TiN薄膜的残余应力调控及力学性能研究

残余应力是制约物理气相沉积（Physical vapor deposition,PVD）硬质薄膜厚度的关键因素。采用多弧离子镀技术在高速钢基体上制备了厚度从3.7 m到15.5 m的TiN薄膜,结合曲率法和有限元法研究残余应力及结合性能随膜厚的变化规律。结果表明,随着膜厚的增加,基片弯曲程度加剧,而薄膜平均残余应力降低;膜层内残余应力的整体水平决定了界面切应力大小,薄膜结合性能随界面切应力的增加而降低。增加基体偏压、降低工作气压均导致薄膜内部残余应力的升高。当残余压应力较高时,TiN薄膜具有细小、致密的柱状晶结构,并呈现（111）择优取向,薄膜硬度及断裂韧度较高,耐磨性能良好。研究结果提示我们,通过残余应力的调控可提高硬质薄膜的力学特性。     

Residual stresses in TiN, DLC and MoS2 coated surfaces with regard to their tribological fracture behaviour

Thin hard coatings on components and tools are used increasingly due to the rapid development in deposition techniques, tribological performance and application skills. The residual stresses in a coated surface are crucial for its tribological performance. Compressive residual stresses in PVD deposited TiN and DLC coatings were measured to be in the range of 0.03-4 GPa on steel substrate and 0.1-1.3 GPa on silicon. MoS₂ coatings had tensional stresses in the range of 0.8-1.3 on steel and 0.16 GPa compressive stresses on silicon. The fracture pattern of coatings deposited on steel substrate were analysed both in bend testing and scratch testing. A micro-scale finite element method (FEM) modelling and stress simulation of a 2 μm TiN-coated steel surface was carried out and showed a reduction of the generated tensile buckling stresses in front of the sliding tip when compressive residual stresses of 1 GPa were included in the model. However, this reduction is not similarly observed in the scratch groove behind the tip, possibly due to sliding contact-induced stress relaxation. Scratch and bending tests allowed calculation of the fracture toughness of the three coated surfaces, based on both empirical crack pattern observations and FEM stress calculation, which resulted in highest values for TiN coating followed by MoS₂ and DLC coatings, being K_C = 4-11, about 2, and 1-2 MPa m^1/2, respectively. Higher compressive residual stresses in the coating and higher elastic modulus of the coating correlated to increased fracture toughness of the coated surface.     

Influence of layer thickness on hardness and scratch resistance of Si-DLC/CrN coatings

Abstract

Multifunctional coatings, widely used in tribological applications, have their properties strongly influenced by the interaction of the system coating/substrate. The use of multilayered coatings has been pointed out as a solution for the problem of high internal stresses that can be generated in coated systems, in particular in the case of soft substrates. In multilayered coatings, a decrease in the stress gradient between substrate and coating improves adhesion. Moreover, the thickness of the coating has shown a strong influence on the tribological behaviour of the coated system. This paper, through widely used and efficient techniques, seeks to assess the influence of the thickness of different layers (DLC and CrN) on the response of a multifunctional coating. Si rich DLC and CrN coatings with different thicknesses were deposited on a steel substrate (AISI 1020) by Plasma Enhanced Magnetron Sputtering (PECVD). Scanning electron microscopy (SEM) and Raman spectroscopy (RS) were used in order to characterize the chemical composition and microstructure of the coatings. Instrumented indentation and scratch test techniques were used to measure hardness, elastic modulus, and adhesion of each layer. Critical loads were determined by visual analysis, using SEM in conjunction with the curves obtained in the scratch tests. The evaluation of the effect of the thicknesses of the layers allowed an optimised design of the multifunctional coated systems with improved durability.     

Load-carrying capacity evaluation of coating/substrate systems for hydrogen-free and hydrogenated diamond-like carbon films

Diamond-like carbon (DLC) coatings have shown excellent tribological properties in laboratory tests. The coatings have also been introduced to several practical applications. However, the functional reliability of the coatings is often weakened by adhesion and load-carrying capacity related problems. In this study the load-carrying capacity of the coating/substrate system has been evaluated. The DLC coatings were deposited on stainless steel, alumina and cemented carbide with two different deposition techniques: the tetrahedral amorphous carbon (ta-C) coatings were deposited by a pulsed vacuum arc discharge deposition method and the hydrogenated carbon (a-C:H) films by radio frequency (r.f.) plasma deposition method. The load-carrying capacity of the coated systems was evaluated using a scratch test, Rockwell C-indentation test and ball-on-disc test. The effect of substrate material, substrate hardness, coating type and coating thickness was studied. An increase in substrate hardness increased the load-carrying capacity for the coated systems, as expected. The two coating types exhibited different performance under load due to their different physical and mechanical properties. For the load-carrying capacity evaluations the ball-on-disc configuration was found to be most suitable. This revised version was published online in July 2006 with corrections to the Cover Date.     

MECHANICAL PROPERTIES AND ANTI-WEARABILITY STUDIES OF MULTILAYER THIN COATINGS ON CUTTING TOOLSS

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Effect of hydrogenated DLC coating hardness on the tribological properties under water lubrication

Hydrogenated diamond-like carbon (DLC) coatings were deposited using unbalanced magnetron sputtering (UBM) equipment with different hardnesses. Effects of coating hardness on tribological properties were investigated with tribo-tests under water lubrication. Results showed that the wear volume increased rapidly during the initial running-in process, but remained nearly constant after the running-in process. The ball wear rate increased as the hardness of the DLC coating increased when metals (stainless steel and brass) were used as counter parts. In contrast, the UHMWPE ball wear rate was independent of the DLC hardness. TEM analysis and nano-indentation measurements were conducted of the transfer layer on the counter bodies’ contact surfaces. The transfer layer consisted mainly of Fe, O and C. The low friction of DLC coating is attributed to this low hardness transfer layer, which acts as a boundary-lubricating layer with low shear strength.     

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.     

Tribological studies of coated pistons sliding against cylinder liners under laboratory test conditions

The presence of coatings and surface topography play an important role in the tribological performance of sliding components. Depending on the coating used, it is possible to reduce friction and/or reduce wear. However, although there may be low friction and wear‐resistant coatings suitable for use in pistons, some coatings may hinder the tribological performance by changing the lubrication regime or by preventing additives from their intended function through chemical mechanisms. In this work, piston skirt segments extracted from a commercial aluminium alloy piston were coated with a diamond‐like carbon (DLC) coating, a graphite–resin coating or a nickel–polytetrafluoroethylene (Ni–PTFE) coating and were tribologically tested using a reciprocating laboratory test rig against commercial grey cast iron liner segments. The tribological tests used commercial synthetic motor oil at a temperature of 120 °C with a 20 mm stroke length at a reciprocating frequency of 2 Hz. Results showed that the graphite–resin coating, although it may serve as a good break‐in coating, wears rapidly. The Ni–PTFE coating showed friction reduction, whereas the DLC coating wore off quickly due to its small thickness. Furthermore, the higher hardness of the DLC coating relative to the cast iron liner surface led to pronounced changes on the liner counterface by polishing. In contrast with the uncoated piston skirt segments, all of the coatings prevented the formation of a visible tribochemical film on the cast iron surface. Copyright © 2012 John Wiley & Sons, Ltd.     

Finite element modelling of brittle fracture of thick coatings under normal and tangential loading

This paper addresses coating fracture in hard brittle coatings subjected to combined normal and tangential loads through a finite element based methodology. The coating is modelled as an elastic layer perfectly bonded to an elastic substrate with a pre-microcrack, assumed to initiate at the contact trailing edge due to high tensile stress. The predicted results are consistent with previously published coating fracture results. The model predicts a significant effect of coating thickness on crack propagation for coatings with large elastic mismatch and the final propagated crack profile is predicted to depend on friction coefficient, coating fracture toughness and sliding displacement.     

Characterization of DC magnetron sputtered diamond-like carbon (DLC) nano coating

Development in vapor deposition techniques over the last two decades has led to the introduction of many advanced coatings for metal-cutting tools. This paper examines the characteristics of multilayer Ti, TiN, and diamond-like carbon (DLC) coatings deposited on standard tool substrates at varying sputtering parameters and conditions, such as power density, partial pressure, substrate temperature, and reactive gases. The characteristics of films were examined using an X-ray diffractometer, Raman microscope, surface profilometer (to measure the thickness of the coating), Rockwell hardness tester (to test adhesion), and a micro hardness tester. The pin-on-disc test setup was used to find the coefficient of friction of the coatings. The results indicated that a graded multilayer coating showed better adhesion to the substrates. It was observed that higher target power density resulted in an increase of micro hardness and crystalline planes of coating. Lattice constant matching among layers of coating, proper substrate preparation, and a sequence of cleaning processes are the crucial factors for the enhancement of adhesion strength.     

MECHANICAL PROPERTIES AND ANTIWEARABILITY STUDIES OF MULTILAYER THIN COATINGS ON CUTTING TOOLS

The nanoindentation fracture of multilayer hard coatings, such as TiN, TiN/Ti(C,N)/TiC, TiN/Ti(C,N)/TiC/Ti(C,N)/TiC and TiN/Ti(C,N)/TiC/Ti(C,N)/TiC/Ti(C,N)/TiC coatings, deposited on cemented carbide using a CVD technique are studied. It is found that these coatings have high hardness. Based on the analysis of the energy release in cracking, the fracture toughness of these coatings are calculated. The observations clearly establish a step occurs in the forcedisplacement curves at the onset of coating fracture and a straigh t line segment in the loadpenetration depth squared curves to identify the interfacial failure of coatings. The hardness, fracture toughness and antiwearability of these coatings are clearly compared. The results show that with the layes increasing, the fracture toughness and antiwearability are getting larger.     

纺织机钢丝圈表面类金刚石膜的制备及摩擦特性研究

为提高纺织机高速纺纱工况下钢丝圈表面的磨损性能,采用直流等离子气相沉积法在钢丝圈表面制备类金刚石涂层(DLC),采用原位扫描探针显微镜观测涂层表面形貌,测量并计算涂层硬度。结果发现,DLC涂层颗粒粒径约为100 nm,呈岛状聚集分布,硬度约为18 GPa。采用球-盘式摩擦试验机研究DLC涂层在不同载荷(20~100 N)和不同转速(100~600 r/min)条件下的摩擦特性。结果表明,在低载高速的条件下,DLC涂层具有良好的耐磨特性,符合钢丝圈的实际工况。采用傅里叶变换红外光谱分析涂层的磨损机制,结果发现,在摩擦磨损过程中从薄膜中释放出来的氢和涂层的剪切变形引起了DLC薄膜的石墨化SP^3/SP^2转变,从而降低了摩擦因数和磨损率。     

Tribological Dependence of the High-Performance Ferrous-Based Coating on Different Coating Counterparts in Engine Oil

A ferrous-based coating with significant chromium was fabricated on aluminum alloy substrate using a plasma spray technique. The tribological performance of the as-fabricated ferrous-based coating sliding against different coatings including Cr, CrN, TiN, and diamond-like carbon (DLC) in an engine oil environment were comparatively studied. Results showed that the high hardness of the sprayed ferrous-based coating was achieved due to the dispersion strengthening effect of Cr₇C₃ phase embedded in the austenite matrix. The ferrous-based coating exhibited low friction coefficients when coupled with these four coating counterparts, which could be attributed to the boundary lubricating effect of engine oil. However, both friction and wear of the ferrous-based coating were different when sliding against these different coating counterparts, which might be closely related to the surface roughness, self-lubricating effect, and mechanical properties of the coupled coatings. Ferrous-based coating sliding against CrN and DLC coatings exhibited good tribological performance in engine oil. The best coating counterpart for the ferrous-based coating in an engine was DLC coating.     

涂层材料热特性参数对点接触弹流润滑的影响

为探究涂层材料热特性参数对点接触弹流润滑的影响,选择3种不同方法制备的类金刚石（DLC）涂层和氧化锆陶瓷涂层,构建考虑涂层热特性的点接触弹流润滑模型,分析涂层材料、涂层厚度和润滑剂的流变性对接触区润滑性能的影响。结果表明：在弹流润滑状态下具有不同热特性的4种表面涂层导致了膜厚的差异,固体表面温度及润滑区温度场会随涂层热惯性变化;热惯性最小的DLC涂层加在快速运动表面能获得更高的膜厚;随着涂层厚度的增加,会引起固体表面的温度升高,使摩擦因数降低;非牛顿流体对压力、膜厚的影响很小,但与牛顿流体相比,能获得相对较低的温度。在弹流润滑状态下,涂层覆在快表面对于减小摩擦、提高膜厚是有益的。     

Influence of DLC coating thickness on tribological characteristics under sliding rolling contact condition

Diamond-like-carbon (DLC) coating of thickness 3 and 10 μm were developed with and without radical nitriding pretreatment on steel rollers and spur gear pair. The friction coefficient and wear amount were evaluated under sliding rolling contact condition in vacuum and under oil lubrication. Delamination of coatings was observed at the interface of the substrate. The wear resistance of coatings improved with the thickness of the coating. In vacuum both the roller and the gear pair of 10 μm coating thickness with radical nitriding showed identical wear behavior. The radical nitriding seemed to enhance the life of DLC coatings.     

Substrate dependence of the scratch resistance of CrN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> coatings on steel

Hard CrN_x coatings were sputter deposited on hot work tool steel (HWTS) and high speed steel (HSS) in an industrial PVD reactor. Coatings were deposited under various nitrogen flows. The thickness, density, hardness, elastic modulus, composition, and stress were determined for the coatings. The specimens were subjected to scratch testing. Two different failure mechanisms were investigated: chipping and complete coating removal. For all specimens, the coating-to-substrate adhesion was that good that adhesion did not limit the scratch resistance. Therefore, the minimum loads at which a given type of failure was initiated were not a measure for the coating-to-substrate adhesion. The scratch resistance was better for coatings on HSS than for coatings on HWTS. This is due to the higher hardness of the HSS. Substrate independent measures for the scratch resistance of the coating were obtained by considering critical track widths instead of critical loads. The hardening of the coating--substrate systems due to the coating was investigated. The uncoated substrates exhibited track width independent scratch hardness. For the coated specimens the scratch hardness increased with increasing track width until chipping of the coating occurred. Complete coating removal coincided with a decrease in hardness. Although the elastic properties, hardness, and thickness of all coatings were more or less equal, CrN_1.0 coatings outperformed CrN_0.6 coatings in scratch tests both on HSS and on HWTS.     

Effect of oil additives on the durability of hydrogenated DLC coating under boundary lubrication conditions

Diamond-like carbon (DLC) coatings have became accepted non-ferrous coatings for automotive tribo-components as they offer excellent tribological properties resulting in improved fuel economy and reduced dependence on existing lubricant additives which can be harmful to catalytic converters and ultimately to the environment. Obtaining optimum durability (wear) as well as high fuel economy (low friction) using DLC-coated parts relies in part on the compatibility between surface and lubricant additives. The objective of this study is to understand the role of friction modifiers and antiwear additives on the durability of DLC coating under boundary lubrication conditions. Experiments were performed using a pin-on-plate tribotester using plates coated by 30 at.% hydrogen containing DLC (HDLC) sliding against cast iron (CI) pins. The physical observation of the wear scar, formed on the HDLC coating by low friction and/or antiwear additives, was performed using both optical and scanning electron microscopes. X-ray photoelectron spectroscopy analysis was performed on the tribofilms to help to understand the tribochemical interactions between oil additives and the HDLC coating. Based on the physical observations and tribochemical analysis of the wear scar, the mechanisms of failure/wear of the HDLC coating are proposed and the requirement for designing optimal additive packages for the HDLC coating is discussed.     

Tribochemistry of silicon and oxygen doped,hydrogenated Diamond-like Carbon in fully-formulated oil against low additive oil

Diamond-like Carbon (DLC) coatings are increasingly used to reduce wear and lower friction in many applications. Doped DLCs are being produced with the goal of further enhancing the friction and wear profile as well as increasing the coating reliability.Silicon is often incorporated into DLC as it is known to affect the sp²/sp³ ratio which in turn can affect the hardness of the film. It can also improve adhesion of the DLC coating to the substrate and lower internal stress.In this study, investigations into the wear behaviour, tribochemistry and oil-formulation dependence of Si, O-doped DLC (Si-DLC) were conducted. The oxidative stability of Si-DLC was also examined.Silicon-doped DLC is able to form a protective tribofilm when a fully-formulated lubricant is used. The tribofilm is composed of S, P, Ca and Zn which are widely recognised as being important to wear reduction.A mechanism of wear repression facilitated by oil additives is proposed.     

Studies of the tribological and mechanical properties of laminated CrC–SiC coatings produced by r.f. and d.c. sputtering

Multilayer CrC–SiC coatings were produced by both direct current and radio frequency sputtering of Cr and Si targets in an Ar–C₂H₂ atmosphere. Coatings of constant thickness of about 4 μm, but with the number of layers varying between two and 200, were prepared and studied. Investigations of coating morphology were performed by scanning elecron microscopy (SEM), and coating composition was investigated using Glow Discharge Optical Spectroscopy (GDOS) and Auger Electron Spectroscopy (AES). Microhardness measurements, scratch adhesion, pin on disc, ASTM rubber wheel and impact wear tests were performed and the results were related to the individual layer thicknesses. It is shown that the improvement observed in hardness tests does not necessarily result in the improvement of other mechanical properties, e.g. adhesion and toughness. When two comparatively hard materials are combined in a multilayer coating, the result can be an increase in brittleness due to an absence of plastic release mechanisms for dislocation accumulation at layer boundaries. Thus, it is necessary to seek a compromise in the hardness of multilayer films in order to achieve optimal behaviour across a range of different surface contact conditions.     

Ti/TiN复合膜工艺优化及性能研究

利用正交试验和极差分析方法，分析了多弧离子镀Ti／TiN复合膜中工艺参数（弧电流、氮气分压、基体负偏压、钛过渡层厚度）对Ti／TiN复合膜的纳米硬度和膜与基体的结合力的影响及主次关系，并通过正交试验对工艺参数进行了优化。研究表明，氮气分压和弧电流是影响Ti／TiN复合膜纳米硬度的2个最主要因素，膜层与基体的结合力随着弧电流的增加而下降；升高基体负偏压，虽然可以提高Ti／TiN复合膜纳米硬度和膜与基体的结合力，但是高负偏压将急剧升高基体温度，可能导致基体退火；沉积一定厚度的钛过渡层可以显著提高TiN膜层与基体的结合力。