Effect of Incidence Angle on the Impact-Wear Behavior of Silicon Nitride

Experiments were conducted by repeatedly hammering a silicon nitride (Si₃N₄) ball on a polished different Si₃N₄ material at different incidence angles: 45°, 60°, 75°, and 90°. Material removal during impact was dominated by the delamination process, which involved abrasion and fracturing of the parent material, compaction of pulverized debris into a film, nucleation of cracks along the film/substrate interface, and lateral propagation of the interfacial cracks. The dependency of repeated impact wear on the incidence angle involved a combination of the effects of normal and tangential stresses on surface deformation. The contrasting influences of these stresses resulted in a minimization of impact wear of the polished Si₃N₄ material at incidence angles in the range of 60°-75°.     

Spherical-Impact Damage and Strength Degradation in Silicon Carbide Whisker/Silicon Nitride Composites

The silicon carbide (SiC) whisker reinforcement of silicon nitride (Si₃N₄) improves fracture strength and toughness, hardness, and Young's modulus, resulting in higher resistance of the composites to sphere penetration and crack initiation at spherical impact. Sintered Si₃N₄ shows an elastic/plastic response and initiates median/radial cracks at 100 m/s impact velocity. SiC-whisker/Si₃N₄ composites, on the other hand, demonstrate an elastic response, with Hertzian cone crack initiation, only when impact velocity exceeds 280 m/s. The SiC-whisker/Si₃N₄ composites thus exhibit improved strength degradation versus critical impact velocity characteristics because of improved mechanical properties provided by the SiC whiskers.     

Evaluation of Mechanical Stability of a Commercial Sn88 Silicon Nitride at Intermediate Temperatures

Dynamic fatigue and stress rupture tests in four-point bending were conducted on a commercially available SN88 silicon nitride ceramic at temperatures in the range 700°–1000°C in air. The objective of the present study was to elucidate the failure of SN88 silicon nitride ceramic nozzles arising from a critical crack initiated at the intermediate temperature airfoil region during an engine field test. Results of dynamic fatigue tests indicated that SN88 silicon nitride tested at a stressing rate of 30 MPa/s exhibited little change in characteristic strength at the various test temperatures. However, SN88 silicon nitride exhibited a significant degradation in mechanical strength when tested at 0.003 MPa/s at temperatures indicative of a great susceptibility to slow crack growth, especially at 850°C. SEM and XRD analyses indicated that the mechanical instability of SN88 silicon nitride at intermediate temperatures resulted from the transformation of secondary phase(s) from oxidation. These phase transformations were accompanied by a large volume change, which led to the generation of large local residual tensile stresses. As a result, extensive damage zones were formed, which led to a substantial degradation of mechanical strength and reliability. Microstructural examination of failed SN88 airfoils indicated that a similar damage zone was formed in the regions exposed to intermediate temperatures during engine testing. Consequently, the ultimate failure of these vanes was attributed to the loss in mechanical strength from the damage zone formation.     

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.     

Mixed-Mode Fracture in Soda-Lime Glass Using Indentation Flaws

Mixed-mode failure of soda-lime glass under inert and fatigue test conditions was studied using Knoop indentation flaws. For annealed cracks (residual stress-free) crack extension (catastrophic or subcritical) is by an abrupt transition from the initial crack plane to a noncoplanar crack plane followed by a reorientation of the crack normal to the applied stress. Although fatigue strength of these inclined flaws increased linearly with respect to orientation of the flaws to the applied stress up to an angle of 60°, this increase was considerably less than what was predicted by existing theories. It is believed that subcritical crack growth causes the crack to be realigned perpendicular to the applied stress before failure for all orientations; hence, fatigue strength does not show the dramatic increase at orientation angles as predicted by theory. For as-indented cracks the contact residual stress causes the crack extension to be less inclined to the initial crack plane than for annealed cracks, but in this case also, the crack realigns itself perpendicular to the applied stress. Again, fatigue strength is relatively insensitive to the orientation angle as predicted by theory and subcritical crack growth is believed to play a primary role in determining this strength dependency.     

Strength of Silicon Nitride after Thermal Shock

A water-quenching technique was used to evaluate the thermal-shock strength behavior of silicon nitride (Si₃N₄) ceramics in an air atmosphere. When the tensile surface was shielded from air during the heating and soaking process, the quenched specimens showed a gradual decrease in strength at temperatures above 600°C. However, the specimens with the air-exposed surface exhibited a ∼16% and ∼29% increase in strength after quenching from 800° and 1000°C, respectively. This is because of the occurrence of surface oxidation, which may cause the healing of surface cracks and the generation of surface compressive stresses. As a result, some preoxidation of Si₃N₄ components before exposure to a thermal-shock environment is recommended in practical applications.     

Particle Impact Damage in Silicon Nitride

The evolution of particle-impact-induced fracture damage in hot-pressed (HP) silicon nitride was established by accelerating single 2.4-mm-diameter tungsten carbide spheres against polished HP Si₃N₄ surfaces. Threshold velocities for ring, cone, and radial cracks were determined and the corresponding threshold stress for ring cracking was obtained from an elastic stress analysis. Particle size had significant effects on the threshold velocities for the inelastic impression and the various crack types. Loading rate had little effect on the threshold stress for ring cracks; rate effects on other crack types could not be assessed because the quasistatic indenter failed at stresses less than those required to invoke other crack types. A 20-μm-thick oxide scale had little influence on morphology and extent of damage but was removed easily at low velocities, suggesting higher erosion rates for Si₃N₄ in oxidizing environments. Damage phenomenology in 85% dense reaction-bonded Si₃N₄ was similar to that in HP material; however, all stages of damage occurred at substantially lower velocities.     

High-Temperature Toughness and Tensile Strength of Whisker-Reinforced Silicon Nitride

The R -curve behavior of hot-pressed silicon nitride reinforced with silicon carbide whiskers is investigated from room temperature to 1300°C using the chevron-notch bend test. The bridging stress, estimated from increment of fracture resistance in the rising R -curve, is discussed in relation to tensile strength measured with various displacement rates at 1300°C. The reinforcing whiskers provide most of the tensile strength in the creep-deformation range at 1300°C. The whiskers appear to bear a great deal of the applied tensile stress during slow crack growth.     

Strength of Slip-Cast, Sintered Silicon Nltrlde

The flexural strength of slip-cast, sintered silicon nitride was evaluated as a function of temperature (20° to 1300°C in air), applied stress, and time. The flexural strength was independent of temperatures from 20° to 800°C, and the mode of crack propagation was primarily transgranular. Above 800°C, the flexural strength decreased because of viscous flow of the glassy phase present in the material resulting in subcritical crack growth (SCG). The mode of crack propagation during SCG was essentially intergranular. Flexural stress-rupture evaluation in the temperature range 800° to 1000°C was used to identify the stress levels for time-dependent and time-independent failures.     

Elevated-Temperature Mechanical Properties of Silicon Nitride/Boron Nitride Fibrous Monolithic Ceramics

A unique, all-ceramic material capable of nonbrittle fracture via crack deflection and delamination has been mechanically characterized from 25° through 1400°C. This material, fibrous monoliths, was comprised of unidirectionally aligned 250 μm diameter silicon nitride cells surrounded by 10 to 20 μm thick boron nitride cell boundaries. The average flexure strengths of fibrous monoliths were 510 and 290 MPa for specimens tested at room temperature and 1300°C, respectively. Crack deflection in the BN cell boundaries was observed at all temperatures. Characteristic flexural responses were observed at temperatures between 25° and 1400°C. Changes in the flexural response at different temperatures were attributed to changes in the physical properties of either the silicon nitride cells or boron nitride cell boundary.     

Mechanical Behavior of a Continuous-SiC-Fiber-Reinforced RBSN-Matrix Composite

The results of a detailed study are presented on the toughening of reaction-bonded silicon nitride reinforced with large-diameter SiC monofilaments at ambient and elevated temperatures. Composite stiffness, strength, toughness, and R -curve behavior were investigated at ambient temperature, with strengths measured up to 1400°C. At elevated temperature, toughening mechanisms were explored by investigating crack initiation and growth under creep conditions. The results show that, at ambient temperature, the composite exhibited noncatastrophic failure with substantial toughening associated with contributions of both fiber pullout and elastic bridging of fibers in the crack wake, consistent with predictions using available models. Limited R -curve measurements suggest that large-scale bridging effects may be present. At elevated temperature, crack initiation occurred in the matrix at about 1000°C, but in the fiber at higher temperatures. Growth of cracks is governed by time-dependent bridging of unbroken fibers in the crack wake, consistent with a model based on fiber pullout by viscous sliding of fibers out of the matrix along amorphous interfacial layers.     

Theoretical Model of Impact Damage in Structural Ceramics

This paper presents a mechanistically consistent model of impact damage based on elastic failures due to tensile and shear overloading. An elastic axisymmetric finite element model is used to determine the dynamic stresses generated by a single particle impact. Local failures in a finite element are assumed to occur when the primary/secondary principal stresses or the maximum shear stress reach critical tensile or shear stresses, respectively. The succession of failed elements thus models macrocrack growth. Sliding motions of cracks, which closed during unloading, are resisted by friction and the unrecovered deformation represents the "plastic deformation" reported in the literature. The predicted ring cracks on the contact surface, as well as the cone cracks, median cracks, radial cracks, lateral cracks, and damage-induced porous zones in the interior of hot-pressed silicon nitride plates, matched those observed experimentally. The finite element model also predicted the uplifting of the free surface surrounding the impact site.     

Dye Impregnation Method for Revealing Machining Crack Geometry

The palladium nitrate dye penetrant method for revealing surface microcracks was investigated and applied to display the geometry of machining cracks in silicon nitride flexure test specimens. This method used elemental mapping with an electron probe microanalyzer to detect the presence of the dye and, thereby, display the crack geometry. A previously used bending method and a method developed in this study in which the specimen surface is exposed to the dye under pressure were used to facilitate dye penetration. Prior to applying the method to study machining cracks, carefully controlled Knoop indentation cracks introduced into flexure specimens were used to verify penetration of the dye to the crack tip. During these experiments it was found that the palladium nitrate dye resulted in a reduction in flexure strength, which, on further study, was attributed to the dilute nitric acid solution used to formulate the dye. Exposure to carbon tetrafluoride plasma etching prior to applying the pressurized dye method also resulted in a detectable decrease in flexure strength. Although there was clear evidence that exposure to dye and plasma etching resulted in a small but measurable decrease in flexure strength for the silicon nitride material studied, there was no detectable change in observed crack geometry. The reduction in flexure strength was apparently caused by a decrease in resistance to initiate crack propagation. It was concluded that the palladium nitrate dye method is an accurate and useful means for determining the geometry of small, otherwise difficult to observe surface microcracks. Nevertheless, caution should be exercised with the use of this method during strength measurements. When applied to machining cracks, the complex nature of these shallow, elongated, sometimes joining cracks was unambiguously revealed.     

Intermediate-Temperature Environmental Effects on Boron Nitride-Coated Silicon Carbide-Fiber-Reinforced Glass-Ceramic Composites

The environmental effects on the mechanical properties of fiber-reinforced composites at intermediate temperatures were investigated by conducting flexural static-fatigue experiments in air at 600° and 950°C. The material that was studied was a silicon carbide/boron nitride (SiC/BN) dual-coated Nicalon-fiber-reinforced barium magnesium aluminosilicate glass-ceramic. Comparable time-dependent failure responses were found at 600° and 950°C when the maximum tensile stress applied in the bend bar was 60% of the room-temperature ultimate flexural strength of as-received materials. At both temperatures, the materials survived 500 h fatigue tests at lower stress levels. Among the samples that survived the 500 h fatigue tests, a 20% degradation in the room-temperature flexural strength was measured in samples tested at 600°C, whereas no degradation was observed for the samples tested at 950°C. Microstructure and chemistry studies revealed interfacial oxidation in the samples that were fatigued at 600°C. The growth rate of the Si-C-O fiber oxidation product at 600°C was not sufficient to seal the stress-induced cracks, so that the interior of the material was oxidized and resulted in a strength degradation and less fibrous fracture. In contrast, the interior of the material remained intact at 950°C because of crack sealing by rapid silicate formation, and strength/toughness of the composite was maintained. Also, at 600°C, BN oxidized via volatilization, because no borosilicate was formed.     

A modified indentor technique for generating cone cracks in diamond and new results obtained from the technique

The traditional Hertzian method of generating cone cracks in diamond by using a spherical diamond indentor is costly in terms of preparation time of the indentor and the fact that very few indents can be made before the indentor fails. A new technique of indentor strength testing was developed which utilises a polycrystalline diamond (PCD) tip shaped into a flattened cone of included angle 120° with a flat tip of diameter 90 μm. To reduce the stress intensifying-effect of asperities on the PCD tip, a 50 μm thick, 316 L annealed stainless steel shim was inserted between the indentor tip and the diamond sample. Atomic force microscopy and micro Raman spectroscopy of cone cracks on synthetic 1b {100} polished diamond surfaces have provided additional information on the deformation and fracture mechanisms of diamond. Deformation of material near the inner edge of the ring crack appears to be larger due to friction along the crack interface preventing complete relaxation of the indented cone. Depressions within the ring crack are believed to be evidence for the microplasticity of diamond at room temperature without associated fracture.     

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.     

Evaluation of the Strength and Creep–Fatigue Behavior of Hot Isostatically Pressed Silicon Nitride

The strength of a commericially available hot isostatically pressed silicon nitride was measured as a function of temperature. To evaluate long-term mechanical reliability of this material, the tensile creep and fatigue behavior was measured at 1150°, 1260°, and 1370°C. The stress and temperature sensitivities of the secondary (or minimum) creep strain rate were used to estimate the stress exponent and activation energy associated with the dominant creep mechanism. The fatigue characteristics were evaluated by allowing individual creep tests to continue until specimen failure. The applicability of the four-point load geometry to the study of strength and creep behavior was also determined by conducting a limited number of flexural creep tests. The tensile fatigue data revealed two distinct failure mechanisms. At 1150°C, failure was controlled by a slow crack growth mechanism. At 1260° and 1370°C, the accumulation of creep damage in the form of grain boundary cavities and cracks dominated the fatigue behavior. In this temperature regime, the fatigue life was controlled by the secondary (or minimum) creep strain rate in accordance with the Monkman–Grant relation.     

Structural Reliability of Yttria-Doped Hot-Pressed Silicon Nitride at Elevated Temperatures

The strength of yttria-doped hot-pressed silicon nitride was investigated as a function of temperature, time, and applied load. Data collected at 1200°C are presented in the form of a strength-degradation diagram for an applied stress of 350 MPa. At this temperature, the behavior of yttria-doped hot-pressed silicon nitride is found to be superior to that of magnesia-doped hot-pressed silicon nitride, in which creep results in the formation of microcracks that lead to strength degradation. By contrast, the yttria-doped material does not suffer from microcrack formation or strength degradation at 1200°C. Strength degradation does occur at higher temperatures and, as a consequence, an upper limit of 1200°C is recommended for yttria-doped hot-pressed silicon nitride in structural applications.     

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.     

Research on material removal mechanism and radial cracks during scribing single crystal gallium nitride

A diamond conical indenter with the cone angle of 60° and a tip arc radius of 5 μm was used to perform the gradual loading scribing experiments on gallium nitride (0001) crystal plane along the (Li et al., 2019; Wang et al., 2018; Yao et al., 2020; Fang and Zhang, 2013; Feng et al., 2009; Zhang et al., 2007; Jing et al., 2007; Yonenaga et al., 2018; Nowak et al., 1999; Shimada et al., 1998) [11-20] crystal orientation, and the material removal mechanism and machining properties of gallium nitride are studied. Single crystal gallium nitride will have the surface and sub-surface damage during machining, and the study of the stress field during scribing can effectively analyze and control the surface and sub-surface damage. Therefore, the stress field for scribing gallium nitride is acquired by superposing the boussinesq field, cerruti field and sliding blister field which have been led-in anisotropy parameters. Then on the basis of the distribution of the maximum tensile stress on the scribing surface, the nucleation and initial spread angle of radial cracks under our experimental conditions are analyzed, and for gallium nitride, the more severe the brittle spalling accompanying radial crack spread is, the greater the initial spread angle of radial cracks is.