In vitro electrochemical corrosion behavior of functionally graded diamond-like carbon coatings on biomedical Nitinol alloy

The anti-corrosion property plays an important role in determining the biocompatibility of metal implants. In this study, functionally graded diamond-like carbon coatings were deposited on the Nitinol substrate by hybrid magnetron sputtering and plasma enhanced chemical vapor deposition. A scratch test was adapted to study the adhesion strength of the coatings. The Si/SiC graded layer (up to 150 nm thick) provided good adhesion between the coating and the substrate, up to approximately 47.17 ± 2.1 N. The effectiveness of corrosion protection for the coated specimen was investigated in Tyrode's simulated body fluid. It was found that adhesion strength had a great influence on the effectiveness of corrosion protection, and the better adhesion strength, the better corrosion resistance. Compared to the others, the coating with a 150 nm thick Si/SiC graded layer provided better corrosion protection, and there existed no large amount of debonding and cracking of the coating around corrosion pits after the potentiodynamic polarization tests.     

高强涂层结合强度的评价--楔形加载法

提出用楔形和载法评价高强度涂层材料与基体的结合强度。该方法利用楔形压头置于有楔形槽的试样中,使楔形压头中心线与涂层基体界面重合,施加静态载荷至试样沿涂层界面开裂,根据试样受力边界条件,给出涂层与基体结合强度的公式。对三种不同涂层的基体材料进行了结合强度试验。结果表明,用楔形加载法可对高强涂层与基体的结合强度进行测试,所得数据分散度与ASTMC633－79标准相同,试验数据不受非随机因素的影响。     

Effect of ultrasound on the adhesion of metal coating to nonferrous metals

The effect of ultrasound on the adhesion of metal coating to nonferrous metals has been studied. It is established that the ultrasonic treatment of metal coatings increases their adhesion strength. This effect is explained by the fact that the momentum imparted by ultrasound to atoms of the coating is sufficient to cause their migration over a certain distance from the surface into depth of the nonferrous metal substrate.     

铜基体上CVD金刚石厚膜的制备

用MWCVD方法在无预处理铜基体上获得了金刚石薄膜和厚膜。所得金刚石膜从基体上自动脱落,但形貌分析和Ramman分析表明,所得金刚石膜具有较好的质量。因此铜是制备金刚石厚膜的理想基体材料。     

Adhesion and vacuum forming properties of tin-zinc thin films deposited on polyester film substrate

Sn-Zn alloy thin films were deposited on a polyester (PET) film substrate by co-evaporation and evaluated their surface, tensile and adhesion properties with a vacuum forming test and pull test.Relationship between the surface roughness and elemental composition of these thin films was evaluated. The surface roughness decreased with increase of the Sn content.The tensile property was estimated by observations of micro-cracks of the thin films due to a vacuum forming test. Sn-Zn alloy thin film, whose elemental composition is 85:15 (wt%), had high vacuum forming durability.The adhesion strength between the Sn-Zn alloy thin films and PET substrate was measured with a pull test apparatus. The pull strength decreased with increase of the Sn content.     

Thin film fracture: Ti-coating-Be-substrate bond failure

The role of microstructure in the fracture of the bond between vapour-deposited thin film coatings and substrates will be modelled to a first approximation using classical fracture mechanics principles. Vapour-deposited coatings are composed of a grain structure with varying orientations. The effect of differing degrees of texture on the bond strength between the coating and its substrate will be considered in this analysis. Incorporated in stress calculations will be the residual stresses arising from the thermal contraction of the coating, an applied tensile stress normal to the coating surface (as that in an adherence pull test) and the critical stress needed for coating-substrate bond failure.     

An investigation on the bonding of hot-hollow cathode deposited silver layers to type 304 stainless steel

The conditions prevailing during the deposition of thick metallic films by the hot-hollow cathode method were investigated. The spacial distribution of temperature, surface roughness, and depth of etch during the pre-deposition etching cycle was investigated for two different sample holder designs. The influence of substrate temperature and the angle of incidence of the vapour on the structure and adhesion of thick silver films to type 304 stainless steel was determined. The nature and strength of adhesion were evaluated on the basis of microscopic examinations and tensile test determinations. Similar evaluations were made on tensile test samples formed through electroplating copper and nickel onto the silver films instead of the joining of two silver-coated samples by hot-isostatic pressing.     

Nanocrystalline Pd alloy films coated by electroless deposition

The structures of palladium and palladium alloys thin films deposited from organic electrolytes onto metallic substrates by electroless plating method have been investigated. The coatings are dense, pore-free 0.005-1 microm thick films with high adhesive strength to the substrate surface. EDX, XRD, SEM and TEM methods were used to determine the composition and structure of alloy coatings of the following binary systems: Pd-Au, Pd-Ag, Pd-Ni, Pd-Pb, and ternary system Pd-Au-Ni. The coatings of Pd-Au, Pd-Ag and Pd-Ni have a solid solution structure, whereas Pd-Pb is intermetallic compound. It has been found that the deposited films consist of nanocrystalline grains with sizes in the range of 11-35 nm. Scanning and transmission electron microscopy investigations reveal the existence of clusters formed by nanocrystalline grains. The origin for the formation of nanocrystalline structures of coating films is discussed.     

Measurement of Interfacial Fracture Toughness of Surface Coatings Using Pulsed-Laser-Induced Ultrasonic Waves

This research develops a new technique for the measurement of interfacial fracture toughness of films/surface coatings using laser-induced ultrasonic waves. Using pulsed laser ablation on the bottom substrate surface, strong stress waves are generated leading to interfacial fractures and coating delamination. Simultaneously, a laser ultrasonic interferometer is used to measure the normal (out-of-plane) displacement of the top surface coating in order to detect coating delamination in a non-destructive manner. We can thus determine the critical laser energy for delamination, yielding the critical stress (that is, the interfacial strength). Subsequently, to examine the interfacial fracture toughness, additional pulsed laser irradiation is applied to a pre-delaminated specimen to show that the delamination area expands. This type of interfacial crack growth can be visualized using laser ultrasonic scanning. Furthermore, the calculation of elastic wave propagation was carried out using a finite-difference time-domain method) in order to accurately estimate the interfacial stress field. In this calculation, the stress distribution around the initial delamination is calculated to obtain the stress intensity factor. Based on the experimental and computational results, interfacial fracture toughness can be quantitatively evaluated. Since this technique relies on a two-laser system in a non-contact approach, it may be useful for a quantitative evaluation of adhesion/bonding quality (including both interfacial fracture strength and toughness) in various environments.     

Hot-Dip Coating of Lead-free Aluminum on Steel Substrates with Ultrasonic Vibration

Hot-dip coating has been practically employed in manufacturing zinc alloy coated steel sheets.However,it is difficult to coat aluminum alloy on a bulky steel substrate without sufficient preheating,because a rapidly solidified layer containing gas babbles is formed on a substrate surface.A variety of iron-aluminides are also formed at the interface of a steel and aluminum hot-dip coating system,which is the main difficulty in joining of steel with aluminum.Ultrasonic vibration was applied to a steel substrate during hot-dip coating of aluminum alloy to control a rapidly solidified layer and a brittle reaction layer.Hot dipping of columnar steel substrates into molten aluminum alloy （Al-2.7 mass fraction Si-4.6 mass fraction Sn） was carried out through the use of a Langevin oscillator with resonant frequency of 19.5 kHz.The application of ultrasonic vibration is quite effective to control a rapidly solidified layer and a surface oxide layer from a substrate surface by the sonocapillary effect based on a cavitation phenomenon,so that the intimate contact is achieved at the beginning of hot-dip coating.The application of ultrasonic vibration to hot-dipping is effective to control a reaction layer with less than 5 #m in thickness.An impact test exhibits that the good adhesive strength is approved in hot-dipped aluminum coatings with a thin reaction layer of approximately 5μm.     

A coatings application for fusion research

A large stainless steel toroidal inner vacuum liner, with a major radius of 79.5 cm and minor radius of 23.5 cm, has been coated with gold to minimize gas adsorption and to provide an inert surface for the plasma in ORMAK, a fusion experiment. A thin platimum film, deposited prior to the gold coating, acts as a diffusion barrier and prevents many of the constituents of the steel from diffusing through the gold to the surface.In a fusion reactor the walls exposed to the plasma will be bombarded by highly energetic neutrals and a gold surface may not be suitable because of its relatively high sputtering yield. Several other materials, some of which may be more suitable for use as a “first wall” surface than gold, have been identified but many of these candidates are expected to be impractical to use in their bulk form. A solution is a composite wall consisting of a suitable coating on a substrate chosen for its strength and ease of fabrication. The techniques employed to deposit several of these candidate materials onto stainless steel substrates as thick coatings are described.     

Preparation and Properties of Plasma Spraying Cu-Al2O3 Gradient Coatings

In order to overcome the limitations of low adhesion strength and poor thermal-shock resistance of pure ceramic coatings, Cu-Al2O3 gradient coatings were fabricated by plasma spraying. The microstructure and distribution of Cu-Al2O3 gradient coatings were analyzed. The adhesion strength, thermal-shock resistance and porosity of the coatings were tested. The results show that the composition of the gradient coatings has a gradient distribution along the thickness of coatings. As copper has a relatively low melting point and the molten copper has good wettability on the surface of Al2O3, it can be melted sufficiently and could fill the interstices and pores among the spraying particles effectively, thus improves the adhesion strength, thermal shock resistance and reduces the porosity. The adhesion strength of the gradient coating is 15.2 MPa which is two times of that of the double-layer structure coating.     

Effects of substrate temperature on the surface of polymer thin films prepared by R.F. sputtering with a polyimide target

In this study, we characterized polymer thin films deposited by a conventional radio frequency sputtering apparatus introduced into argon and nitrogen gases with a polyimide target onto copper substrates.Heating effects due to heating the copper substrate at 250 °C in the sputtering on tribological and adhesion properties of thin films were investigated with measuring friction coefficient, wear durability and pull strength. Surface roughness of the nitrogen sputtered thin film decreased by the heating. Friction coefficient of argon and nitrogen sputtered thin films prepared at 250 °C was almost the same level as that prepared at room temperature, respectively. Wear durability of these thin films and adhesion strength between these thin films and copper substrate decreased by the heating.     

Fatigue effect of WC coatings thermal sprayed by HVOF and laser treated, on medium carbon steel

Tungsten carbide coatings thermal sprayed by High Velocity Oxy-Fuel (HVOF) could be employed to obtain very hard surfaces and for the recovery of shafts or pieces with worn zones, like bearing supports, retaining rings, and so on. The aim of this study is to analyse the effects of fatigue in the WC thin coatings (about 0.1 mm thick) sprayed by HVOF on medium carbon steel substrate.Rotational bending fatigue tests were carried out with test specimens, having applied the coating over the raw materials (without anchorage layer), and the different resistance to fatigue was analysed on the uncoated material, laser surface hardened samples, the samples with the WC coating thermal sprayed by HVOF and last with the same WC coating treated by laser. Results show the possibility of predicting the fatigue life of materials with this coating and these treatments and the huge differences between them.     

Control of the bias voltage in d.c. PVD processes on insulator substrates

In plasma-PVD processes, ion bombardment during the growth of thin films has a strong influence on films properties such as morphology, composition, structure, stress, electrical conductivity, and others. Therefore, an accurate control of substrate bias voltage is needed in order to deposit films with the desired properties. For insulator substrates, dc biasing the substrate holder is useless, since the surface shall not follow the applied bias but it will be at the non-controlled floating potential.In this work we present a simple method for the effective control of the substrate bias in dc PVD processes with insulator substrates, based on placing a metallic grid at a certain distance from the non-conductive surfaces to be coated. The desired negative bias is applied to this metallic grid which accelerates ions from the plasma and directs them to the surface to cover. This method has been successfully applied to the deposition of TiN coatings on glass and decorative ceramics by Cathodic Arc Deposition. The deposited films showed good adhesion and gold color, in contrast with the bad adhered and brownish films deposited without the grid. The dependencies on the color and on the mechanical properties of our TiN films deposited on insulating substrates with the value of the bias voltage applied to the substrate are similar to those typically reported in the literature when conductive substrates are used.     

铜铁界面含氧层对镀层结合强度的影响

镀层与基体界面间含氧层的存在,影响电镀层与基体之间金属键的形成,进而影响镀层的结合强度.用氰化物和焦磷酸盐两种工艺在铁片上镀铜,对镀层进行恒电流极化,并以氩离子深度刻蚀和X射线光电子能谱相结合的方法进行检测.电沉积初始电位-时间曲线显示,两种镀铜工艺,结合强度好的出现了基体表面的还原活化电位平阶,结合强度差的焦酸盐镀铜未出现活化就发生了金属的电沉积,而且铜镀层与基体界面含氧量有增高的现象.铜镀层与铁基底界面间含氧层的存在是影响镀层结合强度的主要原因.通过控制金属电沉积初始电位可提高镀层的结合强度.     

Mechanical properties and adhesion strength of TiN and Al coatings on HSS, steel, aluminium and copper characterized by four testing Methods

TiN and Al coatings on substrates of high-speed steel, steel, aluminium and copper have been used to study mechanical properties of coating systems, especially the adhesion of the coating. The quantities measured are internal stress of the coating, determined by X-ray diffraction, the critical load of the scratch test, the microhardness obtained by the indenter technique, and the interface fracture energy, determined by a three-point bend test developed recently by the authors. The fracture energy, G_c, is a measure for the adhesion strength of a coating system. The effect of bias voltage, sputter cleaning and contamination of the substrates on the adhesion strength and other mechanical properties are investigated with the four methods mentioned. Each of the testing methods reveal only specific aspects of the behaviour of the coating systems. The data obtained depend on bulk properties of the film and the substrate material and on properties of the interface. Variation of the bias voltage can change them in quite different ways. In addition, the inter-relations between the adhesion strength of the coating and the failure behaviour of the three-point bend test samples are discussed.     

Studies of nanocrystalline Pd alloy films coated by electroless deposition

The microstructures of thin coating films of pure palladium and palladium alloys deposited from organic electrolytes onto different metallic substrates by electroless plating method have been investigated. The coatings are dense, pore-free 0.005–1 μm thick films with high adhesive strength to the substrate surface. X-ray spectral analysis, X-ray phase analysis, transmission and scanning electron microscopy were used to determine the composition and structure of alloy coatings of binary systems: Pd–Au, Pd–Ag, Pd–Ni, Pd–Pb, and ternary system Pd–Au–Ni. The coatings of Pd–Au, Pd–Ag, and Pd–Ni have a solid solution structure, whereas Pd–Pb is intermetallic compound. It has been found that the deposited films consist of nanocrystalline grains with sizes in the range of 11–35 nm. Scanning and transmission electron microscopy investigations reveal the existence of clusters formed by nanocrystalline grains. The origin for the formation of nanocrystalline structures of coating films is discussed.     

薄膜结合强度的刮剥式测量方法

本文介绍了一种薄膜结合强度的新型测量方法～刮剥法的测量原理、特点以及应用范围。所谓薄膜结合强度的刮剥式测量方法（简称刮剥法），实际上是一个使用特制刮剥刀具、以类似于薄层金属刮削方式工作，并以从基底上剥离薄膜所需能量作为其结合强度量度的测量方法。刮剥法与现广泛使用的划痕式结合力测量法的区别主要表现为：（１）该法是一个能量测量方法，即以薄膜的剥离能量作为薄膜与基体的结合强度的描述；（２）测量时采用的是对膜／基界面的切向加载方式；（３）测量结果是对膜／基结合强度的直接描述，并对基底表面状态、材质等不敏感；（４）可以对其它方法无法进行测量的超硬材料薄膜，如金刚石和立方氮化硼等，进行结合强度测量。本文通过对刮剥过程中刮剥刀刃附近区域的受力分析，得出了成功实现沿界面剥离薄膜的必要条件以及刮剥法测试对试样的要求和应用范围。