WO2006009522A1

WO2006009522A1 - Ceramic piston for hydraulic brakes

Info

Publication number: WO2006009522A1
Application number: PCT/SI2005/000020
Authority: WO
Inventors: Tomaz Kosmac; Ales Dakskobler; Zmagoslav Stadler
Original assignee: Institut Jozef Stefan; Ms Production Miklavz Zornik S.P.
Priority date: 2004-07-21
Filing date: 2005-07-14
Publication date: 2006-01-26
Also published as: SI21859A

Abstract

The present invention pertains to a piston for hydraulic brakes to be preferably used in vehicles, which is made of transformation-toughened mullite ceramics (ZTMceramics), consisting of a mullite matrix and of particles of generally tetragonal Zr02 dispersed therein.

Description

Ceramic Piston for Hydraulic Brakes

The present invention relates to a piston for hydraulic brakes, to be preferably used in vehicles, which is made of transformation-toughened mullite ceramics (ZTM- ceramics) . The inventive piston protects the brake calipers and the hydraulic fluid contained therein from overheating, is resistant to quick temperature variations, and does not break down with a disastrous rupture when subjected to mechanical and/or thermal overloads. Thereby an efficient and safe operation of the brake system is achieved even in conditions of aggressive and prolonged braking, as may commonly occur in modern brake systems.

The problem of heat generation and transfer in brake calipers between the brake pads and the brake disk has been long known in the art. In the process of braking high temperatures develop. Most of the heat is dissipated through the disk, the remaining heat being conveyed via the friction pads and the pistons to the calipers and to the hydraulic brake fluid contained therein. Although the brake pad is positioned inside the brake calipers so as to have as few points of contact with the caliper casing as possible, overheating of the brake fluid, which fluid is in direct contact with the brake piston, may occur after prolonged and/or aggressive braking, leading in turn to a loss of braking power. The problem is particularly critical in brake systems with disks made of new composite ceramic materials based on C/C-SiC or C/SiC which tend to get considerably hotter than steel disks or disks of grey cast iron. Therefore, the quantity of heat stored by the brake pads in brake systems with ceramic brake disks is considerably greater as compared to the conventional brake systems, and in order to ensure secure operation of such brake systems appropriate thermal protection against overheating of the brake calipers and the hydraulic fluid contained therein must be provided.

Several methods for protecting the brake calipers and the hydraulic fluid contained therein against overheating are proposed by the prior art. WO 02/44580 discloses the employment of metallic annular bands between the brake piston and the brake pad in order to achieve cooling. The manufacture of such cooling spacer elements is limited to metals, such as stainless steel or titanium, thereby determining their thermal characteristics. For said cooling spacer elements to be built in, more space in the axial direction is required, which may be an impediment in smaller brake systems (as in motorcycles) and may negatively affect the rigidity of the brake calipers in larger ones.

An alternative prior-art solution is represented by ceramic brake pistons. As a rule, ceramics conduces less heat as compared to metals. In accordance with the teachings of GB 2250800 A, "Brake Piston", the utilization of Si₃N₄-ceramics is known which exhibits a relatively small elongation, a small specific mass and density and good mechanical properties, especially strength and fracture toughness, but has elevated thermal conductivity, and is tedious and expensive to manufacture.

US 5480847, "Brake Assembly", provides for the possibility to employ porous ceramic pistons made of clay or kaolin materials. Said materials exhibit an extremely low thermal conductivity, and ceramics is impermeable to hydraulic fluid but permeable to water, enabling the water condensing in the brake fluid to be removed. Another advantage is represented by comparatively cheap raw materials and low fabrication costs. However, it has low strength, which is reduced even further by the porosity. It is brittle at low temperatures, and there is danger of plastic deformation at higher temperatures, since it contains a vitreous intergranular phase with a low melting point.

A solution with plastic brake pistons is also known in the art. Plastic pistons have several advantages: they are not brittle and have good mechanical properties at low temperatures. However, they can not withstand high temperatures. In addition, plastic is unstable in hydraulic fluid and is accordingly envisaged by the invention only as an insert within a metal piston.

In contrast, ceramic inserts, fitted between the piston and the friction pad (as per GB 2089914 A, "Disc Brake Piston") , possess a greater thermal and chemical stability. As far as we are aware of, this solution is the only one to be practically employed, specifically in Porsche sports cars equipped with C/SiC-composite disks brake systems. The aforementioned patent protects the basic geometrical characteristics of the ceramic insert and the method for fixing said insert within the metal piston, whereas the material - according to the invention - may be any ceramics having a low thermal conductivity. In practice, tetragonal ZrO_∑-ceramics is utilized which has a low thermal conductivity as required, good mechanical properties at lower and higher temperatures, but is relatively sensitive to thermal shocks and has an elevated density. It is also comparatively expensive. Additionally, the ceramic inserts in the pistons are an additional component which is difficult to manufacture and makes the system substantially more expensive. The addition of ceramic inserts also requires precision finishing of the inner surfaces of the piston which commonly only require rough machining.

The technical problem which, according to known information, is not yet adequately solved, is a hydraulic brake piston which will protect the brake calipers and the hydraulic fluid from overheating, will be resistant to quick temperature variations, will not break down with a disastrous rupture when subjected to mechanical and/or .thermal overloads, will exhibit a small elongation during operation, will make a good seal, and will be fabricated at low cost and from comparatively inexpensive raw materials.

The aforesaid prerequisites for the material for producing brake pistons are very well fulfilled by transformation-toughened mullite ceramics. Mullite (3Al₂O₃.2SiO₂) is the only stable compound of the Al₂Oa-SiO₂ system. It melts at 1920 ⁰C, is a good electrical insulator, has low thermal conductivity (λ = 3.5 W Hf²K^"1) and is one of the ceramic materials with a comparably low thermal expansion (a = 5.3 x 10^'6K^"1) . As for engineering purposes, it is only of limited use because of its poor mechanical properties, specifically its low strength value which seldom exceeds 200 MPa, and its low fracture toughness which varies between 1.8 MPa m^1/2 and 2.5 MPa m^1/2. The strength and the fracture toughness of mullite ceramics may be improved by activating one of the toughening mechanisms. One of the frequently employed mechanisms is the so-called transformation toughening, achieved by embedding dispersed particles of tetragonal ZrO₂ into a brittle ceramic matrix (N. Claussen, "Fracture Toughness of Al₂O₃ with Unstabilized Zrθ₂ Dispersed Phase", J. Am. Ceram. Soc. 59 (1976), 49-51) . Under the influence of the external tension state such particles may be transformed into a monoclinic modification, thus partially absorbing the energy of the fracture, while compressive stresses arise within the matrix due to the volumetric change accompanying the phase transformation, and inhibit the crack propagation (F. F. Lange, "Transformation Toughening", J. Mat. Sci. 17 (1982), 225-262) . Transformation-toughened mullite ceramics (ZTM, Zirconia-Toughened Mullite) may be formed from various natural or synthetic materials, for example from a mixture of synthetic mullite powder and ZrO₂ powder which usually also contains small quantities of Y₂O₃, CeO₂, or some other oxide which stabilizes the tetragonal phase of ZrO₂ (T. Fandel, K. Kroenert, "Investigations of Mullite- Zirconia Composites", Proc. VIII^th German-Yugoslav Meeting on Mat. Sci. Dev., KFA Jullich, 1988, 73-80, ISBN 3-89336- 001-8; S. Prochazka et al., "Microstructure of Sintered Mullite-Zirconia Composites", J. Mat. Sci. 19 (1984), 2909- 2914), or from a mixture of zirconium silicate powder and alumina powder which react during the sinter treatment, yielding mullite and ZrO₂ (N. Claussen, J. Jahn, "Mechanical Properties of Sintered In-Situ Reacted Mullite- Zirconia Composites", J. Am. Ceram. Soc. 63 (1980), 228- 229) ; manufacturing is also possible from a mixture of powders wherein metal aluminum, metal silicon, or SiC is one of the components (S. Wu, N. Claussen, "Reaction Bonding and Mechanical Properties of Mullite/Silicon Carbide Composites", J. Am. Ceram. Soc. 77 (1994), 2898- 2904; J. Brandt, R. Lundberg, "Processing of Mullite-based Long Fibre Composites via Slurry Routes and by Oxidation of an Al-Si Alloy Powder", J. Europ. Cera. Soc. 16 (1996), 261-267); EP 0 531 378 Bl; US 6,083,861) . In all these cases improvements in mullite ceramics strength and fracture toughness have been reported, which may be further improved by inserting SiC monocrystalline fibers (known as "whiskers") (US 4,657,877, Apr. 14, 1987) . Volume percentages of ZrO₂ additions for improving mechanical properties, i.e. strength and fracture toughness, are generally between 3% and 50% relative to the mullite basis.

In accordance with the present invention the problem is solved with a hydraulic brake piston and with the employment of ceramics according to the independent patent claims.

To produce brake pistons according to the present invention synthetic mullite powder obtained by remelting, sintering, co-precipitation, sol-gel reaction or any other reaction in solid, liquid or gaseous state, and ZrO₂ powder optionally doped with Y₂O₃ or CeO₂ or some other oxide stabilizing the tetragonal ZrO₂ phase may be employed as precursors, a mixture of constituents in adequate mol proportions or compounds thereof may be used, combined with natural minerals which react in the process of sintering, yielding mullite and tetragonal ZrO₂; for shaping the green bodies any known process of dry, wet, or plastic forming may be employed, such as pressing, casting, injection molding, or extrusion in combination with a subsequent mechanical finishing; after shaping, the green bodies are dried or freed from the organic additives needed for shaping, and sintered at temperatures at which this type of ceramics compacts to closed porosity or to the maximum obtainable relative density.

The invention shall now be described with relation to particular preferred embodiments thereof.

First Embodiment. The fine fraction (< 100 μm) of milled electro-melted mullite and powder of tetragonal ZrO₂, stabilized with a 12% mol fraction of CeO₂ were utilized as precursors for producing pistons from transformation-toughened mullite ceramics. 200 g of powder mixture was weighed, containing 180 g of mullite powder and 20 g of tetragonal ZrO₂ powder. The mixture was attrition milled for a period of 3 hours in a liquid medium (acetone, to which 1.6 g of stearic acid had been admixed) . After drying, the powder was dispersed in melted paraffin and the brake piston green bodies were formed by the process of low-pressure injection molding. After expelling the organic medium in a filling of active alumina at 200 °C, the green bodies underwent sinter treatment at 1550 °C in atmospheric air for 4 hours. The density of ZTM-ceramics pistons after the sinter treatment was 3.34 g/cm³, the flexural strength was 262 MPa, the fracture toughness was 2.9 MPa m^1/2, the linear thermal expansion coefficient a = 5.5 * 10^"6K^"1, and thermal conductivity λ = 3.5 W/mK.

Second Embodiment. Synthetic mullite powder with sub- micrometer particles, synthesized with a sol-gel process, unstabilized (monoclinic) ZrO₂ powder, and Y₂O₃ powder were utilized as precursors for producing pistons from transformation-toughened mullite ceramics. 160 g of mullite powder, 38 g of monoclinic ZrO₂ powder and 2 g of Y₂O₃ powder were weighed. This mixture of powders was then milled in an attrition mill in isopropanol for 3 hours. After drying, the powder was granulated by hand with the addition of 2 % of camphor, previously dissolved in acetone. Brake piston compacts were formed in a cold isostatic pressing process. The compaction pressure was 120 MPa. After pressing, the compacts underwent sintering at 1520 °C in air for a duration of 2 hours. The density of ZTM-ceramics pistons after the sinter treatment was 3.44 g/cm³, their flexural strength was 345 MPa, the fracture toughness was 3.6 MPa m^1/2, the linear thermal expansion coefficient a = 5.7 x 10^"6K^"1, and thermal conductivity λ = 3.4 W/mK.

Third Embodiment. Zirconium silicate ZrSiO₄ powder, Al₂O₃ powder, Y₂O₃ powder, and the fine fraction (< 100 μm) of milled electro-melted mullite were used for producing pistons made of transformation-toughened mullite ceramics. The mass proportions of the precursors were as follows: ZrSiO₄ 42.5 %

Al₂O₃ 35.5 %

Y₂O₃ 1.5 %

Electro-melted mullite powder 20.5 %

200 g of powder mixture was weighed and attrition milled in water for 3 hours. As deflocculant polyacrilic acid was utilized in the proportion of 0.5 % by mass relative to the mass of the powder. After milling, the powder mixture was dried and again dispersed in water, containing 15 vol. % of hydroxy-methyl-cellulose as binder and 5 % of petroleum oil as lubricant. After homogenization in a three-roller mill the plastified mass, containing 50 vol. % of dry substance, was extruded with a pressure of 10 MPa through a circular hole with a diameter of 40 mm. The extruded bar was dried and cut into shorter cylinders that were subsequently shaped into brake piston green bodies by cutting off on a lathe. Said green bodies underwent a slow (2 °C/min) progressive heating to 500 ⁰C in a filling of active alumina, and organic components were removed therefrom. The green bodies underwent a sinter treatment following a specific regime: they were first heated to 1400 ⁰C at a rate of 5 °C/min, then kept at said temperature for 60 min, whereupon they were heated to 1580 °C at a heating rate of 10 °C/min. After 4 hours of sintering at said temperature the samples were cooled down to room temperature and finely ground off on a diamond-tool lathe to required tolerances and to specified roughness. The density of the pistons manufactured from this reaction- sintered ZTM-ceramics was 3.82 g/cm³, the flexural strength was 320 MPa, the fracture toughness was 3.7 MPa m^1/2, the linear thermal expansion coefficient a = 5.8 x 10^"6K^"1, and thermal conductivity λ = 3.2 W/mK.

The hydraulic brake piston made of ceramics according to the invention is therefore manufactured from transformation-toughened mullite ceramics consisting of a mullite matrix and ZrO₂ particles dispersed therein, the volume percentage thereof being between 3 % and 50 %. Synthetic mullite powder obtained by remelting, sintering, co-precipitation, sol-gel reaction or any other reaction in solid, liquid or gaseous state, and Zrθ₂ powder optionally doped with Y₂O₃ or CeO₂ or some other oxide stabilizing the tetragonal ZrO₂ phase may be employed as precursors for manufacturing ceramics. For producing ceramics a mixture of constituents in appropriate mol proportions or compounds thereof may be used, combined with natural minerals which react in the process of sintering, yielding mullite and ZrC>2 which may optionally be doped with Y₂O₃ or CeO₂ or some other oxide stabilizing the tetragonal ZrO₂ phase.

The process for manufacturing pistons according to the invention is characterized in that for shaping the green bodies any known process of dry, wet, or plastic forming may be employed (such as pressing, casting, injection molding, or extrusion in combination with a subsequent mechanical finishing) ; after shaping, the samples are dried or freed from the organic additives needed for shaping, and sintered at temperatures at which this type of ceramics compacts to closed porosity or to the maximum obtainable relative density.

Transformation-toughened mullite ceramics is utilized for fabricating a hydraulic brake piston.

The brake pistons as components of motorcycle brake systems were tested in a test laboratory and on a motorcycle racing track. The results were adequate and fully satisfied the object and purpose of the invention.

Claims

Patent Claims

1. Ceramic piston for hydraulic brakes, characterized in that it is made of transformation-toughened mullite ceramics.

2. Piston for hydraulic brakes according to Claim 1, characterized in that the transformation-toughened mullite ceramics consists of a mullite matrix and dispersed ZrO₂ particles, the volume percentage thereof being between 3 % and 50 %.

3. Piston for hydraulic brakes according to Claim 1, characterized in that synthetic mullite powder obtained by remelting, sintering, co-precipitation, sol-gel reaction or any other reaction in solid, liquid or gaseous state, and ZrO₂ powder optionally doped with Y₂O₃ or CeO₂ or some other oxide stabilizing the tetragonal ZrO₂ phase are employed as precursors for producing ceramics.

4. Piston according to Claim 1, characterized in that a mixture of constituents in adequate mol proportions or compounds thereof, combined with natural minerals which react in the process of sintering, yielding mullite and ZrO₂, optionally doped with Y₂O₃ or CeO₂ or some other oxide stabilizing the tetragonal ZrO₂ phase, are used for producing ceramics.

5. Process for manufacturing ceramic pistons according to Claim 1, characterized in that any known process of dry, wet, or plastic forming is employed for shaping the green bodies (such as pressing, casting, injection molding, or extrusion in combination with a subsequent mechanical finishing) ; after shaping, the green bodies are dried or freed from the organic additives needed for shaping, and sintered at temperatures at which this type of ceramics compacts to closed porosity or to the maximum obtainable relative density. Utilization of transformation-toughened mullite ceramics in the production of a hydraulic brake piston according to Claim 1 for manufacturing hydraulic brakes.