The effect of substrate properties on tribological behaviour of composite DLC coatings

The unique features of DLC coatings in providing low friction and low wear and, at the same time, causing low wear to the counterface make them very attractive in industrial applications, in improving tribological performance of mechanical components on various substrates. In this study, composite DLC coatings have been deposited on sintered ferrous alloy, M42 tool steel, 2618 aluminium alloy, and 6063 aluminium extrusion substrates using the combined CFUBMS–PACVD technique. The effect of mechanical properties of substrate materials on tribological behaviour of the composite DLC coatings has been investigated at various loads on a ball-on-disk wear machine in dry air. A transition load was usually observed for coatings on the various substrates except for the aluminium extrusion; above the transition load the coating was completely destroyed via some spallation/fragmentation process after 2 h sliding, and the wear rate increased dramatically with further increase in load. The coating system on sintered ferrous alloy substrate exhibited the highest transition load among the four types of substrates studied. This is considered to have resulted from the combined effects of the lower elastic modulus of the porous sintered ferrous alloy substrate, which decreases the stress concentrations in the contact region, and the surface roughness and porosity, which enhance the bonding strength between the coating and the substrate under multi-contact conditions. The high elastic modulus of the tool steel substrate leads to tensile stress conditions in the sliding contact region and therefore makes coatings deposited on such a substrate more prone to breakdown/fragmentation, resulting in a transition load close to that for coatings on the soft 2618 aluminium alloy substrate. For coatings on the 6063 aluminium extrusion substrate, significant plastic deformation occurred in the substrate at loads above 1.5 N. However, despite the heavy deformation in the substrate, coatings on this substrate were not scraped off, as were coatings on the 2618 aluminium alloy substrate, even at a load as high as 20 N. The specific wear rate increased continuously with load, no apparent transition load being explicitly identifiable. This study shows that hard DLC coatings can be applied on both hard and soft substrates for improvement of the tribological behaviour of mechanical components.     

Method and surface roughness aspects for the design of DLC coatings

The design of coatings for highly loaded component contacts, such as bearings, gears and valve train components involves several important factors, including load, friction, lubrication, surface characteristics and material parameters. This paper presents an investigation of the influence of the material, coating thickness and surface roughness on tensional stress levels for coatings that are more compliant than the substrate material. Specifically the effect of multiple asperity contact is studied in three dimensions. The simulation is based on a finite element model where the load is applied as several interacting Hertzian pressure distributions.The results show that the surface structure, in combination with the elastic properties of the coating, has a large influence on the tensional stress level in the coating. The highest tensional stress level in the coating occurs when contact spots almost overlap neighbouring cells and at the same time the size of the contact spots is in the same order of magnitude as the coating thickness.     

金属基/陶瓷复合双涂层的正接触应力分析

以球形压头为模型，采用I—deasCAD／CAE软件对Hertz弹性接触状态下金属基／陶瓷双涂层系统的应力分布情况进行了理论建模，并基于模型计算了在不同的涂层厚度／接触半宽度比和外涂层／过渡层／基体弹性模量比情况下的应力分布情况。双层系统具有相同的厚度和不同的弹性模量。论述了无涂层弹性半空间体的有限元分析结果与经典的Hertz接触力学解析解结果的一致性，计算结果有助于工程中陶瓷涂层的设计与应用。     

Modelling the Deformation Behaviour of Multilayer Coatings

In tribological contacts involving relatively thin vapour-deposited coatings, it is the substrate which provides the main load support. The coating must therefore be able to deflect along with the substrate without cracking if it is to function satisfactorily. Although the potential benefits of multilayer coatings in such contacts have been well documented, their deformation behaviour has not been properly understood. Since it is this response to deformation that provides their benefit (compared to single-layer coatings), we have undertaken studies into this aspect, both by simulation modelling and finite element analysis. These studies conclude that, for multilayer coatings having repeated hard/soft (or low/high modulus) layers, the main shear deformation takes place in the low modulus (soft) layers. This ensures that the harder (more brittle) layers effectively slide over each other, and do not experience high bending stresses. This helps to explain why such multilayers can deflect with the substrate under load without cracking, and therefore survive in heavily loaded tribological contacts.     

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.     

Nano-scratching process and fracture mechanism of amorphous carbon films

The nano-scratch behavior of amorphous carbon films prepared on Ti alloy substrate by rf PECVD was evaluated using a nano indenter system with an attachment for lateral force measurement. The scratching processes were analyzed to understand the deformation behavior and the failure mechanism of the films. It was concluded that, three processes containing fully elastic recovery, asynchronous recovery, and delaminating of the film, successively occurred with the increase of load during scratching; in the first regime, no damage could be found on the surface of the film, in the second regime, due to asynchronous recovery of the film and the substrate, a trace like fish bone was formed, which was composed of tiny cracks along the track of the indenter. It was only in the third regime that the critical load was reached and partial spalliation of the film was detected with careful examination. It was determined that the film spalliation originated right from the fish bone cracking, and the fish bone was just a profiling of the Berkovich indenter. By our model, the film was delaminated from the substrate due to combining effect of parallel cracks and the separation of the interface between the film and the substrate.     

Failure mode maps in the thin film scratch adhesion test

The scratch test has been used to assess thin coating adhesion for some time now. In this test a diamond indenter is drawn across the coated surface under an increasing load (either stepwise or continuous) until at some load, termed the critical load, L_c a well-defined failure event occurs; if this failure event represents coating detachment then the critical load can be used as a qualitative measure of coating-substrate adhesion. However, it is well known that a range of possible failure modes can occur and only some of these are dependent on adhesion. Other failure modes which depend on plastic deformation and fracture within the coating, rather than any adhesive failure at the coating substrate interface, may be just as useful in the assessment of coating quality particularly for tribological applications. In this paper the load regimes in which the main adhesion-related failure modes (spallation and buckling) occur as a function of coating thickness will be presented for thermally grown oxide and sputtered nitride coatings. The origin of the observed failure modes and the use of the scratch test to assess coating/substrate adhesion in a more quantitative fashion is discussed in the light of these observations.     

小能量多冲法对物理气相沉积膜层机械性能的评价

利用自行研制的小能量多冲试验装置,对物理气相沉积(PVD)方法制备的CrMo合金膜和TiA lN陶瓷膜的结合强度、韧性、内聚强度等机械性能进行综合评价;探讨了膜层性质、冲头几何形状、基材硬度等因素对PVD膜层在小能量多冲条件下失效行为的影响规律;比较了膜层在承受动态多冲与静态压入载荷条件下失效行为的异同;分析了膜层在小能量多冲条件下的失效机制。结果表明:小能量多冲法能够很好地模拟膜层承受动态冲击载荷的服役工况。该方法既可以用于评价硬质陶瓷膜层的机械性能,也可用于合金膜层的力学性能表征;冲击力比冲击周次对膜层多冲失效行为的影响更为显著;四棱锥冲头较球型冲头更易于造成膜层的剥落失效,因而更有利于评价膜层的结合强度;承载能力高的基材有利于提高膜层的多冲抗力;膜层在四棱锥冲头多冲条件下主要发生膜基结合型破坏,而在静载压入条件下的失效则以开裂为主,反映了膜层的韧性及内聚强度。     

Mechanical Modeling of Scratch Behavior of Polymeric Coatings on Hard and Soft Substrates

An ASTM standard scratch test is utilized to study the scratch behavior of polymeric coatings on soft and hard substrates. Depending on the different combination of polymeric coatings and substrates utilized, various damage modes can take place, which include coating delamination, transverse cracking, and buckling failure. A soft coating on a hard substrate will give rise to an entirely different scratch damage pattern from those of a hard coating on a soft substrate. The stress and strain responses of scratch on polymeric coating are analyzed using three-dimensional finite element (FE) simulation. The analysis provides mechanistic insights for the observed polymer coating deformation mechanisms and failure modes. Usefulness of the ASTM scratch method and FE modeling to evaluate polymer coating scratch behavior is discussed.     

A numerical/experimental study of contact damage in non-planar porcelain/metal bilayers

In this report, we investigate and visualize the effect of shape irregularity on contact damage in a brittle coating on a stiff metal substrate. Hertzian contact damage in a dental porcelain layer of thickness between 0.25 and 0.75 mm, fused onto a Ni–Cr alloy substrate in both curved and planar geometries was studied with the aid of the finite element method and experimental investigation. Three failure modes were examined with varying porcelain layer thickness: cone cracking at the upper surface of the porcelain, median or interface cracking at the layer/substrate interface and plastic deformation below the contact area in the substrate. It is shown that curvature has very little effect on the initiation of surface cone cracks in this system, but substantial effect on the initiation of interface radial cracks. In particular, curvature reduces the critical load for the onset of interface cracks.     

Characterisation of DLC coating adherence by scratch testing

In order to characterize the adherence of DLC coatings (Diamond Like Carbon), scratch testing was performed on a unit equipped with sensors for normal and tangential forces, and an acoustic detector to detect the nucleation and the propagation of cracks. The system is also equipped with a microscope permitting observation of each event on the scratch according to the friction tangential force signal or the acoustic signal. The local microscopic observation allows identification of the damage with respect to the normal load. The test was performed with a Rockwell C indenter at the relative displacement speed, v=10 mm/min under a progressive normal load from 5 to 55 N.Coating failure appears in various modes, particularly the following: propagation of the cracks along the longitudinal edges of the scratch; propagation in front of the indenter; rupture along the maximum principal stress lines; and, detachment in the subsurface by shearing of the coating. The microscopic analysis of the evolution of the scratch under a progressive normal load permits identification of the various traces and the damage mechanisms of the coating.In this study, experimental results are shown for the scratch tests on bulk glass and DLC coating. Various modes of crack initiation, damage and rupture of these materials according to the critical normal load are presented. The analysis of the contact stress field distribution in bulk glass enables identification of the crack initiation and its propagation in the coating.     

Two-body abrasive wear of electroless nickel composite coatings

The two-body abrasive wear of electroless nickel (EN), EN-silicon carbide, and EN-alumina composite coatings have been investigated using a scratch test with a diamond indenter. The coatings were heat treated at temperatures of 100–500° C. The hardness of the coatings increased with heat treatment temperature from 500 HV100 for the as-deposited condition to 1008 HV100 when fully hardened. Scratch testing showed that the as-deposited coating had scratch tracks with a high degree of plasticity, signs of microploughing and tensile cracking and was characterised as a ductile failure. On the other hand, the heat-treated coatings showed chipping and cracking on the edge of the scratch tracks, failing in a brittle manner. The heat-treated EN-silicon carbide coatings, however, exhibited no cracking nor chipping, believed to be due to its higher fracture toughness than the other heat-treated coatings, attributable to its lower phosphorus content. The volume of material removed from the silicon carbide scratch track was 1/3 of the volume removed from the steel substrate at a 20 N load, and showed the best wear/ scratch resistance of any of the coatings tested.     

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.     

A contact problem for an elastic half-plane with rigid coating

Solution of contact problems on soft or rigid coatings of an elastic half-plane is of great practical interest. The present paper is aimed at considering the problem of interaction between a rigid biquadratic die and an elastic half-plane through a thin rigid coating. We have derived an integral equation for the problem under study and constructed its approximate solution by an asymptotic method.     

The study of the adhesion of a TiN coating on steel and titanium alloy substrates using a multi-mode scratch tester

A titanium nitride (TiN) coating was deposited by magnetron sputter ion plating onto steel and titanium alloy polished substrates. The adhesion of the coating on each substrate material was investigated using a newly developed multimode scratch tester. Progressive loading scratch tests, constant load scratch tests, multiple scratch tests in the same track and indentation tests were all performed. It was shown that the modified scratch tester can be used to identify not only coating detachment during progressive load scratch tests, but also other failure events such as cracking and cohesive damage to the coatings. By using the additional modes of operation, it was possible to study the fracture mechanisms in more detail i.e. chipping in the scratch track was cohesive for the TiN coated steel and adhesive for the TiN coated Ti alloy.     

Deposition of duplex Al2O3/TiN coatings on aluminum alloys for tribological applications using a combined microplasma oxidation (MPO) and arc ion plating (AIP)

Microplasma oxidation (MPO) has recently been studied as a cost-effective plasma electrolytic process to provide thick and hard ceramic coatings with excellent surface load-bearing capacity on aluminum alloys. However, for sliding wear applications, such ceramic coatings often exhibit relatively high friction coefficients against many counterface materials. Although coatings deposited by physical vapour deposition (PVD) techniques such as TiN coatings are well known for providing surfaces with a high hardness, in practice they often exhibit poor performance under mechanical loading, since the coatings are usually too thin to protect the substrate from the contact conditions. In this paper, these challenges were overcome by a duplex process of microplasma oxidation and arc ion plating (AIP), in which an alumina layer Al₂O₃ was deposited on an Al alloy substrate (using MPO as a pre-treatment process) for load support, and a TiN hard coatings were deposited (using AIP) on top of the Al₂O₃ layer for low friction coefficient. Microhardness measurements, pin-on-disc sliding wear tests, and antiwear tests using a Timken tester were performed to evaluate the mechanical and tribological properties. Scanning electron microscopy (SEM) was used to observe coating morphology, and to examine wear scars from pin-on-disc test. The research demonstrates that a hard and uniform TiN coating, with good adhesion and a low coefficient of friction, can successfully be deposited on top of an alumina intermediate layer to provide excellent load support. The investigations indicate that a duplex combination of MPO coating and TiN PVD coating represents a promising technique for surface modification of Al alloys for heavy surface load bearing application.     

Mechanical characterization of nanostructured TiB2 coatings using microscratch techniques

TiB₂-based nanostructured coatings were fabricated on high-speed steel by magnetron sputtering technique. Mechanical characterization of the resultant coating-substrate systems, such as coating adhesion, friction and scratch resistance, was conducted by microscratch technique. The linearly increasing load mode of microscratch test was studied to determine the most effective and informative testing conditions and to determine the critical load (Lc) for coating failure. The mode of failure was examined by high resolution SEM and AFM. In order to gain a better understanding of the scratch behaviour during the test, a three-dimensional finite element (FE) model was developed to simulate the scratch process. The developed FE model was able to demonstrate the elastic and plastic behaviour of the coating and substrate around the contact area during scratch test. Good agreement has been observed between the FE analysis results and experimental investigations.     

Microstructure and wear behaviour of Ni-based surface coating on copper substrate

The Ni-based surface coatings were prepared by a vacuum infiltration casting technique on copper substrate. The surface coatings were fabricated through copper melt penetrating into thin preforms whose thickness could change. By optimizing the processing parameters, compact surface coatings were achievable as confirmed through SEM observation. The surface coating was mainly composed of solid solution of Ni, solid solution of Cu and CrB. The macro-hardness of the coating was about HRC 58, and the micro-hardness of the coating shows a gradient distribution. The average micro-hardness of the coating was about HV450. Wear behaviour was investigated by using block-on-ring dry sliding linear contact at several loads (50 N–300 N) and two different sliding speeds (0.424 m/s and 0.848 m/s). Wear rate and friction coefficient were estimated using a method founded upon the PV factor theory. The surface oxidation predominated as the principle wear mechanism at low load. Meanwhile, adhesion and oxidation mechanism were observed when the coatings were tested at higher load more than 200 N. Friction coefficient decreased with increasing load and sliding speed.     

Application of atomic-force microscopy when measuring microhardness of thin coatings by scratching

The method is proposed for determining the microhardness of thin coatings by scratching. Atomic force microscopy was applied to determine the scratch width and the optimal load on the indenter. Combined coatings 4 μm thick based on Ti, Cr, Mo₂N, and alloy ZrHf obtained by the combination of electric-arc evaporation and ion nitridizing were studied. The combined coatings are shown to have better micromechanical characteristics compared to the nitride coatings deposited by the electric-arc evaporation of these refractory metals and alloys.     

Effect of elastic modulus mismatch on the contact crack initiation in hard ceramic coating layer

Effect of elastic modulus mismatch on the contact crack initiation is investigated to find major parameters in designing desirable surface-coated system. Silicon nitride coated soft materials with various elastic modulus mismatch,E _c /E _s = 1.06 — 356 are prepared for the analysis. Hertzian contact test is conducted for producing contact cracks and the acoustic emission detecting technique for measuring the critical load of crack initiation. The implication is that coating thickness and material strength are controllable parameters to prevent the initiation of contact cracks resulted from the elastic modulus mismatch in the hard ceramic coating layer on the soft materials.