2013_krenkel_xian_htcmc_8

CMC Braking Materials:

Current Status and Perspectives

Walter Krenkel

Ceramic Materials Engineering

University of Bayreuth

Germany

8th International Conference on High Temperature Ceramic Matrix Composites (HT-CMC 8)

September 22nd 26th, 2013 Xian, China

Courtesy of Porsche AG

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University of Bayreuth Lehrstuhl Keramische Werkstoffe

Development History and Fundamentals of Braking Performance

C/SiC Composites: Processing, Microstructure, Properties

Specific Requirements on Braking Materials

Design Aspects of Disk Brakes

Applications in High Performance Transportation Systems

Challenges and Outlook

Outline

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The brakes (along with the streering system) are the most safety-critical accident avoidance

components of a transportation system

Brakes convert the kinetic and potential energy of a vehicle into heat at the friction surface

Limits: 1. Product of tire normal force and tire-road coefficient of friction

2. Ultimate thermal stability of the

braking materials (rotor, pads)

Ventilated brake disks show considerably higher cooling effectiveness compared to

solid disks

Fundamentals of Braking Performance

Brake malfunctioning (London, 22.10.1895)

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Status quo: Grey Cast Iron Brake Disks

Local overheating of a metallic brake disk

Ten

sil

e s

tren

gth

Temperature

Th

erm

al

sh

oc

k r

esis

tan

ce K

Temperature

Crack

Hot spot

Established in series cars since more than 50 years

Casting process results in low costs

Worldwide production > 350 Mio. rotors per year

Limited thermal and corrosive stability

High density of 7.2 g/cm3

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Train Aircraft Automotive Elevator (emergency)

Crane (emergency)

ICE 1 Boeing 777 Porsche GT2 Schindler 700 Mayr roba-stop

Max. speed [m/s] 91.7 72.2 88.9 13.8 30

Mass [103 kg] 440 208 1.7 18 3.1

Deceleration [m/s2] 1.3 2.4 14.5

Brake energy [MJ] 1850 542 6.7 1.7 1.4

No. of brake disks 192 48 4 8 1

Energy per brake disk

[MJ]

7.21 4.52 / 203 1.7 0.21 1.4

Train (Knorr Bremse) Aircraft (Goodrich) Automotive (Ferrari)

Elevator (Schindler)

Crane (Mayr)

1 75 % of brake energy 2 40 % of brake energy 3 emergency (RTO)

Braking Energy of Different Transportation Systems

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Brembo SGL Ceramic Brakes GmbH

PANOX-based Aircraft Brake System

First use in aircraft (Concorde, 1970s) and racing cars

(Formula 1, 1980s)

+ Low density of less than 2 g/cm3

+ High mass-specific energy absorption

+ High thermal shock resistance

- Friction coefficient highly dependent on

temperature and humidity

Not usable for road vehicles

Carbon/Carbon Composites for Aircraft and Racing Cars

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1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012

Start of LSI

process

development

(DLR)

Development

of disks for

train brakes

(DLR, DASA) First C/SiC

brake pads for

passenger

cars (Basic

patents, DLR)

First prototypes

of ventilated

brake disks

(DLR)

Limited editions

in passenger

cars (Daimler)

Joint venture

Brembo/Daimler

Series

production of

pads (FCT)

Start of industrial

series production

(SGL Brakes/Porsche) Options in different

models of Audi, Ferrari,

Mercedes, Porsche, etc.

DLR SGL Brakes Brembo

Joint venture

Brembo/SGL

Schunk Kohlenstofftechnik

Development History of C/SiC Friction Pads and Disks

Standard

equipment in

premium cars

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Fiber Resin Additives (opt.)

Conditioning

Compounding/Mixing

Warm Pressing and Curing

CFRP

Pyrolysis

C/C

First Machining (opt.)

Joining (opt.)

Siliconizing

C/SiC

Final Machining

In-process coating (opt.)

Reaction zone Diffusion of

Si-atoms

F. Gern, Research Report DLR, 95-26

Infi

ltra

tio

n h

eig

ht [m

]

Melt-Infiltration of Silicon into C/C-Preforms (Three Step LSI-Process)

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SEM micrograph of a 2D fabric-reinforced

composite (cross section)

Composition 25.1% SiC

(by weight) 72.5% C

2.4% Si

Three different interphases

Fiber/Matrix CF-SiC (strong)

Fiber/Matrix CF-C (weak)

Matrix/Matrix SiC-Si (strong)

C-fiber

Amorphous

C-matrix

C/C-Segment

SiC

Residual silicon

embedded in SiC

Strong bondings in

the CF-SiC interphase

Weak bondings in

the CF-C interphase

LSI-C/SiC Composites with Different Interphases (No Fiber Coating)

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Continuous fibers (fabrics)

Density 1.8-2.3 g/cm3 SiC-fraction 25-50 %

Porosity < 6 % Flex. strength 130-290 MPa

Chopped fibers

Density 2.0-2.4 g/cm3 SiC-fraction 25-70%

Porosity < 5 % Flex. strength 65-140 MPa

Si

SiC

C-fibers

Longitudinal section

(friction surface)

Cross section

Longitudinal

section

(friction surface)

Cross section

Typical Microstructures of C/SiC Composites

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Short-Fiber C/SiC

(Sigrasic SGL) GG-20

Al-MMC (SiC-Particles)

C/C

Density kg/dm3 2.3 2.45 7.25 2.7 1.7 - 1.8

Mass-Specific Heat Capacity J/kg K 800 500 820 - 886 700

Volume-Specific Heat Capacity J/dm3 K 1800 3600 2350 1200

CTE (in-plane) 10-6 1/K 1 (RT)

2 (300 C)

9 (RT)

12 (300 C) 14 - 21 0.3

Thermal Conductivity (transverse) W/m K 40 54 160 - 185 13

Tensile Strength (in-plane) MPa 20 - 40 150 - 250 310 - 370 70 - 100

Youngs Modulus (in-plane) GPa 30 90 - 110 86 - 125 40

Bending Strength MPa 50 - 80 150 - 250

Strain (in-plane) % 0.3 0.3 - 0.8 0.4 - 1.2

Thermal Shock Resistance W/m > 27000 < 14000

Maximum Temperature C 1350 700 400 > 1350

Source: D. Neudeck, A. Wllner, H. Dietl: Bremsen mit nichtmetallischen Bremsscheiben, in: B. Breuer, K.H. Bill (editors), Bremsenhandbuch, 2006

Comparison of Braking Materials (Typical RT-Properties)

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Specific Requirements on Braking Materials for HP Transportation Systems

1. High thermal shock stability K (avoiding rupture and deformation)

2. High transverse thermal conductivity (low thermal stresses)

3. High and stable coefficient of friction (short stopping distance)

4. Low wear rates (lifetime extension)

5. High degree of freedom in the design (NNS manufacture,

short fiber reinforcement)

7. Novel inorganic pads (NVH improvement)

6. Strain-compatible joining techniques with the metallic substructure

(compensation of different CTE)

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'K

Courtesy of SGL Group, Germany

E

)1(R'K m

Thermal Shock Stability of C/SiC

Thermal shock parameter

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Specific Braking Power P/A = pv [W/m]

Low thermal conductivity results in high surface temperatures

and a decrease in the coefficient of friction

DLR Stuttgart

Effect of Low Transverse Thermal Conductivities

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Short Fiber Reinforced C/C-SiC Composites

20

22

24

26

28

30

20 30 40 50 60

Fiber Content (CFRP Stage) [Vol.%]

Tra

nsvers

e T

herm

al

Co

nd

ucti

vit

y

[W/m

K]

W. Krenkel

Adv.Eng.Mat., 2002

Thermal Conductivity (at 50 C) Versus Fiber Volume Content

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0,0

5,0

10,0

15,0

20,0

25,0

30,0

35,0

40,0

1,80 2,00 2,20 2,40

Density [g/cm]

Tra

ns

ve

rse

Th

erm

al

Co

nd

uc

tiv

ity

[W/m

K]

W. Krenkel

Adv.Eng.Mat., 2002

Thermal Conductivity (at 50 C) as a Function of Density

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C/SiC (HT fibers)

C/SiC (HM fibers)

C/SiC (high transverse fiber fraction)

C/SiC (high SiC fraction)

Average sliding speed m/s

Co

eff

icie

nt

of

fric

tio

n

Test conditions:

n = 3000 min-1

E = 145 kJ

p = 0.34 MPa W. Krenkel

Adv.Eng.Mat., 2002

Effect of Transverse Thermal Conductivity on CoF

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Rotor

Friction

surface

Stator

MPA Stuttgart

Test of C/C-SiC Disks in a High Energy Test Facility

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0 20 40 60 800

0,2

0,6

0,8

1

Time [s]

0 20 40 60 800

0,2

0,6

0,8

1

0 20 40 60 800

0,2

0,4

0,6

0,8

1

Time [s]

C/C-SiC

SiC

C/C

Coeff

icie

nt

of

Fri

cti

on

low energy

Performance of C/SiC composites as a superposition of the tribological behavior of SiC and C/C

W. Krenkel

Adv.Eng.Mat., 2002

CoF of C/SiC Composites (No Additives)

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- High surface temperatures result in higher wear rates

- SiC coatings are extremely wear-resistant

75

45 41

5

95

76 73

12

0

20

40

60

80

100

Standard C/C-SiC

(orthotropic)

C/C-SiC of

high conductivity

(orthotropic)

C/C-SiC with

optimized

fibre orientation

C/C-SiC with

CVD-SiC coating

Wear

[m

m/

MJ]

Disk

Pads

W. Krenkel

Techn. Keramische

Werkstoffe, 2000

Wear Behavior of C/SiC (Disk and Pads of Identical Materials)

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Silicon granulate

Intermediate carbon layer

(open porosity 40% - 95%) C/C-Substrate

Silicon granulate

RB-SiSiC layer

C/C-SiC Composite

Siliconizing

SiC boundary

layer

Silicon SiC Relaxation crack

T Tmelt

RB-SiSiC layer

36% wt. SiC

64% wt. Si

C/SiC substrate

SiSiC Coated C/SiC Composites (Process-Integrated Technique)

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radial cracks

randomly

oriented cracks

circumferentially

oriented fibers

randomly orien-

ted fibers

SiCralee-

Coating

Hub-

Attachment

Pads

Friction surface coated

with SiSiC

Uncoated Coated

W. Krenkel:

Ceramic Matrix Composite Brakes,

TECHNA, 2003

SiSiC Coatings Improve the Wear Stability

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friction layer

load-bearing body

(chopped carbon fibers, Si and

SiC matrix)

reaction bonded joint

Separate friction layer

Ansprechverhalten

B CA

diameter

Wear indicators in the friction layer

A: wear indicator arrangement

B: detail wear indicator

C: oxidized wear indicator

wear

indicator

friction

surface

Courtesy of Porsche AG, Germany

Friction layer and joining

Load-bearing body

Wear indicator

Wear Indicators in the Friction Surface

Wear indicator

Friction layer and joining

Load-bearing body

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SGL Group

The lower heat conductivity and heat storage capacity of C/SiC necessitate optimized cooling channels

Weight reduction of 30 50 % in unsprung mass compared to a typical gray cast iron disk

Internal Ventilation Requires High Degree of Freedom in the Design

Porsche AG

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First generation Current design

One-part design

Removable cores CFRP FORMING Pressing

T up to 250C

PYROLYSIS T up to 1000C

Inert gas

SILICONIZING

Si + C

T 1600C

Under Vacuum

SiC (Matrix) Internally ventilated

brake disk

Symmetric parts

Joining of pyrolized parts Cut-out ring

Solid brake disk

Prototype

Square plate

Brake disk with involute-

shaped cooling ducts

From Solid to Internally Ventilated Brake Disks

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Short Fibers (6mm)

Fiber Orientation

0 15 30 45 60 75 90 Mean

Value Isotropic

Flexural Strength

[MPa] 67 62 48 43 30 28 25 44 45,6

Youngs Modulus [GPa]

45 40 30 25 22 19 16 28 25

Strain [%] 0,17 0,16 0,17 n.a. 0,17 0,16 0,16 0,16 0,17

Flexural Strength

[MPa]

Fiber Length MV Max Min SD Var

3 mm ISO 43 57

6 30 30

6 mm ISO 44 57

8 80 26

9 mm ISO 88 115

12 197 63

Influcence of Fiber Length and Fiber Orientation on the 3-Pt-Bending Strength

Fle

xu

ral S

tren

gth

in

MP

a

90 oriented

67

46

25

0 oriented Random

distribution

Flex. Strength with 6 mm fibers

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The lower heat absorption of C/SiC results in an overall increase of the temperature at the contact surface

between disk and pads

The tribologically active layer (third body layer, TBL) significantly influcences the CoF

Replacement of organic-based NAO (non-asbestos) or Low-Met pad materials by ceramic materials

A. Stenkamp, Eurobrake 2013

Main components of a brake pad (m = 1.1 kg) Composition of a Low-Met pad for C/SiC brake disks

N. Langhof et al., The Tribological Investigation of C-fiber

Reinforced Ceramic Brake Pads Manufactured by Liquid

Silicon Infiltration (LSI) And Chemical Vapour Infiltration (CVI)

S 9, Wednesday, Hall C, 16:50

Wiaterek: Bremsenhandbuch, 2012

Comfort Behavior (NVH Noise, Vibration, Harshness)

Nonferrous metals: 25,0%

Steel wool: 15,0%

Alumina: 5,0%

Silicon carbide: 3,0%

Glimmer: 4,0%

Barite: 2,0%

Sulphide: 10,0%

Graphite: 4,0%

Petrol coke: 12,0%

Fiber (e.g. PAN): 2,0%

Rubber: 1,0%

Resin: 5,0%

Confidential: 12,0%

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Material Low density (1.8 2.4 g/cm3)

Extreme thermal stability (up to 1300 C)

High thermal shock stability

High corrosion stability (e.g. de-icing salt)

Low thermal expansion (no distortion)

Tribology Low wear rates (lifetime brakes)

High coefficients of friction (0.4 to 0.5)

High stability of CoF under dynamic and static conditions

No influence of humidity (no early morning effect)

Construction/Design High degree of freedom (NNS-technique, integral or modular design)

Joining by reaction bonding (non-detachable)

Lightweight design (50 % weight saving)

Costs About 2,000 per brake unit (disk, pads, calliper, bell, fasteners)

Characteristics of C/SiC Friction Materials

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Increasingly used in

premium cars and sports

cars

Production volume about

150 000 disks per year

Organic pads, ceramic pads

under development

More than 250 patents cover

all aspects of design,

material composition and

process parameters Brembo SGL Carbon Ceramic Brakes

Carbon/Ceramic Automotive Brake Disks

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Left: ceramic clutch PCCC

Right: conventional clutch (Turbo)

Porsche AG

Dual disk clutch of Porsche Carrera GT

Clutch plate in titanium, clutch lining in C/SiC

Maximum torque > 1000 Nm

Diameter 169 mm

High wear resistance

Small size (lower gearbox mounting) and low

mass (improved motor dynamic)

C/SiC Clutches for Passenger Cars

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High and stable coefficients of friction Low wear rates

High material costs result in hybrid brake

systems (C/SiC rotor, cast iron stators)

C/SiC

stators

C/SiC

rotor

Courtesy of Chr. Mayr GmbH, Germany

Bra

kin

g T

orq

ue

[N

m]

Bra

kin

g T

orq

ue

[N

m]

Braking Time [s]

Organic

linings

C/SiC

rotor and

C/SiC

stators

Braking Time [s]

Emergency Brakes for Conveying Systems

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Schindler / CH

vmax: ~ 13,5 m/s

Mass: ~ 18 000 kg

Tmax: ~ 1200 C

0,20

0,25

0,30

0,35

0,40

0,45

0,50

0 200 400 600 800

p . v [W/mm]

D

yn

am

ic c

oeff

icie

nt

of

fric

tio

n

DLR11 DLR14 DLR16 DLR19 DLR20 DLR15 DLR17 DLR18 DLR21 DLR22C-shaped

Spring Pack

C/SiC

Friction

Pad Carrier

Plate

Friction pad: Graded C/SiC materials

L x B x T: 142 x 34 x 6 mm

Friction partner: Metallic guide rail St 44

Emergency Brakes of High-Rise Lifts

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Through-thickness gradient of SiC SiSiC coatings

Two Approaches for Lifetime Brake Pads

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Prognosis:

Cost reduction potential

due to volume effects and

due to the implementation

of new technologies

D. Neudeck, A. Wllner, H. Dietl:

Bremsen mit nichtmetallischen Bremsscheiben, in:

B. Breuer, K.H. Bill (editors), Bremsenhandbuch, 2006

Technology I Technology II Technology III

Laboratory scalediscontinuous processing,expensive fibers

Interlinking productionprocessesone-piece manufacture,heavy tows

Continuos processesnew carbon fibers,continuous furnaces

2005

10 000 20 000 40 000 80 000 160 000 1 000 000

Prognosis

Effectivetrend

Effective trend:

Only marginal cost reductions

because of volume effects (high

diversity of geometries)

Standardization of brake

disks necessary

Cost Reduction Potentials (Brake Disks)

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Reibring

Kreissegmente

Reibring

Friction ring

Friction ring

Cooling duct segments

Manufacture of standardized and optimized C/C friction rings and cooling

duct segments separately

Subsequent siliconizing after joining

Modular Design Concept Using in-situ Joining Techniques

Cooling duct segments

Friction ring

Friction ring

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Cost estimation based on the series production of C/SiC rotors

(without metallic bell and fasteners)

Share of Total Costs

33%

45%

11%

11%

Raw Materials

Green Body Shaping

Final Machining Fibers

CFRP warm pressing

Long processing time

Matrix

Near net shape manufacture

High tooling costs

High amount of manual work

100% inspection

Thermal Processes

Long processing times

High investment costs

High energy costs

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High manufacturing costs, high amount of manual labor

Development of automated and fast processes of all manufacture steps

Simplier design for new applications (design is still metal-like) CFRP manufacture

- Infiltration of preforms of chopped fibers (instead of warm pressing of

fiber/matrix compounds)

- Injection molding or injection compression technologies Pyrolysis

- Microwave assisted heating

- Continuously operated processes instead of batch based furnaces Siliconizing

- Electric field assisted processes

- Continuously operated silicon infiltration Quality assurance

- Standardization of engineering guidelines and test methods

- Development of a closed simulation process chain (including relability and

lifetime prediction)

Limiting Factors and Main Challenges

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The breakthrough for mass production has not yet been attained (niche applications in high-end transportation systems)

Possible scenario: Key technology for brake systems in high performance cars (worldwide production of about 600 000 vehicles/year in the luxury class)

New developments of brake systems (brake-by-wire) and the increasing electrification of future generations of vehicles (e.g. weight sensitive wheel hub motors) widen the

field of applications

Mid-term goal is a cost reduction to

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