The present invention relates to a method and an apparatus for making a wear- resistant coating on a treatment blade, such as a coating blade, doctor blade or creping blade, intended for the treatment of a paper web. The invention also relates to a treatment apparatus having a blade provided with a wear-resistant coating, and to a treatment blade.

In the coating of paper, the paper is generally coated with a paste-like additive containing e.g. a pigment and binding agents. The aim of coating is to improve the properties of the paper surface. The coating is performed in the coating unit of the paper machine, where a coating material is applied to the surface of the raw paper and smoothed out. The coating may be performed e.g. in a trailing blade coater, where the coating material is applied to the paper surface and smoothed by means of a coating roll and coating blades arranged in conjunction with it. The coating blade edge facing towards the paper web is beveled. To increase the wear resistance of coating blades, the edge of the coating blade facing towards the paper web is coated with a wear-resistant material, such as a ceramic material. With a coated blade, a longer service life is achieved, leading to fewer blade changes, less waste paper, more paper for the same machine time and a better paper quality.

Coating blades can be coated with a wear resistant plating e.g. as disclosed in published patent application Fl 71794. In this Fl application, ceramic materials, the coating is performed using metal oxides or metal carbides. The coating is implemented using plasma or flame spraying, by passing a carrier material strip through one or more plating stations, each of which is provided with one or more metal spraying guns adjustable relative to the strip and arranged to deliver a predetermined amount of molten coating material. A wear resistant coating is produced in a stepwise manner by bringing several layers one over the other, which makes it possible to avoid especially the degradation of the properties of the blade, such as its flexibility, during the coating process. Before being coated, the carrier material strip is pretreated e.g. by grinding or brushing it by means of a grinding disc or steel wire brush rotating in its longitudinal direction. The strip material can be caused to pass back and forth through the coating

station to gradually increase the coating material thickness by supplying it from reels placed on either side of the coating station

The problem with prior-art technology is how to optimize the surface qualities and adherence of the coating so as to allow the properties of the coating material, which often consists of several components, to be so selected that a sufficiently wear-resistant surface and yet a good adherence of the coating will be achieved.

The object of the present invention is to overcome the drawbacks of prior art and achieve a new type of coating, a so-called FG (Functional Graded) coating, as well as a method and a system for making a wear resistant coating on the edge of a coating blade or equivalent. The coating and method of the invention are based on the idea of changing the mixing ratio of the components of the coating material as selected during the coating process.

The details of the features characteristic of the method, apparatus and treatment blade of the invention are presented in the claims below.

In the following, the invention will be described in detail by means of an example with reference to the attached drawing, wherein

Fig. 1 presents a coating blade produced by an apparatus according to the invention, fitted in conjunction with a coating roller, and

Fig. 2 presents a longitudinal section of the coating of a coating blade.

Fig. 1 presents a blade designed for the coating of paper in a trailing blade coater, where a coating material 1 is applied to the surface of a paper web 2 running between rollers and smoothed by means of a coating roller 3 rotated in the direction indicated by the arrow and a coating blade 4 arranged in conjunction with it. The coating blade edge 41 facing towards the paper web 2 is beveled. To improve the wear resistance of coating blades, the coating blade edge 41 facing towards the paper web is coated with a coating 42 resistant to wear in the direction of web motion.

The wear-resistant coating is produced via thermal spraying, such as plasma, flame or detonation spraying, by using a spray gun.

The wear-resistant coating 42 is a mixture of two powdery coating materials, wherein a first coating agent A may be e.g. a hard metal or a metal alloy, e.g. wolfram carbide, chrome carbide, titanium carbide, possibly containing additives, while a second coating agent B correspondingly contains some other material, such as titanium oxide, or aluminum oxide, AI ₂ O3, e.g. so that agent A has good properties of adherence to the blade metal while agent B has good surface properties, agent A being placed alone against the blade and agent B alone on the outer surface. The agents are sprayed thermally via two different nozzles.

The coating may consist of e.g. five layers as shown in Fig. 2, where the sur- face layer is 421 and the base layer 425, gradated e.g. according to the table below:

TABLE

The composition of the mixture may vary in other respects as well, and the composition of the mixture may also vary in a stepless manner, i.e. continu- ously.

The wear-resistant coating is produced according to the invention by the new coating technique of the invention, via thermal spraying, such as atmospheric plasma spraying (APS) or high-velocity oxy fuel (HVOF) spraying.

It is obvious to a person skilled in the art that different embodiments of the invention are not exclusively limited to the embodiment example described above, but that they may be varied within the scope of the claims presented below. Besides the coating blade, the invention can also be applied to other blades used in the treatment of a paper web, such as doctor and creping blades.