The invention relates to a thermal sprayable coating powder, to a method based on spray-drying techniques for making a thermal sprayable coating powder as well as to the use of a coating powder in paper machine applications such as coating of paper machine rolls and components.

Thermal sprayable coatings are generally used in paper machines, mainly as coating material for rolls. In paper machine applications, the development of coating materials and methods has made it possible to produce coatings in which by suitable blending of the base component one unites good mechanical properties on the one hand, such as especially resistance to abrasion, and optimal functional properties on the other hand. To the most important of the latter mentioned prop- erties, especially when roll coatings of paper machines are in question, belong optimal adhesion of paper web to the roll surface and its optimal release there- from. Its importance is crucial in order to avoid web breaks, and as running speeds of the paper machine increase, control of the adhesion/release properties is even more crucial. As a weakness, the thermal sprayable blended ceramic coatings show especially in certain applications, a relatively weak resistance for corrosion.

Coatings and/or coating material solutions applicable to thermal spraying are de- scribed among other things in publications FI 971543, WO 95/07767 and EP91 112585. 4.

From Patent Application FI 971543 is known a calender roll and a coating com- position where on the roll body is produced by spraying a ceramic coating which comprises from 50 % to 92 % of Cr203, from 3 % to 5 % of Ti02 and possibly a third metal oxide.

In Patent Application WO 95/07 767 there is described a thermal sprayable pow- der as well as a method for producing the same. The powder particles comprise a core which may consist of nickel, iron, cobalt, copper and chromium or their combinations, as well as a layer surrounding the core which consists of finely di- vided particles which may in the first place be of aluminium or of an aluminium alloy, or in addition to aluminium also silicon, magnesium and titanium may be used whilst in any case aluminium stands for at least 80 % by weight of the finely divided surface particles. The size of the core particles is from 38 J. m to 125 pm in diameter, and on the core particle there is agglomerated, preferably with the aid of a binding agent such as polyvinylpyrrolidone, the finely divided particle mate- rial where the particle size is from 0,2 llm to 10 J. m in diameter.

In Patent Application EP 91 112 585.4 is described coating of a roll by spraying of a smooth glass layer. The thermal sprayable finely divided glass powder con- tains from 30 % to 73 % silica and in addition lesser amounts of some other ox- ides.

Attempts have been made to improve the tightness of the coatings among other things by means of tight ceramic layers situated under the coating as is de- scribed in Patent Application FI 971541 or by treating the surface and/or the in- termediate layers with sealing compounds in ways described in Patent Applica- tions FI 955040 and FI 990298.

The oxide ceramic powders used in thermal spraying are most generally produced by melting and crushing the material to be sprayed. However, hereby the powder particles obtained are of indefinite and unregular shape which causes problems among other things in relation to the fluidity of the powder obtained. Producing a homogeneous coating calls for, in addition to an even feeding of the powder, that is a good fluidity, also inner homogeneity of particles in relation to porosity, phases and the distribution of elements. When using in thermal spraying a coating powder produced by melting and crushing, a coating is obtained that may in

places be very nonhomogeneous indeed, and the tightness of the coating also var- ies. These factors naturally have an effect on the abrasion and corrosion resistance of the coating as well as on other properties of the coating. As producing a homo- geneous powder by melting and crushing requires, besides solubility in molten state, also control of phase reactions during solidification and slow cooling, the possibility of regulating potential powder combinations and microstructure of the coating formed is markedly low. Better modification of the microstructure and of the powder combinations may be achieved by other methods. Other methods used in making ceramic powders are sintering, spray-drying and the so-called"clad- ding", where the particles are coated or their surface is treated with another mate- rial. Coating may be accomplished chemically from solution or from gas phase, by means of fluidized bed drying or spray drying. The so-called powder blends are simple mechanical mixtures of sprayable powders produced by one or more of the aforementioned methods. Nevertheless, spray-dried powders are generally coarse in structure, with the result that their mechanical properties are poor, and the particles may in spite of sintering become apart when feeding the powder, which causes fluidity problems. The coarseness is caused by the large size of the starting powders used in making the spray-dried powder as well as by insufficient sintering of the powder. In addition, starting powders with a larger particle size form a coarser microstructure to the coating and make the formation of unbal- anced structures less likely. The coarsest composite structure is accomplished with the so-called powder blends, in which the interactions between various phases are minimal.

In general, when good resistance to abrasion along with good chemical resistance is required of a thermal sprayable coating, chromiun oxide Cr203 is used. The most general chromium oxide powders produced by melting and crushing are ei- ther pure grades containing more than 90 % of Cr203, or they will contain small amounts (less than 5 %) of other blended oxides, among other things of Ti02 or of Si02, either as impurities or as added. These powders may be made also by sin- tering and spray-drying. The purpose of the blending is to improve the toughness

and tightness of the sprayed coating. Traditionally, attempts have been made to avoid chromium oxide proportions higher than 5 % in order not to loose abrasion resistance properties. About 5 % is regarded as the solubility limit of silica in chromium oxide, which is the reason why silica is generally not blended in con- centrations higher than 5 %. Higher concentrations of SiOz of more than from 5 % to 10 % cause a sizable nonhomogeneity in the powder, because in blending in the molten state a solubility gap is produced between chromium oxide and silica, whereby both oxides are already in molten state as separate phases, and producing a homogeneous powder by melting becomes impossible. The use of a higher Si02 proportion in spray-dried powder has traditionally also been limited by poor sprayability of silica due to the pure silica phase's easy vaporization, as a result of which the porosity of the surface is increased. In a finished coating the secondary Si02 phase is susceptible to dissolving especially when located on the granular or lamellar boundaries, whereby the mechanical properties of the coating as a whole are weakened and the coating disintegrates. This phenomenon is accelerated by extensive porosity of the surface. Silica is especially susceptible to dissolving in conditions, in which the pH is basic.

Due to these shortcomings in prior art there is a continuous need to provide a thermal sprayable chromium based coating composition, in which are united good resistance to abrasion, optimal release properties as well as good resistance to cor- rosion. Furthermore, there is a need to provide a coating which is suitable espe- cially for paper grades made of mechanical pulp and the resistance to corrosion of which is improved even at high pH values.

In the present invention the objective stated above has been reached with a new coating powder, the microstructure of which is more homogeneous due to pro- ducing techniques of the powder.

The coating powder according to the invention, the inventive method of making the same as well as the inventive use of the coating powder are characterized in what has been stated in the claims.

It has been discovered that the problems related to prior art can be avoided or sub- stantially diminished by using the inventive solution. The invention is based on applying the spray-drying technique to the mixtures of chromium oxide and silica while producing oxide ceramic powder especially for coatings of components of paper machines such as rolls where the goals are a homogeneous and tight sur- face, good release properties of the paper web and also in addition cleanliness of the surfaces. Powders produced by spray-drying techniques are extremely well suited to thermal spraying because with this method a special even, round shape is obtained for the powder particles plus a homogeneous distribution of the blended components. The advantage of spray-drying is that it provides, besides a limitless possibility of regulating blending ratios even a possibility of tailoring by a certain standard composition the microstructure of the coating being formed. This is ac- complished by using blend material particles of different sizes and/or by bringing a blend component to the surface of the particles as a salt separating from the so- lution during drying or as particles of the order of nanometers. By using in the spray-dried powder starting material components that are small and reactive enough, one may produce in the coating quickly cooling from molten state various unbalanced structures. These unbalanced structures comprise an increased degree of amorphism of the coating, formation of unstable phases as well as an increased solubility of the solid state. With these the mechanical, chemical and electric properties of the coating may be influenced. Furthermore, the structure of the spray-dried powder and with that the structure of the coating formed may be in- fluenced by sintering.

Thanks to the inventive production technique, an exceptionally high Si02- proportion may be used with a Cr203 based powder, that is from 15 % to 25 % by weight of SiOz without the wearing qualities significantly suffering. The Si02-

blending of this order in the chromium oxide matrix has been found advantageous as to the corrosion qualities and adhesive properties. One reason for this is obvi- ously the extremely good dielectric quality of silica in that the risk of formation of a galvanic couple between the base and the coating is diminished and in that the electric properties of the surface are influenced, among other things its electric charge and possibly even its hydrophilicity. The latter factors affect the release properties of the web and cleanliness of the surface.

Addition of Si02 content renders the surface of pure chromium oxide more hy- drophilic. The reason for this phenomenon is not accurately known, but it may be due to the more hydrophilic character of Si02 in relation to chromium oxide, which is true when Si02 exists as a phase of its own, or in the case of mixed phases the reason is the more hydrophilic character of the amorphous mixed phases being formed in relation to pure chromium oxide. The phenomen might also be due to the increased solubility of Si4+ in the chromium oxide lattice, while this changes the surface properties of the chromium oxide, or the reason might be all of the above. The surface charge behaviour is still dominated by chromium oxide.

The more finely divided microstructure and homogeneity of the surface also affect the improvement of corrosion characteristics. In the invention, homogeneity is accomplished by using a very finely divided Cr203 starting material, the particle size of which is typically 10 J. m at the most and is on the average from 0,1 llm to 2 J. m, and especially by controlling the particle size of Si02 by blending either some of the Si02 or the whole amount of Si02 as particles having a size of the order of a nanometer. The blending of Si02 with the size of the order of a nano- meter, from 1 nm to 300 nm and typically from 10 nm to 150 nm is accomplished from a sol, where the silica is added to the spray-dried suspension as an extremely finely divided water-based dispersion. When added as powder, the particle size of Si02 is typically from 0.3 pm to 10 im and on the average from 0.5 um to 3 um.

Regardless of whether a sol or powdered Si02 is used, by a Si02 blend a coating

containing Cr203 produced from powder is attained that provides for very good electric insulation, as a result of which the corrosion resistance characteristics achieved by means of the coating are also improved.

When referring to Si02, materials are meant in which the Si02 content exceeds 50 % by weight. The invention thus encompasses among other things various sili- cates, glasses and pure silicas. The silica may be amorphous or crystalline. It is also possible to improve the sprayability of the Si02 particles by coating the Si02 particles with chromium oxide or with another suitable metal oxide or by using blended Si02 grades. The coating may take place either before spray-drying (chemical coating, sol-gel) or during spray-drying by using a Cr203 powder with an extremely small particle size of the order of from 1 nm to 200 nm on the sur- face of the relatively larger Si02 particle. The particles or liquid components may be separated on the surface of the particles by means of the pH, temperature or pressure.

In the method according to the invention a suspension is produced preferably by dispersing the starting material powders and a silica sol, if any, into water while optionally using a suitable dispersing agent. A silica sol is composed of a stable colloidal silica solution, in which the amorphous silica particles are of the order of nanometers, from 1 nm to 300 nm and typically from 10 nm to 150 nm. The sol may be used as such as a binding agent, or to the suspension may be added as binder any inorganic or organic binding agent used in the production of ceramic powders or formed pieces such as polyethylene glycol, polyvinyl alcohol or mix- tures of binding agents. If Si02 is added as powder, its particle size is typically from 0.3 m to 10 fim, the average particle size being from 0.5 m to 3 pm. The chromium oxide to be added to the suspension is a finely divided powder, whose particle size is typically less than 10 J. m, the average particle size being from 0,1 llm to 2 pm. The suspension is spray-dried whereby homogeneous, spherical ag- glomerates are obtained as a result, one being able to adjust their particle size dis- tribution by adjusting the suspension parameters and spray-drying parameters as

desired. Any suitable equipment known in the art may be used as spray-drying equipment.

After spray-drying the particles may be sintered at temperatures of 900-1500 °C, preferably under reduced pressure, in an inert gas atmosphere or in normal atmos- pheric conditions, whereby particles with good mechanical strength and resistance are obtained. The particles of the coating comprise from 15 % to 25 % by weight of Si02, from 75 % to 85 % by weight of Cr203 and optionally a binding agent, under 8 % by weight and preferably from 1 % to 2 % by weight, as well as op- tionally a dispersing agent. The inventive coating powder is round in particle shape and, finely divided, the particle size of the coating powder being preferably from 5 J. m to 100 pm.

The shape of the powder particles is illustrated by attached Fig. 1, showing a (SEM) electron microscope photograph of the powder particles. From the picture it may be clearly seen that the particles have the shape of a regular sphere and that they have plenty of inner agglomerate contact surface, with the result that the me- chanical strength of the agglomerate is high.

The thermal sprayable coating according to the invention is illustrated by attached Fig. 2, showing a (SEM) electron microscope photograph of a cross-section of the coating. The lighter area in the photograph is chromium containing matrix. The areas containing pure silica are perceived as dark, the various gray shaded areas being mixed phases. From the picture it may be seen that the components are ho- mogeneously distributed in the coating. Furthermore, it may be seen that the mi- crostructure is extremely finely divided.

Very tight, homogeneous and even ceramic surfaces can be produced by means of the inventive finely divided and homogeneous coating powder.

The finely divided coating powder particles according to the invention may be used in making thermal sprayable coatings for various paper machine applica- tions, such as rolls, especially press rolls, such as the center roll of a press or the backing roll of a long-nip press, or calender rolls, foils and doctor and coating blades and rods. The surface to be coated may be of metal, ceramics, metal ce- ramics or a fibre composite. Preferably the inventive coating powder may be used as coating for rolls used in manufacturing paper grades made of mechanical pulp.