The manufacturing and application of nano-sized silicon nitride powders are emerging as one of the fastest growing segments of the contemporary ceramic industry. Nanoparticles are defined as particles that have at least one dimension less than 100 nm. Given their small size, nanoscale particles have a high proportion of atoms close to their surfaces. Hence, strong deviations from the behavior of the bulk structure including chemical composition and reactivity are to be expected. The size also affects other important bulk properties based on the restriction on wavefunction radius, separation of lattice defects, and interacting strain fields [ 20 ]. To obtain commercial use, nanoscale silicon nitride requires the attainment of a complex set of properties that are listed in .
You can find more information on our web, so please take a look.
The synthesis of silicon nitride powders can be achieved in several ways that involve various silicon carriers, such as elemental silicon, silicon dioxide, gaseous silicon tetrachloride and monosilane, as well as organo-silicon precursors [ 19 ]. The routes toward silicon nitride powders include the following:
In addition, highly mechanically resilient textured sintered silicon nitride (TSSN) monolithic shapes were developed. The α-Si 3 N 4 phase stable at low temperature converts to β-Si 3 N 4 at the high sintering temperatures applied during the densification of powder compacts. The rod-like β-Si 3 N 4 particles yield by so-called texturing a self-reinforcing microstructure with greatly enhanced flexural strength and modulus of elasticity ( ). Seeds of rod-like β-Si 3 N 4 crystals are added to the raw powder consisting predominately of α-Si 3 N 4 . These crystals are aligned by hot working techniques such as hot pressing, hot forging, and sinter forging, or template grain growth involving alignment by cold pressing, extrusion of a slurry, tape casting, and static or rotating magnetic fields. The aligned β-Si 3 N 4 seeds grow during subsequent annealing at high temperature into a reinforcing microstructure [ 25 ]. Although there is a tendency that large, elongated grains act as structural flaws that reduce flexural strength, careful tailoring of size, amount, and orientation of grains can alleviate this drawback [ 26 ].
To reduce the final porosity of RBSN and thus to further improve its mechanical and elastic properties, SRBSN has been developed. More recent developments of sintered reaction-bonded silicon nitride (SRBSN) have shown improvements in toughness and impact resistance through the growth of crack-arresting or -deflecting rod-like silicon nitride grains [ 23 ]. In addition, depending on the nature of the oxidic sintering aids and the processing condition, high thermal conductivities can be attained [ 24 ]. The highest values of thermal conductivity were found by adding AlN to Y 2 O 3 as a sintering aid that together with impurity oxygen dissolved in the lattice of β-Si 3 N 4 and formed a β-SiAlON solid solution, i.e., Si 6&#; z Al z O z N 8&#; z . Increasing the sintering time increased the fracture toughness [ 23 ], as shown in .
To obtain fully dense sintered silicon nitride (SSN) monolithic bodies, hot pressing or hot isostatic pressing of silicon nitride powder with added metal oxides is employed. The oxides most often used as sintering aids are magnesium or yttrium oxides. At temperatures above °C, these oxidic additives form with contaminant silicon dioxide films around individual silicon nitride grains a liquid siliceous binder phase of SiAlON (Si 6-z Al z O 2 N 8-z ) composition in which silicon nitride readily dissolves. This intergranular, essentially glassy binder phase leads to the efficient densification of the sintered ceramic body. However, some precautions must be taken to limit the thermal decomposition of the silicon nitride or the loss of the additives by partial evaporation. To avoid this, the compact is sintered in a bed of silicon nitride powder and/or under a high-pressure nitrogen atmosphere (typically 1 to 8 MPa). The protective atmosphere suppresses the evaporation of silicon, nitrogen, and additives. To obtain fully dense SSN, the use of nanoscale silicon nitride powder and high temperatures of to °C are advised.
Since a volume increase of about 22% is completely accommodated by the inter-particle void space of the compacted silicon powder, no shrinking occurs during nitriding. Hence, the original dimensions of the green ceramics will be faithfully retained. This is the reason for using RBSN to shape components with complex geometry. Since the processing route does not require expensive diamond grinding, parts manufactured from RBSN are economically highly competitive. Although the porosity of RBSN ranges from 20 to 30%, its mechanical performance is remarkable as flexural strengths in the range of 200 to 400 MPa can be achieved. This strength level can be retained to about °C, so that designing with RBSN leads to monolithic ceramics with high Weibull moduli and associated high reliability in service.
To obtain RBSN monolithic bodies, direct nitriding of fine compacted elemental silicon powder is employed that leads to the formation of a mixture of α- and β-Si 3 N 4 by prolonged heating in nitrogen or ammonia atmospheres at temperatures up to °C. The fine silicon powder is nitrided predominately by a vapor phase reaction [ 2 ] that involves the oxidation of gaseous Si(g) to form gaseous silicon monoxide SiO(g) according to Equation (1). This SiO further reacts with nitrogen to form silicon nitride according to Equation (2).
The silicon nitride powders synthesized by one of the routes mentioned above are further processed to form shaped compacts. Depending on the processing routes, there exist several structural and textural variants of silicon nitride that differ markedly in their mechanical, elastic, and thermal properties ( ). The variants most frequently used in engineering applications are porous reaction-bonded silicon nitride (RBSN) [ 21 , 22 ], dense sintered silicon nitride (SSN), as well as a combination of both, sintered reaction-bonded silicon nitride (SRBSN). These processing routes are schematically shown in . Whereas the synthesis of RBSN and SRBSN starts from silicon powders that are sintered in argon prior to nitriding, the production of SSN avoids this reaction step. Furthermore, all three types of silicon nitride require machining of the final parts, but RBSN does not require diamond grinding, making parts fashioned from RBSN economically highly competitive.
For example, gel casting of Si-PMMA suspensions and subsequent nitriding and sintering produced SRBSN foams with a porosity up to 80 vol% [ 37 ]. The foams were found to be whisker-free and showed high pore interconnectivity after sintering. The foam struts consisted of β-Si 3 N 4 grains that were embedded in an amorphous phase. Biocompatible silicon nitride foams may have potential biomedical applications that include key components for tissue substitution for in vitro and in vivo tissue engineering applications, including scaffolds for bone reconstruction [ 29 ] and for biosensing and medical diagnostics [ 38 ]. Based on the remarkable strength of silicon nitride, the application of silicon nitride foams loaded with polymers such as poly(methylmethacrylate, PMMA), poly(etheretherketone, PEEK), or poly(lactic acid, PLA) as intervertebral disk implants may be envisaged.
Porous silicon nitride [ 21 ] excels by a unique combination of various mechanical, thermal, chemical, and electric properties such as its light weight, high strain and damage tolerance, high hardness and wear resistance, low thermal expansion coefficient, high-temperature resistance, good thermal shock resistance, oxidation and corrosion resistance, good biocompatibility, and favorable dielectric properties. Its fabrication routes include pressureless sintering [ 27 ], freeze drying [ 28 ], gel casting [ 29 , 30 ], the addition of sacrificial pore formers [ 31 ], the use of replica templates [ 32 ], three-dimensional printing [ 33 ], slip [ 34 ] and tape casting [ 35 ], microwave sintering [ 36 ], and other techniques.
Surface coatings have emerged as important tools of increasingly sophisticated surface engineering technologies. During the past decades, a reliance on functionality and economic considerations have propelled coatings into the limelight of materials engineering. Cheap but mechanically and thermally strong substrates are being functionally improved, protected, and refined by adding expensive but low-volume materials with exceptional properties that render the tandem substrate coating a synergistic success.
Consequently, many attempts have been made in the past to design silicon nitride coatings. However, the deposition of pure, i.e., unalloyed silicon nitride coatings by conventional thermal spraying techniques has been found to be impossible since Si3N4 does not melt congruently but decomposes and in turn sublimates above °C. It also oxidizes readily at elevated temperatures.
On the one hand, thin amorphous silicon nitride films are successfully deposited by chemical vapor deposition (CVD) and applied as masking layers for semiconductor integrated circuits during profile etching, as diffusion barriers in very large-scale integrated production lines, and for the damage protection of optical fibers. Thin silicon nitride films were also deposited by radio-frequency magnetron sputtering [39] and other synthesis methods.
On the other hand, attempts to deposit mechanically stable thick silicon nitride coatings for non-electronic applications by thermal spraying using metallic [40] or silicate glass binders [41], conversion by a reactive spray process [42], or in situ nitridation in flight [43] were not very promising. It turned out that such coatings contain only a small percentage of unaltered silicon nitride but instead substantial amounts of embrittling metal silicides. More successful were attempts to deposit coatings with enhanced Si3N4 content starting from SiAlON precursor powders [44] or clad-type powder consolidation using alloy bond coats [45].
However, later it was recognized that high particle velocities generated by detonation spraying, Top Gun&#; technology, high-frequency pulse detonation, or atmospheric plasma spraying with axial powder injection were essential to deposit dense and well-adhering coatings with high silicon nitride contents [46,47]. In addition, the optimization of heat transfer into powder particles to control the critical viscosity of the oxide binder phase was found to be one of the most decisive factors [48] that requires sophisticated powder preparation procedures [49].
If you want to learn more, please visit our website Zmdy Ceramics.
Extremely high powder particle velocities up to 3 km/s, generated by an electromagnetically accelerated plasma (EMAP) [50], were applied to deposit dense, homogeneous silicon nitride coatings with desirable mechanical properties that adhere very tightly (adhesion strength > 75 MPa) to polished stainless steel surfaces [51]. The operating principle of EMAPs is shown in . The deposition system comprises an evacuated accelerating channel that forms a pair of parallel electrodes connected to a high current power source by a switch. A separate vessel contains a pressurized gas and the powder mix. The process is initiated by activating a fast-opening valve at the nozzle of the vessel ( a). After introducing the powder and the working gas into the accelerating channel ( b), an arc discharge is initiated at the desired position in the accelerating channel. During discharge, the powder remains suspended in the accelerating channel ( c). The plasma at the arc initiating point then receives an electromagnetic force pulse and forms an electromagnetically accelerated plasma of the working gas that heats and propels the powder particles with supersonic speed towards the substrate ( d,e).
shows dense and mechanically highly stable silicon nitride coatings on stainless steel substrates deposited by detonation (A) and EMAP spraying (B). Both types of coating are dense, although the porosity of the detonation-sprayed coating appears to be higher, presumably related to the comparatively large grain size range of +32&#;45 µm, in contrast to the powder with a grain size range of +8&#;20 µm used for EMAPs. It is remarkable that the EMAP-sprayed coating did not require substrate roughening to adhere very well. Indeed, the SUS304 steel substrate was mirror-polished prior to spraying to yield an adhesion strength exceeding 75 MPa ( B). In contrast, the detonation-sprayed coating needed roughening that resulted in the undulating substrate&#;coating interface shown in A.
The powders used for detonation and EMAP spraying were synthesized by mixing commercial α-Si3N4 powder with oxidic sintering aids such as alumina and yttria, agglomerated by spray drying using an organic binder and subsequently sintered at °C in a nitrogen atmosphere [46,51]. The powders used for detonation ( A) and EMAP spraying ( B) contained 68 mass% Si3N4 + 16 mass% Al2O3 + 16 mass%, whereby the volume of the oxidic binder phase comprised 30%. The approximate composition of the resulting SiAlON binder phase was found to be Si3Al3O3N5 (z = 3) [46] and Si5AlON7 (z = 1) [51] for detonation- and EMAP-sprayed coatings, respectively.
The complex high-temperature reactions are shown in Equations (3) and (4). During spraying, silicon nitride reacts with the oxides of the binder phase at grain boundaries according to
Si3N4 + Al2O3 + Y2O3 &#; ß&#;-Si6&#;zAlzOzN8&#;z + Y-Si-Al-O-N glass (I)
(3)
During cooling, glass (I) devitrifies to form crystalline yttrium aluminum garnet (YAG) and glass (II). For example, for z = 1 this reaction can be expressed by
Si5AlON7 + glass(I) &#; Si5+xAl1&#;xO1&#;xN7+x + Y3Al5O12 + Si-O-N glass (II).
(4)
It ought to be mentioned that Equations (3) and (4) are mere simplified representations of the complex high-temperature reactions occurring in the ternary system Si3N4-Al2O3-Y2O3, and that the actual reaction process is thought to be dependent on several factors, including temperature, composition, grain size of the reacting constituents, and porosity and pore structure, including interconnectivity.
shows the phase composition of EMAP powder #1 (top) and the resulting coating (bottom). The coating consists prominently of α-Si3N4, minor amounts of ß-Si3N4, and traces of YAG and SiAlONs with different degrees of substitution 1 < z < 4, predominately z = 1.
The high adhesion strength of the silicon nitride coatings to the mirror-polished stainless-steel substrate is related to the high-velocity impact of solid particles at the substrate surface that causes a mechanism akin to explosive cladding [52]. Solid or only partially molten particles impacting the substrate with supersonic velocity generate a shock wave that propagates into the substrate. Since the P-V adiabat, i.e., the Rankine&#;Hugoniot equation of the state of the porous particle, lies above that of dense material, its thermal energy increases by compression of the porous material. This is accompanied by an increase in the thermal pressure component. The propagating shock wave causes multiple collisions among crystal grains within the porous particle, and thus a high local pressure is generated that causes additional compression, isentropic heating, and crushing of the particle [53].
However, as such EMAP coatings appear successful from a materials science viewpoint, from a process economy perspective they are in dire need of improvement. The excellent coating performance notwithstanding, the batch-type mode of EMAPs with the required replacement of the powder and gas feeding vessel after each shot renders this technique uneconomical at present. Hence, process scale-up and optimization must be carried out during future development cycles with the aim to provide a continuous injection mode of the pressurized gas and the powder particles.
Contact us to discuss your requirements of Silicon nitride ceramic processing. Our experienced sales team can help you identify the options that best suit your needs.