Microstructural control of boundary region between CVD diamond film and cemented carbide substrate

Effects of the composition, texture and pretreatment of cemented carbide substrates on the microstructure of the boundary region between CVD diamond film and the substrate were investigated using a microwave plasma CVD in the CO-H₂ system. Optimum CVD conditions for a uniform coating on to the edge part of cutting insert were: microwave power, 550 W; total pressure, 30 Torr; total flow rate, 200 ml/min; CO concentration, 5–20 vol%; treatment time, 3–5 h. An adherent and tough diamond coating was prepared by initial coating at lower CO concentrations and by subsequent coating at higher CO concentrations. A cemented carbide substrate in the binary WC-Co system which comprised fine-grained tungsten carbide and low content of cobalt was suited for preparation of adherent diamond coating. De-cobaltization pretreatment of the substrate surface in acid solution followed by an ultrasonic microflawing treatment enhanced the nucleation density and adherence of diamond film to the substrate. The rotation of substrate was found to be effective for increasing the uniformity and decreasing the grain size of diamond film.     

TiN coating on an electrical discharge machined WC-Co hardmetal: surface integrity effects on indentation adhesion response

Electrical discharge machining (EDM) is an alternative-shaping route for manufacturing complex component shapes of hard and brittle materials such as hardmetals (WC-Co cemented carbides). It is well established that in these materials EDM typically induces a heat affected surface layer with poor integrity. This degradation effect may be compensated through specific post-EDM surface treatments either by thermomechanical means or material surface deposition. In the latter case, a key property for optimal performance is the level of coating to substrate adhesion and how this is affected by the EDM-induced surface. The main objective of this investigation was to evaluate the adhesion strength of TiN coatings on EDMed hardmetals. A series of hardmetal samples that had been subjected to different multi-pass sequential EDM levels were coated with TiN. Adhesion behavior was assessed using the indentation adhesion test and comparing the critical load for crack extension (P _c) and the interfacial fracture toughness (K _Ic,interface) to those exhibited by the TiN coating deposited on a ground and polished substrate (used as baseline control). Experimental results indicated that indentation adhesion increased with finer-executed EDM, almost reaching baseline level values. The results are discussed on the basis of the compromising EDM influence on both surface integrity of the substrate and tortuousness at the interface, the latter resulting in mechanical anchoring of the TiN coating to the hardmetal substrate.     

WC-Co硬质合金基体上高附着力金刚石薄膜的制备

采用微波等离子体化学气相沉积(CVD)法在WC-Co硬质合金基体上制备金刚石膜, 研究了TiN_x中间层的引入对金刚石薄膜质量及其附着性能的影响. 结果表明, 在酸浸蚀脱钴处理的基础上, 通过预沉积氮含量呈梯度变化的TiN_x中间过渡层, 可在硬质合金基体上制备出高质量的金刚石薄膜; 压痕法测试其临界载荷达1000N.     

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.     

Delamination under Hertzian cyclic loading of a glass coating on Ti6al4v for implants

In this work, the interfacial response of a glass-based coating on Ti6Al4V to monotonic and cyclic Hertzian (spherical) indentation is investigated. This coating belongs to the SiO₂-CaO-MgO-Na₂O-K₂O-P₂O₅ system and it is specifically designed to be used as the inner layer of a bioactive bilayer coating with an outer layer of lower SiO₂ content to ensure bioactivity. During Hertzian monotonic loading, delamination of the coating occurs, which is revealed in the microscope by the presence of a colour pattern of interference fringes at the interface. Hertzian cyclic loading at maximum loads, P _max, lower than the monotonic delamination load, P _Del, also generates delamination damage. A plot of P _max versus the number of cycles for delamination shows a two-slope curve with a “knee” for P _max close to the critical load to induce a radial crack from the interface, P _Rc. The analysis of the delamination morphology and the results for different load ratios, R = P _min/P _max, confirmed the existence of two different delamination mechanisms with a common feature of plastic deformation of the substrate but with a different dependence with the maximum applied load: for P _max > P _Rc the process is mostly controlled by the presence of the radial cracks (P _max dependent), meanwhile, for P _max < P _Rc radial cracks are not observed and delamination is attributed to the residual stress at the interface induced by the cyclic plastic deformation of the substrate and the elastic recovery of the coating during unloading part.     

Formation of mixed TiC/Al2O3 layers and αa- and κ-Al2O3 on cemented carbides by chemical vapour deposition

Al₂O₃ and Ti(C, O) were codeposited as a mixed chemical vapour deposition (CVD) layer from AlCl₃-TiCl₄-CH₄-CO₂-H₂ gas mixtures on cemented carbides and pure alumina substrates. A thermodynamical approach of this CVD system is presented. The coatings were described by SEM and X-ray diffraction analysis. They consist of large facetted-Al₂O₃ crystals containing some titanium and surrounded by a fine grained Ti(C, O) matrix. Carbon diffusing from the cemented carbide substrate can considerably influence the morphology and the composition of the mixed coating.Methane in a AlCl₃-CO₂-H₂ environment stabilizes the-Al₂O₃ phase which can be deposited as a compact layer without whisker formation on a WC-Co substrate even without a TiC underlayer.     

Mechanical and thermal properties of physical vapour deposited alumina films Part II Elastic, plastic, fracture, and adhesive behaviour

Two mechanical characterization techniques were used to deduce the elastic, plastic, fracture, and adhesive properties of non-reactive physical vapour deposited alumina films of varying thickness on Al₂O₃-TiC substrates deposited at two different substrate biases. Depth-sensing indentation at both nano- and macroscopic load scales was used to determine the elastic and plastic properties of the films. Gravity-loaded Vickers indentation was performed to examine the fracture properties of the film and of the interface. Novel fracture mechanics models were developed to describe indentation-induced film fracture by channel cracks and indentation-induced interface delamination. The former model was used to determine the film toughness and the latter model was used to deduce the interfacial fracture resistance of the films and correctly predicted the effect of changing film thickness. Both models described the measured crack lengths with indentation load well and were used to identify the transition from radial and lateral cracking to channel and interfacial cracking.     

FATIGUE CRACK GROWTH BEHAVIOUR AND FRACTURE RESISTANCE IN CERAMICS

Fatigue crack growth and the fracture resistance curve (R-curve) were investigated in a polycrystalline alumina (AD90) and a silicon carbide whisker-reinforced alumina composite (Al₂O₃-SiC_w) at room temperature in air using a combined loading technique for stabilizing crack growth, and a surface film technique for monitoring crack length. Fatigue crack growth was evaluated successfully with those experimental techniques. Load shedding tests were performed until the crack became dormant, in order to determine the threshold stress intensity factor K_th. Subsequently, the specimens were used for quasi-static crack growth tests under a monotonic loading condition. The R-curves were determined in this experiment; however, fracture resistance did not increase markedly with crack growth. Detailed observations of the crack growth behaviour revealed that the flat R-curve was attributed to the shielding effect of the fatigue crack tip wake. Thus, the fatigue precrack introduced by the load shedding test was not regarded as an ideal crack for determining the R-curve. Fractographic observations were performed to investigate the mechanistic difference between fatigue and quasi-static crack growth. It was found that the cyclic loading produced fretting damage in the wake region and it reduced the shielding effect of the fatigue cracks. Based on the experimental results, the relationship between the fatigue crack growth and the R-curve is discussed as is the significance of K_th as a material parameter.     

Indentation fracture of WC-Co cermets

Indentation fracture of a series of well-characterized WC-Co cermets was studied with a Vickers diamond pyramid indenter. The resulting crack length-indentation load data were analysed in terms of relations characteristic of radial (Palmqvist) and fully developed radial/median (half-penny) crack geometries. The radial crack model gave a better fit to the data on all the alloys studied. Crack shapes determined by repeated surface polishing confirmed the radial nature of the cracks. An indentation fracture mechanics analysis based on the assumption of a wedge-loaded crack is shown to be consistent with the observed linear relation between the radial crack length and the indentation load. The analysis also predicts a simple relation among the fracture toughness (K _lc), the Palmqvist toughness (W) and the hardness (H) of the WC-Co alloys.     

Fracture mechanics analysis of thin coatings under spherical indentation

Spherical indentation of a thin, hard coating bonded to a thick substrate is investigated. The bending of the coating over the softer substrate induces concentrated tensile stresses on the lower and upper coating surfaces, from which transverse cracks may ensue. This work is primarily concerned with ring cracks originating from the top surface of the coating. In-situ indentation tests are carried out on a model glass/polycarbonate bi-layer, with the coating thickness and the indenter radius being the main test variables. As the coating thickness is decreased, the critical load to initiate ring cracks progressively departs from that associated with a critical surface stress, the effect that increases with increasing the indenter radius. A fracture mechanics approach in conjunction with the FEM technique is used to elucidate the onset of cylindrical ring cracks in thin-film bi-layer structures due to spherical indentation. The analysis, conducted as a function of the coating thickness and the indenter radius, reveals the existence of bending-induced compression stress regions ahead of the crack tip, which tend to shield the crack or increase the fracture resistance. The specific behavior is dictated by a complex interplay between the contact radius, a, the coating thickness, d, and the crack length, c. An interesting manifestation of this shielding mechanism is that when the coating surface contains flaws of various sizes, small flaws in this population may be more detrimental than large ones. Incorporation of this aspect into the analysis led to a good correlation with the experimental results. In the limit case of point-load, a closed-form, approximate solution for the stress intensity factors and the critical loads is obtained. This solution constitutes a lower bound for the critical loads, and is furthermore directly applicable to finite size indenters provided da. In the limit c/d/to0, a failure stress criterion may be used irrespective of the ball radius, r. The analysis in this case reveals that decreasing either d/r or the coating/substrate modulus ratio tend to favor ring cracking over radial type cracking. The transition between these two failure modes is identified explicitly as a function of the system parameters.     

Vickers Indentation Fracture (VIF) modeling to analyze multi-cracking toughness of titania, alumina and zirconia plasma sprayed coatings

For massive brittle materials, the fracture toughness in mode I, K_IC, can be determined using various reliable techniques. Besides, Vickers Indentation Fracture (VIF) technique has been developed to locally determine fracture toughness. However, since the indentation test generates a complex three-dimensional crack system around the indent, fracture toughness, K_C, is calculated instead of K_IC. Consequently some authors rightly reject the VIF technique to determine standard fracture toughness by arguing that the literature counts numerous VIF crack equations thus revealing discrepancies of this technique. Nevertheless in some cases (e.g. brittle ceramic coatings) inclusive material techniques are not applicable since presence of the substrate and/or multi-crack network can modify the crack propagation into the coating.In this work, we employed VIF technique to study multi-cracking behavior of titania, alumina and zirconia ceramic oxide coatings obtained by plasma spraying. To calculate VIF toughness, we propose (i) to select two crack equations for radial-median and Palmqvist cracking modes respectively, (ii) to adjust the crack equation of Miranzo and Moya for intermediate cracking mode, (iii) to develop a mathematical approach to determine the cracking mode, (iv) to take into account the multi-crack network by defining an equivalent four-crack system and (v) to propose a universal crack equation applicable independently of the cracking mode.     

The crack resistance in polybutylene terephthalate (PBT) at crack initiation and during steady crack growth

The resistance to crack initiation and quasi-static crack propagation is investigated for a polybutylene terephthalate (PBT) using annealed and unannealed three-point bend specimens. The resistance to crack initiation (R _i) is determined based on the generalized locus method which determines the resistance to crack growth including crack initiation utilizing the locus of characteristic points on the load against load-point displacement curves of specimens which differ only in initial crack length. This generalized locus method also enables us to investigate the invariance of the crack resistance value along the locus line. The steady state crack resistance (R _p) during quasi-static crack propagation is determined utilizing the functional relation between total essential energy (U _f) for complete fracture and the initial ligament length. The total essential energy is the sum of the blunting energy and the integration of the resistance to crack propagation with respect to the cracking area. The invariance of the crack initiation resistanceR _i, and the steady state resistanceR _p is discussed based on the experimental results of the annealed and unannealed specimens which show different sizes of crack tip plastic deformation.     

Analysis and simulation of cracking patterns in coating under biaxial tensile or thermal stress using analysis/FEM strain-accommodation method

The cracking patterns in coatings under biaxial tensile or thermal stress are analyzed by the “analysis/FEM strain-accommodation method” that combines the strain of the substrate with a coating obtained from thermo-elastic analysis with the strain of the substrate calculated by a finite element method. The simulation using this method is effective not only for expressing the cracking patterns observed in punch press tests of disk specimens with WC-Co cermet and Al₂O₃-TiO₂ ceramic coatings but also predicting the cracking process for the coating deposited on a part with a complex shape under thermal stress.     

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.     

Crack-Tip Stress-Strain Fields During Cyclic Loading and Effect of Overload

Finite-deformation elastoplastic analysis of a plane-strain crack subjected to mode I cyclic loading under small scale yielding was performed. The influence of the load range, load ratio and overload on the crack tip stress-strain field is presented. Two independent parameters of cyclic loading, such as ΔK and K _max, both substantially affect the near tip evolutions of cyclic stresses and plastic strains, in agreement with typical experimental trends of fatigue cracking. This implies that the behaviour of cracks is governed by stress and strain fields ahead of the tip, via their control over the key process variables (damage accumulation and rupture, i.e., bond-breaking), so that the coupled process becomes a two-parameter one in terms of fracture mechanics variables ΔK and K _max.     

Indentation-induced subsurface tunneling cracks as a means for evaluating fracture toughness of brittle coatings

When a plate glued to a compliant substrate is subject to indentation, cracks may initiate from its subsurface due to flexure. Upon increasing the load, the damage develops into a set of tunnel radial cracks which propagate stably under a diminishing stress field. This phenomenon is utilized here to extract fracture toughness K _C for brittle materials in the form of thin plates or films. Experiments show that the SIF at the tip of the subsurface radial cracks is well approximated as K ~ P/c ^3/2, where P is the indentation load and c the mean length of the crack fragments. Using a transparent substrate, c can be easily determined after unloading, from which K _C is found. This simple and economic concept is applied to a wide variety of thin ceramic coatings, yielding toughness data consistent with literature values. Because the tip of the tunneling cracks are well removed from the contact site, the method circumvents certain complications encountered in common top-surface radial cracking techniques such as the effect of plastic deformation, residual stresses and crack extension after unloading. Although the present tests are limited to coating thicknesses >150 μm, it is believed that thinner coatings may be studied as well provided that the indenter radius is kept sufficiently small to insure that subsurface radial cracking dominates over all other failure modes.     

Optimising deposition parameters of W-DLC coatings for tool materials of high speed steel and cemented carbide

The paper presents the results of investigations on the selection of optimal deposition parameters for W-DLC coatings produced by pulsed reactive magnetron sputtering in order to obtain the coatings with high adhesion to the high speed steel (HSS) and cemented carbide (HM) substrates. To optimise the deposition parameters for W-DLC coatings Taguchi's method was used. An acetylene flow rate, the substrate bias voltage, the thickness of W-DLC coatings and the thickness of chromium sublayer were selected as deposition parameters and for each of them three levels of values were determined. Adhesion, wear resistance and hardness of coatings were chosen as the criteria for selecting the optimum deposition parameters. The test results showed that all the selected parameters have a significant effect on the adhesion of coatings. The thickness of W-DLC coatings has a very significant effect on the adhesion to HSS substrates and in a case of HM substrates the same effect has the thickness of a chromium sublayer. The wear resistance of coatings increases with an increasing acetylene flow rate and decreases with the increasing substrate bias voltage. High correlation was found between the H/E ratio of coatings and their wear resistance as well as toughness. The W-DLC coating showing the best properties (L_C3 ≈ 90 N, k_V = 3.8 × 10⁻⁷ mm³/Nm) was marked by the nanocomposite structure containing about 40% of nanocrystalline tungsten carbide phase and 60% of amorphous carbon matrix.     

Multi-crack analysis of hydraulically pumped cone fracture in brittle solids under cyclic spherical contact

The evolution of surface damage in bilayers due to cyclic spherical indentation in the presence of incompressible lubricant is studied using an all-transparent glass/polycarbonate system as a model for more practical applications such as dental crowns and rolling contact fatigue. In situ observations and post-mortem material sectioning reveal that inner cone cracks evolve sequentially from the contact edge inward by slow growth in a process controlled by stress shielding from preceding cracks. The embryonic cracks are then accelerated by the action of fluid pressure into the flexural tensile stress at the lower part of the coating, where crossover fracture leading to delamination between the coating and substrate may ensue. A consistent FEM brittle fracture analysis incorporating multiple cracks, rate-dependent toughness and liquid pressure is used to follow the damage evolution in the coating. Crack trajectories are determined incrementally under the dual constraint K _I = K _II = 0, which maximize the tension at the crack tip upon the application of fluid pressure. The latter, evaluated at each increment with the aid of a fluid entrapment model, helps drive the leading crack past the compression zone beneath the contact via a hydraulic pump like action. In the early stages of fracture, the liquid pressure is reasonably well approximated by the Hertzian radial surface stress at the crack mouth. Fluid trapped in secondary cracks accentuate the compression beneath the contact. This helps squeeze more liquid into the tip of the leading crack in a zipping like action, which further enhance the crack driving force in the far field. The analytic predictions generally collaborate well with the tests.     

Investigation on impact toughness and wear resistance of nanocomposite Al2O3/WC-Co hammer bit teeth

The material of the hammer bit tooth is cemented carbide YG8, whose impact toughness and wear resistance are insufficient, leading to tooth fracture and wear. In this paper, cemented carbide YG8 teeth and nanocomposite teeth doped with Al₂O₃ nanoparticles and VC inhibitors were fabricated through a series of processes. Cemented carbide YG8 teeth act as comparison. Two kinds of teeth were analyzed and compared using the scanning electron microscopy (SEM) and the MLD-10 dynamic impact-wear tester. SEM micrographs of samples show that Al₂O₃/WC-Co nanocomposite grains are finer and more uniform, which can improve impact-wear resistance. Impact-wear tests indicate that nanocomposite teeth had 3 to 5 times higher impact toughness and wear resistance than YG8 teeth. The fracture toughness can be significantly improved by adding the proper amount of Al₂O₃ nanoparticles. Based on fracture morphology analysis, it is found that the fracture of the YG8 tooth under impact-wear condition is brittle fracture and the fracture of the nanocomposite tooth is ductile fracture.     

Influence of composition and processing parameters on mechanical properties and erosion response of Ni–TiB2 coatings

Abstract

Sputtered Ni–TiB₂ coatings have been shown to protect Ti–6Al–4V and Inconel 718 substrates from solid particle erosion. However, before new erosion resistant coatings can be efficiently designed, it is essential that the role of mechanical properties in determining erosion resistance be fully understood. In this investigation, nanoindentation techniques were used to quantify the effects of substrate preparation, coating composition, and sputtering process parameters on the elastic moduli and indentation hardness of thin coatings deposited on Ti–6Al–4V and Inconel 718 substrates. The influence of these parameters on coating adhesion was determined using a conventional scratch test. Elastic moduli, indentation hardnesses, and coating adhesion were correlated with erosion behaviour. The erosion resistance of the coatings that exhibited microscopic ductility is dependent on the nodule diameter and coating properties such as hardness, elastic modulus, and fracture toughness.

MST/1697