Roll comprising an abradable coating

Abstract

A protective and abradable coating composition is suitable for application on rolls and more particularly for application on conveyor rolls. The abradable coating is suitable for use in high temperature applications. Rolls incorporating the coating may be produced and used according to disclosed processes and procedures. Application of the composition to rolls reduces corrosion by aluminium melt, and enables the removal of built-up substances by friction. The life time of the roll is thereby increased.

Claims

1. Conveyor roll comprising a cylindrical exterior surface, wherein the conveyor roll comprises at least a layer of a coating, wherein the coating comprises a) 83-98 wt % of an aggregate comprising particles of a material selected from the group consisting of hydrolyzed Si.sub.3N.sub.4, SiC or SiAlON and a mixture thereof, b) 2-17 wt % of submicronic particles comprising a material selected from the group consisting of Al.sub.2O.sub.3, SiO.sub.2, ZrO.sub.2, CeO.sub.2, Y.sub.2O.sub.3 and a mixture thereof, and wherein the coating is heated at a temperature of at least 800 C. for at least three hours.

2. Conveyor roll according to claim 1 wherein the aggregate further comprises particles of Al.sub.2TiO.sub.5.

3. Conveyor roll according to claim 1 wherein the specific surface area (BET) of the aggregate of particles is from and including 0.5 m.sup.2/g to and including 8 m.sup.2/g.

4. Conveyor roll according to claim 1 wherein the aggregate of particles comprises a plurality of types of particles, wherein each type of particle has a different specific surface area.

5. Conveyor roll according to claim 1 wherein the coating comprises a plurality of layers as described in claim 1.

6. Conveyor roll according to claim 1, wherein the average thickness of the coating is at least 150 m.

7. Conveyor roll according to claim 1 wherein the conveyor roll comprises a material selected from the group consisting of fused silica, mullite, sillimanite, alumina, SiC matrix and metal.

8. Conveyor roll according to claim 1 wherein the coating comprises a temporary binder selected from the group consisting of acrylic polymer, epoxy polymer and polyvinyl alcohol.

9. Conveyor roll according to claim 1, wherein the conveyor roll comprises a single lateral radius.

10. Process for manufacturing a conveyor roll comprising the steps of: a) providing a roll made of a material selected from the group consisting of fused silica, mullite, sillimanite, alumina, SiC matrix and metal, wherein the roll comprises a core and a cylindrical exterior surface; b) applying on at least a portion of a surface of the roll, a composition comprising a) 50-75 wt % of an aggregate comprising particles of a material selected from the group consisting of hydrolyzed Si.sub.3N.sub.4, SiC, SiAlON and a mixture thereof, b) 1.5-10 wt % of submicronic particles comprising a material selected from the group consisting of Al.sub.2O.sub.3, SiO.sub.2, ZrO.sub.2, CeO.sub.2, Y.sub.2O.sub.3 and a mixture thereof, and c) 15-48.5 wt % H.sub.2O, characterized in that the particles of Si.sub.3N.sub.4, SiC, SiAlON and a mixture thereof have a specific surface area (BET) in the range from and including 0.5 m.sup.2/g to and including 8 m.sup.2/g; c) drying the roll; and d) heating the roll at 800 C. for at least 3 hours.

11. Process according to claim 10 wherein the composition is coated by a process selected from the group consisting of spraying, dipping, flooding and plasma spraying.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 is a side view of a roll of the invention installed in a testing mechanism.

DETAILED DESCRIPTION OF THE INVENTION

(2) The invention will be now illustrated by way of examples according to the invention and comparative example.

(3) The abradable property is assessed by the wear resistance test and the scratch resistance. A minimum of scratch resistance is required to manipulate the roll without damages. In addition, the wear resistance should not be too low to ensure a minimum of lifetime of the coating but not too high, to allow the build-up substances to be removed by friction. It has been noticed by the inventors that the abradable property, results from a combination of the two resistance values.

(4) The wear resistance is determined according to the following protocol. FIG. 1 depicts a side view of the testing conditions. A fused silica roll (1) of 50 mm length and with a diameter of 50 mm is weighed and then provided with a 200 m coating (4) of the composition to be assessed. Once the coating (4) has dried (60 C. for 2 hours), the coated roll is weighted again and the coating weight determined.

(5) The roll (1) is mounted on an axis (5) and a metal plate (3) (USIBOR1500 steel sheet of 2001001 mm, 150 g) retained horizontally between two legs (8a,8b) of 20(c)60 mm on an axis (6) mounted perpendicularly to a support (2), is arranged to enter into tangential contact with the coated roll. The two axes (5) and (6) are parallel. The plate (3) is retained between the legs (8a,8b) by a bolt (7) so as not be displaced by the rotation of the roll. The plate is tilting progressively on the axis (6) as the coating is worn. The distance (a) between the axis (6) and the contact point plate (3)/coated roll is set to 150 mm. The distance (b) between the contact point plate (3)/coated roll and the end of the plate (3) closest to the support 2 is set to 125 mm. The roll is rotated at 220 r m. The rotation is periodically interrupted and the remaining coating weight is determined. The operations are repeated until 30% by weight of the coating has been abraded.

(6) A coating is marked () if 30% by weight of the coating is worn in less than 250 minutes. A coating is marked (++) more than 500 minutes are required to wear 30% by weight of the coating. A coating is marked (+) if 30% by weight of the coating is worn in a time comprised between 250 and 500 minutes.

(7) The wear resistance represents the strength of the bond between coating components. Scratch resistance represents the adhesive strength to the substrate. It is the load (in Newton) required to scratch the coated surface. It was found surprisingly that scratch resistance can be relatively high while the wear resistance is low.

(8) Below a value of 1 N, the adhesive strength is too weak. The coaling is damaged when the roll is transported or manipulated.

(9) When the scratch resistance is higher than 7N, the coating is considered to have a strong adherence. The coating can however, still be abraded.

(10) As examples, three compositions (Composition 1-3 used to produce examples 1 to 9) are made by mixing silicon nitride powders with a colloidal solution of SiO.sub.2 or Al.sub.2O.sub.3 and water. The roll is then dipped in the composition solution and then dried at 60 C. for 2 hours. Examples of coating compositions are shown in table 1.

(11) TABLE-US-00001 TABLE 1 Composition Composition 1 Composition 2 3 Si.sub.3N.sub.4 wt % 62 62 50 Submicronic SiO.sub.2 wt % 3 0 0 Submicronic Al.sub.2O.sub.3 wt % 0 2 4 Water wt % 35 36 46

(12) The rolls (examples 1-9) are then assessed in three different conditions.

(13) Firstly (examples 1-3), the rolls are not submitted to further heat treatment than the drying temperature.

(14) Secondly (examples 4-6), an organic binder is added in the coating composition at a concentration of 2 wt %. The rolls are dipped in the coating solution then dried and tested.

(15) Thirdly (examples 7-9), the rolls-coaled using a composition comprising the organic binderare healed at 800 C. for 3 hours.

(16) As a comparative example (Example 10), a roll is coated with the composition described in US-A1-2008-260608. Because of the granulometry of the Si.sub.3N.sub.4 particles (the grains are mainly 1 micron), the coaling is formed by spaying. After a drying step, the roll is healed at 1000 C. for 3 hours. The composition is the following:

(17) TABLE-US-00002 Comparative sample Example 10 Si.sub.3N.sub.4 wt % 40 Submicronic SiO.sub.2 wt % 10 Water wt % 45 Polyvinyl alcohol 5

(18) The embodiments give the results shown in Table 2.

(19) TABLE-US-00003 TABLE 2 Firing No Heating 1000 C. heating/temporary 800 C. (3 h) No heating organic binder (3 h) Comparative Composition 1 2 3 1 2 3 1 2 3 sample Example 1 2 3 4 5 6 7 8 9 10 Wear resistance ++ ++ ++ + + Scratch <1 <1 <1 >10 >10 >10 2 4.5 7 <1 resistance (N)

(20) As can be observed in table 2, examples (1-3) which are not fired, have a low wear and scratch resistance. These coated are aluminium corrosion resistant but the rolls are not transportable.

(21) Adding an organic binder to the composition increases the scratch and the wear resistance of all the compositions (Examples 4 to 6).

(22) When heating the rolls at 800 C. for at least 3 hours (Examples 7 to 9), the organic binder disappears without damaging the silicon nitride layer. The wear resistance is slightly improved in comparison with non-heated rolls. With a weak wear resistance (), the built-up substances will tend to fall with the silicon nitride particles when submitted to friction. Increasing the wear resistance (+/) and the scratch resistance, built-up substances will tend to fall with isolated silicon nitride particles but also with the same forming a portion of coating. Higher value of scratch resistance can lead to more adherent built-up substances requiring then further strength to remove them.

(23) A high scratch resistance allows to replace the roll without decreasing the furnace temperature which can be required when maintenance or roll replacement is needed. Despite the healing, the wear resistance and the scratch resistance values of the comparative sample are low (Example 10). The behaviour of this coating is completely different. The reason is that the particle size of the nitride particles is lower than 1 m. Although the coating was heated at 1000 C., a roll with this coating cannot be used in this industrial application as the low wear resistance is leading to a highly abradable coaling. After a short time, no more coating remains on roll surface inducing appearance of build-up and corrosion of roll body.

(24) Numerous modifications and variations of the present invention are possible. II is, therefore, to be understood that within the scope of the following claims, the invention may be practiced otherwise than as specifically described.