GB2073169A

GB2073169A - Spraying Material for Hot and Plasma Spraying

Info

Publication number: GB2073169A
Application number: GB8011553A
Authority: GB
Original assignee: Vysoka Skola Chemicko Technologicka V Praze
Current assignee: Vysoka Skola Chemicko Technologicka V Praze
Priority date: 1980-04-08
Filing date: 1980-04-08
Publication date: 1981-10-14
Also published as: GB2073169B

Abstract

The material comprises agglomerates of at least two main oxides, such as Al2O3, MgO, CaO, BaO, Cr2O3, TiO2 or ZrO2, in an amount of 50 to 99% by weight, and at least one glass-forming oxide the melting point of which is by 50 to 1100 DEG C lower than the melting points of the main oxides, such as SiO2, in an amount of 1 to 50% by weight. In a method for the manufacture of the spraying material the main oxides are brought into the plasma stream either separately or as a mixture prepared in advance, with concentration of charged particles between 2 ô 10<24> and 3 ô 10<22>, are partially or completely melted, and the resulting agglomerates are captured.

Description

SPECIFICATION Spraying Material for Hot and Plasma Spraying and a Method of Its Manufacture The invention relates to a material for hot or plasma spraying, especially non-metallic refractory material suitable for the formation of resistant coatings and to a process of the production of such a material.

At present miscellaneous metallic and nonmetallic materials are known and used for the production of protective coatings; the composition of these materials is varied according to required properties of the coating both as to the chemical composition of starting materials and as to their physical properties. These materials exhibit some of the required properties, especially refractoriness, abrasive wear resistance, minimum porosity, good adhesion to the material being coated and resistance to mechanical and thermal shocks as well as chemical resistance to the influence of surrounding medium, but the problem of finding a single coating exhibiting all these properties has not yet been solved.

For example, metallic, especially expensive materials such as chromium, titanium, or nickel, sometimes with additives or alloying additions, are used. These materials, as follows from the properties of their components, produce coatings having very good mechanical properties but poor heat resistance or chemical resistance.

Another large group of spraying materials includes non-metallic materials especially those on the basis of oxide ceramics where spraying material is made up either from one oxide or from a mixture of several oxides in suitable proportions.

Typical representatives of these materials are materials on the basis of aluminium oxide Altos whose characteristic feature is composition of the coating from a mixture of gamma and alpha Awl203 modification. At temperatures above 1 800C irreversible modification transformation of the Al2O3 gamma modification to the alpha modification takes place accompanied by permanent contraction and increase of mass density.Coatings on the basis of Awl203, the socalled corundum coatings are characterized by extraordinary abrasive wear resistance, high adhesion to the material being coated and by very good electric properties; their corrosion resistance is however lower as a result of open porosity, which amounts to 6 to 8% and after transformation to alpha modification is increased to 9 to 10%.Coatings on the basis of zirconium oxide ZrO2 have especially excellent heat insulating properties, those made from chromium oxide Cr2O are very hard and abrasive wear resistant, coatings of titanium oxide TiO2 are dense and well machinable, and very hard are for example coatings of hafnium oxide He02; a common disadvantage of these one-component spraying materials is however their relatively high porosity and from it resulting smaller resistance to the influence of aggressive media.

This disadvantage is partially overcome by coatings on the basis of silicium oxide SiO2 which forms dense coating with very small coefficient of thermal expansion and with zero porosity. This coating is considerably resistant to corrosion and sudden temperature changes, on the other hand its resistance to mechanical impacts is however entirely insufficient.

It has been suggested to improve the properties of plasma coatings by the formation of mixtures of several oxides exhibiting, in suitable proportions more convenient properties than the properties of their components. It is for example zirconium silicate ZrSiO4 with which in the coating composition ZrO2 is mostly in volume stable tetragonal modification in homogeneous mixture with SiO2 in glass form. At temperatures above 11 500C zircon is reversely synthetized in the coating. The coating has excellent resistance to temperature changes, a good heat insulating power and very good resistance to corrosion by melted glass materials, slags and by non-ferrous metals due to poor wetting of zircon by the above-mentioned melts. The general corrosion resistance is however negatively influenced by the open porosity of the coating being 15 to 25% in spite of the presence of a glass form of SiO2.

As a further multicomponent spraying material for example magnesian spinel MgAI204 may be mentioned; it has low porosity, high electrical resistance and excellent adhesion to the material being coated but its corrosion resistance is substantially lower. Also widely used are multicomponent spraying materials on the basis of Al203 with added TiO2 or Cr203, in which TiO2 increases especially compactness of the coating and simultaneously improves of resistance to temperature changes, and Cr203 ensures better abrasive wear resistance, however other disadvantages are not avoided.

Finally it is also known to use Al203 with the addition of SiO2. This spraying material retains very good mechanical properties of corundum coatings, while the presence of SiO2 enhances also corrosion resistance. With regard to the mechanism of transformation of the gamma and alpha Al203 modifications, however, as a result of the negative influencing of the resulting coating porosity, not even with this material corrosion resistance is obtained which is comparable with protective SiO2 layers, which, together with high refractoriness, abrasive wear and resistance to sudden temperature changes is the main aim of this invention.

A further possible way of lowering porosity of the formed coating and thus increasing corrosion resistance is a suitable choice of granulation of the spraying material or use of an amorphous additive, for example zinc, by which, however, adversely influences the hardness and heat resistance of the coating. Known two-component coatings with an amorphous additive do not enable the resistance of the coating to be sufficiently adapted to a corrosion mediumof a given composition.

Spraying materials for hot or plasma spraying are made traditionally by melting the starting material or a mixture of starting materials in an arc furnace and by subsequent treatment to obtain granularity and shape suitable for application by a plasma burner. These processes are considerably uneconomical in view of the relatively small amounts of material treated, especially with respect to high melting temperatures of the spraying materials usually used. In addition when materials alloyed with small amounts of additives are treated so as to obtain very small grains of the size usual for the spraying by a plasma stream non-homogenities become apparent in the structure of the material, which adversely influence the quality of the formed coatings.

Also known are a substantially powerconsuming alloying of spraying materials by diffusion of corresponding additives at high temperatures, or granulation of the mixture of grains of the individual components suitable only for the production of spraying materials consisting of two or more main components contained in the mixture in relatively high proportion by weight.

Also known is a process in which the relatively large grains of one or more components are coated with very fine additives the grain size of which is smaller than 0.3 micrometer. However not even these processes comply with the requirement of high homogeneity of the spraying material.

The mentioned disadvantages of prior art spraying materials for hot or plasma spraying are largely or fully avoided by a spraying material consisting of several metal oxides at least one of which is a glass-forming oxide, which spraying material is formed, according to the invention, by agglomerates of at least two main oxides, such as Awl 203, MgO, CaO, BaO, Cr203, TiO2 or Zero2, in the amount of 50 to 99% by weight, and by at least one glass-forming oxide the melting point of which is by 50 to 11 000C lower than the melting points of the main oxides, such as SiO2, in the amount of 1 to 50% by weight.The spraying material can preferably contain agglomerates of 50 to 80% by weight of CaO, 1 to 5% by weight of MgO, and 18 to 45% by weight of SiO2, or 50 to 90% by weight of MgO, 1 to 5% by weight CaO, and 5 to 45% by weight of SiO2, or 90 to 95% by weight of Cr203, 2 to 8% by weight of TiO2, and 1 to 3% by weight of SiO2, or 65 to 75% by weight of Cr203, 20 to 30% by weight of MgO, and 2 to 10% by weight of SiO2, or 30 to 40% by weight of Al203, 15 to 25% by weight of CaO, and 35 to 50% by weight of SiO2, or 25 to 30% by weight of Al203, 40 to 45% by weight of BaO, and 25 to 35% by weight of SiO2, or 46 to 51 % by weight of Al203, 33 to 41% by weight of Zero2, and 8 to 21% by weight of SiO2, or 25 to 30% by weight of Al203, 25 to 30% by weight of Cr203, 25 to 30% by weight of Zero2, and 10 to 25% by weight of Six2.

The mentioned disadvantages of prior art processes of the production of spraying materials for hot or plasma spraying consisting of several metal oxides from which at least one oxide is a glass-forming oxide are overcome by the process according to the invention the subject matter of which consists in that main oxides and the glassforming oxide or oxides are brought separately or in mixture (prepared in advance) into the plasma stream with the concentration of charged particles between 2.00 x 1024 and 0.3 xl 023, especially into a water-stabilized plasma, are partially or completely melted and resulting agglomerates are captured for example by water or air screen.The process according to the invention can be preferably carried out so that into the plasma stream is brought a mixture of the main oxides with particle size of 0.01 to 0.2 mm and the glass-forming oxide or oxides with particle size of 0.0002 to 0.04 mm, the particles of the main oxides being surface melted and the particles of the glass-forming oxide or oxides being melted or so that the main oxides and the glass-forming oxide or oxides are brought separately or as an in advance prepared mixture into the plasma stream with concentration of charged particles between 2.00x 1024 and 0.3 x 1023, especially into water stabilized plasma, are partially or completely melted and the partially or completely melted particles are applied directly onto the surface which is to be protected by the coating.

The invention will be further explained with reference to several examples.

Example 1 Into the plasma stream of water stabilized plasma burner adapted for spraying of powdered materials is brought a mixture of 65% by weight of powdered CaO with particle size of 0.04 to 0.06 mm, 3% by weight of MgO with the same size, and 32% by weight of SiO2 with particle size 0.0005 to 0.0008 mm, individual particles are exposed to a temperature between 1 5 000 and 60 0000C and after relevant reactions have taken place they are captured by a water screen.

Resulting agglomerates are formed predominantly by dicalciumsilicate accompanied by a small amount of monticellite as a binding phase and a smaller amount of glass phase.

Example 2 Into the plasma stream are brought 70% by weight of MgO and 2% by weight of CaO in a mixture with 28% by weight of SiO2 under conditions analogous to the preceding example.

Materials are applied onto the surtace of a preheated constructional component and they are allowed to cool slowly. Resulting material of the coating will be formed by forsterite accompanied with small amount of periclase, monticellite and of glass phase.

Example 3 For spraying carried out under conditions analogous to the preceding example 95% by weight of Cr203,3% by weight of TiO2, and 2% by weight of SiO2 are used. Resulting material is formed mostly by eskolaite and by a small amount of glass phase.

Example 4 70% by weight of Cr203, 25% by weight of MgO, and 5% by weight of SiO2 are used.

Resulting material is mostly formed by chrompicotite accompanied by a small amount of forsterite and of glass phase.

Example 5 36% by weight of Al203, 20% by weight of CaO, and 44% by weight of SiO2 are used.

Substantial part of the resulting material is formed by anorthite accompanied by glass phase.

Example 6 27% by weight of Al203, 41% by weight of BaO, and 32% by weight of SiO2 are used.

Resulting material is mostly formed by celsian accompanied by glass phase.

Example 7 46 to 51% by weight of Al203, 33 to 41% by weight of Zero2, and 12 to 16% by weight of sio2 are used. Resulting material is mostly formed by corundum accompanied by baddeleyite, mullite and glass phase.

Example 8 28% by weight of At203, 28% by weight of Cr203, 28% by weight of Zero2, and 16% by weight of SiO2 are used. Resulting material consists of approximately equal parts of baddeleyite, ruby and eskolaite accompanied by a smaller amount of glass phase.

The materials prepared according to the abovedescribed examples give security of high refractoriness, resistance to corrosion by metallic or non-metallic melts, abrasive wear resistance and resistance to sudden changes of temperature.

These are new materials of suitable composition and properties containing always a certain amount of glass phase of SiO2 which substantially increases corrosion resistance of coating. In addition to the glass phase also a crystalline phase is always present, the physical and chemical properties of which co-determine the maximum resistance of the coating to a corrosive medium of a given composition and which is formed at least by two main oxides which enables its properties to be selected with great accuracy.

High homogeneity of the formed coating even when relatively small amounts of some additives are used, is achieved by capturing the resulting agglomerates by a water or air screen and by their new application using plasma burner. All the mentioned spraying materials can be applied, with very good results, directly on to the surface which is to be protected by the coating, practically non-porous coating being obtained by the choice of the proportion and size of SiO2 particles, while very good mechanical properties are retained.

Claims

1. Spraying material for hot and plasma spraying comprising agglomerates of at least two main oxides, such as Awl203, MgO, CaO, BaO, Cr203, TiO2 or Zero2, in an amount of 50 to 99% by weight, and at least one glass-forming oxide the melting point of which is by 50 to 11 000C lower than the melting points of the main oxides, such as SiO2, in an amount of 1 to 50% by weight.

2. Spraying material according to Claim 1 comprising 50 to 80% by weight of CaO, 1 to 5% by weight of MgO and 18 to 45% by weight of Six29

3. Spraying material according to Claim 1 comprising 50 to 90% by weight of MgO, 1 to 5% by weight of CaO and 5 to 45% by weight of SiO2.

4. Spraying material according to Claim 1 comprising 90 to 95% by weight of Cr203,2 to 8% by weight of TiO2 and 1 to 3% by weight of Si02.

5. Spraying material according to Claim 1 comprising 65 to 75% by weight of Cr203,20 to 30% by weight of MgO and 2 to 10% by weight of Si02.

6. Spraying material according to Claim 1 comprising 30 to 40% by weight of Al203, 15 to 25% by weight of CaO and 35 to 50% by weight of SiO2.

7. Spraying material according to Claim 1 comprising 25 to 30% by weight of Al203,40 to 45% by weight of BaO, and 25 to 35% by weight of SiO2.

8. Spraying material according to Claim 1 comprising 46 to 51% by weight of Al203,33 to 41 % by weight of ZrO2, and 8 to 21 % by weight of Six28

9. Spraying material according to Claim 1 comprising 25 to 30% by weight of Al203,25 to 30% by weight of Cr203, 25 to 30% by weight of Zero2, and 10 to 25% by weight of SiO2.

10. Spraying material for hot and plasma spraying substantially as herein described with reference to the Examples.

11. A method for the manufacture of a spraying material according to Claim 1 wherein the main oxides are brought into the plasma stream either separately or as a mixture prepared in advance, with concentration of charged particles between 2 xl 024 and 3 xl 022, are partially or completely melted, and the resulting agglomerates are captured.

12. A method according to Claim 11 wherein the plasma is a water-stabilized plasma.

13. A method according to Claim 11 or 12 wherein the agglomerates are captured by a water screen or an air screen.

14. A method according to any one of Claims 11 to 13 wherein a mixture of main oxides, of a particle size of 0.01 to 0.2 mm, and of the glassforming oxide or oxides, of a particle size of 0.0002 to 0.04 mm, is introduced into the plasma stream, the particles of the main oxide being surface-melted and the particles of the glassforming oxide or oxides being melted.

1 5. A method for the manufacture of a spraying material according to Claim 1 wherein the main oxides and the glass-forming oxide or oxides are either separately or in a mixture prepared in advance brought into the plasma stream with concentration of charged particles between 2x 1 o24 and 3 xl 022, are partially or completely melted, and the partially or completely melted particles are applied directly on to the surface which is to be protected by the coating.

1 6. A method according to Claim 15 wherein the plasma is a water-stabilized plasma.

1 7. A method for the manufacture of a spraying material according to Claim 1 substantially as herein described.

1 8. Spraying material for hot and plasma spraying made by a method according to any one of Claims 11 to 17.

1 9. An article sprayed by a spraying material according to any one of Claims 1 to 10 or 18.