GB2025470A - Machine component having a wear-resistant coating - Google Patents

Abstract

As shown the component is a piston ring 1 with a molybdenum or molybdenum alloy coating 2 in a groove between flanks 3 and 4. By beating to a temperature above the operating temperature fissures 5 running radially of the ring are formed as a result of differential thermal expansion. The fissures 5 are filled with solid products of reaction between the molybdenum and a gas or melt to which it is exposed when heated. Upon cooling the coating 2 is compressed between the flanks 3 and 4 and remains in a state of compression during use. <IMAGE>

Description

SPECIFICATION Machine component having a wear-resistant coating The invention relates to a machine component which is loaded in respect of sliding friction, comprising a wear-resistant protective coating on the working surface, such as in particular a piston ring or a sealing strip for internal combustion engines, comprising a thermal spray coating on the working surface, of a construction which is chambered at one or both sides, and the process for the production thereof.

It is known that machine components which are subjected to a wear loading, such as in particular cast iron sealing strips or piston rings for internal combustion engines, which are exposed to high thermal and mechanical loadings in operation, are provided with coatings or their working surfaces, in particular for the purposes of increasing their wear resistance, their resistance to combustion stresses and their sliding properties. Preferably, these coatings are applied by known thermal spray processes, and thermal spray coatings of molybdenum or molybdenum alloys have been found particularly satisfactory.

However, a disadvantage with such coatings is that their outer edges are relatively brittle and must therefore by chamfered off in order to avoid fractures. This means however that they do not have a sharp outer edge which lies closely against the co-operating surface, so that exhaust gas leakage, so-called blow-by, can occur in internal combustion engines. In order to remedy this, as has long been known, recesses or grooves are formed in the base material of the machine components, and the recesses or grooves are then filled with a spray material. Depending on whether a web portion was left only at one outer edge or at both outer edges, the coatings are referred to as being of a construction which is chambered at one side or at both sides.In other cases, a plurality of parailel grooves are also formed in the base material, and correspondingly filled with the coating material.

Although the machine components which are coated in this way then have sharp outer edges with a good sealing action and at the same time good working qualities, fissures may occur either in the coating itself or between the coating and the base material of the machine component, in the region of the web portions of the chambers, particularly when the machine component is subjected to high thermal and mechanical loadings. The fissures can then extend into the bottom of the coating and from there can extend along the interface between the coating and the machine component, and can then turn upwardly and extend up to the working surface.

Depending on the period of service or the loading, entire regions of the coating can then break off, so that the machines completely break down.

Initially, it was assumed that the cause of such coating fractures was the known low strength and adhesion of the spray materials.

Therefore, in order to remedy the aboveindicated problems, attempts were made to increase the strengths and adhesion of the coating materials by using special spray powder compositions. Although coatings with improved strength and adhesion properties were developed, at considered technical cost, nonetheless fractures of the coatings still occurred.

In principle, it was even found that the number of coating fractures increased. Therefore, it is assumed nowadays that the materials for the coating and base members are subjected to different degrees of expansion, because of the differences in their coefficients of heat expansion, so that stresses which can result in fissures with subsequent coating fractures occur in the operating heat of the engine, particularly at the contact surfaces with the chamber wall.With the combination of materials which are preferably used in piston rings, namely cast iron or steel with molybdenum or molybdenum alloy, the above-described effect is particularly serious in that case,-bec use firstly the coefficient of heat expansion of iron is approximately double that of molybdenum, and because piston rings in internal combustion engines are exposed to fluctuating temperature loadings of some hundreds of degrees.

The problem of the present invention is therefore to provide machine components which are exposed to frictional wear, comprising wear-resistant protective coatings on the working surfaces, in particular piston rings or sealing strips for internal combustion engines, comprising thermal spray coatings on the working surface, of a form which is chambered at one or both sides, which components have improved resistance to fissure formation and fractures when subjected to thermal and mechanical loadings. The process for the production thereof is also to be as simple as possible.

According to the invention, this problem is solved by a machine component whose wear coating in the rest condition at normal temperature is under a compression stress which is reduced or at most nullified in the operating condition at the appropriate temperature. According to the process of the invention, for this purpose the machine components are heat-treated, after application of the wearresistant protective coating in chambered form, wherein the machine components are simultaneously exposed to a medium which at that temperature reacts at least with the coating material, forming solid reaction products which are deposited.

The invention was based on the observation that some piston rings with molybdenum spray coatings of chambered form, whose coatings already have fissures directly after the spraying operation and which therefore were considered to be defective and unsuita ble on the basis of the previously held as sumptions, were surprisingly defect-free in test runs when used in an engine, contrary to the previous assumption, and did not suffer from any fractures.

Polished cross sections under the micro scope clearly showed that the fissures in these rings were solidly filled up, and analysis showed that the filling material was molyb denum oxide which had evidently been formed directly after the spraying operation, while still in the hot condition, by reaction between the molybdenum and the oxygen of the air present. The coatings on these rings can no longer contract when they cool and a compression stress therefore exists in the rest condition. In operation in an engine, at the corresponding temperature, these stresses are then reduced and the piston rings are used in a residual compression stress condition.

The heat treatment which is performed in accordance with the invention now causes fissures to be intentionally produced in spray --coatirrgs, and the fissures open in a slot-like manner when subjected to the action of heat.

When this occurs, reaction between the coat ing material and the surrounding medium which is used in accordance with the inven tion occurs simultaneously in these regions, the reaction products fill the gaps when sub jected to the action of heat and bond them firmly together by diffusion bonding. After cooling, a compression stress is then built up in the coating in-the axial direction.

According to the invention, the heat treat ment is performed in a gas atmosphere, salt bath solution, fused salt baths or powder packings which, according to their composi tion, contain the substances which react with the coating material. In the case of spray coatings of molybdenum or molybdenum al loys, particularly suitable forms af heat treat ment are those carried out in air or pure oxygen or in sulphur-bearing fused salt baths, as molybdenum then forms molybdenum ox ide or molybdenum sulphide respectively as the solid reaction product which fills the fissures. In this case, there are obviously also reaction products on the unprotected side surfaces of the web portions and the rear surfaces of the rings, but these oxide and sulphide coatings additionally improve the wear resistance of the rings, in per se known manner.If required however, these coatings can also be ground away or can be avoided by previously masking the relevant areas.

The temperatures used in the heat treat ment which is carried out in accordance with the invention are above the subsequent operating temperature of the machine components. This ensures that fissures which are then filled with the reaction products are formed in any case in the spray coatings. This manner of procedure ensures that compression stresses are formed in all spray coating, by virtue of cooling of the components, and the compression stress is then at most completely nullified when subjected to the operating heat, according to the heat treatment temperature. The coatings are therefore used almost in a stress-free condition, and the coating of all machine components are thus effectively protected from breaking off.

The invention therefore provides machine components whose wear-resistant coatings are protected from breaking off, by a simple process. The invention is particularly effective for piston rings or sealing strips for internal combustion engines, with spray coating on the working surfaces in a chambered form, as these machine components are exposed to particularly high temperature fluctuations, in operation. In addition, the chambered form of wear-resistanct protective coatings is in practice preferably used in respect to thermal spray coatings. In principle however, the invention can also be used with other wearresistant protective coatings, such as for example galvanic coatings.At the same time, a particular advantage with the process according to the invention is that the equipment which is usually employed for metal hardening and heat-treatment processes can be used without substantial conversion, for carrying out the process of the invention.

The invention will now be described in greater detail with reference to two embodiments and a drawing. The drawing shows a view in cross-section through a piston ring which is produced in accordance with the first embodiment.

In this embodiment, the basic machine component in question is piston rings of rectangular cross-section, an outer diameter of 107 mm, an inner diameter of 97 mm, and an axial height of 3.1 mm. A groove which is 2.7 mm in width and 0.4 mm in depth is formed in the working surfaces so that the two web portions formed, acting as chamber walls, are each 0.2 mm in width. After 20 piston rings have been clamped together on a mandrel to form a cylindrical piston ring pack, the chambers are filled with molybdenum by a plasma spray process. After the projecting portions of the molybdenum coatings have been ground away, the actual heat treatment according to the invention is performed on the rings, by annealing the rings for five hours in air The drawing shows a view in cross-section through a piston ring 1 produced in this way, with a chamber filling comprising a molybdenum spray coating 2, and its two chamber web portions 3 and 4. The thermal treatment has caused fissures 5 to be formed in the coating, which fissures are completely filled with molybdenum oxide. The fissures 5 extend from the working surface 6 radially down to the bottom 7 of the chamber. None of the piston rings produced in this way showed any fractures of their coating, after a test run of 1 80 hours.

In the second example, the basic machine component used is the piston rings which are coated with molybdenum in a chambered form, in accordance with Example 1. After the piston ring pack has been briefly heated in an inert gas atmosphere at a temperature of 400"C, the heated ring pack is treated for ten minutes in a fused salt bath comprising a mixture of potssium and sodium thiocyanate at 1 90 C. The polished cross sections showed fissures in the spray coating, which were completely filled with molybdenum sulphide as the solid reaction product formed by the reaction between the molybdenum spray coating and the thiocyanate. None of the rings suffered from any coating fractures in an engine test lasting for 1 80 hours.

Claims (19)

1. A process for the production of machine components having a wear-resistant coating applied in a groove or recess in the component to withstand frictional forces wherein the coating, after application, is heat treated in the presence of a substance or substances which react with the coating mate riai to deposit solid reaction products in fissures formed in the coating material by the heat treatment, whereby the coating is under compression within the groove or recess when the component is at normal temperature.

2. A process as claimed in claim 1 in which the coating is a molybdenum-containing coating applied by a thermal spray process.

3. A process as claimed in claim 2 in which the heat treatment is carried out in air so that molybdenum oxide is deposited in the fissures.

4. A process as claimed in claim 1 in which the heat treatment is carried out in a fused salt bath.

5. A process as claimed in claim 2 in which the heat treatment is carried out in a fused thiocyanate bath to deposit molybdenum sulphide in the fissures.

6. A process as claimed in any of the preceding claims in which the component is a piston ring with a peripheral groove in which the coating is formed.

7. A machine component with a wearresistant coating produced by the method of any of claims 1 to 6.

8. A machine component having a wearresistant coating applied in a groove or recess of the coating to withstand frictional forces, wherein the coating has fissures filled with a solid reaction product resulting from heating of the component in the presence of a substance which reacts with the coating and wherein at normal temperature the coating is under compression resulting from the introduction of the solid reaction products into the fissures.

9. A machine component as claimed in claim 8 in which the coating is composed of molybdenum or a molybdenum alloy.

10. A machine component as claimed in claim 9 in which the solid reaction products comprise molybdenum oxide or sulphide.

11. A machine component as claimed in any of claims 8 to 10 in the form of a piston ring with a peripheral groove containing the coating.

1 2. A machine component as claimed in claim 11 in which the fissures extend generally radially with respect to the piston ring and the coating is under compression in a direction parallel to the axis of the ring.

1 3. A machine component which is loaded in respect of sliding friction, comprising a wear-resistant protective coating on the working surface, such as in particular a piston ring or a sealing strip in an internal combustion engine, comprising cast iron or steel, comprising a thermal spray coating on the working surface, of a construction which- ischambered at one or both sides characterised in that the wear-resistant protective coatings have fissures which extend in approximately a radial direction and which are completely filled with a solid reaction product which is formed in the fissures from the coating material when heated so that the coating has a compression stress acting in the axial direction in the rest condition at normal temperature.

1 4. A machine component according to claim 1 3 characterised in that the wear-resistant protective coating comprises an alloy containing from 50 to 100% molybdenum, which is applied by a thermal spray process.

1 5. A machine component according to claim 1 4 characterised in that the fissures are filled with molybdenum oxide which is formed by the reaction of the molybdenum with oxygen when heated in the fissures.

16. A machine component according to claim 1 4 characterised in that the fissures are filled with molybdenum sulphide which is formed by reaction of the molybdenum with sulphur-bearing compounds in the fissures when heated.

1 7. A process for the production of the machine components according to any one of claims 1 3 to 1 6 characterised in that, after application of the wear-resistant protective coating in a chambered-type form, with the simultaneous presence of substances which with the coating material form solid deposited reaction products, the machine components are heat-treated at a temperature which is above the subsequent operating temperature of the machine components.

1 8. A process according to claim 1 7 characterised in that the heat treatment is performed in a gas atmosphere.

19. A process according to claim 1 7 characterised in that the heat treatment is performed in a fused salt bath.