DE10057953A1 - Thermally sprayed ceramic composite layers consist of nanocrystalline crystals and a coating of aluminum oxide and silicon carbide, in which the silicon carbide grains are homogeneously distributed in the aluminum oxide matrix - Google Patents

Abstract

Thermally sprayed ceramic composite layers consist of nanocrystalline crystals and a coating of Al2O3 and SiC, in which the SiC grains are homogeneously distributed in the Al2O3 matrix. The coating is applied by high speed flame spraying or detonation spraying. The starting powders are mixed together before spraying.

Description

field of use

The invention relates to the production of coatings according to the preamble of Claim I.

State of the art

Ceramic materials are inorganic, non-metallic, sparingly soluble in water and too at least 30% crystalline. These are almost exclusively heterogeneous, i. H. from many uniform or various crystals that are often surrounded by glass. Next to him they still hold pores sometimes. The amount and type of these components and their mutual Ge arrangement is referred to as structure or microstructure. It has a clear one flow on the properties of the body [1].

Ceramic materials in general are widely used in industrial production due to their excellent wear resistance, temperature and oxidation resistance. Two main ceramics are mentioned here: Al ₂ O ₃ (aluminum oxide) and SiC (silicon carbide). Because of their high binding energy, they have a very high hardness (approx. 2000 HV or 2300 HV) and a high melting temperature (2050 ° C and 2300 ° C ^{(decomposition temperature)} ).

Basically there are many ways to manufacture ceramic parts (following methods are based on the production of solid materials):

a) By sintering

One or more (composite ceramics) ceramic powders are mixed together, then pressed into components and then at high temperature (below the melting temperature) heated.

b) By melting

Ceramic materials are heated above their melting temperature, then cast in mold. Since the melting temperature of the ceramics is very high, large amounts of energy are required is therefore rarely used. In addition, not all ceramics can be turned on be melted, e.g. B. SiC (silicon carbide), which is decomposed at 2300 °. Other procedures are derived from the above. from.

It is often very difficult or even impossible to make full ceramic parts. That's why one pretends that these parts are made from any other, cheaper materials which and are only provided with a ceramic coating.

There are different methods of realizing such coatings, but one group the process stands out particularly well: thermal spraying. The basic principle is that a ceramic powder is melted by thermal energy and onto the Substrate surface is accelerated.

However, they differ depending on the type of energy supply (thermal as well as kinetic) Method. The best known and most widely used process is the plasma spraying process.  

The energy required for melting is added to the powder by plasma. In order to very high (up to 30,000 ° C) temperatures can be reached. Then the Pul ver applied by a carrier gas to the surface to be coated.

The powders used have a grain size of approx. 30-200 µm, depending on the system and Application.

Another process, namely high-speed flame spraying (HVOF: High Velocity Oxygen Fuel) is based on generating very high speeds at Combustion of gases. The resulting thermal energy melts of powder particles used. The maximum temperature depends on the available fuel gases (up to 3000 ° C).

Here, however, significantly finer powders (10-30 µm) are used because the residence time is against the very high speeds is very short.

Ceramic layers produced by high speed flame spraying have one higher density and smoother surfaces than the plasma spray layers [2].

In addition, they adhere much better to the substrate surface.

As already mentioned above, the properties of the ceramic components depend directly on their microstructure. Studies of sintered aluminum oxide Al ₂ O ₃ (grain size approx. 300 nm), which were provided with a nanocrystalline (approx. 200 nm grain size) silicon carbide additive (SiC) of 5 vol.%, Showed that this combination of materials drastically increases the Strength as well as an increase in wear resistance could be achieved [4].

This positive change in properties is based on a very fine crystal structure of these materials and on the other hand on a homogeneous distribution of SiC in the Al ₂ O ₃ matrix [3]. The SiC crystallites thus strengthen the Al ₂ O ₃ matrix and the actually weak grain boundaries.

Disadvantages of the prior art

So far, only such ceramics or ceramic mixtures have been made by thermal spraying processed that are fusible, so no SiC.

• - The layers produced with the standard powders (grain size 30-200 µm) are very brittle, d. H. they break at the slightest deformation.
• - The plasma spraying process is an expensive and energy-intensive process.
• - With the help of the plasma spraying process, very high temperatures can be reached, however, the coatings are very porous. In addition, arise due to the very high temperature fittings amorphous, d. H. irregular glass structures, the excellent properties of the Greatly reduce ceramics.
• - The adhesion of the coatings is poor in most cases.

Object of the invention

The following aims are to be achieved with this invention:

• - Very dense, low-pore coatings should be realized;
• - In addition, the coatings should be low-stress;
• - An increase in strength should be achieved;
• - Wear resistance should be increased;
• - The adhesive strength of the coatings should be significantly improved.

Solution of the task

To achieve the defined goals, proceed as follows:

• - Very fine nanoscale powders Al ₂ O ₃ and SiC are used;
• - These are mixed together in certain proportions;
• - Then are subjected to a grinding process by homogenizing (SiC equal moderately distributed) and ground even finer;
• - Subsequently, these powders may be agglomerated (not a requirement);
• - Then the powders are sprayed using high-speed flame spraying, where Al ₂ O _{3 is} melted and SiC remains in the solid phase.

Advantages of the invention

• - The high speed flame spraying (HVOF) process is much cheaper than that Plasma spray process;
• - The use of high-speed flame spraying (HVOF) enables the manufacturer development of low-pore coatings;
• - The coating is reduced due to lower temperatures compared to plasma spraying tensions are less stressed, and the amorphous (glassy) fragile is reduced Proportion of;
• - By using very fine (approx. 200-300 nm grain size) powders, the texture structure to the extent that the coatings become "more ductile" and that can withstand higher loads;
• - The admixture of the harder SiC to the Al ₂ O ₃ in a very fine distribution will result in an increase in wear resistance;
• - In addition, these coatings will have better friction behavior.

literature sources

1. Hennicke, H. W .: "On the term ceramics and the classification of ceramic materials." Ber. German Ker. Ges 44 (

1967

), Pp. 209-211.
2. Kreye, H .: "Comparison of HVOF systems - material properties and layer properties." Contribution to the 4th colloquium high speed flame spraying, Erding

1997

,
3. Todd, RI: "Alumina / SiC Nanocomposites: Big Things from Small Packges." British Ceramic Proceeding, 55, 1996, pp. 255-238.
4. Davidge, RW, Twigg, PC, Riley, FL: "Effects of Silicon Carbide Nano-phase on the Wet Erosive Wear of polycrystalline Alumina". Journal of the European Ceramic Society, 16, 1996, pp. 799-802.

Example description of the patent application

Here is the process for the production of thermally sprayed ceramic coatings described in detail.

Show it:

Fig. 1: Structure of the mixed powder from Al ₂ O ₃ and SiC;

Fig. 2: Construction of an agglomerate;

Fig. 3: Processes during thermal spraying;

Fig. 4: Layer structure in the finished state.

First, the nanocrystalline powders Al ₂ O ₃ ( 1 ) and SiC ( 2 ) are carefully mixed together.

In the next step, these are homogenized dry or wet in a mill, with the mixed powder also being further reduced ( FIG. 1).

Then these powders can also be agglomerated in a spray drying system ( 1 , 2 ). This creates spherical agglomerates with a size of 1-150 µm depending on the system parameters ( Fig. 2). As a result, the conveyability within the high-speed spraying system can be considerably improved.

These mixed powders (simple or agglomerated) are then heated in the hot jet of a high-speed spraying system ( FIG. 3), Al ₂ O ₃ ( 3 ) being melted and SiC ( 4 ) remaining in the solid state. As the process progresses, the powders are accelerated to supersonic speed and hit the substrate surface ( 5 ). SiC grains ( 6 ) are thus stored in and between the Al ₂ O ₃ grains ( 7 ) and together form a closed, low-pore layer ( FIG. 4).

When choosing the system parameters, care must be taken to ensure that there is sufficient thermal energy available to melt Al ₂ O ₃ crystals, but SiC remains intact (in solid form).

Claims (5)

1. Thermally sprayed ceramic composite layers,
characterized in that the layers ( Fig. 4) are made up of nanocrystalline crystals;
characterized in that the coating ( Fig. 4) consists of two components, Al ₂ O ₃ ( 7 ) and SiC ( 6 ) (in addition to Al ₂ O ₃ , SiC is also added);
characterized in that SiC grains are homogeneously (evenly) distributed in the Al ₂ O ₃ matrix;
characterized in that they are produced by means of high-speed flame spraying (HVOF) or detonation spraying ( FIG. 3),
characterized in that these coatings have a higher wear resistance compared to the standard layers made of Al ₂ O ₃ ,
characterized in that these coatings show better friction behavior compared to the standard layers made of Al ₂ O ₃ ,
characterized in that these coatings have a ductile behavior in comparison to the standard layers made of Al ₂ O ₃ . 2. Coatings according to claim 1, characterized in that the starting powder used for coating Al ₂ O ₃ ( 1 ) and SiC ( 2 ) have a nanocrystalline grain size ( Fig. 1). 3. Coatings according to claim 1 or 2, characterized in that the starting powder used for spraying together are mixed. 4. Coatings according to one of the preceding claims, characterized in that the mixed powder used for syringes in one Grinding process and be homogenized. 5. Spray layers according to one of the preceding claims, characterized in that the powder used for coating are agglomerated ( Fig. 2).