High-Temperature Failure Mechanisms of Hot-Pressed Si3N4 and Si3N4/Si3N4-Whisker-Reinforced Composites

The high-temperature flexural strength of hot-pressed silicon nitride (Si₃N₄) and Si₃N₄-whisker-reinforced Si₃N₄-matrix composites has been measured at a crosshead speed of 1.27 mm/min and temperatures up to 1400°C in a nitrogen atmosphere. Load–displacement curves for whisker-reinforced composites showed nonelastic fracture behavior at 1400°C. In contrast, such behavior was not observed for monolithic Si₃N₄. Microstructures of both materials have been examined by scanning and transmission electron microscopy. The results indicate that grain-boundary sliding could be responsible for strength degradation in both monolithic Si₃N₄ and its whisker composites. The origin of the nonelastic failure behavior of Si₃N₄-whisker composite at 1400°C was not positively identified but several possibilities are discussed.     

Oxidation Behavior of Hot-Pressed Si3N4

The high-temperature chemical stability of hot-pressed Si₃N₄ was studied between 600° and 1450°C. Reactions were followed by X-ray diffraction and scanning electron microscopy. In air, this material begins to oxidize at 700° to 750°C; a distinct amorphous siO₂ surface layer results after 24 h at 750°C-Concomitant formation of cristobalite occurs, depending on exposure time, and is enhanced as temperature is Increased. Magnesium and calcium magnesium silicates form above 1000°C. The data suggest that impurities, e.g. Mg, Ca, and Fe, greatly lower the oxidation resistance of Si₃N₄ in air.     

Impurity Phases in Hot-Pressed Si3N4

Impurity phases in commercial hot-pressed Si₃N₄ were investigated using transmission electron microscopy. In addition to the dominant, β-Si₃N₄ phase, small amounts of Si₂N₂O, SiC, and WC were found. Significantly, a continuous grain-boundary phase was observed in the ∼ 25 high-angle boundaries examined. This film is ∼ 10 Å thick between, β-Si₃N₄ grains and ∼ 30 Å thick between Si₂N₂O and β-Si₃N₄ grains.     

Mixed-Mode Fracture of Hot-Pressed Si3N4

The mixed-mode fracture of hot-pressed Si₃N₄ was investigated using inclined indentation surface flaws in bending and large crack geometries in combined tension/torsion. Non-coplanar fracture was observed in all cases. Values of K_Ic, K_IIc, and K_IIIc stress intensity factors were obtained, with ratios K_IIc/K_Ic= 0.79 and K_IIIc/K_Ic= 1.55 observed. For large cracks, mode II conditions had more of an effect on mode I fracture than mode III conditions. The mixed-mode I-II fracture of surface flaws was significantly different from that for large cracks, suggesting surface flaw shear resistance effects. A model describing these effects was derived, based on the ratio of the crack-opening displacement to the crack surface asperity height.     

Cyclic Fatigue of Hot-Pressed Si3N4

The cyclic fatigue behavior of two grades of hot-pressed silicon nitride was investigated. Flat, cantilever-type specimens were tested at temperatures up to 1300°C, in air, where the load was applied by an eccentric driver rotating at 1800 rpm, with a zero mean stress. The lifetime of the lower purity material at temperatures up to 1200°C was controlled by a stress corrosion mechanism. Above 1200°, and for both grades of material, plastic deformation, probably by grain boundary sliding, was rate controlling.     

Controlled Surface Flaws in Hot-Pressed Si3N4

Surface flaws of controlled size and shape were produced in high-strength hot-pressed Si₃N₄ with a Knoop microhardness indenter. Fracture was initiated at a single suitably oriented flaw on the tensile surface of a 4-point-bend specimen, with attendant reduction in the measured magnitude and scatter of the fracture strength. The stress required to propagate the controlled flaw was used to calculate the critical stress-intensity factor, K _IC, from standard fracture-mechanics formulas for semielliptical surface flaws in bending. After the bend specimen had been annealed, the room-temperature K _IC values for HS-130 Si₃N₄ increased to a level consistent with values obtained from conventional fracture-mechanics tests. It was postulated that annealing reduces the residual stresses produced by the microhardness indentation. The presence of residual stresses may account for the low K _IC, values. Elevated-temperature K_IC values for HS-130 Si₃N₄ were consistent with double-torsion data. Controlled flaws in HS-130 Si₃N₄ exhibited slow crack growth at high temperatures.     

Mixed-Mode Fracture from Controlled Surface Flaws in Hot-Pressed Si3N4

The room-temperature mixed-mode fracture of commercial hot-pressed Si₃N₄ was examined using controlled surface flaws in 4-point bending, oriented at various angles 6 with respect to the outer fiber tensile stress direction. Catastrophic fracture paths were non-coplanar with the initial flaw plane, and the stress intensity factor ratio K_I/K_IC was < 1 for fracture in modes II and III. A non-coplanar maximum strain-energy release rate fracture criterion best described mixed-mode fracture.     

Damage Penetration at Elongated Machining Grooves in Hot-Pressed Si3N4

In hot-pressed Si₃N₄ subjected to single-point diamond grinding, the damage penetration ( c ) at low vertical loads ( Pv ) is approximately related by P_vαc , whereas at high loads the load dependence of the crack length is much greater. The results at low loads are discussed in terms of available analyses. An analysis, based on the combined effect of the residual stresses and contact loads, is presented to model the behavior at high loads.     

Residual Stresses in Hot-Pressed Si3N4 Grooved by Single-Point Grinding

Hot-pressed silicon nitride plates were grooved by single diamond points with varying degrees of flatness mounted on a wheel rotating at varying speeds. The plates were divided into bars and the remaining strengths were measured. The depths of damage were measured and used to calculate the theoretical strengths. For sharp diamonds and in some cases for diamonds with flat tips, the measured strengths were less than the theoretical strengths. The differences between the measured strengths and the theoretical strengths are attributed to the presence of residual loads acting to wedge open the cracks.     

Slow Crack Growth from Controlled Surface Flaws in Hot-Pressed Si3N4

The elevated-temperature slow-crack-growth behavior of HS-130 Si₃N₄ was studied by extending "controlled" surface cracks in bars loaded in 4-point bending. Several such nonin-teracting cracks were produced on the tensile surfaces of bend bars by Knoop microhardness indentation. The stress and dimensions of the subcritically growing cracks were used to calculate the stress-intensity factor, K₁ , from fracture-mechanics formulas for semielliptical surface cracks in bending. The crack-growth velocity, v, was obtained by dividing crack extension by loading time interval. The data indicated very large scatter in measured velocities for given K₁ values, which was interpreted as due to the interaction of the small cracks with local material heterogeneities. No simple functional relation between K _I and v could be established for HS-130 Si₃N ₄ from the v − K ₁ data.     

Hot Isostatic Pressing of Si3N4 Powder Compacts and Reaction-Bonded Si3N4

Hot isostatically pressed silicon nitride was produced by densifying Si₃N₄ powder compacts and reaction-bonded Si₃N₄ (RBSN) parts with yttria as a sintering additive. The microstructure was analyzed using scanning electron microscopy, X-ray diffraction, and density measurements. The influence of the microstructure on fracture strength, creep, and oxidation behavior was investigated. It is assumed that the higher amount of oxygen in the Si₃N₄ starting powder compared with the RBSN starting material leads to an increased amount of liquid phase during densification. This results in grain growth and in a larger amount of grain boundary phase in the hot isostatically pressed material. Compared with the hot isostatically pressed RBSN samples therefore, strength decreases whereas the creep rate and the weight gain during oxidation increase.     

Uniaxial Tensile and Flexural Stress Rupture Strength of Hot-Pressed Si3N4

Uniaxial tensile stress rupture testing of hot-pressed silicon nitride NC-132 was carried out at 1000°, 1200°, and 1300°C in air at various applied stress levels and the corresponding times-to-failure were measured. Fracture occurred from both surface and internally initiated flaws. Fractographic evidence for failure-initiation sites and the associated presence of subcritical (slow) crack growth are presented. Limited flexural stress rupture tests at 1200°C in air were also carried out and the data are compared. The uniaxial tensile and flexural stress rupture tests are much more sensitive in revealing the time-dependent crack-growth behavior than flexural stress rate tests.     

Temperature Dependence of KIC for High-Pressure Hot-Pressed Si3N4 Without Additives

Temperature dependence of K_IC values for Si₃N₄ bodies sintered at high pressures without additives was studied from room temperature to 1400°C. Little change in K_K was found in this range.     

Properties of Si3N4 Produced by Nitriding Slip-Cast Si Containing Si3N4 Grog Additions

The properties of Si₃N₄ compositions produced by nitriding slip-cast Si bodies containing up to 16% Si₃N₄ grog were determined. The introduction of grog consistently lowered the densities, the room- and high-temperature strengths, and the resistance to oxidation. The open structure of the grog-containing mixes favored low-temperature gas-phase reactions leading to α-Si₃N₄ formation. In higher-density compositions containing predominantly Si, gas-liquid-solid reactions at higher temperatures produced a relatively greater content of the β phase.