-Temperature Environmental Effects on Boron Nitride-Coated Silicon Carbide-Fiber-Reinforced Glass-Ceramic Composites

The environmental effects on the mechanical properties of fiber-reinforced composites at intermediate temperatures were investigated by conducting flexural static-fatigue experiments in air at 600° and 950°C. The material that was studied was a silicon carbide/boron nitride (SiC/BN) dual-coated Nicalon-fiber-reinforced barium magnesium aluminosilicate glass-ceramic. Comparable time-dependent failure responses were found at 600° and 950°C when the maximum tensile stress applied in the bend bar was 60% of the room-temperature ultimate flexural strength of as-received materials. At both temperatures, the materials survived 500 h fatigue tests at lower stress levels. Among the samples that survived the 500 h fatigue tests, a 20% degradation in the room-temperature flexural strength was measured in samples tested at 600°C, whereas no degradation was observed for the samples tested at 950°C. Microstructure and chemistry studies revealed interfacial oxidation in the samples that were fatigued at 600°C. The growth rate of the Si-C-O fiber oxidation product at 600°C was not sufficient to seal the stress-induced cracks, so that the interior of the material was oxidized and resulted in a strength degradation and less fibrous fracture. In contrast, the interior of the material remained intact at 950°C because of crack sealing by rapid silicate formation, and strength/toughness of the composite was maintained. Also, at 600°C, BN oxidized via volatilization, because no borosilicate was formed.     

Strength, Fracture Mode, and Thermal Stress Resistance of HfB2 and ZrB2

Zirconium diboride and hafnium diboride were fabricated by hot-pressing at 1800°C and 120,000 psi. Bend strengths were measured on the fully dense materials from 25° to 1400° C in an argon atmosphere. These diboride compounds do not exhibit any gross plastic flow in the temperature range studied. The bend strengths go through a maximum between 700° and 1000°C and vary from 39,000 to 68,000 psi for HfB₂ and 30,000 to 56,000 psi for ZrB₂. The maxima in strength correspond to maxima in the fraction of transgranular fracture. The bend strength and room-temperature elastic modulus measurements were combined with available thermal conductivity and expansion data to calculate thermal stress resistance parameters. Under steady-state heat flow conditions, the calculated thermal stress resistance parameters of the borides are higher than those calculated for other refractory compounds.     

Kinetics and Mechanisms of Oxidation of 2D Woven C/SiC Composites: I, Experimental Approach

The oxidation behavior of a 2D woven C/SiC composite partly protected with a SiC seal coating and heat-treated (stabilized) at 1600°C in inert gas has been investigated through an experimental approach based on thermogravimetric analyses and optical/electron microscopy. Results of the tests, performed under flowing oxygen, have shown that the oxidation behavior of the composite material in terms of oxidation kinetics and morphological evolutions is related to the presence of thermal microcracks in the seal coating as well as in the matrix. Three different temperature domains exist. At low temperatures (<800°C), the mechanisms of reaction between carbon and oxygen control the oxidation kinetics and are associated with a uniform degradation of the carbon reinforcement. At intermediate temperatures, (between 800° and 1100°C), the oxidation kinetics are controlled by the gas-phase diffusion through a network of microcracks in the SiC coating, resulting in a nonuniform degradation of the carbon phases. At high temperatures (>1100°C), such diffusion mechanisms are limited by sealing of the microcracks by silica; therefore, the degradation of the composite remains superficial. The study of the oxidation behavior of (i) the heat-treated composite in a lower oxygen content environment (dry air) and (ii) the as-processed (unstabilized) composite in dry oxygen confirms the different mechanisms proposed to explain the oxidation behavior of the composite material.     

Thermal Stability of Lanthanum Zirconate Plasma-Sprayed Coating

Lanthanum zirconate (La₂Zr₂O₇, LZ) is a newly proposed material for thermal barrier coatings (TBCs). The thermal stability of LZ coating was studied in this work by long-term annealing and thermal cycling. After long-term annealing at 1400°C or thermal cycling, both LZ powder and plasma-sprayed coating still kept the pyrochlore structure, and a preferred crystal growth direction in the coating was observed by X-ray diffraction. A considerable amount of La₂O₃ in the powder was evaporated in the plasma flame, resulting in a nonstoichiometric coating. Additionally, compared with the standard TBC material yttria-stabilized zirconia (YSZ), LZ coating has a lower thermal expansion coefficient, which leads to higher stress levels in a TBC system.     

Influence of Loading Frequency on the Room-Temperature Fatigue of a Carbon-Fiber/SiC-Matrix Composite

The influence of cyclic loading frequency on the tensile fatigue life of a woven-carbon-fiber/SiC-matrix composite was examined at room temperature. Tension-tension fatigue experiments were conducted under load control, at sinusoidal frequencies of 1, 10, and 50 Hz. Using a stress ratio (σ_min/σ_max) of 0.1, specimens were subjected to maximum fatigue stresses of 310 to 405 MPa. There were two key findings: (1) the fatigue life and extent of modulus decay were influenced by loading frequency and (2) the postfatigue monotonic tensile strength increased after fatigue loading. For loading frequencies of 1 and 10 Hz, the fatigue limit (defined at 1 × 10⁶ cycles) was approximately 335 MPa, which is over 80% of the initial monotonic strength of the composite; at 50 Hz, the fatigue limit was below 310 MPa. During 1- and 10-Hz fatigue at a maximum stress of 335 MPa, the modulus exhibited an initially rapid decrease, followed by a partial recovery; at 50 Hz, and the same stress limits, the modulus continually decayed. The residual strength of the composite increased by approximately 20% after 1 × 10⁶ fatigue cycles at 1 or 10 Hz under a peak stress of 335 MPa. The increase in strength is attributed in part to a decrease in the stress concentrations present near the crossover points of the 0° and 90° fiber bundles.     

Residual Stress and Microstructural Evolution in Tantalum Oxide Coatings on Silicon Nitride

Due to its coefficient of thermal expansion (CTE) and phase stability up to 1360°C, tantalum oxide (Ta₂O₅) was identified and investigated as a candidate environmental barrier coating for silicon nitride-based ceramics. Ta₂O₅ coatings were plasma sprayed onto AS800, a silicon nitride ceramic from Honeywell International, and subjected to static and cyclic heat treatments up to 1200°C in air. Cross-sections from coated and uncoated substrates were polished and etched to reveal the effect of heat treatments on microstructure and grain size. As-sprayed coatings contained vertical cracks that healed after thermal exposure. Significant grain growth that was observed in the coatings led to microcracking due to the anisotropic CTE of Ta₂O₅. High-energy X-ray diffraction was used to determine the effect of heat treatment on residual stress and phases. The uncoated substrates were found to have a surface compressive layer before and after thermal cycling. Coating stresses in the as-sprayed state were found to be tensile, but became compressive after heat treatment. The microcracking and buckling that occurred in the heat-treated coatings led to stress relaxation after long heat treatments, but ultimately would be detrimental to the function of the coating as an environmental barrier by affording open pathways for volatile species to reach the underlying ceramic.     

Thermomechanical Fatigue Behavior of a Silicon Carbide Fiber-Reinforced Calcium Aluminosilicate Composite

Isothermal fatigue and in-phase thermomechanical fatigue (TMF) tests were performed on a unidirectional, continuous-fiber, Nicalon®-reinforced calcium aluminosilicate glass-ceramic composite ([O]_16, SiC/CAS-II). Monotonic tensile tests were performed at 1100°C (2012°F) and 100 MPa/s (14.5 ksi/s) to determine the material's ultimate strength (σ_ult) and proportional limit (σ_pl). Isothermal fatigue tests at 1100°C employed two loading profiles, a triangular waveform with ramp times of 60 s and a similar profile with a superimposed 60-s hold time at σ_max. All fatigue tests used a σ_max of 100 MPa (40% of σ_pl), R = 0.1. TMF loading profiles were identical to the isothermal loading profiles, but the temperature was cycled between 500° and 1100°C (932° and 2012°F). All fatigued specimens reached run-out (1000 cycles) and were tested in tension at 1100°C immediately following the fatigue tests. Residual modulus, residual strength, cyclic stress-strain modulus, and strain accumulation were all examined as possible damage indicators. Strain accumulation allowed for the greatest distinction to be made among the types of tests performed. Fiber and matrix stress analyses and creep data for this material suggest that matrix creep is the primary source of damage for the fatigue loading histories investigated.     

Monazite Coatings on Fibers: II, Coating without Strength Degradation

Washed and unwashed rhabdophane (LaPO₄· x H₂O) sols were used to apply monazite coatings to 3M Nextel 720 and 610 fibers. This precursor was designed to minimize stress corrosion from gaseous decomposition products at high temperature. The coatings were heat-treated in-line at 900°–1300°C, in air, using a continuous vertical coater with immiscible liquid displacement. Coatings were characterized by optical microscopy, scanning electron microscopy, and transmission electron microscopy. The sol was characterized with light-scattering and zeta-potential measurements. Precursor phase evolution was studied with differential thermal analysis/thermogravimetric analysis and X-ray diffractometry. The washed sol had a higher pH and lower weight loss than the unwashed sol. The as-coated fibers were tensile tested, along with coated fibers heat-treated in air at 1200°C for 100 h. The precursor was slightly phosphate-rich, and this excess phosphate reacted with alumina in the fiber to occasionally make very small (<10 nm) pockets of AlPO₄ at the coating–fiber interface after 100 h at 1200°C. Both washed and unwashed sols made coated fibers with higher tensile strengths than those of coated fibers made from other precursors, and the washed sol may actually have slightly increased fiber strength when in-line heat treatments at <1200°C were used. A small amount of AlPO₄ may also have helped seal preexisting flaws. Degradation mechanisms during fiber coating are discussed in this paper.