Nanoindentation-induced deformation behaviour of diamond-like carbon coatings on silicon substrates

The deformation behaviour of diamond-like carbon (DLC) coatings on silicon substrates induced by indentation has been investigated. DLC coatings, deposited by a plasma-assisted chemical vapour deposition technique, were subjected to nanoindentation over a range of maximum loads from 100 mN to 300 mN. The resulting load-displacement plots displayed pop-ins for maximum loads of 200 mN and above, with no distinct pop-out for any of the loads studied. Compressive deformation of the coating, up to a strain of ∼ 9%, was observed. The coating-substrate composite was devoid of cracks at lower loads, but at the maximum load of 300 mN, ring cracks in the coating and a median crack in the substrate were observed. Furthermore, cracking, {111} slip and localized phase transformations were observed in the silicon substrate. The onset of these structural changes was correlated to the mechanical behaviour during indentation.     

Analysis of plastic deformation in diamond like carbon films-steel substrate system with tribological tests

The influence of plastic deformation of the substrate on the tribological properties of diamond like carbon (DLC) films was investigated in DLC films-steel substrate system. The tribological properties of DLC films deposited on different hardness steel were evaluated by a ball on disk rotating-type friction tester at room temperature under different environments. In dry nitrogen, DLC films on soft steel exhibited excellent tribological properties, especially obvious under high load (such as 20 N and 50 N). However, DLC films on hard steel were worn out quickly at load of 20 N. Plastic deformation was observed on soft steel after tribological tests. The width and depth of plastic deformation track increased with increase of the experimental load. Super low friction and no measurable wear were kept in good condition even large plastic deformation under high load conditions in DLC films-soft steel system. In open air, DLC films on soft steel exhibited high coefficient of friction and DLC films on ball were worn out quickly. Plastic deformation was not observed on soft steel because the contact area increased and the thick hardened layer on contact surface were formed by DLC films and debris particles together on the steel substrate. The wear track on steel became deep and wide with increase of loads and DLC films were worn out. The experimental results showed that super low friction and high wear resistance of DLC films on soft steel can be attributed to the good adhesion and plastic deformation. Plastic deformation played an active role in the tribological properties of DLC films on soft steel in the present work.     

Three dimensional imaging of deformation modes in TiN-based thin film coatings

A TiN thin film coating, approximately 4 μm in thickness, deposited on a ductile steel substrate, was subject to surface deformation via nanoindentation using a spherical indenter, 5 μm in radius, with loads up to 500 mN. Pop-ins were observed during loading, which are characteristic of the onset of cracking and the formation of shear steps at the coating-substrate interface. Focused ion beam microscopy was used to prepare cross-sections through the indentation that revealed the presence of both intercolumnar and inclined cracks. Three-dimensional reconstructions of the deformation zone beneath the indentation were performed using a dual-beam focus ion beam instrument. These constructions provided more detailed images of the morphology of cracks, which were observed to be consistent with theoretical models of plastic deformation of such brittle coatings.     

In-situ investigation on the deformation and damage behaviour of diamond-like carbon coated thin films under uniaxial loading

In the present work, the failure behaviour of diamond-like carbon (DLC) coatings on thin steel substrate under uniaxial tensile loading is analyzed in-situ in scanning electron microscopy as well as ex-situ using focused ion beam cross section and transmission electron microscopy. Aim of the work is to find correlations between the failure behaviour of the coating system, the adhesion and the stress-strain behaviour of a DLC coating system under tensile loadings conditions. Therefore thin amorphous DLC films were coated onto thin stainless steel foils using a plasma assisted chemical vapour deposition technique. It is found from the in-situ investigations that at increasing strains cracks were formed in the coating, with decreasing spacing at higher strains. By comparing uncoated steels foils with coated systems the stress-strain behaviour of a DLC coating was determined. The DLC coating, although already strongly cracked, bears loads up to a total strain of 15%. Cross section analyses with a focused ion beam and microscopy techniques supported these investigations. During straining the formation of two deformation bands adjacent to the Cr adhesion layer was observed. This deformation bands also indicate a high interfacial adhesion.     

Effect of thick DLC coating on fatigue behaviour of magnesium alloy in laboratory air and demineralised water

Rotating bending fatigue tests have been performed using Diamond‐like carbon (DLC) coated specimens of a wrought magnesium alloy, AZ80A, in laboratory air and demineralised water and the effect of DLC coating on fatigue and corrosion fatigue behaviour was studied. Three film thicknesses of 3.5 μm, 13 μm, and 25 μm (two‐layer film) were evaluated and particular attention was paid to the role of thick DLC coating. In laboratory air, the fatigue strengths of the DLC‐coated specimens were higher than that of the substrate specimen and increased with increasing film thickness. This was because hard DLC coating with good adhesion suppressed the crack initiation due to cracking of inclusions or cyclic slip deformation on the substrate surface. In demineralised water, the fatigue strength of the 3.5‐μm DLC‐coated specimen was the same as that of the substrate specimen due to the penetration of the water through pre‐existing film defects, while the 13‐μm and 25‐μm DLC‐coated specimens showed increased corrosion fatigue strength with increasing film thickness and also exhibited nearly the same fatigue strength as in laboratory air except for a few premature failed specimens, indicating a potential of thick DLC coating or two‐layer coating for complete improvement of corrosion fatigue strength in aqueous environments.     

A study towards improving mechanical properties of sol-gel coatings for polycarbonate

Scratch-resistant coatings based on 3-glycidoxypropyltrimethoxysilane and tetraethylorthosilicate with a cross-linking agent and different amounts of colloidal silica are prepared on polycarbonate substrates by sol-gel technique. The failure mode of this type of coating on soft plastic substrate under pencil scratch test is studied. It is found that the pencil scratch failure contains a gouge failure under the static pressure and a film cracking failure under the sliding of the pencil tip. The gouge failure is due to the early plastic deformation in the substrate, while the film cracking is due to the tensile stress in the film induced by the sliding and friction of the pencil tip. Factors influencing the static gouge failure and sliding cracking failure are investigated. It is found that the cross-linking agent and colloidal silica filler increase the intrinsic cross-linking, hardness, elastic modulus and fracture toughness of the coating material, therefore, reduce the film cracking tendency; whereas the increased layer thickness and multi-layer coating improve the pencil scratch resistance significantly via delayed plastic deformation in the substrate. Based on these analyses, we conclude that the main factors towards improved pencil scratch resistance are: layer thickness, elastic modulus, fracture toughness and intrinsic hardness of the coating material. Pencil hardness is increased from grade 2B to 5H by adjusting these parameters.     

XPS and TEM study of W-DLC/DLC double-layered film

A double-layered film of tungsten-containing diamond-like carbon (W-DLC) and DLC, (W-DLC)/DLC, was investigated. A film of 1.6 µm in thickness was deposited onto silicon substrate. The investigate double-layered coating was deposited by using the combination of PECVD and co-sputtering of tungsten metal target. Structure, interface and chemical bonding state of the investigated film were analyzed by Transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). From the results of the analyses, the structure of double-layered film is that amorphous phase of carbon is continued from DLC to W-DLC and tungsten metal clusters are dispersed in W-DLC layer.     

Contact damage evolution in a diamond-like carbon (DLC) coating on a stainless steel substrate

A diamond-like carbon thin film was coated onto a stainless steel substrate using plasma assisted chemical vapour deposition (PACVD). Instrumented indentation and scratching were used, supported by focused ion beam (FIB) microscopy, to explore deformation and fracture behaviours of this coating system. The formation and growth of ring and radial cracks in the coating, as well as plastic flow in the ductile substrate, were observed to be the predominant deformation processes for this coating system. Lateral cracking occurred at the interface of the coating/substrate following indentation, but in the middle of the coating following scratching. No evidence of plastic flow within the coating was observed. Coating deformation is, therefore, controlled by its fracture energy. An indentation-energy-based model was applied to evaluate the fracture toughness of the coating.     

Preparation of diamond-like carbon films by cathodic micro-arc discharge in aqueous solutions

Diamond-like carbon films (DLC) and silicon doped diamond-like carbon films were deposited on Ni substrate by cathodic micro-arc discharge at room temperature in aqueous solutions. The deposit potential was 130 V. The structure of the films was characterized by a scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Raman spectra and XPS analysis demonstrated that the films were diamond-like carbon clearly. SEM observation showed that the DLC films were uniform and the thickness was about 200 nm. Potentiodynamic polarization curve indicated the corrosion resistance of the Ni substrate was markedly improved by DLC films.     

Scratch adhesion test of reactively sputtered TiN coatings on a soft substrate

Reactively sputtered TiN coatings on the relatively soft substrate, Crofer 1700 (Vickers' hardness about 176 HV), were subjected to a scratch test for adhesion. The critical load of the coating/substrate combination is defined as the load at the onset of coating failure as cracking or loss of the coating that causes an increase in acoustic emission. For example, at the critical load of a thicker coating (approximately 1.5 μm), not only cracking but also loss of the coating were observed with an increase in acoustic emission. However, at the critical loads of thinner coatings (1.2 μm or less), coating loss was not observed in spite of an increase in the emission but coating cracking did occur. A failure model is suggested for the given coating plus soft substrate system. Using this model, the maximum observed in a typical acoustic emission curve is explained.     

Fabrication of titania-coated silica fibers and effect of substrate shape on coating growth rate

Titania coating was fabricated on fused silica glass fibers of 4-6 μm in diameter by the hydrolysis of Ti-tetraethoxide in ethanol at 20 °C. Changes in the coating thickness with the deposition time were examined by scanning electron microscopy and compared with those on flat soda-lime glass substrate examined by atomic force microscopy. Uniform titania coating was obtained, but severe cracking occurred on coatings thicker than 150 nm due to isotropic shrinkage during the drying process. The faster growth rate of coating on fiber than that on flat substrate could be explained by employing an ‘adhesion growth model.’ X-ray diffractometry showed that titania coating on fused silica glass plate was transformed to anatase-phase TiO₂ by annealing in air at 300 °C. This indicates that the titania coating on a fused silica glass substrate becomes crystalline after annealing at that temperature.     

Substrate dependent cracking behavior of CrAlN coatings

The present study investigated the effect of substrate deformation behavior on crack resistance of CrAlN coatings under quasi-static and cyclic loads using nanoindentation. (Cr₄₇Al₅₃)N coatings were deposited on cemented carbide WC-Co and high-speed steel HS652C substrates through physical vapor deposition (PVD) und characterized. In order to study the coating cracking behavior, the coated substrates were subjected to quasi-static nanoindentations with indentation force F_max = 1 N, F_max = 1.5 N and F_max = 2 N. Moreover, the crack resistance under cyclic loading with frequency f = 0.16 Hz was analyzed at F = 1 N and F = 1.5 N after n = 900 cycles. A conical diamond indenter was used for the tests. At the end, the indentation imprints were analyzed by scanning electron microscopy (SEM). The substrate dependency was apparent in cracking behavior of the coating. Albeit the lower indentation depth compared to the variant with HS6-5-2C substrate, the CrAlN coating on WC-Co substrate showed surface cracks under quasi-static and cyclic loading. These cracks on the coated surface were absent in the variant with HS6-5-2C substrate. This could be related to higher resistance of cemented carbide substrates against plastic deformation, prompting earlier crack initiation in CrAlN coating for effective energy dissipation during indentation.     

TEM study of the indentation behaviour of thin Au film on GaAs

Au films of 8.9 nm thickness have been sputter deposited onto a (001) GaAs substrate at room temperature. An average grain size of 10 nm and no texture were obtained. Subsequent, nanoindentation tests were performed on the coated specimens and the mechanical response was compared to that of a bulk GaAs sample with the same crystallographic orientation. Furthermore, the loading–unloading curves were analysed in view of transmission electron microscopy plan-view images obtained on the deformed substrate–film specimens and compared to results previously reported in the literature for bulk sample. Constrained plasticity of the films was observed to occur for residual depth to thickness ratio below 0.67. Further, plastic deformation of the substrate happened on coated specimens at loads less than those required to plastically deform bare substrate.     

Finite Element Analysis of Ceramic Coatings under Spherical Indentation with Metallic Interlayer： Part Ⅰ Uncracked Coatings

Spherical indentation of ceramic coatings with metallic interlayer was performed by means of axisymmetric finite element analysis （FEA）. Two typical ceramic coatings with relatively high and low elastic modulus deposited on aluminum alloy and carbon steel were considered. Various combinations of indenter radius-coating thickness ratios and interlayer thickness-coating thickness ratios were used in the modeling. The effects of the interlayer, the coating and the substrate on the indentation behavior, such as the radial stress distribution along the coating surface as well as the coating interface, and the plastic deformation zone evolution in the substrate were investigated in connection with the above mentioned ratios. The coating cracking dominant modes were also discussed within the context of the peak tensile stresses on the coating surface and on the coating interface.     

Tensile behaviour of as deposited and heat-treated electroless Ni-P deposits

Electroless Ni-P coatings were deposited on mild steel substrates and the effect of heat-treatment on their structure and tensile behaviour was studied, with the following conclusions. The as-deposited electroless Ni-P coating is amorphous and it remains amorphous up to 300 °C. At 400 °C the coating becomes crystalline and consists of a Ni₃P matrix containing areas of metallic nickel. For the selected coating/substrate thickness ratio, the contribution of the coating in the tensile properties of the coating-substrate system is negligible as expressed by the values of yield strength, ultimate tensile strength and fracture strain in mild steel substrates and coated as-deposited and heat-treated specimens. Extensive cracking of the coating accompanied by spalling was occurred during the tensile tests. The density of cracks was found to increase close to the fracture surface of the tensile specimen and with increasing heat-treatment temperature. The cracks observed on the surface of the coatings are believed to form due to the inability of the brittle coating to accommodate the strain generated in the ductile substrate. Their orientation to the tensile axis is in close relation to the structure of the coating and the failure mechanism that is dictated by this structure. The first cracks on the surface of the coatings were found to form after the yield strength of the tensile specimen has been reached and plastic deformation of the substrate takes place. Their density increases with the accumulation of strain up to fracture.     

Improvement of adhesion of DLC coating on nitinol substrate by hybrid ion beam deposition technique

Diamond-like carbon (DLC) films were prepared for a protective coating on nitinol substrate by hybrid ion beam deposition technique with an acetelene as a source of hydrocarbon ions. An amorphous silicon (a-Si) interlayer was deposited on the substrates to ensure better adhesion of the DLC films followed by Ar ion beam treatment. The film thickness increased with increase in ion gun anode voltage. The residual stresses in the DLC films decreased with increase in ion gun anode voltage and film thickness, while the stress values were independent of the radio frequency (RF) bias voltage. The adhesion of the DLC film was improved by surface treatment with argon ion beam for longer time and by increasing the thickness of a-Si interlayer.     

Microstructure of interface for high-adhesion DLC film on metal substrates by plasma-based ion implantation

The diamond-like carbon (DLC) film was prepared on various metal substrates with a plasma-based ion implantation and deposition using superimposed RF and negative high-voltage pulses. The adhesion strength of DLC film was enhanced above the epoxy resin strength by implantation of carbon ions or mixed ions of carbon and silicon to the substrate surface before DLC deposition. In order to clarify the mechanism for improvement in adhesive strength, the microstructure of an interface between DLC film and substrate was examined in detail by transmission electron microscopy (TEM) observations in combination with EDS analysis. As a result, the enhancement in adhesion strength of DLC film by C ion implantation resulted from the formation of amorphous-like phase in the ion-implanted region of substrate, the production of carbon-component graded interface, the destruction of the oxide layer on the top surface of substrate, and the reduction of residual stress in DLC film by ion implantation during the deposition. The production of stress-free DLC film allowed us to demonstrate a supra-thick DLC film of more than 400 μm in thickness.     

Finite Element Analysis of Ceramic Coatings under Spherical Indentation with Metallic Interlayer: Part Ⅰ Uncracked Coatings

Spherical indentation of ceramic coatings with metallic interlayer was performed by means of axisymmetric finite element analysis (FEA). Two typical ceramic coatings with relatively high and low elastic modulus deposited on aluminum alloy and carbon steel were considered. Various combinations of indenter radius-coating thickness ratios and interlayer thickness-coating thickness ratios were used in the modeling. The effects of the interlayer, the coating and the substrate on the indentation behavior, such as the radial stress distribution along the coating surface as well as the coating interface, and the plastic deformation zone evolution in the substrate were investigated in connection with the above mentioned ratios. The coating cracking dominant modes were also discussed within the context of the peak tensile stresses on the coating surface and on the coating interface.     

Investigation of shrinkage and cracking of ophthalmic lens coating by a cycle test of UV radiation and high humidity

This paper presents some ophthalmic lens coating failures such as shrinkage, cracking, and cracking with delamination caused by UV radiation, elevated temperature, and moisture. These phenomena are caused by: 1) interactions at the interface between the plastic substrate and the hard coating (HC), and 2) plastic deformation with stress and relaxation of the HC/substrate interface. This phenomenon leads to an understanding of the mechanical properties of ophthalmic lenses through a QUV test which is found to be a useful method in developing better ophthalmic lens systems. The coating failures were investigated by means of different analysis techniques including optical microscope, scanning electron microscope, atomic force microscope, thermo mechanical analysis, and dynamic secondary ion mass spectrometry.     

A numerical study of contact damage and stress phenomena in curved porcelain/glass-filled polymer bilayers

Finite Element Analysis is used to examine contact damage induced by Hertzian indentation of a porcelain coating on a glass-filled polymeric substrate. Different forms of cracking in the porcelain coating are studied –“Hertzian” cone cracks close to the indenter, more distant “outer” cone cracks, and “radial” cracking at the coating/substrate interface. The effects of porcelain coating thickness and radius of curvature on the critical stresses for initiation of these cracks are examined. The predicted critical load curves suggest that for systems with compliant substrates (relative to the coating) with a given radius of curvature, there is an optimum porcelain coating thickness that maximises the critical load for cone cracking. Conversely, for a given coating thickness, the effects of curvature vary significantly – for thinner coatings, where outer cone cracks are dominant, highly convex surfaces are more resistant to cracking, whereas for thicker coatings, which are more prone to Hertzian cone cracking, concave surfaces produce a higher predicted critical load. Curvature is observed to have little effect on the critical load for the formation of radial cracks, which remains the dominant mode of failure in cases of thin coatings on compliant substrates.