WO2013117241A1

WO2013117241A1 - Micro-torque material strengthening by fiber spot-pinning

Info

Publication number: WO2013117241A1
Application number: PCT/EP2012/052360
Authority: WO
Inventors: Horst Adams
Original assignee: Adamco Ag
Priority date: 2012-02-10
Filing date: 2012-02-10
Publication date: 2013-08-15

Abstract

The invention discloses a method for strengthening a material by stress induced micro-torques; a method for making a compound material which is strengthened by stress induced micro-torques acting at spot-pinned fibers, fibrils or nanotubes; and a compound material consisting of a metallic, ceramic, or polymeric matrix material which is mechanically strengthened by embedding tubular fibers, fibrils or nanotubes in the matrix material, characterized in that the bonding strength between the matrix material and the inner tube walls of the fibers, fibrils or nanotubes is stronger than the bonding strength between the matrix material and the outer hull of the tubular fibers, fibrils or nanotubes, which causes stress induced micro-torques acting at the embedded fibers, fibrils or nanotubes, when the compound material is exposed to external mechanical stress, which in turn causes an increased mechanical strength of the compound material in comparison to the pure matrix material.

Description

MICRO-TORQUE MATERIAL STRENGTHENING BY FIBER SPOT-PINNING

Technical Field

The invention relates to the mechanical strengthening of a material by spot-pinning tubular fibers, fibrils or nanotubes in a metallic, ceramic, or polymeric matrix material, forming a compound material.

Background Art

Strengthening of a material by embedding high aspect ratio fibers has been done in the past. In particular Halloysite nanotubes (HNTs) and Carbon nanotubes (CNTs) have been used for that purpose.

Halloysite nanotubes (HNTs) are types of naturally occurring aluminosilicate clays with nanotubular structures, having the chemical composition Al₂Si₂O₅(OH)₄2H2O+SiO₂ . HNTs are a weathering product of volcanic rocks of rhyolitic or granitic composition and occur in vast natural deposits. Typical characteristics include low specific weight (2.53 g/cm³), high length-to-diameter ratio (aspect ratio 10-200), and low hydroxyl group density on the surface. The unique crystal structure of HNTs resembles that of carbon nanotubes (CNTs) in terms of aspect ratio. But unlike carbon nanotubes, naturally occurring Halloysite nanotubes are inexpensive, readily available in quantity, environmentally benign, and safe and easy to process. Their CAS registry number is 1332-58-7. Halloysite nanotubes do not agglomerate, making them ideal for dispersion in compound applications. Halloysite nanotubes are hollow tubes with outer diameters typically ranging from about 40nm to 150nm, inner diameters from about 10nm to 100nm with lengths typically ranging from about 100nm to over 2 μm. Halloysite nanotubes can be functionalized (coated) with metallic and other substances to achieve a wide variety of electrical, chemical, and physical properties including excellent interfacing to a metallic compound matrix. Chemically, the outer surface of a not functionalized Halloysite nanotube has properties similar to SiO₂ while the inner cylinder surface has an Al₂O₃ - like structure, providing an excellent interface to an aluminum matrix.

There are different methods for dispersing HNTs and other tubular fibers, fibrils or nanotubes in a matrix. In particular methods for the dispersion of carbon nanotubes in a metal matrix by high energy milling have been disclosed before.

Recently a high energy ball milling method for making an aluminum compound material consisting of Al6061 as the matrix material and HNTs as reinforcing particles has been described (L.A. Dobrzański, B. Tomiczek, M. Adamiak, Manufacturing of EN AW6061 matrix compounds reinforced by Halloysite nanotubes, Journal of Achievements in Materials and Manufacturing Engineering 49/1 (2011) 82-89). For the experiments described in this article a standard high energy laboratory ball mill (manufacturer: Fritsch) was used. With 10w% of HNTs in the Al6061 matrix a maximum increase of compression yield strength from 276 MPa to 481.6 MPa (about 175%) was realized according to the article.

US4299628(A) describes 'A composition for compressing and sintering to produce high-wear resistant articles essentially includes between 2% and 10% by weight, preferably 5% by weight, basalt powder and the remainder being metallurgical powder.' Since HNTs can be seen as a form of basalt we are referring to this invention. However, US4299628(A) describes an inhomogeneous distribution of basalt particles in the metal matrix, where the compound material is manufactured in such a way that the 'sintered metal powder [is] having the entire pore space filled with basalt'. In this context 'pore space' means the voids in the metallic preform to be sintered. US4299628(A) does neither refer to any hollow basalt structure nor to HNTs.

US2006147690(A1) describes a 'Metallic layer material, reinforced with basalt fibers, as well as products made thereof'. As the title indicates this patent describes an inhomogeneous distribution of layered basalt fibers in a metal matrix, with the basalt fibers arranged in layers.

US2008219084(A1) discloses 'Fabrication Methods of Metal/Polymer/Ceramic Matrix Compounds Containing Randomly Distributed or Directionally Aligned Nanofibers'. However, claim 1 of this patent describes an alignment of the nanofibers by specifying: 'imparting a directionality to the nanofibers via application of a mechanical mass flowing process to a compound material with the nanofibers uniformly dispersed in the metal, polymer or ceramic matrix.'

US2010068526(A1) discloses 'MATERIALS CONTAINING CARBON NANOTUBES, PROCESS FOR PRODUCING THEM AND USE OF THE MATERIALS'. However, according to the invention 'this is achieved by materials containing at least one metal and/or at least one polymer laminated in layers alternating with layers of CNT'. This means that the CNTs are not homogeneously distributed in the metal matrix but arranged in layers.

Another method for embedding a fiber structure in a matrix material is the infiltration of fiber preforms with liquid matrix material. Essentially, a fiber preform is similar to a sponge made of the fibers that will later form the reinforcing structure in the compound. This 'sponge' is placed in a mold and infiltrated with the liquid matrix material. The preform also provides for the proper fiber orientation, volume fraction, and distribution in the compound. After consolidation of the matrix material the orientation of fibers in the matrix is as defined by the fiber preform. The main challenges to the use of preforms are the high pressures and temperatures used to infiltrate the matrix material in particular when the matrix material is a metal. The high pressures and temperatures can break or otherwise chemically destroy the preform.

Whether the infiltration of a liquid matrix material into a preform, conventional powder mixing technology or a ball milling technology at various energies is used to disperse nanoparticles homogeneously in a polymeric, ceramic or metal matrix there is always the additional necessity to create a good bonding (interfacing) between the embedded nanoparticles and the surrounding matrix in order to improve the mechanical strength properties of the compound material in comparison to the pure matrix material.

Fiber-matrix bonding is a key factor in the effective transmission of mechanical load from the matrix to the fiber. It is essential that this bond is strong for the compound to develop the desired increase in mechanical strength. Otherwise, on the onset of mechanical stress, the fibers could potentially slide within the matrix, and thus not carry any of the load. The result of this would be that the matrix, now effectively reduced in cross sectional area due to the volume occupied by the ineffective fibers, has to support the entire load which would result in a lower strength level.

In order to obtain proper bonding, wetting of the fiber by the matrix material is essential. One method to obtain this wetting consists of subjecting the fiber within the liquid matrix to very high pressures produced by a squeeze casting process. However, squeeze casting has limitations because not all compounds can be produced at these high pressures and temperatures.

There are several chemical ways to condition the reinforcing particles in order to ensure good interfacing with the matrix. Usually these methods try to diminish the pullout potential of fibers from the matrix by bonding the fibers together, in a manner similar to a net. Then, there is a mechanical restriction to fiber sliding as the matrix material is restricted due to the bonds between the fiber strands. Peng et al. have bonded alumina fibers in preforms together (Peng, H. X., Fan, Z., Mudher, D. S., and Evans, J. R. G. (2002). 'Microstructures and mechanical properties of engineered short fiber reinforced aluminum matrix compounds'. Materials Science and Engineering A, 335: 207-216).

They used two different methods: The first method consisted in sintering the fibers together. With this approach, there was substantial degradation of the fibers due to grain growth which reduced fiber strength. In the second method a series of binders based on phosphoric acid solutions with and without aluminum hydroxide were used, with the intent of forming an aluminum phosphate binder to bind the fibers together. When only the phosphoric acid solution was used it damaged and weakened the fibers due to chemical attack, while a mixture with aluminum hydroxide improved the properties of the preform.

WO2011032791(A1) describes a mechanical method for functionalizing CNTs prior to dispersing them in the matrix material by high energy ball milling (here called mechanical alloying) . The functionalization is achieved by roughening the surface of at least some of the CNTs. In the described method CNT-agglomerates are submitted to a high pressure of 100 kg/cm<2> (9.8 MPa). It was found that while the CNTs retain the same inner structure, the outermost layer or layers burst or break, thereby developing a rough surface. With the rough surface, the interlocking effect between CNTs and the metal matrix is increased, which in turn increases the nano-stabilization or strengthening effect.

The present invention describes a method for increasing the mechanical strength of a matrix material characterized in that the embedded particles have a spot-wise mechanical connection to the surrounding matrix and can slide in the other areas relative to the matrix material. Because of that micro-torques acting at the embedded particles are generated when the compound material is exposed to external mechanical stress, which causes an increased mechanical strength of the compound material.

Technical Problem

Both chemical treatment of the nanoparticles and mechanical functionalization methods of the reinforcing particles are additional steps in the production of a compound material. Desirable is a solution in which no functionalization is necessary but still good reinforcing effects can be achieved by embedding reinforcing particles in a matrix.

So far, all strengthening methods using particles for the strengthening of a matrix material rely on a good mechanical interface between the matrix and an area as large as possible of the outer surface of the particle, and in particular try to avoid any sliding of the particle in the matrix. Desirable is a solution in which a spot-like mechanical pinning of the reinforcing particle in the matrix is sufficient. Desirable is further a solution in which a sliding of the reinforcing particle in the matrix does not reduce the reinforcing effect.

So far, all strengthening methods using fiber shaped particles rely on the tensile strength of the fiber and try to transfer the largest possible portion of this tensile strength from the fiber to the matrix through a good mechanical interface between the matrix and an area as large as possible of the outer surface of the particle. Desirable is a solution in which the geometrical properties and the bending strength of the reinforcing particle contribute to the strengthening effect.

Technical Solution

For the purposes of clear understanding of the principles of the invention, reference will now be made to the preferred embodiment illustrated in the drawings and specific language will be used to describe the same. It will, nevertheless, be understood that no limitation of the scope of the invention is thereby intended. In particular, alterations and further modifications in the illustrated product, method, use, and further applications of the principles of the invention as illustrated therein, are contemplated as would normally occur now or in the future to one skilled in the art to which the invention relates.

For the purposes of clear understanding of the principles of the invention, reference will also be made to the preferred embodiment of a method for making the compound material according to the present invention, comprising aluminum as a representation of the matrix material and Halloysite nanotubes (HNTs) as a representation of the reinforcing tubular fibers, fibrils or nanotubes.

In a first step, a mixture of commercially available HNTs and a commercially available aluminum powder, in which at least a fraction of the powder particles have a size smaller than the inner (open) diameter (lumen) of the HNTs, are processed in a powder mixer such that the metal particles enter the HNTs, filling the inner tubular space of the HNTs completely or in part. Figure 1 shows a principle picture where the matrix material powder particles (a) enter the tubular fiber (b).

In a second step the mixture produced in step 1 is compacted by hot isostatic pressing, such as to form a solid block of aluminum / HNT compound material where the filled HNTs are firmly embedded in the aluminum matrix. Figure 2 shows a principle cross section of a compacted material body (b) with embedded HNTs (a).

The formation of a solid block of compound material can also be done by infiltrating a HNT preform with liquid aluminum and subsequent consolidation or by dispersion-stirring the HNTs in an aluminum melt and subsequent consolidation.

The present invention now takes advantage of the fact that the outer surface of HNTs has similar chemical properties as SiO₂, which does not have good bonding properties to the aluminum matrix, while the inner walls of a Halloysite nanotube have an Al₂O₃ like structure which provides for an excellent bonding interface between the aluminum matrix and the inner walls of the HNTs. In simple terms, since the outer layer of any aluminum part exposed to an oxygen containing environment contains always Al₂O₃ , the aluminum matrix and the inner wall of the HNTs can essentially share the same surface. This in turn means, that the ends of the HNTs are much stronger mechanically attached to the aluminum matrix than the rest of the fiber, which can easily slide in the matrix because of the much weaker bonding between the outer hull of the fiber and the surrounding aluminum matrix. Figure 3 shows a schematic solid block of a matrix material (a) containing a fiber (b) which has been conditioned in the above described way. The fiber is spot-pinned to the surrounding matrix in the areas marked with concentric circles (b), while the outer hull (c) can slide in the matrix, symbolized by the opposing arrow rows.

The present invention takes further advantage of the fact that, if a mechanical tensile or compression stress is applied to the compacted compound material, the mechanical stress is transferred from the metal matrix much stronger to the ends of the HNTs than to the fiber hull because of the different bonding interfaces. This in turn causes a torque force acting on the HNTs.

Figures 4 shows schematically a sold block of matrix material (c), in which a tensile stress (a) causes a torque (b), which tries to align the fiber (d) parallel to the external force vector (a).

Figures 5 shows schematically a sold block of matrix material (c), in which a compression stress (a) causes a torque (b), which tries to align the fiber (d) perpendicular to the external force vector (a).

However, since the HNTs are completely enclosed by the aluminum matrix the stress induced torque is transferred back from the fiber to the matrix and dissipated as heat in the matrix material. This in turn leads to an increase in tensile and compression force necessary to cause any deformation of the compound material. The increment in force necessary to cause a deformation, or with other word the increment in mechanical strength, is proportional to the fraction of the applied stress which is converted into multiple internal micro-torque actions of the embedded HNTs and thus proportional to the heat generated in the material.

Description of Drawings

Figure 1: The matrix material powder particles (a) enter the tubular fiber (b)

Figure 2: Cross section of a compacted material body (b) with embedded HNTs (a).

Figure 3: Schematic solid block of a matrix material (a) containing a fiber (b). The fiber is spot-pinned to the surrounding matrix in the areas marked with concentric circles (b), while the outer hull (c) can slide in the matrix, symbolized by the opposing arrow rows.

Figure 4: Schematic picture of a sold block of matrix material (c), in which a tensile stress (a) causes a torque (b), which tries to align the fiber (d) parallel to the external force vector (a).

Figure 5: Schematic picture of a sold block of matrix material (c), in which a compression stress (a) causes a torque (b), which tries to align the fiber (d) perpendicular to the external force vector (a).

Figure 6: The particle size distribution of the HNTs as received.

Figure 7: Morphology and the tubular structure of the HNTs as received, showing the diameter of the tube openings (a) and the outer diameter (b) of the HNTs.

Figure 8: Particle size distribution of aluminum powder as received

Figure 9: Particle size distribution of aluminum powder after high energy milling

Figure 10: Micrograph of the HNTs after the mixing showing that the HNTs (b) are at least partly filled with aluminum particles (a).

Description of a Preferred Embodiment

In the following, a processing strategy for producing a compound material according to the invention from exemplary constituent materials will be explained. Also, exemplary use of the compound material in different ways of compacting will be discussed.

In the preferred embodiment, the method for making the compound material according to the present invention comprises the following steps:

1. Procurement of Halloysite nanotubes (HNTs)

2. Preparation of fine grain aluminum powder

3. Filling the HNTs with aluminum powder

4. Compacting the aluminum powder / HNT mixture

It is to be understood that these steps represent an embodiment of the production method, in which a compound material according to an embodiment of the invention is obtained.

1. PROCUREMENT OF HNTs

In a preferred embodiment, commercially available Halloysite nanotubes (HNTs) are used, provided by NaturalNano Inc. (Rochester, NY, USA). The particle size distribution of the HNTs as received is shown in Figure 6.

A SEM micrograph showing the morphology and the tubular structure of the HNTs as received is depicted in Figure 7. By a geometrical analysis of the tube openings (a) visible in the picture, and the outer diameter (b) of the HNTs, it was found that the outer diameter of the HNTs was in the range of about 100-150nm, while the inner tube diameter of the HNTs was in the range of about 50-100nm.

2. PREPARATION OF FINE GRAIN ALUMINUM POWDER

Commercially available, air atomized, fine grain aluminum powder (EN AW2024) from ECKA (Germany) was used as matrix material. The particle size distribution of the aluminum powder as received is shown in Figure 8.

The aluminum powder was then milled for 24 hours using a high energy planetary ball mill (Fritsch, planetary micro mill PULVERISETTE 7). After the milling process the resulting powder had a much finer grain size than the starting powder. The particle size distribution of the smaller size end of the spectrum of the aluminum powder after high energy milling is shown in Figure 9. As can be seen, a substantial amount of particles has a size smaller than 50nm, which means, that they fit into the inner tubes of the HNTs described in the previous paragraph.

FILLING OF THE HNTs WITH ALUMINUM POWDER

The HNTs were then added to the pre-milled aluminum powder containing the increased fraction of particles having a diameter smaller than the inner tube diameter of the HNTs, and the mixture was mixed for 1 hour at low energies. Figure 10 shows a high resolution micrograph of a sample of the HNTs after the mixing. It is clearly visible that the HNTs (b) are at least partly filled with aluminum particles (a) sitting inside the inner tubes of the HNTs.

4. COMPACTING OF THE ALUMINUM POWDER / HNT MIXTURE

The mixed powder material manufactured in steps 1-3 can now be used as a source material for forming semi-finished or finished articles by powder metallurgical methods. In particular, it has been found that the mixed powder material of the invention can advantageously be further processed by cold isostatic pressing (CIP) and hot isostatic pressing (HIP). Alternatively, the compound material can be further processed by hot working, powder milling or powder extrusion at high temperatures up to the melting temperature of some of the metal phases. Also, the powder can be directly processed by continuous powder rolling or powder extrusion.

Table 1 shows exemplary tensile test results of the compound material manufactured according to steps 1-3 after hot isostatic pressing (HIP). The tensile strength was increased by more than a factor of five at a concentration of 10 weight % of HNTs in the aluminum matrix. At lower (5w%) and higher (15w%) concentrations of HNTs in the matrix the achieved strength levels were lower but still at least a factor of two higher than that of the pure matrix material.

Although a preferred exemplary embodiment is shown and specified in detail in the drawings and the preceding specification, these should be viewed as purely exemplary and not as limiting the invention. It is noted in this regard that only the preferred exemplary embodiment is shown and specified, and all variations and modifications should be protected that presently or in the future lie within the scope of protection of the appending claims. It is also noted that the term 'compound' can also be read as 'composite' in the context of the present invention.

TABLE 1

Aluminum powder type	HNT content [%]	Yield strength [MPa]	Increase [%]
EN AW2024	0	95	0
EN AW2024	5	262	276
EN AW2024	10	515	542
EN AW2024	15	485	510

Claims

Method for strengthening a material by generating stress induced micro-torques in the material, which convert mechanical energy into heat.
Compound material with embedded fibers, fibrils or nanotubes in a matrix material, wherein the bonding strength between the matrix material and limited sections of the fiber surfaces, fibrils or nanotubes is stronger than the bonding strength between the matrix material and the rest of the surfaces of the tubular fibers, fibrils or nanotubes.
Compound material according to claim 2 with embedded tubular fibers, fibrils or nanotubes in a matrix material, wherein the bonding strength between the matrix material and the inner tube walls of the fibers, fibrils or nanotubes is stronger than the bonding strength between the matrix material and the outer hull of the tubular fibers, fibrils or nanotubes.
Compound material according to claim 2, wherein the matrix material is a metal, or a ceramic, or a polymer.
Compound material according to claim 2, wherein the difference in bonding strengths is caused by inhomogeneous chemical or mechanical properties of the surfaces of the fibers, fibrils or nanotubes.
Compound material according to claim 2, wherein stress-induced micro-torques are generated at the embedded tubular fibers, fibrils or nanotubes when the compound material is exposed to external mechanical stress.
Compound material according to claim 2, wherein the mechanical transfer of the stress-induced micro-torques to the surrounding matrix causes an increase in mechanical strength of the compound material in comparison to the strength of the pure matrix material.
Method for making a compound material according to claim 3 characterized in that a filling process is used which fills the inner tubular space of the fibers, fibrils or nanotubes completely or in part with the matrix material and in that a subsequent solidification process is used to form a solid material body from the fiber, fibril or nanotube / matrix material mixture.
Method according to claim 8 wherein the filling process is infiltration of liquid matrix material into the fibers, fibrils or nanotubes.
Method according to claim 8 wherein the filling process is dispersion of the fibers, fibrils or nanotubes in a matrix material.
Method according to claim 8 wherein the filling process is powder mixing of a mixture containing the matrix material in a powder form and the fibers, fibrils or nanotubes.
Method according to claim 8 wherein the filling process is mechanical powder milling of a mixture containing the matrix material in a powder form and the fibers, fibrils or nanotubes.
Method according to claim 8 wherein the solidification process is compacting of a powder material.
Method according to claim 8 wherein the solidification process is solidification of a melt.
Method according to claim 8 wherein the solidification process is curing of a polymer.