uhtc ppt

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