IMPROVEMENTS IN METAL COMPONENTS

Abstract: A steel component, for example the piston rod (6) of an hydraulic damper or shock absorber for a vehicle, has an external surface covered with a protective layer. The protec­ tive layer is formed by the steps of nitro-carburising the steel to an extent sufficient to produce an epsilon layer on the surface of the steel, and then applying a protective finish layer of a material, suitably a base/resin binder, on the epsilon layer.

Description: IMPROVEMENTS IN METAL COMPONENTS This invention relates to improvements in metal components of the kind having a surface covered with a protective layer. It is known to apply to exposed surfaces of metal components protective layers in the form of chromium plating. Such a process involves the use of poisonous chemicals and is expensive since, after plating, the protective layer must be polished and inspected for thickness, quality and the like. Also it is expensive to avoid the production of hard brittle nodules of chrome along with the desired smooth protective layer. Attempts to brush off such nodules can scratch the layer but if the nodules are left on, they can break off later, becoming embedded in any guide through which the component may be guided to slide. In addition prevention of rusting beneath the layer of plating cannot be guaranteed and rusting can occur as a result of the surface of the layer being scratched, for example during a finishing operation following plating. It is known that paints can be applied to a metal surface, suitably steel, for example by spraying or dipping in order to provide the component with a protective layer. However it is also known that surface preparation of the metal is essential if the paint is to adhere to the steel with sufficient reliability. For components to be used in applications in which they are guided to slide through guides, with high bearing loads, for example in the case of a piston-rod guided to slide through a bushing in one end of a cylinder, improved adhesion is required. Such surface preparations as are known are not good enough to adhere to the surface of components of that kind and are therefore not usually provided with a protective layer of paint. According to our invention a metal component of steel has a surface covered with a protective layer by the steps of nitrocarburising the steel to an extent sufficient to produce an epsilon layer on the surface of the steel, and then applying a protective finish layer of a suitable material on the epsilon layer. Conveniently the protective finished layer may be applied by spraying, dusting or dipping. The material to form the protective finish layer can be chosen from any of the compounds shown in the Tables below, and the epsilon layer is covered with the chosen compound in the finished component. An epsilon layer is very hard and since it is also porous, the porosity of the layer provides a means whereby the material adheres to the metal with a reliability sufficient to satisfy most requirements. The porosity causes the material to be "keyed" to the surface sufficiently to resist peeling and other damage. Preferably, after the formation of the epsilon layer, the steel is quenched in air before the formation of the protective finish layer. The formation of the epsilon layer can be achieved by any convenient known process. Our invention is particularly applicable to the manufacture of piston-rods for use in hydraulic dampers and shock-absorbers of the kind in which a piston working in a bore in a cylinder is carried by a piston-rod and the piston-rod itself is slidably guided through a bearing, suitably a seal or other sealing guide forming a closure, or a part of a closure, for one end of the cylinder. The rod may be guided through a seal of elastomeric material and at least the exposed portion of the rod, which projects from the cylinder, is covered with the protective finish layer as described above. This process is cheap and is easy to carry out. The protective finish layer, besides protecting the rod against corrosion, also serves to reduce friction. Our invention is particularly applicable to those components which exhibit, or are subjected to, a reciprocable motion in operation, for example to cylinders in which pistons work, and to the pistons themselves. A longitudinal section through an hydraulic suspension damper strut or shock absorber for a vehicle is illustrated in the single Figure of the accompanying drawings of which the external surface of the piston rod is covered with a protective layer. The suspension damper strut or shock absorber illustrated in the drawings is adapted for connection between sprung and unsprung parts of the vehicle, for example between the chassis or frame and an axle carrying a wheel. The strut comprises a cylinder 1 which is provided at its lower end with a bracket 2 by means of which the cylinder is connected to an axle of a vehicle. The cylinder 1 is also closed at that end by an end closure 3. A piston 4 adapted to work in the bore 5 of the cylinder 1 is carried by a piston-rod 6 manufactured from steel of the bar-type and the piston-rod 6 is itself slidably guided through a bearing 7 constituted by a closure for the other, upper, end of the cylinder 1. The closure 7 includes an axially extending skirt 8 which surrounds the rod 6, and a seal 9 of elastomeric material carried by the skirt adjacent to its inner end seals against the rod 6. Axially spaced seals 10 and 11, also of elastomeric material, are also provided in the closure 7 adjacent to its upper, outer, end, and with which the rod 6 also seals. The cylinder 1 is filled with hydraulic fluid to a level, indicated by the line 12, above the seal 9, and the space 13 above the hydraulic fluid is filled with gas or air under pressure to act as a, or to augment any other, suspension spring, and to accommodate the hydraulic fluid displaced by the volume of the pistonrod 6 as it enters, and is withdrawn from, the cylinder 1. The upper end of the piston-rod is provided with a connection, suitably a bushed eye, for connection to the chassis or frame of the vehicle. The piston 4 is provided with oppositely acting one-way valves or restrictors, not shown, through which hydraulic fluid can be displaced through the piston 4 in both directions in order to damp out oscillations between the sprung and unsprung parts. At least the portion of the external surface of the piston-rod 6 which projects from the closure 7 when the strut is fully extended is provided with a protective layer. The protective layer is produced by the steps of nitrocarburising the rod 6 to produce an epsilon layer on the said external surface, quenching the rod 6 in air, and then spraying, dusting or dipping the rod 6 with or in a suitable material to provide a protective finish layer on the epsilon layer. The material to form the protective finish layer, heat curing or air drying synthetic or natural resins, are shown in Table 1. These may be used alone, or in apporpriate combinations as solutions in organic solvents. TABLE 1 Base resins/binders 1. Acrylics 8. Polyvinyl chlorides 2. Alkyds 9. Polyamides 3. Maleic esters 10. Polyimides 4. Epoxides 11. Polyurethanes 5. Melamine formaldehydes 12. Silicones 6. Phenolics 13. Polyvinyl ethers 7. Polyvinyl butyrals 14. Ureaformaldehydes Corrosion protection may be enhanced by the incorporation into the resin solution(s) of Table 1, materials listed in Table 2 below. TABLE 2 Corrosion inhibiting additives 1. Zinc phosphate 2. Zinc chromate The frictional properties of the cured films can be suitably modified by the use of the film friction modifiers listed in Table 3 below. TABLE 3 Film friction modifiers 1. Silicone 4. Molybdenum disulphide 2. Waxes 5. Graphite 3. Polytetraflouroethylenes 6. Zinc stearate In a preferred embodiment, the protective finish layer is formed by application of a cured film of an acrylic/epoxide/amino resin system containing zinc chromate or zinc stearate or a wax. A silicone spray is then applied in order to reduce friction. We have found that in practice the finish layer may not soak into the epsilon layer. Rather, in the case of a piston-rod described above, the finish layer is pushed into the epsilon layer by the action of the rod being pressed against the bearing in use. It may therefore be preferable to "roll'r the finish layer into the epsilon layer on the rod during treatment.

Claims: CLAIMS 1. A metal component of steel having a surface covered with a protective layer characterised by the steps of nitrocaburising the steel to an extent sufficient to produce an epsilon layer on the surface of the steel, and then applying a protective finish layer of a suitable material on the epsilon layer. 2. A metal component according to claim 1, characterised in that the finish layer is applied by spraying. 3. A metal component according to claim 1, characterised in that the finish layer is applied by dusting. 4. A metal component according to claim 1, characterised in that the finish layer is applied by dipping. 5. A metal component according to any of claims 1 to 4, characterised by including the step of quenching the steel in air after formation of the epsilon layer but before the provision of the protective finish layer. 6. A metal component according to any preceding claim, characterised in that the protective finish layer is formed by application on the epsilon layer of a base resin/binder comprising any one of acrylics, alkyds, malaic esters, epoxides, melamine formaldehydes, phenolics, polyvinyl butyrals, polyvinyl chlorides, polyamides, polimides, polyurethanes, silicones, polyvinyl ethers and ureaformaldehydes. 7. A metal component according to claim 6, characterised in that corrosion protection is enhanced by incorporation into the resin/binder zinc phosphate or zinc chromate. 8. A metal component according to claim 6 or claim 7, characterised in that the friction properties of the protective finish layer are modified by the addition of a film friction modifier comprising any one of a silicone, waxes, polytetrafluoroethylenes, molybdenum disulphide, graphite and zinc stearate. 9. A metal component according to any preceding claim characterised in that the protective finish layer is formed by the application to the epsilon layer of a cured film of an acrylic/epoxide/amino resin system containing zinc chromate or zinc stearate or a wax. 10. A metal component according to claim 9, characterised in that a silicone spray is applied to the finish layer to reduce friction. 11. A metal component according to any preceding claim, characterised in that the component comprises a piston rod which carries a piston of an hydraulic suspension damper or shock absorber for a vehicle.

Inventors: GARFIELD, GLEN EDWARD, POWELL, GEORGE BERTRAM
Applicants: LUCAS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1981
Main classification: C23F17
IPC codes: C23C8, B05D3, C23C28, F16F9