Determining the Toughness of Ceramics from Vickers Indentations Using the Crack-Opening Displacements: An Experimental Study

Recently, a method for evaluating the fracture toughness of ceramics has been proposed by Fett based on the computed crack-opening displacements of cracks emanating from Vickers hardness indentations. To verify this method, experiments have been conducted to determine the toughness of a commercial silicon carbide ceramic, Hexoloy SA, by measuring the crack-opening profiles of such Vickers indentation cracks. Although the obtained toughness value of K _o= 2.3 MPa·m^1/2 is within 10% of that measured using conventional fracture toughness testing, the computed crack-opening profiles corresponding to this toughness display poor agreement with those measured experimentally, raising concerns about the suitability of this method for determining the toughness of ceramics. The effects of subsurface cracking and cracking during loading are considered as possible causes of such discrepancies, with the former based on direct observations of lateral subsurface cracks below the indents.     

Hertzian Contact Response of Tailored Silicon Nitride Multilayers

The nature and degree of damage accumulation beneath Hertzian contacts in silicon nitride-based laminates are studied. Specimens with alternating homogeneous and heterogeneous layers are fabricated by a tape-casting route, with strong interlayer bonding. Homogeneous material consisting of relatively pure fine-grain silicon nitride is used as the overlayers. Heterogeneous material containing 10 to 30 wt% boron nitride platelets in a silicon nitride matrix, with weak platelet/matrix interphase boundaries, forms the underlayers. Contact tests with spherical indenters are used to monitor the stress-strain response of the laminates and to investigate the damage modes within the individual layers. The heterogeneous layer exhibits a distinctive "softening" in the stress-strain curve, indicating a quasi-plasticity in the silicon nitride associated with local microfailures at the platelet/matrix interfaces. In contrast to the welldefined cone cracks that develop within the tensile zone outside the contact area in bulk homogeneous silicon nitride, the damage in the laminates is widely distributed within the shear-compression zone below the contact. Fractures form incompletely in the homogeneous layers, as downward-propagating partial cone cracks and upwardpropagating stable cracks. Comparatively extensive, diffuse microscopic damage occurs in the heterogeneous layers, culminating in a macroscopic failure that traverses these layers at higher loads. A strong synergism between the interlayer damage modes is apparent. Implications concerning the design of composite laminates for improved damage tolerance, with retention of strength and wear resistance, are considered.     

Finite elements based approaches for the modelling of radial crack formation upon Vickers indentation in silicon nitride ceramics

By having superior properties silicon nitride ceramics can be considered as the state-of-the-art material in the bearing industry. Vickers indentation of this material is typically accompanied by formation of cracks visible on surface. Two Finite Elements models are developed in the current work: the first model is based on fracture mechanics and the second on cleavage stress criterion. Plastic behavior of silicon nitride is included in the modeling, and since little is known on the plasticity of this material, the Drucker-Prager model (used for non-metallic materials) along with the classical J₂-plasticity are explored. The results of the fracture mechanics based model correlate well with experimental results in terms of surface crack length. The numerical results in terms of the morphology of the indented zone (including cracks and plastic zone) are provided by the stress criterion based model, and these results correlate well too, with the experimental data.     

Contact-Induced Transverse Fractures in Brittle Layers on Soft Substrates: A Study on Silicon Nitride Bilayers

An analysis of transverse cracks induced in brittle coatings on soft substrates by spherical indenters is developed. The transverse cracks are essentially axisymmetric and geometrically conelike, with variant forms dependent on the location of initiation: outer cracks that initiate at the top surface outside the contact and propagate downward; inner cracks that initiate at the coating/substrate interface beneath the contact and propagate upward; intermediate cracks that initiate within the coating and propagate in both directions. Bilayers consisting of hard silicon nitride (coating) on a composite underlayer of silicon nitride with boron nitride platelets (substrate), with strong interfacial bonding to minimize delamination, are used as a model test system for Hertzian testing. Test variables investigated are contact load, coating/substrate elastic-plastic mismatch (controlled by substrate boron nitride content), and coating thickness. Initiation of the transverse coating cracks occurs at lower critical loads, and shifts from the surface to the interface, with increasing elastic-plastic mismatch and decreasing coating thickness. This shift is accompanied by increasing quasi-plasticity in the substrate. Once initiated, the cracks pop in and arrest within the coating, becoming highly stabilized and insensitive to further increases in contact load, or even to coating toughness. A finite element analysis of the stress fields in the loaded layer systems enables a direct correlation between the damage patterns and the stress distributions: between the transverse cracks and the tensile (and compressive) stresses; and between the subsurface yield zones and the shear stresses. Implications of these conclusions concerning the design of coating systems for damage tolerance are discussed.     

Indentation-Induced Cracks and the Toughness Anisotropy of 9.4-mol%-Yttria-Stabilized Cubic Zirconia Single Crystals

The crack systems associated with room-temperature Vickers indentations on {001} surfaces in 9.4-mol%-Y₂O₃-stabilized cubic ZrO₂ single crystals are examined. The indentation-induced radial cracks are not always perpendicular to the free surface, as is usually assumed, and the surface traces are therefore not a reliable guide to the nature of the cracking. In fact, indentations with 〈100〉 diagonals do not form cracks on {010}, but form secondary radial cracks on inclined planes and noncoplanar median cracks on (110) or (1 1 0).     

Anisotropic Properties in Hot Pressed Silicon Nitride—Silicon Carbide Platelet Reinforced Composites

The anisotropic properties (microstructure, mechanical properties) of a hot-pressed platelet reinforced silicon nitride composite were compared with those of the monolithic material. The platelets appeared to be orientated with their basal plane in the compressive plane, and to be embedded in a silicon nitride matrix consisting of interlocked elongated β-Si₃N₄ grains with their c axis orientated in this plane. TEM analysis showed an interface, consisting of glassy phase and graphite at the platelet–matrix grain boundary. Moreover the interfacial tensile stresses are in favour of a crack deflection mechanism. It was shown by TEM analysis that crack deflection occurs not only at the silicon nitride–platelet interface, but also at silicon nitride–silicon nitride grain boundaries. The efficiency of this reinforcing mechanism is highly orientation dependent. Because of their two dimensional geometry compared to the one-dimensional β-Si₃N₄ grains, platelets increase the toughness in two dimensions.     

Effect of Load on the Hardness of Hot Isostatically Pressed Silicon Nitride

The hardness values of five hot isostatic pressed silicon nitride materials, with varying densities, were measured at loads between 1 and 200 N. For the fully dense materials, the calculated hardness decreased from about 30 to 15 GPa as the load increased to about 10 N, and the hardness remained constant at higher loads. For the samples that showed indentation size effect (ISE), cracks formed at the corners of the indentation, starting at the lowest load of 1 N. Materials with lower densities had lower hardness values, displayed a very small or no ISE, and formed corner cracks only at high loads. For the samples that displayed an ISE at low loads, the formation of cracks was analyzed using the Niihara et al . criterion for Palmqvist cracks.     

Thermal expansion and P-V-T equation of state of cubic silicon nitride

We performed in-situ X-ray diffraction measurements of polycrystalline cubic silicon nitride samples at high temperatures under atmospheric pressure and at simultaneous high-pressure-temperature conditions. In air, cubic silicon nitride survives metastably up to 1733 K without oxidation. The temperature dependence of the thermal expansion coefficient was determined to be α(T) = a₁ + a₂T – a₃T⁻² where a₁ = 1.34(6) × 10⁻⁵ K⁻¹, a₂ = 5.06(44) × 10⁻⁹ K⁻², and a₃ = 0.20(10) K. Using all the experimental data obtained under atmospheric and high pressures, a complete set of parameters of the high-temperature third-order Birch Murnaghan equation of state was obtained: K₃₀₀,₀ = 303(5) GPa, K′_300,0 = 5.1(8), and (∂K_T,0/∂T)_P = –0.017(1) GPa K⁻¹, where K₀, K′₀, and (∂K_T,0/∂T)_P are the isothermal bulk modulus, its pressure derivative, and its temperature derivative, respectively. These parameters are necessary to calculate the equilibrium phase boundary between the β and cubic phases in silicon nitride.     

Cracking and the Indentation Size Effect for Knoop Hardness of Glasses

The Knoop hardnesses of five glasses decreased with increasing load in accordance with the classic indentation size effect (ISE). At moderate loads, cracking dramatically altered the indentation sizes and the ISE trends in three of the five glasses. Cracked indentations were as much as 10 μm longer than uncracked indentations made under identical conditions. Diagonal length readings must be corrected for optical resolution limitations if low power lenses are used.     

Crack profiles under a Vickers indent in silicon nitride ceramics with various microstructures

The effect of microstructure on crack morphology under a Vickers indentation was studied using 20 various silicon nitride ceramics including bearing-grade silicon nitrides. The indentation load was decreased from 98 N to 9.8 N and a transition of the crack types from half-penny crack to radial one was observed with both decoration method and serial sectioning technique. All of the indented samples possessed the half-penny cracks at the load of 98 N. The transition of crack profiles in the samples with coarse microstructure occurred when the load decreased from 49 N to 19.6 N, whereas the transition load for the sample with fine microstructure was ～9.8 N. Half-penny cracks were formed regardless of the microstructures when the ratio of the half of crack length to the half of diagonal size of an indentation, c/a, was above ～2. The dependence of the transition load on both Vickers hardness and fracture resistances was analyzed using Pajares's equation.     

Cathodoluminescence of the a-C:N films deposited by a filtered cathodic arc plasma system

Amorphous carbon nitride (a-C:N) films were successfully synthesized on silicon using a 90°-bend magnetic filtered cathodic arc plasma (FCAP) system. ESCA analysis demonstrated that the N/C ratios reached 1.08. Many investigations on photoluminescence (PL) spectra of diamondlike carbon films have been performed, but cathodoluminescence (CL) spectra have seldom been discussed. This work investigated a-C:N films using the CL spectra at 300 K. This work presents CL spectra of a-C:N films obtained at 1.5–3.5 eV and verifies luminescence from the a-C:N films in the visible region. The most prominent luminescence in the CL spectra from the a-C:N films has two peaks centered ∼2.67 eV (blue light) and ∼1.91 eV (red light), but the a-C film yielded only the band at ∼1.91 eV. These optical emissions are intrinsic and extrinsic to the a-C:N film and are sensitive to the content of nitrogen.     

Fatigue Threshold R‐curves Predict Fatigue Endurance Strength for Self‐Reinforced Silicon Nitride

There is a need for methods that can help predict and avoid fatigue failures of silicon nitride ceramic components. The fatigue threshold R‐curve has been proposed as potential solution to this problem. In this study, the fatigue threshold R‐curve for small, semielliptical surface cracks was calculated for a silicon nitride ceramic using the published bridging stress distribution developed from fatigue threshold tests on macroscopic crack specimens. To test the accuracy of the endurance strengths predicted using the fatigue threshold R‐curve, fatigue tests were conducted using four‐point bend beams of silicon nitride containing semielliptical surface cracks introduced by Knoop indentation. The effectiveness of the methodology was verified; indeed, 77% of the beams tested at stress levels above the predicted endurance strength failed within 10⁷ cycles and 0% of the beams tested below the predicted endurance strength failed within 10⁷ cycles. Furthermore, using the bridging stress distribution, which is thought to be a material property, the need for prohibitively difficult fatigue threshold experiments on small surface cracks is avoided. Accordingly, this methodology is potentially quite practical for use in the engineering design of ceramic mechanical components.     

Contact Fatigue in Silicon Nitride

A study of contact fatigue in silicon nitride is reported. The contacts are made using WC spheres, principally in cyclic but also in static loading, and mainly in air but also in nitrogen and water. Damage patterns are examined in three silicon nitride microstructures: (i) fine ( F )-almost exclusively fully-developed cone cracks; (ii) medium ( M )-well developed but smaller cone cracks, plus modest subsurface quasi-plastic damage; (iii) coarse ( C )-intense quasi-plastic damage, with little or no cone cracking. The study focuses on the influence of these competing damage types on inert strength as a function of number of contacts. In the F and M microstructures strength degradation is attributable primarily to chemically assisted slow growth of cone cracks in the presence of moisture during contact, although the M material shows signs of enhanced failure from quasi-plastic zones at large number of cycles. The C microstructure, although relatively tolerant of single-cycle damage, shows strongly accelerated strength losses from mechanical degradation within the quasi-plastic damage zones in cyclic loading conditions, especially in water. Implications concerning the design of silicon nitride microstructures for long-lifetime applications, specifically in concentrated loading, are considered.     

A new approach to protect and extend longevity of the thermal barrier coating by an impermeable layer of silicon nitride

A sample representation of a gas turbine engine blade, consisting of a nickel superalloy substrate with a deposited thermal barrier coating (TBC), was covered with silicon nitride, Si₃N₄, as an impermeable layer using plasma enhanced chemical vapor deposition (PECVD). The silicon nitride layer was used to seal the topcoat of yttria-stabilized zirconia (YSZ) surface of the TBC to mitigate calcium–magnesium–aluminum–silicon oxide (CMAS) attack. CMAS testing was carried out on the covered and uncovered surfaces by melting a ratio of 25 mg/cm² of CMAS powder onto the surface of each sample in a furnace at 1100°C for 1 h. The conformal surface reaction of the sealed layer confirmed no cracking or delamination at high temperatures. Scanning electron microscopy (SEM) micrographs confirmed that the surface of YSZ was successfully sealed. The new coating of silicon nitride was shown to be a viable solution and technique to significantly block CMAS infiltration in porous thermal barrier coatings.     

Finite element analysis of the contact stresses in diamond coatings subjected to a uniform normal load

Finite element analysis is used to predict the stress distribution in a diamond-coated substrate. The model examines the effect of coating thickness in detail, and compares and validates the finite element predictions with experimental results obtained from the soft impressor method applied to a series of diamond coatings on a silicon substrate. The results demonstrate that the performance of a coating can be related to the ratio of coating thickness to contact radius (t/a), so for example, for the silicon substrate most of the protection is achieved when the t/a ratio is about 0.1, and for the diamond coating there is no advantage in using coatings with t/a ratios grater than 1.3.     

Thermal diffusivity in diamond,SiCxNy and BCxNy

Thermal diffusivity (α) of free standing diamond, amorphous silicon carbon nitride (a-SiC_xN_y) and boron carbon nitride (a-BC_xN_y) thin films on crystalline silicon, has been studied using the travelling wave technique. Thermal diffusivity in all of them was found to depend on the microstructure. For a-SiC_xN_y and a-BC_xN_y thin films two distinct regimes of high and low carbon contents were observed in which the microstructure changed considerably and that has a profound effect on the thermal diffusivity. The defective C(sp)N phase plays a key role in determining the film properties.     

High temperature ceramic radomes (HTCR) – A review

An electromagnetically transparent, structurally robust and environmentally resistant enclosure of radar antenna for ground based systems to modern avionics in military aircraft and missiles is called as radome. Radome materials are classified based on: (i) type of function - surface-based or flight-mode and (ii) speed of operation - subsonic, supersonic to hypersonic. The desired properties of these materials are low dielectric constant and low loss factor in addition to its capacity to withstand the high temperature of operation. Composite laminates of glass or aramid fibre reinforced polymeric resins are radome material candidates for applications in subsonic range. However, ceramics are the only viable option for military aerospace applications such as a fighter jet travelling at Mach 3 or an advanced hypersonic missile speeding up to Mach 5. This review outlines the hand-full of ceramic materials already in application as radome materials like high-purity-alumina, pyroceram, slip-cast-fused-silica, their processing technology, electromagnetic and mechanical properties, advantages and disadvantages with respect to advanced military vehicles. Use of silicon nitride based radome materials, that has exceptional mechanical strength and thermal stability up to 1400 °C is illustrated with respect to reaction bonded silicon nitride, hot pressed silicon nitride, silicon oxynitride, sialon and their composites. Design of new generation radome materials was conceptualized and discussed as applicable to silicon nitride and related ceramics, wherein incorporation of varied degree of porosity improves electromagnetic properties, simultaneously, maintaining the required mechanical strength. Multilayer and graded porosity and its influence on electromagnetic properties were briefly discussed. Si₃N₄ ceramics having controlled porosity leading to optimum electromagnetic and mechanical properties produced through systematic processing is proposed as the futuristic high temperature radome material for supersonic applications.     

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

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