Hot-pressed Silicon Nitride Research Articles

Sintering Behavior and Properties of Reaction-Bonded Silicon Nitride

The influence exerted by the conditions of the preliminary synthesis of Si3N4 by nitridation at 1350 and 1400°С and additional high-temperature sintering at 1800 and 1900°С on the properties of reaction-bonded silicon nitride was studied. Secondary silicon nitride (β-Si3N4) formed in the course of nitridation consists of needle-like grains, which reinforce the material and impart to it additional mechanical strength. The microstructure and phase composition of the silicon nitride material at different initial ratios of silicon and silicon nitride were studied. The materials obtained approach liquid-phase-sintered and hot-pressed silicon nitride in the mechanical properties.

Fractography of silicon nitride based ceramics to guide process improvements

Fractography is an important tool to understand and identify the cause of the failure in materials. This understanding can be used to make changes in raw materials selection and processing to increase the strength of brittle materials. This study reports the fracture behavior of hot-pressed silicon nitride based ceramics, with focus on dominant flaw identification with respect to material and process parameters. Silicon nitride is an important material for structural applications which require high strength and wear resistance, such as bearings, nozzles, and cutting tools. Silicon nitride with a target base composition of Si6-zAlzOzN8-z (z = 0.5), along with varying boron dopant levels, was explored in this work. Detailed fractographic analyses revealed that the majority of fracture origins were internal flaws due to the foreign impurities introduced at various stages of processing. All materials were found to have reasonably high strength (800−1100 MPa). Strength was inversely proportional to the square root of the flaw size, however no correlation was found between measured flexural strength and fracture origin types. Mirror constants calculated from fracture mirror measurements ranged between 5.8 and 9.8 MPa.m1/2.

Computational and Experimental Investigation of Mechanical and Thermal Contacts of Ceramic Blades with the Metal Disk in Gas Turbine Engines

Models of thermal and mechanical contacts of ceramic and metal parts in lock joints of turbines of aircraft gas turbine engines have been developed which take account of the character of operating loads in them. Experimental data on the strength and thermal conductance of a ceramics–metal contact joint have been presented. The investigations were carried out with ceramic specimens based on hot-pressed silicon nitride and silicon carbide reinforced with diamond particles.

Study on surface roughness in ultrasonic vibration mill-grinding of hot-pressed Si<SUB align="right">3N<SUB align="right">4

In order to investigate the influence of machining parameters on surface roughness in ultrasonic vibration mill-grinding of hot-pressed silicon nitride, the mechanism of the ultrasonic vibration mill-grinding is analysed. Kinematic analysis of abrasive grain indicates that grains and workpiece separates periodically which is beneficial for coolant entering into grinding zone to reduce grinding temperature, so the surface quality is improved in certain extent. The theory of indentation of brittle material is used to analyse the mode of material removal. Orthogonal experiments were then carried out and machined surfaces were observed by SEM and detected by roughmeter. The detected results indicate that spindle speed has the most significant influence on surface roughness and the following are feeding speed, cutting depth and amplitude in turn. From the experimental data, an empirical formula was obtained through regression analysis. Finally, the effect of machining parameters on surface roughness is analysed.

Effect of Sintering Additive on the Strength and Fractography of Hot-Pressed Silicon Nitride Ceramics

Several silicon nitride ceramics with different compositions and amounts of sintering additive were fabricated by hot-press method. The effect of sintering additive on the sinterability, flexural strength and fractography was examined. The flexural strength of silicon nitrides sintered with alumina as an additive were not high at approximately 400MPa, although the sinterability was good in that bulk densities of 3.05-3.18 × 103kg m-3 were obtained. The sinterability of silicon nitrides sintered with yttria as an additive resulted in low bulk densities of approximately 2.7 × 103kg m-3, with flexural strength ranging from 370 to 650MPa. The sinterability of silicon nitrides that were sintered with both yttria and alumina additives resulted in relatively high bulk densities of approximately 3.21 × 103kg m-3 and extremely high flexural strength in the range of 900 to 1140MPa. The fractography of fracture origin was dependent on the type of sintering additive used; an inclusion size of approximately 30μm for the alumina additive, pores for the yttria additive and small secondary sintering defects for both yttria and alumina as additives were observed at the respective fracture origins.

Characterization of rare-earth doped Si3N4/SiC micro/nanocomposites

Influence of various rare-earth oxide additives (La2O3, Nd2O3, Sm2O3, Y2O3, Yb2O3 and Lu2O3) on the mechanical properties of hot-pressed silicon nitride and silicon nitride/silicon carbide micro/nano-composites has been investigated. The bimodal character of microstructures was observed in all studied materials where elongated ?-Si3N4 grains were embedded in the matrix of much finer Si3N4 grains. The fracture toughness values increased with decreasing ionic radius of rare-earth elements. The fracture toughness of composites was always lower than that of monoliths due to their finer Si3N4/SiC microstructures. Similarly, the hardness and bending strength values increased with decreasing ionic radius of rare-earth elements either in monoliths or composites. On the other hand, the positive influence of finer microstructure of the composites on strength was not observed due to the present defects in the form of SiC clusters and non-reacted carbon zones. Wear resistance at room temperature also increased with decreasing ionic radius of rare-earth element. Significantly improved creep resistance was observed in case either of composite materials or materials with smaller radius of RE3+. .

Fracture Toughness of Si<sub>3</sub>N<sub>4</sub> Based Ceramics with Rare-Earth Oxide Sintering Additives

Fracture toughness of hot-pressed silicon nitride and Si3N4+SiC nanocomposites prepared with different rare-earth oxides (La2O3, Sm2O3, Y2O3, Yb2O3, Lu2O3) sintering additives have been investigated by Chevron Notched Beam, Indentation Strength and Indentation Fracture techniques. The fracture toughness values of composites were lower due to the finer microstructures and the lack of toughening mechanisms. In the Si3N4 with higher aspect ratio (Lu or Yb additives) crack deflection occurred more frequently compared to the Si3N4 doped with La or Y, which was responsible for the higher fracture toughness.

Micro-Hole Drilling on Silicon Nitride Ceramics in Ethanol

Micro-hole drilling of engineering ceramics has potential for application to fuel injection nozzles and fiber spinning nozzles, etc. To date among engineering ceramics, less study has been reported on the micro-hole drilling of silicon nitride. We therefore conducted micro-hole drilling of hot-pressed silicon nitride using a chemical vapor deposition (CVD) diamond-coated tool with ethanol as the cutting fluid. As a result, the successful drilling of silicon nitride was achieved with a smooth drilled surface and sufficient accuracy of the hole size. Wear damage of the CVD coated diamond tool and drilling debris of silicon nitride were investigated using scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX). As a result, it is suggested that tribochemical reaction of silicon nitride effectively contributed to the micro-hole drilling characteristics; that is, smoother hole surface was obtained due to the polishing action of wear debris and tribochemical reaction products. The hole inlet size, varied during successive drilling process, could be explained in relation to the wear damage of the diamond-coated drill tip.

Brazing of Ti-sputtering Activated Hot-pressed-SiN with Prior Metallized Ferritic Stainless Steel

The brazing of hot pressed silicon nitride (HPSN) with ferritic stainless steel using high temperature brazing material has been studied. The effect of prior brazing metallization of stainless steel and activation of faying surface of HPSN by titanium sputtering on characteristics of the joint have been analysed. The characteristics of the HPSN-steel brazed interface have been studied by energy dispersive X-ray spectroscopy (EDS) under SEM to understand the mechanism of joining through chemical interaction in different procedures used for the brazing. The bend strength of the brazed joints produced by different procedures of brazing process has been studied and compared. It is observed that the use of Ti activated HPSN and metallized stainless steel is beneficial for improvement of joint strength.

The Relationship Between Multiple Scratch Tests and Wear Behavior of Hot‐Pressed Silicon Nitride Ceramics with Various Rare‐Earth Additive Systems

The wear behavior of Si3N4ceramics sintered with various rare earth additives was studied for nonlubricated sliding under different conditions, and scratch tests carried out in an attempt to correlate the wear behavior. When multiple scratch testing is used the results can be used to indicate the initial wear behavior under fracture‐dominated wear of the materials. The additive system used in the sintering of the Si3N4ceramics affected the specific wear rate under nonlubricated sliding conditions, and under high load conditions, where fracture is dominant, the specific wear rate was shown to increase in samples sintered with lutetium as a consequence of a strong bonding strength between the grains and grain boundary resulting in a higher degree of brittle fracture.

Friction and Wear Characteristics of SiC and Si<sub>3</sub>N<sub>4</sub> Ceramics Against ZrO<sub>2</sub> Ceramic under Dry Friction

The friction and wear of the silicon carbide (SiC) and hot pressed silicon nitride (Si3N4) against zirconia (Y–TZP) sliding under dry friction and room temperature conditions were investigated with pin-on-disk tribometer at sliding speed of 0.56 m·s-1 and normal load of 50 N, 80 N and 120 N, respectively. It was found that, the coefficient of friction and wear rate are dependent on the test duration as well as the normal load. Through analyzing and comparing, the wear rates of the two frictional couples both are in the 10-6 mm3 (N·m)-1. Based on the variety regulation of the wear maps, the wear mechanisms of the two couples were analyzed. Between the two couples, the friction and wear characteristics of the SiC/ZrO2 couple are better than the Si3N4/ZrO2 couple’s.

Bending Strength of Ceramics Implanted by Titanium, Zirconium and Chromium Ions with MEVVA Source

ZTA (alumina toughened by 20 wt.% zirconia), hot-pressed silicon nitride (with totally 10 wt.% Y2O3 and Al2O3 as additives) and TZP (pressureless-sintered yttria stabilized zirconia) ceramics were implanted by various doses (5 × 1016 ions/cm2 ~ 1 × 1018 ions/cm2) of Ti, Zr, and Cr ions with a MEVVA (metal vapor vacuum arc) source implanter. The bending strength of these ceramics was investigated. It was discovered that, for different ceramics, different behaviors were presented with the same doses of implantation ions. For alumina and zirconia ceramics, the bending strength increased with increasing implantation doses of Ti and Zr ions, but decreased with high dose of Cr ions. For silicon nitride ceramics, however, the bending strength originally increased with smaller doses of metals implanted, and decreased with higher doses of metals of Ti, Zr, and Cr ions. The different behaviors are correlated to the different variations in compositions and microstructures of ceramics after ion implantation.

Solid particle erosion of hot-pressed silicon nitride

Solid particle erosion of hot-pressed silicon nitride

Anisotropy of destruction viscosity of hot-pressed silicon nitride

The paper discusses the study of destruction viscosity and hardness of silicon nitride samples produced by hot pressing from powder and rolled ribbons. Anisotropy of crack resistance and a crystallographic texture are identified in both types of samples. The studies establish that the destruction viscosity in the direction parallel to the hot pressing force is higher than in the perpendicular direction. Acomparative analysis of anisotropy reveals its weak dependence on preliminary rolling.

Hot Pressing of Y<sub>2</sub>O<sub>3</sub> Doped Si<sub>3</sub>N<sub>4</sub> Ceramics

The effect of seeding on the microstructure and mechanical properties of hot-pressed silicon nitride was investigated by introducing large, elongated b-Si3N4 particles into an α-Si3N4 matrix. Densification and fracture toughness were followed in the presence of Y2O3 as sintering aid. The concentration of seeds varied from 0 to 5 wt%. The maximum fracture toughness was 8.0 MPam1/2 for a material with 3 wt% seeds, hot pressed at 1750oC, for 4h.

Observation of delamination wear of lubricious tribofilm formed on Si 3N 4 during sliding against WC-Co in humidity air

Hot pressed silicon nitride that was exposed to high (90%) and low (32%) relative humidity was examined in ball-on-disc geometry against cemented carbide ball at various normal loads. The study indicated that Si 3N 4 tested at high R.H. gave less specific wear rate compared with Si 3N 4 at low R.H. The friction coefficient of Si 3N 4−WC-6% Co tribopairs was found in the range of 0.32–0.39 and 0.05–0.17 at low humidity and high humidity respectively. It is suggested that adsorbed moisture markedly affected the wear and friction properties of silicon nitride. Following the tests, SEM was used to elucidate the wear mechanism and particularly to delineate the effects of relative humidity on the wear and friction. SEM micrographs showed that the main wear mechanism at low relative humidity (32%) was caused by mechanical wear including abrasive grooves, large holes and polishing, whereas at high relative humidity (90%) the main mechanism was highly influenced by a tribochemical reaction related to the moisture adsorption from the environment. It is concluded that the removal of lubricious tribolayer was occurred by delamination induced crack propagation.

Metal–ceramic interfaces: joining silicon nitride–stainless steel

Joining of hot pressed silicon nitride using three types of stainless steel (AISI 304, 316 and 321) as interlayer was done by diffusion bonding at 1100°C for 120min. An extensive reaction zone of about 7μm was formed in the contact region, where Cr2N, FexSiy and α-Fe were observed, outside that region the austenitic phase with precipitates of chromium nitride was observed. In the Mo-containing stainless steel (AISI 316) formation of Mo3Si was also detected. Moderate strengths were measured by shear testing for these joints.

Erosion of a reaction bonded silicon nitride

Erosion of a reaction bonded silicon nitride

Mechanical and Thermal Properties of Hot-Pressed Si<sub>3</sub>N<sub>4</sub> with Added AlN

As hot pressing silicon nitride, sintering additives such as MgO, Al 2 O 3 and Y 2 O 3 are generally used. The use of sintering additives changes the physical properties of hot pressed silicon nitride markedly. In the present paper, silicon nitride was hot pressed with various amount of AIN as a sintering additive. Also mechanical properties such as density, hardness, Young's modulus, fracture toughness, flexual strength and thermal conductivity of hot pressed silicon nitrides were measured with regards to the content of AIN. Also microstructure such as grain-boundary phase was observed using a TEM. The effect of α/β phase fraction on the mechanical and thermal properties of silicon nitride was investigated as a function of the amount of AIN additive.

Hot‐Pressed Silicon Nitride with Lu2O3 Additives: Oxidation and Its Effect on Strength

The oxidation behavior and effect of oxidation on room‐temperature flexural strength were investigated for hot‐pressed Si3N4 ceramics, with 3.33 and 12.51 wt% Lu2O3 additives, exposed to air at 1400° and 1500°C for up to 200 h. Parabolic oxidation behavior was observed for both compositions. The oxidation products consisted of Lu2Si2O7 and SiO2. The Lu2Si2O7 grew out of the surface silicate in preferred orientations. The morphology of oxidized surfaces was dependent on the amount of additive; Lu2Si2O7 grains in the 3.33 wt% composition appeared partially in a needlelike type, compared with a more equiaxed type exhibited in the 12.51 wt% case. The high resistance to oxidation shown for both compositions was attributed to the extensive amounts of crystalline, refractory secondary phases formed during the sintering process. Moreover, after 200 h of oxidation at 1400° and 1500°C, the strength retention displayed by the two compositions was 93%–95% and 85%–87%, respectively. The strength decrease was associated with the formation of new defects at the interface between the oxide layer and the Si3N4 bulk.