Fracture Toughness Of Coatings Research Articles

Microstructure and fracture toughness of in-situ nanocomposite coating by thermal spraying of Ti3AlC2/Cu powder

The low fracture toughness of ceramic coatings has always hindered their wide application. In this study, an in-situ nanocomposite coating was prepared by the atmospheric plasma spraying of a 50 wt% Ti 3 AlC 2 -50 wt% Cu mixed powder. The in-situ nanocomposite coating was found to have an unusual microstructure with a nano-micrometre phase synergistic enhancement, which consisted of submicrometre-thick layers of Cu and nanoparticles of Cu(Al), Ti 4 O 5 , TiO 2 , and Al 2 TiO 5 . Thus, in the spraying process, Al was delinked out of Ti 3 AlC 2 , forming a large amount of plastic Cu(Al) with Cu. The delinked channel provided a path for Cu to diffuse into Ti 3 AlC 2 , which a spatial Cu network structure was formed in the coating. The in-situ nanocomposite coating has high fracture toughness and crack growth resistance by a three-point bending test. This paper reports a new method to prepare a high-fracture-toughness composite ceramic coating.

Preparation and toughening of a-CuZr/c-ZrN nano-multilayer hard coatings

Hard coatings are prone to crack and failure when subjected to large external impact loads due to poor toughness. In this paper, a-CuZr/c-ZrN nanomultilayers were designed to improve the toughness of ZrN coatings. Two groups of a-CuZr/c-ZrN nanomultilayers were prepared: CuZr (10 nm)/ZrN (34, 51, 85 nm) as Group 1 and CuZr (4, 8, 16, 25 nm)/ZrN (46 nm) as Group 2. The microstructures were analyzed by Grazing incident X-ray diffraction (GIXRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). Hardness, elastic modulus were tested by nanoindentation; scratch tests and Vickers indentation methods were adopted to characteristic the toughness. The relationship between parameters of layer structure and mechanical properties were analyzed, the toughening effect and related mechanism were investigated. The conclusions were: In Group 1, both of the hardness and fracture toughness of multilayer coatings increase with the decrease of ZrN layer thickness. In Group 2, the hardness decreases first and then increases with the CuZr thickness increases. The toughness increased with the CuZr thickness increases, especially the a-CuZr/c-ZrN = 25/46 nm has the significant toughening effect, which toughness reached 0.5468 MPa·m0.5 (about 43% higher than the ZrN single-layer film). This is mainly affected by the a-CuZr layer thickness of the multilayer film which alter the deformation mechanism.

Evaluation of failure properties of a DLC/steel system using combined nanoindentation and finite element approach

In the current work, the failure properties of a DLC/steel system are quantitatively characterized by nanoindentation using a conical indenter. The DLC coating with thickness of ~1.7 μm is obtained by physical vapor deposition on a 20CrMnMo gear steel substrate. During the nanoindentation, it is noticed that the circumferential cracking and delamination occur in the coating and interface, respectively. In terms of the energy release theory, the coating fracture toughness and interfacial bonding energy are calculated from the steps in the loading curves. The results show that the interfacial adhesion energy and the fracture toughness are individually <137.113–548.454 J/m2 and ~1.579 MPa·m1/2 for the DLC coating on steel substrate, which accords well with the published data. Furthermore, based on the mechanical properties and cohesive energies of materials obtained above, finite element simulations under the ideal situation (defect-free and residual stress-free) including cohesive zone model are carried out to acquire coating fracture and interfacial bonding strengths as well as evolutions of failures in the coating-substrate system. The present model may find useful application not only for DLC but also for other coatings.

The influence of multiple fillers on friction and wear behavior of epoxy composite coatings

Optimized design of polymeric composites that are comprised of polymers and commonly used fillers forms one of the key focuses in this community since it may enhance the feasibility of real applications in industries. In this work, we produced three different epoxy composite coatings through filling the diamond, SiC, MoS2 and graphite. Then, the influence of these fillers on friction and wear performance of the composite coating is investigated. Comparison of the net epoxy (EP), the fracture toughness and young's modulus of the epoxy composite coatings (GME) can be 37.4 GPa and 4.3GPa, respectively, much higher than those of unfilled EP, i.e. 25.45 GPa and 3.75 GPa. More importantly, inclusion of multiple fillers can significantly reduce the wear rate and frictional coefficient of the GME. They are mainly because the graft treatment of the diamond and SiC dispersed uniformly in EP leads to a strong interface between the particles and matrix, making GME showing higher mechanical properties. Furthermore, due to the lubricant fillers-MoS2 and graphite, a continuous and thick lubricant film were observed on the worn surface of the GME, which can effectively decrease the friction heating and further damage to the matrix. Therefore, the strategy presented here is a valid method to create positive synergetic effect of different fillers, which can serve as an important guidance for rational design of the epoxy composite coatings towards in further industrial applications.

Enhanced mechanical and tribological properties of V-Al-C coatings via increasing columnar boundaries

Grain size, growth morphology, and grain boundary were considered to affect hardness and fracture toughness of hard coatings. An effective approach is currently developed to improve their hardness and toughness via tailoring their architectures through grain boundary control. V-Al-C coatings consisting of variable architectures were prepared by reactive sputtering V2AlC target with different CH4 flow rates in this study. The results indicated that, when the carbon content increased from 37.32 at.% to 71.4 at.%, the microstructure of V-Al-C coatings was tailored to span a wide changes from coarse columnar grain to fibrous columnar grains and finally to nanocomposite structure consisted of the (V, Al)C nanocrystallites and sp2-rich a-C. Especially, the maximum hardness of 28.74 GPa and the excellent toughness with high H/E value of 0.11 were obtained once the coating displayed fibrous columnar structure, such good mechanical properties benefited the lowest wear rate of 2.8 × 10−16 m3/Nm. Instead, the nanocomposite structure showed the lower fracture resistance due to the dominated amorphous carbon matrix. All coatings with nanocomposite structure exhibited low friction coefficient of ∼0.18, which was attributed to the coupling lubrication originated from both V2O5 Magnéli phases and amorphous carbon formed during friction process. The enhanced mechanical and tribological properties were also discussed in terms of the columnar boundaries evolution as a function of carbon content.

A physics-based model of temperature-exposure-dependent interfacial fracture toughness of thermal barrier coatings

A physics-based model of temperature-exposure-dependent interfacial fracture toughness of thermal barrier coatings was developed using Arrhenius-type formulae and experimentally measured interfacial toughness at ambient temperature. The crack delamination occurring at the thermally grown oxide (TGO)/bond coat (BC) interface was assumed in the interfacial fracture toughness evaluation. To evaluate the interfacial toughness at elevated temperatures, the interfacial plastic zone, the interface crack tip opening displacement (ICTOD) and the interface crack density at the thermally grown oxide (TGO)/bond coat interface were specified. The temperature-exposure-dependent Young’s modulus of the topcoat was formulated using the experimentally measured data, and its effect on the interfacial toughness at elevated temperature was investigated. As an application, the proposed interfacial toughness model was then used to study toughness variation versus exposure temperature and time. The trend of interfacial toughness versus temperature and exposure was explained in terms of microstructural changes of topcoat and TGO.

Evaluation of the fracture toughness of Ti1-xZrxN hard coatings: Effect of compositions

The objectives of this study were to evaluate the fracture toughness of Ti1-xZrxN hard coatings using the internal energy induced cracking (IEIC) method and to investigate the compositional effect on the fracture toughness, from which the optimum composition for fracture toughness could be attained. Ti1-xZrxN was selected to be the model system, because Ti1-xZrxN remained single phase structure in the entire compositional range at deposition temperature below 500 °C. Three compositions of Ti1-xZrxN coatings, x = 0.25, 0.55 and 0.85, were deposited by unbalanced magnetron sputtering. The parameters of IEIC method included the residual stress determined by the laser curvature method, Young's modulus obtained from nanoindentation and the film thickness measured from SEM cross-sectional image. The residual stress and film thickness before specimen fracture were used to calculate the elastic stored energy (Gs), from which the fracture toughness (Gc) could be derived. The resultant Gc of the Ti1-xZrxN coatings varied with Zr fraction, ranging from 26.0 to 48.7 J/m2, and reaching a maximum at a composition of Ti0.15Zr0.85N. The results showed that adding Zr atoms into TiN could effectively increase the fracture toughness. The maximum increase of fracture toughness lay in the intermediate range of composition (Zr = 0.55–0.85), suggesting that different properties of Ti and Zr atoms may play an important role on the fracture toughness of single phase Ti1-xZrxN thin films. The increase of Gc with Zr composition may be correlated with the change of configurational entropy of the Ti-Zr-N system. The atomic size difference of Zr and Ti may be crucial on increasing fracture toughness. The increase of fracture toughness for Zr-dominant Ti0.15Zr0.85N coating was higher than that for Ti-dominant Ti0.75Zr0.25N coating. This asymmetrical behavior could be attributed to the difference in lattice constants between Ti-rich and Zr-rich compounds, where the capability of increasing Gs may be higher for a smaller Ti atom incorporated into a larger Zr site in ZrN lattice.

High temperature fracture toughness of ceramic coatings evaluated using micro-pillar splitting

Micro-pillar splitting at temperatures up to 500 °C was used to evaluate the toughness for a series of thin physical vapour deposited ceramic-nitride coatings. When compared to ion beam notched geometries, this testing approach reduces the likelihood of ion impregnation within a material volume where fracture may initiate. A toughness increase with elevated temperature was observed for the nanostructured ceramic coatings; the magnitude of which varied between the coatings. A transmission electron microscopy investigation showed deposition-specific periodic nano-layering within the films as a result of the deposition process, which may explain the observed differences in the toughness trends.

FEM of Cracking during Nanoindentation and Scratch Testing in the Hard W-C Coating/Steel Substrate System

Finite element modelling (FEM) and eXtended FEM (XFEM) combined with the experimental nanoindentation and scratch tests have been used to simulate the process of cohesive cracking in W-C coating on softer and more ductile steel substrate during nanoindentation and scratch testing. The formation of single and multiple circular “frame” cohesive cracks in the sink-in zone during nanoindentation were explained by the development of high local tensile stresses in the coatings controlled by the plastic deformation of the substrate. Analogous mechanisms were successfully applied to the simulation of multiple Chevron type cracking during scratch testing. Thus, the ability of XFEM to predict the formation of different types of cohesive cracks was confirmed. It was also demonstrated that both nanoindentation and scratch tests in combination with XFEM can be used as the methods to determine the strength and fracture toughness of thin coatings.

Elevated temperature solid particle erosion behaviour of carbide reinforced CoCrAlY composite coatings

CoCrAlY+WC-Co and CoCrAlY+Cr3C2-NiCr coatings are deposited on nickel based alloy using atmospheric plasma spray technique. Mechanical properties such as microhardness, adhesion strength and fracture toughness of coatings are evaluated. Elevated temperature solid particle erosion behaviour of these coatings are investigated at 600 °C using alumina erodent at 30 and 90° impact angle. Coatings are characterized utilizing Scanning electron microscope (SEM), x-ray diffraction (XRD) and Energy dispersive spectroscopy (EDS). CoCrAlY+WC-Co coating shows higher hardness, adhesion strength and fracture toughness than CoCrAlY+Cr3C2-NiCr coating. CoCrAlY+WC-Co coating exhibited approximately 3 times higher erosion resistance than CoCrAlY+Cr3C2-NiCr coating at 90° and 30° impact angles. SEM images of eroded surfaces of coatings reveals the combination of ductile and brittle fracture. CoCrAlY+Cr3C2-NiCr coating shows severe cracks, craters, carbide pull out and chipping than CoCrAlY+WC-Co coating. High temperature erosion is a combination of simultaneous building up of material by oxidation and removal of material by erosion process. Thus reforming the erosion process to oxidation modified erosion process.

Influence of Heat Treatment on Fracture Toughness and Wear Resistance of Nicral-Zro2 Multilayered Thermal Barrier Coating

AbstractThe chemical composition and fracture toughness of thermal barrier coatings (TBCs) before and after heat treatment were characterized, and the cracks around the interface between the coating and the substrate could be successfully eliminated and meanwhile the porosity of the coatings tended to reduce. The XRD analysis revealed the coatings were composed of non-transformable tetragonal t’ phase of ZrO2 and $\gamma $-(Ni, Cr) with minor Ni3Al ($\gamma ^' $) precipitates. Additionally, the relationship between the heat treatment and wear resistance was systematically studied. The results indicated that both the hardness and fracture toughness increased after quenching process. The oxidation wear became more prominent after heat treatment, which probably resulted from the better bonding strength of coatings. Dense and homogeneous microstructure introduced by vacuum oil-quenching improved stabilization of the weight gain during thermal cycle test.

Microstructure and mechanical properties of plasma sprayed Al2O3-YSZ composite coatings

Alumina-yttria stabilized zirconia (Al2O3-YSZ) composite coatings are recognized as a potential alternative material for thermal barrier coatings. In this paper, Al2O3-YSZ composite coatings were deposited using atmosphere plasma spraying. The effects of alumina addition and annealing treatment on the microstructure and mechanical properties of the plasma sprayed Al2O3-YSZ composite coatings were investigated by XRD, SEM and an instrumented indentation testing system. Owing to the existence of YSZ in the composite coatings, the formation of amorphous alumina was suppressed. The quenching stress is high enough to trigger microcracking, these microcracks will further propagate or close under the post deposition thermal mismatch stress. Young's modulus of the coatings keeps an upward tendency from 50.56 GPa to 55.93 GPa and hardness increase from 5.36 GPa to 5.93 GPa with alumina contents varying from 10 wt% to 55 wt%. After an annealing treatment, Young's modulus and hardness increase compared with the as-sprayed ones. Fracture toughness of Al2O3-YSZ composite coatings is higher than the YSZ one, and it increases firstly and then decreases with alumina addition varying from 10 wt% to 55 wt%. The fracture toughness of coatings is related to the amount of heterogeneous specific interface between Al2O3 and YSZ splats and the high microcracks density in the composite coatings.

Enhanced thermal stability and fracture toughness of TiAlN coatings by Cr, Nb and V-alloying

The effect of metal alloying on mechanical properties including hardness and fracture toughness were investigated in three alloys, Ti~0.33Al0.50(Me)~0.17N (Me = Cr, Nb and V), and compared to Ti0.50Al0.50N, in the as-deposited state and after annealing. All studied alloys display similar as-deposited hardness while the hardness evolution during annealing is found to be connected to phase transformations, related to the alloy's thermal stability. The most pronounced hardening was observed in Ti0.50Al0.50N, while all the coatings with additional metal elements sustain their hardness better and they are harder than Ti0.50Al0.50N after annealing at 1100 °C. Fracture toughness properties were extracted from scratch tests. In all tested conditions, as-deposited and annealed at 900 and 1100 °C, Ti0.33Al0.50Nb0.17N show the least surface and sub-surface damage when scratched despite the differences in decomposition behavior and h-AlN formation. Theoretically estimated ductility of phases existing in the coatings correlates well with their crack resistance. In summary, Ti0.33Al0.50Nb0.17N is the toughest alloy in both as-deposited and post-annealed states.

A correlation between fracture toughness and cohesion strength of molybdenum thermal sprayed coatings

The Molydenum thermal sprayed coatings on steel substrate using electric arc, atmospheric plasma spray and high velocity oxy-fuel spray were investigated. The tensile bond strength tests were made in order to examine the degree of adhesion/cohesion strength of coatings. The Vickers indentation fracture toughness tests were made on parallel and perpendicular direction to the coating-substrate interface in order to examine the fracture toughness of the material in relationship with the microstructure, indentation direction, hardness and the length of microcracks developed under different loads. It was found that the fracture toughness is influenced by the direction and load of indentation. It was established a correlation between the fracture toughness and cohesion strength of Mo coatings and a general trend was described

Improved toughness of layered architecture TiAlN/CrN coatings for titanium high speed cutting

Fracture toughness is an important characteristic for coatings for high speed cutting of titanium. This study focuses on improving the fracture toughness of the coatings using a layered design method. TiAlN/CrN multilayer coatings with bilayer periods between 12nm and 270nm were prepared by reactive magnetron sputtering. XRD results show the multilayer coatings have face-centred cubic structures. When the bilayer period of multilayer coating decreases from 270nm to 25nm, the coating hardness decreases from 24.5GPa to 18.1GPa. Increased fracture toughness and adhesion strength of multilayer coatings are observed with decreasing bilayer periods. A maximum fracture toughness (KIC) of 3.6MPam1/2 and adhesion strength of 107 N were obtained with a 25nm bilayer period. The inhibition of crack propagation is the main reason for the high fracture toughness values in multilayer coatings. However, when the bilayer period is decreased further (12nm), the coating hardness begins to increase rapidly to 23.4GPa, and the fracture toughness and adhesion strength begin to decrease. Titanium cutting results show that an improvement in cutting performance is obtained with a TiAlN/CrN_25 multilayer coating.

Influence of cobalt content and HVOF deposition process on the cavitation erosion resistance of WC-Co coatings

Cavitation is a wear process that occurs in aqueous environments in which water vapor bubbles on the metal surface of a component implode, leading to erosion. The phenomenon can cause significant damage to structural metal alloys, and many components are therefore covered with coatings to protect against this damage. Thermal spray coatings have long been used for this purpose. With the advent of the HVOF (high velocity oxy-fuel) spray process it became possible to produce denser, harder, tougher coatings, which can be used in applications where fracture toughness is more important. Among the materials most frequently used, tungsten carbide-cobalt (WC-Co) cermet coatings offer a combination of high hardness, toughness and adherence and consequent wear resistance. In this work, two WC cermets with different Co (binder) contents were used to investigate the influence of Co content on the fracture toughness and cavitation resistance of WC coatings deposited by the HVOF process using different fuels. The influence of the HVOF process (specifically the type of fuel used) on the residual stresses in these coatings and their microstructure and properties was also studied. An increase in Co content led to an increase in the fracture toughness and cavitation resistance of the coatings. The primary wear mechanism appears to involve carbide particles becoming detached from the Co matrix. The greater compressive residual stress when liquid fuel was used in the HVOF process resulted in improved properties, including fracture toughness, and a concomitant improvement in cavitation resistance.

Measurement of Interfacial Fracture Toughness of Surface Coatings Using Pulsed-Laser-Induced Ultrasonic Waves

This research develops a new technique for the measurement of interfacial fracture toughness of films/surface coatings using laser-induced ultrasonic waves. Using pulsed laser ablation on the bottom substrate surface, strong stress waves are generated leading to interfacial fractures and coating delamination. Simultaneously, a laser ultrasonic interferometer is used to measure the normal (out-of-plane) displacement of the top surface coating in order to detect coating delamination in a non-destructive manner. We can thus determine the critical laser energy for delamination, yielding the critical stress (that is, the interfacial strength). Subsequently, to examine the interfacial fracture toughness, additional pulsed laser irradiation is applied to a pre-delaminated specimen to show that the delamination area expands. This type of interfacial crack growth can be visualized using laser ultrasonic scanning. Furthermore, the calculation of elastic wave propagation was carried out using a finite-difference time-domain method) in order to accurately estimate the interfacial stress field. In this calculation, the stress distribution around the initial delamination is calculated to obtain the stress intensity factor. Based on the experimental and computational results, interfacial fracture toughness can be quantitatively evaluated. Since this technique relies on a two-laser system in a non-contact approach, it may be useful for a quantitative evaluation of adhesion/bonding quality (including both interfacial fracture strength and toughness) in various environments.

Influence of microstructure on the wear and corrosion behavior of detonation sprayed Cr2O3-Al2O3 and plasma sprayed Cr2O3 coatings

Cr2O3-20wt% Al2O3 cermet composite and Cr2O3 coatings were deposited by the detonation spray coating (DSC) and air plasma spray techniques on Ti-6Al-4V substrates. The microstructure and phase formation in the coatings were analysed. Micro hardness and fracture toughness of coatings were measured using Vickers diamond pyramid indenter. Owing to the well-defined layered structure with individually richer Cr2O3 and Al2O3 phases together with oxide-based solid solution at the splat boundaries, the DSC Cr2O3-Al2O3 composite coating exhibits dense microstructure and better indentation fracture toughness than the well established plasma sprayed Cr2O3 coating. Although the plasma sprayed Cr2O3 coating also exhibits layered structure; the relatively poor indentation fracture toughness is attributed to the significantly higher porosity and weaker inter-splat bonding. Towards understanding the relative tribological behavior, the sliding and three-body abrasive wear tests were conducted. The material removal mechanism and associated degradation behavior of detonation sprayed Cr2O3-Al2O3 coating has been correlated with coating microstructure comparatively against the plasma sprayed Cr2O3 coating. Electro-chemical corrosion tests of coatings in salt solution (artificial sea water) reveals an improved corrosion protection by Cr2O3-Al2O3 composite coating as compared to Cr2O3 coating. Enhanced tribological performance and corrosion resistance was attributed to the dense and crack free structure of Cr2O3-Al2O3 composite coating than its counterpart.

Effect of arc\u2013current and spray distance on elastic modulus and fracture toughness of plasma-sprayed chromium oxide coatings

In this experimental study, chromium oxide powder was sprayed on a low-carbon steel substrate using the atmospheric plasma spray process. The current and standoff distances (SODs) were varied to study their effect on the fracture toughness of the coatings. Theoretically, as the arc current increases, the melting of the ceramic oxide should increase and this in turn should lead to the formation of a dense coating. However, it was observed that if the arc power is too high and because the particle size of the powder is small (approximately 30 μm), the particles tend to fly away from the plasma core. Similarly, an appropriate SOD should provide the particles with more melting time, thus resulting in a dense coating. On the other hand, a larger SOD leads to the solidification of the molten particles before the droplets can reach the substrate. All these effects may lead to substantial variation in the fracture toughness of the coating. The present paper attempts to correlate the plasma spraying parameters and microstructure of the coating with the fracture toughness and other primary coating properties.

The effect of substrate bias on the characteristics of CrN coatings deposited by DC-superimposed HiPIMS system

Chromium nitride coatings were prepared by reactive DC-superimposed high-power-impulse magnetron sputtering (HiPIMS) system. The influence of substrate bias on the microstructure and mechanical properties of CrN coatings was investigated. XRD and cross-sectional SEM were utilized to characterize the film structures. Mechanical properties were characterized by nanoindentation and Vickers indentation test. The results revealed that the microstructure and mechanical properties of CrN coatings were affected by bias voltage. The CrN coatings exhibited dense and fine columnar grain structure with the hardness of about 18.7 GPa. The fracture toughness of CrN coatings was around 3.16 MPa ⋅ m[Formula: see text]. However, further increase of the bias voltage from −250 V to −300 V led to the degradation of coating properties.