SE519494C2 - Composite for coating piston rings, piston ring and method of making the composite - Google Patents

Abstract

A composite powder and a method of producing said powder, which powder comprises metal ceramic composite that consists of 25-75 per cent by volume of NiCr and of a hard phase, said hard phase being Cr2O3. A composite coating, especially intended for piston rings, which coating comprises metal ceramic composite consisting of 25-75 per cent by volume of NiCr and of a hard phase of Cr2O3. A piston ring which is coated with said composite coating.

Description

D.) Ul 519 494 2 properties to create resistance to the different types of wear are therefore increasingly being tested with varying success.

Another way to achieve a durable coating layer material is to use a composite material to form a layer with the desired properties. For example, US-3,914,507 describes a method for forming a variety of composite powders. Using the method in the above-mentioned patent document, a composite powder is formed by sintering an existing powder, which has a core of, for example, diatomaceous earth, chromium oxide or calcium fluoride and a coating of nickel, with an additional powder of, for example, chromium, which forms an alloy with the existing nickel coating using heat. A process that, according to the above-mentioned document, takes about 40-50 hours, which is of course a major disadvantage. Furthermore, the sintering method results in limited possibilities for alloying between nickel and chromium, whereby a large part of the nickel and chromium are found as separate phases even after sintering. This incomplete alloying means that the thermal stability of a powder-based layer of the powder material in question is not optimal.

Normally, thermal spraying of a powder mixture is used to coat a substrate with a layer. This method produces a structure on the composite layer with larger continuous streaks of a substance than is desirable. This also causes major problems with the adhesion of the particles in the powder to form the layer. The poorer adhesion due to the continuous streaks gives rise to crack indications, which lowers the strength of the layer. The thermal stability is also negatively affected. The problem of creating a composite layer for coating a substrate, which layer exhibits thermal stability during both manufacture and use and has high wear and corrosion resistance, therefore remains to be solved. 10 15 20 25 30 35 519 494 3 Summary of the invention The above disadvantages are initially solved by providing a homogeneous composite powder, which composite powder is described by the independent patent claim 1. The composite powder is provided by a method according to patent claim 9. Said composite powder is applied as a layer on a substrate. The obtained composite layer is characterized by the independent patent claim 3. The substrate coated with the composite layer preferably consists of a piston ring in accordance with claim 6. Preferred embodiments and methods according to the invention are apparent from the dependent patent claims.

By the above-described inventive method, a composite powder is produced, which consists of 25-75 volume percent NiCr and hard phase, which hard phase is Crjh. In accordance with the invention, NiCr refers to an alloy consisting of Ni and Cr.

Prior to coating the substrate, the composite particles are screened to select the desired size fraction of the composite powder for coating the layer. Depending on the process used to coat the substrate, a certain amount of the particles is removed or converted during the coating process, which is why the concentrations of the phases in the layer may differ compared to the primary concentrations of the phases for producing the composite powder.

Thus, the invention relates to a composite layer, particularly intended for piston rings, comprising ceramic composite characterized in that the ceramic composite consists of 25-75 volume percent NiCr and hard phase, which hard phase is Crgh.

The layer according to the invention thus consists of a metallic phase and a hard phase or ceramic phase. The layer provides an optimal combination of the properties of the constituent materials from a mechanical and tribological point of view. The layer formed has a corrosion resistance that is comparable to a layer of hard chromium. However, by combining NiCr with a hard phase from the above-mentioned group, a layer is obtained with significantly better thermal properties than, for example, hard chromium, which is a common wear layer on piston rings, which makes the layer according to the invention particularly suitable for coating said piston rings. The combination of a ductile phase and a hard phase in the layer provides very good resistance to abrasion, and at the same time offers a certain ductility. This ductility of the layer is a very great advantage, for example when used as a coating for piston rings.

In a preferred embodiment of the inventive layer, the hard phase in the composite layer consists of Cr2O3.

Chromium oxide has very good tribological properties. Another advantage of a layer of NiCr and Crfib is that a strong metallurgical bond is formed between the chromium in the metal phase and the oxide phase.

In a preferred embodiment of the invention, the composite layer has a porosity of 2-10 volume percent. The porosity in the layer means that there is a certain capacity of the voids in the layer to accommodate deformations in the material and thus avoid some cracking in the layer.

The layer is thus perceived as tougher and one speaks of a false ductility. An additional advantage of the pores is that they can act as lubricating oil buffers.

The composite layer can be applied to a substrate in various ways in accordance with known technology. Examples of known technology for coating a substrate with a layer are laser or other welding methods, plasma spraying and HVOF methods, which will not be described in more detail here.

The invention also relates to a piston ring, which is provided with a coating layer, comprising ceramic composite characterized in that the ceramic composite consists of 25-75 volume percent NiCr and the hardening phase is Crgh. The proportion of pores in the coating layer of the piston ring is 2-10 volume percent.

The invention also relates to a method for producing a composite powder, characterized in that an aqueous suspension, comprising a mixture of 25-75 volume percent NiCr and a hardening phase, which hardening phase consists of 10 l5 20 25 30 U) Uï 519 494 5 Crgh is produced, particles are obtained from said suspension and that said particles are processed with a drying method to form the composite powder.

An advantage of the above method is that by producing the composite powder, a powder is produced which, after being applied as a composite layer on a substrate, forms a composite layer which has good thermal and corrosion resistance and high wear resistance. Subsequent application of the composite powder to the substrate, so that a layer with all three desired properties is formed, is of course simplified when it can be achieved with a single material. In addition, the method according to the invention enables the powder to be formed from a stable slurry so that homogeneous and good particles or granules are formed.

According to the inventive concept, a method is intended in which the particles are obtained from the suspension by dropping the suspension into a cooling medium, whereby cooled particles are formed. This method is hereinafter referred to as freeze granulation. This freeze granulation is achieved, for example, by spraying the formed slurry directly into liquid nitrogen, whereby the sprayed droplets instantaneously form frozen spherical particles. According to the inventive method, the drying method for forming the composite powder of the particles comprises freeze drying.

Freeze granulation of the powder material has a number of advantages over conventional methods, of which the following advantages deserve mention.

The frozen droplets in freeze granulation have the same composition as the original slurry. This avoids migration of fine particles and any binder during drying, which is a problem with alternative drying methods such as evaporation. The migration gives rise to inhomogeneous granules. Very high yield of material in the process is another advantage of the freeze granulation method. In addition, the particle density can be controlled by means of the dry content (amount of powder) in the slurry.

According to a further aspect of the method, the method for preparing the composite powder of NiCr and the hardening phase comprises spray drying. The method of spray drying the particles is a further alternative for preparing the powder. The above-mentioned preparation methods should only be seen as examples of preferred methods for preparing the composite powder, which methods should not be considered limiting to the inventive concept, but also other methods for preparing the composite powder according to the invention are conceivable.

The invention also relates to a method for producing said composite powder, in which a dispersant in the form of a charged polymer is added to the suspension. In order for the particles to have the best possible properties, it is important that the product on which the process is based is homogeneous and well dispersed. An example of a negatively charged polymer that is suitable as a dispersant is polyacrylic acid. The advantage of using polymer as a dispersant during the production of the powder in question is that it is heated away from the material when the layer is applied as a coating on the substrate.

The invention also relates to a method for producing said composite powder, in which a binder is added to the suspension. The choice of binder is made with a view to making the particles as strong as possible. An example of a binder is polyvinyl alcohol (PVA). In order to prevent the binder from becoming too hard and brittle when the powder dries, various plasticizers or (PEG) can be added to a plasticizer. A further advantage of using plasticizers is that they are heated away from the material in connection with heating, for example when the layer is applied as a coating on the substrate. lO 15 20 25 30 u) UI 519 494 7 To avoid the chromium oxide forming continuous streaks in the layer, in which streaks cracks can arise and grow due to low ductility, the dried particles can be pre-sintered at a temperature of 1000-1500° C for 30-90 minutes. These streaks can partly arise due to the composite particles disintegrating during the spraying process itself. The process of pre-sintering the dried particles at a temperature of 1000-1500° C for 30-90 minutes improves the internal strength of the composite powder, which is a way of reducing the risk of disintegrating in connection with coating the composite powder into a layer.

Description of a preferred embodiment A preferred embodiment of the inventive method and products produced by means of the inventive method will be described in the following. The described embodiment is not to be seen as limiting but is given by way of example only.

As a starting point in the production of a composite powder according to a method according to the invention, an aqueous suspension is suitably used. A preferred method comprises, in simplified form, the following steps. A powder mixture with the distribution of 60 vol% NiCr and 40 vol% Crgh is added to the aqueous suspension. In this case, NiCr refers to an alloy consisting of about 80 wt% Ni and about 20 wt% Cr. Dispersant is added to the aqueous suspension. Stirring is then carried out to obtain a homogeneous slurry. Binder is added. In a preferred method, plasticizer is added. The suspension obtained is further mixed. According to a preferred method of the invention, the suspension is then spray-frozen and freeze-dried, whereby particles are formed. In order to obtain the best possible results when coating a substrate with the obtained composite powder, the powder is sieved so that a suitable size fraction of the powder is obtained. This size fraction is preferably 40-100 µm. Particles larger than 150 pm cannot withstand plasma spraying using today's methods.

The invention will now be described in more detail with the aid of a preferred embodiment, which relates to a method for producing said composite powder, in which a dispersant in the form of a charged polymer is added to the suspension. In order for the particles to have the best possible properties, it is important that the product on which the process is based is homogeneous and well dispersed. An example of a negatively charged polymer that is suitable as a dispersant is polyacrylic acid.

The advantage of using polymer as a dispersant during the preparation of the powder in question is that it is heated away from the material when the layer is applied as a coating on the substrate. The amount of dispersant should be adjusted to achieve as many particles as possible in the size fraction 45-95 µm, which is the most optimal size for plasma spraying.

In a preferred embodiment for the production of said composite powder, a binder is added to the suspension. The choice of binder is made with a view to making the particles as strong as possible. An example of a binder is polyvinyl alcohol (PVA). In order for the binder not to become too hard and brittle at the same time when the powder dries, a plasticizer is added. Polyethylene glycol (PEG) is an example of a plasticizer.

In accordance with the inventive concept, a method is intended in which particles are obtained from the suspension by dropping the suspension into a cooling medium, whereby cooled particles are formed. This method is called freeze granulation and also includes subsequent freeze drying, whereby the water content in the frozen granules is sublimed. This freeze granulation is achieved, for example, by spraying the formed slurry directly into liquid nitrogen, whereby frozen spherical particles are formed instantaneously.

The composite powder according to the invention consists of 25-75 volume percent NiCr before sieving, but more preferably 50-70 volume percent NiCr, and most preferably about 60 volume percent NiCr. The remaining portion of the composite powder consists of a hard phase, as a complement to the first portion of NiCr, which hard phase is Cr2O3.

To coat a substrate with the inventive composite material, a known technique such as an HVOF method, laser, welding or plasma spraying is suitably used. During the coating process, the composite powder is exposed to a very high temperature which may change the chemical composition somewhat.

Optionally, the piston ring can be preheated and/or post-heated in a conventional manner in an ordinary oven, before and after the coating process is carried out, respectively.

After sieving the composite powder and coating a substrate, a layer is obtained in accordance with the invention. The distribution in the layer is suitably 2-10 volume percent pores, but more preferably 3-8 volume percent pores and most preferably 4-7 volume percent pores. The proportion of NiCr is suitably between 25 and 75 volume percent, but more preferably between 30 and 55 volume percent and most preferably between 35 and 45 volume percent. The substantially remaining part of the layer containing other than pores and NiCr is made up of the hard phase.

A composite layer according to a preferred embodiment of the invention comprising NiCr and chromium oxide has been shown to have a dry abrasion resistance equivalent to or better than hard chromium. The layer thickness in one embodiment is between 0.1-1.2 mm thick, but more preferably between 0.5 and 1.0 mm and most preferably between 0.7 and 0.8 mm.

Adhesion to the substrate has been found to be equivalent to or better than hard chrome, as has its passivity in highly corrosive environments. The porosity of the layer is 2-10%. Particularly well suited for coating piston rings.

All the above properties show that the layer is Hardness measurements according to the Rockwell method show a hardness of the layer of about 65 HR 62.5 kg. After heat treatment at 500 °C for 24 hours, the layer surprisingly shows an increase in hardness to about 70 HR 62.5 kg.

In this application, powder material refers not only to conventional powder materials but also to those that have been mixed with a binder to obtain a creamy consistency, which can be an advantage when applying the powder material to the piston ring blank.

The inventive method thus provides a ceramic chrome coating whose properties are equivalent to or better than the coatings currently available. It is also understood that the features described above can be combined in various ways to provide further embodiments within the scope of the invention.

The process according to the invention and the products formed are thus not limited to what has been described.

For example, the coating of the substrate can be achieved in a number of ways. The choice of dispersant, binder and plasticizer can also be varied without departing from the inventive concept. The invention can thus be further modified in a number of ways within the scope of the inventive concept and the following patent claims.

Claims (12)

lO 15 20 25 30 h) UI 519 494 ll PATENT REQUIREMENTS Composite layer, specially designed for piston rings comprising ceramic composite kšànzietzecïkziat: in that the ceramic composite consists of 25 to 75% by volume of NiCr and hard phase, which hard phase is Cr fl b. A composite layer according to claim 1, which has a porosity of 2 to 10% by volume. 3. Piston ring, which is provided with a coating layer, comprising ceramic composite} <ä11n 75% by volume of NiCr and hard phase, which hardening phase is Cr2O¿ Piston ring according to claim 4, which has a coating layer with a porosity of 2-10% by volume. Process for the preparation of composite powder, characterized in that an aqueous suspension, comprising a mixture of 25-75% by volume of NiCr and hard phase, which curing phase is Cr is formed and that said particles are processed with a drying method to form the composite powder. A method according to claim 5, wherein the drying method for forming the composite powder of the particles comprises freeze-drying. A process according to any one of claims 5-6, wherein the suspension is added to a dispersant in the form of a charged polymer. A method according to any one of claims 5-7, in which a binder is added to the suspension. A method according to any one of claims 5-8, wherein the plasticizer is added to the suspension. A method according to any one of claims 5-9, wherein the dried particles are pre-sintered at a temperature of 100-1500 ° C for 30-90 minutes. 10 519 494 12 11. ll. Composite powder characterized in that it is prepared according to a process according to any one of claims 5 to 10. A composite layer, in particular for piston rings, comprising the ceramic composite characterized in that the composite layer is formed of a composite powder prepared by a process according to any one of claims 5-10.