A Nickel-Alloy-Based High-Temperature Self-Lubricating Composite with Simultaneously Superior Lubricity and High Strength

It is a challenge to design self-lubricating materials that exhibit and maintain reduced friction coefficient as well as high strength over a wide range of temperatures. A high-temperature self-lubricating nickel-alloy-based composite was created using the hot pressing technique. The composite exhibited high relative density, and simultaneously superior lubricating properties, average friction coefficient below 0.25 from room temperature to 800 °C, and high strength, 470 MPa of tensile strength and 1500 MPa of compressive strength. The composite was very promising in high-temperature tribology.     

Study on Tribological Performance of NiAl Matrix Self-Lubricating Composites Containing Graphene at Different Loads

This study aimed to explore the possibility of improving the tribological performance of NiAl matrix composites by graphene addition. Friction and wear experiments of as-prepared specimens were conducted under different conditions using a pin-on-disk wear testing machine. NiAl matrix composites containing graphene showed satisfactory performance in friction coefficient and wear resistance compared to NiAl matrix composites without graphene. For the active effect of graphene, the friction coefficient and wear rate of NiAl matrix composites were maintained at relatively lower values. The beneficial antifriction and antiwear effects of graphene gradually failed when the applied load was above 8 N. Graphene in NiAl matrix composites played an active role in the formation of a friction layer, which was beneficial to the lower friction coefficient and wear rate. In light of this research, graphene plays an active role in reducing the friction coefficient and wear rate. Hence, graphene has great potential in applications as an effective solid lubricant to promote tribological behavior.     

Effect of Temperature on Tribological Properties and Wear Mechanisms of NiAl Matrix Self-Lubricating Composites Containing Graphene Nanoplatelets

Studies have been carried out to explore the friction and wear behaviors of NiAl matrix self-lubricating composites containing graphene nanoplatelets (NG) against an Si₃N₄ ball from 100 to 600°C with a normal load of 10 N and a constant speed of 0.2 m/s. The results show that NG exhibits excellent tribological performance from 100 to 400°C compared to NiAl-based alloys. A possible explanation for this is that graphene nanoplatelets (GNPs) contribute to the formation of a friction layer, which could be beneficial to the low friction coefficient and lower wear rate of NG. As the temperature increases up to 500°C, the beneficial effect of GNPs on the tribological performance of NG becomes invalid due to the oxidation of GNPs, resulting in severe adhesive wear and degradation of the friction layer on the worn surface of NG. GNPs could hold great potential applications as an effective solid lubricant to promote the formation of a friction layer and prevent severe sliding wear below 400°C.     

Improving the Tribological Behavior of Graphite/Cu Matrix Self-Lubricating Composite Contact Strip by Electroplating Zn on Graphite

Studies to explore the nature of friction, and in particular thermally activated friction in macroscopic tribology, have lead to a series of experiments on thin coatings of molybdenum disulfide. Coatings of predominately molybdenum disulfide were selected for these experiments; five different coatings were used: MoS₂/Ni, MoS₂/Ti, MoS₂/Sb₂O₃, MoS₂/C/Sb₂O₃, and MoS₂/Au/Sb₂O₃. The temperatures were varied over a range from −80 °C to 180 °C. The friction coefficients tended to increase with decreasing temperature. Activation energies were estimated to be between 2 and 10 kJ/mol from data fitting with an Arrhenius function. Subsequent room temperature wear rate measurements of these films under dry nitrogen conditions at ambient temperature demonstrated that the steady-state wear behavior of these coatings varied dramatically over a range of K = 7 × 10⁻⁶ to 2 × 10⁻⁸ mm³/(Nm). It was further shown that an inverse relationship between wear rate and the sensitivity of friction coefficient with temperature exists. The highest wear-rate coatings showed nearly athermal friction behavior, while the most wear resistant coatings showed thermally activated behavior. Finally, it is hypothesized that thermally activated behavior in macroscopic tribology is reserved for systems with stable interfaces and ultra-low wear, and athermal behavior is characteristic to systems experiencing gross wear.     

Formation of Friction Layers in Graphene-Reinforced TiAl Matrix Self-Lubricating Composites

The friction layer structure has been proved to be formed during severe plastic deformation and markedly improves the tribological properties of material. The dry friction and wear performance of graphene-reinforced TiAl matrix self-lubricating composites (GTMSC) at different sliding velocities are systematically researched. GTMSC show the best tribological properties and special friction layer structure containing a wear-induced layer and a grain refinement layer with a nanocrystalline (NC) structure under surface after sliding at a sliding speed of 1.1 m/s. Nanoindentation results show that the grain refinement layer has a higher hardness and elastic modulus than the wear-induced layer. This special microstructure of friction layers beneath the surface after sliding leads to a low coefficient of friction and high wear resistance of GTMSC. Moreover, it is deduced that the appearance of an NC structure results in hardening of the material. The formation mechanisms of friction layers are researched in detail. It can be concluded that the formation of a wear-induced layer results from frictional heat and fracture of the counterpart. The formation of a grain refinement layer is due to severe plastic deformation and dynamic recrystallization. Severe plastic deformation results in the formation of an NC structure and dynamic recrystallization leads to grain refinement.     

Effect of Fluoride Content on Friction and Wear Performance of Ni<Subscript>3</Subscript>Al Matrix High-Temperature Self-Lubricating Composites

The self-lubricating composites Ni₃Al–BaF₂–CaF₂–Ag–Cr, which have varying fluoride contents, were fabricated by the powder metallurgy technique. The effect of fluoride content on the mechanical and tribological properties of the composites was investigated. The results showed that an optimal fluoride content and a balance between lubricity and mechanical strength were obtained. The Ni₃Al–6.2BaF₂–3.8CaF₂–12.5Ag–10Cr composite showed the best friction coefficients (0.29–0.38) and wear rates (4.2 × 10⁻⁵–2.19 × 10⁻⁴ mm³ N⁻¹ m⁻¹) at a wide temperature range (room temperature to 800°C). Fluorides exhibited a good reduced friction performance at 400 and 600°C. However, at 800°C, the formation of BaCrO₄ on the worn surface due to the tribo-chemical reaction at high temperatures provided an excellent lubricating property.     

Ekonol对Ekonol—石墨—PTFE自润滑复合材料的摩擦学性能的影响

本文研究了聚对－羟基苯甲酸酯（Ekonol)的含量对Ekonol-石墨－PTFE自润滑复合材料的摩擦磨损性能的影响,试验结果表明：随着Ekonol含量的增大,复合材料的摩擦系数有所上升,但其均在0．19以下,当Ekonol含量低于20％时,其上升幅度较大,而当Ekonol含量高于20％时,其上升幅度较平缓;复合材料的磨损量最初随着Ekonol含量的增大呈现急剧减少,在Ekonol含量为25％时出现了一个极小值,此后又呈现缓慢的上升。扫描电子显微镜（SEM）对复合材料磨损后表面形貌的观测表明：当Ekonol含量较低时复合材料发生的是严重粘着磨损,而当Ekonol含亘较高时,复合材料发生的是疲劳磨损,说明了Ekonol的填加可明显地提高此复合材料的抗粘着能力。     

等离子喷涂制备镍基自润滑复合涂层的研究

为提高车轴用材料35CrMo的耐磨性,在Ni60A中添加粒度均为(-280～340)目的二硼化钛TiB2钴Co和铬Cr粉;将等离子喷涂和均匀设计方法引入轴减磨抗磨设计中,在35CrMo上等离子喷涂制备200μm镍60A基二硼化钛TiB2、钴Co、铬Cr复合自润滑涂层。研究结果表明:由SEM可看出涂层与基体结合良好且涂层呈层状结构分布,由EDS可看出各元素渗透到了Ni60A基体里并产生了冶金结合;将验证组结果和神经网络预测值对比,磨损误差在12%之内,显微硬度误差在11%之内,涂层相比基体耐磨性提高了6倍,显微硬度提高了3倍;可从人工神经网络的预测结果中选出具有优良性能的镍60A基二硼化钛TiB2、钴Co、铬Cr复合自润滑涂层的配比范围。     

Coupled Effects of Fractal Roughness and Self-Lubricating Composite Porosity on Lubrication and Wear

The lubricant characteristics of porous self-lubricating composites with a realistic rough surface are incorporated into an improved elastohydrodynamic model. The evolved model demonstrates that the wear rate can be measured by examining the lubricant distribution at various fractal dimensions and porosities. The results show that the physical nature of the rough surface topography and the composite's physical properties must be understood, because the relative contact area is enlarged and friction forces are increased by the increase in the fractal dimension and the porosity. It is obvious that the method can significantly improve the lubricant properties to avoid wear by controlling these two coupled effects. The research also indicates that optimization of the design the microstructure of the porous self-lubricating composite should focus on the porosity based on the wear rather than the amount of lubricant.     

Thrust-Washer Evaluation of Self-Lubricating PS304 Composite Coatings in High Temperature Sliding Contact

PS304 self-lubricating composite coatings were successfully deposited on steel substrates at various plasma spray facilities using mixtures blended from commercially obtained constituent particles. Coatings were evaluated in thrust-washer tests against Inconel X-750 at low contact pressures to 40kPa, sliding speed of 5Amis, and either ambient temperature or 500 °C chosen to simulate conditions in airfoil bearings during startup and shutdown contact. Wear factors for all PS304 coatings tested, regardless of contact pressure and temperature, ranged from 1–3*10^?4 mm³/Nm while coefficients of friction of approximately μ =0.5 were measured in all cases. While wear and friction behavior of PS304 in air foil bearings appear to have been simulated, surface roughening was observed in these thrust-washer tests which used continuous sliding contact, as opposed to the evolution of smoother surfaces observed in high-temperature foil bearings experiencing cyclic startup/shutdown. Wear-induced surface smoothening of PS304 was additionally simulated in thrust-washer tests with sliding contact instead imposed intermittently.     

High Velocity Oxyfuel Deposition for Low Surface Roughness PS304 Self-Lubricating Composite Coatings

Efforts were made to achieve lower (R _q < 0.1 μm) initial RMS roughnesses of PS304 coatings so that they may be considered for foil bearings operating under increasingly severe conditions that result in smaller air film thicknesses. Attainable roughness of conventional plasma-sprayed PS304 coatings has been typically R _q > 0.25 μm, as limited by porosity in the deposited coating and surface irregularities correspondingly formed upon finishing. Initial attempts at achieving dense coatings by instead using a high-velocity oxyfuel (HVOF) flame-spraying process failed due to insufficient heating and softening of the NiCr and Cr ₂ O ₃ constituents of the PS304 feed powder, which rebounded from the steel target substrate and resulted in low deposition yield. Efficient HVOF deposition by a hydrogen-fueled system was achieved using NiCr and Cr ₂ O ₃ constituent particles of reduced size that were more effectively heated. The resultant dense coatings provided roughnesses as low as R _q = 0.05 μm upon polishing. Tribological performance of these HVOF coatings was evaluated against Inconel X-750 in thrust-washer tests at a sliding speed of 5.4 m/s, with contact pressures of 20 and 40 kPa, and ambient or 500°C temperatures. The wear and friction performances in direct sliding contact, as would exist upon loss of separating air-bearing film, of the PS304 coatings produced by HVOF are found to be similar to those deposited by the plasma spray process.     

原位合成不同基体组织7715D钛基复合材料的高温力学性能

利用真空电弧熔炼技术,通过热加工原位合成了以TiB纤维和La_2O_3颗粒为增强体的7715D钛基复合材料,然后分别在β相区与(α+β)相区退火,获得层片和等轴两种基体组织;并对不同组织复合材料进行了高温拉伸试验、蠕变试验和不同温度下的热暴露试验。结果表明:与等轴组织相比,层片组织显著提高了复合材料的高温抗拉强度及蠕变性能,能有效阻止增强体断裂后微裂纹的扩展;在600℃下热暴露时,复合材料的热稳定性最差,主要由在等轴组织初生α相中和层片组织的α片边界处析出颗粒状脆性相所致,其对复合材料室温塑性均有不利影响。     

Cold-Sprayed Ni-hBN Self-Lubricating Coatings

Feedstock preparation strategies were explored to produce composite admixed, milled, and precoated (encapsulated) powders of nickel–hexagonal boron nitride (Ni-hBN) for cold-sprayed self-lubricating coatings. The resulting cold-sprayed coatings were then examined for microstructural homogeneity and composition, as well as bond strength, microhardness, and relevant wear behaviors. Though admixed powders were easy to prepare and economical, milled and precoated formulations provided the advantage of aiding contact between Ni and lubricant powders prior to spraying that ultimately improved deposition and properties. The maximum amount of hBN that could be effectively built into the cold-sprayed Ni coatings was approximately 6 wt%. Results of the study also indicated that the composite coatings exhibited slightly higher hardness and reduced adhesive strength relative to a baseline of pure Ni layers. Moreover, some reductions in friction and expected decreases in bond strength and lubricant uniformity were observed when more than 4 wt% of lubricant was retained in the coatings. Given these findings, the most promising path to improve the amount, uniformity, and influence of the lubricant may be to encapsulate smaller particles with thicker levels of Ni to “trick” the composite particle to bond as pure Ni.     

石墨含量对铜基石墨自润滑复合材料摩擦过程中石墨润滑膜的影响

为研究石墨含量对铜基石墨自润滑复合材料摩擦过程中形成石墨润滑膜的影响,采用粉末冶金法制备了不同石墨含量的铜基石墨自润滑复合材料,测试了复合材料的力学性能,用自制环-块摩擦试验机测试评估了材料的耐磨性能,用光学显微镜实时原位观察了摩擦表面组织形貌的变化,用扫描电镜对磨痕进行观察和分析,通过能谱仪成分扫描分析接触面石墨润滑膜的覆盖率。结果表明:随着复合材料中石墨含量的增加,材料的力学性能逐渐降低,石墨润滑膜的覆盖率先升高后降低,磨损量先减小后增大;当石墨体积分数为14%时,石墨润滑膜的覆盖率最高,磨损量最小,耐磨性能最好。     

Tribological Properties of TiAl Matrix Self-Lubricating Composites Containing Multilayer Graphene and Ti3SiC2 at High Temperatures

An investigation is conducted on the unexplored synergistic effects of multilayer graphene (MLG) and Ti₃SiC₂ in self-lubricating composites for use in high-temperature friction and wear applications. The tribological properties of TiAl matrix self-lubricating composites with different solid lubricant additions (Ti₃SiC₂-MLG, MLG) are investigated from room temperature to 800°C using a rotating ball-on-disk configuration. Tribological results suggest the evolution of lubrication properties of MLG and the excellent synergistic lubricating effect of MLG and Ti₃SiC₂ as the testing temperature changes. It can be deduced that MLG has great potential applications as a promising high-temperature solid lubricant within 400°C, and a combination of MLG and Ti₃SiC₂ is an effective way to achieve and maintain desired tribological properties over a wide temperature range.     

高温环境复合材料螺栓连接振动的防松试验

具有耐高温、轻质、高强等特点的复合材料及其连接结构可满足航空航天领域的迫切需求,已广泛应用于工程中。为比较高温振动环境下不同复合材料螺栓连接形式的防松效果,进行了常/高温条件复合材料搭接板的随机振动试验研究。以搭接结构固有频率为评判量,通过对比试件承受振动载荷作用前后的固有频率变化,研究了常/高温环境下高温胶、双螺母防松形式,以及沉头螺栓张角等因素在给定随机振动载荷下对搭接结构连接刚度的影响。研究结果表明:常温随机振动条件下双螺母防松、60°沉头螺栓的防松性能更好;高温环境下,高温胶防松、30°沉头螺栓连接形式性能较优。     

铝基复合钎料钎焊机理

采用Cu/Al-Si/Cu式复合钎料,在不同温度下钎焊铝及其合金,通过钎焊接头金相图片和电子探针对焊缝线扫描,分析接头反应结合情况,研究钎焊接头在此条件下共晶反应特征以及铜和硅的扩散特点。研究表明,铜的加入能够显著降低钎焊温度,从而有效保护钎焊母材。不同的钎焊温度,钎焊接头表现出不同的共晶反应,且共晶液相的产生极大地促进了铜和硅的扩散能力。首先内侧发生Al-Si-Cu三元共晶反应,随着钎焊温度超过548℃,铝和铜之间发生二元共晶反应,接头反应能够充分进行,得到较为理想的接触反应区。     

Fabrication of Metal Matrix Composite Automotive Parts

Driven by the need to build lighter weight, quieter and more fuel-efficient engines, the automotive industry has recognised the need for materials substitution, offering outstanding proper-ties in a number of engine components, including engine poppet valves. Since the valves control the gas flow, valve size, timing and overlap, they all influence volumetric efficiency. The valve train is designed to achieve maximum volumetric efficiency at the desired level of engine performance. Also engine valves are subjected to high operating temperatures and stress con-ditions which affect durability. The application of finite element analysis resulting in a Weibull failure theory analysis is a valid tool in predicting the probability of failure of the valves. The present work envisages carrying out some studies on Al-SiC and Al-TiC composites as possible alternative materials for engine poppet valves. The present trend to make parts at or near net shape has brought powder metallurgy (PM) to the forefront and is being vigorously pursued by automotive design and materials engin-eers who are finding an increased application for this energy-and cost-saving process. Mixtures of four different compositions (15, 20, 25, 30% by weight) of SiC were prepared. By the PM technique, valves were fabricated by placing these powder mixtures in layers (one weight per cent along the stem and one along the base) in a die. Specimens ( φ15 × 30) were also prepared by the PM technique so that properties like compressive, tensile strength, etc. could be studied. A die was fabricated to cast valves through a liquid metallurgy route.     

SiCP/SiC陶瓷基复合材料连接件的研制

采用ＳｉＣ和聚合碳硅烷为主要原料,研制开发应用于某些热机上的陶瓷连接件,由于陶瓷基复合材料是高硬脆性材料,在研制过程, 除了要考虑连接件的机械力学性能和热震性能必须符合要求外,还必须考虑连接件的成型性能和机械加工性能,本文将着重讨论连接件的机械加工问题。     

镍基复合刷镀层研究

在快速镍电刷镀技术的基础上,研究了在镀液中加入石墨微粒,通过改变镀液浓度、电压、pH值以及镀速等条件,达到改变镀层的耐磨性。通过正交实验法优化确定了Ni-石墨复合电刷镀镀液较佳的浓度配制。并分析了电刷镀镀液各成分在刷镀中的作用以及它们对镀速的影响。从而提出了一套较完整的Ni-石墨复合电剧镀工艺方案。