Interfacial mechanical properties of TiN coating on steels by indentation

The elastic theory of indentation on nitride films/steel systems showed distribution of stresses (shear stress, radial stress and circumferential stress) near the interface and in the film. The difference in values for each stress along the distance to the load center increased with increasing Poisson’s ratios of steels. The shear stresses (σ_rz) had the maximum value at a distance to the load center and the difference became more significant with increasing Poisson’s ratios of steel substrates (from 0.2–0.3 of Poisson’s ratio for high-speed steels to 0.3–0.35 for stainless steels), which accounted for the large amount of cracks inside the indent cavity of nitride films/stainless steel in spite of the smoothness outside the cavity. The calculation of σ_r and σ_z showed that the differences in nitride films/steel stress increased with increasing Poisson’s ratios of steels, which also facilitated the formation of ring cracks in the film of nitride films/stainless steel composite. Indentation examination revealed the large amount of cracks inside the indent cavity of nitride film/stainless steel but smooth surface outside the cavity. These were formed under the high sinusoidal shear stress and circumferential radial stress due to the higher Poisson’s ratio of stainless steel and the plastic deformation due to the lower yield stress of stainless steel (SS), which induced more local residual stresses, whereas some cracks or spalling observed around the cavity and no cracks inside the cavity were attributed to the edge effect when the conical indenter strained the surface downward for nitride film/high-speed steel (HSS) system.     

Influence of feed rate on surface integrity and fatigue performance of machined surfaces

This study investigates the influence of the feed rate on the surface integrity and fatigue performance of machined surfaces. The results demonstrate that a higher feed rate increases crack initiation life and crack propagation life. A higher feed rate induces more compressive residual stresses and a more softened layer. The feed rate influences crack initiation life up to 45% and crack propagation life up to 149%. Consequently, the feed rate affects fatigue life up to 132%. The fatigue tests substantiate that the feed rate influences fatigue life significantly and that the effect increases significantly if the loading is reduced.     

The phase composition of a hardmetal coating deposited by spark discharge on a carbon steel substrate

The quantitative phase composition of a hardmetal coating deposited during the spark discharge treatment of U8 steel using a T15K6 hardmetal electrode has been studied by the X-ray diffraction method. It is demonstrated that the quantitative ratio between phase components of the coating is governed by the WC dissociation on the electrode surface and the TiCN synthesis in the spark gap due to nitrogen absorption from air.     

Effects of process parameters on fatigue behavior and surface integrity of tool steel produced by electrical discharge turning

Electrical discharge machining is used in the production of countless parts with complex geometries and micro dimensions, from many elements of industrial molds to parts of motors and pumps. Also, most of these parts are cylindrical and it is always more meaningful to study their rotational fatigue behavior to predict their response during their operation. This study concentrated on the impacts of machining parameters on the surface quality and fatigue behavior of tool steel shaped by electrical discharge turning. The results based on Taguchi methodology have shown that discharge current affects R_a and R_z more, and pulse duration more affects the mean spacing of profile irregularities, S_m. As a result of the heat affected depth in the machined region, which changes in proportional with the processing parameters, the microhardness decreased from the sample surface to the core. The maximum hardness was measured at current of 12 A, pulse duration of 3 μs and pulse interval of 7 μs. According to the fatigue tests, it was found that the fatigue life decreased with the increase in R_z and S_m values. Moreover, high microhardness and thick recast layer reduced the fatigue strength of the samples with relatively smooth surface topography.     

Improving WC-Co coating adhesive strength on rough substrate:Finite element modeling and experiment

The adhesive strength in a coating-substrate system is of primary importance for the coating lifetime in service. However, the underlying mechanism is not fully understood due to the complex internal structure of composite coatings. In this study, the effect of substrate roughness on the adhesive strength of WCCo coatings was investigated by experiment and simulation. Results show that the adhesive strength is significantly affected by the roughness. In the case of the Ra2 μm, the adhesive strength is approximately 35–46 MPa. When the Ra is 4 μm, the adhesive strength increases to nearly 60 MPa. A finite element model was developed to correlate the roughness with adhesive strength. It is found that the predicted values are well consistent with the experimental data. In addition, with the increase of the roughness,the residual stress would be changed from concentrated state to widespread state, which decreases the critical stress to result in crack propagation. That's why a larger roughness can cause a higher adhesive strength. This study gives understanding on the mechanism of adhesive strength affected by roughness,which contributes to the parameter optimization with better performance.     

镁合金表面功能涂层制备与界面表征技术的研究进展

随着低碳经济的发展,汽车轻量化的呼声愈来愈高。镁合金作为最有前景的轻量化材料之一,其表面防腐功能涂层的研究已经越来越受到人们的重视。简要介绍了物理方法和化学方法制备镁基表面防腐功能涂层的工艺技术特点,重点讨论了衡量涂层质量的现行标准以及现有的各种测量界面结合强度的方法,特别是界面应变能释放率定量分析的方法,文中结合自己的实验结果进行了分析。     

Formation of a crater in the workpiece on an electrical discharge machine

A steel molding of ASTM A681 developed a large crater while being fabricated on an electrical discharge machine. Most probably, the fault was caused by inadequate flushing of the dielectric/coolant.     

Numerical study on the dynamics of an electrical discharge generated bubble in EDM

In this paper, the influence of hydrodynamic behaviour of dielectric liquid media around a rapidly growing small spherical bubble in the process of electrical discharge machining (EDM) between the tool and the workpiece on material removal from the workpiece is investigated. The tool and the workpiece are assumed as two parallel rigid boundaries with dielectric liquid between them. The boundary integral equation method is applied for numerical solution of the problem. Results illustrate the time dependent shapes of the bubble generated between the two parallel rigid boundaries due to the electrical discharge. Results also indicate that continuous growth of the gas bubble leads to a sharp drop of pressure within the bubble which results in the sharp pressure drop over the surfaces of the tool and the workpiece. This pressure drop over the surfaces of the tool and the workpiece causes expelling of the gas dissolved in the molten metal and helps the molten material to escape.     

Thromboresistant and endothelialization effects of dopamine-mediated heparin coating on a stent material surface

Heparinization of surfaces has proven a successful strategy to prevent thrombus formation. Inspired by the composition of adhesive proteins in mussels, the authors used dopamine to immobilize heparin on a stent surface. This study aimed to assess the thromboresistant and endothelialization effects of dopamine-mediated heparin (HPM) coating on a stent material surface. The HPM was synthesized by bonding dopamine and heparin chemically. Cobalt–chromium (Co–Cr) alloy disks were first placed in the HPM solution and applied to surface stability then underwent thromboresistant tests and human umbilical vein endothelial cells (HUVEC) cytotoxicity assays. The results showed not only thromboresistant activity and a stable state of heparin on the surfaces after investigation with toluidine blue and thrombin activation assay but also proliferation of HUVEC in vitro. Studies on animals showed that the HPM-coated stent has no obvious inflammation response and increasing of restenosis rate compared to the bare metal stent (BMS) indicating good biocompatibility as well as safety in its in vivo application. Moreover, improving the endothelial cell (EC) proliferation resulted in a higher strut-covering rate (i.e., endothelialization) with shuttle-shaped EC in the HPM-coated stent group compared to that of the BMS group. These results suggest that this facile coating approach could significantly promote endothelialization and offer greater safety than the BMS for its much improved thromboresistant property. Moreover, it may offer a platform for conjugating secondary drugs such as anti-proliferative drugs.     

Effects of nitrogen ion implantation and implantation energy on surface properties and adhesion strength of TiN films deposited on aluminum by magnetron sputtering

The performance of tribological coatings depends greatly on the adhesion strength between the coatings and substrates. In this work, we investigated the influence of the ion implantation energy of nitrogen on the adhesion and surface properties of TiN deposited on aluminum substrate. Aluminum samples were implanted with 15 keV, 30 keV and 40 keV nitrogen ions before TiN films were deposited using magnetron sputtering in a custom-designed multi-functional ion implanter. The adhesion properties of the implanted TiN films were assessed using nano-scratch tests and were observed to vary with the nitrogen ion implantation energy. Our frictional test results show that an appropriate ion implantation energy and dose can improve the frictional behavior of TiN films deposited on aluminum.     

Effect of a thin surface coating layer on thermal stresses within an elastic half-plane

Summary The paper studies the dilatational eigenstrain problem of a thermal inclusion of any shape within an elastic half-plane coated with a thin stiff surface layer. The emphasis is on the effect of the surface coating layer on internal stresses within the inclusion and interfacial shear stress between the coating layer and the surface of the elastic half-plane. The mixed boundary value problem is reduced to a first-order differential equation for an analytic function in the elastic half-plane, and the exact solution is obtained explicitly in terms of an auxiliary function constructed from the polynomial conformal mapping which maps the exterior of the inclusion onto the exterior of the unit circle. The exact solution is used to study the conditions under which the surface coating layer can be negligible or treated as an inextensible coating layer. In particular, when the surface coating is inextensible, the exact solution shows that the mean stress is exactly uniform inside the thermal inclusion of any shape and vanishes outside the thermal inclusion in the whole elastic half-plane. Detailed results are shown for the mean stress and the interfacial shear stress caused by a circular or elliptical thermal inclusion. The results show that the surface coating layer could have a significant effect on the internal stress field within the thermal inclusion and the interfacial shear stress between the coating layer and the elastic half-plane especially when the inclusion is close to the surface coating layer.     

The energy balance and the mechanism of discharge transport on the surface of a thermionic cathode in an arc discharge

Modes of burning of an electric arc are treated under conditions in which the thermionic current from the cathode is much lower than the arc current. The conditions on the cathode surface are refined; it is demonstrated that the hypothesis of ion transport of discharge does not contradict the energy balance on the cathode surface.Translated from Teplofizika Vysokikh Temperatur, Vol. 43, No. 1, 2005, pp. 011–020. Original Russian Text Copyright © 2005 by V. P. Polishchuk.     

Geometry effects of substrate and coating layers on the thermal stress response of FGM structure

Summary Due to transient temperature change, the plane strain elastic-plastic problem for a functionally graded material (FGM) bonded to a homogeneous coating layer and a metal substrate is considered by the use of the finite element method (FEM). The substrate and the coating are assumed to be aluminum and partially stabilized zirconia, respectively. The FGM layer is a particulate composite of aluminum and partially stabilized zirconia with volume fractions continuously varying through the thickness. Generally in high temperature applications, the FGM system is sandwiched between a substrate layer and a coating layer. The coating layer increases the protection from heat but decreases the thermal shock resistance while the substrate layer increases the rigidity of the structure and decreases strength-related properties at high temperature. In order to compromise the thickness of both the coating and substrate layers, different values of the substrate and coating thickness are studied in order to evaluate their effects on the thermal stress response of the FGM structure. Since the main objective of the FGMs is using them in different applications with severe thermal loading conditions, the thermal stresses may be so high that some reinforcements may be fractured and/or debonded from the matrix giving a weakening effect instead of a reinforcing one. Hence, the behaviors of the reinforcements and the matrix are essential to be studied. In this regard, microscopic constitutive equations along with the temperature-dependent properties of the constituent materials are considered to enable us obtaining more realistic results of thermal stresses. Since the FGM structures are fabricated at high temperatures, thermal residual stresses are produced. In order to find out the importance of the consideration of the residual stresses arising from the fabrication process, the FGM structure with stress-free conditions is heated to the operating temperature, and its thermal stress response is compared with that one where the residual stresses are taken into account. Also, several functional forms of gradation of the constituents in the FGM layer are examined to reach the optimum profile giving the minimum stress level for the FGM structure under thermo-elasto-plastic behavior.     

Study of the aging process of corona discharge plasma effects on low density polyethylene film surface

A study of the durability of corona discharge plasma effects on a polymer surface was investigated in this work. We used the corona discharge plasma technique to modify the wettability properties of low density polyethylene (LDPE) film and evaluated the influence of relative humidity and temperature on the aging process with three different storage conditions. The effects of the aging process on the plasma-treated surface of LDPE film were quantified by contact angle measurements, Fourier-transformed infrared spectroscopy, and X-ray photoelectron spectroscopy. The results obtained with these techniques have allowed us to determine how the aging process promotes changes in the plasma-treated surface by decreasing its wettability and taking place a remarkable hydrophobic recovery process.     

The progress on antifouling organic coating: From biocide to biomimetic surface

The advancement in material science and engineering technology has led to the development of antifouling(AF) coatings which are cheaper, durable, less toxic, and safe to the environment. The use of AF coatings containing tributyltin compounds was prohibited at the beginning of 2003, this necessitated the development of environmentally friendly coatings. The fouling release coating(FRC) lacks biocides and has low surface energy, low elastic modulus with smooth surface properties, hence a better release effect to fouling organisms. Several functional coatings have been recently developed based on fouling release(FR) technology to combat the effects of biofouling. Here, we provide a brief overview of innovative technologies and recent developments based on FRCs, including silicone, modified fluorinated polymer,cross-linked coatings, amphiphilic copolymer coating, hydrogel coatings, and biomimetic coatings. We also highlight the key issues and shortcomings of innovative technologies based on FRCs. This may give new insights into the future development of marine AF coatings.     

Influence of weathering and substrate roughness on the interfacial adhesion of acrylic coating based on an increasing load scratch test

An increasing normal load scratch test was employed to study the scratch behavior of acrylic coatings. The effects of weathering and substrate surface roughness on the interfacial adhesion of acrylic-coated thermoplastic polyolefin (TPO) systems were investigated. The increasing load test gives valuable information regarding the onset location to failure and the critical normal load for interfacial delamination. Both flatbed scanner and scanning electron microscope are utilized to characterize the scratch-induced damage mechanism of the coated TPO system. An increase in weathering time and in surface roughness of the virgin TPO leads to the increase in scratch resistance of acrylic coating. In addition, with increasing weathering time there is an increase in discoloration, while a decrease in gloss occurs. The correlation between the change of scratch coefficient of friction and the onset location of interfacial delamination is discussed.     

The effect of surface treatments on the adhesion of electrochemically deposited hydroxyapatite coating to titanium and on its interaction with cells and bacteria

The effect of different mechanical and chemical pre-treatments on the adhesion strength of hydroxyapatite (HAp) coating on a commercially pure titanium (CP-Ti) substrate was studied by means of a standard tensile test followed by microscopic and chemical analysis to determine the locus of fracture. In addition, the effects of either these pre-treatments or post-treatment by low-energy electron irradiation, which allowed tuning the wettability of the surface, on both osteoblast progenitor attachment and S. aureus bacteria attachment were investigated. A dedicated program was developed for unambiguous identification and count of stained cells. A single-phase HAp coating was formed by electrodeposition. A series of surface pre-treatments consisted of grinding down to P1000, etching in HNO₃/HF solution, grit blast, soaking in NaOH and subsequent heat treatment provided the highest adhesion strength to the HAp coating. Osteoblast progenitors derived from rats may be attached preferentially to a hydrophilic surface (post-treatment to θ = 30°), while the bacteria seemed to be less attached to hydrophobic surfaces (post-treatment to θ = 105°). However, the results were not statistically different. The bacteria seemed to be less attached to the smoother, uncoated surfaces.