液相烧结法制备钢结硬质合金覆层材料

以WC、Cr3C2、Fe、Co和Ni为原料,加入有机粘结剂PVB,制备出成形料浆;利用料浆喷涂法在Q235钢基体上成形覆层坯体;在真空烧结炉中通过液相烧结工艺制备出钢结硬质合金覆层材料。覆层材料表面硬度远远大于Q235钢。在覆层受压应力的状态下,覆层材料的抗弯强度大于Q235钢;在覆层受拉应力的状态下,覆层材料的抗弯强度小于Q235钢。覆层中硬质相与粘结相分布均匀,无气孔等缺陷存在。覆层与钢基体之间没有清晰的结合界面存在,形成了冶金镶嵌结构,产生了致密的冶金结合。     

机械密封覆层端面开裂有限元分析

机械密封覆层端面是获得高性价比密封环的重要方向,然而覆层端面的开裂是其主要的失效形式。采用有限元分析软件,建立了机械密封热-结构耦合模型,综合考虑了端面变形、液膜反压和端面温度共同作用下对密封覆层端面的影响,得到了覆层表面和覆层与基体界面的应力分布,分析了覆层端面开裂的原因。研究结果表明:热载荷对覆层应力分布有显著影响,不容忽略;阻封流体的冷却作用有利于降低热载荷对覆层应力的影响;覆层表面最大拉应力、主界面最大切应力、侧界面最大切应力和最大法向拉应力是引起覆层端面开裂的主要因素。     

氩弧熔覆铁基梯度熔覆层的显微组织与性能

利用钨极氩弧熔覆技术在45钢上堆焊了由工作层和M2过渡层组成的铁基梯度熔覆层,研究了梯度熔覆层的显微组织、热膨胀性能、显微硬度、残余应力和抗热震性能,并与单一熔覆层的进行了对比。结果表明:与单一熔覆层相比,梯度熔覆层结构致密无孔洞;单一熔覆层的截面硬度在距表面2mm时突然降低,而梯度熔覆层截面硬度由表面向基体呈梯度降低;梯度熔覆层热膨胀系数匹配度更优,截面残余应力更低,抗热震性能更好。     

Q235D激光熔覆实验研究

在Q235D钢表面激光熔覆Fe基合金粉末,通过对熔覆层外观形貌、表面硬度、金相组织和显微硬度的对比分析得出了双层熔覆时的最优工艺参数,当两层工艺参数相同且均为:激光功率600W、扫描速度2mm/s、搭接率24.2%、送粉电压10V时,所得熔覆层的表面较为平整均匀,通过金相组织分析发现,基体与熔覆层的冶金结合性较好,无裂纹,并且基本无气孔出现,熔覆层的显微硬度显著高于基体且从熔覆层→过渡区→基体呈梯度降低。在两熔覆层交界处,显微硬度从界面处往第一熔覆层方向先减小后增加直到最高值,从界面处往第二熔覆层方向显微硬度呈阶梯状上升逐渐增加到熔覆层硬度的最高值,尽管两熔覆层交界处显微硬度有所降低,但是仍然大大高于基体的显微硬度,对熔覆层性能基本无影响,在工业生产中有着较好的发展前景。     

钢板表面热浸镀锌层的拉伸断裂机理

采用热浸镀技术在Q235钢板表面制备了镀锌层,研究了浸镀时间对镀锌钢板拉伸性能的影响,通过扫描电镜观察了拉伸断裂后镀层表面、截面和断口处的形貌,分析了镀层的断裂机理。结果表明:与基体相比,Q235钢板热浸镀锌后的抗拉强度和塑性变形能力均下降;镀件抗拉强度的下降是由表面镀锌层强度低于基体的强度造成的;而塑性的下降则是由热浸镀时在基体表面形成的微裂纹造成的;由于残余应力的存在,微裂纹首先在σ-FeZn10层中形成,在受到拉应力时,裂纹沿晶界在σ-FeZn10层中向上扩展,然后在ζ-FeZn13层中裂纹沿着FeZn13/Zn的相界面扩展,最后在η-Zn层沿晶界扩展直至镀层发生完全断裂。     

离焦量对激光熔覆Ni/WC的影响模拟与实验研究

利用ANSYS生死单元技术模拟在冷作模具钢Cr12MoV表面上熔覆Ni/WC粉末时改变离焦量对温度场与熔覆后残余应力的分布情况的影响。通过表面探伤剂、倒置金相显微镜等方式检测对应实验制备出的熔覆层，进一步阐述离焦量对熔覆层质量的影响。结果表明，改变离焦量会影响熔覆时温度场的分布以及熔覆后残余应力，而这些因素决定着制备出熔覆层质量的高低；最大残余应力主要出现在熔覆层与基体间的结合区域的两侧，其值会随着离焦量的增加而增加；当离焦量低于13mm时，熔覆层与基体之间会形成一条较为明显的过渡带，过渡带的产生不仅能够抑制裂纹扩展，还能够保证熔覆层与基材的牢固性。     

铸渗法制备的活塞环硬质表层材料微观结构研究

研究采用铸渗法在活塞环表面制备出硬质合金层,确定了以铬铁粉、FeB合金粉、WC粉、1710粉和Cu粉为基本原料,加入一定量的粘结剂与熔剂,混合均匀。采用铸渗法,在以ZG230-450为基体的活塞环表面制备硬质合金覆层;利用金相显微镜研究了铸渗覆层的金相组织,利用硬度计测量了铸渗覆层的硬度分布,利用SEM研究了铸渗覆层的微观结构和元素分布。铸渗层成分分析表明,在界面结合处实现了合金元素的扩散与渗透,硬质合金覆层与铸钢基体形成了良好的冶金结合。     

铜表面激光熔覆镍基合金的显微组织与硬度

用超音速火焰喷涂方法在铜基体表面预置一层镍基合金涂层后,再用高功率密度的激光进行熔覆;用OM、SEM、XRD、显微硬度计对熔覆层进行了表征.结果表明:熔覆层组织致密、无裂纹,气孔、夹杂等缺陷明显减少;在熔覆层底部形成了高熔点、高密度钨相带,其显微硬度明显升高;熔覆层与基体的界面处有铜、镍互渗的冶金结合层形成,其厚度约为13μm;熔覆层的显微硬度是铜基体硬度的8倍以上.     

液压支架激光熔覆不锈钢合金涂层的实验研究

采用激光熔覆技术在硅锰钢样件表面制备了不锈钢涂层,用SEM和能谱仪分析了基体与熔覆层界面以及熔覆层之间的微观组织及成分,开展了试样弯折和冲击强度测试实验。结果表明:在激光功率为6kW、扫描速度为8mm/s、光斑直径为5mm、搭接率30%的工艺条件下,基体与熔覆层界面以及熔覆层之间出现小亮带,形成冶金结合;熔覆方向垂直于观察面的覆层形貌呈蜂窝状,熔覆方向与观察面平行的覆层形貌呈竖直条状;在功率6kW、扫描速度16mm/s、光斑直径5mm、搭接率30%时,熔覆层与基体以及熔覆层之间会产生夹渣;熔覆层具有较好的韧性和冲击强度,熔覆后的试样的冲击强度提高了7.7%。     

激光熔覆涂层在多冲载荷下的力学行为分析

根据多次冲击载荷的特性,从应力波传播、多冲磨损、多冲硬化与软化和多冲塑性变形方面,对带有激光熔覆层试样在多冲冲击载荷下的部分力学行为进行了试验研究。结果表明：在多冲载荷作用下,应力波在涂层和基体的;台金接合面发生反射,形成拉伸波造成涂层的纵裂和角裂;在涂层表面由于应力集中,涂层表面发生微观点蚀和深层剥落;冲击载荷能量的积累．造成涂层试样硬度变化和塑性变形。     

梯度结构对氧化铝陶瓷涂层抗冲击载荷性能的影响

梯度结构陶瓷涂层以其优异的抗热震性表现出巨大的工程应用前景。为推动梯度陶瓷涂层在机械零件表面强化上的应用,采用“三明治”式梯度结构形式,建立镍基氧化铝梯度陶瓷涂层在冲击载荷作用下有限元模型,分析冲击载荷作用下涂层的力学性能,以及梯度层的结构形式、厚度及层数等参数对涂层的力学性能影响。结果表明：较无梯度结构陶瓷涂层相比,梯度结构能有效减缓涂层与基体结合面上的应力突变,涂层内部最大Mises应力明显降低,合理的梯度结构能改善涂层内部Mises应力分布,改变应力分布特征,减缓表面陶瓷涂层的冲击应力,从而防止陶瓷涂层在冲击载荷作用下脱落。最后对制备层状结构梯度陶瓷涂层时,如何进行梯度层结构设计进行了探讨,并提出了采用0.25次方幂指数梯度结构,得出10层中间层就可有效减缓冲击载何、降低Mises应力突变的结论。     

Finite Element Analysis of Multilayered and Functionally Gradient Tribological Coatings with Measured Material Properties

A model has been developed to study the stress distribution in Ti_{1 ? x}C_x multilayered functionally gradient (FG) coatings, with a top coating of diamond-like carbon (DLC), on 440C stainless steel substrates. Using the finite element method, these gradient coatings were assumed as a series of perfectly bonded layers with unique material properties and layer thickness. In addition, a matrix of nanoindentation experiments were performed to measure material properties of each Ti_{1 ? x}C_x layer on separate coating blocks. The yield strength of the coating materials was then determined by coupling the finite element analysis model in connection with the nanoindentation technique. Once developed, this model was used to examine the threshold of plasticity and identify the plastic deformation zone inside the multilayered coatings and substrate. This work shows how the multilayered FG Ti/TiC/DLC coating system improves the coating integrity under heavy loading conditions.     

铀表面铝镀层热应力的有限元分析

对铀表面磁控溅射沉积铝镀层的热应力进行了热弹塑性有限元分析。结果表明：镀层内的热应力较大,达到铝的屈服强度,镀层界面两侧存在明显应力梯度,试样侧边存在由于边缘效应引起的应力分布不均匀性,至侧边约4倍镀层厚度处,应力分布不均匀性逐渐消失;沉积温度升高,界面塑性应变明显增大,镀层弹性模量和泊松比对镀层界面热应力和塑性应变的影响较小,而屈服强度的影响较大,减薄镀层厚度有利于改善镀层界面的热应力和塑性应变。     

Analysis of the dissimilar interface frictional constraints during the upsetting process

A thermo-elastic-plastic coupling model combined with frictional functions is proposed to analyse the plastic upsetting of a cylinder, accounting for dissimilar interface frictional constraints. The flow stress of the deforming workpiece is considered as a function of strain, strain rate and temperature, and the properties of the interface frictional constraints are approached through the use of two continuously varying functions which are presented in terms of the amount of the workpiece deformation.The effects of various combinations of dissimilar interface frictional constraints at the top and bottom die surfaces on the temperature, strain and strain rate distributions are investigated. The asymmetrical flow inducing uneven billet profiles, together with the variations in top and bottom die surface loads, are predicted. The results obtained are consistent with the observations of upsetting experiments.     

Three-dimensional finite element modelling of the scratch test for a TiN coated titanium alloy substrate

A three-dimensional (3D) finite element (FE) simulation of a rigid Rockwell C indenter scratching a TiN/Ti-6Al-4V coating/substrate system is presented. Coulomb friction between the indenter and the surface of the coating/substrate system was considered. The material properties of the coating and substrate were assumed to be elastic–plastic following a bilinear law with isotropic strain hardening. The von Mises yield criteria was used to determine the onset of plastic deformations. The scratch depth profiles at different moving distances were studied. The distributions of the stress field at the contact surface, in the coating, and at the interface of the coating/substrate system were investigated. The finite element results can be used to explain the failure modes of coated materials at the scratch test.     

Computer Simulation of Orthogonal Cutting using a Tool with Multiple Coatings

The development and evaluation of an orthogonal cutting simulation model for carbide tools with multiple coating layers (1 µm-TiN/3 µm-Al₂O₃/6 µm-TiC) is presented. The chip geometry, cutting forces, tool temperatures and stresses were predicted using the Finite Element Method (FEM). The results were analyzed with a focus on the understanding of the thermal influence of coating upon tool temperatures at the tool-chip interface and in the substrate. In the simulation model used, the thermal effect of tool coating was considered by using two different models: (a) use of individual coating layers defined with intrinsic thermal properties and (b) use of a composite coating layer defined with equivalent thermal properties. The proposed models were evaluated by comparing the predictions with the experimental data available in the literature under the same cutting conditions. The steady state tool temperature solution was obtained by adopting a three-step simulation scheme. It consisted of an initial Lagrangian-type simulation until a stable chip shape was formed and a subsequent Eulerian-type calculation with update of the free surface and plastic strain field of the workpiece. The predicted results indicated that for the coated tool considered the thin-film surface coatings with an Al₂O₃ intermediate layer did not significantly alter the steady state temperature gradients between the chip and the tool substrate and provided little thermal insulation effect to the tool substrate. However, the modified thermal response of the coated tool surface caused lower cutting temperatures at the tool-chip interface as compared to that for an uncoated tool under the same conditions. Although these results are consistent with similar experimental observations in the literature, further validation work needs to be conducted.     

Surface stresses in coated steel surfaces—influence of a bond layer on surface fracture

Thin hard coatings in the thickness range of only a few micrometers deposited by physical vapour deposition (PVD) on components or tools can improve the friction and wear properties by several orders of magnitude. A 2 μm thick TiN (E=300 GPa) coating on a high-speed steel substrate with a bond layer at the interface between the coating and the substrate was modelled by micro-level three-dimensional finite-element method (3D FEM) in order to optimise a coated surface with regard to coating fracture. Both compliant low modulus (E=100 GPa) and stiff high modulus (E=500 GPa) bond layers at the coating/substrate interface of 200 and 500 nm thickness were investigated. First principal stresses were simulated for scratch test geometry in the load range of 7.5-15 N. Very high stress concentrations of above 5700 MPa tensile stresses were observed in the bond layer just behind the contact zone for the stiffer bond layer. The stiff bond layer generated 5 times higher tensile stress maxima compared to the compliant bond layer. There was approximately 3.5 times larger strain in the compliant bond layer compared to the stiff bond layer. The general coating design advice based on this exercise is that when a bond layer is used e.g. for coating/substrate adhesion improvement should the bond layer be less stiff than the coating not to generate high and critical tensile stresses. The thickness of the bond layer may vary and is not critical with respect to generated stresses in the surface.     

Mechanical properties of thin carbon overcoats

Components used in magnetic storage systems (hard discs, tape heads and drums) are often very small and lightweight, and operate under very low loads (of the order of a few micrograms to a few milligrams). As a result, friction and wear processes occur on a nanometre scale and conventional tribological test methods and assessment tools are usually not appropriate. Furthermore, the assessment of the mechanical properties of the coatings or surface treatments used to protect these components from wear is complicated by the low thickness of the layers generally used. This paper details the problems associated with the assessment of the mechanical properties of thin diamond-like carbon coatings used to protect hard discs, tape heads and air bearings. Whereas thick coatings (>1 μm) are relatively easy to assess, even if the substrate has a low hardness and offers little support to the coating, there are many more problems when it comes to measuring the properties of the 5–10 nm layer on a hard disc. In many cases there is no plastic deformation of the coating which merely flexes and bends into the hole produced by plastic deformation of the substrate. Deformation of the coating is then limited to localised plasticity at the indenter edges, and/or fracture along the same edges and at the edge of the contact. The limits for use of Nanoindentation to assess the plasticity of the coating are discussed for such cases.     

Finite element analysis of coating adhesion failure in pre-existing crack field

Abstract

The influence of a pre-existing crack field on coating adhesion failure in a steel surface coated with a 2 μm thick titanium nitride (TiN) coating was investigated by finite element method modelling and simulation. The stress and strain fields were determined in contact conditions with a spherical diamond tip sliding over the coated surface at a loading of 8 N. One crack in or at the coating increased the maximum tensile stresses with six times from 82 to 540 MPa when the crack was vertical through the coating or L shaped and with nine times when the crack was horizontal at the coating/substrate interface. A simulated multicrack pattern relaxed the tensile stresses compared to single cracks. The results indicate that a cracked coated surface needs to have about five to nine times higher adhesive and cohesive bonds to resist the same loading without crack growth compared to a crack free surface. For optimal coated surface design, the strength of the adhesive bonds between the coating and the substrate in the vertical direction needs to be 50% higher than the cohesive bonds within the coating and the substrate in the horizontal direction. The first crack is prone to start at the top of the coating and grows vertically down to coating/substrate interface, and there it stops due to the bigger cohesion within the steel material. After this, there are two effects influencing that the crack will grow in the lateral direction. One is that steel cohesion is normally bigger than the coating/interface adhesion, and the second is that there are higher tensile stresses in the horizontal than in the vertical cracks. Several vertical cracks can stop the horizontal crack growth due to stress relaxation.