The relationship between Palmqvist indentation toughness and bulk fracture toughness for some WC-Co cemented carbides

The Palmqvist indentation toughness (W) has been determined for a series of WC-Co hardmetals containing up to 10 wt % cobalt. Using a correlative parameter (the cobalt mean-free path) the Palmqvist toughness has been compared with true fracture toughness data (G _IC values) for similar materials, obtained by conventional testing techniques. It has been shown that a linear relationship betweenW andG _IC can be predicted, on the basis of a recent theory of the fracture process in hardmetals. The validity of such a relationship has been experimentally confirmed, over a limited range of hardmetal compositions and microstructures. The experimental difficulties and expense associated with fracture toughness determination for brittle materials such as hardmetals, by quasi-conventional techniques, are well known. It is concluded that the Palmqvist test provides a useful method of measuring fracture toughness on a routine basis, since it is simple to perform, non-destructive, and does not require a specialized testpiece geometry. Its application in hardmetal quality control is, therefore, indicated.     

Determination of residual stresses in cathodic arc coatings by means of the parallel beam glancing X-ray diffraction technique

A method based on the parallel beam glancing X-ray diffraction geometry has been applied to the measurement of the residual stresses present in cathodic arc plasma (Al_0.66Ti_0.34)N coatings deposited on hardmetal substrates. This procedure avoids the problems associated to the strong overlapping between the diffraction peaks of the coating and the substrate. The method has been validated by comparison with the results obtained with sin²ψ technique on other combinations of coatings and substrates in which no important overlapping occurs (i.e. (Al_0.66Ti_0.34)N on steel and TiN either on steel or on hardmetal substrates). The elastic moduli of the different coatings, required for the calculation of the residual stresses, have been obtained from nanoindentation experiments.     

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.     

Diamond film adhesion onto sub-micrometric WC-Co substrates

Different chemical etching procedures (CF₄ plasma, Aqua regia, and two-step Murakami/Aqua regia) were compared as surface pre-treatments for CVD diamond deposition onto sub-micrometric WC-Co hardmetal grades with cobalt contents of 3.5 and 5.75 wt% Co. Adhesion tests by static indentation up to 1100 N revealed the superior behaviour of the latter when pre-treated by the combined two-step etching for which no film spalling-off is observed at the maximum applied load. An interfacial crack resistance of about 1.6 kgf/μm was estimated anticipating high adhesion levels.     

Crack spacing statistics and spalling area fractions for indented silica-coated bismaleimide (BMI)

Indentation loading of thin, continuous silica coatings adhered to Bismaleimide (BMI) polymeric substrates induces a concentric array of cracks in the silica coating. For Vickers indentation, the array consists of diamond-shaped concentric cracks, while Hertzian indentation gives circular concentric cracks. This paper characterizes the indentation-induced crack damage in the coating in terms of: (1) f _s, the area fraction of the coating (within the indentation-cracked region) that spalls off the substrate due to the indentation and (2) the spacing between the cracks in the crack array. For a given indentation crack field, the crack spacing was uniform as a function of radial distance outward from the center of the indentation. One of the key results of this study was that the curing temperature for the coating dramatically affected both the coating spalling area fraction, f _S, and the manner in which the crack spacing changed as a function of the applied indentation load.     

In vitro corrosion testing of PVD coatings applied to a surgical grade Co–Cr–Mo alloy

Toxic effects and biological reaction of metallic corrosion and wear products are an important concern for metal on metal artificial joints. Corrosion tests were conducted to study the susceptibility to pitting and localized corrosion, with three coatings, CrN, TiN and DLC, applied to a wrought high carbon Co–Cr–Mo alloy substrate material. Corrosion testing involved the measurement of potential time transients during immersion in a physiological solution and cyclic polarization of specimen potentials into the transpassive range followed by reversal of the potential to scan in the cathodic direction to regain the rest potential E_rest. Resistance to pitting and localized corrosion was assessed by determining the transpassive breakdown potential E _bd and if any hysteresis generated during the reverse cyclic scan may have caused crossover with the original anodic scan. Three different surface coating conditions were tested namely: (1) as-coated, (2) polished, and (3) indented to penetrate the coating by diamond pyramid hardness indentor. Results showed that all three coatings produced significant improvements in corrosion resistance compared to performance of the wrought cobalt alloy but that some corrosive attack to both the CrN and TiN coatings occurred and some risk of attack to the cobalt alloy substrate existed due to coating defects or when damage to the coating occurred. TiN coatings were highly effective in preventing corrosion provided they were thick enough to produce complete coverage. Thin TiN coatings displayed some tendency to encourage localized attack of the cobalt alloy at coating defects or where the coating suffered mechanical damage. CrN coatings underwent transpassive breakdown more easily and some degree of pitting at defects within the coating was observed, especially when the CrN coating was polished before the test. No corrosive attack of the cobalt alloy substrate was observed when the CrN coating was mechanically damaged by indentation. DLC coatings produced were much thinner than either of the other two coatings and proved to be rather fragile. They were less effective in preventing apparently high corrosion currents and possibly high rates of corrosion.     

A wear-resistant coating—Oxidized graded multilayer TiN/W coating

Combining sputtering technology using an industrial-scale four-target DC closed-field unbalanced magnetron sputtering ion plating system (CFUBMSIPS™) with post heat treatment, a graded multilayer TiN/W coating, consisting of five layers, was synthesized and its outmost W layer was transformed to lubricious WO₃ successfully. The coatings were characterized by using GDOES, GXRD, a Rockwell C indentation tester, a nanoindentation tester, and a scratching tester. Wear behavior of coatings was evaluated by using a pin-on-disc tribometer. Through proper post heat treatment, the multilayer TiN/W coating, in spite of having a lower nano-hardness, showed good adhesion, much better wear performance and lower friction coefficient compared with the reference monolayer TiN coating.     

On bending strength of superhard composite materials based on WC–Co hardmetals

We present analytical algorithms for computing the ultimate bending strength of superhard composite materials based on WC–Co hardmetals. The study is performed for fine-grained materials (where mean particle size of the dispersed superhard phase d _C and that of carbide grains d _WC are of the same order of magnitude) and coarse-grained materials (with d _C ≫ d _WC ). The strength of the composite is assumed to be governed by the strength of its hardmetal matrix. The stressed state of the matrix is assessed through volume-average microstresses for fine-grained materials and interface-average stresses for coarse-grained composites. The calculated results presented in the form of tables and graphs have been analyzed. The strength has been found to decrease drastically with increasing particle size of the superhard phase and its concentration in the composite.     

The Effect of Substrate Pretreatment on PVD TiN Hard Coating Performance

This work comprises a study of the deposition and characterization of TiN coatings of different thicknesses on AISI M2 substrates heat-treated to different hardnesses. The effect of both substrate hardness and coating thickness on coating tribological performance was evaluated. The characterization tests included surface roughness measurement, coating thickness, micro-hardness, scratch adhesion, pin on disc, impact and corrosion tests. New findings on the impact wear behavior of TiN tempered M2 substrates were highlighted. For example, using a thin coating and tempering the substrate at 650^°C to slightly reduce the substrate hardness gave improved impact wear resistance. A maximum surface composite hardness value was obtained at 'full' substrate hardening (i.e. non-tempered) and the maximum TiN coating thickness as expected. The maximum critical load (79.2 N), in scratch adhesion tests was obtained from the fully hardened substrate with minimum TiN coating thickness. The results from corrosion tests show that tempering has an adverse effect on corrosion resistance.     

Analytical and Finite Element Analysis of Thermal Stresses in TiN Coatings

The thermal residual stress and associated effects in TiN coatings with planar and nonplanar surface roughness on silicon (100) substrates were analyzed using both analytical and finite element (FE) modeling. The effect of growth temperature (T_s), thickness, and the modulus of elasticity on the stress evolution in TiN coatings is reported. The results indicate that the variable thermal stress in the TiN coatings is due to the existence of both positive and negative temperature gradients and thus the resultant existence of disparity of the coefficient of thermal expansion between the substrate and the coating.     

Effect of soft substrate on the indentation damage in silicon carbide deposited on graphite

Indentation-induced damage is investigated in silicon carbide (SiC) deposited on graphite substrate. The SiC films have been grown by LPCVD (Low Pressure Chemical Vapor Deposition) method using MTS (CH₃SiCl₃) as a source gas and H₂ as a diluent gas to provide highly dense deposited layer and strong interfacial bonding. The elastic-plastic mismatch is very high to induce distinctive damages in the coating and the substrate layer. The specimens with various coating thicknesses are prepared by changing CVD condition or mechanical polishing. Indentation damages with different sizes are introduced by controlling indentation load in Nanoindentation, Vickers indentation and Hertzian indentation test. Basic mechanical properties such as hardness, toughness, elastic modulus are evaluated against coating thickness. Mechanical properties are sensitive to the indentation load and coating thickness. The results indicate that coating thickness has a vital importance on the design of hard coating/soft substrate system because the soft substrate affects on the mechanical properties.     

Studies of calcium-precipitating oral bacterial adhesion on TiN,TiO2 single layer,and TiN/TiO2 multilayer-coated 316L SS

Titanium nitride (TiN), titanium oxide (TiO₂) single layer, and TiN/TiO₂ multilayer coatings were deposited on a 316L stainless steel substrate using reactive magnetron sputtering process with the aim of preventing bacterial adhesion. The crystal structures of as-prepared coatings were evaluated using X-ray diffraction analysis. The cubic structure of TiN, anatase, and rutile structure of TiO₂ was noticed. Atomic force microscopy images exhibited a relatively smooth surface for all coatings. The surface wettability studies confirmed that the coatings were hydrophilic in nature. The rate of bacterial adhesion was evaluated using scanning electron microscopy and epifluorescence microscopy. These results demonstrated that the coated substrates could help to effectively reduce the bacterial adhesion and biofilm formations.     

Interfacial fracture for TiN/SiNx nano-multilayer coatings on Si(1 1 1) characterized by nanoindentation experiments

Fracture behavior for TiN/SiN_x nano-multilayer coatings on Si(1 1 1) substrates, deposited using magnetron sputtering Ti and Si, is characterized by nanoindentation experiments, and the morphologies of the indentations are revealed by scanning electron microscopy, along with in situ atomic force microscopy (AFM) in nanoindentation experiments. During nanoindentation experiments, under the condition that the displacement limit mode is used and a strain rate is kept at 0.05/s, an interfacial (between the coating and substrate) fracture is observed as the maximum indenter displacement into the coating reaches 2500 nm, and the corresponding unloading segment in the load–displacement curve shows an obvious discontinuity. This discontinuity is attributed to the rebound of the detached film during unloading. The interfacial fracture toughness for TiN/SiN_x nano-multilayer coating on Si(1 1 1), which is strongly dependent on the preferred orientation for the TiN layer as well as the interfaces between TiN and SiN_x layer in the multilayer stack, is calculated.     

Author index

Films of TiC, TiN and their composite were prepared on molybdenum by a reactive sputtering method with CH₄ and N₂ as the reactive gases and argon as the sputtering gas and applying bias potentials to the substrate material.The films were characterized by X-ray photoelectron spectroscopy and Auger electron spectroscopy. The quantitative chemical composition of the TiC and TiN coatings was determined as a function of the partial pressures of CH₄ (P_CH4) and N₂ (P_N2) during the reactive sputtering. For the TiC coating the most suitable P_CH4 range which gives the stoichiometric composition (carbon-to-titanium ratio, 0.8–1.0) without impurities was found to be (2–5) × 10^?4 Torr (substrate temperature, 300 °C; bias potential, ? 300 V). For the TiN coating the structure and composition of the films prepared by reactive sputtering were observed to depend greatly on the condition of applying the bias potential. The suitable P_N2 range which gives golden films of the stoichiometric composition was higher than 1 × 10^?4 Torr (substrate temperature, 200–300 °C; bias potential from ?75 to ?200 V).On the basis of these experimental studies of TiC and TiN coatings successive coatings of TiC and TiN were deposited onto a molybdenum substrate to achieve higher thermal stability and better adhesion to the substrate. The successive coating method is a promising technique for use in fusion reactors.     

Evaluation of Hard TiN Coatings by Depth Sensing Indentation and Scratch Testing Methods

The IAR Nanomechanical Probe (NMP) was used in the depth sensing indentation mode to evaluate the Vickers hardness (HV) and elastic modulus E of five commercial TiN coatings applied to a 17-4 PH stainless steel substrate. The HV values of the TiN layers at depth to thickness (DTT) ratios of less than 0.1 varied between about 28 and 39 GPa, depending on the deposition process. The elastic modulus was within the range of 300 to 400 GPa, corresponding to published values, at DTT ratios of less than or equal to 0.05. At higher DTT values, the elastic modulus decreased with increasing DTT ratio due to larger and larger interference from the less rigid stainless steel substrate. Depth sensing scratch tests were also performed on the samples to determine the critical load _crvalues needed to cause spallation of the coatings. For each coating, the interfacial fracture toughness K_ic was calculated from _crand used to describe the adhesive strength. Distinct differences between the K_ic values of the different samples were observed, reflecting differences in adhesive strength. The significance of these results is discussed in terms of the application of titanium nitride coatings to gas turbine engines.     

In situ synthesis of TiN/Ti3Al intermetallic matrix composite coatings on Ti6Al4V alloy

The aim of this paper was to develop an in situ method to synthesize the TiN reinforced Ti₃Al intermetallic matrix composite (IMC) coatings on Ti6Al4V alloy. The method was divided into two steps, namely depositing pure Al coating on Ti6Al4V substrate by using plasma spraying, and laser nitriding of Al coating in nitrogen atmosphere. The microstructure and mechanical properties of TiN/Ti₃Al IMC coatings synthesized at different laser scanning speeds (LSSs) in laser nitriding were investigated. Results showed that the crack- and pore-free IMC coatings can be made through the proposed method. However, the morphologies of TiN dendrites and mechanical properties of coatings were strongly dependent on LSS used in nitriding. With decreasing the LSS, the amount and density of TiN phase in the coating increased, leading to the increment of microhardness and elastic modulus and the decrement of fracture toughness of coating. When the LSS was extremely high (i.e., 600 mm/min), only a thin TiN/Ti₃Al layer with thickness around of 100 μm was formed near the coating surface.     

Effect of nano-CeO2 on microstructure properties of TiC/TiN+TiCN-reinforced composite coating

TiC/TiN+TiCN-reinforced composite coatings were fabricated on Ti–6Al–4V alloy by laser cladding, which improved surface performance of the substrate. Nano-CeO₂ was able to suppress crystallization and growth of crystals in the laser-cladded coating to a certain extent. With the addition of proper content of nano-CeO₂, this coating exhibited fine microstructure. In this study, Al₃Ti+TiC/TiN+nano-CeO₂ laser-cladded coatings have been studied by means of X-ray diffraction and scanning electron microscope. X-ray diffraction results indicated that Al₃Ti+TiC/TiN+nano-CeO₂ laser-cladded coating consisted of Ti₃Al, TiC, TiN, Ti₂Al₂₀Ce, TiC_0·3N_0·7, Ce(CN)₃ and CeO₂, this phase constituent was beneficial in increasing microhardness and wear resistance of Ti–6Al–6V alloy.     

CVD nanocrystalline diamond coatings on Ti alloy: A synchrotron-assisted interfacial investigation

Diamond coating on Ti-6Al-4V alloy was carried out using microwave plasma enhanced CVD with a super high CH₄ concentration, and at a moderate deposition temperature close to 500 °C. The nucleation, growth, adhesion behaviors of the diamond coating and the interfacial structures were investigated using Raman, XRD, SEM/TEM, synchrotron radiation and indentation test. Nanocrystalline diamond coatings have been produced and the nucleation density, nucleation rate and adhesion strength of diamond coatings on Ti alloy substrate are significantly enhanced. An intermediate layer of TiC is formed between the diamond coating and the alloy substrate, while diamond coating debonding occurs both at the diamond-TiC interface and TiC-substrate interface. The simultaneous hydrogenation and carburization also cause complex micro-structural and microhardness changes on the alloy substrates. The low deposition temperature and extremely high methane concentration demonstrate beneficial to enhance coating adhesion strength and reduce substrate damage.     

Investigating the wear characteristics of engineered surfaces: low-temperature plasma nitriding and TiN + MoS x hard-solid lubricant coating

Significant progress has been made in the past decade in plasma nitriding with a majority of the research work focusing on improving hardness and wear resistance of the nitrided surface through the reduction of nitriding temperature, pressure or time. Hard-solid lubricating coatings have also been extensively studied for lowering the wear rate and coefficient of friction of traditional hard coatings such as TiN by the combined effect of hardness and solid lubrication. In this study, the wear characteristics of low-temperature plasma-nitrided steel substrate performed using a Saddle-field fast atom beam source and TiN + MoS _x hard-solid lubricant coating deposited by a closed-field magnetron-sputtering technique have been investigated. The thin hard layer in plasma-nitrided substrates exhibited much higher hardness and lower wear compared to the untreated substrate in pin-on-disc wear testing. In addition, the study of the wear track morphology of the nitrided samples evidenced significant reduction of deeper ploughing and plastic deformation due to higher hardness and load supporting of the nitrided layer. On the other hand, due to the incorporation of MoS₂ in TiN coating, the wear resistance and coefficient of friction were greatly improved in TiN + MoS _x coating compared to pure TiN coating. In contrast to TiN coating, a relatively smoother wear track with less abrasive wear also supported the beneficial effects of adding MoS₂ in TiN coating.     

Adhesive and cohesive properties of nanostructured Zr02 coatings by the original Vickers Indentation Cracking technique

In order to characterize the adhesion of nanostructured plasma-sprayed Y₂O₃-ZrO₂ coatings presenting some geometrical specificities, it is here suggested to apply the Vickers Indentation Cracking (VIC) technique, which is capable of initiating a crack at the interface by performing the indentation test close to the interface within the substrate. This method renders it possible to overcome the difficulties concerning the characterization of the adhesion of nanostructured Y₂O₃-ZrO₂ (YSZ) coatings. It was suggested to calculate an adhesive stress parameter by employing two physical parameters, i.e., the critical load to initiate the crack and the indentation distance measured between the interface and the indent center. In addition, for the case of brittle coatings, the crack was found to deviate from the interface toward the coating. Under such conditions and on the basis of the critical point (given by the two parameters: load and indentation distance), a stress parameter representative of the cohesive properties of the coating was defined according to the crack deviation conditions.