uhtc ppt
TRANSCRIPT
Development of Ultra High Temperature Ceramics for Aerospace Applications
Dr. V JayaramSSCU Department
Indian Institute of Science Bangalore - 560012
Outline
• Fundamental Aeronautics Program
• UHTC background
• Current experimental approaches– Morphology and composition– Grain boundary phases
• Summaries and conclusion
Fundamental Aeronautics Program• Long-term, multidisciplinary investment in critical research of core
areas in aeronautics technology– Evaluate new concepts and technology– Accelerate new technology applications– Not tied to specific vehicle/mission, but to tool development
• Hypersonics element covers all hypersonic regimes – Planetary missions (crewed and probes) – LEO (including commercial access to space)
• Ames materials effort addresses wide range of vehicle types
MER Orion SHARP Shuttle
Materials Development Approaches
• Comprehensive readiness versus specific application
• Families of materials — trade space for rapid tailoring to mission needs:– Consistently desired properties:
• Strength• Thermal conductivity
– Properties defined by mission
• Goal for all TPS materials is efficient and reliable performance during entry
Sharp Leading Edge Technology
• For enhanced aerodynamic performance– Improvements in safety
• Increased vehicle cross range• Expanded launch window with safe abort to ground
– Applicable to out-of-Earth-orbit missions• Aerogravity assist missions (solar exploration at Venus)• Accurate placement of probes where rapid deployment is necessary
• Require materials with significantly higher temperature capabilities– Current shuttle RCC leading edge materials: T~1650°C– Materials for vehicles with sharp leading edges: T>2000°C
• UHTC compositions are candidate materials
Sharp Leading Edge Energy Balance
Sharp Nose
Ý q conv Ý q rad Ý q cond
UHTC
High Thermal Conductivity
• Insulators and UHTCs manage energy in different ways:– Insulators store energy until it can be eliminated in the same way it entered– UHTCs conduct energy through the material and re-radiate it through cooler surfaces
UHTCs for Sharp Leading Edges
• Properties required– High thermal conductivity (directional)– Fracture toughness/mechanical
strength/hardness – Oxidation resistance
• Current approach– Combining our experimental process with
computational methods to achieve desired property improvements
– Exploring the design space (processing/ properties)
Previous Work on UHTC’s
• Optimization tied to particular vehicle development• Required early selection of baseline material — hot
pressed HfB2/ 20v%SiC • Focused on improving homogeneity and characterization
of properties• SiC
– Required for processing densematerial
– Promotes refinement ofmicrostructure
– Decreases thermal conductivityof HfB2
– >20v% may not be optimal for oxidation behavior
Current Experimental Approaches• Approach 1: Morphology and composition
– High-aspect-ratio grains– Growing SiC acicular grains– Processing approaches
• Hot-pressing • Spark Plasma Sintering (SPS)
• Approach 2: Grain boundary phases– Third-phase additions– Powder-coating– Processing approaches
• Hot-pressing• Spark Plasma Sintering (SPS)
Morphology and Composition
• Acicular grains for mechanical improvements
• Conventional source for SiC is powder
• Preceramic polymer route– Preceramic polymer will affect densification
and morphology – May achieve better distribution of SiC source
through HfB2– Previously added Si3N4 preceramic polymer
to Si3N4 powder to promote formation of acicular Si3N4 grains*
Effects of SiC Precursor Amounts on Final Microstructure
• Samples processed with 5 to >20 volume % SiC• Can adjust volume of SiC in the UHTC without losing the high l/d
architecture• Amount of SiC affects number and thickness (but not length) of
rods — length constant (~20–30m)
Microstructures at Higher SiC Loadings
• 3D interconnected network of SiC observed at > 20% SiC volume fractions
• Evidence of HfB2 grains trapped in SiC high-aspect ratio grain• Majority of SiC coalesced and formed larger grains — some finer
acicular SiC grains still evident • High aspect ratio architecture of the SiC phase is preserved
Coalesced high aspect ratio SiC
High aspect ratio SiC phase
Effect of Heating Schedule on Formation of Acicular Grains
Heating schedule 1 results in limitedhigh aspect ratio phase
Heating schedule 2 yields larger volume fraction of high aspect ratio phase
Investigating the range of possible microstructures
Comparison of Processing Methods:SPS and Hot Pressing
Materials processed by SPS
10% SiC 10% SiC
Materials processed by hot-pressing
Spark Plasma Sintering (SPS) results in a very refined microstructure — no evidence of acicular grains
Large Grain Growth
Poorly processed HfB220v%SiC
100 m
Areas deficient or rich in SiC result in large grains of HfB2 or SiC. This behavior is echoed in the preceramic polymer work.
Large HfB2 agglomerate Large SiC-rich agglomerate
SiC derived from polymer
5% SiC>20% SiC
Arc Jet Testing
Provides sustained conditions that simulate aerothermal environment of reentry for understanding thermal performance of materials and systems under controlled heating conditions
SiC Depleted During Arc Jet Testing
SiC Depletion LayerSiC Depletion Layer
OxideLayer
SiC DepletionLayer
qCW = 350 W/cm2, Pstag = 0.07 atm
• In baseline material, SiC depleted during arc jet testing — amount of SiC near percolation threshold
•Preceramic polymer route possible way to achieve good mechanical properties and lower amounts of SiC
Post-test arc jet model
ZrB2-SiC System
Nominally 15% SiC high aspect ratio phase in a ZrB2 matrix
Preliminary work on the ZrB2-SiC system indicates possibility of obtaining high aspect ratio SiC phase.
Third-Phase Additions
• Explore effect of additional refractory phases on oxidation resistance / fracture toughness (ductile-phase toughening)*
• Investigate additions of refractory metals
• Focus on– Effect of additives on microstructure– Evaluation of thermal conductivity– Evaluation of mechanical properties
Third-Phase Additions
• Processing and compositions
• Two different hot pressing schedules
• SPS an alternative consolidation approach — short processing times
• Two variants of baseline material (HfB2-20 v% SiC):– Ir– Ir with TaSi2
Sample Consolidation Process
HfB2 SPS
HfB2-SiC
(baseline material)
SPS & Hot Press
HfB2-SiC-Ir Hot Press
HfB2-SiC-TaSi2-Ir
Hot Press
Effect of Processing Parameters on Microstructure
HfB2/20v%SiCHot Pressed (Schedule 2)
HfB2/20v%SiC Spark Plasma Sintered
• Schedule 2 results in finer grain structure than Schedule 1.
• SPS results in finer grain structure than hot pressing.
HfB2/20v%SiCHot Pressed (Schedule 1)
Effect of Additives on MicrostructureHfB2-SiC (hot press)
HfB2-SiC (SPS)
Addition of Ir and TaSi2
Similar microstructureSimilar microstructure
Addition of Ir
Samples processed withadditional phases show
less grain growth
HfB2-SiC-TaSi2-Ir (hot press)HfB2-SiC-Ir (hot press)
Density and Hardness
Sample ProcessDensity(g/cm3)
Density(% Theoretical)
Vickers Hardness
(GPa)
HfB2-SiC HP—schedule 1 9.6 100 16.5
HfB2-SiC SPS 9.6 100 20.3
HfB2-SiC HP—schedule 2 9.6 100 17.8
HfB2-SiC-Ir HP—schedule 2 9.9 100 18.3
HfB2-SiC-TaSi2-Ir HP—schedule 2 9.7 100 18.8
Hardness increases with: • Processing route — SPS processing and hot pressing schedule 2 are beneficial.• Additional phases
Thermal Conductivity
0
20
40
60
80
100
120
140
0 100 200 300 400 500 600 700
Temperature ( oC)
Con
du
ctiv
ity (
W/(m
*K
))
Pure HfB2
SPS
HfB2-SiCHot PressMethod 1
HfB2-SiCSPS
HfB2-SiCHot PressMethod 2
HfB2-SiC-TaSi 2-IrHot Press - Method 2
HfB2-SiC-IrHot PressMethod 2
SHARP-B2
• Schedule 1 hot pressing — lowest thermal conductivity• Schedule 2 hot pressing — significant increase in thermal conductivity• SPS — similar increase in thermal conductivity• Addition of Ir or Ir and TaSi2 to HfB2/SiC (modified HP) — lowers thermal conductivity
Powder-Coating Approach• Advantages of coatings over particles to
introduce additives:– Uniformly distribute and control coating composition– Bypass traditional sources of processing
contamination– Improve oxidation and creep resistance (less O2
contamination)– Control thickness (amount of additive)– Reduce hot-press temperature, pressure, and time
• Use of fluidized bed reactors to deposit controlled, thin, adherent, uniformly dispersed coatings (HfB2, ZrB2, SiC).
Fluidized Bed Reactor — Chemical Vapor
Deposition Technique (FBR-CVD)
Quartz Frit
450 kHz Copper Induction Coil
Reactant Gases
UHTC Fluidized Powder Bed
Vent
Examples: HCL + CH4 or TiCl4 or SiCl4
Quartz Reactor
Fluidizing Gas(Ar)
Summary: Morphology
• Forming acicular SiC grains in both HfB2 and ZrB2 by adding preceramic polymer
• Adjusting volume of SiC in UHTC without losing high aspect ratio grains
• Processing samples with 5 to >20 volume % SiC from polymer:– Amount affects diameter of acicular grains, but not length– At >20% groups of interconnected acicular grains form
• Processing method affects formation of acicular grains• Modified microstructure does not have significant effect on hardness• Mechanical properties in preliminary results:
– Comparable fracture toughness in reinforced systems with lower SiC volume fractions
– Implications for oxidation behavior — arc jet testing for verification– Indications that acicular SiC phase is improving toughness
Summary: Grain Boundaries
• Addition of Ir and of Ir with TaSi2 to baseline material appears to:– Further improve the microstructures of hot-
pressed materials (SPS more refined and more marked effect on hardness)
– Reduce thermal conductivity• The FBR-CVD technique:
– Can be used to deposit controlled uniformly-dispersed phase additions
– Avoids grain boundary contaminants introduced during mixing and milling operations
Conclusion
• Exploring large design space has yielded potential for tailoring material for both:– Comparable or improved mechanical properties – Good oxidation behavior in entry conditions
• Future directions: – Continue modification of morphology, composition,
and grain boundaries to understand influence on properties
– Modeling/computation for efficiency in experiment
– Arc jet testing to evaluateperformance in relevantenvironment