New method for extracting fracture toughness of ceramic materials by instrumented indentation test with Berkovich indenter

Applying finite element analysis, a method is proposed for evaluating fracture toughness of ceramic materials by instrumented indentation with Berkovich indenter. The crack-tip K_I (Stress intensity factor) of Berkovich-produced crack is numerically calculated by using virtual crack closure technique, in particular, three kinds of crack pattern, i.e., radial crack, transition crack and half-penny crack are identified and their crack fronts meet the equi-K_I requirement. The validity of the proposed method is verified by instrumented indentation tests on standard SRM2100 (Si₃N₄) and CRM156 (Fused Silica) samples. Comparison with six representative conventional indentation methods indicates that the proposed method has advantages including wide application range, high accuracy and applicability to different crack patterns. Additionally, it’s revealed that the conventional indentation fracture toughness formulae derived from Lawn-Evans-Marshall formula tend to exhibit larger test error when applied to materials of relatively high indentation work ratio W_e/W_t.     

A comparative investigation of the tribological behavior and scratch response of polyester powder coatings filled with different solid lubricants

Employing coatings is one of the most effective methods to reduce friction and protect contacting surfaces from wear. The deposition of protective coatings from thermosetting polymer powders has witnessed a rapid growth as an ecological, economic and energy efficient technology. During the last few decades, many new deposition techniques have been developed, and more and more tribological coatings have been made available. In this context, our present investigation tried, firstly to analyze the friction and wear behavior of electrostatically sprayed polyester powder coatings deposited on an aluminum substrate and secondly to focus on the response of these thermosetting coatings to micromechanical deformation under scratch test loading. The effect of graphite and hexagonal boron nitride (hBN) solid lubricant fillers on the friction and wear behavior of polyester composite coatings was evaluated using a reciprocating tribometer under dry friction condition. The experimental findings show that the additions of graphite or hBN are effective in enhancing the wear life of polyester powder coatings. Meanwhile, under the same sliding conditions, the wear results revealed that the polyester coating filled with only 10 wt.% of graphite has a higher anti-wear ability compared to the polyester coating filled with the same weight fraction of hBN. Thus, the two reinforcing polyester matrix fillers play an important role in reducing the plastic deformation of the coatings and enhance the formation of thick third body between the sliding parts as the fraction of solid lubricant increases from 0 wt.% to 10 wt.%. From the scratch analyses, we deduced that coatings scratch behavior is severely affected by the kind and amount of fillers inside the polyester matrix. In fact, the best friction characteristic and scratch resistance are observed in the case of polyester coatings filled with very low amount of hBN (5 wt.%).     

Reliable evaluation of fracture toughness in ceramics via nanosecond laser notching method

As a new type of notching approach, laser notching method always neglecting the significant impact of equivalent notch angle (θ) on fracture toughness results, although it has successfully solved the problem of introducing sharp enough V-notches in ceramics. Here, porous ZrB₂ and dense ZrB₂-SiC ceramics are taken as the experimental targets, and the effect of θ on the fracture toughness values of these typical ceramics is investigated. To measure the fracture toughness accurately, the θ should be less than 60°, namely, the laser notch depth should be larger than the traditional U-groove root radius. Nanosecond laser notching method shows great advantages in high notch depth (easy to reach 300 μm) and sufficient accuracy (tip radius less than 1 μm), and is deemed to have a good prospect of standardization.     

Artificial neural network investigation of hardness and fracture toughness of hydroxylapatite

Hardness and fracture toughness of hydroxylapatite were investigated by artificial neural network (ANN). Hardness and fracture toughness of hydroxylapatite were predicted by using its sintering temperature, sintering time, relative density, and grain size with ANN. It was found that prediction results of its hardness and fracture toughness closely matched with the experimental results.     

Elasticity,hardness, and fracture toughness of sodium aluminoborosilicate glasses

Due to an increasing demand for oxide glasses with a better mechanical performance, there is a need to improve our understanding of the composition-structure-mechanical property relations in these brittle materials. At present, some properties such as Young's modulus can to a large extent be predicted based on the chemical composition, while others—in particular fracture-related properties—are typically optimized based on a trial-and-error approach. In this work, we study the mechanical properties of a series of 20 glasses in the quartenary Na₂O–Al₂O₃–B₂O₃–SiO₂ system with fixed soda content, thus accessing different structural domains. Ultrasonic echography is used to determine the elastic moduli and Poisson's ratio, while Vickers indentation is used to determine hardness. Furthermore, the single-edge precracked beam method is used to estimate the fracture toughness (K_Ic) for some compositions of interest. The compositional evolutions of Vickers hardness and Young's modulus are in good agreement with those predicted from models based on bond constraint density and strength. Although there is a larger deviation, the overall compositional trend in K_Ic can also be predicted by a model based on the strength of the bonds assumed to be involved in the fracture process.     

Notch radius effect on fracture toughness of ceramics pertinent to grain size

Unlike fracture toughness, the notch fracture toughness of a ceramic is not a constant; rather, it increases with the notch-root radius ρ in a notched specimen. In this study, by analyzing the fracture measurements of eight different notched ceramics with an average grain size G of 3–40 μm, a simple model describing the relation between the notch fracture toughness and fracture toughness is proposed as a function of the relative notch-root radius ρ/G. The normal distribution is incorporated to consider the inevitable scatter in measurements where fracture mechanisms and errors are present. The results demonstrate that the model can effectively predict the quasi-brittle fracture variation trend for ceramics, including the upper and lower bounds, with 96% reliability, from a normal distribution; thus, it can address virtually all of the experimental data. We also determined that the notch fracture toughness approximates the fracture toughness if ρ ≤ G.     

Thermally enhanced dislocation density improves both hardness and fracture toughness in single-crystal SrTiO3

Dislocation-tuned functionality in ceramic oxides for potential versatile applications gains increasing attention. As the widespread chemical doping suffers from poor temperature stability, dislocations in well-controlled mesoscopic structure may be an alternative to thermally stable intrinsic doping features. To this end, the dislocation density in plastic zones introduced by cyclic Brinell indentation is considered under thermal annealing conditions. The considerably enhanced dislocation density due to thermal treatment is found to impact both microhardness and fracture toughness, albeit only to a modest degree. The mechanistic understanding centers around enhanced mobility and multiplication of the pre-engineered dislocations at elevated temperatures driven by the residual indentation stress, as well as the strengthened interaction of point defects and dislocations at high temperature.     

Accurate measurement of fracture toughness in structural ceramics

The measured values of fracture toughness for ceramics are closely correlated with the sharpness of notch tips, which in turn influences the accurate measurement of fracture toughness. Here, typical structural ceramics, i.e., 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP), ZrB₂, ZrB₂-SiC and ZrB₂-SiC-Grapite, were used for the measurement of fracture toughness, and the effect of notch tip radius on the fracture toughness values of these typical structural ceramics was investigated. Ultra-sharp notches with a tip radius less than 1 μm can be fabricated by laser, lower than the critical notch tip radius in ceramics below which the fracture toughness value almost remains constant, and improved accuracy and consistency of fracture toughness measurement can be obtained by this method compared with traditional method.     

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% Ti₃AlC₂-50 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), Ti₄O₅, TiO₂, and Al₂TiO₅. Thus, in the spraying process, Al was delinked out of Ti₃AlC₂, forming a large amount of plastic Cu(Al) with Cu. The delinked channel provided a path for Cu to diffuse into Ti₃AlC₂, 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.     

High hardness and high fracture toughness B4C-diamond ceramics obtained by high-pressure sintering

The B₄C-diamond composite with high hardness and toughness was first prepared by high-pressure sintering of B₄C and diamond powders at 5 GPa and 1600 °C. The effect of the diamond fraction on the densification, microstructure and mechanical properties of B₄C-diamond composite were investigated. The results indicated that the hardness of the as-prepared composite ceramics increased gradually with the increase in diamond content. The composite having 40 vol% diamond exhibited excellent comprehensive mechanical properties with a relative density of 98.3%, a density of 2.86 g/cm³, Vickers hardness of 39.8 GPa and fracture toughness of 8.1 MPa·m^1/2. The use of superhard diamond enhanced the fracture toughness of the B₄C while maintaining its lightweight and high hardness. The main toughening mechanisms were crack bridging, crack deflection and pull-out of homogeneously dispersed diamond grains. Superhard second phase dispersion high-pressure sintering provides a new technical route to improve the properties of advanced composites.     

Evaluation of damage in front of starting notches induced by ultra-short pulsed laser ablation for the determination of fracture toughness in zirconia

Machining a very sharp notch on the surface of ceramics for fracture toughness testing has been a critical issue during many years. In this work, we explore a novel method capable of inducing sharp features with negligible damage with laser pulsed ablation. We study the effect of different laser ablation parameters in the notch length and damage induced in 3 mol% yttria doped zirconia. In front of the notch it is found a very narrow microcracked zone which spreads in the notch direction, whose length only depends on the laser pulse energy; meanwhile the length of the notch is proportional to the energy deposited during the process.     

Data-driven optimization of hardness and toughness of high-entropy nitride coatings

High coating hardness and toughness are mutually contradicting properties and are challenging to be achieved simultaneously. Combining the vast component space of high entropy systems and the powerful high-dimensional data processing tools is expected to be the best solution to this problem. In this paper, high-entropy nitride coatings data for quinary and hexagonal systems were collected and machine learning prediction models were trained. Using a new material system combined with multi-objective optimization, high-entropy nitride coatings with the optimal hardness and elastic modulus combination were successfully obtained and verified by experiments. In addition, the partial dependence heatmaps were used to visualize how elemental content affects mechanical properties prediction in this system. This approach helped to better interpret the optimization results and discover the unknown mapping relationships between elemental content and the mechanical properties of high-entropy nitrides in machine learning models.     

PVD hard coatings on ceramic tiles for aesthetic applications: surface characterisation and corrosion properties

The development of innovative ceramic tiles looks for materials with improved mechanical and tribological properties as well as a higher corrosion resistance (high relative humidity, daily watering, household chemical cleaners). In addition, a greater durability leads to lower environmental impact. Along with their improved functionality and recyclability, ceramic tiles should also provide aesthetic properties. Ceramic tiles can be treated to modify the physico-chemical properties of the surface by metal coatings or metallic compounds, also providing an attractive metallic sheen appearance. In the present paper, titanium nitride (TiN) and zirconium nitride (ZrN) coatings were deposited on glazed porcelain stoneware by an industrial PVD multicathode arc deposition system under a reactive nitrogen atmosphere. After the process, the tiles showed a gold-like colour, a smooth surface and a coating thickness between 0.7 and 1.6 μm. The coating composition, scratch resistance and corrosion behaviour have been evaluated. It can be concluded that both coatings are suitable for use in domestic environments due to their stability and resistance to aggressive conditions. Few references have been found regarding these coatings on ceramic tiles for domestic and industrial applications, but it has been proved that they bring added value to traditional ceramics, giving new functional properties of ceramics both decorative and highly corrosion and mechanical resistance.     

The fracture toughness of natural fibre- and glass fibre-reinforced SMC

The fracture toughness properties, in terms of stress intensity factor K_Ic and strain energy release rate G_Ic, of hemp fibre mat-reinforced sheet moulding compound (H-SMC) are measured using the compact tension (CT) method and compared with those of glass fibre-reinforced SMC (G-SMC). Three material parameters were considered for composite optimisation: fibre volume fraction, CaCO₃ filler content and hemp fibre surface treatments using either alkaline, silane or a combination of these two treatments. The highest fracture toughness for H-SMC composites was obtained at a fibre loading of around 30?vol.-%, while it was also shown that the fracture toughness properties of H-SMC are sensitive to mineral filler content. Surface treatment of the hemp fibres using a combined alkaline-silane treatment resulted in a significant improvement in fracture toughness of H-SMC composites. Optimised H-SMC composites exhibited fracture toughness properties similar to those of G-SMC at fibre contents of 20?vol.-%, with K_Ic values of around 6?MPa.m^?1/2.     

Experimental evaluation of plowing and scratch hardness of aqueous two-component polyurethane (2K-PUR) coatings on glass and polycarbonate

Scratch resistance of aqueous two-component (2K-PUR) polyurethane coatings deposited on glass and polycarbonate was investigated by constant mode scratch tests. Penetration and residual depths as well as scratch widths were experimentally evaluated. A first analytical model was applied to estimate plowing and scratch hardness of the polyurethane coatings according to contact pressure and load rate and the corresponding 3D maps were drawn out. The experimental findings allowed mapping the scratch response the polyurethane coatings came through varying the applied load, load rate and substrate compliance, thus establishing the basis for the development of useful tools for failure prediction of the investigated coating systems in a broad range of operational conditions.     

Effect of molten zone ablated by femtosecond laser on fracture toughness of oxide ceramics

In this paper, an ultra-sharp V-notch was produced by femtosecond laser on the green bodies of 3Y-TZP, Al₂O₃ and 8Y-FSZ, respectively. After sintering, the fracture toughness of those ceramic samples was tested by single-edge V-notched beam (SEVNB) method. For comparison, the fracture toughness of ceramic samples with an ultra-sharp V-notch ablated by femtosecond laser directly on the sintered test bars was also determined as a regular testing route. The results reveal that the two different preparation methods of ultra-sharp V-notches can obtain the actual fracture toughness of 3Y-TZP, Al₂O₃ and 8Y-FSZ. It proves that the influence of thermal effect of ablation process caused by femtosecond laser in front of the notch tip on the fracture toughness can be negligible.     

Interface controlled micro- and macro- mechanical properties of aluminosilicate fiber reinforced SiC matrix composites

Novel Nextel™ 440 aluminosilicate fiber reinforced SiC matrix composites, with/without chemical vapor deposited carbon interphase were fabricated by polymer derived ceramic process, and they were studied by a combination of micro- and macro- mechanical techniques such as nanoindentation, micropillar splitting, fiber push-in, digital image correction and high temperature three point bend tests. Specifically, micropillar splitting test was firstly employed to measure in-situ the localized fracture toughness. The results revealed that the carbon interphase can effectively hinder the interfacial reactions between Nextel™ 440 fiber and SiC matrix, thus remarkably weakening the composite interfacial shear strength from ∼293 MPa to ∼42 MPa, and enhance the composite fracture toughness from ∼1.8 MPa√m to ∼6.3 MPa√m, respectively. This is mainly a consequence of weak interface that triggers crack deflection at the fiber/interphase interface. Finally, this novel composite showed stable mechanical properties in vacuum at temperature range from 25 °C to 1000 °C.     

Evaluation of the parameters of titanium surface micro-pits on the titanium-PEEK joint fracture toughness

The present work is to investigate fracture toughness of titanium-PEEK by linking the interface fracture toughness to surface parameters of micro-pit on titanium panels during delamination. The relationship between geometrical parameters and failure mechanisms is also explained. The regular micro-pits were fabricated on pure titanium panels by Lithography. The effects of micro-pattern on fracture toughness were systematically evaluated by changing geometrical parameters of surface topography. The results showed that by changing the parameters of surface pattern, the fracture toughness can be manipulated through plastic deformation and molecular bond breaking of PEEK. The optimized wetting angle and surface free energy, having influenced on the real contact area and the stress regions, were obtained by adjusting the geometrical parameters. Finally, it was found that the failure mechanism transition from interface to cohesive was because of the attributed optimal parameters, showing more energy dissipation.     

Hybrid CO2 laser waterjet heat (LWH) treatment of bindered boron nitride composites with hardness improvement

Boron nitride (BN) material is chemically and thermally stable which makes it desirable for high speed machining in demanding chemical and thermal environments. Although the hardness of BN material is well below that of single polycrystalline diamond (PCD), a laser waterjet heat (LWH) treatment process provides a new potential approach to achieve hardness values that are comparable to diamond hardness. This study investigates the hardness change of LWH-treated bindered cBN/TiN and cBN/AlN composites. Results indicate that measured hardness increase is dependent on the laser beam pass and the distance from the beam center.     

A detailed analysis of the determination of fracture toughness by nanoindentation induced radial cracks

The correlation between the choice of reference material and model utilized in determining fracture toughness from indentation-induced radial cracks was critically investigated with six commonly used reference materials. Initially, the empirical constants in Anstis’s and Laugier’s equations were calculated, compared and analyzed. According to the values of the constants, the reference materials were categorized into three groups. This classification was further verified by evaluating the corresponding constants of 13 additional equations. To account for the classification, FIB technique was employed to examine the crack morphology in the reference materials – Si (100) features an almost Half-penny-shaped crack while the cracks in SiC (0001) display nearly as a rectangle, both far from the assumptions employed by the models. Subsequently, an improved and more reliable procedure to determine fracture toughness is proposed. Finally, with this procedure, the fracture toughness of a diamond/SiC composite film is determined to be 11.1 ± 1.1 MPa m^1/2.