WO2009033435A1

WO2009033435A1 - Nanocrystalline composite material based on al2o3 - zro2 - sio2 and its production method

Info

Publication number: WO2009033435A1
Application number: PCT/CZ2008/000102
Authority: WO
Inventors: Tomas Chraska; Karel Neufuss
Original assignee: Institute Of Plasma Physics As Cr, V.V.I.; Institute Of Inorganic Chemistry As Cr, V.V.I.; Eutit, Ltd.
Priority date: 2007-09-12
Filing date: 2008-09-11
Publication date: 2009-03-19
Also published as: CZ300602B6; CZ2007625A3

Abstract

The merits of the present invention is production of commercially utilizable three- dimensional articles with nanocrystalline composite structure from a material based on Al2O3 - ZrO2 - SiO2. The nanocrystalline composite material contains 45 - 58 wt.% Al2O3, 28 - 38 wt.% ZrO2, 9 - 25 wt.% SiO2, its total porosity is below 5% and contains two levels of internal structure. At the micrometer level, the material is made up from mutually overlapping, thin and wavy discs (called splats) with thickness of up to 3 μm. The splats are formed by thermal spraying process and their respective chemical composition varies slightly. Inside of each splat, nanometer sized crystallites are found with average sizes ranging from 8 to 25 run and narrow size distribution. The nanometer sized crystallites are solely of one phase, which is the solid solution of tetragonal ZrO2 with Al2O3 and SiO2. The method producing the above material is as follows. Material containing Al2O3, ZrO2, and SiO2, is melted in arc furnace, the melt is casted, cooled, and crushed into powder with particle size below 120 μm. A coating is produced by thermal spraying of the powder onto a model preheated to 100-400 °C. The amorphous coating is removed during cooling and the temperature of crystallization in solid state (Tk) is determined by differential thermal analysis of the coating sample. The coating is then heated at a minimum heating rate of 5 K/s to a temperature from Tk -10 °C to Tk +80 °C, and after a dwell time of up to 60 minutes cooled at a minimum cooling rate of 5 K/s to the room temperature.

Description

Nanocrystalline composite material based on Al₂O₃ - ZrO₂ - SiO₂ and its production method

Technical field

The present invention relates to a new nanocrystalline composite material based on Al₂O₃ - ZrO₂ - SiO₂ and also to a method for production thereof. Three-dimensional articles of this material are produced by thermal spraying and successive heat treatment.

Background art

Material

Ceramic material based on Al₂O₃ - ZrO₂ - SiO₂ is currently produced by melting appropriate raw materials in arc furnaces followed by casting into sand molds and controlled cooling during which the material crystallizes. The raw material may come from worn-out linings of glass melting furnaces, which contains 45 - 58 wt.% Al₂O₃, 28 - 38 wt.% ZrO₂, 9 - 25 wt.% SiO₂ and small amount of oxides of alkali metals or it may be mechanically mixed from relatively pure oxide constituents (Al₂O₃, ZrO₂, SiO₂). The final product is a thick-wall casting possessing eutectic microstructure of corundum and baddeleyite lamellae and only a small amount of a glassy phase. The products exhibit very good properties namely high hardness, high abrasion resistance, chemical resistance and are refractory. The melt casting production route, however, limits the final product shape into thick-wall articles that can be further finished by diamond tools only. Nanocrystalline ceramics

In general, formation of nanocrystalline structure brings about substantial improvement of materials mechanical properties. It is assumed and to some extent also verified that structural products of nanocrystalline ceramics will exhibit significantly increased hardness, strength, wear resistance. Nevertheless, limited ability to fabricate large three-dimensional parts (e.g. pipes, tiles etc.) of compact nanocrystalline materials is the major obstacle for their greater use. There are many different techniques of fabricating nanocrystalline materials from solids, liquids, and vapors. Most of these techniques produce materials in the form of nanocrystalline powders (nanoparticles). Synthesis of large quantities of ceramic nanoparticles has been mastered but the consolidation of nanoparticles into useful mesoscopic structures and large bulk parts remains a challenge and prevents their fabrication. The powder consolidation process must allow retention of the nanometer grain size and at the same time bring residual porosity levels to a minimum. For the successful consolidation of ceramic nanomaterials to densities where the improvements in physical and mechanical properties can be exploited, processing techniques that rely on application of high pressure and raised consolidation temperatures are required. Raising the temperature causes undesirable grain growth and microstructure coarsening whereas low temperature is often not sufficient to achieve full inter-particle bonding and thus fully dense samples [2]. Moderate success has been achieved by so called Transformation Assisted Consolidation (TAC) [3], which takes advantage of a pressure-induced phase transformation to suppress grain growth during consolidation. TAC has been used to fabricate only small compact samples, which are of no commercial value. Thermal spraying

The thermal spraying (TS) technology, which has been used for many decades, is able to achieve rapid solidification in sprayed materials. In the conventional TS process, powder particles (10-120 μm) are injected into a high temperature plasma jet generated by plasma torch. Individual particles are quickly melted by the plasma and propelled onto a substrate. Upon impact, the molten particles spread and rapidly solidify due to high heat extraction by the relatively cold substrate. The solidified discs are called splats and they represent the basic building blocks of a TS coating. Repetitive passings of the plasma torch over a substrate produce coatings by layering splats on top of each other in a stochastic manner. The extremely high cooling rates (10³-10⁵ K/sec) in the impinged particles give rise to microstructures of narrow columnar grains of nanometer dimensions or amorphous phases due to complete suppression of crystallization. Thermal spraying is used for a variety of applications from thermal barriers coatings to delicate coating in electronic industry. It can be used to fabricate free-standing ceramic articles, functionally graded materials, or amorphous materials.

Disclosure of the invention

The merits of the present invention is origin of commercially utilizable three- dimensional articles from a material based on Al₂O₃ - ZrO₂ - SiO₂ possessing a novel nanocrystalline composite structure and improved mechanical properties and then a novel method leads to production of such material utilizing thermal spraying and successive heat treatment. The novel noncrystalline composite material based on Al₂O₃ - ZrO₂ - SiO₂ contains 45 - 58 wt.% Al₂O₃, 28 - 38 wt.% ZrO₂, 9 - 25 wt.% SiO₂. The material has total porosity below 5% and contains two levels of internal structure. At the micrometer level, the material is made up from mutually overlapping, thin and wavy discs (called splats) with thickness of up to 3 μm. The splats are formed by thermal spraying process and their respective chemical composition varies slightly. Inside of each splat, nanometer sized crystallites are found with sizes ranging from 8 to 25 nm (according to heat treatment conditions and individual chemical composition) and with narrow size distribution (standard deviation is equal to 15% of the mean size). The nanometer sized crystallites are made up solely from one phase, which is the solid solution of tetragonal ZrO₂ with Al₂O₃ and SiO₂. Individual nanocrystallites are rounded, are not in direct contact and do not form standard grain boundaries. Between the nanocrystallites, there is a thin layer of the original amorphous matrix from thermal spraying which in certain areas partially crystallizes as γ-Al₂O₃ or 5-Al₂O₃ phases.

The novel material exhibits microhardness (using Vickers indentor) 16,5 - 17,5 GPa, which amounts to more than 50% increase in comparison with the cast material and the microhardness values are also higher than those of conventional one-component materials, i.e. pure Al₂O₃, ZrO₂, and SiO₂. The slurry abrasion response test carried out according to the ASTM G75 standard demonstrates one third improvement of abrasion resistance to volume loss of 2,9 mm³ for 1000 m of sliding distance. The novel nanocrystalline composite material based on Al₂O₃ - ZrO₂ - SiO₂ can be fabricated in the form of macroscopic three-dimensional articles, such as tiles with thickness ranging from 1,5 to 6 mm, or pipes of 30 mm and larger diameter, wall thickness from 2 mm up and length of over 1 m, which have significantly higher hardness and abrasion resistance than that of conventionally cast products of the same chemical composition. These articles can also take on shapes that are not possible to produce by casting technique.

The fundamental aspect of the production method is to apply thermal spraying and successive heat treatment to an appropriate ceramic feedstock material that is transformed to a product whose entire volume consists of nanocrystalline composite material and possesses significantly improved mechanical properties.

The raw feedstock material is based on Al₂O₃ - ZrO₂ - SiO₂ and contains 45 - 58 wt.% Al₂O₃, 28 - 38 wt.% ZrO₂, 9 - 25 wt.% SiO₂ and is located near the ternary eutectic point of the equilibrium phase diagram. The material needs first to be processed into a powder form suitable for thermal spraying. This can be done by fusing the three oxide components, casting, cooling, crushing it, and sieving to the right particle size below 120 μm for the selected thermal spraying device. After the powder preparation it is essential to ensure that each individual particle of the processed feedstock powder contains all of the three oxide constituents and their chemical composition is not too far from the ternary eutectic point. For example, a simple mechanical mixture of powders of the three oxide constituents does not meet this requirement and thus cannot be used in this novel production method.

The selected thermal spraying device must ensure complete melting preferably of all the powder particles before their impact on the surface of a model. The model (e.g. a mandrel) defines the shape of the sprayed coating, which in turn becomes a free-standing article. Spraying parameters (e.g. model surface temperature, distance of the model from the thermal spraying device, deposition rate etc.) must be adjusted for the given thermal spraying device to achieve three goals: to achieve very high cooling rates in impacting particles, to attain low total porosity, and to enable removal of the coating from the model. The very high cooling rate leads to rapid solidification with high melt undercooling and suppression of diffusion processes that are essential for developing the equilibrium eutectic crystal microstructure. Therefore, the rapidly solidified particles (splats) do not undergo crystallization but remain in amorphous state. The result of the thermal spraying step is an amorphous coating that can be removed from the model during cooling to obtain a free-standing amorphous article.

The free-standing article is then subjected to heat treatment to induce controlled crystallization. Initially, the temperature of crystallization in solid state (Tk) is must be determined by means of differential thermal analysis. The free-standing article is then heated (at a minimum heating rate of 5 K/s) to the proximity of the crystallization temperature (T_k - 10 °C to T_k +80 °C) and after a short dwell time (up to 60 minutes) cooled (at a minimum cooling rate of 5 K/s) down to room temperature. The heat treatment sets off crystallization process and results in nanocrystalline composite structure with crystallites sizes ranging from 5 to 60 run. An essential prerequisite for the success of this novel production method is a low value of crystallization shrinkage that permits the heat treated material to maintain its permanency of form and structural integrity in its nanocrystalline nature. This prerequisite is met by the material, for which the patent protection is claimed. The exact value of the selected temperature for heat treatment and the amount of dwell time determine the final size of nanocrystallites and the quantity of the residual amorphous matrix.

The result of the invented production method is a three-dimensional article from a novel nanocrystalline composite material in shapes and sizes, which for the composition according to the claims was not possible to prepare by methods for nanocrystalline materials preparations available up to date. The resulting material exhibits very high hardness and high abrasion resistance.

Example

The nanocrystalline composite material based on Al₂O₃ - ZrO₂ - SiO₂ charcterized is that it contains 45 - 58 wt.% Al₂O₃, 28 - 38 wt.% ZrO₂, 9 - 25 wt.% SiO₂ and is made up from wavy discs (splats) with thickness of up to 3 μm that are mutually overlapping and the total porosity is below 5%. The splats contain residual matrix, in which rounded nanocrystallites with average diameter of 8 to 25 run are thickly and evenly dispersed. The nanocrystallites are solid solution of tetragonal ZrO₂ with Al₂O₃ and SiO₂ .

The starting cast material with fine eutectic microstructure had the following composition: 51,5 wt.% Al₂O₃, 34 wt.% ZrO₂, 13 wt.% SiO₂ a 1,5 wt.% other oxides. The feedstock powder was prepared by mechanical crushing and sieving to obtain powder particles of 40-63 μm. Thermal spraying was done by WSP ^®500 plasma torch with water stabilized plasma jet in ambient air and power input of 16OkW. The powder was fed to the plasma at a rate of 250 g/min in a distance of 30 mm from the torch nozzle and the mandrel was positioned 350 mm away from the torch nozzle. These spraying parameters provided molten feedstock particles with average temperature of 245O⁰C and standard deviation of approximately 100⁰C, which is sufficiently above the melting point temperature (around 1800⁰C) and ensures that majority of the powder particles impacts the mandrel in molten state. Average velocity of the impacting particles was between 85 and 95 m/s with standard deviation of 17 m/s. The as-sprayed amorphous coatings contained around 4 vol.% of unmelted feedstock particles and their open porosity was 1,5%. The coatings were removed from the mandrel during cooling.

The onset temperature of solid state crystallization for the as-sprayed material was determined to be 958⁰C and crystallization shrinkage amounts to 1,8%. Two examples of heat treatment are now given with identical heating and cooling rates of 10 K/s. In the first example, the material dwelled for 2 minutes at 955°C and in the second example the material dwelled for 1 minute at 96O⁰C . In the first example nanocrystalline composite structure with average crystallite size of 11 nm resulted in and in the second example with average crystallite size of 13 nm. The crystallites size was determined by direct measurement on transmission electron microscope micrographs and by line width analysis of X-ray diffraction patterns. Industrial applicability

Products made from the nanocrystalline composite material based on Al₂O₃ - ZrO₂ - SiO₂ ceramic can be exploited in a number of industrial applications, in which high hardness and high abrasion resistance are of great importance, as various shapes such as protective tiles, pipes for hydraulic or pneumatic transport systems.

Claims

1. Nanocrystalline composite material based on Al₂O₃ - ZrO₂ - SiO₂ characterized is that it contains 45 - 58 wt.% Al₂O₃, 28 - 38 wt.% ZrO₂, 9 - 25 wt.% SiO₂, is made up from wavy discs with thickness of up to 3 μm which are mutually overlapping and the total porosity is below 5%, discs contain residual amorphous matrix, in which rounded nanocrystallites of solid solution of tetragonal ZrO₂ with Al₂O₃ and SiO₂ and with average diameter of 8 to 25 run are thickly and evenly dispersed.

2. Method of preparation of nanocrystalline composite material based on Al₂O₃ - ZrO₂ - SiO₂ according to Claim 1, characterized in that material containing Al₂O₃ - ZrO₂ - SiO₂ is melted in arc furnace, the melt is casted, cooled and crushed into powder with particle size below 120 μm, coating is produced by thermal spraying of the powder onto a model preheated to 100-400 ⁰C, amorphous coating is removed during cooling to the room temperature, the temperature of crystallization in solid state (T_k) is determinated by differential thermal analysis of the coating sample, the coating is then heated at a minimum heating rate of 5 K/s to a temperature from T_k -10 °C to T_k +80 °C, and after a dwell time of up to 60 minutes the coating is cooled at a minimum cooling rate of 5 K/s to the room temperature.