GB2227755A

GB2227755A - Improving the wear resistance of metallic components by coating and diffusion treatment

Info

Publication number: GB2227755A
Application number: GB8828706A
Authority: GB
Inventors: Allan Matthews
Original assignee: University of Hull
Current assignee: University of Hull
Priority date: 1988-12-08
Filing date: 1988-12-08
Publication date: 1990-08-08
Anticipated expiration: 2008-12-08
Also published as: GB2227755B; GB8828706D0

Abstract

A process for improving the wear and corrosion resistance of metallic components entails forming a compound phase within the metallic component by a RF plasma assisted technique and depositing a ceramic coating on the compound by means of a DC or RF plasma assisted technique. The compound phase may be a nitride, carbide, oxide or boride compound. The deposition of the ceramic coating may follow formation of the compound phase. Alternatively the compound phase may be formed after deposition of the ceramic coating, provided.

Description

A PROCESS FOR IMPROVING THE WEAR AND CORROSION RESISTANCE OF METALLIC COMPONENTS The present invention relates to a process for improving the wear and corrosion resistance of metallic components. More specifically the present invention relates to a process for forming a compound phase within a metal substrate and depositing a ceramic coating on the substrate There have been many reports commissioned by government and industry which demonstrate the cost of wear and corrosion of metallic components, and as energy and material resources become increasingly scarce much attention is being given to improving the wear and corrosion resistance of metallic components.

Furthermore, many technological developments are surface-limited, i.e. it is friction, wear and corrosion which limit improvements in machine efficiency and working life in many areas of industry and progress is very much dependent upon the availability of special wear and corrosion resistant surfaces. Applications such as this have increased interest in the branch of technology known as "Surface Engineering" which seeks to maximise both the bulk properties and the surface properties of materials - requirements which are sometimes mutually exclusive. For example, toughness may be required in the bulk and hardness at the surface, properties which are not usually found in a single material.

There has been much progress in recent years in the development of techniques to produce hard coatings on surfaces, principally by ionisation assisted physical vapour deposition (PVD) methods. However, the coatings produced by these methods are typically quite thin (usually < 0.5mm) and they thus require sufficient support from the underlying substrate material to ensure that major surface deformation does not occur under load.

Nitriding of steel is an established technique and produces hard, wear resistant surface layers. Traditional nit riding by gases containing ammonia, in the temperature range 500 to 1000 C, is a well known industrial process, but recently much attention has been directed to ion and plasma nitriding. These allow close control over the nature and structure of the nitrided layer and especially with the plasma method high rates of nitriding have been claimed. Conventionally the component is exposed as the cathode to a dc glow discharge in nitrogen.

Unfortunately, a problem commonly encountered with dc nitriding is surface contamination. This could cause a problem if one wished to coat the substrate following nitriding as the spurious deposits would have a detrimental effect on the adhesion properties of the coating.

It is an object of the present invention to provide a process for producing a good load supporting layer within the surface of a metallic substrate and a wear resistant coating which has good adhesion properties.

According to the present invention there is provided a process wherein a compound phase is formed within a metallic substrate by an RF plasma assisted technique and a ceramic coating is deposited on the substrate by means of a DC or RF plasma assisted technique.

The compound phase may be a nitride, carbide, oxide or boride compound. Typically deposition of the ceramic coating follows formation of the compound phase, but it is possible to forum the compound phase after deposition of the ceramic coating provided the ceramic compound does not prevent diffusion of the compound phase-forming gas into the substrate.

Preferably, the coating is formed using an RF plasma assisted technique.

The process of the present invention will now be described, the examples used to illustrate the process being non-limitive.

The metallic samples to be coated are connected to an electrical feedthrough within a vacuum chamber, which is evacuated to a suitable "base" pressure (typically < 10-5 Torr). The pumping speed is then reduced and an inert gas such as argon is introduced into the chamber to increase the chamber pressure to within the range 10 to 20 mTorr. A negative electrical RF bias is then applied to the samples which initiates a glow discharge within the chamber. In conventional and well understood fashion this glow discharge sputter cleans the samples and removes any contaminants at the surface of the samples.

In the process of the present invention the glow discharge within the chamber is enhanced, typically by thermionic means, to increase the ion current to the samples. This facilitates substrate temperature control, and also allows the process to be carried out at a relatively low chamber pressure, thus providing relatively long mean free paths for the evaporative material, while still retaining a high ionisation efficiency. Indeed, using this arrangement it is possible to shorten or even dispense with the initial thermionically assisted or enhanced sputtering stage. It is still beneficial however to carry out the initial heating under inert gas conditions to assist in contaminant removal. In some cases, the use of hydrogen as the support gas has been found to be beneficial in removing unwanted layers.Cleaning of the samples is important in that it can remove layers which can act as diffusion barriers in the next stage in the process.

The next stage in the process is the admission of a suitable gas, such as nitrogen, to produce a compound layer or phase in the sample surface. By way of example, where the sample is comprised of a ferrous material then iron nitrides will be produced, where it is titanium or a titanium alloy then nitrides of titanium or other alloy elements can be produced or the gas may diffuse into the metal to remain at interstial or substitutional sites within the metallic crystals. The benefit of the thermionic support arrangement is that it allows close control of the discharge power to the samples. The ion current can be increased and the bias potential on the samples can be reduced (even to less than lOOV) which allows temperature control.Also the cathode fall distance or "sheath" thickness around the sample which determines the uniformity of the electrical field can be reduced. This leads to more uniform bombardment of the sample and thus more consistent layer formation. In nonenhanced plasma systems the pressure would have to be increased to achieve the same effect and this can lead to increased contamination of the sample. It has been shown that the use of an RF bias on a sample avoids the formation of so called "white layer" compound on the surface of ferrous materials during nitriding. This, of course, is very important for subsequent coating of the samples as such a layer would adversely affect coating adhesion.

For the most effective discharge enhancement during compound layer formation the thermionic electron emitter should be at or near earth potential, with a further electrode placed in the chamber biased positively with respect to the chamber walls (e.g. at lOOV). This arrangement has been found to provide the greatest increase in ion current to the samples.

Typically, the production of a suitable compound layer or phase will take more than two hours. For example, at a sample temperature of 500 C in a typical nitriding steel such as HlD a compound layer of over locum thickness was produced over this period of time using this arrangement. The sample bias voltage was 100V, and the current density was 5mA/cm2. The chamber pressure was 5mTorr.

After the samples have been prenitrided a coating is formed on them. For the coating deposition stage, the most appropriate arrangement is to have the thermionic source biased negatively with respect to the chamber walls, again to maximise the ion current to the sample.

Coating deposition will normally follow the production of the compound layer, but the latter can be grown through the coating if it offers a sufficiently low diffusion barrier to the appropriate gases, (e.g. nitrogen, oxygen, acetylene, methane, etc.). The method used to produce the coating need not, of course, be limited to the thermionically supported system described above (which often is used with an electron beam gun to vaporise the metallic element within the coating). Other vaporisation systems such as the hollow cathode gun or the arc source can equally be used. RF or DC bias can be used.

The benefits of the process of the present invention can best be described with reference to the use of titanium components. Titanium has very good corrosion resistance and is lightweight. However, it has generally poor wear resistance and low hardness. By producing a 200um nitrided layer in a titanium substrate and then coating this with a 5um layer of titanium nitride, remarkable and significant improvements in wear and corrosion resistance are achieved. The friction coefficient is reduced from 0.5 for titanium rubbing against titanium to less than 0.2 for two surfaces treated in accordance with the process of the present invention rubbing together. The process is particularly beneficial as it allows the entire duplex treatment to be carried out in one coating chamber - thereby avoiding rejigging complications and problems associated with recontamination. Furthermore, samples do not need to be reheated twice, thereby saving on energy consumption.

Claims

1. A process wherein a compound phase is formed within a metallic substrate by an RF plasma assisted technique and a ceramic coating is deposited on the substrate by means of a DC or RF plasma assisted technique.

2. A process according to claim 1, wherein the compound phase is a nitride, carbide, oxide or boride compound.

3. A process according to claim 1 or 2, wherein the deposition of the ceramic coating follows formation of the compound phase.

4. A process according to claim 1 or 2, wherein the compound phase is formed after deposition of the ceramic coating, provided the ceramic coating does not prevent diffusion of the compound phase forming gas into the substrate.

5. A process according to any preceding claim wherein the coating is formed using an RF plasma assisted technique.

6. A process according to any preceding claim, wherein the process is carried out within a vacuum chamber and the glow discharge within the chamber is enhanced to increase the ion current to the metallic substrate.

7. A process according to claim 6, wherein the thermionic electron emitter of the vacuum chamber is at or near earth potential, and a further electrode is provided in the vacuum chamber biased positively with respect to the chamber walls.

8. A metallic component treated according to the process of any of claims i to 7.