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Reaction Bonded Silicon Carbide: SFF, Process Refinement and Applications



Reaction bonded silicon carbide (RBSiC) has a wide variety of industrial applications and a manufacturing process based on Selective Laser Sintering (SLS) has been demonstrated in previous research at the University of Texas. That study was directed toward semiconductor manufacturing applications and was based on prior indirect SLS methods. Several key research questions were addressed for three main manufacturing phases: preform SLS, binder burnout and reactive infiltration. The current research is focused on development of material systems and manufacturing capability and is directed toward a broader set of potential applications. Preform formation utilizes SiC powder of an appropriate average particle size mixed with a multi-component binder. The preform or green part is then placed in a vacuum furnace to carbonize the binder. The details of the binder chemistry must support accurate SFF shapes and acceptable surface roughness, a strong green part and maintenance of the part shape during the first furnace operation. Finally, the physics and chemistry of the infiltration process, based on the microstructure of the initial green preform, determine the viability of the manufacturing process and the characteristics of the final composite material. The functionality of metal, polymer and ceramic matrix composites can support the growing SFF industry desire to move beyond functional prototyping and into manufacturing arenas. This project is being explored for more general application to matrix composite materials, especially highly functional systems tailored specifically for SLS. The goal is to establish the governing principles of binder function, carbonization and infiltration as well as to understand the interdependence of these phases in terms of manufacturing application. With this understanding new applications and special SLS composites can support the development of new products and a greater SFF manufacturing presence.
R. S. Evans, D. L. Bourell, J. J. Beaman and M. I. Campbell
Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712
Reaction bonded silicon carbide (RBSiC) has a wide variety of industrial applications and
a manufacturing process based on Selective Laser Sintering (SLS) has been demonstrated in
previous research at the University of Texas. That study was directed toward semiconductor
manufacturing applications and was based on prior indirect SLS methods. Several key research
questions were addressed for three main manufacturing phases: preform SLS, binder burnout and
reactive infiltration. The current research is focused on development of material systems and
manufacturing capability and is directed toward a broader set of potential applications. Preform
formation utilizes SiC powder of an appropriate average particle size mixed with a multi-
component binder. The preform or green part is then placed in a vacuum furnace to carbonize
the binder. The details of the binder chemistry must support accurate SFF shapes and acceptable
surface roughness, a strong green part and maintenance of the part shape during the first furnace
operation. Finally, the physics and chemistry of the infiltration process, based on the
microstructure of the initial green preform, determine the viability of the manufacturing process
and the characteristics of the final composite material.
The functionality of metal, polymer and ceramic matrix composites can support the
growing SFF industry desire to move beyond functional prototyping and into manufacturing
arenas. This project is being explored for more general application to matrix composite
materials, especially highly functional systems tailored specifically for SLS. The goal is to
establish the governing principles of binder function, carbonization and infiltration as well as to
understand the interdependence of these phases in terms of manufacturing application. With this
understanding new applications and special SLS composites can support the development of new
products and a greater SFF manufacturing presence.
This paper provides an introduction to the material, a look at basic rapid manufacturing
trends, an overview of the previous work, a review of relevant RBSiC material science issues,
and an outline of the current study.
Why Silicon Carbide?
Material Properties
SiC is an extremely hard, heat resistant, abrasion resistant, chemical resistant, and
thermally conductive material. However, it is very difficult to manufacture. Fully dense, sintered
varieties of SiC can cost $400 per cubic inch just for raw material. The powdered variety,
however, may be purchased by the boxcar for around $2.00/pound. Discovered over a century
ago, SiC has been the subject of extensive materials research for many decades [Taylor and
Laidler]. Over 140 microstructural variations of the material have been identified, each
associated with certain formation parameters and subtle property differences [Babula]. For this
paper, two main variations are important; high temperature a-SiC, formed above 2000°C and
relatively low temperature b-SiC with synthesis at temperatures as low as 1150°C, probably as a
precipitate or gas-phase deposition. Table 1, compiled from several standard sources, illustrates
several advantages of SiC material and puts it in context with aluminum and steel. As with all
ceramics, SiC is brittle. However, some fiber-reinforced SiC composites have shown promising
impact strength.
Material Density Tensile Modulus Flexural Strength
Thermal Cond.
Melt/ Soft Temp.
Silicon Carbide 3.1 kg/m
>400 GPa 550 MPa
120 W/m/K
RBSiC 2.9 kg/m
200-375 GPa 40-450 MPa
110 W/m/K
Aluminum 2.7 kg/m
62-70 GPa 240 MPa
150-210 W/m/K
Steel 7.8 kg/m
~195 GPa 750-2500 MPa
15-35 W/m/K
Table 1 : Material Properties (for basic comparison only)
Figure 1 : RBSiC from Laser Sintered Preform
Typical manufacturing processes for SiC are pressureless sintering, gas pressure
sintering, hot pressing, hot isostatic pressing, chemical vapor deposition, re-
crystallization and reaction sintering
-Suyama et al., p.1201
The temperatures, pressures and material preparation of these processes make them costly
and impractical for large scale operations. Reaction sintering (pursuing fully dense SiC) and its
close relative, reaction bonding (creating a matrix composite) as seen in Figure 1, are considered
a-SiC Initial Powder
Si Matrix Material
b-SiC Reaction Formed
the most promising fabrication strategies for SiC materials due to their net-shape capabilities,
speed and low cost [Suyama et al., Rajesh and Bhagat]. Suyama, et al., add that the process
imparts almost negligible dimensional changes. Figure 1 shows the main elements of RBSiC; a-
SiC from the starting powder, b-SiC formed during the infiltration process and remaining Si.
Additional details about reactive infiltration are discussed below. Several variations on these
processes are found in industry. The process employed by Poco Graphite (
involves machining graphite to final part dimensions and a chemical vapor infiltration process to
convert the carbon structure to SiC. Rohm and Haas (
employs a chemical vapor deposition process directly into SiC. Other companies create
SiC/carbon powder slurries which are molded into part shapes and directly sintered or reaction
Product Applications
A wide variety of SiC products exist today, as seen in Table 2, but not all of them
represent potential for SLS manufacturing. Many have simple shapes due to the difficulty
involved in making complex shapes with SiC. Others require fully dense SiC, rather than
RBSiC. Finally, those made using certain molding operations are mass produced, which leaves
SLS at a disadvantage. However, the ability of SLS to make nearly any shape could put SiC into
service for entirely new applications.
Turbine components
Burner Nozzles
Metal working equipment
DC Magnetron Sputtering
Faucet Washers
Mechanical Seal Faces
Pulp and Paper
Wear & Corrosion Resistant
Heat Exchanger Tubing
Valve & Valve Trim
High Temperature /
Thermal Components
Thermocouple Tubes/Prices
Kiln Systems
Posts and Rollers
Ball valve parts
Wear plates
Kiln furniture
Foundry equipment
Heat exchangers
Table 2 : Current Applications of SiC Components
Rapid Manufacturing
Rapid manufacturing (RM) may not soon replace mass production, but the economic
model advanced by Hopkinson and Dickens, as an example, indicates SLS is a viable alternative
to injection molding at product runs up to 14,000 units and with some lower cost RM materials
that number would increase significantly. The fit with foundries has been one of integration into
previously existing manufacturing standards [McDonald, et al., p.87]. The opportunity for
parallel fabrication in SFF also tends to favor smaller parts. Yet, complex parts even now are
more cost effectively produced via RM methods [Pham and Dimov]. As an example, the heat
exchanger assembly for the Pratt and Whitney PW6000 engine is manufactured using SLS to
create the shape of each part before casting in aluminum. The reduced costs of tooling, design
freedom and flexibility have been addressed widely in SFF literature, but what may be more
critical is the importance of materials research for realizing the anticipated impact of RM [Kai &
Fai. p.201, Hague et al.].
Rapid manufacturing will become more of a reality when the properties of the
materials that are produced become more acceptable and consistent. This
materials research is one of the main stumbling blocks to (RM)…
-Hague, et al., p.30
Even though SLS has the largest material set among SFF techniques [Ryder, et al.] there is a
long way to go before designers have the range of materials necessary to address a critical mass
of manufacturing tasks [McDonald, et al., p.240]. During this material development a move
from low-volume applications toward higher volumes could proceed with improved RM
processes, increasing the design of products to be made specifically by SFF techniques.
Previous Project Overview
The previous work was driven by a desire to leverage the capabilities of SFF to develop a
new fabrication scheme for RBSiC, particularly for the manufacture of wafer carrier boats. The
resistance of SiC materials to thermal shock and corrosion fits well with the challenging
constraints of high-temperature semiconductor processing. The coefficient of thermal expansion
(CTE) matches well with Si, reducing the particulate contamination and wafer defects when
compared to the more traditional quartz fixtures, and contributes to the estimated 20x service life
over quartz. Wang’s focus was on the physics and chemistry of pressureless liquid Si infiltration
of the porous preforms. She established the basic indirect process used to make SiC from SLS
performs; laser sintering a SiC powder mixed with binder compounds (green part), carbonization
of the binder (brown part) and reactive infiltration of liquid silicon. The novelty of this research
was the use of SLS for the creation of SiC performs for reaction bonding. The main
developments during Wang’s work were an examination of wicking mechanisms in porous
media, an analysis of the SiC forming reactions present and ultimately a fabrication process that
produced viable parts. Figure 2 shows a part made during the previous study.
Figure 2 : (a) A Rendering of a Slotted Horizontal Boat. (b) A Silicon Carbide/Silicon
Composite Boat Produced by Selective Laser Sintering and Silicon Infiltration. Also shown
is a 6-Inch Scale.
Roughly concurrent to the UT work was a study conducted in Germany
[]. The purpose
of that work was to create a high-SiC content final part. Although SLS was used to form the
performs several differences existed especially in initial powder and additional resin infiltrations,
presumably to enhance the carbon structure after carbonization. Figure 1 was taken from
information published about that work.
Several key questions from this previous work provide avenues of exploration for the
current research. First, the green strength of the preforms was barely adequate for handling the
parts and preparing them for the furnace operations. A viable manufacturing operation would
need a significant improvement in this area. The density of parts increased both with infiltration
temperature (1450-1600°C) and with dwell time (0.1,1 and 5hrs). The flexural strength was at a
maximum when infiltration was at 1500°C for 1hr and at 1600°C and 0.1hrs. Relatively low
strengths were found at all temperatures for the 5 hr dwell time. The greater final density of
parts that underwent longer infiltration dwell times suggests more SiC growth; however, this
does not necessarily correlate with better final part performance. The differing strength versus
temperature [Wang, p.134] indicates the likelihood that the shorter infiltration (0.1h) has not
developed significant regions where the b-SiC has established a fragmented structure, a
phenomenon discussed below. In other words, although the density of the parts increased, it is
possible that the microstructure supports a weaker final part. Although changing the particle size
is expected to improve surface roughness, as Wang suggests, it can have greater influence on
other parameters such as part strength or infiltration success. Finally, the coefficient of thermal
expansion (CTE) was found to be closer to silicon (~3.5x10
@ 500°C) than pure SiC
@ 500°C). The lowest CTE was from samples formed with the 5 hr dwell which
paradoxically suggests lower SiC content.
Successful development of the best composite materials will require deeper
understanding of the formation of SiC during infiltration. The structure of the “green part” and
the “brown part” after binder decomposition need to be improved and correlated to the structure
of the final material. Ultimately, the characteristics of the initial binder and SiC powder mixture
will need to be optimized for the best system of processing and final part characteristics.
Key Issues in New Research Effort
Binder Development
The polymer chemistry and function of each constituent in the previous binder system is
being examined. A new formulation is being developed based on the knowledge from the
previous work, a literature review of liquid infiltration and a focused research effort. The specific
elements of the binder study, including constituent chemistry, rheology and green and brown part
structures are expected to be published separately and are beyond the scope of this paper. It is
anticipated that the uniformity and consistency of the finished parts will be greatly supported by
this work. There is a correlation between surface conditions, particle sizes and the packing
density achievable in the part bed as may be inferred from the work of Paik et al. The final
strength of the material is related to this packing density as well as the dispersion of the
constituent materials. Further, the microstructure of the green preform is intimately related to the
final properties of the infiltrated part. In addition, this work will be a springboard for studying
the mechanics of unsupported, pressureless, reactive infiltration as well as the microstructural
elements that evolve through the entire SFF-based manufacturing process.
Material Formation
When liquid Si meets carbon it reacts to form SiC. When a porous (<200 µm pore size)
SiC preform is placed in contact with liquid silicon, the silicon fills the preform to at least a
height of 2m [Wang]. In the following paragraphs a few challenges are discussed that undermine
the simplicity of the previous statements.
A rather significant ~116kJ/mol is released when this reaction occurs [Rajesh and
Bhagat]. Wang observed a local temperature increase of 400°C. This heat of reaction, which
changes based upon the structure of the carbon, influences the formation of SiC and the
characteristics of the final parts. This combined with the significant changes in permeability
observed by Rajesh and Bhagat indicate an influence on wicking kinetics and therefore final
composite microstructure based on temperature at the onset of wicking. It may be possible to
tailor preform characteristics for more rapid or more effective infiltration. Enhanced SiC
formation from “solution reprecipitation” due to the increased exposure of C to Si melt which is
likely due to carbon diffusing from higher temperature regions of the Si melt and supersaturating
others. Favre et al., observed significant SiC growth at the liquid atmosphere boundary, away
from the interface between carbon and silicon which they attributed mainly to the diffusion of
to the SiC grains within the Si melt. The effects of reaction heating, dissolved carbon and
other transport phemomena need to be addressed.
A thorough understanding of the wetting characteristics and the driving capillary action
from the previous work coupled with a newly available model of transient permeability within
preforms for reaction-formed SiC materials [Rajesh and Bhagat] should provide insight into the
infiltration mechanics. If the Si is introduced to the preform as a melt, the main SiC formation
occurs within the first minute of contact and proceeds to a relatively final thickness of 10-12 µm
at carbon surfaces [Favre, et al.]. After this thickness has been reached further growth is
inhibited by the extremely low diffusion of carbon or Si through SiC. High compression forces
at the grain boundaries cause crystals to break away, causing periodic growth and local breaks
[Favre, et al.]. These breaks cause a sudden exposure of carbon to Si and also cracks in the
carbon surface itself. In small capillary channels the growth of SiC can choke off the subsequent
flow of Si. SiC growth can also occur in other regions where liquid saturation or gas-phase
(specifically Si
, SiO
CO and CO
) transport supports growth. The quick initial boundary
growth and the liquid surface growth are both promising SiC formation avenues, but must be
understood in practice.
Residual silicon has a detrimental effect on the mechanical properties and reliability of
the finished parts [Paik et al.] with fracture of RBSiC dominated by the failure of the Si matrix
[Fernandez et al.]. This is especially apparent in maximum temperature, acid resistance and
fracture toughness. The sintering study by Suyama et al. generated a remarkably strong sintered
material by, “controlling the residual Si size under 100 nm.” Fernandez, et al., also observed a
significant strength when a continuous SiC structure (from carbonized wood) was a feature of the
infiltrated material. This was corroborated by the work of Dyban where larger SiC particles
(~100µ) maximized the connection of SiC to SiC within the structure of the infiltrated material.
Again, it seems likely that a very capable material may be made, but what is possible within this
manufacturing strategy is unclear.
Basic Research Tasks
It is known that the infiltration reaction occurs and that fully dense parts may be created.
Yet, we need to explore the transient permeability, the local temperature effects, the reaction
kinetics, the kinetics of various chemical reactions, and the evolution of the composite
microstructure. These elements must be linked to the nature of the preforms that can be produced
via SLS. We want to create a final material system with an interconnected microstructure of SiC
that supersedes the limitations of a silicon matrix. One key to this may be in the tailoring of the
carbonized brown part which may help to create an interstitial b-SiC structure similar to that
formed in RBSiC created from charcoal as discussed by Fernandez et al. and others. This will be
challenging due to the complexity of SiC formation coupled with the need for a binder material
and initial powder to work within the constraints of SLS processing.
There is a desire to move from proof of concept to a far more refined process and better
finished product. Based on this greater understanding of each manufacturing phase we seek
methods of part design as well as a deeper understanding of this technology in terms of Rapid
Manufacturing. In other words, we want to tie the potential of the microstructure to a system for
understanding the tradeoffs across the manufacturing process and incorporating the design of
finished parts. More generally, we see the development of new materials as a key driver to the
future development of SFF technology in general and expect that this project will generate a
template for the more rapid deployment of additional powder-based composite materials in SLS
In this paper we have discussed SiC material, specifically in a reaction bonded form and
its applications. The previous work conducted at UT has provided a good foundation for the
current effort. The desire within the SFF industry to support more manufacturing presence will
require more extensive research into highly functional materials that may be manipulated
effectively into useful shapes. This particular material system is complicated but we are
confident that it will yield a viable manufacturing alternative for a variety of applications and
serve as a basis for additional SFF materials research.
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... Already in 2003 Evans et al. recognized the industrial interest for the AM of SiSiC for a wide range of applications ranging from armor ceramics to the semiconductor industry. [3] SiSiC materials are characterized by a high stiffness and strength, low thermal expansion coefficient and high thermal conductivity. A further advantage is that the shrinkage during silicon infiltration is close to zero, therefore a near net shaping process is possible, which is particularly relevant for AM parts. ...
... The density of LSD-printed green parts was 2.11 ± 0.01 g/cm 3 . The density of LSD-printed and infiltrated SiSiC parts of various geometries was > 3.05 g/cm 3 . ...
... The density of LSD-printed green parts was 2.11 ± 0.01 g/cm 3 . The density of LSD-printed and infiltrated SiSiC parts of various geometries was > 3.05 g/cm 3 . This value corresponds to the internal standard at H.C. Starck Ceramics for isostatic pressed SiSiC parts. ...
The current work presents for the first time results on the Additive Manufacturing of SiSiC complex parts based on the Layerwise Slurry Deposition (LSD)process. This technology allows to deposit highly packed powder layers by spreading a ceramic slurry and drying. The capillary forces acting during the process are responsible for the dense powder packing and the good joining between layers. The LSD process can be combined with binder jetting to print 2D cross-sections of an object in each successive layer, thus forming a 3D part. This process is named LSD-print. By LSD-print and silicon infiltration, SiSiC parts with complex geometries and features down to 1 mm and an aspect ratio up to 4:1 could be demonstrated. The density and morphology were investigated for a large number of samples. Furthermore, the density and the mechanical properties, measured by ball-on-three-balls method, were in all three building directions close to isostatic pressed references.
... In the latter, the liquid phase binding the ceramic particles together can be either a sacrificial polymer or a metal that is melted and re-solidified. At the University of Texas in Austin, various authors have proven and expanded on polymer-binder assisted laser sintering of RBSC (Vail, Barlow, and Marcus 1993;Birmingham and Marcus 1993;Evans et al. 2003;Evans et al. 2005). Combined with LSI as a densification step, fully dense RBSC components were obtained (Stevinson, Bourell, and Beaman 2006). ...
Additive manufacturing (AM) technologies for technical ceramics are rapidly emerging. Many of these processes rely on a polymer binder-assisted printing approach followed by de-binding and furnace sintering for densification. However, the required de-binding step is long and sensitive, and the presence of densification shrinkage requires a compensation in design. This study explores laser powder bed fusion (LPBF) as an additive manufacturing method for full net-shaping of reaction bonded silicon carbide (RBSC) and reaction bonded boron carbide (RBBC). During LPBF, silicon is used as a structural binder instead of traditional sacrificial polymer binders. By combining this with liquid silicon infiltration (LSI) as a densification method, long de-binding times and densification shrinkage are avoided. This leads to a net-shaping, additive manufacturing process for RBSC and RBBC ceramics with high Young’s modulus (285 and 308 GPa respectively), flexural strength (220 and 168 MPa) and hardness (2045 and 2242 HV).
... All previous devices used straight channels, and further investigation into the influence of channel geometry was impossible due to traditional material and machining constraints. A new additive manufacturing (AM) method called in-direct laser sintering (LS) with metal infiltration (Evans et al. 2003) allows for the fabrication of the required narrow passages and complex shapes while maintaining compatibility with a combustion environment (Ernstberger et al. 1983). The resulting material is a ceramic-metal composite of siliconized silicon carbide (Si-SiC) with an operational temperature of up to 1414°C. ...
Combustion at small scales is inhibited by the large amount of external surface area leading to excessive heat losses and extinguishment. This limitation is diminished by transferring heat from the products to the reactants through solid surfaces leading to the development of heat recirculating reactors. A promising design is the counterflow configuration in which reactants in one channel are preheated by energy transferred through the wall from the products in the adjacent channel. In the lean equivalence ratio limit of the reactor, this arrangement encourages the stabilization of the flame. In the high equivalence ratio limit, the enhanced heat transfer may encourage blow-off. Therefore, it is important to understand the relationship between heat transfer and the operating range of the reactor. In this paper, the importance of the channel surface area-to-volume ratio is investigated analytically and experimentally in a mesoscale combustor. Reactors with different channel shapes were fabricated via additive manufacturing and studied. The change in heat transfer area affected the stable operating range, maximum temperature, and location of flame stabilization. The emission measurements showed low CO and NOx emissions. For all reactors, the stable range was limited by flashback when the burning rate exceeded the flow velocity and by blow-off when the burning rate was insufficient. This work quantifies the importance of heat transfer surface area to the operation of the counterflow reactor and guides the optimization of reactor design.
... This is a common defect in materials processed by RMI process. Previous studies attributed this phenomenon mainly to CTE mismatch between the formed SiC and the unreacted graphite [48,60]. The different trends in bending strength curves can be correlated to the different microstructure between samples which evidences the presence of Si-filled cracks with respect to those in which the crack is absent. ...
A novel microstructure of graphite-Si-SiC ceramics was successfully prepared by liquid silicon infiltration of graphite-based preforms; instead of using conventional methods, the reactive infiltration process was assisted by microwaves. The effects of microwave power variation on the microstructure and the mechanical properties of infiltrated materials were studied. X-ray diffraction and Raman investigations showed the presence of both unreacted graphite and Si in addition to SiC formed at their interface. The graphitic and silicon phases were separated by a SiC network, which results more homogeneous as microwave power was increased. The amount of SiC was found to be higher in function of the growing power level; a higher conversion of graphite into SiC yielded a more dense material. The bending strength measurements confirm this, showing higher values for the samples processed using a power level of 75% of the full power compared to those obtained with 30% and 60%.
... In recent years, some investigations have been conducted by using 3D printing technology to generate green parts, and then combining with LSI process to fabricate SiC based composite parts. Stierlen et al. [9] first employed selective laser sintering (SLS), a powder bed fusion 3D printing process, in order to fabricate porous preforms using the mixture of SiC powder and reactive polymer binder, and subsequently infiltrated the preforms with liquid silicon to form reaction bonded SiC composites [10][11][12]. Tian et al. [13,14] used high carbon yield and photo-curable resin for stereolithography (SLA) 3D printing process to fabricate resin precursors, which are then pyrolyzed and converted to porous carbon preforms for LSI process. It has been a challenge for 3D printing to fabricate CFRP composites, especially with the polymeric matrices of high carbon yield, such as phenolic resin or other aromatic polymers [15]. ...
A novel method has been developed to fabricate carbon fiber reinforced SiC (Cf/SiC) composites by combining 3D printing and liquid silicon infiltration process. Green parts are firstly fabricated through 3D printing from a starting phenolic resin coated carbon fiber composite powder; then the green parts are subjected to vacuum resin infiltration and pyrolysis successively to generate carbon fiber/carbon (Cf/C) preforms; finally, the Cf/C preforms are infiltrated with liquid silicon to obtain Cf/SiC composites. The 3D printing processing parameters show significant effects on the physical properties of the green parts and also the resultant Cf/C preforms, consequently greatly affecting the microstructures and mechanical performances of the final Cf/SiC composites. The overall linear shrinkage of the Cf/SiC composites is less than 3%, and the maximum density, flexural strength and fracture toughness are 2.83 ± 0.03 g/cm³, 249 ± 17.0 MPa and 3.48 ± 0.24 MPa m1/2, respectively. It demonstrates the capability of making near net-shape Cf/SiC composite parts with complex structures.
... With powder bed fusion processes, both mixed powder/binder systems [e.g., 83,259] and slurry approaches [277] have been used. Ceramic AM parts may be post-infiltrated to create full density parts in lieu of high-temperature furnace post-sintering [100][101][102]. ...
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Critical to the selection requirements for additive manufacturing (AM) is the need for appropriate materials. Materials requirements for AM include the ability to produce the feedstock in a form amenable to the specific AM process, suitable processing of the material by AM, capability to be acceptably post-processed to enhance geometry and properties, and manifestation of necessary performance characteristics in service. As AM has matured, specific classes of material have become associated with specific AM processes and applications. This paper gathers this information for each of the seven categories of ISO/ASTM AM categories. Polymers, metals, ceramics and composites are considered. Microstructural features affecting AM part properties are listed. Service properties of AM parts are described, including physical, mechanical, optical and electrical properties. An additive manufacturability index is proposed.
High hardness and superior thermal stability of silicon carbide (SiC) are advantageous for engineering parts but they make it difficult to manufacture complex products with full density. Reaction-bonded SiC are effective solutions for the paradox. SiC-Silicon (Si) composite materials are manufactured by molten Si infiltration through porous SiC and/or Carbon (C) preforms owing to superior wetting of molten Si on SiC and reactive wetting of molten Si on C. Cold isostatic pressing of mixed powders feedstock, green machining and joining are a typical manufacturing pathway for green-body parts. Multiple processes with tools and significant materials loss are regarded as wastes from the viewpoint of lean manufacturing principle. In the present study, feasibility of binder jetting additive manufacturing (BJT AM) technology was assessed for green-body part manufacturing stage in the course of SiC/Si parts production. Adoption of BJT AM showed opportunities for process savings, tool savings, materials savings and design freedom. However, strength of brown-body parts as well as density of as-built green-body parts are challenges. To overcome the limitations, phenolic resin solution impregnation through as-built green parts was introduced. It is highlighted that green-body strength, brown-body strength and infiltrated-body strength are modified with evolutionary phase transformations from cured phenolic resin to reaction-synthesized SiC by way of decomposed carbon through the conventional post-AM densification manufacturing pathway.
Heat recirculating reactors have many potential applications as thermal oxidizers, combustors, and fuel reformers due to their extensive operating range. Low emissions and fuel flexibility make such devices highly desirable as heat sources as well as chemical reactors. The dependence on the solid/gas heat transfer implies that wall characteristics and operating conditions significantly influence the stable range. In this paper, the importance of various combustor parameters is examined through an analytical model, and an experimental reactor is fabricated from a new ceramic-metal composite using additive manufacturing. A full range of possible operation modes, from flashback to blow-off, was observed together with characteristic temperature distributions at various firing rates. The new combustor showed improved operational flexibility as compared to a traditionally assembled counterpart. Low CO and NOx emission levels were observed together with an audible sound in the range between 825 Hz and 1000 Hz. The combustor operated for over 70 hours without visible damage to the material. The overall thermal performance, low emissions, and high power density make the heat recirculating reactor a viable solution for combustion applications.
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This work is devoted to an analysis of the composition of a silicon carbide based ceramic brazed seam as a result of its interaction with rapidly quenched titanium-zirconium-niobium-beryllium filler metal. Structural-phase studies based on EDX and EBSD analysis, mechanical shear tests, and microhardness measurements of brazed joints were carried out. It was shown that titanium, zirconium, and niobium silicides as well as particles of titanium carbosilicides and silicon carbides in the silicon matrix are formed in the brazed seam, probably because of the presence of free silicon in the base material, which leads to increasing joint microhardness and unstable shear strength results.
Additive manufacturing (AM) of ceramics is in its infancy. Compared to conventional shaping processes, AM is more economical for small series and comes with a higher degree of design freedom. AM technologies like laser sintering can be used to produce preforms for reaction bonded silicon carbide (RBSC) parts. After laser sintering, liquid silicon infiltration is carried out and the preforms are transformed into dense RBSC parts, also known as Si-infiltrated SiC (SiSiC). These parts, however, contain residual Si which has a detrimental effect on mechanical and thermal properties. The current work focusses on creating an RBSC component with high SiC and low residual Si contents. This is done by impregnating the laser sintered preforms with a carbon precursor phenolic resin. The carbon provided by the resin reacts with the Si melt during infiltration to form a secondary (reaction formed) SiC. The result is a fully dense RBSC component with up to 75 vol% SiC.
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A thermal model of the interaction of pulsed near-infrared laser radiation from a Nd:YAG laser was made, taking the measured powder properties such as reflectance, optical penetration depth and thermal conductivity into account. It allows an estimation of the evolution of two different temperatures: the average temperature of the powder (taken over the grains in a volume given by the laser beam diameter and the optical penetration depth) and the temperature distinction within a single grain. It showed that in pulsed mode consolidation can be achieved at much lower average power as the surface of the powder particles are molten but their cores remain at nearly room temperature. This leads to a much lower average temperature and therefore a dramatic decrease in residual thermal stresses in the finished piece. The results of the model were experimentally tested and confirmed.
Rapid prototyping (RP) technologies that have emerged over the last 15 years are all based on the principle of creating three-dimensional geometries directly from computer aided design (CAD) by stacking two-dimensional profiles on top of each other. To date most RP parts are used for prototyping or tooling purposes; however, in future the majority may be produced as end-use products. The term ‘rapid manufacturing’ in this context uses RP technologies as processes for the production of end-use products. This paper reports findings from a cost analysis that was performed to compare a traditional manufacturing route (injection moulding) with layer manufacturing processes (stereolithography, fused deposition modelling and laser sintering) in terms of the unit cost for parts made in various quantities. The results show that, for some geometries, it is more economical to use layer manufacturing methods than it is to use traditional approaches for production in the thousands.
We have studied the effect of particle sizes in the starting powders in the silicon carbide ― carbon system, and also effects on the structure and phase composition of self-bonded silicon carbide (SBSC) that are associated with structural interactions between the starting components.
In a binary system of silicon carbide (SiC) and carbon black (C), various green bodies having different porosity and pore size distribution were prepared from suspensions with different dispersion conditions controlled by varying pH, and by the type and concentration of polyelectrolyte dispersant. The green bodies showing different green microstructures were fabricated by slip casting, and the reaction bonding process was then carried out on these green bodies at 1600 °C for 20 min under vacuum atmosphere using the infiltration process of molten silicon. SEM analysis was applied to investigate the effect of green microstructure on that of reaction-bonded silicon carbide (RBSC) and subsequently of which strength was measured. It was found that green microstructure was highly dependent on the dispersibility of starting powders, which influenced sintered microstructure as well as strength of RBSC. RBSC prepared from the well-dispersed suspension showed an average four-point flexure strength of 310±40 MPa, compared to the RBSC from the relatively agglomerated suspension showing that of 260±50 MPa. The bimodal microstructure, which is frequently seen in a typical RBSC, was not observed in the present study, indicating that the ratio between SiC and C was responsible for the formation of bimodal microstructure.
Reaction sintering is one of the most attractive manufacturing processes of silicon carbide (SiC), because of dense structure, low processing temperature, good shape capability, low cost and high purity. However, mechanical properties of reaction-sintered SiC (RS-SiC) were typically much lower than normal sintered one. Particularly, the bending strength was approximately 300 MPa. In this study, in order to develop the high-strength RS-SiC, effect of the microstructure on the bending strength was examined. The bending strength of RS-SiC was recognized to be increased with decreasing the residual silicon (Si) size, and high-strength RS-SiC has been newly developed. The strength over 1000 MPa was obtained to control the residual Si size under 100 nm. Some other properties of developed high-strength RS-SiC were also evaluated.
The presence of cubic β-SiC has been identified by X-ray photographs when graphite and silicon are heated together at temperatures as low as 1 150°C, and when vitreous silica is heated with graphite the carbide is formed at 1 450-1 475°C, probably by a vapour phase reaction. No matter how the starting materials may be varied in nature and in proportion, face-centred cubic carborundum is always formed, unless the temperature is in the region of the decomposition point. Near 2 000°C face-centred cubic carborundum begins to decompose, as shown by the presence of graphite lines in an X-ray photograph, and lines corresponding to the hexagonal modification II appear. Light green commercial carborundum shows faint graphite lines when heated at 2 000°C, while at 2 050°C it turns black and a large amount of macrocrystalline graphite is formed. The long soaking periods, high vapour pressure, and high temperature conditions occurring in commercial processes favour the formation of large crystals of modification II. The evaluation of the Debye-Scherrer photographs from commercial silicon carbides shows that the elementary tetrahedra which form the basis of the structures are slightly distorted, being expanded in the c direction and compressed in the direction of the a-axis. A possible wurtzite type, not so far reported in the literature, was looked for, but without success.
Reactive infiltration is a fast and cost-effective technique for manufacturing ceramic-matrix composites (CMCs). CMCs are used in elevated temperature applications like rocket engine casings, jet nozzles, gas turbine blades and nuclear cladding. There is an urgent need for minimizing experimental costs as well as optimizing process parameters during manufacture, so that we have minimized manufacturing costs and reduced infiltration times. Towards this end, the objective of this research was to develop an integrated micro-macro model of reactive flow of molten silicon in a porous preform consisting of carbon-coated silicon carbide fibers and then optimize process parameters computationally. The overall objective of the research was to arrive at a modified equation of Darcy's law for flow through a porous medium with the help of numerical/computational modeling. This paper deals with the flow of silicon through porous carbon at the macro level. The macro flow of silicon was integrated with an available micro model by determining the transient porosity from the micro model and using it in Darcy's law written for the macro flow of silicon. From the results of this study, we recommend suitable process parameters such as initial temperature of the solid reactant and the specific kind of reactants to be used for achieving complete infiltration. These conclusions are drawn after observation of the rate of decrease of permeability with more reaction.
A study is made of the effect of particle dimensions of the initial carbon and silicon carbide powders on the structure of compacts in the SiC carbon system. It is established that carbon can be distributed among the SiC grains by three different schemes: interstitial, substitutional, and displacement. The factors which determine the carbon distribution scheme are the relative concentrations of the components in the semifinished products and the dimensions of the particles of the initial powders. Specimens in which carbon is distributed in accordance with the interstitial scheme have the best characteristics.