high-strength graphites for carbon piston applications

Pergamon

PII: MOOS-6223(98)00081-5

Carbon Vol. 36, No. 7-8, pp. 107991084, 1998 0 1998 Else&r Science Ltd

Printed in Great Britain. All rights reserved 0008-6223/98 $19.00 + 0.00

HIGH-STRENGTH GRAPHITES FOR CARBON PISTON APPLICATIONS

J. SCHMIDT,~** K. D. MOERGENTHALER,’ K.-P. BRBHLER~ and J. ARNDT~ “DaimlerBenz AC, Research Center, Wilhelm-Runge-Str. 11,89013 Ulm, Germany

bLuK GmbH and Co, Buehl/Baden, Germany ‘Free University of Berlin, Berlin, Germany

(Received 17 October 1997; accepted in revised form 18 December 1997)

Abstract-Recent studies have shown that carbon pistons are highly thermal-resistant in comparison to pistons made of aluminium alloys and can contribute to lower hydrocarbon (HC) emissions. A graphite piston material has to meet requirements which will be discussed in this paper. The near-net-shape manufacturing process is regarded as a cost-effective approach to producing carbon pistons. This process will be discussed in connection with results of our own material studies. The results include studies on forming, sintering and evaluation of mesophase carbon materials. The mechanical and physical properties of a crucible acting as a piston showed properties comparable to those of a commercial graphite block. Flexural strength of up to 116 MPa and a Weibull modulus m > 20 were found. The thermal conductivity was determined to be ca 105 W mK-’ with an open porosity of 10 ~01%. The properties of the powder, the densification and the controlled release of volatile matter during the temperature treatment appear to be the key factors to achieve the high performance of the graphite. Further results are reported on the successful cold isostatic pressed piston blank with a completely reproduced near-net-shape inner contour. 0 1998 Elsevier Science Ltd. All rights reserved.

Key Words-A. Mesophase, B. carbonization, B. graphitization, D. mechanical properties.

1. TARGET

The demand for environmentally friendly motor vehi- cles must lead to further development of today’s engines with the aim of increasing combustion effi- ciency in the engine and of reducing emissions, Tests providing guidelines and material developments have shown that carbon is suitable for use as a structural component in the engine [ 1,2]. Engine pistons made of carbon can sustain higher thermal loads than those made of conventional aluminium alloys and in comparison to the latter they also offer significant benefits with regard to HC exhaust emissions [3].

Graphite piston materials must meet a number of requirements [4]: at a flexural strength of at least 100 MPa, graphite must have a sufficiently high strength to be able to sustain the cyclic tensile and pressure loads in the engine. In addition, the material must also show low scatter of the strength coefficients. High thermal conductivity, in excess of 50 W mK_‘,

and low open porosity are required to avoid any potential problems during the engine combustion process. Up to now, pistons made of high-strength graphite have had to be machined from solid-body materials, which means that the high machining costs lead to a product which is not economically competi- tive (Fig. 1). So far it has not been possible to press carbon powders into carbon pistons that match the near-net shape. The complex geometry with a variety of different radii and wall thicknesses, especially in the area of the internal contours, makes it necessary

*Corresponding author.

to use a manufacturing process which is tailored to carbon powder.

One way of obtaining high-strength graphite with the required properties is to process carbon meso-

phase [ 5,6]. The production of high-strength components from this powder is characterized by the following process steps: pressing; carbonizing; and graphitizing [ 7- 111. The key parameters of the manufacturing process are the degree of densification and the release of volatile matter during the carbonizing process. Extensive further development of the piston design, material and manufacturing technology is required so that pistons can be produced in cost- efficient manner using this basic material. For these reasons, this paper will report on the development of a manufacturing process for producing carbon pis-

Fig. 1. Cylindrical graphite blank and carbon piston manufactured from this blank.

1079

tons on the basis of carbon mesophase powder. The graphite components produced from carbon powder will be characterized on the basis of their physical and mechanical properties.

2. TESTING PROCESS

2. I Basic materials A carbon mesophase powder with a pycnometri-

tally determined powder density of 1.44 g cm-j and a mean grain size of 8 nrn was used for the purpose of material development. Initially, simple geometrical shapes such as cylinders and plates were manufactured from this powder, while later, near-net-shape carbon pistons were produced.

2.2 Shaping The production of the green bodies from carbon

powder was performed by cold isostatic pressing ot the powder (cold isostatic press Quintus QIC 39, Mannheim, Germany) in cylindrical rubber moulds at a moulding pressure of 100 MI%. At this pressure, the powder used was moulded into green bodies which were then carbonized in a crack-free process and graphitized. To produce the complex inner contours, moulds were used that include a centre steel core. To achieve a near-net final shape. a round. rotationally symmetric steel core was adopted initially, whereas other tests were carried out using a complex section as the core (Fig. 2). This section allowed a variety of radii and wall thicknesses to be represented in one single body. Near-net-shape pistons were produced by using a pressing ram which constitutes the negative mould of the inner piston contours. The relevant shrinking characteristics of the green body which occur during the carbonizing and graphitizing process were taken into account in the design of the pressing ram. To ensure clean release of the green body from the mould after the pressing process, the pressing mould was given a mould release bevel of 2”.

Fig. 2. Pressing tools for cold isostatic pressing. Cylindrical rubber mould (dia. 150 mm) and metal cores for producing

bodies with complex inner contours.

2.3 l‘emperuture lreutmrnt On the basis of thermogravimetric as well as

dilatometer tests, a temperature programme for car-

bonizing and graphitizing the test samples was devel-

oped. When the results from preliminary tests were

applied to the carbonizing process of large bodies.

the heating speed and holding time parameters had

to be adapted to the body geometry and body size.

Low heating speeds of 0.1 -0.2 K min ’ were selected

for the carbonizing process and holding times of

several hours were incorporated in the process.

During the graphitizing process, heating speeds of

up to 1OKmin~’ were specified and holding times

were also included. The carbonizing process took a

total of 4 days, and the graphitizing process took

3 days.

Carbonizing of the cold isostatically pressed green

bodies and graphitizing of the carbonized bodies

were performed in two separate furnaces (supplied

by IBV, Hamburg, Germany). The carbonizing pro- ccss was performed in a hot-wall tube furnace (with

a useful volume of 75 1) at temperatures of up to IOOOC under nitrogen. The tests in the graphitizing

system (useful volume I5 I ) were performed in an

argon atmosphere up to a final temperature of 2500 c.

2.4 Methods jbr solids charuclrrizution The physical-mechanical properties of high-

strength graphites were determined. A four-point

flexural test was performed to check the strength.

The measurements were carried out on a general-

purpose testing apparatus supplied by Zwick (Ulm.

Germany). The preload was set to 10 N while the

test speed was set to 0.1 mm min-‘. Rectangular-

section bending test samples of the following sizes were used: length 45 mm, width 4 mm, height 3 mm.

The support width of the sample supports was 40

and 20 mm, respectively.

To evaluate the strength measurement results, the

Weibull statistical process [ 121 was used since this

process takes non-homogeneities in the material into

account. The regression straight line through the

measuring points was calculated using the least- square method [ 131.

Open porosity was determined by infiltration with helium in a gas pycnometer (Model Accu F’yk 1330, Monchengladbach, Germany) in accordance with DIN 51913.

The density of the graphites was determined by a

buoyancy process in water according to the Archimedes method, with the sample surface being

sealed with a coat of protective lacquer. Thermal conductivity was determined according to

the photo-flash method at room temperature [14]. The samples consisted of plane-parallel cylinders with a length of 25 mm and a diameter of 8 mm that were turned from the graphite blocks.

High-strength graphites for carbon piston applications 1081

strength of 102 MPa, a density of 1.93 g cmm3, a thermal conductivity of 105 W mK_’ and an open porosity of 10 ~01% and thus met the properties required for pistons.

Fig. 3. Testing samples produced from carbon mesophase powder to cylindrical (dia. 55 mm), disc (dia. 100 mm) and

crucible shapes (dia. 100 mm).

3. RESULTS

Simple testing samples such as cylinders (dia. 55 mm, height 60 mm) and plates (dia. 100 mm, height 20 mm) were produced from the mesophase powder by cold isostatic pressing, carbonizing and graphitizing (Fig. 3). The samples showed a mean

The process used for the production of simple process samples was also used to manufacture a piston dummy (Fig. 3, right) with the following dimensions: wall thickness 20 mm; diameter 100 mm; and height 52 mm. Linear shrinkage of the dummy during the carbonizing and graphitizing process was virtually the same at 12% in the wall area and 13% in the crown area. The strength of the crucible was determined by performing 40 flexural tests with 20 samples each from the wall and from the crown. During those tests, the mean strength of the samples from the crown area was found to be 107 MPa, with peak values of 116 MPa being reached (Table 1). The strength figures show only minimum scatter so that a high Weibull modulus of 20 was obtained (Fig. 4). The density figures also show some scatter, with the highest value of 1.96 g cm-’ being found in the crown area, whereas the lowest value of

1,000 1

Y m = 20 -4,000

4,430 4,480 4,530 4,580 4,630 4,680 4,730 4,780

insigmrb

Fig. 4. Weibull distribution of strength figures of 20 samples taken from the bottom of a graphite crucible produced to near- net-shape. Mean flexural strength: 107 MPa.

Table 1. Physical-mechanical properties of high-strength fine-gram graphites

Property

Mean flexural strength CT (MPa) Weibull modulus m Thermal conductivity i ( W mK- ‘) Density (g cm-3) Onen norositv (vol%)

Commercial fine-grain graphite (Poco)

Cylindrical blank

126 6

109 1.77

18

Mesophase-based graphite Crucible shape

Wall Bottom

97 107 39 20

111 105 1.94 1.95

10 10

1082 J. %‘HMIUI ef cd.

2 .

I,99

1.98 j

1,9J Bottom (5-9)

i y 1,96 * l

e

.p 1,95 . * * *

.z 5 134 Comer

Cl 1.93 * CONM

x

1,91- -~ I I , t -~ ~~ I

1 2 3 4 5 6 7 8 9 10 11 12 13

Sample Number

Fig. 5. Density distribution in graphite crucible of carbon rncsophase pressed to near-net-shape.

1.92 gem-3 was found at the outer edge of the

crucible wall (Fig. 5). Table 1 gives an overview of further characteristics of the graphite crucible of carbon mesophase and a cylindrical solid body for a

commercially available graphite product (Poco. Decatur, TX, USA). With figures of 18 ~01%. commercially available graphite shows very high open porosity and also very large scatter of the strength figures. This is evident from the low Weibull modulus of m=6. The mean strength and thermal conductiv-

ity, on the other hand, are sufficient for use as piston material.

Following pressing tests with a cylindrical steel

core to represent the use of crucibles, additional tests were performed with a geometrically complex press-

ing core. Apart from representing different radii and surfaces. the purpose of the pressing tests on this core was to find out if different wall thicknesses can be pressed in a homogeneous manner within one single component. All bodies produced by pressing

around this core showed more or less pronounced cracks in the individual stages. The contours of the radii, on the other hand, were shaped correctly and

the surfaces were reproduced accurately. All green bodies broke into two parts along the longitudinal axis. Pressing shrinkage becomes more pronounced as the wall thickness decreases (Fig. 6). As a result, compression is non-homogeneous inside the green

body. Further pressing tests with the basic powder were

carried out in an attempt to mould pistons to a near- net final shape. For this purpose. the piston skirt was given an oval shape with a uniform wall thickness

(20 mm) and thus matched the oval pressing core. The crown. on the other hand, was given a round

shape (dia. 132 mm) and a gradual transitional radius to match the piston skirt. The overall height of the pressed blank was 90 mm. The input powder quantity

used for the pressing process was ca 800 g. Figure 7 shows a piston pressed in a crack-free process with

5;

Wallthickness in mm

70

Fig. 6. Pressing shrinkage of carbon mesophase powder as a function of the wall thickness in a piston-like cross-section

High-strength graphites for carbon piston applications 1083

bodies selected were a graphite crucible and a cylin-

drical specimen with internal contours that closely

match those of a piston. Pressing near-net-shape carbon pistons without any cracking was found to be possible.

Fig. 7. Crack-free piston pressing with inner contours of carbon mesophase powder.

accurately reproduced inner contours. Because of the

homogeneous pressing process around a core, and the use of a uniform wall thickness, the surfaces of

the external contours are very smooth and do not show any deformations. The pressing tests showed that it is possible to press and shape the internal contours of a carbon piston in a fault-free manner. The uniform density distribution in the green body (Fig. 8) confirms that the homogeneous pressing

process can be applied to shaping green bodies from carbon mesophase with the specified piston dimensions.

4. CONCLUSIONS

The carbon pistons currently manufactured and tested in engine applications are made of cylindrical solids of commercially available fine-grain graphites. The properties required for carbon pistons, for exam- ple, high strength > 100 MPa and thermal conductivity > 50 W mK_‘, are met. With regard to homo- geneity, however, this material is not yet suitable for large-scale production. Additional drawbacks of solids due to the production process are the long sintering times and the high cost incurred during final machining of solids into pistons.

Mesophase-based graphite blocks produced within the scope of this work also meet the requirements

for use as piston materials in the same way that

commercial graphite does. The properties relevant

for pistons were obtained in the cylinders and plates

made of carbon mesophase, and it was found to be

possible to reproduce them in a crucible mould. The graphite produced in a crucible is characterized by

high mean strength and low scatter of the actual

strength figures. One problem encountered was the production of a more complex body with stepped

internal contours that match those of pistons and

result in variable wall thicknesses. The internal contour with its different radii was reproduced with a

high degree of accuracy and high surface finish quality. The reason for the deformation of the outside

contours of the bodies as a result of cold isostatic

pressing can be traced to the differences of compression. Powders used in thinner wall thicknesses are

compressed more than powders in larger wall thick-

nesses. This generated pressing tensions in the entire

body and, hence, relief cracking became apparent as a cracked crown. In addition, differences in compres-

sion lead to different mechanical and physical proper-

ties after the carbonizing and graphitizing process.

This became evident upon examination of the crucible

form (crown and wall samples). A homogeneous green body that allows a piston blank with homogen-

eous characteristics to be produced is only obtained if the body to be pressed has the same wall thicknesses

throughout.

The mesophase powder used was found to be suitable for producing plates and cylinders in prelimi-

nary tests and subsequently for producing complex shapes by pressing on non-elastic metal cores. The

To produce a body accurate to size, a pressing

mould closely adapted to the contours of the core

shape should be used. Taking the internal contours required for a specific piston design into account, it

should be attempted to achieve a constant wall

thickness in order to offset pressing tensions and to

achieve homogeneous compression. The studies carried out showed that near-net shape

2-

1,8 -~

116 ~-

“E 1.4 -~

s 1,2 -* .E E 1

2 0,8

: 0,6 --

0,4 --

0,2 --

OT

0 10 20 30 40 60 60 70 80

Sample Number

Fig. 8. Density distribution in a carbon piston of carbon mesophase pressed to near-net-shape

1084 J. Sctrwu,r YI trl

carbon pistons can be pressed and sintered in a homogeneous manner by adopting this design prin- ciple. Due to the reduced wall thickness, a reduced amount of pyrolysis gases is generated which must diffuse towards the upper component surface. As the gas diffusion paths are shorter, the time required for thermal treatment can be reduced. especially during

the carbonizing process. A faster and improved diffusion process contributes to reducing the number of large pores and other structural deficiencies. In

addition, the near-net shape offers significant cost benefits and time savings during the manufacturing process as well as reduced expenditure during the

final machining stage.

To achieve higher load stability and durability of

carbon pistons based on carbon mesophase. further

improved material specifications, and a piston design

adapted to the material, are required.

Acknowledgemenls The above work was carried out withm the scope of a project on “Development of a Technology for Manufacturing Internal-combustion Engine Pistons of High-strength Ultra-tine Grain Carbons” funded by the German Federal Ministry of Education and Research (BMBF). Funding Code: 03M 1065. The article is based on a dissertation by Jens Schmidt [ 151.

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high-strength graphites for carbon piston applications

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