DLC Coated Research Articles

Micropitting performance and friction behaviour of DLC coated bearing steel surfaces : On the influence of Glycerol-based lubricants

A better understanding about the rolling contact fatigue and micropitting performance of machine component surfaces lubricated with environmentally friendly lubricants is critical to designing and further formulating new lubricants intended to be used in rolling–sliding contacts such as those found in gear and bearing applications. In this work, the frictional behaviour and rolling contact fatigue (RCF) performance of DLC, Cr/a-WC:H/a-C:H and a-C:Cr coatings under glycerol-based lubrication in rolling sliding contact conditions have been investigated. Traction maps, Stribeck curves, and fatigue plots have been generated by using a micropitting test rig (MPR). The initiation and progression of micropitting was monitored by means of white light optical interferometry and scanning electron microscopy (SEM). Results indicated that glycerol-based lubricants exhibited a significant friction reduction as the hydrodynamic effect is enhanced at higher rolling-speeds. Under boundary lubrication the friction coefficient was significantly higher compared to the values obtained with a commercial mineral-based transmission oil. Compared to uncoated steel surfaces, DLC coatings effectively reduced the volume loss and micropitting progression. Irrespective of the coating thickness, DLC showed an excellent tribological behaviour when the base lubricant favours the onset of mild-wear, over micropitting. When the lubricant formulation favoured the onset of micropitting, the coatings tended to prematurely fail due to debonding from the substrate, and local micro-spallation. The experiments demonstrated that friction reduction does not necessarily correspond with a reduction of micropitting.

Silicon wafer as a feasible candidate for tribological characterization of thin coatings under high contact stress?

Silicon (Si) wafer is generally used as the substrate for structural analysis and residual stress measurement of thin coatings. A few studies can be found for mechanical and tribological characterization wherein Hertzian contact stress never exceeded 1 GPa. Si, being single crystal, pure, and having low surface roughness (Ra) is an ideal potential candidate for thin coating analysis. However, Si wafer breaks immediately under higher contact stress. The current study aims to push the contact stress limit to test thin coatings on Si wafer especially for friction and wear testing. To serve the purpose, different bias DLC coatings (−120 V, −80 V, −40 V, Zero V, +40 V, +80 V, +120 V) were deposited on Si wafer employing closed field unbalanced magnetron sputtering (CFUMS) system. Using conveniently available experimental approach, Hertzian contact stress in tribological tests has been pushed up to 2.6 GPa successfully on DLC coated Si wafer. Thus, it is feasible to test thin coatings on Si wafer at high contact stress which will provide useful and consistent friction and wear data such that further test using the steel substrate will be reduced by a significant margin. A thorough mechanical, tribological and structural (Raman) analysis is presented. It was found that −120 V possessed the highest hardness (H: 17.6 GPa), modulus (E: 184 GPa) and residual stress (4.5 GPa). The same coating produced the lowest sp2/sp3 ratio, wear rate using contact stress (0.092 × 10−13 mm4/N) and wear/friction energy (0.071 × 10−13 mm3/J2) in comparison to other coatings used in the study. Counter body analysis for −120 V shows the minimum scratch area depicting friction between transfer film and DLC hence the lowest coefficient of friction (0.061) was obtained. Moreover, surface topography is explained with the aid of atomic force microscope (AFM) images and surface roughness (Ra) data. AFM revealed craters and bumpy like surfaces for positive and negative biased samples respectively.

Combined effect of laser texturing and carburizing on the bonding strength of DLC coatings deposited on medical titanium alloy

To improve the bonding strength of diamond-like carbon film (DLC) on Ti6Al4V alloy (TA) substrate, the effect of texturing, carburizing and their combination was investigated. The different surface treatments were carried out on the Ti6Al4V substrate surface by laser texturing and carburizing before the deposition of DLC coatings. Thus, three different DLC coated samples were prepared: 1. DLC coatings with a laser textured substrate, named TDLC; 2. DLC coatings with a laser carburized substrate, named CDLC; 3. DLC coatings with a hybrid laser textured and carburized substrate, named TCDLC. Meanwhile, the only mechanical polished DLC coatings (ODLC) were also prepared for comparison. The adhesion and tribological performance of different DLC coatings were evaluated by using variable depth nano-scratch tests and ball-on-disk wear tests . Among the four kinds of DLC coatings with different surface treatments, the bonding strength of hybrid pretreated coatings (TCDLC: 12.25 N) were highest compared with other three samples (ODLC: 5.73 N; TDLC: 8.72 N; CDLC: 9.28 N). The coupling of the mechanical-physical-chemical effects accompanying laser texturing and carburizing reaction can enhance the wettability and specific surface area of the Ti6Al4V substrate surface, thereby providing a mechanical self-locking and matching chemical property between the Ti6Al4V substrate and DLC coatings. • A hybrid pre-processing method of laser texturing and carburizing was proposed. • The combined treated DLC coated sample presented the best coatings adhesion. • Matching chemical property between Ti6Al4V substrate and DLC coatings was obtained.

Seizure and Friction Properties of the DLC Coated Journal and Aluminum Alloy Bearing

Seizure and Friction Properties of the DLC Coated Journal and Aluminum Alloy Bearing

A methodology for characterizing the electrochemical stability of DLC coated interlayers and interfaces

DLC coatings are often deposited on Si and Cr based adhesion-promoting interlayers to mitigate stress at the DLC/substrate interface. Interlayers reduce the likelihood of mechanical coating delamination. However, their chemical stability at miniature DLC coating defects, such as pores and micro-pinholes, may be adversely affected in a corrosive environment, such as the human body where DLC coated implants are envisaged. An experimental methodology is presented for accessing and characterizing the electrochemical reactivity of buried interlayers and interfaces. The interlayer, with its interfaces, is revealed by ion beam polishing at an angle, forming a wedge-like profile of the substrate/interlayer/DLC system. The chemical binding and composition of the interlayer/interface is determined by Auger electron spectroscopy (AES), and the susceptibility of the different interfaces to carbide formation and oxidation is identified. It is shown that pure interlayer/interface materials can be obtained for the model Co interlayer. On the other hand, for more reactive materials such as Si, Ti and Cr, the formation of new interface phases was observed. These different new materials are characterized for their corrosion susceptibility with a local-electrochemical microcapillary technique both at open circuit potential (OCP) and under potentiodynamic polarization. Cr and Si-DLC based interlayers presented good passive behavior, while a Si interlayer corroded in bovine-based wear test fluid (HyClone® WTF). An analogous degradation is found after a long-term immersion experiment and failed implant cases. The influence of coating defects distribution on the current response of DLC surfaces during electrochemical measurements was also investigated by varying the exposed area (from the cm2 to the μm2 range). Microscale characterization allowed for a better representation of the intrinsic reactivity of DLC, while larger areas were highly dependent on the underlying interlayer.

Investigation of diamond-like carbon coated steel corrosion: Enhancing the optical detection of defects by a controlled electrochemical activation

It is often reported that DLC thin films have good corrosion properties due to chemical inertness and low electrical conductivity. Nevertheless, the performance of these coating is highly sensitive to the presence of growth defects. An in situ optical microscopy coupled to an electrochemical test was then developed to evaluate corrosion protection performance of DLC coated steel. A square wave voltammetry was applied to increase the sensitivity of electrochemical techniques based on the detection of the dissolution of the bare metal surface triggered by the presence of uncoated spots due to inherent defects. With this in situ electrochemical test, a clear improvement in optical detection “natural” defects is obtained. SEM coupled with FIB milling technique was used to generate artificial defects with controlled dimensions down to 2.5 μm diameter to define the sensitivity of the proposed counting procedure. Results showed that it was possible to detect cylindrical defects with diameter down to 2.5 μm. Moreover, the electrochemical approach allowed to characterize the propagation of the damage under the DLC coatings.

Tribological and corrosion performance of DLC coating on sintered Al alloy

DLC formation on sintered Al alloys and its purity is obtained for automotive applications. In this work, initial evaluation of DLC coatings was performed using scratch, micro-hardness, fracture toughness and surface roughness tests on sintered Al. Raman spectra analysis confirms the DLC formation. The DLC coated sintered Al samples exhibits micro-hardness in the range of 2000 to 2300 kg mm−2 when measured using a Vickers diamond pyramid indenter of 0.5 N load applied on it. In some localized regions, even more hardness (>5000 kg mm−2) has been obtained. Scratch test results shows a fairly homogenous coating with strong adhesive nature having almost four times the average critical force compared to electro-statically deposited sample. Fracture toughness and surface roughness are well within the acceptable ranges of DLC coatings. The capability of laser sintering to produce thick DLC coatings with outstanding mechanical and tribological properties and excellent bonding with aluminium offers the possibility to tailor an extreme lightweight, strong and wear-resistant material. Nano indentation result indicates the information of plastic and elastic deformation for industry based applications.

Experimental Study on Surface Element Composition and Hydrophobic Properties of DLC Coating

Precision rubber mold is an extremely important basic process equipment in industrial production, however, there are some classic problems, such as viscose, coke accumulation and difficult demoulding, can not be effectively resolved. In order to solve this problem, in this paper, a new process of preparing high performance DLC coated precision rubber mold is proposed by using the excellent anti-sticking properties, self-cleaning properties and hydrophobic properties of DLC coating. In the experiment, steel ( S35C ) substrate with the same material as tire mold is used. Hydrophobic of DLC coating surface mainly depends on the surface element composition and the roughness. All the substrates are exactly processed, so as to ensure that the roughness has minimal impact on hydrophobic. Plasma enhanced chemical vapor deposition (PECVD) technology is employed to prepare fluorine doped diamond-like carbon (F-DLC) coating and hydrogenated diamond-like carbon (a-C:H) coating. Arc ion plating (AIP) technology is applied to produce Non-hydrogenated diamond-like carbon (a-C) coating. The coating is deposited on the surface of steel ( S35C ). Then, the roughness, the element composition and the hydrophobicity of the prepared coating are tested. Experimental results demonstrate that the water contact angle of a-C coating only 73.8 degrees, water contact angle of a-C:H coating up to 80.16 degrees, however, water contact angle of F-DLC coating could reach to 89.01 degrees, meanwhile, the surface energy can be reduced to 22.62 mN/m. To some extent, the hydrophobicity of the coating enhances with the increase of H content, and the content of weak polarization group CF2 becomes stable with the incorporation of F element. As a consequence, the surface of coating becomes smoother and the wettability of coating and water becomes poorer, therefore the performance of hydrophobic of the coating is further enhanced.

Tribological behavior of thick DLC coatings under lubricated conditions

DLC coatings, with properties like high wear resistance, corrosion protection and low friction coefficient under unlubricated conditions, are becoming more and more popular in different fields every year, and offer a wide application spectrum in many industrial areas. A new approach is the deposition of thick DLC films with thickness up to 55μm. With this level of thickness, it is possible to create interesting tribological systems with high mechanical load carrying capacity even on softer steels, which are used in general engineering. However, under lubricated conditions, DLC films can show very different behavior compared to dry conditions. In this work three different DLC coatings were investigated. A “Hard” and a “Soft” coating, as well as a Multilayer DLC film. As substrate material, samples of steel grade 4140 were used. In order to determine the influence of the surface condition, each coating was deposited on a rough (sandblasted) and a polished substrate. The friction coefficient (COF) was determined by a pin-on-disc tribometer with a 100Cr6 (AISI 52100) ball counterpart and a sliding distance of 2000m. In addition, the wear track and the surface defects were investigated by confocal microscopy. The used lubricants were water and engine oil.The experiments showed that under same tribological parameters all lubricated DLC coatings on polished samples suffered under strong fatigue wear on the surface. By using thick coatings, it could be realised that this is not caused by an adhesion problem, because the resulting defects do not reach the steel interface. Fatigue wear was found for lubrication with oil as well as for lubrication with water. In contrast DLC coated rough surfaces showed no fatigue wear, also under lubricated condition. By using rough DLC coatings the wear of the counterpart is obviously remarkable increased. If the tribological experiments were done under excessive use of oil (not on a thin oil film), no fatigue wear on the surface could be found as well. On smooth surfaces the adhesion forces caused by the thin oil or water film seem to be responsible for the observed fatigue wear defects. It is also shown that with a special topography of the DLC coating fatigue wear under slightly lubricated condition can be eliminated and simultaneously the wear of the counterpart can be kept very low.

Effects of PVD DLC Coating on 7075-T6 Fatigue Strength at High and Low Number of Cycles

Despite its high tensile strength and its brilliant fatigue behaviour, 7075-T6 aluminium alloy usage is dramatically reduced, due to its sensitivity to corrosion and its poor surface characteristics. In order to be adopted for advanced applications in the aeronautic, automotive and maritime fields, as well as in further innovative applications, 7075-T6 resistance must be improved. Controversial results are found in literature on coated 7075-T6 fatigue strength, due to the complex mechanical interaction between the substrate and the coating, and the high temperatures involved in the coating process. In the present work, testing on rotating bending fatigue (R = -1) 7075-T6 uncoated and PVD DLC coated specimens has been conducted. Different stress levels were considered, to assess the impact of the applied stress on the fatigue life of the substrate-coating system. SEM characterization of the fracture surfaces has been performed, to investigate the influence of the coating on the fracture mechanism

Study on failure mechanisms of DLC coated Ti6Al4V and CoCr under cyclic high combined contact stress

In this study, pin-on-disc (POD) tribotest and cyclic impact-sliding wear tests were used to study contact failure wear mechanisms of DLC coated and uncoated Ti6Al4V and ASTM F-75 CoCr bio-materials. The tests were operated in a laboratory dry environment and in Hank's balanced salt solution (HBSS), which may act as a lubricant against sliding wear or cause corrosion degradation to metal substrates. Uncoated CoCr was superior to Ti6Al4V in the cyclic impact-sliding tests in either dry or wet test conditions, which was believed due to the higher stiffness and strong strain-induced work hardening of CoCr and inferior wear resistance of Ti6Al4V. As a widely used antifriction coating, DLC did not benefit but decreased the durability of the samples in impact-sliding tests under dry condition due to high impact loads which deformed coating surface quickly and invited hard particles to sliding couples. A cushion effect against impact forces by HBSS and a better support provided by stiffer CoCr contributed to an improved base for DLC coatings, compared to the Ti alloy; so the DLC-coated CoCr sample outperformed the DLC-coated Ti6AlV, especially under HBSS condition.

Mechanical analysis and high temperature wear behaviour of AlCrN/DLC coated titanium alloy

The present investigation focused on the mechanical and the tribological behaviour of PVD coated Ti-6Al-4V for elevated temperature applications. The effects of AlCrN and DLC coatings on the surface of the titanium alloys produced by arc evaporation method were studied. Nano indentation, tensile and high temperature wear tests were carried out to evaluate the hardness, strength and tribological properties of coated Ti-6Al-4V. The DLC coating on the titanium alloy exhibits a higher hardness value of 1,125 HV. The results of tensile test made clear that both AlCrN and DLC coatings can improve the plasticity of uncoated Ti-6Al-4V alloy. High temperature sliding wear characteristics of AlCrN and DLC coatings was studied under lubricated condition for elevated temperature applications. The wear tests showed that the wear rates and friction coefficients of AlCrN and DLC coatings, both increases with the increasing temperature. The wear results were compared with those of uncoated Ti-6Al-4V titanium alloys, which exhibited a lower coefficient of friction (COF) due to the change of wear mechanism to slight abrasion wear from severe delamination. The superiority of the DLC coated Ti-6Al-4V was confirmed by the final results.

Reduction in EHL Friction by a DLC Coating

This work evaluates the influence of DLC coatings on the frictional behaviour of lubricated line contacts in the fluid friction regime. Experiments on a twin-disc machine showed a significant reduction in friction using DLC coated discs instead of uncoated ones. The experimental results are compared by calculations using a three-dimensional thermal elastohydrodynamic simulation model. According to the calculations, the lower heat conductivity of DLC coatings is the main reason for the reduction in friction. This is because of the higher temperature level in the lubricating gap, which results in a reduction in the effective lubricant viscosity and consequently in reduced frictional shear stress. Lower bulk temperatures of the discs are a further consequence of the low heat conductivity of DLC coatings. The resulting lower temperature at the inlet zone of the contact in the steady-state conditions also leads to larger lubricating gaps and to a further reduction in friction.

A model for single asperity perturbation on lubricated sliding contact with DLC-coated solids

In lubricated sliding contact systems with DLC coated solids, recent studies have shown that DLC coatings are highly sensitive to asperities breaking through the lubricant film within the contact area. Those asperities produce damages similar to those obtained from scratch tests, from where coating delamination can initiate and propagate. To better understand the link between scratches and coating delamination, endurance tests can then be performed on coating with controlled initial scratches. To ensure the scratches’ representativity with respect to the actual asperities’ contact condition, one has to estimate the load transmitted by such asperities as well as the induced perturbation on surface and subsurface stresses. In this paper, a simplified numerical model corresponding to such problems is presented. It is consistent with elasto-hydrodynamic (EHD) lubrication approximations and can be used on coated or uncoated systems. As a simplified model, it can be run quickly on multiple configurations, enabling the creation of representative scratch maps for the given lubricated contact conditions.

DLC Coated Textile Vascular Prostheses Tested in Sheep

Textile vascular prostheses ARTECOR were coated by laser with amorphous diamond-like carbon layers (DLC) with thickness up to 200 nm. Layers were created in 0.25 Pa of Argon at laser energy density of 8 or 22 Jcm-2. Depending on the deposition conditions, DLC properties moved from soft „graphitic“ to more „diamond“ (53 % of sp3 bonds). Coated prostheses of various DLC thickness and sp3 content were implanted into carotid artery of Merino sheep. The prostheses were extirpated after 100 days (~180 days). From preliminary results follows that prostheses coated with DLC layer thickness of 20 nm and higher sp3 content showed the best results.

The effect of a DLC coating adhesion layer on the corrosion behavior of titanium and the Ti6Al4V alloy for dental implants

The properties of DLC layers provide for their broad use in medical applications. Their tribological properties are frequently utilized in big joint implants, and their barrier effect offers another benefit. The present work studied corrosion behavior of DLC coatings formed on titanium and Ti6Al4V alloy with a titanium or chromium inter-layer, in environments to which dental implants may be exposed. Electrochemical impedance spectroscopy, XPS surface analysis, ICP/MS chemical analysis method and a set of standard biological tests were employed in the study.The behavior of both coated systems, regardless of the basic material, was comparable in an environment that did not contain fluoride ions. An addition of fluorides revealed the occurrence of pores as deep as the DLC layer even in specimens with a surface polished prior to coating. Porosity of layers was clearly evident on jet-blasted specimens. The best corrosion behavior was recorded in specimens with a chromium inter-layer on both types of the basic material. With the titanium inter-layer applied, coatings on TiAlV exhibited higher corrosion resistance than those on commercial-pure titanium. The conducted biological tests indicated applicability of a chromium inter-layer on DLC coated implants.

S134022 LCコーテッドドリルを用いたプリント配線板の微小径穴あけ加工に関する研究 : DLCコーティング膜表面のドロップレッドが穴あけ加工性能に及ぼす影響

S134022 LCコーテッドドリルを用いたプリント配線板の微小径穴あけ加工に関する研究 : DLCコーティング膜表面のドロップレッドが穴あけ加工性能に及ぼす影響

The Effect of Maximum Normal Impact Load, Absorbed Energy, and Contact Impulse, on the Impact Crater Volume/Depth of DLC Coating

In this work, the influence of maximum normal impact load, absorbed energy, and contact impulse, on the impact crater volume/depth of a hydrogen-free diamond-like carbon coating (commonly known as DLC) has been studied. The tungsten high speed steel (SKH2) specimen discs were coated with DLC using the Physical Vapour Deposition (PVD) method. The 90o impact test was performed using a self-developed impact tester, where the DLC coated disc was impacted by a chromium molybdenum steel (SCM420) pin, at 400 impact cycles, under lubricated conditions. The results show that the most crucial factor, affecting the impact crater volume/depth of DLC coating under impact, is the maximum normal impact load.

アルミニウムに対するDLC膜のトライボロジー特性とドライしごき加工用DLCコーティッド金型の寿命(機械要素,潤滑,設計,生産加工,生産システムなど)

アルミニウムに対するDLC膜のトライボロジー特性とドライしごき加工用DLCコーティッド金型の寿命(機械要素,潤滑,設計,生産加工,生産システムなど)

Erosion–corrosion and corrosion properties of DLC coated low temperature gas-nitrided austenitic stainless steel

Low temperature nitriding of stainless steel leads to the formation of a surface zone of so-called expanded austenite, i.e. by dissolution of large amounts of nitrogen in solid solution. In the present work the possibility of using nitrogen expanded austenite “layers” obtained by gaseous nitriding of AISI 316 as substrate for DLC coatings are investigated. Corrosion and erosion–corrosion measurements were carried out on low temperature nitrided stainless steel AISI 316 and on low temperature nitrided stainless steel AISI 316 with a top layer of DLC. The combination of DLC and low temperature nitriding dramatically reduces the amount of erosion–corrosion of stainless steel under impingement of particles in a corrosive medium.