Damage resistance of SiC and Si3N4 coatings with control of microstructure and thickness

We investigated the contact damage and indentation stress–strain behavior of silicon carbide (SiC) coatings and binary coatings consisting of SiC and silicon nitride (Si₃N₄), synthesized on graphite substrates with porosities of 10 and 13% by a solid–vapor reaction, in order to determine the coatings’ damage resistance. The coating thickness was affected by the porosity of the substrate. The coatings on the substrate with 13% porosity showed a graded interface structure below the top dense layer. The SiC coatings were thicker than the SiC/Si₃N₄ composite coatings. The SiC coatings made the substrates hard, and SiC-coated substrates exhibited higher stress–strain curves than the substrates alone, but the SiC/Si₃N₄ composite coatings appeared unaffected. The coating thickness played an important role in limiting the effect of damage. The hardness values of the SiC coatings were higher than those of the substrates and the SiC/Si₃N₄ coatings. These corresponded well with the indentation stress–strain curves. The values of each coating showed saturated points depending on the applied load. This indicated that the substrate itself influenced the damage resistance of the coatings because of the layered structure of a harder coating with a softer substrate. The coatings enhanced contact damage and transmitted the damage to the substrates at a high load of P = 2000 N. Both coatings showed an extensive subsurface damage, independent of the porosity of the substrate. In cyclic indentation tests, the contact diameters linearly increased with the number of cycles and depended on the porosity of the substrate, showing smaller contact diameters by coating the substrate.     

Nanoindentation-induced deformation behaviour of tetrahedral amorphous carbon coating deposited by filtered cathodic vacuum arc

The nanoindentation-induced deformation behaviour of a ta-C (tetrahedral amorphous carbon) coating deposited on to a silicon substrate by a filtered vacuum cathodic vapour arc technique was investigated. The 0.17-μm-thick ta-C coating was subjected to nanoindentation with a spherical indenter and the residual indents were examined by cross-sectional transmission electron microscopy. The hard (~ 30 GPa) ta-C coatings exhibited very little localized plastic compression, unlike the softer amorphous carbon coatings deposited by plasma-assisted chemical vapour deposition. However, neither through-thickness cracks nor delamination was observed in the coating for the loads studied. Rather, the silicon substrate exhibited plastic deformation for indentation loads as low as 10 mN and at higher loads it showed evidence of both phase transformation and cracking. These microstructural features were correlated to the observed discontinuities in the load-displacement curves. Further, it was observed that even a very thin coating can modify the primary deformation mechanism from phase transformation in uncoated Si to predominantly plastic deformation in the underlying substrate.     

Structure-scratch properties relationships of acrylate photo-polymerizable protective coatings for thermoplastic substrates

The relationships between the composition and the scratch resistance of clear photo-polymerized protective coatings for thermoplastic substrates were studied in relation with their thermomechanical properties. For this purpose, dynamic mechanical analyses of free-standing films were compared to micro-scratch tests of thick or thin coatings deposited on polycarbonate. In these experiments, the depth indented by the tip, the elastic recovery of the material, the residual depth of the scratch, and the load at which the first crack appears, were analyzed. Different coatings close in formulation were studied. First, the proportion of a specific difunctional monomer featuring a hard structure was varied in order to change the crosslinking density of the polymer network. The thermomechanical properties were consequently modified at high temperature, but remained similar at 23 °C, whereas at this temperature, the scratch properties of the coating evolved with its composition. The addition of 5 wt% alumina or silica nanoparticles did not modify the thermomechanical properties or the scratch resistance of the coatings, even if a more concentrated filler layer was observed near the surface of the coating. Nevertheless, the consequent incorporation of a new diacrylate monomer in the polymer matrix delayed the ductile-brittle transition. Finally the substitution of petro-based monomers by slightly different bio-based compounds led to a change of the scratch behavior of the thickest coatings (150 μm-thick), and increased the critical load for the thinnest coatings (15 μm-thick). It comes out that micro-scratch tests allow a finer comparison of the samples.     

Adhesion of diamond coatings on steel and copper with a titanium interlayer

Adherent diamond coatings on steel and copper were obtained by using a titanium interlayer. The adhesion of the coatings was evaluated by scratch tests and micro-indentation tests. The diamond coating on steel exhibited a much higher critical load than on copper, as revealed by the scratch tests. However, an observation on the back of the scratch-delaminated film and on the corresponding substrate surface showed that the detachment occurred between the diamond film and the titanium interlayer. Therefore, the difference in the critical scratch load is due mainly to a substrate effect, making it difficult to compare the adhesion of different coatings.On the other hand, Knoop indentation tests showed interesting results: a small indentation load causes round spallation in the film with no observable crack. An exponential sink-in deformation under the indentation is proposed, y=−a exp(−bx). The coating adhesion is considered to be equivalent to the deformation stress at the edge of the spallation zone. The adhesion of diamond coatings on steel and copper with a titanium interlayer is evaluated quantitatively using this model. Furthermore, a thermal quench method is proposed to estimate the coating adhesion. The results found are in agreement with the indentation model.     

Understanding the mechanical properties of hot pressed Ba-doped S-phase SiAlON ceramics

In this contribution, we report the analysis and interpretation of the mechanical property measurements for a new class of SiAlON ceramic. The hardness and indentation fracture toughness were measured on the hot pressed Ba-doped S-SiAlON ceramic using Vickers indentation at varying loads (up to 300 N). An important observation was that all the investigated S-SiAlON exhibited the characteristic rising R-curve behavior with a maximum toughness of up to 10–12 MPa m^1/2 for ceramics, hot pressed both at 1700 and 1750 °C. Crack deflection by large elongated S-phase grains and crack bridging by β-Si₃N₄ needles has been found to be the major toughening mechanisms for the observed high toughness. Theoretical estimates, using a toughening model based on crack bridging and deflection by platelet shaped ‘S’-phase grains and β-Si₃N₄ needles, reveal the interfacial friction of around 200 MPa. Careful analysis of the indentation data reveals the average (apparent) hardness modestly increases with indent load in all S-SiAlON samples, with more significant effect for S-SiAlON, hot pressed at 1600 °C. This effect has been analyzed in the light of the established model of ‘indentation-induced cracking’ phenomenon. Our experimental results suggest that a modest combination of average hardness of 15 GPa and indentation toughness of around 12 MPa m^1/2 could be achieved in Ba-S-SiAlON ceramic and further improvement requires microstructural tailoring.     

Synthesis and mechanical properties of carbon nanotube/diamond-like carbon composite films

Diamond-like carbon (DLC) coatings were successfully deposited on carbon nanotube (CNT) films with CNT densities of 1 × 10⁹/cm², 3 × 10⁹/cm², and 7 × 10⁹/cm² by a radio frequency plasma-enhanced chemical vapor deposition (CVD). The new composite films consisting of CNT/DLC were synthesized to improve the mechanical properties of DLC coatings especially for toughness. To compare those of the CNT/DLC composite films, the deposition of a DLC coating on a silicon oxide substrate was also carried out. A dynamic ultra micro hardness tester and a ball-on-disk type friction tester were used to investigate the mechanical properties of the CNT/DLC composite films. A scanning electron microscopic (SEM) image of the indentation region of the CNT/DLC composite film showed a triangle shape of the indenter, however, chippings of the DLC coating were observed in the indentation region. This result suggests the improvement of the toughness of the CNT/DLC composite films. The elastic modulus and dynamic hardness of the CNT/DLC composite films decreased linearly with the increase of their CNT density. Friction coefficients of all the CNT/DLC composite films were close to that of the DLC coating.     

Investigation of the tribological and biomechanical properties of CrAlTiN and CrN/NbN coatings on SST 304

This study examines the influence of thin layer coatings of CrAlTiN and CrN/NbN, deposited via physical vapor, on the biocompatibility, mechanical, tribological, and corrosion properties of stainless steel 304. The microstructure and morphology of the thin CrAlTiN and CrN/NbN layers were characterized by scanning electron microscopy (SEM), EDX, and X-ray diffraction. The pin on disc wear test was performed on bare and metal-nitride coated SST 304 under a 15 N load at 60 rpm and showed that the wear rates of the thin CrAlTiN and CrN/NbN film coatings were lower than the bare substrate wear ratio. The coefficients of friction (COFs) attained were 0.64, 0.5, and 0.55 for the bare substrate, CrN/NbN coating, and CrAlTiN coating, respectively. Nano indentation tests were also performed on CrAlTiN-coated and CrN/NbN-coated SST 304. The nanohardnesses and Young's moduli of the coated substrates were 28 GPa and 390 GPa (CrN/NbN-coated) and 33 GPa and 450 GPa (CrA1TiN-coated), respectively. For comparison, the nanohardness and Young's modulus of the uncoated substrate were 4.8 GPa and 185 GPa, respectively. Corrosion tests were conducted, and the behaviors of the bare and metal nitride-deposited substrates were studied in CaCl₂ for seven days. The corrosion Tafel test results showed that the metal-nitride coatings offer proper corrosion resistance and can protect the substrate against penetration of CaCl₂ electrolyte. The CrN/NbN-coated substrates showed better corrosion resistance compared to the CrAlTiN-coated ones. In evaluating the biocompatibility of the CrAlTiN and CrN/NbN coatings, the human cell line MDA-MB-231 was found to attach and proliferate well on the surfaces of the two coatings.     

The properties of hydroxyapatite ceramic coatings produced by plasma electrolytic oxidation

Calcium phosphate coatings produced on the surface of Ti6Al4V by plasma electrolytic oxidation (PEO) using different concentrations of hydroxyapatite (HA) in a 0.12 M Na₃PO₄ (NAP) electrolyte solution was investigated. It was found that the amount of calcium phosphate particles infiltrated into the coating layer as well as the thickness and the surface roughness of the coating increased with increasing HA concentration. The porosity of the ceramic coatings indicated an inverse relationship with the concentration of HA particles dispersed in the NAP solution. The result also demonstrates that higher scratch adhesive strength was achieved using 1.5 g/L HA solution, producing a critical load of 2099 mN, while 0 g/L HA only produced a critical load of 1247 mN. The adhesion becomes independent of thickness when the concentration of HA exceeds 1.5 g/L. The failure of the coating was characterized by large periodic hemispherical chipping, while intermittent delamination was noticed with the coating embedded with HA particles. This study demonstrate the viability of using PEO to produce a thin layer of HA ceramic coating on Ti6Al4V suitable for biomedical applications.     

Nanotribological and nanomechanical properties of 5–80 nm tetrahedral amorphous carbon films on silicon

Nanoscratch testing has been used to investigate the tribological behaviour of 5, 20, 60 and 80 nm tetrahedral amorphous carbon (ta-C) thin films deposited on silicon by the filtered cathodic vacuum arc method. The nanoscratch behaviour of the films was found to depend on the film thickness, with 60 and 80 nm films undergoing border cracking and then at higher critical load a dramatic delamination event. 5 and 20 nm films have a lower critical load for onset of border cracks but do not undergo a clear dramatic failure, and instead are increasingly worn/ploughed through until film removal as confirmed by microscopic analysis. This is consistent with the thinner films having lower stress and reduced load-carrying ability. Nanoindentation confirms that the thicker films have enhanced load support and higher measured composite (film + substrate) hardness. The 80 nm film in particular can retain appreciable load support whilst deformed during indentation, as shown by its ability to alter the critical loads for contact-induced phase transformations in the Silicon substrate during unloading.     

Characterization of thick and soft DLC coatings deposited on plasma nitrided austenitic stainless steel

Thick and soft a-C:H:Si coatings containing more than 45% hydrogen (thickness: 25–27 μm, hardness: 6 GPa, Young's Modulus 38 GPa and low ratio of sp³ bonds) were deposited by PACVD with a DC pulsed discharge on nitrided (duplex sample) and non-nitrided austenitic stainless steel (coated sample). After deposition, the chemical, microstructural and tribological properties were studied. Finally, the adhesion and the atmospheric corrosion resistance of a-C:H:Si coatings were also investigated.In pin-on-disk tests, the friction coefficient using an alumina pin of 6 mm in diameter as counterpart, under 0.59 GPa Hertzian pressure was 0.05 for the coated samples and 0.076 for the duplex samples. These values were more than one order of magnitude smaller than the friction coefficient of the nitrided sample without coating, which was around 0.65. In the coated samples, the wear loss could not be measured. In ball-on-disk tests under dry sliding conditions, the coatings were tested under different Hertzian pressures (1.29, 1.44 and 1.57 GPa) using a steel ball with a diameter of 1.5 mm as counterpart. Using a normal load of 9 N, the a-C:H:Si coating of the coated samples was broken and detached thus leading to a coefficient of friction of around 0.429. However, in contrast to that, the friction coefficient of the duplex samples remained stable and reached as maximum a value of 0.208.In abrasive tests, mass loss was undetectable in both duplex and coated samples. Furthermore it could be seen that the a-C:H:Si film showed only some smaller grooves and no severe damage or deformation. On the contrary, severe damage was observed in the only nitrided sample. With respect to adhesion, the critical load to break the coating was higher in the duplex sample (27 N) than in the only coated sample (16.3 N). By chemical analysis using the salt spray fog test, the duplex sample remained clean, but the coated sample failed and presented film delamination as well as general corrosion.     

Nanohardness and tribological properties of nc-TiB2 coatings

The work is devoted to the investigation of nanohardness and tribological properties in TiB₂ coatings deposited on austenitic steel substrates using an unbalanced magnetron sputtering with the focus on the coatings prepared under small negative bias to reduce compressive stresses. The coating prepared under floating potential exhibited nanocomposite microstructure with the size of TiB₂ (hcp) nanocrystallites in the range of 2–7 nm. It is in contrast with the textured microstructure typically developed under higher negative bias. The reduction of the compressive stresses up to ?0.4 GPa while keeping the nanohardness >30 GPa and the coefficient of friction of 0.77 were obtained in this coating. The highest nanohardness of 48.6 ± 3.1 GPa and indentation modulus of 562 ± 18 GPa were achieved at ?100 V bias in the textured coating. The friction mechanisms include mechano-chemical formation of a tribological oxide film between the sliding partners combined with an abrasive wear.     

Microstructural and electrical characterization of Mn-Co spinel protective coatings for solid oxide cell interconnects

Electrophoretic deposition, thermal co-evaporation and RF magnetron sputtering methods are used for the preparation of Mn-Co based ceramic coatings for solid oxide fuel cell steel interconnects. Both thin and relatively thick coatings (1–15 μm) are prepared and characterised for their potential protective behaviour. Mn-Co coated Crofer22APU samples are electrically tested for 5000 h at 800 °C under a 500 mA cm⁻² current load to determine their Area Specific Resistance increase due to a growing chromia scale. After tests, samples are analysed by scanning and transmission electron microscopy. Analysis is focused on the potential chromium diffusion to or through the coating, the oxide scale thickness and possible reactions at the interfaces. The relationships between the coating type, thickness and effectiveness are reviewed and discussed. Out of the three Mn-Co coatings compared in this study, the one deposited by electrophoretic deposition presents the best protection against Cr diffusion and offers long term stability.     

Mechanical properties and deformation mechanisms of B4C–TiB2 eutectic composites

Samples of B₄C–TiB₂ eutectic are laser processed to produce composites with varying microstructural scales. The eutectic materials exhibit both load dependent and load independent hardness regimes with a transition occurring between 4 and 5 N indentation load. The load-independent hardness of eutectics with a microstructural scale smaller than 1 μm is about 31 GPa, and the indentation fracture toughness (5–10 N indenter load) of the eutectics is 2.47–4.76 MPa m^1/2. Indentation-induced cracks are deflected by TiB₂ lamellae, and indentation-induced spallation is reduced in the B₄C–TiB₂ eutectic compared to monolithic B₄C. Indentation-induced amorphization in monolithic B₄C and the B₄C phase of the eutectic is detected using Raman spectroscopy. Sub-surface damage is observed using TEM, including microcracking and amorphization damage in B₄C and B₄C–TiB₂ eutectics. Dislocations are observed in the TiB₂ phase of eutectics with an interlamellar spacing of 1.9 μm.     

Analysis of spherical indentation of porous ceramic films

Spherical indentation of a porous brittle La_0.6Sr_0.4Co_0.2Fe_0.8O₃ ceramic film (porosity = 39.7%) on a stiffer elastic Ce_0.9Gd_0.1O_1.95 substrate is simulated by finite element modelling incorporating the Gurson model to account for densification. The simulated load-displacement curves, apparent elastic modulus E, indentation hardness H and densification profile are all in good agreement with experimental data for the film. The simulations show that E and H are not sensitive to film residual stress. However E is very sensitive to the indent depth-film thickness ratio f, although H is less so for f < 0.3. The simulated dependence of E and H on f are highly consistent with experimental data, supporting the extrapolation of E and H measured for 0.1 < f < 0.3, to zero depth for good estimates of the film-alone properties. The inclusion of densification in the simulation makes only a small difference to E, but has a large influence on H as a function of indentation depth.     

Nanoindentation with spherical tips of single crystals of YBCO textured by the Bridgman technique: Determination of indentation stress–strain curves

The mechanical properties of superconductor ceramics are of interest in the manufacture of superconducting devices. The current trend is to produce smaller devices (using, e.g., thin films), and the correct characterization of small volumes of material is critical. Nanoindentation is used to assess mechanical parameters, and several studies determine hardness and Young's modulus by sharp indentation. However, studies on the elasto-plastic transition with spherical indentation are scare. Here we used, spherical diamond tip indenter experiments to explore the elasto-plastic transition and to measure the yield strength of the orthorhombic phase of YBa₂Cu₃O_7?δ (YBCO or Y-123) at room temperature. The study was carried out for a range of monodomains on the (1 0 1)-plane for Bridgman samples. Inspection of the load–unload curves for penetration depths lower than 200 nm allows for observation of the elasto-plastic transitions. Focused ion beam (FIB) trenches showed no cracking due to the indentation, although oxygenation cracks were apparent. The mean pressure for the onset of elasto-plastic deformation is 3.5 GPa, and the elastic modulus, E, calculated using the Hertzian equations is 123.5 ± 3.4 GPa.     

A study on the elastic modulus of silicone duplex or bi-layer coatings using micro-indentation

A Fischerscope continuous microindenter with a spherical indenter was used to obtain maximum indentation load and depth data for a 2.2 mm sheet of RTV11 (a silicone elastomer), a 1.6 mm sheet of J501 (an elastomer containing 60% silicone and 40% butyl acrylate styrene) as well as six duplex elastomeric coatings. The duplex coatings consisted of RTV11 top coat and J501 bond coat. The Waters’s empirical relationship was used to determine the modulus of elasticity E for the RTV11 and J501 sheets. The Waters’s relationship was then used to determine the equivalent modulus, E_c, for duplex coatings from maximum indentation load versus elastic indentation depth data. The values of E_c as determined from the Waters’s model (and experimental data) were in good agreement with the values obtained by an equivalent stiffness method. By being able to determine E_c from the equivalent stiffness method and using this value in the Waters’s model, one may determine the load versus elastic depth of indentation for duplex coatings.     

Multiple cohesive cracking during nanoindentation in a hard W-C coating/steel substrate system by FEM

Stress evolution and subsequent cohesive cracking in the hard and stiff W-C coating on steel substrate during nanoindentation have been investigated using finite element modelling (FEM) and eXtended FEM (XFEM). The FEM simulations showed that the maximum principal stresses in the studied system were tensile and always located in the coating. They evolved in several stages. At indentation depths below 15% of the relative indentation depth, the maximum principal tensile stresses of ∼3 GPa developed at the top surface of the coating along the indenter/coating interface. At relative depths range 15–60%, the maximum tensile stresses of ∼6–8 GPa concentrated under the indenter tip in the coating along the interface with the substrate. At relative depths exceeding 60%, the maximum stresses gradually increased up to 10 GPa and they were located in the sink-in zone outside the indent as well as below the indenter tip. The first and subsequent cohesive cracks developed when the maximum tensile stresses in the sink-in zone at the top surface of the coating (and at the coating/substrate interface under the indenter) repeatedly reached the ultimate tensile strength of the coating. The hardness profile as well as cohesive cracking is controlled by the deformation of the substrate defined by the ration of the yield stresses of the coating and substrate. Very good correlation between the experimentally obtained cracks and multiple cracks predicted by XFEM confirmed the ability of the applied modelling in the prediction of fracture behavior of the studied coating/substrate system.     

Effect of interface roughness and coating thickness on interfacial shear mechanical properties of EB-PVD yttria-partially stabilized zirconia thermal barrier coating systems

The effect of interface roughness and thickness of thermal barrier coating (TBC) on the interfacial shear mechanical properties of electron beam-physical vapor deposited (EB-PVD)-TBC was examined using as-sprayed and polished bond coats (BC) 200 μm and 500 μm TBC thickness systems, by using a barb test method. The residual compressive stress in the TBC layer from the interface to the top surface was measured, by using Raman spectroscopy. The interface toughness related to the interface roughness and the thickness of the TBC. The interface toughness was larger for the BC as-sprayed TBC system than for the BC polished TBC system. The delamination of the TBC propagated within the TBC layer adjacent to the interface for the BC as-sprayed TBC; for the BC polished TBC, this occurred at the interface between the TGO and the BC. Moreover, the interface toughness was larger in the 500 μm thickness TBC than in the 200 μm thickness TBC. The relation of interface toughness to interface roughness and thickness of the TBC was associated with the interface residual compressive stress and with the interface sliding friction during the delamination of TBC.     

Comparison of microstructure and oxidation behavior of La-Mo-Si coatings deposited by two approaches

To investigate the effect of preparation methods on the La-Mo-Si (LMS) coatings, we developed a new LMS coating system using pack cementation (PC) and supersonic atmospheric plasma spraying (SAPS). Microstructure analysis showed that the SAPS-LMS coating possessed a higher porosity than that of the PC-LMS coating. Higher porosity can provide more channels to the oxidative and corrosive gasses to permeate the SAPS-LMS ceramic top-coat. After static oxidation for 150 h under 1773 K, the mass loss of SAPS-LMS coating (3.12 wt%) was much higher than that of the PC-LMS coating (0.05 wt%), and the parabolic rate constants presented faster oxidation kinetic in SAPS-LMS coating with respect to the PC-LMS coating. These results revealed that the protection effectiveness of SAPS-LMS coating was inferior to PC-LMS coating. Compared with SAPS-LMS coating, microscopic pores and cracks appeared in the PC-LMS coating with a thicker oxide film, which benefits from the formed La-Si-O-Al glass oxide with an excellent ability for crack healing. The reasons for poor antioxidant performance of SAPS-LMS coating are the higher volatility of La-Si-O glass containing Mo₅Si₃ and a weak interfacial interaction between coatings and substrate.     

Influence of diamond particles content on the critical load for crack initiation and fracture toughness of SiOC glass–diamond composites

The crack initiation load and fracture toughness were characterized as a function of diamond particle content, up to 25 vol%, in silicon oxycarbide glass matrix by means of Vickers indentation and single edge notch beam (SENB) technique, respectively. The larger fracture toughness value of 3.21 ± 0.3 MPa m^1/2 was reached for 20 vol% diamond content composites and the value was 4 times higher than that of the unreinforced glass. The addition of diamond particles greatly influenced the crack initiation load, which increased from 2.9 to 49.0 N. The enhancement in the fracture toughness and crack initiation load can be explained by both the intrinsic mechanical properties of diamond (especially the elastic properties; E ～ 1100 GPa) and the diamond/SiOC glass interfacial bonding. A clear correlation was found between the fracture energy, the reinforced interparticle spacing and the residual stress arising upon cooling due to thermal expansion mismatch between the matrix and the diamond particles.